Page 151 - Plant design and economics for chemical engineers

COMPUTER-AIDED DESIGN 125
The ethylbenzene feed needs to be heated and the reactor effluent cooled.
Heat exchange between them is feasible and desirable. Setting an approach
temperature for a countercurrent exchanger (temperature difference between
heated feed and reactor effluent) at lO”C, fixes the heated feed temperature at
590°C (1094°F). An energy balance for the heated feed and steam mixture
establishes the required steam temperature:
[m*C*(T- 1237)], + [m*C*(1094 - 1237)],, = 0
where the subscripts s and EB signify steam and ethylbenzene, respectively. Using
C, = 0.5, C,, = 0.6 Btu/lb-“F, and the flow rates calculated above gives
T(of superheated steam) = 1309°F (709°C)
An energy balance around the feed-effluent heat exchanger,
[m*C*(1112 - T)]eraue”t = [m*C*(1094 - 140)],,,
gives
T = 792°F as the outlet temperature of the effluent
The reactor effluent must reach about 40°C (104°F) for condensation. Thus,
further cooling is required, by cooling water or other process loads. Heating and
cooling for the distillation columns represent such possible loads.
Figure 4.5 shows the styrene process that has been devised as a result of this
analysis.
Step 5. Economic evaluation. Not shown in this example, but included in a
process synthesis program such as PIP, are algorithms for costing and economic
evaluation of the process. Process equipment is sized and priced and total plant
investment is estimated. Requirements and costs for utilities and raw materials,
other operating costs, and product values are estimated. These values are used to
evaluate the profitability of the proposed process and can provide a sound basis for
a more detailed design.

FLOW-SHEETING SOFTWARE

Flow-sheeting has been defined as “The use of computer aids to perform
steady-state heat and mass balancing, sizing, and costing calculations for a
chemical process.“? Flow-sheeting and process design are analogous activities.
An objective is specified, typically the desired annual production rate of the
principal products, as is the process flow sheet. The process flow sheet defines
the reactors, unit operations, and utilities needed, along with the streams of
material and energy into and out of the process and between all the process
units. Sufficient information is specified so that a unique solution exists to the
posed problem. Feed and product compositions, temperatures, and pressures
typically are specified, as are performance requirements and some operating
characteristics of the process equipment.

?A. W. Westerberg, H. P. Hutcbison, R. L. Motard, and P. Winter, “Process Flowsheeting,”
Cambridge University Press, Cambridge, England, 1979