🦫Intro to Chemical Engineering Unit 7 – Mass Transfer

Key Concepts and Definitions

• Mass transfer involves the movement of a substance from a region of higher concentration to a region of lower concentration
• Diffusion is the spontaneous movement of molecules from a high concentration region to a low concentration region driven by a concentration gradient
• Convection is the transport of mass due to bulk fluid motion, which can be natural (buoyancy-driven) or forced (externally-driven)
• Mass transfer coefficient quantifies the rate of mass transfer across a boundary layer and depends on factors such as fluid properties, flow conditions, and geometry
• Fick's law describes the relationship between the diffusive flux and the concentration gradient in a system
  • Fick's first law applies to steady-state diffusion
  • Fick's second law describes transient diffusion
• Equimolar counterdiffusion occurs when the molar fluxes of two species are equal in magnitude but opposite in direction, resulting in no net molar flux
• Mass transfer rate is the amount of mass transferred per unit time and is influenced by factors such as concentration gradient, interfacial area, and mass transfer coefficient

Fundamental Principles of Mass Transfer

• Mass conservation principle states that mass cannot be created or destroyed in a closed system, and the total mass entering a system must equal the total mass leaving the system plus any accumulation
• Concentration gradient is the driving force for mass transfer, with molecules moving from regions of high concentration to regions of low concentration
• Interfacial mass transfer occurs at the boundary between two phases (gas-liquid, liquid-liquid, or solid-fluid) and is influenced by the properties of the interface and the bulk phases
• Equilibrium is reached when the chemical potentials of a species are equal in all phases, and there is no net mass transfer
• Mass transfer resistance arises from the presence of boundary layers, which are regions of reduced mixing near interfaces and can limit the rate of mass transfer
• Analogies between heat, mass, and momentum transfer allow the use of similar mathematical models and dimensionless numbers (such as Sherwood, Schmidt, and Reynolds numbers) to describe and analyze mass transfer processes
• Multicomponent mass transfer involves the simultaneous transfer of multiple species and requires consideration of interactions between the species and their effects on the overall mass transfer rate

Types of Mass Transfer Processes

• Absorption is the transfer of a gas into a liquid phase, often used in gas purification and separation processes (removing CO2 from flue gas using amine solutions)
• Adsorption is the adhesion of molecules from a fluid onto a solid surface, commonly employed in water treatment and gas purification (activated carbon for removing organic contaminants)
• Distillation is a separation process that relies on differences in volatility between components, involving the vaporization and condensation of a liquid mixture (crude oil fractionation)
• Extraction is the transfer of a solute from one liquid phase to another immiscible liquid phase, based on the solute's relative solubility in the two phases (recovering penicillin from fermentation broth using butyl acetate)
• Drying is the removal of moisture from a solid material by evaporation, often using hot air or vacuum (drying of food products, pharmaceuticals, and chemicals)
• Membrane separation processes use selective permeability of membranes to separate components from a mixture, driven by pressure, concentration, or electrical potential differences (reverse osmosis for water desalination)
  • Ultrafiltration and microfiltration are pressure-driven membrane processes used for particle removal and protein concentration
  • Pervaporation is a membrane process that combines permeation and evaporation for separating liquid mixtures (dehydration of ethanol)
• Crystallization is the formation of solid crystals from a supersaturated solution, melt, or vapor, used for purification and separation of solid products (production of salt, sugar, and pharmaceuticals)

Mass Transfer Coefficients

• Mass transfer coefficients quantify the rate of mass transfer across a boundary layer and have units of length per time (m/s)
• Overall mass transfer coefficient ($K$) accounts for the resistances to mass transfer in all phases and the interface, and is determined by the individual mass transfer coefficients and the equilibrium distribution of the solute between phases
• Gas-side mass transfer coefficient ($k_G$) describes the rate of mass transfer in the gas phase boundary layer and is influenced by factors such as gas velocity, viscosity, and diffusivity
• Liquid-side mass transfer coefficient ($k_L$) describes the rate of mass transfer in the liquid phase boundary layer and is affected by liquid properties, flow conditions, and interfacial area
• Solid-side mass transfer coefficient ($k_S$) represents the rate of mass transfer in the solid phase boundary layer and is relevant in processes such as adsorption and heterogeneous catalysis
• Correlations and empirical equations are used to estimate mass transfer coefficients based on system properties and flow conditions, such as the Sherwood number correlation for forced convection mass transfer
  • Sherwood number ($Sh$) is a dimensionless number that relates the convective mass transfer to the diffusive mass transfer
  • Schmidt number ($Sc$) is a dimensionless number that compares the momentum diffusivity (viscosity) to the mass diffusivity
• Enhancement of mass transfer coefficients can be achieved through techniques such as increasing turbulence, reducing boundary layer thickness, and increasing interfacial area (using packed columns or trays)

Diffusion and Fick's Law

• Diffusion is the spontaneous movement of molecules from a region of high concentration to a region of low concentration, driven by the concentration gradient
• Fick's first law states that the diffusive flux ($J$) is proportional to the negative of the concentration gradient ($-dC/dx$), with the proportionality constant being the diffusion coefficient ($D$)
  • $J = -D \frac{dC}{dx}$
• Diffusion coefficient ($D$) is a measure of the ease with which a species can diffuse through a medium and depends on factors such as temperature, pressure, and molecular size
• Steady-state diffusion occurs when the concentration gradient is constant with time, resulting in a linear concentration profile and a constant diffusive flux
• Fick's second law describes the transient diffusion process, relating the change in concentration with time to the spatial variation of the diffusive flux
  • $\frac{\partial C}{\partial t} = D \frac{\partial^2 C}{\partial x^2}$ (for one-dimensional diffusion)
• Equimolar counterdiffusion is a special case where the molar fluxes of two species are equal in magnitude but opposite in direction, resulting in no net molar flux
• Effective diffusivity ($D_{eff}$) is used to describe diffusion in porous media, accounting for the effects of porosity, tortuosity, and constrictivity on the diffusive transport
• Diffusion limitations can significantly impact the overall rate of mass transfer, especially in processes with slow diffusion or large diffusion lengths (e.g., in heterogeneous catalysis or membrane separation processes)

Convective Mass Transfer

• Convective mass transfer involves the transport of mass due to bulk fluid motion, which can be natural (buoyancy-driven) or forced (externally-driven)
• Natural convection occurs when fluid motion is induced by density differences caused by temperature or concentration gradients (e.g., in the atmosphere or in crystal growth processes)
• Forced convection is driven by external means, such as pumps, fans, or stirrers, and is commonly encountered in industrial mass transfer equipment (e.g., packed columns, heat exchangers)
• Convective mass transfer coefficient ($h_m$) relates the convective mass flux to the concentration difference between the bulk fluid and the interface
  • $N_A = h_m (C_{A,bulk} - C_{A,interface})$, where $N_A$ is the molar flux of species A
• Dimensionless numbers are used to characterize convective mass transfer and develop correlations for estimating mass transfer coefficients
  • Sherwood number ($Sh$) represents the ratio of convective mass transfer to diffusive mass transfer
  • Schmidt number ($Sc$) relates the momentum diffusivity (viscosity) to the mass diffusivity
  • Reynolds number ($Re$) characterizes the flow regime (laminar or turbulent) based on the ratio of inertial forces to viscous forces
• Analogies between heat and mass transfer allow the use of heat transfer correlations (e.g., Nusselt number correlations) to estimate mass transfer coefficients by replacing the Nusselt number with the Sherwood number and the Prandtl number with the Schmidt number
• Enhancement of convective mass transfer can be achieved by increasing turbulence, reducing boundary layer thickness, and optimizing flow geometry (e.g., using baffles or packing materials)
• Convective mass transfer is often the dominant mechanism in industrial mass transfer processes, such as gas absorption, distillation, and extraction

Mass Transfer Equipment and Applications

• Packed columns are widely used for gas-liquid and liquid-liquid mass transfer processes, such as absorption, distillation, and extraction
  • Packing materials (e.g., Raschig rings, Pall rings) provide a large interfacial area for mass transfer and promote turbulence
  • Column diameter and height are designed based on the desired separation performance and mass transfer requirements
• Tray columns are another common type of mass transfer equipment, particularly for distillation processes
  • Trays (e.g., sieve trays, valve trays) create a series of stages where vapor and liquid phases interact, promoting mass transfer
  • Tray efficiency and column capacity are important design considerations for tray columns
• Membrane contactors combine the advantages of membranes and conventional mass transfer equipment, providing a large interfacial area and avoiding phase dispersion
  • Hollow fiber membrane contactors are commonly used for gas-liquid mass transfer applications, such as CO2 capture and removal of volatile organic compounds
• Adsorbers are used for the selective removal of components from a fluid phase using solid adsorbents
  • Fixed-bed adsorbers are widely used for gas and liquid purification processes (e.g., air separation, water treatment)
  • Pressure swing adsorption (PSA) and temperature swing adsorption (TSA) are cyclic processes that regenerate the adsorbent by changing the pressure or temperature
• Spray columns and bubble columns are simple mass transfer devices that create a dispersed phase (droplets or bubbles) in a continuous phase to enhance mass transfer
  • Spray columns are used for gas-liquid mass transfer applications, such as flue gas desulfurization and gas scrubbing
  • Bubble columns are employed for gas-liquid and gas-liquid-solid mass transfer processes, such as fermentation and wastewater treatment
• Heat exchangers can also serve as mass transfer devices when a phase change occurs, such as in condensation or evaporation processes
  • Plate heat exchangers and shell-and-tube heat exchangers are commonly used for these applications
• Mass transfer equipment is designed and selected based on factors such as the desired separation performance, mass transfer rate, phase properties, and operating conditions

Problem-Solving Techniques

• Identify the type of mass transfer process (e.g., diffusion, convection, absorption, extraction) and the relevant phases involved
• Determine the appropriate mass transfer model based on the system geometry, flow conditions, and assumptions (e.g., steady-state, one-dimensional, equimolar counterdiffusion)
• Write the mass balance equations for the system, considering the mass transfer mechanisms, chemical reactions (if any), and boundary conditions
  • For steady-state problems, the mass balance equation simplifies to $\nabla \cdot (\mathbf{J}_A + C_A \mathbf{v}) = R_A$, where $\mathbf{J}_A$ is the diffusive flux, $C_A$ is the concentration, $\mathbf{v}$ is the velocity, and $R_A$ is the reaction rate
  • For transient problems, the mass balance equation includes an accumulation term: $\frac{\partial C_A}{\partial t} + \nabla \cdot (\mathbf{J}_A + C_A \mathbf{v}) = R_A$
• Use Fick's law to express the diffusive flux in terms of the concentration gradient and diffusion coefficient
• Apply the appropriate boundary conditions, such as known concentrations, fluxes, or equilibrium relationships at the interfaces
• Solve the resulting differential equations analytically or numerically, depending on the complexity of the problem
  • Analytical solutions are possible for simple geometries and boundary conditions, such as one-dimensional, steady-state diffusion with constant boundary concentrations
  • Numerical methods (e.g., finite difference, finite element) are required for more complex problems involving multidimensional geometries, variable properties, or coupled phenomena
• Interpret the results and calculate the desired quantities, such as mass transfer rates, concentration profiles, and separation efficiencies
• Perform sensitivity analyses to investigate the effects of key parameters (e.g., diffusion coefficients, mass transfer coefficients, flow rates) on the mass transfer performance
• Validate the results using experimental data, literature values, or empirical correlations, and refine the model if necessary