12
Oct
Reactors: Heart of Chemical Process
Comments
- Chemical reactors are often regarded as the nucleus of any chemical plant, housing the chemical reactions that transform raw materials into high-value products.
- In the realm of chemical engineering, a chemical reactor represents a fundamental unit of operation, crucial to process analysis.
- Design of a chemical reactor is pivotal to maximizing the net present value of a given chemical reaction. Designers aim to ensure that the reaction proceeds with optimal efficiency, resulting in a high yield of the desired product while minimizing financial investment.
BATCH Reactors
Batch reactors are used for most of the reactions carried out in a laboratory. The reactants are placed in a test-tube, flask or beaker. They are mixed together, often heated for the reaction to take place and are then cooled. The products are poured out and, if necessary, purified.
This procedure is also carried out in industry, the key difference being one of size of reactor and the quantities of reactants.
- Following reaction, the reactor is cleaned ready for another batch of reactants to be added.
- Batch reactors are usually used when a company wants to produce a range of products involving different reactants and reactor conditions. They can then use the same equipment for these reactions.
- Examples of processes that use batch reactors include the manufacture of colorants and margarine.
Continuous reactors
- An alternative to a batch process is to feed the reactants continuously into the reactor at one point, allow the reaction to take place and withdraw the products at another point. There must be an equal flow rate of reactants and products. While continuous reactors are rarely used in the laboratory, a water-softener can beregarded as an example of a continuous process. Hard water from the mains is passed through a tube containing an ion-exchange resin. Reaction occurs down the tube and soft water pours out at the exit.
- Continuous reactors are normally installed when large quantities of a chemical are being produced. It is important that the reactor can operate for several months without a shutdown.
- The residence time in the reactor is controlled by the feed rate of reactants to the reactor.
Types of Continuous Reactors
Tubular reactors
- In a tubular reactor, fluids (gases and/or liquids) flow through it at high velocities. As the reactants flow, for example along a heated pipe, they are converted to products.
- At these high velocities, the products are unable to diffuse back and there is little or no back mixing. The conditions are referred to as plug flow. This reduces the occurrence of side reactions and increases the yield of the desired product.
- With a constant flow rate, the conditions at any one point remain constant with time and changes in time of the reaction are measured in terms of the position along the length of the tube.
- The reaction rate is faster at the pipe inlet because the concentration of reactants is at its highest and the reaction rate reduces as the reactants flow through the pipe due to the decrease in concentration of the reactant.
Types of Continuous Reactors
Fixed bed reactors
- A heterogeneous catalyst is used frequently in industry where gases flow through a solid catalyst (which is often in the form of small pellets to increase the surface area). It is often described as a fixed bed of catalyst
- Among the examples of their use are the manufacture of sulfuric acid (the Contact Process, with vanadium(V) oxide as catalyst), the manufacure of nitric acid and the manufacture of ammonia (the Haber Process, with iron as the catalyst).
- A further example of a fixed bed reactor is in catalytic reforming of naphtha to produce branched chain alkanes, cycloalkanes and aromatic hydrocarbons using usually platinum or a platinum-rhenium alloy on an alumina support.
Types of Continuous Reactors
Continuous stirred tank reactors, CSTR
- In a CSTR, one or more reactants, for example in solution or as a slurry, are introduced into a reactor equipped with an impeller (stirrer) and the products are removed continuously. The impeller stirs the reagents vigorously to ensure good mixing so that there is a uniform composition throughout. The composition at the outlet is the same as in the bulk in the reactor. These are exactly the opposite conditions to those in a tubular flow reactor where there is virtually no mixing of the reactants and the products.
- A CSTR reactor is used, for example in the production of the amide intermediate formed in the process to produce methyl 2-methylpropenoate. Sulfuric acid and 2-hydroxy-2-methylpropanonitrile are fed into the tank at a temperature of 400 K. The heat generated by the reaction is removed by cooling water fed through coils and the residence time is about 15 minutes.
