Continuous Distillation Column

CHAPTER ONE 1.0 INTRODUCTION Distillation columns are one of the main units used for separation processes in industry. The basic theory behind them is very simple and relies on a mixture being made of components of different boiling points. As a feed enters the column and some fractions may vaporise and rise up the tower. The vapour components will condense and leave the column at different levels as the temperature decreases up the tower. Based on a binary mixture, the more volatile component will come out at the top of the tower, and the less volatile component will leave at the bottom as a liquid. The less volatile component will have a higher boiling point so it will be a liquid in the column. Continuous distillation is used widely in the chemical process industries where large quantities of liquids have to be distilled. Such industries are the natural gas processing, petrochemical production, coal tar processing, liquor production, liquefied air separation, hydrocarbon solvents production and similar industries, but it finds its widest application in petroleum refineries. In such refineries, the crude oil feedstock is a very complex multicomponent mixture that must be separated and yields of pure chemical compounds are not expected, only groups of compounds within a relatively small range of boiling points, which are called fractions. These fractions are the origin of the term fractional distillation or fractionation. It is often not worthwhile separating the components in these fractions any further based on product requirements and economics. In continuous distillation, feed constantly is charged to the column at a point between the top and bottom trays. The section above the feed point rectifies the more volatile component while the column section below the feed point strips out the more volatile component from the less volatile liquid.(Wikipedia, 2015) .

1.1

AIMS AND OBJECTIVES

  

To demonstrate the use of a continuous distillation column To investigate the steady state distillation of a binary mixture To generate the equilibrium data for a binary mixture and obtain the refractive index data for both overhead and bottom products.

CHAPTER TWO 2.0 LITERATURE SURVEY 2.1 Distillation: Description And Brief History Distillation industry for This is done miscible and components.

process is widely applied in the engineering mass transfer and separation operations. by vaporization of a liquid mixture of volatile substances into individual

Today it is particularly well suited for high purity separations since any degree of separation can be obtained with a fixed energy consumption by increasing the number of equilibrium stages.

2.2

FUNDAMENTALS

2.2.1 The Equilibrium Stage Concept The equilibrium (theoretical) stage concept (see Figure 2.1) is central in distillation. Here we assume vapour-liquid equilibrium (VLE) on each stage and that the liquid is sent to the stage below and the vapour to the stage above. For some trayed columns this may be a reasonable description of the actual physics, but it is certainly not for a packed column. Nevertheless, it is established that calculations based on the equilibrium stage concept (with the number of stages adjusted appropriately) fits data from most real columns very well, even packed columns.

One may refine the equilibrium stage concept, e.g. by introducing back mixing or a Murphee efficiency factor for the equilibrium, but these “fixes” have often relatively little theoretical justification, and are not used in this article. For practical calculations, the critical step is usually not the modelling of the stages, but to obtain a good description of the VLE. In this area there has been significant advances in the last 25 years, especially after the introduction of equations of state for VLE prediction. However, here we will use simpler VLE models (constant relative volatility) which apply to relatively ideal mixtures.

2.2.2 RAOULT’S LAW In a solution of two miscible liquids (A & B) the partial pressure of component “A” (PA) in the solution equals the partial pressure of pure “A” (PAo) times its mole fraction (XA) Partial Pressure of A in solution PA = (PAo) x (XA)

(2.1)

Partial Pressure of B in solution PB = (PBo) x (XB)

(2.2)

When the total pressure (sum of the partial pressures) is equal to or greater than the applied pressure, normally Atmospheric Pressure (760 mm Hg), the solution boils Ptotal = PA

+

PB

=

PAo.XA

+

PBo.XB

(2.3)

If the sum of the two partial pressures of the two compounds in a mixture is less than the applied pressure, the mixture will not boil. The solution must be heated until the combined vapor pressure equals the applied pressure Not all mixtures obey Raoult’s law. Some components that have high solubility with each other form azeotrope. An azeotrope is a mixture that has either a higher or lower boiling point than the boiling point of any of the pure components. This means that when a mixture reaches an azeotrope, such as ethanol and water at 95.6 % water, it behaves as a pure substance.

2.2.3

THE BOILING POINT DIAGRAM

The boiling point diagram shows how the equilibrium compositions of the components in a liquid mixture vary with temperature at a fixed pressure. Consider an example of a liquid mixture containing 2 components (A and B) - a binary mixture. This has the following boiling point diagram. The boiling point of A is that at which the mole fraction of A is 1. The boiling point of B is that at which the mole fraction

of A is 0. In this example, A is the more volatile component and therefore has a lower boiling point

than B. The upper curve in the diagram is called the dew-point curve while the lower one is called the bubble-point curve. The dew-point is the temperature at which the saturated vapour starts to condense. The bubble-point is the temperature at which the liquid starts to boil. The region above the dew-point curve shows the equilibrium composition of the superheated vapour while the region below the bubble-point curve shows the equilibrium composition of the subcooled liquid.

Figure 2.2 Boiling Point Diagram

2.2.4 VAPOUR LIQUID EQUILIBRIA Distillation columns are designed based on the boiling point properties of the components in the mixtures being separated. Thus the sizes, particularly the height, of distillation columns are determined by the vapour liquid equilibrium (VLE) data for the mixtures. CurvesConstant pressure VLE data is obtained from boiling point diagrams. VLE data of binary mixtures is often presented as a plot, as shown in the figure on the right. The VLE plot expresses the bubblepoint and the dew-point of a binary mixture at constant pressure. The curved line is called the equilibrium line and describes the compositions of

the liquid and vapour in equilibrium at some fixed pressure.

FIGURE 2.3 VLE Plot For a Binary Mixture (Ideal System) This particular VLE plot shows a binary mixture that has a uniform vapour-liquid equilibrium that is relatively easy to separate. There are other nonideal systems such as the azeotropic systems.

2.2.5 RELATIVE VOLATILITY Separations of components by distillation process depends on the differences in volatilities of components that make up the solution to be distilled. The relative volatility indicates the ease or difficulty of using distillation to separate the more volatile components from the less volatile components in a mixture. The greater difference in their volatility, the better is separation by heating (distillation). Conversely if their volatility differ only slightly, the separation by heating becomes difficult. For a Binary mixture, the relative volatility of compound A to Compound B is an indication of how volatile compound A is, compared to compound B

(2.4) Where; Subscrips ‘A’ and ‘B’ indicates properties of compound A and compound B respectively X and y represents the mole fraction of in liquid and vapour phases respectively

2.3

TYPES OF DISTILLATION

The various types of distillation are: Simple Distillation Molecular Distillation Fractional Distillation Vacuum Distillation Extractive Distillation Azeotropic Distillation Flash Distillation Batch Distillation Continuous Distillation

2.3.1 SIMPLE DISTILLATION This is a Single Vaporization/Condensation cycle of a mixture that produces a distillate that is always impure Therefore, it is impossible to completely separate the components in a mixture with Simple Distillation

Relatively pure substances can be obtained from a mixture with Simple Distillation if the boiling points of the components differ by a large amount (>100oC) If a small increment of the initial distillate is separated and redistilled and this process is repeated many times, effectively producing multiple sequential Vaporization/ Condensation Cycles, an increasingly pure solution can be attained.

FIGURE 2.4 SCHEMATIC DIAGRAM OF SIMPLE DISTILLATION

2.3.2 MOLECULAR DISTILLATION A special application of the simple distillation is molecular distillation, known also as evaporative distillation or short path distillation. The mean free path of a molecule is defined as the average distance through which a molecule can move without coming into collision with another. For material that are regarded as non volatile under ordinary conditions of temperature and pressure are generally removed by this by increasing the mean free path. Molecular distillation process is characterized by Very high vacuum, and the evaporating surface must be close to the condensing surface.

The liquid area is usually large to avoid boiling and evolution of the vapors is from surface only. Some applications of molecular distillation are in the Purification of oils and Separation of vitamins.

2.3.3 FRACTIONAL DISTILLATION As the solution to be purified is heated, its vapours rise to the fractionating column. The Fractionating Column, of which there are many types containing a variety of packing materials, subjects the mixture to many Vaporization/Condensation Cycles as the material moves up the column toward the Distillation Head, which is attached to the Condenser. With each cycle within the column, the composition of the vapor is progressively enriched in the lower boiling liquid. This process continues until most of the lower boiling compound is removed from the original mixture and condensed in the receiving flask. Each component with different boiling points would be separated at each different stage and condensed to obtain a liquid. At the bottom there would be highest melting point component’s residue. Example- Fractional distillation of petroleum.

FIGURE 2.4 SCHEMATIC DIAGRAM OF FRACTIONAL DISTILLATION

2.3.4 VACUUM DISTILLATION Some compounds have very high boiling points. To boil such compounds, it is often better to lower the pressure at which such compounds are boiled instead of increasing the temperature. Once the pressure is lowered to the vapour pressure of the compound (at the given temperature), boiling and the rest of the distillation process can commence. This technique is known as vacuum distillation. Example-Dimethyl sulfoxide usually boils at 189 °C. Under a vacuum, it distilles off into the receiver at only 70 °C

2.3.5 EXTRACTIVE DISTILLATION Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture. The method of extractive distillation uses a separation solvent, which is generally nonvolatile, has a high boiling point and is miscible with the mixture, but doesn't form an azeotropic mixture. The solvent interacts differently with the components of the mixture thereby causing their relative volatilities to change. The solvent must alter the relative volatility by a wide enough margin for a successful result. The quantity, cost and availability of the solvent should be considered. The solvent should be easily separable from the bottom product, and should not react chemically with the components or the mixture,

or cause corrosion in the equipment. A classic example is of aniline as suitable solvent.

2.3.6 AZEOTROPIC DISTILLATION An azeotrope is a mixture of two or more liquids (chemicals) in such a ratio that its composition cannot be changed by simple distillation. This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture. Because their composition is unchanged by distillation, azeotropes are also called constant boiling mixtures. Azeotropic distillation usually refers to the specific technique of adding another component to generate a new, lower-boiling azeotrope that is heterogeneous (e.g. producing two, immiscible liquid phases), such as the example below with the addition of benzene to water and ethanol. In actual fact, this practice of adding an entrainer which forms a separate phase is a specific sub-set of (industrial) azeotropic distillation methods, or combination thereof. In some senses, adding an entrainer is similar to extractive distillation. Example - distillation of ethanol/water, using normal distillation techniques, ethanol can only be purified to approximately 96% . Some uses require a higher percentage of alcohol, eg. when used as a gasoline additive.

2.3.7 FLASH DISTILLATION The Flash can be seen as a distillation with only one equilibrium stage. The operation stops, when the liquid and vapour streams reach the equilibrium compositions defined by temperature and pressure, and the two streams can easily be separated.

FIGURE 2.5 FLASH DISTILLATION

The incoming liquid is first heated and pressurised, before being fed into the drum. Due to the large pressure drop, the liquid evaporates very quickly(hence "flash"). Usually, flash distillation cannot achieve a large degree of separation, and is therefore employed as an auxiliary operation to prepare streams for further processing. In some cases however, like the desalination of sea water, complete separation can be achieved. If only two components are present in the feed, the ash is called binary, while more than two components in the feed define a multicomponent flash.

2.3.8 BATCH DISTILLATION Batch distillation refers to the use of distillation in batches, meaning that a mixture is distilled to separate it into its component fractions before the distillation still is again charged with more mixture and the process is repeated

Two liquids(A & B) are heated in a distillation tower. The ratio between A and B in the vapour will be different from the ratio in the liquid. Now A will be more in the vapour phase and would be separated and will be obtained back on condensation. Also B be will be more in the retaining liquid. Finally component A is distilled off and the remaining component is enriched in B. There is at least one volatile distillate fraction, which has boiled and been separately captured as a vapour condensed to a liquid. There is always a residue, which is the least volatile residue that has not been separately captured as a condensed vapour. In batch distillation, the composition of the source material, the vapours of the distilling compounds and the distillate change during the distillation.

FIGURE 2.6 BATCH DISTILLATION

2.3.9 CONTINUOUS DISTILLATION This is a form of distillation, is an ongoing separation in which a mixture is continuously (without interruption) fed into the process and separated fractions are removed continuously as output streams as time passes during the operation.

A distillation produces at least two output fractions. These fractions include at least one volatile distillate fraction, which has boiled and been separately captured as a vapor condensed to a liquid, and practically always a bottoms (or residuum) fraction, which is the least volatile residue that has not been separately captured as a condensed vapor. In a continuous distillation, each of the fraction streams is taken simultaneously throughout operation; therefore, a separate exit point is needed for each fraction. In practice when there are multiple distillate fractions, each of the distillate exit points are located at different heights on a fractionating column. The bottoms fraction can be taken from the bottom of the distillation column or unit, but is often taken from a reboiler connected to the bottom of the column.

FIGURE 2.7 CONTINUOUS DISTILLATION

In a continuous distillation, the system is kept in a steady state or approximate steady state. Steady state means that quantities related to the process do not change as time passes during operation. Such constant quantities include feed input rate, output stream rates, heating and cooling rates, reflux ratio, and temperatures, pressures, and compositions at every point (location). Unless the process is disturbed due to changes in feed, heating, ambient temperature, or condensing, steady state is normally maintained. Since a continuous distillation unit is fed constantly with a feed mixture and not filled all at once like a batch distillation, a continuous distillation unit does not need a sizable distillation pot, vessel, or reservoir for a batch fill. Instead, the mixture can be fed directly into the column, where the actual separation occurs. The height of the feed point along the column can vary on the situation and is designed so as to provide optimal result.

2.4

CONTINUOUS DISTILLATION COLUMN TYPES The continuous distillation column can be operated as packed or plate type.

2.4.1 PACKED COLUMN Packed columns are used for distillation, gas absorption and liquid-liquid extraction. The gas-liquid contact in a packed column is continuous, not stage-wise, as in a plate column. The liquid flows down in the column over a packing surface and the vapor (or the gas) moves countercurrently, up the column. The performance of a packed column is very dependent on the maintenance of good liquid and gas distribution through the packed bed, and this is an important consideration in packed column design Packings are passive devices that are designed to increase the interfacial area for vapour-liquid

contact. The following pictures show 3 different types of packings.

FIGURE 2.8 SOME TYPES OF PACKINGS USED IN DISTILLATION COLUMN These are some advantages of packed columns over plate columns. 1. For corrosive liquids a packed column will usually be cheaper than the equivalent plate column. 2. The liquid hold-up is lower in a packed column than in a plate column. This can be important when the hold-up of toxic or flammable liquids must be kept as small as possible for safety reasons. 3. Packed columns are more suitable for handling foaming systems. 4. The pressure drop can be lower in a packed column than the equivalent plate column. The packing types can be divided into two broad classes: structured and random packings. Earlier the random packings were more commonly used in the industry (Raschig-, Pall- and HyPack rings, and Berl- and Intallox saddles).

2.4.2 PLATE COLUMN Plate contractors/ towers are vertical cylindrical columns in which a vertical stack of trays or plates are installed across the column height as shown in Figure 7.1. The liquid enters at the top of the column and flows across the tray and then through a downcomer (cross-flow mode) to the next tray below.

The gas/vapor from the lower tray flows in the upward direction through the opening/holes in the tray to form a gas-liquid dispersion. In this way, the mass transfer between the phases (gas/vaporliquid) takes place across the tray and through the column in a stage-wise manner.

FIGURE 2.9 A typical tray in a distillation column. A is the tray itself, B are the holes in the tray that allow the vapor to pass through the tray, C and D are the tubes that allow liquid to pass from one tray to another. As the vapor moves up the column it gets progressively cooler which allows some of the mixture to condense and further concentrates the vapor with the lightest component(s).

These are some advantages of plate columns over packed columns; 1. Plate columns can be designed to handle a wider range of liquid and gas flow rates than packed columns. 2. The efficiency of a plate can be predicted more accurately than the equivalent terms of packings (HETP or HTU). 3. Packed columns are not suitable for very low liquid flow rates, in such a case, the plate column is the convenient choice, as they can easily handle wide variations in flow rates. 4. They are lighter in weight. It is easier and cheaper to install. 5. Maintenance cost is reduced due to the ease of cleaning.

2.5

BASIC DISTILLATION EQUIPMENT AND OPERATION

Distillation columns are made up of several components, each of which is used either to tranfer heat energy or enhance materail transfer. A typical distillation unit contains several major components: a vertical shell where the separation of liquid components is carried out column internals such as trays/plates and/or packings which are used to enhance component separations a reboiler to provide the necessary vaporisation for the distillation process a condenser to cool and condense the vapour leaving the top of the column a reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column

The vertical shell houses the column internals and together withthe condenser and reboiler, constitute a distillation column (see figure 2.7) The liquid mixture that is to be processed is known as the feed and this is introduced usually somewhere near the middle of the column to a tray known as the feed tray. The feed tray divides the column into a top (enriching or rectification) section and a bottom (stripping) section. The feed flows down the column where it is collected at the bottom in the reboiler.

Heat is supplied to the reboiler to generate vapor. The source of heat input can be any suitable fluid, although in most chemical plants this is normally steam. In refineries, the heating source may be the output streams of other columns. The vapor raised in the reboiler is re-introduced into the unit at the bottom of the column. The liquid removed from the reboiler is known as the bottoms product or simply, bottoms.

The vapour moves up the column, and as it exits the top of the unit, it is cooled by a condenser. The condensed liquid is stored in a holding vessel known as the reflux drum. Some of this liquid is recycled back to the top of the column and this is called the reflux. The condensed liquid that is removed from the system is known as the distillate or top product.

2.6

BASIC EQUATIONS FOR CONTINUOUS DISTILLATION OF BINARY SYSTEMS

Sorel (1899) first derived and applied the basic stage equations to the analysis of binary systems. Figure 2.10a shows the flows and compositions in the top part of a column. Taking the system boundary to include the stage n and the condenser, gives the following equations:

Rectification section

Stripping section

FIGURE 2.10 SECTIONS OF A DISTILLATION COLUMN

Where: V is the molar flow rate of lighter component in vapour phase L is the molar flow rate of lighter component in liquid phase D is the molar flow rate of lighter component in the distillate Y is the vapour phase mole fraction of lighter component X is the liquid phase mole fraction component

of lighter

N refers to the plate number

Combining equations 2.5 and 2.6 gives

(2.7) An analogous equation for the stripping section is

(2.8) Where: V’ is the boil up rate B is the molar flow rate of bottoms X, y and n have retain their usual meanings

2.6.1 NUMBER OF PLATES REQUIRED IN A DISTILLATION COLUMN In order to develop a method for the design of distillation units to give the desired fractionation, it is necessary, in the first instance, to develop an analytical approach which enables the necessary number of trays to be calculated. First the heat and material flows over the trays, the condenser, and the reboiler must be established. Thermodynamic data are required to establish how much mass transfer is needed to establish equilibrium between the streams leaving each tray. 2.6.1.1

LEWIS-SOREL METHOD (EQUIMOLAR OVERFLOW)

For most distillation problems a simplifying assumption, first proposed by Lewis (1909), can be made that eliminates the need to solve the stage energy-balance equations. The molar liquid and vapour flow rates are taken as constant in the stripping and rectifying sections. This condition is referred to as equimolar overflow: the molar vapour and liquid flows from each stage are constant. This will only be true where the component molar latent heats of vaporisation are the same and, together with the specific heats, are constant over the range of temperature in the column; there is no significant heat of mixing; and the heat losses are negligible. These conditions are substantially true for practical systems when the components form nearideal liquid mixtures. If a unit is operating as shown in Figure 2.11, so that a binary feed F is distilled to give a top product D and a bottom product W, with xf , xd, and xw as the corresponding mole fractions of the more volatile component, and the vapour Vt rising from the top plate is condensed, and part is run back as liquid at its boiling point to the column as reflux, the

remainder being withdrawn as product, then a material balance above plate n, indicated by the loop I in Figure 2.11 gives: Vn = Ln+1 +D

(2.9)

FIGURE 2.11 MATERIAL BALANCE AT TOP AND BOTTOM OF COLUMN Carrying out a mass balance for the more volatile component in top part of the column, a straight line equation is obtained

(2.10) And for the bottom part of the column

(2.11) This equation, which is similar to equation 2.10, gives the corresponding relation between the compositions of the vapour rising to a plate and the liquid on the plate, for the section below the feed plate. These two equations are the equations of the operating lines.

In order to calculate the change in composition from one plate to the next, the equilibrium data are used to find the composition of the vapour above the liquid, and the enrichment line to calculate the composition of the liquid on the next plate. This method may then be repeated up the column, using equation 2.11 for sections below the feed point, and equation 2.10 for sections above the feed point. 2.6.1.2

McCABE METHOD

In 1925, McCabe and Thiele [5] published a graphical method for combining the equilibrium curve with material balance operating lines to obtain, for a binary-feed mixture and selected column pressure, the number of equilibrium stages and reflux required for a desired separation of feed components. The “McCabe-Thiele Assumptions” are; 

Both components have equal and constant molar

enthalpies of vaporization (latent heats).   

Sensible heat, CpΔT, is negligible compared to latent heat. Column is insulated (no heat loss on each stage). Column pressure is constant (thermodynamics can be

done at a single pressure). These are big assumptions, but allow for simple analysis, since L and V are constant under these assumptions. For the rectifying section, A material balance for the LK over the envelope for the total condenser and stages 1 to n is as follows, where y and x refer, respectively, to LK vapor and liquid mole fractions. (2.12) Solving equation 2.13 for yn+1 gives the equation for the rectifying section operating line:

(2.13)

FIGURE 2.12 MASS BALANCE FOR RECTIFYING SECTION

FIGURE 2.13 McCABE THIELE OPERATING LINE FOR RECTIFYING SECTION

FIGURE 2.14 MASS BALANCE FOR STRIPPING SECTION

FIGURE 2.15 McCABE THIELE OPERATING LINE FOR STRIPPING SECTION The optimum feed location is normally determined by the point of intersection of the rectifying section operating line and the stripping section operating line. The feed stage may be varied from one position to another to suit certain distillation conditions.

2.7

REFRACTIVE INDICES OF BINARY LIQUID MIXTURES

Refractive index measurements in combination with density, boiling point, melting point and other analytical data are very useful industrially also for common substances which include oils, waxes, sugar syrups etc. Literature survey reveals that its general applicability in chemical analysis and industry. Number of mixing rules has been proposed in the literature for the refractive index measurements. Some of them are not suitable when there is a large change of volume on mixing.

FIGURE 2.16 SKETCH OF REFRACTIVE INDEX VS MOLE FRACTION PLOT FOR METHYLCYCLOHEXANE A refractometer measures the extent to which light is bent (i.e. refracted) when it moves from air into a sample and is typically used to determine the index of refraction (aka refractive index or n) of a liquid sample.

The refractive index is a unitless and 1.7000 for most compounds, and to five digit precision. Since the depends on both the temperature of wavelength of light used these are reporting the refractive index:

number, between 1.3000 is normally determined index of refraction the sample and the both indicated when

The italicized n denotes refractive index, the superscript indicates the temperature in degrees Celsius, and the subscript denotes the wavelength of light (in k ] this case the D indicates the sodium D line at 589 nm).

The refractive index is commonly determined as part of the characterization of liquid samples, in much the same way that melting points are routinely obtained to characterize solid compounds. It is also commonly used to:

  

Help identify or confirm the identity of a sample by comparing its refractive index to known values. Assess the purity of a sample by comparing its refractive index to the value for the pure substance. Determine the concentration of a solute in a solution by comparing the solution's refractive index to a standard curve.

2.8

OVERALL COLUMN EFFICIENCY

CHAPTER 10 REFERENCES Geankoplis, Christie J., “Transport Processes and Unit Operations,” 3 rd ed., Prentice Hall (1993).

Perry, Robert H., and Don W. Green. “Perry’s Chemical Engineers’ Handbook” 7 th ed. New York: McGraw-Hill Inc., (1997)

Robert E. Treybal, Mass Transfer Operations, McGraw-Hill, Inc., 3 rd ed. 1981. Perry’s Chemical Engineers’ Handbook, McGraw-Hill, Inc., 8 th ed. 1997. R. K. Sinnott, Coulson & Richardson’s Chemical Engineering: Chemical Engineering Design (vol. 6), Butterworth-Heinemann, 3rd ed. 1999. Perry’s Chemical Engineers’ Handbook, McGraw-Hill Companies, 7 th ed. 1997. Henry Z. Kister, Distillation Design, McGraw-Hill, Inc., 1st ed. 1992. King, C.J. (1980). Separation Processes (2nd ed.). McGraw Hill. Kister, Henry Z. (1992). Distillation Design (1st ed.). McGraw-Hill.

Seader, J. and Henley, E. Separation Process Principles. John Wiley & Sons, 1998, . McCabe, Warren. Unit Operations of Chemical Engineering, Fifth Edition. McGraw-Hill, Inc, 1993 Vargaftic, N.B., Y.K. Vinogradov and V.S. Yargin, 1996. Handbook of Physical Properties of Liquids and Gases, Pure Substance and Mixtures. 3rd Edn., Begel House Inc., New York.