Heat Transfer Prof. Dr. Aloke Kumar Ghosal Department of Chemical Engineering Indian Institute of Technology, Guwati

Transcription

1 Heat Transfer Prof. Dr. Aloke Kumar Ghosal Department of Chemical Engineering Indian Institute of Technology, Guwati Module No. # 08 Evaporation Lecture No. # 1 Introduction to Evaporation & Evaporators Welcome to the first lecture of module 8. In module 8, we are going to discuses on evaporation, and to start with that means the first lecture, we are going to discuses on introduction to evaporator and introduction to the evaporation and evaporator types. (Refer Slide Time: 00:38) Now, to start with the evaporation so what we mean by, what we mean by evaporation? In generally, mean that evaporation is nothing but vaporization of a solvent from a solution to make it concentrated, that is what is in general means mean by that, so that is solvent is volatile or it is evaporated, and other part is nothing but is called the solute which is almost non volatile in nature. And that product that we are getting after evaporation is nothing but a thick liquor, so we can say evaporation is nothing but concentration of a solution through vaporization of the solvent. And what we are left with is that a thick liquor, so vapor is gone so residue is called thick liquor, and that solute is non volatile that mean its volatility is very small, solid will not be a solid

2 substance that is dissolve in the in the liquid it can be even a gelatinous substance or may be a liquid substance which is very having very low volatility, that time also we can think of that is method of evaporation. For example, we know that drying of gelatin aqua s gelatin is an example of that evaporation either heat gelatin means drying of gelatin or glycerin this kind of substances which are highly viscous and very low volatility is therefore, there they are not they are not solid substances they are many viscous and highly viscous and semi solid jealous gel type of substance. Now typical example of evaporation are that we know in sugar industry concentrating cane sugar juice considering in fertilizer industry in similarly, in food industry that is many other type food juices is also important similarly, in fertilizer in that seed is concentration of say ammonium sulphate then in alumina plant it is concentrated or recycle and concentrating sodium hydroxide solution so these are the typical examples of this. (Refer Slide Time: 04:23) So, the question comes, what are the difference between drying and evaporation. So, drying verses evaporation and we know that incase of drying also that solvent is evaporator that means evaporation also the solvent is vaporized, so then what is the basic difference in case of the basic difference here is that objective of this operation in case of drying the material the solvent is totally vaporized to have a solid residue. So, here so we can have a solid residue here and in case of evaporation we will have concentrated

3 solution which is called thick liquor. So, this is the major difference between the drying and evaporations this is also you can called as a viscous solution is obtain so this is due to concentrating. Now, next part is that, another question comes is that typical example is that drying of milk to produce milk powder. So for example, drying of milk to produce milk powder; so here we are getting a solid substance that is in case of evaporation we are not getting the we are not interested for the solid substance we are interested for getting heat concentrative. Now, second part is like what is the deference between the distillation verses evaporation, and we know that distillation is also is that vaporization of the volatile liquid part which is getting vaporized. So in case of distillation, we know that it is basically the two liquids or two or more liquids have some differences in their relative volatility they are being vaporized and then in the in the vapor part once they are vaporized in the vapor part they are getting separated. So, they are getting not only vaporized then to the vapor phase are getting separated by distillation, but in case of evaporation there is no attempt in this case so incase of evaporation, what is happening is the mostly a pure component is that is the solvent which is getting vaporized. So, in case of here, in case of evaporation, is the a pure component is getting evaporated; and that vapor, no attempt is being taken to get the vapor separated; because is there is a mixture of some components, because that if the solvent is the mixture of another liquid component, which also have some kind of vapor percentage in the vapor; so that is not attempt been taken those components that is why that is the difference between distillation and evaporation. So, we can say that so next part of distillation is as we know that its mostly as I told you betweens two liquids, and it is liquid and solid can be liquid or it can be a solid portion that reduce a non volatile almost a almost zero volatility. As I was telling a typical example of dilute solution, glycerin is an example, but dilute solution of glycerin that when we think of a distillation we can never do because the vapor of the glycerin in the vapor phase when with the water vapor phase is almost there is almost nil almost negligible so there is no separation is possible by that way so this is the difference between the distillation and the evaporation.

4 (Refer Slide Time: 08:24) And then comes crystallization. During the crystallization, we required the evaporation to take place, because through evaporation what we are doing is we are concentrating the solution and that means we are taking out the solvent as much as possible we are concentrating the solution and we are trying to make it super saturated by evaporation of solvent from solution and then crystals grow in super saturated solution so that mean we can say that crystallization, for crystallization evaporation is a first and information first requirement. So, we can understand from there that evaporation is one important unit operation for chemical engineering particularly for chemical engineers and for any chemical process industry and we should know that different type of evaporator and what are the basic concept and how to calculate or do design operation for evaporation evaporator. Now, so before we discuss further about the different types of point there are some important issue like that what are the properties and how this properties would have an impact on the design of the evaporator, so when we say that the following properties of evaporating liquids the properties of the evaporator I am going to list out they have impact on the design of the evaporator the properties of the concentration of the liquid or the solution and then forming properties, then temperature sensitivity, then scale formation, so these are the four important properties we should understand clearly while going for any type of evaporator design.

5 (Refer Slide Time: 11:15) Now, if we just see this concentration part so usually we usually thin leaker surface that means very dilute having property equivalent to water this is one thing and, but what happen is that in increased concentration changes the viscosity, boiling point we know that call the elevation of boiling points then there are many other issue like specific heat etcetera. So, we should be pretty careful while calculating this that what happen to the properties change and how to take care of this thing so this is that concentrating effect of the concentration on the evaporation design then so we have to see the that salting out affect or I should say that scale formation sorry scale formation we should discuses later so salting out affect at higher concentration that means at higher concentration weather it is getting crystallized weather any crystal formation is there or not so all this things are very important. So, if any crystal formation, because this crystal formation what it does it blocks the tubes. So, there will be a problem in the evaporation phenomena. So, this are the typical things that has to been seen and also we have to see that to what exchanger the viscosity is increased, so that whether there is flow ability or not we know that increase viscosity will create what that change in big transfer barrier big transfer coefficient. So, this things all this thing are related to the concentration of the evaporating liquid so we have to be pretty careful about that how is this concentration and how is the properties being change and how to these are taken to the consideration.

6 (Refer Slide Time: 14:08) Second part is the foaming and we know that the foaming is one important issue that when the boiling phenomena comes and then there will be a foam formation and then foam means that it fine partials will entrained will be getting entrained along with the vapor so that there will be lot of liquid loses and this particular happens with organic substances. So, we should see that sufficient length of the tube is there to take care of form and so sufficient length of tubes to take care of foam or also we have to see that any plate what is the impingement plate to detach the intern droplets. So, these are the important issues relative to the evaporator and then temperature sensitivity as we know that temperature sensitivity is very important because the heating is there evaporation means the heating and the liquid gets boiled up. So, if the liquid material is highly temperature sensitive then we should be calculable, because in the high temperature if we put the exposure at high temperature is very important if we give long exposure then there can be damage to the materials. So, that is one very important things that should be a particularly sympathy particularly for say fine chemicals, pharmaceutical, then we could have say any food product engines so this are highly temperature sensitive. So, we have to see that it is not given as a long exposure at higher temperature.

7 (Refer Slide Time: 16:36) And then say next part is the scale formation as I was telling this is called nothing but deposition of the scales on solid heating surface by some solutions and we say deposition of scales on the solid surface and this scale deposition maybe due the dissolve solid these solids are just getting deposited sometimes it is due to the salting out effect also that means the source comes out, it gets precipitated or deposited this is called salting out affect so this may be of that reason also solute get dissolve on the surface and know the thermal conductivity of the solutes are very low and therefore, the heat transfer coefficient will be drastically reduced, coefficient will be drastically reduced, therefore, this has to be taken into consideration while going for any such heat exchanger design.

8 (Refer Slide Time: 17:51) Now, come to this next part which is called material of construction, what kind of materials of construction we should use in case of evaporation designed, it is usually or mostly made of some kind of steel that is just because know that it is operating at some high temperature under boiling liquid is there and the other side there is the steam usually the condensing steam is used for heating purpose it can be any other heating medium, but usually the condensate steam is used and the liquid is which is being evaporated is basically at a higher temperature at a boiling point so at high temperature the correction chances are much more and therefore, some kind of steel are generally used but for highly corrosive fluids special materials like copper, nickel, some stainless steel, aluminum in impervious graphite etcetera. Are used so we have to see that, because these are used these are very costly. So, we should see that we should look into the fact that heat transfer coefficient is maintain high this is, because higher hear transfer coefficient means we need to have less surface area that means less consumption of this materials. So, that is one important character criteria we should have high heat transfer coefficient to reduce area or size so actually this is the intention of anybody would be that we should see the high heat transfer coefficient to reduce the situation to reduce that area of with that solid that is automatically it is always reduce the cost capital investment, but sometime it may not be possible due to the external constant, but here when we are having some highly corrosive material that is to be used and that time and if you are using very highly costly material like copper, nickel,

9 ammonium, such kind of material are used as a material construction that case it is advisable that special attention should be this kind of things so that size recommended is in drastically reduced. So, in that case the capital investment should be reduced to a considerable extend and then many other characteristics also that we should know in case of evaporation design these are specific heat of concentration, freezing point and then of liquid defiantly, gas levitation, proximity level, then exploitation hazard, radio activity and then and so etcetera. So, these are the support certain information that we should have regarding the solution which we are going to concentrate particularly for evaporating design so thus we should be pretty carful about that liquid characteristic while going for evaporator deign as we have discussed in many ways. (Refer Slide Time: 22:25) Now, is we just to see the type of evaporator, we will concern of the different type of evaporators that there are different types of evaporator has of has come and mostly the evaporator are of it is the kind of shell and tube heat exchanger thing so that tubular surface are there, tubular surface are the heating surfaces and that can happen in two ways one is called natural circular evaporator another one is called four circular evaporator in the natural circular evaporator means that the natural circular mean that that liquid which is getting evaporated it is flowing with the help of density differences it is the gradient of density, because of the changes in temperature there is density, because

10 of the density radian that flow the moves that is called natural circular another case is that four circular where that density difference though it works, but it is needed to have the liquid to be flowing in a greater speed higher speed that situation is the necessary of putting a pump which will give a flow to a additional energy to the fluid for the flow. So, that is called the flow circulation and natural circulation there are typically there are two type one is call short tube vertical which is called calandria type and another is called long tube vertical. This is short tube vertical, another is called long tube vertical and for circulation we have normally long tube v circular time which is similar to the long tune vertically only and this as I told you that this four circulation is needed particularly for highly viscous liquids for heat transfer coefficient that is to increase the heat transfer coefficient, because for low velocity they are viscous velocity low and for solution condensate suspended solid. So, what happen is suspended solid they may clock the post tubes not post clock the tube they may get separated from the main flow. So, if you have high velocity, then the suspended solution would not be separated from the flow region, they would not clock this will be allows, will be remaining suspended in the fluid in the higher velocity condition, because of the drags and for heat sensitive material, for heat sensitive material, they can be able to can allow to stay in the evaporator in the longer time. So, we need to be have a higher velocity so that directory in the evaporated is less, so that is why this four circulation kind of thing and then there are various other types of evaporators are falling film, climbing film or rising film and auditing film and fluid types.

11 (Refer Slide Time: 25:08) So, it will be this will be quickly discussing the different aspect of the evaporators just as I told you that if we start with to start with that the first one is that the natural circular innovation and it is happening due to as I told you it is basically evaporated density difference and here this is typical example of natural circulation evaporation which is the short tube vertical and it is also to call Calandria we can see this portion this is the shell. So, you have the tube bundle that is placed in the shell and it is here it is flange and this shell is called this one shell is called Calandria. So, the tube is places over here so in this case what we can see is that the feed is coming from here and you can see that that it comes like this and then it goes like this. So, this is the circulation that it takes place and this portion this is the Calandria you can see here this is the thick liquor is going out like this, so this is the fifth coming. So, this is the smaller tube and we have this is the vapor region the vapor is coming like this and so then we have a the thick liquor will be coming like this from this through the hallow or this is the liquid will flow like this and then it will go to the tube and then finally, the thick liquor will be drain out from this. So, then is what has been done in the case of short or Calandria type of evaporator and through the tube the liquid goes up. So, it goes up due to the density difference so the high temperature liquid at high temperature will have lower density and like that and then there will be while there will be vaporization. So, there will be affective density will be reduced and it will come it will go upward, so this is called that radian go upward and so this portion is called this

12 portion where it is called the down the down tape or the down comma this portion is called down tap or down comma. (Refer Slide Time: 27:59) Now what happen is that typically that in this case of Calandria operator that diameter is 1 to few millimeter for Calandria and shell dia and tube dia is usually 2 to 3 inch and length tube length is usually 1.2 to 2 meter and down comma area is nearly 5200 percent of tube flow. (Refer Slide Time: 29:02)

13 And that as you see that this is there will be some interim separator here, this interim separation is here just to see that there will be no vapor is no liquid barbless are entrained in this region with the vapor; and so if we see that the uses are typically to concentrate variety of solutions typically sugar solution. (Refer Slide Time: 30:04) That Calandria is not suitable for solutions in which passepied and salting out of as I told you earlier salting out of solid may occur and then what will it why it is not suitable then what will happen why it is not suitable then what will happen then, because it is a natural circulation force of the liquid is not that much high. So, what will happen is that it will not allow it will not take the deposited solid material along with itself. So, there will be staying and there will be clogging the force of the tubes solid particles cannot be in suspension and so clogging will take place. So, this problem to some extend can be overcome by putting some propeller.

14 (Refer Slide Time: 31:43) So, we show you the propeller arrangement when there is the propeller here putting the this impeller to the whole thing is the impeller this propeller, Calandria using propeller Calandria what we can do is here you can put some circulation here we are putting some circulation and using the circulation when increase in the speed of the liquid passing through the tube speed of the liquid passing through the tube that were able to this is the tube through passing to speed of this is this passing through the tubes. So, we by this is the down common sector by putting the propeller kind of thing we can make it go through the tubes and then salting of affect can be push and extend avoided. So, this is called propeller Calandria and we can see that it is similar to that short tube vertical these are all called short tube vertical evaporator.

15 (Refer Slide Time: 33:00) So, short tube vertical evaporators are called vertical this is called short tube vertical this short tube vertical evaporators are these are also called the standard evaporators. That standard operation evaporators and there advantages are they are of low cost, then easy cleaning, then low head room requirement by disadvantages they are it has got some requirement and deserve able for viscous liquid then, because the reason is offer low heat transfer coefficient, because the flow is very small let us say, because of natural circulation that called liquid will be not be allowed and also require larger flow space. So, what we can understand that that tube bundle should be of bigger size, because the tube are of smaller size in case of cylindrical the tube length are smaller. So, tube bundle should be bigger in size and therefore, the flow area should be will be relatively high.

16 (Refer Slide Time: 34:55) There is another kind of short tube vertical evaporator is that is the basket type vertical evaporator and what we can see here in this case it is just arrangement is little bit change what you can see here is that the steam is inside this portion and fluid is in this regime where fled is so fled is the annual as and steam is inside this regime. So, this part is like a basket which can be just taken out for cleaning. So, this give us a very easy cleaning operation and so here what is happening here that the hot liquid flow down the annulus so and then goes to the tube to the upwards and through the annulus and steam enters the chest steam enters in this regime chest here, and that through this pipe which is entering here it is here, the steam is entering this is what is happening here and finally, thick liquor is being collected from this from this thick liquor is coming out. Now the here it is mainly used, because it can better handle scale formation, because of easy cleaning possibilities this is the advantage of this, advantage of basket type is that basically easy cleaning and maintains so, this is that short tube vertical evaporator that is again that basket type evaporator which is being used for natural circular purpose and you have seen then there are three type of Calanrdia it is a normal simple Calandria types and another one is with a propeller arrangement and other one is basket arrangement. So, in case of propeller arrangement the propeller is just to increase the speed of circulation that takes care of any kind of scales being deposited over there. So, they allow them to be suspended and another case is here, is the basket types for that just flow direction is reversed for that reveres in that case is keeping that steam region central portion and

17 liquid be evaporated in the annulus problem and it is, because of that tubes tube bundle remains like baskets and it can be taken out easily and can be cleaned. (Refer Slide Time: 38:27) Now another type is as you have seen is nothing but the long tube vertical evaporator this is also goes by the natural circulation mode and what happens here again that we can see here. The feed is coming here and then feed goes through this tube feed goes up to this tube feed goes through upward what is being targeted in case of this. (Refer Slide Time: 38:57)

18 Here, these are long vertical bundle tube bundle heated with a shell and feed enters with the bottom and flow to the tube once, and it is going seen that the important think of this is that as we have seen that feed goes like this and then in the separated plate. And now what the entrainment is to be stop and then vapor goes to this and this is the entrainment separation and then vapor comes out like this, and any changes liquid as we see that entrain liquid comes out like this, and there is the thick liquor that is coming out this is called steam trap. And there is the veined is for any kind of condensable. So, non condensable dashed is there that will be that is allow to come and seen chest the steam is going for heating and the resistance in the now important point over this resistance it is of condensing steam on shell side that is always there is small that is always true what happen is that in case of long tube initially heat transfer coefficient is less and then increases due to boiling once. The boiling starts turbulent starts heat transfer coefficient increases and then, so liquid at lower point in the in this case that liquid of lower point will boil at higher temperature. Liquid at higher point will boil at lower temperature that is an easier due to the hydro static hair, and frictional drop all the things are there. So, the pressure in this liquid is this portion is high pressure in this portion is less. So, the boiling point is more and the boiling point is the less, and then usual things are that l t v long tube vertically evaporator they have tubes of 3 to 10 meter length and 1 to 2 inch dia tube bundle dia is less than short tube vertical cases. And sometimes baffles are also used for baffles in shell side to have more uniform flow of steam.

19 (Refer Slide Time: 42:43) And now the use for this cases we can say that you say a paper and pulp industries to concentrate black liquor, then concentrating any white solution then foaming solution so, because is the long tube the foaming solution use will be easy and corrosive solution also used now similar kind of one is that recycler a long tube vertical recirculation type, it is similar kind of things here the flow is not once through. So, that means in case of simple long tube vertical efficient of flow is just going through once, but in this case flowing is not once through that means it is re circulated flow is circulated. So, circulation is there and the reason is the flowing circulation. (Refer Slide Time: 44:26)

20 The reason is to get high transfer coefficient and low cost these are advantages low liquid hold up less; these are all advantages less flow space requirement, but this advantages are high heat rate requirement and unsuitable for viscous and scale forming materials. (Refer Slide Time: 45:37) So, this is what is that it is a long tube vertical you can see here with a recirculation. So, things are getting re circulated here mountain vertical recirculation heat exchanger. (Refer Slide Time: 45:55) Now, there is another kind of heat exchanger is there is the forced circulation. So, here in this case the force circulation is when that when that natural circulation does not succeed

21 already I have told succeed what are the cases that means the cases of viscous liquid, then suspended solids, heat sensitive material, under those situation natural circulation does not succeed. So, then it is better to use four circulation on using four circulation what is being done is that that velocity is increased. So, for viscous liquid that it will be high heat transfer coefficient can be obtained and then suspended solids suspended solids kept suspended, because of high velocity and for heat sensitive material the reason is that once velocity is it will go through the evaporator once for the smaller time it will pass through the evaporator and so exposal time will be less per the heat sensitive materials. So, that is this for force circulation is the singular type of whatever is being discuss for natural circulation cases, but only thing is here, but only some pump is needed just to give that energy to this thing energy give some energy to the fluid. So, that it passes through now there is there are two more, but only problem with this four circulation is that so advantage these are the advantages of the four circular but disadvantages of this four circulation in it use that it is high capital cost, because of the pumps high capital cost and high head room requirement. This high head room requirement cases is that cases when that that tubes is not allowed the thread is not allowed to boil in the tube and then what is happening that it is taken to the vapor flashing zone while in the head room that that reduced pressure is the area is reduced the volume is increased. So, and the temperature will be less and that that liquid can flash over there and then therefore, that flashing part the vapor is to the vapor that high room is recommends there that is needed. (Refer Slide Time: 48:59)

22 And there are some other types of one thing is that one is falling film evaporator and we know that incase of falling phase evaporator we have mostly that what is happen is that the liquid is allow to fall like a film. And this falling film evaporator is most important fact in this case is that most important factor is liquid distribution. So, from the name itself we can say that that liquid is falling down and in the form of film this will be the so if we have this is the tube. So, liquid will falling as a film here, so falling film this is the film this portion will be the vapor. So, vapor from the in the poor it will go upward and the liquid will fall down. So, vapor will be generated then they should come from the liquid side vapor will be generated and it will come to the upward direction. So, what is important here is that is the liquid distribution and to maintain the film and the advantages is and for that the to maintain a film Renoylds number defined as 4 gamma by mu where gamma is liquid rate per circumference of a tube. So, this Renoylds number is this is greater than maintain greater than 2000 so this is what is being done. (Refer Slide Time: 51:00) And the advantages is of this following film is that heat transfer rate is high and good for heat sensitive material and then minimum viscosity upper level limit of viscosity is a mu is equal to 100 to 500 c p so it is so not good for high viscous, because in that case that that film will be very thick then another one is the climbing film evaporator incase of climbing film evaporator is like that. There will be the boiling liquid will flow out flow in through the tube. So, here in this case climbing film the vapor will go. So, there will be liquid initially it will be liquid then bubble then salt of bubble and then there will be

23 this is the vapor zone and this is the film liquid film. So, what will happen that the vapor will take along the liquid. So, liquid climb upward that is why it is called climbing film and here also this is also similar kind of utility advantages it can get similar advantages and that can be obtained using climbing film evaporator. (Refer Slide Time: 53:04) And then there are some other kind of evaporator is called additive film thin evaporator this is also used for highly viscous this is used for highly viscous liquid and this viscosity is range is we can it is a very high may be 10,000 to 20,000 c p viscosity. So, what is happening here, because of this rotation rooter blade is there, because of the rotation over here this here you can have that liquid film as we shown liquid film will be and there will be just thin liquor will come down. This is what is being used so for high liquid viscous liquid, and there will be some hesitation. So, we will maintain a film and the film thickness is maintained.

24 (Refer Slide Time: 54:01) Here, and this is heat transfer coefficient or heat transfer coefficient is increased and then we can say that fouling of surface is reduced and and keep film thickness small and so blade clearance nearly equal to 1.5 millimeter. So, based on that film thickness will be maintain over there. So, this are the advantages that we get and this is used for agitated thin film is used for evaporator is this that advantages of that agitated thin film, you have and is used for re highly viscous material, and then if you see here, that it goes to that this is the regime this is called the vapor regime and here it is of high size. So, that vapor can be reduced here the pressure is reduced and that liquid come down. (Refer Slide Time: 55:42)

25 And then another one is the plate type, heat exchangers here, it is similar to it is the plate type similar to the plate type evaporator. (Refer Slide Time: 55:55) And here in this case also, it is we can say that can be rising film, falling film mode or a combined mode that is what it can be used for high viscous liquid of says 20,000 cp of this range viscosity, this is used and is also good for heat sensitive and foaming solutions. So, in the plate of heat exchange in one side there will be a steam and another side there will be the liquid that will be there this is the plate type heat exchangers that is being used. So, typical example plate type evaporator here example is concentration of molt extract, gelatin, glue, emulsion etcetera, so these are being a using the a plate type of operator.

26 (Refer Slide Time: 57:32) So, plate type of operator is similar to plate to heat exchanger, these are the typical examples of various types of operators and the product being handles that already, we have discussed and we can see that that plate type and the differences are there in this cases. If you see that for four circulation that salting and scale forming sodium, calcium, sodium sulphate solution with that hesitated film there is high viscous material gelatin in a glue similar it is for plate type heat exchangers and rising film and volume film these are low to medium viscosity materials and these are used and all. So, this is used for this kind of operator detail analysis study can be made, but I should stop here regarding this type of evaporator type of evaporator descriptions. So, this is all for this lecture on in the next lecture we will discuss on the different principles involve in the performances of heat exchangers, thank you very much.