CHAPTER 1-CHE112P - C HE112P CALCU LATIONS OF ChE C HAPTER...

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CHE112P – CALCULATIONS OF ChE [ CHAPTER 1: MASS BALANCE WITH REACTION ] CHAPTER 1: MASS BALANCE WITH REACTION 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
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CHE112P – CALCULATIONS OF ChE [ CHAPTER 1: MASS BALANCE WITH REACTION ] 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 O o
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