what i'm going to do is talk through and introduce a distillation column and describe the how it works and why it's quite useful to us in practice and the situations in which we can use it and in this situation i've looked up the tabulated values of our reactant a and our product p and found that reactant a has a boiling point of 100 degrees c and our product pre p has a boiling point of 50 degrees c and so what this means is that product p is going to vaporize much more readily and it will be referred to as a lighter component because it is much more likely to be in the vapor phase than component a and so what will happen is we're going to take the effluent stream called f and we're going to put it into this distillation column that i've drawn to the left here and it's going to enter a feed plate and to work through what's going on at a distillation column i think it's best to start at the bottom and then examine what happens as we ascend the distillation column and so what we're going to find is that at the bottoms or at the bottom tray the heavier component is concentrated and the reason for this is because we have a liquid that is descending our column and the heavier component component a is more likely to exist in that liquid phase that is falling down our distillation column and so when we reach the bottom tray of our distillation column it's going to be very concentrated in our heavier component which is our reactant called egg and so once we enter the bottom tray what will happen is we're going to be having a heater here we're going to have some kind of reboiler and in practice this would just be a copper coil and we could be running electric current through it for instance or perhaps we're burning methane and having a fire but the point is we're supplying heat into our bottom plate and as we heat this mixture that just entered the bottom tray some of it's going to vaporize and the part of it that vaporizes is going to exist more likely it's going to have your lighter component in it but you're also going to have some of your um heavier component and so we are going to vaporize this we're going to get v prime which is a vapor phase that is going to go back up into our distillation column and v prime will be somewhat concentrated or will have a higher concentration of your lighter component or product p than did the liquid phase that entered and it is very important to note that at each tray we reach a vapor liquid equilibrium and what this means is we've given sufficient residence time for the two phases to reach equivalent chemical potentials and this is uh thermodynamic lingo but what it really means is that we have allowed whatever wants to exist in the vapor phase to vaporize and enter the liquid the vapor phase and whatever wants to stay in the liquid phase can stay in the liquid phase and we've given them enough time to they can do that and that is how we are separating our mixture is by relying on this temperature difference between our two components and so this is a take home message for distillation columns it revolves almost entirely around this difference in boiling points between species that we have in an effluent stream that is entering our distillation column from a reactor and so with this information this vapor phase v prime that we've just sent back up into our column is now going to enter into another plate another area in which we're going to reach vapor liquid equilibrium again and as we ascend the column each tray is progressively cooler and so as we move to the next tray above the bottoms tray the temperature is going to be slightly less and because of that whatever heavy component that existed in the vapor phase that entered that tray is going to want to now exist in the liquid phase to go back down the column and whatever lighter component that existed in the vapor phase is going to want to stay in the vapor phase and so we reach this vapor liquid equilibrium at each tray as we move up our column and depending on the number of trays that we have we can get better and better extraction but we do get diminishing returns and this is modeled in some very important equations and we can see this when we are working with solutions or mixtures that have azeotrope compositions and so to continue with this discussion what we'll find is that we're going to have a temperature gradient a temperature gradient must exist to have mass transfer and what that means is that if you're operating a distillation column in practice and you are seeing that each tray is the same temperature that's a bad sign because that tells you that you're not having any you're not getting any better extraction by having more trays and more trays means you're paying more money and you're having to supply more energy in your reboiler to operate your distillation column and so you have an inefficient process and so you need to go back to the drawing board change the number of trays you have lower your reboiler duty before you realize a nice temperature gradient and so to kind of get a back of the envelope calculation as to what kind of temperature gradient you would expect uh in this binary system that we're working with with our unreacted reactant and our product p we would expect to find a temperature gradient like this so it would take the boiling point of a so we'd have 100 degrees c minus 50 degrees c divided by and if we had 10 trays in our distillation column we would expect to see that our trays would have a 5 degrees c temperature gradient per tray and in other words what this means is that it's 5 degrees c colder for each tray working with this binary system and so working with binary systems is nice because i can do this kind of calculation you can get an idea of what kind of temperature gradient you want to see but it gets quite a bit more complicated when you're working with multi-component systems more than two components but for the sake of an introduction this binary system gives us an idea that we're going to want to see if we had a distillation column with 10 trays and we had two components and the boiling points of each component was independent of pressure and quite often the pressure difference is negligible or has a negligible effect on the boiling point then we can say that we want to see a 5 degree c temperature drop as you ascend the distillation column at each tray so this is a very important note just so you have an idea and this what will happen is at each tray the colder temperature will get us a new vapor liquid equilibrium and by the time we reach our condenser at the top of our distillation column the vapor phase is going to be concentrated in your lighter component and this is v so the vapor phase is concentrated in the wider component which was our product p and when we enter the condenser as the name suggests we're going to be extracting or removing heat from this stream which is going to cause the incoming vapor to condense and we're going to get a liquid that will be rich in our product p that we can sell to market because it's now purified and we can divert some of the liquid back down the tray or back down the column if we wanted to get better performance and there are mathematical models that tell us what kind of ratio we want to have of sending that liquid back down the tray and that is referred to as a reflux ratio and so reflux ratio is one of the key parameters that chemical engineers adjust in practice and i have other videos that discuss that but this is an introduction and so what we'll find is that the distillate has a mole ratio of the lighter component that is greater than the feed mole ratio of the light component and so this is the take home message of a distillation column and we ended up with a distillate stream that has a product we can sell to market and we also have this bottom stream here that we can send right back into our reactor to get better conversion of our reactants an important note to make here is what's powering distillation in other words if i wanted to double the amount of feed that i could send to my distillation column if i wanted to make more product or purify more product what parameter do i have to change and the answer to that is q reboiler and so if we are doubling the mole flow rate that we're processing we're going to need to double the amount of heat that we're supplying in our reboiler to vaporize double the number of moles at our bottom plate and this is how distillation works and then a final note is in terms of sizing what should your distillate flow rate be in other words the letter d which denotes the mole flow of your distillate well we want to make sure that d is at least as big as the flow rate of our more volatile component in our feed stream in other words if we had 50 moles of our product which is a light component entering our distillation column we want to make sure that our distillate is at least as big or has at least as big of a molar flow rate as our light component p and the reasoning behind this should be intuitive because if our distillate is less than the mole flow rate of your lighter component so f is the total mole flow and z is the composition we get accumulation of your product which is the light component and lose the light component unnecessarily in the bottom stream so you're sending your product back into your reactor and you're going to be getting a worse conversion because you're ruining the equilibrium conditions inside of your reactor by le chatelier's principles and so this is another take-home message with a distillation column you need to have a distillate flow rate that is at least this big and when we start discussing reflux ratios this will hint at what kind of distillate flow rate we're going to want to go with and reflux ratio i call rr and that is equivalent to l over d which is the amount of liquid that we send back down our tray divided by the distillate flow rate and this is the conclusion of my introduction to distillation columns and i hope this helps people who are interested in distillation uh as well as the chemical engineers out there who are having to learn this for the first time it did take me quite a while to figure this stuff out and i hope you guys find it useful let me know if you have any questions and thanks for watching