Homework answers / question archive / The answer will be : Clear summery and Paraphrasing inclode references CCE Harvard style Provide some figures/ pictures and diagrams with reference Each paper summary should be minimum in 2 pages (including the discussed problem or issue ,methodology, result and finding , recommendation ) Bryan Research and Engineering, Inc

The answer will be : Clear summery and Paraphrasing inclode references CCE Harvard style Provide some figures/ pictures and diagrams with reference Each paper summary should be minimum in 2 pages (including the discussed problem or issue ,methodology, result and finding , recommendation ) Bryan Research and Engineering, Inc

Sociology

Share With

The answer will be :

• Clear summery and Paraphrasing
• inclode references CCE Harvard style
• Provide some figures/ pictures and diagrams with reference
• Each paper summary should be minimum in 2 pages (including the discussed problem or issue ,methodology, result and finding , recommendation )
• Bryan Research and Engineering, Inc. - Technical Papers Design Glycol Units for Maximum Efficiency VINCENTE N. HERNANDEZ-VALENCIA, MICHAEL W. HLAVINKA, JERRY A. BULLIN, Bryan Research & Engineering, Inc., Bryan, Texas ABSTRACT In designing dehydration units for natural gas, several critical parameters exist which can be varied to achieve a specified dew point depression. This paper studies the effects of varying the glycol flow rate, number of stages in the contactor, reboiler temperature, and stripping gas rate on water content in glycol dehydration units. The effect of high carbon dioxide composition in the feed is also presented. Finally, the emissions of aromatic (BTEX) and other VOC's from the regenerator and flash will be considered. Proceedings of the Seventy-First GPA Annual Convention. Tulsa, OK: Gas Processors Association, 1992: 310-317. Bryan Research & Engineering, Inc. Visit our Engineering Resources page for more articles. INTRODUCTION A common method to remove water from natural gas is glycol dehydration.1 In this process, triethylene glycol (TEG) or diethylene glycol (DEG) is used to remove the presence of water in the gas stream. Water vapor can cause hydrate formation at low temperatures and high pressures or corrosion when it is in contact with hydrogen sulfide (H2S) or carbon dioxide (CO2), components regularly present in the gas stream. Glycol dehydration units are typically represented by a contactor, a flash tank, heat exchangers, and a regenerator, as shown in Figure 1. The glycol, usually TEG, enters at the top of the contactor and absorbs water as it progresses toward the bottom of the column. A dry gas exits at the top of the contactor and may be used for cooling the incoming lean glycol. Figure 1. Typical glycol dehydration unit. Copyright 2006 - All Rights Reserved Bryan Research and Engineering, Inc. Page 1 of 12 Bryan Research and Engineering, Inc. - Technical Papers The rich stream flows to a separator or flash tank where gaseous hydrocarbons that were absorbed along with some of the water in the contactor are liberated and used as fuel. Finally, the glycol flows to the stripper where it is regenerated by boiling off the water and returned to the contactor. For processes requiring gas with very low water dew points, a stripping vapor will most likely be needed to aid the regeneration process. This technique is illustrated by the region enclosed in the dotted line in Figure 1. For maximum stripping, this vapor is normally injected into a short column at the bottom of the reboiler. However, the gas may also be introduced directly into the reboiler. In the past years, glycol dehydration plants have been designed using rule-of-thumb procedures. While still sufficient for many applications, today more efficient designs are often required. In many cases the plant feed will contain small quantities of aromatic hydrocarbons that are quite soluble in the TEG. The aromatics are primarily comprised of benzene, toluene, ethylbenzene, or xylenes (BTEX). These aromatics are carried to the flash tank where a small fraction are released along with other volatile organic compounds (VOC’s). The remaining VOC’s and aromatics travel to the regenerator where the application of heat will remove virtually all volatile gases. Since the regenerator is usually vented to the atmosphere, the plant may have serious environmental impact, even in small plants where the total aromatic emissions can easily exceed 100 lb/day.2 Other problems have also caused special concern in glycol dehydrator design over the past few years. Enhanced oil recovery using CO2 and dehydration of refinery gases have required glycol units that can properly dehydrate gases containing a few percent CO2 to pure CO2. In addition to the effect on dehydration, high quantities of CO2 in the feed can accelerate corrosion in the regenerator. On the other hand, the CO2 can act as a stripping vapor in the regenerator.1 This paper presents the progress of current work to modernize simulation software used in design and optimization of dehydration units. The results provide an analysis of the dehydration effectiveness at a variety of common operating variables for a typical dehydration facility. Next, the effect of carbon dioxide in the plant feed will be presented. Finally, emission calculations of aromatic and non-aromatic hydrocarbons absorbed in the glycol contactor are presented. PROCESS DESCRIPTION All calculations provided in this report are based upon calculations made with Bryan Research & Engineering’s general purpose process simulator, PROSIM. The results are obtained in an effort to update and expand the capabilities of the BR&E dehydration program, DEHY. PROSIM allows the user to draw the process flow diagram on the computer screen and enter the operating parameters on "pop-up" forms. The program is flexible to handle almost any glycol dehydration scheme. Additionally, the program has heavy ends/crude characterization, complex heat exchanger, tray rating, and a variety of utility calculational operations. Dehydration results obtained by PROSIM have been compared to plant data obtained by Worley in the GPSA Engineering Data Book and other pertinent experimental vapor-liquid equilibrium data as noted throughout this report.3 In order to provide the results needed for this paper, a base set of operating parameters was selected as presented in Table I. Except when noted, these parameters are held constant throughout the analyses presented here. The process flow scheme is basically that presented in Figure 1 except for cases where stripping gas was not utilized. In all runs, the amount of water in the feed was determined by PROSIM so that the entering gas would be at the water dew point. Table I Base dehydration unit operation parameters used in this analysis. Inlet gas temperature Inlet gas pressure Inlet gas composition: Methane Ethane Propane n-Butane 90oF 500 psia 85.1 mol% 8.5 mol% 3.8 mol% 1.9 mol% Copyright 2006 - All Rights Reserved Bryan Research and Engineering, Inc. Page 2 of 12 Bryan Research and Engineering, Inc. - Technical Papers n-Pentane Lean glycol temperature Rich glycol flash pressure Regenerator pressure Equilibrium trays in contactor 0.7 mol% 90oF 65 psia 1 atm 2 DESIGN OF DEHYDRATION UNITS When optimizing the design of dehydration facilities, the impact of the following parameters should normally be considered: z z z z z Number of trays in the contactor Glycol circulation rate through the contactor Temperature of the reboiler in the regenerator Amount of stripping gas used, if any Operating pressure of the regenerator Of the above parameters, only the first four are normally considered as variable parameters. The first two parameters affect the approach to equilibrium at the top of the absorber while the third and fourth dictate the value of the equilibrium water content by limiting the purity of the lean glycol to the absorber. The last parameter affects the lean glycol purity in a manner similar to reboiler temperature. However, the vast majority of units are vented to the atmosphere so this parameter is beyond control. In addition to the design parameters listed above, several other factors influence the residual water content of the sales gas. However, often these factors are fixed and cannot normally be changed when optimizing a unit. First, the temperature of the inlet gas will dictate the total amount of water fed to the unit. Lower plant inlet gas temperatures will require less water to be removed by the glycol. Second, lean glycol temperature at the top of the contactor will affect the water partial pressure at the top stage. Consequently, high glycol temperatures will result in high water content in the overhead gas. However, this temperature is normally no cooler than 10oF above the inlet gas to prevent hydrocarbons in the feed from condensing in the solution. This limit is normally maintained by a gas/glycol exchanger that cools the lean glycol to approximately a 10 oF approach using the dry gas. Other parameters in the plant have limited or no effect on the dry gas water content. The number of equilibrium stages in the regenerator has only a slight effect on the lean glycol purity. Equilibrium at the reboiler temperature and pressure is approached in the reboiler so that additional stages have no effect. Operating temperature of the lean/rich glycol exchanger only significantly impacts the reboiler heat duty. ANALYSIS OF PERTINENT DESIGN PARAMETERS AND OTHER FACTORS In order to investigate the important design variables, plots of the residual water content versus circulation rate are presented in Figures 2 to 6 for common values of the parameters discussed in the previous section. Case 1. Effect of Number of Equilibrium Stages in Absorber Figure 2 illustrates the effect of the number of equilibrium contact trays on residual water content using a 400 oF reboiler. Figure 3 presents a similar figure comparing dew point depressions instead of actual water content. Increasing the number of trays allows the gas to approach equilibrium with the lean glycol at a lower glycol circulation rate. Considering a typical glycol circulation rate of approximately 3 gal TEG/lb water removed, Figures 2 and 3 illustrate that a three equilibrium-stage contactor is virtually at equilibrium with the inlet glycol. In a two stage contactor, a circulation rate of 5 to 6 gal TEG/1b water would be required to approach equilibrium. Significantly higher flow rates would still be required when only one ideal stage is used. Copyright 2006 - All Rights Reserved Bryan Research and Engineering, Inc. Page 3 of 12 Bryan Research and Engineering, Inc. - Technical Papers Figure 2. Effect of the number of equilibrium stages in the contactor on the water content of a stream of sweet natural gas. Reboiler temperature 400oF. Figure 3. Effect of the number of equilibrium stages in the contactor on the dew point depression of natural gas. Reboiler temperature 400oF. Case 2. Effect of Reboiler Temperature Figures 4 to 6 illustrate the overhead water content using a fixed number of equilibrium stages at reboiler temperatures of 360, 380, and 400 oF. For two or three equilibrium stages, pipeline quality gas containing less than 7 lb water/MMscf gas could be produced using either a 380 or 400 oF reboiler. At 380 oF, approximately 4.5 Copyright 2006 - All Rights Reserved Bryan Research and Engineering, Inc. Page 4 of 12 Bryan Research and Engineering, Inc. - Technical Papers gal TEG/1b water circulation would be needed with two stages as opposed to approximately 2 gal TEG/lb with three stages. Similarly, at 400 oF, approximately 3 gal TEG/1b water circulation would be needed with two stages as opposed to 1.5 gal TEG/lb with three stages. Note that these results are for the inlet gas temperature of 90 oF. Higher temperatures would result in higher residual moisture at the same circulation rate. The reboiler temperature influences the overhead water content by changing the purity of the lean glycol. Glycol purities of 98.0, 98.5, and 98.8 wt % are obtained at 360, 380, and 400 oF, respectively, at one atmosphere pressure. Figure 4. Water content for a natural gas stream treated by a one equilibrium-stage contactor. Figure 5. Water content for a natural gas stream treated by a two equilibrium-stage contactor. Copyright 2006 - All Rights Reserved Bryan Research and Engineering, Inc. Page 5 of 12 Bryan Research and Engineering, Inc. - Technical Papers Figure 6. Water content for a natural gas stream treated by a three equilibrium-stage contactor. Case 3. Effect of Stripping Gas As stated earlier, applications requiring high dew point depressions will virtually always utilize stripping gas in the regenerator. Low dew points simply cannot be achieved using the maximum 98.8 wt % glycol obtainable with a 400 oF reboiler at atmospheric pressure. These low dew points will need up to 99.9 wt % glycol in the absorber. Increasing reboiler temperature is not an option due to the thermal decomposition temperature of 404 oF for TEG. Even a 400 oF reboiler can result in glycol decomposition due to higher film temperatures. Further, stripping gas has a much greater effect than increasing reboiler temperature. For maximum efficiency, stripping gas should be introduced in a short column after the hot glycol is removed from the reboiler. Stripping gas may be place directly in the reboiler, but the high water partial pressure in the vapor space limits the mass transfer driving force. Figures 7 and 8 illustrate the effect of stripping gas on residual water content and dew point depression of the dry gas. As can be seen, even small stripping gas rates of 1 scf/gal circulated solution have a pronounced difference. With this stripping gas rate, the dry gas will contain about half the water of the same process without stripping gas. Increasing the stripping gas rate beyond 2 to 3 scf/gal will have little impact on dew point depression. Copyright 2006 - All Rights Reserved Bryan Research and Engineering, Inc. Page 6 of 12 Bryan Research and Engineering, Inc. - Technical Papers Figure 7. Effect of stripping gas rate on water content of a natural gas stream. Figure 8. Effect of stripping gas rate on dew point depression, at three gal TEG/lb H2O. Case 4. Impact of Carbon Dioxide In this phase of the work, a plant with significant CO2 in the feed was modeled using PROSIM. The parameters in Table I were used as in the previous section but a total 50 mol % CO 2 was added to the plant feed. The amount of water in the feed was calculated by the program so that the gas remained at the dew point. Figure 9 illustrates the effect of this CO2 concentration on the dehydrated gas water content. The results indicate Copyright 2006 - All Rights Reserved Bryan Research and Engineering, Inc. Page 7 of 12 Bryan Research and Engineering, Inc. - Technical Papers that the addition of CO2 slightly increased the water content about 1 lb water/MMscf gas at all glycol circulation rates. The most substantial data compilations for CO2 - TEG - Water systems are that of Jou, Deshmukh, and Mather, and Takahashi and Kobayashi in GPA report TP-9.4,5 In the GPA report, the water content of a natural gas mixture and of CO2 is presented at various temperatures, pressures, and glycol concentrations. These data indicate that pure CO2 water content above glycol solutions is slightly less than in natural gas at 865 psia and 565 psia. At 565 psia and 10 oF, the difference in water content of CO2 and natural gas above glycol solutions of less than 5 wt % water is insignificant. At 765 psia, the report indicates that the water content of CO2 is greater than that of natural gas for temperatures greater than 75 to 100 oF. For the most part, the difference in water content between pure CO2 and natural gas is very small at conditions presented in the report. Therefore, the slight discrepancy between the program and the data is probably within the accuracy that can be expected for such complex systems. However, work is still being performed to investigate this difference. Figure 9. Water content of a natural gas stream with 50 mol % CO2, treated by a two equilibriumstage contactor. Case 5. Aromatic and VOC Emissions in TEG Dehydration In the last phase of the work, estimates of aromatic and VOC emissions from the regenerator and flash tank were calculated. According to other work, the total aromatic concentration in natural gas streams is normally well less than 0.05 to 0.1 mol %.2 In order to give an approximation for the maximum amount of aromatics emitted per day, the parameters in Table I were used with the addition of 0.1 mol % total aromatic compounds. The actual ratios used for benzene:ethylbenzene, toluene:ethylbenzene, and o-xylene:ethylbenzene were 8:8, 13:3, and 12:6, respectively. These ratios were based on a feed analysis obtained for an operating plant. For this comparison, a feed flow rate of 20 MMscfd was taken. The total non-aromatic and aromatic hydrocarbon emissions versus glycol circulation rate for the flash tank are presented in Figures 10 and 11, respectively, for contact pressures of 200, 500, and 900 psia. Figure 10 also includes the total non-aromatic emissions for the case with the same feed composition without aromatics at 500 psia to illustrate the slight increase in solubility in the presence of aromatics. Copyright 2006 - All Rights Reserved Bryan Research and Engineering, Inc. Page 8 of 12 Bryan Research and Engineering, Inc. - Technical Papers Figure 10. Total non-aromatic VOC emissions from flash tank. Contactor temperature 90oF. Figure 11. Total aromatic emissions at the flash tank. Contactor temperature 90oF. The total non-aromatic emissions at the regenerator are presented in Figure 12. The emission rates for benzene, o-xylene, and total aromatics from the regenerator are given in Figures 13 to 15, respectively. Figure 13 indicates that the emission rate for benzene increases with glycol circulation rate and contact pressure. However, Figure 14 illustrates that the o-xylene emission rate at 900 psia and circulation rates greater than 3 gpm is smaller than at 500 psia for the same scenario. The only substantial data that exist on aromatic solubility in TEG are those of Robinson. In this compilation, the equilibrium K ratio (K=y/x) is shown to decrease with pressure for benzene and toluene as is the case with most volatile components. However, for ethylbenzene and o-xylene, the ratio is shown Copyright 2006 - All Rights Reserved Bryan Research and Engineering, Inc. Page 9 of 12 Bryan Research and Engineering, Inc. - Technical Papers to increase with pressure at certain temperature/glycol concentrations and to decrease with pressure at others. Most notable is that at temperatures in the range of the current scenario and relatively dry glycol concentrations where the K value increases with pressure. Since drier glycol would be present in higher circulation cases, the increase in K with pressure causes a decrease in solubility for ethylbenzene and o-xylene at high pressure. Since the aromatic portion of the feed is primarily o-xylene on a molar basis (and even higher on a mass basis), the total emission rate presented in Figure 15 will also exhibit this phenomenon at higher circulation rates. Figure 12. Total non-aromatic VOC emissions from regenerator. Contactor temperature 90oF. Figure 13. Benzene emissions at the regenerator. Contactor temperature 90oF. Copyright 2006 - All Rights Reserved Bryan Research and Engineering, Inc. Page 10 of 12 Bryan Research and Engineering, Inc. - Technical Papers Figure 14. o-Xylene emissions at the regerator. Contactor temperature 90oF. Figure 15. Total aromatic emissions at the regenerator. Contactor temperature 90oF. These data also indicate that in systems with larger amounts of water in the glycol and higher temperatures (122 the K values for ethylbenzene and o-xylene decrease with pressure so that solubility would increase at the higher pressure range. Therefore, several cases at this temperature were modeled, and PROSIM indicated the solubility to increase with pressure except at high glycol circulation rates. Again, at these high circulation rates, the water content in the glycol would be much lower where the solubility decreases with pressure. Comparison of Figures 10, 11, 12, and 15, indicate that a significant fraction of the dissolved non-aromatic VOC’s are released in the flash tank. However, only a small fraction of the aromatics will be liberated without the application of heat. oF), Copyright 2006 - All Rights Reserved Bryan Research and Engineering, Inc. Page 11 of 12 Bryan Research and Engineering, Inc. - Technical Papers Special attention should be made to the fact that the majority of the aromatics released in the regenerator can be recovered using an aerial cooler operating about 120 oF and a three phase separator. Simple calculations yield that up to 97 % of the aromatics emitted at the high glycol circulation rates can be recovered. Often economics makes the return on this recovery system feasible. REFERENCES 1. Kohl, A. and Riesenfeld, F., "Gas Purification", Gulf Publishing Co., Houston, 1985. 2. Fitz, C. W., and Hubbard, R. A., "Quick, Manual Calculation Estimates Amount of Benzene Absorbed in Glycol Dehydrator," Oil & Gas J., p. 72, Nov. 8, 1987. 3. Gas Processors Suppliers Association, Engineering Data Book, 1987. 4. Jou, F. Y., Deshmukh, R. D., Otto, F. D., and Mather, A. E., "Vapor-Liquid Equilibria for Acid Gases and Lower Alkanes in Triethylene Glycol," Fluid Phase Equilibria, 36, p. 11, 1987. 5. Takahashi, S., and Kobayashi, R., "The Water Content and the Solubility of CO2 in Equilibrium with DEG-Water and TEG-Water Solutions at Feasible Absorption Conditions," Technical Publication TP-9, GPA, 1982. 6. "The Solubility of Selected Aromatic Hydrocarbons in Triethylene Glycol," D. B. Robinson Research LTD., API/GPA Progress Report, March 1991. copyright 2001 Bryan Research & Engineering, Inc. Copyright 2006 - All Rights Reserved Bryan Research and Engineering, Inc. Page 12 of 12 See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/279063450 Natural Gas Dehydration Chapter · October 2012 DOI: 10.5772/45802 CITATIONS READS 13 8,814 2 authors, including: Pavel Ditl Czech Technical University in Prague 82 PUBLICATIONS 430 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: PSA Serie View project Macroinstability View project All content following this page was uploaded by Pavel Ditl on 08 October 2015. The user has requested enhancement of the downloaded file. Chapter 1 Natural Gas Dehydration Michal Netušil and Pavel Ditl Additional information is available at the end of the chapter http://dx.doi.org/10.5772/45802 1. Introduction The theme of natural gas (NG) dehydration is closely linked with storage of natural gas. There are two basic reasons why NG storage is important. Firstly, it can reduce dependency on NG supply. With this in mind, national strategic reserves are created. Secondly, NG storage enables the maximum capacity of distribution lines to be exploited. NG is stored in summer periods, when there is lower demand for it, and is withdrawn in the winter periods, when significant amounts of NG are used for heating. Reserves smooth seasonal peaks and also short-term peaks of NG consumption. Underground Gas Storages (UGS) are the most advantageous option for storing large volumes of gas. Nowadays there are approximately 135 UGSs inside the European Union. Their total maximum technical storage capacity is around 109 ms3. According to the latest update, over 0,7?109 ms3 of additional storage capacity will come on stream in Europe by 2020 [1]. There are three types of UGSs: (1) Aquifers, (2) Depleted oil/gas fields, and (3) Cavern reservoirs (salt or hard rock). Each of these types possesses distinct physical characteristics. The important parameters describing the appropriateness of UGS use are storage capacity, maximum injection/withdrawal performance, and gas contamination during storage. Generally, the allowable pressure of stored gas inside a UGS is up to 20 MPa. The pressure inside increases as the gas is being injected, and decreases when gas is withdrawn. The output gas pressure depends on further distribution. Distribution sites from UGS normally begin at 7 MPa. The temperature of the gas usually ranges from 20 - 35°C. The exact temperature varies with the location of the UGS and with the time of year. 1.1. Water in the gas A disadvantage of UGSs is that during storage the gas become saturated by water vapors. In the case of depleted oil field UGSs, vapors of higher hydrocarbons also contaminate the stored gas. The directive for gas distribution sets the allowable concentration of water and concentration of higher hydrocarbons. In the US and Canada, the amount of allowable water © 2012 Netušil and Ditl, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 4 Natural Gas – Extraction to End Use in the gas is specified in units: pounds of water vapor per million cubic feet (lbs/MMcft). This amount should be lower than 7 lbs/MMcft [2], which is equivalent to 0,112 gH2O/mS3. In Europe, the concentration of water and higher hydrocarbons is specified by their dew point temperature (Tdew). Tdew for water is -7°C for NG at 4 MPa, and Tdew for hydrocarbons is 0°C for NG at the operating pressures [3]. This value for water is equivalent to roughly 0,131 gH2O/mS3 of NG at 4 MPa. As was stated above, the distribution specifications depend on the geographic region in which they are applied. For example, in Nigeria water Tdew should be below 4°C for NG at 4 MPa, which means that the NG can contain more than twice as much water vapors as in Europe. The water content of NG at saturation is dependent on temperature and pressure. With increasing pressure of the gas the water content decreases, and with increasing temperature the water content in the gas increases. This is well presented in Figure No. 20, Chapter 20, in the GPSA Data Book, 12th Edition. The water content of the gas can be calculated using the following equation [4, 5]: = 593,335 ? 0.05486 ? ? . (1) Where wwater is in kilograms of water per 106 ms3 of NG, tG is temperature of NG in °C, and PG is pressure of NG in MPa. The average value of water in NG withdrawn from UGS is 2 - 5 times higher than required. An NG dehydration step is therefore essential before further distribution. 1.2. Problems with water in the gas If the temperature of pipeline walls or storage tanks decreases below the Tdew of the water vapors present in the gas, the water starts to condense on those cold surfaces, and the following problems can appear. ? ? ? ? ? NG in combination with liquid water can form methane hydrate. Methane hydrate is a solid in which a large amount of methane is trapped within the crystal structure of water, forming a solid similar to ice. The methane hydrate production from a unit amount of water is higher than the ice formation. The methane hydrates formed by cooling may plug the valves, the fittings or even pipelines. NG dissolved in condensed water is corrosive, especially when it contains CO2 or H2S. Condensed water in the pipeline causes slug flow and erosion. Water vapor increases the volume and decreases the heating value of the gas. NG with the presence of water vapor cannot be operated on cryogenic plants. 2. Dehydration methods 2.1. Absorption The most widely-used method for industrial dehydration of NG is absorption. Absorption is usually performed using triethyleneglycol sorbent (TEG). Absorption proceeds at low Natural Gas Dehydration 5 temperatures and the absorbed water is boiled out from TEG during regeneration in a reboiler at high temperatures. Some physical properties of pure TEG are given in the following text. Viscosity data vs. temperature is shown in Table 1 and is shown in a graph in Figure 1 [6]. [°C] [m2/s 10-5] [°C] [m2/s 10-5] [°C] [m2/s 10-5] 4 9,53 71 0,672 138 0,171 10 7,094 77 0,577 143 0,159 16 5,367 82 0,5 149 0,147 21 4,124 88 0,436 154 0,138 27 3,214 93 0,384 160 0,129 32 2,539 99 0,34 166 0,121 38 2,032 104 0,303 171 0,115 43 1,646 110 0,272 177 0,109 49 1,348 116 0,245 182 0,103 54 1,116 121 0,222 188 0,099 60 0,934 127 0,203 193 0,095 66 0,788 132 0,186 199 0,091 Kinematic viscosity [10-5 ? m2/s] Table 1. Kinematic viscosity of TEG according to temperature 10 9 8 7 6 5 4 3 2 1 0 0 20 40 60 80 Temperature [°C] 100 120 140 Figure 1. Kinematic viscosity of TEG as a function of temperature It follows from Figure 1 that the kinematic viscosity of TEG increases dramatically with low temperatures. The temperature of TEG during a process should never decrease below 10°C. The reason is to prevent pump damage or even clogging of the flow. For temperatures above 100°C, the viscosity changes just slightly and the average kinetic viscosity value 5?10-6 m2/s can be used. For a description of the dependency of viscosity on temperature, the following polynomial interpolation coefficients have been calculated. + 1,347 ? 10 = 1,159 ? 10 − 5,655 ? 10 −7,820 ? 10 + 2,116 ? 10 − 2,393 ? 10 + 1,872 ? 10 + 1,572 ? 10 (2) 6 Natural Gas – Extraction to End Use The density of TEG at various temperatures is shown in the following table [7]. [°C] [kg/m3] [°C] [kg/m3] [°C] [kg/m3] 10 1132 82 1076 154 1019 16 1128 88 1071 160 1015 21 1124 93 1066 166 1010 27 1119 99 1063 171 1007 32 1114 104 1058 177 1002 38 1111 110 1053 182 997 43 1106 116 1050 188 993 49 1101 121 1045 193 989 54 1098 127 1041 199 984 60 1093 132 1036 204 980 66 1089 138 1032 210 975 71 1084 143 1028 216 972 77 1080 149 1023 221 967 Table 2. Density of TEG according to temperature Table 2 shows that in the range of working temperatures the density of TEG is a linear function of temperature. The following equation can be used for determining the density at a certain temperature. = −0,7831 ? + 1140 (3) Finally, the thermal conductivity of TEG does not change in the range of working temperatures and has a value of 0,194 W/m2/°C. For determining the physical properties of TEG solutions with water (concentrations cTEG above 95 wt.%), the activity coefficient of water in TEG can be approximated by the following equation. = −0,0585 ? + 6,2443 (4) The industrial absorption dehydration process proceeds in a glycol contactor (a tray column or packet bed). In a contactor, a countercurrent flow of wet NG and TEG is arranged. During the contact, the TEG is enriched by water and flows out of the bottom part of the contactor. The enriched TEG then continues into the internal heat exchanger, which is incorporated at the top of the still column in the regeneration section of the absorption unit. It then flows into the flash drum, where the flash gases are released and separated from the stream. The TEG then runs to the cold side of the TEG/TEG heat exchanger. Just afterwards, the warmed TEG is filtered and then runs into the regeneration section, where is it sprayed in the still column. From there, the TEG runs into the reboiler. In the reboiler, water is boiled out of the TEG. The regeneration energy is around 282 kJ per liter of TEG. The temperature inside should not exceed 208°C, due to the decomposition temperature of TEG. Regenerated (lean) TEG is then pumped back through the hot side of the TEG/TEG and NG/TEG heat exchanger into the top of the contactor. The entire method is depicted in Figure 2 [8]. The circulation rate (lTEG/kgH2O) and the purity of the regenerated TEG are the main limiting factors determining the output Tdew of NG. The amount of circulating TEG is around 40 times the amount of water to be removed. The minimal TEG concentration should be above 95 wt.%, but the recommended value is higher. However, to obtain TEG concentration above 99 wt.% enhanced TEG regeneration has to be implemented. The simplest regeneration enhancing method is gas stripping. Proprietary designs DRIZO®, licensed by Poser-NAT, and COLDFINGER®, licensed by Gas Conditioners International, Natural Gas Dehydration 7 Dry Gas Flash gases Water vapor Lean TEG Still column Flash drum Glycol Contactor Reboiler Filter Wet Gas Rich TEG Inlet scrubber Figure 2. TEG absorption dehydration scheme have been patented as an alternative to traditional stripping gas units. The Drizo regeneration system utilizes a recoverable solvent as the stripping medium. The patent operates with iso-octant solvent, but the typical composition of the stripping medium is about 60 wt.% aromatic hydrocarbons, 30 wt.% naphthenes and 10 wt.% paraffins. The three-phase solvent water separator is crucial for this method. The Coldfinger regeneration system employs a cooling coil (the “coldfinger”) in the vapor space of the surge tank. The cooling that takes place there causes condensation of a high amount of vapors. The condensate is a water-rich TEG mixture, which is led to a further separation process [9]. Figure 3 depicts enhanced regeneration systems which replace the simple reboiler in the regeneration section shown in Figure 2. Vent Gases to Flare or Recycle Vent Gases to Flare or Recycle Cooler Vent Rich TEG Still Column Rich TEG Still Column Solvent Vaporizer Reboiler Lean TEG Surge Tank Rich TEG 3-Phase Separator Water Reboiler Stripping gas Stripping Solvent Lean TEG Surge Tank STRIPPING GAS Figure 3. Enhanced TEG regeneration systems Blanket Gas Lean TEG DRIZO Still Column Reboiler Surge Tank Cooling Medium Water Rich TEG Mixture COLDFINGER 8 Natural Gas – Extraction to End Use 2.2. Adsorption The second dehydration method is adsorption of water by a solid desiccant. In this method, water is usually adsorbed on a mole sieve, on a silica gel or on alumina. A comparison of the physical properties of each desiccant is shown in Table 3 [4,10]. Properties Specific area [m2/g] Pore volume [cm3/g] Pore diameter [Å] Design capacity [kg H2O/100 kg desiccant] Density [kg/m3] Heat capacity [J/kg/°C] Regeneration temperature [°C] Heat of desorption [J] Silica gel 750 – 830 0,4 – 0,45 22 7-9 721 920 230 3256 Alumina 210 0,21 26 4-7 800 - 880 240 240 4183 Mol. sieves 650 – 800 0,27 4-5 9-12 690 – 720 200 290 3718 Table 3. Comparison of the physical properties of desiccants used for dehydration of NG The amount of adsorbed water molecules increases with the pressure of the gas and decreases with its temperature. These facts are taken into account when the process parameters are designed. Adsorption dehydration columns always work periodically. A minimum of two bed systems are used. Typically one bed dries the gas while the other is being regenerated. Regeneration is performed by preheated gas, or by part of the dehydrated NG, as depicted in Figure 4. Separator Wet Gas Adsorption Dry Gas Cooler Regeneration Heater Figure 4. Scheme of the temperature swing adsorption dehydration process Water Natural Gas Dehydration 9 This method is known as temperature swing adsorption (TSA). Regeneration can also be performed by change of pressure - pressure swing adsorption (PSA). However, PSA is not industrially applied for NG dehydration. Further details about PSA can be found in [11,12]. A combination of those two methods (PSA and TSA) seems to be a promising future option for adsorption dehydration of NG. This idea is still in the research process. In classical applications, the TSA heater is realized as an ordinary burner or as a shell and tube heat exchanger warmed by steam or by hot oil. The regeneration gas warms in the heater and flows into the column. In the column passes through the adsorbent and the water desorbs into the regeneration gas. The water saturated regeneration gas then flows into the cooler. The cooler usually uses cold air to decrease the temperature of the regeneration gas. When the water saturated regeneration gas is cooled, partial condensation of the water occurs. The regeneration gas is led further into the separator, where the condensed water is removed. A downstream flow of wet NG through the adsorption column is usually applied. In this way, floating and channeling of an adsorbent is avoided. Regeneration is performed by countercurrent flow in order to provide complete regeneration from the bottom of the column, where the last contact of the dried NG with the adsorbent proceeds. The typical temperature course for 12 h regeneration of molecular sieves is shown in Figure 5 [13]. inlet temperature of regeneration gas 300 TH Temperature [°C] 250 TD 200 150 TC TB 100 30 TA TE A start 0 C B 3 D 6 E end 9 Time [h] Figure 5. Typical temperature course for 12 h TSA regeneration of molecular sieves 12 10 Natural Gas – Extraction to End Use The shape of the curve representing the course of the outlet regeneration gas temperature is typically composed of four regions. They are specified by time borders A, B, C and D with appropriate border temperatures TA, TB, TC and TD. Regeneration starts at point A. The inlet regeneration gas warms the column and the adsorbent. At a temperature around 120°C (TB) the sorbed humidity starts to evaporate from the pores. The adsorbent continues warming more slowly, because a considerable part of the heat is consumed by water evaporation. From point C, it can be assumed that all water has been desorbed. The adsorbent is further heated to desorb C5+ and other contaminants. The regeneration is completed when the outlet temperature of the regeneration gas reaches 180 - 190°C (TD). Finally, cooling proceeds from point D to point E. The temperature of the cooling gas should not decrease below 50°C, in order to prevent any water condensation from the cooling gas [13]. Part of the dehydrated NG is usually used as the regeneration gas. After regenerating the adsorbent the regeneration gas is cooled, and the water condensed from it is separated. After water separation, the regeneration gas is added back to the inlet stream or alternatively to the dehydrated stream. The total energy used for regeneration is composed of heat to warm the load (30%), heat for desorption (50%) and heat going into the structure (20%). With proper internal insulation of the adsorption towers, the heat going to the structure can be minimized and around 20% of the invested energy can be saved. So-called LBTSA (Layered Bed Temperature-Swing Adsorption) processes are an upgrade of the TSA method. Here, the adsorption column is composed of several layers of different adsorbents. Hence the properties of the separate adsorbents are combined in a single column. For example, in NG dehydration a combination of activated alumina with molecular sieve 4A is used. Alumina has better resistance to liquid water, so a thin layer is put in first place to contact the wet NG. This ordering supports the lifetime of the molecular sieve, which is placed below the alumina layer. The effect of adsorbent lifetime extension is shown for two cases in Figure 6. It can be seen that contact with liquid water dramatically decreases the lifetime of the molecular sieve [14]. 25 °C 5 - 6 MPa water saturated NG Adsorption capacity [wt %] 22 20 18 16 14 12 10 8 0,2 0,5 1 1,5 2 2,5 Adsorption capacity [wt %] 3 Number of adsorber regenerations [thousands] 22 20 18 16 14 12 10 8 25 °C 7,2 MPa NG with water drops 0,2 0,5 1 1,5 2 2,5 3 Number of adsorber regenerations [thousands] Figure 6. Effect of layered bed adsorption on the lifetime of the adsorbent Natural Gas Dehydration 11 2.3. Condensation The third conventional dehydration method employs gas cooling to turn water molecules into the liquid phase and then removes them from the stream. Natural gas liquids and condensed higher hydrocarbons can also be recovered from NG by cooling. The condensation method is therefore usually applied for simultaneous dehydration and recovery of natural gas liquids. NG can be advantageously cooled using the Joule-Thompson effect (JT effect). The JT effect describes how the temperature of a gas changes with pressure adjustment. For NG, thanks to expansion, the average distance between its molecules increases, leading to an increase in their potential energy (Van der Waals forces). During expansion, there is no heat exchange with the environment, and no work creation. Therefore, due to the conservation law, the increase in potential energy leads to a decrease in kinetic energy and thus a temperature decrease of NG. However, there is another phenomenon connected with the cooling of wet NG. Attention should be paid to the formation of methane hydrate. Hydrates formed by cooling may plug the flow. This is usually prevented by injecting methanol or monoethylenglycol (MEG) hydrate inhibitors before each cooling. Figure 7 depicts an industrial application of dehydration method utilizing the JT effect and MEG hydrate inhibition. Dry Gas flash 1° Wet Gas Air cooling flash 2° External cooling Condensate MEG Condensate Figure 7. Dehydration method utilizing the JT effect and hydrate inhibition The wet NG is throttled in two steps inside the flash tanks. The lower temperature (due to the JT effect) of the gas stream in the flash tanks leads to partial condensation of the water vapors. The droplets that are created are removed from the gas stream by a demister inside the flashes. In cases where cooling by the JT effect is insufficient (the usable pressure difference between the inlet and outlet of the gas is insufficient), the air pre-cooler and the 12 Natural Gas – Extraction to End Use external cooler are turned on. Since dehydration is normally applied to large volumes of NG, the external coolers need to have high performance, so this type of cooling is very energy expensive. For dehydration of low pressures NG the external coolers consumes up to 80% of total energy of dehydration unit. However, if the usable pressure difference is high, the JT effect inside the flashes is so strong that internal heating of the flashes is required to defreeze any methane hydrate or ice that may form. A condensation method is applied when suitable conditions for the JT effect are available. 2.4. Supersonic separation The principle of this method lies in the use of the Laval Nozzle, in which the potential energy (pressure and temperature) transforms into kinetic energy (velocity) of the gas. The velocity of the gas reaches supersonic values. Thanks to gas acceleration, sufficient temperature drops are obtained. Tdew of water vapor in NG is reached, and nucleation of the droplets proceeds. Figure 8 depicts the basic design of a supersonic nozzle [15]. Static blades Cyclone separation Diffuser Wet Gas Dry gas Swirl flow Inner Body Liquids and slip gases Figure 8. Design of a supersonic nozzle for NG dehydration At the inlet to the nozzle there are static blades which induce a swirling flow of the gas. The water droplets that form are separated by the centrifugal force on the walls. The centrifugal force in the supersonic part of the nozzle can reach values up to 500 000 g [16]. The thin water film on the walls moves in the direction of flow into the separation channel. The separation channel leads into the heated degas separator. From here, the slip gas is returned back to the main stream and the water condensate is removed. After separation of the water it is important to recover the pressure of the gas from its kinetic energy. A shock wave is used to achieve this. Generally, shock waves form when the speed of a gas changes by more than the speed of sound. In supersonic nozzles, the shock wave is created by rapid enlargement of the nozzle diameter. This part of the nozzle is called the diffuser. Thanks to the diffuser 65 - 80% of the inlet pressure is recovered [17]. This section might also include another set of static devices to undo the swirling motion. The profile of pressure, temperature and velocity of a gas passing through the supersonic nozzle is depicted below. Natural Gas Dehydration 13 velocity nozzle throat shock wave temperature pressure nozzle length Figure 9. Profile of pressure, temperature and velocity of a gas passing through the supersonic nozzle The scheme of a supersonic dehydration line working on the principle introduced here is depicted in Figure 10. Laval Nozzle Wet Gas Condensated water Separator Free Water Shock wave Dry Gas Returned slip gases Separator Water Figure 10. Scheme of a supersonic dehydration line The gas residence time in the supersonic nozzle is below two milliseconds [18]. This time interval is too short for any methane hydrate formation, so no inhibitors are needed. To obtain supersonic velocity of the gas, the inlet diameter should be minimally √5 times higher than the nozzle throat. The geometry of the tapered section of the Laval nozzle is calculated by the following equations [16] 14 Natural Gas – Extraction to End Use =1− = ≤ 1− (5) > (6) Where D1, Dcr, L, Xm stand for the inlet diameter, the throat diameter, the length of the tapered section, and the relative coordinate of tapered curve, respectively; x is the distance between an arbitrary cross section and the inlet, and D is the convergent diameter at an arbitrary cross section of x. A model of a supersonic dehydration unit was analyzed with the use of numerical simulation tools, and the separation efficiency in respect to lost pressure was evaluated. The simulations were performed on water saturated NG at 30 MPa and 20°C. The results are presented in Table 4 [19]. Pressure lost in nozzle (%) Water separation efficiency (%) 17,3 40 20,0 50 27,6 90 49,0 94 51,5 96 Table 4. Supersonic water separation efficiency in respect to pressure lost in the nozzle The supersonic separation is a promising new technology. The main advantage of the method is the small size of the supersonic nozzle. For example, a nozzle 1,8 m in length placed in a housing 0,22 m in diameter was used for dehydrating 42 000 ms3 per hour of water-saturated NG at 25°C compressed to 10 MPa to output water Tdew < -7°C [20]. The corresponding absorption contactor would be 5 m in height and 1,4 m in diameter, and the corresponding adsorption line would be composed of two adsorbers 3 m in height and 1 m in diameter. A further advantage is the simplicity of the supersonic dehydration unit. The supersonic nozzle contains no moving parts and requires no maintenance. The operating costs are much lower than for other methods. The only energy-consuming devices are the pumps for removing the condensate and the heater for the degas separator. However, during supersonic dehydration a pressure loss occurs. However, if the same pressure loss were used for the JT effect, the temperature drop would be 1,5 – 2,5 times lower [21]. Supersonic separation enables simultaneous removal of water and higher hydrocarbons from the treated gas, and can be used as pretreatment method before NG liquefaction. This method could also be usable for other applications of gas separation. The application of supersonic separation has some disadvantages. The most limiting condition of use is the need for stationary process parameters. Fluctuations in temperature, pressure or flow rate influence the separation efficiency. However, it is in many cases impossible to achieve constant process parameters. For example, this is the case when withdrawing NG from UGS. However, supersonic dehydration can be used even in this case. The problem is solved by arranging several nozzles into a battery configuration with a single common degas separator. The battery configuration enables an optimal number of nozzles to be switched on, depending on the inlet parameters of the gas. The scheme of a possible arrangement is depicted in Figure 11 below [22]. Natural Gas Dehydration 15 Flow meter Wet gas Splitter Slip Gas Degas separator Dry gas Water Figure 11. Arrangement of supersonic nozzles for unsteady inlet parameters of NG Nozzle A is designed to process 80% of the nominal gas flow. Nozzles B, C, D are in the proportion 4:2:1 with respect to the processed gas flow. Nozzles B, C, D together can process 40% of the nominal gas flow. This arrangement therefore enables ± 20% deviation of the nominal flow to be covered. With appropriate switching of the nozzles, the maximal deviation between the real gas flow and the designed flow for the combination of nozzles is below 4%. A further disadvantage of the supersonic dehydration is its novelty. The appropriate nozzle design is complicated, and “know how” is expensive. The geometry of the nozzle ranges in the order of micrometers. In addition, the construction material has to withstand abrasion and the impacts of a shock wave. 3. Comparison of conventional dehydration methods 3.1. General comparison Each of the methods presented here has its advantages and disadvantages. Absorption by TEG is nowadays the most widely used method. Outlet Tdew around -10°C is usually reached and this water concentration is sufficient for pipeline distribution of NG. Indeed, with improved reboiler design (Vacuum Stripping, Drizo, Coldfinger), the outlet Tdew is even 2 - 3 times lower. However TEG has a problem with sulfur, and with gas contaminated with 16 Natural Gas – Extraction to End Use higher hydrocarbons. The TEG in the reboiler foams, and with time it degrades into a “black mud”. BTEX emissions (the acronym stands for benzene, toluene, ethylbenzene and xylenes) in the flash gases and in the reboiler vent are a further disadvantage. Adsorption dehydration can achieve very low outlet water concentration Tdew < -50°C, and contaminated gases are not a problem. Even corrosion of the equipment proceeds at a slower rate. However, adsorption requires high capital investment and has high space requirements. The adsorption process runs with at least two columns (some lines use three, four, or as many as six). Industrial experience indicates that the capital cost for an adsorption line is 2 - 3 times higher than when absorption is used [5]. In addition, the operating costs are higher for adsorption than for absorption. Expansion dehydration is the most suitable method in cases where a high pressure difference is available between UGS and the distribution connection. However, the difference decreases during the withdrawal period and becomes insufficient, so that an external cooling cycle is needed. A cycle for regenerating hydrate inhibitor from the condensate separated inside the flashes is also required. The general overview of areas suitable for application of target dehydration method is depicted on the following Figure 12. 16000 Wet Gas Water Content [kg/106m3] Condensation 1600 160 Adsorption (Molecular Sieves) Adsorption (Alumina, Silica gel) & Enhanced Absorption Absorption 16 -50 -40 -30 -20 -5 10 30 Dry Gas Water Dew-Point [°C] Figure 12. Overview of areas suitable for application of target dehydration method 60 Natural Gas Dehydration 17 3.2. Energy comparison based on own analyses and available data The energy demand of the conventional methods presented here is compared on the basis of a model, where a volume of 105 ms3/h of NG from UGS is processed. The NG is water saturated at a temperature of 30°C. The pressure of the gas is varied from 7 to 20 MPa, but in the case of the condensation method the pressure range starts at 10 MPa. The required outlet water concentration of in NG is equivalent to dew point temperature -10°C at gas pressure 4 MPa [23]. The calculation of TEG absorption is based on GPSA (2004) [24]. The results are compared with the paper by Gandhidasan (2003) [5] and with industrial data provided by ATEKO a.s. The total energy demand is composed of heat for TEG regeneration in the reboiler, energy for the pumps, filtration and after-cooling the lean TEG before entering the contactor. Enhanced regeneration is not considered. The basic parameters for the calculation are: regeneration temperature 200°C, concentration of lean TEG 98,5 wt.%, and circulation ratio 35 lTEG/kgH2O [24]. For calculating adsorption dehydration, molecular sieve 5A is considered to be the most suitable adsorbent. The total energy demand is directly connected to the regeneration gas heater, and no other consumption is assumed. The calculations are again based on GPSA (2004) [24]. The results are compared with the paper by Gandhidasan (2001) [4] and also with the publication by Kumar (1987) [7]. The calculation procedure for GPSA and Gandhidasan arises from the summation of the particular heats, i.e. the heat for adsorbent warming, the heat for column warming (insulation of the adsorption towers is considered), and the heat for water desorption. Kumar’s calculation procedure runs differently. The regeneration step is divided into four regions (as depicted in Figure 5). Afterwards, we determine what individual phenomena proceed in each region, what the border and average temperatures are, and how much energy is required to cover these phenomena. Finally, the demands for each region are added. The basic parameters for all procedures are: temperature of the regeneration gas 300°C, time of adsorption/regeneration 12 h, and two column designs. The condensation method was calculated on the basis of industrial data provided by TEBODIN s.r.o. and supplementary calculations of the JT effect. The key parameter influencing energy demand is the pressure of NG. Because it is not feasible to apply this method for low pressures, and because the provided data starts at 10 MPa, the pressure range was adjusted. The total energy demand consists of the air pre-cooling unit, the external cooling, the pumps for MEG injection and condensate off take, the heat for MEG regeneration, and flash heating. 3.3. Results of the analyses The results obtained for the TEG absorption method are the same for each of the calculation procedures, and good agreement with industrial data was also obtained. However, the calculation procedures for the adsorption method lead to different results. Hence the 18 Natural Gas – Extraction to End Use average energy demand value was taken as the reference. The maximum deviation from it is below 20% for all calculation procedures. The source of the deviation lies in the “loss factor and the non-steady state factor”. In the case of the condensation method, the calculated values for the JT effect were in good agreement with the industrial data, but the amount of data was limited, resulting in limited representation of the condensation method. The final energy consumption results for each dehydration method are summarized by the graph in Figure 13. For low pressures (pressure of NG from UGS < 13 MPa), the condensation method is the most demanding. Its demand decreases linearly with pressure to a value of 145 kW for 13 MPa. At this point, the energy demand for the condensation method is roughly the same as for the adsorption method. When the NG pressure is further increased from 13 MPa to 16 MPa, the energy demand for the condensation method still decreases, but with a lowering tendency. For a high pressure of NG (> 16 MPa), the energy demand of the condensation method is at its lowest, and it remains nearly constant, with an average value around 36 kW. Energy consumption [kW] Absorption TEG 300 Adsorption MS Two stage expansion 200 100 0 7 8 9 10 11 12 13 14 15 16 Pressure of the NG [MPa] 17 18 19 20 Figure 13. Energy consumption results for conventional dehydration methods The course of the energy demand for the adsorption and absorption methods is quite similar: with increasing pressure of dehydrated NG the energy demand slowly decreases. The absorption method is less demanding on the whole pressure scale, and begins with consumption of 120 kW at 7 MPa. The adsorption method starts with 234 kW at 7 MPa, but the energy demand decreases slightly more as the pressure of NG in UGS rises. This leads to a gradual decrease in the difference between these methods, and the energy demand at the final pressure value of 20 MPa is equal to 54 kW for absorption and 103 kW for adsorption. Natural Gas Dehydration 19 3.4. Conclusions of the analyses By far the highest energy demand of the condensation method at low pressures of NG from UGS is due to the pressure being close to the distribution pressure, so that pressure cannot be used for the JT effect in flashes. Cooling is then compensated by the air pre-cooler and the external cooling device, which are unsuitable for large volumes of processed NG. However, as the pressure difference between UGS and the distribution site increases, the space for expansion rises and the JT effect proceeds with increasing impact. This is projected into a linear decrease in the energy demand of the air pre-cooler and the external cooling device. From the point where there is a pressure of NG > 14 MPa, flash heating is gradually turned on to prevent any freezing caused by the strong JT effect. The energy demand of flash heating is reflected in the total energy consumption. Finally, for pressures of NG > 16 MPa, total cooling and subsequent condensation is achieved by the JT effect. The total energy demand remains constant, and consists of flash heating and inhibitor injection and regeneration. In case of the adsorption and absorption dehydration method, the similar falling course of the energy demand with increasing pressure of NG can be explained by the fact that with increasing pressure within a UDG the amount of water present in the NG decreases. The absorption method generally consumes less energy, because the regeneration of TEG is less demanding than adsorbent regeneration. The composition of the total energy demand of the adsorption method can be divided into three parts. The heat for water desorption is approximately 55%, for warming the adsorbent it is 31%, and for warming the column it is 14%. It also has to be assumed that just part of the heat in the regeneration gas transfers to the adsorbent, the column and heat loss leaves to the atmosphere, and the balance leaves with the hot gas. In brief, in cases of high pressure the most appropriate dehydration method from the energy demand point of view is the stored NG condensation method. This holds for NG from UGS with pressure > 15 MPa and distribution pressure requirements 7 MPa. For lower pressures, the condensation method is used if the objective is to recover NGL and remove water simultaneously. However, this is usually not the case when storing NG in a UGS. In cases where insufficient pressure difference is available, the absorption method is therefore favored over the adsorption method in terms of energy demand. TEG absorption is almost twice less demanding. However, if a gas contaminated with sulfur or higher hydrocarbons is being processed, the TEG in the reboiler foams and degrades with time. This can occur when a depleted oil field is used as a UGS. Adsorption is preferred in cases where very low Tdew (water concentration lower than 1 ppm can be achieved) of NG is required, for example when NG is liquefied. It is worth to note that the power comparison can be used as a measure of the technical excellence. From power data the specific energy consumption was calculated and its values indicate that the energy cost is much lower than the investment cost (depreciations). On the other hand the energy cost represents more than 60 % of the total operating cost. 4. Conclusions and recommendations The chapter should help in the selection of a proper dehydration method and in calculating NG dehydration. The following methods are available as options: absorption, adsorption 20 Natural Gas – Extraction to End Use and condensation. Absorption is used in cases when emphasis is not placed on the water content of the output stream, and when low operating and capital investment are required. Adsorption is used in cases when bone dry NG is required. Low temperature separation employing the JT effect is used in cases where a sufficient pressure drop is available between the input and the output of the dehydration unit. Supersonic nozzles are a promising method that will in future displace these three conventional methods. We have selected for citation here articles and procedures that we consider to provide reliable results. Author details Michal Netušil and Pavel Ditl Czech Technical University in Prague, Faculty of Mechanical Engineering, Department of Process Engineering, Prague, Czech Republic Acknowledgement The authors are grateful for the financial support provided by Ministry of Industry and Trade of the Czech Republic (program TIP nr. FR-TI1/173). Abbreviations NG - natural gas UGS - underground gas storage lbs - pounds MMcft - millions of cubic feet TEG - triethyleneglycol MEG - monoethylenglycol TSA - temperature swing adsorption LBTSA - layered bed temperature swing adsorption JT effect - Joule-Thompson effect BTEX - benzene, toluene, ethylbenzene and xylenes Symbols Tdew - dew point temperature gH2O - grams of water mS3 - standard cubic meters of gas (293,15 K; 101,325 kPa) wwater - kilograms of water per 106 ms3 of NG tG - temperature of NG in °C C5+ - pentane and higher hydrocarbons PG - pressure of NG in MPa ρ - density of NG in kg/m3 γ - activity coefficient dimensionless Natural Gas Dehydration 21 cTEG - weight concentration of TEG in TEG/water solution lTEG - liters of TEG kgH2O - kilograms of water D1 - inlet diameter of nozzle mm Dcr - throat diameter of nozzle mm L - length of tapered section of nozzle in mm Xm - relative coordinate of tapered curve in mm D - convergent diameter in mm x - distance between arbitrary cross section and the inlet in mm g - standard gravity 5. References [1] Gas infrastructure Europe (2011) Map Dataset in Excel-format Storage map. Available: http://www.gie.eu/maps_data/storage.html. Accessed 2011 Mar 8. [2] Foss M (2004) Interstate Natural Gas Quality Specifications and Interchangeability. Center for Energy Economics. [3] NET4GAS (2011) Gas quality parameters. Available at: http://extranet.transgas.cz/caloricity_spec.aspx. Accessed 2011 Mar 8. [4] Gandhidasan P, Al-Farayedhi A, Al-Mubarak A (2001) Dehydration of natural gas using solid desiccants. Energy 26: 855-868. [5] Gandhidasan P (2003) Parametric Analysis of Natural Gas Dehydration by Triethylene Glycol Solution. Energy Sources 25: 189-201. [6] CHEM Group, Inc. (2012) Triethylene Glycol - Liquid Density Data. Available at: http://www.chem-group.com/services/teg-density.tpl. Accesed 2012 Mar 6. [7] CHEM Group, Inc. (2012) Triethylene Glycol - Kinematic Viscosity Data. Available at: http://www.chem-group.com/services/teg-viscosity.tpl. Accesed 2012 Mar 6. [8] Bahadori A, Vuthaluru H.B (2009) Simple methodology for sizing of absorbers for TEG gas dehydration systems. Energy 34: 1910–1916 [9] Hubbard R.A, Campbell J.M (2000) An appraisal of gas dehydration processes. Hydrocarbon Engineering 5: 71-74. [10] Tagliabue M, Farrusseng D, Valencia S, Aguado S, Ravon U, Rizzo C (2009) Natural gas treating by selective adsorption: Material science and chemical engineering interplay. Chemical Engineering Journal 155: 553-566 [11] Roušar I, Ditl P (1993) Pressure swing adsorption: analytical solution for optimum purge Original Research, Chemical Engineering Science 48: 723-734. [12] Roušar I, Ditl P, Cekal M (1993) Pressure swing adsorption – the optimization of multiple bed units, Precision Process Technology: perspectives for pollution prevention: 483-492. [13] Kumar S (1987) Gas Production Engineering. Houston: Gulf Professional Publishing 239 p. [14] Jochem G (2002) Axens Multibed Systems for the Dehydration of Natural Gas, PETEM. 22 Natural Gas – Extraction to End Use [15] Schinkelshoek P, Epsom H.D (2008) Supersonic gas conditioning – commercialization of twister technology. 87th Annual Convention. Grapevine, Texas. [16] Wen C, Cao X, Zhang J, Wu L (2010) Three-dimensional Numerical Simulation of the Supersonic Swirling Separator. Twentieth International Offshore and Polar Engineering Conference. Beijing, China. [17] Okimoto F, Brouwer J.M (2002) Supersonic gas conditioning, World Oil 34: 89-91 [18] Ma Q, Hu D, Jiang J, Qiu Z (2010) Numerical study of the spontaneous nucleation of self-rotational moist gas in a converging–diverging nozzle. International Journal of Computational Fluid Dynamics: 29–36. [19] Karimi A, Abdi M.A (2006) Selective dehydration of high-pressure natural gas using supersonic nozzles, Chemical Engineering and Processing 48: 560–568 [20] Twister BV (2012) Twister supersonic separator – Experience. Available: http://twisterbv.com/products-services/twister-supersonic-separator/experience/. Accesed 2012 Mar 7. [21] Betting M, Epsom H (2007) High velocities make a unique separator and dewpointer, World Oil, 197-200. [22] Netušil M, Ditl P, González T (2012) Raw gas dehydration on supersonic swirling separator, 19th International Conference Process Engineering and Chemical Plant Design, Krakow. [23] Netusil M, Ditl P (2011) Comparison of three methods for natural gas dehydration, Journal of Natural Gas Chemistry 20: 471 - 476 [24] GPSA (2004) Engineering Data Book. 12th ed. Tulsa: GPSA Press. View publication stats IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 OPTIMIZATION OF TRIETHYLENE GLYCOL (TEG) DEHYDRATION IN A NATURAL GAS PROCESSING PLANT Nmegbu Chukwuma Godwin Jacob Department of Petroleum Engineering, Rivers State University of Science and Technology, Port Harcourt Abstract In optimizing glycol dehydration system for the natural gas processing, several critical parameters that can be varied to achieve specific dew point depression exist. From HYSYS simulation, a gas plant was placed under investigation with the aim of studying the effects of variation in glycol flow rates, number of stages (4, 8 and 12 trays) of the absorber, reboiler temperature ( 180oC, 190oC and 200oC) and stripping gas rate on the water content of the gas in a glycol dehydration unit in the Niger Delta. Results show that an increasing reboler temperature above 200 oC led to the thermal decomposition of glycol and introduction of stripping gas had a significant effect than increasing the reboiler temperature. Also, the design of the TEG system was adequate to achieve a dew point of about 5oC at 95bars. This study therefore seeks to proffer solutions for the optimization of the Niger Delta gas plant dehydration process using the HYSYS simulator. Keywords: Dehydration, Glycol, Optimization TEG.. --------------------------------------------------------------------***---------------------------------------------------------------------1. INTRODUCTION 2. MATERIALS AND METHODS Natural gas is a combustible mixture of hydrocarbon gases and a vital component of the world’s supply of energy. It is one of the cleanest, safest and most useful of all energy sources, formed primarily of methane, ethane, propane, butane and pentane [1]. In Its processing, the presence of water it poses a great problem and vapors of its water content can lead to hydrate formation at low temperatures and high pressures and/or corrosion when it is in contact with hydrogen sulphide (H2SO4) and carbon dioxide (CO2) components which are all present in the gas stream [2]. The removal of the water vapor that exists in these natural gases requires a complex treatment consisting of treatments with varying degrees of efficiency involving gas dehydration processes [3]. Glycol dehydration is the most common dehydration process used to attain pipeline sales specifications and field requirements [2], [4]. [5]. For this process, a liquid desiccant dehydrator serves to absorb water vapor from the gas system within the contactor. This desiccant having glycol as its principal agent has the chemical affinity for water, stripping most water content of the travelling gas stream. Regeneration of this glycol occurs in a specialized reboiler designed to vaporize only the water from the solution [6]. For the past few years, the design of glycol plants utilizes the rule-of thumb procedures. The problem of pigging in export plants reveals a significant wall thickness loss up to about 36% at some points in pipeline systems.[4], [6], [7] Presence of water in these systems in reaction with CO2 can cause corrosion. Routine pigging assessment showed that 4 - 9% of the liquid recovered from plant in the Niger Delta spur of the gas transmission system (GTS) as free water. This research is aimed at optimizing gas plant dehydration systems to meet the specified sales of gas-water dew point requirements of 5oC at 95bar. 2.1 The Contacting Unit (HPLT) A glycol contactor is divided into two sections, the smaller lower section known as the scrubber section (integral scrubber) and the larger upper section known as the absorber section consisting of a wire mesh mixed extractor and separation of these two trays is done by installing a special section called the Chimney tray. The wet inlet gas stream enters the integral scrubber tangentially through the gas inlet for centrifugal separation of liquid accumulation and then passes through the high capacity, high efficiency mist extractor for extra trapped liquid contents. The gas stream undergoes several processes before it comes in contact with the glycol liquid flowing counter-currently down the column. As the gas passes through the trays water is absorbed by these glycol from the gases until the gases become extremely dehydrated. Incoming glycol from surge tanks is cooled in a heat exchanger before it enters the top of the contactor through the contactor vessel by passing across the trays, stripping more water till it becomes increasingly enriched with water during the counter flow. Operating efficiency of these contactors depends on the gas flow rate, temperature, pressure and also on the lean glycol concentration and its circulation rate. 2.2 The TEG Regeneration Unit (HTLP) This unit is conditioned with removal of dissolved gases, liquid hydrocarbon, solids and water from the glycol. Rich glycol leaves the contactor and reaches the lean-rich glycol heat exchanger to warn the glycol in order to its viscosity and accelerate glycol-liquid-gas separation in the flash drum, proceeding to the cartridge filters solid particles are removed to prevent plugging in heat exchangers foaming, pigging/fouling, cell corrosion and hot spots on the fire tubes downstream of the cartridge filters. The cooled filtered _______________________________________________________________________________________ Volume: 03 Issue: 06 | Jun-2014, Available @ http://www.ijret.org 346 IJRET: International Journal of Research in Engineering and Technology glycol solution is routed through shell side of the HE to warm the glycol in order to reduce fuel consumption in the reboiler before entering the Still Column which operates at atmospheric pressure mounted on the reboiler at a fixed point. Glycol flows downward through the still column towards the reboiler contacting hot glycol vapor, water vapor and stripping gas. The temperature control (heater) in the reboiler operates a fuel gas motor valve to maintain the proper temperature in the glycol to reduce potential glycol degradation at about 374 oF on the condition that TEG decomposes at a temperature of 404oF. After a series of other glycol purification process, stripping gas is added to the reboiler to provide extra-dry glycol. TEG recovered from the reboiler which also increases glycol concentrations by providing counter-current contact of the glycol and the stripping gas.. hot regenerated glycol passes through a tube from the stripping column to cool thereby increasing its absorption capacity and its tendency to flash in the contactor. 2.3 Model Formulation Optimum Operation (Design of the Dehydrating Unit) The following steps are requisite for the proposed design; 1. Obtain design information. 2. Select an appropriate combination of lean glycol, circulation rate absorber trays or packing balance. 3. Establish the required balance i.e material balance, energy balance. The maximum and minimum gas flow rates were taken into account, among others are the gas composition and expected water dew point/ water content of the outlet gas. Obtaining the above parameters, the minimum concentration of TEG in the lean solution entering the top of the absorber required to meet outlet gas water specification. The required lean TEG rate needed to pick-up the necessary amount of water from the gas to meet the outlet gas water content specification. Also, the amount of absorber contact required to produce the necessary equilibrium in the above at any circulation rate will be considered. Other important considerations such as safety, utilities and environmental regulations for discharging stripper overhead are also evaluated. 2.4 Process Simulation of Gas Plant-X using HYSYS This study analyses the results of a plant simulation performed using the HYSYS simulator. This all-purpose simulator incorporates objective oriented programing techniques to provide an objective linking and embedded (OLE) technology responsible for creating a common interactive interface between HYSYS and other applications. Also providing increased functionality framework in integrating a broad range of technologies under a common look and feel. This simulator offers a unique attribute to support the implementation and solution of a wide range of applications in the chemical, petrochemical, oil and gas industry. eISSN: 2319-1163 | pISSN: 2321-7308 3. RESULTS AND DISCUSSION In order to investigate the important design variables, we obtained plots or residual water content versus circulation rates and they were presented. Among the plots made were for common values of such parameters like numbers of trays in the contactor, glycol circulation rate through the contactor, temperature of the recoiled in the regenerator and the amount of stripping gas used. 3.1 The Effects of Number of Equilibrium Stages in the Absorber Figure 1 illustrates the effect of the number of equilibrium contact trays on residual water content using a 200oC reboiler. Fig 2 presents a similar response comparing the dew point dispersion in place of actual water content. Increasing water content allows the gas to attain equilibrium with the lean circulation rate. Considering a typical glycol circulation rate of approximately 3galTEG/lb of water removed, fig 1 and 2 illustrates that a 12 tray contactor is virtually in equilibrium with the inlet glycol. For an 8-tray contactor a circulation rate of about 5 – 6 gal TEG/lb water could be required to attain equilibrium. 3.2 Effects of Reboiler on Temperature Increasing reboiler temperature to 204oC will result in a thermal decomposition of TEG. Even at 200 oC reboiler, glycol decomposition can occur due to higher film temperature and as such, temperature of about 180 oC, 190oC and 200oC were simulated Fig 3, 4 and 5 illustrates the overhead water content using a fixed number of trays at reboiler temperatures of 180oC, 190oC and 200oC for 8 or 12 contactor contactor trays, pipeline quality gas containing less than 71lb water/MMSCF gas could be produced using either the 190oC or 200oC reboiler temperature. At 190oC approximately 4.5gal TEG/lb water circulation would be needed with 8-tray contactor as opposed to approximately 2gal TEG/lb-water with 12-tray contactor. Similarly, at 200oC, approximately 3gal TEG/lb water circulation would be needed with 8-tray contactor as opposed to 1.5gal TEG/lb water with 12-tray contactor, noting that the deduced results for the gas inlet temperature of 35, 36, 37, 38 and 39 oC were also carried out. This gave a temperature differential ( lean glycol temperature – inlet gas temperature) for 3oC. reboiler temperature influences the overhead water content by changing the purity of the lean glycol. Glycol purity values of 96.0, 97.0 and 98.0wt% were obtained at 180oC, 190oC and 200oC respectively at pressure of about 1atm. 3.3 Effects of Stripping Gas Applications requiring high dew point depressions will always utilize stripping gas in the regenerator. A low dew point cannot simply be achieved by using the minimum 98% obtainable with 200oC reboiler temperature at atmospheric pressure. These low dew points will require up to 99.99wt% glycol in the absorber. Stripping gas poses a much greater effect than just increasing reboiler temperature. For a maximum efficiency, stripping gas must be introduced in a _______________________________________________________________________________________ Volume: 03 Issue: 06 | Jun-2014, Available @ http://www.ijret.org 347 IJRET: International Journal of Research in Engineering and Technology short column after the hot glycol is removed in the reboiler. They may be placed directly in the reboiler but high water partial pressures in the vapor phase limits its mass transfer driving force. Fig 6 and Fig 7 illustrates the stripping gas effects on the residual water content and dew point depression of the dry gas. As seen, at a small stripping gas rate of 1sc/gal circulation rate, dry gas will contain about half the water of the same process without stripping gas. Increasing the stripping gas rate beyond 2 - 3scf/gal will have a minimal impact on the dew point depression. Table 1 Deduced Values for water content and circulation rates for 4, 8 and 12 trays Outlet gas water content (lb/MMSCF) TEG rate(gal/lb water) N=4 N=8 N = 12 7.6 9.4 7.2 9.6 5.2 5.0 6 10 5.2 5.0 5 4 11 12 5.2 5.8 5.0 5.0 3 14 7.0 5.0 2 19 9.0 6.0 1.5 23 11.2 7.5 13 9.0 eISSN: 2319-1163 | pISSN: 2321-7308 Table 2 Deduced dew point depression for 4, 8 and 12 trays Dew point depression (oC) TEG rate(gal/lb water) 7.6 N=4 N=8 N = 12 7.2 14.16 22.50 23.06 6 13.33 522.10 23.06 5 11.94 21.589 22.89 4 10.00 20.88 22.77 3 7.778 19.44 22.50 2 3.889 16.389 21.38 1.5 0.2778 12.77 18.89 10.00 16.38 14.444 1.25 1.25 0.6 7.50 dew point depression (C) 25 N=4 20 N=8 15 10 5 0 0 0.6 5 10 15 TEG circulation rate (gas/lb-water) Fig 2 Effects of the number of contactor trays on the dew point depression of natural gas with reboiler temperature 200oC Table 3 Overhead water content using a Fixed number of contactor trays at different rebioler temperature (4 trays) Outlet gas water content (lb/mmscf) TEG Treb Treb =190oC Treb= 200oC o rate(gal/lb =180 C water) Fig 1 Effects of number of equilibrium stages in the contactor on water content of a stream of natural gas with reboiler temperature of 200oC 7.6 11.8 10 9 7 12 10 9.2 6 12.7 11 10.2 5 14 12 11.1 4 15.7 14 12.8 _______________________________________________________________________________________ Volume: 03 Issue: 06 | Jun-2014, Available @ http://www.ijret.org 348 IJRET: International Journal of Research in Engineering and Technology Table 5 Overhead water content using a Fixed number of contactor trays at different rebioler temperature (12 trays) Outlet gas water content(lb/mmscf) 17.8 16 15 2 21.5 20 19 1.5 25 24 23.6 Treb =180C water content (lb water/MMscf) 25 Treb =190C 20 15 10 5 0 0 2 4 6 8 TEG circulation rate (gal/lb-water) Fig 3 Water content of a natural gas stream heated by a 4tray contactor Table 4 Overhead water content using a Fixed number of contactor trays at different rebioler temperature (8 trays) Outlet gas water content (lb/mmscf) TEG Treb Treb =190oC Treb= o rate(gal/lb =180 C 200oC water) 7.8 8.2 7.5 8.22 6.8 7.2 8.3 6.82 5.35 6.0 8.5 6.95 5.5 5.0 8.8 7.0 5.65 4.0 9.0 7.4 6.0 3.0 9.8 8.0 6.85 2.0 11.5 9.5 8.45 1.2 14.8 13.4 12 Treb=180 14 Treb=190 12 Treb=200 water content 16 10 8 6 TEG rate(gal/lb water) Treb =180oC 7.8 8.0 7.4 8 6.2 7.2 8 6.2 5.0 6 8 6.25 5.0 5 8 6.25 5.0 4 8.15 6.4 5.1 3 8.3 6.52 5.25 2 8.89 7.1 5.7 1.6 9.5 7.6 6.7 Treb =190oC Treb= 200oC 10 water content (lbwater/MMSCF) 3 30 eISSN: 2319-1163 | pISSN: 2321-7308 8 6 4 Treb =180C 2 Treb =190C Treb= 200C 0 0 5 10 TEG ciculation rate (gal/lb-water) Fig 5 Water content of a natural gas stream heated by a 12tray contactor Table 6 Deduced values showing the effects of stripping gas rate on water content and glycol circulation rate Outlet gas water content (kg/h) TEG rate(gal/mmscf) 0scf/gal 1scf/gal 3scf/gal 6scf/gal 6.6 5.35 2.1 1.4 0.9 6 5.4 2.2 1.5 1 5 5.5 2.5 1.7 1.1 4 6 3.0 2 1.5 4 3 6.8 4 2.8 2.2 2 2 8.5 5.5 4.7 4.5 0 1.2 13.0 10.5 9 9 12.4 11 11 0 2 4 6 8 10 TEG circulation rate (gal/lb-water) Fig 4 Water content of a natural gas stream heated by a 8tray contactor 1 _______________________________________________________________________________________ Volume: 03 Issue: 06 | Jun-2014, Available @ http://www.ijret.org 349 water content (lb-water/mmscf) IJRET: International Journal of Research in Engineering and Technology 14 0scf/gal 12 1scf/gal 10 3scf/gal 8 6scf/gal 6 4 2 0 0 2 4 6 8 TEG circulation rate (gal/lb-water) Fig 6 Effects of stripping gas rate on water content of natural gas streams Table 7 Relationship between dew point depression and stripping gas rate Stripping gas rate Dew point depression (oC) (gal/mmscf) 6 32 3.2 30 1 26 0 18 35 Dew point depression (C) 30 25 20 15 10 5 0 0 2 4 6 8 Strpping gas rate (scf/gal TEG) Fig 7 Effects of stripping gas on dew point depression 4. CONCLUSIONS The design review involved a review of the TEG dehydration systems generally and in comparison to the proposed design, the Niger Delta plant is adequate and robust enough to condition the gas to comfortably achieve a water dew point of 5oC at 95bar. It is highly recommended that regular inspection of glycol contactor internals are conducted at intervals so as to check components of the system if they are in good condition and still operates properly. Also regular dew point measurements and validation from the moisture analyzers installed on the gas processing trains will be a lot easier with reverse eISSN: 2319-1163 | pISSN: 2321-7308 calculations using HYSYS. Operational review process involved spot checks of key process parameters in the TEG dehydration systems and plant is operated in a satisfactory manner within the design envelop. NOMENCLATURE ??? GTS HPLT LPHT Treb Triethylene Glycol Gas Transport Systems High Pressure Low Low Pressure High Temperature reboiler Temperature Temperature ACKNOWLEDGEMENTS The author highly appreciates the insurmountable efforts of Daniel Dasigha Pepple and Ohazuruike V. Lotanna in the fruitiness of this Study. Among other Such as Family, Friends and colleagues highly recognized. REFERENCES [1]. Arnold, K and Stewart, M. “Surface Production Operations”, Vol 2, Design of Gas Handling Systems and Facilities, 1999. [2]. Lyons, W. C.: “Standard Handbook of Petroleum and Natural Gas Engineering”, Vol. 3, 1994 [3]. Katz D.L “Handbook of Natural Gas Engineering, Production and Storage ”, McGraw-Hill Book Co., Inc 1990. [4]. Hernandez, V., Hainvinka, M. W. and Bullin, J. A.: “Designing Glycol Units For Maximum Efficiency”. Bryan Research and Engineering, Inc. 2001 [5]. Campbell, J. M.: “Gas Conditioning and Processing” Vol 2, Campbell and Company 2001. [6]. McLeod, W.R : Prediction and control of natural gas pipeline. SPE8137 presented at the European offshore petroleum conference and exhibition in London 24-27 October, 1978, pp.1-8 [7]. Rossi, L.F. and Gas paretto, C. A .1991 “Prediction of hydrate formation in natural gas system, paper SPE 22715 presented at the 66th Annual Technical Conference exhibition of the SPE held in Dallas, TX ,October 69,1991.pp. 1-7.. [8]. Sloan, E.D. “ Natural gas hydrate”, SPE 23562, Journal of Petroleum Technology, 1991. pp 1-4. [9]. Ikoku, C. U” Natural gas Engineering” Pennwell Publishing Company, Tulsa, Oklahoma 1980 [10]. Bhangole, A. Y. , Zhu, T., Mc Grail, B. P. and White, M. D. 2006 “A model to predict gas hydrate equilibrium and gas hydrate in the process media including mixed CO2 – CH4 Hydrates”, paper SPE 99759 paper presented at the 2006 SPE/DOE symposium on improved oil recovery held in Tulsa, Oklahoma,22-26,April 2006, Pp. 1-7. _______________________________________________________________________________________ Volume: 03 Issue: 06 | Jun-2014, Available @ http://www.ijret.org 350 Ministry of Higher Education & Scientific Research University of Technology Chemical Engineering Department A study of the dehydration process of natural gas in Iraqi North Gas Company and the treatment methods of molecular sieve problems A Research Submitted to the Department of Chemical Engineering University of Technology in Partial Fulfillment of the Requirements for the Degree of Higher Diploma in Chemical Engineering/Petroleum Refining and Gas Technology By Farman Saeed Abd. Zangana (B.Sc. in Chemical Engineering 2004) Supervised by Assist.Prof. Dr.Anaam A.Sabri 2012 CERTIFICATE We certify that we have read this research entitled " A study of the dehydration process of natural gas in Iraqi North Gas Company and the treatment methods of molecular sieve problems " by Farman Saeed Abd. Zangana and as an Examining Committee examined the student in its contents and that in our opinion it meets the standard of a research for the degree of Higher Diploma in Chemical Engineering /Petroleum Refining and Gas Technology. Signature: Assistant Prof. Dr. Anaam A.Sabri (Supervisor) Date: / / 2012 Signature: Signature: Assist. Prof. Dr.Mohammed F.Abid Prof. Dr. Mumtaz A. Zablouk (Member) Date: / (Chairman) / 2012 Date: / / 2012 Approved for the University of Technology Signature: Prof. Dr. Mumtaz A. Zablouk Head of Chemical Engineering Department Date: / / 2012 CERTIFICATION This is to certify that I have read the research titled " A study of the dehydration process of natural gas in Iraqi North Gas Company and the treatment methods of molecular sieve problems " and corrected any grammatical mistakes I found. The research is therefore qualified for debate. Signature: Prof. Dr. Mumtaz A. Zablouk University of Technology Date: / / 2012 Acknowledgments First of all, praise is to Allah for every thing. Without his great assistance the work wouldn't have been finished. I would like to express my sincere appreciation and thanks to my supervisor Assist.Prof. Dr.Anaam A.Sabri, for his constant guidance and valuable comments, without which, this research would not have been successfully completed. My grateful thanks to Prof. Dr. Mumtaz A. Zablouk, the Chairman of the Department of Chemical Engineering at the University of Technology for the provision of research facilities. My deep thanks go to Assist. Prof. Dr. Mohammed I. Mohammed, the head of post graduate committee for all the help and encouragement, also I wish to express my sincere gratitude to my family for them encouragement and helpful. Also I would like to convey my sincere appreciation to any one that helped me in Iraqi North Cas Company. Also I would like to convey my sincere appreciation to all staff of Chemical Engineering Department in the University of Technology and the workshops unit. Finally, to all that helped me in one way or another, I wish to express my thanks. Farman Abstract The purpose of this research is to study the dehydration process of natural gas by adsorption using molecular sieve as it is in the North Gas Company. Dehydration of natural gas is needed to remove the water that is associated with natural gas in vapor form. The natural gas industry has recognized that dehydration is necessary to insure smooth operation of gas transmission lines, dehydration prevents the formation of gas hydrates and reduces corrosion. Unless gases are dehydrated, liquid water may condense in pipeline and accumulate at low point along the line and reducing its flow capacity. Several methods have been developed to dehydrate gases in an industrial scale . The four major methods of dehydration are direct cooling , indirect cooling , absorption and adsorption. This study focuses on the adsorption method which is used to dehydrate the natural gas in North Gas Company and also focuses on the problem of breaking up and aging of the molecular sieve before ending its real life time .Some testing were made on the aging and the new molecular sieve to show the difference between them. Some suggestions are made to overcome the problem of aging of the molecular sieve, like improve the efficiency of gas separator,using antifoaming to prevent liquid carryover, choose the regeneration conditions carefully, using multi bed technology and using glycol dehydration unit with molecular sieve dehydration unit to reduce the water content of natural gas. Contents Subject 1 Page Chapter One Introduction 1-3 1-1 Problem Statement 3 1-2 Scope of the Study 3 2 Chapter Two 4 Literature Review 4 Types of Dehydration of Natural Gas 4 2-1-1 Direct Cooling 4 2-1-2 Indirect Cooling 4-5 2-1-3 Dehydration by Absorption 5-9 2-1-4 Dehydration by Adsorption (Solid Desiccant) 10 2-2 Types of Adsorbents 11 2-2-1 Alumina 11 2-2-2 Silica Gel and Silica-Alumina Gel 11 2-2-3 Molecular Sieves 2-3 Some Types of Molecular Sieves 14 2-3-1 3A Molecular Sieve 14 2-3-2 4A Molecular Sieve 14-15 2-3-3 5A Molecular Sieve 15-16 2-3-4 13X Molecular Sieve 2-1 11-13 16 2-4 Solid Desiccant Adsorption Kinetics 16-17 2-5 Fundamentals of Adsorption 17-18 2-6 Development of Adsorption Isotherms 18-19 2-6-1 Freundlich Isotherms 19- 20 2-6-2 Langmuir Isotherm 20-21 2-7 Some previous Work on Dehydration of Natural Gas 3 Chapter Three 23 3-1 Design and Theoretical Considerations Design Consideration 23 23 3-1-1 Solid Desiccant Dehydration 23 3-1-1-1 Allowable Gas Velocity 23-24 3-1-1-2 Bed Length to Diameter Ratio 24-25 3-1-1-3 Desiccant Capacity 25 3-1-1-4 MTZ Length 25 3-1-1-5 Breakthrough Time 25 3-1-2 Glycol Dehydration 25 3-1-2-1 Absorber (Contactor) 3-1-2-2 25-27 Still (Stripper) 27 3-1-2-3 Reboiler 27 3-1-2-4 Surge Tank (Accumulator) 28 3-1-2-5 Heat Exchanger 28 3-1-2-5-1 Glycol/Glycol Exchanger 28 3-1-2-5-2 Dry Gas/Lean Glycol Exchanger 28 3-1-2-6 Phase Separator (Flash Tank) 3-1-2-7 Glycol Circulation Pumps 29 3-1-2-8 Filters 29 3-1-2-8-1 Fabric (Particulate) Filters 29 3-1-2-8-2 Carbon Filters 29 Operational Problems 30 3-2-1 Solid Desiccant Dehydration 30 3-2-1-1 Bed Contamination 30 3-2-1-2 High Dew Point 3-2-1-3 Premature Breakthrough 31 3-2-1-4 Hydrothermal Damaging 31-32 3-2-1-5 Liquid Carryover 32 3-2-1-6 Bottom Support 32 3-2-2 Glycol Dehydration 33 3-2 U 28-29 30-31 3-2-2-1 Absorber 33 3-2-2-1-1 Insufficient Dehydration Causes of insufficient dehydration 33 3-2-2-1-2 Foaming 33 3-2-2-1-3 Hydrocarbon Solubility in TEG Solution 33 3-2-2-2 Still (Stripper) 3-2-2-3 Reboiler 34 3-2-2-3-1 Salt Contamination 34 3-2-2-3-2 Glycol Degradation 34-35 3-2-2-3-3 Acid Gas 35 3-2-2-3-4 Surge Tank 35 3-2-2-3-5 Heat Exchanger 3-2-2-3-6 Phase Separator (Flash Tank) 36 3-2-2-3-7 Glycol Circulation Pump 36 Chapter Four 37 Case Study 37 4 33-34 35-36 4-1 Dehydration Process 37-38 4-2 Regeneration Process 38-39 4-3 The Molecular Sieve Dehydrate 39-42 5 Chapter Five 43 5-1 Result and Discussion 43-49 5-2 Overcome the Breaking Up and Aging Problems of the Molecular Sieve Chapter Six Conclusions and Recommendations 49-50 Conclusions Recommendations 51-52 53 6 6-1 6-2 51 51-53 Chapter One Introduction Introduction Today, natural gas is one of the most important fuel in our life and one of the principle sources of energy for many of our day-to-day needs and activities. It is an important factor for the development of countries that have strong economies because it is a source of energy for household, industrial and commercial use, as well as to generate electricity. Natural gas, in itself, might be considered a very uninteresting gas - it is colorless, shapeless, and odorless in its pure form, but it is one of the cleanest, safest ,and most useful of all energy sources. Natural gas is a gaseous fossil fuel. Fossil fuels are essentially, the remains of plants and animals and microorganisms that lived millions and millions of years ago. It consists primarily of methane but including significant quantities of ethane, propane, butane, and pentane. Methane is a molecule made up of one carbon atom and four hydrogen atoms, and is referred to as CH4. Natural gas is considered 'dry' when it is almost pure methane, having had most of the other commonly associated hydrocarbons removed. When other hydrocarbons are present, the natural gas is 'wet'.The natural gas used by consumers is composed almost entirely of methane .However, natural gas found at the wellhead, although still composed primarily of methane, is by no means as pure. Raw natural gas comes from three types of wells: oil wells, gas wells, and condensate wells. Natural gas that comes from oil wells is typically termed 'associated gas'. This gas can exist separately from oil in the formation of (free gas),or dissolved in the crude oil (dissolved gas). Natural gas from gas and condensate wells ,in which there is little or no crude oil, is termed non associated gas. Gas wells typically produce raw natural gas by itself, while condensate wells produce free natural gas along with a semi-liquid hydrocarbon condensate. Whatever the source of the natural gas, once Page 1 Chapter One Introduction separated from crude oil it commonly exists in raw natural gas or sour gas.The raw natural gas contains water vapor, hydrogen sulfide (H2S),carbon dioxide, helium, nitrogen, and other compounds .In order to meet the requirements for a clean, dry, wholly gaseous fuel suitable for transmission through pipelines and distribution for burning by end users, the gas must go through several stages of processing, including the removal of entrained liquids from the gas, followed by drying to reduce water content. In order to remove water content, dehydration process is used to treat the natural gas. Dehydration is the removal of water from an object. In Physiologic terms, it entails a relative deficiency of water molecules in relation to other dissolved solutes. Gas dehydration is one of the most prominent unit operations in the natural gas industry(1). There are five major reasons for natural gas treating from water contents: • Liquid water and natural gas can form hydrates which causes plug the pipelines and other equipments such as valves, collectors etc. (Gas Hydrate: are solids formed by the physical combination of water and other small molecules of hydrocarbons. They are icy hydrocarbon compounds of about 10% hydrocarbon and 90% water). • Natural gas containing H2S and/or CO2 is corrosive when liquid water is present (corrosion often occurs when liquid water is present along with acidic gases, which tend to dissolved and disassociate in the water phase, forming acidic solutions). • Water content of natural gas decreases of it is heat value. • Liquid water in natural gas pipelines potentially causes slugging flow conditions resulting in lower flow efficiency of the pipelines. • In most commercial hydrocarbon processes, the presence of water may cause side reactions, foaming or catalyst deactivation(2). Page 2 Chapter One Introduction There are several methods of dehydrating natural gas. The refrigeration (direct cooling and indirect cooling). liquid desiccant (glycol) dehydration and solid desiccant dehydration, the first two methods employ cooling to condense the water molecules to the liquid phase with the subsequent injection of inhibitor to prevent hydrate formation. The other two methods utilize mass transfer of the water molecular into a liquid solvent (glycol solution) or a crystalline structure (dry desiccant). However, the choice of dehydration method is usually between glycol and solid (3). 1-1 Problem Statement Gas dehydration is a common process in gas treatment plant because water and hydrocarbons can form hydrates ,which may block valves and pipelines .Also water can cause corrosion in present of acid compounds in natural gas .In Iraqi North Gas Company, the molecular sieve of type 4A is used to remove water from the natural gas, the main problem is the breaking up and aging of the molecular sieves (1.5-2 year) before ending its service life time (3years). 1-2 Scope of the Study The aim of this research is 1.To conduct literature survey of different methods for dehydration of natural gas . 2.To describe the process of dehydration of natural gas using molecular sieve as it is in Iraqi North Gas Company and try to study the problem of breaking up and aging of the molecular sieve which is used in the dehydration process . 3.Suggesting solutions for this problem (aging of molecular sieve) . Page 3 Chapter Two Literature Review Literature Review 2-1 Dehydration Methods of Natural Gas Dehydration of natural gas is the process removal of the water that is associated with natural gases. The natural gas industry has recognized that dehydration is necessary to ensure smooth operation of gas transmission lines. Several methods have been developed to dehydrate gases on an industrial scale. 2-1-1 Direct Cooling The ability of natural gas to contain water vapor decreases as the temperature is lowered at constant pressure. During the cooling process, the excess water in the vapor state becomes liquid and is removed from the system. Natural gas containing less water vapor at low temperature is output from the cooling unit. The gas dehydrated by cooling is still at its water dew point unless the temperature is raised again or the pressure is decreased. It is often a good practice that cooling is used in conjunction with other dehydration processes. Glycol may be injected into the gas upstream ahead of the heat exchanger to reach lower temperatures before expansion into a low temperature separator(5). 2-1-2 Indirect Cooling Expansion is a second way of natural gas cooling. It can be achieved by the expander or Joule-Thomson valve. These processes are characterized by a temperature drop to remove condensed water to yield dehydrated natural gas. The principal is similar to the removal of humidity from outside air as a result of air conditioning. Gas is forced through a constriction called an expansion valve into space with a lower pressure. As a gas expands, the average distance between molecules increase. Because of intermolecular attractive forces, expansion causes an increase in the potential energy of the gas. If no external work is extracted in the process and no heat is transfered, the total energy of the gas remains the Page 4 Chapter Two Literature Review same. The increase in potential energy thus implies a decrease in kinetic energy and therefore in temperature(5). 2-1-3 Dehydration by Absorption The basis for gas dehydration using absorption is the absorbent; there are certain requirements for absorbents used in gas treating. ? Strong affinity for water to minimize the required amount of absorbent (liquid solvent). ? Low potential for corrosion in equipments, low volatility at the process temperature to minimize vaporization losses. ? Low affinity for hydrocarbons to minimize their loss during the process. ? Low solubility in hydrocarbons to minimize losses during treating. ? Low tendency to foam and emulsify to avoid reduction in gas handling capacity and minimize losses during regeneration. ? Good thermal stability to prevent decomposition during regeneration and low viscosity for easily pumping and good contact between gas and liquid phases. Off course, the major critical property for a good absorbent is the high affinity for water. The others are used to evaluate potential absorbents practical applicability in the industry (2) . There are numbers of liquids that can be used to absorb water from natural gases such as calcium chloride, lithium chloride and ...