 Usually it is always desirable to subject your VLE data to a thermodynamic consistency test and therefore this additional data can come in handy. So this is all briefly about in general what are the different methods some pros and cons of it, advantages disadvantages, likely fields of application for which they have been used so far. Let me illustrate all this by taking actual referring to three different studies, one of them is my own and the other two are by other different people. So these are the three cases that I will briefly discuss. The first one is an example in which dynamic recirculation steel is there, we will discuss it in three parts, experimental setups, steel design and details, operation and procedure. The first one is an experimental, first let us see the experimental setup, this is the experimental setup that a sketch of the experimental setup that one can have for a VLE determination using a dynamic steel. So this, if you have the nodes then when I discuss it you can probably keep that sketch open. There are three major things that we can see here, one thing is, one this is the cell, the heart of the equipment setup as I said along with this which is the ebulliameter. So ebulliameter and the dynamic VLE steel, this is the main part of your experimental setup. To make it work you must have heating arrangements, so a heater here as well as heater here. To measure temperature these are the two locations for measuring temperature rather than measuring and sensing equipments. Along with that we can consider this condenser as part of the VLE cell. In addition to that you have an important thing which I have so far not referred to that is given here. An empty glass bottle of a very large volume relative to the volume that you can have here as well as here that is the total volume that your equilibrium cell or an ebulliameter or a few of them connected in the same can have. Another reason for this is this being a dynamic steel actual boiling is taking place and we know that whenever boiling takes place there are sudden changes of so many things, right. Vapor is suddenly generated from liquid and therefore there are going to be disturbances. The disturbances are likely to be transported to the other parts and therefore they have to be nullified. You have to ensure that none of the things which we measure or are trying to maintain are disturbed by the dynamic things which are taking place inside that is the boiling which is taking place inside. And therefore that is ensured by keeping a very large volume connected to the whole system such that any flashing or boiling or vaporization here or here is dampened by a large volume present here. So all the pressure fluctuations which are likely to take place because of boiling and spurting and etc. etc. are controlled or totally dampened by a large volume available here. So this is referred to as ballast. So a ballast volume has to be present which is a primary requirement of all the dynamic steel experimental setups. That is other part. These two are of course for so many other things vacuum pump or nitrogen cylinder to ensure that you are able to maintain the pressure that you want either below atmospheric or slightly above atmospheric or near atmospheric whatever you want plus these are also required for evacuation, cleaning, flushing etc. of the equipment setup. So if we look at it from this point of view therefore these are the important things which you will find in the experimental setup. Steel and the bilimetric as I have already indicated pressure and temperature sensors. Thermocouples for temperature sensor that I have showed and manometer for the pressure. Hitters and condensers. Hitters for boiling, condensers for generating distillate from the vapor. Pressure monitoring as well as control. In fact so whereas monitoring is concerned for example I have shown here an Ibulometer. I am not using the Ibulometer for generating VLE data. I am using the steel for generating the VLE data but this Ibulometer by using a pure component in the Ibulometer I am able to monitor whether the pressure in the system is what I want it to be because I know the vapor pressure data for this particular liquid which is getting circulated in the Ibulometer. For example in my case it was acetone. So I know at a particular pressure what is the boiling point of acetone. So long as the thermocouple here is indicating the same temperature I know that the pressure is what I desire in the system. So therefore that in that case Ibulometer was working as a monitoring equipment rather than a measuring equipment of the setup. Then you have as I said evacuation, flushing and inert systems. Your vacuum pump, nitrogen cylinder and related equipment is what ancillary equipment as I have referred to and then of course the important connection between all of these in the form of manifold. So therefore the whole setup will consist of these. The dynamic steel of this particular equipment setup or experimental setup. One particular design which is a modification of Erasunis design as I said has some of these features but before I discuss these features let us have a look at the actual steel. This is as I said the heart of the steel but the actual steel is completely including the condenser as I have already earlier mentioned. So this is the condenser, this is where boiling, contacting, etc., etc. occurs and this is the link between the condenser and the location where heat input is going to be. The most important component of this is the contactor that is present here which effectively functions as what we call as a bubble cap. The function of a bubble cap in distillation tray is nothing but to maintain a liquid and allow the vapor to flow through or bubble through to ensure intimate contact and long enough contact for mass transfer to take place and therefore here this part is your contactor whereas this part as you see here some of the liquid levels shown and the arrows drawn this is the boiler. So if I refer to two components boiler and contactor in this case they are these. This is the boiler part of the equipment and this is the contactor part of the equipment. The transfer of material from boiler to the contactor takes place through many concentric cylinders which automatically provide some kind of buffers outside the main contactor so that the system inside the contactor, temperature, etc., is maintained there are no heat losses from that because heat loss can change both the temperature or affect both the temperature as well as compositions. As I have written here there are 8 triangular slots of 2 mm depth and located 45 degrees apart on the outer shell that is here and the inner shell there are 4 such slots present. So they are what I called as the slots in the bubble cap design that we have. Therefore there will be a liquid level present here. Vapor will be passing through it will disengage and then leave go from here to the condenser get condensed condensate will come back and mix with the liquid here. I do not have to bother much about this part because I am not making any measurements here. What I have to only ensure is that boiling occurs here because in short vapor is generated here. This is only vapor generated from my point of view. Vapor and liquid come in contact with each other and establish equilibrium here they are separated here vapor is taken here liquid is left here and therefore I can sample the liquid and vapor from here the liquid sample and from here the vapor sample. When I take the vapor sample I can use that 3-way caulk valve to ensure that no liquid from this limb comes in only the distillate comes into this. So this is in brief what it is these are some of the additional features or information about it. There are 4 concentric cylinders made of Pyrex glass and therefore it can usually withstand that cell which I used can withstand 3-4 atmospheres although preferably it is used for atmospheric or sub-atmospheric data generation. It can also therefore go to as high temperature as 400 to 500 degrees centigrade theoretically. The contactor and the boiler which are referred to have these capacities. Contactor has a capacity of 30 ml and the boiler has a capacity of 50 ml. The contactor has these features part of them I have already discussed. Vapor bubble through the liquid hold up, vapor jacket makes mass transfer adiabatic and the temperature measured is at the vapor and liquid disengagement location. So that is the location where they are supposed to be in equilibrium with each other. The vapor is generated in the boiler and the vapor and liquid are sampled from the contactor as I have already indicated. This is just a sketch for the billiometer which I said I will show. This is sketch for the differential billiometer. You can probably imagine that this is removed from here and then it will become the simple billiometer. So instead of a two stage billiometer in which case if it is two stage this will be measuring the boiling point and this will be measuring the condensation temperature. If it is a single stage billiometer then this will be absent only this will be present and the vapor disengaged from here will be directly connected to the condenser. Condense liquid comes back and mixes here. If sample is to be taken then sample is taken from here rather than from here. So there is a separate sampling port usually made available here sometimes with a rubber safety so that you can use an injection to draw a sample without disturbing the system. So that is your billiometer. For this case I have just given some idea about the procedure to indicate what is the kind of effort the time involved and what are the important things to be taken care of in this. System has to be completely evacuated using your vacuum pump. Usually you can go up to 10 raise to minus 3 or 10 raise to minus 4 mm of hg pressure which is good enough for majority of situations. It generally takes it can take anywhere between 3 and 4 hours. You have to do this repeatedly because you have to evacuate it first then flush it with the hydrogen again evacuate it flush it with the hydrogen and so on to ensure that nothing which is not required is present in the experimental setup. Then next important thing is feeding the boiler and the contactor with the mixture which you want to use for taking the vapor liquid equilibrium data. In this particular case you have to provide both the boiler as well as the contactor with the liquid. Otherwise of course one can provide everything in the boiler let the vapor get generated vapor will go to the condenser will condense can also get condensed in the contactor and slowly it will build up from there. But it is preferable that part of the liquid feed is directly given to the contactor instead of only to the boiler. It can hasten or reduce the time that you require for getting the equilibrium. So you have to feed boiler and contactor by ensuring that the pressure inside is lower than what actually you are going to maintain for the run. If it is isobaric at one atmosphere then you have to maintain somewhere around 700 mm Hg or so suck in the liquid to ensure that nothing else gets into it along with the liquid. And then boiling and condensing as I have already said you use a droplet, drop counter or droplet counter. This is the ideal range actually if you see one of the books that talks in detail about experimentation. It maintains that anywhere between 5 and 40 droplets per minute or drops per minute there is hardly any difference in the condensate rate or heat rate. This is true if you have a particular type of design. So the condensate which is coming out coming down from the condenser is not coming through a simple open tube like this. It is a constriction given on the way so that the liquid comes like this and does not fall vertically down. When it is coming out from a pipe or smaller constriction outlet given you maintain a small piece of glass hemisphere. It will come on that and then fall. This design ensures that it falls only in drops. There is no stream of liquid which is generated because the whole idea is to only ensure that the droplet the condensate comes in as drops and not as a stream. Once you have that design then you can calculate rather count the drops. This is the ideal range but as I said anywhere between 5 and 40 is good enough for ensuring that there is no overheating or condensate rate is almost the same. Then of course goes on till you reach a steady state which we call as equilibrium and it takes anywhere between 1.5 to 3 hours. So as you can see here the total time required for one sample will be this of course. The later sample if you continue working without stopping will take lower time because you can then reduce this time requirement that is evacuation and you can go on adding one of the components and change the composition and so on. The second one is from literature and is an example of non-recycle flow still. As I said this is of course of particular importance to Dr. Mahajani's expertise also that is reactive distillation. If you have systems or components which are reactive in nature then you cannot allow a long contact time between them because they will then react and the whole composition will change. True vapour liquid equilibrium will not be achieved. It will be a combined VLE plus R that is RVLE reactive vapour liquid equilibrium. You are not interested in that from simulation point of view while during simulation what you want is separate reaction equilibrium and a vapour equilibrium equilibrium model. So so that you can use it in simulation together therefore you need to generate vapour liquid equilibrium data without allowing the reaction to go in. So this is a case study I have taken from literature from that. The cell which they have is usually is generally designed for binary VLE let us have a look at that first. This is the experimental setup. These are the reservoirs. These are the pumps. These are the heaters. This is the mixer. This is this part we had already seen in the conceptual thing. After the mixer it goes into the flow through steel as it is called. Flow through steel therefore will allow vapour and liquid to be in contact with each other for mass transfer and establishing true equilibrium and then allow them to separate from each other. Therefore once they get separated they are flowing like this they get separated here they are then separately taken out as vapour to be condensed and liquid to be cooled. Since it is a flow there will be continuous flow you can of course take sample for this from this as well as from this and there will have to be usual instrumentation necessary for the measurement of temperature, maintenance of pressure etc. Some of the features of these experimental setup are these. There will be reservoir, there will be a metering pump, there will be heaters separately for each component because you do not want them to get in touch with each other before they have to be before so that they can exchange mass that is undergo mass transfer. Then static two phase mixer for pure vapour and pure liquid, pure component vapour for one pure liquid for the other component. The flow steel with separation of mixed V and L as I have just now discussed and then the condenser for the vapour and the cooler for the liquid. The actual pressure which you want to maintain suppose you want to carry this out at a lower pressure a vacuum has to be therefore supplied at locations here as well as here because these are so to say the coolest spots in the equipment setup. Before that everywhere as you see majority of locations temperatures are quite high the pressure tapings to manometer or manostat or whatever else that you have has to be taken at locations which are the lowest temperature in the setup so that is what it is done. In fact the details are given there they have it has been described what kind of actual instrumentation and controls that they have provided for this that part probably I will not discuss. Let us see some more details about the flow through steel. Let us read through this and then go to the stage, go to the sketch. The vapour liquid separator whose dimensions are these 8 cm diameter and 20 cm long has two components. One is two-phase two-component coterelle tube and second the vapour liquid disengagement on the thermo valve or thermo valve placed at the location where vapour and liquid are getting disengaged. This coterelle tube provides enough time because as I said it is slugs of liquid separated by bubble so there is good area provided between vapour and liquid for mass transfer to take place. In the static mixer you have already mixed them without any moving part that is because it is a static mixer. It is laid into the actual steel with the help of a coterelle tube so that you have enough area also available. The reason being this is a flow through steel and therefore you are not providing a long enough time for them to be in contact with each other. It will depend on what is the volume available for flow through coterelle tube and the flow rate with which the vapour and liquid are moving through. In other words the concept of residence time that we usually use in reactors is very much applicable here not from reaction point of view but from mass transfer point of view and therefore you have this coterelle tube design available there. Then there is a double mantle one in which thermostatic oil is present another which is a complete vacuum to prevent any heat losses and provide adiabatic system inside. And there is a liquid seal provided for siphoning off the condenser and cooler or counter current coil type. What we have seen just now is this from the static mixer the mixture comes here it goes through this as what I have called as coterelle tube so right up to here is the coterelle tube when it comes out as you see here there is a thermo well located right in front of the coterelle tube. The vapour and liquid separate here vapour goes up liquid comes down what I said as liquid seal is provided by this design to ensure that there is some liquid present in this only to prevent any vapour getting into this line there will always be some liquid present here to prevent the vapour vapour which is separated here will pass through to the condenser and liquid which is separated at the bottom will pass through this cooler and then they will flow out from this so this is in a sense what the design is this complete design that they have made is made in glass not in any metal or non glass. These are some of the operating features of the same steel mass flow rates in the range of 500 to 1000 grams per hour can be handled if you go if one goes beyond this for the actual design given then you will not be giving them enough time for mass transfer to occur and therefore vapour liquid that you are likely to get are not possible to be in equilibrium so to be on the safe side one should not that is that design should not exceed these flow rates. The individual mass flow rate is based on the actual concentration desired and is obtained by as I said metering pumps so exactly metering pumps accurate metering pumps are used to flow in the two components at the desired concentration that is the only way you can ensure that the concentration is what you want so what is given what is taken in is the overall concentration that you can confirm that is feed composition or feed concentration the vapour and liquid concentration has to be measured by sampling the vapour and liquid the component with higher demand for heat is the one which is vapourized by maintaining a temperature almost 10 to 15 50 degrees above the boiling point of that particular component whereas the one the liquid which is only heated is heated up to about 5 degrees below the boiling point of its own so these are the individual boiling points but the mental oil is usually thermostat for thermostating the oil is kept at temperature plus minus 5 degrees of the equilibrium temperature that is likely to be present once you get a particular measurement of the liquid temperature or liquid vapour separation temperature at the separating point then that also gives you an idea about how much temperature the oil should be maintained at these are some of the features of this. The third case this is actually a paper which describes in detail how automation has been carried out for a differential static cell I am not going to go in any details or even refer to how the automation part of it is taken care of but I will take advantage of this and refer to the static cell or I will look at this as an illustration of the static cell measurement of VLE as given here it is called a differential static cell because there are two cells here one is called the equilibrium cell that is the one which is actually going to handle your mixture for which equilibrium is to be established and the other one is a reference cell in reference cell you will have a liquid of known composition or known properties so to say this whole thing is kept in a thermostated environment in many designs it is kept in an air bath temperature is measured rather maintained with the help of air being circulated rather than what is normally used as water bath instead of water bath what is done here is air bath for the simple reason that nothing of this will then come in contact with any liquid either water or anything else air is safe enough for any of these four contact and therefore an air bath design has to be available for this there is then once you know what is this supposed to be you will know what is the pressure generated here and what is the pressure generated here will is likely to be different and therefore this provides you with the differential pressure measurement therefore what you do is since it is an air bath that I am talking about you decide upon the temperature that you are going to maintain and therefore this run is going to be an isothermal run what you will measure will be pressure at a particular fixed temperature rather than pressure rather than temperature or boiling point as is done in the previous dynamic or flow through steel so this is in that sense different another the another advantage of this is here as I said again you are not going to bother about sampling the system but still although not shown here sometimes as probably I had shown in the previous sketch schematic sketch that I had shown conceptual sketch many times to hasten the equilibration that is the time required to reduce the time required for equilibrium to be achieved vapor and liquid are circulated through each other the vapor is taken by a pump and sparsed into the liquid liquid is taken by a pump and spread into the vapor space in that sense you could call this recirculation but this is not recirculation in the form of phase change whereas other recirculations were in the form of phase change liquid was boiled vaporized and then condensed and brought back so therefore this is called or this is a true static cell for automation or for precise measurement what they had done was they had used this injection pump which can be controlled with the help of a computer or a program if I remember correctly it was around it was a file it is a stepper motor it is as you see here this is a stepping motor that can I think it had about 5000 or 50,000 step provision for changing or moving the piston and therefore this piston which is used as the pressure generating equipment can be used to get precise pressure adjustment there and then find out what is the pressure at which you get the temperature or when is the temperature maintained at a particular pressure so either way it can be done this whole thing there was minimum manual component involved in this majority of the operations after everything was flushed etc etc was controlled by an automation program of course this was quite old I think it was Rayry and Meiling group contribution method expert they had generated this and carried out this they had used IBM XT at that time now of course we have a lot of sophistication available but I am not aware and lately I have not seen the literature whether any further advance or what further advancement there must have has taken place so far as automation of the VLE generation is