13.1 Distillation and absorption

Distillation and absorption are key separation processes in chemical engineering. They rely on differences in volatility or solubility to separate mixtures into their components. These methods are crucial for purifying products and recovering valuable materials in many industries.

Understanding the principles behind distillation and absorption is essential for designing efficient separation systems. We'll explore vapor-liquid equilibrium, mass transfer, and equipment design to grasp how these processes work and how to optimize them for various applications.

Distillation Fundamentals

Fractional Distillation Process

Top images from around the web for Fractional Distillation Process

• 

  Raoult's law and distillation View original

  Is this image relevant?

• 

  Fractional distillation - Wikipedia View original

  Is this image relevant?

• 

  Distillation - Wikipedia View original

  Is this image relevant?

• 

  Raoult's law and distillation View original

  Is this image relevant?

• 

  Fractional distillation - Wikipedia View original

  Is this image relevant?

1 of 3

Top images from around the web for Fractional Distillation Process

• 

  Raoult's law and distillation View original

  Is this image relevant?

• 

  Fractional distillation - Wikipedia View original

  Is this image relevant?

• 

  Distillation - Wikipedia View original

  Is this image relevant?

• 

  Raoult's law and distillation View original

  Is this image relevant?

• 

  Fractional distillation - Wikipedia View original

  Is this image relevant?

1 of 3

• Fractional distillation separates liquid mixtures into their component parts based on differences in boiling points
• Involves heating the mixture to vaporize components, then condensing and collecting them at different heights in a distillation column
• Repeated vaporization and condensation steps create a gradient of composition from bottom to top of the column
• More volatile components with lower boiling points are collected at the top, while less volatile, higher boiling point components are collected at the bottom (crude oil refining)

Vapor-Liquid Equilibrium Principles

• Raoult's law describes the vapor pressure of an ideal solution as proportional to the vapor pressure of each component and its mole fraction in the liquid phase
  • $P_i = x_i P_i^*$, where $P_i$ is the partial pressure of component $i$, $x_i$ is its mole fraction in the liquid, and $P_i^*$ is its vapor pressure as a pure component
• Relative volatility $\alpha_{ij}$ measures the ease of separating two components $i$ and $j$ based on their vapor pressure ratio: $\alpha_{ij} = \frac{y_i/x_i}{y_j/x_j}$
  • Higher relative volatility indicates easier separation (ethanol-water $\alpha \approx 2$, benzene-toluene $\alpha \approx 2.5$)
• Theoretical plates represent the equilibrium stages in a distillation column where vapor and liquid phases are assumed to reach equilibrium
  • More theoretical plates lead to better separation but increased column height and cost

Distillation Design and Analysis

McCabe-Thiele Graphical Method

• McCabe-Thiele method is a graphical approach to analyze and design distillation columns at constant pressure
• Plots equilibrium curve (vapor-liquid) and operating lines (rectifying and stripping sections) on an xy-diagram
• Stepping off stages between the operating lines determines the number of theoretical plates required
• Minimum reflux ratio can be found from the intersection of the q-line and equilibrium curve
• Optimum reflux ratio is typically 1.2 to 1.5 times the minimum reflux to balance separation and cost

Reflux Ratio Optimization

• Reflux ratio $R$ is the ratio of liquid reflux $L$ returned to the top of the column to the distillate product $D$ withdrawn
• Higher reflux ratios improve separation but require larger columns and more energy input
• Total reflux ($R = \infty$) achieves maximum separation corresponding to the number of theoretical plates
• Optimizing reflux ratio balances product purity and recovery with capital and operating costs (typical values range from 1.1 to 5)

Absorption Fundamentals

Absorption Process Principles

• Absorption is a mass transfer operation where a soluble component is removed from a gas stream by dissolving it into a liquid solvent
• Absorption column contacts gas and liquid phases counter-currently to promote mass transfer
• Solute transfers from the gas phase to the liquid phase based on concentration gradients and solubility
• Henry's law relates the equilibrium partial pressure of a solute $p_A$ to its liquid phase concentration $c_A$: $p_A = H c_A$, where $H$ is Henry's constant

Mass Transfer and Separation Efficiency

• Mass transfer coefficient $k_L$ quantifies the rate of solute transfer across the gas-liquid interface per concentration driving force
• Overall mass transfer is determined by gas and liquid side resistances: $\frac{1}{K_L} = \frac{1}{k_L} + \frac{H}{k_G}$
• Absorption factor $A = \frac{L}{mG}$ relates liquid $L$ and gas $G$ flow rates with slope of equilibrium line $m$
• Absorption efficiency depends on contact area, residence time, concentration driving forces, and solvent selection (water for ammonia, amines for acid gases)

Absorption Equipment

Packed Absorption Columns

• Packed columns are filled with packing materials that provide high interfacial area for gas-liquid contact
• Common packings include Raschig rings, Berl saddles, and structured sheet metal packings
• Packing characteristics like size, shape, surface area, and void fraction affect column performance
• Pressure drop and liquid holdup are key design considerations for packed columns (typically 3-8 theoretical stages)

Tray Absorption Columns

• Tray columns use a series of perforated plates or trays to create distinct stages for gas-liquid contact
• Liquid flows across each tray and down the column while gas bubbles up through the perforations
• Tray types include sieve, valve, and bubble-cap, each with different flow patterns and characteristics
• Tray spacing, weir height, and downcomer design affect tray efficiency and capacity (typically 5-15 theoretical stages)
• Tray columns can handle higher liquid rates and are less prone to maldistribution compared to packed columns