Page 83 - 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 47

capital investment to a level which will render economical in less useful time, since a new product can be started
a process for a single-product plant. One or several trains only when the complete plant is empty and clean. It is
of equipment may have to be replaced by less expensive convenient to produce only one product in a given week.
trains. The new process is expected to be safe and environ- The number of batches produced for each product depend
mentally clean and to meet proper product specifications. on the time allocated to the product. In this task, comput-
The utilization of standard, used, or idle equipment may erization may help to minimize storage of raw materials
bring an economic solution to the design. and reduce the cost of working capital, while customer
When a single-product scheme cannot be sustained, the orders are filled on time.
costofcapitalinvestmentmaybesplitamongseveralprod- Multipurpose plants are adequate for the simultaneous
ucts. A multiproduct or a multipurpose plant may be de- manufacture of many products on a small scale. This is
signed at a minimum capital cost for the yearly production typical of the pharmaceutical or specialty chemicals in-
rates needed. This procedure may lead to a lower product dustries. A structure housing general-purpose equipment
cost. Raw material and operating costs can be minimized can be set up and used as required. The trains available
simultaneously. Also the operation of the multiplant may should be sufficient so that a large number of products
be scheduled to attain the minimum operating costs of the can be produced at a time. Their size—small rather than
various products handled. large—should be adequate for the production volumes
A cost equation may be written to include all the costs, of the products handled. If it is economical, one product
which are expressed in terms of the capacity of the flow- may have more than one production route and procedure.
scheme components. The selection of equipment sizes that The various products compete for raw materials, utilities,
minimize capital investment is (1) complicated by inter- manpower, and production facilities. With no interaction
relations between pieces of equipment, (2) limited by the between products, the plant may operate with overlap-
discontinuity in size of standard equipment, (3) fixed by ping or nonoverlapping cycles and product changeovers.
theavailabilityofidleorusedequipment,and(4)restricted When products interact, the situation becomes extremely
bythehighercostofcustom-madeequipment.Writingone complex.
equation for a complete plant is a complex task. It is more Production planning studies the amounts of the various
likely that it may be done for small sections of the plant products which should be manufactured in a given plant
which can be operated as interrelated trains. when the production requirements are known. This prob-
The cost equations thus written are discontinuous func- lem must be considered during the plant design stage.
tions of the size of the units which compose the trains. Scheduling decides in which equipment each product
A mathematical minimization of any of these equations should be manufactured, when the operation starts, and
may not lead to a practical minimum. It may indicate only when it is expected to finish.
the domain where less-expensive solutions may exist. The Production planning and scheduling problems arise in
practical alternative is to draw flow schemes which are multiproduct plants because the production time avail-
equivalent to the process under investigation. The eco- able on each train must be allocated to the manufacture
nomic analysis of these schemes terminates with the se- of a large number of products within certain production
lection of one which requires the minimum capital invest- constraints. Short-term industrial scheduling is a com-
ment and operating costs. plex problem because the day-to-day plant status, cus-
tomer needs, raw materials, and personnel availability are
changing continuously. The demand for computer-aided
C. Batch Process Management
scheduling tools is increasing because production sched-
Inamultiproductplant,allproductsfollownearlythesame ulersarelinkedtotheirsalesandservicepersonnelthrough
path through the process. Only one product, or in a com- computerized reports on customer orders, inventory lev-
plexcaseafewproducts,isproducedatonetime.Although els, and plant status. Using such information would reduce
it is feasible to alternate batches of different products to the complexity of the scheduling job, speed up operations,
reduce idle time of equipment, this is rarely, if ever, done and produce more economical selections.
to minimize operational error and cross-contamination. Production planning problems result from substantial
Product changeovers are not frequent, especially if exten- changes in market demands, the need to introduce new
sive cleanout between changeovers is required. processes, and the increasing pressure on existing pro-
The operating decisions to be made for a multiprod- duction capacity. On these accounts the operating plan
uct plant are more complex than for a single-product may require radical changes in the use of equipment. A
plant. The useful production time of making several prod- computerized algorithm, BATCHMAN is useful in deter-
ucts is diminished by the time for overlapping product mining the best equipment configuration for the optimal
changeovers. Operation without product overlap results product combinations and assigns the available equipment