Page 143 - Plant design and economics for chemical engineers

118 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

large. For instance, Perry’s Handbook shows 15 different ways just to separate a
solid from a liquid.?
Chemical engineers have traditionally synthesized process flow sheets
using experience, insight, invention, and sequential evaluation of alternatives.
Recently there has been some analysis and computerization of this process.
Douglas and co-workers have prepared a program, PIP-Process Invention
Program-available through CACHE, that performs process synthesis.* The
core of this approach is to build up a process in steps of increasing complexity,
applying scientific principles and heuristics (rules of thumb) at each step. It is
anticipated that computer-aided process synthesis will see a rapid growth in the
future. Computer-aided process synthesis is illustrated by the following example
using a process that is available on the PIP program.

Example 2 Synthesis of a styrene process. Styrene, the monomer of polystyrene,
has enjoyed strong market growth over the past two decades. It is prepared
starting with benzene and ethylene which react to form ethylbenzene; the
ethylbenzene is dehydrogenated to yield styrene. Further information about styrene
manufacture, properties, and uses is available.3 In this example, the steps involved
in synthesizing a process to produce styrene from ethylbenzene will be illustrated.
The procedure followed is analogous to that followed by the PIP program.
Experience will be cited frequently in this example for guidance of the
synthesis procedure. In a computer process-synthesis program the guidance is
provided by a set of heuristics summarizing the experience. Douglas and co-workers
provide information about such heuristics.ll The PIP program displays, at the
request of the user, the heuristics used in selecting processing steps.

Solution
Step 1. Inputs and outputs. Establish whether the value of the reaction
products exceeds the value of the reactants. The chemical reaction is written and
balanced, as
C,Hs----C,H, + C,H,-C,H, + H, (1)
ethylbenzene styrene hydrogen
Values of $0.42/lb for styrene, $0.25/lb for ethylbenzene, and $0.30/lb for
hydrogen are used (the hydrogen value is based on its heat of combustion and a
heat value of $5/million Btu). 1 lb-mol each of styrene (104 lb) and hydrogen

tR. H. Perry and D. Green, eds., “Perry’s Chemical Engineers’ Handbook,” 6th ed., Fig. 19-133, pp.
19-108, McGraw-Hill Book Company, New York, 1984.
SJ. M. Douglas and R. L. Kirkwood, Design Education in Chemical Engineering, Part 2: Using
Design Tools, Chem. Eng. Ed., 23(2):120-127 (Spring, 1989); R. L. Kirkwood, J. M. Douglas, and
M. H. Locke, A Prototype Expert System for Synthesizing Chemical Process Flowsheets, Comput.
and Chem. Erg., 12(4):329-343 (1988).
SM. Grayson and D. Eckroth, eds., “Kirk-0thmer Encyclopedia of Chemical Technology,” 3rd ed.,
Vol. 21, pp. 770-801, 1983.
15. M. Douglas and R. L. Kirkwood, Design Education in Chemical Engineering, Part 1: Deriving
Conceptual Design Tools, Chem. Eng. Ed., 23(1):22-25 (winter, 1989).