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:·.::.:: .. :: --.·. ~·;_·., -----------------------~---------~ Minichapter 4 Selecting Options for the Separation of Components from Homogeneous Phases We often need to separate or conccntratc species. They may 0ccur in a single homogcneous phase {like a glass of Coke that ís to be separated ínto components); they may occur as an intimate mix of two or more different phascs (like a mixture of sand and gold or a mixture of sand anel water). If the starting mixture is a hornogencous phasc, we start with rhe ideas givcn in Lhis chaptcr. I f thc initial mix- wre is heterogeneous. wc start with -thc ideas in Chaptcr 5. 4.1 SELECTING OPTIONS FOR SEPARATI NG A HOMOGENEOUS PHASE To scparatc a homogencous mixture, wc primarily exploit the diffcrc:1ccs in properties among thc diffcrcnl spccics In actunlly sclecting thc typc of cquipmcnt, wc also study lhe fcccl conccntration, thc product rcquircmcnts, and uniqlll: charactcristics of thc equipmcnt itsclf. In this chaptcr we foc.: u~ on how to sclcct fcasiblc cq uipmcnt options. 1 fow to rough-si1c the options is gtvcn clsewhcrc (\\'oods, 199 3a). 4.1 -1 Overvicw to Selccting Options for Separations In :;dccling scp;u"tinn (lptiorh, wc shouhl tcq' an O(l('ll mim!. 1t.ldition, as cx~mphfi:-d by thi~ t.:.xt, m,ty :mggL· ·t cc1 tatn nptinn<:. I lll\W\'N, m·w d~vdopmcnt' UI C' l'Cl'llll ing rapidly. Individuais and companies have their "favorite" separation techniques and thus may prefer to work with lhe familiar. T he purpose of this textbook is to providc short- cut methods rhat will allow you to rough-size many options beforc deciding. Hence, in selecting options, choose about three to consider in more detail for any separation. Ncxt think of combinations of options; perhaps ion exchange followed by solvem cxtracrion followed by dis- tillation is an appropriate choicc. Thc gcnaal cri teria for selecting options include the propcrties of the spccics, thc feed and product constraints, and thc char,tcierist ic.s o f t11e equipment. A. Exploit the Properties o( the Species. Jf lhe origmal mixturc appears as one homogcncous phase, a gu idcline to thc propcrties that can bc cxploitcd and thc namc o f lhe tcchniquc or unit opaation th<lt might bc used to Sl'pnratc thc SJWCics are listeu in Tablc 4 1. I f the orig i nal mixturc is hc1~·rogcneou~ (suei! as solid particfes di s pcrscd in liquiu). thc propcny's cliffcrcncl''> thal mirht h· exploitcd and th~: 1tame of thL' unit opet.lllon fl1r physical ~cparalion .uc Ji,tL'<I in Tablc 5- 1. J hu.;, lhe fu .,l 111 lÍil COrt,idt:r.l!ÍOil i'~H :til)' ~.?p:tr:tlÍllll i'> thcn lllli H I• 11 ,/ t/jcrc·nc tJ 111 pltyçic<rl or cht'r•: 1"-of •· ,·uin tlttu c a•! I•·' n p/nirc,/. lf,.t ~ l l~ th,· 1 li \l tlll s \'.c ar~ l l)' ln · tos 'fl·'''L ha\'e :1 .&> ~ i -- Table 4-1. General criteria for selecting options for separation I Ir 1here is a difference in ... and its phase Equilihrium R ate I is •.. crente two physic:IIJ>hases 10 gh·e differenl equilibrium concenlr.tlions in Exploil differences 11 i1J1in a singlc phase (WO l>y ± Agcnl by ± Agem h} :1dding byusing Agcnl barrier nonuniformilics in conccntration V:1por prc:ssurc: T• L E vapor a1ion I.ICJurd >nll'eOI or AL~ntrop11: dtsullauon (G · L) Di>tillation Stcam Stearn distillation Mokcuhor dislill tolion E\lr3Cllle d"liiiJlinn Cryogcnic di!>irllalion Freezing pornl anti T, L Mch Cf)'>HIIIJL:olion ~lubrli1y !L·SJ s. Prcelc co nccn1ra1ion s , s Znnc rclining Solul>íli1y J..s K .. L Sohuion cryMallinolion Pr~cipinll ion G·L s. s. L Sleam/gas nc~nrption V. T• G Soh•enl Absorp1ion ~ -- .L....t._à..L I a r ,. ... "L,taif,.d.el.'''" ., · ·~~-· """M*a&i'zo;(..;!tp.tt) .. 'tcõ"""'S·« 'io.dtti:si.i:CA"•::",-i.t--;,:.ê:kttl;~st.::J r,-.. ..: .. ·._;. ; ... ..!!_,~ ... ~·""• l ... ·. fi"• ... .. . ·:_~ __ . ,- - -- - ------ l'anioioo Codf. L·L õ~ ô, L lrnrniscible solvem Solvent Extraction v E:\changc G-S ó~ ôp G Solid absorbent Adsorption l!l.Juilihrium V Tb L-S ~I L Solid absorberu Adsorption L-S K •• L Rcsin lon Exchangc valcncc Suof.ot·c t\ ctl\·oty M.' L lon Exchangc Foam Frac.:toonatoon L·G Adsorption lon r1otation t.lolc:cular G- M G Zeolitcs Adsorption 1\lcmbranc Sizc E;~.clusion Gcomctry GS pcrmcation Chrom:11ogr.1phy ! L .M L Oialysis Ultrafiltrntion MK . L Reverse Osmosis Electrodialysis Elcctromigration K . L .: Electrodialysis Electrophoresis ;\lolccular K.E. Thcrmal Diffusion I I RcadÍ\ÕI) BOD/COD L t\ I icro-organisms Biological rcactions :t" """ w 4-4 Selecting Options for the Separation of Components 'rom Homogeneous Phases large number of componems. For example, a cup of coffee contains water, dissolved sugar, cream, and the host of components that have been ''perked" from the coffee bean to give it aLaste. The water conLains dissolved oxygen and some iron, sodium, chloride, calcium, and magnesium. Where do wc stop in listing Lhe spccics? In keeping with the engineer' Principie of Optimum Sloppiness, we focus first on the two major things that we are trying to sep- nrate; wc callthese Lhe key components. ln thc above cof- fee cxample, we might want to separatc thc "sugar" from ali of Lhe rcst. We would characteriz.e ali the rcst as "watcr." llcnce, sugar anti watcr bccome thc kcy compo- ncnts. K.ing ( 197 1) defines the separation factor <X,J. which is thc ratio of the conccntration ratio of lhe two key compo- ncnts in one separated product slream to thc conccntration r:uio in thc othcr product strcam. That is where x =mole or mass composition i = one key component j = othcr key component (4- 1) For any differences in properties, we can calculate the separation factor. This dcfinition is illustrated in Figure 4-1. Naturally, the largcr the separation factor, the more allractive the separation process and the easicr the separa- Lion. Figure 4-2 relates the separation factor to Lhe feed conceulraLion and shows lhe usual regions of application for most of the equipment options given in Table 4-l. Be- cause different equipment exploits differcnces in different propenies, Lhe separalion factor has different definitions. Table 4-2 provides lhe appropriate definition of the separa- tion factor for the different unit operations. Three other important property guidelines are the ac- tual size of the species, the temperamre sensitivity of the species (will Lhe species decompose at high temperawre?), and whether ít is organic or inorganic. The size may be ex- pressed as the ionic o r molecular ·'size." as the molar mass, or as the molar volume (density divided by the molar mass). The approximatc relationshi p between the size and the molar mass is given in Figure 4-3a. Thus, a lriglyceride oi! wiLh a molar mass of about 900 would havc a "size" of about 1.3 nm. Conceming temperature sensitivity, the big- ger the molecule, Lhe ea<>ier it is to break down at highcr temperatures. Tn general, for organic species, as thc si7c in- creascs thc more sen itive thc species is to tcmperature. Most materiais degrade whcn heated to a high temperature, and especially i f they are kcpt ata high tempcr:uurc. l lick- man and Embree (1949) suggest thnt the ratc of degrada- tion varie linearly with time and doublcs for each 10°C lise in temperaturc. Thus, a dccompo!.ition hazard indcx can be defined as thc product of the absolutc tcmperature and the number of' &cconds thc !.pecies is held at that tcm- perature. They found thm it was more convenient to ex- pres:. thc temperature effcct in tcnns of the operating pres- surc aL which Lhe material is boiling. Thus, they define a clecomposition haLard indcx as the product of the rcsidencc time and thc absolute operating pressurewhen the species is boiling. Some example data are givcn in Figure 4-3b. Thus. thc lriglyceride oil (whose boiling point at 0.7 Pais abouL 300°C) could noL be kept boiling at 0.7 Pa pressure for more Lhan about lO minutes before it would start to de- com pose. We note thaL the decomposition index is not di- rectly rclaLed Lo lhe molar mass; some small, fragile mole- cules decompose more readily than larger, more robust spccics. A guideline is thaL pharrnaceutical and biological spccies are usually very fragilc. \'fany proLeins denature if ti1e temperature ÍS raised to 80°C; Certain polymers undergo transitions if the tempcrature exceeds 60°C and so on. The data in Figure 4-3c can be used as a guide and a re- minder of how sensitive species are to temperature. This becomes a major guideline because one of rhe more popu- lar options for separation is evaporation or distillation, 2 (xi/xih ------~ _ ... /,_,.... I ' ,. ' ' Feed X x, I I Separator \ ,. \ >-\1 __ ;, ·• Heat ---- • Material (t (x i/xj)2 (xi/xj)3 ' I I I I / l'lgu rc -1- J D(:finition of thc Scp;trJtion Factor et 3 (xi/xjh .,.. 4.1 Selecting Options for Separating a Homogeneous Phase I!! a: ~ u <{ u.. z o i= <{ a: ;t w (f) 1051 I I 't==~t~~=t=t=tl +-r-_--+-+-t-tl=t:!-11--+-1--11-----+-+--+-1-+++-+-'i~~ 'i d I : -~ 104 1---t-!--l-1-+-1---!-l--+-l-- --f-1-1~-1---+- f\ - I ·b~t~,l--+-'-+--+--l' --J-t--1----r-=-J\,) 1=1~ ~,. l _ _ crf-== 1---t--•-1--i--t-~--l,.-1-- -1----1- --1-- 1-=!- / -~ "- - -1---+-·1- -1--1-- ---- . . t-- \ r--1~-H-1--+-_;:1--1--1--i ---- _,_ +=trl j~~ ls~?l_ c-~~ 103 -+--1-f+--J-,1---1--1--H-~-+--1-i-1- -i--- I ~ r-- - ,___ ,-1,-t--t--+--t--1--t-1: --r-r- - - I -- - =F--- ... : 1---+-+--H -1---1--1- -t--- -r- ·- ...... -..,-H ·~ - ,. ,.- - H\ ,~~- l---+--l--1--i-+-t--t-+--1--l--1-+-- '-- - -;-~ :::_ -- . . I f- l,-"'"" \ I I I / ' .-- I .' fL I ~=::::.::_ . -r+ I I . ,.-, j_ Spl~ent 1 l-t-===1==;:·=:r:t==t=t==j=::t=t::t=::t=t===t=t/ /' _.,.Ui~r" "CJd:rn-çttcrn I 1 1 . / -/fi~ ' I I li 1 1/ I/ ever e : 1 "'-' I 1J I i osmo is I Di"'IY is I 10 r-~-+~+-+-r-~~-+~;-+--~~;--+-T--r-Hrl-r-r;--~J~r--·•~·r--~r+-r~rY' l---~'~l~l-l-+1--+!-+--+-+--+·-l---l--l\\~li~==~=tt=t4==;711ll'=±_l~~ri~~:i' li I '-L __ .,. n-~r--.. I I I _,.- - -- /J 11 I I _L IL ./.; 12Sl_ 2 f-----i----+--+--r---!--+1 -+--+--+-i~-+--+-+-++-+--,-=-- -+:1.:' -?--' "'f--o!~-;-+---r1 ..,_-r-r-~-"' l ~I 1.1s I I I 1 I I I I ( I \ ":>1 I I 't A~sor~tion '-..I \' '" u'F'r" 1.5t--~~~~~~~-+--~-+-~--+-+--r-r~--t~--r~,l--rt--r~-~--r-,~~~ ~. A~-._~-+~/~I 1.~ \ I 1.3 r---+-+-++-'~'·-+---r~++--+-+--~-r-+-+--+~---+-r-rr-~---+-l-~--~1---*-'1 +-~-+1 I I \D1stll~ti qn 12 G.~so~us 1 V V ., dtffuSIOn ~ 1.1 !---;~-++-~+-~~-+,_-+-+---r-1~,_,_,_ __ +-~<r-1-,_--+-+-1---H'~--+-+-~-+~ 1.075 I I I V rl.a,s--,.- 1 ; l 1 I! c n•ri ugb 1 05r--1-t-T7-t-t--41i-ti-~t---~ri~~1---t-~~~~~~~==~J. =L~t-t-'~n· t~~4 1.04 l--t--t-t+1-t--t--++-H-t-+-->--++1-i'--+--t-t-9'F==t-' +~"~+-:!=...._::r-r=~~:r-t-11~ '-r-ri 1.03 1---t--t-++ 11--rl-t---~,_-r-t-t f I ...._r- -;- )I -f!.-.. I -t- lpn I r 1 T-+ ~~--' . ./ 1 ' 02 r-~-r-+-+~-+~-~j_ 1 -r-...__ C>< 1an '"ft1ãtT s/th~rr 1a~ '\. R ~acti1ns --.I-- ~iffu1or 1.01 --t-+-H--+-1---l--t-H-+-1---1- 1- · -1-t-T"t-+_._--*f'-.-'--tJ.--;1-r 1---!-- 7 s -!--H!--1- -~i--1-~ 1---1-+-+-+-+-tl _ _,__,__,l!-!---·1---1-t·-- - 1- --+--1---t--r-~ '-i- i 2 ..___ -t--1-t--+·-1---+--!--+-1 -- ·-r- -, ~ I I l -I 8iolo~1ic< I -++-+-.;---+-+-t-+-!-+--1 .001 I J '- -~--'--'--"-''-'--'-"--'--'--'--.__. _ _._--'--'-"-"--' 2 3 4 ~ 7.S 10-11 1o-s 10 4 10 3 10 2 10 INITIAL CONCENTRATION, mass fraction Figure 4-2 General Regions of Applicnhil ity (from KodJt'>~y. 1978) 4-5 2 4-6 Setecting Options for the Separatron of Componems from Homogeneous Phases TABLE 4-20 Some definitions of the separation factor a For differencc in o o o A delin ilion of a o o o Appl ication Vapor pressure a,P:: vp•/vp•2:: P"/P•, distillatron; cvaporation (( :: p• Jí\1 "P 2 I /p0 , l.\ l 2 molecular clistillation Freezing point and solubiliry a.,c"'T2 /T, rnclt CI)'Stallit.ation Solubrlrty o:, - pK,P2 I pK,,, solution cry~tallilatron Partition cocfficicnt usx = "-/ "-, solvcnt CAtracuon ((Ad odsorption Exchangc equilibrium o:,x = r· o< J.c• o)/( I r 'n)c',, ron cxchan~e uc, chromato~;raph) Surface actr' uy cx5A = I'A/cV fonrn fractionauon Molecular gcornctry CXM = K2 rzbl K, (}b I <o rnembr<mc a= <r2>1 <tr> sizc cxclu~ion chrornatography <Xo = cl, rzbi c" 9>, dialysis Elcctromigration <Xu = ll·lt ll', electrophore~i~. ion rnigration Molecular l..inctic cnergy ex = (l\1 / M / 12 a= exp ((M 2 • M1) w2 r/12 RT gas centrifugc a= exp((z N,RT/pJ(It<!il2 - lt<!il ,) l mass drffusron Ct = <Jy> In (T too.1 Tcold) thcrmal diffusion S) mbols: A = arca c2 = conccmrarion of specics 2 in the feed 9J = diffusivity; cm2/s r = surface conccntration )(,. = thermal diffusivity K :: distribution coefficicnt or partition cocffic ient; moi/moi; or mol/Umoi!L K = solubility product 'P :\f = molar mas; N ' = molar fluA of separating agent; mol/cm2os p• = saturation \'apor pressure, kPa rl = outcr radius, em R = ideal gas constant ~~· = elcctric rnobility, cm2/V s T = absolute tempcrature, K <l> = pcal.. rcsrdencc rime in a chromatograph v = volume vp• = saturation ''apor prcssurco I..Pa w = angular velocity z = diffusion path lcngtho em which boils maleria lso Some materiais degrade if lhcy are boiledo Thus , lo ob1ain a scparalion thcrc musl be some dif- fc rcncc in propcrty bet \\ cen lhe spccics, and we a1.k aboul lhe siz.e, lhe tempcralure sensilivity, and whrthcr lhe spe- c ies are organic or inorgantco Wc look to lhe physical prop- c rlics li ~ tcd in T able 4-1 (and Data Pa!l D)o Whcn considcring thc propcrt1cs o f spccics, il is u~cful to list the spccies 111 rhe order o f lhe cha ngc in lhe properlyo Thus. if wc dccidc 10 exploi l vapor pre~sure differe nc.e 10 separmc four spcc•cs, \\ 'C would li 5t thcrn rn lhe ordcr o f dccreasing \ ola1i lity o I f wc c lcc1 10 consider ion exchange, wc wo uld lis l thc spccJcs in lhe orcler of va- lc ncco • · R•l• ting Sltt to M oi• r Mau _L__ 0.1 1 lO 102 tOl Size of molecules, specles. nm ___,_ lO tOl tOl 101 10' to• Molar ma .. Vlrus IOOfQIOÍC Enzymes t Encephalit•• lona ~ 1----l Vollow fevc r Antobiotoca Prorolns t Poliomyollrla 1---i I I te. coll b. Rtlat lno Decompoaltlon Haurd lnd .. to Spociu lond to alzol .__._._ I .i-.1 -1-.1... 1....1.-L.. .L-!-L. .1.-.1- -L-i- '...1...--1- to•• 10U 1012 1010 t ot tot 101 101 t 10 l 10 I tO • 10 I Hlckman Embroe Oocomposltion Haurd lnde•. Po s Oecomposes lf specoes boils at IOOkPa for -" ld Ih lOmln 1s ai 1Po for 1d Ih IOmin 1s / / / ; ,.,.~ Penicillin estor tM-400) , ,. ~ ActJvated storofs / /' ~ Natural vitomin O IM- tlOOI Oecomposes / ~ Votamin A esters ,' Triglycerode oiiiM-9001 ,' ~ Nllturel cruda glyeorino / ,' ~ Refinod glycarine oil / ; ; / ; ; ; ; ; ; ; ; ; ; ,~ PotroJeum lube oils ; ; ; "' "' "' "' "' "' Ew&DOlat.lcn Oloillalloo ....., ........ "' "' "' ; - w.tloryttll!tu tlon "' FrMJe oonc:.,ttalion lonereld~ 6ot.lllon ...,.,.~ .. ~~ ... Pooclllltalloll Abaoo~llon Ooaor!>lloll "' Solvt11l • • tt atdon Allwllion. O nu d bod ~ ......... ..,t.IIII!J L: tufW L.~ ... lllll'l IX.Iuod bod ........ "' '"""'"""""""" ; ; "' Sb• ud~ab-1 c:NOtNt001a»'lr ~•r•• O L: .,., • .,,,. L,..,., .. ..... L. ....... ., .. A" tiO" booloQI<&I ~ C, to C1 hydrocarbons Slze of molecule - - ···-l-I- - .. ... .. ... ... ·200 ., 00 o 100 200 300 400 500 600 Usual operating temperatura, OC Figure 4-3 Sr1c, ~1ol3r M ass, Decomposition Hatard, Opcratrng Temperature co Choicc o f Opcion~ .L.... 4-7 •. A-8 Selecting Options for the Separation of Components from Homogeneous Phases Examplc ~J: \V e wish to separate acetic acid from water. What might the options be? The bcginning state is a homogcncous liquid. An Answer: From Tablc ~I we see that it is hclpfulto compare the properties o f thc two spccies and thcn \CC where they are diffcrent. Tablc 4-3 is a summary from Data Part Da. Wc ai o note that both spccics are rclativcly small; as such. it is unlikcly that tcmpcraturc sensitivity or dccomposition would bc an issuc for cithcr o f thcsc spccics. llcncc, from Tablc 4-1 some options might bc: vapor prcssure? diffcrencc in boiling point: converting th.is to vapor prcssurc differcncc wc obtain an ex= 1.4. This is a rclativcly low valuc. Although distilla- tion is fcasiblc wc should ccrtainly con- sidcr other options. freeLtng point? modcst diffcrence in frcczing point. Perhaps melt crystnlliza- tion. • solubility? no K,P valucs, so solution crystallization not considcrcd. solubility in absorbing solvem? reason- able differences in molar volumes and Hildebrand solubility parameters. How- ever, this difference, combined with the rclatively high boiling point do not sug- gest that desorption is a strong possibil- iry. Usually for desorption, thc species in the smallest concentration is a gas at room temperature. • solubility parameters for partition coeffi- cient? rcasonable differences in molar volumes and Hildebrand loolubility pa- rameters. \V e should bc ablc to cxploit this diffcrence via solvem extraction. • exchangc cquilibrium? adsorption is a pu:.:.ibility bccau:.e of the differences in molar volumes and Hildebrand solubil- ity parameters. • dissociatíon constam and valcnce? al- though differences occur. dissociation TABLE 4-3. Exploiting propcrty differences !olor rrcctc Uoil ~ l a\ Tcmp. Tcmp. Accuc actd 60.06 16.6 117 .9 \V ater 18. o 100 for acetic acid is relatively small. Pcr- haps ion exchange. • surfacc activity? with only two carbons in thc organic chain length, acetic acid is not surface active. Acctic acid dissoci- atcs to some degrcc. Perhaps, íon flota- tion. molecular gcometry? the molar masses are diffcrcnt and so pcrhap~ dialysis and ultrafiltration. Bccause acctic acid ion- izes, rcvcrsc osmosis and elcctrodialysis might bc possiblc. Thc guidclines of Table 4-1 are vcry general. Later wc will sec how feed nnd product conccntration and char- actcristics of the equ ipment help refine thcse initial consid- erations. B. Feed and Product Conditions. A sccond major consideration is thc feed and product conditions. Some scparation dcviccs work best for dilute feed concen- trations l.css than 1 %; othcrs thrive on concentrated feeds o f grcater than 1%. Sometimes we need to rccover both com- poncnts into two separate streams; sometimes we focus on only one product strcarn. Thc amount o f species recovered (or the rccovcry) and thc purity rcquired are other consider- ations. Figure 4-2 relates the feed concentration to the sep- aration factor: Figure 4--4 shows how the purity requirc- ments relate to cquipmcnt sclcction. Another way of cxpressing this information is with a fced-product diagram such as illustrated in Figure 4-5. In this diagram, lhe feed concentration is given on the diago- nal. The product concentrations in the exit streams {the overhead and bottoms) identify a point on the diagrarn that is joined to the feed poinr. Thus, in Figure 4-5a, line "A" represents a separator operating on a feed concentration of the species of 50% that produces an exit ovcrhcad strcam o f concentration 90% with an exit underflow concentration of 2%. The slope of the line is approximately the ratio of the mass of the total stream taken ovcrhead to thc mass in the underflow. Thc diagram is slightly differcnt when a scparating agcnt is addcd, as for example in solvem extraction whcrc an immiscible solvent is added. ow, the concentration in the exit "solvem stream" depends on the amount of the ~t ola r ô, ôh pK . K ,, Volume 57.2 12.2 I .9 4.76 - 18 22.8 40.4 14 ·' j -~ ~ ii. :::> 0.. >- u ::> o o a: 0.. o w >- u w 0.. X w a) For Processing Liquids Zone refonong I I I I / , 9999 -- ------------------------~ 9!) 9 ,::.::;.c. 99 1 1/ li ! I RO " i ~ 90 I I 1 I : I \ : " \ ' Ul trallltlation --- ,. .... ------ IX :I Dialysís proteins -- - - - 7- / .... ___ ., , .... , ---- --- '{ .... ' \ I I / \ I I I I I ... I I I I I I I I I IX I I· I I I I protein I I I I I I I I I I I I I I I I \ I I >, / , .... , , - ---- L- '-~" ..l..........ill'll' t•tt, l .l......_ . l tJ•,, ,!I I 1111!1,1 -1..J..1._ 0.1 1 10 102 10) 10 lnorganoe lons f--i SIZE OF MOLECULES, nm L _I t 10: tOl 10' 10~ 106 Approx .. mo101r mass Enlymcs 1--1 Virus Anhbiotics Prote•ns 1----1 I I ~ r:· ii. ::> 0.. >-u :J o o a: a. o w >- u w 0.. X w b) For Proce:.sing Gases 99.99 99.9 99 95 r 90 80 0.1 .... ----- ..... .... " \ Adsorption 1 I I ,---------..... 1 ,..., I I I \ I I I \ f I f I f I f I f I Absorption or f I : : membrane : : I I f f I I I I \ \ I I ' ' / / .... ' , / '-~~--- Relrigeration ' ...LL_ 10 102 SIZE OF MOLECULES. nm Figure 4-4 llow Expccted Product Purity Affects Choice • <-1 o Selecting Options for the Separation of Components from Yomogeneous Phases agcnt added. The more that is added, the smaller the con- centration will be. This is shown in Figure 4-Sb. As the ratio of separating agcnt to feed increases, the concentra- tion decreases. This can mean that bolh the two exit streams núght beco me more di lute t11an lhe feed. This is not unusual in many options; the diagram reminds us that streams are both dilutc and asks us to question whether this is indced what we want. Idcally, we would like lhe "ltne" to be downward to thc right. Thi s form of prescnting the information is uscful bc- cause wc can: quickly see 1he approximate range of fecd conccntra- tions traditionally U'>cd for a picce of equipmcnt. • use thc two output conccntrations to be the fceds to other separation deviccs. Thus, !ater we can sec how lo hook up separation options. This is illustraLCd in Figure 4- 5c, wherr membrane separation is followed by distillation. I lere, an initial feed of 20% is up- graded by the mcmbranc lo producc a mcmbranc rctcntatc conccntration of 80%. This becomes the fecd to lhe disúllation column whcre thc stream is concentrated further to 99%. Thcre also are two other streams, the permeatc from the membranc and the bottoms from the distillation, with solute concentra- tions of I anel 5% respecrively. Thus, options can be combined with each other. Figure 4-5d illustrates lhe combinarion of íon exchange with solvent extraction to pro\·ide a systems approach. In this illustration, feed oí concemration 0.05% is fed to an ion exchange unit. The exit eluant, afler recovery, is 5% solut.e. This stream then becomes lhe fccd to a solvem ex- lraction unir, which yields an extract of concentration 60% solute. In this illustralion, we followed only lhe solute. The other exit streams (0.001 o/o from the íon exchange unir and 0.0l o/o from thc solvem extracúon unit) could also be considercd. Indeed, in lhis illustra- tion, we núght wish to operate the solvent extraction unit so that the raffinate concentration is 0.05%. In this way, thc raffinate could then bccome part of the feed to the ion exchange. Thus, this form of diagram helps to link together combinations of options anel to take a systems view of ali o f the strcams. Wc can use this graphical fonn to help us visualize the big picture or the separation ystem. • relate performance qualitatively with expcnse. Thc longer the line, the more expensive the operation and thc capital cquipmcnr. For examplc, Figure 4-5e shows a distillation column to obtain 90% purity. This nught neecl 20 trays and a rcnux ratio of 1.2. lf wc then wem to 99% purity, wc might necd 30 trays and a reflux rauo of I .4. !f the target was 99.9% purity. we might ncecl 50 1rays and a renux ratio of 2. Care is nccded in selccting the variablcs for thc axis. In general , Lhe a.xis should bc chosen so that thc diagram is downward to the right. In orher words, the concentration of the species that becomes more "concentrated" should bc on the abscissa. With lhis choicc, thc concentration of the same species (shown on lhe ordinate) should becomc more di lute. The diagram cannot be used when the scparation op- tion selected is reaction. This is un fortunate, becausc thc rcaction option is often forgouen in our search. To illustrate both the usefulness of rcactions and the implica1ions of spccifying purity, considcr the following example. Examplc 4-2: Wc need to remove about I moi % I 12S and 18 moi % col from a hydrocarbon gas s1rearn, recover thc ~ulfur and producc a ta i I gas strearn that \\e can re !case to 1 h c atrnosphcre thm has less than 500 ppm sulfur. llow might wc do it? An Answcr: Although wc should think of many options baseei on the propcrty differenccs, this examplc is to tllustrmc thc role o f rcactions as a separation option. Hence, we use existing technology. Oflen lhe first stcp is to use an absorption/scrubbing process to remove the "sour" componcms from the hydrocarbon stream. First, thc H2S and co2 might bc absorbed by a liquid agenl and recovered in the recovery still, as illustrated for co2 removal!recovery in lhe ammonia piam in PID-4. The strcam might now be 30% H2S. \Vhat to do now? Part of the "acid gas" might be fed, together \\ ith molten sulfur, to a furnace where both are bumed in ai r to pro- duce S02• This in tum goes to a catalytic reactor, where elemental sulfur is produced by the reaction between H2S and S02. This "process" includes reaclion and separations and is called a Claus plant. However, even a three-stage Claus plant can remove about only 95% o f Lhe sulfur in the feed. Thus. the tailgas from Lhe Claus plant needs to be processed furlher, by say a Shell Claus Offgas Trcater. SCOT, which wlll make lhe overall recovery of sulfur O\ e r 99%. The exit gas stream has less than 500 ppm. The SCOT proeess also includes a reaction step as parto f the separmion scheme. Thus, three complete processes are addcd in ~cqucuce (and intcgraled) to achicve 95% and then 99% recovcry. Thc separations illustratcd tn thts e.xample are "absorb, rcact, and rcact" as viable separation Opllon~. f hc challcnging faCllS that the capital co~t o f each of the~c thrcc plants is about thc samc l'hu\, to mo\'e from 95% to 99% \\C are doubling the cosr. ., . a. c. (n ~-"' ox ~i' o . CXl(/) .z oQ Zl- <(<( ,a: wl-1-Z <(W wu ~z a: O wu ~ "'"' ..... ..... "" .... 1i .. .. .. .. 0001 0001 000<1 b . u. o z o ~ ~X' z . ww Ul- z::> O-' uO ~(/) <l z u::: ~ a: "-·r-· ..u .. s r .. ... .. ... '"' 'hJ .. to .. .. .. ,. 10 I ' 8! o o• 001 .... 0001 0- i I I I i E><trect Solvcnt ., ... -rJ_ F, F, _.L__j .. . , Raffinate 00001 V-~~~~~~~~~~~~~~~~~ 00001 ...... ...... ..... ..... "" .... :J .. .. .. r.o ,, ,. 10 • ' .~ OI o .. 001 000> OCOI ..... 00001 OVERHEAD CONCENTRATION. x2 or more volatilc I I I li 111111 i i li I 17 "' ([ "'' ··~ Q.::, .. .. , /r-/ Distillation feed x2 Membrane LfÍ feed x1 Al±l= ~ Al / / l /I 111 I I I I VI 111 I 111 T I - -o o::~: .,."'o~: S:S:ftJ=~!1~ $ !! § - ~ Bortoms Exit ~~:::: ! Permeate from Exit membrane solute from dis:tillation ><', OVERHEAD CONCENTRATION, x2 of more volatile e. ...... I d. ttm& "'" z ..... o .... i=w .... ... <(_. 1i a:-~3 .. .. 1- ~:-( 1:: 1- wo .. ~> .. •• OW 10 ua: (/)o 10 • ~~ ' ou. d ~o o o• 001 o .... ID . ~· ~= :: - - """ nttM z "'" o ""' ~ .... "" a: ::· 1- "' u zx •• w . .. uw zl- .. o3 .. •• uo ,. •(/) " ~u. <(0 1 z 8t u::: ... u. 001 <( O.C05 a: . 0.001 ..... .... , I - I a '''~"-s:u s '"U:l EXTRACT CONCENTRATION, x2 or solutc 1 ~~~~~~~~--- / '~Qfo I /~ IX . ~:?- Solven/ X 1 X) / ., / x, Eluete /I I sx solvent extraction /.-! . " ' A rx IX ion exchang~ "'/ I X •'\ I 1 I IL J 1'.. I ., / I I 71 I 11 .. li I• 11 L VIl I I 111 li I I 11, I I o:::~:- '"""'õ ~~i: 13: 1:::111~ :;g ª a: -• Exit Exit -•ss ~ from from the the IX, SX, x2 x·2 ELUATE & EXTRACT CONCENTRATIONS . / /f- /'-f- / / - - - - - :-:::= - J ~or~en;J= ~~ ~li tiMit d•d= -t-. - ~ f- I-· - - .... ~f - - -UOt'l 00001 /I I I r- ;_i • •••••-~·•• &118ll~U:S:S :s S :s ~~ s ! ! H·' ~~u ~ ~ f i OVERHEAD CONCENTRATION OF MORE VOLATI LE SPECIES Figurl' 4 -5 Fccd-Pre>dud Oingrams 4-11 ' 4-1 2 Selecting Options for the Separation of Components from Homogeneous Thus, once again through this example, we see that lhe sclection of the recovery or the purity of the product will have a major impact on thc costs. In summary, the feed concentration and the expected product specifications affcct thc options available. Examplc 4-3: Rcconsider case Example 4-1 for the sepa- ration o f :tcetic acid from w:ttcr. Assume the feed concemration is 15% and that we want 90% acetic acid as product. An An ·wcr : From Figure 4-4, uhmfihrntion secrns innppropnate because it works on larger sitcd moleculcs. C. Beware of Trace Materiais and Too Narrow a View. In the previous sections wc illusu·atcd the usc- fulness of identifying a pair of kcy components whcn con- sidcring any scparation. I lowever, we must not forget tl1e cffccts of othcr species on Lhe separation; othcr spccics can completely rui e out an option. For example, we could focus on scparating two kcy components of ethanol and watcr and forget tllat salt is prcscnt-salt would wreck some very possiblc options. Or, we might be exploiting the difference bctwcen tl1e valence in ions (like removing divalent calcium ions via ion exchange in water "softening") and neglect to realize that small quanlities o f trivalent would dominate thc ion exchange oplion. This reminds us that we really need to understand the fundamental principies upon which each option is based and ensure that we consider the effect of species othcr than the two key cornponents on tlle option. Other common pitfalls are: • to focus on finding one opúon to do the task. Some- times we should consider a systems approach and team up sequences of different options . This has been illustrated in Figures 4-Sc and d , where membrane- distillation combinations and íon exchange-solvent extraction combinations are considered. to consider only homogeneous phase options; werrtight find it more appropriate to create a second phase and use Lhe tcchniques from Chapter 5 to give the scparation. to consider only cparation oplions when we might be better served by dcstroying or reacting the spe- cies This was illustratcd in Examplc 4-2. D. Equipment Consideratíons. A fourth major fae tor is the special concem s imposed by thf' equip- mellt. From Table 4-1 \\e note that some de v ices require the nddition or ublraction of energy. That is usual ly rela- tively easy and so thcse usually represent the casicr and most cconomical devices to use. Othcrs bascd on cquilib- rium considerations requirc the addi tion of agcnts. This cau cause complications. Still otller methods are based on rat~ phenomena. These tend to be our third choice because of ' the economics and the quality o f separation. 1. Effect of Addi11g Material Separati11g Age111. Whenever another agent is added, it may contaminate the original phase (bccause of solubility and vapor pressure) and it will have to be removed . ln addition, we have to ci- ther recovcr/regenerate or discard the agent. I f Lhe agcnt is recovercd, tllis adds an additional scparation stcp. Tn general, when thc agent added is a solid phase, more expensive difficulties occur bccause for continuous proccssing, it is more difficu lt to transport solids than it is fluids and we must separate the solids after the contaeting is complete. The solids thcn nccd to be regencratcd. I f wc use a discontinuous or batch opcration, wc incur thc pcn- alty o f rcgcncration cquipmem and cyclic operations. Basi- cally wc nccd at lcast 1wo units. One is activc; thc other is being regenerated. Thc purpose of thc regeneration can be t wo-fold: to regeneratc the agent so that it can bc rcused, and to reeover tlle solutc that the agent attracted. Some ex- ample ways of doing this include change Lhe prcssure, change tl1c temperaturc, backwash Lhe solids with an elut- ing agent that recovcrs thc solute. The use of a pai r of nu- idized bed columns performs tllc two functions in separate columns but has the solids moving continuously between the two colurnns. Another variation is to keep the solids fixed in the bed, but lhrough astute changes to tlle liquid Oow pattems, pcrform thcse functions as a continuous sweep through lhe column. An impression of how this rrtight be done is given in Figure 4-6. A variation on this is a chromatographic style operation where a pulse of the process fluid is sent tllroug h a bed of the solids and this is followed by a pulse of lhe eluting fluid. Some of these ideas are illustrated in Figure 4-6. Ali in all , handling solid agents means more complex and expensive opera- tions. 2. Effect of Temperature, Pressure, and MaJerials ofCoustruction. Excursions in temperature and prcssure away from "usual" operating conditions can be very expen- sive. Thus, although tlle chemistry says that a separation is possible, lhe equipment may cost too much lo operate under thc requircd conditions. Similarly, if solvents or other materiais are added, the materiais of construc tion re- quired lo prcvent corrosion may bc prohibitivcly cxpcnsivc. llazard excursions require the addition o f fireproof- ing, watcr spray, spccial instrumentation anel monitors, vent'ilation, blowdown o r remova! systems, safe ty gar- rnents, and possibly diking or cxplosion barriers. To assess the hnzard, two semi quantitative measurcs are available in Data Part D. First, the NFPA rating~ are on a scale from O to 4. A rating of O means ncgligiblc hazard; a rating of 4 means extreme haz.ard. Thrce diffcrcn t aspects are consid- crcd: hcallh 11, nanunability F. and stability or likclihood to react, explode, or combust spontancously, S. Thus, ace- tonc has a rating of 1,3,0; acetylene, a rating of 1,4,3 and T • Batch Operation: Fixed Bed Process feed . I ~ ·•· o o • • • o •• o • • •• · ~o . .. o . o . ••••• ••••• · ~ • o Fixcd bcd is "on stream" bcing loaded with adsorbed spccies "x" until the bod is full o r worn out Load • • •• • •• • • • * Eluont or :+--o- regenerating fluid Process feed is stoppcd. Recovery of "x" is dono via cluent fluid or/and bed is regencratcd to go "on stream" again Recover/ Rcgeneratc • Continuous: Moving Bed Process feed Separator :i·-- • • ~ o~· Solids move • O ! O• • • o o • • Fluidized t ~!JU l-~ Ft Batch-fixed Adsorb Regenerate Adsorb Regenerate Ft t t t @-------® § -------® ~ t Eruont t F t Eluont Downflow with: bed brought out·of servicc Adsorb Elucnt Ft t Eluatc Upflow with: to rcgenerato Fixed bcd but bod moves from one location to another (Porter-Arden) Resin r :::r • ... • ~o ... • o • • o • o • • • r . Fluidized pri ncipie illustrated Higgins continuous Pulses of resin move counterclockwise A~ Resin I F Multistage Permunit CCIX resin flow counterclockwise • • • • • o • o • ~ • .. ,. ~.t M• :t= o o Sep~rator Eluent or regcnerating fluid ASAHI: Fixed with pulse resin flow is counterclockwise (Powdered) Single stage t Regeneratcd [~~ [J\[] Ft Fluidized or expanded bed with continuous regoneration 'i>fí_rf Throwaway Pulp or "powdered" method Eluent Eluate Elucnt Cloete-Streat fluidi zed with pulse resin flow Himsley continuous column Powdercd mu ltistage F Two stago divided Countcrcurrent Fi~;ure 4-6 Opuons for Handling Solid Agents 4-13 4-14 Selecting Options for the Separation of Components from Homogeneous • Continuous-Simulated Moving Bed Process Separators Cycle lnactive Recover regen. /nactive I r ---:~t __ _ ....••••••••..•... I GL ... .. fi·~· -· --l----1 .• , .• , ....... . • Continuous-Chromatographic-Style Eluent ••• • •• •••• •••• ••• • • •• 4----------------- Eluent recycle Pulse feed Distance Rotating valvo exit Figure 4-6 (Continucd) Time Adsorbing species get "delayed" but not loaded/caught in the bcd 4.1 Selecting Opt1ons for Separat1ng a Homogeneous Phase butyronitrile, a rating of 3,3,0. A second measure has bcen dcvcloped by Dow Chemical Co. (1966) and pertains on ly to flammability. They assign a "material hat.ard" rating to each spccies. The values for ali the speeies proccssed on one section of the piam are summed and the total providec; guidance as to the safety components that should be put in placc. Thc implicntions for the fonr excursions (tcmpera- ture, pressure, materiais of construction, anel hazard) are il- lustrated for thc co~t of purchasing the equipmcnt and for opcmting the cquipmcnt in Figures 4- 7a, h, c, and d. lt is not that the excursion may not be wonh it; rather, we should anticipate thc cost implications. 3. Ease in lncreasing tlle Purity by Multistaging. Sometimes we cannot obtain the degree of scparation we want by using the separator once. This is particularly truc o f the equil ibrium typc separations listed on the LI IS o f Tablc 4-1, but it is also applicablc to some of Lhe rate sep- arations describcd on thc RHS of this table. Often wc scnd the process stream through a whole series of units; we call each un.it a stage. Some methods are easy 10 stage; others are not. Figure 4-8 illustratcs mulústaging. 4. Energy and Ol'era/1 Economics. Other rules of thumb based on the economics of processing and sequcnc- ing options affect the ultimatc choice o f separating devicc. Details will be discussed in Section 4.1-3. 5. Economies of Scale. Sometimes bigger is bet- ter; sometimes small is beautiful. Some of the equipmenl optionsare best suited for large-scale applicarions; others for small. Figure 4-9 illustrates some idea o f how well dif- ferenr options are suited for lhe amount of material pro- cessed. This information pertams to the type of equipment. In general, most options havc three regions: the largest-or industrial scalc, the intermediate or pilot plant scale, and the small or lab scale. Even as wc approach the small scale, therc often is a limit that we get around by operating the unit hatchwi e and only parto f thc time. For adsorption, for ion cxchange, and for precipitalion/crystallization the ca- pacity rangeis continuous. For others, the size range occurs in bands. For example, for the large-scale operation for dis- ti llation, thc size range is relatively narrow. The diamctcr of thc dcvice mighl range from I em to 12m for continuous opcration (corrc~ponding approximatcly to capacities of lO-' to I 02 kg/s). The "usual" continuous operation is. how- evcr, a narrowcr band of about I to 50. Simi larly, mcm- branes are built in modules that havc different capacitics. Thc "usual" are O. I to I kg/s. Some options apply primarily to small capacities. For such options, larger capacities are achieved by doubling ancltnpling thc small-sizc unit. For examplc, 7one rcfining processes a vcry small amount, about lO~ kg/s. lf a capacity of 1000 kg/a is rcquired, we achieve this by installing five units. Sometimes increasing 4-15 lhe capacity by using mulliples of a hase size can be pro- hibitively expensivc. On the other hand, we note lhat some options scem most applicable at largcr sizes (for example, cryogenic distillation, clcctrodialysis, and simulatcd mov- ing-hed adsorption). This is because thcsc require a collec- tion of specializcd equipment that is cxtremely expensivc to put togcther on a small cale. Some options are extcn- sions o [ laboratory scale equipmcnt and can gencrally bc built for any capacity. Another formo f information relatcd to this is the pro- duction scalc for diffcrcnt typec; of product . Table 4-4 shows t11c "usual" si te of a plant that is constructed. Examplc ..t-t: Reconsider the case of Example 4-3, where we wish to scparatc a 15% acctic acid, water mixture. Thc capacily is !O kg/s. What options might bc pertinent? An Answcr: Ali options still remain pertinent. Atthis stage we should n01 restrict the options. llowever, melt crystallization, dialysis, reverse osmosis, and electrodialysis might be less appealing because, from Figure 4-9, this capacity is grcater than lhat for single-size process units. I f we had li mited time, wc might consider distillation, ion exchange, and solvent extraction as the "prime" candidates. Example 4-5: A colleague has the task of evaluating cryogenic distillation, pressure swing adsorption, and gas membranes as possible options for producing 98% purity oxygen from a ir. The oxygen production rate is to bc 0.05 kg/s at 400 kPa. Time is limited. She asks which option might be most appropnate i f only one can be considered in the time available. What do you say? A n Answcr: Thc first caution is to reprcss the haste to considcr only onc option atthis time. Severa! should bc considcrcd. lf onc musr bc considcrcd. then pcmse Figure 4-9 for thc "feed" to the unit. The amount of ai r that must go to producc an oxygcn output o f 0.05 kg/s is 0.05/0.2 or 0.25 kg/s (sincc ai ris about 20% oxygen). Cryogcnic di s ullation sccms to be more applicable nt h1ghcr fecdrates. Eithcr adsorption or mcm- brane scparation might be the opuons to considcr. r.torc dctat!s aboutthe subtlctie~ o f the equipment options are nccded bcfore any distinction betwcen adsorpllon and rncmbrane separation can be madc. 4-16 Selecting Options for the Separation o f Components 'rom Homogeneous Phases Fig. 4-7a: Cost of Pressure Excursions § uH+----+-1 ~1--/1--1 ~ 1.2 \ Effect of prouuro on the capital cost o r a completaly in5tollod plant. (The cost factor for an individual p1ece of tqu•pmen1 is much largor than the data glvcn hora: thls la bccauso tho ~ '·',u __ \ / 1 - _ cfv'- . 10·> 0.01 0.1 1 10 102 101 co11 of e complotaly onstalled plant includoo tha cost o f labor, tnstnllauon, concrote wor ... e~c. tha1 •I •clatlvoly lndopondont of tho preuuro and tomporaturo.) ~ <11 o u ~ 5 ... "' PRESSURE. atmospho••• Ba .. d on Allcn and Pagoi1975) Purchaso Co11 of Equopmont Only f1~~:,~~~~~3:1 pumpa / I Sholl and tuba hoat ~ oxchangore t---+--1--1--v- f-;j : : , PrOCOOO YOII OIB 1.7 I 0 1.5 f- I. 1.3 : f • .; 1-- Olroct hxad process : : furn1co . . 1-i i:: i-- •High p1011Uro• : • ...........- ctnHlflJ{)BI · : comprenor 1---1--1--1--H i- : : : 1.2 .. ~-+--~--+--+!! j~ '·' I 10· > 1o·> 10"' 1 tO tO' PRESSURE. atmosphoros Based on Woods (1975a) Fig. 4-7c: Cost of Alloy Excursions Selectmg exotic materiais of construction costs moncy Cost •mpliea:tion Matenal for installed o f consuuction complete plant Carbon steel 10 Bronze t05 Carbon/ 1.065 Molybdenum Aluminum 1.075 Cast lron l.lt Sta•nless &teci 1.28 1304) WOtthlt• allov t" Sta•n•us ateai 1.5 13t61 Hastalloy C t .54 Mooel 1.65 r'óde~ltonal 1./t Tttonturn 20 Cost implicetion on purchase cost of uninstalled equipment Heat Vessels exchangcrs 2.75 4.0 &3 8.2 8 Fig. 4-7b: Cost of Temperatura Excursions On Ec;uipment lii 1.3 J-1.--j.--l.--T-~- 8 UJ 1.2 L: 7 Effcct of temperatura on l~t cap,tal coSI of a completely instat:ed plant ~ ~ 11 7 1\.. 1-- I /tJ L "'-_~- ~ From Allan and Pago 119751 100 10 1 10 10) to> 10' ~ .. ISO i.;; 8 100 : ~ ~ :3 50 f- ... "' TEMPERATURE. 'C On Oporat ons l Opero1lng costJDTU of temporaturo oxcur1iont --- r. .. p., 119681 Audd, Powo11 • '·' • • & S11rollo ( 19731 ~· ·· ... 1 I K1110 ( t9711 _ _J_ _ _..~.._-._,. ~-~--'-__ _JI -- p.7sa tOO -10 t tO tO' to> to• TEMPERATURE, "C Fig. 4·7d: lmplications of Hazard Excursions From ·oow 's F~ro and Exploslvelndex Haza1d Classi f•catoon Guode" 1966. (Uscd w1th pormitsion from Amencan Jnstuute of Chemical Engincor~ 1966 AIChE. Ali roghts rescrvcd I Preventauve or protective reature A FIREPROOFING B WATER SPRAV a directional b area c curtain C SPECIAL INSTUMENTATION a temperatura b pressure c flow control O OUMP BLOWOOWN SPILL CONTAOL E INTERNAL EXPLOStON F COMBUSTIBLE GAS MO'IIrTORS a signal ahum b actunto oquipment G REMOTE OPERATION H BUILDING VENTILA110N I BUfLOING EXPLOSION RELIEF J OIKING K OUST EXPLOSION CONTROL L BLAST ANO BAARIEA WALLS SEPARATION Legend Lowhnard Exttomo hezard FIRE ANO EXPLOSION lr.OEX NUMBERS 0·20 20·40 4Q-60 6o-75 75·90 90 up t 2 2 3 4 4 I 2 3 3 • • t 2 3 3 4 4 I t 2 2 2 • t 2 3 3 • 4 t 2 3 3 3 • t 2 3 4 • 4 t I 2 3 3 4 t 2 3 3 • 4 1 t 2 3 3 4 t 1 2 2 3 4 t 1 2 3 3 4 Feeturo optlnnal ............................................................................................ 1 Feoturo au~go>ted. • ............................................................................................. 2 roaturo rocommended . ................................................................................ . . 3 Footuro roquirtJd ............................................................................................ • f.ignrc ~ 7 lrnplic.ttiom of Prc~'urc. Tcmpcraturc, Materiais of Comtrucuon, nnd l lntard~ E~curSt<lfl~ 4.1 Selecting Options for Separating a Homogeneous Phase Multistaging ,-----·~11,.. li~ Single "stage· lf a 2 for this stage but we nccd greatcr than 2 .. )loL ' I ,. I 'I' ... ,. )lo .... I ,. , I ... \lf,.. ,. ..,,~ ~ I Some multistage configurations Figure 4-8 llow Muhistaging Works ........ . l . . ·-+-- .... .-:· ...... ·r-- -1'"'11 .......... ,.., ~ Evaporatlon DlaUIIatlon oriAwy ldOQ.W .,_... .. .. . .. .. .. . --+- --+--+--- -+-...... ~---+--€> MeU ayllalllzallon Fr .. ze concentratlon Zone reflnlng Solutlon cryatallza!lon Preclpltatlon Abaorptlon Otaorptlon Solvanteatractlon Adtorptlon: G flud bed Q:c/woiUlOO' I p/1 l:lllf4bod l: ·~toei "'"Wlo bod l:-•IOO'If'll IX: t lrld b.cl c.lto•ul•;upll F o alllflon I lo ta tJon Slzt ucfualon chrornatography 1---+--+--+---+--'-+€) Mtmbnnu: G l:clllyoll l:~IIIU&IIon l: rnene temo1l1 l:t'ICIIOdA!)o't Reactor: blolook;ll ...... .. .. :---~,....~:) "'" '"' ~I' - 10·7 10·6 10·5 10·4 10·3 10·2 10·1 100 101 102 103 104 Feedrate, kg/s Fib'llrc ~9 Typ1cal Fccdrates to Equipment Opuons 4-17 ... .. 4-18 Selecting Options for the Separation of Components from Homogeneous Phases TABLE 4-4 Some production capacity data for single plants ~·lty chtmlcalt TI r 1 r 11 111 I 1'11 ohtmlealt ~ltnneclata ohe~Noala HN vy ohtmleall - Acataldehydl Acatlc acld Acatlc anhydricM Acetont -Acetylene Acryloacld Acrylc 1'11« Aay\oritl'tt Aclplc acld f- - Aclponltrle • a·Aicohola Altyl ben.Zent ~d) A ltyl btnzene ) Alylehloriôt • Atum - Alumlna - ~ Alumln~.m 1- -AlumlnLm ~ate Ammonla AlllnONum ~ate Anwnonium phosp/1111 AlllnONum aW!att 1- - Anlllne Arornatlca AlbMtOI • BNt 1- - Blnr.wol Blnzolo aold BllplwdA 1- -Bromlna Butadllot Butano~ ButyiiOt 1- - 1- - c.pt'cúctatn -Carbon blac k Carbon dloxlde - Carbon dllulf'ldt Carbon tetracNOIIde 1- - carboxy methy1 celluloae • c.IUott actt&te • Cemtnt ~ 1- - C~o.cetlc ack! • CNo!OilllpMNeol • CNoloform CNoloprene 1- • Cokt - eopp. Cycloheune Cyâooctadlene 1- • - 2,4.0 • OOT • - D!amnonl~.m phoaphate D!anmon!Lm aullate 1- - D!cyardamlcle D!methy11111~t - • D!methy1 taceplrthalate 1111 I I I I • ~ 111 I I I 1111 I I 1 0-4 1 0·3 1 0·2 1 0·1 1 QO 1 01 1 Q2 1 03 Feedrate, kg/s 4.1 Se>ecttng Options for Separating a Homogeneous Phase 4.1-2 Equipment Options So far we have sccn that in cons1dering equipmcnt options, we should keep in mind the unique differences in proper- ties between thc speeies, typical fecd/product conccnt ra- tions, the effcct of trace componente;, and the need to kcep thc overall ~ystem in mind .. cx t, we should bc aware o f thc unique characteristics of the equipment. In thi s section, cach of thc classes of options is dc- scribed bricny. Thc empha~is is on the key propcrty diffcr- cnccs that should exist for that option to work and the prin- cipie of opcration. Skctchcs are givcn. (No detaiJs are presented about selecting from among the options within a class. For example, cnough dctai l is given so that we can decide on cvaporation as a candidate. I !owevcr, wc wi llnol ex plore which type of evaporator to sclcct. Oetail s of se- lecting and sizing the type are given by Woods ( I993a). Thc number, puri ty, and fonn of the product streams are describcd. For each option, we also summarize charac- tcrisrics that might affect our willingness to select thi s op- tion as an initial trial. Table 4-5 is an overall summary of the characteris- tics o f each option. A. Exploiting Differences in Vapor Pressure. Two options are in this category: evaporation and distilla- tion. 1. Evaporation. The key physical property differ- ence must be a difference in the vapor pressurc (or more simply in the normal boiling tcmperatures) between Lhe two species. This starrs with a liquid phase. The principie is that heat is used to volati ze the more volatile component and thus create a vapor that has a composition rich in the more volatile component. Some skctches of equipment are given in Figure 4-10. Th is is basically ave sei with a boiler for the tiquid in the vessel and a condcnscr for the vapor. (Dc- tails about how ro select the best configuration of an cvap- orator is explored elscwhere (Woods, 1993a).) The product may be eithcr the overhead phase or the bottoms. For saline waler treatment, the product is the over- hcad vapor; for glycerol concentration, lhe producl is lhe bouoms. Figure 4-1 O shows lhe feed-product diagram. For cvaporators, we usually wanl lo know lhe enrichmenl in the non-volatilc species that concentralcs in thc underflow s1rcam. Usually pure solvem i takcn overhead. Because wc do not know the purity of that strcam, the fines on this figure are shown as dotted lincs. Usually about onc stage i uscd; howcver, to con- serve cnergy, somctimes up to three to five multistages are uscd. The design procedurc are well developed. Large- scale units can be built; the larger the amount to be pro- cessed, u ually, thc more attractive lhis option is. Large 4-19 scale usually means processing I to 40 kg/s of fced. Smaller capacily specializcd evaporators, such as agi tated falling film evaporators, have feedratcs in thc range 0. 1 to I O kg/s. Lower feedra1es are usually handled by batch evaporation. Evaporalion is an ex tension of a simple labo- ralory operalion. The main drawback to cvaporat ion is the amounl of encrgy required. To try to minimize the cnergy used, some- times lhe overhead vapor is uscd as the heating medi um for lhe boiler. To providc thc rcquircd temperature driving force, the vapor is compresscd. This approach is called vapor rccornpression. Evaporation may bc a qucslionablc oplion if solids are prcsent, the liquid foams readily, the species degrades Hl high tempcratures, is thcrmally unstable, or if thcre is a Iow conccmration of the less volatile specics so that we have to boi I just about ali o f the feed. Another concern is i f thc species fom1 an azcotropc. An azcotrope is a mixture that, as far as vapor prcssure is concemed, appears to be a "pure" compound. When this occurs, we cannot separate these species further. For evaporation, lhis is not usually a concern; for distillation it can be a major challenge. 2. Condensalion. Condensation is a possible op- tion i f Lhe feed is a gas. 3. Distillation. The key physical property differ- cnce is in the vapor pressure (or more simply in the nor- mal boiling temperatures). The phase distribution occurs between the liquid and the vapor created. Thc principie is thal heat is used to volatize the more volatile componenr and thus create a vapor that has a com- position rich in that more volatile component. The equip- ment is sketched in Figure 4-11. Each column consists of many ·'stages" that each represem a new Iiquid-gas equilib- rium condition. The vapor is created at the bottom of lhe column in the reboiler. The overhead vapor is condensed at lhe top. Part is rcturned at the top to ilow down through the stages as "rcilux." The ovcrall purity of the ovcrhead and bottoms products can bc increased by increasing the num- bcr of stagcs and by increa ing the renux. Usually, we cx- press the rcilux as a ratiO O f the reflux ilowrate lO thc ilow- rate of the overhead product. Trays or packing are placed inside thc column to promete thc gas-liquid contacting. I ligh performance or stmcturcd packi ng are oflen bcing used in placc o f thc tradi tional trays. Usually thc fced enters about the middle of the col- umn with stagcs abovc and below. Thc stagcs above lhe fccd are callcd thc enriching or rectification stagcs bccause thcir purpose is thc ensure that the less 'olatilc specics do not go ovcrhead; thc slages bclow the fced are callcd the stripping stagcs because the more volatilc species are beingstripped to go ovcrhead. For some sep:mnions, where the fccd has very volatilc species, we may nccd very few recti- fication stages and vice versa. .. ~ . 1\) o TABLE 4-5. Specific criteria and comments fo r selec ting options for separations Usuall) considcr Yicld No. or l'urity Commcnts i\lultistaging Ease in Scale Dc~i,::n T radc-orr \\ hCn • .• ltcco••cr y Prod. Si~e Kno>~ -ho" Easc in ~0- E~apor.uion a"" > 20; product JlQl I ll.igh enl!rgy rcquired p <lO L WK tcmperaturc: sensillvc; cspccially whcn o:,P is Condcnsation negligiblc solids; liquid small and purity needed phasc; high cone. of less volatilc species I Di.,lillation 0:'1'>1.5, product ~ 2 99+ High energy rcquired E 100 B WK tempcrature senstttve, especially when o:,l' is negligible solids small and high purity necded; difficult if azcotropes fonn 1\lch Heat scnsitive species; I 99 Cannot obtain complete p s D Washmg to remove Crystalliwtiun Iower energy; difference in separation; watch for ~ liquor vs. rcdissolve frcczing temperatures eutcctic formation; washing to remove liquor l'rcc.t.c Primarily to conccmrate by 1 99 Cry~tallization lrcczing out watcr Zune Rehmng I hgh purity solid rcqutred; 95 1 99.999 thc contaminant to bc rcmovcd Iowers thc lrcczing point Suluuon llc:u scnsittvc; o; small I 99 Cannot recover ali p Washmg to givc purity ., Ct)·~tallttatiun w;~nt solid product, no nced beca use of solubility Iim11s; decrcases yicld for multicomponcnt low encrgy requircd; scparations difficult to avoid contamination Prc.:tpitation Good, sharp fir~t cut to Move into solid-Iiquid remove specte~ from liquid. separauon technology Spcc1es ts temperaturc sensitivc; Olher options do not seem viable - --- - ----- - --- - ·- · --- TABLE 4-5 (Continued) ~ u~ually consider Yicld i\ o. or l'urity Commcnts Addcd Recovcry \lultistaging Scale Design Trade-orr "hen ... Reco\ c r y Prod. Agcnt regcnc ration S izc Kno\l·ho" Ease Numher in Ab~Of'JlliOII Spcc1cs 1S difficuh to Cross contumination of Add Distillauon E <40 B condcn\C (solublc gas); vapor from absorbing liquid o r Dc~orption low conccmration liquid must bc acceptablc Prcssurc Add gas Rcduc1ion or s1eam Solvem Prime option for di lute 2 Solids should bc removcd Addcd Distillauon E Sclcctivily vs C><lr<1Cl1011 conccn1ra1ion fecds; lo <I 00 ppm 10 preveni solvenl o r capacll)' (SX) J>nmc aherna1ive for crud formalion; difficull SX wash <hstillation lo a void producl stages contaminalion (extension: I supcrcri1ical cx1rac1ion) Adl'Of'ptíon Fccd cone. of more 1 901o Addsolid Discard or Sclectivil)' vs (Ga~) volatiles <10%: aad>2. 99 adsorbcnt thermal capacity pruduct diflicult to regeneralion condense; or largc or tcmp. swing amount ncc<b to bc press. swing boiled inert purgc or displacement Ad"><>rption Prime option for di lute I l f particles prescnt. use Add solid (Liqu1d) cone. fccd~ cspeci ali y fluidi7.ed bed or pulp adsorbent "'hcn nccd more ~electi\ ity than SX lon E~o.:hangc Fccd cone. small <0.5 Necd to con1ro l bacteria Add solid Batch -elute s D Thc more ( IX) %w/w, high valence anti algae growth; IX rcsin with highcr selcctivc thc 1011.ç; ha,,c ~ for sensitivc to ch lorine; can't cone. of lowcr rcsin the more dcsin .. 'tl solute. Relali\'C remove ali thc dcsircd \'alence or d1fficuh it isto pcrnlitti\'ÍI)' of solution is íons. IX costs incrcases changc pH clute or >35. Thc highcr the proportional to ionic fecd abou1 tpc rcgcncratc ;nomic number. Lhe concentrai ion mure cconomtcal i1 is 10 I u~c IX IX Chromato !'rolem~ with d1ffcrcn1 Add sohd Change pll graph11: ch;u-gc depcnding on IX resin and elute pH ~ N .,. i-J N TABLE 4-5 (Continued) lj Usuall) consider Yield No. of Purity Commcnts Added R eco ver) :\ fultistagjng Scale Dcsign T radc-off when ... Recovery Prod. Agent r egeneration Size KnO\\·how Ease ;\umber in Fwm Trace quantiltes of I Rccovcry limitcd by Add F r ;,.;tionation organic~ with carbun solubihty nnd drainagc of bubblcs chaiu lcngth > 12 foam. Vcry scnsitive to ionic strcngth lou Hoeatron Solutc is only cation or I Add countcrcharged Add Recover anion present surfactant surfactan surfactant via t and SXplus bubbles distillaúon Chrumato- Nccd short residencc 96 but 60 2 99.99 Nccd to eliminatc flow Addsolid p 500; use s D gmphic lime and ha'c toSO% in maldistribution and adsorbcnt about 10 temperature sensitive singlc pass capacity-limiting lhermal and liquid times lhe specres; a,l' <1.2; a c, gradients; prevent bacterial carrier thcoretical >2 and tcmp <300"C. comamin:uion; usually use trays for Usually last stcp in dilute buffer as carrier to di~tillaúon pu ri fication beca use o f minimize IX effects ( 0.02 small volume rélativc to moi!L). Pcrhaps kccp carrier and column concentration <10% to \Olume (I to 2% minimize viscosity effcct .• column) recommendcd )I diflcr by >20% lor su.:cess 1\lcmbr:~nt: Whcn product spcctcs 80 to 95 I 86to Nccd to mmimizc the <3 s Selectivity vs (Gas) difficult to condense; :1 96 fouling of mcmbrane. pcrmcability; sh~rp cut ts not necdcd Functions best whcn Power use and modcratc purity and product is rctcntatc. Best tf incrcascs rccovcry rcqu1rcd. Gas (pcrmcatc/ rctcntatc) <0.4. whcn improvc fccd cone.> 5'7·, gas is Economic permcate ratc~ = purity via ~'·ai lable at pressurc; if 0.25 to 2.5 gls.m2 multiswging a,1 >20. thcn prcssurc rauo dominatcs ~:ost :-Jcmbr~nc h1gh conccntrations of Add (Liquid: small sizcd spccies or mem- Oio~Jy,is) >5% conccntratton of brane small spccies in a plus S)"stcm that is fragi lc dialy- and damaged by shear satc liquid ------- TABLE 4-5 (Conlinued) ., U~uall) COilSidcr Yield No. or Purity Comments Added R eco\ery -'lultistaging Sc:lle Design Trade-orr \\hcn .. R eco• ery l'rod. Agenl regcneration Si7.c Know-how Easc i\umbcr in Mcmlmmc h<1uid with low solutc 50 to90; I 95 limitcd rctcntatc Add p s D (Ltqut<.l con..:cntration 0 1 90 to95 conccmration cannot mem- liltraftltraliun) sp.:c:ics wíth molar ma\~ Wtlh excccd gcl conccntralion branc > 1111 to lOs and rccycle which is usually 20 lo 30%; tcmpcralurc 4 lo IOO''C fouling of mcmbranc \lcmbranc f-c.:d conccntration 2 50to90 I 99 limitcd prcssurc as thc Mcm- p s D (Ltqui<.l: ppmto 10%; lfionic rctclllate conccnlration branc that Rcvcr~c 'ulule. thcn rcasonablc mcreases, bccausc thc h as o~mo~"· RO) selcclivity or pressure must be I 0% hydro- .. rcjcction". grc:ner than the osmotic phobic Ot bt: rw iJ>e: r:. ir; not pressurc = 2.4 to 3.6 MPa; and hydo· clcan cuts; cannot climinalc fouling and philic ' climinatc ali íons. OK chlorinc; cosls independem parts for urganic~ M >300 of ionic feed conccntration anti tempcrature <.l5•C. for conccntration <1500 Thc higher lhe valcncc, ppm., rctentate b.:ttcr the rejection conccntration <10% ;\lembram: J>n•all iomc fcc<.l I cannot remove org,anics; Add p s D (Liqmd: con~entration {c a I 000 eliminalc fouling; usually mem- Elcctwdialys) ppm); and whcn nccd 10 <700 ppm is economic bane plus ~cparate ionized from power un1onit.cd specics BtOiogil:al hJ .. dcgrad;~blc organtcs I deslroys species Add species not s D R~action~ (BOD!COD >0.5) in micro- recovcrable com:emration range orgamsms 100 to 1000 mg/L; pH 6 toS; tcmpaaturc 35 to 65'C ~ ~ (,) 4·24 Separation: (Liquids) Evaporation Externai c i rculation Internai circulation fi::: -v- ~ Internai Internai Calandría Natural circulation (short tubc externai vertical) Forced circulatíon (short tube externai vertical) Short tube oxternal kettlo type horizontal tube type or short vertical tube ina basket Long tubo ovaporators Wipcd film evaporator Jacketted Fecd Rising film 99.9999 99.9995 99.999 99.995 99.99 99.95 "' 99.9 C1> 99.5 ·c:; 99 - C1> ~a. 98 -- "' ~ C1> 95 ~ == o - 90 N~ )( o 80 - > :2:(/) 60 <{"' w~ 40 a:Q) t-.C cn:::. 20 o o <{ c 10 wo :c ·::: 5 a:m w!:: 2 >c 08 1 c 0.5 o 0.1 (.) 0.05 0.01 0.005 0.001 0.0005 0.0001 I I I I I I I I I I 11 I I 11 - X2 G to I I 11 I I Jacketted glass-lined vessels / A r- / r-r- X1 • --L r- solute r- ~~ r- ._e r- c,~ r- y ~ XJ 'llo - «e / ;- - q,o ,e - ·,~ . I ~ I I ~o - ... ~ ~~ - ,,o~vq, j< t- ~k - I -:).._0 / 1\i\. I - ~o~ L [1\ r\ I - /~\ \ r- / \ \ \ I r- I r- // 1\ \ r-r- / \ !- . / ~, J\ '" !-/ \ \ \ ~ --· Llu t\ 1 l o o o o O 000_. N t./1_. N S>o Ol ()) (!)(!) =:g:g~ o o o o o o:...u. o o o o OOtJ1 o o o o ... (J1 in ÍD ÍD ÍD o o ..... tJ1 SOLUTE·RICH STREAM, x3 %w/w (J1 (!) ... tJ1 conccntrate of thc less volatile spocies Figure 4-10 F.quipmcnt Options for E\".tporation: Conligurations and Fccd Produ•t Dtnsr.un I I I I L I i (!) 10 (!) (!) (!) 10 (!) (!) ÍD ÍD ÍD ÍD (!) (D (D (D U1 (D (D (D tJ1 (D ~~<~-~~-------------------------------------------------------- 4.1 Selecting Options for Separating a Homogeneous Phase ~Ove•head (c__\.. liquod tn t :::::::.,. for mov•no Feed -~ liquld f1om ono t••v to the ne_.t Tray -+- Vapot 1n r f1om l •eboolo• eonoms Tray Oistillatlon Unit Configuration Column ortower J;=~~:-;:;O::v:c,::h::a:::.••vor, drum 9ottoms tops, di,tlllltt Bonoms Reboiler, calandria, kettte Usual uni t Sream ejector Packed column fo r distillation ~Overhead Gos ou]~-loquld on os •effux ~~. llquod dls tllbutoon plate .l'•• Pocklno hold rv~d-+ ~ downplotoo ~· ......- Pockong oupport plote LlQUid ' radlttrobutlon r._ Gas In from rebollor pinto Fcccf Tower ~ LIQutd out Pockod Stoom C!OCtor +-Barometrtc leg L..:.J.."+ Ovorheads Bottoms Vacuum unit Recovery tower Steam Feed 1~·::::: ... , .. ._111'L ~"'"' ~r-v•P•• ~ Bonoms Structured pocking Feed Steam stripping unit Extractivc distillation unit Azeotropic distillation unit Molecular distillation F••d-~e;i[{"" Stoam ~- Hogn vocuum ,r;!./ rheodt L;:.-..--- ~.'~o;:;•!,~~~ Dcgossifier Ocgo.,oflnd fttd to st111 Centrifugai molecular still Sponnlng rotor Figure ~li Equipmenl Options for Disllllation: Configurations Folling film molecular Still 4-25 4-2G Selecting Options for the Separation of Components from Homogeneous Phases Distillation comes in various forms: batch, \\herc small amounts are processed and lhe diffcrent species are collected as time progresses; • ordinary, continuous (with the maximum diameter about 12m corrcsponding to a fced rate of about 300 kg/s); a.~:eotropic and extractive, whcrc a solvent is added to changc the properties of lhe system; • steam, wherc steam (or an incrt) is adcled direct ly to thc column to alter the pressures withtn lhe systcm so that an apparently high vacuurn can bc achicvcd for the scparation; • molecular, where very high vacuum is applied to the !>pccially dcsigned system (the fecd capacity is usu- ally in the range 5 g/s to 200 g/s; thc uppcr limit is becausc the column must have no leaks. This means that ali joints are wcldcd.) c1yogcnic, where the column operalcs at very low lem- peratures (bccausc o f the availabilily o f refrigcration units, usually lhis op1ion is applied to relatively high feedrates. At lower feedrates, it becomcs very expen- sivc to obtain a small refrigeration packagc anel the othcr components needed for cominuous disti llation.) \Ve note that the firsl two operalions rcquire energy; the olher operations also require the addition o f a solvent or incrt, or excursions in pressurc and temperature. Skctches o f some o f lhese configurations are given in Figure 4-1 I. The product is usually both the overhead and thc bot- toms. Douglas (1988) suggests that reasonable expccta- tions for the purity o f both the product streams be 99%. The feed-product diagram is shown in Figure 4-12. Distillation is lhe workhorse for separating low molar mass, concentraled organic, or organic/water mix- lures that do not degrade when exposed to high lempera- tures. The equipment is an extension of a laboratory tecb- niquc. Thus, ordinary distillation should be possible for any feedrate. For smallcr feedrates in lhe range O. l LO 5 kg/s, columns are usually operatcd batchwise. For evcn smaller fcedrales, small laboratory-size units are used. Before structured pacJ..ings became so popular, for tray columns lhe diamcler usually was not lcss than 0.6 m lo allow acccss lo the column in between the plales. Smaller diameter, con- tinuously operaled columns are being inslalled. Thc energy requiremenl is rebtively high because thc latent hcat o f vaporization (approximalcly 500 kJ/kg for mos1 organic materiais) is the energy driving force. ~lultiMaging is very easy; we can ob1ain aboul I 00 stages easi ly; a limitation is the hcigh1 we can build onc column. The design and scalc-up proccdurcs are well dc- vclopecl. We can achie'c an cconomic advantage by build- ing large-~i;e units. l'hu s, cvcn fo1 ~mall-scale operalion, dbtillation is often thc bc~l oplion. f·or omc condi11ons, anolher ~eparation tcchnique migh1 be prcfcJrcd at smal l seale; however, as the throughput or capacity increases, distillation may become lhe favorcd option. Hence, we usu- ally slart by considering dislillation. As with evaporation, vapor recompression is some- times used to try lo minimize lhe energy use. Distilla1ion may be a questionable option i f solids are present, the liquid foams readily, the species degrades at high temperatures or is thermally unstable, or i f thcre is a low conccntralion of the lcss volalile species so lhat wc have to boi I just about ali o f thc fccd. Another conccrn is if the spccies fonn an azeo1ropc. Examplcs of sys1cms whcre azeotropcs form include e1hanol-w:11er (al 89.4 moi% cth- anol), ncetone-chloroforrn (at 3..J. moi % acc1onc), carbon dioxidc-cthane (at 65 mol % carbon dioxidc), and cthnnol- bcnzcne (at 44.8 moi % e1hanol). B. Exploitlng Differences in Melting Temper- ature or Liquid-Solid So/ubility. Cr)stallization: 111e ncxl five options are confusing because in thc literature lhe word cr)•stallizmion or "freezing" may be uscd to dcscribe any o f 1hese. llence, we start with a few dcfinitions. Fi gure 4-13 illuslrates lhe chemical charactcristics lhat are impor- tant. This shows thc behavior of a liquid-solid system of sodium sul f ate and watcr. At temperatures below Lhe frecl.- ing point, solid forrns because of freezing. This identifies rcgion "A," where processes called mell crystaUizalion, freeze concentration, or .~:one refining might occur. For temperatures well above lhe freezing point but concentra- lions ncar the solubility lirnil, solid forms because il prccip- itates at location "B" in Figure 4--13. Here we have solution crystallization or precipitation. Table 4-6 summarizes other kcy differences. 1. Melt Crystalli:;ation. For melt cryslallization, the subject of lhis section, lhe key physical property is the freezing tcmperatures of 1hc species. The principie of operation for melt crystallization is analogous 10 dislillation except lhat the energy expcnditure relates to the freezing and melting lalent heat (which is about 20% of the vaporization larem heal). The configura- tion is illustrated in Figure 4-14. Crystals are fo rmed from lhe melt at thc bouom,move through lhe washer stages and the "pure, wa~ht::d'" crystals are melted to crente the over- hcad pure product and thc reflux. Typical reflux ratios are aboul 2 10 411. Typical washing s1ages are fivc. Because il is difficult to wash ali of 1hc solution from thc crystals, the purity of the stream is usually aboul 99%. The feed strcam I caves as an "impure" conccntrate. Thc temperalure diffcrcnce across lhe washer (be- tween 1hc freezcr and the mellcr) is in lhe range 20 to I 00°C. Although thc differencc in frccLing tcmperalures is lhe major criterion to consider, wc mighl look at the 50iu bility products of othcr !>pccie~ 10 ensure 1ha1 thcy do no1 precipitalc out. Skclches ot the conf1gurations for mclt crystalh.~:a tion, givcn 10 hgurc -1-1-lb, illu~lrate thc \\ide range of ~----------------------------------------------------------- - 4.1 Selecting Options for Separating a Homogeneous Phase 4-27 99.9999 99.9995 99.999 99.995 99 99 99.95 99.9 99.5 99 ~ 98 o E 95 õ "' 90 ., )( § ot 80 ~ -~ 60 ., ... 5 8. 40 ="' Cl ~ 20 ,;.; ·.: "' ..... -uo 10 ::> > Cl 5 Cl a: 0... 2 1 0.5 0.1 0.05 0.01 0.005 0.001 0.0005 0.0001 I I I I 1 I I I I I I I I I I I I I I 11 I I / '-~: Usual targct 2 I x2 I A I I - I / I t= - I - I I - XI~ I - - r- }../t:. - ---- l 3 ~ - , x3 c,· / ~ ·~\ I .. . I «,lb'li /)'-~~~ i\ . " t /~'r--'~ .... ~~ :-,... ....... ~~ ~ i""-- :-/ f\ ~r-- """' ~ ~ .... "' Mo e reflux ~ // '\'\r- r- ................ "'-'~ ~ me re trays ....... ' / _I'\ // r-- 1- / I I ,__ / /I [/1 I I I l I I . o o o ~ o o o o <11 9 o o !=' O O O O _. ,..., Ul _. N ~ tn CO tD W CDC.OC.O CD C.O <D CD o Q 0 :_. (.n O O O O O O Ul CDCDC.O C.O CD <D f.D o -" Ul ê,n(o(D(o to U1 U1C.O CD PRODUCT 2: "Concentrate• of more volatile species, %, x2 <11 <D "' (o "' to <D <D (o <D <D <11 "' to (o "' ~ Figure 4-12 Equ ipme nt Options for Di;,tillation: Fced-Product Dingram 40 r--r-,-,-~-~--r--.------~~-~-.-. 35 f----1----+- 30 25 f----1-- ~ 20 CD 5 Cií 15 1---l./ a; ~ 10 CD ..... ,~~-1---+---~--~~f-~----~--~-~ ·5 -Eutectic ·1 O I o 10 20 30 40 50 60 70 80 Fi ~:ure 4- 13 ;\lelt Jnd Soluuon Cr)'~tJIIi z:IIIOn Concentration Na2S04, w/w ~ ~ (X) T ABLE 4·6. Options that are called " crystallization" Mel L I Freezc Zonc Soluuon Prccipllallon CrystalliL.aLion ConcentaLion Refining CrystalhzaLion Start wilh liqu1d: me h or mohen soluLe solid hqutd: soluLc in a sohent liquid: potcoúal solutc in "mother hquor" a solvcnt Key solidify by rcducing Lhe tcmp. tcmp. increasclde- cause soluLc to become cause soluLc to be crease Lo meltlsoli<hfy insolublc by producing insolublc: by supcrsaturation condiúon cbemical reaction 1\tam conLrol Heat Transfer :O.lass Transfer Adjust TemperaLurc Temperature Temp., Prcssure, salt cone., or ~T =-50 Lo too•c from freezing ~T =- 2 Lo 3''C away from chernical composiúon in solvem usually SLOp beforc cuLecLic solubilit) line I Key Parameter T K K I free7.int ., lp I Driving Force t.T Dcgrcc of supcrsaturation relative 10 solubility Aclion solute sohdifics I solvem solidifies solutc melts Lhen solutc solidifies as Cf)·stal soliJifics LímiLcd by Eutccllcs by solubihty location "A" in Figure 4-13 location "IJ'' in Figure -1-13 -------------------------------------------------------------------------------------- 4.1 Selecting Options for Separating a Homogeneous Phase a. Configuration for Melt Crystallizer: Overview b. Configuration for Melt Crystallizer: Details Roflux "Puro" ,------.----- molten product ---~ I I I I I I I I I I I I I I Rotary drum: mult istage w 1th product as solid on drum that is cooled Column crystalhzor, Schildknecht. Arnold, Phillips z ____ ; CJcomprossor I I I I I I "Impuro• conccntrato Batchwise Proabd, Ncwton·Ct1ambcrs c. Configurations for Zone Refining Czochralski cell Puro xl drawn out Co oi Typical aluminum solidification Heater ~~tt ~ o c::=:!--+ Batch-zone moving heaterftube horizontal Vertical d. Feed·Product Diagram ...... ..... ..... ""' w .... f- .... <1: "' cr "d f- .. z .. w .. u z .. o .. u " . " w • cr: ' ::::» ri 0.. ~ .. ... '"" .... . ..., ..... ~ I I li li i 1 1 1 111111 1J5~;a: t~r~eÇ1 forzo/f---0 1-+ t- o~ 0 'V v..~/ \0 / --- ~" " ~<t'/ H· " ~ . . - -=-~ «\ (f. I --= - 1- .. 1"1 ·- - : . - - 1-: - l . ~ -- -# / 71·--/1 I I ti li . . • • ""!"" • ~, ~ t r 8 111>ltiUI I ~ • • ··n g J I I "PURE" PRODUCT F~~F~I~tolrl ~ Xllization P Newton-Chambers Continuous partia! frcczo ~p Flat zone fixed heater, no tube Void Zone-void Figure .t- 14 F.qUJpment Op11ons for ~1eh and Frcctc Crystallization and for Zone Rcfimng ConfiFurations and Fecd Product1J1agr.1m a) Configuration for lllelt Cry~talli7er· Overv~ew b) Configuratlons for \ 1cl1 Cry~tallizer: Details c) Conflgurations for Zone Refini ng d) Feed-Prcxluct Diagram 4-29 4-30 Selecting Options for the Separation o f Components irom Homogeneous Phases cquipment configura1ions that have been devcloped. On thcse ske1ches lhe ·'pure·· product is given the symbol "P" and 1he concentrareis callcd "impure." The product is usually the single, '"overhead" phase. The feed-product diagram is given in Figure 4-14d. Usu- ally the fecd conccmra1ion is reialively high. Multi taging is po ible ahhough usually fcwer Lhan 10 s1ages are ins1alled. Thc design and scale-up procedurcs are developing. This used to be a smal)-fced capacity op- tion. With lhe developmenl of direct comact rcfligeration for thc frcctc cycles, fccd capaci tics as largc as 3 kg/s can be ltandled. Using indirec1 refrigcra1ion, lhe feed capacity is about one-fifth o f lhis or 0.6 kg/s. Melt crystall it.ation is an aurac1ivc option whcn lhe spccics are Lcmpcra1ure sensiti ve and/or whcn thc boiling points of the spccics to bc scparated nrc similar. For mclt crystallitalion, onc of thc main limitations is that many mixturcs form eutectics, as illustrated in Fig- ure 4 13. Oncc the mclt has been cooled to lhe cutectic tcmpcrature ( - 1.1 C on Figure 4-13 ), al i o f thc species bcgin 10 solidify together. For the system in Pigure 4- 13, this means t11atLhe solid is a mixture o f ice and sodium dec- ahydrate. Although for some systems ihe individual specics will rctain thcir idenlily and can be physically separaLed, for rnost sysLerns the eutectic condition is to be avoidcd. The euteclic condition can be estimatcd by tl1e use o f van' t Hoff's iaw: lnxz= ~f(~ -1 J (4-2) For a binary, tbis law is applied for bolh of the spe- cies and will result in the freezing temperaturc-concentra- tion curves that are projectcd until tl1ey intersect at the eu- tectic. Because of the eutectics, separation of the feed into two "pure" phases is usually impossible. 2. Freeze Concentration. Freeze concent.ration is closcly related to meit crystallization in that the principies and 1hc equipment are similar. Thc key physical properties diffcrcnces for scparation are lhat there must be a differ- ence (for the specics to be separated) in the freczing tcm- pt:ratures. Wc start with a Iiquid phase; the principieis that heat is removed to freezc or solidify one of thc componems (usually thc solvent) and leave behind a liquid phasc rich in thc othcr cornponents. Thus, in rnelt crystalliLation we usu- ally frcezc lhe solwe; in frec;e conccntrarion, thc solvellf. ln mclt cry.c,tall ization the product is u ~ually Lhe
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