CO2 Capture from Gas Mixtures Using Membrane-Liquid Absorbent in Hollow Fiber Membrane Contactors

Abstract

CO₂ concentration of the earth's atmosphere because of excessive CO₂ emission a great consumption of fossil fuels has been risen in the last 100 years. The excessive CO₂ in the atmosphere has been significantly contributing to the global warming. CO₂ emission has become a subject of worldwide concern. It is necessary to explore cost-effective, low energy-consumption, available and efficient technologies for capture and removal of CO₂ generated by the industrial sources.

Conventional techniques such as absorption, adsorption, conversion and cryogenic separation for the removal and capture of CO₂ are energy-consuming and not easy to operate because of the frequent problems including flooding, foaming, channeling, entrainment and high capital and operating costs.

Membrane contactor as a hybrid process is a promising alternative which has been one of research focuses for decades because of its advantages over the traditional processes. Membrane contactor can provide much higher specific surface area, independent controllable gas and liquid flowrates, a compact and energy efficient process and an easier scale-up design. It has also high selectivity and high mass transfer coefficients.

In this technology, a good absorbent should be the liquid with high selectivity, good gas solubility, good reactivity, high absorption rate and high capacity, low volatility, low regeneration energy, no degradation, no corrosiveness

A membrane contactor usually consists of a bundle of hydrophobic microporous symmetric hollow fiber membranes where the fluids flow on each side of the membrane without direct any contact. For gas–liquid absorption operations. In these processes, the membrane contactor mainly acts as a physical barrier between two phases (gas and liquid) without significant effect in selectivity, i.e. the membrane does not change the partition coefficient. Being the two phases separate by the membrane, there is no phases mixing and dispersion phenomena. The gases are transferred from one phase to the other only by diffusion. Hydrophobicity of gas-liquid membrane contactors increases mass transfer rates and separation efficiency. The membranes do not allow the liquid phase to penetrate through the membrane pores and thus the gas phase fills the membranes pores.

In this study chemical absorption of CO₂ from gas mixtures (especially CH₄) was carried out in a polypropylene porous asymmetric hollow fiber membrane contactor (HFMC). The absorption medium used was an aqueous solution of methyldiethanolamine (MDEA). Effects of gas flow rate, liquid flow rate and CO₂ concentration on the CO₂ removal percentage were investigated.

This research aims at studying an efficient removal of CO₂ from gas streams containing high concentration of CO₂ using a hollow fiber membrane module, so as to reduce the amount an as a result the re-generation process capacity of absorbent used.

The experimental results showed that, CO₂ removal performance using MDEA solution were much higher than those of reported for packed towers. The increase of evaluated parameters including CO₂ feed concentration and gas flow rate, significantly decreased the CO₂ removal percent. A reverse effect observed for liquid flow rate.

The results showed that the removal of CO₂ with aqueous solution of MDEA was very high and indicated almost total removal of CO₂. Experimental results also indicated that the membrane contactor was very efficient in the removal of trace CO₂ at high gas/liquid flow ratio. The removal of CO₂ was almost complete with recovery of higher than 99%. Using feed gas mixtures containing 20.0 vol. % CO₂, the outlet CO₂ concentration of less than 1.0 vol. % was attained.