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