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Multiple Reaction Conversion Notes

Multiple Reaction Conversion Notes - ChE ChE 400 Reactive...

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L11 - 1 ChE 400 - Reactive Process Engineering ChE ChE 400 400 - Reactive Process Engineering Reactive Process Engineering Multiple Reactions We have largely considered single reactions so far in this class How many industrially important processes involve a single reaction? The job of a chemical engineer is therefore to design
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L11 - 2 ChE 400 - Reactive Process Engineering ChE ChE 400 400 - Reactive Process Engineering Reactive Process Engineering Multiple Reactions We need to develop tools that will allow us to quantify how well (or how poorly) we are doing at producing a desired product There are three concepts that we need to develop for multiple reactions: Conversion (similar to single reactions) X j = – Selectivity S j = – Yield Y j =
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L11 - 3 ChE 400 - Reactive Process Engineering ChE ChE 400 400 - Reactive Process Engineering Reactive Process Engineering C 2 H 2 2 - > 2 > 2 C (s ) + H + H 2 2 (VII) coke formation (explosion!) (VII) coke formation (explosion!) Multiple Reactions So far, we have exclusively looked at simple system with only one reaction occuring. However, many reaction systems of practical relevance involve many reactions occuring at the same time, either in parallel or in series (i.e. sequentially). Let’s look at steam cracking of ethane steam cracking of ethane (~ 80 bio. to/a world production!): C 2 H 6 -> C 2 H 4 + H 2 (I) ethene (ethylene) formation C 2 H 6 -> C 2 H 2 + 2 H 2 (II) acetylene formation C 2 H 4 - > C > 2 H 2 + H H 2 (III) acetylene formation from ethylene acetylene formation from ethylene C 2 H 6 + H 2 -> 2 CH 4 (IV) methane formation 3 C 2 H 6 -> C 6 H 6 + 6 H 2 (V) benzene formation C 2 H 6 -> 2 C (s) + 3 H 2 (VI) coke formation C (s ) + H + H 2 O - > CO + H > CO + H 2 2 (VIII) coke gasification (VIII) coke gasification We distinguish between parallel reactions and series reactions .
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L11 - 4 ChE 400 - Reactive Process Engineering ChE ChE 400 400 - Reactive Process Engineering Reactive Process Engineering Conversion, Selectivity, Yield Conversion: j X = Simple example: A -> B; A -> C B S = Selectivity: ( remark : you must be sure that you know all products!) or: B S = preferred form ! Yield: Y B = X A . S B Typically, selectivity selectivity is the crucial quantity! ( caveat : note the formulation with mol numbers, not concentrations! -> Why?!) Production rate: F B = Y B F A0 (and similar for C) Why?
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