2-Amino-2-ethyl-1,3-propanediol 수용액의

이산화탄소 흡수속도 연구

윤상준, 윤지호*, 엄희문*, 이흔

한국과학기술원 화학공학과

한국전력연구원*

A Study of CO2 Absorption Rate in Aqueous 2-Amino-2-ethyl-1,3-propanediol Solutions

Sang Jun Yoon, Ji-Ho Yoon*, Hee-Moon Eum*, Huen Lee

Department of Chemical Engineering, KAIST

Korea Electric Power Research Institute*

Introduction

Acid gas absorption using chemical reaction with alkanolamine solution is widely used technique in the chemical industries such as the processing of natural gas. Widely used alkanolamines are monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), N-methyldiethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP). Especially, research and employment for sterically hindered amines are increased because of its improved characteristics of high absorption capacity, selectivity, and low regeneration energy compare to conventional amines[1]. This stripping characteristic results in a great saving of energy and reduces the total operating cost of acid gas treating unit[2]

In the present research the AEPD was selected, which is of course one of the hindered primary amino alcohol. The absorption kinetics of CO2 into aqueous AEPD solutions was carried out using a wetted wall column. The physical properties, density and viscosity, were also correlated and measured. The values of the diffusivity and Henry's constant relation required for analysis of reaction kinetics were measured using N2O analogy method.

Experimental Apparatus and Method

2-Amino-2-ethyl-1-3-propanediol with a minimum purity of 97% was obtained from Acros Organics and was used without further purification. Aqueous AEPD solutions were prepared using triple-distilled water. A surface active agent, Tween 80, was purchased from Sigma-Aldrich Chemical Co.

A schematic diagram of wetted wall column absorber was shown in Figure 1. Kinetics measurement apparatus and procedures are similar to those of other researchers.[3] Experimental apparatus consists of three parts. Mass flow controller (MFC) (Brooks, 5850E) to supply the CO2 and N2 with a constant ratio, wetted wall column which is the reaction was occurred, and gas chromatograph (Hewlett Packard, 5890A) equipped with a thermal conductivity detector (TCD).

WWC was made of 316 stainless steel and temperature was maintained by air bath equipped with temperature controller, heater, and air circulation fan. Temperature of air bath was also controlled by a refrigerator/heater (Jeio Tech, RBC-20) with an accuracy of ±0.1 ℃. Aqueous amine solution was supplied using gear pump (Cole-Parmer, P-07002-17) with a flow rate range of 50∼1150 ml/min. Liquid flow rate was calibrated for each condition. Input gas pass through the saturator and the pre-heater to reach an experimental temperature.

Outside diameter and height of WWC are 2.54 cm and 8.6 cm. To analyze the composition of exhaust gases, 6 ft by 1/8 in. diameter column packed with Porapak Q. The temperatures of the oven, injector, and detector were 40, 120, and 120 ℃, respectively. To prevent the ripple on the liquid surface, 0.04 vol% of surface active agent, Tween 80, was added. Absorption rate of CO2 and N2O was measured by using soap-film meter.

Results and Discussion

The absorption kinetics of carbon dioxide into aqueous AEPD solutions were measured at temperatures ranging from 303.15 to 318.15 K and concentration range of 0.41 to 2.15 kmol/m3. Most of experiments were performed under the atmospheric pressure and CO2 partial pressure was also maintained to measure the order of reaction to carbon dioxide. The density and viscosity data to require the calculation of the reaction kinetics were calculated and measured by our previous work[4]. Additional measurements of viscosity at experimental conditions were carried out.

To check the validity of the experimental apparatus and procedures, diffusion coefficient of CO2 into water was measured at 298.15 K and 7.55, 9.05, and 10.05 cm length of WWC. Averaged calculation value of diffusion coefficient was 2.03(±0.01)×10-9 m2/s. Tang and Sandall[5] got the similar value of 1.98×10-9 m2/s. By this result, we confirm the apparatus and the procedures and can estimate the stagnant film height which is absorption of carbon dioxide does not occur. Absorption rate per unit area, NA, can be expressed as the total absorption rate, NAT, and the surface area (contact with gas phase) of WWC. When the height of column and (NAT/PA)2 are plotted as x and y axis, the crossover point to x axis is stagnant film height. For our experimental apparatus, stagnant film height is 2.36 cm

In order to determine the order of carbon dioxide for CO2-AEPD reaction, absorption rates of carbon dioxide were measured at CO2 partial pressure range of 2 to 80 kPa with AEPD concentrations of 0.41, 0.84, and 1.27 kmol/m3. The plot of ln NA versus ln PA at constant temperature of 303.15 K was shown in Figure 2. These three straight lines are almost parallel with a slop of unity. It can be estimated that the reaction is first order for CO2.

The absorption rates of carbon dioxide and N2O into aqueous AEPD solution were measured to determine the order of reaction for amine and reaction rate constant at temperatures of 303.15, 313.15, and 318.15 K and AEPD concentrations of 0.41, 0.84, 1.27, 1.70, and 2.15 kmol/m3. N2O absorption rate results were used to calculate the value of DA1/2/HA. The value of DA1/2/HA for CO2 was estimated by using N2O analogy method suggested by Versteeg et al.[6]. The plot of (NA/PA)/(DA1/2/HA) vs. the concentration of AEPD is shown in Figure 3. In this plot, CO2-AEPD reaction rate constants were obtained the values of 193.85, 388.43, 542.29 m3/(kmol·s) at 303.15, 313.15, 318.15 K. Order of reaction for AEPD was found respect to 1.5, as well. Figure 4 shows an Arrhenius plot of the second-order rate constants. A fit of these data gives

[eqn: lnk_2 ~=~ 27.08 ~-~ 6611.81/T (K)]

Reference

(1) Sartori, G.; Savage, D. W. Sterically Hindered Amines for CO2 Removal from Gases. Ind. Eng. Chem. Fundam. 1983, 22, 239-249.

(2) Chakravarty, T.; Phukan, U. K.; Weiland, R. H. Reaction of acid gases with mixtures of amines. Chem. Eng. Prog. 1985, 81(4), 32-36.

(3) Yih, S.-M.; Shen, K.-P. Kinetics of Carbon Dioxide Reaction with Sterically Hindered 2-Amino-2-methyl-1-propanol Aqueous Solutions. Ind. Eng. Chem. Res. 1988, 27, 2237-2241.

(4) Yoon, S. J.; Lee, H.-S.; Lee, H.; Baek, J.-I.; Yoon, J.-H.; Eum, H.-M. Densities, Viscosities, and Surface tensions of Aqueous 2-Amino-2-ethyl-1,3- propanediol Solutions. to be submitted for publication in J. Chem. Eng. Data.

(5) Tang, A.; Sandall, O. C. Diffusion Coefficient of Chlorine in Water at 25-60 ℃. J. Chem. Eng. Data. 1985, 30, 189-191.

(6) Versteege, G. F.; van Swaaij, W. P. M. Solubility and diffusivity of acid gases (CO2, N2O) in aqueous alkanolamine solutions. J. Chem. Eng. Data. 1988, 33, 29-34.

Figure . Kinetics measurement

apparatus

Figure . CO2 partial pressure effect

on absorption rate

AEPD weight percent (●) 5 wt%;

(□) 10 wt %;  (▲) 15 wt%

Figure . Determination of reaction rate constant and order with respect to AEPD

(●) 303.15 K; (■) 313.15 K; (▲) 318.15 K

Figure . Arrhenius plot for CO2-AEPD

system