Page 47 - Chemical engineering design

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CHEMICAL ENGINEERING
plus non-productive periods.
8760 ð plant attainment
Batches per year D 1.5
batch cycle time
where the “plant attainment” is the fraction of the total hours in a year (8760) that the
plant is in operation.
Annual production D quantity produced per batch ð batches per year.
annual cost of production
Cost per unit of production D 1.6
annual production rate
With many batch processes, the production rate will decrease during the production
period; for example, batch reactors and plate and frame filter presses, and there will
be an optimum batch size, or optimum cycle time, that will give the minimum cost per
unit of production.
For some processes, though they would not be classified as batch processes, the period
of continuous production will be limited by gradual changes in process conditions; such
as, the deactivation of catalysts or the fouling of heat-exchange surfaces. Production will
be lost during the periods when the plant is shut down for catalyst renewal or equipment
clean-up, and, as with batch process, there will be an optimum cycle time to give the
minimum production cost.
The optimum time between shut-downs can be found by determining the relationship
between cycle time and cost per unit of production (the objective function) and using one
of the optimisation techniques outlined in this section to find the minimum.
With discontinuous processes, the period between shut-downs will usually be a function
of equipment size. Increasing the size of critical equipment will extend the production
period, but at the expense of increased capital cost. The designer must strike a balance
between the savings gained by reducing the non-productive period and the increased
investment required.

1.11. REFERENCES

ALLEN, D. H. (1971) Brit. Chem. Eng. 16, 685. Linear programming models.
ARIS, R. (1963) Discrete Dynamic Programming (Blaisdell).
BAASEL, W. D. (1965) Chem.Eng.,NY 72 (Oct. 25th) 147. Exploring response surfaces to establish optimum
conditions.
BAASEL, W. D. (1974) Preliminary Chemical Engineering Plant Design (Elsevier).
BEIGHTLER,C.S. and WILDE, D. J. (1967) Foundations of Optimisation (Prentice-Hall).
BELLMAN, R. (1957) Dynamic Programming (Princeton University, New York).
BERGE, C. (1962) Theory of Graphs and its Applications (Wiley).
BEVERIDGE, G.S.G.and SCHECHTER, R. S. (1970) Optimisation: Theory and Practice (McGraw-Hill).
BOAS, A. H. (1963a) Chem. Eng., NY 70 (Jan. 7th) 95. How to use Lagrange multipliers.
BOAS, A. H. (1963b) Chem. Eng., NY 70 (Feb. 4th) 105. How search methods locate optimum in univariate
problems.
BURKLIN, C. R. (1979) The Process Plant Designers Pocket Handbook of Codes and Standards (Gulf).
CASEY,R.J. and FRAZER, M. J. (1984) Problem Solving in the Chemical Industry (Pitman).
CHADDOCK, D. H. (1975) Paper read to S. Wales Branch, Institution of Mechanical Engineers (Feb. 27th).
Thought structure, or what makes a designer tick.