이산화탄소 분리 회수룰 위한 중공사막 접촉기 hybrid 공정

연 순 화, 서 봉 국, 이 기 섭, 박 유 인, 이 규 호

한국화학연구원 분리막다기능소재 연구센터

Hybrid Process Using Hollow Fiber Membrane Contactor

for Carbon Dioxide Removal

Sun-Hwa Yeon, Bongkuk Sea, Ki-Seop Lee, Yu-In Park and Kew-Ho Lee

Membrane & Separation Research Center, Korea Research Institute of

Chemical Technology, Teajon 305-600, Korea

Introduction

The greenhouse effect is one of the important factors which influence the energy flows in the atmosphere. The gases responsible for this effect are greenhouse gases such as carbon dioxide, methane, nitrous oxide and chlorofluorocarbons. In particularly, carbon dioxide is the largest contributor to the greenhouse effect and is produced whenever fossil such as oil, natural gas or coal are burned.

The most common process for the removal of CO2 from gas streams is gas absorption using a conventional absorption column such as packed or plate columns. An aqueous solution such as sodium hydroxide, sodium carbonate, monoethanolamine (MEA) or diethanolamine (DEA) is often employed as an absorbent. Although the traditional packed bed absorbers have been used in the chemical industry for decades, there are several disadvantages such as flooding, channeling and foaming, which leads to difficulty of mass transfer between gas and liquid. An alternative technology that overcomes these disadvantages and also offers more interfacial area than conventional approaches is non-dispersive contactor via a microporous membrane[1, 2].

There are many advantages membrane contactors can offer over conventional absorption system. The available contact area remains undisturbed even at high and low flow rates because the two fluid flows are independent. In contrast, column is subject to flooding at high flow rates and unloading at low ones. Scale-up is more straightforward with membrane contactor. Membrane operations usually scale linearly, so that a predictable increase in capacity is achieved simply by adding membrane modules. Modular design also allows a membrane plant to operate over a wide range of capacities. Interfacial area is known and is constant, which allows performance to be predicted more easily than with conventional system. With packed columns the interfacial area per unit volume may be known, it is often difficult to determine the loading, that is, what fraction of the available interfacial area is actually used.

In principle, conventional absorption process can be replaced by a hollow fiber membrane contactor. Therefore, it is necessary to test the CO2 absorption capacity using various type of membrane modules combined with conventional stripper.

In this study, hybrid process equipped with the membrane contactor instead of the packed tower as the absorber in the conventional process is applied for carbon dioxide removal. The combined process was operated continuously recycling absorbent through membrane contactor and thermal stripper. PTFE, PVDF and PP hollow fibers are used as the membrane contactor and compared in the performance of CO2 absorption.

Experimental

Absorption modules.  The properties of the hollow fibers used in this study were shown in Table 1. PTFE (Polytetrafluoruethylene, Sumitomo Co. Japan), PVDF (Polyvinylidinefluoride, Krict Korea), PP (Polypropylene, Memtech Co. Australia) hollow fibers are used as membrane contactor. These hollow fibers are widely used as membrane contactor for gas absorption. PTFE and PP hollow fibers are symmetric, while PVDF hollow fiber is asymmetric. Cylindrical module of 0.02m i.d. and 0.23m length was used as the hollow fiber membrane contactor. The packing density was 0.4 for PP and PVDF module and 0.64 for PTFE module. For comparison, a packed column was used as the conventional process. The 1/4" PTFE rasching rings were used as packing material and the column was 0.027m i.d. and 0.2m length.

Hybrid process.  The experimental apparatus for CO2 recovery by hybrid process using membrane contactor was shown in Fig. 1. A gas mixture containing 25 vol.% CO2 in balance of N2 was fed into the fiber lumen and was in countercurrent contact with a absorbent solution fed in the shell side of the modules. 5wt.% monoethanolamine (MEA, Junsei chemical co.) aqueous was used as CO2 absorbent. The carbon dioxide in the feed gas reacts chemically with the absorbent while the purified gas is vented to the atmosphere, and the absorbent enriched by CO2 is pumped from the contact tower to a heat exchanger. The rich absorbent is preheated in the lean/rich exchanger by the hot lean solution returning from the regenerator/stripper on its way back to the absorber. The rich absorbent solution enters the top of the stripper where it flows down through the vessel countercurrent to the stripping steam generated in the solution reboiler. Steam and absorbent vapours move up the regenerator column, condensing as CO2 is liberated and the absorbent solution is heated. Uncondensed steam and carbon dioxide leave the top of the regenerator and then enter the reflux condenser. The condensate is returned to the system while the carbon dioxide is removed to further processing. The lean absorbent solution is pumped from the bottom of the regenerator directly to the lean/rich exchanger. The absorbent leaves the lean/rich exchanger after giving up heat to the rich solution and then enters a cooler, where its temperature is further lowered before being returned to the absorber. The absorbent and feed gas temperatures were maintained as 313K and 298K, respectively.

The outlet CO2 concentration of absorber was measured using the gas chromatography (Shimadzu GC-14B) equipped with thermal conductivity detector at an interval of 20 min. All data were obtained at steady state after sufficient operating time was allowed for system stabilisation. Ten samples were taken under the same operating conditions and the average value was calculated. The deviation between the sample values and the average value is less than 5%.

Results and Discussion

Fig. 2 shows CO2 absorption rate per unit volume for absorber modules when CO2 removal efficiency is 90-95%. Comparing to packed column, membrane contactor module exhibited higher absorption rate and particularly, absorption capacity of PVDF hollow fiber module was 2 times higher than that of conventional packed column. It may thus be presumed that membrane contactor possess a large amount of available liquid-gas contact area accelerating mass transfer of CO2. Because of the independent flow path in the hollow fiber membrane module without flooding and channeling, the membrane contactors can be provide considerably higher CO2 absorption efficiency.

The overall resistance to mass transfer is the sum of three individual resistance; gas phase, membrane and liquid phase. Thus, liquid phase overall mass transfer coefficient, KL can be represent as [1, 2]

(1/KL) = (1/Ek0L) + (1/kM)(RT/He)(di/do) +(1/kG)(1/He)(di/do)

and its value can be evaluated from individual mass transfer coefficients using appropriate correlations. The value of KL experimentally was determined using the following equation

KLa = Q/(△Plm/He)

Where Q is the CO2 absorption rate per unit volume of the contactor, △Plm is the log-mean partial pressure of CO2 calculated from the log-mean of the inlet and outlet CO2 concentration in the gas phase and, the average column shell side pressure. Membrane contactors show higher mass transfer rate than packed column. In the membrane contactors, Particularly, the PVDF hollow fiber module exhibit the highest mass transfer coefficient as 0.562m/s, that was as much as three times that typically obtained for conventional packed towers.

Economic assessment for the hybrid absorption process was conducted by comparison of the cost for absorbers. The CO2 recovery cost was calculated from the following equation

Capital cost of absorber[｀]

Recovery cost[｀/kg-CO2] = ---------------------------------------------------------

{Recovered CO2[kg-CO2/yr]}{Life time[yr]}

The capital cost of absorbers was estimated from the cost of hollow fibers and their housing. Fig. 3 shows the CO2 recovery cost with life time of the absorption modules. PTFE hollow fiber module shows very high recovery cost because of low absorption capacity and high fiber cost. In other hand, PVDF hollow fiber module was able to recover CO2 with cheaper cost when the life time of the fiber was 1 year, and this correspond to the recovery cost when the conventional packed column is used for 20 years.

References

1. Gabelman, A and Hwang, S.-T.: J. Membrane Sci., 159, 61(1999).

2. Noble, R. D. and Stern, S. A.; "Membrane Separations Technology

Principles and Applications", Elsevier, Netherlands p.467 (1995).