13.4 Adsorption and ion exchange

Adsorption and ion exchange are key separation techniques in fluid systems. These processes rely on the interaction between molecules and solid surfaces to remove specific components from mixtures.

Understanding adsorption isotherms, fixed-bed operations, and regeneration methods is crucial for designing efficient separation systems. These concepts help engineers optimize processes like water treatment, gas purification, and chemical separations.

Adsorption Isotherms and Materials

Adsorption Isotherms

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• Adsorption isotherms describe the equilibrium relationship between the amount of adsorbate on the surface of an adsorbent and the pressure or concentration of the adsorbate in the gas or liquid phase at a constant temperature
• Commonly used to characterize the adsorption capacity and behavior of different adsorbent materials
• Provide insights into the interaction between the adsorbate and adsorbent surface, such as the strength of adsorption and the surface heterogeneity
• Two widely used adsorption isotherm models are the Langmuir isotherm and the Freundlich isotherm (activated carbon, zeolites)

Langmuir Isotherm

• Assumes monolayer adsorption on a homogeneous surface with a finite number of identical adsorption sites
• Adsorption occurs without interaction between adsorbed molecules, and each site can accommodate only one molecule
• Mathematically expressed as: $\frac{q}{q_m} = \frac{K_L C}{1 + K_L C}$, where $q$ is the amount adsorbed per unit mass of adsorbent, $q_m$ is the maximum adsorption capacity, $K_L$ is the Langmuir constant, and $C$ is the equilibrium concentration of the adsorbate
• Applicable to systems with strong adsorbate-adsorbent interactions and limited multilayer adsorption (gases on solid surfaces, dyes on activated carbon)

Freundlich Isotherm

• Empirical model that describes adsorption on heterogeneous surfaces with a distribution of adsorption sites having different adsorption energies
• Assumes that the adsorption capacity increases with increasing adsorbate concentration, following a power-law relationship
• Mathematically expressed as: $q = K_F C^{1/n}$, where $q$ is the amount adsorbed per unit mass of adsorbent, $K_F$ and $n$ are Freundlich constants, and $C$ is the equilibrium concentration of the adsorbate
• Suitable for systems with multilayer adsorption and a wide range of adsorbate concentrations (organic compounds on activated carbon, heavy metals on zeolites)

Adsorbent Materials

• Activated carbon is a highly porous carbonaceous material with a large surface area (500-1500 m²/g) and a wide range of pore sizes
• Prepared by pyrolysis and activation of organic precursors (wood, coal, coconut shells) to create a network of micropores and mesopores
• Exhibits excellent adsorption capacity for a variety of organic compounds, gases, and heavy metals due to its high surface area and surface functional groups
• Zeolites are crystalline aluminosilicate materials with a uniform pore structure and high surface area (300-800 m²/g)
• Consist of a three-dimensional framework of SiO4 and AlO4 tetrahedra, forming channels and cavities of molecular dimensions
• Display selective adsorption properties based on the size and shape of their pores, making them useful for gas separation, catalysis, and ion exchange (molecular sieves, catalytic cracking)
• Ion exchange resins are synthetic polymeric materials with functional groups that can exchange ions with a solution
• Consist of a cross-linked polymer matrix (polystyrene, polyacrylate) with attached functional groups (sulfonic acid, quaternary ammonium)
• Used for the removal of ionic contaminants from aqueous solutions, such as water softening, demineralization, and wastewater treatment (softening of hard water, removal of heavy metals)

Fixed-Bed Adsorption and Regeneration

Fixed-Bed Adsorption

• Fixed-bed adsorption is a continuous process where a fluid (gas or liquid) containing the adsorbate is passed through a stationary bed of adsorbent particles
• As the fluid flows through the bed, the adsorbate is removed from the fluid and adsorbed onto the surface of the adsorbent
• The adsorption process continues until the adsorbent becomes saturated, and the concentration of the adsorbate in the effluent reaches a predetermined level (breakthrough)
• The performance of a fixed-bed adsorption system is characterized by the breakthrough curve, which represents the concentration of the adsorbate in the effluent as a function of time or the volume of fluid treated (activated carbon beds for gas purification, zeolite beds for water treatment)

Breakthrough Curve and Mass Transfer Zone

• The breakthrough curve is a plot of the ratio of the effluent concentration to the inlet concentration (C/C0) versus time or the volume of fluid treated
• Initially, the effluent concentration is near zero, as the adsorbent effectively removes the adsorbate from the fluid
• As the adsorbent becomes saturated, the effluent concentration gradually increases until it reaches the breakthrough point (C/C0 = 0.05-0.10)
• The mass transfer zone (MTZ) is the region of the bed where the adsorption process is actively occurring, and the concentration of the adsorbate changes from the inlet to the effluent level
• The MTZ moves through the bed as the adsorption progresses, and its width depends on factors such as the fluid velocity, adsorbent particle size, and mass transfer rates (adsorption of volatile organic compounds on activated carbon, removal of heavy metals by ion exchange resins)

Regeneration

• Once the adsorbent in a fixed-bed system becomes saturated, it needs to be regenerated to restore its adsorption capacity for subsequent adsorption cycles
• Regeneration can be achieved by various methods, depending on the nature of the adsorbate and the adsorbent
• Thermal regeneration involves heating the saturated adsorbent to a high temperature (200-500°C) to desorb the adsorbed species and regenerate the surface
• Chemical regeneration uses a regenerant solution (acids, bases, or salts) to displace the adsorbed species and regenerate the adsorbent
• Pressure swing adsorption (PSA) employs a reduction in pressure to desorb the adsorbed species, followed by repressurization for the next adsorption cycle
• Regeneration is a critical step in the adsorption process, as it determines the overall efficiency and economics of the system (regeneration of activated carbon by steam, regeneration of ion exchange resins by sodium chloride solution)