Chapter 5 - Review of CO 2/CH 4 Separation Membranes

Abstract

Carbon dioxide separation from natural gas streams is an important industrial process where membrane systems are expected to play a more significant role in the coming years. In this chapter, we shall review the current state-of-the-art membranes for CO 2 /CH 4 separation: inorganic (carbon, zeolite, or silica), polymeric (pure or blend), and composite (mixed-matrix or room-temperature ionic liquid). The best performance in terms of CO 2 permeance and CO 2 /CH 4 selectivity was achieved using inorganic zeolite molecular sieve membranes that had CO 2 permeance in the range of 5.0 × 10 − 8 –2.0 × 10 − 6 mol m − 2 s − 1 Pa − 1 (1000–30,000 Barrer) with CO 2 /CH 4 selectivity ranging from 30 to 400. Carbon molecular sieve membranes followed closely with CO 2 permeance in the range of 2.0 × 10 − 9 –3.4 × 10 − 7 mol m − 2 s − 1 Pa − 1 (20–1000 Barrer) and selectivity of 30–150. Amorphous silica membranes displayed excellent separation properties with CO 2 permeance of around 7 × 10 − 8 mol m − 2 s − 1 Pa − 1 and CO 2 /CH 4 selectivity of up to 380. Organic polymeric membranes and composite organic–inorganic (mixed matrix) membranes exhibited reasonable CO 2 /CH 4 selectivity of 10–80 but poor CO 2 permeance in the range of 2 × 10 − 12 –1 × 10 − 8 mol m − 2 s − 1 Pa − 1 (1–400 Barrer). Supported ionic liquid membranes displayed CO 2 permeance comparable with the polymeric systems in the range of 1 × 10 − 11 –2 × 10 − 9 mol m − 1 s − 1 Pa − 1 (3–700 Barrer) but with inferior CO 2 /CH 4 selectivity between 7 and 40. In order to find industrial application, new membrane materials must offer dramatic improvements in CO 2 permeance and CO 2 /CH 4 selectivity compared to existing (polymeric) membrane systems. New membranes must have excellent thermal and chemical stability (particularly in hydrocarbon or oxygen rich environments), resistance to plasticization (for polymeric membranes), resistance to aging, low cost, and ease of scale-up in order to justify the inevitable increase in cost.