Essential Concepts in Chemical Separation Processes

Physical Adsorption (Physisorption)

• It occurs due to weak van der Waals forces between the adsorbent and adsorbate.
• The enthalpy of adsorption is low, typically between 20–40 kJ/mol.
• It is usually non-specific and can occur on many types of surfaces.
• It is reversible in nature and can be undone by changing pressure or temperature.
• Physisorption is favored at low temperatures.
• It can result in the formation of multiple layers of adsorbed molecules.
• Little to no activation energy is required for physisorption.

Chemical Adsorption (Chemisorption)

• It involves the formation of strong chemical bonds (covalent or ionic) between the adsorbent and adsorbate.
• The enthalpy of adsorption is high, ranging from 40–400 kJ/mol.
• It is highly specific and depends on the chemical nature of both substances.
• Chemisorption is generally irreversible under normal conditions.
• It is favored at higher temperatures.
• It results in the formation of only a monolayer of adsorbate.
• Chemisorption often requires activation energy for the reaction to occur.

Here’s a concise explanation of key terms related to distillation column operation:

Reflux Ratio (R)

• Ratio of liquid returned to the distillation column to the distillate collected.
• R = Reflux / Distillate
• Controls the separation efficiency in a distillation column.
• Higher reflux ratio increases purity but also increases energy cost.
• Affects the number of theoretical stages needed.

Minimum Reflux Ratio (R_min)

• The lowest reflux ratio at which desired separation can just be achieved.
• At R_min, the number of stages required becomes infinite.
• Determined using the intersection point in the McCabe-Thiele method.
• It is not practical for operation due to excessive column height.
• Used as a reference to estimate practical (optimum) reflux ratios.

Optimum Reflux Ratio

• The reflux ratio that gives the most economical operation.
• Balances energy cost (reboiler duty) and equipment cost (number of stages).
• Usually lies between 1.2 to 1.5 times the minimum reflux ratio.
• Selected to minimize total cost (capital + operating).
• Often determined using economic analysis or design heuristics.

Tray Efficiency

• Definition: It is a measure of how effectively a real tray performs compared to an ideal (theoretical) tray.
• Expression: Efficiency = (Actual Separation / Theoretical Separation) * 100
• Types: Includes Murphree efficiency, overall tray efficiency, and point efficiency.
• Factors Affecting: Depends on liquid/vapor contact, tray design, flow rates, and foaming/flooding.
• Use in Design: Used to convert theoretical stages into the actual number of trays needed in a column.

Key Distillation Methods

Simple Distillation

• Simple distillation separates components with large differences in boiling points.
• It involves one vaporization and one condensation step.
• It is commonly used for purifying liquids from non-volatile impurities.
• It cannot separate azeotropic mixtures.
• The process is easy to set up and economical.
• It is suitable for laboratory and small-scale applications.

Azeotropic Distillation

• Azeotropic distillation is used to separate azeotropic mixtures.
• It involves adding a third component called an entrainer.
• The entrainer forms a new azeotrope with one of the original components.
• This changes the volatility and enables separation.
• The method is more complex than simple distillation.
• It is commonly used to separate ethanol-water mixtures.

Extractive Distillation

• Extractive distillation separates components by adding a high-boiling solvent.
• The solvent changes the relative volatility of the components.
• It does not form azeotropes with the original mixture.
• The solvent interacts more with one component to aid separation.
• The solvent is recovered and reused after the process.
• It is effective for close-boiling or azeotropic mixtures.

Key Factors for Solvent Extraction Selection

• The distribution coefficient determines how well the solute partitions between the solvent and feed phase.
• Selectivity of the solvent is crucial for separating the desired component effectively.
• Solvent immiscibility with the feed phase ensures proper phase separation.
• The density difference between phases affects ease of separation after extraction.
• Solvent recovery and recyclability impact the economic feasibility of the process.
• Chemical stability of the solvent is important to avoid degradation during operation.
• Temperature influences solubility and mass transfer rates.
• pH can affect the solubility and ionization of solutes, especially in aqueous systems.
• Viscosity of the solvent affects mixing and mass transfer efficiency.

Single-Stage Leaching Process

• Single-stage leaching involves contacting the solid and solvent only once to extract the desired solute.
• It is the simplest form of leaching, often used in lab-scale or low-value processes.
• The extent of solute recovery depends on solvent-to-solid ratio, temperature, and contact time.
• Complete extraction is usually not achieved in one stage due to limited mass transfer.
• The process is economical and easy to operate, but often less efficient than multistage systems.
• After leaching, the residue and solution are separated by filtration or settling.