Conference Proceedings Paper Exergy Destruction and Entropy Generation in Desalination Systems

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1 Confrnc Procdngs Papr Exrgy structon and Entropy Gnraton n salnaton Systms Vra Gnanswar Gud Cvl & Envronmntal Engnrng partmnt, Msssspp Stat Unvrsty, Starkvll, MS 39762, USA; gud@c.msstat.du; Tl.: Abstract: Th mptus for nw tools for a comprhnsv and accurat analyss of nrgy utlzaton and ndustral systms coms from th nd for sustanabl dvlopmnt that could b mpdd by xhaustng nrgy sourcs and dtroratng nvronmnt. Exrgy valuaton provds nsght to achv hghst tchnologcal ffcncy at th lowst cost whl mtng th socal and lgal condtons. salnaton procsss ar known as major nrgy consumrs and xrgy dstructon and ntropy of ths procsss s ssntal for thr sustanabl dvlopmnt. Ths papr dntfs mportant aras of xrgy dstructon and ntropy gnraton n dsalnaton procsss ncludng mult-ffct dstllaton (ME), multstag flash (MSF) and rvrs osmoss (RO) wth cas studs and offrs rcommndatons for futur fforts n rsarch and dvlopmnt. Kywords: nrgy; xrgy; ntropy; dsalnaton; dstllaton; mmbrans; osmoss 1. Introducton salnaton has now bcom a promsng altrnatv for frshwatr supply du to rapdly ncrasng dmands for frshwatr throughout th world. Howvr, snc wll-stablshd dsalnaton tchnologs ncludng thrmal and mmbran procsss dmand larg quantts of nrgy, provdng dsalnatd watr can plac a concomtant dmand on th lmtd nrgy sourcs. Enrgy producton also nvolvs dgradaton of nvronmntal qualty. Thrfor, watr, nrgy, and nvronmnt bcom th most mportant componnts for sustanabl human dvlopmnt. salnaton tchnologs utlz varous forms of nrgy to produc frshwatr. Whl th procss ffcncy can b rportd, n gnral, by th frst law of thrmodynamc analyss, ths s not a tru masur of th procss prformanc snc t dos not account for all th forms of nrgy. Accordngly, th scond law of thrmodynamcs has bn mor usful to valuat th prformanc of th dsalnaton systms. Scond law of thrmodynamcs (xrgy analyss) accounts for th avalabl forms of nrgy n th procss strams and nrgy sourcs wth a rfrnc nvronmnt and dntfs th major losss of nrgy/xrgy dstructon. Ths ads n dvlopng ffcnt dsalnaton procsss by lmnatng th hddn losss [1]. Ths papr laborats on xrgy analyss of dsalnaton procsss to valuat th thrmodynamc ffcncy of major componnts and procss strams and dntfs sutabl opratng condtons to mnmz xrgy dstructon. Wll-stablshd MSF, RO, and Solar dstllaton wr dscussd wth cas studs to llustrat th xrgy prformancs. 2. Matrals and Mthods Fundamntal mass, nrgy and xrgy balancs ar dscussd blow. fntons for xrgy dstructon and xrgy loss n ndvdual componnts and complt systms. Th 3rd Intrnatonal Elctronc and Flppd Confrnc on Entropy and Its Applcatons (ECEA 2016), 1 10 Novmbr

2 Exrgy Analyss: A complt stady-stat analyss of nrgy convrson/utlzaton procsss can b mad basd on mass, nrgy, and xrgy balancs. Ignorng kntc and potntal nrgy trms, th thr balancs for a control volum ar [50]: Mass balanc: 0 = m m (1) Enrgy balanc: ( ) ( ) 0 = Q W + m h m h (2) Exrgy balanc: T 0 0 = 1 Q j W + ( m ) ( ) m E (3) j Tj Th varabls ar dfnd n th Appndx. Th trms on th rght hand sd of th xrgy quaton rprsnt th xrgy assocatd wth hat transfr, j at tmpratur Tj; th work transfr; th xrgy nflow; th xrgy outflow; and th xrgy dstructon, rspctvly. Th xrgy nflow and outflow assocatd wth th strams ntrng and lavng th control volum ar quantfd n trms of th spcfc xrgy,, dfnd as follows: ( ) ( ) = h h T s s (4) o o o For a gvn st of opratng condtons and th corrspondng proprts of th workng flud, th rats of xrgy dstructon and xrgy loss for ach componnt of th procss can b computd from th abov quatons. Th followng masurs can now b dfnd to assss th thrmodynamc prformanc of th componnts of a systm and th ntr systm [1]: Exrgy dstructon rato for componnt c of th systm, y,c: y c, Exrgy dstructon n componnt, E = Exrgy dstructon n systm, E Exrgy dstructon rato for complt systm, y: Exrgy loss rato for th complt systm yl: y Exrgy dstructon n systm, E = Exrgy of ful suppld, E (6) y L F Exrgy loss n systm, E L = Exrgy of ful suppld, E (7) F c, (5) Exrgtc ffcncy, ψ: ψ E + E E F L = 1 Th xrgy dstructon rat (rrvrsblty) s rlatd to th ntropy gnraton rat by th Gouy Stodola quaton as follows: y = T0 Sgn (8) 2

3 3. Rsults and scusson Exrgy prformanc of mult-stag flash dstllaton, rvrs osmoss and solar stll dsalnaton procsss ar dscussd wth cas studs n ths scton followd ntropy gnraton n ME, MSF and RO systms MSF salnaton A larg MSF dstllaton plant (Al-Jubal) n th Araban Gulf ara was analyzd thrmodynamcally by Kahraman and Cngl [2] usng actual plant data and opraton data. Exrgy flow rats wr valuatd throughout th plant to dtrmn th locatons of th hghst xrgy dstructon. Th hghst xrgy dstructon occurrd wthn th MSF unt, as xpctd. Th dtals of th MSF dsalnaton plant ar as follows: watr producton capacty of m 3 /d wth an ntgratd powr producton capacty of 1295 MW. Ths plant has 40 MSF unts ach consstng 22 flashng stags. Each MSF unt has a capacty of 23, 500 m 3 /d. A gnral dagram of a MSF unt s shown n Fgur 1. Sawatr (a flow of 2397 kg/s) at 35 C and a salnty of 46,500 ppm ntrs th dsalnaton plant. A major porton of th ncomng sawatr s usd as th coolng watr and rjctd nto th sa whl th rst (808 kg/s) s suppld through th MSF stags. Th saln watr s thn flashd through th MSF stags at lowr prssurs n ach stag succssvly wth a total prssur drop of 627 kpa. Th saln watr (brn) s dschargd at a flow rat of 536 kg/s and a salnty of about 70,000 ppm. Fgur 1. Schmatc of a typcal MSF dstllaton unt and xrgy balanc of th Al-Jubal MSF plant. Th total scond law ffcncy was 4.2%. Majorty of th xrgy dstructon (78%) occurrd n th MSF dstllaton unts, followd by 8.3% of xrgy dstructon n hat xchangrs. Pumps and motors contrbutd 5.3% and coolng watr dscharg had an xrgy dstructon of 4.8%. Th rmanng 3.8% of th xrgy dstructon occurrd n throttlng valvs and dscharg of th brn and product watrs Rvrs Osmoss Mmbran Procss A rvrs osmoss dsalnaton plant wth a tratmnt capacty of 7250 m 3 /d was valuatd for ts xrgy prformanc [3]. Th plant rcvd a bracksh watr sourc wth a salnty of 1550 ppm and th product watr had a salnty of lss than 500 ppm. Th flow schmatc of th rvrs osmoss plant s shown n Fgur 2. Th bracksh watr s suppld by a low prssur pump from whch a partal flow s bypassd as blnd. Th raw watr passs through statc mxr to add chmcals to prvnt potntal scalng and foulng and thn t passs through fltrs to rmov th contamnants and suspndd solds bfor t s pumpd (hgh prssur pumps) through th rvrs osmoss unt. Prmat from th RO unt wll thn b blndd wth th bypass saln watr to produc product watr wth a rqurd salnty of lss than 500 ppm. Th xrgy analyss of th unt s shown n Fgur 2. Th RO mmbran unt accounts for th maxmum xrgy dstructon n ths unt whch accounts up to 74.1%. Throttlng valvs contrbut to othr sgnfcant xrgy losss and th smallst 3

4 xrgy losss occur n th statc mxr and fltr. Th scond law ffcncy of th plant was 4.3%. Th xrgy ffcncy was mprovd to 4.9% wth addton of a prssur xchangr whch rsultd n savngs of 19.8 kw. Fgur 2. Flow schmatc and xrgy balanc of a rvrs osmoss unt rcvng bracksh watr. In anothr study, a bracksh watr dsalnaton plant n Jordan wth a salnty of 2500 ppm had two RO unts whch producd prmats of 90 and 70 ppm rspctvly [4]. As shown n Fgur 2, ths unt had prssur fltrs and cartrdg fltrs followd by RO unts, dcarbonator and a hydrostatc watr tank. Majorty of xrgy dstructon occurrd n th throttlng valvs whch accountd up to 56.8% followd by mmbran unts wth 21% of total xrgy dstructon. Th pumps and motors causd 19.6% xrgy dstructon. Th scond law ffcncy of ths plant was 4.1% whch was smlar to th prvous study rportd by Crc [3]. Ths analyss showd hghr xrgy ffcncs of fltrs and suggstd that rplacmnt of throttlng valvs wth nrgy rcovry dvcs and pumps quppd wth varabl frquncy motors mght rduc th xrgy dstructon n th ovrall RO unt Solar Stlls and Solar salnaton Systms Th ssntal componnts of a passv solar stll ar collctor plat (absorbr), saln watr (brn) and glass covr. Passv systms collct solar nrgy ncdnt on th glass covr drctly to caus vaporaton of pur watr. Th xrgy ffcncy of a passv solar systm was rportd at 5% [5], that of th absorbr and saln watr wr 12.9% and 6%. urng non-sunlght hours, th snsbl hat stord n th saln watr causs furthr vaporaton of frshwatr du to lowr ambnt tmpraturs ncrasng xrgy ffcncy of saln watr to as hgh as 93%. Th hghst xrgy dstructon of 615 W/m 2 was notd for th collctor out of a solar xrgy nput of 935 W/m 2 [5]. Incrasng th numbr of ffcts n solar stlls may mprov th xrgy ffcncy. Sow t al [6] rportd xrgy analyss of a sngl, doubl, and trpl ffct solar stlls. Thr mportant crtra wr appld whch wr watr rjcton lmt of 50%, maxmum salnty lmt of 5.5% and a mnmum solar nrgy utlzaton. Th xrgy ffcncy of th trpl ffct systm was btwn 19 and 26% and that of th doubl-ffct systm was btwn 17% and 20%. Th sngl-ffct systm had an xrgy ffcncy lss than 4%. Enrgy storag may also mprov th nrgy and xrgy ffcncs n solar dsalnaton systms and low grad hat sourcs [7]. Twar t al [8] conductd a comparatv study on th actv and passv solar stlls. Th actv solar stll was supportd by flat panl solar collctors whl th passv stll was standalon rcvng drct solar nrgy. Th frshwatr ylds wr hghr for actv solar stll whn compard wth passv solar stll. Th ylds for th actv stll wr 3.08 L and 2.85 L for qual and unqual nnr and outr glass covr tmpraturs rspctvly whl th sam for th passv solar stll wr 1.14 L and 1.09 L rspctvly. Th ffct of th numbr of solar collctors on th nrgy and xrgy 4

5 ffcncs of th actv solar stll was valuatd. Snc th dpth of th saln watr n th solar stll would hav sgnfcant ffct on th procss prformanc, calculatons wr prformd at dffrnt saln watr dpths as shown n Fgur 3. Fgur 3. Enrgy and xrgy ffcncs of an actv solar stll. Hghr numbr of solar collctors rsultd n lowr nrgy and xrgy ffcncs whl th optmum numbr of solar collctors was dtrmnd as 3. Th dpth of th saln watr n th solar stll also showd a ngatv ffct on th nrgy and xrgy ffcncs of th actv solar stll. Lowr dpth of saln watr at about 5 cm was favorabl n vw of both th frst law and scond law ffcncs. Solar stlls and smlar confguratons can b drvn by low grad wast hat sourcs such as th hat rjctd from domstc ar-condtonng unt and othr ndustral procss wast hat sourcs [9,10] Entropy Gnraton n salnaton Systms Exrgy dstructon can also b shown as ntropy gnraton n dsalnaton procsss [11]. Physcal modls can b dvlopd to stmat th magntud of ntropy gnraton by componnt and ndvdual procsss. Tabl 1 shows th total ntropy gnraton n multpl ffct dstllaton (ME), multstag flash (MSF), and, rvrs osmoss (RO) dsalnaton systms. 4. Conclusons Tabl 1. Entropy gnraton n ME, MSF, RO dsalnaton systms. Procss componnts ME! MSF* Procss componnts RO Thrmal dsqulbrum 2.5% 0.2% - - Chmcal dsqulbrum 6.2% 10% Chmcal dsqulbrum 15.9% Evaporators - - Fd through PX 3.2% Condnsr 21.8% - Brn through PX 3.3% Flash boxs 0.6% - Fd pump 0.7% Fd hatrs 12.3% 73.9% Boostr pump 2.1% Brn hatrs % Hgh prssur pump 20% Effcts 56.5% - RO modul 54.8%! forward fd ME systm; * onc-through MSF systm; sngl stag RO systm Exrgy analyss provds nsght nto th thrmodynamc losss n dsalnaton systms. Ths nformaton can b usd to modfy procss confguratons to optmz th xrgy prformanc. For 5

6 thrmal dsalnaton systms th xrgy losss occur n th condnsr whr th latnt hat s lost to th nvronmnt or rjctd to th coolng watr. For mmbran systms, th mmbran barrr tslf s th major xrgy dstructor ndcatng opportunts for dvlopmnt of hgh prmablty and low nrgy consumng mmbrans. In rnwabl powrd dsalnaton systms, th xrgy losss ar th hghst for th solar nrgy basd systms. Ths s bcaus th solar xrgy s vry hgh. It s also advsabl to utlz hgh xrgy sourcs for hghr qualty product dvlopmnt. For dsalnaton purposs, consdrng th unavodabl dstructon n both mmbran and thrmal systms, low xrgy sourcs should b utlzd as drvng forcs whr possbl. Ths may lad to ffcnt utlzaton of avalabl nrgy rsourcs and sustanabl nvronmnt and procss dvlopmnt. Acknowldgmnts: Ths rsarch was supportd by th USEPA grant SU Th author acknowldgs th support rcvd from th partmnt of Cvl and Envronmntal Engnrng (CEE), th Bagly Collg of Engnrng (BCoE), and th Offc of Rsarch and Economc vlopmnt (ORE) of Msssspp Stat Unvrsty. Author Contrbutons: Vra Gnanswar Gud concvd, prformd analyss, analyzd th data; and wrot th papr. Conflcts of Intrst: Th author dclars no conflct of ntrst. Rfrncs 1. Gud, V.G.; Nrmalakhandan, N.; ng, S.; Magant, A. salnaton at low tmpraturs: An xrgy analyss. saln. Watr Trat. 2012, 40, Kahraman, N.; Cngl, Y.A. Exrgy analyss of a MSF dstllaton plant. Enrgy Convrs. Manag. 2005, 46, Crc, Y. Exrgy analyss of a rvrs osmoss dsalnaton plant n Calforna. salnaton 2002, 142, Aljund, I.H. Scond-law analyss of a rvrs osmoss plant n Jordan. salnaton 2009, 239, Torcha-Nunz, J.C.; Porta-Gandara, M.A.; Crvants-d Gortar, J.G. Exrgy analyss of a passv solar stll. Rnw. Enrgy 2008, 33, Sow, O.; Sroux, M.; smt, B. Enrgtc and xrgtc analyss of a trpl-ffct dstllr drvn by solar nrgy. salnaton 2005, 174, Gud, V.G.; Nrmalakhandan, N.; ng, S.; Magant, A. Low tmpratur dsalnaton usng solar collctors augmntd by thrmal nrgy storag. Appl. Enrgy 2012, 91, Twar, G.N.; mr, V.; Chl, A. Paramtrc study of an actv and passv solar dstllaton systm: Enrgy and xrgy analyss. salnaton 2009, 242, Gud, V.G.; Nrmalakhandan, N. salnaton usng low-grad hat sourcs. J. Enrgy Eng. 2008, 134, Gud, V.G.; Nrmalakhandan, N.; ng, S. Sustanabl low tmpratur dsalnaton: A cas for rnwabl nrgy. J. Rnw. Sustan. Enrgy 2011, 3, Mstry, K.H.; McGovrn, R.K.; Thl, G.P.; Summrs, E.K.; Zubar, S.M.; Lnhard, J.H. Entropy gnraton analyss of dsalnaton tchnologs. Entropy 2011, 13, by th authors. Submttd for possbl opn accss publcaton undr th trms and condtons of th Cratv Commons Attrbuton (CC-BY) lcns ( 6