Functions of Tray In Distillation Column

     One of the most prominent hardware used for mass transfer is tray. Tray columns are widely used in various types of mass transfer operations. All the simulation results, which predict a certain number of theoretical stages, can be converted to actual trays depending upon tray efficiency for a particular service.

     Basic functioning of a tray/plate is mass transfer. It actually brings about vapor-liquid contact. More the vapor-liquid contact, more would be the mass transfer. This is practically achieved when a liquid is held on a perforated tray and vapors pass through this liquid layer from below the liquid depth through the perforations. It should be noted here that, flow rate should be adjusted such as the liquid "may not" come down from the tray above "through perforations". If this happens, then vapor-liquid contact would be less which will result in low vapor liquid contact and hence it lowers the efficiency of tray. This condition is said to be weeping.

     Similarly, vapor velocity must not be so high, since it may take the liquid over the tray to the tray above it, in form of droplets. This will again reduces the efficiency of the tray in the similar manner. This process is called entrainment.

     In any conventional tray vapour rises through the liquid pool on the tray deck and then disengages from the liquid in the space above the deck. Liquid enters the tray from a downcomer above and leaves via a downcomer below.

Conventional Tray has three functional zones:

1. Active area for mixing vapour and liquid: This is the zone where mass transfer occurs.
2. Vapour space above the active area: This is the zone in which liquid is separated from vapour.
3. Downcomer between trays. This zone has two functions, first moving liquid from one contacting tray to another and second disengaging vapour from liquid.

What is Turndown Ratio And It's Significance In Distillation Column

Turndown Ratio:
                              The ratio between minimum vapor load to maximum vapor load, thus it indirectly defines both the operating range (low or high vapor flow rates) and also the minimum capacity before trays start to leak. A low turndown ratio depicts the tray to be less flexible in operation i.e, it cannot handle a range of vapor flow rates. 

Operational Problems In Distillation Column

Major operational problems in distillation column due to adverse vapour flow conditions can cause

• foaming
• entrainment
• weeping/dumping
• flooding

Foaming

                Foaming refers to the expansion of liquid due to passage of vapour or gas. Although it provides high interfacial liquid-vapour contact, excessive foaming often leads to liquid buildup on trays. In some cases, foaming may be so bad that the foam mixes with liquid on the tray above. Whether foaming will occur depends primarily on physical properties of the liquid mixtures, but is sometimes due to tray designs and condition. Whatever the cause, separation efficiency is always reduced.

Entrainment

                      Entrainment refers to the liquid carried by vapour up to the tray above and is again caused by high vapour flow rates. It is detrimental because tray efficiency is reduced: lower volatile material is carried to a plate holding liquid of higher volatility. It could also contaminate high purity distillate. Excessive entrainment can lead to flooding.

Weeping/Dumping

                                This phenomenon is caused by low vapour flow. The pressure exerted by the vapour is insufficient to hold up the liquid on the tray. Therefore, liquid starts to leak through perforations. Excessive weeping will lead to dumping. That is the liquid on all trays will crash (dump) through to the base of the column (via a domino effect) and the column will have to be re-started. Weeping is indicated by a sharp pressure drop in the column and reduced separation efficiency.

Flooding

               Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the vapour up the column. The increased pressure from excessive vapour also backs up the liquid in the downcomer, causing an increase in liquid holdup on the plate above.  Depending on the degree of flooding, the maximum capacity of the column may be severely reduced. Flooding is detected by sharp increases in column differential pressure and significant decrease in separation efficiency.

Plate Columns and Comparison of Tray Types

    For purpose of distillation, plate columns and packed columns can be used. In plate columns each plate constitutes a single stage, or in packed columns where mass transfer is between a vapor and liquid in continuous countercurrent flow.

     In order to design a plate type distillation column, following factors must be considered:

1) The type of plate or tray
2) The vapor velocity, which is the major factor that determines the diameter of the column.
3) The plate spacing, which is the major factor fixing the height of the column when the number of stages is known.

Types of Trays:
     The main requirement of a tray is that it should

a) provide an intimate contact of vapor and liquid phases, because more the contact of these two, more will be the mass transfer which brings about more enrichment.

b) it should be capable of handling more desired flow rates of vapor and liquid without excessive entrainment or flooding.

c) be stable in operation or have flexibility in operation.

d) be reasonably easy to erect and maintain.

Arrangement of Flow:
     The arrangements for the liquid flow over the tray depend largely on the ratio of liquid to vapor flow. Three layouts are shown here, of which the cross-flow array is much the most frequently used.

a) Cross-Flow: Normal, with a good length of liquid path giving a good opportunity for mass transfer.

b) Reverse: Downcomers are much reduced in area, and there is a very long liquid path. This design is suitable for low liquid -vapor ratios.

c) Double-pass: As the liquid flow splits into two directions, this system will handle high liquid-vapor ratios. 

     The liquid reflux flows across each tray and enters the downcomer by way of weir, the hight of which largely determines the amount of liquid on the tray. The downcomer extends beneath the liquid surface on the tray below, thus forming a vapor seal. The vapor flows upwards through risers into caps, or through simple perforations in the tray. Weir and downcomer is shown in second figure as follows:

Types Of Trays:
    Purpose of tray is to provide an intimate contact of liquid and vapor, and to make a low drop of pressure. So far in industry, following three types of trays are usually used:

a) Seive or Perforated Trays:
                                              These are much simpler in construction, with small holes in the tray. The liquid flows across the tray and down the segmental downcomer. This type of tray offers a very low pressure drop and is cheaper than the rest of the two, but it brings about less vapor-liquid contact as compared to the other two.

     The general form of the flow on a sieve tray is typical of a cross-flow system. With the sieve plate the vapor velocity through the perforations must be greater than a certain minimum value in order to prevent the weeping of liquid stream down through the holes. At the other extreme, a very high vapor velocity leads to excessive entrainment and loss of tray efficiency.

b) Bubble Cap Trays:
                                   This is the most widely used tray because of it's range of operations, but is now-a-days unable to compete with the third type which offers more flexible operation. The individual caps are mounted on risers and have rectangular or triangular slots cut around their sides. The caps are held in position by some form of spider, and the area of the riser and the annular space around the riser should be about equal. With small trays, the reflux passes to the tray below over two or three circular weirs, and with the larger trays through segmental downcomers.

     This type of tray provides good efficiency than seive tray, flexible in operation (i.e, can be used for a range of liquid-vapor flow rates) but  is most costly and offers great pressure drop as compared to the other two. Bubble cap trays are capable of dealing with very low liquid rates and are therefore useful for operation at low reflux ratios.

c) Valve Trays:
                        These may be regarded as a cross between a bubble-cap and a sieve tray. The construction is similar to that of cap types, although there are no risers and no slots. It may be noted that with most types of vlave tray the opening may be varied by the vapor flow, so that the trays can operate over a wide range of flowrates (i.e, it provides more flexibility in operation than a bubble-cap tray).
 
     It is low is cost than a bubble- cap tray since it is simple in construction. Because of their flexibility and low price, valve trays are tending to replace bubble-cap trays. It operates at the same capacity and efficiency as sieve trays. It has high turn-down ratio, i.e, it can be operated at a small fraction of design capacity.

Steam Distillation

     Steam distillation is used where a material to be distilled has a high boiling point, and particularly where decomposition might occur if direct distillation is employed. In this distillation type, steam is passed directly into the liquid in the still, but it should be kept in mind that the solubility of the steam/water must be very low otherwise it would contaminate the product and one will have to bear more cost to separate water from the product. Steam disillation is perhaps the most common example of differential distillaion.

Explanation:

     Two cases may be considered in distillation using steam. The steam may be superheated and so provide sufficient heat to vaporize the material concerned, without itself condensing. The second case will exist when some of the steam may condense, producing a liquid water phase.

     If there is no liquid phase present, then from the phase rule there will be two degrees of freedom,

                                             F = C - P + 2
                                    =>     F = 2 - 2 + 2
                                    =>     F = 2

F = degrees of freedom (tells about the number of parameters to be essentially specified to specify a system)
C = no. of components of the solution (binary solution considered here, hence 2 components)
P = no. of phases (2 in this case i.e, steam and liquid solution phase)

     Both the total pressure and the operating temperature can be fixed independently, and partial pressure of high volatile component, which must not exceed the vapor pressure of pure water, if no liquid phase is to appear (since at higher partial pressure than water will cause steam to condense, as at high pressure boiling point increases. See Effect of pressure on boiling point of liquid )

     When a liquid water phase is present (3 phases will exist , there will be only one degree of freedom),
                                             F = C - P + 2
                                       =>  F = 2 - 3 + 2
                                      =>   F = 1

      here, water will form a separate phase since being polar it is not miscible with the organic solution which is a non-polar one.

     Since, degree of freedom for this case is 1, therefore, selecting one variable (temperature or pressure) fixes the system, with the water and the other component each exerting a partial pressure equal to tis vapor pressure at the boiling point of the mixture. In this case, the distillation temperature will always be less than that of boiling water at the total pressure in question. Consequently, a high boiling organic material may be steam-distilled at temperature below 373K at atmospheric pressure. By using reduced operating pressures, the distillation temperature may be reduced still further, with a consequent economy of steam.

Comparison of the two cases:

     Where there is no liquid water phase present, the steam consumption will be high unless the steam is very highly superheated. With a water phase present, the boiling point of the mixture will be low, and consequently partial pressure of the component will have a low value. Thus, on a molar basis the steam consumption will again be high, although due to the relatively low molecular weight of the steam, the consumption may not be excessive. Steam economy may be effected by using indirect heating of the still, having no liquid water phase present, or by operating under reduced pressure.

Properties Of Solvent Used For Extractive Distillation

The solvent to be used is selected on the basis of :

1) Selectivity

2) Volatility

3) Ease of separation from the top and bottom products

4) Cost of separation

The selectivity is most easily assessed be determining the effect on the relative volatility of the two key components of addition of the solvent. The volatile the solvent, the greater the percentage of solvent in the vapor, and the poorer the separation for a given heat consumption in the boiler.

Extractive Distillation And It's Comparison With Azeotropic Distillation

Exractive Distillation:
   
     Exractive Distillation is carried out for such azeotropic solutions in which relative volatility is very low. In this case continuous distillation of the mixture to give nearly pure products will require high reflux ratios with correspondingly high heat requirements. In addition, it will necessitate a tower of large cross-section containing many rays.

     Basic principle for separation of this type of solutions is to add a substance that will alter the relative volatility of the original constituents, thus permitting separation. The added solvent should be of lower volatility as compared to the components and hence it does not appreciably vaporizes in the fractionation column. This solvent must be continuously fed near the top of the column and it runs down the column as reflux and is present in appreciable concentrations on all the plates.

     Actually, this extractive agent differentially affects the activities (activity coefficients) of the components, and hence alters the relative volatility of the mixture. It is important to note that the solvent must not form an azeotrope with any of the components.

Comparison With Azeotropic Distillation

     Extractive distillation is usually more desirable than azeotropic distillation since no large quantities of solvent have to be vaporized. In addition, a greater choice of added component is possible since the process is not dependent upon the accident of azeotrope formation. It cannot, however, be conveniently carried out in batch operation.

Azeotropic Distillation

     Azeotrpic Distillation is carried out for the separation of component/s from an azeotropic solution. The technique for this kind of separation is to add a third substance to the solution which will forms an azeotrope with one or more components in the mixture, creating a good enough relative volatility difference for the component to be separated. The added component  will be present on most of the plates of the column in appreciable concentration.

Example:
     Let's take an example of an azeotropic solution of ethanol-water. To separate water from ethanol, add a third substance benzene. A termary azeotrope is formed with a boiling point of 338K, that is less than that of the binary azeotrope, 351K. This addition of relative non-polar benzene entrainer serves to volatilize water, a highly polar molecule, to a greater extent than ethanol, a moderately polar molecule and a virtually pure ethanol product may be obtained.

Azeotropic Solution, It’s Role In Distillation and It’s Types

Azeotropic solution is defined as such a solution in which the components to be separated have nearly equal or very close boiling points. In other words, their relative volatility is unity or near to that.

Another way of expressing an azeotropic solution is that in an azeotropic solution, the composition of vapor becomes equal to that of the liquid and no enrichment of vapor occurs. It may be at the start of distillation process or during the distillation operation.

This type of solution cannot be separated by usual distillation method. It requires a special type of distillation called “Azeotropic Distillation” to separate azeotropic solution.

Example:

            If a mixture of ethanol and water is distilled, the concentration of the alcohol steadily increases until it reaches 96 percent by mass, when the composition of the vapor equals that of the liquid, and no further enrichment occurs. This mixture is now called an azeotrop.

In non-azeotropic solutions, during distillation the vapor becomes steadily richer in the more volatile component on successive plates. But, in azeotropic types of mixtures this steady increase in concentration of more volatile component in vapor, either does not takes plate, or it takes place so slowly that an uneconomic number of plates is required.

Types Of Azeotropic Distillation:

      1. Minimum Boiling Azeotropes

      2. Maximum boiling azeotropes

All that depends on “activity coefficient”. Activity coefficient is similar to relative volatility concept, but it is for non-ideal (real) systems. Activity coefficient approaches unity as the liquid concentration approaches unity and the highest values of this coefficient occurs as the concentration approaches zero.

       1. Minimum Boiling Azeotrops:

                                                     In this type of solution, components of the azeotropic solution boils off, at a lower temperature, as compared if they were ideal. Hence called minimum boiling azeotrops.

When the activity coefficient is greater than unity, giving a positive deviation from Roult’s Law , the molecules of the components in the mixture repel each other and exert a higher partial pressure than if their behavior were ideal. This higher partial pressure is the indication of lower solubility of components in each other. This leads to the formation of Minimum boiling azeotrops.

             2. Maximum Boiling Azeotrops:

                                                          In this type of solution, components of the azeotropic solution boils off, at a higher temperature, as compared if they were ideal. Hence called maximum boiling azeotrops.

When the activity coefficient is less than unity, giving a negative deviation from Roult’s Law, the molecules of the components in the mixture attract each other and exert a lower partial pressure than if their behavior were ideal. This lower partial pressure is the indication of higher solubility of components in each other. This leads to the formation of Minimum boiling azeotrops.

Reflux Ratio, Importance And It's Effect On Distillation Operation

Reflux Ratio: 
                      It is the ratio of liquid flow rate (L) from the reflux drum to the flow rate of distillate (D) (also called top product).

 R = L/D

Importance Of Reflux Ratio:
                                               Any change in reflux ratio will modify the slope of operation line, as can be seen in figure, this will alter the number of plates required for a given separation to be achieved.

     If R is known the top line is most easily drawn by joining point A (mole fraction of distillate) to B. If no product is withdrawn from the still, that is D=0 (R is infinite), then the column is said to operate under conditions of total reflux and, as seen from the figure above, and coincides with the line x=y. 

     

     If reflux ratio is reduced (or if distillate rate D is increased compared to L) the slope of the operating line is reduced and more stages are required to pass from xf to xd or to achieve desired concentration (i.e, from point K to A). Furthur, reduction in R will eventually bring the operating line to AE, where an infinite number of stages is needed to pass from xd to xf. This arises from the fact that under these conditions the steps become very close together at liquid compositions near to sf, and no enrichment occurs from the feed plate to the plate above. These conditions are known as minimum reflux. Any small increase in R beyond this reflux will give a workable system, although a large number of plates will be required. 

     Two important deductions may be made.

1) The minimum number of plates is required for a given separation at conditions of total reflux.

2) There is a minimum reflux ratio, at and below which it is impossible to achieve desired enrichment, no matter many plates are used.

Effect of Reflux Ratio On Distillation Operation:

                                                                       Distillation process is done to achieve a specific level of enrichment. To achieve this enrichment level, one must specify a reflux ratio. Decreasing that reflux ratio (i.e, increasing D), has an advantage that the duty of condenser and reboiler decreases, since the load in distillation column decreases (more mass goes out as D increases) . In other sense, the operating cost of distillation column decreases. However, it would be at expense of less enrichment achieved than required.

     Increasing reflux ratio would act in reverse i.e, it would have a disadvantage of increase in duty of condenser and reboiler as load increases. This increases the operation cost of distillation column, however, more enrichment is achieved.

Vapor (ammonia) Absorption Refrigeration System and It's Difference With Vapor Compression System

     This article describes what the absorption refrigeration system is, parts of the this system, how it works and it's difference with Vapor Compression Refrigeration System.

• What is Absorption Refrigeration System?

       The vapor absorption refrigeration system comprises of all the processes in the vapor compression refrigeration system like compression, condensation, expansion and evaporation. In the vapor absorption system the refrigerant used is ammonia, water or lithium bromide. The refrigerant gets condensed in the condenser and it gets evaporated in the evaporator. The refrigerant produces cooling effect in the evaporator and releases the heat to the atmosphere via the condenser.
• Parts of Simple Absorption System and How it Works?
• • 1) Condenser: Just like in the traditional condenser of the vapor compression cycle, the refrigerant enters the condenser at high pressure and temperature and gets condensed. The condenser is of water cooled type.

    2) Expansion valve or restriction: When the refrigerant passes through the expansion valve. Due to this throttling of valve, the pressure reduces and hence the boiling point reduces as well and this way ammonia partly turns to vapor state This refrigerant (ammonia in this case) then enters the evaporator.

    3) Evaporator: The refrigerant at very low pressure and temperature enters the evaporator and produces the cooling effect. In the vapor compression cycle this refrigerant is sucked by the compressor, but in the vapor absorption cycle, this refrigerant flows to the absorber that acts as the suction part of the refrigeration cycle.

    4) Absorber: The absorber is a sort of vessel consisting of water that acts as the absorbent, and the previous absorbed refrigerant. Thus the absorber consists of the weak solution of the refrigerant (ammonia in this case) and absorbent (water in this case). When ammonia from the evaporator enters the absorber, it is absorbed by the absorbent creating a vacuum above the absorbed solution that create suction hence producing more flow of refrigerant (ammonia) from the evaporator to the absorber. At high temperature water absorbs lesser ammonia, hence it needs cooling by the external coolant (e.g water) to increase it ammonia absorption capacity.
  • 
    5) Pump: When the absorbent absorbs the refrigerant strong solution of refrigerant-absorbent (ammonia-water) is formed. This solution is pumped by the pump at high pressure to the generator. Thus pump increases the pressure of the solution to about 10bar.
  • 6) Generator: The refrigerant-ammonia solution in the generator is heated by the external source of heat because desorption occurs when temperature is increased. This is can be done using steam, hot water or any other suitable source. Due to heating the temperature of the solution increases. The refrigerant in the solution gets desorbed and vaporized, and it leaves the solution at high pressure and higher temperature. The high pressure and the high temperature refrigerant then enters the condenser, where it is cooled by the coolant, and it then enters the expansion valve and then finally into the evaporator where it produces the cooling effect. This refrigerant is then again absorbed by the weak solution in the absorber.
         When the vaporized refrigerant leaves the generator weak solution is left in it. This solution enters the pressure reducing valve and then back to the absorber, where it is ready to absorb fresh refrigerant. In this way, the refrigerant keeps on repeating the cycle.
         The pressure of the refrigerant is increased in the generator, hence it is considered to be equivalent to the compression part of the compressor.
  •      This part of the article describes how the absorption refrigeration system works. The absorption refrigeration system comprises of condenser, expansion valve, evaporator, absorber, pump and generator. The refrigerant leaving the evaporator enter the absorber, where it is absorbed by the absorbent. The strong solution of refrigerant-absorber enters the generator with the help of the pump. The refrigerant then enters the condenser while the remaining weak solution enters back to the absorber and the cycle is repeated.
  • Driving Force for Vapor Compression Cycle:
  •      The initial flow of the refrigerant from the evaporator to the absorber occurs because the vapor pressure of the refrigerant-absorbent in the absorber is lower than the vapor pressure of the refrigerant in the evaporator. 

         When the refrigerant entering in the absorber is absorbed by the absorbent its volume decreases, thus the compression of the refrigerant occurs. Thus absorber acts as the suction part of the compressor. The heat of absorption is also released in the absorber, which is removed by the external coolant.

    Difference b/w Vapor Compression And Vapor Absorption Refrigeration System:
  • 1)     The major difference between the two systems is the method of the suction and compression of the refrigerant in the refrigeration cycle. In the vapor compression system, the compressor sucks the refrigerant from evaporator and compresses it to the high pressure. The compressor also enables the flow of the refrigerant through the whole refrigeration cycle. In the vapor absorption cycle, the process of suction and compression are carried out by two different devices called as the absorber and the generator. Thus the absorber and the generator replace the compressor in the vapor absorption cycle. The absorbent enables the flow of the refrigerant from the absorber to the generator by absorbing it.
     2)    Another major difference between the vapor compression and vapor absorption cycle is the method in which the energy input is given to the system. In the vapor compression system the energy input is given in the form of the mechanical work from the electric motor run by the electricity. In the vapor absorption system the energy input is given in the form of the heat. This heat can be from the excess steam from the process or the hot water. The heat can also be created by other sources like natural gas, kerosene, heater etc. though these sources are used only in the small systems.

Fractionation Vs Distillation

     Distillation and Fractionation often used interchangeably. The goal of these two is the same i.e, separation of the components of a liquid mixture.

     Distillation is the technique of separation of components of a liquid mixture based on difference in their boiling points. Whereas, fractionation is one step ahead of distillation. Fractionation means enrichment of a particular component, which is obviously through distillation.

Differential Distillation, Flash / Equilibruim Distillation and Rectification Difference

Differential Distillation:
                                        It is a batch and a single stage distillation that starts with a still pot, initially full, heated at a constant rate. In this process the vapor form on boiling the liquid is removed at once from the system. Since this vapor is richer in the more volatile component than the liquid, it follows that the liquid remaining becomes steadily weaker in this component, with the result that the composition of the product progressively becomes weaker in more volatile component (i.e, impurity starts dominating).

     The special distinction of differential distillation is that whilst the vapor formed over a short period is in equilibrium with the liquid, the total vapor formed is not in equilibrium with the residual liquid. Then, at the end of the process the liquid that remains (and is concentrated in less volatile component) is removed as bottom product.

Flash / Equilibrium Distillation:
                                                    This is type of distillation is the continuous process. It's main distinction is that the definite fraction of the liquid feed is vaporized in such manner that the vapor must be "in equilibrium" with the residual liquid. Hence called "equilibrium distillation".

     The feed is usually pumped through a fired heater but is still a liquid and not in vapor phase. This feed enters the still through a valve where the pressure is reduced. This sudden reduction in pressure reduces the boiling point of the feed. The hot feed becomes vaporized at once as it passes through the valve (hence also named "flash distillation") and enters in the still. The still is essentially a separator in which the partly liquid and vapor produced by the reduction in pressure have sufficient time to reach equilibrium. The vapor is removed from the top of the separator and is then usually condensed, while the liquid leaves from the bottom.

Rectification:
                      In the two process above, the vapor produced in still is in equilibrium with the liquid remaining, but only a small increase in concentration of more volatile component is achieved. Major distinction b/w rectification and the above two distillation processes lies here, that this process achieves comparatively more 

Melting Point, Freezing Point and Boiling Point

Melting Point:

     Pure, crystalline solids have a characteristic melting point, the temperature at which the solid melts to become a liquid. The transition between the solid and the liquid is so sharp for small samples of a pure substance that melting points can be measured to 0.1^oC. The melting point of solid oxygen, for example, is -218.4^oC.

Freezing Point:

         Liquids have a characteristic temperature at which they turn into solids, known as their freezing point. In theory, the melting point of a solid should be the same as the freezing point of the liquid. In practice, small differences between these quantities can be observed

      It is difficult, if not impossible, to heat a solid above its melting point because the heat that enters the solid at its melting point is used to convert the solid into a liquid. It is possible, however, to cool some liquids to temperatures below their freezing points without forming a solid. When this is done, the liquid is said to be Supercooled.

Boiling Point:

     The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.

     OR

     When a liquid is heated, it eventually reaches a temperature at which the vapor pressure is large enough that bubbles form inside the body of the liquid. This temperature is called the boiling point. Once the liquid starts to boil, the temperature remains constant until all of the liquid has been converted to a gas.

     

     See Effect of pressure on boiling point of liquid

Understanding Supercool Liquid

     If you cool a liquid at it's freezing point, the liquid phase will change to solid phase. But, it is possible to cool a liquid below it's freezing point without changing it to solid phase. This cooled liquid is now called the Super Cooled liquid.

     An example of a supercooled liquid can be made by heating solid sodium acetate trihydrate (NaCH₃CO₂ 3 H₂O). When this solid melts, the sodium acetate dissolves in the water that was trapped in the crystal to form a solution. When the solution cools to room temperature, it should solidify. But it often doesn't. If a small crystal of sodium acetate trihydrate is added to the liquid, however, the contents of the flask solidify within seconds.

     A liquid can become supercooled because the particles in a solid are packed in a regular structure that is characteristic of that particular substance. Some of these solids form very easily; others do not. Some need a particle of dust, or a seed crystal, to act as a site on which the crystal can grow. In order to form crystals of sodium acetate trihydrate, Na⁺ ions, CH₃CO₂^- ions, and water molecules must come together in the proper orientation. It is difficult for these particles to organize themselves, but a seed crystal can provide the framework on which the proper arrangement of ions and water molecules can grow.

Effect of Pressure On Boiling Point Of Liquid

Boiling point of a liquid depends strongly and directly on the Pressure above the liquid. If the pressure above the liquid is increased, then the boiling point of that substance increases as well and you have to apply more heat to vaporize that liquid.

Let's take and example of water. Water at ground level (1 atm) boils at 100 degree C. If you take this above ground (let's say on a mountain), where the pressure is low, the boiling point would decrease below 100 degree C. Going above in the atmosphere decreases pressure and going down the surface increases the surrounding pressure.

Difference Between Latent Heat And Sensible Heat

     Sensible heat is the heat provided to a fluid that changes (increases) it's temperature, but does not effects the phase. 

     

     Let's take and example of water. If you start heating water at atmospheric pressure (1 atm) and at 25 degree C, the temperature of water starts increasing, but the water will remain in liquid phase upto 100 degree C. This heat supplied would be called as sensible heat.

     Latent Heat is the heat provided to the fluid that changes it's phase and it does not changes the temperature of the substance. It is worth to note that phases change always take place at "constant" temperature.

     Let's take same example as above. Now, water is at 100 degree C and at 1 atm pressure and you continue to heat it. At that instant, after absorbing some heat the water changes from liquid phase (at 100 C) to vapor phase (at 100 C). Look, the temperature does not change for a phase change. This "some heat" provided is the latent heat for water.

     Every substance has it's own value of latent and sensible heat.

Effect Of Temperature On Relative Volatility

Relative Volatility vary somewhat with temperature, it remains remarkably steady for many systems, and a few values to illustrate this point are given in the following table:

It may be seen that relative volatility decreases as the temperature increases, so that it is sometimes worth while reducing the boiling point by operating at reduced pressure.

Understanding Vapor Pressure and Measure it

     Vapor Pressure of a pure liquid is the pressure exerted by the vapors of that liquid when it is heated in a closed container and allowed the liquid and its vapors to reach equilibrium, at that temperature.

     It must be noted that the liquid must not contain any impurity along with it to measure it's vapor pressure.
By the help of a pressure guage in that closed container, one can measure vapor pressure by above defined method.

Roult's Law and Henry's Law

For an ideal mixture, the partial pressure is related to the concentration in the liquid phase by Roult's law which may be written as:

Pi = Pi* Xi

where,  

          Pi = partial pressure of component i in gas/vapor mixture

          Pi* = pressure of pure component i (vapor pressure)

          Xi = mole fraction of component i in liquid mixture

     It must be noted that this relation is usually found to be true only for high values of  Xi as compared to mole fraction of other components in the liquid mixture.

    For low values of Xi, a linear relation between Pi and Xi again exists, although the proportionality factor is Henry's constant (H') and not the vapor pressure of the pure material. i,e

Pi = H' Xi

     This is the Henry's Law.

Concept of Partial Pressure / Dalton's Law of Partial Pressure

     Partial pressure is the pressure of component A, which it would have if it alone occupied the volume at the same temperature as now present in that "ideal" gas mixture. Greater the concentration of vapors of that component (A) in mixture, greater will be it's partial pressure.
   
     Total pressure of this gaseous mixture is the sum of partial pressure of all components. This is what called Dalton's Law of Partial Pressure. This is because in and ideal mixture, there are no forces of attraction or repulsion b/w molecules. Mathematically

     Since in an ideal gas or vapor the partial pressure is proportional to the mole fraction of the component in vapor phase, therefore 

     The partial pressure of a gas dissolved in a liquid is the partial pressure of that gas which would be generated in a gas phase in equilibrium with the liquid at the same temperature. The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. Gases will always flow from a region of higher partial pressure to one of lower pressure; the larger this difference, the faster the flow. Gases dissolve, diffuse, and react according to their partial pressures, and not necessarily according to their concentrations in a gas mixture.

    

Separation process of Components Inside Distillation column / What Happens Inside Distillation Column

Phenomenon of partial vaporization and partial condensation brings about enrichment in the column.
If a mixture of benzene and toluene is heated in a vessel, closed in such a way that the pressure remains atmospheric and no material can escape and the mole fraction of  the more volatile component in the liquid, that is benzene, is plotted as abscissa, and the temperature at which the mixture boils as ordinate, then the boiling curve is obtained as shown by ABCJ in the following figure. The corresponding dew point curve ADEJ shows the temperature at which a vapor of composition y starts to condense.

     If a mixture of composition x2 is at a temperature T3 below its boiling point, T2, as shown by point G on the diagram, then on heating at constant pressure the following changes will occur:

a) When the temperature reaches T2, the liquid will boil, as shown by point B, and some vapor of composition y2, shown by point E, is formed.

b) On further heating the composition of the liquid will change because of the loss of the more volatile component to the vapor and the boiling point will therefore rise to some temperature T'. At this temperature the liquid will have a composition represented by point L, and the vapor a composition represented by point N. Since no material is lost from the system, there will be a change in the proportion of liquid to vapor, where the ratio is

Liquid / Vapor = MN / ML

c) On further heating to a temperature T1, all of the liquid is vaporized to give vapor D of the same composition y1 as the original liquid.

     It may be seen that partial vaporization of the liquid gives a vapor richer in the more volatile component than the liquid. If the vapor initially formed, as for instance at point E, is at once removed by condensation, then a liquid of composition x3 is obtained represented by point C. The step BEC may be regarded as representing an ideal stage, since the liquid passes from composition x2 to a liquid of composition x3, which represents a greater enrichment in the more volatile component than can be obtained by any other single stage of vaporization.

     Starting with superheated vapor represented by point H, on cooling to D condensation commences, and the first drop of liquid has a composition K. Further cooling to T' gives liquid L and vapor N. Thus, partial condensation brings about enrichment of the vapor in the more volatile component in the same manner as partial vaporization. The industrial distillation column is, in essence, a series of units in which these two processes of partial vaporization and partial condensation are effected simultaneously.

Types Of Refrigerants / Refrigerant Fluids

TYPES OF REFRIGERANTS

Following type of refrigerants are used for refrigeration process:

1) halocarbons or freons.
2) azeotropic refrigerants.
3) zeotropic refrigerants.
4) inorganic refrigerants like carbon dioxide, ammonia, water and air.
5) hydrocarbon refrigerants 

     Halocarbons are generally synthetically produced. Depending on whether they include chemical elements hydrogen (H), carbon (C), chlorine (Cl) and florine (F) they are named after as follows:

CFCs (Chlorofluorocarbons): R11, R12, R113, R114, R115
HCFCs (Hydrochlorofluorocarbons): R22, R123
HFCs (Hydrofluorocarbons): R134a, R404a, R407C, R410a

     Amoung these types, the one containing chlorine (CFCs) are not used because of their potential harm to ozone layer that protect life on earth by blocking harmful radiations coming from the sun.

Relative Volatility And It's Significance in Separation Process

     Relative Volatility of a liquid is the ratio of volatility of the two components.

                 Relative Volatility = volatility of component A / volatility of component B

     Where, "volatility" of a component A is the ratio of partial pressure (P_A) of that component in the vapor mixture to mole fraction of that component in liquid mixture (x_A). Therefore,

Relative volatility = P_Ax_B / P_Bx_B

     Using Dalton's Law of Partial pressure (click here), we can write it in the vapor phase mole fraction and liquid phase mole fraction. Subsitute Py_A for P_A and Py_B for P_B :

Relative Volatility = y_Ax_B/y_Bx_A

     From the definition of the volatility of a component, it is seen that for an ideal system the volatility is numerically equal to the vapor pressure of the pure component (P_A*), i.e,

Relative Volatility = P_A*/ P_B*

Significance of Volatility in Separation Process:

                                                                      For separation to be achieved, relative volatility must not equal 1. Considering the more volatile component (in the numenator), as relative volatility increases above unity, y increases and the separation becomes much easier.

Distillation Definition

     Distillation process is the separation of components of a liquid mixture based on difference in their boiling points or relative volatility (click here). This process is usually achieved in equipment name "distillation column" by application of heat (using steam or hot oil).

Vapour Compression Cycle / How Household Refrigerator Works

Vapor Compression Cycle:

     This is the thermodynamic cycle that is usually used in household refrigerators and air conditioning units. The basic theme is that you have to achieve cooling below 0 Celcius. If you are to make cooling above and near to 0 celcius, then you can make use of water evaporation (as water cooling fans are used) since evaporation causes cooling.

     Vapor Compression cycle starts with a refrigerant (gas) which is

1)  Compressed to increase it's pressure and hence temperature (since P is directly proportional to T) when passed through "compressor". This process takes place at constant entropy. The gas turns to superheated vapors.

2)  This superheated vapors then passes through "condenser". The condenser removes the superheat and then removes the latent heat as well to bring it down to liquid form. The refrigerant is now at high pressure but at low temperature.

3)  The refrigerant in condition of low temperature and high pressure is passed through a throttling valve (or through a nozzle) that causes a sudden pressure drop. This drop in pressure results in flash evaporation (since decreases in pressure decreases the boiling point) and hence a vapor-liquid mixture is formed.

4)  This vapor-liquid mixture at very low temperature and pressure is now ready to transfer heat of the refrigerator. For that purpose it is passed through that chamber in which you have to bring heat transfer (here called "Evaporator"). After gaining heat of the warm air, the refrigerant (now at high temperature and at same low pressure) boils up and turn into vapor (gas) state. This vapor re-enters in compressor and the process continues.

     All that just happened is the heat is transfer from low temperature area (the evaporator) to high temperature area (condenser), which is against the natural heat transfer gradient. Therefore, one have to apply energy for this transfer (the electrical energy transfer to the compressor). This conforms with the second law of thermodynamics.   Click here for refrigerants and it's types.

Lewis Number

     Lewis number (Le) is a dimensionless number defined as the ratio of thermal diffusivity to mass diffusivity. It is used to characterize fluid flows where there is simultaneous heat and mass transfer by convection.

It is defined as:

where  is the thermal diffusivity and  is the mass diffusivity.

     The Lewis number can also be expressed in terms of the Schmidt number and the Prandtl number :

.

Schmidt Number

     Schmidt number (Sc) is a dimensionless number defined as the ratio of momentum diffusivity (viscocity) and mass diffusivity, and is used to characterize fluid flows in which there are simultaneous momentum and mass diffusion convection processes.

     Schmidt number is the ratio of the shear component for diffusivity viscosity/density to the diffusivity for mass transfer D. It physically relates the relative thickness of the hydrodynamic layer and mass-transfer boundary layer.

It is defined as:

where:

•  is the kinematic viscosity or (/) in units of (m²/s)
•  is the mass diffusivity (m²/s).
•  is the dynamic viscocity of the fluid (Pa·s or N·s/m² or kg/m·s)
•  is the density of the fluid (kg/m³).

The heat transfer analog of the Schmidt number is the Prandtl number.