Separation and enrichment of carbon dioxide by capillary membrane module with permeation of carrier solution

https://doi.org/10.1016/S1383-5866(02)00144-2Get rights and content

Abstract

A novel facilitated transport membrane for gas separation using a capillary membrane module is proposed in which a carrier solution is forced to permeate the membrane. Both a feed gas and a carrier solution are supplied to the lumen side (high pressure side, feed side) of the capillary ultrafiltration membrane and flow upward. Most of the carrier solution which contains dissolved solute gas, CO2 in the present case, permeates the membrane to the permeate side (low pressure side, shell side), where the solution liberates dissolved gas to form a lean solution. The lean solution is circulated to the lumen side. This type of capillary membrane module was applied to the separation of CO2 from model flue gases consisting of CO2 and N2. Monoethanolamine (MEA), diethanolamine (DEA) and 2-amino-2-methyl-1-propanol (AMP) were used as carriers or absorbents of CO2. The feed side pressure was atmospheric and the permeate side was evacuated at about 10 kPa. CO2 in the feed gas was successfully concentrated from 5–15% to more than 98%. The CO2 permeance was as high as 2.7×10−4 mol m−2 s−1 kPa−1 (8.0×10−4 cm3 cm−2 s−1 cmHg−1) when the CO2 mole fraction in the feed was 0.1 and temperature was 333 K. The selectivity of CO2 over N2 was in the range from 430 to 1790. The membrane was very stable over a discontinuous one-month testing period.

Introduction

Facilitated transport membranes have been attracting attention since very high selectivity is obtained compared to conventional polymeric membranes [1]. One of typical facilitated membranes is a supported liquid membrane (SLM), which is usually prepared by impregnating a microporous membrane with a carrier solution. While this type of membrane is very easy to prepare, the membrane is not stable enough to be used commercially. The instability of SLMs is mainly caused by the loss of the carrier solution from the pores of the membrane due to its evaporation and also due to the transmembrane pressure.

In order to overcome such instability problems of the SLM, we have already proposed a novel facilitated transport membrane in which a carrier solution is forced to permeate the membrane and is circulated between the feed and the permeate side of the membrane [2], [3], [4]. As shown in Fig. 1, a carrier solution, an aqueous amine solution in the present case, is continuously supplied to the feed side (high pressure side) of the membrane and absorbs CO2 to form carbamate or bicarbonate ion. The carrier solution is allowed to permeate the membrane to the receiving side (low pressure side) where the solution releases CO2 and the carrier is regenerated. The transferred carrier solution is recycled to the feed side by a pump. In this operation, since the membrane is always wetted with the carrier solution, the membrane becomes very stable with no open pores or liquid-unfilled pores through which gas flows to the low pressure side of the membrane unselectively. Therefore, very high selectivity can be obtained. Furthermore, since gas permeation accompanied by the convective transport of the carrier solution dissolving a solute gas mainly contributes to gas permeation, very high gas permeance is obtained compared to conventional SLMs in which gas permeates by the molecular diffusion mechanism. We named this type of membrane a ‘Bulk Flow Liquid Membrane’ (BFLM). In our previous papers on the separation of CO2 from CH4 using water [2] and aqueous diethanolamine (DEA) solutions [3] as membrane liquids as well as the separation of C2H4 from C2H6 using aqueous silver nitrate solutions [4] as the membrane liquid, we showed that the BFLM is very stable and has high permeability compared to conventional SLMs.

In our previous studies, we used flat membranes to make the data analysis easier, and the permeate side was swept by a sweep gas to shorten experimental time. However, for practical use, hollow fiber or capillary membrane modules are desirable due to their much larger specific membrane area than flat membrane modules. In addition, gas cannot be enriched by the sweep gas method. Another problem of our previous permeation cell is that gas–liquid contacting in the feed side is not enough for a solute gas to be absorbed effectively in the carrier solution. This disadvantage should be overcome since the proposed membrane system has high efficiency only when the liquid absorbs sufficient amount of gas in the feed side of the membrane.

In the present study, capillary membranes modules were fabricated to overcome the disadvantages of our previous permeation cell since capillary tubes have been recognized as very effective gas absorbers [5]. As shown in Fig. 2, both a feed gas and a carrier solution are supplied to the lumen side (high pressure side, absorption side) of the capillary membrane module and the carrier solution is forced to permeate the membrane to the shell side (low pressure side, stripping side) maintained at a reduced pressure to strip and enrich a solute gas. We applied this type of capillary membrane module to the separation and enrichment of CO2 contained in simulated flue gas.

Section snippets

Capillary membrane module

We fabricated two types of capillary membrane module, i.e. module A and B, which are schematically shown in Fig. 3. The polyethersulfone capillary ultrafiltration membranes (Daicel Chemical Industries, Ltd, inner diameter: 0.8 mm, outer diameter: 1.3 mm, molecular weight cut-off: 150 000, water permeability: 3×10−7 m3 m−2 s−1 kPa−1) were used. To obtain a regular packing in the module, two acrylic resin discs were regularly perforated (staggered arrangement with triangular pitches of 2.7 mm)

Characteristics of the reaction of CO2 with amines

In order to discuss the experimental results using various amines as carriers or absorbents, the information of the reaction between CO2 and amines, i.e. kinetics and chemical equilibria are necessary.

According to the zwitterions mechanism [6], CO2 reacts with primary and secondary amines to form an intermediate zwitterion.CO2+RR′NHk1k2RR′NH+COO

The zwitterions is deprotonated by bases such as amine itself and H2O existing in the solution to form carbamate.RR′NH+COO+RR′NH kAMRR′NCOO+RR′NH2+

Results and discussion

The experimental results are summarized in Table 1. The liquid circulation rate per capillary, vL, was 1.94 ml min−1 and the feed gas flow rate per capillary, vG, was 100 ml min−1 in all experiments. The number of capillary membrane was 18 and the liquid volume in the capillary module including connecting tubes and liquid reservoirs was about 135 ml. A steady state was reached within about 40 min after the start of each experimental run. The module A was used in most of experiments unless

Conclusion

A novel gas separation method using capillary membrane modules was proposed for simultaneous separation and enrichment of CO2 from simulated flue gases. Both a feed gas and a liquid absorbent were supplied to the lumen side of the membrane module and the liquid containing dissolved gas permeates the membrane to the shell side maintained at a reduced pressure, where stripping occurs. Preliminary experiments were performed in the temperature range from 323 to 353 K, and the following conclusions

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