Page 78 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: LLL Revised Pages
Encyclopedia of Physical Science and Technology EN002E-49 May 17, 2001 20:13

42 Batch Processing

an element the magnitude of which is much larger than selected to maintain smooth response of the materials and
that of the other elements. the equipment. The chances for thermal decomposition of
Train Section of a plant, or group of pieces of equip- the reagents, products, or certain intermediates and other
ment, which operates relatively independently and in safety risks increase significantly, since the startup con-
the same time length. ditions, where safety is more critical, are produced every
time the operation cycle of the plant is repeated. Thus,
in batch processing, safety is of greater concern than in
BATCH PROCESSING is a chemical engineering continuous processing.
methodology for producing chemicals in cycles. In en- One of the challenges in the development and construc-
gineering practice, the term often refers to the analysis tion of new processes is the scaleup of chemical reactors.
of the methodology. Industrial plants are run batchwise Initially,thesereactorsarebatch,sincemostnewprocesses
or continuously. There are economical or practical ad- are conceived in the laboratory. Development of continu-
vantages to each method. The large size of many chem- ous processes has to face the conversion from batch to con-
ical and petroleum industries precludes the use of batch tinuous, and the impressive growth from the test tube and
processing. The petrochemical, pharmaceutical, and food the pilot plant to industrial sizes hundreds of times larger
industries are batch when production levels make batch than the experimental size. Hence, the scaleup to conti-
processing economical. Batch processing is suitable for nuous commercial units is always full of surprises and un-
processes involving multiple grades, variable market de- certainties. In the continuous plant, the chemicals often
mands, or products with a short profitable life, unusual behave differently than in the batch reactor. Secondary
specifications, or requiring subjective tests, such as taste reactions, which are unseen in the pilot plant, become
in the food industry; for products made at low or moder- important in commercial scale, causing the undesired ef-
ate rates, such as pharmaceuticals, fine organic chemicals, fect of reducing product quality and specifications. These
or specialty chemicals; when annual requirements can be and other challenges, which are inherent in the develop-
manufactured in a few batches; or when production can ment of continuous plants, forced designers to return to
be scheduled together with other products in an existing the construction and operation of batch processes. Many
or in a new plant. uncertainties are eliminated using batch reactors, since a
lack of knowledge can be covered by adjusting operating
conditions until satisfactory results are obtained.
I. DESIGN AND OPERATION

Capital and operating costs are optimized by minimizing II. DISCONTINUOUS PROCESSING
the capital investment of modular sections of a plant, by
reducing the operation cycle of one product, or by making The concept of batch production is as old as the history
several products in the same plant to use idle equipment. of humanity. The manufacture of soap, paints, fermented
The process may be operated for the whole year or dur- beverages, drugs, dyes, and metals are examples. The con-
ing an optimum period, having idle equipment thereafter. cept of large-scale manufacture, which developed after
The use of idle equipment may be resolved with multiple the industrial revolution, produced a change to continuous
production, which is accomplished in multiproduct plants. production. The goal achieved was making larger quanti-
The production of specialty chemicals has led to the use ties of goods at a lower price. The large sizes of chemical,
of the same plant for the manufacture of several products. petroleum, and metallurgical plants could never have been
The idea is an extension of long practice at pharmaceutical possible otherwise. Interestingly, batch processing was re-
installations. Simultaneous production with the purpose of born in the same facilities of the large continuous plants.
sharing equipment and reducing capital costs has led to the Batches of raw materials and products had to be stored to
establishment of multipurpose plants. Certain aspects of meet the trends of the market demand and supply. Opti-
design and operation become critical, due to the produc- mization of batch storage acquired importance for the cost
tion in cycles. The reactants, products, and equipment are of the equipment and the value of the materials held. Con-
subjected to drastic changes in operating conditions due to tinuity was often disrupted by variability of the quality of
the working routine of the plant. These changes may pro- the raw materials due to the need for processing stocks
duce significant pressure or thermal stresses or situations from different origins in the same plant. The use of cata-
promoting localized corrosion; the cyclic operation may lysts became another cause of discontinuity, since the cat-
cause fatigue of the materials. The combined result may alytic beds had to be dumped or regenerated periodically.
lead to materials failure. These problems are not current Now discontinuous processing is used when continuous
in continuous plants, where the operating conditions are production is not economical because the reactions and