THE EFFECT OF IMBIBITION AND CANE QUALITY ON THE FRONT END MASS BALANCE

Transcription

1 Procdings of Th South AfricanSugar Tchnologists' Association - Jun 1995 THE EFFECT OF IMBIBITION AND CANE QUALITY ON THE FRONT END MASS BALANCE A WIENESE Sugar Milling Rsarch Institut, Durban Abstract It is gnrally accptd that th fibr % can, bri % can and imbibition % fibr affct th prformanc of th front nd of a factory. In an attmpt to quantify ths ffcts a modl was dvlopd giving front nd mass balancs on fibr, bri and watr as a function of ths paramtrs. Th modl is basd on th assumption ofconstant ratios of [fibr % juicl/lfibr % bagass], [bri % juic]/[bri % bagass] and [watr % juic]/[watr %bagass]. Th modl was tstd against ral factory data with surprisingly good rsults. As far as traction is concrnd th rsults wr vry similar to th corrctd rducd traction (CRE) quation as usd in th South African sugar industry. Introduction Th composition of th can and th lvl of imbibition influnc th prformanc of th front nd of th factory. Som knowldg of th ffct of ths paramtrs is of intrst for two rasons, firstly to compar prformancs with similar paramtrs and scondly to prdict th ffct on th prformanc du to a chang in ths paramtrs. A litratur survy shows a multitud ofpaprs on this topic. Drr (1933), Hugot (1986), Mittal (1969), Rin (1975) and many othrs hav studid th ffct of fibr and pol % can on traction to driv what is now known as a corrctd rducd traction (CRE). Sullivan (1985), Wins (19)and othrs hav lookd at th ffct of imbibition on traction. Thr ar howvr two distinct disadvantags of th approachs followd by ths authors. Th first is that prformanc is masurd only in trms of traction. Both th amounts of bagass and juic and thir compositions ar howvr also affctd and ar important for diffrnt rasons, th bagass from a ful point of viw and th juic from an vaporator point of viw. Th scond disadvantag is that thy dal ithr with th composition ofth can or with th lvl ofimbibition but nvr with th combination of th two. On would prfr an approach in which th output in all its aspcts is givn as a function of all th rlvant inputs. Such an approach lads to front nd mass balancs on fibr, bri and watr. Thortical modl Th input for th front nd of a sugar factory consists of can and imbibition. This combind input has a quantitativ and a qualitativ aspct. Th quantity of th input is gnrally considrd to b th throughput in tons of fibr. Th quality can b prssd in trms of fibr, bri and watr prcntags. Similarly th output of th front nd consists of bagass and juic with thir rspctiv compositions of fibr, bri and watr. Monthly factory figurs covring th priod btwn 1979 and 1993 wr usd to dvlop a standard mass balanc using th avrags of ths figurs and is shown in Tabl 1. For a givn input thr indpndnt output variabls ar rquird in ordr to complt mass balancs on fibr, bri and watr around th traction plant. Ths could b, for Tabl 1 Standard mass balanc Fibr (%) 1,22 46,2,46 Bri (%) 9,67 1,98 lj,76 Watr (%) 8,11 51,82 87,78 ampl, bri % bagass, moistur % bagass and fibr % juic. It is now assumd that any chang in concntration of fibr, bri or watr in th input affcts th concntration of ths componnts in th output, in th sam dirction. This mans that at a constant fibr throughput an incras in bri % input rsults in an incras in both bri % bagass and bri % juic. Equally, a dcras in bri % input lads to a dcras in bri % bagass and in bri % juic. If this is tru thn a constant bri % input must giv a constant bri % bagass and a constant bri % juic. This applis qually to fibr and watr. Hnc th imbibition watr is tratd th sam as th watr in can. This is obviously not ntirly corrct but maks provision for changs in imbibition. Any qualitativ chang in th composition of th input (Tabl 1 to Tabl 3) can b sn as th sum of two changs. Th first is a chang in bri % input at a constant fibr % input and th scond a chang in fibr % input at a constant bri % input. Sinc th first chang dos not affct th fibr % input, th fibr % bagass and th fibr % juic stay constant. This givs two output variabls and only on mor variabl is rquird in ordr to calculat a full mass balanc for this particularchang. If that variabl is th bri %juic (12,58) thn Tabl I changs to Tabl 2. Tabl 2 Effct of bri on mass balanc Can + Imbition Bagass Juic Fibr (%) 1,22 46,2,46 Bri(%) 1,32 2, 12,58 Watr (%) 79,46 51,8 86,96 Similarly th scond chang dos not affct th bri % input, and th bri % bagass and th bri % juic stay constant. Again two output variabls ar known and on mor is rquird to stablish a full mass balanc. If that variabl is th fibr % juic (,44) thn Tabl 2 changs to Tabl

2 Procdings of Th South African Sugar Tchnologists' Association - Jun 1995 Tabl 3 Effct of fibr on mass balanc Fibr (%) 9,78 44,24,44 Bri (%) 1,32 2, 12,58 Watr (%) 79,9 53,76 86,98 pl assumption of constant output concntration ratios, th rsults wr surprisingly good. As far as tons imbibition, tons bagass and tons juic ar concrnd thr ar virtually no diffrncs btwn calculatd and actual data. Figur I shows tons of bagass , , 12 Th nt rsult of th sum of ths two changs should howvr still satisfy th condition that a chang in a concntration in th input is followd by a chang in concntration in th two outputs in th sam dirction. So far no solution has bn found to mt this rquirmnt and lad to ralistic answrs at th sam tim. Th forgoing discussion pavs th way howvr for a slightly diffrnt approach. Whn a chang in concntration in th input rsults in a chang in concntration in th output in th sam dirction thn obviously th chang in concntration in th bagass and th juic ar in th sam dirction. On way to achiv this is to assum a constant ratio btwn th concntration in bagass and th concntration in juic. Sinc bagass and juic consist ofthr componnts, fibr, bri and watr, this rsults in thr ratios [fibr %juic]/[fibr % bagass], [bri % juic]/[bri % bagass[ and [watr % juic]/[watr % bagass]. This is actly th numbr of output variabls that ar rquird to calculat mass balancs givn a crtain input. Th ratios basd on Tabl 1 ar,1; 5,954 and 1,6 for fibr, bri and watr rspctivly. Using ths ratios th ffct of th prvious chang in input (Tabl 1 to Tabl 3) is now shown in Tabl FIGURE 1 Tons bagass. Th sam can b said about tons bri in juic, tons fibr in bagass, tons watr in juic and tons watr in bagass. This is not rally unpctd sinc ths quantitis ar rlativly larg. Whn considring small quantitissuch as tons bri in bagass and tons fibr injuic thr ar indd gratr diffrncs btwn calculatd and actual data but thy still follow th sam trnd. Figur 2 shows tons bri in bagass. Tabl 4 applid to standard mass balanc , 3 Fibr (%) 9,78 46,48,46 Bri (%) 1,32 2,9 12,43 Watr (%) 79,9 51,43 87,11 This "constant ratio" modl applis to milling tandms and to diffusrs alik. It provids a mass balanc of th front nd as a function of fibr %can, bri %can and imbibition at a constant fibr throughput vrification Tsting ofth modl can obviously only b don against ral factory data. Th problm with such data is howvr that not only th quality of th input changs but also th quantity. Nvrthlss th modl was usd to calculat monthly front nd mass balancs for th priod btwn 1977 and 1993 using concntration ratios basd on th avrag mass balanc for that priod (Tabl 1). Ths ratios for [fibr % juic]/[fibr % bagass], [bri % juic]/[bri % bagass] and [watr % juic]/[watr % bagass] ar,1; 5,954 and 1,6 rspctivly. Considring th rlativly sim- FIGURE 2 Tons bri in bagass. Whn looking at prcntags rathr than tonnags thr ar two trms. Th bri % juic and moistur % juic show prfct fits btwn calculatd and actual data. As far as th formr is concrnd this is not unusual whn considring th high lvls oftraction that ar achivd. Figur 3 shows th bri %juic. By far th gratst discrpancy is found in th fibr % juic. Th actual fibr '% juic fluctuats much mor than th calculatd on. Sinc on would pct this figur to b 182

3 Procdings of Th South African Sugar Tchnologists' Association - Jun , , 13 l12 Ql :2-.5::. 11 m Ql 2.2 >. '"!'" FIGURE 3 Bri % Juic. FIGURE 5 Bri % bagass. rasonably stady it is possibl that ths fluctuations ar , du to othr factors than th quality of th input in trms of its fibr and bri. Sand which is considrd part of th fibr could wll playa rol hr. Fortunatly th fibr %juic has only a small ffct on th othr variabls of th mass 96 balanc. Figur 4 shows th fibr % juic. (' i ; 2?i: Ql :2,.S.c.8 u:: 1.6 a \ \ tl, 1! 95 i i Q).!S ai ,--, , , ,,._--..- l FIGURE 6 Bri traction. \1.4 FIGURE 4 82 Fibr % juic Othr prcntags such as bri % bagass, fibr % bagass and moistur % bagass do show som diffrncs. Ths diffrncs ar howvr small and th calculatd valus crtainly follow th actual valus quit wll. This is vn tru for a small and thrfor difficult to dtrmin variabl lik bri % bagass which is shown in Figur 5. On ofth most important variabls is obviously th bri traction. With a consistntly high traction ofabout 97% with small variations this figur is vry difficult to calculat accuratly. Taking this into considration th agrmnt btwn actual and calculatd valus is still vry good. Th actual traction rsponds howvr mor strongly to changs in th input than th calculatd on. Figur 6 shows th bri traction. Corrctd rducd traction dpndnt of can quality. At th sam tim ths modls can b usd to dmonstrat th ffct of can quality on traction, particularly th ffcts of fibr and pol in can on pol traction. Th CRE modl usd in South Africa is a statistical approach dvlopd by Rin (1975). Whn basd on bri rathr than on pol this CRE modl has th following form: Thr ar various modls for corrctd rducd traction (CRE) which is a comparativ traction that should b inwhr th subscripts 1 and 2 rfr to two diffrnt conditions. E = bri traction F = fibr % can B = bri % can Using th data from Tabl 1 as th standard mass balanc th ffct of fibr % can on bri traction was calculatd using th "constant ratio" and th CRE modls. Around th standard fibr % can th two modls produc vry similar rsults. Only at vry high fibrs, abov 2%, th traction calculatd by th CRE modl dcrass at a fastr rat. This is howvr outsid th normal fibr rang princd in South Africa. Figur 7 shows th bri traction as calculatd by th two modls. 183

4 Procdings of Th South African Sugar Tchnologists' Association - Jun c: U 95 Q). ld Fibr in can (%) FIGURE 7 CRE and th ffct of fibr on bri traction. FIGURE 9 CRE and bri traction. Basd again on th data from Tabl 1 as th standard th ffct ofbri % can on bri traction was calculatd using both modls. Although moving in th sam dirction this ffct is much mor prominnt for th CRE modl than it is for th "constant ratio" modl. Within th normal bri rang th diffrncs ar howvr within accptabl lvls. Figur 8 shows th bri traction as calculatd by th two modls. l c:. s. III 99T" , Briincan(%) FIGURE 8 CRE and th ffct of bri on bri traction. I.I Th combind ffct of fibr and bri on bri traction was calculatd using th CRE modl on th monthly data of 15 yars. At th sam tim th "constant ratio" modl was usd on th sam data. Although th lattr modl taks th imbibition into considration as wll as fibr and bri, th rsults wr vry much th sam. This mrly indicats that th imbibition lvls hav not changd much ovr th last 15 yars. Figur 9 shows th bri traction as calculatd by th two modls. Applications of th modl Th modl would b much mor usful if th bri traction could b rlatd to th sucros traction. Lionnt (1981) dvlopd an mpirical rlationship btwn can 25 3 purity, mid juic purity and sucros traction. Basd on this rlationship an quation of th following gnral form was drivd: Sucros traction = a * Bri traction' + b * Bri traction + c whr "a", "b" and "c" ar constants. In ordr to calculat ths constants thr bri tractions and thir corrsponding sucros tractions must b known. Th first is th % bri traction corrsponding to a % sucros traction. Th scond is th 1%bri traction which coincids with a 1% sucros traction. Th third is th standard bri and sucros traction. With a standard bri traction of 95,65% and a corrsponding standard sucros traction of 97,33%th constants "a", "b" and "c" ar,44; 1,4428 and, rspctivly. With ths thr constants known, matching sucros tractions can now b calculatd from non-standard bri tractions. Th forgoing applis to milling tandms as wll as to diffusrs. With this addition th modl provids a mans to calculat mass balancs onfibr, bri, watr and sucros whrby th output is a function of th input. Changs in th input can com from two sourcs, th amount of imbibition and th composition of th can. Whil changs in th formr ar just straightforward incrass or dcrass in th lvl ofimbibition, most of th changs in th lattr ar th rsult of sasonal ffcts and/or variations in harvsting practics, i.. burning or trashing. Th ffct of ths changs on th output touchs on th prformanc of subsqunt procsss. Th amount of juic togthr with th bri impact on vaporator capacity. On its own th bri is a masur oftraction and is dirctly rlatd to sugar output. Th suspndd solids in juic control th load on th clarifir and mud filtrs. Bagass normally srvs as ful for th boilrs and both its quantity and quality influnc boilr opration. Th quality is mainly dtrmind by its calorific valu on which moistur and bri hav an advrs ffct. Too littl or bad bagass could man th nd to bum additional ful in th form of coal. Th modl could prov its usfulnss in th study of all ths ffcts. Som ampls ar th ffct of imbibition on traction and th moistur % bagass, th ffct of can burning on th quantity of bagass and th influnc of sasonal changs in fibr and bri in can on traction. Figur 1 shows an ampl of th possibl ffct of th imbibition % fibr on sucros traction and moistur % bagass basd on th figurs givn abov. 184

5 Procdings ofth South African Sugar Tchnologists' Association - Jun 1995 Tabl 5 Standard mass balanc at imbibition of 186% on fibr % Fibr % Watr % Bri % Purity Mass FI Can 14,15 72,27 13,58 82;7 1, Juic,9 85,97 13,13 83,6 95,22 Bagass 45,5 51,3 3,65 78,43 29,51 Imbibition, 1,,, 24,73 Tabl 6 Calculatd mass balanc at imbition of 25% on fibr % Fibr % Watr % Bri % Purity Mass F1 Can 14,15 72,27 13,58 82,7 1, Juic,89 86,95 12,15 83,6 13,5 Bagass 44,73 51,89 3,38 78,43 29,57 Imbibition, 1,,, 33,6 197 s. 98T""""' ,/ g / (/)./ / Etraction Moistur 9S.J--.,.---,----,-----,,-----,--, ,---r soo 6 lmbibltion% fibr FIGURE 1 Th ffct of imbibition on sucros traction and moistur % bagass. Th modl was rcntly usd to invstigat th ffct of an incras in imbibition on th mass balanc from a particular factory. Th standard mass balanc ofthat factory at 186% imbibition on fibr is givn in Tabl 5 showing a sucros traction of 92,47%. This standard mass balanc has ratios for [fibr % juic]/ [fibr % bagass], [bri % juic]/[bri % bagass] and [watr % juic]l[watr % bagass] of,2; 3,593 and 1,676 rspctivly. Th constants rlating bri traction to sucros traction ar a= -,56; b= 1,568 and c=o,o. Basd on ths figurs th nw mass balanc at 25% inbibition was calculatd and is givn in Tabl 6. Th sucros traction at that imbibition lvl is 93,2%. All this is howvr only maningful undr constant opration conditions. Changs such as a chang in fibr throughput, th addition of a mill, bttr can prparation, diffrnt mill sttings, tc. will most likly chang th concntration ratios and thrfor diminish th validity of th modl. Th modl should also b applid within ralistic limits. An imbibition lvl which causs flooding in th diffusr is typically outsid ths limits. Apart from bing usd for individual factoris th modl can also b mployd for th industry as a whol to compar factoris, in which cas on nds to agr on common standards. Conclusions Using th "constant ratio" modl on historical data of 15 yars to calculat front nd mass balancs was surprisingly succssful, surprising bcaus of th rlativly simpl assumption of constant ratios of [fibr % juic]/[fibr % bagass], [bri % juic]/[bri % bagass] and [watr % juic]/ [watr % bagass]. All variabls showd good agrmnt btwn calculatd and actual valus with th cption of th fibr % juic. Whn comparing th modl with th CRE on, th ffct of fibr was vry similar in th usual fibr rang whil th ffct of bri was somwhat lss. Th bri traction calculatd with both modls did howvr giv vry much th sam rsults. As th modl dscribs th ffct of can quality and imbibition on th amount and composition of bagass and juic, it can b usd to' prdict th ffct of changs in th input and to valuat front nd prformanc. REFERENCES Drr, N (1933). Th rduction of sugar factory rsults to a common basis of comparison. Int Sugar Journal 35: 214. Hugot, E (1986). Hand Book of Can Sugar Enginring. 3rd dition Elsvir, Amstrdam, Lionnt, GRE (1981). Th ffct of th lvl of traction on mid juic purity. Proc S AfrSug Tchnol Ass 55: Mittal, BL (1969). Critical survy of formula for assssing can milling capacity and can milling fficincy.int Sugar Journal. 71: Rin, PW (1975). A statistical analysis of th ffct of can quality on traction prformanc. Proc S Afr Sug Tchnol Ass 49: Sullivan, MD (1985). Imbibition optimisation. In! Sugar JournaI8?: 141, Wins, A (19). Imbibition optimisation at Mount Edgcomb. Proc S Afr Sug Tchnol Ass 68: