Membranes

Question 1

How to increase ultrafiltration solvent flux?

Answer 1

1. Cross-flow of fluid past the membrane can be used to sweep away part of the polarised layer, thereby increasing kc
2. Higher velocities can also be used to increase turbulence, and hence kc

Question 2

What is diffusion flux simplified to in ultrafiltration?

Answer 2

Nw = Aw change in P

Question 3

How would you increase yield in a thermodynamically limited reaction?

Answer 3

Operating thermodynamically near/at reaction equilibrium. The reaction must be sufficiently fast compared to the mass transport through the membrane (kinetic compatibility)
Typically enhances conversion

Question 4

What is a distributor/contactor type membrane?

Answer 4

Operating under reaction kinetics controlled conditions
The desired product is usually an intermediate in consecutive reactions or is a product in parallel reactions.
Typically enhances selectivity.

Question 5

What is an extractor type membrane?

Answer 5

Operating thermodynamically near/at reaction equilibrium. The reaction must be sufficiently fast compared to the mass transport through the membrane (kinetic compatibility)
Typically enhances conversion

Question 6

What is a membrane reactor?

Answer 6

A bifunctional device combining a membrane based separation with a catalytic reaction

Question 7

Draw complete mixing model

Answer 7

x

Question 8

What is the molecular weight cut off?

Answer 8

Defined as the molecular weight of globular proteins which are 90% retained by the membrane

Question 9

How does concentration polarisation occur in UF?

Answer 9

Since solute is rejected by membrane it accumulates and starts to build up at the surface of the membrane. As concentration of the solute is increased concentration polarisation occurs, which is much more severe than in RO.
Increasing P drop gives a higher convective transport of solute to the membrane.
The concentration (cs) increases and gives a large back molecular diffusion of solute from the membrane to the bulk solution .
Further increase in P drop increase the value of cs to a limiting concentration where the accumulated solid forms a semi solid gel cs = cg.
With inc in P drop the gel layer increases in thickness and this causes solvent flux to decrease because of added gel layer resistance

Question 10

Find kc from graph

Answer 10

Plot of Nw/p vs ln(c1) is a straight line with the negative slope of Kc