 Good morning. We are going to learn some advanced topics in reaction engineering as part of this particular course. You have already learnt basic aspects of reaction engineering, where we talk about what is reaction kinetics, what is its role in reactor design or simulations for, then how to come up with a rate law based on the laboratory data that is given, what are the effects of heat transfer, like for example, like these reactions exothermic or endothermic. So, how the temperature would change and accordingly how the reactor design will get affected. So, there are so many basic aspects of reaction engineering that you have learnt in the first part of this course. So, in the second part we are going to look at, we are going to go next step, where we talk about what is the role of catalyst and if the catalyst is in solid form, how the reactor design will get affected, what are the different procedures to take into account effect of or the presence of the catalyst. Then if the flow patterns are non-ideal, how do we deal with them? There are many other possibilities where one reactant is sitting in one phase that is say gas phase and the other reactant is in liquid phase and they are going to react either in liquid or gas phase, then in that case how do we do the reactor design? Like for example, we have a multi phase case where two reactants are sitting in two phases, like how to account for something called as mass transfer effects. So, in this particular course we are going to look at all these additional effects that will come in while dealing with reactor design. So, let me go back and talk about what is reaction engineering. So, normally what we deal with is the reactor design. So, that is at the centre reactor design or it can be simulation as well. So, reactor design needs some input and where it comes from the main input is the reaction kinetics is a reaction kinetics. Now, this reaction kinetics we can derive it from laboratory data or it can be based on some theoretical aspects like for example, density functional theory or then you have some additional effects of this reaction kinetics. If the reactions are taking place in multi phase systems, this reaction kinetics that is normally R is equal to say minus k C A or something which will not remain as it is because it will be affected by something called as mass transfer effects. This is something that you have not done in reaction engineering part one. Then apart from this you may have heat effects as well. So, you may have energy effects or heat effects where the reaction is affected by the heat evolved or heat absorbed. So, the temperature of the reaction reactor may change and you as you know rate of the reaction increases exponentially with temperature and that is why we need to take into consideration the change in temperature. So, heat effects what else then they can be pressure drop. So, pressure drop in the reactor especially in the case of gas phase reactions if there is a change in pressure drop then there is a change in concentration and that will again affect the rate of reaction. So, delta P is again very important what we need to do here is to write a momentum balance take into consideration the frictional pressure drop and so on. In the case of gas phase reaction is very important. Anything else we have something called as non-ideal flow patterns. This is what you are going to learn at later stage in this particular course non-ideal flow patterns. Now, most of the reactors that you have studied in the part one of reaction engineering course they are ideal reactors. So, they are a CSTR or plug flow reactor where we assume certain flow pattern and accordingly define the concentration in the reactor. For example, CSTR the concentration is uniform everywhere it is constant does not change with respect to spatial coordinate. Whereas, in plug flow reactor it changes from in lead to outlet, but there is no back mixing that we assume and we assume that the concentration profile or the velocity profile along the radial direction is uniform. So, these are the assumptions which make the reactors ideal in the sense we assume that the behavior is specific, but in reality it may not be that way. So, in reality the flow patterns may be non-ideal depending on the internals. For example, in the packed bed reactor you have channeling in a CSTR you have stagnant pockets. So, all these effects are to be considered while designing the reactor. So, if you see reaction engineering it is all reactor design and around which you have so many effects. And what you have learned in reaction engineering one is the reaction kinetics for simple reactions for say homogeneous reactions r is equal to minus k C A or C A C B or C A raise to n C B raise to m and so on. So, this is what you have learned what reaction kinetics is all about and how to design an ideal reactor based on the reaction kinetics given. Now, you may have multiple reactions also you may have heat effects also. So, this is what you have learned to some extent in part one. Then you have learned in this one also in part one and to some extent how to design a reactor for these particular cases, but when it and of course probably the pressure drop constraints. So, these three things you have already taken into account while designing the reactor you know the procedure how to design the reactor. But now what you are going to look at is the effect of mass transfer if the reactants are sitting in two different phases or product comes out of the reaction phase the multi phase effects or if the catalyst that you are using is in solid phase or in a different phase. So, you have mass transfer effects then you may have the flow pattern effects where there is a non-ideality. So, it does not follow ideal CSTR or plug flow reactor. So, these are the things that we will learn in this particular course. Now in case or in the case of mass transfer effects you may have the reactants sitting in two phases or you may have even the catalyst sitting in a separate phase. And this forms almost 25 to 30 percent of this course the catalytic reactions and catalytic reactors. In fact, you will see industrial reactors almost all of them are catalytic react very few reactions are uncatalyzed reactions. And in these catalytic reactors most of the reactors are where the catalyst is used in the solid form. So, which is which forms a separate phase most of the times the reactions are taking place or the reactants and products are in either gas or liquid phases. Whereas, a solid catalyst phase is a solid phase. So, we need to consider the mass transfer effects apart from that there are so many other aspects to be learned in catalysis. So, what we are going to start now in today's lecture is the first part of this course that is catalysis catalysis and catalytic reactors. What do we learn in this course? First we know what we try and understand what is catalysis? What are catalysts? Something that you already know, but then we just try and review that information in context of reactor design. Then different steps involved in catalysis when a reaction takes place on the catalyst. What else? Rate law from the data laboratory data. Now, again this is something that you learn before as well, but now the rate law will be different if you are using a catalyst. The rate law will have a different expression. A normal expression is R is equal to minus KCA whereas, in the case of catalytic reaction there are so many steps that are taking place while the reaction is happening. So, in that case the rate law would have a different form. So, you are going to spend some time understanding what kind of rate equation one can encounter in catalytic reactions especially when the catalyst is solid. Then catalyst has again a very important, I would say property or it is not a property as such, but then there is something that is undesired where when the catalysis is happening. What is that? It is catalytic deactivation or is the catalyst deactivation. If a catalyst is solid then the catalytic sites they are deactivated during the course of the reaction and the many reactions where the deactivation rate is much faster than the actual reaction. So, in that case how to deal with deactivation. So, catalyst deactivation and the last one of course is the design of catalytic reactors. So, these are the different things that we are going to learn as part of this particular chapter. So, let us start with what is catalyst? Very simple, most of you know about it. What is catalyst? Catalyst is a substance that increases the rate of the reaction, but is not consumed during the reaction, does not undergo a change, but increases the rate. It may undergo a change temporarily, but at the end of the reaction you see the catalyst is coming out in a form that you are already started with. Of course, deactivation is a different thing, but otherwise if the deactivation is not taking place the catalyst comes out of the reactor as such. Catalyst remains in the reactor as it is in the form that we already taken. So, what is it doing? It is increasing the rate of the reaction and there are many types of catalysts. It can be homogeneous and it can be heterogeneous depending on what is the phase in which it is. If it is in the same phase as the reaction mixture, same phase then it is homogeneous. If it is in different phase then it is heterogeneous. So, as an example if you have a solid catalyst which is catalyzing the reactions of the reactants which are present in gas or liquid phase say for example, ion exchange resin for esterification reaction which is a liquid phase reaction. Fluid catalytic cracking zeolites they are used for catalyzing hydrocarbons cracking which is a gas phase mixture. So, that is heterogeneous reactions and as I said before the heterogeneous reactions are quite common in chemical industry. There are many advantages of heterogeneous reactions. What are the advantages? Why we prefer heterogeneous catalysts over homogeneous? Of course, not that see there are many other reactions also still practicing industry with homogeneous catalysts, but the drive is towards developing a heterogeneous catalyst for a given reactions because there are many engineering benefits if we use heterogeneous reactions. So, before we go into details of the catalysts let us try to understand what are the advantages of heterogeneous catalyst. As I said it is mostly in a different phase and most of the reactions it is solid and reactants and products are sitting in either gas or liquid phases. So, the heterogeneous catalysts the advantages are first of all you do not need catalyst separation or catalyst separation is easy. If it is a fixed bed reactor the catalyst remains in the reactor and we do not need to separate it. Whereas, in the case of homogeneous catalysts it goes with the reaction mixture and we need to separate it. So, you have disposal problems also in the case of homogeneous catalyst. So, this separation is easy sometimes you have you avoid disposal issues or there is less load on the effluent treatment. If the catalyst is corrosive for example, sulphuric acid it is liquid phase catalyst it corrodes the material of construction whereas, if you immobilize those acid sites on solids and if you have acid catalysts in the solid forms is zeolite or ion exchange resin. In that case it does not come in contact with the material of construction. So, corrosion problems are avoided. So, you get rid of corrosion problems and the material cost goes down. So, like this the many advantages apart from sometimes you may get enhancement in selectivity and rates. So, these are only practical advantages from process point of view whereas, sometimes you get advantages in terms of selectivity yield or the performance. So, in the case of heterogeneous catalysts I will just continue. There may be a rate enhancement not always in fact, some most of times it goes down, but still we prefer it because there are many other engineering benefits, but sometimes I can increase the rate more than that it is a selectivity towards a particular product that goes up. I can design a catalyst as per my requirement and get higher selectivity. We will talk about it later more. So, these are the advantages of heterogeneous catalysts. So, I will just quickly revise what we have learned. First I told you what is the importance of learning mass transfer effects and non-ideal flow patterns which is going to constitute major portion of this particular course from reaction engineering point of view from reacted design point of view. Then I told you the importance of catalyst most of the times the catalyst is present in another phase. So, which makes it heterogeneous and because of that there are many aspects need to be studied while designing the reactor. So, in this particular chapter we are going to learn what is catalyst? What are the different steps involved when reaction happens on the catalyst surface? Coming up with a rate law based on the laboratory data which is a natural extension of what we have learned in reaction engineering 1, but now for catalytic reactions then catalyst deactivation and catalytic reactor design. Now, what is catalyst is a substance that increases the rate of reaction, but does not undergo a change it and may undergo a change during the course of reaction, but at the end of the reaction it comes out as it is that is nothing, but the catalyst and catalyst can be homogeneous or heterogeneous and heterogeneous catalyst are advantages because of no disposal problem separation is easy no corrosion issues sometimes you may get higher rate and it can be designed appropriately to increase the selectivity towards a particular component. So, let us go ahead and try understand what catalyst is. So, if you see a normal reaction say path of the reaction on one side you have reactants on the other side you have products and this is energy. This is a very famous plot that you have learned long back let us try and understand it again. You start from somewhere here that is where your reactants are the energy level goes up and then comes down and this is where your product is and this is where your reactants are. Now, it needs to go up and then it comes down what is it. So, it goes to a transition a peak will be obtained at which you have an intermediate form which has a very high energy it is not stable there. So, it rolls down and attains a stable value where your product is. So, what is this called as this is nothing but activation energy delta E of the reaction and this particular energy level is very important this particular energy level. This can be either above this point or it can be below this point depending on depending on whether the reaction is exothermic or endothermic and the way I have shown here this reaction is this reaction is endothermic. Why because this difference the energy difference here to here is positive. So, I have there is a net absorption of energy by the reactions because of which it becomes endothermic. If this point goes down further compared to the reactants then it is exothermic and this is nothing but a transition state where you have some intermediate form. So, every reaction either it is catalytic or non catalytic it has this energy curve or energy diagram where you have transition from reactants to product through an intermediate stage which is at high energy level. So, I need to provide this much energy. So, that it goes here and then comes down. So, that the reaction takes place and this energy level comes because of the molecules they have this energy that energy comes because of the temperature. Now, if I use the catalyst what happens to this particular graph or before that the rate of the reaction would depend on which factor it will depend on this activation energy higher the activation energy lower is the rate. So, as simple as that and you have learned something called as Arrhenius law or Arrhenius equation where we say that R is equal to k C A or minus k C A right this k the rate constant depends on temperature right. We will talk about it more later, but this rate constant depends on temperature is the temperature increases then the rate constant increases and higher the activation energy higher is the sensitivity of this rate constant to the change in temperature. So, if you increase temperature the rate increases and for higher activation energy the rate would be lower right. So, now you tell me if I use a catalyst what happens to this particular graph if I use a catalyst this activation energy is going to go down because the rate increases right. So, you may have a different graph for a catalytic reaction. So, what is catalyst doing catalyst is changing the path. So, now the transition state here is different from the transition state here the intermediate formed here that may be t 1 and this is different from t 1. So, you have a different intermediate which has less energy compared to the earlier one right and because of that the activation energy has reduced. So, the rate increases what will happen to this catalyst cannot change this always remember that most of the time we just concentrate on this, but forget this this comes out of thermodynamics this comes out of thermodynamics and this is not decided by the catalyst. This means this energy level the heat of reaction catalyst cannot change the heat of reaction catalyst cannot make endothermic reaction and exothermic reaction. So, that is the property of the reaction catalyst only changes the path it only changes the activation energy and it only changes the rate it will not change the equilibrium always remember that. So, the rate of the reaction is changed by the catalyst by virtue of reduction in activation energy. So, we have learned what is catalyst. So, in this we had considered the rate law which is nothing, but r a is equal to minus k c a the c a can be c a raise to n c b whatever depending on the type of reaction and stoichiometry. Now, this k is a rate constant. So, let us have a closer look at this rate constant this is nothing, but k 0 exponential minus e by r t or delta e by r t into c a. So, what is this k is equal to k 0 exponential minus delta e by r t this is Arrhenius equation. So, as you see from this equation sorry this is minus yeah. So, from this equation the rate is proportional to temperature that means if I increase the temperature rate increases it appears in denominator, but this is negative and the sensitivity of the rate constant depends on this factor that is delta e activation energy alright and this is what you have already learned Arrhenius law. Now, let us get back to the catalyst at I was telling you about the types of catalyst the different types of catalyst homogenous and heterogeneous. Most of the times the homogenous catalyst like whatever you have learned so far like we can directly extend that analysis to homogenous catalyst whereas, for heterogeneous catalyst the things may change. Now, what are the different types of heterogeneous catalyst heterogeneous catalyst again they come in different forms. So, one of them is a porous catalyst. Now, what is porous catalyst is a very important concept we need to understand it at in detail. Now, if I see a single particle of the catalyst this is a catalyst particle. Now, the size or dimension of this particle can be in the order of 1 millimeter or even less than that can be of the order of few microns. It is very small 1 millimeter and this small particle itself will have large surface area within it large surface area. How do I get this surface area? Now, it is a porous catalyst. So, it will have pores inside. Now, what are pores? Let me draw them. Now, this red ones are the solid this is a solid space. This is a solid phase and the space between these red matrices these are nothing but pores. The typical pore size can be of the order of few angstroms say 10 angstroms 20 angstroms or it can be nanometers 1 nanometer or so. So, this is open this is open to the reaction mixture. So, what happens is the reactant molecules say A is a reactant that goes inside through this pores and then the so much surface available inside the catalyst particle and on this surface there are catalytic sites which are responsible for catalysis on which the reaction takes place and then the product will come out say B. So, you may have reaction taking place A going to B. So, A goes inside it diffuses through these pores. There are catalytic sites present we will talk more about the catalytic sites on which the reaction takes place. So, I was talking about this transition state intermediate north all that will happen inside the catalyst or rather inside a pores and the products will be formed here. So, this are small tiny reactors on where in the reaction would take place and the product will come out. So, slowly I am we are developing a picture about how the catalysis happens. What are the different steps I am not told you all the steps, but you can understand diffusion reaction and the product coming out, but more than that there are other steps as well we will come to that. Now, so what is happening here now you as I said so much surface area available can you imagine the order of magnitude of the surface area. This surface area can be as high as say 1000 meter square per gram can you imagine. So, 1000 meter square per grams gram is a very small quantity and get a feel for this number 1 gram is very small quantity and what is the surface area 1000 meter square 30 by 30 meters it is like a ground such a high such a large surface area in such a small amount of the catalyst it is all because of these pores. So, I am not necessary that every time you get this much surface area depends on the material, but just to give you some idea I wrote this number it can be 500 it can be 200 it can be even 10 also. But a porous catalyst are the materials which provide very large for surface area in a small amount in a small volume and that is a main advantage of using porous catalyst for catalysis. Now, where are the sides present sides are inside. So, if you take example of say zeolites which are alumina silicates the sides are inside. So, say acid catalysis by zeolites the H plus or protons are present here of course, there are small amounts of sides on the external surface as well, but these sides the number is much lower compared to what you have inside. So, the surface area is mainly inside external surface area is much smaller compared to inside if the catalyst particle size is of the order of few millimeters. So, if you go to nano level then that case the external surface area also increases and it becomes comparable with the surface area inside a pores, but we are not talking about that surface area or that particle size most of the times the catalyst particle size is of the order of few millimeters or probably less than a millimeter, but not less than 0.1, 0.01 millimeter. So, in that particular range it is the interior surface area which is important. So, the best example of the porous catalyst is zeolites and which are used in many applications say catalytic cracking of hydrocarbons in refinery. So, this is one type of catalyst porous catalyst then there are other forms of catalyst they are supported catalyst. Now, what are supported catalyst the same thing a porous material which is non catalytic in nature otherwise same porous material, but that is not acting as a catalyst. Now, the main catalytic species the component is different and not this material say for example, alumina or silica activated carbon these are the materials which are porous in nature, but may not be the good catalyst for the reaction, but I still use these materials because they have the property to provide so much surface area. Now, I use them as supports I use them as supports supports for what supports for the catalyst. Now, for example, I have platinum being used as a catalyst what I will do is I will disperse or I will put this platinum on the support at various locations. So, how is this catalyst different from the earlier one the earlier one was the porous catalyst porous material itself acts as a catalyst the sides are part and parcel of that material itself. Now, what I am doing here is I am putting catalyst on a support that means the material which is porous in nature is support otherwise non catalytic on which I am putting this catalytic species. How to do it is something not of concern at present, but then if I am able to put this catalytic species on the surface say platinum, ruthenium, nickel whatever cobalt, iron which are all good catalyst see the D block elements transition metals they are good catalyst because of their variable valency that we know. So, we are able to put them on the porous support and because of which I get so much surface area and many atoms of the catalytic species are available for the reaction. So, these are supported catalyst and quite common in industry many supported catalyst as I said like all this metals when supported on alumina, silica, carbon they are all supported catalyst. The third type of catalyst gauze or foil the reaction is so fast that I do not need to expose the metal ions to that extent. So, even if I take a wire or a gauze that is good enough to catalyze the reaction. The catalyst is so active that even the external the atoms present on the external surface of the catalyst they are able to catalyze the reaction fast. So, I do not need to go for a porous material. So, these are called as monolith catalyst. So, example is of course, this oxidation of ammonia to nitric acid they use platinum gauze oxidation of ethanol to acetaldehyde they use silver gauze why gauze and why not supported on something because I do not need very large surface area catalyst itself is very active only the atoms which are present on the external surface of the wire or gauze are good enough for catalysis. So, monoliths these are non porous and non supported. So, these are different forms of catalyst. So, how do we define catalysis or how do we compare different catalysts? How does the reaction take place on the catalyst surface when the reactant molecule approaches the catalyst that is something we have to learn now. Now, let us look at the catalyst surface. So, what I am going to do is I am just going to magnify that pore on molecular level. So, as I said like we have this pores inside a catalyst these are the pores inside a catalyst. Now, I am going to just magnify this. So, I will have a pore through which the reactant is flowing and of course the product is also flowing in the sense through diffusion there is no velocity inside because the concentration gradient they would be a flow. Now, this if you just look at this surface now I am going to magnify this further I am just going to open this. So, I will have a surface or which A is going to react because it will come in contact with A with surface rather A will come in contact with surface. Now, on surface what do you have on surface you have catalytic sides what are these sides remember I had shown those metals supported on the catalyst. So, typically if you have a supported catalyst then you will have a sides present. If you have wire gauze or if you do not have supported catalyst the porous material itself acts as a catalyst in that case which what are the sides. So, it is hypothesized that in such cases sides are nothing but those places where there are defects or irregularities or which the activity is very high compared to other atoms. So, these are the sides. So, in general whether the catalyst is porous or non porous supported or non supported on the surface on the surface I have some places where the catalytic activity is more. And that is where the reaction is going to take place. So, A is going to get adsorbed it has to get adsorbed on the surface. So, that you have an intermediate form and then the reaction takes place. So, adsorption is a very important phenomena as far as heterogeneous catalysis is concerned catalysis with solids is concerned. We need to understand adsorption in detail in a normal reaction without a solid catalyst adsorption is not important. Whereas, for heterogeneous catalyst this is an important step in catalysis. Now, we have learned what is adsorption adsorption is of two types it can be physical adsorption it can be chemical adsorption. This is because of van der Waals forces and other long range forces. And this is mainly because of strong interactions bond formations. So, it is more like reaction whereas, this is a kind of affinity it is more like condensation a gas molecule is going on solid changing its phase. So, whatever heat is released heat of condensation is the heat of physical adsorption whereas, in this case it is much more than that it is more like a reaction is a bond formation. So, heat evolved here is say less than 10 kilo calorie per mole whereas, in this case it is much higher than 10 kilo calorie it can be even of the order of 100 kilo calories this is chemical adsorption. And what we are looking at as far as catalysis with solids is concerned is the chemical adsorption. Where the substrate or the reactant will form a bond with the catalyst will form a bond with catalyst. So, these are the two types of adsorptions physical and chemical. So, what is going to happen on the catalyst surface is for example, I have a catalytic site and there is a reactant which is coming to the surface. So, it is going to get adsorbed on it it is going to get adsorbed on it. So, it is going to form a complex. So, this is s it is going to form a complex. So, the reaction can be represented or the step can be represented at a plus s gives a s adsorption. And there is this interaction through the bond formation of course, it may be unstable and may break within no time. So, it is possible that this is just a transition state or intermediate stage and further the reaction will take place to form a product which is say b or c whatever ok.