Chemical engineering is partially about taking things apart to separate desirable products from undesirable products, and partially about putting things together to make new substances. These are loosely called “separations” and “reactions” and there are dozens of ways to do either of these.

The continuous stirred tank reactor (CSTR), used to combine chemicals in a vessel, is certainly among the later. This device gradually meters reactants in the right ratio, contains the reaction and permits removal of the new product, often without disrupting the reaction.

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A bench-scale laboratory CSTR for mixing two liquids. The reactants are stored in the brown bottles, the actual CSTR is in the center (with several probes and a heating coil), and the heating water circuit is on the right. Source: Seth PriceA bench-scale laboratory CSTR for mixing two liquids. The reactants are stored in the brown bottles, the actual CSTR is in the center (with several probes and a heating coil), and the heating water circuit is on the right. Source: Seth Price

Batch processing vs. continuous processing

In high school chemistry class, reactions are performed in small beakers or reaction vessels. The reactants are dumped from separate vessels into a single vessel and the reaction occurs. Then, the product is removed. This is called batch processing, because a “batch” of material is produced at discrete times.

If instead of making 200 ml of a new chemical in a chemistry laboratory, there is a need to make 200 gallons per hour, the solution is not to buy lots of beakers. Instead, equipment is modified to make a continuous reaction, meaning a continuous stream of reactants enters a reactor and a continuous stream of product leaves the reactor.

Not sure if a process is batch or continuous? Batch processes are in units of volume, parts or some other discrete quantity. Continuous processes are discussed in terms of rate, such as liters per hour, gallons per minute and so on.

What is a CSTR?

A CSTR is a reaction vessel that produces chemicals continuously, as the name implies. In the CSTR, two or more reactants are pumped into a vessel and allowed to react, with the product skimmed or drained away at the opposite end of the reactor.

Parts of the CSTR

The simplest CSTR will have a minimum of two feed ports for reactants to enter the vessel and one exit port for the product to leave the vessel. It will also have a motor with a stirring auger to mix the products. More complex designs will have baffles to help improve the mixing of the reactants, a heating/cooling jacket or coil to control the rate of reaction, and two or more pumps for moving chemicals to the feed ports.

A schematic of a simple CSTR. Source: Daniele Pugliesi/CC BY-SA 3.0A schematic of a simple CSTR. Source: Daniele Pugliesi/CC BY-SA 3.0

There are only a few controls on a CSTR, but they can drastically alter the throughput and the purity of the product. The most obvious impact is the feed rates of the pumps used to inject reactants into the CSTR. Chances are, the pumps will move a certain volumetric flow rate, and the engineers will need to know the stoichiometry of the reaction to ensure proper flow rates — both the ratio between reactants and the minimum and maximum for predictable reactions and heat management (for exothermic reactions). Other controls are the reactor temperature, which may involve changing the flow rate of a cooling fluid or heating fluid flow rate, the stirring rate and so on.

In terms of instrumentation, a CSTR may have a thermocouple or other temperature sensor that is used in a feedback loop to verify the operation of the temperature control. They may also have a means to ensure the quality or the steady-stateness of the reaction. This could mean measuring conductivity, pH or some other similar method.

For system surety, there may be level sensors to monitor the volume of material in the tank. This can be used to ensure the pumps are operating within specifications, or to prevent the temperature controller from heating an empty tank, check for leaks and other such problems.

Advantages of a CSTR

A CSTR is a relatively straight-forward device that can replace batch reactions for increasing throughput. The simplicity of design means there are only a few options when troubleshooting is required, and little maintenance and cleaning is needed.

The residence time of chemicals in the chamber and the mass balances are easy to calculate, as the CSTR will eventually run at steady-state operation. Batch reactions are rarely steady-state, as the reactions themselves are using up reactants and changing the concentration gradient between products and reactants.

With only a few controls and some watchdog processing monitoring variables, the CSTR is easy to explain and train to new employees.

Common assumptions

The idealized CSTR meets several assumptions. First, it is assumed that the agitator does its job and that the reactants are well mixed. When reactants are well mixed, this maximizes the surface area between the two substances, increasing the likelihood and completion of reaction. This also means the agitator is mixing enough to overcome density differences between liquids, preventing them from separating due to settling. Finally, it also leads to the assumption that the temperature will be constant across the reactor, as the mixing ensures that no hot pockets or cold pockets of chemicals develop during mixing.

The next assumption is that the feed rates and extraction rate are constant. This means that the rate of reaction will not change throughout the process, and that the residence time that any atom sits in the reactor will simply be the total reactor volume divided by the feed rate.

Another assumption is that the reactor is at steady-state. Obviously, during start-up and shut-down, the reaction is in a transient period, and so engineers must account for this when performing calculations. Once the reactor is running and reaches steady-state, certain measures of changes approach zero, and some of these equations are simplified. The temptation is to use these equations all of the time, and that is not appropriate during non-steady-state times.

Another important note is that CSTRs are useful when there is only one major reaction, and that it is non-reversible. If the reaction can run in both directions, or if there are several side reactions that occur, the final product’s composition must be constantly evaluated. Depending on the circumstances, a CSTR may not be a good fit for reactions with multiple side reactions or reactions that are reversible, and another reactor type should be used.

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A CSTR is just one type of continuous chemical processing equipment, and just one type of reactor. Even so, they see extensive use in the chemical processing industry due to their simplicity and versatility.

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