Equilibrium & Multiple Reactions
Chemical Equilibrium
Some chemical reactions are irreversible -- the reaction proceeds from
reactants to products, and given enough time will eventually stop. If
the reactants are present in stoichiometric amounts, these reactions may
proceed to completion.
Many other reactions are reversible -- the reaction conditions determine how far,
and which direction, the reaction proceeds.
If the reaction mixture is held under controlled conditions, eventually it will
balance out to a fixed composition. This "long time" condition is called
equilibrium, and the "equilibrium composition" ("steady state composition") is
of great importance.
We won't spend much effort defining or studying equilibrium -- that can wait for
kinetics -- but we will make a lot of use of the equilibrium
constant. For ideal mixtures of ideal gases, an equilibrium
constant can be written in terms of the mole fractions.
Other forms of the equilibrium constant are written in terms of partial
pressures -- and in later classes we'll prefer those forms.
This information can be used to specify a relationship between two stream
compositions; the equilibrium constant imposes a composition specification on a
problem.
Multiple Reactions
In many processes, you put your reactants together and try to make a product.
Often, there are "side reactions" and "byproducts" to worry about.
For instance, consider the combustion of methane:
The complete combustion reaction to form carbon dioxide is more desirable than
incomplete combustion to carbon monoxide.
Yield and Selectivity are defined to help us quantify the
impact of competing reactions:
Usually, high yields and selectivities are good.
WARNING!: Different authorities use different definitions of
some of these terms. If you bring information in from an outside source, be
sure you know how it defines selectivity, etc. For instance, some authors
(Himmelblau?) define yield as "moles product/moles reactant".
Approaches to Multiple Reaction Problems
I can think of four approaches to use in solving problems with multiple
reactions.
- Blind Faith and Fortune. The student's favorite. Hope that you can guess
the right thing to do. Success does not correlate with skill.
- Direct application of stoichiometry. Best used when reactions are distinct
(not coupled). You need specific information on stoichiometry, conversion, and
full compositions of at least one stream.
- Atomic Species Balances. You need to be able to completely track one
species throughout the system. Particularly useful when a species enters in a
single component and leaves in several (or vice versa), as with
carbon in complete combustion of multiple fuels.
- Extent of Reaction. Good if you know one side of the process (the feed or
product stream) completely and hove one constraint (conversion, yield,
equilibrium constant, etc.) for each reaction.
You should already know how to do the first three. We'll develop the extent of
reaction approach as the next topic.
Other hints:
- Look for streams that are missing components. They are often useful in
deciding which balances to write.
- Look for inerts and "fellow travelers". Components which go through the
reactor without changing often provide good clues to total stream flowrates.
- Do NOT pick a method and apply it to a problem. Let the problem determine
the method.
References:
- Felder, R.M. and R.W. Rousseau, Elementary Principles of Chemical Processes, 2nd
Edition, John Wiley, 1986, pp. 124-29.
- Felder, R.M. and R.W. Rousseau, Elementary Principles of
Chemical Processes, 2005 3rd Edition, 2005, p. 121-23.
R.M. Price
Original: 6/14/94
Modified: 10/6/98; 1/7/2005
Copyright 1998, 2005 by R.M. Price -- All Rights Reserved
