equilibrio dehydration

8/12/2019 Equilibrio Dehydration

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Water Equilibr ium in the Dehydra t ion of Na tura lG a s Wit h Triethylene G lycol

A. ROSMAN

ABSTRACTTo develop reliable design data foT glycolcontractors, gas-liquid equilibna in the system water-

methane- trietbylene glycol TEG tuere investigatedexperimentally.

Eqrii[ibrium va[ues vury little at theTEG concentrations used in moderndesign, but increase significantly withuwter concentration in the contactingwith increasing equilibr ium ternperoture.

very Aighcorrtactor

increasingTEG, and

\Jarious methods of data correlation are describedand compared with experimental data, Thecorrelation provides the means /or extending therrstilts o/ this investigation to other pressures andtcmp~raturcs.

INTRODUCTIONWater removal is a fundamental operation in

natural gas processing. Hydrate formation, corrosion,and the formation of liquid water that might separatein the transmission lines are some of the problemscaused by an excess of water in the gas. Of themethods available for gas dehydration, waterabsorption is by far the most generally used.Glycols, especially triethylene glycol (TEG), arethe preferred absorbents.

A survey of the literature on the water dewpoint of natural gas over glycol solutions revealssignificant disagreements. A sampling of publisheddewpoint data for gas in equilibrium \vith TEG(Fig. 7) illustrates the prevailing confusion. Scant,but still contradictory, information was publishedfor glyco] concentrations in excess of 99.8 weightpercent. Data in that range are needed in designingmodern glycol contractors where the water dewpointtemperature must be reduced by more than 100 F.

The main reason for discrepancies in experimentalresults is the difficulty of measuring accuratelyvery small amounts of water in gas. Water is easilyadsorbed on the surfar. s of experimental apparatus.Normally acceptable data scatter looms large inrelation to the low water concentrations that mustPaper SPE 4040 WJS presented at SPE-AIME 47th AnnualFall Meeting, held in San Antonio, Tex., Oct. 8-11, IQ72. ~Copyright 1973 American In. titute of Mining, Meta Ilurg~cal,and Petr oleum Engineer s, Inc.pre fe rences g iven t end Of PaP~r.This paper will be printed in ?ransactions volume 2S5, whi chwi ll cover 1073.

oCTOBER. 1973

CHEVRON OIL FIELD RESEARCH CO.LA HABRA, CALIF.

be measured. Attempts ro establish water dewpoints on the basis of plant performance have beenmore successful]. However, accuracy is limited bythe difficulty in establishing the relativecontribution of various factors that interrelate inplant operation.

Faced with these doubts, contactor designershave chosen to provide for lEG circulation ratesthat are overly high so as to insure more thanadequate water removal. Such a practice isundesirable, however, where space and power areat a premium, as on offshore production platforms.Thus, the range of this investigation was governedby the need to extend equilibrium information to thecontact temperatures and TEG concentrationsnecessary to optimize glycol contractors on offshoreproduction platforms.

New procedures were developed for sampling andanalyzing very small concentrations of water in gasand in TEG. To avoid experimental difficultiesencountered by previous authors, equilibrium wasreached and samples were takml under dynamicconditions.

Experimental equilibrium results were smoothedand correlated by several methods. Thermodynamicequations were used to check on the internalconsistency of data and to calculate equilibriumconstants at conditions outside the range of theinvestigation itself. The White expression, l fittedto the COFRC experimental data, adequatelydescribes the results within the range oftemperatures and concentrations studied.

DEFINITIONS AND METHODSAt water dewpoint temperature, the water

contained in a natural gas reaches saturation. Partof that water will condense if the gas is broughtto a lower temperature or to a higher pressure.Thus, the dewpoint temperature describes thewater content of the gas.

When dewpoint gas contacts TEG, the watercontent of the gas decreases. The lower watercontent corresponds to saturation water at a lowertemperature; that is, the dew point will be lower.The initial dewpcint temperature is the contactingtemperature. The temperature corresponding to thelowered water content is the equilibrium dewpointtemperature, and the difference between the twotemperatures is the dewpoint depression.

297


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The above definitions apply to a system at anypressure. The effect of pressure on the dewpointdata was not studied, since most authors agree thatdata obtained at atmospheric pressure are validfor the design of glycol contractors operating athigher pressures. For instance, Scauzi110,2 whomade a study of the TEG-water-gas equilibrium dataup to 1961, found that dewpoint values for any onecontact temperature agree within 1 F throughoutpressures ranging from 14.7 to 2,500 psia.

Assuming that equilibrium dew points areindependent of pressure, data obtained atatmospheric pressure can be used to forecast thedewpoint depression attainable in a glycol contactoroperating at a higher pressure. Although the amountof water in gas will be lower at the higher pressure,the equilibrium dew point will still be the same ifthe saturation temperature and the TEG concentrationremain the same.

Any serious attempt to optimize contactor designmust make use of gas-liquid equilibrium ratios,Ku, as defined by Eq. 1:

YKw= oo..osoos..(l)w

Although the equilibrium investigated is for atwo-phase, three-component system, the study waslimited to the distribution of one component, water,between two mutual Iy noninteracting phases. Thisis justified by the very low TEG vapor pressure inthe temperature range considered here and by thenegligible methane solubi]ity at atmosphericpressure. Water concentrations were determinedexperimentally in the gas and liquid phases.

Because the approach to equilibrium in therecirculating system is essentially asymptotic, gasequilibration was approached from two directions atevery selected temperature to make sure thatequilibrium had been reached. This was achieved(1) by using overly dry gas so that water wouldshift from the liquid TEG to the gas and (2) bycontacting the TEG with gas containing excessmoisture, thereby reversing the driving force.

For the first approach, the gas was previouslydried at a lower temperature with glycol of aboutthe same concentration as that used for the pointstudied. To reach equilibrium from the oppositedirection, we used gas from a higher contactingtemperature, Experimental results from the twoapproaches were usually closer than the expectedanalytical variation at the corresponding concen-tration level. Where a larger discrepancy wasobserved, the steps for a point were repeated.

This equilibration method is amenable toconducting experimental determinations at severaltemperatures using essentially the same TEG-watersolution and the same gas.experimental sequence for anywould involve determinationstemperatures:

Schematically, theTEG concentrationat the following

40 +600+800+ 1005+1200+1400+(1500)400


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mixed the incoming gas with the bulk.THE SAMPLING SYSTEM

The sampling system consisted of two subsystems,one for liquid and one for gas.

To sample the TEG prior to water analysis, thecontactor was pressurized with equilibrium gas,which forced the liquid through an enclosed rubbertube mounted over the injecting system. To reachthe sample, the syringe needle went through anouter septum, and once the liquid was sampled theneedle was pushed further through the injectionport septum. Thus, the sample was injected withoutallowing the TEG to come in contact with thehumidity in the air.

To sample the equilibrium gas, a large sample(up to 55 cc was passed from the equilibratedsyringe into a freezing coil, where the water in gasaccumulated as ice. As too large an amount ofmethane would have flooded the chromatographiccolumn, the volume of the gas sample was measuredby mercury displacement in a twin burette thatcould be read to fO.01 cc. The water was thenreleased from its frozen state by heating androuted to the gas chromatography. The resultingpeak was measured against an absolute waterresponse standard.THE ANALYTICAL SYSTEM

The analytical system consisted of a chromato-graphy with thermal conductivity detector, amultirange recorder, andan Instron electromechanicalintegrator. The recorder permitted the development

of large peaks by switching from the standard I-revrange to a range of 0.1 mv. Both the liquid and thegas samples were separated in columns packedwith Porapak Q.

PRECISION OF THE EXPERIMENTAL DATAA common difficulty of previous work in this area

is the inability to reproduce analytical datareliably. Consequently, investigators must adoptself-imposed constraints as precision criteria forexperimental repeatability. Examples of theprecision achieved are presented in Table 1.Experiments where repeatability was exceptionallygood were not considered for inclusion in the table.Good precision is more significant for the analysisof low concentration components, since anydeviation apparent there will have more relativeimpor~ than if applied to the abundant component.

Different sample sizes could reproduce relativeconcentrations well. However, efforts were made tomaintain constant sample volume. As the volumeof the liquid sample was only 2 microliters,maintaining constant sample size was difficult. Inthe example on Table 1, assuming that the countsper microliter are constant throughout the series ofruns, a maximum deviation of 3.3 percentcorresponds to an absolute sample-size variationof 0.000066 cc. In many runs, the count variationwas less than 3.3 percent.

Because Kw is the ratio of two concentrations,analytical accurac, and precision cannot exceedthe worst of the two analyses involved. The gas

~ __, ~____ . . .i r m-_ _pI TO :: ONSTANT TEMP BATHI I WITH TEG-GAS CONTACTORr [ANOMETER cIIII1

JT,[T, >TIJ

1

1IRCULATINGPUMP

t

)ASSPACEII 1 I

3 1;Ill SEBLED SYRINGE III SEPTUM SYSTEM20CC 1 ISYRINGE 1, I I PORAPAK O

? % F d

COLUMNSI I I I, 1 D;I I I I II I

. _ . J

III II I @FREEZING1 I SAMPLE LOOP~11 o.GASBURETTE J IL_II _ .CHROMATOGRAPHYlL _. ___ J--JI SAMPLI NGYSTEMII

CONTACTI NGYSTEMFIG. 1 SCHEMATIC OF TEG-WATER-GAS EQUILIBRIUM MEASUREMENT APPARAT\JS,

OCTOBER, 1973


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analyses at-e likely to show the larger variationsbecause of the small amount of water present.Thus the most severe analytical limitations on thestudy of equilibria exist at high TEG concentrationsand low temperatures, where water concentrationsare in the ppm range.Experimental Run 65, listed in Table 1, illustratessome of the least favorable conditions encounteredin our work, That run studied water distributionbetween gas and 99.95 weight percent TEG at60 F. Water in the gas phase was analyzed fourtimes, The results averaged 0.0034 mol percentwith a standard deviation of 0.00029. Water in theliquid phase was analyzed three times. Results(not shown in Table 1) averaged 0.4314 mol percent,with a standard deviation of 0.01087. The range ofwater equilibrium constant values spreads from aminimum Kw of 0.007032 to a maximum K w of0.008775. Average K is 0.00790420.000872,How much of an achievement is a variation of311 percent in the calculated K-value? While notdirectly comparable, many studies were made onthe precision of hydrocarbon analysis bychromatography. One study S shows that relativevariation between consecutive runs performed by

the same operator on the chromatography, increasesfor any one component with its decreasingconcentration in the sample. Accordingly, therelative deviation expected at a true concentrationof 0.003 percent is f35 to 70 percent. This workreports water concentrations of 0.003 mol percent(30 ppm) with a deviation of t8.5 percent.Analytical accuracy is harder to demonstratesince an analysis might be repeatable but wrong.When data were suspect, a decision was made onthe basis of consistency checks. A study wasconducted to establish possible sources ofexperimental error, and where consistency checkswere inconclusive the experiments were pursuedfurther.

EXPERIMENTAL RESULTSEQUILIBRIUM DEWPOINT DEPRESS1ON

Water equilibrium dew points were establishedfor TEG concentrations ranging from a nominal 94to 99.95 weight percent and equilibrium temperaturesof 40, 600, 80, 100, 120 and 140 F. Fig, 2 showsthe relationship between dewpoint depression andTEG concentration at 80 F equilibrium temperature.

-. ... TABLE 1 - PRECISION OF THE EXPERl d ENTAL DATA

PercentFxneriment Stondfird Standard. .-. . .

Number Type of Dots RUI-I 1 Run 2 Run 3 Run 4 Average - -eviation Deviation Observations.Mol %H. O in aas 0.0030 0.0034 0.0035 0.0037 0.0034 0.00029.-Mol % CH4 in gasWt %H20 in liquidWt %TEG in liquidSample size, ml gosWater counts, gasTEG counts, liquidWater counts, liquidMo] % HaO In gasMol % CH4 in gasWt %H20 in liquidWt %TEGin liquidSample size, ml 90sWater counts, gasTEG cou nts, liqu idwater counts, liquidMol % HZO in gasMol % CH4 in gasWt % H20 in liquidWt % TEG in liquidSample size, ml gasWater counts, gasTEG counts, liquidWater counts, liquidMol % H20 in gasMol % CH4 in gosWt % H20 in IiqujdWt % TEG in liquidSample size, ml gasWater counts, gasTEG counts, liquidWater counts, liquid

99.99700.0.533

99.9A6718.743.4

185,681,212.0

0.014599,985S0.0633

99.936719.32249,

194,853.264.10,090299.9098fl.0712

99.928818.

1,477.190,621.291.

0.308399,69175.5S6694.4434

3.5499Q160,33%17,383.

99,99660.0519

99.948154.38143.4192,043.

213,50.0140

99.98600.060199.9399

19.32239,5189,118,243.7

0,082299.91780.079099.9210

17.861,329.186,618.316,0.2949

99.70515.781394.2167

3,72997.152,487.17,242.

99.9965 99.99630.050799,9493

54,9 36.52150,2 108.189,313.205.5

0.014LI 0.014499.9855 99.98560.0588

99.941219.2 19.26

246. 3 245,5189,852. (197,988. )239.2 (212.0 )

0.0852 0.088299.9148 99.91180.1838 0.0837999162 99.9)63

18. 18.1,391. 1,434,189,262. 189,003.340. 339.

0.2825 0.298699.7175 99.70145,511994.4881

4.08 3.501,050. 951.152,761.16,421.

99.9966 0,0720.0520 0.001399.9480 0.088

189,012. 3,192.210.3 4,250.01435 0.00024

99.9856 0.0720.0607 0.002399.9393 0.063

19.275 0,057245.1 4,03191,274. 3,114,249. 13.3

0.0864 0.003599.9136 0.110.0794 0.005999.9205 0.063

17.965 0.071,407,8 63.2188,876. 1,659,

321.5 23.160.2961 0.010799.7039 0.1305,6166 0.14494.3834 0.165

3.71 0.265999.3 40.45155,194. 4,456.17,015, 519.4

65

67

70

101

8,50.072.50.09

(1) (2)(1)1.7 (4)2.0 (4)1.70.073.80.060.3 (2)l,rj1.6 (3)5.3 (3)4.00.117.40.060.4 (2)4,50.97.23.60.132.50.177.1 (2)4,02.93.1

1)(2)(3)(4)

300

No direc+ comparison because of different sample sizes.Before calculation volume is corrected for barometric pressure and ombient temperature.Figures in parentheses not used for calculation.Sample size: 2 microliters = 0.002 cc.

SOCIETY OF PETROLEUM ENGINEERS JOURNAL


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Similar plots can be drawn for other equilibriumtemperatures. Dewpoint depression, measured inequivalent degrees F, decreases with decreasingcontacting temperature. The rate of the decreaseappears to be a function of the equilibriumtemperature. Everything else being equal, thehigher the contacting temperature (i.e., watersaturation) of a gas, the larger the amount of waterremoved at equilibrium. The amount of water removedfrom the gas decreases with increasing dilution ofthe contacting TEG. Presumably, the TEG-watersolution ceases to be very effective as a dehydratingagent once it reaches a certain composition. Byextrapolation on Fig. 2, that composition is foundat approximately 60 weight percent water.The semiIogarithmic relationship between dewpoint

160MO K%120-100- \

80 - 60 - \

\40 - \ \*oo~%84q 10 50 ;

CONcEN(RA1lON0S I+ZOINT~G ~ %2 DEWPOINT DEPRESS1ON VS TEG CONCEN-TRATION, 80% EQUILIBRIUM TEMPERATURE.

110100 -90 -80 -70 -: 60

g 50 -~ 40r~ 301-= 20: 10 -Ho~ -,0 .>~-zo +s -30

-40 -5o-6o-7o

WI %TEG

/

90

x?:

-80t-90 - ~. 60 80 100 120 I40 160

EQUILIBRIUMTEMPERATURE..FFJIG. 3 WATER DEWPOINT TEMPERATURE OFMETHANE IN EQUILIBRIUM WITH TEG. LEAST-SQUARES FIT OF EXPERIMENTAL DATA (LINEAR).6)CTO~ER, 1973

depression and TEG concentration points up thefact that small increments in TEG concentrationcorrespond to increasingly larger dewpointdepressions and significant gains in absorberperformance.

Values interpolated from plots like Fig, 2were used to present the experimental data as Equilibrium Dew Point vs Equilibrium Temper-ature on Fig. 3, and as Equilibrium Dew Pointvs Water Concentration in TEG on Fig. 4. Theyalso form the basis for an empirical correlationthat will be further discussed in the section onData Interpretation.

Experimental data on Fig. 3 confirm that theequilibrium water dew point of a natural gasdecreases with increasing concentration of thecontacting TEG, and with decreasing contactingtemperature. The family of concentration isogramswas extended below the lowest TEG concentrationactually studied (94 weight percent). However,data are not shown beyond the highest experimentalconcentration of 99.97 weight percent.Fig. 4 is useful as a consistency check for datagenerated experimentally. The equilibrium temper-atures are operating parameters, so Fig. 4 isexpected to show a family of isotherms whereequilibrium dew point at 100 percent water is theequilibrium temperature itself. In effect, theisotherms may be drawn as straight lines up toapproximately 60 percent water, after which theycurve slightly toward the expected temperature.

Water Distribution Between TEG and MethnneFig. 5 presents a plot of gas-liquid equilibriumratios, Kw, as a function of TEG concentration, for

the temperature range 40 F to 140 F and atmosphericpressure. Logarithmic coordinates were chosen asa matter of convenience because they compress awide range of data into one figure. To emphasizethe important role of even small amounts of liquidwater, the abscissa shows weight percent water inthe contacting TEG rather than the concentrationof TEG itself.

On first approximation, K-values show little

-:70., ;2 ~5 ,0 ~. I0 IN 200 50Q {000 20 50 10004W1H3MT % WAT R I N1 G

FIG. 4 WATER VAPOR DEW POINT VS WEIGHTPERCENT WATER IN EQUILIBRIUM TEG (NON-SMOOTHED EXPERIMENTAL DATA).301


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dependency on contacting TEG concentration,throughout the concentration range of this study.Horizontal isotherms, drawn through the averagevalue of all K-data at ~ne temperature, present areasonable fit. However, a statistical analysis ofthe data, confirmed by thermodynamic fitting,indicated that K increases slightly with increasingTEG concentration.Equilibrium values roughly doubled for every20F increase in equilibrium temperature. Fig. 6,which shows a plot of K vs equilibrium temperaturefor a contacting 99.0 weight percent TEG is typical.Regardless of the method used to smooth the data,the relationship is quasilinear within the studiedrange of TEG-water compositions. If the relativelysmall effect of TEG concentration is ignored, onesingle line applies to the temperature range 400 to140 F. Its empirical equation offers an easy-to-usefirst approximation of water equilibrium values atatmospheric pressure:

Kw=e 0. 035129 L23. 082f55. . (2)For more accurate calculations the curvature of

the plots (Fig. > and 6) cannot be ignored, however.Comparison wltb Pub Iisbed Data

The Chevron Oil Field Research Co. (COFRC)results reveal some general similarities in shapewith published data. However, differences in valueare significant. Closest to our observations aredata published by Union CarbideG and by Worley,7Fig. 7 presents several typical isograms ofequilibrium dew points vs equilibrium temperaturesfrom the two sources mentioned above. Generally,COFRC equilibrium dewpoint temperatures arelower than Union Carbides for corresponding TEGconcentrations. On the other hand, both sets of datashow a very similar dependence on the contacttemperature. The similarity is particularly apparentfor TEG concentrations around 99 weight percent.

The equilibrium dew points in Ref. 7, which?0 1 , , I , I

1001 I , [ , ,( 1 , , J01 L% *O . 10 60 10

WATCR W+4CLNTRA110N IN TCO M IG .TFIG. 5 WATER EQUILIBRIUM CONSTANTS INTHE TEG-GAS-WATER SYSTEM; 14.7 PSIA EXPERIMENTAL,

were obtained from operating glycol contractorsand extrapolated to an infinite number of trays andglycol rates, coincide very well with COFRC valuesat the extremes of the concentration range. Thesimilarity does not hold true for intermediateconcentrations, where differences in dewpointtemperature range up to 16 F, the COFRC databeing the more conservative.

I

wJ

01

00

xCOFRC ExPERIMENTALoATA 990wT%ltG$ /

e /

/

/

+

~ , ~0035129 1 - 2308265Tm OR

)0) (520) (540) (5601 (580) (600) R40 bb 8( Icc ,X 4 F

61)un,I+K:R). 11,p IIAIIJR~ p vFIG. 6 VARIATION OF WATER EQUILIBRIUMCONSTANT WITH TEMPERATURE.

110- wEIGI+T %TEC100

[90 -80101- /

, . .. . -w

/

/

//

/ /--71/ / - / / ///A /.-. /:=

0 60 80 [M 120 140 160ECIUILKIRIUM TEMPERA1uR6. t F

FIG. 7 WATER DEWPOINT TEMPERATURE OFNATURAL GAS IN EQUILIBRIUM WITH TEG.aoz SOCIETY OF PETROLEUM E~Cl~EE~S JouR~~L


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Comparisons of COFRC equilibrium dew pointswith other values in the literature, for examplethose published by Dingman and LeBas,8 or DowChemical Co.,g~10 are much less satisfactory.Data published in 1961 by Scauzillo2 deservespecial mention since they are calculated on thebasis of older experimental fugacity values ratherthan determined experimentally. Scauzillos dataare very similar to Worley s, which they precede by5 years.

Few papers on the subject allow a directcomparison with the gas-liquid equilibrium values,K, originated in this investigation, In mostinstances, K-values must be calculated indirectly,from equilib~ium dewpoint data for natural gas orfrom vapor pressure over aqueous TEG solutions.Calculations are limited by some impliedassumptions: in the case of dewpoint data we needto know what moisttire charts, if any, were usedby the author in translating water content of gasinto equilibrium dewpoint temperatures.TEG-water so]utions have two partial vaporpressure components. TEG vapor pressure is verylow at experimental temperatures. Townsends 11estimated value, 0.01 mm Hg at 68 F, is in line.with the Union Carbide and Dow Chemical publisheddata. Wise etal.~2 indicated that TEG vapor pressure(PTEG) between 69 and 84F obeys the relationship

log PTEG = ~; f i + 7. 758 s (3)In either case we conclude that, within ourexperimental range, errors that result from neglectingthe TEG vapor pressure would not be significant.

Despite differences in value, most shapes ofK-vs-TEG concentration plots obtained frompublished data are similar to those found in ourinvestigation. However, some isograms have morepronounced curvatures and exhibit a minimum forcertain TEG concentrations.In conclusion, water equilibrium constants arevirtually independent of TEG concentrations at thehigh TEG concentrations required in gas dehydration(98 to 99.5 weight percent TEG). With increasingconcentrations of water in TEG (95 percent TEG),equilibrium constants do increase. The differencein K-values between the conditions at the top andat the bottom of a glycol contactor could thereforebe significant.

INTERPRETATION OF DATADATA SMOOTHING AND CORRELATION

Least-squares smoothing methods were appliedwhere data appeared to be in a linear orsernilogarithmic relationship. In most of the datasets the correlation coefficient was very close tounity. The smoothed data were then correlated byan empirical equation, based on the variation ofthe equilibrium dewpoint depression with TEGconcentration, at various temperatures. Given twovariables within our range of investigation, thethird can be calculated.

WATER VAPOR DEWPOINT DEPRESSIONThe variation of dewpoint depression with glycolconcentration and temperature, one example of

which is shown on Fig. 2, can be fitted to anequation of the typeW=aebD). . . oo(4)4)

where a is the intercept on the water concentrationcoordinate and b is the slope of dewpoint depressionvariation with weight percent water in TEG. Inphysical terms , a is the weight percent of water inan aqueous glycol solution where the glycol is toodiluted to remove water from natural gas. Both aand b are temperature dependent and can becorrelated by semilogarithmic exponential equations.

Eq. 4 can then be rewritten in a generalized form:w= ( o0074f1I n T-6. 7932 )exp [( 6)]

i n T- 6. B7 36~D . . . . .(5)11. 364When TEG concentration and equilibrium temperatureare known, dewpoint depression can be calculatedfrom the following form of Eq. 4:

D= in WI n a77- - (6)where all terms are either known or can becalculated.

Eq. 4 is not amenable to an explicit solution forthe contacting temperature, T, even if both ~~andD are known. However, T can be easily calculatedwith a computer by trial and error.EQUILIBRIUM DEWPOINT CALCULATION

Water vapor dewpoint temperature, TD(OF) canbe calculated from the following equation:

D =T - (D + 459. 7). . . . . . (7)Within the range of concentrations and temperaturesinvestigated, the equations above permit a quickevaluation of attainable water dew points, providedthe natural gas and TEG come close tc equilibrium.Since the attainable dew point is assumed to beindependent of pressure, the above equations canbe applied to real systems.

Fig. 8 is a plot of the equilibrium dewpointtemperatures calculated from Eqs. 6 and 7. TEGconcentrations were extended below experimentalrange to account for the lowest iikely compositionat the bottom of a glycol contactor. The resultingconcentration isograms exhibit a slight curvaturethat becomes more pronounced as the equilibriumtemperature increases.

To test the validity of the generalized dewpointequation, data calculated from individually smootheddewpoint depression plots (Fig. 2, for instance)were plotted for comparison along the 90.0, 99.0and 99.9 weight percent TEG lines, With one

oCTOBER, 1973 903


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exception at 40 F, the individual data points fallwithin 3F of the predicted dewpoint temperature.The fit is even better in the high-temperature,high-concentration range where exact data arescarce.

Another validity test is to check how well thecurves on Fig. 8 can predict values beyond therange of this ifivestigation. An equilibrium dewpoint was originated experiment~lly at 1600F and99.906 weight percent TEG. Tht solid circle showsthe experimental results, the open circle thepredicted dewpoint temperature. The difference of1.44 F is only 0.9 percent of the 159.1F totaldewpoint depression.

BASIC EQUILIBRIUM RELATIONSHIPSFOR THE SYSTEM TRIETHYLENEGLYCOL-WATER-NATURAL GAS

Several previous attempts to describe theTEG-water-gas system by means of an equation ofstate were limited by the disparity in the nature ofthe components and the uncertainty of theexperimental data. The thermodynamic approachwas used by Wise et al lzto measure equilibria inthe binary system TEG-water. Townsendl 1 andlater Scauzillo2 considered TEG-water-nattual gassystems from a thermodynamic point of view.Results were not too satisfactory, however.

To extend correlating applications, variousthermodynamic equations were fitted to the COFRCexper imental data. Among them, the White equationl

11010090 [ /80 70: n 601-

g 50 -~ a 40u

~ 20 - EG2 lo -1s 90.80 ///

g -30-40

-60 gg~

-80 ~-90- 1 140 60 60 100 120 140 160

EWILIBRIUM TEMPERATuRE.f FFIG. 8 WATER DEWPOINT TEMPERATURE OFMETHANE IN EQUILIBRIUM WITH TEG (EXPONENTIALLEAST-SQUARES FIT OF EXPERIMENTAL DATA).sol

was chosen because of its successful application. .to binary systems of widely different boilingpoints. 13 The equation calculates water activitycoefficients, which in turn can be defined in termsof the equilibrium constant.APPLICATION OF THE WHITE EXPRESSIONTO THE EXPERIMENTAL DATA

The White equation describes vapor-liquidequilibria in terms of temperature, pressure, liquidcomposition, and two independent constants. Similarto the Van Laar equation, the expression includesa temperature factor and possesses other correlatingadvantages that will be discussed later. For abinary system the equation takes the following form:(), 1. 5 1[T l o9 yl l -0*5 = ~ $( )0. 5bl be, . . . . . . . .+ a21 q

(). 5b2 2CT l og y2]- 05 = ~ ~( )(). 5b2+ bl . . . . . . ..(8b)a12 qThe ratios of constants in Eq. 8a are also constants:()1, 5~ =m= constant . . . .(9)G

b2 n= = const ant . ( 10)blTheir values can be calculated from the slope andintercept of the linear plot:[T log yl]-05 VS X, 1X2

This is a significant advantage when experimentalvalues for the equilibrium of the second component(TEG) are very difficult to evaluate. A numericalsolution of the expression (T log yl )-05 is likelyto be imaginary, but that fact appears to have noeffect upon the applicability of Eq. 8a, providedthat xl/x2


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XwT I n ~w)- o. 5 = 0000g2 XTEG

+0. 0535 (11)when fitted to nonsmoothed experimental data, andXw

(T I n ~w)- 005 = o*0063 xTEf j+0. 05449 (12)

when fitted to data smoothed by the least-squaresmethod.Small differences in the constants obtained for

the two cases do not amount to a significantdifference in the calculated values. AS xw/xTEG


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CONCLUSIONSWater-vapor dewpoint temperatures can be lowered

to a greater extent than predicted by most publishedglycol equilibrium data.

Experimental dew points are dependent 0~equilibrium TEG concentration and temperature andcan be described by means of a thermodynamicrelationship. Close first approximations may beobtained from an exponential least-squarescorrelation that allows calculation of either dewpoint, TEG concentration, or equilibrium temperatureif the two other quantities are known.

Water equilibrium constants are virtuallyindependent of TEG concentration in the highcon centration range required for advanced gasdehydration. If TEG contains more than a nominal3 weight percent water, equilibrium constantsincrease significantly with increasing concentrationsof water. The difference in TEG concentrationsbetween the top and the bottom of a glycol contactorcould therefore be significant.

The dependence of the equilibrium constant oncontact temperature is exponential. The equilibriumwater content of the natural gas roughly doubles forevery 20 F increase in temperature. The relationshipprovides a useful first approximation for all TEGconcentrations studied.

The effect of contacting pressure on water vapordewpoint temperature and water distribution constantwas not invest igated. A consensus of published dataindicates that equilibrium dewpoint depression isindependent of pressure and that the waterequilibrium constant decreases with increasingpressure. The thermodynamic expression of White,found valid for our data, indicates that the latterassertion is correct.

The White correlation fitted to our smoothedexperimental data (Eq. 12) is recommended as thepreferred method for obtaining design data ontriethylene

D=

PTEG =T=TDw=a12 =

61,62 =p =p; =

glycol contractors.NOMENCLATURE

dewpoint depression = saturation temper-ature - equilibrium dewpoint tempera-ture, F

system pressure, psiaTEG vapor pressure, mm Hgequilibrium temperature, Rwater-vapor dewpoint temperature, Fweight percent water in equilibrium TEG- a21 = constantsconstants in White equationpartial pressure of pure watervapor pressure of the pure component iat system temperature

Xw = mole fraction of water at equilibrium inthe liquid phase

yi =yw .

y.

mole fraction of component i in vaporphasemole fraction of water at equilibrium in

the gas phaseactivity coefficient

SUBSCRIPTSw = water

TEG = triethylene gIycolACKNOWLEDGMENTS

I wish to express my thanks to Chevron Oil FieldResearch Co. for permission to publish this paperand to C. A. Johnson for his competent andmeticulous handling of the demanding experimentalprogram.I.2.

3.

4.

5.

6.7.8.

9.10.11,

12.

13.

14.

REFERENCESWhite, R, R.: Vapor-Liquid Equilibria in Non-IdealSolutions, Trans., AIChE (1945) Vol. 41, 539.Scauzillo, F, R.: Equilibrium Ratios of Water inthe Water-Triethylene Glycol-Natutal Gas System,})J, Pel. Tech. (July 1961) 697-702; Trans., AIME,vol. 222.Bukacek, k, F.: Equilibrium Moisture Content ofNatural Gases, IGT Research Bull. 8, Chicago(1955); Charts of the Equilibrium Moisture Contentof Natural Gases, J Supplement to IGT ResearchBull. 8 and ASTM Method D 1142-58, Chicago (1959),Sharma, S. C.: Equilibrium Water Content of GaseousMixturea, PhD thesis, U. of Oklahoma, Norman(1969),Millhone, R, S. and Fett, E. R.: ChromatographicAnalysis of Natural Gas Liquids, toc 41stAnnual Fall Meeting of WGP and ORA, Anaheim,Calif. (Oct. 1966 ) 91 -109 .GIYCOIS, Bull. F-4763H, Union Carbide Corp.(1964) Table 29, (Page 46).Worley, M. S.: TEG Still the Least ExpensiveRoute, Cdn Petroleum (June 1967) 34.Dingman, J, C. and LeBas, C. L.: NOW NewDew Point Data for Triethylene Glycol Solutions, Oil and Gas ] (Feb. 3, 1964) 7.5.Gas Conditioning Fact Book Dow Chemical Co.,Midland, Mich, (1962).GIYcoIs, Properties and Uses, Form No. 125-285-61, Dow Chemical Co., Midland, Mich. (1961).Townsend, F, M.: ~~Equilibrium Water Content S ofNatural Gas Dehydrated by Aqueous Diethylene andTriethylene Glycol Solutions at Varioua Temperaturesand Press ures, PhD thesis, U. of Oklahoma, Norman(1955).Wise, H., Puck, T. T. and Failey, C. F,: Studiesin Vapor-Liquid Equilibria: The Binary SystemTriethylene Glycol-Water, j, Pbys Chern (1950)vol. 54, 734.Ha-la, E., Pick, J., Fried, V, and Vil~m, O.: Vapour-Liquid Equilibrium, 2nd English Ed., t rans by G.Standart, Pergamon Press, Oxford (1967).Sharma, S. C, and Campbell, J. M.: Water Contentof Natural Gas, paper presented at 1969 NGPANational Convention. ***

S06 SOCIETY OF PETROLEUM ENGINEERS JO CRNAL

equilibrio dehydration

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