What Are Reactors?
In a nutshell a reactor is a vessel whereby chemical changed occur. 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 operations.
Chemical reactors are at the heart of any process system. This is the place where raw materials are converted into useful products or sub-products destined for refinement, purification, and resell.
In this complete guide we will uncover everything there is to consider with regards to Reactor Design,and provide you with a taste of what you can expect in our fantastic Reactor Design Course!
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 therefore 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 to be a “reactor”. This can be form a simple vessel like a glass jar or plastic bottle. However as engineers, we are more concerned with the five most common types. These include…
- Batch Reactors
- Continuously Stirred Tank Reactors (CSTR)
- Tubular Plug Flow Reactors (PFR)
- Fixed Bed Reactors (FBR)
- Fluidised Bed Reactors
Its important to also define the terms of a Homogeneous and Heterogeneous system, but we will discuss this later. We will also discuss the key differences between each type and discuss the design equations and principles that are generally applied.
Before designing any piece of equipment, it is important you clearly define any assumptions you make. These assumptions will determine how complex or true-to-life your design will actually be. There are a set of assumptions that most engineers will consider, there are of course others depending on the industry and nature of the design.
Here is a list of the most common assumptions to consider before setting out to design your reactor.
- 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 will determine 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 of course we know that there will always be an element of accumulation as 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 involves the modelling of the material using a “mass balance”, and the other models the energy using an “energy balance”.
Both equations are illustrated as follows, starting with the mass balance…
Accumulation = (In flow) – (Out flow) +/- (Reaction)
Accumulation = (Enthalpy In) – (Enthalpy Out) +/- (Work) +/- (Transfer)
We go into the full details of each term and how to model specific reactors in our Reactor Design & Mas & Energy Balance Courses! There are several key parameters that form the backbone of these equations and they are: Concentration Volume, Temperature, Pressure, Enthalpy, Residence Time, and Heat Transfer Coefficients. A good physical property find can be found here.
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. 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 homogenous 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-gas and liquid.
Generally, heterogeneous reactors exhibit a greater variety of configuration and contacting pattern than homogeneous reactors. Initially, therefore, we shall be concerned mainly with the simpler homogeneous reactors, although parts of the treatment that follows can be extended to heterogeneous reactors with little modification.
Characteristics of Reactors
Each type of reactor has a different set of characteristics. While we cant describe in detail all the fundamental and advanced principles of each, we have highlighted the main characteristics of each type of reactor.
- Reactor is charged (i.e., filled) through the holes at the top while reaction is carried out.
- Nothing else is put in or taken out until the reaction is done.
- Tank easily heated or cooled by jacket.
- Most basic type of reactor, used typically in pharmaceutical industry.
- Run at steady state, the flow rate in must equal the mass flow rate out, otherwise the tank will overflow or go empty (transient state).
- The feed assumes a uniform composition throughout the reactor, exit stream has the same composition as in the tank.
- The reaction rate associated with the final (output) concentration.
- Reactor equipped with an impeller to ensure proper mixing.
- Dividing the volume of the tank by the average volumetric flow rate through the tank gives the residence time, or the average amount of time a discrete quantity of reagent spends inside the tank.
- 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 (concentration).
- Concentration changes with length down the reactor.
- As the concentrations of the reagents decrease and the concentration of the product(s) increases the reaction rate slows.
- A PFR typically has a higher efficiency than a CSTR of the same volume. That is, given the same space-time, a reaction will proceed to a higher percentage completion in a PFR than in CSTR.
FBR & Fluidised Beds
- FBR is essentially a tubular reactor that is packed with solid catalyst particles.
- Is analogous to the CSTR in that its contents.
- Heterogeneous reactor are well mixed.
- Versatile systems however can be difficult to model.
- Requires use of advanced techniques and often ideal systems aren’t 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 can help make the process super easy.
As a general recommendation, carry out the follow in this defined order. This will of course deviate from one design to another, but in general this will be a good place to start.
- Define your objectives clearly and state what you wish to find.
- Gather all appropriate data required for both the mass and energy balances. Ensure you get specific information about the compounds involved.
- List your set of assumptions and prepare the general balance equations to suit them and your chosen reactor.
- Begin with the mass balance, and start by looking at the stoichiometric equation to determine the order.
- Start to determine the relationship between the concentrations as a function of either conversion or time.
- Then move to the energy balance to determine the key enery input or outputs that you require.
- Depending on the complexity of your system you may have to use equations such as the Arrhenius equation to relate particular variables together.
- Collect and present your data in tabular form and begin the mechanical design of your vessel.