Page 70 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

2.2 Step 1—Batch versus Continuous Process

It should be pointed out that there is a difference between a batch process and a batch (unit) operation.
Indeed, there are very few, if any, processes that use only continuous operations. For example, most
chemical processes described as continuous receive their raw material feeds and ship their products to
and from the plant in rail cars, tanker trucks, or barges. The unloading and loading of these materials are
done in a batch manner. Indeed, the demarcation between continuous and batch processes is further
complicated by situations when plants operate continuously but feed or receive material from other
process units within the plant that operate in a batch mode. Such processes are often referred to as
semibatch. A batch process is one in which a finite quantity (batch) of product is made during a period of
a few hours or days. The batch process most often consists of metering feed(s) into a vessel followed by a
series of unit operations (mixing, heating, reaction, distillation, etc.) taking place at discrete scheduled

intervals. This is then followed by the removal and storage of the products, by-product, and waste
streams. The equipment is then cleaned and made ready for the next process. Production of up to 100
different products from the same facility has been reported [3]. This type of operation is in contrast to
continuous processes, in which feed is sent continuously to a series of equipment, with each piece
usually performing a single unit operation. Products, by-products, and waste steams leave the process
continuously and are sent to storage or for further processing.

There are a number of considerations to weigh when deciding between batch and continuous processes,
and some of the more important of these are listed in Table 2.1. As this table indicates, there are many
things to consider when making the decision regarding batch versus continuous operation. Probably the
most important of these are size and flexibility. If it is desired to produce relatively small quantities, less
than 500 tonne/y [1], of a variety of different products using a variety of different feed materials, then
batch processing is probably the correct choice. For large quantities, greater than 5000 tonne/y of product
[1], using a single or only a few raw materials, then a continuous process is probably the best choice.
There are many trade-offs between the two types of processes. However, like most things, it boils down
to cost. For a batch process compared to the equivalent continuous process, the capital investment is

usually much lower because the same equipment can be used for multiple unit operations and can be
reconfigured easily for a wide variety of feeds and products. On the other hand, operating labor costs and
utility costs tend to be much higher. Recent developments in batch processing have led to the concept of
the “pipeless batch process” [4]. In this type of operation, equipment is automatically moved to different
workstations at which different processes are performed. For example, a reactor may be filled with raw
materials and mixed at station 1, moved to station 2 for heating and reaction, to station 3 for product
separation, and finally to station 4 for product removal. The workstations contain a variety of equipment
to perform functions such as mixing, weighing, heating/cooling, filtration, and so on. This modular
approach to the sequencing of batch operations greatly improves productivity and eases the scheduling of
different events in the overall process.

Table 2.1 Some Factors to Consider When Deciding between Batch and Continuous Processes