 Okay, so in the last class we have discussed about these chemical kinetics reactions, heterogeneous and homogeneous, we also had a list of the combination of heterogeneity like gas liquid that is one heterogeneity or liquid solid, liquid liquid all that and we also listed out taking only one reactor called packet bed reactor, it is catalytic packet bed reactor then we have written some 3, 4 different rates for the same reactor, rate of reactions but is there a connection between these 4 or 5 rates, one can be converted into other rate, they can be easily converted if you look at those equations but let me write because it is not obvious to everyone, I cannot only say see one or two people who say yes, yes, yes we know but all there are other many people who may not know that at all, so that is why let me write that and without writing the complete definition we have written yesterday yes this is equal to 1 minus V I have written you can correct that one V epsilon wide H correct wide we have told you, the first definition R1 dash was based on wide, wide of the bed okay packet bed yes, so this is dNi by dt then we also have, this is volume of particles other one right, so this is again dNi by dt then we also have weight and the other one is for surface area dNi by dt right, so the common thing here is dNi right, so using this particular one I can now convert one rate into the other that means if I know rate based on W weight of the catalyst then I can also convert that into per unit surface area if you know the surface area of course you should know that, so that is why general expressions can be written here, so now taking this dNi as I mean equal because dNi dNi dNi we can write these equations very easily this is V epsilon V P W s, so for example if I want to take these two that means I know based on, rate based on W I would like to find out yeah this R i for strikes okay, so then this will be simply this R i 3 strikes W divided by yes okay, so that is why one I know it is a big problem you may not know now okay in fact yesterday Ranganathan also was telling me about the same thing because he was trying to have some gasification I mean papers he was reading, every paper gives a different one some people give surface area some people give weight of the coal and some people give on the surface I mean voidage of the bed so there are so many things are I think it is very difficult to convert so that is why unless you have the clarity here then you know you will make lot of mistakes in defining rate itself which is the basic parameter for our design expression okay that which gives me kinetics and indirectly that gives me rate of reaction okay, so finally our conclusion is that rate of reaction we know okay I do not want to write an equation is a function of concentration and temperature that is all these are the two terms you will have if it is heterogeneous reactions this concentration term this term will be a function of both physical kinetics and chemical kinetics I will again discuss with you once we understand the basic reactors then we will go to again kinetics and I will also derive one or two equations for heterogeneous reactions homogenous reactions can be simplest one zero order reaction okay what is zero order reaction equation yeah zero order means it is not a function of concentration actually it is C concentration raised to the power of zero so fortunately that has become one so that is there only k right so that is the easiest one simplest one then first order but you should know onwards never should think about this order of reaction at all it can be any reaction rate like if I am talking about one particular instant it may be k into C A divided by some constant again capital K 1 C A plus C A square it can be it can be anything right because that is why you cannot have a single idea in your mind that you should have some order of reaction what is the order of reaction for this please remember order of reaction you can tell only for elementary reactions order of reaction we have one okay but you know even empirical order of reactions where you find out sometimes C A to the power of 1.2 1.5 that is only possible if you have concentrations only raised to the power of something okay so it should be only in this form that is the yeah okay sorry rate as some k C A to the power of A C B to the power of B etc right so that is name I have forgotten how that name so this is C A to the power of A only here you will have rate but the moment you have something else there like this you cannot tell about rate that is why even Michael S. Manton equation which is some format like this correct know some format like this again you have to specifically say that at very high concentrations okay so this k may be neglected if the concentration C A is 1000 this k is 1 then it becomes 0 order so on the other hand when this constant is very large and this constant is very small that becomes 1st order so that means unnecessarily you are trying to simplify things and then say that this is the order of reaction it is not required right so that is why whatever reaction you have here reaction rate you have here somehow you have to introduce this in the reactor design expression that is what what we are going to derive and then integrate that and sometimes it is possible to integrate analytically sometime it is not possible so you have to go for graphical integration or numerical integration and then finally calculate either volume if conversion is known or either conversion if volume is known that is all the entire you know reactor design good so that is the one so now I think this is here till now I just wanted to give an idea of what is rate of reaction and then how do you define it the definition of rate is actually moles per time but we would like to have that one as an intensive property so that means whoever talks we will talk you know all of us together we would like to talk the same language so that is why take some parameter volume of the reaction mixture or weight of the catalyst or volume of slurry or this volume of slurry means volume of solids plus liquid together but some base which is uniform for all of us so then automatically all rates whoever do it I mean you may do in IIT Madras or Anna University or somewhere Coimbatore or somewhere you know Nagapur University you do same experiment based on the same volume you should get exactly the same rate so that is the reason why that intensive property is very very important for us because all of us talk about the same language otherwise I take in IIT Madras 2 liters Abdul takes I think he is there now currently he is in IIT Madras only but I think he would have taken in his AC take AC take only you okay so 5 liters 5 liters if you are talking about only moles per moles per time moles per second he will have a different value I will have a different value but if you normalize that with the volumes which you have taken I will divide by 1 liter or 2 liters here and he will divide by 10 liters A A A A and B then automatically our answer should be same if it is same reaction conducted at the same conditions so that is the reason please remember all these are simple things many people may not teach you or they would have taught you but you would have never cared but you know these simple things also are very important and do not try to forget the moment you leave this lecture okay that is happy because why unnecessarily taxing our brain so I think only at the time of examination we will see all that but this is not burden at all because this is very very simple language I am talking you do not have to really feel or struggle to record the data in your mind what is that to struggle I think very simply I am trying to explain okay good very nice so now we will go to contacting this contacting will give us what kind of reactor we are talking about and we are talking about ideal reactors non-idealities you know you see ideal behavior of human being anywhere in the world on this planet we do not know this ideal behavior in some other planet may be different okay we should not harm people there may be the raw I mean the other planet it may be only the law that okay you should harm everyone we do not know I mean what kind of life they have and we only have what is called you know life based on carbon if you live based on silica or some other element then the behavior may be totally different I do not know whether you have seen some old movies that is why you have to see movies I say and also observe the science in that there were movies called aliens I do not know whether you have seen a site yeah I think he is my friend I think most of the movies which I see he is telling yes yeah see this alien does not have blood you know it has only highly concentrated acid okay if some small you know the spaceship goes there and then you know the alien enters and they they try to kill the moment they started you know one limb will cut and afterwards highly concentrated acid will ooze out from the body then that concentration that is so concentrated the flight you know yeah the spacecraft has some three four stairs okay and then through all that it puts the whole something goes down that much concentrated and it falls also on one person I think immediately his face will become almost dissolved and then he dies so that is the kind of thing that means the planet you know on that planet life is developed in that way so that is why at least on our planet when you talk about good people we should not harm others we should try to help you take any religion these are the only things every religion tells that is why I do not have a religion because all religions are same everyone tells the same thing do not harm anyone do not I mean try to help what else you have be good to others that means all same thing only I am telling many times okay so like that that means is very simple to identify good people with these three rules easy helping good right that is all only one rule that is exactly the same reason why also we would like to assume ideal reactor because if I have something ideal in my mind it is very easy for me to remember that is the most ideal case non-ideality means you know to deal with non-ideal people it is very difficult we do not know when he will get angry when he when he will harm you when he will come and just stab you back so all these things why so many say problem if all of us are good then absolutely there is no problem on this planet you need not fear about anyone and then things will be very very happy you do your thing and they do their thing and it is excellent so that is the reason why we first think about contacting ideal contacting where we have some definitions for ideal reactors so these ideal reactors happen to be in chemical engineering only two and that is batch and continuous reactors and in continuous reactors again we have plug flow and mixed flow okay these are again two extremes and this is one where there is no continuous feeding and continuous removal of the product you just put all the things in the reactor and wait for some time that time is now you have to calculate from the design expression and then after that time discharge and then pack it and then send it if it is pure product otherwise you have to send it to discharge and column absorption column or whatever okay so that is what is the one for ideal batch reactor and now the first question I would like to ask you is when do you choose a batch system and when do you choose a continuous system do you have any rules for choosing continuous system and you know for example after mtech or after phd you got a job in flow rates for slow reaction rates you choose what why you take continuous reactor and it will take more volume in terms of batch react why you should take more who will discuss your thing will be clear later ... దారిలు ఎోవాలి దారారా... ర౗ి దార్లు దార్ పా పళిందదాకినిందోందా దింనిందోమి త౪ర్యర్ దానిందాయ౉దా పర్లెమీయదోతిందార్వన ఈన౻నిందారెనలోనలలోతిందానినిత్తోట if you wait sufficient time you may get 100% conversion or equilibrium conversion because 100% conversion is not possible for all the reactions if it is equilibrium reactions, divisible reactions then maybe 60% is the maximum afterwards what will happen to the rate? 0 when you are designing the reactor you need again infinity when you reach 0 because minus r a will come in the denominator that is the reason why you cannot right? okay anyway I think that is not correct this time you see here we are only discussing about we are not talking about the timings we are talking about there are three ideal reactors and I mean of course first batch and continuous two systems I am just asking you a question when do you use a batch system, when do you use a continuous system okay that means if you take batch in between you have to stop and again wait that time is lost in the production so that is why if you have the continuous system so then continuously you feed and continuously you get all large scale productions like for example ammonia, sulphuric acid, nitric acid, HCl all these heavy chemicals what we call okay so large amount you produce so for those things the best design is only continuous design okay small production rates normally pharmaceuticals I mean you won't produce millions and millions of tons of headache tablets okay there is one of the drugs so like that because it depends on because everyone is not getting headache someone may get stomach ache someone may get headache okay so if you for every disease if you have only one medicine that is excellent that is poison okay anyway so that is why I think pharmaceuticals for example paints okay but you know like that depending on the seas and depending on the demand you can have the flexibility in batch system that is the main criteria that is why pharmaceuticals, food industries, pesticides, herbicides all these industries if you go and look you know I think even the spence line and all that you know pharmaceutical industries all these industries will have only batch systems but you may see one particular factory where continuous system is used even in pharmaceuticals but there must be a specific reason because they want to produce very large quantities or they would like to have very good control advantage of using a continuous system is you will have a good control because it is flowing if the flow rates are fixed nothing is going to change but whereas in batch system every time you are feeding every time you are discharging then you are starting okay and also batch system is it unsteady state system or is it steady state system batch system intrinsically all batch systems are unsteady unsteady system unsteady state so during unsteady state you will not get uniform product because in batch reactor if you use good mixing and all that you will get it but anyway there is some uncertainty whereas in continuous systems once you design you will get beautifully with the steady state and also the steady product within plus or minus some small range which industry you work which industry you work what are they producing they are producing chemical reactions are there chemical reactions or chemical reactors so I was in contact with the previous company before that I was working in chemical company that company what were they producing potassium metal so what are the production rate production rate is actually 900 kg that is nothing for a chemical company that is nothing so that is why probably they are going for what is the reaction reaction is sodium metal that is KCL that is in solid and this is melted that is gas liquid reaction which is normally very soft okay and I think one answer was there what he told what is your name probably okay what you said that one point is right if the reactions also rates are very very slow then we will go to batch systems the reason is it is not the volume and all that you can give whatever amount of time that is required in a batch system simply if I do not allow you to go out of this room whole day I can arrest you here and then talk theory so really no that is all why you have that time to to keep the material till the reaction is over because it is slow it takes more time so in batch reactor you can provide that much time similarly of course in continuous reactors also you can provide the length will be very large okay and you can also provide the same thing in a mixed flow reactor correct no mixed flow reactor if you cut off both limbs it is only batch both limbs means entering leaving cut both then it becomes because there is also stirring right then automatically that becomes so we will see which is efficient which is not efficient why and all that okay efficient in the sense that for a given volume which will give you more conversion or for a given conversion which will give you more less volume here less volume less volume is our criteria for a given conversion and more conversion for a given volume those are the main objectives for the reactor design okay good so that is why batch system and continuous system you know now how to choose okay but normally if I want to give the names I mean not names the numerical values for how large I can tell you definitely 1000 tons per day you will go for only continuous okay but I have a problem to tell that you know 10 tons 50 tons model L this is that based on obeisance I can obviously comparing how many times you will get smaller for testing you are only 4 days if you are in the where we cannot tell. But definitely as he said, you know 900 kg, 99 percent of the time we go for only batch system. So that is how you can say that, okay, around one ton or less, definitely we can go for easily batch systems. Or 100 and above per day, again you can go for continuous. But in between somewhere you have to check the economics and decide which react is the best. That is why we have the computers now. Calculating power is very high. So just, you know, you know, XL you can use for finding out XL sheet, there is a program where you can use. Automatically if you change one, everything will change. One parameter you change, all the other things will change and finally your parameter in the last column is either react, either conversion if volume is known or volume if conversion is known. See how the volumes are changing or conversions are changing given the other one, okay. So that why you can try to simulate and then try to find out which one is the best system, right. But general thumb rule is for low production rates, flexibility in production rates and very slow reactions, batch reactor is used. And continuous reactor is used for very high production rates and also good quality and which one will be costly, initial cost, continuous definitely because it needs more instrumentation, all that. But still we go for that because that is why you have done your project, right, by taking depreciation cost and also break even. Yeah, break even, all that you calculate. You break even should be as quickly as possible. So that is why after calculating, after designing the reactor and also taking all other components, all other equipment, then you have to again calculate using your economics, then finally say break even point, break even point cannot take 100 years, okay. It should be as short as possible. If you are able to as short as make the system as short as possible, then your profits start coming for you, right. So that is the overall picture of this batch and continuous systems, okay, good. So now in continuous now we have two again, plug flow and mixed flow. Let me write that I think for the sake of others who do not know about this, plug flow and mixed flow, okay. So this plug flow is normally tubular reactors, okay. So it can have of course any length depending on the system, it comes out, okay. That is plug flow. And mixed flow generally you will have tank reactors where it is continuously entering and continuously leaving. As I told you, if I cut off these two, then that will be nothing but batch reactor, okay. So we have to, we are supposed to use plug flow reactor and mixed flow reactor. I am hesitating to ask the question because I have not explained what is plug flow, what is mixed flow, I do not know how do you give the answer because you have exposed it to two courses earlier in your B Tech and M Tech. So that is why I am now asking this question, okay. So given a chance to you in the continuous system, continuous system you have chosen, now there are two alternatives. So you got job in some industry and then your boss ask, okay, now choose the reactor, okay, good. Yeah, so I think of course I have not told many complicated things yet but I think only simple things only we are discussing, right? Yeah, anyone? I think when can you choose plug flow reactor, when can you choose mixed flow reactor? Why? Pressure? Why high pressure? There are many reactions without much pressure also. There are many gas phase reactions occurring yet only one atmospheric pressure, normal pressure. High pressure is not the criteria. Why you want, why you do not want concentration to fall rapidly? We want because when concentration is falling rapidly, rate of reaction is high, when rate of reaction is high, time is very small and then I will get very small volume. No, please tell, I mean please tell your mind because these are the misconceptions in the mind that you should not keep them. That is why I told you know copy-ups, copy-ups there, you have to remove them. So that is why please without any fear please talk, if you have anything. And also it is worse to have a wrong answer than not having any answer. If you do not have any answer in your mind that is very bad after two courses of CRE. But even if you have wrong answer, no problem. So that is why you have to tell something. Very jai hana vi. Again repeat. For all reactions, as you said always mixed flow will give you more value. So that means always we should take only plug flow. But we do not do that. We also use mixed flow, we also use plug flow. That means there is a chance left. No, this is the problem. Do not worry, you are in the good company of our brothers and sisters. Ok, in this course reactor theory I am again starting all the basics. The reason is for this also no one gave the answer. And even for batch and continuous also many people do not know the answer. When do you choose batch system, when do you choose continuous system? Now I think at least you know. And please remember only this, this kind of questions only are asked in the interviews. No one will ask you very, very complicated mathematical equation in the interview. They start only with this basic questions. Ok, what is the reactor? That is why the questions which I gave, they are very simple and very nice questions for in terms of Gopis language, general knowledge. General knowledge in CRE. Ok, that is what is the first you know, I think you would have, you would have definitely attended some interviews before coming here. Ok, anyone asked you what is the dispersion equation? To write? If he has asked you really, he does not know how to interview. Really, no one will suddenly start and then ask dispersion equation. They will start slowly, Ok, what is the reactor? And then what is plug flow reactor? In plug flow reactor is the dispersion. Now you tell me what is dispersion equation? So many steps are required. Ok, so that is why even this question also, when do you choose plug flow, when do you choose mixed flow is many people cannot, may not able to answer. One answer was there, who said gas phase reactions? Oh, you said gas phase reactions, that is right. Not easy pumping. The gas phase reactions, the reaction time is very, very small. Most of the time it happens within minutes. If you calculate ammonia residence time in that reactor or SO2 to SO3 residence time in that reactor or any gas phase reaction, there are many, many gas phase reactions. The rate of reaction is so fast for gas phase reactions because molecules are easily moving, they are colliding and so easily rate of reaction can occur. So when the reaction time is very less, we choose plug flow reactor because it is very easy to provide short residence times in a plug flow. Ok, why? It is a pipe, right? And you know, of course, because already you know what is plug flow, right? Yeah, in terms of Reynolds number, can you tell me under what Reynolds numbers you can say that you have plug flow, the value of the Reynolds numbers? Good. Ok, now I will try to give the answer in indirect way. You know fluid mechanics, right? Ok. Now I will draw the laminar flow velocity profile. Ok, so this is the pipe. How I have to draw the laminar flow velocity profile? Parabolic. Ok, that is the one. And turbulent. This is only fluid mechanics knowledge, what we are using. Now turbulent. Ok, I have Reynolds number may be 40,000. What kind of profile I get? Yeah, so it is not still flag. Initially boundary, yeah. Like this. Ok. Good. So what is that you have written the answer in your first examination that you know, the definition of plug flow? Those you have written. Those you have not written, I think it is okay. It is, theoretically speaking, the Reynolds number should be equal to infinity. Very good, very good. Infinity. Then only you will get ideal plug flow, what you call, you know, with flat velocity profile. Otherwise whatever Reynolds numbers you use may be 70,000 if I go, what will happen? It may be like this. As long as you have the walls, still you will have some molecules clinging to the wall and then you will have zero velocity profile there. From zero to flat velocity there must be some continuity, right? It suddenly cannot jump. So that is the reason why, theoretically speaking, R equal to infinity for this profile. Ok. Yeah. What is that? Yeah, by definition of mixing I think you know you should have infinite mixing there. Infinite, infinite turbulence. Where is mixing? No, I think that I will explain when you come to definition of plug flow. Ok, yeah. I think all these problems are there, I know. That is why I think I have again teach all these things. Ok, now we are only talking about plug flow and mixing flow. I am now, you know, how do you choose? So that is the reason why I have given this example, Reynolds number. Reynolds number is a form of velocity, right? You should have fixed, for fixed liquid, that means properties, rho and diameter, then only as you, as you are increasing velocity, you are increasing the Reynolds number, Ok. As velocity is increasing, what will happen to the residence time? Residence time is defined as this length divided by velocity, right? So I told you know, I think as when you said gas phase reactions are because of the low residence time required for the gas phase reactions, we go for plug flow. The reason is for and when you want to maintain plug flow, you need very, very high Reynolds numbers, right? Ok, not theoretically, theoretically you cannot go to infinity, but you need very high to maintain that plug flow, ideal plug flow. So that means you should use very, very high velocities and if the reaction time is 10 hours, then what is the length of the pipe you require? 10 hours. There are many reactions with 10 hours and 10 hours no gas phase reaction is there, it is only 10 seconds, 10 hours liquid phase, Ok. So that means if I am taking a liquid phase reaction where the time required is 10 hours for the reaction, if I want to provide 10 hours in a pipe with velocities almost, Ok, I mean reaching infinity velocities because plug flow, you have to maintain plug flow, otherwise you cannot call it as a plug flow reactor, if you do not maintain. So you have to go to very high velocities, then you need a pipe which will go around the world once, Ok. Or if it is not enough, it has to go to moon, from there Mars, Ok, starting from earth. So that length of reactors you have, but that is why I do not know whether you have seen any plug flow reaction in industry, most of the industrial plug flow reactors are designed in this fashion. And you know sometimes in industry you will have 2 kilometres, 3 kilometres length of plug flow. That means like this many, many, many, many, each one will have 30 feet or 40 feet length, Ok. Like that you can now count how many numbers you have to put and then join at these ends and you can guarantee that there will not be plug flow. Even though we say that it is plug flow reactor. That is why you have non-ideality coming there, Ok. So the ideal plug flow is only existing when you have velocity equal to infinity. That means you cannot provide even zero second, correct, no? Infinity, L by U, this U equal to U equal to infinity. Then it becomes zero. And with zero reactions I think you know zero time, if some reaction is happening you do not need any reactor at all. You simply do not need any reactor. Because even before you mix them if they are reacting, why do you need a reactor? You do not need anything there. So that is why you have to choose the plug flow reactor for residence times which are very, very small and most of the time that happens only for gas phase reactions. And if there are some liquid phase reactions whose reaction times are very, very small, there also you can go. But criteria is time. That is one thing. Ok. Next one is when do you choose mixed flow reactor? Opposite of that. That means whenever you want large amounts of residence times, large amount of residence time, ten hours, twenty hours because it is a tank, it is a tank reactor, so then you can definitely provide that much time, right? Again we have an equation, we do not know who told you that, a term equal to volume by volumetric flow rate, this is volumetric flow rate, this is volumetric flow rate, Ok? This is volumetric flow rate. I want to provide two years residence time, two years, Ok? Exaggerating to just make a point. That is all. So I can still provide that in that reactor. I may take, let us say one meter cube and I want two years residence time. Can I provide? Is it in my hands? How? Cut off the outlet becomes batch. That is all. Drop by drop, drop by drop I will add. Then I can provide not only two years, ten years residence time also. Two years I told that because you are empty tank, Ok? I know, you would have seen that big wastewater treatment plants. Why do they need that big size? Have you over thought? Because of exactly the same reason. The residence time required for wastewater treatment is three weeks, four weeks, five weeks, Ok? That is why and also depending on the effluent sometimes they will have my god big swimming pools five or six together. This goes from there, this goes from there, this goes, that is tanks in series, Ok? Because the liquid phase reaction there, it is very slow and the top of it they are all microorganisms. They are also as lazy as us, Ok? If the temperature is very hot, they will sleep. Like, you know, but we will put AC and then we will sleep, Ok? But microorganisms. If the food is not good, what do you do? If two days I think if the mess is closed you never come to the class. You take rest. Same thing, microorganisms also like us because we are also part of this planet and microorganisms also part of this planet, life wise. So our behaviour is same. So give them, that is why you put biochemical reactions, right? And also you give correct condition, Ok? 30 degrees exactly maintain 30 degrees Like we are maintaining in our rooms for our body comfortability is 25 degrees, Ok? So that is why AC and all that we use. So exactly this microorganism is also like us. And if you make them anger by not giving food or by heating it because if the sun is very hot they will say that, Ok, just sleep for some time in the afternoon. So all this we have to take into account that is why biochemical reactions are very very slow because exactly like us but the molecules are different chemical molecules. They do not behave like microorganisms. Fortunately this molecules they do not have life. They cannot think. Given a chance they will go and react. That is all, what they know. So increase the temperature, more vigorously they will not go for AC. They will go for more and more collisions and more and more reactions because you are giving them more energy. That means more food by giving the temperature. So that is why as you increase the temperature the rate of reaction is very very fast in the actual chemical reactions whereas up till some point may be 25 to 30 in many biochemical reactions it increases afterwards, Savita if you go to 40 it starts decreasing. Not for many but there are specially designed by God, specially designed microorganisms where even at 60, 70, 80 degrees also they are surviving. You know hot springs, you know sulfur heating and all that, you know, those things that is fine. That is only few but those are designed only for some specific purposes. The other organisms have been designed for some other purposes. Ok. So that is why biochemical reactions is one of the examples where always we go for tank, either batch or mixture plug flow reactors are very rarely used Yeah, you can use provided that No, you can use provided that tube length is one diameter of the earth earth diameter earth diameter, yeah the length of the, not diameter, sorry, yeah the length, the length of the reactor will be one around, you know, one circumference of earth What is the earth diameter? We have a encyclopedia Britannica Excellent, I think we have a gram for you Very good So that is multiplied by pi with the diameter of radius radius, you see that 2 pi d 2 pi r See, yesterday tank columns is also very big in comparison You can use multiple Yeah, but you know the length, there the diameter is fixed most of the time Ok, you cannot go to so big diameter in plug flow Ok, what is the diameter you can guess in any plug flow reactors? Diameter Huh? In terms of centimeters Or inches 5 centimetres 15 centimetres, yeah Any other value? 1 inches 15 centimetres 5 inches 5 inches 1 centimetre 1 centimetre 50 centimetres 5 inches 5 inches 2 inches 5 centimetres Package diameter also What you are saying is right You know, but I think why we are calling that also as a plug flow, I will explain to you later that diameter, big diameter Yeah, so that is the reason why most of the time we won't go beyond 6 inches or 8 inches maximum 8 inches means 200 mm 20 centimetres or 15 centimetres most of you told 15 centimetres So that is fixed plug flow the velocity required is almost very very high so the residence time is to provide that much residence time then you have to the length will be very very long so that is the reason we do not go okay your doubt is then why the same thing is happening in the mixed flow also right that is correct what she asked me is right but here I have a flexibility in increasing the diameter and also length you can do okay why the reason is that the concentration of gas inside that will be very very less diluted unless otherwise you go for very high pressures in the CSTR high pressures you can go to very high 100 atmospheres 200 atmospheres then concentration is P A by R T as partial pressure if it is a pure reactant then you will have total pressure itself is that one so that pressure by R T will give you the concentration but at room temperature if you calculate that concentration that will be very small as concentration is very small you have rate of reaction very yeah very small and then you will again take more time so that is the reason why you do not go for most of the time but we use mixed flow CSTRs in the laboratory to find out kinetics for gas phase also for kinetics because you are using a small reactor and then small amount of gas at different pressures or at atmospheric pressure and you just find out what is entering what is leaving perfect mixing is there then you can calculate what is the rate of reaction for gas phase reactions okay homogeneous only we are talking here when I am talking about these two reactions I mean these two reactors so now at least you have an idea when do you choose plug flow plug flow is chosen when you have the reaction times are very small so most of the time gas phase reactions will have small residence times required so gas phase reactions is one answer and liquid phase reactions most of the time we conduct in yeah then I will ask another question what will happen which reactor is the best if I have highly exothermic reactions is it mixed flow reactor or is it plug flow reactor your question also there is valid you know I am not saying that whether you have gas phase or liquid phase gas phase also may be very high exothermic highly exothermic okay either exothermic either liquid or gas phase which reactor is preferred and why plug flow reactor is preferred for highly exothermic reactions okay mixed flow why mixed embryo yeah you have to put only jack and the around is okay which is sorry this one is going inside oh here yeah so this enters cooling coolant enters and coolant comes out there yeah Savitha you are telling something yeah this exchange is never perfect you can never maintain isothermal condition in a plug flow reactor and you will have what are called hot spots okay that is why plug flow reactor is a lozy reactor for heat exchange remember that really lozy you can never have uniform temperature throughout you can have theoretically in your mind okay if you have infinite number of heat exchangers so that means I will put here one heat exchanger here one heat exchanger here one heat exchanger so almost every I do not know 1 mm 2 mm 1 heat exchanger if you put probably you may get almost isothermal and will you design so many heat exchangers so many feeds so many outlets so many pumps no one will do okay so that is the reason why whenever you have exothermic reactions we go for only even for very high exothermic reactions gas phase reactions because there I do not worry about the timing I do not worry about conversion I worry about the heat where heat may it may explode after sometime if the heat is building up or when the hot spot is reaching it may reach the flash point and then everything may burn so safety is very important that is correct no definitely safety is very very important so that is the reason why even gas phase I have to go sometimes for mixed flow reactors if it is continuous if it is highly exothermic so these are the main two criteria for choosing between plug flow reactor and mixed flow reactor now can you repeat when do you choose mixed flow reactor that is all okay any other criteria except this so that is why when you go for clogging problem I have the gas phase reaction because most of the time gas phase reactions we are using okay plug flow when do you get clogging catalyst particles when you have that that we will discuss later okay when you come to heterogeneous reactions heterogeneous reactors that we will discuss yeah so it can happen it can happen but whenever you are using even packet bed reactors a gas phase clogging is very rare unless you design your catalyst loosely I told you know you may go for large surface area but what you do you will only finally get only powder if you want to use very very large surface area so there powder may form that powder may be transported from one place to the other place and then may may block okay so those are the problems which you have to separately deal with when you are talking about catalyst design right so that is why these two are the criteria which normally we take for choosing between continuous I mean continuous and batch you know how to choose and also in continuous between plug flow and mixed flow when do you choose okay how do you choose so I think this is very good information you should remember and again you know you you discuss among yourself otherwise you will forget very easily