[Music] [Music] [Applause] [Music] [Applause] [Music] hello everyone I welcome you all in 27th lecture of the course Process Equipment design and here we are in sixth week of this course okay in this course we are starting a new topic and that is the reboiler we will cover this topic in four next lectures okay so let's introduce the reboiler what is the reboiler and where we use this okay being a chemical engineer you must have some idea but let's recall that with the introduction of reboiler so if you see a reboiler is a heat exchanger that is used to generate Vapor supplied to bottom tray of a distillation column okay so that uh so as far as reboiler is concerned this is an important unit in distillation column okay as far as type of equipment is concerned this is basically the heat exchanger and it is specifically used to generate vapor from the liquid which is exiting the distillation column from the bottom and and uh it enters into the reboiler where it is converted into the vapor and Vapor returns back to distillation column right so boiler are used in distillation columns to vaporize a fraction of bottom product okay so as far as bottom liquid is concerned it is entering into reboiler where some section of this is converted into the vapor and that Vapor liquid mixture returns back to distillation column now if I consider the reboiler as vaporizer okay reboiler basically converts liquid into vapor okay however if I convert the whole feed which is entering to the reboiler to Vapor we call this as vaporizer otherwise as far as function is concerned design is concerned reboilers and vaporizer both are same okay but their product are slightly different both give Vapor but but reboiler converts a fraction of liquid into product and vaporizer converts all feed to the vapor okay and further the liquid from the bottom of the column is partially vaporized in an exchanger which is usually of shelling tube type so here you see reboilers are again a shelling tube type assembly as we have found the condenser okay so here we will classify the reboiler but those are especially shell and Tube type okay as far as heating media in reboiler is concerned that is usually the steam because we have to convert the liquid into vapor okay so liquid will take heat from a source and then it is converted into vapor okay so obviously we have to provide the heating media at higher temperature in comparison to the in comparison to the feet to the reboiler right so mostly we are using a steam but if in the plant any other stream is available which is having higher temperature which is having higher temperature than the liquid of the distillation column we can use that as heating media okay so when I'm speaking about reboiler it is basically involved boiling mechanism right and because feed is continuously entering into this okay some portion of it is converted into vapor so Vapor liquid mixture recycles back to the distillation column okay and again feed is entering to the reboiler so basically boiling process is going on but that is going on continuously same feed may enter twice or Thrice in reboiler till it will be converted into vapor and therefore it is called as reboiler Okay so boiling takes place either in tube side or in Shell side of this unit okay further exchanges that Supply Vapor for other unit operations are referred as vaporizer but are similar to most respects to reboiler so as far as design is concerned operation is concerned construction is concerned both units are same but purpose is slightly different reboiler only provide vapor to distillation column however vaporizer provide Vapor to any unit operation right so that is the only difference as far as thermal analysis of reboiler are concerned it is much more complicated in comparison to singlephase exchangers okay and the reason is very simple that here phase change occurs so we have multiphase system in reboiler which is not the case with usual shell and Tube heat exchanger okay and further you can see that physical properties of liquid and Vapor fractions can exhibit large variation throughout the reboiler because distillation bottom liquids are often mixtures having substantial boiling ranges okay so if you consider the distillation column we usually consider multiple component system okay so if the components vary in Vapor as well as liquid stream we should consider that while Computing the physical properties okay so that may be Mass fed right so this is about the introduction so these are some of the points about the reboiler and now we will classify the reboiler okay and as far as classification is concerned the choice of type of reboiler depends on different factors such as nature of the process fluid particularly its viscosity and propensity to fowling okay if fing tendency is more if viscosity of the liquid is more we have to choose a special kind of reboilers okay so this is one criteria to choose the reboiler second the operating pressure whether it is at vacuum or a pressure and lastly we have equipment layout particularly the head room available okay so in this way we consider the so these are some of the factors on Which choice of reboiler Depends and uh now we will see that what are the different types of reboilers available so as far as classification of reboilers are concerned these are classified according to the orientation and the type of circulation considered right so as we have discussed that condensers are usually placed horizontally or vertically right so in the same line reboilers can be placed either horizontally or vertically okay and wherever the boiling will take place accordingly we can have the reboiler and along with that the type of circulation because liquid should enter into reboiler vapor should be formed in reboiler Vapor liquid mixture should return back to the distillation column so there must be some circulation okay and B based on this circulation we have different reboilers okay so usually we have most commonly three types of reboiler the first one is the kettle reboiler second is natural circulation or thermosyphon reboiler and thirdly we have four circulation reboiler okay so these are main type of reboiler and we are going to discuss each type of in subsequent slide in detail so let's start with the kettle reboiler okay now as far as this Kettle reboiler is concerned what you can understand from this reboiler you can understand it is used to generate the vapor but what is the meaning of kettle okay Kettle means we have some heating media and over that we keep the fluid which will be heated up right so you all know how the cattle works okay so in the same line we consider Kettle reboiler because in this whatever heating media is available and whatever liquid is Avail and whatever liquid is available which is to be converted into vapor this is entirely covered the heating media okay and as you see in Kettle the heating media is merged in fluid okay so therefore it is named as Kettle reboiler so Kettle reboiler basically consists basically consists of horizontally mounted tea K shell okay now if you remember the nomenclature for the shell we have EFG type and we have another shell that we call as stea K shell and K comes from the kettle right so Kettle reboiler has a special kind of shell and it is only used in Kettle reboiler not in other shell and Tube exchanger right so along with kshell reboilers have a tube bundle comprised of either u tube or straight tube with the pull through floating head okay so the kettle reboiler has basically the tube bundle which includes either u tube or straight tube but that should be internal floating head type because whole bundle should be taken out from the shell and then it will be cleaned or maintenance will be done and then further it should be inserted fine so it is basically internal floating head type where it is placed in Shell right so as far as es schematic is concerned here you can see that it is connected to the distillation column this is the distillation column and here I'm having the kettle reboiler and if you see the bottom liquid of the distillation column enters to the reboiler okay and it is usually enters into shell side okay and if you consider the shape of the shell it is a special kind of shape and therefore it is called as Kettle reboiler okay whatever vapor is generated it recycles back to the distillation column fine and further we can visualize it more clearly from this schematic here feed enters from the bottom okay and uh start filling this section okay so feed whatever is entering into the reboiler it is entering to Shell side there is no option in s there is no option in such reboiler where feed enters to the tube side in Kettle reboiler feed always enters to the Shell side and in this exchanger we have a straight tube we can also use YouTube over here and in this YouTube or in this T and in this tube side is steam is basically entering which is us used as a heating media and from here it exits as a condensate okay so here you can visualize that shell shape so here you can visualize that shape of shell is totally different than whatever we are using in either condenser or in normal shell and Tube heat exchanger okay and the reason behind this is that when the liquid is when the liquid enters into this and uh all bundle complete bundle merg in the liquid right and after that when heat transfer takes place from Steam to liquid Vapor formation takes place okay Vapor formation takes place and because of the natural movement of vapor to Upward Vapor comes to this region right and here we have the space for vapor and when vapor enters to this space it also it also includes some leak liquid droplets okay so Vapor liquid separation occurs in this region and Vapor whatever is generated it recycles back to the distillation column right so in this way what we can say that the kettle reboiler generates the vapor and also separates the and also separates the vapor from the liquid right so this is the only reboiler which Supply Pure Vapor to the distillation column other reboilers we other reboilers which we are going to discuss in subsequent slide there you can find that Vapor liquid mixture both will enter to the distillation column okay so in that case we can consider Kettle reboiler as vaporizer also because it is having Vapor liquid separation in the unit itself okay now further you see here we have VAP weer here we have V okay and uh this Veer is basically this Veer is basically type of baffle okay it is basically the plate with it is basically the plate which is welded at the periphery of the shell from inside right now what is the purpose of this wear the purpose of wear is that it ensures that all tubes in the tube bundle should merge in the liquid okay so whatever feed is entering from the bottom until unless it will not reach to this level it will not come out from here it will not enters in this region and from this region I can collect the bottom product right right so the purpose of V is it must so the purpose of V is all tubes available in the bundle is inside the liquid okay and uh as far as design of reboilers are concerned we will discuss each and every details whatever I'm going to whatever I'm sharing right now okay what is the tube bundle like what is the tube bundle Dia and what is the height of verer number of tubes Etc and what is the space I'm providing as Vapor liquid separator what is the area of that right so in this way we can com so in this way we complete the design of reboiler okay but that is not a simple step it is it includes further many it includes different steps okay so that we will discuss later on fine now as far as shell and Tube heat exchanger and condenser are concerned there we have discussed that buffle placement is different in both type of either it is a exchanger or it is a condenser okay so when I'm focusing on reboiler okay we consider baffles in we consider baffles in these type of exchangers also okay now if you focus on this es schematic what will happen here liquid is coming from the bottom and start Rising upward through these tubes okay now in this case movement of liquid is what movement of liquid is upward okay here I'm not considering this this type of flow or this type of flow fine as we have discussed in heat exchanger as well as condensers so the purpose of reboiler is so the purpose of baffle in Kettle reboiler is different because it is only used to support the tubes otherwise support otherwise tube may have vibrations okay and which is not allowed during the operation so baffles are specifically used here as support plate okay and in this type of reboiler baffles do not have any cut okay whatever plate I'm considering as baffle it is basically circular plate and which is entering or which is inserted to all tubes okay because here movement of liquid is not decided by the baffle fine because movement of liquid is simply upward so whatever would be the cut it doesn't matter at all okay so usually no cut is provided in the baffles which I'm using in Kettle reiler right so so here you should consider some different construction as well as operation right so now as far as movement of liquid is concerned from bottom of the distillation to reboiler it is only because of gravity fine so liquid is fed by gravity from column and enters at the bottom of the Shell through one or more nozzle okay so in this way circulation takes place in reboiler but feed is liquid and product is vapor okay it is not the vapor liquid mixture right and product of the distillation column we collect from reboiler not the distillation column if you remember the condenser the product of distillation column from Top it is collected from condenser not the distillation column directly right so these are some points about the kettle reboiler let's have more discussion about this okay so it has low circulation it has low circulation rates horizontal configuration and all Vapor return flow make kettle reboiler relatively insensitive to system Hydraulics okay so whatever would be the pressure Vapor generates and Vapor returns back to to the distillation column because natural movement of vapor is upward Direction only right so we do not have to provide any unit or any assembly to circulate the liquid as well as Vapor it happens automatically okay so as a result they tend to be reliable even at very low vacuum or high pressures where thermosyphon reboilers are more where thermosyphon reboiler ERS are most prone to operational problem okay so here we can have so here we consider feed and Vapor so here we consider feed and we provide vapor and we also maintain the circulation okay because it is not very sensitive towards the Hydraulics fine and further you can see that kettle can be operated effectively or efficiently with a small temperature driving force Es as high heat flux can be obtained by increasing the tube pitch okay so in this reboiler tube pitch is not always 1.25 into D okay as we have already discussed in previous equipment so here we have variation in tube pinch so here we have variation in tube pitch okay now it has one negative aspect also that because of low circulation we have because of low rate of circulation what will happen it has more residence time so liquid will be available for longer time in this Kettle reboiler and because of that scale formation in Kettle reboiler are significant in comparison to other reboilers and at the same time we are providing tea K shell right so if you consider the diameter of the kettle it is much more than the usual diameter of the shell we consider in other type of reboiler condenser and usual condenser and normal heat exchangers okay so it is more expensive so these are some of the negative points about the kettle reboiler and now we should focus on natural circulation or thermosyphon reboilers okay so you see here I'm having the schematic of thermosiphon reboiler when it is placed horizontally and here we have the schematic when the re boiler is placed vertically right so what will happen bottom liquid of distillation column it enters to the it enters to the reboiler okay and it enters to the Shell side of the reboiler right if I'm considering the horizontal reboiler boiling will take place at Shell side right and if I'm considering vertical reboiler boiling is always taking place place in tube side okay so we have only two combination if horizontally placed shell side will be the boiling space and if vertically placed tube side will be the boiling space okay and the reason is very simple because here we have to circulate the liquid automatically we are not providing any assembly to circulate the liquid okay so it means when the liquid is entering into the reboiler and ex exiting from the reboiler it has least hurdle fine so if feed enters to the horizontal reboiler thermosyphon reboiler it is when it is entering to the tube side it has to travel long distance okay and when I'm considering shell side feed comes from the bottom converts into the vapor and Vapor liquid movement will be taking place upward Direction okay so horizontal Al shell side vertical tube side that is the only combination we have so as far as natural circulation is concerned the liquid circulation through the exchanger is maintained by difference in density between two phase mixture of vapor and liquid in the re boiler and the singlephase liquid in the base of the column okay so if I'm considering this section or if I'm considering this section we have only single phase that is the liquid however inside the EVAP however inside the reboiler either it is this or this we consider two-phase flow okay so natural circulation means so how we make the natural circulation if I consider this schematic whatever pressure whatever pressure drop available to us it is available from this to this height right so this we call so this we call as the pressure drop available to us okay and when liquid enters into this it starts converting into the vapor and Vapor liquid mixture moves up and then it is entering into the re then it is entering into the distillation column so in this way Vapor continuously form and moves upward because of the density difference right so that Vapor also include certain amount of liquid which also moves upward upward because from the bottom continuous feed is entering right now how this circulation is maintained when I am Computing pressure drop in this Zone that pressure drop should be less than the pressure drop which is available in this region then only the liquid which is available in distillation column is able to push or it or is able to enter into the re boiler okay so in this way circulation is maintained in thermosyphon reboiler right and as it is circulating continuously it has less residence time and therefore it does not have much fowling problem okay because more circulation gives more velocity which will reduce scale formation right but here one disadvantage is there that it does not have any vapor liquid separation unit that separation occurs in a distillation column itself okay and if I have to use this as a vaporizer I have to provide a vapor liquid separator above this fine and now we will discuss Force circulation reboiler as it is forced circulation we provide an we provide an assembly which we provide an assembly which circulate the liquid and Vapor within the reboiler and distillation column okay so it is not done automatically fine and that assembly is basically called as pump okay so you see here I'm having the schematic of it so this is basically the reboiler and here I'm providing the pump okay so Vapor liquid mixture again enters into the distillation column in the similar line we can consider feet to the tube side in this ES IC so in this way we basically consider both side boil we consider both side boiling either in Shell side or in tube side because pump is used so whatever difficulty in flowing occur that will be taken care by pump okay so this so this is about Force circulation reboiler we have another type of reboiler which we consider as internal reboilers okay so internal reboiler if you see in this SCH schematic here we have this reboiler okay and this reboiler complete bundle and the complete bundle of tubes it is and the complete bundle of the tube enters to the distillation column or it is fixed in the distillation column and here we have the liquid which is converted into vapor and move directly to Upward Direction so in this case I'm not providing any shell okay only bundle is there which is inserted in distillation column fine so so as far as so as far as this type of reboilers are concerned it is not taking much it it does not have much heat Duty okay because bundle is inserted into the distillation column and what it shows it shows that length of the distill it shows that length of the reboiler is fixed and that will depend on the diameter of the column right right so the amount of heat transfer area that can be accommodated is severely limited and formation of froth and foam in the column can cause operational problem and therefore this type of reboilers are infrequently used fine so that is about the internal reboiler now now we have some points about recirculating versus once through operation okay once through operation you understand it is liquid is liquid is converted into liquid is converted into vapor and whatever liquid is remain that exits as a product right so if you see this schematic here we take the liquid from this side okay and it is completely converted into vapor and then Vapor enters from here okay so the thermosiphon reboiler system can be either the recirculating type or once through type in the letter case that is once through type liquid from the bottom tray is collected in a trap out from which it flows to the reboiler so it is basically this assembly the liquid fraction of the return flow collects in the column sump from which it is drawn as a bottom product thus the liquid passes through the reboiler only once as with the kettle reboiler okay so in this way once true operation is done and how recirculating operation is done that we have already discussed in thermosyphon reboiler right so here I'm having one important point about the reboiler that for Reliable design and operation The Vapor weight fraction in thermosyphon reboiler should be limited to 25 to 30% for organic compounds and 10% of water or Aqua Solution okay and if this limit and if these limits cannot be obtained with Once through operation then a recirculating system should be used okay so in recirculating system what will happen only fraction of the liquid is converted into vapor and that is around 25 to 30% for organic compound right and this point we will consider in designing of thermosyphon reboiler I'll speak about that at that time okay and and now we will discuss some fundamentals about the heat transfer in reboiler as we already have seen that reboilers basically includes boiling operation fine either shell side or tube side so the mechanism of heat transfer from submerged surface to a pool of liquid depends on temperature difference between the heated surface and the liquid okay so when I considered the boiling mechanism I should also Focus focus on the boiling curve okay and here we have the boiling curve for an example purpose okay if you see the boiling curve varies from here and then it starts reaching to the critical point and then drops down and then finally increases so if you see here I'm having the heat flux on x-axis so here I'm having the heat flux on y AIS and surface temperature is available at and surface temperature is available on x-axis so if you see here I'm having different regimes if I consider 0 to a it is basically free convection boiling okay so at this time the heat is transferred to the liquid and uh liquid droplets takes the Heat and recirculate in the pool so in this way free convection boiling is taking place and further if we move Beyond a so a to so A to B so A to B and then to see we consider that as a nucleate boiling because then um bubbles will start forming and uh in that case because bubble starts forming over here okay and after that what will happen after that from C to D we have transition boiling okay so here basically what happens when bubbles are formed those bubbles basically covers the boiling surface right so the these bubbles join and make a film at the bottom of the pan or at the bottom of the container from which I'm taking the heat right so when this bubbles join and make the blanket heat transfer is not taking FL heat transfer is not taking place from bottom to the liquid because between that we have the vapor blanket okay in that condition heat transfer reduces drastically and and after some time when film will be there film of bubbles will be there the heat will start reaching to the bulk of liquid through radiation okay and in that case we call that as and in that case we call it film boiling as we can see from D to e right so all these points we have to consider while designing reboilers okay now as far as type of boiling is concerned we usually have pool boiling which includes nucleate boiling within the pool of the liquid because bundle is completely merged in the liquid if I speak about the kettle reboiler okay so in that case Nuclear So in that case nucleate boiling will occur in a pool of liquid next I'm having convective boiling or film boiling it occurs when the vaporizing feed is flowing over the heating surface okay so when the fluid is available over the heating surface it takes the Heat and then molecules of the liquid will start moving in a bulk in a circular motion okay so that basically we call as the convective heat transfer and also heat transfer takes place by force circulation and nucleate boiling in such cases okay so as far as pool boiling is concerned we have a specific heat transfer coefficient expression which is shown over here and here we have basically H&B which we call as the nucleate boiling and all parameters are defined over here and if I have to use this equation for the mixture we consider one factor and that is basically FM and H&B hnb which is this for single component because I'm having too many properties of the compounds because I'm having too many properties of a single compound right so this FM is equal to exponential bracket minus 0.83 TBO minus TBI it means boiling point outlet and boiling point Inlet okay so these based on these temperatures we can find out we can find out heat transfer coefficient in pool boiling right similarly in pool boiling we can have another correlation which is based on critical pressure so if you know the critical pressure if you know the heat flux you can find out pool boiling heat transfer coefficient using this expression you can find out pool boiling heat transfer coefficient using this expression right and next I'm having the critical heat flux as we also have as we have already discussed in boiling curve that Beyond critical heat flux or beyond the critical point heat transfer reduces significantly so we have to ensure that in the reboiler critical heat flux should should not be maintained or the boiling should take place well below than the critical heat flux so that check is also required which is a part of design and you can use this expression to calculate critical heat flux for Kettle reboiler where different uh parameters are explained over here and now we have the convective boiling when convective boiling is taking place we have different regions and these regions are single phase slug flow anular flow anular Mist flow and dry wall Mist flow okay so as the vapor formation increases accordingly we have different regions okay and to account this we basically consider chain method okay and in force C con and in force convective boiling the effective heat transfer coefficient HCB can be considered to be made up of convective as as well as nucleate boiling component h- FC and h- NB okay so this is basically convective boiling heat transfer coefficient so this is basically convective boiling heat transfer coefficient which which is based on which is based on hfc as well as hnb okay with prime okay and what are the terms and what are the expression of these terms that we will discuss in subsequent lectures and uh I'm just stopping this lecture we will start design in next lecture okay so that's all for now thank [Music] [Applause] [Music] [Applause] [Music] you [Music] a