Page 380 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

should also be developed. These choices often have dramatic effects on the other parts of the GBFD. The
earlier these effects are understood, the better the final design will be.

At this stage, the utility needs of the reactor should be considered. If heating or cooling is required, the
design of an entire additional system may be required. The choice of heating or cooling medium must be
made based on strategies described in Chapter 15, the heuristics presented in Chapter 11, and the costs of
these utilities.

The trade-offs of different catalysts, parallel versus series reactors, and conversion versus selectivity
should be considered, even though the optimization of these choices occurs after the base case is
developed. Again, early identification of alternatives improves later detailed optimization.

Once the base-case reactor configuration is chosen, the duties of the reactor feed preparation and
separator feed preparation units are partially determined.

For the reactor, important questions to be considered include the following.
1. In what phase does the reaction take place (liquid, vapor, mixed, etc.)? The answer will affect
the reactor feed section. For example, it will determine whether a vaporizer or fired heater is
required upstream of the reactor when the feed to the plant is liquid.
2. What are the required temperature and pressure ranges for the reactor? If the pressure is
higher than the feed pressure, pumps or compressors are needed in the reactor feed preparation
section. If the required reactor feed temperature is greater than approximately 250°C, a fired
heater is probably necessary.
3. Is the reaction kinetically or equilibrium controlled? The answer affects both the maximum
single-pass conversion and the reactor configuration. The majority of gas- and liquid-phase

reactions in the CPI are kinetically controlled. The most notable exceptions are the formation of
methanol from synthesis gas, synthesis of ammonia from nitrogen and hydrogen, and the
production of hydrogen via the water-gas shift reaction.
4. Does the reaction require a solid catalyst, or is it homogeneous? This difference dramatically
affects the reactor configuration. For enzymes immobilized on particles, for example, a
fluidized bed reactor or packed-bed reactor could be considered, depending on stability of the
enzyme and mass-transfer requirements. The immobilization may also impart some temperature
stability to the enzyme, which provides additional flexibility in reactor configuration and
operating conditions.
5. Is the main reaction exothermic or endothermic, and is a large amount of heat exchange
required? Again, the reactor configuration is more affected by the heat transfer requirements.
For mildly exothermic or endothermic gas-phase reactions, multiple packed beds of catalyst or
shell-and-tube reactors (catalyst in tubes) are common. For highly exothermic gas-phase

reactions, heat transfer is the dominant concern, and fluidized beds or shell-and-tube reactors
with catalyst dilution (with inert particles) are used. For liquid-phase reactions, temperature
control can be achieved by pumping the reacting mixture through external heat exchangers (for
example, in Figure B.11.1). For some highly exothermic reactions, part of the reacting mixture
is vaporized to help regulate the temperature. The vapor is subsequently condensed and
returned to the reactor. External jackets and internal heat transfer tubes, plates, or coils may
also be provided for temperature control of liquid-phase reactions. (See Chapters 20 and 21.)
6. What side reactions occur, and what is the selectivity of the desired reaction? The formation
of unwanted by-products may significantly complicate the separation sequence. This is