HUNGARIAN JOURNAL OF INDUSTRY AND CHEMISTRY Vol. 45(2) pp. 45–49 (2017) hjic.mk.uni-pannon.hu DOI: 10.1515/hjic-2017-0020 APPLICATION OF A HYDROPHOBIC POLYMERIC MEMBRANE FOR CARBON DIOXIDE DESORPTION FROM AN MEA-WATER SOLUTION ZENON ZIOBROWSKI * , ADAM ROTKEGEL Institute of Chemical Engineering of the Polish Academy of Sciences, ul. Balycka 5, 44-100 Gliwice, POLAND Carbon dioxide desorption from a monoethanolamine (MEA) solution using a hydrophobic polydimethylsiloxane (PDMS) tubular membrane on a ceramic support is presented. The effects of operating parameters such as feed temperature, liquid flow rate and MEA concentration on mass transfer were examined. The mass transfer of CO2 from the liquid to gaseous phase was predicted by a multilayer film model with an accuracy of ±25%. Research into new selective materials is needed to develop more efficient and environmentally friendly CO2 capture tech- nology Keywords: MEA, desorption, carbon dioxide, hydrophobic membrane, PDMS 1. Introduction Fossil fuel combustion from power plants is one of the most significant sources of CO2 emissions [1]. The sep- aration of carbon dioxide from gases can be realized by processes such as adsorption, absorption, low tempera- ture distillation and membrane separation. The absorp- tion of carbon dioxide in amine based solutions is cur- rently the most widespread method in industry for the post-combustion capture of CO2 [2]. The advantage of chemical absorption in amine so- lutions is the fact that at higher temperatures the chemi- cal reaction can be reversed and the amine recycled. On the other hand, obstacles include a relatively low CO2 capture capacity, solvent losses caused by evaporation, thermal stability, highly corrosive characteristics, eco- toxicity and biodegradability in the natural environment [2-4]. It was shown that MEA and diethanolamine (DEA) might promote potential long-term toxicity ef- fects towards living organisms [5,6]. In addition the regeneration step may increase the total operating costs of the capture plant by up to 70%, especially for prima- ry and tertiary amines where the heat of reaction is quite high [7]. The amine scrubbing processes carried out in packed columns are currently most widely used in in- dustry for the post-combustion capture of CO2. Limiting factors for the application of this technology are its size and large capital costs. The mass transfer performance of this solution can be reduced by flooding, foaming and entrainment conditions. *Correspondence: zenz@iich.gliwice.pl In comparison to the studies on CO2 absorption in MEA solutions there are only a few concerning CO2 desorption, despite the fact that the stripping unit is re- sponsible for most of the separation cost of the process [8]. It is important that materials used in the processes concerning post-combustion capture of CO2 exhibit low or no environmental effects. Various tubular membranes were operated as catalyst supports [9]. Recently a new type of ceramic hollow fiber membrane contactor has been studied [10]. This kind of membrane can be modi- fied to be hydrophobic which enables it to be applied for CO2 absorption-desorption in amine solutions. In this study the process of CO2 removal from an MEA solution using a hydrophobic polydimethylsiloxane (PDMS) tubular membrane on a ceramic support was investigated. 2. Experimental 2.1. Experimental setup The experimental setup shown in Fig.1 consisted of a membrane module, reactor vessel, cooling system, as well as circulation and vacuum pumps. The hydropho- bic PDMS membranes on ceramic support (ceramic tubes with an outer diameter of 0.01 m and length of 0.25 m using a PVM 250 membrane module made by Pervatech BV) was studied. The feed was circulated by a pump and the flow rate was controlled by a flowmeter. In all experiments the feed temperature was stabilized by a thermostat (1C). The permeate was condensed and collected in cold traps immersed in liquid nitrogen. The vacuum pump was used to maintain the pressure between 7 and 10 mmHg on the permeate side. The concentration of ZIOBROWSKI AND ROTKEGEL Hungarian Journal of Industry and Chemistry 46 carbon dioxide in the permeate was calculated by meas- uring the mass of carbon dioxide and water in the ana- lyzed permeate sample. The pressures on the feed and permeate sides were measured by pressure gauges. The temperatures of the feed in the reactor vessel, before and after the membrane module were measured by thermo- couples. Pure monoethanolamine (MEA) and deionised wa- ter were used to prepare the liquid-feed solution. After- wards the obtained solution was loaded with CO2 by bubbling pure CO2 in a magnetically stirred vessel until the required carbonation ratio, , was achieved. In our experiments the carbonation ratio was determined by measuring the mass of absorbed CO2 in the amine solu- tion at a given temperature. Additionally, independent pervaporation experi- ments with the same PDMS membrane under similar thermal and hydrodynamic conditions for a 2-propanol – water mixture were performed to estimate the mem- brane resistance (1/kM). 2.2. Experimental results The performance of the PDMS membrane was exam- ined experimentally. The operating temperature was between 323 and 348K (50 and 75°C), liquid flow rate between 20 and 600 l/h and the MEA concentrations were 5, 10 and 15 wt%. The effect of liquid flow rate on the CO2 mass flux and selectivity is presented in Figs.2 and 3 for the tem- perature of 323K (50°C) and 10% MEA concentration. The selectivity of the process is defined as follows: 2 2 2 2 CO CO CO CO ( (1 )) ( (1 )) p f w w S w w    (1) The measured fluxes increase with the Reynolds num- ber. The highest values were obtained for Re>10,000 (turbulent flow). This can be explained by the CO2 mass transfer increase in the liquid phase for turbulent re- gime. The measured selectivities rise with the Reynolds number and for turbulent flows reach the value of 10. The operating temperature is an important parame- ter as far as the efficiency of the membrane is concerned as shown in Fig.4. For a given turbulent liquid flow rate the measured CO2 mass fluxes rise with the feed tem- peratures due to the increased driving force in favour of CO2 mass transfer. The selectivity does not change sig- nificantly with the operating temperature, Fig.5. The effect of the MEA concentration on mass flux and selectivity is presented in Figs.6-7 at an operating temperature of 323K (50°C) and turbulent flow (Re of about 40,000). The measured mass fluxes do not change signifi- cantly with MEA concentration (Fig.6), because of the Figure 1. The experimental setup: 1 – membrane contactor, 2 – feed tank, 3 – cold traps, 4 – circulation pump, 5 – vacuum pump, 6 – heater Figure 3. The effect of Re number on selectivity (T = 50°C and wMEA = 10 wt%) Figure 2. The effect of Re number on CO2 mass flux (T = 50°C, wMEA = 10 wt%) Figure 4. The effect of feed temperature on CO2 mass flux (wMEA = 10 wt%) APPLICATION OF HYDROPHOBIC POLYMERIC MEMBRANE FOR CO2 DESORPTION ... 45(2) pp. 45–49 (2017) 47 relationship between equilibrium constants of the CO2 - MEA reaction and the CO2 solubility in water at a given temperature. The selectivity decreases with MEA con- centration as a result of the rising amount of CO2 ab- sorbed in the MEA solution and the constant CO2 flux in the permeate, see Fig.7. 3. Mathematical model and calculation results When CO2 is absorbed in aqueous monoethanolamine (MEA) solution, the following reactions can be written as [11]: slow 2 2 2CO RNH RN H COO    (2) fast 2 2 3RN H COO RNH RNH RNHCOO       (3) The formation of carbamate is well understood and the rate of the forward reaction has been determined as first order with respect to both CO2 and RNH2: CF 2 2[CO ][RNH ]r k (4) During the desorption process the differences in the concentration of the component and the temperature between the inlet and outlet in the liquid phase are very small. Therefore, the desorption rate may be simply calculated using the arithmetic mean value of CO2 in the liquid phase. With this assumption we can calculate the mass fluxes of CO2 can be calculated as follows: 2 2 2 * LCO CO CO( )N K x x  (5) where NCO2 [kmol/s] is the flux of CO2 and KL [kmol/m 2 s] is the overall mass-transfer coefficient of the liquid phase. The overall mass-transfer coefficient for CO2 can be evaluated by a resistance-in-series model [12]. The numerical calculations based on model equa- tions were performed and estimated values of mem- brane resistance (1/kM) used. In the calculations the ex- perimental values of the Henry’s constant for CO2 in water and MEA under standard conditions are 1.2456 and 1.5732, respectively [13]. The enhancement factor of the chemical reaction of CO2 in the liquid phase, as defined by DeCoursey [14], was between 20 and 60. The viscosity of the water–MEA mixture was calculated according to a Grunberg and Nissan equation [15]. Cal- culated and experimental values of CO2 mass fluxes are Figure 5. The effect of feed temperature on selectivity (wMEA = 10 wt%) Figure 6. The effect of MEA concentration on CO2 mass flux Figure 7. The effect of MEA concentration on selectivity Figure 8. Comparison of calculated values of CO2 fluxes with experimental ones ZIOBROWSKI AND ROTKEGEL Hungarian Journal of Industry and Chemistry 48 shown in Fig.8. The scattering of calculated and exper- imental values of CO2 mass fluxes was within the range of ±25% . The experimental values of CO2 mass fluxes were compared with those obtained from the literature for CO2 stripping in a ceramic hollow fiber membrane con- tactor [10]. In spite of the different types of membrane type and hydrodynamic conditions the measured values of CO2 mass fluxes were comparable in both cases. Conclusions The application of a membrane in the process of CO2 stripping from MEA solutions avoids some technical problems that are encountered in industrial practices. The PDMS hydrophobic tubular membrane on a ceramic support can be applied for the removal of CO2 from MEA solutions. In developed turbulent flows the measured CO2 mass fluxes and selectivities do not change significantly with Re number (Figs.2-3). The measured CO2 mass fluxes increase as the feed tempera- ture rises (Fig.4) and slightly depend on the MEA con- centration (Fig.6). The measured and calculated CO2 mass fluxes are in good agreement with each other (Fig.8). The ±25% variation in scattering can be ex- plained by the accuracy of the correlations, experi- mental precision and simplification of the model. 4. 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