Mass transfer characteristics of CO2 absorption into 2-amino-2-methyl-1-propanol non-aqueous solution in a microchannel

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Abstract

The gas–liquid two-phase flow and mass transfer performance of CO2 absorption into 2-amino-2-methyl-1-propanol (AMP) with ethylene glycol (EG) non-aqueous solution in a microchannel were investigated under different gas–liquid two-phase flow rates and AMP concentrations. A new correlation was proposed for accurately predicting the liquid side volumetric mass transfer coefficient by considering the enhancement factor. It was verified that the volumetric mass transfer coefficient of CO2 absorption in the microchannel is obviously higher than that in the traditional macroscopic column. Therefore, with the usage of microchannel for AMP-EG non-aqueous solution, both low energy consumption and high efficient absorption could be reached.

Introduction

Currently, near eighty percent of global energy comes from fossil fuels such as coal, oil and nature gas. The greenhouse effect stemming from the combustion of fossil fuels has received extensively social concern [1]. Among all greenhouse gases, CO2 is a primary emission source. Actually, CO2 has shown the potential to change the earth’s climate in a short time [2,3]. According to the distribution of the carbon emissions, the industrial emission of CO2 is the largest emission contributor [4]. Consequently, the industrial engineering faces significant emission reduction problem [5]. The capture of CO2 is a necessary environmental-protection demand to prevent air pollution. Practically, novel and effective CO2 capture technologies are always desired urgently during the last decades.

Today, chemical absorption method has become an effective technology and been widely used in acid gas separation [6], in which chemical absorption by alkylol amine aqueous solution is one of the most efficient methods for CO2 capture with larger CO2 loading and higher absorption rate, and has been used extensively in the purification processes of natural gas and syngas [7]. However, the disadvantages of high energy consumption for generation, strong corrosion, high volatility and potential environmental pollution limit the application of alkylol amine aqueous solution [8,9]. To be more commercial, ethylene glycol, as a green solvent, has been considered as a potential solvent due to its excellent thermal stability and low vapor pressure [10]. The absorption of CO2 via ethylene glycol combining with alkylol amine not only has the advantage of high absorption efficiency, but also could overcome the defects of high energy consumption for regeneration, corrosivity and volatility of the alkylol amine aqueous solution [11,12]. However, the enhancement of the absorption efficiency of non-aqueous solvent is still a challenging problem for CO2 capture in industry because of its relatively high viscosity.

In the past decades, micro-chemical technology has aroused worldwide attention in both academia and industry [13]. The microchannel could provide high specific surface area and improve the heat and mass transfer rate, thereby the energy utilization efficiency could be augmented and the volume of the reaction system could be reduced, which makes it possible to enhance the absorption of CO2 [[14], [15], [16]]. The absorption of CO2 in microchannel has been investigated extensively. Li et al. [17] investigated experimentally the volumetric mass transfer coefficient of CO2 absorption into MEA aqueous solution in a rectangular microchannel. Ganapathy et al. [18] used DEA aqueous solution as the continuous phase and CO2/N2 mixture as dispersed phase, and found that the liquid side volumetric mass transfer coefficient was 2–4 orders of magnitude higher in microchannel than that in traditional equipment. Yue et al. [19] explored the physical and chemical absorption processes of CO2 in the microchannel by using water, NaHCO3/Na2CO3 buffer solution and NaOH solution as absorbents respectively. The measured liquid side volumetric mass transfer coefficient kLa was from 0.3 s−1 to 21 s−1, which is 1–2 orders of magnitude higher than the traditional gas–liquid reactor. Zhu et al. [20] investigated the CO2 chemical absorption into MEA solution in a microchannel, and a mass transfer model for predicting the volumetric mass transfer coefficient was established. Moreover, Yao et al. [21] investigated the CO2 absorption into aqueous ethanol solution in a microchannel. The evolution of bubble length in absorption process was observed, and the dependency of the mass transfer coefficient on the ethanol concentration was determined. Nevertheless, few studies on the CO2 absorption into non-aqueous solutions in microchannels could be found in the literature so far.

In this paper, the mass transfer characteristics of CO2 absorption into AMP-EG solution in a microchannel was studied. The effects of gas–liquid two-phase flow rates and AMP concentration on the bubble length, the specific surface area and the liquid side volumetric mass transfer coefficient were investigated. A new correlation was proposed to predict the liquid side volumetric mass transfer coefficient by considering chemical enhancement factor.

Section snippets

Experimental method

Pure CO2 gas was used as dispersed phase and AMP-EG solution as continuous phase. AMP concentration was 0.5 mol L−1, 1.0 mol L−1, 1.5 mol L−1 and 2.0 mol L−1 respectively. Gas and liquid phases were driven into the microchannel by syringe pumps (Harvard PHD2000, USA) respectively, and contacted at a microfluidic T-junction by crossing. The depth and width of the microchannel are 400 μm, while the length of the main channel is 30 mm. Pressure sensors (Honeywell ST3000, USA) were used to measure the

Evolution of bubble length in the microchannel

Once gas enters the T-junction microchannel, it would be squeezed and sheared by liquid phase to form a string of stable slug bubbles [26]. The initial length of bubbles varied with the gas and liquid flow rates. By processing the pictures, the coordinates of the front and rear of the bubble could be determined as xf and xr respectively. The length and the coordinate of the bubble were calculated by LB=xfxr and x=(xf+xr)/2 respectively, and then the evolution of a single bubble length could be

Conclusion

The absorption of CO2 into AMP-EG non-aqueous solution accompanying by chemical reaction in a microchannel was explored. The effects on the bubble length LB, the specific area a and the liquid side volumetric mass transfer coefficient kLa were obtained. The results showed that, for a fixed concentration of AMP solution, the specific surface area, the liquid side mass transfer coefficient and the liquid side volumetric mass transfer coefficient increase gradually and tend gradually to a constant

Acknowledgement

This work was supported by the National Nature Science Foundation of China (No. 21776200, 21576186, 91634105, 91434204), the aid of Opening Project of State Key Laboratory of Chemical Engineering (No. SKL-ChE-13T04, SKL-ChE-16B03) and the Programs of Introducing Talents of Discipline to Universities (Grant No.B06006).

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      From Fig. 4, it is evident that the initial bubble length decreases with an increase in the liquid flow rate and viscosity, but becomes larger with an increase in the gas injection pressure, which is in agreement with the bubble length laws without gas absorption under the conditions of the gas constant-pressure driven method [50]. Regarding the absorbent concentration effect, Guo et al. [37] found that the initial bubble length slightly decreased as the AMP concentration was increased by the gas constant-flow-rate driven method, and they indicated that this was mainly due to the stronger squeezing and shearing forces and higher CO2 loading ability. However, the initial bubble length increases with increasing AMP concentration via the gas constant-pressure driven method in this study (Fig. 4(d)).

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