Improving performance of absorption tower in natural gas dehydration process. Abstract

Transcription

1 Improvng performance of absorpton tower n natural gas dehydraton process N.Kasr *, Sh.Hormozd Computer Aded Process Engneerng Lab, Chem.Eng.Dept., Iran Unversty of Sc. & Tech.,Narmak, Tehran, Iran Telfax: Capepub@cape.ust.ac.r, 9 th Iranan Chemcal Engneerng Congress Iran Unversty of Scence and Technology November, 2004 Abstract Absorpton tower s the most mportant secton of a natural gas dehydraton plant. In ths paper the effect of some nfluental factors such as temperature and flow rate of TEG enterng the tower, number of equlbrum stages and lqud fractonal entranment n gas phase on performance of dehydraton absorpton tower s nvestgated. Entranment s one of the most effectve factors on tray tower performance, partcularly absorpton towers where the lqud concentraton s much hgher than the vapor n equlbrum wth t. The effect of ths parameter has not been consdered n commonplace smulatng softwares. Havng extracted an equaton for the fractonal entranment factor wth the ad of Far emprcal correlaton, a smulator s prepared usng VC ++ program language n 9000 lnes of source code whch ncludes subprograms for smulatng absorpton, desorpton and dstllaton towers, heat exchangers, condensers and flash drums. Input data corresponded to TEG dehydraton unt of Asaloye gas feld n Iran. The smulaton results show that ncreasng TEG nlet temperature decreases water absorpton effcency whle ncreasng TEG flow rate and number of stages ncreases ths effcency as well as absorpton of pollutants lke benzene and toluene. Fnally t was observed that ncreasng entranment decreases water absorpton effcency as expected. Keywords: Dehydraton, Absorpton Towers, TEG, Entranment, Smulaton Correspondng author

2 Part 5: Separaton Processes 2045 Introducton All gasses have the capacty to hold water n a vapor state. Ths water vapor must be removed from the gas stream n order to prevent the formaton of sold ce-lke crystals called hydrates. Hydrates can block ppelnes, valves and other process equpment. The dehydraton of natural gas must begn at the source of the gas n order to protect the transmsson system. The source of the gas moved through the transmsson lnes may be producng wells or developed storage pools. Ppelne drps nstalled near well heads and at strategc locatons along gatherng and trunk lnes wll elmnate most of the free water lfted from the wells n the gas stream. Mult stage separators can also be deployed to nsure the reducton of free water that may be present [, 2]. Water vapor moved through the system must be reduced to acceptable ndustry levels. Typcally, the allowable water content n gas transmsson lnes ranges from 4 lb. to 7 lb. per MMSCF. There are bascally three methods employed to reduce ths water content. These are:. Joule-Thomson Expanson 2. Sold Desccant Dehydraton 3. Lqud Desccant Dehydraton Joule-Thomson Expanson utlzes temperature drop to remove condensed water to yeld dehydrated natural gas. The prncpal s the same as the removal of humdty from outsde ar as a result of ar condtonng n your house. In some cases glycol may be nected nto the gas stream ahead of the heat exchanger to acheve lower temperatures before expanson nto a low temperature separator. Sold desccant dehydraton, also known as sold bed, employs the prncpal of adsorpton to remove water vapor. Adsorbents used nclude slca gel (most commonly used), molecular seve (common n NGV dryers), actvated alumna and actvated carbon. The wet gas enters nto an nlet separator to nsure removal of contamnants and free water. The gas stream s then drected nto an adsorpton tower where the water s adsorbed by the desccant. The thrd method of dehydraton whch has been dealt n ths study s va lqud desccant and s most common n the Northeast Unted States. Ths method removes water from the gas stream by counter current contact n a tray type contactor tower wth tr-ethylene glycol (TEG). Natural gas enters the unt at the bottom of the adsorber tower and rses through the tower were t ntmately contacted wth the TEG soluton flowng downward across bubble trays. Through ths contact, the gas gves up ts water vapor to the TEG. The water laden TEG s crculated n a closed system, where the water s boled from the TEG. The regenerated TEG then s recrculated to the contactng tower [,2]. A schematc vew of a dehydraton process wth the ad of TEG s represented n Fgure. Absorpton tower s the most mportant secton of a dehydraton unt. Prncpal role of regeneraton secton s TEG consumpton optmzng. In ths study the effect of temperature and flow rate of TEG enterng the tower, number of equlbrum stages and fractonal entranment parameter on absorpton tower performance has been nvestgated. Logcally ncreasng TEG flow rate and number of stages wll ncrease amount of mosture absorpton. Snce absorpton s an exothermc process temperature ncrease, decreases dehydraton effcency. The effect of entranment factor dd not consder n common smulaton software of 9 th Iranan Chemcal Engneerng Congress (), Iran Unversty of Scence and Technology (IUST), Nov., 2004

3 Part 5: Separaton Processes 2046 chemcal processes. Therefore t gave us the mpresson of developng a software to take ths parameter nto account. Entranment n a plate column s that lqud whch s carred wth the vapor from a plate to the plate above. It s detrmental n that the effectve plate effcency s lowered because lqud from a plate of lower volatlty s carred to a plate of hgher volatlty, thereby dlutng dstllaton or absorpton effects. Entranment s also detrmental when nonvolatle mpurtes are carred upward to contamnate the overhead product from the column. Many expermental studes of entranment have been made, but few of them have been made under actual dstllaton condtons. The studes are often questonable because they are lmted to the ar water system, and they do not use a realstc method for collectng and measurng the amount of entranment. Tray spacng, lqud heght on trays and lqud and vapor velocty between trays are the most mportant factors whch nfluence amount of entranment n tray columns. It s clear that the domnant varable affectng entranment s gas velocty through the two-phase zone on the plate. Entranment has a mnor mpact on close separatons when the dfference between vapor and lqud concentraton s small, but ths factor can be domnant for systems where the lqud concentraton s much hgher than the vapor n equlbrum wth t (.e., when a component of the lqud has a very low volatlty, as n an absorber) [3]. The man part of ths study s focused on the mpact of entranment factor on tray effcency of dehydraton absorpton columns due to naccessblty to ths parameter n other smulatng softwares. Smulator descrpton Four sets of equatons, mass balance, equlbrum relatons, sum of mole fractons of each phase and heat balance (the so called MESH equatons) are used to descrbe tray columns treatment. Materal balance of component at tray, for lqud phase wthout sdestream s as follows: l L x L x F 0 (), +, +, = Where, L s Lqud molar flow rate, x s Mole fracton n lqud phase, F s feed molar flow rate and the superscrpt l refers to lqud phase. Consderng the mpact of entranment factor n vapor phase the followng equaton wll be obtaned [4]: l l l ( + E )L x, L+ x, + E L x, F, = 0 (2) As can be seen, a part of component whch enters tray from - and also enters tray + from, along wth vapor n lqud form s taken nto account n the above equaton. It s obvous that as fractonal entranment ncreases, calculated values of Equaton (2) become more dfferent than that of Equaton (). The same equatons could be wrtten for gas phase. Equlbrum relaton for component at tray s gven by: K = y / x (3),,, Where, K s equlbrum constant. Sum of mole fracton of each phase are wrtten as follows: N = N y,.0 = 0, x,.0 = 0 (4) = Energy balance equaton s as follows: L + H + V H + F H L H V H Q = 0 (5) L + V F L V 9 th Iranan Chemcal Engneerng Congress (), Iran Unversty of Scence and Technology (IUST), Nov., 2004

4 Part 5: Separaton Processes 2047 Saturaton pressure of TEG s evaluated by the followng equaton: sat 36 6 p ( ) ( ) = Pc exp 4.92ω Ln(Tr ) ω Tr + 42Ln(Tr ) (6) Tr Where, ω s acentrc factor and subscrpts c and r refer to crtcal and reduced (T r = T/T c ), respectvely. Saturaton pressure of components other than TEG s evaluated by: sat b f p = a + + dlnt + et (7) T + c ( ) Where a, b, c, d, e and f are constants whch are avalable n lteratures for each component [5]. Real enthalpy of components s calculated by combnaton of deal gas enthalpy and resdual enthalpy of gases and lquds. Ideal gas enthalpy s evaluated by: g H = a + b T + c T + d T + e T + f T (8) a, b, c, d, e and f could be found n lteratures [5]. General form of gas and lqud resdual enthalpy are as follows: H H RT Lnf T Where l g = P,x RT H and P T T 0 p H H = 2 RT + dp (0) + H are fractonal molar enthalpy of component n lqud phase and deal gas state, respectvely. Regardng to the type of appled equlbrum relatons, lqud and gas resdual enthalpy could be extracted by equaton of states and actvty models relatons. Peng Robnson (PR) equaton s the most common equaton of state appled for hydrocarbons. In the present work standard PR, HYSYS extracted PR (HYSYS-PR) and PR Stryek-Vera (PRSV) equatons are used. Wlson actvty model s utlzed for predcton of lqud phase treatment. Evaluatng fugacty coeffcent of gas phase ( ϕ ) from PR equaton of state and actvty coeffcent ( γ ) from Wlson actvty model, the followng equlbrum relaton could be appled: v sat sat y ϕ P = xγ p ϕ () Relevant relatons of entranment factor Applyng Far dagram s the most usual method for extracton of entranment factor. In ths dagram wth the ad of floodng percentage and ( L / V )( ρ ρ ) 0. 5 v V / L (9), fractonal entranment wll be found out. In order to use entranment factor n materal balance relatons, an equaton should be determned for t usng Far dagram. For ths, the relevant dagram s scanned and ts lnes coordnate s extracted usng dagram processng software (Grafula). Then some equatons have been ftted for extracted ponts. The most accurate equaton whch has been obtaned for floodng percentage, z, s gven by: 2 2 a + cα + eβ + gα + β + kαβ = (2) 2 + bα + dβ + fα + hβ + αβ z 2 9 th Iranan Chemcal Engneerng Congress (), Iran Unversty of Scence and Technology (IUST), Nov., 2004

5 Part 5: Separaton Processes 2048 Where, y s fractonal entranment of lqud n vapor phase, α = Ln( x), β = Ln( y ) and ( L / V )( ρ ρ ) 0. 5 x =. Parameters values of Equaton (2) are as follows: V / L a = b = c = d = e = f = g = h = = = k = y n Equaton (2) s a dependent varable whch should be extracted. Usng numercal root defnng method the best real root s as follows: ( z( + bα + dβ + fα + hβ + αβ ) ( a + cα + gα + β kαβ ) + y = exp (3) e Varables and parameters of ths equaton are smlar to Equaton (2). Evaluaton of gas n lqud fractonal entranment s more dffcult than that of lqud n gas phase and no dagram or equaton s presented for estmaton of ths parameter untl now. Takng entranment factor nto account, a smulaton program has been developed for the dehydraton of natural gas. Ths smulator s prepared va VC ++ program language n 9000 lnes of source code and ncludes subprograms for smulatng absorpton, desorpton and dstllaton columns, heat exchangers, condensers and flash drums. Input data correspond to TEG dehydraton unt of Asaloye gas feld n Iran (Table ). Results and dscusson TEG flow rate Absorpton of toluene and benzene whch cause envronmental polluton s nvestgated besdes water n ths secton. Absorbed toluene and benzene n absorpton column are exhausted to the envronment n regeneraton secton. Snce dsposal of such pollutants should be kept at a specfed level, TEG flow rate s regulated n an optmum value whch not only dehydraton effcency becomes maxmum (economcal aspects) but also ar polluton crtera are consdered (envronmental aspects) [8]. Water, toluene and benzene absorpton rate varaton as a functon of TEG flow rate s depcted n Fgure 2. Accordng to ths fgure, ncreasng TEG flow rate ncreases absorpton rate of water (dehydraton effcency) as well as toluene and benzene. As can be seen, standard PR results concde wth PRSV at all TEG flow rate whle HYSYS-PR result show some devaton at lower flow rates. TEG temperature Water absorpton rate as a functon of lean TEG temperature s represented n Fgure 3. Snce absorpton s an exothermc process, ncreasng nput TEG temperature decreases water absorpton rate. In real dehydraton plant temperature of TEG entered to absorpton tower s adusted to 5-6 C more than enterng gas temperature [6,7]. Fgure 3 shows that, HYSYS- PR, standard PR and PRSV results are almost the same at dfferent lean TEG temperature. Number of stages The effect of number of stages on water absorpton rate s shown n Fgure 4. As can be seen, usng HYSYS-PR equaton of state, the smulator gves dfferent results from that of standard PR and PRSV. Increasng number of equlbrum stages ncreases water absorpton rate and n other way ncreases manufacture and mantenance costs. Therefore an optmum number of stages based on requred water absorpton amount and economc aspects should be specfed. 9 th Iranan Chemcal Engneerng Congress (), Iran Unversty of Scence and Technology (IUST), Nov., 2004

6 Part 5: Separaton Processes 2049 Fractonal entranment factor The effect of floodng percentage on water concentraton n leavng dry gas and on TEG flow rate n the lqud leavng the column are llustrated n Fgures (5) and (6), respectvely. Floodng percentage s utlzed n these fgures due to ths fact that varyng ths parameter can change entranment factor value based on Far emprcal correlaton and Equaton 3. Fgure (5) shows that, ncreasng floodng percentage decreases water concentraton n leavng dry gas and consequently performance of absorpton tower mproves. But ths result s not n concordance wth real cases. Accordng to Fgure (6), ncreasng floodng percentage decreases TEG flow rate whch means that TEG concentraton n gas phase has been ncreased because of entranment phenomena. Eventually t can be concluded that water concentraton decrease s due to leavng gas flow rate ncrease, but not column performance mprovement. Concluson In the present work the effect of some process parameters on dehydraton performance of an absorpton tower were nvestgated. Entranment of lqud n gas phase was one of these parameters. Ths parameter was not found n any of the accessble smulaton softwares. Hence for studyng the effect of ths parameter a smulator was developed. Frst of all, subprograms related to thermodynamcs equatons were wrtten. Then the routnes related to mass and energy balances and equlbrum relatons were provded. Fnally, usng these subprograms and exstng models, programmng was accomplshed for absorpton column of dehydraton plant and a smulator was developed. TEG dehydraton unt of Asaloye gas feld n Iran was smulated usng the present smulator. The results showed that ncreasng flow rate and number of stages ncreases dehydraton effcency whch contradcted wth the behavor of process as temperature and fractonal entranment ncreased. It has also been found that usng HYSYS- PR equaton of state gves almost dfferent results from that of standard PR and PRSV. Notaton L Lqud molar flow rate (kmol/hr) x Mole fracton n lqud phase V Vapor molar flow rate (kmol/hr) F Feed molar flow rate (kmol/hr) E Fractonal entranment K Equlbrum constant H Lqud/vapor molar enthalpy (J/kmol) Q Molar heat rate (J/hr) P Pressure (bar) T Temperature ( C) H + Pure component enthalpy n deal gas state (J/kmol) H Fractonal molar enthalpy n lqud phase (J/kmol) ρ Densty (kg/m 3 ) z Floodng percentage y Mole fracton n gas phase, Fractonal entranment of lqud n gas phase R Unversal gas constant (J/kmol C) Volume (m 3 ) 9 th Iranan Chemcal Engneerng Congress (), Iran Unversty of Scence and Technology (IUST), Nov., 2004

7 Part 5: Separaton Processes 2050 ω f ϕ γ Acentrc factor Fugacty (bar) Fugacty coeffcent Actvty coeffcent Subscrpt Component Tray V Gas phase L Lqud phase c Crtcal r Reduced Superscrpt l Lqud phase v Vapor g Ideal gas sat Saturaton Reference. Company of royal Dutch/shell group, Glycol type gas dehydraton systems manual, December M.Ikhlaq, Glycol dehydraton of natural gas, The Chemcal Engneer, June 992, pp R.H.Perry, D.W.Green, Perry s chemcal engneers handbook, Mc Graw Hll publcaton, 7 th ed., J.D.Seader. E.J.Henley, Separaton process prncple, John Wley & Sons, J.M.Prausntz, R.N.Lchtenthaler, E.G.de Azevedo, Molecular Thermodynamcs of Flud-Phase Equlbra, Prentce Hall PTR, 3 rd edton, W.P.Mannng, H.S.Wood, Gudelnes for glycol dehydrator desgn, part, Hydrocarbon processng, January 993, pp W.P.Mannng, H.S.Wood, Gudelnes for glycol dehydrator desgn, part 2, Hydrocarbon processng, February 993, pp W.R.True, Federal, state efforts force reexamnaton of glycol-reboler emssons, Glycol-reboler emssons, Ol &Gas Journal, May 7, 993, pp th Iranan Chemcal Engneerng Congress (), Iran Unversty of Scence and Technology (IUST), Nov., 2004

8 Part 5: Separaton Processes 205 Table - Smulator nput data from Asaloye gas feld n Iran Wet Gas Components mole fracton Water CO H 2 S N CH C 2 H C 3 H C 4 H n-c 4 H C 5 H n-c 5 H C Benzene Toluene Flow rate (kmol/hr) Pressure (bar) Temperature ( C) Lean TEG Components mole fracton TEG.00 Flow rate (kmol/hr) Pressure (bar) 7.20 Temperature ( C) th Iranan Chemcal Engneerng Congress (), Iran Unversty of Scence and Technology (IUST), Nov., 2004

9 Part 5: Separaton Processes 2052 Off Gas Condenser Glycol Contactor Dry Gas Stll Column Wet Gas st Heat Exchanger 2 nd Heat Exchanger Stahl Column Reboler Glycol Pump Strppng Gas Flash Drum Water absorpton rate (kmol/hr) Fgure - Schematc vew of TEG dehydraton process Lean TEG rate (kmol/hr) Benzene absorpton rate (kmol/hr) (a) 4.20E E E E E E-0 HYSYS-PR Standard PR PRSV 3.00E Lean TEG rate (kmol/hr) HYSYS-PR Standard PR PRSV Fgure 2- Varaton of (a) water (b) toluene and (c) benzene absorpton rate wth lean TEG flow rate Toluene absorpton rate (kmol/hr) (c) (b) Lean TEG rate (kmol/hr) HYSYS-PR Standard PR PRSV 9 th Iranan Chemcal Engneerng Congress (), Iran Unversty of Scence and Technology (IUST), Nov., 2004

10 Part 5: Separaton Processes Water absorpton rate (kmol/hr) 4.00 HYSYS-PR 3.90 Standard PR PRSV Lean TEG temperature (C) Fgure 3- Water absorpton rate as a functon of lean TEG temperature Water absorpton rate (kmol/hr) HYSYS-PR 3.70 Standard PR 3.65 PRSV Number of stages Fgure 4- Water absorpton rate as a functon of number of stages Water concentraton n dry gas (kg H2o/kg total).30e-03.25e-03.20e-03.5e-03.0e-03.05e-03.00e E E Floodng percentage Fgure 5- Varaton of water concentraton n dry gas wth floodng percentage 9 th Iranan Chemcal Engneerng Congress (), Iran Unversty of Scence and Technology (IUST), Nov., 2004

11 Part 5: Separaton Processes 2054 TEG flow rate n leavng lqud (kmol/hr) Floodng percentage Fgure 6- Varaton of TEG flow rate n the lqud leavng column wth floodng percentage 9 th Iranan Chemcal Engneerng Congress (), Iran Unversty of Scence and Technology (IUST), Nov., 2004