DIFFERENT TYPES OF CHEMICAL REACTORS FOR THERMAL APPLICATIONS

I -  DIFFERENT TYPES OF CHEMICAL REACTORS

In chemical engineering (the branch of chemical engineering which deals with chemical reactors and their design, especially by application of chemical kinetics), chemical reactors are vessels designed to contain chemical reactions. The design of a chemical reactor deals with multiple aspects of chemical engineering. Chemical engineers design reactors to maximize net present value for the given reaction. Designers ensure that the reaction proceeds with the highest efficiency towards the desired output product, producing the highest yield of product while requiring the least amount of money to purchase and operate. Normal operating expenses include energy input, energy removal, raw material costs, labor, etc. Energy changes can come in the form of heating or cooling, pumping to increase pressure, frictional pressure loss (such as pressure drop across a 90° elbow or an orifice plate), agitation, etc.

The number of types of reactors is very large in the chemical industry. Even for the same operation, different types are used. Chemical reactors can be designed as either tanks or pipes, depending on the needs, and they can vary in size considerably. Small benchtop [1] chemical reactor designs are intended for use in labs, for example, while large tanks can be used to make chemicals on an industrial scale. The design also includes a variety of features which can be used to control conditions inside the reactor.

Here, we will state all reactors type and will describe in the following paragraphs, the advantages and disadvantages. So a comparison could be easier.

Ø  The Batch and Semi-batch Reactors.

Ø  The Plug Flow Reactor (PFR).

Ø  The Perfectly Mixed Flow Reactor (PFR).

Ø  The Continuous Stirred-Tank Reactor (CSTR).

Ø  The Fixed Bed Reactor (FiBR).

Ø  The Fluidized Bed Reactor (FBR).

Ø  The Multiphase Flow Reactor (MFR).

1.     The Batch and Semibatch Reactors

Batch reactors (isothermal and nonisothermal) are generally used for liquid phase reactions. When a solid has to be kept in suspension or when there are two liquid phases. The batch reactor is generally considered to be spatially uniform in composition and temperature. The composition changes with time, however. Temperature sequencing may be favorable for the selectivity or for achieving complete conversion in a safe way. With a batch chemical reactor, the components of the reaction are added to the reactor and a controlled reaction is allowed to take place. When the reaction is finished, the batch can be removed and the reactor can be prepared for another round. This type of reactor works best when people need chemicals on a small scale, as for example when research chemists are preparing compounds for pharmaceutical research.

In pure batch operation the reactants are completely fed into the reactor at the beginning. For better control of temperature this type of operation may not be advisable and the reactant(s) may have to be added progressively to the contents of the vessel. The reactor is then said to operate in the Semibatch mode. Or, a product may be withdrawn, for instance, water in an esterification subject to equilibrium, to reach complete conversion of the reactant(s).

Batch and Semibatch reactors are most often used for low production capacities, where the cost of labor and dead time are only a small fraction of the unit cost of the product. They are generally encountered in the area of specialty chemicals and polymers and in pharmaceuticals, in particular, in plants with a wide variety of products.

2.     The Plug Flow Reactor

The plug flow reactor (PFR) model is used to describe chemical reactions in continuous, flowing systems. The PFR model is used to predict the behavior of chemical reactors, so that key reactor variables, such as the dimensions of the reactor, can be estimated. PFR's are also sometimes called Continuous Tubular Reactors (CTR's). Plug flow is a simplified and idealized picture of the motion of a fluid, whereby all the fluid elements move with a uniform velocity along parallel streamlines. This perfectly ordered flow is the only transport mechanism accounted for in the plug flow reactor model, and generally for irreversible reactions in first or two order reaction.

Fig1: Plug Flow Reactor

In a PFR, one or more fluid reagents are pumped through a pipe or tube. The chemical reaction proceeds as the reagents travel through the PFR. In this type of reactor, the changing reaction rate creates a gradient with respect to distance traversed; at the inlet to the PFR the rate is very high, but as the concentrations of the reagents decrease and the concentration of the product(s) increases the reaction rate slows. Some important aspects of the PFR:

• All calculations performed with PFRs assume no upstream or downstream mixing, as implied by the term "plug flow".
• Reagents may be introduced into the PFR at locations in the reactor other than the inlet. In this way, a higher efficiency may be obtained, or the size and cost of the PFR may be reduced.
• 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 a CSTR.

For most chemical reactions, it is impossible for the reaction to proceed to 100% completion. The rate of reaction decreases as the percent completion increases until the point where the system reaches dynamic equilibrium (no net reaction, or change in chemical species occurs). The equilibrium point for most systems is less than 100% complete. For this reason a separation process, such as distillation, often follows a chemical reactor in order to separate any remaining reagents or byproducts from the desired product. These reagents may sometimes be reused at the beginning of the process.

3.     The Perfectly Mixed Flow Reactor

This reactor type is the opposite extreme of the plug flow reactor. The essential feature is the assumption of complete uniformity of concentration and temperature throughout the reactor, as contrasted with the assumption of no intermixing of successive fluid elements entering a plug flow vessel. Therefore, in the perfectly mixed flow reactor, the conversion takes place at a unique concentration (and temperature) level which, of course, is also the concentration of the effluent. To approach this ideal mixing pattern, it is necessary that the feed be intimately mixed with the contents of the reactor in a time interval that is very small compared to the mean residence time of the fluid flowing through the vessel.

The stirred flow reactor is frequently chosen when temperature control is a critical aspect, also chosen when the conversion must take place at a constant composition, or when a reaction between two phases has to be carried out. Finally, several alternate names have been used for what is called here the perfectly mixed flow reactor. One of the earliest was “continuous stirred tank reactor,” or CSTR, which some have modified to “continuous flow stirred tank reactor,” or CFSTR. Other names are “backmix reactor,” “mixed flow reactor,” and “ideal stirred tank reactor.

4.     The Continuous Stirred Tank Reactor

The continuous flow tank reactor is the most common type of reactor, and consists of a tank, a stirring mechanism, and feed pumps. Components are continuously feed into the reactor, and the product is continuously removed. After initial startup, steady state must be reached before a quality product is produced. It is preferentially used for reactions involving rather large ratios of liquid to gas for rather exothermic reactions, because the agitation improves the heat transfer and internal heat exchangers are easily built in. They also permit achieving high interfacial areas.

Fig2: Isothermal CSTR

Its main characteristics are to run at steady state with continuous flow of reactants and products; the feed assumes a uniform composition throughout the reactor, exit stream has the same composition as in the tank. This reactor allow good temperature control, easily adapts to two phase runs, low operating (labor) cost, easy to clean. However, we have a lowest conversion per unit volume.

5.     The Fixed Bed Reactor

 In chemical processing, a fixed bed is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig rings (pieces of tube approximately equal in length and diameter) or else it can be a specifically designed structured packing. Fixed beds may also contain catalyst particles or adsorbents such as zeolite pellets, granular activated carbon, etc. Generally use to carry out solid- catalyzed reactions and in heterogeneous phase reactions.

        Fig3: Fixed Bed Reactor

This type of reactor has a high conversion per unit mass of catalyst, low operating cost and is a continuous operation. However, an undesired thermal gradients may exist in the reactor, the temperature control is poor, channeling may occur and unit may be difficult to service and clean.

6.     The Fluidized Bed Reactor

For many industrial applications nowadays fluidized bed reactors are used. Floating on the upward gas flow the particles inside the reactor start to behave like a bubbling fluid. Several aspects of fluidized beds are not yet fully understood and therefore a lot of research is dedicated to these reactors [2]. A fluidized bed reactor (FBR) is a type of reactor device that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a granular solid material (usually a catalyst possibly shaped as tiny spheres) at high enough velocities to suspend the solid and cause it to behave as though it were a fluid. This process, known as fluidization, imparts many important advantages to the FBR.

The solid substrate (the catalytic material upon which chemical species react) material in the fluidized bed reactor is typically supported by a porous plate, known as a distributor. The fluid is then forced through the distributor up through the solid material. At lower fluid velocities, the solids remain in place as the fluid passes through the voids in the material. This is known as a packed bed reactor. As the fluid velocity is increased, the reactor will reach a stage where the force of the fluid on the solids is enough to balance the weight of the solid material. This stage is known as incipient fluidization and occurs at this minimum fluidization velocity.

                                                        Fig3: Fluidized Bed Reactor

The FBR seems to be important due to the inherent advantages as: a uniform particle mixing in the reactor, a uniform temperature gradients (Many chemical reactions require the addition or removal of heat. Local hot or cold spots within the reaction bed, often a problem in packed beds, are avoided in a fluidized situation such as an FBR), and also the ability to operate in continuous state. However, FBR does have it draw-backs: an increased reactor vessel size because of the expansion of the bed materials in the reactor; dealing with pressure drop associated with deep beds; pressure loss scenarios have to be understand because if fluidization pressure is suddenly lost, the surface area of the bed may be suddenly reduced.

7.     The Multiphase Flow Reactor

Multiphase flow is simultaneous flow of materials with different states or phases (i.e. gas, liquid or solid), of materials with different chemical properties but in the same state or phase (i.e. liquid-liquid systems such as oil droplets in water).  Depending of the type of phases flow (gas-liquid flow, liquid-solid flow, gas-liquid-solid flow, gas-solid flow, liquid-liquid flows) and physics, we can use fixed bed reactor and granular flows, stirred vessel coupled to plug flow reactor, fluidized bed reactor or packed bed reactor.

II – REACTORS DESIGN

An overview of chemical reactors is very necessary to describe ideal reactors and indicate important aspects. Sometimes the decisive element about the form of the reactor can be very high heat effects connected with the reaction system, which demands a large surface area of heat transfer. It seems that a very useful and practical classification of the chemical processes and reactors can be given as follows:

·        The reaction system is homogeneous (one phase system) or heterogeneous (multiphase).

·        The process is continuous or periodic (without inflow of reactants).

·        The process is conducted isothermally, adiabatically or by means of heat exchange with the surroundings.

 Let us supposed a reactor which is feeding by two reactants A and B and some fractional reactants will make products C and D. We can have two reactions:

A + B à C + E    (1)          or             A + 2B à D      (2)

So, to justify the appropriate chemical reactor, some factors have to be taking into account like mass balances, energy balances, selectivity, conversion, and reaction rate.

Selectivity is the ratio of isomeric reaction products formed in a reaction where more than one product may be formed. Or the capacity factor ratios of two substances measured under identical chromatographic conditions. In general, its represents the ratio of reaction rates and in this case (1) will be defined as:

                                    Selectivity = Fc/(Fc+Fd)       where F (mol/s) is the molar flow rate

By looking previous detailed reactors types, we can define four ideal reactors (Batch, CSTR, Semibatch and PFR) generally used for large scales and just mentioned that real reactors are sometimes a combination of ideal reactors.

The first (Batch) is a simple scale-up laboratory battery where concentrations and temperature can change with time (high reactant concentration), so this is not steady state and can assumed to be a closed system.

The CSTR really quite different looks like a battery reactor where we can change reactants and removing materials and this change things little bit (adapt for low reactant concentration), that means its works in steady state ( ) and then Semibatch mean a batch reactor where for example it might like one component like A and then continuously feeding B, so it is not a steady state reaction. Another steady state reactor is the Plug Flow. So for the three first reactors, it’s assumed all uniform spatially, that means no concentration gradient and temperature gradient across the reactor. That not the case for the PFR, so the concentration is continuously changing as he move in the reactor.

For the mass balances analogy, we have to deal with the rate of reaction (r_A = rate of formation of A and if A is reactant, r_A < 0). One thing important is to keep in mind that in general  (there can be an equivalence in a special circumstance), because in the cases of CSTR and PFR     but   . That’s why it is defined in the literature   where k is the rate coefficient or rate constant, came into formula by thermodynamic consideration (Arrhenius formula). By this way, we can write down the basis mass balance equations:

Batch:    dN_A/dt=-K*C_A*V case of equimolar species, V = volume of reactor, N_A number of moles. 

CSTR: 0 = F_Ao - F_A + r_A*V 

Semibatch:  dN_A/dt = F_Ao - F_A + r_A*V 

PFR:     dF_A/dt = r_A

Another important term for the reactor design is the conversion to determine how much is reacted  in general. But, if mass density is constant, . Conversion increases when reaction takes long times, or where higher temperature involved.

III – REACTORS MODELLING

1.     Isothermal Batch Reactor

This model deals with a reacting system comprising an equimolar mixture of water gas and TCM which is allowed to react and form a hydrate TCM. The reaction runs under isothermal conditions, and the temperature is kept at 400 K throughout. The experiment takes place in a perfectly mixed batch reactor.

By using the reaction engineering module, we define this reversible equation: MgCl₂ + 6H₂O ßà MgCl₂H₁₂O₆ at a reactor constant volume V_r = 1 m³.

                                                        Fig5: Species concentration evolution

2.     Nonisothermal Plug Flow Reactor

The plug flow reactor is used to model the nonisothermal decomposition of MgCl2H12O6 (TCM). Process considerations are investigated as the reaction is run adiabatically and as heat exchanger supplies energy to the reacting flow. The system is heterogeneous and the reaction is irreversible here:  MgCl₂H₁₂O₆ à MgCl₂ + 6H₂O at a reactor constant volume V_r = 1 m³.

We suppose an inflow of N2 when the reactant is flowing into the reactor. The parameters used can be found in the table below.

UA	165000 W/m3.K	Heat transfer coefficient
Tx	1150 K	Temperature of heat exchanger medium
TCM_frac	1	Inlet mole fraction of TCM
X_tcm	100 * (FA0 – FA)/FA0	Conversion of TCM
P	165325 Pa	Pressure in reactor
Qext	UA * (Tx –T)	External heat source

                                                        Fig6: Species concentration evolution

IV – REFERENCES

1.      http://www.wisegeek.com/what-is-a-chemical-reactor.htm visited 06 März 2012 Copyright © 2003 - 2012 Conjecture Corporation

2.      http://caes.ewi.utwente.nl/index.php/education/assignments/available/72-characteristics-of-a-chemical-reactor-model

3.      www.wikipedia.org

4.      Bookgoogle: “Chemical Reactor, Analysis and Design 3^rd edition– Froment, Bischoff, De Wilde. ©2011.