Batch Process or Continuous Process?
Miri Kesner1
In general terms, we can relate to a production process in the chemical industry (as we
can to any other production process) as the processing of inputs to outputs in a
defined series of actions.
The inputs include reactants, auxiliary materials, energy etc.
The outputs include products, by-products, energy, etc.
The series of actions is actually the conditions of reaction and activation of the
chemical reaction.
Production processes in the chemical industry can be carried out in several ways: batch,
continuous or semi-continuous (in which certain parts are done continuously and some in
batch form).
The batch process is a single- or multi-stage process in which a certain quantity of inputs
(raw materials, auxiliary materials, energy, etc.) are fed into the chemical reaction unit (of
the entire reaction) under conditions suitable for obtaining the desired reaction
(temperature, pressure, required time, etc.). In the batch process, in the reactor and at
any given period of time, various actions take place in the wake of which a concentration
of reactants and products varies so long as the reaction progresses. At the conclusion of
the process the mixture is removed from the reactor and it then undergoes the appropriate
separation and processing stages (either physical or chemical) at the required level of
cleanliness. This is generally dictated by the customers for whom the specific product is
manufactured.
In the batch process, so long as the batch has not undergone the entire series of actions,
there is no possibility of preparing a further batch. The batch process can be undertaken
in one reactor in which all the actions are carried out one after the other, or in a series of
reactors in each of which a different stage of the process is carried out. The quality of the
end product can also be controlled by the addition of appropriate separation stages
between the various other stages as required. Reactants that do not react and which are
separated from the reaction mixture can be returned for a further reaction (usually after
1
I am grateful to Mr. Yigal Antonir and Dr. Haim Strage of “Bromium Compounds” for their
constructive remarks and help in the writing of this article.
they have undergone treatment or cleaning for by-product contamination), thus fully
exploiting the process.
A good example of an “installation” working in batches is the washing machine, into which
a certain quantity of dirty washing is put. The required inputs are water, electrical energy,
washing powder, etc.
A “batch” of laundry goes through various stages that are
programmed as required: soaking, washing, various rinses, and extraction. All the actions
take place in one receptacle and at the conclusion of the process we obtain wet laundry
that is clean and ready for drying.
From the washing machine the wet laundry is
transferred to the drier, in which a “batch” process of separation/purification of the “end
product” from water is also carried out, until dry laundry is obtained.
This process can be illustrated with a flow chart showing the series of actions:
Batch of dirty laundry
 Soaking  Washing  Rinse  Extraction
Washing machine
Batch of
clean, wet laundry
Drying
Drier
Batch of
clean, dry
laundry
There are washing machines in which the drying process is also carried out and then the
entire process takes place in a single receptacle.
The continuous process is one in which inputs are fed into the system at a constant rate
and at preset ratios (raw materials, auxiliary materials, energy, etc.), and at the same time
a constant extraction of outputs is done (products, by-products, energy, etc.).
This
process is characterized by a constant process taking place in each section of the facility
and during the time of its action a constant process takes place. Thus, the concentration
of reactants and products at every location in the system is in a durable state and control
of the process is done by maintaining these concentrations.
The batch and continuous processes can be compared to the production of the cocoa
drink, known as “chocolate milk” and various other trade names, using two methods.
When the drink is produced in “batch” form, measured amounts of cocoa, sugar and milk
are added to the reactor. The materials are mixed and heated, and after a suitable delay,
chocolate milk, whose color and taste are constant in accordance with the program, is
obtained. Only after the conclusion of the process is the drink put into storage tanks and
then the process can be restarted and a further batch of chocolate milk prepared. If no
faults occur and the instructions are strictly observed, the second batch will be identical to
the first.
When chocolate milk is produced using the continuous method, cocoa, sugar and milk are
injected into the reactor at the desired and constant ratios through the entry points. These
concentrations will, in the end, determine the quality of the product obtained. At the same
time, the finished product is extracted at a constant rate through the exit points. The
process is planned so that while the ingredients are flowing through the reactor, they are
heated and mixed. Because of the different way in which the process is carried out,
chocolate milk that is not completely homogeneous will be obtained, and this is liable to
contain a certain amount of undissolved sugar and cocoa that came out with the chocolate
milk. In addition, there is a great probability that the color and taste of the drink will be
subject to certain variations between the place at which they entered the dissolving and
mixing unit, and the exit point of the chocolate milk at the conclusion of the process.
These variations can, of course, be minimized to an agreed point (according to the prearranged permitted amount of cocoa and sugar that did not react), by setting the ratios of
feeding, mixing rate, flow conditions, extraction, etc. In order to obtain chocolate milk
identical to that obtained in the batch process, a further action on the product of the
continuous process must be carried out, like, for example, filtration of the product and
separation of the cocoa and sugar that did not dissolve in the milk. Their reintroduction to
the process will not be carried out directly into the reactor (mixing and dissolving), but they
will be combined with the flow of raw material entering the process. Their return will
improve the process’s efficiency without harming product quality.
Each of the production methods (batch/continuous) has its own characteristics. In the
batch process, the shift from one stage to the next is carried out in series and so the
overall time of the process is, in fact, the sum of the times required for the various stages,
and it is relatively extensive.
In the continuous process, all the stages are carried out simultaneously (although possibly
in different parts of the system), and so the overall time required for the process is
shortened. In contrast, the required volume of the tanks for a specific batch process is
greater than that required for a parallel continuous process.
For example, if a batch process takes one hour and the required output is one cubic meter
per hour, a reactor whose volume is one cubic meter will be required. If we carry out a
continuous process instead of a batch process, it will be shorter.
Thus, if the continuous process takes half an hour and the required output is still one cubic
meter per hour, a reactor whose volume is half a cubic meter will be required. Hence, the
volume of the reaction systems in the batch process will be greater, and in consequence
the investment in equipment will also be greater.
In general terms, a batch installation requires more manpower while a continuous
installation requires greater computerized and automated control. Experience shows us
that over time, an initial investment in control is more feasible that the high day-to-day
costs of manpower.
Each method has its advantages and disadvantages, and so the decision on whether to
build a specific installation using either the batch or continuous method depends on a
complex system of considerations that is governed mainly by technological and economic
considerations. The economic factors usually tilt the balance.
The initial financial investment in the establishment of continuous installations is generally
higher than that of batch installations. This is due to the automated control systems and
despite the fact that the reactors in the batch installation are bigger.
As a result of
economic and technological considerations, products required in large quantities and for
which there is a year-round demand, are usually manufactured in continuous installations.
On the other hand, in the case of materials required in small quantities, or alternatively, for
which the annual demand varies, it is unfeasible to build a continuous system. Production
processes in chemical industry plants are often planned so that certain sections are
continuous while others use the batch method. Planning of this kind is also derived from
technological and economic considerations.
The following table of comparisons between batch and continuous processes will be of
assistance:
Batch Process
Types of materials
Can be used with all types of materials
(with non-flow materials, it is easier to
use the batch process).
Installation size
Relatively large installations. Very big
investment in land and installations.
Reactor
Changes occur in the concentrations of
materials over time.
Feeding raw materials
Raw materials are fed before the start
of the reaction.
Control of the set of
actions in the system
Simple control. It is easier to control
reaction conditions (pH, pressure,
temperature). Manual control can also
be done.
Product(s)
Extraction of materials only after all the
actions are finished with the conclusion
of the reaction.
A fault or dealing with a batch requiring
“repair” does not cause problems in the
other stages. Appropriate tests are
conducted after each stage.
Trouble shooting
Quantities produced
Variety of products in
the plant
Product development
stage
Preferable when production of small
quantities of a specific material are
planned.
Preferable when the plant produces a
wide variety of materials and when the
product is likely to be changed now and
again, while using the same reactor.
Preferable when the process is
relatively new and still unfamiliar. In
this case the initial investment is in a
smaller batch reactor, and thus the
economic risk is smaller.
Continuous Process
Easier for use with flowing
materials (today, almost any
material can be produced with the
continuous process; investment
cost is the decisive factor).
Relatively small installations.
Significant savings in land and
installations.
At all locations, conditions are
constant over time (durable
conditions).
Constant feeding of raw materials
during the entire reaction
process.
Complex control. Automatic
control must be used. Control of
reactor conditions is more
difficult. Control must be
exercised over the rate of flow of
the materials.
Continuous extraction of products
at all times during the reaction.
The installations are
interconnected, so a fault in one
causes a stoppage in all the
others. Material that has been
damaged cannot be repaired
under the same working
conditions. It must be isolated
and the process restarted.
Preferable for large scale
production.
Preferable for a central and
permanent product.
Preferable after the conclusion of
all the stages of grossing-up and
economic feasibility tests.
Note!
In both methods:

Recycling of raw materials according to economic feasibility can be carried out.

Required product quality can be attained in accordance with customer
requirements and economic feasibility.

Optimal efficiency can be attained according to reaction conditions and
economic feasibility.
Typical students’ errors:
A number of difficulties have come to light with students with regard to their grasp of
concepts linked to the batch and continuous processes. One of the most typical
errors is the tendency to link the flow chart to the continuous and not the batch
process. The error originates in the fact that the flow chart is perceived only as a
succession of installations and not as a succession of actions that may take place in a
batch installation, and also in one reactor.
Another mistake is to link the possibility of recycling materials only to the continuous
process. This is also apparently related to an erroneous perception of the flow chart.
These students view the flow chart as describing only the continuous process, and
therefore recycling can only take place in the continuous process.
Reactant(s)
 Reaction  Separation  Product(s)
Recycling
(reactants that did not react)
In addition, there are students who link product quality (its level of cleanliness) or the
process efficiency with the method used. The students generally link better quality
with the product manufactured using the batch process, and better efficiency with the
continuous process.
Apparently, the batch process, because it is non-automatic, is perceived by these
students as a process that can be easily controlled.
On the other hand, the
continuous process has a higher efficiency because the reactants that did not react
can be recycled.
While teaching this subject it is important to note these errors and emphasize the
characteristics of each of the methods and the differences between them. The table
on the previous page will be of assistance in this.