Lecture 11 Flashcards
(21 cards)
What is the membrane separation process?
In a membrane-separation process, a mixture of two or more components is partially separated by means of a semipermeable barrier (the membrane) through which some species travel faster than others. The most general membrane process is shown in digital notes, where the feed mixture is separated into a retentate (that part of the feed that does not pass
through the membrane) and a permeate (that passes through the membrane). The feed, retentate, and permeate are usually liquid or gas.
To maintain selectivity, the barrier must not dissolve, deform, disintegrate, or break. The optional swee is a liquid or gas used to facilitate the removal of the permeate
What are the 3 conditions that define membrane separations?
(1) the two products are usually
miscible, (2) the separating agent is a semipermeable barrier, and (3) a sharp separation is often difficult to achieve.
What are permeance and permeability?
Permeance is a measure of how easily a specific species (e.g., gas or liquid molecules) can pass through a membrane of a given thickness. It is typically expressed as the flow rate per unit area per unit driving force (e.g., pressure or concentration difference).
Permeability is an intrinsic property of the membrane material itself. It tells us how easily a substance (like a gas or liquid) can pass through the membrane, regardless of its thickness
permeance depends upon the specific thickness of the membrane material and is defined as the (IMP:) ratio of the permeability of the membrane material to the membrane thickness
What is the molar transmembrane flux (flow rate per unit cross-sectional area of
membrane) of species i?
Ni =(P(Mi)/l(M))*(driving force) = P̄(Mi) *(driving force)
where P̄(Mi) is the permeance. l(M) is the membrane thickness, and PMi is its permeability.
What are the characteristics of a good membrane?
Digital notes
What are the two types of membranes?
- Dense (amorphous) membrane
- Microporous membranes
What are dense (amorphous) membranes
In dense (amorphous )membranes, pores of microscopic dimensions may be present, but they are generally less than a few Å (angstroms) in diameter. For this case, diffusing species must dissolve into the membrane and then diffuse through the membrane between the segments of the macromolecular chains. Diffusion can be difficult
What are Microporous membranes?
Microporous membranes contain interconnected pores and are categorized by their use in microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF).
The MF membranes, which have pore sizes of 200–100,000 Å, are used primarily to filter bacteria and yeast and provide cell-free suspensions.
UF membranes have pore sizes of 10–200 Å and are used to separate low-molecular-weight solutes such as enzymes from
higher-molecular-weight solutes like viruses or cell debris.
NF membranes have pore sizes from 1 to 10 Å and can retain smaller molecules. NF membranes are used in reverse osmosis
and pervaporation processes to purify liquids.
Permeability for microporous membranes is high, but selectivity is low for small molecules, due in part to pore-size distributions that can be variable and broad. However, mixtures of molecules smaller and larger than the Pore size may be separated almost perfectly by size.
What is a permselective layer?
A permselective layer is the thin, dense surface layer of a membrane (on top of it) that controls which molecules can pass through while blocking others. The permselective layer was developed to solve the problem of high permeability but low selectivity in membranes. The flux rate of a species is controlled by the permeability of
the very thin, permselective skin.
An example of it is shown in digital notes
What is the attack plan of this lecture, cause the above was low-key an introduction
Now we are going to talk about the mass transfer through the membranes, starting off with bulk flow in MACROporous membranes, followed by the mass transfer in liquid diffusion, and then gas diffusion, and finally ending off with nonporous (dense) membrane solution-diffusion transport, for liquid and gas mixtures.
External mass-transfer resistances in the fluid films on either side of the membrane are treated where appropriate.
What are three important things to mention before beginning?
- MACROporous membranes are just membranes used to support thin microporous and dense membranes when significant pressure differences across the membrane are necessary to achieve high flux. In other words, the pores are MACRO (duh,) allowing for bulk flow
- Only microporous or dense membranes are permselective. Bulk flow is not permselective, so no separation occurs
- Because of the range of pore sizes
in membranes, the distinction between porous and nonporous membranes is not always obvious. The distinction can be
made based only on the relative permeabilities for diffusion through pores and diffusion through the solid, amorphous
regions of the membrane, respectively.
What are the different transport mechanisms of liquid and gas molecules through a porous membrane (Super imp, also repetitive, also long srry)
The different transport mechanisms are shown in digital notes
(a) - If the pore diameter is large
compared to the molecular diameter and a pressure difference exists, bulk, convective flow through the pores occurs, as in Digital notes. Bulk flow is undesirable because it is not permselective; therefore, no separation between feed components occurs.
(b) - Permselective diffusion of components through the pores takes place if fugacity, activity, chemical-potential, concentration, or partial-pressure differences for components exist across the membrane, but the total pressure is the SAME on both sides of the membrane, so no bulk flow occurs. Then a separation occurs, as shown in digital notes
(c) - If the pores are of the order of molecular size for at least some of the components in the feed mixture, the diffusion of those components will
be restricted (hindered) as shown in digital notes, resulting in an enhanced separation. Molecules larger than the pores are prevented from diffusing through the pores. This special case is highly desirable and is referred to as size exclusion or sieving
(d) solution diffusion through dense
membranes - Expanded on in the end of the lecture
What is the Knudsen Diffusion
It is Another special case exists (the other being size exclusion) for restricted gas diffusion in which the pore size and/or pressure (typically a vacuum) is such that
the mean free path of the molecules is greater than the pore diameter, resulting in ballistic mass transfer via Knudsen
diffusion, which depends on molecular weight.
What is the derivation for Pore Resistance to Bulk Flow?
It is in the book, important things to note:
- Depiction of straight cylindrical pores and real pores is shown in written notes
- Tortuosity is a measure of how convoluted or twisted the pathways are through a porous medium. It essentially compares the actual path length a molecule or fluid must travel through the medium (non-ideal pores) to the straight-line distance (or the shortest path, ie straight cylindrical pores. This means that Tortuosity will always be larger than 1
- We compare with the Ergun equation to obtain the upper limit of the tortuosity as travel through a packed bed is considered to be the longest fluid pathway meaning:
1 < t < 2.08
What is Liquid Diffusion Through Pores?
In the book, Important things to note:
- For the respective factor when 1» dm/dp, Kr = 1 giving us bulk flow
- The selectivity ratio is found by dividing the two fluxes, with the assumption that the concentration gradient is the same
What is Gas Diffusion Through Porous Membranes?
It is in the book, important things to note:
- Already mentioned, but I want to double highlight on it: For a gas, diffusion through a pore occurs by ordinary diffusion, and/or in series with Knudsen diffusion when the pore diameter is very small and/or total pressure is low
- Separation of gas molecules of equal size often doesn’t work well, and we need to use transport through non-porous membranes
What is the mechanism for the mass transport through a nonporous (dense) membrane?
As indicated in digital notes (Flashcard 12 under D), gas or liquid species
absorb at the feed side of the membrane, diffuse through the membrane, and desorb at the permeate side.
How do diffusivities compare in transport through nonporous (dense) solid membranes?
Liquid diffusivities are several orders of magnitude less than gas diffusivities, and diffusivities of solutes in solids are
a few orders of magnitude less than diffusivities in liquids. Thus, differences between diffusivities in gases and solids are
enormous. Example shown in digital notes. As might be expected, small molecules fare better than large molecules for diffusivities in solids
What is the solution-diffusion
model?
It is a model based on Fick’s law for diffusion through solid, nonporous membranes based on the driving force, ci0 − ciL shown in digital notes, where concentrations refer to solute dissolved in the membrane.
for Nonporous membranes, the concentrations in the membrane are related to the concentrations or partial pressures in the fluid adjacent to the membrane faces by assuming thermodynamic equilibrium for the solute at the fluid–membrane interfaces
for POROUS membranes, the concentration profiles are continuous from the bulk-feed liquid to the bulk-permeate liquid because liquid is present continuously from one side to the other, as shown in digital notes
What is the Solution-Diffusion for Liquid Mixtures
In the book
What is the Solution-Diffusion for gas Mixtures
In the book, Important to note:
for POROUS membranes, a continuous PARTIAL PRESSURE profile is shown, while for non-porous a CONCENTRATION profile is shown WITHIN the membrane
