INTRODUCTION TO MULTICOMPONENT DISTILLATION

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INTRODUCTION TO MULTICOMPONENT DISTILLATION

• LECTURE 4

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OVERVIEW

• INTRODUCTION
• PHASE EQUILIBRIUM
• FLASH DISTILLATION OF MULTICOMPONENT MIXTURES
• FRACTIONATION OF MULTICOMPONENT MIXTURES

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INTRODUCTION

• In industry, distillation processes involve the
  separation of more than 2 components
• Principle same as binary distillation
• Mass balance for each components
• Equilibrium data is used to calculate boiling
  points and dew points

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INTRODUCTION

• In binary distillation, only 1 column is needed
• Multicomponent of n mixtures, n-1 column is
  required
• Phase equilibria for multicomponent more complex
  because of the several components
• Equilibria depend on temperature, which changes
  from stage to stage

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PHASE EQUILIBRIUM
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PHASE EQUILIBRIA

• V-L equilibria for mixture are describe by
  distribution coefficient, K
• It is the ratio of mole fraction in vapor and
  liquid phase in equilibrium

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PHASE EQUILIBRIA

• Assuming Raoults law and Daltons law is valid,
  Ki may be calculated from vapor pressure of the
  system

Pi Vapor pressure of component i pi partial
pressure of component i
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PHASE EQUILIBRIA

• K factors are temperature dependent because of
  change in vapor pressure.
• But relative values of K for 2 components change
  only moderately with T
• Ratio of K factors is the same as relative
  volatility of the components

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PHASE EQUILIBRIA
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BUBBLE POINT CALCULATION

• Both bubble and dew point are required in flash
  distillation calculation
• Bubble point initial boiling point of a liquid
  mixture

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DEW POINT CALCULATION

• Dew point initial condensation temperature

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PROCEDURE

• Assume a flash temperature, T
• Determine K values at that temperature
• Compute the summation
• If the summation is not equal to 1.0, adjust the
  temperature and repeat
• The final values of T and the Ks are used to
  determine the product compositions.

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PROCEDURE

• If summation is close to 1.0, the vapor
  composition can b e determined with little error
  from the relative contribution of each term

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EXAMPLE 1

• Find the bubble point and dew point temperatures
  and the corresponding vapor and liquid
  compositions for a mixture of 33 mole percent
  n-hexane, 37 mole percent n-heptane, and 30 mole
  percent n-octane at 1.2 atm total pressure
• Vapor pressure plot of the three components as
  log P vs Tabs

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EXAMPLE 1
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EXAMPLE 1

• To calculate bubble point
• Guess T1050C.
• From previous graph, find vapor pressure Pi
• For hexane, Pi 2.68 atm
• For heptane, Pi 1.21 atm
• For octane, Pi 0.554 atm

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EXAMPLE 1

• a) Hexane
• Calculate for hexane from

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EXAMPLE 1

• Calculate vapor compositions, yi from

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EXAMPLE 1

• Calculate for the rest of the compositions

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EXAMPLE 1

• Since is too large, guess a lower T.
• Pick T where Ki is lower by a factor of 1/1.24.
• T 960C, where Pi 2.16 atm

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EXAMPLE 1

• By interpolation, the bubble point is 970C.
• The vapor in equilibrium with the liquid is
• 60.4 mole hexane
• 29.2 mole heptane
• 10.4 mole octane

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EXAMPLE 1

• To calculate dew point, guess T 1050C

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EXAMPLE 1

• Guess another T since the summation is gt 1
• Choose T1100C

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EXAMPLE 1

• By extrapolation, the dew point is 110.50C
• Composition of L in equilibrium with vapor is
• 13.00 mole hexane
• 31.70 mole heptane
• 55.30 mole octane

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FLASH DISTILLATION OF MULTICOMPONENT DISTILLATION
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FLASH DISTILLATION

• Flash is a single equilibrium stage
• Vaporizing a fraction of liquid feed
• Separating vapor in equilibrium with remaining
  liquid (residual)
• Condensing the vapor

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FLASH DISTILLATION

• Equation for each component in a flash
  distillation
• Since D B in equilibrium, it can be rearranged
  to

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FLASH DISTILLATION

• And in terms of xBi
• This equation is solved by iteration and
  interpolation in the same way as the dew point
  and bubble point.