Design of adiabatic flash unit

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1 Design of dibtic flsh unit Henrik Svensson Ekberg Deprtment of Chemicl Engineering, Lund University, P.O. Box 24, SE Lund, Sweden Abstrct Concentrted sulfuric cid is used s homogenous ctlyst in the lkyltion process. Due to side rections the cid becomes diluted nd must be removed from the process s the concentrtion reches 90 % w/w H 2 SO 4. Regenertion of the cid is necessry to mintin efficiency nd it is desirble to concentrte the cid to s high concentrtion s possible. Unfortuntely the wter-sulfuric cid system shows mximum boiling zeotrope t ~98.5 % w/w H 2 SO 4. It is impossible to exceed the zeotrope with equilibrium seprtion process such s distilltion. In this pper the pressure dependence of the zeotrope is investigted together with the possibilities for concentrting the cid to ~99.0 % w/w H 2 SO 4. Publictions on the thermodynmics of the wter-sulfuric cid system show tht the position of the zeotrope is uncertin. Dt found in literture, on the pressure dependence of the system, shows tht t totl pressures between 5 nd 0 mbr the position of the zeotrope is close to 99.0 % w/w H 2 SO 4. It is in this pper reported tht flsh vessel or severl flsh vessels in series cn provide the desired product concentrtion. Mtlb simultions on single stge flsh shows for very low seprtion power with gret losses of vluble sulphuric cid. The seprtion power is enhnced by connecting severl flsh units in series. The number of units depends strongly on the feed concentrtion. For producing product of 98.9 % w/w with recovery of 99 % nd feed concentrtion of 98.0 % w/w H 2 SO 4 two single stge flsh units re required nd with feed concentrtion of 96 % w/w H 2 SO 4 four is required for the sme requirements. To get complete picture of the economicl fesibility of the developed process the het demnd nd the mteril costs must be further investigted.. Introduction In the lkyltion process highly concentrted (~98 to 99 % w/w H 2 SO 4 ) sulfuric cid is used s homogenous ctlyst. Low moleculr weight hydrocrbons re upgrded into C 8 -isomeres producing high octne fuel mixture. The produced lkylte is mixed with gsoline or diesel to produce fuel of high qulity. As side rection polymers soluble in the queous cid phse re produced. This cn be considered s loss of C 8 -isomeres on the ccount of polymers. As the cid gets weker, more polymers re produced. When the cid reches 90 % w/w H 2 SO 4 it hs to be removed from the process. It is for economicl nd environmentl resons importnt to regenerte the spent cid. The sulfuric cid wter system shows n zeotrope just bove 98 % w/w H 2 SO 4, mening tht t tmospheric pressure it is not possible to exceed this concentrtion by equilibrium seprtion like distilltion [,2]. Currently, n cid concentrtion of between 98.0 to 98.5 % w/w H 2 SO 4 is used in the lkyltion process. An cid of higher concentrtion (~99.0 % w/w H 2 SO 4 ) would increse the efficiency of the process. Publictions on the wter-sulfuric cid system show tht the position of the zeotrope is pressure dependent. In this pper the thermodynmics of queous sulfuric cid nd process lterntives for the concentrtion of cid re evluted. The results from these studies re combined to investigte the possibilities for producing product of ~99.0 % w/w H 2 SO 4 with high recovery nd low equipment costs. 2. Theory Azeotropes re found in mixtures showing devition from idelity. When the devitions re lrge enough the mixture shows common minimum or mximum in dew-point nd bubblepoint t certin composition. At equilibrium, the composition in the vpour is the sme s in the liquid in this extreme point nd by conventionl distilltion this point cn not be exceeded. In this pper focus will be on concentrtions between 96.0 nd 99.0 % w/w H 2 SO 4 since this concentrtion rnge is of industril interest [2]. At high concentrtions nd tempertures H 2 SO 4 is prtilly dissocited into wter nd sulfur trioxide ccording to rection [3]. H SO g) H O( g) + SO ( ) (Rection ) 2 4( 2 3 g In vpour with wter in surplus, ll of the produced SO 3 rects with wter to produce H 2 SO 4 when condensed. The sulfuric cid wter system cn therefore be considered s binry system, simplifying the equilibrium clcultions considerbly [2]. 2. Thermodynmic study Prtil pressure dt in the concentrtion rnge 0 to 00 % w/w hs been published in Perry s

2 Chemicl Engineer s Hndbook [4]. The equilibrium dt is used to crete LE digrms where the position of the zeotrope cn be determined t different pressures (see tble ). Pressure [br] Azeotrope composition [mole- %H 2 SO 4 ] Azeotrope composition [% w/w H 2 SO 4 ] Tble : Azeotrope dt for constnt boiling sulfuric cid from Perry [4] Comprison with the most cited investigtors re mde nd presented in figure below. vessel vporizing frction of the fed liquid (see figure2). The vpour is richer in the more voltile component (see figure 2). The vpour nd liquid leving the flsh vessel is optimlly equilibrted. To chieve equilibrium the seprtion requires sufficient time nd re of contct between vpour nd liquid. All single stge distilltion opertions require high reltive voltility nd most often the seprtion power is very poor [5]. Heter lve F, x F L, x, y Figure 2: Principle sketch for flsh distilltion, F = liquid feed flow [kmole/h], = vpour flow [kmole/h], L = liquid product flow [kmole/h], x i = mole frction of the most voltile component in liquid flow i, y = mole frction of the most voltile component in the vpour flow, Ti = the temperture in strem i [ o C] [8] Figure : Azeotropes t totl pressures rnging from 5 to 000 mbr for constnt boiling sulfuric cid [4, 5, 6] 2.2 Distilltion theory Distilltion is bsed on prtil vporiztion of mixture. The vpour becomes richer in the more voltile compound nd the liquid becomes richer in the less voltile. All the compounds re voltile but for seprtion to occur the compounds must show different voltility. The two min types of distilltion re single stge distilltion nd multistge distilltion with reflux [9] Multistge distilltion Seprtion cn be enhnced considerbly if number of flsh units re connected in series. The method illustrted in figure 3 is bsed on the equilibrium-stge concept tht ssumes tht vpour nd liquid re brought to equilibrium t every stge, the pressure is constnt nd every stge nd divider is dibtic. The totl heting duty needed to evporte the entire feed is provided by boiler before the first stge of the rectifying unit. The liquid in ech step is return to the previous nd the vpour is fed into the following step without condenstion. The vpour leving the column is totlly condensed nd prtilly refluxed into the top of the column. The product is retrieved continuously from the bottom stge without recircultion. Countercurrent direct contct between liquid nd vpour provides optiml conditions for intensive mss trnsfer. This rrngement, clled rectifying section, increses the recovery of the hevy component nd purifies the lighter by contcting it with successively richer liquid [8, 9] Single stge distilltion There re three commonly used single stge methods; equilibrium flsh, btch nd stem distilltion.the simplest continuous distilltion process is the flsh seprtor. Liquid under pressure is heted nd the pressure is reduced in flshing 2

3 Totl condenser 3. Clcultion methods D, x D A simple wy of evluting distilltion process is grphiclly using LE digrms nd mteril blnces over the system. An x-y digrm with n operting line, corresponding to mteril blnce over the system, is used to determine the composition of vpour nd liquid in equilibrium. 3. Single stge distilltion The eqution for the operting line in single stge process is clculted through mteril blnce over the system in figure 3 for the most voltile component (see eqution ). Boiler F, x F L, x Figure 3: Flow sheet for multistge distilltion, rectifying unit with 3 equilibrium steps, F = feed flow [kmole/h], D = distillte flow [kmole/h], L = liquid product flow [kmole/h], x i = mole frction of the most voltile component in liquid flow i [8]. In distilltion column the section bove the feed is the rectifying section, sme s described for series of flsh units bove. Stges below the feed increse the recovery of light product nd the purity of the hevy product. This section is clled the stripping section. Combining these two sections provides better seprtion nd purity then either section lone. ( /F) x F () y = x + ( /F) ( /F) where the symbols re described in figure 2. The slope of the operting line depends on the frction vporized (f =/F). The frction vporized depends on the enthlpy (or temperture) of the liquid feed nd the system pressure. For given feed condition more vpour cn be produced by flshing to lower pressure nd for given system pressure more vpour is produced by incresing the enthlpy of the feed. The intersection between the operting line nd the equilibrium line gives the compositions of the vpour nd liquid products. The compositions cn be red from n x-y digrm s illustrted below (see figure 4) [9, 0]. 2.3 Equipment recommendtions The choice of process rrngement depends on the physicl nd chemicl properties of the wter sulfuric cid system. A single step seprtion gives only the equilibrium composition t the existing condition while multistge distilltion with reflux theoreticlly cn produce ny desired product concentrtion depending on the number of steps nd the reflux rtio. The lter system demnds more equipment nd optimiztion nd is suitble for industry with lrge scle production [, 2]. Multistge distilltion with low number of flsh vessels in series is n lterntive not s complex nd expensive s column nd with better seprtion thn single flsh vessel. In this cse high recovery of the high boiling sulfuric cid is of high importnce mking rectifying unit suitble. Figure 4: x-y digrm for the more volitile compound in n idel binry system, x B = mole frction of low boiler in the bottom liquid, x D = mole frction of the low boiler in the distillte, x F = mole frction of the low boiler in the feed, y* = y = mole frction of the low boiler in the vpour [9]. 3.2 Multistge distilltion Multistge flsh distilltion cn be evluted by grphicl method developed by McCbe nd Thiele. A mteril blnce for the more voltile component 3

4 over the system, defined by the dshed line in figure 5 below, furnishes the eqution for the operting line. Using n x-y digrm with the operting line plotted in it is clled McCbe- Thiele digrm. The stging nd how to red the compositions is illustrted in figure 6 [9]. = y = y L x Stge Figure 5: Principle sketch over rectifying unit, L b = liquid product flow [kmole/h], x = mole frction of compnent in the liquid product, b b = vpour flow entering the bottom stge [kmole/h], y = mole frction of component in the feed vpour, b y = mole frction of component in the vpour leving stge, L = moll flow rte of liquid leving stge n [kmole/h], n = moll flow rte of vpour leving stge [kmole/h], = mole frction of component in the liquid leving stge n, x n = moll flow rte of vpour leving the top of the column [kmole/h], y = mole frction of component in, L = moll flow rte of liquid entering the top of the column s reflux [kmole/h], x = mole frction of component in [0] The eqution for the operting line is: y b y b = y N+ L = n x n + L n- x n- L n x n y L L x (2, [0]) where the symbols re described in figure 5. The slope of the operting line is L n nd the externl reflux rtio is defined: D L (see figure 3, 5). n y n Stge n- Stge n y Stge Stge N L b = L N x b = x N Figure 6: McCbe-Thiele digrm for binry system, x B = mole frction of low boiler in the bottom liquid, x D = mole frction of the low boiler in the distillte, x F = mole frction of the low boiler in the feed [9]. In the McCbe-Thiele digrm stging begins t the distillte composition of the most voltile component (point A in figure 6). Stging continues, between the operting line nd the equilibrium line, s shown in figure 6 until the point on the operting curve where the composition in the feed is equl to the composition in the bottom liquid is pssed (point B in figure 6). In figure 6 the number of steps needed to rech the desired bottom liquid composition (x B ) B is less thn four [9]. In most multistge distilltion processes the moll flow rte of vpour nd liquid is nerly constnt throughout the system. The ssumption of constnt moll overflow fcilittes the eqution for the operting line (see eqution 3). L D x D y = xf = x + (3) F F where the symbols re described in figure Simultions nd results Simultions re mde in Mtlb where the equtions nd LE digrms re mnul progrmmed nd interpreted. All the simultions re bsed on the dt from Perry s Chemicl Engineers Hndbook becuse this is the most comprehensive dt source vilble. 4. Single stge distilltion For single stge flsh the slope of the operting line depends on the vpour/liquid flow rtio. The vporized frction of the feed is vried between 0 nd. The simultions will determine the reltion between product concentrtion (x, y), product flows 4

5 (L, ) nd cid losses. Simultions re mde on how the feed concentrtion ffects the product concentrtion nd recovery. The results from the simultions re shown in tble 2-4 below. Frction vporized Acid conc. vpour Loss [%] Acid conc. liquid Tble 2: Results from simultions on single stge flsh distilltion t P tot = 5 mbr, feed concentrtion 96.0 % w/w H 2 SO 4. Frction vporized Acid conc. vpour Loss [%] Acid conc. liquid Tble 3: Results from simultions on single stge flsh distilltion t P tot = 5 mbr, feed concentrtion 97.0 % w/w H 2 SO 4. Frction vporized Acid conc. vpour Loss [%] Acid conc. liquid ~ Tble 4: Results from simultions on single stge flsh distilltion t P tot = 5 mbr, feed concentrtion 98.0 % w/w H 2 SO 4. The high losses in the single stge process derive from the low reltive voltility between wter nd sulfuric cid. Figure 7 below shows tht in the relevnt concentrtion rnge, bove 90 mole-%, the reltive voltility is well below. This cn be compred with the rule of thumb of vlue of 0-20 for effective seprtion []. 4.2 Multistge distilltion For multistge distilltion the simultions illustrte the reltion between product losses, feed composition nd number of units. In ll simultions the product concentrtion nd feed flow is kept constnt while the cid loss nd feed concentrtion is vried. The results from the simultions re shown in tble 5-7 below. Loss [%] 0 5 Number of stges Acid conc. in vpour Acid conc. in liquid [% w/w H 2 SO 4 ] Liquid flow[kmole/h] pour flow[kmole/h] Tble 5: Results from simultion on multistge distilltion with feed concentrtion 96 mol-% H 2 SO 4 nd vrible cid losses Loss [%] 0 5 Number of stges Acid conc. in vpour Acid conc. in liquid [% w/w H 2 SO 4 ] Liquid flow[kmole/h] pour flow[kmole/h] Tble 6: Results from simultion on multistge distilltion with feed concentrtion 97 mol-% H 2 SO 4 nd vrible cid losses Loss [%] 0 5 Number of stges Acid conc. in vpour Acid conc. in liquid [% w/w H 2 SO 4 ] Liquid flow[kmole/h] pour flow[kmole/h] Tble 7: Results from simultion on multistge distilltion with feed concentrtion 98 mol-% H 2 SO 4 nd vrible cid losses The dt in tble 5 to 7 indictes tht product holding concentrtion of 98.9 % w/w H 2 SO 4 is theoreticlly possible to obtin. For process with 99 % recovery, product concentrtion of 98.9 % w/w H 2 SO 4 nd feed concentrtion of 98 % H 2 SO 4 two single stge steps re required ccording to the simultions. Figure 7: The reltive voltility (lph = prtil pressure wter /prtil pressure sulphuric cid), between wter nd sulfuric cid (y-xis) s function of the mole frction sulfuric cid in the liquid phse (x-xis), totl pressure = 5 mbr [4] 5. Conclusions nd discussion It is reported in literture tht the thermodynmics of queous sulfuric cid re very complex. The existent of n zeotrope is confirmed 5

6 in numerous sources, though the position of it is disputed. Depending on the investigtor the position of the zeotrope for queous sulfuric cid vries from to % w/w H 2 SO 4. The boiling temperture t the zeotrope does not differ significntly in literture. Most of the more recent publictions refer to erlier investigtions, such s Perry (98.5 % w/w H 2 SO 4, 337 o C), Kunzler (98.48 % w/w H 2 SO 4 ) nd Luchinskij (98.32 % w/w H 2 SO 4, o C) [4, 5, 6]. According to dt published in Perry s Chemicl Engineer s Hndbook, the position of the zeotrope is very close to 99.0 % w/w H 2 SO 4 t pressures below 0 mbr. The desired product concentrtion cn ccording to the thermodynmic study be obtined in n equilibrium seprtion process operting t totl pressure of 0 mbr or lower. The simultions on single stged flsh distilltion demonstrte how the low reltive voltility between wter nd sulfuric cid leds to wek seprtion power. To obtin product concentrtion of 98.9 % w/w H 2 SO 4 gret mount of vluble cid will be lost in the distillte. With feed concentrtion of 98.0 % w/w H 2 SO 4 nd vporiztion fctor of 0.3 the concentrtion in the distillte is close to 95.6 % w/w H 2 SO 4 compred to 98.9 % w/w H 2 SO 4 in the liquid product. Almost 30 % of the fed cid will be lost in tht cse. Using more thn one flsh unit, it will be possible to produce product with ny desired composition nd high recoveries, ccording to literture on distilltion theory. The results from the simultions showed tht product holding concentrtion of 98.9 % w/w H 2 SO 4 with recovery of 99 % is theoreticlly possible to obtin. However it requires high feed concentrtion which demnds more pretretment of the feed. With feed concentrtion of 98.0 % H 2 SO 4 w/w two single stge flsh units re required nd with feed concentrtion of 96.0 % w/w H 2 SO 4 four is required to fulfill the requirements. There hve been no estimtions on the energy demnd of the process in this pper. porizing the feed would demnd gret mount of energy for the highly concentrted cid. Neither hve there been ny mteril cost estimtions. Expnding the vpour to pressure of 5-0 mbr would require very lrge dimeter of the distilltion vessel which is followed by gret mteril costs. These issues must be tken in ccount in order to estimte the economicl fesibility of the multistge distilltion process fully. References. M. Thellefsen, Kemiteknik exmensuppslg flshdestilltion, E-mil converstion with M. Thellefsen Hldor Topsoe A/S 3. H. Müller, Sulfuric Acid nd Sulfur Trioxide,Ullmnn s Encyclopedi of Industril Chemistry, John Wiley & Sons, Published online: 5 June, R. H. Perry, D. W. Green, Perry s Chemicl Engineers Hndbook, McGry Hill Interntionl Edition, Seventh Edition, pp , J. E. Kunzler, Absolute Sulfuric Acid, Highly Accurte Primry Stndrd, Anlyticl Chemistry, volume 25 no., pp 93 03, J. I. Gimitro, T. ermeulen, pour liquid equilibri for queous sulfuric cid, pp , A.I.Ch.E Journl, C. J. Genkoplis, Trnsport Processes nd Unit Opertions, Prentice-Hll ECS Professionl, Third Edition, pp , E. J. Henley, J. D. Seder, Equilibrium-Stge Seprtion Opertions in Chemicl Engineering, John Wiley & Sons, pp , pp , J. Stichlmir, Distilltion nd rectifiction, Ullmnn s Encyclopedi of Industril Chemistry, John Wiley & Sons, Published online: 5 June, McCbe, Smith, Hrriot, Unit opertions of Chemicl Engineering, Seventh Edition, McGrw Hill, pp , pp , 200. N. Kukl, J. P. Grverholt, UIC symposium: Short Pth nd Thin Film Distilltion, BioCentrum-DTU (Lyngby/Denmrk), UIC GmbH, 29 November,

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