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378 Chapter 7

transfer area consists of the cylindrical surface of the reactor and the dished
bottom. Only 80% of the total surface area of an STR is available for heat trans-
fer. The upper head contains nozzles, a port for the mixer, lugs for support, and
usually a sight glass, as shown in Figure 7.2.
We will use a spiral coil to illustrate the calculation procedure. First, con-
sider a jacketed STR. If the jacket heat-transfer area is insufficient, then con-
sider an internal heat exchanger and finally a shell-and-tube external heat ex-
changer. For the latter case, the reacting solution is pumped out of the reactor
continuously, cooled in a heat exchanger, and then returned to the reactor. If the
jacketed reactor does not provide sufficient heat-transfer area, then try using
internal helical coils. If more than one coil is used, then the heat transfer coeffi-
cient must be reduced by 30% for each additional coil [14]. Thus, if the reaction
requires three coils, then the coil near the reactor wall will only have 40% of the
heat-transfer coefficient of the coil closest to the impeller. Frank [33] believes
that this reduction in the heat-transfer coefficient may be too pessimistic. Each
coil requires spacing between the reactor wall and other coils. To minimize in-
terfering with liquid recirculation, the coils should not extend completely to the
surface of the liquid or the bottom of the tank. Hicks and Gates [14] recommend
locating the top of the coil at least one sixth of the diameter of the reactor below
the liquid surface. They also recommend locating the bottom coil' at one-sixth
the coil diameter above the bottom of the STR.
The jacket temperature, Tj, in Equation 7.4.8, equals the average of the
jacket inlet and outlet temperatures. For a coil also use the average of the inlet
and outlet temperatures. First, determine if there is sufficient heat-transfer area
by assuming a simple jacket. The area of the jacket is given in Table 7.3. The
area will be about the same for simple, pipe coil, and dimple jackets. If the
jacket area is insufficient, then determine if coils will provide the additional sur-
face area. The reactor volume should be compensated for the volume occupied
by the coils.

Example 7.1 Sizing a CSTR for Synthesizing Propylene Glycol________

This problem is an adaptation of a problem taken from Fogler [16]. Propylene
glycol is produced by hydrating propylene oxide using a solution of 0.1 % sulfuric
acid in water as a catalyst. The reaction is

CH 2 —— CH — CH 3 + H 2O -» CH 2 — CH — CH 3
I O__| ]_OH |_OH

An equi-volumetric solution of methanol and propylene oxide flows into a
CSTR. At the same time, a 0.1% sulfuric acid solution also flows into the CSTR at
a rate of 2.5 times the combined flow rate of propylene oxide and methanol. The

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