Novel adsorbents for natural gas separation and purification

Abstract

This thesis explores novel adsorbents for separation of three different types of gas mixtures found in liquified natural gas processes (a) CO2 from CH4 and N2, (b) CH4 from N2, and (c) helium from N2 and CH4. These separations represent challenging operations in natural gas processing because the conventional technologies to remove CO2 such as amine absorption units, reject N2 and recovery helium by cryogenic distillation are capital and energy intensive. Cyclic adsorption processes such as pressure swing adsorption (PSA) have potential as alternative technologies to reduce equipment costs and improve energy efficiency in liquified natural gas (LNG) production facilities, especially for small-scale plants. However, there are several challenges related to the development of adsorbents required to advance PSA technologies for natural gas processing. These include the development of adsorbents with excellent selectivity, low-pressure drops, and low production costs.Chapters 4 and 5 of the thesis report novel carbon foam monoliths that have several potential advantages such as lower pressure drops, better heat transfer properties, lower void fractions and higher mechanical strength in fixed-bed adsorption processes over pellets or granular adsorbents. In Chapter 4 I report pitch-derived carbon foam monoliths, and in this chapter I investigated the effects of using coal as a filler particle and the amount of potassium hydroxide on the stability of tar pitch during the foaming process, the product's density, and the micropore structure. These pitch-derived carbon monoliths featured an open-cell structure and a well-developed microporosity that presented a BET specific surface area of 1044 m 2 .g -1 . At 298 K and pressures close to 3500 kPa the adsorption capacities of the carbon monolith prepared with 50 %wt coal to pitch were 7.398 mmol.g -1 CO2, 5.049 mmol.g -1 CH4, and 3.516 mmol.g -1 N2.The second type of carbon foam developed in this thesis was a monolithic carbon foam with an open cellular structure that was synthesized from banana peel using a soft-template method with zinc nitrate, furfural, and 2-aminophenol (Chapter 5). I extended the experimental methods to investigate the effects of (a) carbonization temperature and (b) post-carbonization CO2 activation to enhance the microporosity of the carbon foams as adsorbents for CO2 capture. The CO2-activated carbon foams featured BET surface areas up to 1426 m 2 .g -1 . The effect of surface chemistry and N-containing functional groups on the CO2 uptake was also investigated and it was showed that both nitrogen functional groups and microporosity are critical parameters in equilibrium CO2 absorption of the carbon foams. To evaluate the potential of these carbon foams as adsorbents for gas separation, the adsorption capacity of the carbon foams for CO2 and N2 were measured by a gravimetric sorption. At 298 K and pressures of 3990 kPa, the carbon foam synthesized at 1273 K in CO2 adsorbed 9.21 mmol.g -1 of CO2 and 3.29 mmol.g -1 of N2.II Although the synthes...