Page 33 - Plant design and economics for chemical engineers

1 6 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

Valuable information on material and energy balances can be obtained, and
process conditions can be examined to supply data on temperature and pressure
variation, yields, rates, grades of raw materials and products, batch versus
continuous operation, material of construction, operating characteristics, and
other pertinent design variables.

Design
If sufficient information is available, a preliminary design may be developed in
conjunction with the preliminary feasibility survey. In developing the prelimi-
nary design the chemical engineer must first establish a workable manufacturing
process for producing the desired product. Quite often a number of alternative
processes or methods may be available to manufacture the same product.
Except for those processes obviously undesirable, each method should be given
consideration.
The first step in preparing the preliminary design is to establish the bases
for design. In addition to the known specifications for the product and availabil-
ity of raw materials, the design can be controlled by such items as the expected
annual operating factor (fraction of the year that the plant will be in operation),
temperature of the cooling water, available steam pressures, fuel used, value of
by-products, etc. The next step consists of preparing a simplified flow diagram
showing the processes that are involved and deciding upon the unit operations
which will be required. A preliminary material balance at this point may very
quickly eliminate some the alternative cases. Flow rates and stream conditions
for the remaining cases are now evaluated by complete material balances,
energy balances, and a knowledge of raw-material and product specifications,
yields, reaction rates, and time cycles. The temperature, pressure, and composi-
tion of every process stream is determined. Stream enthalpies, percent vapor,
liquid, and solid, heat duties, etc., are included where pertinent to the process.
Unit process principles are used in the design of specific pieces of
equipment. (Assistance with the design and selection of various types of process
equipment is given in Chaps. 14 through 16.) Equipment specifications are
generally summarized in the form of tables and included with the final design
report. These tables usually include the following:

1. Cofumns (distillation). In addition to the number of plates and operating
conditions it is also necessary to specify the column diameter, materials of
construction, plate layout, etc.
2. Vessels. In addition to size, which is often dictated by the holdup time
desired, materials of construction and any packing or baffling should be
specified.
3. Reactors. Catalyst type and size, bed diameter and thickness, heat-inter-
change facilities, cycle and regeneration arrangements, m?terials of construc-
tion, etc., must be specified.