Quiz 5 Separations

Permeate

The portion of the feed mixture that passes through the membrane.

Retentate

The portion of the feed mixture that does not pass through the membrane (also called reject, concentrate, or residue).

Semipermeable Barrier

The separating agent in membrane processes that allows certain species to pass while blocking others.

Sweep

An optional stream used on the permeate side to help carry away the permeated species.

Porosity (ε)

The fraction of the total membrane volume that is empty or void space.

Tortuosity (τ)

A factor representing the actual path length a molecule travels through a porous membrane compared to the membrane thickness.

Sieving (Microporous Separation)

A mechanism where separation occurs by molecular filtration based on size and shape.

Solution-Diffusion

A mechanism where species dissolve in the membrane material and move through it; separation is based on differences in solubility and mobility.

Knudsen Diffusion

Occurs when the mean free path of molecules is large compared to the pore diameter, causing molecules to collide more often with pore walls than with each other.

Bulk Flow

Pressure-driven flow of fluid through a membrane where the pore diameter is large enough to pass all molecules.

Driving Force

The difference in chemical potential across the membrane, which can be caused by gradients in pressure, concentration, temperature, or electrical field.

Permeability (P_M)

A fundamental material property of the membrane that is independent of its thickness.

Permeance (\bar{P}_M)

The ratio of permeability to membrane thickness (P_M / l_M); it is analogous to a mass-transfer coefficient.

Selectivity

The ability of a membrane to separate two components, measured by the different rates at which they pass through the membrane.

Barrer

A common unit for gas permeability, defined as 10^{-10} cm^3 (STP)*cm /(cm^2*s*cmHg)

Concentration Polarization

The accumulation of solute at the membrane surface, creating a concentration gradient between the surface and the bulk solution.

Flux (N_i)

The molar flow rate of a species per unit cross-sectional area of the membrane (N_i = \bar{P}_{M_i} \times \text{Driving Force}).

Anisotropic (Asymmetric) Membrane

A membrane consisting of a very thin, dense permselective 'skin' supported by a thicker, micropourus layer.

Thin-Film Composite (TFC):

A type of asymmetric membrane where the top selective layer and the support layer are made of different polymers.

Caulking

A process used to seal defects in the permselective skin of a membrane to maintain high selectivity.

Membrane Fouling

The accumulation of unwanted materials on the membrane surface or within its pores, leading to a decrease in flux over time.

Gas Permeation Applications

Includes hydrogen production, carbon capture, natural gas purification, and air separation

Advantages

Low Energy Consumption: No phase changes (like boiling) are required.

Modular/Scalable: Easy to expand by adding more units in parallel.

Small Footprint: Compact equipment compared to traditional columns.

Gentle Processing: Operates at ambient temperatures, protecting heat-sensitive materials.

Continuous Operation: Easy to automate and integrate into flow processes.

Shortcomings

Fouling/Clogging: Accumulation of particles reduces efficiency and requires cleaning.

Chemical Sensitivity: Some membrane polymers degrade when exposed to specific solvents or pH levels.

Concentration Polarization: Build-up of solutes at the membrane surface creates resistance.

The "Upper Bound" Trade-off: High permeability often comes at the cost of lower selectivity.

Mechanical Limits: High pressure can lead to membrane compaction or physical failure.

Industrial Applications

Gas Separation: Nitrogen generation, hydrogen recovery, and CO_2 removal.

Reverse Osmosis (RO): Desalination for drinking water.

Ultrafiltration (UF): Removing macromolecules and bacteria (e.g., in dairy or wastewater).

Dialysis: Medical blood purification and removal of metabolic waste.

Electrodialysis: Using electric fields to remove ions from water.

Pervaporation: Dehydration of solvents (like ethanol) and organic removal.