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Gas Dehydration Processes

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    -Mo/Sieve

    I TEG DRIZOTemperature - Methanol Injection< -40CI Temperature> -40C .--- MEG InjectionNGL Extraction l- f----TEG wi EnhancedPipeline Regeneration

    Specification I ~ Methanol InjectionI Refrigeration. Ambient .:

    Temperature ErrGI Sil ica GelMembranes

    AN; APPRAISAL OF GASDE,H ,YD ,RATION PROCE:SS :E :SRobert A. Hubbard, John M. Campbell & Co., USA, provides anevaluation of the most popular dehydration processes available

    today, including a number of applications.

    D ehydration is a required step in the processingof natural gas for transportation and sale, or forthe extraction of NGLs. Several dehydrationprocesses are available. Selection of the optimumprocess depends on several factors including dew-point specification, processing temperature and gascomposition. This article reviews the most populardehydration processes available today, outl iningadvantages and disadvantages as wel l as appl icationconsiderations.

    ,. - "'\ .~_:_,iferVleWThe removal of water from natural gas can be accom-plished in a number of ways. Commercial methodscurrently include: Absorption (glycol dehydration). Adsorption (dry desiccant). Condensation (glycol/methanol inject ion). Permeation (membranes).

    The first two methods use mass transfer of the watermolecule into a liquid solvent (glycol solution) or a crystallinestructure (dry desiccant). The third method employs coolingto condense the water molecule to the liquid phase with thesubsequent injection of inhibitor (glycol or methanol) to pre-vent hydrate formation. The fourth takes advantage of thedifferent permeation rates of water and hydrocarbonsacross a semi permeable polymer membrane. Figure 1shows a flow diagram, which simplifies the selection of corn-mercial gas dehydration processes.Glycol dehydration

    rout question, glycol dehydration is the most widelyused method to dehydrate natural gas' worldwide. Sizesvary from 2800 m3 (std)/d [100 mcfd] to 40 x 106m3 (std)/d[1400 mcfd]. Glycol dehydration units are simple, provenand relatively inexpensive. They can easily meet mostpipeline specifications, but can also dehydrate to very lowwater dewpoints if required. When equipped with enhancedregeneration capability they can dehydrate natural gas towater dewpoints below -40 C.

    A typical glycol unit is shown in Figure 2. Contactors aretypically packed with structured packing or trayed with bub-ble cap trays. A new, high capacity Swirl Tube tray hasbeen introduced by Shell. In applications where the waterdewpoint specification is set by the sales gas contract (typ-ically 0 to -10 C) the contactor will usually contain 2 - 2.5theoretical stages. This is equivalent to approximately 8bubble cap trays or 3 to 3.5 m (10 to 11.5 ft) of packing. Forvery low dewpoint applications, the contactor wi ll typicallybe designed with 3 to 3.5 theoretical stages which is equiv-alent to 12 to 14 trays or 5 to 6 m (16.5 to 20 tt) of packing.The critical parameter in the design of high dewpointsuppression glycol units is the lean TEG concentration.

    Figure 1.Decision chart for selection of gas dehydrationprocesses.

    When the water dewpoint requirement is below -40 C, therequired lean TEG concentration frequently exceeds 99.95wt%. A standard regenerator operating at atmosphericpressure and 204 C (400 F) can produce a lean TEG con-centration of about 98.7 wt%. Higher concentrations neces-sitate some type of enhanced regeneration system.

    The three commercial enhanced regeneration systemsmost commonly used are listed below: Stripping gas. Drizo. Coldfinger.

    Each of these is shown in Figure 3.Stripping gasStripping gas is the most common method employed toincrease lean TEG concentration. It is simple, cheap andeffective. Dry natural gas is introduced into the regenera-tor either through a small packed column below the reboil-er or into the reboiler itself. Stripping gas rates are low,typically less than about 50 m3 (std) per m3 TEG. Mostdesigns incorporate a small packed stripping columnbelow the reboiler to increase the effectiveness of thestripping gas.

    Stripping gas is very popular because it is simple, inex-pensive and nonproprietary. However, the required strip-ping gas rates increase dramatically as the required leanTEG concentration goes up. For concentrations above99.95%, the stripping gas rates can exceed 75 m3 (std)/m3TEG (10 set/US gal). This creates several problems: If the stripping gas is vented or flared, operating costs

    are increased If the stripping gas is compressed and recycled backinto the process, this increases system complexity

    'This article is based on a paper presented by the author at the Annual Continental European GPA meeting in Antwerp, September 1997.

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    CARBON FIL TERSSOCK FlLTERSTO HeDRAIN

    WATER VAPORre-_"'~~V~~DA~;.F O~AS

    INCINERA TOR

    H.M.SUPPL Y&RE71JRN

    LEAN TE GBOOSTER PUMP(IF REQ'O)

    (RICH TEG )

    Figure 2.Basicglycol dehydration unit.

    The ability to condense BTEX components from theregenerator vent is compromised by high stripping gasrates.

    It not only increases hydrocarbon emissions from theTEG unit, which in itself can raise environmental issues,but in countries (such as Norway) where CO2 or carbontaxes are imposed, stripping gas can incur heavy taxes.

    DrizoAs an alternative to traditional stripping gas units, manycompanies use the Drizo regeneration system. This is apatented proprietary design licensed by Proser-NAT, whichis essentially a stripping gas system, but utilises a recov-erable solvent as the stripping material. The solvent isvaporised and introduced into the stripping column belowthe regenerator. It is then condensed with the water leav-ing the regenerator, separated and recirculated back to thevaporiser and stripping column. In the original Drizo patentthe solvent was iso-octane.

    In actual practice, the solvent used consists primarily ofthose hydrocarbons co-absorbed from the gas in the TEGcontactor and subsequently recovered in the regenerator.As would be expected, the solvent is predominately aro-

    matic hydrocarbons. A typical composition isabout 60 - 70% aromatics, 20 - 30% naph-thenes and about 10% paraffins. Any excesssolvent recovered in the system is removed .atthe three phase solvent-water separatordownstream of the condenser.The Drizo process was originally marketedas a regeneration scheme for very high dew-point suppression TEG units. While a fewinstallations of the systems have indeed beenapplied in this service, the most popularapplications have been for outlet water dew-point requirements >-40 C where the recovery

    of BTEX and other heavy hydrocarbons from the regenera-tor overhead is desired. A brief summary of advantages anddisadvantages of Drizo is shown in Table 1.ColdfingerA third enhanced regeneration system for glycol dehydra-tion units is Coldfinger, a proprietary process licensed byGas Conditioners International (GCI). The designemploys a cooling coil (the 'coldfinger') in the vapourspace above the hot lean glycol in the surge tank. Thisvapour is rich in water, about 85 - 90 mol%. Consequently,any cooling that takes place in this vapour space willresult in the condensation of a TEG water mixture with ahigh concentration of water. This water rich liquid isdrained from the coldfinger into a blowcase where it isperiodically recycled back to the regenerator to recoverthe TEG and drive off the water.

    Since water is continuously removed from the surge tankvapour space, the partial pressure of water in the vapourphase is decreased, resulting in a non-equilibrium systemand creating the stripping effect which concentrates the TEG.

    GCI claims that the Coldfinger process can achievelean TEG concentrations of 99.9 wt%. The author has seen

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    such installations producing lean TEG concentrations of99.6 - 99.7 wt%.

    The process is marketed as a regeneration option thatdoes not require stripping gas. This is of significance sincesmaller volumes of hydrocarbons will be present in theregenerator vent. A small amount of gas is, however, intro-duced into the surge tank to replace the condensatedrained from the coldfinqer. Any additional gas (over andabove that required to replace the condensed liquid) leavesthe regenerator vent and effectively serves as traditionalstripping gas. It is sometimes difficult to determine howmuch of the lean TEG concentration enhancement is due tothe Coldfinger effect and how much is due to the strippinggas effect. Nonetheless, the design is quite popular withover 60 units instal led worldwide". Advantages and disad-vantages of the Coldfinger process are shown in Table 2.Condensation processesCondensation processes employ cooling to remove thewater from the gas by condensation. This requires the inhi-bition of the condensed water to prevent the hydrate and/orice deposition. The most common inhibitors are monoethyl-ene glycol (MEG) or methanol.~--col injectionGlydol (MEG) inject ion processes have been used in thegas processing industry for nearly 50 years. They are sim-ple, proven and inexpensive. A flow diagram for a typicalMEG injection system is shown in Figure 4. Glycol injectioncan be used in all types of refrigerated processes: mechan-ical refrigeration, valve expansion and (in the case of theKollsnes onshore facility in Norway) turboexpanders.

    The concentration of the lean MEG solution injectedupstream of the exchangers isusually 75 - 80 wt%. The con-centration of the rich solutiondrained from the cold separa-tor is usually 70 - 79 wt%.

    MEG injection systemsare effective to temperaturesof about -40 C. Below thistemperature, the viscosity ofthe MEG-water mixture is toohigh for efficient separation oft"",rich solution from the con-

    .ised . hydrocarbons. Inaddition, the range of richMEG concentrations that pro-vides effective inhibition isvery narrow.

    Circulation rates are typi-cally low (comparable to aTEG unit) and depend on thewater content of the feed gasentering the process and themixing efficiency of the MEGand the gas. The MEG is intro-duced into the gas by injectionthrough spray nozzles thatatomise it into a fine mist. Thismist is carried with the gasthrough the heat exchangers,valves, expanders, etc. andprovides the inhibition of thewater as it condenses fromthe gas.

    A common misconceptionabout glycol injection systems

    is that the MEG 'absorbs' the water from the gas. This is nottrue. The water condenses due to cooling and the MEGdepresses the water hydrate/freezing point. At the concen-trations typically employed, the equil ibrium water dewpointof the vapour leaving the cold separator is only a fewdegrees lower than the separator temperature.

    One of the disadvantages of glycol injection systems isthe separation of the rich MEG solution from the condensedhydrocarbons in the cold separator. This is a dif ficult separa-tion to achieve, particularly in the glycol phase because ofthe high viscosi ty of the solution. Signi ficant entrainment ofhydrocarbons in the MEG is frequently encountered. Thiscan result in increased MEG losses at the regenerator due tothe 'stripping' effect of these hydrocarbons.

    A second disadvantage is the solubility of aromatichydrocarbons in the MEG. These soluble hydrocarbons arereleased in the regenerator overhead much like in a TEGunit. The solubility of BTEX components in MEG is lessthan in TEG. Solely on the basis of solubility, the BTEXemissions from a MEG regenerator will be about 10 - 15%of the emissions from a TEG regenerator. However, hydro-carbon entrainment in the glycol leaving the cold separatorcan increase the amount of BTEX in the rich MEG; henceincreasing emissions.Some operators have also reported significant prob-lems with salt accumulation in the MEG. This problem isnot inherent to the glycol injection system, but it is causedby inadequate separation upstream of the glycol system.Similar problems occur in TEG absorption systems whensalt water is entrained in the gas leaving the upstream sep-arator. Advantages and disadvantages of MEG injectionsystems are summarised in Table 3.

    Flue Gas

    D 1 > & C tFl'ed

    a) Strippinggas

    Flue GasVent Gases to Flare

    or RecycleFlue Gas

    Glycol Pump

    Water R ichTEG MIXtUreTo Blowcase

    b ) DRIZO c) GOLDFINGER"

    Figure 3. TEG regeneration alternatives.

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    Table 2. Advantages and dIsadvantages of Coldf lngerAdvantages DIsadvantagesCan increase the lean TEGconcentrat ion Limited to lean TEG concentration less thanhigher levels than can be achieved 99.9 wt% which Is generally not adequate to achieveIn a conventional regenerator without water dewpolnts less than about 30 C [ -40 C.

    BTEX emissions In regenerator vent. '".- - . '.C an result In_high MEG l0,;ses i f three phase separat ion. In .cold separator i s Inadequate . .

    . Imposes slightly higher refrigeration duties to account for thelatent heat of water . and sensible heat of inhibitor .- ..

    column at BP Amoco's P-15D platformin the Dutch sector of the North Sea is I'24 m high and is equipped with an exter- rnal reboiler, condenser, reflux drum,reflux pump, etc-. Compare this to a gly-col regenerator in a MEG injection sys-tem and one begins to see why 9IYCOll

    1

    , . ~

    injection is more popular than methanolinjection.IFPEXOLIFPEXOL was introduced to the gas pro-cessing industry in 1991 by InstitutFrancais du Petrole (IFP). It is amethanol injection process whichemploys a novel regeneration scheme.The rich methanol is regenerated not byfractionation, but by str ipping with a por-tion of the feed gas entering the process.A simplified flow diagram for the processis shown in Figure 5. As discussed previ-ously, methanol is 3 to 4 times morevolatile than water. Methanol will betherefore preferentially stripped from amethanol-water solution in the presenceof a stripping medium such as naturalgas. The required stripping gas rate istypically some portion (30 - 60%) of thefeed gas rate. Higher stripping gas ratesincrease the diameter of the strippingcolumn, but decrease the methanol con-tent of the water leaving the bottom ofthe column. The process is simple androbust. Problems associated withmethanol-water disti llation systems areavoided. In addition, there are no emis-sions of aromatic hydrocarbons as foundin glycol regenerators.

    The disadvantages of the processare essential ly those disadvantages thatare inherent to methanol injection sys-tems in general: contaminated productstreams and methanol losses.Depending on process temperaturesand pressures, the methanol content ofthe outlet gas stream can bequite high.At temperatures on the order of -20 'Cand pressures of 40 - 70 barg (580 -1015 psig) the methanol content of thegas leaving the cold separators can be

    on the order of 300 - 400 ppm (vol).In addition to vapour phase losses, methanol is also

    soluble in the liquid hydrocarbon phase. This methanol isoften recovered by using a water wash on the liquid hydro-carbon stream. The water wash step is simple; but theresult is a liquid hydrocarbon stream that is water saturat-ed. If the hydrocarbon stream is required to meet typicalNGL dryness specifications it must be dehydrated. Thisobviously increases the cost and complexity of themethanol injection system. Despite these disadvantages,IFPEXOL is an attractive inhibition option in applicationswhere the gas is refrigerated. A summary of advantagesand disadvantages is shown in Table 5.

    Methanol injectionIn condensation systems, methanol can be used as an alter-native to MEG for inhibition of the condensed water. In prac-tice, methanol is not as popular as MEG as an inhibitor ingas processing facilities. This is primarily due to the com-plexity of regeneration. A brief summary of the relativeadvantages of each inhibitor is given in Table 4.

    Methanol is not commonly used as an inhibitor in con-junction with gas processing plants due to the difficulty ofregeneration. The relative volatility of methanol to water isapproximately 3 to 4:1 compared to the relative volatility ofwater to MEG of about 30:1. In addition, the product spec-ifications in a methanol-water distillation are much morestringent than in a MEG-water system. Methanol puritiesare typically 98 - 99 wt% and water specifications are typi-cally on the order of 99.9 wt% or higher.

    The difficulty of this separation is manifested in the sizeof the equipment. As an example, the methanol recovery

    AdsorptionThe use of adsorption systems has been primarily confinedto the following applications: Dehydration to very low water content 1 ppm).

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    Dehydration of gases contain-ing H2S (with or without simul-taneous H2S removal). Simultaneous water and Cs+hydrocarbon extraction.Applications 1 and 2 employ

    molecular sieve as the desiccant,application 3 uses silica gel.Activated alumina can also beused for dehydration of natural gasbut it is not common.Without question, the predomi-nant application of adsorptiondehydration in natural gas systemsis to produce 'bone dry' gasupstream of low temperature gasprocessing plants such as deepNGL extraction and LNG faci li ties.A two tower dry desiccant unit isshown in Figure 6. The most com-mon desiccant is molecular sieve.It has a relatively high usefulcapacity, minimises hydrocarbonadsorption and produces the lowest water dewpointsof any commercial desiccant.

    Molecular sieve has another advantage as well - i t iseffective in dehydrating gas containing H2S. Glycol sys-tems will absorb a portion of the H2S from the gas. Thisrequires handling of an H2S contaminated vent streamleaving the regenerator. Adsorption systems can bedesigned to dehydrate the gas with simultaneous H2Sremoval or without.

    The first application is seen in situations where asour gas must be dehydrated in the field for trans-portation to another facility such as a gas treatmentplant. In these applications the H2S breakthroughoccurs long before the water breakthrough. The bed isoperated to water breakthrough (i.e. dehydration) andthe regeneration takes place with dry sour gas that istypically recycled back to the inlet. There are typical lyno emissions of H2S from the facili ty.As an alternative, the unit can be operated to simul-taneously remove water and H2S. The equilibriumcapacity of the desiccant for H2S is much lower than forwater (often less than 1 - 2% for H2S compared to 15 -20% for H20). In addition, the mass transfer zone forH2S is much longer {2 - 3 times) than that for water.These factors mean that H2S breakthrough occursmuch faster than water breakthrough. Because of therapid breakthrough of H2S, simultaneous water andH2S removal is only viable when the H2S concentrationis low - usually less than 200 - 300 ppm. Two examplesof simultaneous H2S and water removal with molecularsieve are Esso Australia's Longford facili ty and AmocoSharjah's Saaja facility. Another advantage of dry desiccantsystems is that there is no emission of BTEX such as thatencountered in glycol systems.

    The primary disadvantage of dry desiccant systems ishigh capital costs and the weighVspace requirements of theunit. A minimum of two adsorption towers are required (sev-eral units use three or four) and these towers will be largerand heavier than the contactor in a TEG system. The allow-able gas velocities in a glycol absorber containing structuredpacking are about three times higher than the velocities in anadsorber containing 4 - 8 mesh (nominal ly 3 mm) molecularsieve beads. This means the adsorber diameter will beroughly 70% larger for the same gas handling capacity.

    Figure 4.Typical MEG injection system.

    Processf----.----lMechanca RelrigerationV a lv e Exp ans i onTurbo ExpandBr

    Make-upMethanol

    Wei RawG a s F ee d

    StrippingGas

    \ i\ j

    \!,\ \\ \

    r----OyGas

    Cold

    Cold"--~--r""'" Separator

    DecantedMethanol-WaterS o lu ti on P ump

    WetNGLProduct

    Water

    T o D i sp o sa l

    Figure 5.Simplified IFPEXOLdehydration process flowdiagram.In addition, regeneration temperatures in a molecular

    sieve system (using 4A sieve) are about 300C. Thismeans that the regeneration gas heater is frequently adirect fired heater, which will be significantly more expen-sive than a glycol reboiler.

    Finally, each adsorber requires a minimum of four andfrequently six switching valves per tower. These are typi-cally remote actuated ball valves.

    These factors all help explain the cost differentialbetween an adsorption system and a glycol system. Basedon the author's experience, a dry desiccant unit will have acapital cost about 2 - 3 times the cost of a glycol system ofcomparable capacity. Table 6 summarises the advantagesand disadvantages of dry desiccant dehydration systems.

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    FEEDPRESSURE VESSEL

    SWEEPGAS

    HOLLOW FIBER ....MEMBRANE PRODUCT

    Figure 7. Prism membrane separator for natural gasdehydration. Copyright Air Products NS 1996.Table 7.Advantages and disadvantages of dehydration membranesAdvantages DisadvantagesLight and compact Membranes not commerc ially proven In high pressure

    natural gas dehydration applications.Simple, easy to operate. Compression required to recycle sweep gas.No solvent losses. Scale up to higher throughputs is l inear.No BTEXemissions. Feed gas pretreatment Is critical.High turndown. Not applicable to very low water dewpoints -40 C).\

    ~JembranesThe most promising new technology for dehydration of nat-ural gas appears to be membranes. Air Products (PermeaDivision) has field demonstrated the application of mem-branes to gas dehydration in at least three European loca-tions, including BP Amoco's West Sole field.

    Water permeates polymer membranes very rapidly rel-ative to methane. Membranes are therefore very selectiveto the permeation of water from natural gas. The perme-ability of a component through the membrane materialdepends on its solubility in the material as well as its diffu-sivity. The driving force for permeation is partial pressure,i.e., the molecule will permeate from high partial pressureto low partial pressure. As a simple analogy, the permeabil-ity is similar to a heat transfer coefficient and the partialpressure difference similar to a temperature difference.

    The problem with the application of membranes to nat-ural gas dehydration is not the permeabili ty of water but thedriving force. The concentration of water in natural gas

    yeting a typical -10 C at 70 barg dewpoint is 0.0072. .. 0 1% . At a pressure of 70 bargthe partial pressure of H 0. 2IS 0.0051 bar(a). The partial pressure of water in the per-meate stream must be less than this to faci li tate mass trans-

    fer of water across the membrane.This can be accomplished by creating a vacuum or

    by using a dry sweep gas on the low pressure perme-ate side of the membrane. The sweep gas option issimpler and more straightforward.

    In the units demonstrated thus far, the sweep gasrates have varied from about 3 - 5% of the processstream", A simple diagram of a membrane unit is shownin Figure 7. The high pressure wet gas enters the unit onthe 'shell ' side and the water permeates across the hol-low fibres from the outside to the inside. Dry sweep gasflows through the hollow fibres countercurrent to thehigh pressure process gas. The permeate gas pressureis typically 1 - 2 barg.

    One source of sweep gas is dry process gas leav-ing the membrane. The gas is let down to permeatepressure and the low pressure sweep/permeate gas isused for fuel, flared or recompressed back into the wetfeed gas either with a dedicated compressor or by usingexisting compression in the facility. A second option is touse a 'closed' permeate gas loop in which the low pres-sure permeate product gas is compressed, dehydratedthrough a dedicated membrane and recycled back tothe permeate side of the primary dehydration mem-brane. Approximately 0.5% of the feed gas (higher if

    CO2 is present) wi ll copermeate with the water. This gas willaccumulate in the 'closed' permeate loop and must be con-tinuously purged from the loop.

    Membranes offer a simple, lightweight option to tradi-tional gas dehydration methods and the technology lookspromising. These units would appear to be favoured indehydration of associated gas offshore where low pressurecompression (from the crude stabilisation separators) isalready required. The membrane sweep gas can easily becombined with this low pressure flash gas, avoiding theinstallation of a dedicated sweep gas compressor. Someadvantages and disadvantages of membrane systems arelisted in Table 7.References1 Private communication with T SKIFF, Houston, USA.2 Private communication with D DIBA, Houston, USA.3 FESTEN, LJFM, et al., 'Gas treatment installed on Dutch North Sea

    platform' OGJ, 20 March 1995, p. 954 CICCARELLI, S.D., JOHANNESSAN, T, JONES, K, 'Natural Gas

    Dehydration Field Testing of a Novel Membrane Separator' OffshoreMediterranean Conference 97, Ravenna, Italy, 19-21 March 1997.

    Enquiry no: 33

    w w w . j m c a m p b e l l . c o me-mail: [email protected] fax: 1-405-321-4533 phone: 1-405-321-1383Enquiry no: 34HYDROCARBON ENGINEERING FEBRUARY 2000 77

    http://www.jmcampbell.com/mailto:[email protected]:[email protected]://www.jmcampbell.com/

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