Page 392 - Chemical process engineering design and economics

372 Chapter 7

heat transfer [12]. Markovitz [12] has given the following rules for selecting the
jacket type:
3
for < 500 gal (1.89 m ) use the simple jacket
3
for > 500 gal (1.89 m ) use the dimple or half-pipe coil
if the reactor pressure is greater
than twice the jacket pressure use the simple jacket
for a jacket pressure < 300 psi (20.7 bar) use the dimple
for a jacket pressure > 300 psi (20.7 bar) use the half-pipe coil jacket
but < 1000 psi (68.9 bar) use the half-pipe coil jacket
for steam the pressure is < 750 psi (51.7 bar) use the half-pipe coil jacket

Besides heat transfer and structural considerations, pressure drop across the
jacket is also important because it affects both pump and power costs. For the
dimple and partial-pipe-coil jackets, the pressure drop will be higher than in the
simple jacket because of the increased turbulence. The pressure drop in the
dimpled jacket is approximately 10 to 12 times higher than in the simple jacket
[12]. For this reason, the liquid velocity in the dimpled jacket is limited to
about two feet per second. There is no limitation on the number of inlet and
outlet connections for the partial-pipe coil. Thus, to reduce the fluid velocity
and hence the pressure drop, the process engineer will split the heat-transfer
fluid into zones, as shown in Figure 7.2. The partial pipe-coil jacket is more
versatile - it can be used with both high and low temperature heat-transfer flu-
ids. If the heat-transfer coefficient inside the reactor is small compared to the
jacket heat-transfer coefficient, then consider using the simple jacket. Because
it is difficult to clean dimple jackets, they should not be used with dirty fluids.
Also, do not use the dimple jacket for applications requiring high temperature
organic heat-transfer fluids, which may degrade to form solids. The solids will
deposit on the dimples, fouling the jacket.
The most frequently used internal heat exchanger is the spiral coil. Manu-
facturers fabricated internal coils by bending straight lengths of pipe. The num-
ber of coil banks that can be placed in a reactor depends on the minimum coil
radius, which is about 8 to 12 in (0.203 to 0.305 m). Below the minimum coil
radius, the pipe will crush during coiling. A common pipe diameter is 2 in.
(50.8 cm). The outer coils are less efficient in transferring heat than the inner
coils, which are close to the impeller, because the heat transfer coefficient de-
creases from the inner coil to the outer coil. Hicks and Gates [14] described the
design of polymerization reactors using three banks of coils.

CONTINUOUS STIRRED-TANK REACTOR SIZING

Sizing continuous stirred-tank reactor (CSTR) requires selecting a standard reac-
tor, given in Table 3, from a manufacturer. Table 7.4 lists the relations for calcu-
lating the reaction volume, heat transfer area, and the mixer power for CSTRs.

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