Page 161 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

• In general, pressures between 1 and 10 bar and temperatures between 40°C and 260°C do not
cause severe processing difficulties.

The rationale for the conditions given in the second generalization are explained below.

6.1.1 Pressure

There are economic advantages associated with operating equipment at greater than ambient pressure
when gases are present. These result from the increase in gas density and a decrease in gas volume with
increasing pressure. All other things being equal, in order to maintain the same gas residence time in a
piece of equipment, the size of the equipment through which the gas stream flows need not be as large
when the pressure is increased.

Most chemical processing equipment can withstand pressures up to 10 bar without much additional
capital investment (see the cost curves in Appendix A). At pressures greater than 10 bar, thicker walled,
more expensive equipment is necessary. Likewise, operating at less than ambient pressure (vacuum
conditions) tends to make equipment large and may require special construction techniques, thus
increasing the cost of equipment.

A decision to operate outside the pressure range of 1 to 10 bar must be justified.

6.1.2 Temperature

There are several critical temperature limits that apply to chemical processes. At elevated temperatures,
common construction materials (primarily carbon steel) suffer a significant drop in physical strength and
must be replaced by more costly materials. This drop in strength with temperature is illustrated in
Example 6.1.

Example 6.1

The maximum allowable tensile strengths for a typical carbon steel and stainless steel, at ambient
temperature, 400°C, and 550°C are provided below (Walas [1]).

Determine the fractional decrease in the maximum allowable tensile strength (relative to the strength at
ambient conditions) for the temperature intervals (a) ambient to 400°C and (b) 400°C to 550°C.
a. Interval: ambient to 400°C: