Page 204 - Elements of Chemical Reaction Engineering 3rd Edition

176 Isothermal Reactor Design Chap. 4

The operating costs include such costs as energy, labor, overhead, and depre-
ciation of equipment. You will learn more about these costs in your senior
design course. While most if not all of the streams from the separators could
be recycled, lets consider what the profit might be if the streams were to go
unrecovered. Also, let's conservatively estimate the operating and other
expenses to be $8 million per year and calculate the profit, Your design
instructor might give you a better number. The prices of ethane, sulfuric acid,
and ethylene glycol are $0.04, $0.043, and $0.38 per-pound, respectively. See
"http://www.chemweek.com/" for current prices.
For a feed of 4 million pounds per year and a production rate of 2 mil-
lion pounds of ethylene glycol per year:

(Ethylene Glycol Cost) - (Ethane Cost)

(Sulfuric Acid Cost) - Operating Cost
1

X 2.26 X 106 $8,000,OO0

= $76,000,000 - $16,000,000 - $54,000 - $8,000,000
= $52 million
Using $52 million a year as a rough estimate of the profit, you can now make
different approximations about the conversion, separations, recycle streams,
and operating costs to learn how they affect the profit.

4.6 Using C, (liquid) and FA (gas) in the Mole
Balances and Rate Laws
Multiple rxns There are a number of instances when it is much more convenient to work in
Membranes terms of the number of moles (NA, NB) or molar flow rates (FA, FBI etc.) rather
than conversion. Membrane reactors and multiple reactions taking place in the
gas vhase are two such cases where molar flow rates rather than conversion are
- I
preferred. In Section 3.4 we described how we can express concentrations in
terms of the molar flow rates of the reacting species rather than conversion. We
will develop our algorithm using concentrations (liquids) and molar flow rates
(gas) as our dependent variables. The main difference is that when conversion
is used as our variable to relate one species concentration to that of another
species concentration, we needed to write a mole balance on only one species,
our basis of calculation. When molar flow rates and concentrations are used as
our variables, we must write a mole balance on each species and then relate
the mole balances to one another through the relative rates of reaction; for
b c d
-C+-D
A+-B __j a a
a