Page 73 - Chemical engineering design

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CHEMICAL ENGINEERING
500,000
1000 C
0.5 D
45,000 ln 1000 100
1000 C

0.045 D ln
900
e 0.045 ð 900 D 1000 C
C D 140 ppm
So the maximum allowable concentration will not be exceeded.
2.18. GENERAL PROCEDURE FOR MATERIAL-BALANCE
PROBLEMS

The best way to tackle a problem will depend on the information given; the information
required from the balance; and the constraints that arise from the nature of the problem.
No all embracing, best method of solution can be given to cover all possible problems.
The following step-by-step procedure is given as an aid to the efficient solution of material
balance problems. The same general approach can be usefully employed to organise the
solution of energy balance, and other design problems.

Procedure

Step 1. Draw a block diagram of the process.
Show each significant step as a block, linked by lines and arrows to show the
stream connections and flow direction.
Step 2. List all the available data.
Show on the block diagram the known flows (or quantities) and stream compo-
sitions.
Step 3. List all the information required from the balance.
Step 4. Decide the system boundaries (see Section 2.6).
Step 5. Write out all the chemical reactions involved for the main products and by-
products.
Step 6. Note any other constraints,
such as: specified stream compositions,
azeotropes,
phase equilibria,
tie substances (see Section 2.11).
The use of phase equilibrium relationships and other constraints in determining stream
compositions and flows is discussed in more detail in Chapter 4.
Step 7. Note any stream compositions and flows that can be approximated.
Step 8. Check the number of conservation (and other) equations that can be written, and
compare with the number of unknowns. Decide which variables are to be design
variables; see Section 2.10.
This step would be used only for complex problems.