CH701 - Chemical Reaction Engineering - II

1. CH701 - Chemical Reaction Engineering - II • Ref. Books: • Chemical Reaction Engineering – Leven Spiel, 3rd Edition, Wiley. • Introduction to Chemical Reaction Engineering – H. Scott Fogler, 4th Edition, PHI Pub. • G.F. Froment and K.B. Bischoff, Chemical Reactor Analysis and Design, Jhon Wiley & Sons

2. Your suggestions to improve class room teaching – learning process • Feedback • Audibility in class • Visibility of slides and board work • Theory and fundamental discussion • Discussion part for applications and others • Practice of examples • Your suggestions to learn better

3. Think and Reply for following Question: Question: Write various most attractive and inspiring reactions from natural phenomena. • To go further to learn in the field of Reaction Engineering, I would like to draw your consciousness for the reactions occurring naturally and they are the most attractive and inspiring all of us to think in different direction. • Can you take this challenge and give your reply to this question with smallest possible answer with suitable example…We will discuss answer in class today… • Are you inspired to think?

4. Why we need to learn CRE? • How CRE is useful during your professional career? • What is the significance of reactor in overall plant or process? • How reactor performance effect overall performance of the plant or process? • How can we improve the reactor performance? • How lab scale reactor differ from industrial reactor? • Is rate of reaction dependent on type of reactor configuration? • How different contacting pattern effects the performance of the reactor? • Suggest basis for selection of PFR/MFR?

5. Before we move forward, can you tell me, at least one new observation from Nature where reaction engineering is doing wonderful job, before starting of every lecture of CRE-II? • Oxygen generation from plant…Some thing different? Can you take a lead to grow a plant in this semester to neutralize our own CO2 generation..Can any body calculate our carbon footprint in a day or month or year? How many such plants or trees required to neutralize our own CO2 footprint? Homework.. • Our comfort is a major problem for environment………Is it not true?

6. Industrial Examples of types of reactors • Ammonia production • Hydrogen production through steam reforming • Sulfur dioxide to sulfur trioxide • Removal of CO2 from gas mixture (like in ammonia synthesis) • ??? • ?? • ?

7. How can we design a reactor? • Which information is required to start reactor design?

8. Introduction to CRE-II • What you have learn in CRE-I? • Why we need to learn CRE-II? • What we are going to learn in CRE-II? - Non-ideality of flow patterns - Heterogeneous systems - Without catalyst - Fluid-Fluid reactions - Solid-Fluid reaction - With catalyst - Heterogeneous solid catalytic reactions • Which are different types of reactors you have seen/learned/observed during Industrial Training?

9. Question and Answer based Discussion – A true Dialogue rather than Monologue A Vedic Perspective – Bhagvad Gita and Srimad Bhagvatam were taught in Question and Answer fashion…A better of learning.. I believe, if you have question, probably I may or may not be able to answer, but you can find answer from anywhere and keep your mind open for it. But if you don’t have question, your learning may not be complete.. Are you ready to learn?

10. 3.1 Self Study: The self-study contents will be declared at the commencement of semester. Around 10% of the questions will be asked from self-study contents. Self-study contents are given as follows.To study BET, XRD, TPD-TPR-TPO, EDX, SEM, TEM characterization methods for heterogeneous catalysts. To study heterogeneous catalysts preparation by wet-impregnation, co-precipitation, sol-gel, ion-exchange, grining, combustion etc. methods.

11. Non Ideal Flow

12. Characteristics of ideal Plug flow reactor (PFR) • No axial dispersion • No over taking • No back mixing • Concentration changes gradually along the length • Flat velocity profile throughout one particular cross section • All fluid elements spent same amount of time within the reactor • No bypassing • Steady state reactor

13. Characteristics of ideal mixed flow reactor (MFR) • Complete mixing. Uniform concentration within reactor. • Concentration within the reactor is same as of leaving the reactor • No bypassing • Steady state reactor

14. Factors affecting contacting pattern • Residence Time Distribution (RTD) : Different fluid element spent different amount of time within the vessel. • Degree of aggregation : Tendency of fluid to clump and for a group of molecules to move about together. • Earliness or lateness of mixing

15. 1. Residence Time Distribution (RTD) Deviation from the two ideal flow patterns can be caused by channeling of fluid, by recycling of fluid, or by creation of stagnant regions in the vessel.

16. E, EXIT AGE DISTRIBUTION, RTD It is evident that elements of fluid taking different routes through the reactor may take different lengths of time to pass through the vessel. The distribution of these times for the stream of fluid leaving the vessel is called the exit age distribution E, or the residence time distribution RTD of fluid. E has the units of time-l. It is convenient to represent the RTD in such a way that the area under the curve is unity.

17. E, EXIT AGE DISTRIBUTION, RTD (cont….) • The fraction of exit stream of age between t and t + dt is, E dt. • Restriction of E curve : fluid only enters and only leaves the vessel one time. This means that there should be no flow or diffusion or eddies at the entrance or at the vessel exit. We call this the closed vessel boundary condition. • Where elements of fluid can cross the vessel boundary more than one time we call this the open vessel boundary condition.

18. Experimental Methods for Finding EXIT AGE DISTRIBUTION (E) • The simplest and most direct way of finding the E curve uses a physical or nonreactive tracer. • For finding ‘E’ curve some sorts of experiments can be used, which is shown in Figure below. • Because the pulse and the step experiments are easier to interpret, the periodic and random harder, here we only consider the pulse and the step experiment.

19. ‘E’ curve from pulse experiment Let us find the ‘E’ curve for a vessel of volume V m3 through which flows v m3/s of fluid. For this instantaneously introduce M units of tracer (kg or moles) into the fluid entering the vessel, and record the concentration-time of tracer leaving the vessel. This is the ‘Cpulse’ or ‘C’ curve.

21. From ‘C’ curve to ‘E’ curve To find the E curve from the ‘C’ curve simply change the concentration scale such that the area under the curve is unity. Thus, simply divide the concentration readings by M/v.

22. ‘F’ curve (Cumulative age distribution curve) ‘F’ curve directly give idea about the what fraction of fluid has spent time less than t and greater than t. For ex. Consider the ‘F’ curve shown in figure. As per ‘F’ curve fraction of fluid spent time less than 8 min is 0.8, means 80% of fluid has spent time less than 8 minute and 20% of fluid has spent time greater than 8 minute.

23. ‘F’ curve (Cumulative age distribution curve) from step experiment Let us find the ‘F’ curve for a vessel of volume V m3 through which flows v m3/s of fluid. Now at time t = 0 switch from ordinary fluid to fluid with tracer of concentration Cmax, and measure the outlet tracer concentration Cstep versus t. This is called ‘Cstep’ curve.

24. ‘F’ curve from ‘cstep’ curve Simply divide Cstep by Cmax

25. Convolution integral theorem Convolution means Intricacy, Complexity, Involvedness, Sophistication It shows relation between Cin, Cout and E curve

26. We say that Cout is the convolution of E with Cin and we can write, OR

27. Relation between ‘E’ AND ‘F’

28. Application of convolution integral theorem If the input signal Cin, is measured and the exit age distribution functions Ea , E,band Ecare known, then C1 is the convolution of Eawith Cin and so on, thus

30. EXAMPLE 11.3 CONVOLUTION Let us illustrate the use of the convolution equation, Eq. 10, with a very simple example in which we want to find C,,, given Ci, and the E curve for the vessel, as shown in Fig. E11.3a. Figure E11.3

31. SOLUTION First of all, take 1 min time slices. The given data are then Now the first bit of tracer leaves at 8 min, the last bit at 13 min. Thus, applying the convolution integral, in discrete form, we have

35. 2. Degree of aggregation Flowing material is in some particular state of aggregation, depending on its nature. In the extremes these states can be called microfluids and macrofluids

36. Single-Phase Systems Two-Phase Systems These lie somewhere between the extremes of macro and microfluids. A stream of solids always behaves as a macrofluid, but for gas reacting with liquid, either phase can be a macro- or microfluid depending on the contacting scheme being used.

37. 3. Earliness or lateness of mixing The fluid elements of a single flowing stream can mix with each other either early or late in their flow through the vessel.

38. Conversion of non-ideal reactor To evaluate reactor behavior in general we have to know four factors: 1. the kinetics of the reaction 2. the RTD of fluid in the reactor 3. the earliness or lateness of fluid mixing in the reactor 4. whether the fluid is a micro or macro fluid

39. Conversion in non-ideal reactor for macrofluid For macrofluids, imagine little clumps of fluid staying for different lengths of time in the reactor (given by the E function). Each clump reacts away as a little batch reactor, thus fluid elements will have different compositions. So the mean composition in the exit stream will have to account for these two factors, the kinetics and the RTD.

41. Examples for RTD A pulse input to a vessel gives the results shown in Fig. l. (a) Check the material balance with the tracer curve to see whether the results are consistent. (b) If the result is consistent, determine , V and sketch the E curve.

42. Example : Find conversion in real reactor and compare with ideal reactor

43. Example :

44. Models for non-ideal reactors

46. Chapter 12Compartment Models

47. COMPARTMENT MODEL In the compartment models, we consider the vessel and the flow through it as follows: By comparing the E curve for the real vessel with the theoretical curves for various combinations of compartments and through flow, we can find which model best fits the real vessel.

48. Notations used are: • M = kilograms of tracer introduced in the pulse • M = v (area of curve). • V is volume of reactor and v is volumetric flow rate