 Okay so in the last class we have been discussing about that beautiful diagram I will draw it again and I have to draw many many times a reactor I think by the end of this course you will all by heart this output and it is not a bad diagram definitely if you remember this diagram you will know the entire picture of C R E chemical reaction in it right so that is why you have to remember this okay this is kinetics and here we have contacting in kinetics we have again chemical and physical and in contacting we have batch continuous and in continuous we have P F and M F okay yeah I think P F means plug flow M F means mixture flow for those who are not who are not familiar with this notation okay so then the performance equation equation is given by this output is a function of kinetics no first input yeah input kinetics and contacting yeah so this is the performance equation yeah in fact if this tells that there are 4 variables okay what we should know is kinetics which gives you a rate expression so that is why it is no way different than biochemical reactor or normal chemical reactor because if you are able to get a particular rate in biochemical reaction like monodes rate and they are very likely they have only 2 rates right either monodes are Michael Simenton right and slight variations of that you know in the denominator or numerator you add 1 term or remove 1 term then you will get different forms of those 2 rates only but whereas in actual chemical engineering there are many many many forms of rates okay so you have this kinetics giving you a rate expression a rate and contacting will tell you what kind of reactor you have to choose and fortunately for us we have only 3 reactors oh no 3 reactors okay yeah 3 reactors so one is batch reactor and we have 2 continuous reactors ideal reactors by name plug flow and mixed flow if you are able to understand the meaning of this contacting what is batch what is plug flow what is mixed flow when do you use batch when do you use continuous that means lifelong you have all the information with respect to reactors only variation comes if they are not ideal what is that you want to do what is that that extra information that comes into picture okay that is why in my opinion contacting is much easier to understand and also to use all the time whereas kinetics much much difficult right and I do not know how many of you really understood what is the meaning of chemical and physical what is the meaning of this chemical and physical what do you write there I think what do you mean by physical kinetics what do you mean by chemical kinetics reaction rate and mass transfer okay reaction rate right where does that come where does mass transfer come in chemical reaction rates where where do the heterogeneous reactions right in all heterogeneous reactions you wrote very nicely that question everyone wrote uniformly right okay heterogeneous reactions and homogeneous reactions what is the extra information we require in the heterogeneous reactions when compared to homogeneous reaction that mass transfer that is extra step okay but in fact that is much more difficult because for every reaction you have to first visualize how the reaction is taking place that we will discuss a little bit later but now first the contacting pattern is easiest so that is why I thought I will do first contacting but before that when you are writing an equation for either batch or continuous reactors either this or this we need a rate expression so that is why we have to now come back to kinetics again and then question this chemical and physical these things will come in homogeneous and heterogeneous reactions right so we can now divide this chemical reactions now short form r x n like action T we have no a x n so like this here reaction right r x n means reaction okay now chemical reactions can be divided into homogeneous and heterogeneous okay now we have to identify homogeneous how many phases we have homogeneous means what is the definition you already know you are experts you should have one phase okay everything should be happening in one phase so how many phases we have so shall I list all these three here we have gas phase okay a gas phase and then liquid and solid okay ya this is never called as homogeneous right because what is the meaning of homogeneous when you are talking in chemical reactions for solid also density remains constant this is the problem no because we know only the definitions but homogeneous we know but I think beyond that when you ask we do not get any answers most of the time okay and what is that requirement in homogeneous reaction why do you say that okay is homogeneous homogeneous means single phase but so what in what is happening in that phase we react in a different way that is different okay molecules only definitely react okay but still why do you call it is homogeneous but do not get deserted I mean we are only just learning so that is why you do not get next time you should not say no this man is laughing so that is why no no I think it is only for joke that is all ya molecules definitely react but then actually what is the requirement for homogeneous phase properties should remain same yeah I mean still what is the meaning properties ya okay they are remaining same so what sir determining kinetics will be much more easy why property is remaining at one location it will be different and that some other location it will be different that comes as mixing but I think you know ya properties same means I think we are talking about concentration same throughout but I think see the basic requirement in homogeneous phase is that the molecules should be readily available for reaction right so that means if I have A and B together in a homogeneous phase A need not search for B B need not search for A both of them are there very near so they come together and then collisions and all that we can imagine no then immediately you will get the reaction so that is why the basic requirement for homogeneous phase definition is that you have the molecules together and sufficient amount whenever A wants to react with B B wants to react with A it is not only that many times you will have only A decomposing to some other reactions so that means all the A molecules should be just present there and when you create this reaction environment they should be able to easily react that in the other sense the molecules of particular reactant in the in that phase should be freely available there is no extra effort that is required for the reaction to take place right okay so then that is what is the definition and that cannot happen in solid phase okay and even though if you take the purest form of solid the molecules are not able to freely move okay so that is why this is a question mark always whether we have to call solid as homogeneous or heterogeneous okay but most of the time you never get purest form of solid anywhere right so that is why when the molecule wants to react with another molecule that has to diffuse extra okay within the solid itself so that is the reason why when compared to liquid and gas where the molecules are very free to move I mean the kinetic velocities are 300 meters per second 500 meters per second and what is your diameter of the reactor maximum 3 meters and that is moving 500 meters per second the molecules particularly gas phase so you see 500 meters means from here to your hostel in one second it goes can you go if you are able to go when you are hungry you may go okay one second half second also so that much faster that means you can just imagine how easily they are available whereas solids by definition they would not move the molecules will not move so that is why you have to apply high temperatures where more energy comes for the molecules they try to move and then you know that self diffusion is there so then other molecules will come together and then both will react all that information is required all that all those steps are required so that is the reason why normally we do not call them as homogeneous that is why all the time we have gas and liquid as heterogeneous I mean homogeneous systems where that means there is no effort extra required at all for the molecules to require except you temperature you are providing sufficient concentrations you are providing so that anytime the reaction can occur that is what is the definition of homogeneous reaction so I want to tell these basic things in in a number of times the reason is that those are the things which we have to understand we know only the meaning of homogeneous but beyond that if you ask one more question I think if you are not able to answer means that is not the knowledge that is only the parrot knowledge where parrot repeats homogeneous homogeneous 10 times if you tell the throughout its life it will just repeat if you ask the parrot okay tell me explain what is the meaning of homogeneous it cannot so we should not become parrots so that is the reason why extra information is required for us so the basic requirement for homogeneous phase when you say that I have a homogeneous reaction is that absolutely there is no mass transfer limitation molecules are freely able to move and at any time if there are two A and B these molecules are easily available to each other for the reaction to take this okay so that is what is homogeneous and the heterogeneous this we will remove out we have more than one phase right yeah so that means now we have a totally three phases the combination of all these three phases will give us the overall number of types of yeah the overall types of reaction rates or reactions okay so I do not know whether you understood or not we have three phases combination of these three that means we have gas liquid solid right three so first we will write here gas liquid is one combination right and then liquid liquid is another combination right and then we have liquid solid is another combination yeah so then we have another combination is gas solid yeah I will come I will come I will come to that yeah okay so then yeah so gas solid is another combination then what is others one you are telling solid solid is there any other combination gas gas there is no combination because gas gas molecules are so fast because because I have written liquid liquid no so he is adding gas gas we have now added liquid liquid gas gas and solid solid also okay yeah gas is always homogeneous because that is very very free happy to move okay so that is why that will not come and yeah gas liquid solid now usually I will give the first exercise first assignment is this now find out under each category under gas liquid reactions it was in a system five minimum five actual reactions you can not even tell one gas liquid systems can you tell one reaction actual reaction I am talking about see how much time we are thinking yeah I know where to hit exactly that is very bad okay now tell me at least liquid liquid reactions we do not know at least liquid solid reactions there are few which are easy for you to guess yeah very good leaching most of the time right that means you take a solid for example if you want to remove purest form of copper you take copper ore add sulphuric acid or HCl and then now you will have copper sulphate as I told copper no if you take sulphuric acid copper sulphate now you take copper sulphate to electrolytic cell and then separate SO4 and separate see you you will get the purest form of copper which will get deposited on one of the you know yeah cathode or anode okay yeah so that is what I think you will have this one gas solid systems many this you should be able to tell burning of coal copper oxide formation every day you see rusting yeah rusting Fe plus O giving you Fe O okay you leave your cycle there and before you go after M tech you will see only rusted cycle most of you could have purchased good cycles you know new cycles yeah lock them properly in IIT all new cycles will go once at least once so that is why I think you buy more costlier than cycle keys locks yeah cycle now how much is cycle now 4,000 my god you know when I joined here PhD how much I have purchased can you guess huh good guess beforehand just 400 I purchased at that time also no thefts happy okay no one was stealing our no one was stealing our cycles cycles used to be there but nowadays I think smart people know that is why what else they do I think doing this kind of things they think it is very very smart okay anyway so rusting of iron and all that will come but what you told all these things examples like coal combustion carbon plus oxygen giving you CO2 I mean if you assume that only that reaction is present all those reactions are called non-catalytic reactions what is the difference both the reactants carbon plus oxygen both of them are reacting or reacting participating in the reaction to give you CO2 but now you see I will give an example so that kind of even FEO FE solid plus O2 oxygen will give you to FEO I mean that I will write now I think I can write here C solid plus O2 gas better write these things earlier you are not writing this the state of the reactant so better write now so that you will immediately know whether the system is homogeneous or heterogeneous because you have now two phases if I don't write yes yes here don't write G here you don't know what is that you don't know which phase they are of course carbon and oxygen we know but some other chemicals we will go we don't know that that's why better write that if you don't know you have to find out and write this is CO2 gas this is a non-catalytic reaction and also I have FE solid plus O2 gas giving us FEO solid correct to yeah so now it is balance 2 to a yeah so that is one and now the same FE I can also use in other reaction for example N2 gas plus H2 gas yeah plus FE solid giving us NH3 gas plus FE solid right yeah so now I have to balance again this one this is 2 this is 2 this is 3 what is the difference between this and this here also I have FE here also I have FE yeah here it is not actually participating in the reaction and getting transformed to something else but it is retaining its iron form as it is but only thing is it helps the rate of reaction to increase right yeah so that is why this is a catalytic reaction this is catalytic reaction and these 2 are non-catalytic reactions good very nice yeah so this we have to first identify because the design of catalytic reaction is totally different when compared to the design of non-catalytic reaction right but here catalyst is there you have the packed bed for example that one of the examples we can give you is packed bed ammonia they always use packed beds only all iron is loaded in this reactor and then hydrogen and nitrogen it is sent continuously so you will get continuous things and most of the time this will not get deactivated you have deactivation of catalyst so that is why for lifelong you can run that if you if you do not have deactivation that is why packed beds are very good in the sense that in packing when you have packed beds the solids are not moving when you have movement of solids then we have lot of problems okay how they move where they move and when they are moving they are also rubbing towards the walls so after sometime ten years if you run walls may get eroded thickness may go then it may burst all these things are there but if they are not moving happily sitting there then that is why packed bed is called work horse of chemical engineering chemical industry work horse you know there are 2 types of horse work horse running horse okay running horse will be useful where only in race courses okay and they run only maybe how many minutes I do not I have never gone there anyone has gone for races nothing wrong I say to go and see also no problem how many one round I think may be one round or ten rounds I do not know how much but maximum you take that may maximum half an hour it runs and you know preparation for half an hour run one year good amount of food some injections many things they give so it is not profitable to industry because we need a horse which will work all through not that half an hour and then you know the remaining 395 days 365 days minus half an hour so it will be eating eating eating eating all the time that is waste of time for us waste of money but we need work horses where all the time they work and in between they also eat right so that is why the chemical industry the packet bed without any problems all the time it works that is why the name is given for packet beds work horse of chemical industry okay so that is a very nice name okay so that is why in the catalytic reaction what happens is the solids are there all the time and you have to find out what is only the total amount of solids in the reactor okay based on that is why your design will be to find out what is the total amount of catalyst that is required whereas here I do not have FE at the end I do not have FE but it will become FEO at least this is becoming a solid but you see here I start with solid and I start with gas now everything is gas what volume you calculate is it solid volume or gas volume yeah we will see it is not gas volume okay so that is why the design of non-catalytic reactors and non-catalytic reactions is totally different or reactors totally different and now we extend our definition these are chemical reactions the same thing is duplicated if I write here chemical reactors that means wherever these reactions are taking place you you can call them as heterogeneous chemical reactors right so that means I may have GL gas liquid reactors liquid liquid reactors liquid solid reactors all combinations and this one GLS is called as slurry reactor most of the time the solids are used in the form of slurry and when you have liquid and put solids and you do not put you know 1 kilometer 2 kilometer diameters okay you will put only may be 10 microns 20 microns 30 microns you know you have the idea of microns yeah how much I mean I do not have here to say but how much you are here thickness 0.5 microns so that is why anyway as engineers you better have you know that kind of estimation in the mind is excellent what he has done is really good we should have that kind of estimates like for example if I ask what is the diameter of this this pen what is the diameter of this yeah may be 10 to 12 yeah that means you know 6 inches almost okay so like that you know for example when you are writing yeah the smokers cigarette what is the diameter what is the length in fact that is non-cateratic reaction correct no you are burning for tobacco okay and that is becoming like carbon becoming ash yeah carbon also becomes ash if you have some other you know silica and all that so this also becomes ash no every time you do like this it falls all that okay so you can also calculate from non-catalytic reaction rates what is the time required for normal smoking of a cigarette normal smoking is simply taking and then we are taking one breath or so but some people you know for them the rate of reaction is very very high why it is very very high not you know there is some step there in fact there is a mass transport step coming I don't whether you are when I do when I am when I am doing like that what I am trying to do I am sucking the air you know that oxygen will come through this pores because that is porous you will not have solid you know cigarette solid cigarette means you cannot you know smoke will not go come it should be porous and the beautiful question is what is the porosity of a cigarette yeah general knowledge what will be the porosity units I say meter cubed or kilometer cubed just guess no guessing chemical engineering is no guessing I think yeah that will be around 50 percent it is a packet bed I say yeah it is a packet bed simply because if you make the porosity point two point one your lungs will go much quicker anyway they will go okay it is only the time it is only the time whether you choose ten years or maybe one year so that is why the design of you see for everything there is wonderful design really for everything there is wonderful design and also to stop to stop all that nicotine and all that they will have another filter on the top actually that is bad for the lung okay because it will extend your life a little bit but lung will go because that porosity is small you know the filter porosity right that may be only point two point three like that so there you have to apply more pressure drop the pressure drop across that is more see all chemical engineering we are talking even though we are talking about cigarettes so every problem you can convert into as a chemical engineering problem and then discuss about pressure drops flow rates mass transfer all that so that is why this person like that is doing that means he is taking a lot of oxygen so burning rate is more so cigarette is burnt very very quickly right so that is why the design of non-catalytic reactions not because of cigarette the when these two reactants are participating in the reaction the design is different whereas in this case the design is different and and this is also the reason why the rate of reaction is different here the rate of reactions are different here so for every reaction you have to find out rate of reaction for heterogeneous system that is why I told you this portion is very very difficult every reaction you have to derive equation whereas whatever reaction you bring you have only three reactors correct no any reaction you bring any heterogeneous homogenous are all three phases there nowadays biochemical engineers have actually four phases also it is gas liquid liquid solid okay yeah someone was telling you know that this immediately when I wrote this someone was telling that no no sir this is not correct liquid liquid is homogeneous it is not homogeneous yes it is homogeneous if the both liquids are thoroughly miscible if they are not miscible then definitely like take for example kerosene and water or some oil and water in this oil you have some reactant and water is another reactant let us say so now for the reaction to take place the reactant in the oil should come to the surface interface okay similarly from water side the reactant has to come or what itself if it is a reactant that has to contact there that particular reactant which is in the oil so then only the reaction that means what is that that extra step what is that extra step mass transfer not only mass transfer even heat so heat and mass transfer both should be present that is why we call this as physical reactions I mean you know physical steps that are required for chemical reactions because mass transfer is a chemical step physical step and reaction also sorry heat transfer also is a physical step so those are the two extra things that are required for heterogeneous reactions you know for the reaction for the reactor to be designed that information you require why because that information only tells me what kind of rate expression I have whereas in homogenous reactions what is the general rate expression just guess one first order rate for example minus r a equal to k into c a that c a contains all the molecules you have the concentration how do you express concentration moles per unit volume so in that unit volume how many moles are there in one mole how many molecules are there that is all so that is the reason but whereas that is not guaranteed in heterogeneous the concentrations okay so that is why unless okay let me give you this example I have this this reaction carbon plus oxygen going to CO2 okay I keep carbon here and then next room we keep oxygen where is the reaction it has to necessarily contact that is why the term contacting has come and this contacting is really very very suitable for heterogeneous systems because in homogenous there is no contacting in the sense that you know individual phases we are not contacting but here you have many many possibilities how do you put gas liquid into the reactor if it is batch both you put and then wait if it is continuous because you can choose either continuous or gas yeah counter current yeah that is only possibility why not co-current is that only possibility why not cross current and beyond that we do not have any currents okay yeah so all these things are possible the moment we have co-current counter current cross current your rate of reactions will change your reactor design will change that is why heterogeneous systems are very very complicated heterogeneous rates are very very complicated but the beauty in chemical engineering is whatever contacting use cross current counter current or co-current the movement of that particular phase can be visualized as either plug flow or mixture that is all that is the beauty that is really my god I will get really excited when I see the reactor design because any kind of reactor you bring you know from even from other planets also or from our planet we can reduce them to two continuous systems either plug flow or mixture flow but we have not decided I mean discussed what is this plug flow and what is mixture flow yet we will do that right so but I have to talk to talk to you about this kinetics first so that we will have a meaning for the rate no because unless these molecules come together chemical reaction happens but before that okay now again same example carbon I put there and oxygen I put this side there is no contact right but what is the actual condition for the reaction to take place I have to take the carbon lump or this is the carbon over this we are sending oxygen and fluid mechanics will tell me that whenever I have this kind of situation no because always we draw the spherical particles even though they are not spherical particles right so then this goes like this so this is oxygen this is carbon right and fluid mechanics again tell me that whenever I have a solid and gas phase or liquid phase around this solid we have what is called stagnant yeah stagnant place which we call it as which we call it as is it meter which we call it as there is the vertex there not heard of boundary layer I say simply you know this is the boundary okay that is a stagnant I mean stagnant layer what you say okay stagnant layer is normal word but boundary layer is high funda word the people will get impressed when you say boundary layer so now this boundary layer is stagnant now you can say boundary layer is stagnant actually that is not stagnant it is dynamic but you know the because the oxygen molecules are just diffusing and CO2 molecules are oxygen is going out CO2 also has to come so that is why again you know that is why you read fixla counter correct diffusion and all that if correctly one mole is going inside and one mole is coming outside then you have equimolar counter and if you do not have that then you have to go for that stiffen equation correct for multi-component diffusion where the mole balance is not proper all that okay so that is why that will become much more complicated that is why we will say that okay let us assume only equimolar counter diffusion it is only because we are lazy to do that kind of mathematical work so we will assume we will many assumptions we make so that our mathematics will be simpler but we cannot make stupid assumptions it should be realistic assumptions where finally what your theory and what your experiment both are corresponding that is why what we have to do so now this is the one so here oxygen as you said even with boundary layer if there is not much oxygen diffusion to the surface temperature is there will the reaction takes place you will not have if the oxygen concentration is very very very low you may not have sufficient reaction so that is why the reaction between C and O2 is a chemical reaction oxygen molecule must be there and carbon should be there then only the reaction takes place but what is that physical step coming this extra mass transfer step not only that if I keep I mean you know carbon we just store in the atmosphere is it burning it is not burning why what is lacking there temperature so that is why heat transfer also is important how do you heat this particle and you send very hot oxygen okay RA where oxygen is there then initially it takes time for this particle to get heated up that depends on thermal conductivity of carbon correct no yeah so then once it reaches sufficient temperature then only starts burning that is why heat transfer is also a must step not only that I think you can also take this catalytic reaction FE and nitrogen and hydrogen these things will go to the reactor at what temperature 350 or 550 550 degree centigrade how do you bring the catalyst to 550 degree right that is heat transfer unless the all the particles are uniformly heated around that temperature okay you may not get uniform reaction rates on the other hand let us say that even this yeah I have kept the reaction at 550 overall macroscopically but now I will just look at one particular iron particle okay what is the size they use normally 2 to 5 meters 2 to 5 centimeters 2 2 means 1 inch slightly less than 1 inch yeah 2 5 5 means 2 2 inches yeah okay any other guess I am not saying is right or wrong less than half an inch yeah any other guess powder form yeah any other powder means how much because powder means in anything can be powder right yeah yeah I think that is vague only I think you know powder anything can be powder you know even of course more than once one mm we do not call powder see you know what is the talcum powder site which we apply for phase you should tell that nowadays they should also tell because it is not only girls boys also use yeah how much you know yeah general knowledge what is your name Gopinath yeah Gopinath any guess is there yeah that is also one micro just imagine that you you are applying that powder with one mm skin will come out when you are putting like this so one m that is why it is highly dangerous you apply may be if you are taking 5 grams and then applying here 2 grams will go inside because you are not stopping your breathing and applying you are breathing normally and then applying so when you put like this and then pour something will go that is why nanotechnology is highly dangerous really I know that is a silent killer without knowing that you are doing nanotechnology you may be knowing that you know you may be doing nanotechnology but that kill you that will kill you depending on what kind of particles you are using this those particles will happily go inside and deposit in the lungs and everywhere throughout the body so if they are toxic then that is why there is a separate research going on how to estimate the toxicity in nanotechnology we are very happy to write you know nanotechnology and also talk about nanotechnology but these are the dangers there right okay so normally you know the 2 to 5 2 centimetres to 5 centimetres normally no one will use normally it is often itchy and particularly here nitro I mean ammonia they use I asked you know I think why they use heat exchanger type reactor for ammonia that is one of the questions which I asked in the first day question paper you may not remember I will remember okay yeah there is a special reason for that which you also you will learn okay so they use heat exchanger type that heat exchanger type means it is like heat exchanger where inside you have tubes inside the tubes you have the packed bags and those tubes are not normally more than 2 inches when you have tube diameter itself for 2 inches what will what should be the particle size it may be one fourth inch okay otherwise if you use again big particles you have what are called wall effects you heard of wall effects okay so wall effects means when you have large particle and small diameter your flow will not be uniform so that is why we say that it is wall effects and all that that means your porosity inside that bed is not uniform one particle this side another particle this side another particle this side and when one particle is here one particle is here all this is vacant space this is the diameter of the tube so that is the reason why that uniform flow will not be there so that is why wall effects minimum number of particles you should put across the minimum across a diameter will be 8 to 10 particles okay along the length so many particles all that I will tell you when you are come when you are coming to RTD and all that I mean these things will come there okay so all these things also we have to keep in mind when you are finally designing so yeah what is that I was telling I was telling about this iron where packed bags why do we use and I think you asked a question for that who asked about that yeah physical kinetics I think I already explained that because the physics the two steps that I asked you that particle because that question because I was telling you the entire reactor is at 550 but now I will take one particle out will this particle will have 550 or will it have different why it why different yeah you are right but we see this is what the thinking you have to do all the time you know with your mind when you are thinking about any process it is a metal so I think a conductivity is almost uniform but it is not non-uniformity of the particles there is a very good reason whether it is exothermic reaction or endothermic reaction okay where the reaction is taking place actually why surface it is a porous iron you can also make porous iron it is in fact porous iron because we are not satisfied with only external surface external surface is very very small inside the surface you know if I tell you you will get really astonished in some catalysts even if I take one gram of catalyst can you guess what is the surface area yeah 200 meter square that is minimum in fact it can go to even 2400 meter square in fact we produced one my PhD student who was making activated carbon in one sample he got 2800 or 2700 meter square per gram of activated carbon okay that means you can imagine this what is the size of football ground Olympics just now over 2000 meters also you can tell 100 meters and then this one is not more if I remember correctly that is I think 80 160 or something 80 meters and 160 meters because it is length is large you know this is small yeah how much if you multiply this oh for easy calculation you can take no hundred and 200 and 2000 you can take right yeah and you know what is the catalyst per gram of catalyst how much you can have good catalyst will have even 2000 2400 this 200 is very very normal simple catalyst and that is one of the objectives of chemical engineers to produce as much surface area as possible you do not have you know infinite opportunity to go for any any area why because if you are putting more and more pores then after sometime everything will be pore there is no solid okay where is the particle that means this mechanical strength of the solid will be that is why you have to take into mechanical strength into account and also what is the good surface area what you can get without sacrificing the mechanical strength why should I have why should I worry about mechanical strength see so many questions I say I can talk talk talk talk I am not able to start theory is also theory what you are talking why why should I worry about mechanical strength as if I do not have any I am taking one particle size you know I told you half an inch how can they run out and also they do not run out you put a sieve at the top of the reactor at the bottom and also top you put on that only packet bed all these solids are supported you will have a perforated plate above that if you are not confident then you can put a sieve and over that only you will keep all these catalyst top and bottom both places you put this why I should worry because in industry in a one reactor you will have two tons three tons of catalyst and the bottom most layer should also withstand for that entire weight it has to carry that entire weight so if it is not able to carry what will happen it will crush when it is crushing what will happen you will have fine powder it is good when it is fine becoming fine powder you will get very good surface area that is very good but problem is that will block all the force then whatever compressory use it is not able to push that is why we calculate pressure drop across the packet beds what is the pressure drop that is required when you have all powder I think you are someone was telling you powder we cannot use powders in packet beds because of this pressure drop limitation when you put all the powder in a packet bed then they are very there is no porosity when all of them are very fine powder you take and then pack right then whatever compressory use the gas cannot go through that and also you are fixing that between two good distributor plates very fine distributor plates if the distributor plate diameter is slightly larger than the diameter of the particle then all the powder will come out so all these problems are there that is why we take either you know one fourth inch one one one half inch that means maybe 12 mm 10 mm 8 mm 6 mm that is what the particles we use but if the diameter of the particle is I mean reactor is slightly bigger then normally we go for maximum 18 mm that is 3 4th inch okay one inch also as far as I know in the industry know we will go for catalyst because we are losing surface area is one but other than that also you know but we would like mainly it is losing the surface area right so that is there is and also as the diameter of the particle is increasing we talked about mass transfer step you know the particles the gas has to diffuse into the particle so if you have bigger particle then the diffusion is not very fast when you have small particle diffusion is very that means through you know you heard of effectiveness factors right yeah the diameter of okay if you are using this size particle literally this size particle so the gas molecules have diffuse from here to here so the diffusion length is very very large right but whereas if I use one mm particle the diffusion length is very small so that is why it is easy for the gas to diffuse and that is why I asked you the question you have heard of effectiveness factor right yeah so what does that tell you if I say that I have effectiveness factor equal to 1 what is the meaning of that but what is the condition in the particle right what you said is right yeah so that means throughout the particle I have the same concentration of bulk that is the reason so bulk concentrate that means practically this is not I mean offering any resistance at all that is how you have to design your catalyst it should be porous but that pore should be sufficiently large it should give sufficient surface area but less resistance that is why that I know idea of effectiveness factor that is why you manufacture catalyst and then try to find out what is the effectiveness factor of the catalyst so if I say that I have 50% only that 0.5 effectiveness factor we are talking about isothermal non isothermal it will go much higher okay so because same definition you said the rate of reaction divided by rate at bulk conditions right so when I have highly exothermic reaction the rate of reaction on the numerator is very high but because the exothermic reaction the temperature is more only inside the particle so when the temperature is more rate of reaction is more so that rate of reaction and your bulk rate where the temperature is less when compared to the particle it is small that is why you will have effectiveness factors more than 1 in non isothermal case but it should be only exothermic reactions and in fact I can tell you in the next semester you will get even 500 effectiveness factor even 500 that means the rate of reaction inside the particle is 500 times larger than the rate based on bulk which is very good for us but the again problems are there if that is stores so much heat is generated inside then the particle will get what is called thermal sintering sintered so when sintering occurs naturally you will not have that effective sides active sides of the catalyst they will die it becomes like glass surface if it is becoming like smooth glass surface you know then you will not have that defects where catalytic activity is going on that is why we say that is called thermal sintering and we also have the thermal deactivation the other thing is called chemical deactivation that means these fellows like sulphur and all that will go and then block all the active sides and also even coal when you burn coke will form and that coke will completely block the surface where there is diffusion but the actual catalytic surface is not available for oxygen to go and sit there or some other component I mean active reactant to go and sit there that is called chemical deactivation you see how many things we have to think when you are designing heterogeneous catalytic reactor whereas homogenous catalytic I mean homogeneous reactors very simple we have the liquid A and B you put them together make good mixing they are missable 100 percent missable so then A is always finding B B is always finding it is very easy to get married in the U S because A is available B is available one side I will love you I will love you both go and then marry next day they may divorce that is different okay but you know that is very easy but here our this procedure is very very complicated know so the parents from boy side, parents from girls side both you have to meet they have to high level discussions okay you know submit discussions they call okay so those discussions and now this parents will take another five people with them these parents will take another ten people with them because girls will be always more careful you know so that is why more people means proportional to more number of questions so how is the boy and all that so that is why he takes a lot of time for us so that is mass transfer limitation so reaction is not very fast okay so but in the U S no parents direct molecule striking yes okay happy to marry happy to marry church is open go there get get married come out next day you may divorce and all that is different okay or they may stay together you know so that is why that is what is the you have to imagine for our molecules that resistance only we have to remove okay so that is why homogeneous reactions American society heterogeneous reactions Indian society okay good you remember that you will never forget what the difference between these two what I have to do in the next class is I have to give actual definition of what is the rate right that means how do you define homogeneous rates okay actually rate is something per unit time correct no anything from some unit time right your speed also is rate so many kilometers per second but in chemical reactions what we do is so many moles per second reacted that is the rate but always we use something else there what is that volume how do you use that rate is simply only that I would like to know only moles per time so many moles have reacted or vice versa from stoichiometry you can say so many moles have been formed correct no I mean left hand side right hand side of the stoichiometric equation if you see so many reacted so many form but I think always we can say in terms of reaction you know rate of reaction oh yeah now tell me what do you use that volume there you said volume even without volume also I know how much reacted depending on the rate you can represent in terms of concentration is always unit per you know I can take one later one later and that means one mole per later one mole per later but why do you use this one for expressing the rate see these are all simple things where you know these are called cow-bebs in the mind we know rate we know definition but if you want to discuss further we don't know that means there are cow-bebs you know cow-bebs you can see in the house so if you clear them then you have the clarity in the house similarly you have also removed that cow-bebs from our brain then you will have clarity that is why so many so much discussion I do there is one very important reason why we do that okay if I take one later and one later and react my rate will be different she takes five liters five liters her rate will be different okay because okay yeah so like that you know you have different people now expressing different rates if I take different concentrations different volumes then how do I simplify this because all of us should talk only one language that is why you heard also in thermodynamics what are called extensive properties and intensive properties what is the beauty with intensive properties independent of size that is what what we want to talk here the rate is expressed like that you know moles converted per unit time per unit volume then you take ten liters I take five liters he will take point five liters all of us when you convert per unit volume we should get the same that is the reason that is the reason why we express moles per unit time per unit volume otherwise I may say that okay my rate is 100 moles per second he will say that my rate is only 0.1 mole per second it is same reaction right so that is why the definition of rate of reaction is very important because of that reason right and now the same thing if I extend for heterogeneous systems now this volume is there that volume is what you know homogeneous reactions I am talking that you moles per unit time per unit volume what is that volume volume of reaction mixture where both are miscible right if it is liquid or gas and we are talking about that volume right and now if I take this liquid liquid they are not miscible simplest example so then how do I express that rate sorry in terms of surface area that is the only way so that is why there are many possibilities for heterogeneous systems that is right very good thing is to express based on surface area why that is where actual reaction is taking place but you know how difficulty it is to estimate correctly the surface area particularly in a large column where I have two liquids one is called disperser phase because you are dispersing you cannot have two continuous phases so your limitation of contact is very very low when I have two liquid I mean two continuous phases that is why we disperse one liquid into other we will make very very fine droplets of one liquid into other why this fine droplets will give me lot of interfacial area that is why I told you know sometime back chemical engineering definition is that it is bubbles drops and particles if you are able to really understand the behavior of all these three okay and combinations then you know all chemical engineering so that is why what we do is okay it is very difficult to find out what is interfacial area let me find out at least volume of one reactant that we know I am charging or otherwise you say why one reactants are why not two reactants yes no problem or you may say that why only one two reactant volumes let me take weights so I will say that moles converted per unit weight of the liquid both the liquids together or one liquid together may be continuous liquid continuous phase so that is why you have many many possibilities to define the rates that is where the confusion starts when you are designing heterogeneous reactors the definition itself is not clear you should have seen the design of catalytic reactors right so definitely some of you right so then what is the rate how they express the rate weight of catalyst is it the only way need not be you can express surface area of the catalyst but again which very difficult to find out surface area because there are two surface areas for a catalyst what are the two surface areas one is the external the other one is all together external plus internal together which surface area you are talking what you measure using BET BET method know BET method or some mercury porosimators what you measure is the entire surface area of the particle inside the pores outside surface all together you estimate estimation is again difficult that is why the simplest parameter which I can use is weight of the catalyst because I can simply put weight 10 kgs 20 kgs or 1 kg and then find out the rate based on weight and use the same rate so that you will get weight of the catalyst in the actual reactor that is why the reactor design whatever you express the rate of reaction on that means you can express as volume of reactor weight of reactor and sorry weight of catalyst or weight of surface area you will get directly in the final when you are going for this design equation this design equation you will get that particular part if I use my rate based on weight of the catalyst I will get weight of the catalyst if I use my rate based on volume of the catalyst I will get volume of the catalyst but this volume of the catalyst again there are many you know misconceptions there what volume we are talking volume of the catalyst means this volume has the catalyst has inside again wide volume are you talking about only pure solid volume or that means everything crushed together and then you will have no pores at all are you talking about that or are we talking about the volume of the particle or you are talking about volume of the reactor correct no reactor has now additional voidage packed bed you have the particles this particle is porous some void volume there so now all these particles are kept in a reactor where you have between the particles there is some more void volume which volume we are talking that is why we have to clearly say okay moles converted per volume of the reactor that means you are talking about the entire reactor or volume of solid or volume of solid solid that means without any voids okay yeah so like that only or volume of voids only volume of voids only which voids again reactor voids or particle voids you see how much confusion is there that is why we should have very very clear picture of what we are talking when you are talking about heterogeneous