Page 34 - Plant design and economics for chemical engineers

PROCESS DESIGN DEVELOPMENT 17

4. Heat exchangers and furnaces. Manufacturers are usually supplied with the
duty, corrected log mean-temperature difference, percent vaporized, pres-
sure drop desired, and materials of construction.
5. Pumps and compressors. Specify type, power requirement, pressure differ-
ence, gravities, viscosities, and working pressures.
6. Instruments. Designate the function and any particular requirement.
7. Special equipment. Specifications for mechanical separators, mixers, driers,
etc.

The foregoing is not intended as a complete checklist, but rather as an
illustration of the type of summary that is required. (The headings used are
particularly suited for the petrochemical industry; others may be desirable for
different industries.) As noted in the summary, the selection of materials is
intimately connected with the design and selection of the proper equipment.
As soon as the equipment needs have been firmed up, the utilities and
labor requirements can be determined and tabulated. Estimates of the capital
investment and the total product cost (as outlined in Chap. 6) complete the
preliminary-design calculations. Economic evaluation plays an important part in
any process design. This is particularly true not only in the selection for a
specific process, choice of raw materials used, operating conditions chosen, but
also in the specification of equipment. No design of a piece of equipment or a
process is complete without an economical evaluation. In fact, as mentioned in
Chap. 1, no design project should ever proceed beyond the preliminary stages
without a consideration of costs. Evaluation of costs in the preliminary-design
phases greatly assists the engineer in further eliminating many of the alternative
cases.
The final step, and an important one in preparing a typical process design,
involves writing the report which will present the results of the design work.
Unfortunately this phase of the design work quite often receives very little
attention by the chemical engineer. As a consequence, untold quantities of
excellent engineering calculations and ideas are sometimes discarded because of
poor communications between the engineer and management.?
Finally, it is important that the preliminary design be carried out as soon
as sufficient data are available from the feasibility survey or the process-devel-
opment step. In this way, the preliminary design can serve its main function of
eliminating an undesirable project before large amounts of money and time are
expended.
The preliminary design and the process-development work gives the
results necessary for a detailed-estimate design. The following factors should be

.
tSee Chap. 13 for assistance in preparing more concise and clearer de&n rebrts.