Membranes 4 Flashcards
(27 cards)
What influences the economics of gas separation processes
Selectivity and permeability
What is permeability
The rate at which any compound permeates through a membrane
depends on thermodynamic factor and kinetic factor
What is selectivity
The ability of a membrane to accomplish a given separation
Why might uranium need to be enriched for nuclear fission
It increases the amount of atoms that can be split by fission
Why is enriched UF6 an expensive product
It uses separation based on knudsen diffusion which is an uneconomical process
How is UF6 enriched
It is pumped into membranes with pores so small that there is barely enough room for UF6 molecules to pass through
The lighter 234 and 235 atoms flow through faster leaving the 238 behind
It takes many membranes in series to achieve enough U235 to be used in reactors
What is the mean free path equation
kT/(sqrt(2)(pi)(sigma^2)P)
What is the effecive diffusivity equation
Deff = eD/t
What is the knudsen number equation
Mean free path/ pore diamter
What is the Diffusivity in knudsen diffusion if kn > 5
D = (d/3)*(8RT/(pi)M)^0.5
What is knudsen flux
N = Deff (p1-p2)/RTL
For binary gas mixtures how are the fluxes related
Nb/Na = -(Ma/Mb)^0.5
What do industrial polymeric membranes require
High permeability thus rapid separation and high selectivity result in high yields
In reality there is a trade off in membranes between what
Permeability and selectivity
Polymers can be devided into two groups, what are they
Rubbery or glassy
how is the gas transfer behaviour in rubbery and glassy polymers explained?
In rubbery it is simple solution diffusion mechanism, in glassy it is more complex
Why are polymers that are used to separate the gas pairs CO2 and CH4 glassy polymers
They offer Improved permeability and selectivity combinations
What advantages to glassy polymers have over rubbery ones
Higher mechanical stability
High gas selectivity
What does sorption describe
The interactions between gas molecules and the membrane surface
Can occur physically or chemically
What does diffusion describe
The rate of gas passage through the membrane
Of the steps involved in solution diffusion mechanism which is the rate limiting step and why
Diffusion, because adsoprtion/desorption occurs rapidly in comparison
What is the equation for permeability
Permeability = Solubility x Diffusivity
What is the selectivity equation (define components)
(Alpha) = (Ya/Yb)/(Xa/Xb)
Y is concentrations in permeate
X is concentrations in feed
Selectivity is also defined as Pa/Pb
Where P is permeability
Explain Tg
Below a Tg a polymer is glassy, and above it, it is rubbery
If a polymer has a Tg above room temperature, it is a glassy polymer, if it has one below room temperature it is a rubbery
