Optimum Surface Area Distribution of Multiple-effect Evaporator for Minimizing Steam Use in Raw Sugar Manufacturing

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1 805 A publaton of CHEMICAL ENGINEERING TRANSACTIONS VOL. 61, 2017 Gust Edtors: Ptar S Varbanov, Rongxn Su, Hon Loong La, Xa Lu, Jří J Klš Copyrght 2017, AIDIC Srvz S.r.l. ISBN ; ISSN Th Italan Assoaton of Chal Engnrng Onln at DOI: /CET Optu Surfa Ara Dstrbuton of Multpl-fft Evaporator for Mnzng Sta Us n Raw Sugar Manufaturng Sohart Chantasrwan Faulty of Engnrng, Thaasat Unvrsty, Rangst Capus, Pathu Than 12121, Thaland sohart@ngr.tu.a.th Th sugar u vaporaton pross onssts of u hatr, ultpl-fft vaporator, and rystallzr. Th u hatr nrass u tpratur fro th abnt tpratur to th bolng pont bfor ntrng th ultpl-fft vaporator. Watr vaporaton n th vaporator taks pla n any stags at drasng prssurs. Th output of th vaporator s onntratd sugar u or syrup. Th vaporaton of th ranng watr ontnt n syrup ours n th rystallzr. Both u hatng n th u hatr and watr vaporaton n th rystallzr rqur hgh-tpratur vapor, whh s bld fro th vaporator. Thrfor, th odl of th u vaporaton pross onssts of sub-odls of th thr oponnts and ntraton btwn th through ass and nrgy balans. Th nrgy ffny of th pross an b asurd fro th sta nput rqurd to oprat th pross rlatv to th produton of raw sugar by th pross. Th proposd odl shows that th sta rqurnt dpnds on dstrbutons of fxd vaporator and u hatr surfas. Th odl s usd to fnd th optu vaporator and u hatr surfa ara dstrbutons that nz th rqurd sta nput for a spfd valu of prossd u flow rat. 1. Introduton Multpl-fft vaporator s on of th thr oponnts of th u vaporaton pross n raw sugar anufaturng. Th othr two oponnts ar u hatr and rystallzr. Th u hatr s usd to ras th tpratur of nong u to th bolng pont bfor th u s snt to th vaporator. Th rystallzr s usd to vaporat th ranng watr ontnt of onntratd u lavng th vaporator. Th output of th rystallzr s raw sugar. Th vaporaton pross rqurs a supply of low-prssur sta fro a turbn. Pnh analyss has dntfd th ultpl-fft vaporator as th ost ntnsv thral nrgy onsur n raw sugar anufaturng pross (Corts t al., 2010). Thrfor, prvous suggstons to prov th pross prforan hav ostly fousd on th ultpl-fft vaporator. Urban t al. (2000) usd Pross Intgraton thodology to donstrat that rtrofttng sugar vaporaton pross ld to nrgy savng. Hluan t al. (2007) nvstgatd th probl of optzng sugar vaporaton pross through opraton shdulng of th vaporator. Ensnas t al. (2007) usd a thro-ono produr to optz th dsgn of ultpl-fft vaporator. Hga t al. (2009) showd that nrasng th nubr of vaporator ffts and usng vapor bldng for u hatng ld to rdud sta onsupton. Sharan and Bandyopadhyay (2015) proposd an analytal thod for ntgratng th ultpl-fft vaporator wth bakground pross to nz sta onsupton. Enrgy ffny an also b nrasd by optzng th dstrbuton of hatng surfa ara aong th ffts of th vaporator. Crtra for optu surfa ara dstrbuton proposd by prvous nvstgatons ar basd on splfyng assuptons. Hugot (1986) assud that bolng tpratur rs of sugar u was proportonal to th tpratur drop of th u. Jays (2004) assud that th latnt hat of vaporaton of saturatd lqud watr dd not vary wth prssur. Rntly, Chantasrwan (2015) proposd a or ralst odl of ultpl-fft vaporator for dtrnng th optu surfa ara dstrbuton aong vssls of th vaporator afftd by foulng. Howvr, vapor bldng s gnord undr th assupton that vapor suppld to th u hatr and th rystallzr os fro anothr sour. A or Plas t ths artl as: Chantasrwan S., 2017, Optu surfa ara dstrbuton of ultpl-fft vaporator for nzng sta us n raw sugar anufaturng, Chal Engnrng Transatons, 61, DOI: /CET

2 806 ralst odl should tak nto aount vapor bldng fro th vaporator as norally arrd out by sugar fators. In ths papr, a odl of sugar u vaporaton pross s prsntd. Ths odl taks nto aount ntratons btwn th thr oponnts of th pross through ass and nrgy balans. It s thn usd to fnd th optu dstrbuton of a gvn total hatng surfa ara aong th ffts of th vaporator that axzs th sta onoy, whh s dfnd as th rato of watr ontnt of th nong sugar u to th aount of sta rqurd to run th pross, subtd to a st of spfd onstrants. 2. Evaporaton pross Fgur 1 shows th shat rprsntaton of th sugar u vaporaton pross. Sold ln rprsnts sta and vapor, dashd ln rprsnts sugar u, and dottd ln rprsnts ondnsat. Th ultplfft vaporator rqurs a supply of saturatd sta at prssur p0. Th thral nrgy rlasd by th ondnsaton of th sta rsults n th vaporaton of watr n sugar u at a lowr prssur p1 n th frst fft (E1). Vapor lavng all ffts xpt th last fft (E4) s usd to vaporat watr n sugar u n th sudng fft. Condnsat fro E1 s usd as fd watr for th bolr. Flash tanks ar plad aftr E2 and E3. F1 rvs ondnsat at prssur p1 fro E2 and H1, and produs vapor and ondnsat at prssur p2. Th vapor s usd for vaporaton of sugar u n E3. Slarly, F2 rvs ondnsat fro thr sours. Vapor produd by F2 s snt to E4. Th ondnsat lavng F2 s olltd n a storag tank. gs Th,0 Th,1 Th,2 Th,3 H1 H2 H3 f,n, xn C a b,2 b,3 v,4 E1 E2 E3 v,0 v,1 v,2 v,3 E4 FC v, F1,1 F2,2 HC f,0 f,1 f,2 f,3 f,4 Fgur 1: Sugar u vaporaton syst. Th u hatr onssts of four tubular hat xhangrs (HC, H1, H2, and H3). Inong dlutd u at th flow rat of f,n and th tpratur of Th,3 passs sussvly through H3, H2, and H1, rsultng n th tpratur nras fro Th,3 to Th,0. Vapor usd to nras u tpratur n H1, H2, and H3 s bld, rsptvly, fro th 1 st, 2 nd, and 3 rd ffts of th vaporator. Th u lavng H1 s at a prssur slghtly largr than th atosphr prssur so that dssolvd gass n th u ar rovd n FC. Bfor ntrng th frst fft of th vaporator, th u prssur s rasd fro th atosphr prssur to p1, and th u tpratur s nrasd to th bolng pont n HC, whh uss low-prssur sta as th hatng du. Th rystallzr (C) also rqurs bld vapor fro th 1 st fft of th vaporator. Enrgy rlasd by vapor ondnsaton s usd to vaporat th watr ontnt of th onntratd u fro th 4 th fft. Outputs fro C ar raw sugar and ondnsat. 3. Mathatal odl Th athatal odl of th vaporaton pross onssts of sub-odls of vaporator, u hatr, and rystallzr. Equatons of ths odls ar ass balan quatons, nrgy balan quatons, and quatons of thro-physal proprts. Th nrgy balan quatons of th vaporator ar

3 807 n out h h h out h h 1 v,0 vl,0 f,0 f,1 f,1 a v,1 v,1 f, 1 n out out 1 h h h h h v,1 vl,1 f,0 a v,1 f,2 f,2 v,2 b,2 v,2 f, 2 n out out 1 h h h h h v,2,1 vl,2 f,0 a v,1 v,2 b,2 f,3 f,3 v,3 b,3 v,3 f, 3 n out out 1 h h h h h v,3,2 vl,3 f,0 a v,1 v,2 b,2 v,3 b,3 f,4 f,4 v,4 v,4 f, 4 (1) (2) (3) (4) whr hvl, s th latnt hat of vaporaton at saturaton tpratur T, hv, s th saturatd sta nthalpy at T, and hf, s th sugar u nthalpy n fft, whh s th produt of spf of hat apaty of sugar u (pf) and u tpratur (Tf). Equatons for hvl, hv, and pf ar provdd by Rn (2007). It s assud that th hat-loss fraton n vaporator s n th 1 st and 2 nd ffts, and n th 3 rd and 4 th ffts. Mass and nrgy balans of th flash tanks yld th followng xprssons for vapor ass flow rats (,1 and,2) fro flash tanks F1 and F2. f T T f T T 1, v, b, v, b,, 1, 1 1 whr f T, 1 T h v T 1 hv T hvlt 1 hvlt h T vl (5) (6) Ju tpratur (Tf) s assud to b th saturaton tpratur. It s largr than th bolng pont of saturatd lqud watr at th sa prssur du to th onntraton of dssolvd solds n u (Hong, 1963). It should b notd that bolng tpratur rs du to hydrostat prssur had s not takn nto aount n ths odl baus th vaporator s assud to b of a dsgn n whh th fft of hydrostat prssur had on bolng tpratur s nglgbl. Th u onntraton (x) lavng fft of th vaporator s dtrnd fro th ass balan of dssolvd solds. x f,0 a f, n x n 0 v, b, 1 (7) 1 whr 0 s th Kronkr dlta. Not that sn thr s no vapor bld fro th 4 th fft, b,4 = 0. Addtonal quatons ar obtand fro th rqurnt that th rat of hat transfr aross vaporator surfa (A) n fft s qual to th rat of hat transfr rlasd by ondnsng sta n that fft. n out U A T 1 0.5T f, Tf, 1 v, 1, 1h v, 1 (8) Th vaporator s assud to b of th fallng-fl typ, for whh th orrlaton of hat transfr offnt s provdd by Paho and Fron (2004) x 1x U (9) For th u hatr, th rqurnt that th latnt hat of ondnsaton of th bld vapor quals th u nthalpy nras n H1, H2, and H3 ylds 1 h T T (10) h b, vl, f, n p, h, 1 h, whr p, s th avrag hat apaty of th u, and th hat-loss fraton h n u hatr s In addton, th rqurnt that th hat transfr aross th surfas of H1, H2, and H3 quals th nras n nthalpy of th u ylds T h, 1 T Uh, Ah, f, np, T T h, (11) If u vloty s 2.0 /s, th ovrall hat transfr offnt of th u hatr s (Hugot, 1986).

4 808 U, T h (12) Aftr lavng H1, th u prssur (pn) s a lttl abov th atosphr prssur (pout). Th u s allowd to flash n FC, rsultng n a rdud ass flow rat (f,0) that s dtrnd fro 1 f T, T f, 0 f, n n out Th u prssur s nrasd to p1 bfor ntrng th frst fft. Furthror, t s hatd n HC by th xhaust sta. Th followng quatons for HC ar slar to orrspondng quatons for H1, H2, and H3. 1 h T T h v, vl, f,0 p, 1 out (13) (14) T T 1 Uh, Ah, f,0p, T T out (15) Uh, s approxatly 1.0 kw/ 2.K (Paok and Lov, 2003). Th sta prssur n HC (p) s assud to b ontrolld n suh a way that th u tpratur s xatly T1 at th xt of HC. Th surfa ara of HC (Ah,) s 800 2, whh s larg nough that p dos not xd p0. Th rystallzr ay b odlld as a sngl-fft vaporator. It uss th vapor bld fro th frst fft to vaporat th ranng watr ontnt n th syrup lavng th vaporator. Idally, th aount of watr to b vaporatd s th watr ontnt of th syrup. In prat, howvr, or watr s rqurd, and a orrton fator (C) ust b nludd. Prvous works ndat that C s n th rang of 2.0 to 2.2 (Rd and Rn, 1983; Rn, 2007). In ths papr, th rato of 2.1 s assud. Thrfor, 4 2.1f, 0 a v, b, a h vl,1 f,0 x0 h vl,4 (16) 4. Dtrnaton of prforan paratrs It should b notd that, baus th u tpratur at th xt of HC s assud to b T1, HC s unoupld fro th rst of th syst as far as th soluton to th syst s onrnd. For th rst of th syst, thr ar 28 varabls (xn, f,n, f,0, v,0 v,4, b,3, Th,0 Th,3, A1 A4, and Ah,1 Ah,3) and 16 quatons Eqs(1) (4), (8), (10), (11), (13), and (16)]. By posng th valu of u onntratons (x4) at th outlt of th vaporator, Eq. (7) ylds an addtonal quaton. Two or quatons ar obtand f th total vaporator surfa (Atot = A1 + A2 + A3 + A4) and th total u hatr surfa (Ah,tot = Ah,1 + Ah,2 + Ah,3) ar gvn. Furthror, t s a oon prat n th sugar vaporaton pross to spfy valus of xn, x4, p0, p4, Th,0, and Th,3. As a rsult, thr ar 4 fr paratrs. For a gvn st of ths paratrs, th soluton of th syst of quatons govrnng th opraton of th u vaporaton pross an b found. Two prforan paratrs ar obtand fro th soluton. Th frst paratr s f,n, and th sond paratr s th sta onoy (SE), dfnd as SE x v, 0 n v, f, n Th frst paratr s rlatd to rvnu arnd by th sugar fatory, whras th sond paratr s rlatd to nrgy us by th fatory. 5. Rsults and dsusson In th followng sulaton rsults, th u onntraton at nlt of th u hatr (xn) s 12%, th u onntraton at outlt of th vaporator (x4) s 65%, th sta prssur (p0) at th nlt of th vaporator s 200 kpa, th vapor prssur (p4) at th outlt of th vaporator s 16 kpa, th u tpratur (Th,3) at th nlt of th u hatr s 30C, and th u tpratur (Th,0) at th outlt of th u hatr s 103C. Th total vaporator surfa (Atot) s 12,000 2, and th total u hatr surfa (Ah,tot) s 2, If vapor s bld fro only th 1 st and 2 nd ffts of th vaporator, Ah,3 = 0, and thr ar only 3 fr paratrs, whh ar A1, A2, and A3. Fgur 2 shows that, for fxd valus of A1 and A2, nrasng p1 rsults n drasng f,n and nrasng SE. In prat, p1 s not allowd to b lowr than a spfd valu baus (17)

5 bld vapor fro th 1st s rqurd for th rystallzr, whh s dsgnd to oprat at a dsgn prssur. It s assud that p1 s 150 kpa. By spfyng th valu of p1, thr ar only 2 fr paratrs lft - A1 and A f,n (kg/s) SE p 1 (kpa) Fgur 2: Varatons of nput u ass flow rat and sta onoy wth 1 st -fft prssur for th vaporator surfa ara dstrbuton n whh A1 = 4,800 2 and A2 = 2, Fgur 3 shows that, for a gvn valu of A1, thr xsts th optu valu of A2 that axzs f,n. Th orrspondng vaporator surfa ara dstrbuton s known as th A2-optzd vaporator surfa ara dstrbuton. f,n (kg/s) A1 ( 2 ) A 2 ( 2 ) Fgur 3: Varatons of nput u ass flow rat wth 2 nd -fft surfa and thr 1 st -fft surfas for th vaporator surfa ara dstrbuton that ylds th 1 st fft prssur of 150 kpa. Fgur 4 shows that, for th A2-optzd vaporator surfa ara dstrbuton, f,n nrass wth A1, whras SE drass wth nrasng A1. It an b sn that t s possbl to axz both paratrs. A typal sugar fatory puts th prorty on th nu u prossng apaty of th vaporaton pross baus th ntr raw sugar anufaturng pross wll b afftd f th ass flow rat of prossd u s too low. Thrfor, t s pratal to pos a spfd valu of f,n as anothr onstrant of th syst f,n (kg/s) SE A 1 ( 2 ) Fgur 4: Varatons of nput u ass flow rat and sta onoy wth 1 st -fft surfa for th A2- optzd vaporator surfa ara dstrbuton. By posng th rqurnt that f,n = 180 kg/s, thr ar unqu vaporator and u hatr surfa ara dstrbutons for a gvn valu of Ah,3. If Ah,3 = 0 2, t s found that A1 = 4, , A2 = 3, , A3 =

6 810 1, , A4 = 2, , Ah,1 = , Ah,2 = 1, , and SE = Tabl 1 shows vaporator and u hatr surfa ara dstrbutons for 4 valus of Ah,3. It an b sn that SE drass onotonally wth nrasng Ah,3. Th ost nrgy ffnt vapor bldng sh uss vapor bld fro only th frst two ffts of th vaporator. Tabl 1: Optu vaporator and u hatr surfa ara dstrbutons that rsult n th ass flow rat of nput sugar u of 180 kg/s. Ah,3 ( 2 ) A1 ( 2 ) A2 ( 2 ) A3 ( 2 ) A4 ( 2 ) Ah,1 ( 2 ) Ah,2 ( 2 ) SE 0 4, , , , , , , , , , , , , , , , , , , , Conlusons Th proposd odl of th vaporaton pross for raw sugar anufaturng ndats that, f th total vaporator and u hatr surfa aras ar fxd, u onntratons ar spfd at th nlt and outlt of th pross, u tpraturs ar spfd at th nlt and outlt of th u hatr, and sta prssurs ar spfd at th nlt and outlt of th vaporator, th pross prforan dpnds on 4 paratrs. Pross prforan s asurd by th ass flow rat of prossd sugar u and th rato of sta rqurd to run th pross to th prossd sugar u. If th frst-fft prssur s spfd, th nubr of fr paratrs s rdud to 3. Two paratrs ar rlatd to th dstrbuton of th fxd vaporator surfa ara aong th ffts of th vaporator, and th othr paratr s rlatd to th dstrbuton of th fxd u hatr surfa ara aong th hat xhangrs of th u hatr. Sulaton rsults ndat that th nu sta rqurd to pross a spfd aount of sugar u s nu whn vapor s bld fro only th frst two ffts and th vaporator surfa ara s optally dstrbutd aong th 4 ffts. Rfrns Chantasrwan S., Optu surfa ara dstrbuton n o-urrnt ultpl-fft vaporator, Journal of Food Engnrng, 161, Corts M.G., Vrlst H., Suarz E.G., Enrgy ntgraton of ultpl fft vaporators n sugar pross produton, Chal Engnrng Transatons, 21, Ensnas A.V., Nbra S.A., Lozano M.A., Srra L., Dsgn of vaporaton systs and hatrs ntworks n sugar an fators usng a throono optzaton produr. Intrnatonal Journal of Throdynas, 10, Hluan H., Blano A.M., Hrnandz M.R., Bandon J.A., Sultanous r-dsgn and shdulng of ultpl fft vaporator systs, Coputrs & Opratons Rsarh, 39, Hga M., Frtas A.J., Bannwart A.C., Zp R.J., Thral ntgraton of ultpl fft vaporator n sugar plant. Appld Thral Engnrng, 29, Hong P., Prnpls of Sugar Thnology, Vol. III, Elsvr, Nw York, USA. Hugot E., Handbook of Can Sugar Engnrng, 3 rd Ed., Elsvr, Astrda, Nthrlands. Jays W.E.G., Optu dstrbuton of hatng surfa n a ultpl fft vaporator tran, Intrnatonal Sugar Journal, 106, Paho C.R.F., Fron L.S.M., Exprntal rsults for vaporaton of suros soluton usng a lbng/fallng fl vaporator, Journal of Food Engnrng, 64, Paok S.D., Lov D.J., Clar u hatrs do w nd th? Prodngs of South Afran Sugar Thnologsts Assoaton, 77, Rd M.J., Rn P., Sta balan for th nw Flxton II ll, Prodngs of South Afran Sugar Thnologsts Assoaton, 57, Rn P., Can Sugar Engnrng, Vrlag, Brln, Grany. Sharan P., Bandyopadhyay S., Intgraton of ultpl fft vaporators wth bakground pross, Chal Engnrng Transatons, 45, Thaval O.P., Broadfoot R., Knt G.A., Rakann D.W., Dtrnaton of optu tub dnsons for Robrt vaporators. Prodngs of th Intrnatonal Soty of Sugar Can Thnologsts, Intrnatonal Soty of Sugar Can Thnologsts, Chang Ma, Thaland, Urban K., Zalwsk P., Zhu X.X., A doposton approah for rtroft dsgn of nrgy systs n th sugar ndustry. Appld Thral Engnrng, 20,

ANALYSIS: The mass rate balance for the one-inlet, one-exit control volume at steady state is

ANALYSIS: The mass rate balance for the one-inlet, one-exit control volume at steady state is Problm 4.47 Fgur P4.47 provds stady stat opratng data for a pump drawng watr from a rsrvor and dlvrng t at a prssur of 3 bar to a storag tank prchd 5 m abov th rsrvor. Th powr nput to th pump s 0.5 kw.