1 Chemical Reactors

A chemical reactor is a closed container where a chemical reaction is carried out. Process designers have to make sure that the reaction progresses as efficiently as possible towards the intended product, resulting in the best yield while needing the least money to buy and operate. Energy intake, energy disposal, cost of raw materials, labor, and other charges are all part of regular operational expenses.

The reactant and products are usually fluids (gases or liquids). Continuous reactors are usually operated at a constant state, whereas batch reactors are operated in a transient state. A reactor is transient when it is turned on for the first time or after a shutdown, and critical process variables vary over time.

Three idealized models are used to determine the basic process variables of diverse chemical reactors:

  • Batch reactor
  • Continuous stirred-tank reactor (CSTR)
  • Plug flow reactor (PFR)

Many reactors in the process industry can be characterized as combining these fundamental types. Main process variables associated with the reactors include:

  • Volume (V)
  • Residence time (t)
  • Temperature (T)
  • Pressure (P)
  • Concentrations of species (C1, C2, … Cn)
  • Heat transfer coefficients (U, h)

A packed bed is frequently used in tubular reactors. The channel or tube comprises particles, generally solid catalysts in this scenario. The reactants are pushed through the catalyst bed in liquid or gas form. A fluidized bed can also be used as a chemical reactor.

Chemical processes in a reactor can be exothermic (gives out heat) or endothermic (absorbs heat). Tubular reactors can be built as heat exchangers if the reaction is highly exothermic.

2 Types of Reactors

2.1 Batch Reactor

A batch reactor is a non-continuous form of reactor consisting of a closed vessel wherein reactions occur. Initially, all of the reactants are added to the reactor simultaneously. Batch reactors usually have an agitator that mixes the reactants thoroughly to execute the reaction and synthesize the product effectively.

Exothermic reactions are typically treated using cooling coils in batch reactors. The batch reactor is a non-stable, transitory reactor. It means that the quantity of conversion in the reactor varies with time. The batch reactor nature remains quite uniform because of the agitation. It implies that the degree of conversion is unaffected by the placement of the reactor. The extent of reaction in any part of the reactor remains equal at all times.

2.1.1 Advantage

The versatility of a batch reactor is its most significant advantage. A single batch reactor can deliver different products. Batch reactors are very effective when a reaction produces a large number of products.

2.1.2 Disadvantage

A batch reactor has the drawback of requiring a lot of effort to charge reactants, release products, and clean the reactor for the next batch.

2.2 Continuous Stirred Tank Reactor

A continuous stirred tank reactor (CSTR) is also known as a mixed flow reactor. In this reactor, the reaction takes place in a closed tank. An agitator is also included in the tank to make sure that the reactants are well mixed.

The reactants enter the reactor at a constant flow rate, react within the vessel for a time indicated by the space-time of the reactor, and then produce products. All products flow out of the reactor at the same time. The time it takes to execute one reactor volume is equivalent to one space-time.

The agitator maintains a constant concentration throughout the reactor. It also implies that the amount of conversion is unaffected by position in the reactor. The volume of the reactor determines the conversion extent.

2.2.1 Advantage

The main advantage of employing a CSTR in the industry is generating a massive product volume. It is a continuous steady-state reactor that can run for an extended length of time.

2.2.2 Disadvantage

CSTRs are unsuitable for reactions with extremely slow kinetics that often need a very large volume reactor. The operation costs of the reactor may render it unfeasible. In this situation, a batch reactor is employed.

2.3 Plug Flow Reactor

A plug flow reactor (PFR) is also known as a continuous tubular reactor (CTR). The plug flow reactor model describes chemical reactions in cylindrical, continuous flows.

2.3.1 Advantage

The benefit of PFR over CSTR is that the PFR has a lower volume than a CSTR for the same space-time and conversion level. It indicates that the reactor requires less space and that the amount of conversion is higher in PFR than in CSTR for the same reactor volume. The PFR is frequently used to determine the kinetics of the gas-phase catalytic process.

2.3.2 Disadvantage

Performing an exothermic reaction in a PFR is difficult to control because of the wide range of temperature profiles. Operational and maintenance expenditures for a PFR are more expensive than the CSTR, which is less expensive.

2.4 Semi-Batch Reactor

A semi-flow reactor is a batch reactor modification. It is a closed vessel with an agitator to mix the reactants. One reactant is completely charged in the reactor initially, while the other is charged after regular time intervals. So, one chemical reactant is filled into the reactor, and the other chemical is added slowly (e.g., to prevent side reactions), or a product formed by a phase transition is continuously separated, such as gas formation during the reaction, precipitation of solids, or formation of hydrophobic product.

2.4.1 Advantage

Using a semi-batch reactor while executing many reactions provides better control over the yield and product selectivity. This reactor comes in helpful when executing an exothermic reaction since the continuous flow of the other reagent may be regulated to control the exothermic reaction better.

2.4.2 Disadvantage

Clean reactors, clean blades, and more labor are all needed to charge and discharge a reactor properly.

2.5 Catalytic reactor

Catalytic reactors are frequently deployed as plug flow reactors; however, their calculations demand a more complicated technique. The amount of catalyst that the reagents come into contact with and the concentration of the reactants determine the catalytic reaction rate. A catalytic reaction pathway frequently happens with chemically bound intermediates in numerous phases. The kinetics may be affected by the chemical bonding to the catalyst, which is itself a chemical process. Catalytic processes commonly display so-called faked kinetics, in which the perceived kinetics vary from the true chemical kinetics due to physical transport factors.

Catalysts are deactivated by coking, poisoning, and sintering, especially in high-temperature petrochemical processes. There are vast process operations in industries where catalyst reactors are required to carry out sophisticated and complex reactions.

2.5.1 Advantages

Some chemical reaction needs a specific catalyst to execute the reaction with suitable kinetics. Without a suitable catalyst, some reactions cannot proceed with economic kinetics. In general, the catalyst’s function is to provide an alternate, low-energy route for a process. Cracking of petroleum, hydrogenation of unsaturated hydrocarbons, ammonia synthesis, sulfuric acid synthesis, etc., are all examples of reactions that require some sort of catalyst.

2.5.2 Disadvantages

Catalyst reactors require special maintenance, and it is difficult to control. A very controlled environment and experience are needed to carry out the specific reaction at an economical rate. There is also a chance of degradation of the expensive catalyst if the conditions are not optimized.

3 References

  1. Doble, M. (2007). Perry’s chemical engineers’ handbook. McGraw-Hill, New York, USA.
  2. Prud’homme, R. (2010). Flows of reactive fluids(Vol. 94). Springer Science & Business Media.
  3. Allen, D. T. (1999). The Engineering of Chemical Reactions. American Institute of Chemical Engineers. AIChE Journal, 45(8), 1836.
  4. Levenspiel, O. (2013). Chemical Reactor Omnibook-soft cover. Lulu. com.
  5. Jakobsen, H. A. (2008). Chemical reactor modeling. Multiphase Reactive Flows.
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