(480g) Liquid-Liquid Extraction: Comparison in Micro and Macro Systems

Liquid – liquid extraction operation in micro channels and capillaries received attention with a focus on intensification on mass transfer. Yuechao Zhao et.[1] reported  mass transfer characteristics of water, succinic acid and n-butanol in case of counter and cross-flow T-junction micro-channels and proposed correlations to predict the volumetric mass transfer coefficients, which are two or three orders of magnitude higher than those of conventional liquid-liquid contactors.  Kashid et al [2] have investigated the mass transfer coefficient in liquid-liquid slug flow through a capillary micro-reactor for three different non-reacting systems. They have observed enhanced rates of mass transfer in micro-reactors as compared to the conventional reactors reported in literature. Yoshihito Okubo et al [3] have investigated three extraction operations, contact flow in a Y-shaped micro channel, segmented flow and emulsification. They noted that extraction using segmented flow is dependent on mass transfer by molecular diffusion and its rate is enhanced by the internal circulation flow generated inside slugs and extraction rate can be controlled precisely by adjusting operational parameters such as the flow rate and the flow rate ratio. Nobuaki Aoki et al.[4] have studied that to increase the maximum flow rate that enables slug flow formation in miniaturized channels, gas phase slugs were added in a liquid- liquid slug flow to form a gas-liquid –liquid slug flow. Effects of channel size, void fraction (ratio of volumetric flow rate or gas phase to that of total flow rate), and volume ratio of aqueous phase to organic phase on flow regime were examined.  Usha Rao [5] have studied the flow pattern mapping and also dimensionless analysis of two phase liquid-liquid systems in glass micro channels of sizes varying from 500 pm to 1000um (ID) and varying shapes such as straight and sinusoidal. Thee liquid-liquid systems were studied viz., (i) water methyl ethyl ketone (ii)  palm oil-methanol (iii) ethyl acetate-water, each having different properties. These fluids were fed into the micro channel through different junctions, Y, T and L at controlled flow rates ranging from 0.02-0.08 ml/min and slug flow pattern was seen for all the three systems irrespective of diameters and properties of the systems and concluded that slug flow pattern is good for operations in micro channels, as it gives good mixing and good contact between phases.

In the present study liquid-liquid operation was investigated in micro channels of different diameters and compared the results with that obtained in packed column using Raschig rings and structured sulzer packing. Water-acetic acid-ethyl acetate was the system chosen for the purpose. Ethyl acetate was used as solvent and dispersed into the aqueous phase containing acetic acid. Micro channels of different diameters namely, 0.7, 0.9 and 1.8 mm were used for extraction. The liquids were drawn using peristaltic pumps from reservoirs consisting of feed and solvent. The channel volume was kept constant (0.1708 ml) by decreasing the diameter of the channels and increasing the length. The acetic acid volume was 20 percent in water. Runs were made with flow rates such as 0.04 to 0.08 ml/min and operated with solvent to feed ratio, S/F = 1.

The continuous counter current extraction glass column with an internal diameter of 45mm and a height of 500mm had a packed length of 200mm and disengaging spaces at the top and at the bottom. Two dosing pumps were used to pump the solvent and the aqueous phase. Concentrations of acetic acid in water was 20 volume percent. Solvent flow rate was varied from 4 to 16 ml/min which gives S/F of 1 to 4.

Volumetric mass transfer coefficient, k_La and the number of transfer units, N_tL were found to increase with S/F ratio in the packed columns. k_La, N_tL and percentage extraction were observed to be higher for structured sulzer packing. Slug flow or segmented flow is most promising flow patterns for good mass transfer in micro channels. Slug flow, which occurs at a relatively low flow rates, is characterized by elongated bubble flowing in the axial direction. For a fixed residence time k_La and percentage extraction were observed increase with decrease in diameter for S/F = 1. This is due to increase in surface to volume ratio which reduces the mean distance of the specific liquid volume to the channel walls or to the domain of the second liquid. As a consequence mass transfer performance with the second fluid is enhanced. The values of  k_La were of the order of 3.56 x 10^-4 in the packed columns and 8.98 x 10^-3in the micro channels. In micro channels k_La is almost 25 times than that of packed columns with S/F =1. The percentage extraction obtained at an S/F = 1 in micro channels, was almost same with S/F = 4 in the packed columns.

When different phases are injected as adjacent streams in one channel, one phase often preferentially wets the boundaries and encapsulates the second fluid as discrete drops. However, the flow can also become stable by the generation of a clear interface. With downscaling, the gravitational force becomes less important [6]. Multiphase-fluid dynamical responses are commonly successfully characterized by dimensionless numbers, Reynolds number, Re, Capillary number, Ca and Weber number, We. In micro channels Re obtained were less than 10 and hence laminar profile can be observed. The results indicate the influence of interfacial tension, characterstic of We and is found to be more than the inertial and viscous forces.

Flow patterns by micro Particle Image Velocitymetry indicated internal circulation of liquid in the organic slug caused due to two liquid flows. The diffusion of solute from aqueous to organic slug provides better mixing.       

References

1. Yuchao Zhao, Guangwen Chen and Quan Yuan, “Liquid-Liquid Two-Phase Mass Transfer in the T-Junction Microchannels”, AICHE.J.53.12.3042-3053,2007.
2. Kashid M.N., A.Renken, L.Kiwi-Minsker, “Gas-liquid and liquid-liquid mass transfer in microstructured reactors”, Chem.Eng. Sci. ,66,3876-389,2011.
3. Yashihito Okubo, Taisuke Maki,Nobuaki Aoki, Teng Hong Khoo, Yoshikage Ohmukai,Kazuhiro Mae,” Liquid-liquid extraction synthesis and separation by utilizing micro spaces”. Chem.  Eng.  Sci., 63,4070-4077,2008.
4. Nobuak Aoki, Ryuichi Audo and Kazuhiro Mac, “Gas liquid –liquid slag Flow for improving Liquid-Liquid Extraction in Miniatured Channels”,  Ind.Eng.Chem.Res., 50,   4072-4677,2011.
5. Usha Rao H.,”Study of microscale fluid dynamics an input for micro reactor modeling and design “Ph.D., Thesis submitted to Osmania Universwity,2012.
6. Yuanhai Su, Guangwen Chen, Yuchao Zhao and Quan Yuan, “Intesfication of Liquid-Liquid Two Phase Mass Transfer by Gas Agitation in a Microchannel” AICHE J.,55, 8, 1948-1958,2009.