Page 138 - Plant design and economics for chemical engineers

COMPUTER-AIDED DESIGN 113
ASCII? data files (for easy transportability of data into and out of the
spread sheet)
Iteration (repeat of the calculations automatically or under user control)
Multiple buffers (accommodation of several spread sheets at once)
Virtual memory (allows a microcomputer to act as though it were a much
larger machine)
Macro capability (permits a user to write and use a procedure)
Functions (e.g., statistical, financial, data regression, matrix operations,
and database functions)
Graphical output.

Spread-sheeting has been found to be especially useful for the mass and
energy balancing, approximate sizing of equipment, cost estimating, and eco-
nomic analysis steps of process design. It is less useful for more detailed
equipment design, because the complex algorithms usually necessary for this
step can be difficult to incorporate into a spread sheet. Examples of chemical
engineering design applications of spread-sheeting software may be found in the
1iterature.S Julian illustrates how to draw a process flow sheet using Lotus
1-2-3TM as well as showing several examples of process mass balances.0
A spread-sheet calculation applied to a chemical-process mass balance, along
with some tips for spread sheet use, is shown in the following ‘example.

Example 1 Reactor mass balances. The synthesis of methanol from carbon mon-
oxide and hydrogen by the reversible reaction
CO + 2H, + CH,OH
is to be conducted continuously. The reactor is followed by a condenser and drum
that separate the methanol product from the unreacted carbon monoxide and
hydrogen. A process flow sheet is shown in Fig. 4-1.
Mass balances are to be solved for this process. Perfect separation between
methanol and the reactants is assumed. Unreacted reactants are recycled to the
reactor to improve their utilization. The recycle stream within the process
complicates solving the mass balances, for there is a circularity in the logic of the
solution. The mass balance equations must be solved simultaneously rather than
singly, or solved iteratively, as is done with a spread sheet.
It is recommended that the spread-sheet solution be built up by increments
rather than trying to solve the complete problem in a single step. As an illustration,
this example will be solved first without the complication of the recycle, then the
recycle wil1 be added.

tAmerican Standard Code for Information Interchange.
SE. A. Grulke, “Using Spreadsheets for Teaching Design,” Chem. Eng. Ed. XX(3):128-131 + 153
(summer, 1986); W. P. Schmidt and R. S. Upadhye, Material Balances on a Spreadsheet, Chem.
Eng., 91(26):67-70 (1984).
SF. M. Julian, Flowsheets and Spreadsheets, Chem. Eng. Prog., 81(9):35-39 (1985).