Analysis of Gas Expansion Required for Hydrate Formation

Transcription

1 J. Rajnauth: Analysis of Gas Expansion Rquird for Hydrat Formation 22 ISSN Th Journal of th Association of Profssional Enginrs of Trinidad and Tobago Vol.42, No.1, April/May 214, pp Analysis of Gas Expansion Rquird for Hydrat Formation Jrom Rajnauth Rsrvoir Enginring Dpartmnt, Ptrolum Company of Trinidad and Tobago, Trinidad and Tobago, Wst Indis rajnauth@hotmail.com (Rcivd 5 Novmbr 213; Rvisd 26 March 214; Accptd 12 May 214) Abstract: Natural gas hydrat tchnology provids an attractiv mthod to captur and transport natural gas on a small scal utilising a concptual floating hydrat formation vssl. Expansion of th gas from wllhad conditions is ncssary to triggr hydrat formation and this dpnds on th proprtis of th gas. Thrfor, gas from th wll is passd through an xpandr to lowr th tmpratur and prssur suitabl for hydrat formation. Hnc, for capturing natural gas from th wllbor, xpansion would b usd to xpand th gas to nw prssur and tmpratur valus that rprsnt th inlt conditions to th hydrat formation vssl. Th xpandr is usful to xtract nrgy from gas stram and thrby rducing tmpratur and prssur of th gas stram to hydrat formation conditions (6 psia and 35 F). Th xpansion procss must also nsur th gas sampl rmains in th singl phas rgion of th phas diagram, which is important in th dsign procss. If th gas gos into th two-phas rgion of th nvlop, thn additional gas sparation would b rquird bfor formation of natural gas hydrat. This xpansion procss will b valuatd in th analysis using th slctd sampls: dry gas sampl which is 99% mthan and sampl 2 which has havir componnts up to C6. Rsults show that th xpansion fficincy rquird to obtain th natural gas at 35 F may vary from sampl to sampl. In this cas, for dry gas sampl, 85% fficincy was rquird, whras 9% fficincy was ndd for sampl 2. Som commrcial xpandrs can hav up to 9% xpansion fficincy. Kywords: Natural gas hydrat, formaton vssl charactristics 1. Introduction It may b possibl that gas hydrat can provid an asir to produc, safr and chapr to stor mthod of capturing natural gas whn compard to othr transportation mods such as CNG and LNG (Rajnauth t al., 28). It is th blif of many rsarchrs that it is possibl to form natural gas hydrat from gas as soon as it coms out a wll. This is quit diffrnt from forming natural gas hydrat from a storag sourc or piplin sourc, simply bcaus th prssur and tmpratur of th gas is diffrnt. Natural gas hydrat has nvr bn formd from a flowing natural gas wll. Thrfor, sinc th intntion is to form natural gas hydrat dirctly from th natural gas strams obtaind dirctly from gas wlls, on has to considr if th conditions ar right for this. Th conditions rcommndd for hydrat formation ar 6 psia and 35 F. Hnc, thr is th nd to xpand th gas from th high prssurs and tmpraturs coming from th wll to conditions suitabl for hydrat formation (as shown in Figur 1) with th hydrat formation vssl at th wll sit. Th intntion of this papr is to look at th xpansion procss rquird to form natural gas hydrat. Th proposd xpansion procss will b discussd using two rprsntativ gas sampls ( Dry gas and Sampl 2 ). Th formr is basically pur mthan, whras sampl 2 has th havir C 5 and C 6 componnts (Rajnauth t al., 213). It is assumd that ths gass ar producd from a givn fild, at a givn rat which is typical of offshor gas wlls. Tabl 1 shows th natural composition of th gas sampls usd in this papr (Mol %). Bsids, th PVTSim programm (Calsp, 28) was usd for th valuation of xpansion procsss in this study. Tabl 1. Natural Composition of th gas sampls usd (Mol %) N 2 H 2 S CO 2 C 1 C 2 C 3 Dry Gas Sampl ic 4 nc 4 ic 5 nc 5 C 6 Dry Gas..... Sampl Figur 1. Dsign concpt showing th nd of an Expandr

2 J. Rajnauth: Analysis of Gas Expansion Rquird for Hydrat Formation Expansion Concpt Turboxpandrs (21) offrs th high powr lvl, oprating tmpratur and prssur ratio solutions for nrgy rcovry and rfrigration. Th nd of th xpandr in this study is to xtract nrgy from gas stram and hnc rduc th prssur and tmpratur rquird for hydrat formation. Th work th gas prforms in th xpandr is gaind from its nthalpy and th gas cools in th xpandr. For an idal cas, th isntropic procss (no chang in ntropy and 1% fficint), and For non idal cas, and Th fficincy of th xpandr is rlatd to th nthalpy by th following xprssion: whr T 2 - output tmpratur aftr xpansion T 1 - initial tmpratur - xpansion fficincy For diffrnt xpansion fficincis th output tmpraturs ar diffrnt. Th Hors Powr dvlopd by th xpandr (Simms, 29) can b stimatd by: whr w is th gas flow rat Th xpansion procss must also nsur th gas sampl rmains in th singl phas rgion of th phas diagram, which is important in th dsign procss. From th wllhad, th gas flows through a turbo-xpandr, which causs th gas tmpratur to drop to 35 F, and th prssur to drop to 6 psia, assuming an fficincy of 85% or 9% dpnding on th sampl. tmpraturs ar shown in Tabl 2. Th data wr obtaind from th commrcial simulator (Calsp, 28). An fficincy of at last 85% is rquird to obtain an outlt tmpratur of 35 F, rquird for hydrat formation. This gnrats 1.1 x 1 7 Btu of nrgy. Blow is a sampl calculation of powr from th xpandr: Powr = Δh w η whr: Δ h = chang in nthalpy, btu/lbmol, w = flow rat, lbmol/hr η = xpandr fficincy, %. For an 85% fficincy, Δ h = 93.1 btu/lbmol and w = 55 lbmol/hr Powr xp andr = Btu/hr Som commrcial xpandrs can hav up to 9% xpansion fficincy (TurboExpandr, 21). Tabl 2. Hors Powr Gnratd and Outlt Tmpratur for Various Expansion Efficincis Efficincy 1% 95% 9% 85% 8% Inlt, Enthalpy, Btu/lb-mol Outlt, Enthalpy, Btu/lb-mol Chang Enthalpy, Btu/lb-mol HP, Btu/hr Outlt Prssur, psia Outlt Tmpratur, F Gas Expansion Analysis Blow ar th stps usd in th analysis of gas xpansion prior to hydrat formation: (i) To achiv th hydrat formation conditions (6 psia and 35 F), xpansion procss was valuatd by th commrcial simulator (Calsp, 28). (ii) Phas nvlops wr obtaind for all sampls using a commrcial simulator (Calsp, 28). (iii) Th actual wllhad conditions of two slctd sampls wr usd in this analysis. (iv) Th simulator was usd to dtrmin th tmpratur of gas aftr xpansion with a fixd hydrat formation prssur (6 psia). (v) Prssur and Tmpratur of gas aftr xpansion is plottd on th phas nvlop diagram to indicat th phas at this tim. 3.1 Dry Gas Sampl Analysis Th procss flow in Figur 2 illustrats th xpansion of dry gas from on producing wll to hydrat formation conditions. Th wllhad conditions ar considrd to b 1,75 psia and 168 F. From th wllhad, th gas flows through a turbo-xpandr, which causs th gas tmpratur to drop to 35 F, and th prssur to drop to 6 psia, assuming an fficincy of 85% (not that som commrcial xpandrs can xhibit up to 9% fficincy). Ths nw prssur and tmpratur valus rprsnt th inlt conditions to th hydrat ractor vssl. Figur 3 shows th xpansion procss with corrsponding nrgy xchang for th idal procss (isntropic and 1% fficint) and for th actual procss at various xpandr fficincis. On th scondary axis of

3 J. Rajnauth: Analysis of Gas Expansion Rquird for Hydrat Formation 24 th graph is th outlt tmpratur that corrsponds to a givn fficincy. Th figur prsnts th variation in nthalpy and ntropy for th xpansion procss considring svral xpansion fficincis. At 1% fficincy (Isntropic procss), ntropy is constant but th valu incrass as fficincy dcrass. Th work th gas prforms is gaind from its nthalpy and th gas cools rapidly in th xpandr. Th ky in th xpansion procss is to nsur th gas rmains in th gasous phas. Tabl 3. Hors Powr Gnratd and Outlt Tmpratur for Various Expansion Efficincis for Dry Gas Efficincy Powr Outlt Tmpratur % Btu/h Figur 4 shows th phas diagram for Dry Gas sampl, with th wllhad and outlt conditions for varying xpansion fficincis. It can b sn that gas rmains in th gas phas rgion during th xpansion procss. Not that this sampl is mainly pur mthan and dos not xhibit a phas nvlop. Figur 2. Expansion Procss Flow for th Dry Gas Sampl Prssur, Psia Phas Lin Critical Point Wllhad Conditions Expansion (1% ff) Expansion (95% ff) Expansion (9% ff) Expansion (85% ff) Expansion (8% ff) Tmpratur, F Figur 4. Phas Diagram for th Dry Gas Sampl Figur 3. Expansion Procss for Dry Gas Sampl tmpraturs ar shown in Tabl 3. An fficincy of at last 85% is rquird to obtain an outlt tmpratur of 35 F, rquird for hydrat formation. This gnrats 1.1 x 1 7 Btu of nrgy. Blow is a sampl calculation of powr from th xpandr. Powr = Δh w η Whr: Δ h = chang in nthalpy, btu/lbmol, w = flow rat, lbmol/hr η = xpandr fficincy, % For an 85% fficincy, Δ h = 93.1 btu/lbmol and w = 55 lbmol/hr Powr = Btu/hr 3.2 Analysis of Sampl 2 Th procss flow in Figur 5 illustrats th xpansion of Sampl 2 gas to hydrat formation conditions. Th actual wllhad conditions in this cas ar 1,8 psia and 173 F which is slightly diffrnt from th Dry Gas Sampl. Th xpansion turbin xtracts th potntial hat nrgy from th gas, causing it to cool drastically from 173 F to 35 F. Th xpansion procss is shown in Figur 6. tmpraturs ar shown in Tabl 4. At last 9% fficincy is rquird to hav an outlt tmpratur of 35 F rquird for hydrat formation in this cas. This gnrats 1.29 x 1 7 Btu of usful nrgy. Hnc, Figur 7 shows th phas diagram for th Dry Gas sampl with wllhad and outlt conditions, and with varying xpansion fficincis. It is to not that at 1% fficincy, th gas is vry clos to th two-phas rgion. For 9% fficincy, th gas howvr rmains in th gasous stat.

4 J. Rajnauth: Analysis of Gas Expansion Rquird for Hydrat Formation 25 5 MMSCF Natural Gas From wll 18 psia 173 F N2.78 CO C C C3.74 ic4.14 nc4.21 ic5.1 nc5.8 C 6.25 Hating valu 129 btu/ft x 1 9 btu Expandr Enrgy Gnratd by xpandr (1.29 x 1 7 btu) Hydrat Ract or/ 35 F 422 bbls Figur 5. Expansion Procss Flow for th Dry Gas Sampl Enthalpy, btu/lbmol F Entropy, btu/lbmol Figur 6. Expansion Procss for Sampl 2 1% fficincy 95% fficincy 9% fficincy 85% fficincy 8% fficincy Tabl 4. Hors Powr Gnratd and Outlt Tmpratur for Various Expansion Efficincis for Sampl 2 Efficincy Powr Outlt Tmpratur % Btu/h Outlt T, F 4. Discussion and Conclusions Basd on th study findings, it is concludd that th xpansion procss yilds usful nrgy that can b usd in many ways, including powr gnration. Bsids, th xpansion fficincy rquird to obtain th natural gas at 35 F may vary from sampl to sampl. In this cas, for dry gas sampl, 85% fficincy was rquird, whras 9% fficincy was ndd for sampl 2. Som commrcial xpandrs can hav up to 9% xpansion fficincy and thrfor can b usd for xpansion of both gas sampls prior to formation of a hydrat. If howvr th fficincy of th xpandr is blow 85%, thn this xpandr may not b applicabl for this procss. Furthrmor, th xpansion procss nsurd that both th dry gas sampl and sampl 2 rmain in th singl phas rgion of th phas diagram, which is important in th dsign procss. Th us of an xpandr is vry important if natural gas hydrat is formd dirctly from th natural gas strams of gas wlls. Th xpandr would xtract nrgy from th gas stram thrby rducing tmpratur and prssur to hydrat formation conditions. Commrcial xpandrs hav up to 9% fficincy. Th xpansion fficincy rquird for diffrnt sampls is diffrnt. For sampl 2, 9% fficincy was rquird whil th dry gas sampl rquird an fficincy of 85%. Thrfor, th slctd xpandr fficincy must allow th xpansion of th gas to th rquird hydrat formation conditions. Th powr dvlopd by th xpandr is of th magnitud 4.2 x 1 5 Btu/h. Rfrncs: Calsp (28), PVTSim Softwar, Vrsion 18, Calsp A/S, Copnhagn, Dnmark. Simms, J. (29), Fundamntal of th Turboxpandr: Basic Thory and Dsign, availabl at: Turboxpandrs.pdf. <Citd March 21>. Turboxpandrs (21), TurboExpandr-Gnrator for Natural Gas Applications: GE Oil and Gas, availabl at: rv/prod/turboxpandrs/n/indx.htm. Downloadd March 21. Rajnauth, J., Barruft, M., and Falcon. G. Potntial Industry Applications Using Gas Hydrat Tchnology ISSN Th Wst Indian Journal of Enginring Vol.35, No.2, January 213 pp Rajnauth, J., Ayni, K., and Barruft, M. (28), Gas transportation: Prsnt and futur, Papr SPE prsntd at th 28 CIPC/SPE Gas Tchnology Symposium Joint Confrnc, Albrta, Canada Jun (SPE MS). Figur 7. Phas Diagram for Sampl 2 Author s Biographical Nots: Jrom Rajnauth is prsntly a Rsrvoir Enginr with th Ptrolum Company of Trinidad and Tobago, having rcivd his PhD from Txas A&M Univrsity in Dcmbr 21 and his MSc and BSc dgrs from Th Univrsity of th Wst Indis. Dr. Rajnauth has ovr fiftn yars xprinc in various aras of th nrgy sctor having workd in an oil company, a rgulatory body of GORTT and a srvic-orintd company working offshor

5 J. Rajnauth: Analysis of Gas Expansion Rquird for Hydrat Formation 26 Trinidad, Gulf of Mxico and Vnzula. H has authord and prsntd ovr fiftn SPE paprs as wll as othr tchnical and journal publications on th Oil and Gas Industry. His rsarch intrsts includ unconvntional oil and gas, rsrvs stimation and gas transportation mthods. H was th rcipint of th SPE Young Enginr award in 24, and srvd as Dirctor of Continuing Education on th Trinidad Chaptr from Dr. Rajnauth has bn a committ mmbr on svral committs such as th Joint String Committ to th Unitisation Agrmnt 212, th LACPEC Ptrolum Confrnc 23 and th Fifth Gas Exporting Countris Ministrial Forum 25 in Trinidad.