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2007_6_Nr6_EEMJ

Siminiceanu et al.

Siminiceanu et al. /Environmental Engineering and Management Journal 6 (2007), 6, 555-561 3. Results and discussion The primary experimental results have been interpreted on the basis of the gas- liquid chemical process theory (Siminiceanu, 2004). The rate of the chemical absorption of CO 2 ( = i)is of the form (4): - dn i / A dt = E k o L C e i , mol/ m 2 s, (4) The gas phase is assumed ideal (Pi V g = n i RT), CO 2 is completely consumed by the reaction in the liquid film, and the CO 2 concentration at the interface is replaced by the Henry law ( C e i = P i e / H i ). The partial pressure of CO 2 is obtained by subtraction of vapor pressure of the solution ( P v ) from the total measured pressure (P T ) : P i = P T – P v . By integrating (4) under these assumptions, the equation (5) is derived: ln (P T - P v ) t / (P T – P v ) to = - β (t- t o ) (5) where: β= E k L 0 ART/ V g H i (6) The enhancement factor E can be calculated for each experiment, using the Eq. (6). In order to compare our results with those for other solutions at the same temperature, the overall rate constant (k ov ) of the pseudo- first order reaction has been calculated for the fast reaction regime (E = Ha > 3): k ov = (k L 0 E ) 2 / D i (7) The mass transfer coefficient k L 0 is calculated with the Eq. (8) which was established, using the N 2 O analogy, for the absorber also applied in these new kinetic experiments (Amararrene and Bouallou, 2004): calculated with the Eqs (9) and (10), respectively (Versteeg and van Swaaij, 1988): H 0 i = 2.8249x10 6 exp (-2044/T) (9) D o i = 2.35x10 -6 exp (-2119/T) (10) The presence of the amine in water decreases the gas solubility (“salting out effect”). Taking into account the influence of the ionic strength of the solution on the solubility (Siminiceanu, 2004) with an equation of Sechenow type, the H i for the solution of 1.45 M APEDA was evaluated with (11): H i = 1.113 xH 0 i (11) The diffusivity of CO 2 in the APEDA aqueous solution was evaluated with Eq. (12), tested in a previous work (Siminiceanu et al., 2006): D i = (D o i/ 2.43) ( µ L / µ W ) 0.2 (12) The ratio µ L /µ W has been correlated for the APEDA solutions on the basis of experimental data published in a previous paper (Tataru-Farmus et al., 2007). The results from the Table 3 (first row, for the same loading) can be compared to those obtained for the absorption of CO 2 in a solution of AMP (1.5 M) with different doses of PZ as activator, in a wetted wall column absorber at the same temperature and a loading a= 0.288- 0.031 (Sun et al., 2005). The value obtained in this work with APEDA (k ov =17255.51 s -1 ) is higher than k ov for AMP with 0.1 and 0.2 M piperazine, and inferior to that for larger doses of PZ. It must be noted that he solution AMP- PZ- H 2 O gives the grates absorption rate among the new systems studied in the literature in the last decades. Sh = 0.352 Re 0.618 Sc 0.434 (8) Where the dimensionless Sherwood (Sh), Reynolds (Re) and Schmidt (Sc) numbers have been defined as follows: Sh= k L 0 D c / D i Re= ρ L N d st / µ L ln k ov 12.00 11.50 11.00 10.50 10.00 9.50 9.00 8.50 a=0.05-0.10 Sc= µ L / ρ L D i E being calculated with Eq. (6), using the experimental values of β from the Tables 2, 3, and 4. The Henry constant (H o i) and the diffusion coefficient (D o i) for the system CO 2 - H 2 O have been 8.00 3.00 3.10 3.20 3.30 3.40 1000/T, K -1 Fig. 3. The Arrhenius plot at low loading (a= 0.05- 0.10 mol CO2/ mol APEDA) 558

Kinetics of carbon dioxide absorption into aqueous solutions of 1, 5, 8, 12- tetraazadodecane (APEDA) Table 2. Experimental and calculated data for the absorption of CO 2 in APEDA (1.45 M) aqueous solution at 298 K. a, molCO2/mol APEDA β H i , D Pa.m3/mol CO , m²/s 0 2 k , m/s E=Ha kov , s -1 l 0.012 0.028 2965.85 2.00E-09 1.96E-05 206.04 8 490.91 0.070 0.026 2965.85 2.00E-09 1.96E-05 191.32 7 030.98 0.180 0.025 2965.85 2.00E-09 1.96E-05 183.96 6 500.34 0.295 0.023 2965.85 2.00E-09 1.96E-05 169.24 5 501.79 0.382 0.022 2965.85 2.00E-09 1.96E-05 161.88 5 033.48 0.484 0.021 2965.85 2.00E-09 1.96E-05 154.52 4 586.39 Table 3. Experimental and calculated data for the absorption of CO 2 in APEDA (1.45 M) aqueous solution at 313 K a, H molCO2/mol β i , D , m²/s CO Pa.m3/mol 2 APEDA 0 k , m/s l E=Ha kov , s -1 0.031 0.040 4110.08 2.10E-09 2.16E-05 278.69 17 255.51 0.087 0.036 4110.08 2.10E-09 2.16E-05 250.82 13 125.00 0.208 0.035 4110.08 2.10E-09 2.16E-05 243.85 12 487.19 0.305 0.034 4110.08 2.10E-09 2.16E-05 236.88 11 783.55 0.409 0.030 4110.08 2.10E-09 2.16E-05 209.01 9 173.88 0.508 0.027 4110.08 2.10E-09 2.16E-05 188.11 7 862.10 Table 4. Experimental and calculated data for the absorption of CO 2 in APEDA (1.45 M) aqueous solution at 333 K a, H molCO2/mol i , D , m²/s CO Pa.m3/mol 2 APEDA 0 k , m/s l E=Ha kov , s -1 0.047 0.041 6098.76 2.23E-09 2.49E-05 445.95 55 292.49 0.109 0.041 6098.76 2.23E-09 2.49E-05 445.95 55 292.49 0.222 0.040 6098.76 2.23E-09 2.49E-05 435.07 52 627.42 0.330 0.033 6098.76 2.23E-09 2.49E-05 358.93 35 818.99 0.430 0.029 6098.76 2.23E-09 2.49E-05 315.43 27 663.03 0.518 0.026 6098.76 2.23E-09 2.49E-05 282.79 22 235.57 Table 5.The results for the absorption of CO 2 in 1.5 M solutions of AMP activated with PZ at 313 K (Sun et al., 2005) C o PZ, mol/L ax 10 2 , k o Lx 10 5 , mol/mol m/s D i x10 9 , m 2 /s H i , Pa m 3 /mol N A x 10 6 , kmol/m 2 s k ov , s -1 0.1 3.11 3.97 1.72 4 144 3.46 7 530 0.2 2.88 4.05 1.66 4 047 3.88 13 857 0.3 3.10 3.68 1.57 4 095 4.31 20 572 0.4 3.16 3.64 1.42 4 070 4.52 27 819 ln kov 12.00 11.50 11.00 10.50 10.00 9.50 9.00 8.50 8.00 a=0.40-0.50 3.00 3.10 3.20 3.30 3.40 1000/T, K -1 ln k ov 11.50 11.00 10.50 10.00 9.50 a=0.00-0.05 a=0.05-0.10 a=0.10-0.20 9.00 a=0.20-0.30 a=0.30-0.40 a=0.40-0.50 8.50 3.00 3.10 3.20 3.30 1000/T, K -1 Fig. 4. The Arrhenius plot at high loading (a= 0.40- 0.50 mol CO2/ mol APEDA) Fig. 5. The Arrhenius plots for all experimental loadings 559

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