Membrane Gas Separation

Unshifted
Fuel Gas
Energy
Air
Stack 
Gas
Coal
Steam
Sulfur
Particulates
Gasifier
Water-Gas
Shift
Steam
Shifted
Fuel Gas
0-260°C
Concurrent
Separation
Post-shift
Separation
260-500°C
Turbine
O
2
Figure 10.2 The integrated gasifi cation combined cycle (IGCC) with integral water - gas 
shift reaction incorporated

---

188
Membrane Gas Separation
combustion turbine inlet temperature contributes signifi cantly to costs. Additionally, 
water is condensed from the gas stream during the cooling process and cannot then be 
expanded across the turbine to produce electricity. 
Given the desired high temperature and pressure for CO 
2
capture in IGCC systems and 
the current state of the art, major advances in separation technology will be necessary. 
Membranes are one technology with which such advancement might be possible. The 
National Energy Technology Laboratory (NETL) has pursued an approach based on 
supported liquid membranes (SLMs), which consist of a liquid transport medium and a 
solid support. The liquid allows higher diffusivity, leading to an increase in permeability 
over solid membranes and the potential to add chemical complexes which increase CO 
2
solubility, as shown in Figure 10.3 . There are two signifi cant problems in the development 
of practical SLMs: evaporation of the liquid at high temperatures and forcing of the 
transport medium out of the support due to large pressure differentials. A possible solution 
to evaporative failure would be the use of ionic liquids (ILs). Ionic liquids are organic 
salts with melting temperatures below ambient due to their inability to easily form stable 
crystal lattices and associated delocalized charge
[9] 
. As salts, these materials have 
negligible vapour pressure [10] and are also interesting for their considerable thermal 
stability and high CO 
2
solubility relative to hydrogen (H 
2
), nitrogen (N 
2
), and methane 
(CH 
4
). Proposed explanations for this high solubility include the interaction of CO 
2
and 
the anion occurring through a weak Lewis acid – base interaction and interactions of the 
polar ionic liquid ions with the large quadrupole moment of CO 
2
[10 – 16] .
The exceptional ability of ionic liquids to absorb CO 
2
along with their other chemical 
and physical properties makes them potentially interesting materials for CO 
2
separation 
N
N
C
6
H
13
N
N
NH
2
Permeability = Solubility × Diffusivity
Selectivity =
H
2
Permeability
CO
2
Permeability
Potential Solution:
Formation of Chemical Complexes
Loss of Selectivity at High Temperatures