hemical Reacto esign Y W L Youn-Woo Lee School of hemical and iological Engineeing 55-74, 599 Gwanango, Gwana-gu, Seoul, Koea ywlee@snu.ac. http://sfpl.snu.ac.
hapte 6 Multiple Reactions hemical Reaction Engineeing
Intoduction Seldom is the eaction of inteest the only one that occus in a chemical eacto. Typically, multiple eactions will occu, some desied and some undesied. One of the ey factos in the economic success of a chemical plant is minimization of undesied side eactions that occu along with the desied eaction. In this chapte, we discuss eacto selection and geneal mole balances fo multiple eactions. Fist, we descibe the fou basic types of multiple eactions: seies, paallel, independent, and complex. Next, we define the selectivity paamete and discuss how it can be used to minimize unwanted side eactions by pope choice of opeating conditions and eacto selection.
Objectives efine diffeent types of selectivity and yield hoose a eaction system that would maximize the selectivity of the desied poduct given the ate laws fo all the eactions occuing in the system. Size eactos to maximize the selectivity and to detemine the species concentations in a batch eacto, semi-batch eacto, STR, PFR, and PR, systems.
6. efinition of Multiple Reaction Paallel xns (competing xns) Seies xns (consecutive xns) omplex xns (Paallel + Seies xns) + + + E Independent xns + E + F
Examples of Multiple Reaction Paallel xns (Oxidation of ethylene to ethylene oxide) H =H +O H -H O + H O Seies xns (xn of EO with NH ) O O Monoethanolamine H -H + NH HOH H NH EO EO (HOH H ) NH (HOH H ) N iethanolamine omplex xns (fomation of butadiene fom ethanol) Tiethanolamine H 5 OH H 4 + H O H 5 OH H HO + H H 4 +HHO HO 4 H 6 +HO Independent xns (The cacing of cude oil to fom gasoline) 5 H H 6 + H 6 8 H 8 6 H 4 + H 4
esied and ndesied Reaction (esied Poduct) (ndesied Poduct) The economic incentive Maximize the fomation of Minimize the fomation of ompeting o side xn Total cost M sepaation Reaction cost eaction Sepaation cost, Low High Rxn-sepaation system poducing both & Efficiency of a eacto system
Instantaneous Selectivity, S (esied Poduct) (ndesied Poduct) The ate laws ae Selectivity tells us how one poduct is favoed ove anothe when we have multiple l eactions. S / Rate of fomation of = Rate of fomation of
~ Oveall Selectivity, S Fo Flow Reacto ~ F Exit mola flow ate of desied poduct S / = = F Exit mola flow ate of undesied poduct Fo atch Reacto ~ S / = N = N No. of moles of desied poduct at the end of xn time No. of moles of undesied poduct at the end of xn time
~ Example 6-: ompaison between S / and S / fo a STR ~ Mission: evelop a elationship between S / and S / in a STR Solution (esied Poduct) S / = ~ F (ndesied Poduct) S / = F Mole balance of and yields F = V and F = V, espectively ~ F = V = = S = / S F / V ~ S / = S /
Yields Instantaneous Yield (asis: Reaction Rate) Y = - Oveall Yield (asis: Mole o Mola Flow Rate) N = Fo a batch system: Y = N o -N ~ Mole of desied poduct fomed at the end of eaction Numbe of moles of ey eactant consumed Fo a flow system: ~ F Y = Fo -F
Note iffeent e definitions fo selectivity ect ty and yield hec caefully to ascetain the definition intended by the autho Fom an economic standpoint, oveall selectivities and yields ae impotant in detemining pofits The instantaneous selectivities give insights in choosing eactos and eaction schemes that t will help maximize i the pofit
6. Paallel Reactions (esied Poduct) (ndesied Poduct) W t t The ate laws ae We want to maximize S /. Rate of fomation of / S = Rate of fomation of Rate of fomation of and ae positive eaction odes
6.. Maximizing the esied Poduct fo One Reactant (esied Poduct) (ndesied Poduct) S / ase : >, a = - S / a Maximize S / - eeping the concentation of eactant as high as possible duing the xn -in gas phase xn, we should un it w/o inets and at high pessues to eep high - in liquid phase xn, the use of diluents should be eep to a minimum - use a batch o plug-flow eacto
6.. Maximizing the esied Poduct fo One Reactant (esied Poduct) (ndesied Poduct) S ase : >,b= - S / b Maximize S / - eeping the concentation of eactant as low as possible duing the xn - in gas phase xn, we should un it with inets and at low pessues to eep low - in liquid phase xn, the use of diluents should be eep to a maximum - use a STR o ecycle eacto (poduct steam act as a diluent)
Maximizing S fo one eactant Whethe the eaction should be un at high o low T? S / ~ e E E RT S ase : E > E - ( ) inceases moe apidly with inceasing temp. than does the. - eeping the tempeatue as high as possible to maximize S. S / E E T ase 4: E > E E E - eeping the tempeatue as low as possible to maximize S S / -not so low that t the desied d xn does not poceed to any significant ifi extent. t T
Maximizing S /XY fo the Tambouze Reaction Reactant decomposes by thee simultaneous eactions to fom thee poducts, (desied), X (undesied), and Y (undesied). These liquid phase eactions, along with the appopiate ate laws, ae called the Tambouze eactions. mol 0. 000 dm s X X () (0.005s () ) dm mol s Y 0 008 Y. () How and unde what conditions (e.g., eacto types, tempeatue, concentations) should the eaction be caied out to maximize the selectivity of? The specific eaction ates ae given @ 00K E =0,000, E =5,000, and E =0,000 cal/mole 0 =0.4M, v 0 =.0 dm /s S/(XY) X Y
Maximizing S /XY fo the Tambouze Reaction S /(X+Y) 0 0 0.05 0.574 0.05 0.65 0.075 0.77586 0. 0.8 0.5 0.8 0.5 0.8057 0.75 0.76087 0 0. 0.7486 0.5 0.6687 0.5 0.65 0.75 0.58506 0. 0.54878 S /(X+Y) S /(XY) X Y S /(X+Y) =(0.005* )/(0.000+0.008* ) 0.8 0.6 04 0.4 0.5 0.5587 0.5 0.486 0. 0.75 0.45984 0.4 0.4478 0.45 0.46 045 0.45 0.944 0.475 0.7406 S 0.84 /(XY) 0 X Y 0. 0 0. 0. 0. 0.4 0.5 0.5 0.574 (mol/dm )
Maximizing S /XY fo the Tambouze Reaction s we see, the selectivity eaches a maximum at a concentation *. ecause the concentation changes down the length of a PFR, we cannot opeate at this maximum. onsequently, we will use a STR and design it to opeate at this maximum. To find the maximum *, we diffeentiate S /(X+Y) w..t.,setthe deivative to zeo, and solve fo *. That is, deivative to zeo, and solve fo. That is, Y X Y) /(X S * * * Y) /(X - 0 ds * d * s) 0 000(mol/dm * dm 0.8mol/ s) /mol 0.008(dm s) 0.000(mol/dm 0.885 S * * Y X /XY Y X
Maximizing S /XY fo the Tambouze Reaction We now calculate this STR volume and convesion. The net ate of fomation of fom eactions (), (), and () is - X Y STR volume fo this liquid-phase id eaction, 0 V [ ] [ ] * * 0 0 0 0 567.66 dm * * [ ] STR volume fo maximum selectivity V 0 * 0 * * 0 * * 78.8s 0.000 mol/dm s 0.005 s 0.008 dm /mol 0 0d.0 dm 0.4 0 * 0. /s mol/dm mol/dm s
Maximizing S /XY fo the Tambouze Reaction Maximum the selectivity w..t. tempeatue S /XY ase : ase : E E X E E Y E E * * E E E exp RT Run @ high tempeatue (w/o side xn) Run @ low tempeatue (with insued convesion) E E Fo the activation enegies given above E 5,000 So the selectivity fo this combination of activation enegies is independent of tempeatue!
Maximizing S /XY fo the Tambouze Reaction What is the convesion of in the STR? X * 0-0 * 0.4-0. 0.4 0.7 If geate than 7% convesion of is equied, then the STR opeated with a eacto concentation of 0. mol/dm should be followed by a PFR because the concentation and selectivity will decease continuously fom * as we move down the PFR to an exit concentation f. Hence the system STR * PFR Would be the highest selectivity while foming moe the desied poduct, beyond what was fomed at * in a STR. How can we incease the convesion and still have a high selectivity S /(X+Y)? f *
Maximizing S /XY fo the Tambouze Reaction Optimum STR followed by a PFR. The exit concentation of X, Y, and can be found fom the STR mole balances Species X: Species : Species Y: V V V υ υ 0 X 0 X / X τ 0.000)(78) 0.078mol X υ0 υ υ υ 0 * τ * ( dm ( 0.005)(0.)(78) 0.mol / dm 0 Y 0 Y * / Y τ 0.008)(0.) (78) 0.0786mol * Y ( dm F X 0 X 0. 56 mol / s, F 0. 64 mol / s, F 0 Y 0 Y 0. 57 mol / s Let s chec to mae sue the sum of all the species in solution equals the initial concentation 0 = 0.4. + X + + Y =0.+0.078+0.+0.0786=0.4 //// QE
Maximizing S /XY fo the Tambouze Reaction The eason we want to use a PFR afte we each the maximum selectivity, S /XY,is that the PFR will continue to gadually educe. Thus, moe will be fomed than if anothe STR wee to follow. If 90% convesion wee equied, then the exit concentation would be f =(-0.9)(0.4 mol/dm )=0.04 mol/dm. The PFR mole balances fo this liquid-phase eaction (v=v 0 )ae d d d X d, X, dv d d d, d Y d 0 Y ombining mole balances with thei espective ate laws yields d d d X d d d d Y d ( at 0, then ( at 0, then ( at 0, then ( at 0, then X Y 0. 0. 078 0. 0. 0786 mol / dm mol / dm mol / dm ) ) mol / dm ) ) =0, 0 the enteing concentations to the PFR ae the exit concentations fom STR.
Maximizing S /XY fo the Tambouze Reaction The convesion can be calculated X 0 0 We will use Polymath to plot the exit concentations as a function of and then detemine the volume (V=v 0 ) fo 90% convesion ( =0.04 mol/dm ) and then find X,, and Y at this volume. This volume tuns out to be appoximately 600 dm. ttheexitofthepfr, =0.07, X =0., =0.6, and Y =0.09 all in mol/dm. One now has to mae a decision as to whethe adding the PFR to incease the convesion of fom 0.7 to 0.9 and the mola flow ate of fom 0.6 to 0. mol/s is woth not only the added cost of the PFR, but also the decease in selectivity. This eaction was caied out isothemally; nonisothemal multiple eactions will be discussed in hapte 8.
PFR concentation & selectivity pofiles fo the Tambouze Reaction
PFR concentation & selectivity pofiles fo the Tambouze Reaction
PFR concentation & selectivity pofiles fo the Tambouze Reaction
PFR concentation & selectivity pofiles fo the Tambouze Reaction
PFR concentation & selectivity pofiles fo the Tambouze Reaction
6.. Reacto Selection and Opeating onditions (esied Poduct) Two simultaneous eactions with two eactants + (nwanted Poduct) The ate laws ae S / Rate selectivity paamete Rate selectivity paamete (= Instantaneous selectivity) is to maximized by choosing eacto schemes.
Reacto Selection iteia Selectivity Yield Tempeatue contol Safety ost
Figue 6- iffeent eactos and schemes fo maximizing the desied poduct (a) STR (b) tubula eacto (c ) batch (d) semi-batch (e) semi-batch (f) Tubula eacto with side steams (g) Tubula eacto with side steams (h) Seies of small STRs
Figue 6- iffeent eactos and schemes fo maximizing the desied poduct The two eactos with ecycle shown in (i) and (j) can be used fo highly exothemic eactions. Hee ecycle steam is cooled and etuned to the eacto to dilute and cool the inlet steam theeby avoiding hot spots and un-away eactions. The PFR with ecycle is used fo gas-phase eactions, and the STR is used fo liquid-phase eactions. (i) Tubula eacto with ecycle (j) STR with ecycle
Figue 6- iffeent eactos and schemes fo maximizing the desied poduct The last two eactos, () and (l), ae used fo themodynamically limited eactions whee the equilibium lies fa to the left (eactant side) + + nd one of the poducts must be emoved (e,g., ) fo the eaction to continue to completion. The membane eacto () is used fo themodynamically limited gas-phase eactions, while eactive distillation (l) is used fo liquid-phase eaction when one of the poducts has a high volatility (e.g., ) than the othe species in the eacto. + () Membane eacto (l) Reactive distillation
Example 6-: Minimizing unwanted poducts fo two eactants Fo the paallel eactions, conside all possible combinations of eaction odes and select the eaction scheme that will maximize S /. + (esied Poduct) (ndesied Poduct) ase I : >, >, a = - > 0, b = - > 0 the ate selectivity paamete S a b To maximize the S, maintain the concentation of both and as high as possible a tubula eacto (Figue 6. (b)) a batch eacto (Figue 6. (c)) high pessues (if gas phase), and educe inets
Example 6-: Minimizing unwanted poducts fo two eactants fo the paallel eactions + (esied Poduct) ) (ndesied Poduct) ase II : >, <, a = - > 0, b = - > 0 the ate selectivity it paamete a S b To maximize the S, maintain high and low. a semibatch eacto in which is fed slowly into. (Figue 6.(d)) a tubula eacto with side steam of continually (Figue 6.(f)) a seies of small STRs with fed only to the fist eacto and small fed to each eacto. (Figue 6.(h))
Example 6-: Minimizing unwanted poducts fo two eactants fo the paallel eaction + (esied Poduct) (ndesied Poduct) ase III : <, <, a = - > 0, b = - > 0 the ate selectivity paamete S a b To maximize the S, maintain the concentation of both and as low as possible astr(figue6(a)) 6.(a)) a tubula eacto in which thee is a lage ecycle atio (Figue 6.(i)) a feed diluted with inet mateial low pessues (if gas phase)
Example 6-: Minimizing unwanted poducts fo two eactants fo the paallel eaction + (esied Poduct) ) (ndesied Poduct) ase IV : <, >, a = - > 0, b = - > 0 the ate selectivity it paamete b S a To maximize the S, maintain the concentation of both and as high as possible a semibatch eacto in which is slowly fed to a lage amount of (Figue 6.(e)) a membane eacto o tubula eacto with side steam of (Figue 6.(g)) a seies of small STRs with fesh fed to each eacto (Figue 6.(h))
6. Maximizing the desied poduct in seies eaction In paallel xns, maximize the desied poduct by adjusting the eaction conditions (e.g., ) by choosing the pope eacto In seies xns, maximize the desied poduct by adjusting the space-time fo a flow eacto by choosing eal-time fo a batch eacto
Maximizing the desied poduct in seies eaction esied Poduct If the fist eaction is slow and second eaction is fast, it will be extemely difficult to poduce species. If the fist eaction (fomation of ) is fast and the eaction to fom is slow, a lage yield of can be achieved. Howeve, if the eaction is allowed to poceed fo a long time in a batch eacto o if the tubula flow eacto is too long, the desied poduct will be conveted to. In no othe type eaction is exactness in the calculation of the time needed to cay out the eaction moe impotant than in seies eactions.
Example 6-4: Maximizing the yield of the intemediate poduct The oxidation of ethanol to fom acetaldehyde is caied out on a catalyst of 4 wt% u-wt% on l O. nfotunately, acetaldehyde is also oxidized on this catalyst to fom cabon dioxide. The eaction is caied out in a dilute concentations (ca. 0.% ethanol, % O, and 98.9% N ). onsequently, the volume change with the eaction can be neglected. etemine the concentation of acetaldehyde as a function of space time. H H OH(g) H HO O ethanol O H O acetaldehyde 5 O H O The xns ae ievesible and fist-ode in ethanol and acetaldehyde, espectively.
Solution
0 e τ
e τ e 0 τ
Reaction paths fo diffeent s in seies eaction Fo / >, a Lage quantity of an be obtained Fo / <, a Little quantity of an be obtained ' ' ' ~ st xn is slow nd xn is fast Long xn time in batch o long tubula eacto will be conveted to