Gas Separation Membrane - KOCw

Gas Separation

Yang, Seung Yun

(Biomaterials Science, PNU)

Gas Separation Membrane

2

 Separation in gas mixture is occurred when the gases have different diffusion ability and affinity to membranes

 Gas separation is possible with the two extreme types of

membranes (porous and dense (nonporous) membranes) but transport mechanism are completely different.

 This technology has become a major industrial applications of membranes only during the past 20 years.

Gas Separation

3

 Thomas Graham measured the permeation rates of all the gases then known, through every diaphragm over a period of 20 years

 solution-diffusion model (Graham’s law)

 Graham’s law was exploited for the first time as part of the Manhattan project (concentration of U

²³⁵

F

₆

from U

²³⁸

F

₆

)

Thomas Graham (1805-1869)

m: molecular mass of molecules v: velocity of molecules

 Graham’s law of effusion

First Study in Gas Separation

4

(hydrogen)

Milestones in Gas Separation

5

Poiseuille(viscous) flow (D(pore dia.)>1um) -No separation is occurred

Knudsen diffusion (D>1um)

-Mean free path of the gas molecules may become greater than D

Size exclusion (0.5< D < 2nm)

-Gases are separated by molecular sieving

Transport of Gases in Porous Membranes

6

Poiseulle (D/λ ＞5)Flow

Knudsen (D/λ ＜1)Flow

: Mean free path

P: Pressure

D: Dia. of gas molecule T: Abs. Temp.

D (2r)  Transport in knudsen

flow

D_k : Knudsen Diffusivity (확산계수)

 Selectivity

Ex)

Poiseulle vs. Knudsen Flow

7

 In case that gas molecules to be separated have a similar size

 Gas mixture are separated based on solution-diffusion model

diffusion dissolution

Dense membrane

1^st step: Absorption of gas on feed side (high pressure) and dissolution into the membrane 2^nd step: diffusion of dissolved gas through the free volume in membrane and desorption of

gas at interface of permeate(low pressure) side

feed permeate

Transport in Non-porous Membranes

8

feed permeate

Fick’s law

Applying Henry’s law (S: distribution coefficient) (용해도계수)

Transport Theory in Non-porous Membranes

9

Permeability (P) = diffusivity (D) x Solubility (K)

Membrane Selectivity

 Diffusion coefficient (D_i): mobility of the gas molecules in the membrane material

 Gas sorption coefficient (K_i): number of molecules dissolved in the membrane

Selectivity in Nonporous Membrane

10

Diffusion coefficient as a function of molar volume for a variety of permeants in natural rubber and in poly(vinyl chloride), a glassy polymer.

Diffusion Coefficient of gases in Membranes

11

Larger permeants are more

condensable and have higher sorption coefficients

Gas sorption coefficient as a function of molar volume for natural rubber membranes

Gas Sorption Coefficient

12

For natural rubber, the

permeability increases rapidly with increasing permeant size because sorption dominates

For glassy polymer, the

permeability decreases with increasing permeant size because diffusion dominates

Permeability of Gas Separation Membrane

13

Materials for Gas Separation Membrane

14

 Hydrogen separation

 Nitrogen production from air

 Natural gas separations

 CO₂ separation

 Dehydration

- To control corrosion of the pipeline and to

- To prevent formation of solid hydrocarbon/water hydrates can choke valve

 Vapor/vapor separation

The membrane gas separation industry is still growing and changing.

Applications of Gas Separation Membranes

15

 The first large-scale commercial application

 Hydrogen is a small, noncondensable gas, which is highly permeable compared to all other gases

 the glassy polymers primarily used to make hydrogen-selective membranes

Hydrogen Separation

16

 The largest gas separation process in current use

 Trade-off between permeability and selectivity

Strong inverse relationship between flux and selectivity

Nitrogen Production

17

 Required composition of natural gas (US)

 Size and condensability (boiling point) of natural gases

Glassy membranes generally separate by differences in size; rubbery membranes separate by differences in condensability

 Membrane materials used for natural gas separations

Natural Gas Separations

18

 Used for separation of close-boiling mixtures

 For example, ethylene (bp −93.9 ◦ C) from ethane (bp

−88.9 ◦ C), propylene (bp −47.2 ◦ C) from propane (bp

−42.8 ◦ C), and n -butane (bp −0.6 ◦ C) from isobutane (bp

−10 ◦ C)

 Ceramic or carbon membrane would be useful to achieve a high selectivity

Vapor/Vapor Separation