Distillation

Saperation Tecnique: Distillaton

Distillation It is a separation technique for homogenous mixture in which mixtures are separated by the difference in their boiling point. Types of Distillation: 1. Fractional Distillation 2. Differential Distillation 3. Steam Distillation 4. Vacuum Distillation 5. Azeotropic Distillation

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• Fractional Distillation Distillation in which rectification is used to obtain a product as pure as possible is called fractional distillation.

• Differential Distillation In this process, the vapor formed on boiling liquid is at once (i.e. no reflux) removed form the system. Since the vapor is richer in MVC (most volatile component) than the liquid, it follows that the liquid has a decreased amount of MVC. This results in a progressive change in the composition of the product i.e. a progressive decrease in the amount of MVC in liquid. Thus, the vapor formed over a short period is in equilibrium with the residual liquid. At the end of the process, the LVC (least volatile component is removed as the bottom product.

• Steam Distillation Steam distillation is used for distillation of chemicals (usually organic compounds) which are insoluble in water and which do not disintegrate (decompose) at their boiling point. By reducing pressure substances boil quickly. In steam distillation, the feed, as it enters in the reduced pressure environment, boils up at lower temperature and does not require much heat.

• Vacuum distillation Any pressure below 1 atm is a vacuum. The process used in vacuum distillation is similar to steam distillation but the feed in vacuum distillation is water soluble.

• Azeotropic Distillation A mixture of two or more components having a very close boiling point is called an azeotropic mixture. In azeotropic mixtures the nearly same boiling range causes a difficulty in their separation by normal distillation. In order to overcome this difficulty we use a selective solvent with a high boiling point which dissolves one of the azeotropic components. Thus the solvent increases the boiling point of the dissolved component and decreases its volatility. The other component now becomes the MVC and hence vaporizes separately. Then using normal distillation techniques we can separate components from azeotropic mixtures. A+ B + C (Solvent) → AC + B

• Flash Distillation

In this type of distillation the liquid is converted into vapor at once; in a flash (i.e. with no reflux) but this type of distillation differs from the differential distillation as the flash distillation is a continuous distillation process whereas the differential distillation is a batch process.

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Saperation Tecnique: Distillaton

Types of commonly used plates in distillation / extraction towers: 1. Sieve Plate 2. Bubble cap 3. Turbo grid 4. Molecular sieves Types of Refineries: 1. Fuel refineries 2. Lubricant Refineries (or Lube Refineries) 3. Petrochemical Refineries Azeotropes: Mixtures of substances which have nearly the same boiling point are called azeotropes.

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Partial Pressure: In a gaseous mixture the individual pressure exerted by a gas is known as partial pressure. In an ideal gas the partial pressure due to a component is proportional to the mole fraction of the component

• Dalton’s Law of Partial Pressure According to Dalton's Law of partial pressures: “The partial pressure of a gas i defined as Pi exerted by a single component in a gaseous mixture is equal to mole fraction of the component times the total pressure of the gas.”  n  Pi =  i PTotal = yi PTotal  nTotal 

yi is the mole fraction of vapor and P Total is the total pressure exerted by the gas

• Raoult’s Law PA = x A PAO Here PA is the equilibrium vapor pressure of a component A in a mixture with other components PA, B, C ... as well xA  is the mole fraction of the component A in the mixture PAO is the equilibrium vapor pressure of component A as it exists in pure state at the same temperature

• Henry’s Law Henry law states that: “The partial pressure of a gas is directly proportional to its mole fraction.” Mathematically: Pi ∝ xi Or Pi = H i xi Henry’s law is used primarily for a component whose mole fraction approaches zero, such as a dilute gas dissolved in a liquid. Hi is called Henry’s constant

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Saperation Tecnique: Distillaton • Volatility: The volatility of any substance in a liquid solution may be defined as: “The equilibrium partial pressure of the substance in the vapor phase divided by the mole fraction of the substance in the liquid solution” P VA = A xA • Relative volatility: It may be defined as: “The volatility of one component of a liquid mixture divided by the volatility of another component of the liquid mixture” The relative volatility gives an indication of the ease with which the two components can be separated by distillation. Symbolically the relative volatility may be expressed as: V P /x Px α= A = A A = A B VB PB / x B PB x A From Dalton’s Law for vapor phase: y A P = PA From Raoult’s Law for Liquid phase of component A: x A PAO = PA y A P = x A PAO

Similarly: x PO yB = B B P

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yA =

x A PAO P

In a binary mixture of only A and B the sum of mole fractions is equal

to 1.

x A PAO x B PBO + =1 P P x A PAO (1 − x A ) PBO + =1 P P P − PO xA = O B O PA − PB y A + yB =

Now for relative volatility V Px y Px y x α= A = A B = A B = A B VB PB x A y B Px A y B x A x  yA = α  A  yB  xB  Using ( x A + x B = 1 ) & ( y A + y B = 1 ) and simplifying we get yA α xA yA = xA = & 1 + (α − 1) x A α − (α − 1) y A • 1. Entrainment:

Factors which hamper Distillation

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Saperation Tecnique: Distillaton The composition of the product depends upon the speeds at which the vapors and the liquid are circulated in the tower. If the speed of the vapors is very high, then the vapors, which must contain 90-95% MVC, will mix with vapors of LVC and both shall be condensed in the condenser. This reduces the purity of the product collected as the distillate. This effect is called entrainment. Carrying of vapors of LVC by the vapors of MVC moving upward, due to the high velocity of the vapors of the MVC, is called entrainment. Increasing the reflux minimizes the effects of entrainment. 2. Flooding: If the velocity of vapors is less than optimum values, then the quantity of liquid condensing at every plate increases. After some time the entire plate is topped with liquid and enriched vapor is not easily formed as most of it will condense on the plate. This effect is called flooding. In order to minimize the decreased enrichment due to flooding, the speeds at which the vapor is moving up must be regulated to optimum values. 3. Frothing: If the heat from the boiler is increased, the velocity of the vapors will also increase. If the velocity of the vapors increases form optimum values, then the time of contact of the vapor with the liquid will decrease. This decrease in contact time means poorer enrichment. Due to the continuous motion of the fast vapors, high speed bubbling starts on the plates which can be termed as frothing. Speed should be regulated to optimum values to prevent frothing. 4. Channeling: (only in packed columns) When gas and liquid are made to pass through packings in a packed column, there is a possibility that gas or the liquid may form a channel or stream in the packings and pass through the packings without enrichment.

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Peep Holes: These are holes on the sides of fractionating columns to observe if frothing, flooding or entrainment is occurring. Flow Patterns in distillation Column: 1. Co-Current: If we move in the direction of flow than that type of flow is called co-current 2. Counter current: If we move in the opposite direction of flow than that type of flow is called counter current 3. Cross Current: If we move across the direction of flow than that type of flow is called cross current. C o - c u r r e n t C C

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• Packings: Packing is used in the distillation columns to baffle the downward flow of countercurrent liquid. These are made of metal, glass, plastic, wood or silk. They have geometrical shape. Packed column is made by packings. Types of packings: 1. Berl packing 2. Rasching packing Arrangement of Packing: 1. Random 2. Stacked (arranged) • Assumptions of McCabe-Thiele method General Assumption: Equimolal overflow exists over all plates Other Assumptions: 1. Sensible heat changes throughout the distillation column are negligible in comparison with the heat of vaporization of the component. 2. Heat of vaporization is the same for all components. 3. Heat of mixing of the components are negligible 4. Heat losses form the distillation column are negligible

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Saperation Tecnique: Distillaton

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Overall material Balance around the column: F = D +W Overall material Balance around the Rectifier: Vn = Ln + D Component Material Balance around MVC in rectifier section:

yn+ 1Vn = xn Ln + xD D

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yn+1 =

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Dividing numerator and denominator by D

xn Ln /D xD D / D x R x + = n + D Vn / D Vn / D Vn / D Vn / D

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yn+1 =

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xnR xD xnR xD y n+ 1 = + = + (Ln+D)/ (Ln+D)/ R R++ 11

 Ln  = R ∴  D 

 R  x y n+1 =  xn + D R +1  R +1 R is the reflux ratio. This is the equation of a straight line having slope R/(R+1) and y-intercept xD/(R+1). The line is also called the R-line.

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