Page 398 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

optimization (see Chapter 15).

Pressure may also need to be increased for the reactor (or, infrequently, decreased), and this requires a
pump for liquids or a compressor for gases. When there is a choice, pumps are preferable to
compressors, because the operating, capital, and maintenance costs are all lower for pumps. If pressure is
reduced between the reactor and the separator sections (or anywhere else in the process), an expander
can be considered (for gases), but often it is not economical both because of its high cost and also
because it reduces the controllability of the process. A valve allows control at a modest cost, but energy
is not recovered.

In these temperature and pressure matching sections, the lowest-cost utility should always be used. For
heating the feed to an exothermic reaction, heat integration can be used with the reactor effluent. For low-
temperature heating, low-pressure steam or another low-temperature utility is used. For safety reasons,
exothermic reactions (when reactor runaway is possible) should be run, with the reactor feed coming in at
a temperature high enough to ensure a significant reaction rate. This avoids the buildup of large
inventories of unreacted feed materials, which can happen if cold material enters the reactor and quenches
the reaction. When sufficient heat is later provided, the entire contents of the reactor could react very

rapidly, a process called ignition.

When possible, consider operation between 1 and 10 bar. High pressures increase pumping, compression,
and capital costs, whereas low pressures tend to increase the size and cost of vessels. The temperature of

the feed to the separation unit (at least for the base case) is usually set between the boiling points of the
top and bottoms product for distillation, or based on similar considerations for the other separation
options.

12.5 Recycle Section

This section is relatively straightforward. The stream or streams of unreacted raw materials are sent back
to the reactor to reduce feed costs, to reduce impurities in the product, or to improve the operation of the
process. If the conditions of the recycle streams are close to those of the raw material feed, then the
recycle stream should mix with the raw materials prior to the reactor feed preparation section. Otherwise,
any heating/cooling or pressure increase/decrease should be done separately. Thus, the recycle stream is
combined with the raw material streams when they are all at a similar temperature. Example 12.4
involves a recycle in a biochemical process.

Example 12.4

A recycle is used to return enzyme to the reactor. What particular concerns must be addressed in the
recyle section?

The enzyme must be protected from deactivation and from degradation by microbes during the recyle,
which might include storage times between batches. The recycle must be maintained in aseptic conditions,
at the appropriate temperature and pH.

12.6 Environmental Control Section