Page 36 - Chemical process engineering design and economics

Processes and Process Engineering 23

Figure 1.3 shows the process flow diagram for converting starch into glu-
cose. Table 1.4 identifies the basic process operations in the process, according to
those given in Table 1.3. Sinclair [22] describes the process but it has been modi-
fied after discussion with Leiser [23]. Harness [24] describes the corn wet-milling
process for producing a corn-starch slurry containing 30 to 40% solids, which
flows to the first hydrolyzer, R-l. The first hydrolyzer converts 15 to 25% of the
starch into glucose using alpha-amylase, an enzyme, which catalyzes the hydroly-
sis. Two process operations occur in the hydrolyzer - conversion and mixing - but
the main purpose of the process unit is conversion. After hydrolysis the viscosity
of the slurry is reduced. The centrifuge, PS-1, removes any residual oil and pro-
teins, which were not removed in the corn wet-milling process. This is a phase-
separation operation. The oil and protein will be processed to make animal feed.
The second hydrolyzer, R-2, completes the hydrolysis using glucoamylase,
another enzyme. The reduction in viscosity of the starch slurry in R-l aids in the
mixing of glucoamylase and prevents the formation of a unhydrolyzable gelati-
nous material in R-2. Most of the remaining starch is hyrolyzed to glucose in 48 to
72 h in a batch operation. Aspergillus phoenicis, a mold, produces the glucoamy-
lase enzyme in a fermentation process. The overall conversion of starch in this
two-step hydrolysis is almost 100%. The effluent from R-l is cooled by preheating
the feed stream to R-l, which is an energy transfer operation. After the second
stage of hydrolysis, the solution is decolorized in an adsorber, CS-1, packed with
carbon. Because the hydrolysis is a batch operation, internal storage, S-l, of the
solution is required to keep the next step of the process operating continuously.
After converting the starch into glucose, the rest of the process removes wa-
ter from the glucose to obtain a dry product. The solution is pumped from storage
to the first of three stages of evaporation (called effects) where some water is re-
moved. To conserve steam and therefore energy, the first evaporator employs me-
chanical recompression of the water vapor evolved from the evaporation. Com-
pressing the vapor elevates its temperature above the boiling point of the solution
in CS-2 so that heat can be transferred to the boiling solution. Also, because the
glucose is heat sensitive, the evaporation is carried out in a vacuum produced by
the vacuum pump C-1. Each stage of evaporation is carried out in two steps. In the
first step, a component-separation operation, energy is transferred to the solution
in a boiler to evaporate some water, concentrating the glucose. Thus, the boiler is a
component separator. In the second step, vapor and liquid are separated in a phase
separator. After the first stage of evaporation, the solution is again decolorized in
the adsorber, CS-3, and the small amounts of organic acids are removed in an ion
exchanger. The ion exchanger, R-3, replaces anions with hydrogen ions and
cations with hydroxyl ions, and thus the net effect is to replace the organic acids
with water. Although the operation is a chemical reaction, the overall process is a
separation because the ion exchanger is eventually regenerated and reused.

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