Page 25 - Chemical process engineering design and economics

12 Chapter 1

combined and only one compressor is used. If the conversion is less than 100%, a
recycle compressor will transfer the unreacted gases back to the reactor after sepa-
rating out the products. Since the recycled gases are already at a high pressure, but
at a lower pressure than at the reactor inlet because of frictional pressure losses, a
compressor is needed to recompress the gases to the reactor inlet pressure. This
step would be considered primarily material transfer.
Because raw-material delivery cannot be accurately predicated, on account
of unforeseen events such as bad weather, strikes, accidents, etc., storage of raw
materials is a necessity. Similarly, the demand for products can be unpredictable.
Also, internal storage of chemical intermediates may be required to maintain
steady operation of a process containing batch operations or to store chemical in-
termediates temporarily if downstream equipment fails. Production can continue
when repairs are completed.
The last three process operations; size reduction, agglomeration, and size
separation; pertain to solids. Examples of size reduction are grinding and shred-
ding. An example of agglomeration is compression of powders to form tablets.
Screening to sort out oversized particles is an example of size separation.
The first step in the synthesis, or development and design of a process, is to
construct a flow diagram, starting with raw materials and ending with the finished
product. The flow diagram is a basic tool of a chemical engineer to organize his
thinking and to communicate with other chemical engineers. A selected list of
flow-diagram symbols for the process operations discussed above are given in
Figure 1.2. Other symbols are given by Ulrich [19] and by Hill [20] and have
been collected and reviewed by Austin [21]. The various process operations dis-
cussed above, using the flow-diagram symbols in Figure 1.2, are used to describe a
process for producing glucose from cornstarch, which is illustrated in Example
1.1.

Example 1.1 Glucose Production from Corn Starch_______________

A process flow diagram for the production of glucose is shown in Figure 3. Iden-
tify each process unit according to the process operations listed in Table 3.
Although glucose could be obtained from many different natural sources,
such as from various fruits, it is primarily obtained by hydrolysis of corn starch,
which contains about 61% starch. Starch is a polymer consisting of glucose units
combined to form either a linear polymer called amylose, containing 300 to 500
glucose units, or a branched polymer called amylopectin, containing about 10,000
glucose units. Glucose is a crystalline white solid, which exists in three isomeric
forms: anhydrous cc-D-glucose, oc-D-glucose monohydrate and anhydrous (3-D-
glucose. Most of the glucose produced is used in baked goods and in confection-
ery as a sweetener. It is sold under the trivial name of dextrose, which has evolved
to mean anhydrous a-D-glucose and a-D-glucose monohydrate.

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