operate regenerator top without boiling aq. amine solution.

Transcription

1 HSRemoval-MaterialBalacemcd Rough material balace for removal of H S from shale gas with MDEA ad recovery of elemetal sulfur via the Claus process The solvet has bee dow-selected to be methyldiethaolamie (MDEA) Step remove H S with a absorber Step regeerate solvet by releasig captured H S with distillatio Step covert captured H S to elemetal sulfur S Specificatios rocess iput: Shale gas ~ millio stadard cubic feet/day (MMscfd) shale gas 7 ppm (by volume) H S ad % (by volume) CO Assume shale gas remaiig compositio is 9% (by volume) CH 4, % C H 6 Gas is available at 7 F, 96 psia rocess output: pipelie quality atural gas <4 ppmv H S ad <% (by volume) CO <7 lbs water/ millio scf Gas should be delivered at 6 psia, 7 F recover elemetal sulfur S Istructor: Nam Su Wag Thermophysical data MDEA (-methyldiethaolamie) = CH N-(CH CH OH) = C H NO MW MDEA 96 MW HO 8 boilig poit & freezig poit bp MDEA 47 C mp MDEA C boilig poit & freezig poit bp HO C mp HO C operate absorber i this rage Assume o chage i volume upo mixig specific gravity ρ water g/cm ρ MDEA 4 g/cm operate regeerator top without boilig aq amie solutio The followig specificatio ot used (util gas is throttled to 6psia) Alteratively, throttle sour gas to 6psia ad operate the absorber at 6psia Higher pressure helps gas absorptio, but icreases equipmet cost totalout 6 psia rocess flow diagram Chemical compoets compoet stream 8 ( total CH4 CH6 CO HO HS MDEA SO S O N ) ( i ) compoet ( dummy amier amiel furace claus ) ( ) j stream iitialize storage space for mole umber ad pressure compoet, stream compoet, stream

2 HSRemoval-MaterialBalacemcd Natural gas basis: mole basis 7 ppm (by volume) H S ad % (by volume) CO remaiig compositio is 9% (by volume) CH 4, % C H 6 Assume ideal gas i traslatig volume % to mole % ad partial pressure HS, CH6, total, total, basis = basis 7 6 = 7 basis basis i, total, HS, HS, 9 = 8768 HS, CH6, = 974 = 96 psia i, total, total, total, 4696 total, = a Absorber -- top at T=~4~ C Assume very little pressure drop i the absorber, compared to the absolute pressure total, Assume hydrocarbos CH 4 & C H 6 do ot dissolve i aq MDEA solvet, ad most of this shale gas stream is CH 4 & C H 6 CH6, CH6, Assume CO =% vol CH6, The rest are i very small amouts CH6, = 8768 = 974 = a 98 CH6, Saturated with H O wrt % H i liquid at 4 C; HO =689psia This is H O lost to the product stream ad is to be made-up total, total, = 768 a = = 988

3 HSRemoval-MaterialBalacemcd We ca assume the product gas cotais H S saturated wrt wt% aq MDEA solutio; or we ca assume the specificatio level: H S=4ppmv I the fial desig, we wat to stay ot exactly at this specificatio level but a bit below this specificatio level to allow for process fluctuatios HS, HS, 4 6 HS, HS, = 6469 a HS, Assume at 4 C, vapor pressure of MDEA is mmhg This is MDEA lost to the product stream ad is to be made-up total, 76 total, = = 6664 a = = 994 total, Absorber -- top; amie-lea stream Assume aq amie solutio recirculatio = mole aq amie solutio per mole of gas Sice there is ot as much CO i this problem compared to flue gas or sygas, the amie solutio circulatio could be a bit less i the fial desig MDEA HO basis Assume wt% aqueous amie solvet; wt MDEA :wt HO =: (based o weight) MDEA MW MDEA fractio MDEA MW wt fractio wt MDEA HO MW HO Give the above two equatios, we have: fractio wt MW HO basis = amiel MW MDEA fractio wt MW amiel MDEA fractio wt MW HO amiel fractio wt MW MDEA basis = MW MDEA fractio wt MW amiel MDEA fractio wt MW HO We assume ~% of H S & ~% of CO is stripped off i the solvet regeerator; a small amout of H S ad a small amout of CO may remai i the lea aqueous amie solutio Of course, we ca assume ay other degree of strippig as well, say, 9% removal of acid gas i the regeerator Alterately, we ca substitute this assumptio with oe that assumes the amie solutio is saturated wrt H S i the vapor phase (which meas a loadig of ~mole H S/mole amie i the rich-amie stream) HS, amiel amiel total, amiel amiel = total, amiel

4 4 HSRemoval-MaterialBalacemcd Absorber -- bottom; amie-rich stream at T=~4~ C (more acid gas absorbed but slower rate at low T) material balace o the absorber i, amier i, i, amiel, i = HS, amier 7 = amier Solubility of H S ad CO i aq MDEA solutio At wt% aq MDEA solutio, HS =ka of H S correspods to loadig of ~ mole H S/mole amie ( C~4 C), with higher loadig at lower T (data from Li & She, J Chem Eg Data, 8, -8, 99, Fig ) We ca apply a loadig of mole H S/mole amie at HS =ka (data from Huttehuis et al, J etroleum Sci Egi,, -4, 7, Fig ) Here, we merely check to see if we are withi the saturated loadig limits If the umbers are ureasoable, we adjust the amout of recirculatig amie absorber HS, = 496 a HS, amier saturatio loadig (mole HS/mole amie)=~: = 6 amier check Let us apply a theoretical loadig of mole CO/mole amie at CO =7ka (data from Huttehuis et al, J etroleum Sci Egi,, -4, 7, Fig ) = 64 loadig (mole CO/mole amie): amier amier amier amier amier However, this far exceeds the amout of CO i the ; thus, selective removal of oly H S is questioable at equilibrium with the amout of amie we circulate It is possible if selective removal is kietically govered; that is, kietics of absorptio of CO is sufficietly slow compared to that of H S We ca reduce the amout of amie if we allow a very log time for equilibrium (thus, a very large absorber), or we recirculate a excess amout of amie to offset a fractioal approach toward equilibrium for CO = For ow, we ca assume either all CO i the gas dissolves i amie, or oly the balace amout (after subtractig CO i the product gas) dissolves i amie amier amiel With the latter assumptio, the CO loadig is: amier amier total, amier amier = = 9 oly a fractio of the theoretical loadig of CO :MDEA=: amier = 87 total, amier = bad

5 HSRemoval-MaterialBalacemcd Regeerator -- distillatio at T bottom =~ C, T top =below C (high T to drive off acid gas without boilig the aqueous amie solvet, which is mostly water based o mole fractio ad defiitely above freezig of water at C); pick T=~ C as a first approximatio The off gas from the codeser is saturated with water vapor ad MDEA vapor (which is egliget) Assume bottom =~atm, top =~atm; T top goes ito estimatig the relatively small amout of water vapor ad MDEA vapor; ad top gives a rough estimate of relative error whe igorig water vapor (which is roughly ~% error) material balace o the regeerator i, i, amier i, amiel total, If we igore HO vapor, = Saturated with H O wrt % H i liquid at C; HO =788psia This is H O lost to the off gas stream ad is to be made-up HS, HS, total, = 4 a total, a = We ca go back to the lea amie stream ad correct for the above small amout of water ( mole out of ~87 mole), but that is ot at all sigificat Assume at C, vapor pressure of MDEA is mmhg This is MDEA lost to the off gas stream ad is to be made-up 76 total, = 6664 a = material balace o the regeerator (with a bit of water i off gas stream) total,

6 6 HSRemoval-MaterialBalacemcd Claus reactor Claus Reactio : H S + O SO + H O + heat Claus Reactio H S + SO S + H O + heat Overall (Step + X Step): 6 H S + O 6 S + 6 H O Re-write the above reactios Claus Reactio : / H S + / O / SO + / H O + heat Claus Reactio / H S + / SO S + / H O + heat Overall (Step + Step): H S + / O S + H O ossible udesirable side reactio : SO + / O SO (fid a coditio to miimize this later) Reactio stoichiometry Reactio Reactio HS, HS, O, SO, SO,, S total, total, 6 compoet, Assume we provide stoichiometric amout of oxyge (i the form of ) eeded to react with H S The amout of HS to be reacted is based o equilibrium at the lastlaus step, which i practice is equivalet to ~9% coversio of H S to elemetal S I other type of reactios, we ormally provide extra oxyge (perhaps % excess) to push for complete combustio of the fuel However, i this problem, we wat to avoid geeratig excess SO with excess O, because SO is ot the desired fial product I the fial desig, we should iclude a cotroller for ig O to the furace based o H S measuremet O, HS, N, total, 79 O, O, N, = 7 = 4 = 79 total, Assume complete combustio i the furace at T=flame temperature ε O, = O, material balace for the furace ε 7 extet of Claus Reactio to completely cosume O furace < > < > ε We the cool dow the furace exhaust to as low T as possible to favor equilibrium coversio but ~ C above the dew poit of S i subsequet reactors (dew poit at ~ C) to avoid codesig S o catalyst ad deactivatig catalyst 9 ε HS, ε = S, claus < furace > ε 7 extet of Claus Reactio to covert 9% of H S to elemetal S

7 7 HSRemoval-MaterialBalacemcd Summary of molar flow rates: colums=streams = (iput) = =amier 4=amieL = (output) 6= 7=furace 8=claus (output) rows=compoets =total =CH4 =CH6 =CO 4=HO =HS 6=MDEA 7=SO 8=S 9=O =N ( dummy amier amiel furace claus ) = total CH4 CH6 CO HO HS MDEA SO S O N