The economics of the overall process is largely decided by the chemical step. To carry out a chemical step, best possible reactor is used so chemical reactor may be thought as a heart of the chemical process. Chemical reactors vary widely in size and shape and in method of operation/mode of operation. Based upon the shape of reactor, we have tank reactors and tubular reactors. As per the mode of operation (mode of operation refers to how reactants are fed to and how the products are discharged from reactor), we have batch reactors, semibatch reactors and continuous flow reactors. Batch reactors are used when relatively small amounts of material are to be treated while continuous flow reactors are used when large quantities of material are to be treated. The semibatch reactor is used when good control of reaction is required as the reaction proceeds only when reactant(s) are added.
Types of Reactors
- Batch Reactor
- Semi-Batch Reactor
- Continuous Stirred Tank Reactor (C.S.T.R)
- Plug Flow Reactor (P.F.R)
- Fixed Bed Reactor
- Fludised Bed Reactor
Batch Reactor
In case of batch reactor, the reactants are charged at the beginning into a container, left to react for certain time period in the reactor under agitation during which no material is fed or removed from it, and the resultant product mixture is withdrawn at a subsequent time.
The batch reactor is characterised by the variation of extent of reaction (conversion) and properties of the reaction mixture with time. It operates under unsteady state conditions as in it composition changes with time; however, at any instant of time the composition throughout the reactor is same.
Batch reactor consists of a vertical cylindrical vessel equipped with an agitator/stirrer for stirring the contents. It is provided with either external jacket or cooling coil or both for heating or cooling the reactor contents.
Advantages of batch reactor
- It is simple in construction.
- It is simple to operate.
- It has flexibility of operation (may be shut down quickly and easily).
- Its cost is relatively low.
- It requires small instrumentation and less supporting equipment.
- It can give high conversion that can be obtained by leaving the reactant in the reactor for long periods of time.
Disadvantages of batch reactor
- High labour costs per unit volume of production.
- Requires considerable time to empty, clean out and refill.
- Poorer quality control of product. It is difficult to maintain the same quality in different batches.
- Large scale production is difficult.
Applications of batch reactor
Batch reactors are often used for liquid phase reactions when the required production rates are low i.e. a batch reactor is used for small scale production, to produce many different products from the same piece of equipments, to carry out reactions having long reaction times, for testing new processes that have not been fully developed, for kinetic study, and for manufacture of expensive products such as pharmaceuticals, dyes, dye intermediates, etc.
Semi-Batch Reactor
In semibatch reactor, one of the reactants (say A) is charged initially and the other reactant (say B) is added continuously to the reactor over a certain time period under agitation. After addition is over, holding of reactor mass is done for certain time period and finally the product mixture is withdrawn from the reactor.
This reactor offers good control of reaction speed because the reaction proceeds as the reactant is added. So when the heat of reaction is large, the evolution of heat energy can be controlled by regulating the rate of addition of the reactant (say B). The construction features of semibatch reactor are same as the batch reactor.
Advantages of Semi-Batch Reactor
Semibatch reactor has advantages of good temperature control and the capability of minimizing unwanted side reactions by maintaining a low conversion of one of the reactants.
Disadvantages of Semi-Batch Reactor
The disadvantages of this reactor are essentially same as that of batch reactor.
Applications of Semi-Batch Reactor
Semibatch reactor in used for liquid phase reactions and also for two-phase reactions in which the gas is continuously bubbled through the pool of liquid in the reactor.
Continuous Stirred Tank Reactor (CSTR)
This is probably the most common type of reactor used in chemical industry. It is a vertical cylindrical vessel equipped with mechanical stirrer and provision for heat transfer (either jacket, coil or both).
It is also referred to as backmix or mixed flow reactor. It is very commonly used in industrial processing. It has provisions for continuous inflow of reactants and outflow of the product mixture. A certain pool of reaction mixture is always maintained in it. It normally runs at steady state and is usually operated so as to be quite well-mixed. So there is no variation of concentration, temperature or reaction rate throughout the reactor volume.
Advantages of CSTR
- It is relatively easy to maintain good temperature control with CSTR.
- It is possible to operate CSTR under isothermal condition even when heat of reaction is high.
- CSTR required large volumes they provide long residence time. The isothermal nature together with long residence time permits the use of CSTR at the optimum temperature for long reaction time.
- CSTR may be used in single or in series.
Disadvantages of CSTR
- With these reactors, the conversion of reactant per unit volume of reactor is smallest of the flow reactors and consequently very large reactors are required to obtain high conversion.
- For high pressure reactions CSTR are not recommended because of cost consideration. - CSTR operated at high pressures requires a large wall thickness and complex sealing arrangements required for agitator increases both initial and maintenance cost.
- They are also not suited for reactions with very high heat of reaction due to low area heat transfer is available per unit volume (as compared to tubular reactor) and low heat transfer coefficients.
Applications of CSTR
CSTRs are normally employed on commercial scale, mainly for liquid phase reactions at low or medium pressures. These can be used when the heat of reaction is high only if the temperature level obtained in their isothermal operation is satisfactory from other stand paints.
Plug Flow Reactor (PFR)
Tubular reactor (.e. plug flow reactor) consists essentially of a single continuous long tube or several tubes arranged in parallel (tube bank). The reactants enter at one end and product mixture leaves from the other end with continuous variation of composition of reaction mixture in between (along length). Heat transfer can be accomplished either by use of jacket shell and tube type design.
Advantages and Disadvantages of PFR
Tubular reactors are relatively easy to maintain as there are no moving parts and gives the highest conversion per unit volume of reactor of any of the flow reactors.
It is difficult to control the temperature with the reactor and hot spots can occur in case of exothermic reaction system.
Tubular flow reactors are well suited for high pressure reactions because of cost considerations- as it uses small diameter tubes and for reactions with high heat of reaction as the rate of heat transfer per unit volume of reaction mixture is high (high ratio of heat transfer surface per unit volume of reactor and higher heat transfer coefficients).
A large number of commercially important reactions are of the fluid-solid class and are carried out in tubular reactors. e.g. catalytic cracking of petroleum, oxidation of SO₂ ammonia synthesis, etc.
Most homogeneous liquid phase reactors are CSTRs and most homogeneous gas phase flow reactors are tubular reactors.
Fixed Bed Reactor
A fixed bed (packed bed reactor) reactor is essentially a tubular reactor which is packed with solid catalyst particles. In this reactor, reaction mixture is passed through the mass of small catalyst particles. It consists of a cylinder of large diameter with multiple catalyst beds or many tubes in parallel packed with catalyst. This reactor makes use of relatively large size catalyst particles for lower pressure drop.
Such type of heterogeneous reaction system is used commercially to catalyse gas phase reactions. It is difficult to control temperature and formation of hot spots is there in the reactor when the reaction in exothermic. In case of this reactor, replacement of the catalyst in trouble some i.e. regeneration on continuous basis is not possible. There are also chances of channeling of gas flow-to occur that result in ineffective use of parts of the reactor bed.
Advantages of Fixed Bed Reactor
The advantage of the fixed bed reactor is that it gives the highest conversion per unit weight of catalyst of any catalytic reactors for most reactions.
Fludised Bed Reactor
It is another catalytic reactor in common use and is analogous to the CSTR in that its contents, though heterogeneous, are well-mixed that result in even temperature distribution throughout the bed.
It consists of vertical cylindrical vessel containing fine solid catalyst particles. The fluid stream (usually gas) is introduced through the bottom of the reactor at a rate such that solids are suspended in the fluid stream without being carried out. Under these conditions the entire bed of catalyst particles behaves like a boiling liquid which tends to make the composition of reaction mixture and temperature throughout the bed uniform. As temperature is uniform throughout there are no hot spots in this reactor. With this reactor, it is possible to regenerate the catalyst continuously without shutting down the reactor. This reactor is particularly suitable when the heat effect is very large or when frequent catalyst regeneration is required.
This reactor can handle large amounts of feed and catalyst and has good temperature control and therefore, it is used in a large number of applications. The advantages of ease in replacement of catalyst or regeneration of catalyst on continuous basis without shutting down reactor are sometimes offset by high cost of reactor and catalyst regeneration equipment.