Page 152 - Plant design and economics for chemical engineers

126 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

Flow-sheeting is applied to steady-state conditions. The unsteady-state
features of normally steady-state processes-start up, shutdown, disturbances,
control actions-are usually analyzed separately from the process-design task.
This results in a major simplification of the design problem, because time
variations and integration with respect to time need not be performed (although
integration with respect to space may still be required in models for particular
operations, such as that of a plug-flow chemical reactor). The flow-sheeting
language SPEED-UP is an exception in that it includes unsteady-state capability
in its code.?
The solution to the problem is obtained by solving mass and energy
balances to yield the quantity and state (i.e., composition, temperature, pres-
sure) of all the streams and the utility requirements. Additional parameters for
the process equipment, sufficient so that stream specifications are met and the
cost of the equipment can be estimated, are calculated. The cost of equipment,
raw materials, and utilities is estimated and an economic analysis is carried out.
Methods of cost estimation and economic analysis are presented later in this
text. This entire procedure may be repeated many times to examine modifica-
tions of the process flow sheet or to find optimal values of key process variables.
Computer software can greatly simplify these repetitive calculations for the
engineer. But even without the need for repetition, the software may simplify
the calculations and provide detail and accuracy that would have been impossi-
ble otherwise.
Certain features of chemical process calculations contribute to their dif-
ficulty, complexity, and challenge. These include: large sets of nonlinear, alge-
braic equations, the need for large amounts of physical and chemical property
data, the presence of operations that require very complex models, and the
occurrence of recycle streams. The property issue has been discussed and the
other matters are discussed in the following sections.
The flow-sheeting program FL,OWTRAN is available to universities
through CACHE. This program was developed by the Monsanto Company in
the 1960s and made available for university use in 1973. FLOWTRAN is well
documented with a manual and examples.+ It can be installed on many main-
frame computers, but not on personal computers. FLOWTRAN is available
through Chemical Engineering Departments at most universities. CHEMCAD
IITM is a flow-sheeting program for personal computers offered by
COADE/Chemstations, Inc. PROCESS TM, the product of Simulation Sciences,

t.J. D. Perkins and R. W. H. Sargent, SPEED-UP: A Computer Program for Steady-State and
Dynamic Simulation and Design of Chemical Processes, “Selected Topics on Computer-Aided
Process Design and Analysis,” R. S. H. Mah and G. V. Reklaitis, eds., AIChE Symposium Series,
Vol. 78, No, 214, American Institute of Chemical Engineers, New York, 1982.
$J. D. Seader, W. D. Seider, and A. C. Pauls, “FLOWTRAN Simulation--An Introduction,” 3rd
ed., CACHE, 1987; J. P. Clark, T. P. Koehler, and J. T. Sommerfeld, “Exercises in Process
Simulation Using FLOWTRAN,” 2nd ed., CACHE, 1980.