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Unformatted text preview: CHE112P CALCULATIONS OF ChE [ CHAPTER 1: MASS BALANCE WIT H REACT ION ] C HAP T ER 1: M ASS BA LANCE W I T H REACT IO N Differences Due to Reactions Now we are ready to include reactions in our material balances. There are two different ways to do balances with the reacting systems. One can do balances on chemical compounds or elements. Elements can't be changed (no alchemy here), but chemical reactions do interconvert one compound into Mass Balances with Reaction | Brigham Young University 1 CHE112P CALCULATIONS OF ChE [ CHAPTER 1: MASS BALANCE WIT H REACT ION ] another. If we choose to do element balances, we will have no generation terms; if we do compound balances we must include the generation/consumption terms. You need to become familiar with both methods! 1. Component Balances You can do component balances but now m cons and m gen terms need to be considered. Therefore the general mass balance for steady-state, steady- flow processes becomes: m acc = 0 = S m in + m gen - S m out - m cons (for steady-state flow) 2. Element Balances Element balances on the other hand have no generation or consumption terms and the mass balance simplifies to "in" equals "out" for continuous, steady-state processes. C 5 H 10 C 3 H 6 , C 2 H 4 Element balances are quite simple, but sometimes students get the multiplicative factors in the wrong place. Consider the reactor above which produces propylene and ethylene from pentene. The element balances in the form of "in" = "out" is based upon the number of moles of that element regardless of the compound that it is in. We must multiply the number of moles of each compound by the stoichiometric number for the element. In pentane, for example, there are 5 moles of carbon atom for every mole of pentane. Thus, the element balances become: C balance: 5 n C5H10 = 3 n C3H6 + 2 n C2H4 H balance: 10 n C5H10 = 6 n C3H6 + 4 n C2H4 We will do some element balance examples a little later on. 3. Stoichiometry Stoichiometry relates number of moles of reactants to number of moles of product. It is generally best to use moles, but remember that moles are not conserved , only mass. Thus, if doing an overall mass balance, "in" equals "out" (for continuous process). If doing a mole balance, then the generation or consumption term is governed by the stoichiometric ratios: 2C 2 H 4 + O 2 = 2C 2 H 4...
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This note was uploaded on 04/27/2011 for the course CHM 111P taught by Professor Marquez during the Spring '11 term at Mapúa Institute of Technology.

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