Comparison of three methods for natural gas dehydration

Availableonlineatwww.sciencedirect.comScienceDirectJOURNAL OFNATURAL GASCHEMISTRYELSEVIERJournal of Natural Gas Chemistry 20(2011)471-476Comparison of three methods for natural gas dehydrationMichal Netusil". Pavel ditlDepartment of Process Engineering, Czech Technical University, Prague 6, 16607, Czech RepublI Manuscript received April 6, 2011: revised May 23, 2011 1AbstractThis paper compares three methods for natural gas dehydration that are widely applied in industry: (1)absorption by triethylene glycol, (2)adsorption on solid desiccants and(3)condensation. A comparison is made according to their energy demand and suitability for use. Theenergy calculations are performed on a model where 10 Nmh water saturated natural gas is processed at 30 C. The pressure of the gasvaries from 7 to 20 MPa. The required outlet concentration of water in natural gas is equivalent to the dew point temperature of -10C at gaspressure of 4 MPaKey wordsgas reservoir; underground gas storage; natural gas; gas dehydration1. IntroductionUGSs, vapors of higher hydrocarbons also contaminate thestored gas. The distribution specification sets the allowableThe theme of natural gas(NG ) dehydration is closely con- water concentration in NG by specifying a dew point tempernected with the storage of NG. There are two basic reasons ature(Tdew)of NG. Dew is usually taken to be-7"C for NGwhy storing NG is an interesting idea. First, it can decrease the at 4 MPa(2). This value is equivalent to roughly 5 gH20/mdependency on supply. Second, it can exploit the maximum NG at 4 MPa. The water content in NG at saturation is depencapacity of distribution lines. NG is stored in summer periodsdent on the temperature and pressure within the UGS. This iswhen there is lower demand for it, and is withdrawn in winter well presented in Figure No 20, Chapter 20, in the GPSA Dataperiods when significant amounts of NG are used for heatingBook(12th Edition). The average value of H2O in NG is fiveUnderground Gas Storages(UGSs)are the most advantageous times higher than that of required. a dehydration step of NGoption for storing large volumes of gas. Nowadays there are from UGS is therefore essential before the gas is distributedapproximately 130 UGSs inside the European Union. Their This paper compares industrially applied dehydration meth-total maximum technical storage capacity is around 95 bcmods according to their energy demand and suitability for use.According to the latest update, over 70 bcm of additional storThere are three types of UGSs: (1)aquifers, (2)depleted 2. Dehydration methodsage capacity will come on stream in Europe till 2020[1].oil/gas fields and (3)salt cavern reservoirs. Each of thesetypes possesses its own physical characteristics. Generally,2.1. Absorptionthe allowable pressure of stored gas inside a UGS is up to20 MPa. The inside pressure increases as the gas is injectedFirst dehydration method is absorption of H2O. Absorp-and decreases when there is a withdrawal. The output gas tion is usually performed using triethylene glycol (TEG). abpressure depends on further pipeline distribution. Distribution sorption proceeds in a glycol contactor (a tray column orsites normally begin at 7 MPa. The temperature of the gas packet bed) with countercurrent flows of wet NG and tEGusually ranges from 20-35C. The exact temperature varies During the contact, TEG is enriched by H2O and flows out ofwith the location of UGS and with the time of year. a disad- the bottom part of the contactor. Enriched TEG then continuesvantage of UGSs is that the gas becomes saturated by water flowing into the internal heat exchanger, which is incorporatevapors during the storage. In the case of depleted oil field at the top of the still column. It then flows into the flash drum,中国煤化工Corresponding author. Tel: +420-2243522714: Fax: +420-224310292: E-mail netusilL michalThis work was supported by the Inovation and Optimalization of Technologies for Natural GasCNMHGCopyright@2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserveddoi:l0.1016s10039953(1060218-6472Michal Netusil et al/ Joumal of Natural Gas Chemistry Vol. 20 No 5 2011where the flash gases are released and separated from the temperature should not exceed 208C based on the decompostream. TEG then runs to the cold side of the TEG/TEG sition temperature of TEG Regenerated TEG is then pumpedheat exchanger. Just afterwards, warmed TEG is filtered and back through the hot side of the tEGTEG and NG/TEG heatsprayed into the still column. From there, TEG runs into the exchanger into the top of the contactor. The entire process isreboiler In the reboiler, H2O is boiled out of TEG. The inside depicted in Figure 1 3]FlashFlash drumGlykol contactorReboiler∧RichTEGlet scrubberFigure 1. Scheme of absorption dehydrationThe purity of the regenerated TEG and the circulation aromatic hydrocarbons, 30% naphthenes and 10% paraffins.rate(LTEG/kgH,o) limit the obtained output Idew of NG. Water separator of thephase solvent is crucial for thisGas stripping can be implemented to enhance TEG regener- method. The Coldfinger regeneration system employs a cool-ation. Proprietary design DRIZO, licensed by Poser-NAT, ing coil (the"coldfinger")in the vapor space of the surge tank.COLDFINGER and Gas Conditioners International, have The cooling taking place there causes the condensation of abeen patented as an alternative to traditional stripping gas huge amount of vapors. The condensate is a water rich TEGunits. The Drizo regeneration system utilizes a recoverable mixture, which leads to a further separation process [4]. Ensolvent as the stripping medium. The patent operates with hanced regeneration systems are depicted in Figure 2.isooctane solvent, but the typical composition is about 60%flare of recycleflare or recycleStill columnStill columnStill columnRich TEGRich tEg3-PhasRich TEGReboilerReboilerStripping gasCooling mediumtrippingBlanket gasrge tankSurge tankLcmo←Qcan TEG中国煤化工[TyCNMHGStripping gasDrizoL GoldfingerFigure 2. Scheme of enhanced TEG regeneration systemsJourmal of Natural Gas Chemistry VoL. 20 No. 5 20114732.2. Adsorptionspecified by time borders A, B, C and D with appropriate bor-der temperatures TA, TB, Tc and TD. Regeneration starts atThe second dehydration method is adsorption of H2O by point A. The inlet regeneration gas warms the column and thea solid desiccant. In this method, H20 is usually adsorbed on adsorbent. Around a temperature of 120C(TB), the sorbeda molecular sieve, silica gel or alumina. A comparison of the humidity starts to evaporate from the pores. The adsorbentphysical properties of each desiccant is shown in Table 1 [5,6]. continues warming more slowly, because a considerable partof the heat is consumed by water evaporation From point C,Table 1. Comparison of the physical properties of desiccantsaround the temperature of 140C(Tc), it can be assumed thatused for NG dehydrationall water has been desorbed. Adsorbent is further heated toSilica gel Alumina Molecular sieve desorb C5+ and other contaminants till point D. The regenera-area750-830210650-800tion is completed when the outlet temperature of the regenerPore volume(cm /g)04-0450.210.27ation gas reaches 180-190C(TD). Finally, cooling proceedsPore diameter(A)from point d to E. The temperature of the cooling gas shouldDesign capacity7-99-12not decrease below 50C, in order to prevent any water con-(kEH2 o/100 kgdesiccant)densation from the cooling gas [7]. Part of the processed NG721800-880690-720is sometimes used as the regeneration gas. Then it is cooled,Heat capacity(-kg-k-)920and water condensed when it is separated. After H2O sepa-Regeneration temperature (C) 230ration, the regeneration gas is added back into the processed3718streamource: Tagliabue(2009). Gandhidasan(2001)So-called LBTSA (Layered Bed Temperature-Swing Ad-sorption)processes are an upgrade of TSA method. Here, theadsorption column is composed of several layers of differentThe amount of adsorbed H2O molecules incradsorbents. Hence the properties of the separate adsorbentsthe gas pressure and decreases with its temperature, which are combined in one column. For example, a combination ofare taken into account when the process parameters are de- silica gel with alumina is used in NG dehydration. Aluminasigned. Adsorption dehydration columns always work period- has better resistance to liquid water, so it is put in the firstically. Minima of two bed systems are used. Typically, one place to contact the wet NG. This ordering prolongs the life-bed dries the gas while the other is being regenerated Regen- time of the silica gel, which is placed below the alumina layereration is performed by preheated gas, as depicted in Figure 32.3. CondensationRegenerationThe third dehydration method employs gas cooling to turnH20 molecules into the liquid phase and then removes themfrom the stream. Natural gas liquids(NGLs)and condensedWaterhigher hydrocarbons can also be recovered from NG by cool-ing. The condensation method is therefore usually appliedRegenerationfor simultaneous dehydration and NGL recovery. NG can beadvantageously cooled using the Joule-Thompson effect (JTDry NGeffect). The JT effect describes how the temperature of a gaschanges with pressure adjustment. For NG, owing to expan-igure 3. Scheme of the temperature swing adsorption dehydration process sion, the average distance between its molecules increases,leading to an increase in their potential energy ( Van der WaalsThe heater for tSa can be realized as an ordinary burner forces). During expansion, there is no heat exchange with theor as a shell and tube heat exchanger warmed by steam or hot environment or work creation. Therefore, according to theil.The regeneration gas flows through the adsorbent into a conservation law, the increase in potential energy leads to acooler(usually using cold air) and then further into the sep- decrease in kinetic energy and thus a temperature decrease ofarator. Most of the desorbed humidity from the adsorbentNG. However, there is another phenomenon connected withremoved there. a downstream flow of wet NG through the the cooling of wet NG. Attention should be paid to the forma-adsorption column is usually applied. In this way, floating tion of methane hydrate. Methane hydrate is a solid in which aand channeling of an adsorbent is avoided. The regenera- large amount of methane is trapped within the crystal structuretion is performed by countercurrent fow in order to provide of water, forming a solid similar to ice. The hydrate produc-complete regeneration from the bottom of the column, where tion from a unit amount of water is higher than the ice formathe last contact of the dried NG with the adsorbent proceeds. tion. Hydrates中国煤化工 g the flow. This isThe typical temperature course for regeneration of molecular usually preventH G monoethylsIevesis presented by Kumar(1987)[7). The shape of the col(MEG)hydranto civic caLli cooling. Figure 4curve representing the course of the outlet regeneration gas depicts a dehydration method utilizing the JT effect and hytemperature is typically composed of four regions. They are drate inhibitionMichal Netusil et al/ Joumal of Natural Gas Chemistry VoL. 20 No 5 2011The wet NG is throttled in two steps inside the flash tanks. defreeze any ice that may form. A condensation method is ap-The lower temperature(due to the JT effect)of the gas stream plied when suitable conditions for the T effect are availablein the flash tanks leads to partial condensation of H20 vaEach of the methods presented here has its advantagespors. The droplets created are removed from the gas stream and disadvantages. Absorption by TEG is nowadays the mostby a demister inside the flashes. In cases when cooling by widely used method. Outlet Tdew around -10C is usu-the jT effect is insufficient(the usable pressure difference be- ally reached. Indeed, with improved reboiler design (Vac-tween the UGS and the distribution network is insufficient), uum stripping, Drizo, Coldfinger), the outlet Tdew is even 2-3the air precooler and the external cooler are turned on. Since times lower. However, TEG has a problem of sulfur or gasdehydration is normally applied to large volume of NG, the contaminated with higher hydrocarbons. The tEg in the re-external coolers need to have good performance, so this type boiler foams, and with time it degrades into a "black mudof cooling is very energy expensive. However, if the usable BTEX emissions(the acronym for benzene, toluene, ethylpressure difference is large, the JT effect inside the flashes benzene and xylenes)in the reboiler vent are a further disadis so strong that internal heating of the flashes is required to vantageFlash IFlash 2ExternalCondensateMEGFigure 4. Scheme of dehydration method utilizing the T effect and hydrate inhibitiAdsorption dehydration can obtain very low outlet water 7 to 20 MPa, but in the case of the condensation method theconcentration of Idew<-50C, and contaminated gases are pressure range started at 10 MPa. The required outlet concen-not a problem. Even corrosion of the equipment proceeds at tration of H2o in NG was equivalent to the dew point tempera slow rate. However, adsorption requires high capital invest- ature of -10C at gas pressure of 4 MPament and has high space requirements. The adsorption IThe calculation of TEG absorption was based on GPSacess runs with at least two columns(some lines use three four (2004)[9. The results were compared with the paper byor as many as six). Adsorption columns are taller and heav- Gandhidasan(2003)[8]and with industrial data provided byier than absorption contactors. The allowed flow velocity for ATEKO a.s. The total energy demand was composed of heatTEG contactors is approximately three times higher than that for TEG regeneration in the reboiler, energy for the pumps,for adsorption. This results in an approximately 70% larger filtration and after-cooling the lean TEG before entering thediameter of the adsorption column for the same amount of contactor. Enhanced regeneration was not considered. Theprocessed gas Industrial experience indicates that the capital basic parameters for the calculation were: regeneration tem-cost for an adsorption line is 2-3 times higher than that for an perature of 200C, concentration of lean TEG of 98.5%andcirculation ratio of 35 LTEG/kgH,oExpansion dehydration is the most suitable methodFor calculating adsorption dehydration, molecular sievecases when a high pressure difference is available betw5a was considered to be the most suitable adsorbent. The to-uGs and the distribution connection However the difference tal energy demand was directly connected to the regenerationdecreases during the withdrawal period and becomes in- gas heater, and no other consumption was assumed. The cal-ufficient, so that an external cooling cycle is needed. A cycle culations were again based on GPSA(2004). The results werecompared with the papers reported by Gandhidasan(2001)for hydrate inhibitor regeneration from the condensate sepa- and Kumar(1987)[7]. The calculation procedure for GPSArated inside the flashes is also requiredand Gandhidasan arose from the summation of the particularheats, i.e. the heat for adsorbent warming, the heat for column3. Experimentalat for water desorption. Kumars calculation proced中国煤化工 eneration step wasThe energy demand for the methods presented heredivided into f[7. Afterwards wecompared on the basis of a model, where a volumedetermined wC N MH Goceeded in each re-10 Nm/h NG from UGS was processed. The NG was H20 gion, what the border and average temperatures were, andhowsaturated at 30C. The pressure of the gas was varied from much energy was required to cover these phenomena. Finallyral Gas Chemistry Vol. 20 No 5 2011the demands for each region were added. The basic parame- Under high pressures of NG(>16 MPa), the energy demandters for all procedures were: temperature of the regeneration of the condensation method was at its lowest, and it remainedgas of 300C, time of adsorption/regeneration of 12 h and two nearly constant with an average value around 36 kwcolumn designs.The courses of the energy demand for the adsorption andThe condensation method was calculated on the basis absorption methods were quite similar: with increasing presof industrial data provided by TEBODiN s.r.o. and sup- sure of dehydrated NG, the energy demand slowly decreasedplementary calculations of the JT effect. The key parame- The absorption method was less demanding on the wholeter influencing energy demand was the pressure of NG from pressure scale, and began with consumption of 120 kw atUGS. Because it was not feasible to apply this method under 7 MPa. The adsorption method started with 234 kw at 7 MPa,low pressures, and the provided data start at 10 MPa, the pres- but the energy demand decreased slightly more as the pressure range was adjusted. The total energy demand consisted sure of NG in UGS was risen This led to a gradual decreasefor MEG injection and conde54 kW for absorption and 103 kw for adsorpting a the energyof the air pre-cooling unit, the extemal cooling, the pumps in the difference between these two methods, andoff take, the heat for MEG demand at the final pressure values of 20 MPa were equal toregeneration, and flash heatin4. Results5. DiscussionThe results obtained by TEG absorption method were theBy far the highest energy demand of the condensationsame for each of the calculation procedures, and good agree- method at low pressures of NG from UGS is due to the presment with industrial data was also obtained. However, the cal- sure being close to the distribution pressure so that pressureculation procedures by the adsorption method led to different cannot be used for the jT effect in flashes. Cooling is thenresults. Hence, the average energy demand value was taken compensated by the air pre-cooler and the external cooling deas the reference. The maximum deviation from it was below vice, which are unsuitable for large volume of processed NG20% for all the calculation procedures. The source of the de- HoHowever, as the pressure difference between UGS and the dis-viation lies in the "loss factor and the non-steady state factortribution site increases, the space for expansion rises and theIn the case of the condensation method, the calculated values JT effect proceeds with increasing impact. This is projectedfor the JT effect were in good agreement with the industrial into a linear decrease in the energy demand of the air predata, but the amount of data was limited, resulting in limited cooler and the external cooling device. From the point whererepresentation of the condensation methodthere is a pressure of NG >14 MPa, flash heating is gradu-The final energy consumption results for each dehydra- ally turned on to prevent any freezing caused by the strongtion method are summarized and shown in Figure 5JT effect. The energy demand of flash heating is reflectedin the total energy consumption. Finally, at pressures ofNG>16 MPa, total cooling and subsequent condensation areachieved by the JT effect. The total energy demand, whichconsists of flash heating and inhibitor injection and regeneration, remains constant.For adsorption and absorption dehydration methods, thesimilar falling courses of the energy demand with increasingpressure of NG can be explained by the fact that with increas-ing pressure within a UDG, the amount of H2O present in theNG decreases. Generally, the absorption method consumesless energy, because the regeneration of TEG is less demandng than adsorbent regeneration. The composition of the totalenergy demand of the adnethod can be divided intP/MPasorbent and the column is approximately 55%, 31% and 1496,Figure 5. Final results of energy consumption for each dehydration method respectively. It also has to be assumed that just part of the heatin the regeneration gas transfers to the adsorbent, the columnUnder low pressures (pressure of NG from UGS and heat loss leave to the atmosphere, and the balance leaves<13 MPa). the condensation method was the most demanding with the hot gasone. Its demand decreased linearly with pressure to a value ofIn brief. firov demand. the most145 kw for 13 MPa. At this point, the energy demand for the appropriate中国煤化工 f high pressures iscondensation method was roughly the same as that for the ad- the stored NGCNMHGholds for NG fromsorption method. When NG pressure was further increased UGS with pressure >1and distribution pressure re-from 13 MPa to 16 MPa, the energy demand for the conden- quirement of 7 MPa. Under lower pressures, the condensationsation method still decreased, but with a lowering tendency. method is used if the objective is to recover NGl and remove476Michal Netusil et al Jourmal of Natural Gas Chemistry VoL 20 No 5 2011water simultaneously. However, this is usually not the caseBtEX benzene, toluene, ethy benzene and xyleneswhen storing NG in a UGS. In cases when insufficient pres-sure difference is available, the absorption method is therefore Refavored over the adsorption method in terms of energy de-mand. TEG absorption is nearly twice less demanding. However, if a gas contaminated with sulfur or higher hydrocarbons[1] Gas infrastructure Europe [Internet]. Brussels: GSEMap Dataset in Excel-format Storage map (version: Auis being processed, TEG in the reboiler foams and degrades[cited 2010 Mar 8].Available fromwith time. This occurs when a depleted oil field is used ashttp://www.gie.ew/maps_data/storage.htmla UGS Adsorption is preferred in cases when very low Tdew [2NET4GAS [InternetGas quality pa-(H2O concentration lower than 1 ppm)of NG is required, forrameters[cited 2010 Mar 8]Available fromexample, when NG is liquefiedhttp://extranet.transgas.cz/caloricity-spec.aspx[31 Bahadori A, Vuthaluru H B Energy, 2009, 34(1 1): 1910Abbreviations[4] Hubbard R A, Campbell J M. Hydrocarbon Eng, 2000, 5: 71natural gas[5] Tagliabue M, Farrusseng D, Valencia S, Aguado S, Ravon, U,NGL natural gas liquidRizzo C, Corma A, Mirodatos C. Chem Eng J, 2009, 155(3)UGS553bbillion cubic meter[6]Gandhidasan P, Al-Farayedhi AA, Al-Mubarak AA. Energydew point temperature2001,26(9):855TEGtriethylene glycol[7] Kumar S Gas Production Engineering. Houston: Gulf Profes-MEGmonoethylenglycolsional Publishing. 1987[81 Gandhidasan P. Energy Sources, 2003, 25(3): 189LBTSA layered bed temperature swing adsorption[9] GPSA Engineering Data Book. 12th ed. Tulsa: GPSA Press,JT effect Joule-Thompson effect中国煤化工CNMHG