Page 79 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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

Batch Processing 43

other processing steps are too slow or the manufacturing needed, time losses due to human error or inefficiency, and
sequence is too complex. Many chemicals, petrochem- other operations that may be required. Table I illustrates
icals, drugs, and vitamins are manufactured batchwise. two batch polymerization cycles where the reaction time
The unit operations of continuous processing are prac- is almost 60% of the batch cycle time.
ticed batchwise in batch processing; descriptions of these Aplantoperabilityanalysishelpstoestablishthecorrect
can be found in standard references. Some of the advan- cycle time. In certain situations, the batch cycle of the
tages of continuous processing are retained. For example, entire plant is not determined by the units around the main
solid materials can be charged on a batch basis, while reactor. Additional time may be needed downstream for
the fluid reactants are added continuously, thus minimiz- product finishing in equipment that operates in series with
ing conditions for side reactions, or the feed rate can be the main reactor. The batch cycle time T is a composite
scheduled to favor the kinetics of the main reaction and of three contributions: (1) the sum of the batch residence
increase product yield. In modern batch plants manual op- times t(i)inthe M true batch units of the plant that operate
erations are refined by computer-controlled sequencing, in series; (2) the sum of the residence times t( j) in the N
and specialized criteria for product finishing are applied semicontinuous trains of the plant that operate in series;
from batch to batch. Thus, product quality is more uniform and(3)thesumofthedowntimest(k)inseriesencountered
and not as greatly affected by the differences in the skills in the total batch cycle:
of plant operators. Three types of discontinuous processes
T = SUM t(i) + SUM t( j) + SUM t(k) (1)
are common: batch, when materials do not enter or leave
the unit during the cycle; semibatch, when one or more but
not all components enter or exit the unit during the cycle;
and semicontinuous, characterized by a processing rate IV. BATCH PROCESSING PLANTS
at which the unit runs continuously, subject to periodic
startups or shutdowns. A plant is a battery of equipment for the conversion of raw
materials to products. It consists of three main sections:
storage, process equipment, and utilities. Process equip-
III. BATCH PROCESSING
ment for batch processing is standardized so it can be
adapted to make different products. The major processing
Batch processing is a form of discontinuous processing
units of a batch plant are operated on a batch or semibatch
characterized by operation in uniform, repetitive cycles.
basis. Transfer interunits (pumps) run on a semicontinu-
The batch cycle time is the time elapsed between succes-
ous basis. Heat and mass-transfer operations are contin-
sive batches (Table I). The reaction time is only a fraction
uous, but in some instances they are semicontinuous or
of the total cycle time; additional time must be provided
batch. Storage is provided to keep the raw materials, to
for charging the raw materials, bringing them to reaction
hold the product and rework streams, and to serve as a
conditions, allowing the reaction to take place, treating
buffer between the batch and the continuous sections of
the reaction mass once the desired conversion is attained,
the plant. It provides for surge vessels, which decouple
discharging the products, cleaning the reactor when it is
the upstream and downstream units and maintain conti-
nuity of operation. It may require a substantial portion of
TABLE I Batch Polymerization Cycles a the plant capital investment. Utilities are the section that
supplies the plant with water, fluids (air or nitrogen) for
Cycle time (hr)
instrument operation or to maintain an inert atmosphere,
Batch cycle Cycle 1 Cycle 2 and energy: steam or Dowtherm for heating, refrigeration,
or electricity for power.
Materials charge 0.5 0.5
Plant economics depends on procedures that must
Preheating 1.0 0.0
be carefully engineered to render the process attractive.
Polymerization 8.0 4.0
Equipment size is selected to optimize capital investment.
Stripping unreacted 1.0 0.5
The operating cycle is chosen to reduce equipment idle
Cooling 1.0 0.0
time. Cycle scheduling is programmed to minimize op-
Discharge 0.5 0.5
erating costs. The plant may run for the whole year, or
Reactor flushing and cleaning 1.0 0.2
activities may be reduced to an optimum operating pe-
Maintenance 0.5 0.5
riod beyond which the equipment is idle. Depending on
Inefficiency (time losses) 0.5 0.3
whether one or several products are made in the same
Total cycle time 14.0 6.5
plant, a batch processing plant may be one of the follow-
a Reactor volume, 3700 gal; load, 5300 lb. ing types.