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Absorption rates of CO2 into aqueous mixtures of MDEA (methyldiethanolamine) and HMDA (hexamethylenediamine)
Greenhouse Gas Control Technologies, Volume n M. Wilson, T. Morris, J. Gale, K. Thambimuthu (Eds.) © 2005 Elsevier Ltd. All rights reserved
1901
ABSORPTION RATES OF CO2 INTO AQUEOUS MIXTURES OF MDEA (METHYLDEETHANOLAMINE) AND HMDA (HEXAMETHYLENEDIAMINE) Sungyoul Park'*, Jongsup Leel\ Byoungmoo Min^ Kyungryong Jang^& Heemoon Eum^ 'Korea Institute of Energy Research, Daejeon, Korea ^Korea Electric Power Research Institute, Daejeon, Korea ABSTRACT Absorption rates of CO2 by aqueous mixtures of MDEA(20.5 wt%) and HMDA(0.7, 3.5, 7.0 and 14.4 wt%) were measured by the apparatus for gas absorption equilibrium. The experimental results showed that the absorption kinetics of CO2 into the aqueous mixtures of MDEA and HMDA regarded as a pseudo-first order reversible reaction. As the concentration of mixed HMDA increased, the apparent absorption rate constants were increased from 25 %(0.7 wt% HMDA) to 292 %(14.4 wt% HMDA) comparing with 20.5 wt% MDEA. INTRODUCTION The improvement of conventional absorbents is mainly focused on MDEA (methyldiethanolamine). MDEA has been used for the selective separation of H2S from gas mixture of H2S and C02[l]. Due to the low vapor pressure of MDEA, the loss of absorbent by evaporation is very small. And also, MDEA has strong resistance to thermal and chemical degradation and doesn't have corrosiveness. The small reaction heat of MDEA is useful for the bulk separation of CO2 from synthetic gas and natural gas. Indirect reaction of MDEA with CO2 made the stripping of CO2 by pressure decrease possible and decrease the heat consumption of CO2 stripping by evaporation than those of MEA (monoethanolamine) and DEA (diethanolamine). Therefore, MDEA has been considered as one of the efficient absorbent which solve the economical problem of absorption process. In spite of the above advantages, MDEA has disadvantages in the absorption of CO2. Due to low reaction rate with CO2, high partial pressure of CO2 and circulation flow rate of absorbent is required to absorb more CO2. This made packed column for CO2 absorption with MDEA have higher packing height and larger diameter of packing column as compared with those of MEA and DEA[2]. In order to enhance the slow reaction rate of MDEA, various researches in the mixing of additives which improving the absorption rate of CO2 to MDEA has been performed. As additives for reaction rate enhancement, MEA, DEA[3] and piperazine[4] were mixed with MDEA aqueous solution and the addition of alkyleneamine with MDEA solution enhanced 25 - 200 % of absorption rate and over 70 % of absorption capacity[5]. Considering chemical absorption is based on the chemical reaction according to stoichiometry, HMDA of which molecular weight is similar to MDEA is selected as an additive for absorption rate enhancement. And the variance of absorption rate and capacity of CO2 by the aqueous mixture of HMDA (hexamethylenediamine) and MDEA. EXPERIMENT Absorbent Preparation and Experimental Apparatus Absorbents were prepared by the mixing of 99 % MDEA(Aldrich Co.), 70 % HMDA(Aldrich Co.) and distillated water and the concentrations of absorbents were controlled to have 20.5 wt% MDEA + 0.7 wt% HMDA, 20.5 wt% MDEA + 3.5 wt% HMDA, 20.5 wt% MDEA + 7.0 wt% HMDA and 20.5 wt% MDEA + 14.4 wt% HMDA. Experimental apparatus was made of SS316 and consist of reservoir(5933 cm^) for the measurement of injected CO2, mixer(4044 cm^) for the circulation of CO2 and absorber(428 cm^) for the absorption of CO2. The apparatus was installed in temperature controlled air bath. For the fast absorption and equilibrium by contact with absorbent, CO2 was circulated by cylindrical piston pump. Glass beads were packed in the bottom of absorber to disperse the
*Corresponding author: Tel. +82-42-860-3046, Fax. +82-42-860-3691, Email: [email protected]
1902
injected CO2 and monitored by view cell installed in absorber. The schematic diagram of CO2 absorption equilibrium apparatus was shown in figure 1.
Figure 1: Schematic Diagram of CO2 absorption equilibrium apparatus Operating pressures and temperatures of reservoir, mixer and absorber were measured by three pressure transducers(Walcom) with accuracy of 0.001 kg/cm^ and four K-type thermocouples with accuracy of 0.1 K respectively. Temperatures of absorbent and CO2 in absorber were measured separately. The measured pressures and temperatures were recorded by hybrid recorder(Yokogawa, HR2300) and stored in computer as data file. The concentration of CO2 in gas sample was measured by gas chromatograph(HP5890A). Experimental Methods Before experiment, CO2 was stored in reservoir and nitrogen was supplied to mixer and absorber to remove impurities and air. The gas samples were analyzed by gas chromatograph to confirm that only nitrogen is contained in mixer and absorber. After the injection of absorbent, temperature was increased to 50 °C and equilibrium pressure of nitrogen was measured with the circulation by piston pump. After the equilibrium temperature and pressure, specific amount of CO2 was injected to absorber from reservoir and CO2 absorption reaction was started with the circulation of CO2 by piston pump. Equilibrium pressure before CO2 injection is the summation of nitrogen pressure and vapor pressure of absorbent and the amount of injected CO2 was calculated fi-om the pressure decrease in reservoir. CO2 partial pressure of absorber in initial state was calculated from the pressure increase of absorber by CO2 injection. The state that absorber pressure does not change is defined as equilibrium state and RESULTS AND DISCUSSION Absorption Rate of CO2 The absorption rate of prepared absorbents were measured and compared. The experiments were performed at the same initial condition of 50 °C, 215 kPa, 0.536 of CO2 mole ratio and 14.7 1/min of CO2 circulation flow rate. As the absorption proceeds, CO2 partial pressure of absorber decreased and the results were shown on figure 2. The experimental results showed that CO2 partial pressure was rapidly decreased with the increase of HMDA concentration. This means that the absorption of CO2 is dependant on the concentration of HMDA and can be explained that at first HMDA, which has fast reaction rate, reacts with CO2 at the interface of gas and liquid phase and then the intermediate reacts with MDEA at liquid bulk. At this step, HMDA was regenerated and reused for the reaction with CO2 at the interface. Through these steps, it was found that the mixture of HMDA and MDEA show higher reaction than pure MDEA solution
1903
100
150
Run Time (min)
Figure 2: Absorption rate of CO2 into aqueous mixture of MDEA and HMDA Absorption rates of CO2 with HMDA concentration were quantitatively compared by the apparent rate constant, kapp- Unless very high CO2 partial pressure or very high conversion rate of absorbent, it can be assumed that the concentration of absorbent in bulk liquid is very high and that of absorbent in the interface is constant. Therefore, the reaction rate can be described as the equation (1) and derived to pseudo first order reaction as equation (2). '^app yC02
"
(1)
''^Clh I
-ln{
(2) 'C02
Where /;,^ is the absorption rate of CO2 in absorbent, k^,, is apparent rate constant, p^^^ is CO2 partial pressure of absorber, p^; is initial CO2 partial pressure of absorber, p*^ is equilibrium CO2 partial pressure of absorber and t is time. Apparent absorption rate constant was calculated by Levenber-Marquardt optimization method using equation (2) and the values with HMDA concentrations were shown on table 1.
TABLE 1: COMPARISON OF APPARENT RATE CONSTANT AND ENHANCEMENT OF APPARENT RATE CONSTANT HMDA concentration (wt%) 0 0.7 3.5 7.0 14.4
Apparent rate constant (sec"', X 10^) 0.4583 0.5716 1.0833 1.4133 1.7983
Enhancement of apparent rate constant(%)
24.7 136.4 208.4 292.4
The calculated apparent rate constants showed 18 - 292 % increase comparing with that of 20.5 wt% MDEA aqueous solution. Though the experimental results were different from those of other researches due to the configuration of experimental apparatus that absorption rates were dependant on the circulation rate, the absorption rates with the increase of HMDA concentration could be relatively compared under constant initial condition and CO2 circulation rate.
1904
Absorption Capacity of CO2 The variation of absorption equilibrium with the concentration of HMDA was studied using the same absorbent as absorption rate experiments and the results were shown on figure 3. According to the experimental results, the absorption equilibriums of CO2 in pure MDEA and HMDA showed different trends. While the absorption capacities of HMDA were rapidly increased until the saturation of CO2 and show low increase of CO2 loading at high CO2 partial pressure, those of MDEA showed relatively low increase at high CO2 partial pressure. Therefore, the absorption equilibrium of CO2 in mixture of MDEA and HMDA showed the intermediate trend of pure MDEA and HMDA. 20.5 wt% MDEA 20.5 wt% MDEA + 0.7 wt% HMDA 20.5 wt% MDEA + 3.5wt% HMDA 20.5 wt% MDEA + 7.0 wt% HMDA 20.5 wt% MDEA + 14.4 wt% HMDA 100
14.4 wt% HMDA
40
0.5
1.0
1.5
2.0
2.5
Absorption Capacity ( mol COg / L Solution )
Figure 3: VLE of CO2-MDEA-HMDA system at 50 °C In order to compare the absorption capacity of mixed absorbents with 20.5 wt% MDEA, the enhancement factor, £, was defined as equation (4). Absorption Capacity of Mixed Absorbent - Absorption Capacity of MDEA Absorption Capacity of MDEA The enhancement factors and equilibrium absorption capacities of mixed absorbents were interpolated and showed on table 2. While the enhancement factors were proportional to the concentration of HMDA at same CO2 partial pressure, the enhancement factors were inversely proportional to CO2 partial pressure at same concentration of HMDA. In case that the CO2 partial pressure is 10 kPa which is the same the concentration of CO2 in flue gas, the enhancement factor showed the increase of 13 - 238 % with the increase of HMDA concenU-ation. TABLE 2: THE EFFECTS OF HMDA CONCENTRATION ON CO2 ABSORPTION CAPACITY(A) AND ENHANCEMENT FACTOR(E) Mixed Solution CO2 partial pressure HMDA Concentration (kPa) 0% 0.7 wt% 7.0 wt% 14.4 wt% 3.5 wt% A E A A E A E E A 10 0.5394 238 1.1717 117 1.8238 0.6088 0.8789 63 13 20 0.7747 1.4134 82 2.0608 166 0.8310 7 1.1310 45 30 0.9125 0.9660 41 1.5736 72 2.2180 143 6 1.2897 40 1.0139 1.0847 1.7059 68 2.3395 131 7 1.4158 39 50 1.0963 1.1662 36 1.7904 63 2.4397 123 6 1.5017 60 1.1656 1.2270 36 61 2.5256 117 5 1.5869 1.8769 70 1.2266 1.2803 4 1.6522 34 1.9436 58 2.6018 112 80 1.2814 1.3336 2.6694 108 2.0062 56 4 1.7096 33
1905
CONCLUSION The absorption of CO2 by aqueous mixture of 20.5 wt% MDEA and 0.7, 3.5, 7.0 and 14.4 wt% HMDA showed that the apparent absorption rate constants were increased 25 - 292 % and the relation between apparent rate constant and HMDA concentration was confirmed to have the equation as In(kapp) = - 6.3818 + 0.3704 ln([HMDA(mole/l)]). Comparing the absorption capacity of CO2 in 20.5 wt% MDEA, the addition of HMDA increased the absorption capacity 13 - 238 %. Through this study, it could be expected that the aqueous mixture of MDEA and HMDA can contribute to the solution of economic problems of continuous absorption process by the increase of absorption rate and decrease of absorber size. REFERENCES 1. 2. 3. 4. 5.
Kohl, A. and Riesenfeld, P., 1985. Gas Purification 4'^ ed. Gulf Publishing Co. Ball, T. and Veldman, R., 1991. CEP, January, 67. Glasscock, D. A., 1990. Ph. D. Dissertation. University of Texas, Austin, U.S.A. Xu, G. W., Zhang, C. P., Qin, S. J. and Wang, Y. W., 1992. Ind. Eng. Chem. Res. Vol. 31: 921 Kubek, D. J. and Kovach, D. S., 1989. U.S Patent 4,814,104
1901
ABSORPTION RATES OF CO2 INTO AQUEOUS MIXTURES OF MDEA (METHYLDEETHANOLAMINE) AND HMDA (HEXAMETHYLENEDIAMINE) Sungyoul Park'*, Jongsup Leel\ Byoungmoo Min^ Kyungryong Jang^& Heemoon Eum^ 'Korea Institute of Energy Research, Daejeon, Korea ^Korea Electric Power Research Institute, Daejeon, Korea ABSTRACT Absorption rates of CO2 by aqueous mixtures of MDEA(20.5 wt%) and HMDA(0.7, 3.5, 7.0 and 14.4 wt%) were measured by the apparatus for gas absorption equilibrium. The experimental results showed that the absorption kinetics of CO2 into the aqueous mixtures of MDEA and HMDA regarded as a pseudo-first order reversible reaction. As the concentration of mixed HMDA increased, the apparent absorption rate constants were increased from 25 %(0.7 wt% HMDA) to 292 %(14.4 wt% HMDA) comparing with 20.5 wt% MDEA. INTRODUCTION The improvement of conventional absorbents is mainly focused on MDEA (methyldiethanolamine). MDEA has been used for the selective separation of H2S from gas mixture of H2S and C02[l]. Due to the low vapor pressure of MDEA, the loss of absorbent by evaporation is very small. And also, MDEA has strong resistance to thermal and chemical degradation and doesn't have corrosiveness. The small reaction heat of MDEA is useful for the bulk separation of CO2 from synthetic gas and natural gas. Indirect reaction of MDEA with CO2 made the stripping of CO2 by pressure decrease possible and decrease the heat consumption of CO2 stripping by evaporation than those of MEA (monoethanolamine) and DEA (diethanolamine). Therefore, MDEA has been considered as one of the efficient absorbent which solve the economical problem of absorption process. In spite of the above advantages, MDEA has disadvantages in the absorption of CO2. Due to low reaction rate with CO2, high partial pressure of CO2 and circulation flow rate of absorbent is required to absorb more CO2. This made packed column for CO2 absorption with MDEA have higher packing height and larger diameter of packing column as compared with those of MEA and DEA[2]. In order to enhance the slow reaction rate of MDEA, various researches in the mixing of additives which improving the absorption rate of CO2 to MDEA has been performed. As additives for reaction rate enhancement, MEA, DEA[3] and piperazine[4] were mixed with MDEA aqueous solution and the addition of alkyleneamine with MDEA solution enhanced 25 - 200 % of absorption rate and over 70 % of absorption capacity[5]. Considering chemical absorption is based on the chemical reaction according to stoichiometry, HMDA of which molecular weight is similar to MDEA is selected as an additive for absorption rate enhancement. And the variance of absorption rate and capacity of CO2 by the aqueous mixture of HMDA (hexamethylenediamine) and MDEA. EXPERIMENT Absorbent Preparation and Experimental Apparatus Absorbents were prepared by the mixing of 99 % MDEA(Aldrich Co.), 70 % HMDA(Aldrich Co.) and distillated water and the concentrations of absorbents were controlled to have 20.5 wt% MDEA + 0.7 wt% HMDA, 20.5 wt% MDEA + 3.5 wt% HMDA, 20.5 wt% MDEA + 7.0 wt% HMDA and 20.5 wt% MDEA + 14.4 wt% HMDA. Experimental apparatus was made of SS316 and consist of reservoir(5933 cm^) for the measurement of injected CO2, mixer(4044 cm^) for the circulation of CO2 and absorber(428 cm^) for the absorption of CO2. The apparatus was installed in temperature controlled air bath. For the fast absorption and equilibrium by contact with absorbent, CO2 was circulated by cylindrical piston pump. Glass beads were packed in the bottom of absorber to disperse the
*Corresponding author: Tel. +82-42-860-3046, Fax. +82-42-860-3691, Email: [email protected]
1902
injected CO2 and monitored by view cell installed in absorber. The schematic diagram of CO2 absorption equilibrium apparatus was shown in figure 1.
Figure 1: Schematic Diagram of CO2 absorption equilibrium apparatus Operating pressures and temperatures of reservoir, mixer and absorber were measured by three pressure transducers(Walcom) with accuracy of 0.001 kg/cm^ and four K-type thermocouples with accuracy of 0.1 K respectively. Temperatures of absorbent and CO2 in absorber were measured separately. The measured pressures and temperatures were recorded by hybrid recorder(Yokogawa, HR2300) and stored in computer as data file. The concentration of CO2 in gas sample was measured by gas chromatograph(HP5890A). Experimental Methods Before experiment, CO2 was stored in reservoir and nitrogen was supplied to mixer and absorber to remove impurities and air. The gas samples were analyzed by gas chromatograph to confirm that only nitrogen is contained in mixer and absorber. After the injection of absorbent, temperature was increased to 50 °C and equilibrium pressure of nitrogen was measured with the circulation by piston pump. After the equilibrium temperature and pressure, specific amount of CO2 was injected to absorber from reservoir and CO2 absorption reaction was started with the circulation of CO2 by piston pump. Equilibrium pressure before CO2 injection is the summation of nitrogen pressure and vapor pressure of absorbent and the amount of injected CO2 was calculated fi-om the pressure decrease in reservoir. CO2 partial pressure of absorber in initial state was calculated from the pressure increase of absorber by CO2 injection. The state that absorber pressure does not change is defined as equilibrium state and RESULTS AND DISCUSSION Absorption Rate of CO2 The absorption rate of prepared absorbents were measured and compared. The experiments were performed at the same initial condition of 50 °C, 215 kPa, 0.536 of CO2 mole ratio and 14.7 1/min of CO2 circulation flow rate. As the absorption proceeds, CO2 partial pressure of absorber decreased and the results were shown on figure 2. The experimental results showed that CO2 partial pressure was rapidly decreased with the increase of HMDA concentration. This means that the absorption of CO2 is dependant on the concentration of HMDA and can be explained that at first HMDA, which has fast reaction rate, reacts with CO2 at the interface of gas and liquid phase and then the intermediate reacts with MDEA at liquid bulk. At this step, HMDA was regenerated and reused for the reaction with CO2 at the interface. Through these steps, it was found that the mixture of HMDA and MDEA show higher reaction than pure MDEA solution
1903
100
150
Run Time (min)
Figure 2: Absorption rate of CO2 into aqueous mixture of MDEA and HMDA Absorption rates of CO2 with HMDA concentration were quantitatively compared by the apparent rate constant, kapp- Unless very high CO2 partial pressure or very high conversion rate of absorbent, it can be assumed that the concentration of absorbent in bulk liquid is very high and that of absorbent in the interface is constant. Therefore, the reaction rate can be described as the equation (1) and derived to pseudo first order reaction as equation (2). '^app yC02
"
(1)
''^Clh I
-ln{
(2) 'C02
Where /;,^ is the absorption rate of CO2 in absorbent, k^,, is apparent rate constant, p^^^ is CO2 partial pressure of absorber, p^; is initial CO2 partial pressure of absorber, p*^ is equilibrium CO2 partial pressure of absorber and t is time. Apparent absorption rate constant was calculated by Levenber-Marquardt optimization method using equation (2) and the values with HMDA concentrations were shown on table 1.
TABLE 1: COMPARISON OF APPARENT RATE CONSTANT AND ENHANCEMENT OF APPARENT RATE CONSTANT HMDA concentration (wt%) 0 0.7 3.5 7.0 14.4
Apparent rate constant (sec"', X 10^) 0.4583 0.5716 1.0833 1.4133 1.7983
Enhancement of apparent rate constant(%)
24.7 136.4 208.4 292.4
The calculated apparent rate constants showed 18 - 292 % increase comparing with that of 20.5 wt% MDEA aqueous solution. Though the experimental results were different from those of other researches due to the configuration of experimental apparatus that absorption rates were dependant on the circulation rate, the absorption rates with the increase of HMDA concentration could be relatively compared under constant initial condition and CO2 circulation rate.
1904
Absorption Capacity of CO2 The variation of absorption equilibrium with the concentration of HMDA was studied using the same absorbent as absorption rate experiments and the results were shown on figure 3. According to the experimental results, the absorption equilibriums of CO2 in pure MDEA and HMDA showed different trends. While the absorption capacities of HMDA were rapidly increased until the saturation of CO2 and show low increase of CO2 loading at high CO2 partial pressure, those of MDEA showed relatively low increase at high CO2 partial pressure. Therefore, the absorption equilibrium of CO2 in mixture of MDEA and HMDA showed the intermediate trend of pure MDEA and HMDA. 20.5 wt% MDEA 20.5 wt% MDEA + 0.7 wt% HMDA 20.5 wt% MDEA + 3.5wt% HMDA 20.5 wt% MDEA + 7.0 wt% HMDA 20.5 wt% MDEA + 14.4 wt% HMDA 100
14.4 wt% HMDA
40
0.5
1.0
1.5
2.0
2.5
Absorption Capacity ( mol COg / L Solution )
Figure 3: VLE of CO2-MDEA-HMDA system at 50 °C In order to compare the absorption capacity of mixed absorbents with 20.5 wt% MDEA, the enhancement factor, £, was defined as equation (4). Absorption Capacity of Mixed Absorbent - Absorption Capacity of MDEA Absorption Capacity of MDEA The enhancement factors and equilibrium absorption capacities of mixed absorbents were interpolated and showed on table 2. While the enhancement factors were proportional to the concentration of HMDA at same CO2 partial pressure, the enhancement factors were inversely proportional to CO2 partial pressure at same concentration of HMDA. In case that the CO2 partial pressure is 10 kPa which is the same the concentration of CO2 in flue gas, the enhancement factor showed the increase of 13 - 238 % with the increase of HMDA concenU-ation. TABLE 2: THE EFFECTS OF HMDA CONCENTRATION ON CO2 ABSORPTION CAPACITY(A) AND ENHANCEMENT FACTOR(E) Mixed Solution CO2 partial pressure HMDA Concentration (kPa) 0% 0.7 wt% 7.0 wt% 14.4 wt% 3.5 wt% A E A A E A E E A 10 0.5394 238 1.1717 117 1.8238 0.6088 0.8789 63 13 20 0.7747 1.4134 82 2.0608 166 0.8310 7 1.1310 45 30 0.9125 0.9660 41 1.5736 72 2.2180 143 6 1.2897 40 1.0139 1.0847 1.7059 68 2.3395 131 7 1.4158 39 50 1.0963 1.1662 36 1.7904 63 2.4397 123 6 1.5017 60 1.1656 1.2270 36 61 2.5256 117 5 1.5869 1.8769 70 1.2266 1.2803 4 1.6522 34 1.9436 58 2.6018 112 80 1.2814 1.3336 2.6694 108 2.0062 56 4 1.7096 33
1905
CONCLUSION The absorption of CO2 by aqueous mixture of 20.5 wt% MDEA and 0.7, 3.5, 7.0 and 14.4 wt% HMDA showed that the apparent absorption rate constants were increased 25 - 292 % and the relation between apparent rate constant and HMDA concentration was confirmed to have the equation as In(kapp) = - 6.3818 + 0.3704 ln([HMDA(mole/l)]). Comparing the absorption capacity of CO2 in 20.5 wt% MDEA, the addition of HMDA increased the absorption capacity 13 - 238 %. Through this study, it could be expected that the aqueous mixture of MDEA and HMDA can contribute to the solution of economic problems of continuous absorption process by the increase of absorption rate and decrease of absorber size. REFERENCES 1. 2. 3. 4. 5.
Kohl, A. and Riesenfeld, P., 1985. Gas Purification 4'^ ed. Gulf Publishing Co. Ball, T. and Veldman, R., 1991. CEP, January, 67. Glasscock, D. A., 1990. Ph. D. Dissertation. University of Texas, Austin, U.S.A. Xu, G. W., Zhang, C. P., Qin, S. J. and Wang, Y. W., 1992. Ind. Eng. Chem. Res. Vol. 31: 921 Kubek, D. J. and Kovach, D. S., 1989. U.S Patent 4,814,104