Page 80 - 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

44 Batch Processing

A. Single-Product Plant TABLE II Block Cycle of a Multiproduct Plant
When production volume is sufficient, it is economical to Cycles Product Total time (hr)
build one plant for one product. Batch production in a sin-
24 A-1 61
gle unit may be limited by maximum reactor size. Holdups 19 A-2 49
of greater than 20,000 gal are handled in separate parallel 110 A-3 280
reactors. To use common upstream and downstream facil- 27 A-4 69
ities, the reactors may not be operated simultaneously but 4 Downtime 32
180 All 4 491
on overlapping schedules. When long reaction times can-
not be avoided, the reaction sections operate batchwise;
however, feeding reactants and recovering products may cedure. The products handled are chemically similar, and
be continuous for economic reasons. This practice is typi- product changeovers are relatively simple. Product qual-
cal of many processes, such as the saponification of natural ity can be altered; new products or product grades can be
fats in intermediate quantities. In the production of ethanol tried, and product capacity can be adjusted to meet mar-
by fermentation, two reactions (saccharification and fer- ketdemandbyschedulingchangesratherthanbychanging
mentation) are operated on a batch basis, while hydrolysis equipment. Several products can be scheduled for produc-
(conversion of starch to dextrin) and product recovery by tion in sets of various cycles, one set following the other.
distillation are continuous. The batch cycle of the plant takes into account the various
The interaction between the batch and the continuous products handled by the same plant. Each product has its
parts of the plant is a major concern of plant design and op- own batch cycle. The block cycle is the time needed for the
erating procedures. When the production volume reaches once-through manufacture of all the products at the annual
certain levels, continuous operation may be more econom- production rate. For illustration, consider a multiproduct
ical. More often, batch chemical plants are built to produce plant that makes 100 million pounds per year of four prod-
more than one product. The objective is the use of existing ucts: 13.5% A-1, 10.5% A-2, 61.5% A-3, and 14.5% A-4.
equipment, which becomes idle when market demands are The plant makes only one product at a time in batch cy-
fulfilled. These plants can be classed in one of two broad cles of ∼153 min. The front part handles the reaction on a
categories: multiproduct and multipurpose. The difference batch basis. A second reaction and product recovery take
between the two types is illustrated in Fig. 1. place continuously in the back end of the plant. Since three
products must be stored while the fourth one is manufac-
B. Multiproduct Plant tured, bulk storage must be provided for four products and
their raw materials. A typical block cycle for the four prod-
A given battery of equipment is used to produce only one
ucts consists of 491 hr distributed in 180 cycles (Table II).
product at a time. The same equipment is used for the man-
Each product cycle includes reactants loading, batch re-
ufacture of different products by changing operating pro-
action, residual reactant stripping, product transfer, and
downtime between batches. Eight hours of downtime and
cleanup are allowed to switch from one product to the
next. The number of product cycles per block cycle is set
from market demands. The size and the capital cost of the
bulk storage needed increase in proportion to the block cy-
cle time. The effective operating time decreases when the
block cycle time is reduced, due to the downtime needed
to switch from one product to the next. Hence, the size and
the capital cost of the process equipment (battery limits)
must increase to make the same annual production rate.
There is an optimum block cycle that must be selected to
minimize capital costs of storage and battery limits. The
optimum is between 3 and 5 weeks; it is not largely de-
pendent on downtime and cleanup between products nor
on the extra capacity of product storage, which may be
needed to ensure enough raw material supply.

C. Multipurpose Plant
A battery of process equipment is used in various arrange-
FIGURE 1 Multiproduct batch plant (top) and multipurpose batch
plant (bottom). ments to manufacture more than one product at one time.