Chap6Lec1 - CHAPTER 6 MULTIPLE REACTIONS Chemical plant:...

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CHAPTER 6 MULTIPLE REACTIONS Chemical plant: Economic success: Objective: Minimize undesired side reactions. Need General Mole Balances for multiple reactions In parallel reactions (competing reactions) Example: oxidation of ethylene to ethylene oxide In series reactions (consecutive), reactant forms an intermediate A B C Example: Reaction of ethylene oxide (EO) to form mono-, di-, and triethanolamine: According to the industrial needs, the desired product is diethanolamine: Multiple reactions involving series and parallel reactions: formation of butadiene from ethanol A + B C + D A + C E C 2 H 5 OH C 2 H 4 + H 2 O B k 1 A k 2 C 2CO 2 + 2H 2 O CH 2 =CH 2 + O 2 O CH 2 -CH 2 O CH 2 -CH 2 + NH 3 HOCH 2 CH 2 NH 2 EO (HOCH 2 CH 2 ) 2 NH EO (HOCH 2 CH 2 ) 3 N
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C 2 H 5 OH CH 3 CHO + H 2 (acetaldehyde) C 2 H 4 + CH 3 CHO C 4 H 6 + H 2 O Independent reactions (cracking of oil) A B C D + E In parallel reactions A → 1 k D A → 2 k U The objective is to maximize the desired product, D over the undesired product U. The cost of separation could be very high. Reactor system Separator U D Total cost Reactor cost Separator cost Cost Low High Fig. 6-2 Efficiency of reactor system
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6.1 Maximizing the Desired Product in Parallel Reactions Consider the competing reactions A → D k D (desired) 1 α A D D C k r = A → U k U (undesired) 2 A U U C k r = The rate of disappearance of A 2 1 A U A D A C k C k r + = - We would like to increase the rate r D over r U . Taking the ratio of the two rates, one finds the expression to maximize: 2 1 - = = A U D U D DU C k k r r S 6.1.1 Maximizing S for one reactant S: selectivity parameter, or instantaneous selectivity
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Chap6Lec1 - CHAPTER 6 MULTIPLE REACTIONS Chemical plant:...

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