What is the Difference Between a Continuous and a Batch Process?

In my previous article “What is a Distributed Control System (DCS)?”, I explained that a process is a series of steps that change or refine raw materials to create new products.

In a process, materials, in the form of liquids, gases, or slurries, are transferred through a series of operations where they are measured, mixed, heated, cooled, filtered, and stored to produce an end product.

As a general rule, a process is a one-way street — once an end product has been produced, it cannot be distilled back to the raw materials that it is made from.

What we didn’t talk about in that post was the types of processes that you may encounter in industrial automation. In general, every process can be defined as a continuous or batch process.

In this post, I will explain the difference between a continuous and a batch process and the applications that each type of process is suited for.

To start with, let’s define what a batch process is.

What is a Batch Process?

In manufacturing, a batch process is a process that creates an end product stage by stage over a series of workstations. In a batch process, the manufacturer has a clear start and end point.

The manufacturer starts the batch process with a specific and finite quantity of raw materials and produces a quantity of finished product (also known as a batch).

Once one batch is complete, the manufacturer starts the process again to produce another batch.

Batch Process Example

A mixing tank is an example of a batch process.

In a mixing tank, raw materials are fed into the tank through a feed.

These raw materials are raised to a specific temperature.

Once the raw materials are at the correct temperature, they are mixed using an agitator.

After being mixed for a period of time, the end product is released from the mixing tank.

If another batch of end products needs to be produced, these steps are repeated.

Batch Process Recipes

In the mixing tank example above, you may have noticed that there is a set sequence of steps used in a batch process to produce the end product.

In the mixing tank example above, raw materials were fed into the tank, heated to the correct temperature, and mixed for a defined period of time to produce the end product.

In a batch process, a recipe defines the procedure (or order of steps) and quantities of materials used for producing a batch of end products.

Thanks to the S88 standard for batch control, end users can rapidly change batch recipes to alter the end product without changing any program code. The S88 standard defines a way to separate the recipe from the control of equipment.

In our example, the PLC controlling the mixing tank knows how to heat the tank because a heating routine is defined in the PLC’s application code. In the recipe, an end-user can specify the temperature that the tank should be heated to in order to produce the desired end product.

The use of recipes in batch processes makes it easy for process engineers and operators to adjust a batch process without having to dive into the PLC code.

The most effective batch process control systems are the ones that provide the most flexibility to produce many products using the same equipment through recipe management. Batch process recipes allow companies to produce different flavors of ice creams, different kinds of soft drinks, different styles of beer, and different pharmaceutical products in one factory with standardized equipment.

What is a Continuous Process?

In contrast to a batch process, a continuous process does not have a defined start or end point. In a continuous process, materials are continuously processed as they are transferred through the process.

Each machine in a continuous process operates at a constant state while performing one dedicated function. For example, in a dry pasta production line, an extruder constantly extrudes dough into long strings of spaghetti.

Operators try to run continuous processes with as few breaks as possible. In certain continuous processes, the next planned shutdown of a process may be more than one year away.

To keep the process running continuously, operators work in rotating shifts and provide perpetual support to the process.

Continuous Process Examples

Certain industries tend to operate continuous processes. These industries include chemical, oil and gas, paper, pharmaceutical, water treatment, and power industries.

The individual continuous processes used in these industries include oil refining, paper making, power generation, and, of course, dry pasta production.

Continuous Versus Batch Processes

Now that we know what a continuous and a batch process are, we should ask why an end-user would choose to make their process a continuous or batch process.

Why Design a Continuous Process?

Very often, the main driver in designing a process as a continuous process is cost. Since it is very capital intensive to build a production facility, the management of the facility may want to run the process continuously to maximize the operating time of the facility.

Another driver may be the operating conditions required by a process. If a process requires extreme operating conditions, such as a very high temperature, then it may make sense to reach the temperature once and then continuously run the process while maintaining this temperature.

Why Design a Batch Process?

In contrast, a process that requires flexibility may be best designed as a batch process. For example, a facility that produces ice cream needs to change its production recipe to produce different flavors.

Since the process has to be adjusted regularly, it makes sense to design this process as a batch process.

Where are the Grey Areas?

Some processes may be designed as a continuous process or a batch process. In these processes, a continuous and a batch process will yield similar results so the decision to design the process as a continuous or a batch process is a management decision.

For example, an oven in a bakery may be designed as a continuous or a batch process.

In a batch process, the dough would be loaded into the oven and kept there for a period of time. Once the process is complete, the oven can be opened and the baked bread can be removed.

In a continuous process, the dough would be transported through the oven without stopping on specially-designed conveyor belts. When the bread reaches the end of the oven, it would be baked to perfection.

In cases like these, management will choose to implement the process that best fits the manufacturing facility.

Wrap Up

In this post, I explained the difference between a continuous and a batch process and looked at some of the reasons why a manufacturing facility would choose to design a process as a continuous or a batch process.

In a future post, I will take a deeper dive into the S88 Standard for Batch Control which I mentioned in this post. Be sure to subscribe to the mailing list below to be notified when that post is released.

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