acid gas water content and physical properties

8/2/2019 Acid Gas Water Content and Physical Properties

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Acid Gas Water Content and Physical Properties: Previously UnavailableExperimental Data for the Design of Cost Effective Acid Gas Disposal

Facilities, an Emission Free Alternative to Sulfur Recovery PlantsBy

M.A. Clark, W. Y. Svrcek. W.O. Monnery: University of CalgaryA.K.M. Jamaluddin: Hycal Energy Research Laboratories

E. Wichert: SogaproEngineering td.AbstractThe economicsof recoveringsulfur from sour natural gas have become unfavorable or smallfields. Hydrocarbonproducing companies require a cost effective yet environmentally soundalternative o deal with acid gas. Compressed acid gas re-injection into producing,depleted ornon-producing ormationshas emerged as a viable alternative o traditionalsulfur recovery. Mostinjection schemes nclude dehydration facilities to remove the saturatedwater from the gas,preventing corrosion and hydrate formation. An alternative, less costlyapproach is proposed:keep the water in the acid gas phase throughout the injection circuit, eliminating the need todehydrate.To design an optimized injection strategy, determination of thermodynamic and physicalpropertiessuch as water content, dewpoint, bubble point, hydrateconditions and density of theacid gas is necessary. A comprehensive oint industry/research roject is underway to obtaindata for several different acid gas compositions over a range of operating temperatures andpressures. An online, repeatable and precise method of water content analysis using gaschromatographyhas been designed and tested. Pure CO2 data has been generated from theequipmentand matchesacceptedvalues with an averagestandarddeviationof 5 %. To date thephase behaviorof an acid gas mixture containing89.5%CO2,9.9% H2Sand 0.6% CH. has beenstudied and compared o equation of state predictions. Other mixture data is being generatedand should be completed n six months. Until this or other experimental ata becomes available,operating companieswill be forced to rely on equation of state predictions and over-designaccordingly

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IntroductionThe unfavorable economics of constructing small Claus sulfur recovery plants, due to theirinherent operating difficulties when the H2S feed concentration is low and the decreased demandand oversupply of elemental sulfur, necessitate an altemative technology. Air emission standardsand regulations are increasingly stringent, compounding the need for an environmentally-friendly,cost-effective method to deal with acid gas. Acid gas compression and re-injection into depletedreservoirs or disposal zones, similar to produced water disposal, is a viable alternative totraditional sulfur recovery processes with the added advantages of reducing greenhouse gasemissions and providing pressure support for producing reservoirs. 1.2,3

Acid gas mixtures, separated from hydrocarbons in a sweetening plant, are compressed throughseveral stages, dehydrated and pumped into a reservoir. Phase behavior, water content andphysical properties of the acid gas are required for facility design. Phase behavior data is used toavoid the two-phase region during multi-stage compression to ensure equipment is not damagedby the appearance of a liquid phase. The injection pressure required is dependent on severalfactors including the formation pressure and the density of the acid gas mixture.1.4 Thehydrostatic head of the acid gas column in the well bore aids injection, particularly if the fluid isinjected above its critical point as a dense phase. Water content data determines the need forand aids in the design of dehydration facilities.

Although experimental data is available for pure hydrogen sulfide and pure carbon dioxide, littlework has been done with acid gas mixtures5,6,7,8,9. Equation of state predictions can be used formixtures, but their applicability for these polar/non-polar mixtures has not been proven and liquiddensity calculations are known to be unreliable. An acid gas mixture study is underway to fill theinformation gap, providing the required thermodynamic and physical properties over a range oftemperatures and pressures of a series of acid gas mixtures. This pertinent, practical project is ajoint effort between industry and research institutions.

BackgroundHydrogen sulphide and carbondioxide are removed from sour gas by absorption with aregenerative solvent in an amine plant. The acid gas mixture of H2S, CO2 and a small amount oflight hydrocarbons leaves the sweetening unit saturated with water at the amine still conditions oflow pressure and high temperature, represented by point A in Figure 1. The gas mixture is thencompressed from points A to F in 3 or 4 stages. After each stage, the gas mixture is cooled,

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without entering the two-phase region. Condensed water is removed after each stage. After thelast stage, the mixture travels down a pipeline into the disposal well. Ideally, at the finalcompressor discharge pressure the mixture will be supercritical. Further cooling in the pipelinewill increase the density without a phase change, increasing the hydrostatic head of fluid in thewell and reducing the required injection pressure. The operator must ensure that the mixturedoes not cool below its water saturation temperature, to avoid corrosion and hydrate plugging ofthe pipeline and wellbore.

Corrosion and hydratesmay occurwhen he gas is saturatedwith water. Due to the safetyhazard associated with acid gas equipment failure, most injection schemes currently includedehydration facilities to ensure the acid gas is undersaturated throughout the system.Unfortunately.dehydration acilities and stainless steel comprise a major portion of the capitalcost of re-injection acilities. Methanol njection s an option to combat corrosion and hydrateformation,but can significantly ncreaseoperatingexpenses. When the gas is not in contact witha water phase and is undersaturated, hydrates cannot form until the temperature dropssufficiently hat the gas can no longerhold all the water n solutionand "free" water is available orthe formation of hydrates. If the mixture s below ts saturated hydrate ormationcurve, hydratesform preferentially o a liquidwater phaseonce free water is available.Although there is little experimental data on acid gas mixtures, the solubility of water in purehydrogen sulfide and pure carbon dioxide lead to some interesting hypotheses. The ability of thepure compounds to hold water in the vapor phase decreases as the pressure increases up toabout 3000 kPa (400 psi) for H~ and 6000 kPa (900 psi) for CO2. At higher pressures the waterholding capacity of the gases increases, corresponding to a higher water absorption capacity inthe liquid phase or dense phase compared to the vapor phase. In both cases, increasing thetemperature allows more water to be absorbed in the gas phase. Small amounts of methanesubstantially reduce the water absorption ability of both components.

If it is assumed that solubility of water in the gas mixtures mimics the trend of the individualcomponents,hen a minimumwater holdingcapacityexists at some pressure. Figure 2demonstrateshow this informationcould eliminate he need for dehydration acilities. Points Athrough G in Figures 1 and 2 correspond o the same stages n a re-injection acility. Over eachcompression tage, he pressureand temperature ncreaseand after each compression tage thegas is cooled. Initially he water holdingcapabilityof the gas decreases rom stage to stage, untilthe minimumwater holding capacity s reached. If the condensedwater is removedat this point,the gas will be undersaturatedwith water throughout he rest of compression. Stainlesssteel willnot be required n the compressorsor coolersafter point E. If the temperatureof the compressed

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gas does not drop to the new water saturation temperature, point G, in the pipeline or wellbore,dehydration can be eliminated and stainless steel materials and methanol injection will not benecessary

!~II)II)G)...no

Temperature

c c- 0>-';:'511)- Q)

.Q~=a.OEU)o'aUCC)IV C-.-C ...Q) =-0C II)0 IV

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If it is concluded that dehydrationcannot be completely eliminated due to a particular set ofconditions, or example n an extremelycold climate, he experimentaldata will still be beneficial.The operator will know the inlet water content of point A and the conditionsof the lowest watersolubility of the system. The glycol contactor ower, regeneratorand circulationsystem can thenbe designed appropriately.Currently, estimates of the density, water content and phase behavior of acid gas mixtures arebeing predicted with equation of state models, as experimental data is unavailable. Theequations can produce considerable error and result in over or underdesigned facilities.Experimental data results in proper decisions on the equipment and materials required for eachparticular set of conditions, which can lead to considerable cost-savings.

Apparatusand ExperimentalProcedureThe apparatus consists of a temperature controlled air bath, a high-pressure cell with a sightglass, a positive displacement pump. a Hewlett Packard 6890 gas chromatograph (GC), and anAnton Parr density meter. The sight cell has an internal volume of approximately 80 cm3 (5 in3)and a maximum working pressure of 70 MPa at 150C (10000 psi at 300F). The 2.5 cm (1 in)thick sight glass located on the front and back of the cell allows visual observation of the cellcontents throughout experimentation. As shown in Figure 3. the high-pressure sight cell ismounted in the center of the oven as are the cylinders containing gas mixtures and distilled water.The density measuring cell is mounted at the bottom of the oven and wired to the density meteroutside the oven.

A resistancedetection hermometers located nside he oven next to the sight cell and is wired toa digital temperaturecontroller. The temperature ontrollerswitches he three oven heaters on oroff depending on the difference between he set and detected temperatures. Two fans in theoven circulate he air, preventing emperaturegradientswithin the oven. The oven can be heatedup to 150C (300F) and the temperature s measuredand controlled o within :t 0.1C (O.2F).The temperatureof the sight cell contents s measured o within :t O.1Cwith a second resistancedetection hermometer nstalled n a port on the side of the sight cell.The sight cell volume and pressure are controlled by the addition and withdrawal of mercurythrough a port at the bottomof the sight cell. Sample luids are pumped n and out of a port at thetop of the sight cell. The pump measuresvolume displacementwith a precisionof :f: 0.02 cm3(0.001 in1. A dead-weightcalibrateddigital pressure gauge is connected o the pump outlet.

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The oven is connected o a motor via a steel arm. The motor rotates he arm, rocking he ovenand its contents n a 1800arc. The mercury n the sight cell agitates the fluids and enhancesmixingwhen the oven and cell are rocked.A heated, insulated line runs between the cell and the GC. A metering valve is used to controlthe flow and delivery pressure to the GC. A thermocouple downstream of the metering valveverifies that the temperature of the depressurized gas is sufficiently high to prevent watercondensation.

Figure 3 - xperimental Apparatus for Acid Gas S~dy

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The apparatus s contained n a sour gas laboratory. The lab is continuously lushed with freshair pumped n from the ceiling and drawn out of vents located n the bottom four comers of theroom to an incineratorstack. A permanentH~ monitor s located close to the floor, below theapparatus. If the monitordetects 10 ppm HzS,an alarm soundsoutside the sour gas lab and theincinerator ires up. At 20 ppm the main lights in the lab shut off and emergency ights flash.Personnel worKing n the lab with supplied air breathing apparatus (SABA) may not hear thealarm and the flashing ights ensure their evacuation. The lab is equipped with a video camerafor continuous emotemonitoringof personnelperformingdangerousworK. An -HzSPanicButton-which summonsemergency escue and medical services s located immediatelyoutsidethe sour gas lab.

A gas mixture is synthesized rom pure components n the laboratory using partial pressures.Concentrationsare verified using the GC. The system is purged and gas is transferred o thesight cell. The gas in the cell is saturatedwith water at the desired emperatureand pressureandallowed o equilibratewhile rocking. At equilibrium,usually reachedwithin a few hours, a stablefree water phase should be visible in the cell, ensuring he gas is fully saturated. Equilibrium sverified by consistent GC measurements. Gas samples are then transferred to the densitymeasuring cell and the GC for analysis, while maintaining constant cell pressure andtemperature.The density measuringcell contains a U-shaped ube, electromagnetically xcited by the densitymeter and vibrating at its natural frequency. The addition of sample fluid to the tube causes afrequencychange. The density s calculated rom the change in frequency using two calibrationconstantsand is read directly rom the meter n g/cm3. The density meter has been calibratedatdifferent emperaturesand pressureswith methaneand pentane. Tests with carbon dioxide haverevealedan accuracyof:t 0.5 % of the density eadingcompared o accepted iteraturevalues.Water Content AnalysisThe accurate measurementof the small amount of water (as little as 100 ppm) that is soluble nthe acid gas phases is crucial. Literature on the subject of water content of acid gases andanalytical methods used to determine moisturecontent was researchedand reviewed. In 1984and 1987. Kobayashiand SongS.1Oeasured he water content of carbon dioxide and a carbondioxide/methanemixture using a "tedious modified chromatographic echnique" developed byBlock and Lifland11 The techniqueuses a glycerol packed absorptioncolumn to withdrawwaterfrom a measuredquantity of gas. The amount of water withdrawn s determined n a calibrated

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split column gas chromatograph system. Kobayashi and Song reported an accuracy of 1: 5-6%based on calibration results.

In 1985, Huang et al.9 analyzed two mixtures of H~, CO2, CH4 and water by directchromatography. The GC was equipped with a therrno-conductivity detector (TCD) and a 3.2 mmOD by 1 m long Porapak as column and the experiments were run between 30C and 60C.Calibrations were performed with pure carbon dioxide. The reported repeatability of .t; 0.2 molepercent is reasonable at high temperatures, where the water content is greater than 2 molepercent, ten times the repeatability. At other conditions, the water content measured less than0.2 mole %, the repeatability of the analyses. Carroll et. al.7 reported in 1989 that watermeasurements in H2S performed by iodometric titration were accurate to within 0.1 molepercentage for amounts above 1 mole % water, but were inaccurate for measurements below 1mole % water.

None of these procedures determine water content with the precision, accuracy.epeatability andsimplicity necessary for our project. Consultation and experimentation with Hewlett Packardresulted in the use of direct chromatography, as in Huang's method, with a TCD. A Plot-Qcapillary column, 30 m long by 0.53 mm diameter with a 40Jlm film thickness ofDivinylbenzene/styrene porous polymer is being used. Hydrogen is used as the carrier gas forimproved resolution.

Water content analysis s performedby withdrawinga sampleof gas througha meteringvalve tothe GC sampling oop. The line, valve and loop are heated o preventcondensation. Once theline and the sampling oop are adequately purged with sample, he GC run is started and theswitchingvalve sends he carrier gas through he sampling oop. The componentsare separatedin the column and travel o the TCD. The TCD contains a filament,which s electricallyheated oa constant emperature. The carTierand sample gases flow intermittently ver the filament Thedifference n power required o maintain he filament emperaturewith the sample gas comparedto the carTiergas is measured. The resultant peaks, with a height measured n microvolts (~V),are integratedover time. seconds. o give area counts measured n ~V's.To convert area counts to mole fractions, a response factor is required for each componentanalyzed. The response actor s determinedby running experimentswith known concentrationsof each of the components. The percent area count of a componentdivided by its responsefactor equates to the mole percent of that component present n the sample. Determining heresponse actors or the components o be tested is commonly eferred o as calibrating he GC.

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Between runs the line, valve and loop are purged with heated nitrogen until no water is detectedin the nitrogen stream.

Many tests were run without success until modifications to the gas analysis were perfected. TheGC column was changed from a methyl silicone coated column to the Plot-Q, thus allowing theruns to proceed at higher temperatures and preventing water freeze out in the column. The heattracing and insulation of the line between the sight cell and the GC was increased. Calculationsindicate that for the most severe conditions studied, the gas must be heated to above 200C toensure that water does not condense over the metering valve. The metering valve replaced apressure regulator due to carryover between runs, despite lengthy nitrogen purges, attributed to alarge, unswept dead volume in the regulator.

WaterContentExperimentalResultsWhile testing the apparatus, the water content of a gas mixture of carbon dioxide and 5.06%methane with traces of ethane, nitrogen, propane and butane was determined at 6200 kPa (900psia) and 27C (80.6F). Water area counts of 125.0, 119.6, and 126.6 ~V's were obtained overthree successive runs. The measured data has a standard deviation between runs of less than3% and average water content of 0.135 mole %, proving the repeatability of the equipment at lowwater concentrations. The data is plotted vs. published data for a carbon dioxide and 5.31 %methane gas mixture in figures 4 and 5. Considering the compositional difference between thetwo gases, the measured water contents are very dose.

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A series of experiments were run with pure carbon dioxide to verify the accuracy of results over arange of temperatures and pressures. There is ample experimental data available for pure

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carbon dioxide n the literature. As shown n figures 4 and 5, the applicationof response actorsto the raw data resulted n a close match o publisheddata. Figure4 shows he data plotted n 51units while figure 5 shows the data plotted on a logarithmic scale with field units for ease ofcomparisonwith plots ound n the GP5A EngineeringData Book.The water content of carbon dioxide at 31C (87.8F) was measured at three different pressures.At 2500 kPa (370 psia) and 7200 kPa (1040 psia) the match was within 3% of published data andthe standard deviation between runs at the same conditions was less than 4%. At 5100 kPa (735psia) some difficulty was encountered obtaining repeatable data and only the last data point offive experiments is plotted. The match is within 10%. At 50C, water content was measured attwo different pressures. The match at 10100 kPa (1470 psia) was exact and the repeatability waswithin 1%. Difficulties were again encountered at 7600 kPa (1100 psia), the last three of fIVe runsis plotted, and the match is within 9%.

The poorer matches of 10% and 9% are suspectedo be the result of insufficient pressure controlduring the experiments, affecting the repeatability and accuracy of the results. Since very goodrepeatability is obtained when the experiment is run correctlyany fluctuations in analyzed watercontent of greater than 5% alert the operator to inaccurately obtained samples and theexperiments should be repeated. With proper pressure control results with less than 5% errorcan be obtained.

Phase Behavior Experimental ResultsSeveralsothemls were obtained to establish the phase envelope of a mixture of 89.5% CO2,9.9% H2S and 0.6% CH.. The cell temperature was set and allowed several hours to equilibrate.The cell mercury volume was increased incrementally, resulting in 15-30 psi pressure steps.Transient and stabilized phase behavior was observed and recorded. The change in mercuryvolume was recorded as a function of pressure. At pressures close to the dew and bubble points,the volume/pressure increment was reduced. By taking a series of data points immediatelyabove and below the appearance and disappearance of the two-phase region, the dew andbubble points were established.As recorded n Table 1, dew pointswere observed at 7C/4100 kPa, 9C/4342 kPa, 15C/5045kPa, 26C/6355 kPa, 34C/7479 kPa and 37C/7955 kPa. Bubble points were observed at7C/4528 kPa, 9C/4755 kPa, 16C/5510 kPa, 27C/6900 kPa and 34C/7844 kPa. Volume ofmercury in the cell is plotted in Figure 6 vs. pressure for each isotherm. As seen in figure 6, two

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The slope changes correspond to the dewistinctslope changes occurredor each isotherm.and bubble points, verifying the visual observations.

Figure 6- ressure vs Volume Data9.9% H2S, 89.5%CO2, 0.6% C1

1400

1200.-CI'iDo-e~f)f)eQ.

1000

800

800

400Cell Volume (cc)

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Above 37.5C 99.5F),a stable wo-phase egionwas not observed. Some droplets,andelongatedbubblesappearedduring a volume/pressurehangeand while the system wasstabilizing, ut upon eaching quilibrium,he systemwassinglephase t all pressures.At 37.5C (99.5F) and 8253 kPa (1197 psia) the critical point was observed. At all othertemperatures the contents of the cell were clear and colorless in the vapor, liquid and two-phaseregions. In the critical region, a small change in pressure (3-5 psi) resulted in the entire cellcontents becoming a murky, grey cloud and then stabilizing out into a variety of shades of yellow.Above 8303 kPa (1205 psia) the contents were single phase, clear and colorless. At about 8274kPa (1200 psia), the see-through single phase took on a slightly yellow tint. At the critical point of8253 kPa (1197 psia), two phases appeared with an indistinct thick yellow interface, a darkeryellow color at the bottom of the cell and a lighter yellow color on top. At 8212 kPa (1191 psia)the bottom half of the cell was a distinct dark orange liquid and the top half a colorless vapor. At8198 kPa (1189 psia) the liquid phase faded to yellow and below 7957 kPa (1154 psia) the cellcontents were again a colorless single phase.

In Figure 7, the calculated equation of state phase envelope is plotted along with theexperimental data. The widths of the two phase envelopes are similar, but the calculatedenvelope falls below and to the left of the experimental data. The calculated critical point occursat 34.9C and 7633 kPa (94.8F, 1107 psia), 2.6C and 620 kPa below the experimentallydetermined critical point of 37.5C and 8253 kPa (99.5F, 1197 psia). The deviations betweenactual and calculated phase behavior emphasize the importance of obtaining an experimentaldata set for acid gas mixtures. The equation of state was regressed to fit the phase behaviourdata obtained experimentally. The regressed curves and critical point match the measured datawithin the experimental error. The modified equation of state allows some extrapolation todifferent conditions, but experimental verification will be necessary until more data becomesavailable and a general regression is completed.

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Figure 7 -Pressure vs TemperaturePhase EnvelopeExperimentalvs Equation of State

{!~I

.. 5.0 10.0 15.0 200Temperatu,. (C)

ao 30.0 35.0 40.0

The two-phase egion can be avoided during compressionby cooling the gas to a minimum of37.5C betweencompressionstages. The fluid is in the supercritical,dense phase above 8253kPa and above 37.5C.Conclusions & Future PlansAcid gas re-injectionmay be the optimum solution or producingsmall sour gas fields. As theissue of greenhousegases heats up, re-injectionwill becomeviable for even moderate o largeoperations. The operating company must avoid the two-phase region during compression.Water condensation and hydrate formation in the post-compressionequipment must beprevented o ensure safe, cost-effectiveoperation. Experimentaldata on the water content,density.and phase behaviorof acid gas mixtures s thereforenecessary.A comprehensive joint interest project is underway to obtain data for several different acid gascompositions over a range of operating temperatures and pressures. An online, repeatable andprecise method of water content analysis using gas chromatography has been designed andtested. The reproduction of carbon dioxide data has proven the reliability of the method.

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Experimentalphase behavior of a single mixture has shown that actual behavior does deviatefrom equation of state predictions. Additional mixture data is being generated and should becompleted in six months. Until this or other experimental data becomes available, operatingcompanies will be forced to rely on equation of state predictions and over-design accordingly.

AcknowledgementsThe authors would like to acknowledge the support of staff at both Hycal Energy ResearchLaboratories Ltd. and the University of Calgary.

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References

WIChert,E. and T. Royan, .Sulphur Disposal By Acid Gas Injection-, SPE 35585, GasTechnology Conference, Calgary, May 1996.

2. Keushnig, H., -Hydrogen Sulphide - If You Don't Like It, Put It Back", Journal of CanadianPetroleum Technology, June 1995, 34,6.18-20.

3. Longworth, H.L., G.C. Dunn and M. Semchuck, .Underground Disposal of Acid Gas inAlberta, Canada: Regulatory Concerns and Case Histories", SPE 35584, Gas TechnologyConference, Calgary, May 1996.

4. Carroll,J. and D. Lui, "Phase Equilibrium and Physical Property Considerations for Acid GasInjection Systems", Canadian Gas Processors Association, Calgary, November 1996.

5. Song, K. and R. Kobayashi, "Water Content of CO2 in Equilibrium with Liquid Water and/orHydrates", SPE Formation Evaluation, December 1987, 500-508.

6. Selleck, F.T., L.T. Carmichael and B.H. Sage, .Phase Behavior n the Hydrogen Sulfide-Water System., nd. Eng. Chern.,September,1952,44,9,2219-2226

7. Carroll, J.J. and A.E. Mather, .Phase Equilibrium n the System Water-HydrogenSulfide:ExperimentalDetermination f the LLV Locus.,CanadianJournalof Chern. Eng., June, 1989,67,468-470.

8. Ng, Ho, D. Robinson and A. Leu, .Critical Phenomena in a Mixture of Methane, CarbonDioxide and Hydrogen Sulfide-, Fluid Phase Equilibria, 1985, 19,273-286.

9. Huang, S., A.-D. Leu, H.-J. Ng and D.B. Robinson, The Phase Behaviorof Two MixturesofMethane,Carbon Dioxide, Hydrogen Sulfide, and Water", Fluid Phase Equilibria, 1985, 19,21-32.

10. Song, K.Y., and R. Kobayashi, -The Water Content of CO2-richFluids in EquilibriumwithLiquidWater or Hydrate,"ResearchReportRR-80,GPA,Tulsa, 1984.

11. Bloch, M.G. and P.P.Lifland, "Catalytic Reforming mproved by Moisture Metering",Chern.Eng. Prog. 69(9), 1973,49-52

acid gas water content and physical properties

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