Intro to reactor design

This slide completely describes you about the stuff include in it and also everything about chemical engineering. Fluid Mechanics. Thermodynamics. Mass Transfer Chemical Engineering. Energy Engineering, Mass Transfer 2, Heat Transfer,

1. Intro to Reactor Design Chap # 4 Octave LEVENSPIEL

2. Introduction Introduction to reactor design is given in chapter #1. Here we will deal with some more discussion on the reactor, their types and basic equations needed for a reactor to be designed. The Reader must study chapter 1 before the start of this chapter For a particular type of reaction to be occurred in a controlled manner some type of hardware is needed called “REACTOR”. This reactor, also called heart of Chemical process, is to be designed to provide the conditions necessary for chemical reaction to proceed.

3. In Majority of cases, reactor do three things, it provides Residence Time Transfer Heat Agitates or mix phases Principle Factors Involved in the design of Reactor: The principle factors which must be considered in the design of the reactor are: The Phase involved Temperature Range Operating Pressure Residence Time or Space Velocity

4. Corrosiveness Heat Transfer Temperature Control Agitation for uniformity or temperature control Batch or Continuous operation Production rate

5. Types Of Reactor Chemical Reactor may have a variety of sizes, shapes and operating conditions. We here try to classify them in three different modes. Based on method of Operation Based on shape Based on No. of phases involved Based on Method of Operation : The reactor may be Batch Reactor Continuous flow (Steady Flow) Reactor Semi- Batch(Semi-continuous)

6. Batch Reactor A batch reactor has neither inflow nor outflow of reactants or products while the reaction is being carried out. In such extend of reaction and properties of reaction mixture very with time. Thus composition changes with time. For gas phases batch reactor may be constant volume or constant pressure.

7. Continuous Reactor or Steady State Reactor The reactants are continuously fed and product are also continuously removed from the reactor. In such reactor the extend of reaction may vary with position in reactor not with time. Thus composition at any point is not changed with time( They may be continuous-stirred tank or Tubular Reactor )

8. The steady state flow reactor is ideal for industrial purpose when large quantities of material are to be processed and when the rate of reaction is fairly high to extreme high. Supporting equipment's need are high However, extremely good products quality can be obtained This reactor is widely used in the oil industry

9. Semi-Batch Un-steady State Reactors The Reactants are introduced partially and additional reactants are added progressively until the desired quantity is added. Alternatively, one may charge the reactants at once and continuously remove products as they formed. Semi- Batch Reactors may be either Reactors in which volume and composition changes

10. ii. Reactor in which volume changes but composition is unchanged iii. Reactors in which volume is constant but composition changes

11. Flexible system but is more difficult to analyze than the other reactor types. Good control of reaction speed b/c the reaction proceed as reactants are added. Uses/application from the calorimetric titration in the laboratory to the large open hearth furnace for steel production

12. b) Based on Shape Tank Reactor Tabular Reactor Tank Reactor: An Ideal Reactor is one in which stirring is so efficient that the contents are always uniform in composition and temperature throughout the tank. This type of reactors are called as stirred tank or well-mixed Reactor. The simple tank reactor may be operated in a variety of modes, Batch, semi-Batch or continuous-Flow. When it is continuous its called continuous-Stirred tank Reactor (CSTR) or back mix Reactor.

13. Tubular Reactor: Tubular reactor is one in which there is no mixing in the direction of flow in the reactor. The reactants are continuously consumed as they flow in the axial (Down THE LENGTH OF REACTOR) direction. Thus the concentration varies along the axial direction. They also called plug flow reactor(PFR), Slug Flow Reactor, Piston Flow Reactor or un-mixed flow reactor.

14. Example of Tubular Reactor

15. Example of Tubular Reactor

16. C ) Based on No. Of Phases Involved Homogeneous Heterogeneous In many cases the physical process results from the fact that the reaction mixture is heterogeneous, e.g the reaction of SO2 and O2 on V2O5 catalyst. It would not exist if the reaction is a single phase homogeneous, thus the classification is necessary.

17. Fundamental Design Equation: The starting point of all designs is the material balanced expressed for any reactant/product. A material Balance on a reactant species of interest for an element of volume say Δ V can be written as: In short, INPUT – OUTPUT – LOSS OF REACTION = ACCUMULATION

18. When the composition within the reactor is uniform (Independent of position), we will consider the whole reactor of material balance. On the other hand when the composition within the reactor is not uniform, it must be made over a differential element of volume and then integrated across the whole reactor for the appropriate flow and concentration condition (For Tubular Flow Reactor). For the batch reactor the first two terms are zero. For continuous flow reactors operating at steady –state, the accumulation terms is omitted. Where for unsteady state condition are involved, it will be necessary to integrate over time as well as over volume in order to determine the performance characteristics of the reactor.

19. Since rate of chemical reactions is normally strongly temperature dependent, it’s essential to know the temperature at each point in the reactor in order to be able to utilize the material balance properly. When there are temperature gradients with in the reactor, it is necessary to utilize an energy balance in conjunction with the material balance in order to determine the temperature and composition at each point in the reactor at a particular time. The general Energy balance for an element of volume Δ V over a time Δ t can be written as:

20. For completeness, the term corresponding to the entry (In) of material to the volume element and out, therefore must contain in addition to the ordinary enthalpy of the material, its kinetic and potential energy. However in chemical reactors, only the enthalpy term is significant. Although the heat effects in chemical reactors are significant, shaft work effects are usually negligible. The chemical reaction rates does not appear explicitly in the equation, but its effect are implicit in all terms except third.

21. The first, second and fourth terms reflect differences in temperature and or in composition of the entering and leaving streams. The energy effects associated with composition changes are a direct reflection of enthalpy change associated with the reaction ( i.e heat of reaction) From the summary we can write in short: Heat In – Heat Out – Disappearance by reaction = accumulation

22. For the stirred tank reactor contents are uniform in temperature and composition throughout and it is possible to write the energy balance over the entire reactor. In the case of batch reactor, first and second terms are not there. For continuous flow systems operating at steady state, the accumulation term disappears. For adiabatic operation in the absence of shaft work effects the energy transfer/disappear term will be omitted/zero.

23. For Tubular Flow Reactors, neither the composition nor the temperature need to be independent of position and the energy balance must be written on a differential element of reactor volume. The resultant differential equation then must be solved in conjunction with the differential equation describing the material balance on the differential element. The Purpose of the energy balance is to describe the temperature at each point in the reactor ( or at each time for Batch Reactor) so that the proper rate may be assigned to that point.

24. Symbols and Relationship B/W C A and X A

25. Special Case1: Constant Density Batch and Flow Systems: This includes most liquid reactions and also those gas reactions run at constant temperature and density. Here C A & X A are related as follows: Fractional change in volume of the system b/w no conversion and complete conversion of reactant A, Thus