CHEMICAL REACTOR DESIGN Third Year by Dr. Forat Yasir AlJaberi University of Al Muthanna College of Engineering Chemical Engineering Department

Introduction The chemical reactor is the heart of any chemical process that converts inexpensive chemicals in to valuable products. While separation units, such as distillation columns, are required to purify materials before and after the reactor. Raw materials    Products The by-products, any solvents, and the unconverted reactants must be separated from the desired products before it sold or used as a reactant in other chemical processes. Therefore, the key component in any plant is the chemical reactor while the efficient separation process ensured the savings in any process. Separation process Chemical reactor Separation process

The performance of the chemical reactor: This issue is totally controlling the cost and modes of operation of the expensive separation units that dominated the size. Therefore, the chemical reactor extremely controls the overall economics of most processes. In general, the capital and operating costs of a reactor equal 10 to 25 percent of the total cost of a typical chemical process. This process could be a petroleum refining, food processing, commodity, materials, fine chemicals, pharmaceuticals, etc.

Analysis of chemical reactors: The analysis of a chemical reactor is required various fields in the chemical engineering curriculum such as the following objects: Kinetics Thermodynamics Mass transfer Heat transfer Fluid flow Mathematics Materials Design Economics Process control

Chemical reaction: It is comprised of chemical equilibrium, kinetics and mechanisms. Reactions are generally categorized into the following: 1. According to the energy: Exothermic reactions Endothermic reactions 2. According to the direction: Reversible reactions Irreversible reactions 3. According to phases: Homogeneous Hetrogeneous

Chemical kinetics: It is important for any study or design every type of reactors because they relates to the studies of the rates at which chemical processes occur, the factors on which these rates depend, and the molecular acts involved in reaction mechanisms. Therefore, chemical kinetics and reactor design are extremely essential of producing all industrial chemicals. This aspect distinguished the chemical engineer from other engineers. According to this rule, chemical reactions could be classified into: Elementary reaction which corresponds to the stoichiometric equation. Non-elementary reactions which are not corresponding to the stoichiometric equations.

Combination such the formation of cumene from benzene and propylene. Rate of reaction: It is time-dependent issue of consuming such materials to generate new materials via three basic ways as follow: Decomposition such as the conversion of cumene into benzene and propylene. Combination such the formation of cumene from benzene and propylene. Isomerization which is neither the first way or the second one mentioned above. There several objects of studying the rate of reaction that should be taken into consideration as explained in the following papers.

The fraction conversion: The extent of reaction It is important to evaluate the progress of reaction via the measure of the variation of moles consumed or formed. Where the molar extent of reaction is a time-dependent extensive variable which means it is proportional to the mass of the mass of the system. The fraction conversion: It is the related to the molar extent of reaction and not proportional to the mass of the system, i.e. intensive measurement. The employment of catalyst is so useful to enhance the performance of reactors that used this additive.

Rate constant: The rate of reaction, in general, depends on two important factors which are the temperature of the reaction and the concentration of reactants and/or products. The temperature –dependent factor is characterized as the rate constant (k) which is almost always strong functions of temperature whereas independent on the concentration of materials. It could be estimated by using Arrhenius equation that consists of the activation energy which is directly proportional to the temperature sensitivity of reactions. The units of rate constant should be chosen correctly to make the rate in (moles/volume . time).

Yield: It is the ration of moles of the desired product formed to the moles of the reactant consumed times of the stoichiometric factor of this reactant material. Selectivity: It is the ration of desired product formed to the reactant consumed. It changes as the reaction progresses. Excess reactant: The presence of this type of reactant tend to increase the equilibrium fractional conversion of the limiting reactant in comparison to what obtained in the stoichiometric reactions. Limiting reactant: It disappears first if a reaction proceeded to completion because it is present in less than its stoichiometric proportion relative to every other reactants.

Thermodynamics of chemical reactions: They are very important to study or design each types of reactors. They consist of several parameters such as: 1- Heat of reaction which depends on the temperature and pressure of the system and equals to the difference between the enthalpies of products and reactants for a reaction. The standard value of heat of reaction is estimated at reference temperature and pressure (25oC, 1 atm.). Moreover, the sign of its value refers to the kind of reaction which is an endothermic or exothermic reaction. Furthermore, its value depends on the state of materials either reactants or products, which is gas, liquid, or solid.

Thermodynamics of chemical reactions: 2- Heat of formation refers to that reaction in which the compound is formed from its atomic constituents as they normally occur in nature. The heat of reaction equals the difference between the heat of formation of products and reactants times their stoichiometric individually, i.e. Hess’s law. 3- Heat of combustion means the heat produced from the reaction of a substance with oxygen to formed specified products at 25oC and1 atm. The heat of this reaction equals the difference between the heat of combustion of products and reactants times their stoichiometric individually

Analysis of rate equations: It could be evaluated by estimating the composition of the reaction mixture at various stages along the time of reaction. For that purpose, two methods are employed as follows: The integration method The differential method The former method proceeds via the integrating of the rate expression while the later one depends on the differentiating of the experimental data obtained. The obtained rate equations could be zero, first, second, third, and/or (n) order.

Homogeneous catalyzed reactions: It occurs when one of the reactants acts as a catalyst that caused to increase the extent of reaction. Reactors employed catalysts could be classified into several types such as fixed-bed reactors, fluidized bed reactors, and autocatalytic reactors. Types of reactors: There are several types of reactors such as batch, plug flow, semi-batch, continuous stirring tank, catalytic, electrochemical reactors. Each type possess rate equation where its specification depends on the order of the reaction.

References: Chemical reaction engineering, Octave Levenspiel. An introduction to chemical engineering kinetics and reactor design, Charles G. Hill. Elements of chemical reaction engineering, H. Fogler. Chemical reactor design, Peter Harriott. Chemical engineering kinetics, J.M. Smith.

With Best Wishes