Page 41 - Chemical process engineering design and economics

Processes and Process Engineering 27

in the cooling water, and the air leaking into the system - are compressed to the
pressure of the next stage by a steam-jet ejector before being condensed and com-
pressed again. This operation is material transfer because the main purpose is to
transfer the non-condensable gases and the remaining vapor to the atmos-
phere.
After the evaporation is complete, the glucose solution could be sold as a syrup
or processed further to obtain powdered a-D-glucose monohydrate. To obtain the
powder, the glucose is separated from the solution in horizontal cylindrical crystal-
lizers by cooling and slowly mixing at 1.5 rpm. The concentrated solution is
seeded with glucose crystals to promote crystallization. Approximately, 60% of
the dextrose in the solution crystallizes as the monohydrate. After two days, the
slurry is transferred by a screw conveyor, MT-1, to a perforated-screen centrifuge
where the solution is partially separated from the crystals. The wet crystals, con-
taining 14% water, are then conveyed to a rotary dryer to remove the remaining
water. In this particular case, component separation occurs because water is being
removed from the sugar solution that adheres to the crystals. As the water evapo-
rates further crystallization of the glucose dissolved in the solution occurs. If wa-
ter were removed from a insoluble solid by drying, such as from wet sand, then the
operation is a phase separation.
The powdered glucose from the drier contains some oversized crystals,
which must be removed to obtain a more marketable product of fine crystals. The
oversized crystals are separated by the screen, SS-1, a size-separator. When re-
moving a small amount of oversized crystals (less than 5%) from a feed, which
consists predominately of fines, the operation is called "scalping". The oversized
crystals are recovered by first melting and then pumping the liquid through a leaf
filter to remove any insoluble material that has been carried through the process.
After filtering, the liquid is recycled back to the evaporators for reprocessing.

REFERENCES

1. Wei, J., Russel, T.W.F., Swartzlander, T.W., The Structure of the
Chemical Processing Industries, McGraw-Hill, New York, NY, 1979.
2. Greek, B.F., Petrochemicals Inch Toward Recovery, Chem. & Eng. News,
p. 18, Nov. 22, 1982.
3. Matz, S.A., Modern Baking Technology, Sci. Am., 251, 5, 122,1984.
4. Reisch, M., Aspirin is 100 Years Old, Chemical & Eng. News, p. 12,
Aug. 18, 1997.
5. Stinson, S.C., Bulk Drug Output Moves Outside U.S., Chem. & Eng.
News, p. 25, Sept. 16,1985.
6. Thayer, A.M., Use of Specialty Food Additives to Continue to Grow,
Chem. & Eng. News, p. 25, June 3, 1991.
7. Stinson, S.C., Custom Synthesis Expanding for Drugs and Intermediates,
Chem. & Eng. News, p. 25, Aug. 20, 1984.

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