If there are multiple rxns, use concentrations not conversions. These might occur in combination or by themselves.

Size: px

Start display at page:

Download "If there are multiple rxns, use concentrations not conversions. These might occur in combination or by themselves."

Nigel Terry
5 years ago
Views:

1 hapte 6 MLTIPLE RETIONS If thee ae multiple xns, use concentations not convesions. intemediate. Seies Reactions onsecutive xns. Paallel Reactions. omplex Reactions: Seies and Paallel 4. Independent None of the poducts o eactants ae common. These might occu in combination o by themselves. esied & ndesied Rxns We want to get the desied poduct, and avoid the fomation of undesied one,. In the paallel xn In seies ost Total ost Reacto ost Reacto Sepeato Sepeato ost Low Efficiency High

2 Selectivity and Yield Selectivity and Yield Selectivity tells us how one poduct is favoed ove anothe when multiple xns tae place Exit flow ate o batch tend N N S ~ Reaction yield atio of the xn ate of a given poduct to the xn ate of ey eactant. etemine the instantaneous selectivity, S /, fo the liquid phase eactions: Setch the selectivity as a function of the concentation of. Is thee an optimum and if so what is it? S / [ ] [ ] * * * * d ds ind the optimum concentation

3 Paallel Rxns: o the following competing xns (desied) (undesied ) α α Rate of disappeaa nce of - α α whee α and α ae positive xn odes. S / α α Maximizing the esied Poduct fo one Reactant ase: α If S/ α α > α S/ as ase: If α < α S/ S/ as α α In ase : If gas xn, we should un it without any inets and at high pessues to eep high. If liquid phase xn, the diluents should be minimum. STR should not be used, batch o plug flow should be used. In ase : ilute the feed with inets. STR should be used, because deceases apidly.

4 Effect of Tempeatue on Selectivity It can be detemined fom S / ~ e whee : feq facto, -([E -E ]/RT) E : activation enegy E > E S / Rxn should be opeated at the highest possible T to max. S / T E < E S / T. Seies Reactions This seies eaction could also be witten as Reaction () Reaction () Species : Species : 4

5 sing the integating facto, i.f.: at t, When should you stop the eaction to obtain the maximum amount of? Let's see Then nd 5

6 The Integating acto () If you only had an expession of the fom () things would be much easie, then you could integate with espect to z and find y(z). then you could manipulate equation (): () whee the tem in bacets is the left hand side of equation (). You need du/dz u f(z). Recall (4) (5) If you define and f(z) dq/dz (i.e., then This satisfies the condition that du/dzu f(z). Theefoe, and substituting into equation (4), (6) (7) whee the tem in bacets is the left hand side of equation (). ONLSION: If you poblem is of the fom you can multiply both sides of the equation by the 6

7 (which you should be able to evaluate, since you now f(z)), to yield o, substituting fom equation (7) eivation f(t), so om equation (), the solution is then The constant can be obtained fom the intitial condition that at t, ; 7

8 8. lgoithm fo Multiple Reactions Reacto Selection & Opeating onditions: Reacto Selection & Opeating onditions: onside two simultaneous xns ( ) ( ) / S β β α α β α β α We usually want to max S /. So the eactos should be designed to max S / H>> H>> Themodinamically limited xns <->

9 lgoithm fo Solution of omplex Reactions: In combinations of paallel and seies xns, OE solve pacages mae life easie. Wite the xns Wite the mole balances Net ate laws Stoichiomety ombine & Solve 4. pplications of lgoithm ase : PR () () Mole alances : d d : d d : d d : d d Rate Laws NOTE: The specific eaction ate is defined with espect to species. NOTE: The specific eaction ate is defined with espect to species. 9

10 Species o eaction (): o eaction (): Species Species Species Species - - (- ) Witing the net ates Stoichiomety ombine Species Species Species Species liquid : v v v d d v d d d d v d d v d d v d d d d v d d v v v Evaluate solution using Polymath of MatLab.5. v t;, 4, 4,, f 5 dm

11 ase : STR Species Species Species Species ( - ) v( - ) - ( - ) v( - ) - ( - ) v - ( - ) v - We will specify,, along with the specific eaction ates ij. This fomulation leaves us with fou equations and fou unnowns (,, and ) f( f( f( f( ) ) - - ) [ ] v ( [ ] ) v - ) v v ( - )

12 ase : Semibatch Liquid Phase ase : Semibatch Liquid Phase t v dt d Species dt d Species ] [ dt d Species dt d Species v v v v 8 h t 4 conditions Initial dm dm mol 4 h dm. v h mol dm h mol dm.5 f i i i i 4 6 Paametes PolyMath Solutions

13 THE LGORITHM Mole balances Rate Laws Relative ates Rxn () -> Net ates Rxn () -> Species Species Species Species Species Species Species Species Stoichiomety T P T P T T v P T P T T T

14 4,,,,, ond. Initial.8..5, Paametes d d d d d d d d T T, P P, P f T T T T 5 T T T 5 T T T T T T T T i T i ombine Species Species Species Species T - X ive equations five unnowns : Total - ) - ( Species - ) - ( Species - ) - ( Species - ) - ( Species o a STR o a STR

15 oncentation-time Tajectoies. The following concentation-time tajectoies wee obseved in a batch eacto Which of the following eaction pathways best descibes the data. Setch the concentation-time tajectoy fo the eaction Solution: Pat hoice is the answe. hoices and ae incoect because they show species eventually consumed, which is clealy not the case 5

16 Solution: Pat () vitually consumed so no moe can be poduced in eaction. () Rates of onsumption of and a ae vitually the same. () Rate of consumption of geate than that of owing to Reaction 6

Youn-Woo Lee School of Chemical and Biological Engineering Seoul National University , 599 Gwanangro, Gwanak-gu, Seoul, Korea

Youn-Woo Lee School of Chemical and Biological Engineering Seoul National University , 599 Gwanangro, Gwanak-gu, Seoul, Korea 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