Types of reactor

Types of reactor

• The confines in which chemical reactions occur are called reactors.
• In chemical engineering chemical reaction or set of reactions is carried out in a reaction vessel that is in chemical reactors.
• All chemical processes are centered in a chemical reactor.
• The design of a chemical reactor is the most important factor in determining the overall process economics.
• While doing any operation we have to first decide what reactor type and reactor shape to select and whether it would be advantageous to operate in batch or continuous mode.
• Ideal reactors have three ideal flow or contacting patterns. They are main basic type of chemical reactor

1. Batch reactor
2. Flow reactor

• Continuous Stirred-Tank Reactor(CSTR)
• Plug Flow Reactor (PFR)

Batch reactor

• Whether the system does not exchanges mass with its surroundings, then the system is called a batch reactor.
• The batch reactor is simply a container with an agitator and an integral heating/cooling system.

• The reactants are initially charged into a container, are well mixed, and are left to react for a certain period.
• The resultant product mixture is then discharged.
• This is an unsteady-state operation where composition changes with time; however, at any instant the composition throughout the reactor is uniform
• During the operation, no addition or withdrawal is made.
• The experimental batch reactor is usually operated isothermally and at constant volume because it is easy to interpret the results of such runs.
• This reactor is a relatively simple device adaptable to small-scale laboratory set-ups, and it needs but little auxiliary equipment or instrumentation.
• Thus, it is used whenever possible for obtaining homogeneous kinetic data.

Advantages

• Suitable for small scale production
• Suitable for processes where a range of different products or grades is to be produced in the same equipment
• Suitable for reactions requiring long reaction times
• Suitable for reactions with superior selectivity

Limitation

• Not suitable for large batch sizes.
• It is a closed system in which once the reactants are added in the reactor, they will come out as products only after the completion of the reaction.

Application

• Batch processes are used in chemical (inks, dyes, polymers) and food industry.

Continuous Stirred-Tank Reactor (CSTR)

• The flow reactor is used primarily in the study of the kinetics of heterogeneous reactions.
• The first of the two ideal steady-state flow reactor is called the mixed reactor, the backmix reactor, the ideal stirred tank reactor, CSTR, or the CFSTR (constant flow stirred tank reactor)
• As its names suggest, it is a reactor in which the contents are well stirred and uniform throughout.
• In CSTR the reactants are fed to the reactor and the products or byproducts are withdrawn in between while the reaction is still progressing.

• The CSTR is normally run at steady-state and is usually operated so as to be quite well mixed.
• This type of flow is mixed flow, hence this reactor also called the mixed flow reactor or MFR.
• The exit stream from this reactor has the same composition and temperature as the fluid within the reactor.
• Continuous reactors are usually preferred for large scale production.

Advantages

• Highly flexible device
• By products may be removed in between the reaction.
• It is economically beneficial to operate several CSTRs in series or in parallel.
• Reaction can be carried out in horizontal as well as vertical reactors.

Limitation

• More complex and expensive than tubular units.
• All calculations performed with CSTRs assume perfect mixing.
• At steady state, the flow rate in must equal the flow rate out, otherwise the tank will overflow or go empty.

Application

• Chemical industry especially involving liquid/gas reactions.

Plug Flow Reactor (PFR)

• The other ideal steady-state flow reactors are variously known as the plug flow, slug flow, piston flow, ideal tubular, and unmixed flow reactor.
• It consists of a cylindrical pipe and is normally operated at a steady-state, as is the CSTR.

• This reactor is known as the plug flow reactor, or PFR because the flow in the reactor is highly turbulent and the flow field may be modeled by that of plug flow.
• It is characterized by the fact that the flow of fluid through the reactor is orderly with no element of fluid overtaking or mixing with any other element ahead or behind.
• Actually, there may be lateral mixing of fluid in a plug flow reactor; however, there must be no mixing or diffusion along the flow path.
• Hence in the tubular reactor, the reactants are continually consumed as they flow down the length of the reactor.
• The necessary and sufficient condition for plug flow is for the residence time in the reactor to be the same for all elements of fluid.

Advantage

• Higher efficiency than a CSTR of the same volume
• PFRs may have several pipes or tubes in parallel
• Both horizontal and vertical operations are common
• They can be jacketed
• Reagents may be introduced at locations even other then inlet

Limitation

• Not economical for small batches

Application

• The tubular reactor is especially suited to cases needing considerable heat transfer, where high pressures and very high or very low temperatures occur