7.5 Distillation

Distillation is a key mass transfer process that separates liquid mixtures based on differences in component volatilities. It's crucial in chemical engineering, allowing for the purification of products and recovery of valuable materials in many industries.

Understanding distillation principles is essential for designing efficient separation processes. This topic covers the fundamentals, vapor-liquid equilibrium, column design, and energy considerations, providing a foundation for optimizing distillation operations in various applications.

Fundamentals of Distillation

Principles and Components of Distillation

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• Distillation is a thermal separation process that exploits differences in the volatilities of components in a liquid mixture to achieve separation
• The fundamental principle of distillation is based on the vapor-liquid equilibrium, where the more volatile component(s) preferentially vaporize and are collected as the distillate, while the less volatile component(s) remain in the liquid phase as the bottoms product
• The key components of a distillation column include:
  • Reboiler: Provides the necessary heat for vaporization
  • Condenser: Condenses the vapor back into a liquid
  • Column: Consists of a series of trays or packing material to facilitate vapor-liquid contact and mass transfer
• The feed stream enters the distillation column at a specific location, and the separation is achieved through successive vaporization and condensation steps as the vapor rises up the column and the liquid flows down

Factors Influencing Distillation Separation

• The relative volatility (α) is a measure of the ease of separation between two components in a mixture, with higher values indicating a greater difference in volatility and easier separation
  • For example, a mixture of benzene and toluene has a higher relative volatility than a mixture of ethanol and water, making the former easier to separate by distillation
• The boiling point difference between the components in the mixture is a key factor influencing the feasibility and efficiency of distillation, with larger differences generally resulting in easier separation
  • For instance, separating a mixture of methanol (boiling point: 64.7°C) and water (boiling point: 100°C) is easier than separating a mixture of ethanol (boiling point: 78.4°C) and water due to the larger boiling point difference between methanol and water

Vapor-Liquid Equilibrium in Distillation

Vapor-Liquid Equilibrium (VLE) and Phase Diagrams

• Vapor-liquid equilibrium (VLE) describes the relationship between the composition of a liquid mixture and its corresponding vapor phase at a given temperature and pressure
• VLE data is essential for designing and analyzing distillation processes, as it determines the composition of the vapor and liquid streams leaving each stage of the column
• The VLE behavior of a mixture can be represented using phase diagrams, such as:
  • T-xy (temperature-composition) diagrams: Show the bubble point and dew point curves at constant pressure
  • P-xy (pressure-composition) diagrams: Show the bubble point and dew point curves at constant temperature
• The relative volatility (α) can be calculated from VLE data using the ratio of the vapor-phase mole fractions (y) to the liquid-phase mole fractions (x) for each component: α = (y1/x1) / (y2/x2)

Ideal and Non-Ideal VLE Behavior

• Ideal mixtures follow Raoult's law, which states that the partial vapor pressure of each component is proportional to its mole fraction in the liquid phase
  • For example, a mixture of benzene and toluene closely follows Raoult's law and exhibits nearly ideal behavior
• However, most real mixtures exhibit non-ideal behavior due to intermolecular interactions
  • For instance, mixtures of ethanol and water deviate significantly from Raoult's law due to the formation of hydrogen bonds between the molecules
• Non-ideal VLE behavior can be described using activity coefficients (γ), which account for the deviation from ideality
  • Models such as Margules, Van Laar, and Wilson equations are used to estimate activity coefficients based on the composition and properties of the mixture
• Azeotropes are special cases of VLE behavior where the vapor and liquid compositions are equal at a specific temperature and pressure, resulting in a constant-boiling mixture that cannot be separated by conventional distillation
  • Examples of azeotropes include ethanol-water (95.6 wt% ethanol at 78.2°C) and acetone-methanol (86.5 wt% acetone at 55.7°C)

Distillation Column Design and Optimization

Design of Binary Distillation Columns

• The design of a distillation column involves determining the number of theoretical stages (or trays), the feed stage location, the reflux ratio, and the column diameter
• The McCabe-Thiele method is a graphical technique used for the design of binary distillation columns at constant pressure, utilizing the VLE diagram and operating lines to determine the number of theoretical stages and the feed stage location
  • This method assumes constant molar overflow and neglects heat effects, making it suitable for preliminary design and ideal systems
• The Fenske equation is used to estimate the minimum number of theoretical stages required for a given separation, based on the relative volatilities and desired product purities
  • For example, the Fenske equation can be used to determine the minimum number of stages needed to separate a mixture of benzene and toluene to 99% purity
• The Underwood equation is employed to determine the minimum reflux ratio for a given separation, considering the feed composition and the desired product purities
  • This equation is particularly useful for multicomponent systems and helps establish the lower bound for the reflux ratio

Multicomponent Distillation and Optimization

• For multicomponent systems, the concept of key components is used, where the light key (LK) and heavy key (HK) are the two components that are most difficult to separate
  • The distillation design is based on the separation of these key components, while the other components are distributed between the distillate and bottoms products according to their volatilities
• Shortcut methods, such as the Fenske-Underwood-Gilliland (FUG) method, are used for the initial design of multicomponent distillation columns, providing estimates of the minimum number of stages, minimum reflux ratio, and actual number of stages
  • The Gilliland correlation relates the actual number of stages and reflux ratio to the minimum values obtained from the Fenske and Underwood equations, respectively, allowing for the optimization of column design
• Rigorous simulation tools, such as process simulators (Aspen Plus, HYSYS), are employed for the detailed design and optimization of distillation columns, taking into account complex VLE behavior, mass and energy balances, and equipment specifications
  • These tools enable the optimization of operating conditions, such as reflux ratio, feed stage location, and pressure, to minimize energy consumption and maximize product purities

Energy Efficiency of Distillation Processes

Energy Consumption and Influencing Factors

• Distillation is an energy-intensive process, with the majority of the energy consumed in the reboiler to generate the necessary vapor flow for separation
• The energy efficiency of a distillation column is influenced by factors such as:
  • Relative volatility of the components: Higher relative volatilities lead to easier separation and lower energy consumption
  • Desired product purities: Higher purities require more stages and energy input
  • Reflux ratio: Higher reflux ratios improve separation but increase energy consumption
  • Feed condition (temperature, pressure, and composition): Preheating or introducing the feed as a vapor can reduce reboiler duty, while subcooling may increase energy demand
• The thermal efficiency of a distillation column can be evaluated using metrics such as:
  • Heat duty per unit of product: Lower values indicate better energy efficiency
  • Vapor-to-feed ratio: Lower ratios suggest more efficient use of the vapor flow
  • Thermodynamic efficiency: The ratio of the minimum theoretical energy required for separation to the actual energy consumed

Energy Integration and Advanced Distillation Configurations

• Pinch analysis is a technique used to identify energy integration opportunities within a distillation system or between the distillation column and other process units, aiming to minimize the overall energy consumption
  • This method helps locate the optimal heat integration points and can guide the design of heat exchanger networks
• Heat integration strategies can be employed to reduce the external energy requirements of a distillation process, such as:
  • Feed-effluent heat exchange: Preheating the feed using the hot distillate or bottoms streams
  • Intermediate condensers and reboilers: Providing additional heat transfer points within the column to reduce the load on the main condenser and reboiler
• Advanced distillation configurations can significantly improve energy efficiency by reducing the number of columns required and optimizing heat integration, such as:
  • Divided wall columns (DWCs): Combine multiple separation steps into a single column shell, reducing capital and operating costs
  • Heat-integrated distillation columns (HIDiCs): Integrate the condenser and reboiler duties of two columns, enabling energy savings through heat transfer between the columns