(35e) CO2 Removal from CO2/H2 Mixtures Using Immobilized Liquid Membranes Containing Amine Blends as Carrier

Abstract

Selective separation of carbon dioxide from a wet gaseous mixture of CO₂-H₂ through facilitated transport membranes containing immobilized aqueous solutions of MEA, DEA and EDAH+ and their mixtures was experimentally investigated. The effect of CO₂ partial pressure, amine concentration and amine type in case of equilibrium constant of the reaction between CO₂ and amines on CO₂ and H₂ permeation have been studied. The CO₂ permeance through amine solution membranes decreases with increasing CO₂ partial pressure of feed and H₂ permeance was almost independent of H₂ partial pressure. A comparison of experimental results showed that the mixed or single amines with low viscosity and moderate equilibrium constant (i.e. large forward and reverse reaction rate of CO₂-amine) are proper for effective separation of carbon dioxide. Permeability of CO₂ generally increased with an increase in amine concentration howbeit it may be compromised by the salting effect and decreased species diffusivities. The results obtained indicated that permeation rate of CO₂ across a variety of amines are in order of DEA (2M)> MD (2M)> MD (1M)> MEA (2M)> MEA (4M)> MD (4M)> DEA (1M)> DEA (4M)> MEA (1M) for various concentrations of MEA and DEA mixture and are in order of EDAH⁺ (2M)> DEA (2M)> MH (2M)>DH (2M)> MEA (2M) for various mixture of amines.

1. Introduction

The selective removal of CO₂ form natural gas, refinery and synthesis gas streams is an important and critical process because CO₂ poisons catalysts, it is one of major greenhouse gases and highly corrosive. 

Hydrogen is an environmentally clean energy source since unlike fossil fuels, it produces water as the only by product. Hydrogen is expected to be an energy source in the 21^st century. Hydrogen is usually produced by reforming reaction, followed by the water gas shift reaction. CO₂ associated with hydrogen should be removed since it acts as impurity in case of H₂ production and poisons catalysts when synthesis gas used as feedstock in petrochemical processes such as ammonia plants.

It is therefore to extremely important to develop more efficient methods for CO₂ separation from hydrogen rich mixtures.

Facilitated transport is a well-known process in which the reversible complexation reactions are carried out between solutes and reactive carrier and produced complex diffuses through a liquid membrane. Carrier molecules transport the solutes through a liquid membrane and then release them on the permeate side. This mechanism enhances the permeability and selectivity of liquid membranes. So, due to their very high permselectivity compared with conventional polymeric membranes and also low energy consumption, they have attracted the attention of many industries and researchers.

Recently, the interest in the use of mixed solvents in gas treating processes is increasing due to utilizing the advantages of each amine.

Blends of amine solvents which combine the higher reaction rate of one solvent with higher reverse reaction rate or lower viscosity of another solvent are suggested to employ as liquid films immobilized in micro porous membranes.

The purpose of this study is the experimentally investigation of CO₂ permeation characteristics through immobilized various aqueous mixtures of amines membranes. The specific objective of present study is to investigate the possibility of such membranes for CO₂ separation from hydrogen streams in terms of selectivity and permeability.

2. Experimental

Experimental Setup

The schematic diagram of the experimental apparatus is shown in Figure 1. a double layer polymeric flat sheet membranes system is housed in a holder designed to allow a feed stream and sweep gas to pass counter-current on opposite sides of the membrane.

The hydrophilic membrane was soaked in each aqueous amines solution for more than 12 hr to ensure complete filling of the micro pores with liquid solution. The total exposed area on each surface of the membrane was 100 cm2.

The feed gas was a mixture of CO₂ and H₂ and the sweep gas was N2. The partial pressure of CO₂ in the feed gas was changed from 0.87 kPa to 17.4 kPa since the CO₂ content in industrial synthesis gas is below 15~20 mol%.

The permeated H₂ and CO₂ gases in the sweep gas were analyzed by an online gas chromatograph. The permeation experiments were performed at temperature 25 ± 0.5 ºC.                                                                                                                                   

3. Results and Discussion

The main objective of this research was to characterize the immobilized liquid membrane based process using mixture of amines in order to combine the benefits of each amine for effective removal of CO₂ from H₂.

In this regards the permeation experiments through some conventional primary and secondary amines such as MEA, DEA, EDAH⁺ and their mixtures in various concentrations have been done

The total permeation of CO₂ is composed of physically solution-diffusion plus that of chemically combined CO₂-carrier complex. As it is seen the CO₂ permeance through amine solution membranes decreases with increasing CO₂ partial pressure of feed. This depression may be explained in term of salting effect which is resulted to lesser solubility of CO₂ in amine solution. It also should be noted that the increased complex and other ionic concentrations increase liquid solution viscosity and accordingly decrease the species diffusivities.

All figures show that the permeation rate of H₂ which take place via the physically solution-diffusion mechanism is slightly decreased with H₂ partial pressure and is almost independent of H₂ partial pressure. The results obtained indicated that permeation rate of CO₂ across a variety of amines are in order of DEA (2M)> MD (2M)> MD (1M)> MEA (2M)> MEA (4M)> MD (4M)> DEA (1M)> DEA (4M)> MEA (1M) for various concentrations of MEA and DEA mixture and are in order of EDAH⁺ (2M)> DEA (2M)> MH (2M)>DH (2M)>MEA (2M) for various mixture of amines.

A comparison of experimental results showed that the mixed or single amines with low viscosity and moderate equilibrium constant (i.e. large forward and reverse reaction rate of CO₂-amine) are adequate for improvement of selective removal of CO₂, although the H₂ permeation may be increased. Some of results have been shown in Figures 2 and 3.

Symbols used

Figure 1- Schematic process flow diagram of experimental setup

Figure 2- Effect of feed partial pressure of CO₂ on the CO₂ and H₂ permeances and separation factor for MEA(2M), DEA(2M), MD(1M), MD(2M) and MD(4M) membranes

Figure 3- Effect of feed partial pressure of CO₂ on the CO₂ and H₂ permeances and separation factor for MEA(2M), EDAH⁺(2M) and MH(2M) membranes