Page 162 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Carbon Steel: (1190–970)/1190 = 0.18
Stainless Steel: (1290–1290)/1290 = 0.0
b. Interval: 400°C to 550°C:
Carbon Steel: (970–170)/1190 = 0.67
Stainless Steel: (1290–430)/1290 = 0.67

Example 6.1 has shown that carbon steel suffers a loss of 18%, and stainless steel suffers no loss in
tensile strength, when heated to 400°C. With an additional temperature increase of 150°C to 550°C,
stainless steel suffers a 67% loss while carbon steel suffers an additional 67% loss in strength. At
operating temperatures of 550°C, carbon steel has a maximum allowable tensile strength of about 15% of
its value at ambient conditions. For stainless steel, the maximum allowable strength at 550°C is about
33% of its ambient value. For this example, it is clear that carbon steel is unacceptable for service
temperatures greater than 400°C, and that the use of stainless steel is severely limited. For higher service
temperatures, more exotic (and expensive) alloys are required and/or equipment may have to be

refractory lined.

A decision to operate at greater than 400°C must be justified.

Thus, if we specify higher temperatures, we must be able to justify the economic penalty associated with
more complicated processing equipment, such as refractory-lined vessels or exotic materials of
construction. In addition to the critical temperature of 400°C, there are temperature limits associated with
the availability of common utilities for heating and cooling a process stream.
Steam: High-pressure steam between 40 and 50 bar is commonly available and provides heat at 250
to 265°C. Above this temperature additional costs are involved.

Water: Water from a cooling tower is commonly available at about 30°C (and is returned to the
cooling tower at around 40°C). For utilities below this temperature, costs increase due to
refrigeration. As the temperature decreases, the costs increase dramatically (see Table 8.3).

If cryogenic conditions are necessary, there may be an additional need for expensive materials of
construction.

A decision to operate outside the range of 40°C to 260°C, thus requiring special heating/cooling

media, must be justified.

6.2 Reasons for Operating at Conditions of Special Concern

When you review the PFD for different processes, you are likely to encounter conditions in reactors and
separators that lie outside the temperature and pressure ranges presented in Section 6.1. This does not

mean to say that these are “bad” processes, but rather that these conditions had to be used, despite the
additional costs involved, to have the process operate effectively. These conditions, outside the favored
temperature and pressure ranges, are identified as conditions of special concern.

When you encounter these conditions, you should seek a rational explanation for their selection. If no
explanation can be identified, the condition used may be unnecessary. In this situation, the condition may