Page 208 - Design of Simple and Robust Process Plants

5.7 Simplification and Ranking per Unit Operation 193
Continuous stirred tank reactor (CSTR) The CSTR for homogeneous reactions is the
second best with regard to simplicity of design. This type of reactor needs to condi-
tion mixing and heat transfer.

Continuous Stirred Tank Reactor

Heat transfer is ranked in terms of increasing complexity for exothermal systems:
1. Increase process flow and decrease inlet temperature, as discussed under the
plug flow reactor.
2. Reflux condenser applicable for boiling systems.
3. Internal cooling with spirals (fouling might cause a problem).
4. Jacket cooling of most standard reactors can be provided with jackets, though
scale-up may be difficult at the same length to diameter (L/D) ratio; this is
3
because the volume increases with cube of the diameter (D ), while the sur-
2
face increases with the square (D ).
5. External cooling through a recycle.
Mixing is seldom a problem for a homogeneous liquid system, but this is not to
relevant to this discussion.
The ranking of the alternatives with increasing complexity lists as follows:

1. A jet mixer for the inlet stream; in the case of jacket or internal cooling, this
might limit the heat transfer. To create overall mixing, careful design with a
CFD code is a requirement (specifically for gas reactors).
2. An agitator is often used to enhance heat transfer device, especially in combi-
nation with a jacket or an internal coil. One drawback, however, is that agita-
tors require more power and maintenance.
3. In a recycle system, a jet mixer is often used in combination with an external
heat exchanger; in this case the circulation pump is in shared service.

5.7.1.2 Heterogeneous reactions; L/L
These reactor designs follow the same order in priority as the homogeneous reac-
tion. The difference is made by the mixing. Indeed, it might be said that stirred tank
reactors have bad mixing and mass transfer properties for two-phase systems (as
discussed earlier in Section 5.5.2; see also Figure 4.8 Chapter 4). When the reaction
is transferred from a CSTR to a plug flow reactor, the ranking order will be: