Chapter 1 complete ppt chemical reaction engineering.pptx

Cre

1. Chemical Reaction Engineering (CRE) (303103303)

2. Course Synopsis: This subject covers the knowledge of reaction kinetics, reactor design and separation which distinguishes chemical engineer from other engineers. The course introduces the basic reactor design calculation and design of commercial chemical reactors, emphasizing synthesis of chemical kinetics and transport phenomena. The topics cover in this subject are kinetic rate theory, homogeneous reaction in batch and flow systems, heterogeneous reaction and catalysis, temperature effect, effect of heat transfer and catalytic reactor also reactor design, sizing and modeling of performance

3. Course Objectives: Apply basic fundamentals of chemical reaction engineering such as reaction progress variables, conversion, rate laws, order of reaction and molecularity, reversible reactions and stoichiometry. Acquire the analytical and modeling skills required for the design and operation of industrial reactors for the chemical processes.

4. Typical Chemical Process: Separator Reactor Separator Raw Material Product Undesired Product Typical chemical process consist of these steps: (Physical Process) (Physical Process)

5. What’s involved in reactor design?

6. What is Chemical Reaction Engineering? CRE deals with chemically reactive systems of engineering significance. Chemical reaction engineering is the discipline that quantifies the interactions of transport phenomena and reaction kinetics in relating reactor performance to operating conditions and feed variables. CRE is needed in the development of new and the improvement of existing technologies. search for alternative processes to replace old ones find routes to make a product from different feedstock novel processes for synthesis-gas production Hydrocarbon production from syn gas Biodiesel production reduce/eliminate unwanted byproducts fuel-cells for automobiles NOx reduction

7. CRE: Introduction Every industrial chemical process is designed to produce economically a desired product from a variety of starting materials through a succession of treatment steps. The subject of chemical reaction engineering initiated and evolved primarily to accomplish the task of describing how to choose, size, and determine the optimal operating conditions for a reactor whose purpose is to produce a given set of chemicals in a petrochemical application. The principles of chemical reaction engineering are presented in such accuracy to make possible a comprehensive understanding of the subject. Mastery of these concepts will allow for generalizations to reacting systems independent of their origin and will furnish strategies for attacking different problems. So, Reactor design uses information, knowledge, and experience from a variety of areas-thermodynamics, chemical kinetics, fluid mechanics, heat transfer, mass transfer, and economics. Chemical reaction engineering is the synthesis of all these factors with the aim of properly designing a chemical reactor.

8. CRE: Introduction To find what a reactor is able to do we need to know the kinetics, the contacting pattern and the performance equation. The expression to relate input to output for various kinetics and various contacting patterns, or output = f [input, kinetics, contacting] This is called the performance equation. Why is this important? Because with this expression we can compare different designs and conditions, find which is best, and then scale up to larger units.

9. CRE: Introduction Chemical reaction engineering is that engineering activity concerned with the utilization of chemical reactions on a commercial scale. Its goal is the successful design and operation of chemical reactors, and probably more than any other activity, it sets chemical engineering apart as a distinct branch of the engineering profession. The ingredients of CRE are (i) thermodynamics, (ii) kinetics, (iii) transport processes, (iv) types of reactors, (v) mode of operation and contacting, (vi) modelling and optimization, and (vii) control. Chemical kinetics is a study of rates at which chemical reaction occur and the effect of parameters such as temperature, pressure and reactant concentration

10. Classification of Chemical reaction Classification based on state of reactant and products 1.Homogeneous reactions 2.Heterogeneous reactions Classification based on presence of catalyst 1.Catalytic reaction 2.Non-catalytic reaction Classification based upon the directions 1.Reversible reaction 2.Irreversible reaction

11. Classification of Chemical reaction Classification based on molecularity of a reaction 1.Unimolecular reactions 2.Bimolecular reactions 3. Tri molecular reactions Classification based upon the heat effect 1.Exothermic reaction 2.Endothermic reaction Classification based upon the order of reaction 1.First order reaction 2.Second order reaction etc.

12. Parameters Affecting Rate of Reaction: The Rate Law Rate of reaction depends on a number of parameters, the most important of which are usually 1.The nature of the species involved in the reaction ; Many examples of types of very fast reactions involve ions in solution, such as the neutralization of a strong acid by a strong base, and explosions. In the former case, the rate of change may be dictated by the rate at which the reactants can be brought into intimate contact. At the other extreme, very slow reactions may involve heterogeneous reactions, such as the oxidation of carbon at room temperature. The reaction between hydrogen and oxygen to form water can be used to illustrate both extremes. Subjected to a spark, a mixture of hydrogen and oxygen can produce an explosion, but in the absence of this, or of a catalyst such as finely divided platinum, the reaction is extremely slow. In such a case, it may be wrongly supposed that the system is at equilibrium, since there may be no detectable change even after a very long time.

13. Parameters Affecting Rate of Reaction: The Rate Law 2.Concentrations of species ; Rate of reaction usually depends on concentration of reactants (and sometimes of products), and usually increases as concentration of reactants increases. Thus, many combustion reactions occur faster in pure oxygen than in air at the same total pressure. 3.Temperature; Rate of reaction depends on temperature and usually increases nearly exponentially as temperature increases. An important exception is the oxidation of nitric oxide, which is involved in the manufacture of nitric acid; in this case, the rate decreases as T increases. 4.Catalytic activity ; Many reactions proceed much faster in the presence of a substance which is itself not a product of the reaction. This is the phenomenon of catalysis, and many life processes and industrial processes depend on it. Thus, the oxidation of SO, to SO3 is greatly accelerated in the presence of V2O5 as a catalyst, and the commercial manufacture of sulfuric acid depends on this fact.

14. Parameters Affecting Rate of Reaction: The Rate Law 5.Nature of contact of reactants ; The nature or intimacy of contact of reactants can greatly affect the rate of reaction. Thus, finely divided coal burns much faster than lump coal. The titration of an acid with a base occurs much faster if the acid and base are stirred together than if the base is simply allowed to “dribble” into the acid solution. For a heterogeneous, catalytic reaction, the effect may show up in a more subtle way as the dependence of rate on the size of catalyst particle used. 6.Wave-length of incident radiation : Some reactions occur much faster if the reacting system is exposed to incident radiation of an appropriate frequency. thus, mixture of hydrogen and chlorine can be kept in the dark, and the reaction to form hydrogen chloride is very slow; however, if the mixture is exposed to ordinary light, reaction occurs with explosive rapidity. Such reactions are generally called photochemical reactions.

15. Rate Equations:

16. Kinetics of homogeneous reaction:

17. Single and Multiple Reaction:

18. Elementary and Nonelementary Reaction:

19. Elementary and Nonelementary Reaction: Non-elementary is reaction is not single step reaction and it is a series of elementary reaction and there if formation of intermediates. Different types of intermediates are as follows. Free radical, ions, polar substances, molecules and transition complex. Transition complex is divided into chain and non-chain reaction. Chain reaction involves three steps. Initiation, propagation and termination. Chain reactions are divided into three types. Free radicals chain reaction mechanism, molecular intermediates non chain mechanism and transition complex non chain mechanism.

20. Reaction mechanism of Nonelementary reactions:

21. Reaction mechanism of Nonelementary reactions:

22. Molecularity and Order of Reaction

23. Molecularity and Order of Reaction Concentration can be represented in another form using a term called ‘conversion’ . And it is denoted by X i where N A0 initial amount of moles and N A is amount of moles after some time ‘ t 1 ’. X A = N A0 – N A / N A0 = 1- (N A /V) / (N A0 /V) = 1-C A /C A0 dX A = - dC A /C A0 Where, C A0 concentration at t=0 V = volume of the reactor C A = Concentration at t = t 1

24. Rate Constant

25. Temperature Dependency from Arrhenius’ Law Rate of reaction = f(concentration). g(temperature) = f(concentration).k Rate constant is experimentally found value and it is well represented by Arrhenius’ law k = k exp (- E /RT) Where, k = rate constant k = frequency/pre-exponential factor E = Activation energy R = Universal ideal gas constant, T = Temperature

26. Temperature Dependency from Arrhenius’ Law If we write full rate equation, we obtain, Rate of reaction = f(concentration). g(temperature) = f(concentration). K = f(concentration). k exp (- E / R T) At same composition, but at two temperatures, Arrhenius’ law gives

27. Temperature Dependency from Arrhenius’ Law Rate of reaction is directly proportional to rate constant so we can equate the ratio of rate constant to the ratio of rate terms. Rate equation term is related with time. Rate of reaction is inversely proportional to time because as time increases rate of reaction decreases and vice versa. By relating time and rate of reaction the following equation is obtained

28. Temperature Dependency from Arrhenius’ Law Comparison of theories with Arrhenius’ law General equation for rate constant is represented by k = k T m exp (- E /RT) where ‘m’ is a constant varies from 0 to 1. If m = 0, k = k exp (- E /RT) , Arrhenius’ equation If m = 1/2, k = k T 1/2 exp (- E /RT), collision theory equation If m = 1, k = k T exp (- E /RT), transition theory equation

29. Temperature Dependency from Arrhenius’ Law ln

30. Activation Energy and Temperature Dependency From Arrhenius' law a plot of In k vs. 1/T gives a straight line, with large slope for large E and small slope for small E. Reactions with high activation energies are very temperature-sensitive; reactions with low activation energies are relatively temperature-insensitive. Frequency factor does not influence the temperature sensitivity. It is observed that activation energy is very dependent on temperature.

31. Activation Energy and Temperature Dependency Example: Milk is pasteurized if it is heated to 336 K for 30 min, but if it is heated to 347 K it only need 15 s for the same result. Find the activation energy of this sterilization process. Solution: T 1 = 336 K in 30 min (t 1 ) T 2 = 347 K in 15 sec (t 2 ) As it discussed rate is proportional to rate constant and inversely proportional to time. Following equation can be used to find the activation energy. R = 8.314 J/mole. K So, E= 422 kJ/mole

32. Examples: On doubling the concentration of reactant the rate of reaction triples. Find the reaction order. Solution: Assume rate of reaction is, - r A = k C A n At C A1 rate is, -r A1 = k C A1 n And At C A2 rate is, -r A2 = k C A2 n If C A2 = 2 C A1 Than, -r A2 = 3(-r A1 )

33. 2. The pyrolysis of ethane proceeds with an activation energy of about 75000cal. How much faster is the decomposition at 650 °C than at 500°C. Solution: C 2 H 6 Product E =75000 cal /mol Let k 1 be the rate constant at T 1 and k 2 be the rate constant at T 2 T 1 = 500°C = 773 K, T 2 = 650°C= 923 K

34. 3. Phosphine decomposes when heated according to following reaction: 4PH 3(g) → P 4(g) + 6H 2(g) At a given instant , the rate at which phosphine decomposes is 2.4 Χ 10 -3 (mol/ l.s ). Express the rate in three different ways using differential notation and shows the relation between them. What is the rate of formation of 1) P 4 and 2) H 2 Solution: 4PH 3(g) → P 4(g) + 6H 2(g) From stoichiometry the rate of decomposition of PH 3 , formation of P 4 and H 2 are related as follows:

35. b.

36. 4. For a gas reaction at 400 k the rate is reported as - rp A = - dp A / dt = 3.66 p A 2 , (atm/h) What are the units of rate constant What is the value of the rate constant for the this reaction if the rate equation is written as - r A = (-1/V)x( dN A / dt ) = kC A 2 , (mol/ l.h ) - r A = kC A 2 , (mol/m 3 .s) Solution,

37. 5 . A rocket engine, burns a stoichiometric mixture of fuel (liquid hydrogen) in oxidant (liquid oxygen). The combustion chamber is cylindrical, 75 cm long and 60 cm in diameter, and the combustion process produces 108 kg/s of exhaust gases. If combustion is complete, find the rate of reaction of hydrogen and of oxygen.

38. 6 . A human being (75 kg) consumes about 6000 kJ of food per day. Assume that the food is all glucose, and that the overall reaction is Fin d man's metabolic rate (the rate of living, loving, and laughing) in terms of moles of oxygen used per m3 of person per second.