Complete Guide To Reactor Design

What Are Reactors?

In a nutshell, a reactor is a vessel whereby chemical change occurs. In general, for chemical engineers we refer to these as chemical reactors. While the design principles are similar for non-chemical reactors, these types of vessels are more simplistic in their operation.

Chemical reactors are at the heart of any process system. This is the place where raw materials are converted into useful products, or into sub-products destined for refinement, purification and resale.

In this complete guide we will uncover everything there is to consider with regards to reactor design.

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The Basic Objectives

In chemical engineering, physical operations such as fluid flow, heat transfer, mass transfer and separation processes play a very large part in the overall design considerations of any reactor. In any manufacturing process where there is a chemical change taking place, the chemical reactor is at the heart of the plant.

In size and appearance it may often seem to be one of the least impressive items of equipment, but its demands and performance are usually the most important factors in the design of the whole plant. When a new chemical process is being developed, at least some indication of the performance of the reactor is needed before any economic assessment of the project as a whole can be made. As the project develops and its economic viability becomes established, so further work is carried out on the various other chemical engineering operations involved.

As a general statement of the basic objectives in designing a reactor, we can say that the aim is to produce a specified product at a given rate from known reactants. In proceeding further, however, a number of important decisions must be made, and there may be scope for considerable ingenuity in order to achieve the best result.

What Types Of Reactors Are Available?

Generally speaking, you can term almost anything a reactor. This can be a simple vessel like a glass jar or a plastic bottle. However, as engineers we are more concerned with the five most common types.

• Batch reactors
• Continuously stirred tank reactors (CSTR)
• Tubular plug flow reactors (PFR)
• Fixed bed reactors (FBR)
• Fluidised bed reactors

It is important to also define the terms homogeneous and heterogeneous system, which we discuss below, along with the key differences between each type and the design equations and principles that are generally applied.

Design Assumptions

Before designing any piece of equipment, it is important you clearly define any assumptions you make. These assumptions determine how complex, or how true to life, your design will actually be. There is a set of assumptions that most engineers will consider, and of course others depending on the industry and the nature of the design.

• Reaction kinetic model, i.e. power law
• Elementary reaction
• Steady-state or transient-state conditions
• Isothermal or non-isothermal conditions
• Adiabatic or non-adiabatic conditions

Each assumption determines the level of detail required during the design modelling stage. For example, if we assume steady-state conditions, we are also assuming we have no accumulation within the reactor, therefore neglecting the accumulation term in the general design equation.

With each assumption comes a loss of accuracy, as we know that there will always be an element of accumulation, since some material will spend longer inside the reactor than others, resulting in a different conversion and residence time.

The General Design Equation

When we design any reactor we have two design equations we must consider. One models the material using a mass balance, the other models the energy using an energy balance.

There are several key parameters that form the backbone of these equations: concentration, volume, temperature, pressure, enthalpy, residence time, and heat transfer coefficients.

Within the world of reaction kinetics, the importance of the reaction order is something that needs careful consideration. In simple terms, the order of the reaction is based on the stoichiometric coefficients of the reactants, and the order greatly influences the reaction rate equation.

Difference Between Heterogeneous & Homogeneous

In homogeneous reactors only one phase, usually a gas or a liquid, is present. If more than one reactant is involved, provision must of course be made for mixing them together to form a homogeneous whole. Often, mixing the reactants is the way of starting off the reaction, although sometimes the reactants are mixed and then brought to the required temperature.

In heterogeneous reactors two, or possibly three, phases are present, common examples being gas-liquid, gas-solid, liquid-solid and liquid-liquid systems. In cases where one of the phases is a solid, it is quite often present as a catalyst; gas-solid catalytic reactors particularly form an important class of heterogeneous chemical reaction systems. It is worth noting that, in a heterogeneous reactor, the chemical reaction itself may be truly heterogeneous, but this is not necessarily so.

In a gas-solid catalytic reactor, the reaction takes place on the surface of the solid and is thus heterogeneous. However, bubbling a gas through a liquid may serve just to dissolve the gas in the liquid where it then reacts homogeneously. The reaction is thus homogeneous, but the reactor is heterogeneous in that it is required to effect contact between two phases.

Generally, heterogeneous reactors exhibit a greater variety of configuration and contacting patterns than homogeneous reactors.

Characteristics Of Reactors

Each type of reactor has a different set of characteristics. While we cannot describe in detail all the fundamental and advanced principles of each, here are the main characteristics of each type.

Batch Reactor

• The reactor is charged, i.e. filled, through the holes at the top while the reaction is carried out.
• Nothing else is put in or taken out until the reaction is done.
• The tank is easily heated or cooled by a jacket.
• The most basic type of reactor, used typically in the pharmaceutical industry.

CSTR

• Run at steady state, the flow rate in must equal the mass flow rate out, otherwise the tank will overflow or go empty.
• The feed assumes a uniform composition throughout the reactor, and the exit stream has the same composition as the tank.
• The reaction rate is associated with the final, output, concentration.
• The reactor is equipped with an impeller to ensure proper mixing.
• Dividing the volume of the tank by the average volumetric flow rate gives the residence time, the average amount of time a discrete quantity of reagent spends inside the tank.

PFR

• Consists of a long cylindrical tube, or many short reactors in a tube bank.
• Operated at steady state.
• The rate is very high at the inlet to the PFR.
• No radial variation in reaction rate, and therefore concentration.
• Concentration changes with length down the reactor.
• As the concentrations of the reagents decrease and the concentration of the products increases, the reaction rate slows.
• A PFR typically has a higher efficiency than a CSTR of the same volume, so given the same space-time a reaction will proceed to a higher percentage completion.

Fixed & Fluidised Beds

• An FBR is essentially a tubular reactor packed with solid catalyst particles.
• It is analogous to the CSTR in that its contents are well mixed.
• Versatile systems, however they can be difficult to model.
• They require the use of advanced techniques, and often ideal system assumptions are not accurate.

How To Actually Model Reactors

Designing reactors is one of the best and most interesting parts of being a chemical engineer. However, it can often be a complicated project, so following a systematic approach makes the process much easier.

As a general recommendation, carry out the following in this defined order. It will of course deviate from one design to another, but in general this is a good place to start.

1. 1Define your objectives clearly and state what you wish to find.
2. 2Gather all appropriate data required for both the mass and energy balances, including specific information about the compounds involved.
3. 3List your set of assumptions and prepare the general balance equations to suit them and your chosen reactor.
4. 4Begin with the mass balance, starting by looking at the stoichiometric equation to determine the order.
5. 5Determine the relationship between the concentrations as a function of either conversion or time.
6. 6Move to the energy balance to determine the key energy inputs or outputs that you require.
7. 7Depending on the complexity of your system, use equations such as the Arrhenius equation to relate particular variables together.
8. 8Collect and present your data in tabular form, and begin the mechanical design of your vessel.

Work through that order once and the method transfers to any reactor you are asked to design, whether it is a batch vessel in a pharmaceutical plant or a fixed bed running a catalytic reaction.