(617hc) Experimental and Theoretical Analysis of Carbon Dioxide Absorption in Aqueous Sodium Hydroxide in the Millichannels

Various types of two phase flow patterns (Sobieszuk, P., Aubin, J., & Pohorecki, R. (2012) in gas-liquid systems have been studied by various researchers (Sobieszuk, P., Aubin, J., & Pohorecki, R. (2012). One of the applications of gas liquid flows is in gas absorption. Carbon dioxide absorption is an important step in todayâ??s context since the gas plays an important role in global warming. The results of kinetics reported in the literature have many contradictions in the quantitative kinetics. This is due to the inherent coupling of the physical and chemical phenomena involved in the absorption process in the presence of solutes in the aqueous phase. The physico-chemical effects have been lumped together in a parameter in earlier approaches. This is the primary cause for the discrepancies in the reported measurements where the intrinsic kinetics was not delineated.

In this work, an absorption study of CO₂in an aqueous solution of NaOH is carried out in slug flow in a milli channel. The well defined flow behavior in the micro channel helps to separate out the effects of hydrodynamic mass transfer resistance and the kinetics of absorption. This would help identify the rate determining step to help us better design absorption systems. Slug flow is usually found to be present at low flow rates of gas. (Abadie, T., Aubin, J., Xuereb, C., & Legendre, D. (2012)).Accordingly, in this work also, slug flow is observed at low flow rates and the absorption study is carried out in this flow regime.

Experiments are carried out in a T-junction circular cross-section milli-channels of 2 mm inner diameter using the 99.9 v/v% pure CO_2 and the aqueous solution. The flow rates of gas and liquid are maintained with the help of a mass flow controller (MFC) and a Harvard syringe pump respectively. Liquid phase composition is carried out by Warderâ??s method of titration. This helps us estimate the concentration of various species in the liquid and consequently the rate of gas absorption.

A model is developed for predicting the slug length variation along the channel. This depicts the rate of absorption of CO_2 in the sodium hydroxide solution. The theoretical model and the experimental data are compared for various operating conditions and they are found to be in close agreement.

Keywords: milli-channels, slug flow, circular channels, two phase flow.

REFERENCES:

1. Sobieszuk, P., J. Aubin, and R. Pohorecki (2012). Hydrodynamics and mass transfer in gas-liquid flows in microreactors. Chemical Engineering & Technology, 35(8), 1346â??1358.
2. Abadie, T., Aubin, J., Legendre, D., & Xuereb, C. (2012). Hydrodynamics of gas-liquid Taylor flow in rectangular microchannels Microfluidics and Nanofluidics, 12, 355-369.