Types of Reactors: Basic and Pharma Classification

Every pharmaceutical industry handles chemicals to carry out certain reactions for manufacturing their desired product.

Different processes require different types of reactors. Before directly looking at those types of reactors, let us go through the fundamental classification of reactors according to Chemical Engineering.

Two basic types of chemical reactions are dealt in chemical processing plants.

1. Homogenous Catalysis (Reactant and Catalyst in same phase)
2. Heterogeneous Catalysis (Reactant and Catalyst in different phase)

The types of reactors classified based on chemical kinetics, which deals with the rate of reaction and its mechanisms with the help of catalysts.

Catalysts are substances when introduced in the chemical process, increases the rate of reaction, and drives the reaction at a faster rate.

Types of Reactors per Chemical Aspects

To carry out such chemical reactions, we require extraordinary control for the conversion of reactants into products. We term these controlling pieces of equipment as Reactors.

Therefore, choosing appropriate types of reactors for production depends on the expected output from the reaction mass.

Apart from that, operating conditions like pressure, temperature, and mixing speeds become necessary to run the process optimally.

Based on this, reactors are selected from the following different types.

1. Batch (Batch and Semi-batch)
2. Continuous (Continuous Stirred Tank Reactor)
3. Tubular (Plug Flow Reactor)
4. Catalytic (Fixed and Fluidized Bed)

Remember, these are the fundamental classification of reactors and not the typical ones. Sub-types of these briefed further (Image credits to Wikimedia).

Batch Reactors

Once the reactants added to the reactor, no third component introduced, and also no component is taken out till the reaction completes.

So the process will run as a single batch till it produces the desired level of process conversion.

Semi-batch operations involve the progressive addition of one or more reactants during the reaction till it achieves the desired conversion.

Else, the reactants added at once and progressively removed the converted product. Among all the fundamental reactors, this type used frequently in pharmaceutical industries.

Here are few highlights of the ideal batch reactor.

• Uniform composition throughout the reactor.
• Non-steady state mostly.
• They are cost-effective in terms of instrumentation and construction than the continuous type of reactors.
• The size of the reactor depends on the volume of material that is to be charged instead of reaction time.
• For small capacity plants handling the requirement of 4 to 5 tonnes/day, batch reactors are more economical than other types.

Though being the above highlights, it has some cons too.

• It requires tight temperature control due to the poor handling of highly heat-sensitive reactions.
• Quality from each batch may vary.
• Consumes significant resources.

Typical representation of batch reactor is shown below.

Continuous (Steady-State)

Unlike the batch reactor, a continuous stirred tank reactor carries out the reaction simultaneously, i.e. reactant addition and product removal simultaneously.

The level of reaction mass maintained constant such that the internal composition of the reaction mass is the same as discharged mass. Apart from other types of reactors, this type involves more common use in chemical process plants.

Here are few highlights of the continous flow reactors.

• Being at steady-state, composition at any point does not change with time.
• Optimum quality parameters can be maintained for desired product conversions.

Tubular OR Plug Flow Reactors

Tubular types of reactors such as Plug Flow Reactors have cylindrical tube used to carry out the reaction.

Reactants enter one end, react, harvest the product while traveling through the tube, and exits at the other end.

They are more efficient than CSTR for the same volume because of turbulent flow at the inlet. They generally have a flat velocity profile resulting in radial mixing.

The time the particles of fluid spends in the plug flow is called Residence time and it is the same for all the particles.

Here are few highlights of the ideal plug flow reactor.

• No axial mixing is permitted i.e. no chance of diffusion between the different particles traveling through the reactor.
• The composition of the reaction mass is unchanged with time. Rather, it changes with the length of the reactor.
• Requires less volume to yield the same conversion as that of continuous steady-state reactors for any positive order reaction.
• It does not have any moving parts, therefore, promoting highly pressurized and corrosive reactions.

Catalytic Reactors

The driving force for these types of reactors involves heat transfer, mass transfer, and catalysts. Applications include Chemical Synthesis, Polymerization, Hydrogen Cracking, etc.

Common classification of these reactors decided by movement pattern of catalysts as given below;

1. Fixed Bed
2. Trickle Bed
3. Fluidized Bed

Fixed Bed Reactors

Fixed Bed of catalyst used to carry out heterogeneous catalyzed gas-phase reactions. They can build on single or multiple tubes as per requirement.

Two types of reactors in the Fixed bed category commonly include Packed bed and Multitube reactors.

Trickle Bed Reactors

Used most commonly for hydrogenation reactions, trickle bed reactors have a solid phase as catalyst carrying out gas-liquid reactions.

It introduces liquid reactants from the top and flows downward contacting the solid catalyst bed while it introduces gas reactants from the cross current direction for better contact surface area.

Fluidized Bed Reactors

A bed of solid particles supported with a pull of gas or liquid flowing in an upward direction at a velocity such that the solid bed behaves as a fluid. This phenomenon is known as Fluidized Bed and the reactor used for this called Fluidized Bed Reactor.

This was an overall picture of the fundamental classification of reactors and not an exhaustive one.

Below are the types of reactors based on their material of construction called out commonly in pharmaceutical-related industries.

Pharmaceutical Classification

Pharmaceutical or healthcare-related industries have ordered types of reactors based on the material of construction and process of interest specifically;

1. Stainless Steel Reactors
2. Glass Lined Reactors

The application and selection of these types of reactors depend on the process in scope. Chemically, three types of processes exist;

1. Acidic (pH<7) – Appropriate to use Glass Lined Reactors
2. Alkaline (pH>7) – Appropriate to use Stainless Steel Reactors
3. Neutral (pH=7) – Appropriate to use Stainless Steel Reactors

Stainless Steel Reactors (SSR)

Pharmaceutical and Biotechnological products have different safety requirements. Biotech product manufacturing follows more stringent requirements than bulk drugs.

According to this, there are two common types of reactors based on the material alloy configuration, i.e. SS304 and SS316. You can get more information in ASTM guidelines for this, though we’ll see it in short.

SS304 contains approx. 18% chromium and 8% nickel while 316 contains 16% chromium, 10% nickel and 2% molybdenum. This 2% molybdenum helps it to survive in corrosion promoting environment.

Hence, SS316 is more advanced than SS304. However, visible differentiation is not possible among these two and requires a material test report from the vendor or tester for concluding it as SS316L.

Also, if carbon is a problem, preference is given to SS304L or SS316L. In this, L stands for low carbon content. Regular 304 or 316 contains ~0.09% carbon, while the ‘L’ version has around 0.03% carbon. Carbon also contributes to corrosion during very high-temperature operations.

Bottom-line, SSR is more suitable for neutral or alkaline process operations. Though sometimes, weak acids like acetic acid can satisfactorily be processed in SS316L with a pH value as low as 3 because of less corrosion impact than other strong acids.

Glass Lined Reactors (GLR)

When dealing with strong acids like Nitric, Sulphuric, Hydrochloric, or Phosphoric acids, stainless steel reactors are non-sustainable and hence corrode. To avoid that, the reactors with glass lining installed as thick as 0.2mm because glass can sustain acids.

Glass offers complete protection against strong acids preventing wear and tear along with the time when handled carefully.

Glass Lined Reactors are not suitable for alkaline operations because of borosilicate commonly used in glass, which reacts with high pH solutions.

Alkalis are more corrosive than acids, especially at elevated temperatures. This may cause cracks in glass, which is undesired. Though, GLRs are more sustainable to a pH value of 8.5 i.e. till you use weak bases.

Stainless Steel Reactors possess a better heat transfer effect because of enhanced thermal conductivity. Apart from this benefit, GLRs have restricted operation at high pressures.

Though easy to clean, lightweight, cost-effective and corrosion resistance, the GLRs have delicate structure and components which require careful handling.

GLRs are seen in various colors according to their suitable purpose such as Blue, Light Blue, Green, White. Characteristics of each color in short.

• Blue – Higher thermal conductivity, a wide range of pH acceptance
• Light Blue – Highly resistant to alkalis, easy for visual observation
• Green – Suitable to operate under high temperatures up to 300ºC
• White – Easy visual observations in colorful chemical reactions like photochemical.

Above all, these types of reactors (SSR and GLR) require different specialized instruments according to their MOC like agitators, baffles, temperature probes, pH probes, valves, manhole, etc. as per process suitability and cleanability.

The types of reactors for chemical and pharmaceutical applications described above were focused to give you a fundamental idea.

SSR and GLR: FAQs