water content of sour product

8/13/2019 Water Content of Sour Product

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Water Content; of Sout Hydrocarbon.,/t, ,, ... 1, ,_Systems.. ... .,... k-_.,,,,,,..1

U OF ALBERTA ~1

JoI. UKACS0. B. ROBINSON I%IMONTON,.ALTA. ,; ,14/,, / t,,..., ABS~~AC~ .

A hnowtedge of the equilibrium water copteat of ihydrocarbon systems tinder pressu(e .fs irhportantto the natural gas industrp The information auail-able on the volubility of water in hydrocmbon,hydrogetr ,sulfide,.. and carbon diozide s~stenrs ~~ ,~reviewed in thi; paper. and the inflrle ce of the. ,mwe important var~abtes. such as tehvperature,,Jwessure and rnrilecrflar structure on volubility in.liquid,s [email protected] is discuss~d.A sbitable. cbromatographic technique has beendevelopek for a%itemiriing low concentra~ions ofwater. Tai in& of the water peaks %asbeen etim-r?rated by addin~ water to the cartier gas :[email protected] method: is :applicbb~e for both gas or Jiquid -samples and iseffective in the presence of hydrogen-sulfide. >. . The t?xpe;imenta~ study of water volubility inmetbaue-hydrogen.. sulfide systems izt a ttemperatureoj 160F has shown that the presence of hydrogensul{icfe causek only a modest increase in ~watercbntent at pressures up to 1~400.psi~ Theoreticalconsiderations and data on pure hydrogen sulfideand carbon dioxide suggest that the effect of both

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these ct@ounds wi~l be greater at higher pressuresand .in. the.liquid phase. , ... . ,,, ., ~, INTROtiUCTION -;kefore ,transporting br processing aacural gaseand gas condensate s,, it is usuqlly nedgssary to ;

&y ;hem. using suitable dehydration ,equiprnenc. The design and operation of this equi~ment requiresa knowledge of the amotuit of water present in the fluid at the reservoir.. and operating conditions;This is influenced by temperature, pressure andcomposition, p,artidtiy - when certain non- .,;hydrocarbon components are. presert~. Field expe-rience indicates that hydrogen sulfide and carbonilioxide,, for example, alter the usual water solubiIityrelationships appreciably. EIowever, ~n extensives@irch of tHe Iiterat,pte does not reveal any tpkmti-tative data on buch:s ystems. ,.:::-;L=:-;~-\~Foti5wXFi-:nktw~Egase-sj= gen~iaiized:-~rnfiirical=~.d correlations such as the ~one proposed by Katz,

1--- ...= ..=-. - - . . . . . ., . . --- . . . ----- ._ m ----. Jd..J uTY... lk?~;,. . . , . . . ... . . . .. ... .

et ~ al, 6 can be used to predict water solubilit yswith confidence at most temperatures and. pressuresof interest+ Hqwever, existing theoretical. relation-,shi~s do not permit a calculrttlori of the deviationfrom these curves when polar substances likehydrogen sulfide are present .?:,:the system, Thusone mu%t reso~ to an experuit~: tal, approach .,toobtain the necessary information. The fact that -laboratory experimental methods frequently involvethe use of mercury which reasts with hydrogensulfide in the presence of watert and that hydrogensulfide interferes with many chemical reactionsspecific for water has contributed to rhe difficqlt yof studying water solbbility in systems ,contahting@ydiogen ~sulfide. In this investigation the, watercontent of ..aimited member .of methane-hydrogensulfide mixtures was detetmineii using a specialt~chnique with ~as chromatography.,,RiVIEW OF PUBL&ED DATA

Exprmim&taiIy determined water volubili ty datahave been reported for methane, 14 ethane, -15proptie, 9 n-butane, 16 l-butene, 11 ,hydrokensulfide 18 and carbon dioxide, 1? These stvdiesleport the effect of, pressure; tempeiat&e andmolecular atructt#e on water solubiIity in singlecomponent gases and liqdds, - Fig, 1 compares experimentally determined watersolubilitiee in several hydroc,stbons, carbon dioxideand ,hydrogen ~~ulfide- at l@F as taken from this.. . .

. . . MOLEFRACTIONOF WATERFIG. 1 - SOLUBILITY OF WATER IN FURE COM-PONEN13 AT lW. .. . . . . . . .: . . . . . . ,7 ..,.


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literature. Tle dotted line represents Water con- hydrogen sulfide and water-carbon dioxide mixtures>suggest that the presence of these substances inhydrocarbon systems would increase the equilibriumwater content of the syatem~~ especially at higher ~pressures and pr@cularly in the, hydrocarbon-richliqui~_~@e.

centrations calculated from the vapor pressure ofpure water and ideal soIubiIit y reIationships, Noneof the components can be considered to behave ...ideally and think+%% significa~t. difference betweenthe soltibilitv in saturated and unsaturated hvdro=..

, .carbons , and between hydrocarbons and non-hydrocarbons, .Experimental work o; wa;er volubility: in binaryhydrocarbon sjwtems is Iirnited to that of l$cKetta ,,apd Katz 13on the methane n-butane water,system, They f$und that at a given pressure and ~~temperature, ,$here both the hydrocarbon-rich., liquid and vapor exist in equilibrium, dte ~,atercontent of thti. vapor phase is always higher than, 1,.the. water content of the hydrocarbon-rich liquid

when calculated on a mole fraction basis. Thewater content of th~- hydrocarbon-rich liquid phask,unlike the single component hydrocarbon liquids,increased with increasing pressure. at a constant

:A .DEV~LOPMENT OF ANALYTICALTEC*IQUEIn addition to the many difficulties wh~ch are inlier$nt in che detg~minstion of trace quantitiesof water in any mifiure, the presence of hydrogensulfide renders the problem even more difficult,The, formation of mercury sdfide in the presenceof water,and hydrofietr sulfide tweclud~s che use of.,mercuiy. as -a ~onf~ning fIuid ht equilibrium cells.The use of: solid adscrbants is limited to low-.boiling=poikt. hydrocarbons, Hydrogen sulfideinterferes in die anaiysis for water by the Karl-Fischer method. It was the purpose of this studyCOfind a method that would be rapid and accurate,

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temperat~e. The water vapor content of more com-plex natbal gas systems h~s b$e,tt investi ated byDodson and Standing 4 and others, 3#G~o, ~~ These.

.,that would reqtiire only a small sample, and thatwo@d quantitatively determine water. iit small .concentrations even in the presence d hydrogensulfide or heavier hydrocarbons. With tljs h mindgas chromatography was investigated as a possible., .:-

studias indicatqd that although the water sol-@ility curves fch the. hydrocarbon mixtures deviatefrom ideal behavior, they can be cep~esented by oneaverage cufke. ovkr a wide. range ~f pressure withoutintroduq ing serious error. Several authors ~)s~g: 11,14?15, have -attemptedco. describe the equilibrium water content of hydro-c,srbon systems by . thermodynamic or qn@iricaI

method.Chromatography has been wideiy used for cheanalysis of trace compounds. n the direct deter-mination of traces of wares however, the chromato-graphic technique has been complicated b,y theformaiion of br~ad asymmedic water peaks whichmade the method, inaccurate for quantitative w~rk.In 195S, Knight7 found that if water w?re addedto the carrier gas stream the tailing of water peaks

,, equations. The success of these efforts :hqs beenvery limited and no relationship is available that>~ould be- used for naturally occurring systems atthe pressure and temperatures of interest. . I> ,-{.

EFFECT OF Hl@ROGEN SUL~ID~ AhCARBON DIOXIDEcould be reduced; however, no literature has beenfound on the application of this. method for tracewater analyil~. In W52i Knight and We,iss8 agal yzedfor traces &f water by, reacting water and calciumcarbide and then anal yzing for the resulting acety:lene by chromatography: -A similar procedure b.aqedon the production of hy~gen from water has alsobeen reported. Both of these methods are advtirselyaffected by the presence of. hydrogen sulfide in the,~ampk. .Carlstrom, Spencer and Jghnson2 used, atrapping arrangement to concentrate the water untifthe waier sample was big enough to give a symmetric. ,peak, but this method fras obvious disadvantagesfor Iabortory samples. , ~

The volubility of water-in hydroge~ sulfide andin carbon dioxide differs considerably from thesolubilit y of water .in hydrocarbon system-.as.~ho.wnin Fig. 1, This can probabiy be ex#ained by thedifferent molecular structures of these substances.

.,,.Hydrogen sulfide, for example, poisesseq higherpolarity. than hydrocarbons becaus: of the asym-.metric arrangement of its at9ms. Since water is a (strongly polar materi@ itself ~. it will. have highersoiubilit yin materials with higher polarity. Carbon...

dioxide on the-other hand has zero dipole momentand. one might expect thqt w,ater, wouid be. lesssoluble .in carbon dioxide than. in hydrogen sulfide:The., low water solubi~ity in carbon dioxide at Iow.prei$sure is most Iikely - dtie COthe fact that. waterand carbon dioxide form carbonic acid in the water-rich liquid phase which lowers the vapor pressuieof water in the v@or phase.- At high$r--preswres,

Since the determination of water by chromato-graphy. has.been hind:red in. the past by.asymmetricwate~ peaks, che cause and possibie soiution ofwater ~ailing, were studied. A gas chromatography*was ,-used for the. anal~ical work with. helium ascarrier gas, Several columns were prepared and,, used: in- die ;chromatograph to give the required- -. wher~ the, distanc$s_ between thk individual MOL..-. -. resolution betwqgn methane-? hy~ogen sulf id~ and'ecule 44are=+~eduESd;-'th&'-r@rn%kab1exiri6reas&-Ain- --:=--mtytiFlibr~~Z-8@~arid~~eflon-T-~6 were~kviih.tatkd--=----water scdubility in, both the hydrogen sulfide and as the solid support in the column. They .verecarbon dioxide may be due to some stronger packed in ~M in,,0 D glass spirai tubes made CO, mokuiar fcrces such as hydrogen bonding. ~. - - . . .,-, The sol~ili~y ,,.r$lations .:epo~ced -, in water-,. ... . . . . . .. . .. *Beckrn7~Madel.(3C. 2- .,.- .- ..., . . . . . ... . ... ..,, >x. .. ... . . ..,. .,,,_. J ._... -.- ._- ______ .. ______ ..... ... . - __.* __... . .. . . . . ..:~ _ ~ .. .. ... ... . . . . .. .. .--.7 -----..-. ....... ... +_._. ..._ ---

-- - 294 . . .. .... .- .. .90CIttTY OP PETli OLEUM ENfENB@RS 10 URN.4L.-.. ..... ~.: .,._ -,......--,,... .- . ..-.. .. .. ..... ,.,. ---- ... .. . s..... ~. ...... ... .,.>=...., . ....._..... .... .. . .. .... . .


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fit in the constant temperature compartment of thechromatography, me types of columns used for thepreliminary studies included Ansul Ether onFluoropak 80, Carbowax 1500 on , Fluoropak 80,Diisooctyl phthalate on Fluoropak 80, and Carbowax400 on Teflon T-6, The column len~th was variedbetween ,,4 and 10 ft and the concentration of theIpsrtitionir$ Iiquid on.the solid support{ was variedfrom 1 to 20 ,,per cent by weight. The operatingtemperature of the colurim and the helium flow ratewere SISO varied over a wide range, Most of thesecihrnns gave good sepstation for water andsymmetric water peaks for liquid water samples of0.0002 cc or larger, Small water samples wereprepared for injection with- the chromatographiccolunin by saturating. methane with whter at roomtemperature and atmospheric pressure. All columnsgave asymmetric water peaks with,= jrnolersbletailing,~xamining the .whter peaks for diffqrent samplesizes, - it was found that the tailing was of thesame magnitude for all ssnqple sizes, as shown onFig. 2. The first chromatogram on Fig, 2 wasobtained for a 0,001 cc liquid water sample andthe. second for a 1 cc gas s%ple of appr~~imately3 per cent water and 97 per cent rnethan~. Theattenuator waschanked to a more sensitive posi?ionafter the majority of the 0.001 bc water samplepassed through the detector to show the tailing,T~e tailing ,cif the large water sample. is evidentlythe sqme nature and magtiitude as tke tailing of the-small water s~mpIe introduced witki the meth~e,Fig, 2 i~/licates that the sbme quantity of wareris retarded on the column regardless of the sample

,, :.... . . FIG.. 2 .-... TY.PICAL .CHROMATOGRAM..OR.WATER.

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size. In the, case of. large water. samples, however, the amount of water retarded is relatively smallas compared to the bulk of the sample; and con- sequently the water peak is not significantlydistorted. In the case of a small water sample,such as water in methane, the amount of water ~retarded on the column represents a relativelylarger portio~ of the sampli+, and hence the whole ~peak is distorted. It was aIso noticed that whenone water peak, was, put on the tail of anotherwater. peak there was a remarkable improvementin peak height and shapci.

,.~ The two phenomena described above indicate>that every time a water sample is introduced iqtothk column a constant amount of wa&r will beretained on the column for a Ionger time ~han therest of thd sample, dtie to die interaction ,betweenthe water and the column materials. This suggested; that, if a constant saturation of water was maintained -on the column in sufficient quantity to eliminatethis interaction, the tailing could be reduced,, Following this reasoning, a constant quantity of,,water was introduced continuously, into the heliumcarrier stream. In this way an equilibrium wasreached between the water & the ~olumn And thecarrier gas stream and ii :emained constant, providedthe operating. conditions did not change., Tesrs,simiI iar to that shown on Fig; 2 were repeated.With this procedure the tailing of water was almoatcompletely eliminated. -A. typical. chrornhtogram fora 3,8 cc meth,ane-,water vapor aarnple i+ shown onFig. 3: The water content of the methane s,amplewas approximately 3 per cent. As shown on Fig.

COLUMN - 6 FT. 5 BY W1.ANSUL ETHER E 181,CIN FLUOROPAK 60REGULATORPRE5SURE -25 RCARRIER GAS - H&: WITHFLOW RATE - 45 . C /MIN.COLUMN TEMP.-70 CCURRENT. 390. mA .CHART SPEED-1.0 lN./MlNSAMPLE -3.8 CC METHANEAND WATERSENiTfilTY - 4S NOTED +

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FIG,. 3 - TYPICAL&CHROtiT~RAM OF ~THAN~---- AND WATER-WITHWAT~ l@JCARB~R. ~= .-..


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a minimum of 4 hours, After equilibrium had beenestablished the agitation was, discontinued andthe gas phase was sampled by slowly releasingthe gas through the expansion valve. During thessmplihg period, mercury or water was injectedirsto the cell to maintain the constant., pres sure.Th~ sample from the expansion valve travelled tdthe chromatography tlirough 1/8 in, O D stainiesssteel tubipg, The expansion valve and the stainlesssteel tubing were heated t~ 300 to 400F to reducewater condensation on the walis. The sample wascontinuously purged through the chromatography

3, the constant water saturation on the columneliminated the tailing ,qf the water in the sampleand made it possible to obtain symmetric waterpeaks...Several different tests were then conduct~d toevaiuate the effect of hydrogen sulfide on thechromatographic column, and it was coocluded thathydrogen sulfide did not interfere, The effect ofother factors on the resolution of the samplecomponents aid the optimum operating conditionswere evalticed experirnentaiiy and the optimumwater content of the helirim stream was det&mined.It was found that the amount of, wsyer in the carriergaa stream significantly affect~d the size of water,signals, For example{.: increasing the amoant ofwater in -the helium str~am over, a certain leveldettreased the water peak. Base line, fluctuationswere also increased by inci+sing the amount ofwater in the helium stream. After studying theeffe:ct of the, different variables on the columnperformance, a column. 4 ft iong packed with 5per cent Carbowax 400 on Teflon T-6 ,wiis chosenfor the experimental tiwrk. The water content of

and., the water cotttent. determined at about sixniinu{e int&vals over a period of time, With tifistechnique,~the effect of the lead line and ~onnectionson the water contene of the sample while travelingfrom the cell to the chrofiatograph could readilybe ascertained. As many as 10 td 20 samples wereanalyzed for one conclusive point at each set ofequilibrium conditioris.EXPERIMENTAL RESULTS ~

i In order to verify the sampling tethniqu~, theoperation of the chromatography and thk ~method ofthe helium- stream was 0.23 mol peq cent with Q ~flow rate of 50 cc/ritin, The column temperaturewas held at 97,5C, and a sample size of 3.8 cc ,. .was used. . .2715 .8\ \ 17. Am5 - EXPERIMENT& TECHNI@JE ,.The experimental apparatus consisted, of anequilibrium cell where the gas was saturated with


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caution be exercised when reducing the pressureon die sample and transferring it to the chromato-graphic column, All points in the connection izystemshould b kept at a constant, temperature in the5rder of 00 to 500F to reduce the ad&oiption ofwater on the steel. ,

EQUILIBRIUM WATER CONTENT OFtY?l t4EYYDRo,GEN suLFIDE VAPOR AT 160FAV. Peak H2S iI&ht Crmten+ water No*of #&~~;nPressure ~;: of V~pOr Content Samplespsla iwm Andyzed %. .

1395 116 16 i ~~ 7 - 2.6; t 1971010 199 17, 6160 10 -4.5; +8611 379 ) 19 9300 -6 - 0+?6;+ 1:: 358 ~l? 21 .; 15000 8 - 2,3; + 313?2 140 27.5 5200 ,4: -2; +.2 ,-: 6900995 ,,240 .29 ..: 6 - 1,6; + 0.8

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ACKNOWLEDGEMENT ,. . .The fi&ncial support offered by the Impe$ialOil Co. of Canada and the J3echcei Corp. in supportof this work is gratefully acknowledged..,.>

water content of sour product

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