Liquid Liquid Extraction: Comparision in Micro and Macro Systems. Presentation ID#: Abstract

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1 Liquid Liquid Extraction: Comparision in Micro and Macro Systems Presentation ID#: Prof. Sankarshana Talapuru 1, Ilaiah S 2, Dr. Usha Virendra 3 1, 2 University College of Technology, Osmania University, Hyderabad, India, 3 Indian Institute of Chemical Technology, Hyderabad, India. Abstract Liquid-liquid two phase mass transfer process was investigated in packed column with varying acetic acid concentrations in the feed, solvent to feed flow rate (S/F) ratio, using random and structured packing. The overall volumetric mass transfer coefficient, percentage extraction, number of transfer units were determined quantitatively. A comparison was made between the results obtained for structured and random packings. Extraction operation was investigated in micro channels of different diameters namely 0.7, 0.9 and 1.8 mm having same volume. A comparison was made between the results obtained in the different micro channels and Dimensionless analysis was carried out by using Reynolds, Weber, and Capillary numbers. For the solvent to feed ratio S/F ratio=1, it was found that the K L a values obtained in the micro channels were more than two orders of magnitude than the packed column irrespective of the type of packing employed. Very high percentage extraction was obtained in micro channels when compared with the packed columns. Introduction In large size industrial extractors, large amount of solvent is required, mixing performance is not very effective and as a consequence extraction efficiency decreases. Micro devices are an approved tool for process intensification and are becoming effective platforms for liquid liquid or gas liquid mass transfer [1-3]. The equivalent hydraulic diameters of the micro fluidic devices are up to a few hundreds of micrometers which can provide large surface-tovolume ratio and short mass transfer distance [4-5]. The concentration gradient can thus be increased and the mass transfer resistance can be reduced greatly. Zhao et al. [6] reported the mass transfer characteristics of water-succinic acid and n-butanol in case of counter and crossflow T-junction micro-channels and proposed correlations to predict the volumetric mass transfer coefficients, Kashid et al.[7] have investigated the mass transfer coefficient in liquid liquid slugflow 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. Yoshihito Okubo et al. [8] 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. Usha Rao [9] have studied the flow pattern mapping and also dimensionless analysis of two phase liquid-liquid systems in glass micro channels of sizes varying from 500µm to 1000µm (ID) and varying shapes such as straight and sinusoidal with viz., i) water-methyl ethyl ketone ii) palm oil-methanol iii)ethyl acetate-water systems, each having different properties.

2 Slug flow pattern was seen for all three systems irrespective of diameters and properties of the systems. 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. Experimental A) Packed Column The essential parts of the equipment used for continuous counter current extraction consisted of the extraction column, two feed reservoirs, and two dosing pumps. The column was made two different of 45mm internal diameter and 500 mm long glass tube. The column had a packed length of 200 mm and two disengaging spaces at the top and bottom from where the light and heavy phases respectively were withdrawn. The column was packed with kinds of packings and several experiments were conducted for each of these packings. The complete setup is shown in Figure 1. First set of experiments were carried out with randomly packed 483 Raschig rings made of glass having dimensions 6.5,4.5 and 8 mm od, id and height respectively. Figure 1. Schematic diagram of experimental setup The other set of packing was structured Sulzer packing height 200 mm, under otherwise same conditions. The pore volume of the packed column was was found to be 70 cm 3 for the packing with Raschig rings and 40 cm 3 for the Sulzer packing. The aqueous feed contained varying concentrations of acetic acid namely 10,15,20,25 and 30 volume % of acetic acid in water. Experiments were performed by changing dispersed phase (ethyl acetate phase) flow rates and keeping the continuous phase (aqueous phase) flow rate constant. The packed volume was 192.5cm 3. The aqueous feed solution containing acetic acid was fed from the top of the column and the solvent was fed from the bottom. The solvent was pure ethyl acetate and was dispersed into the aqueous feed solution.the aqueous feed flow V f was maintained at 4 ml/min and the solvent flow V s was varied from 4 to 10 ml/min i.e, the solvent to feed ratio, S/F was varied from 1 to 4. The extract over flows from the extract valve provided at the top of the column and at the same time the raffinate was collected from the raffinate valve provided at the bottom of the column. The analysis was done with simple acid-base titration using sodium hydroxide (NaOH) as the titrant. The samples were collected at different intervals and analyzed until steady state is reached. The distribution coefficient K d for the system was found to be 1.5. The volume of the packing was 192.5cm 3. The rate of extraction, R A was calculated by the expression, V f (X 1 -X 2 ). X 1 and X 2 are outlet and inlet concentrations of acetic acid in extract and solvent.

3 The volumetric mass transfer coefficient, K L a was calculated by dividing the rate of extraction of acid into the organic phase by the volume of the packing and the mean overall driving force. The number of transfer units,n tl was found using the usual expression applied to packed columns [10], assuming the solution to be in dilute condition. B). Micro Channels The experimental set up consisted of two peristaltic pumps one for the feed and other one for the solvent. The liquids were drawn using these pumps from reservoirs consisting of feed and solvent. The feed consisted of 20% acetic acid in water and the solvent reservoir contained pure ethyl acetate. The pump inlets were connected to the respective reservoirs and the outlets of the pumps were connected to the adjacent branches of Y- junction micro channel. The outlet of this micro channel was connected to a phase separating funnel in which both raffinate and extract were collected and were analyzed for acetic acid concentration. The complete schematic and experimental setup is shown in Figure 2. Figure 2. Schematic assembly of liquid-liquid extraction in micro channels Three different circular cross section Y- junction micro channels were used for the extraction process. The channel volumes were kept constant at cm 3 by decreasing the diameter and increasing the length of the channels. The dimensions of these channels are as follows: 1.8 and 67, 0.9 and 268 and 0.73 and 408 mm diameters and length respectively. Different experiments were performed with flow rates such as 0.04 ml/min, 0.06 ml/min and 0.08 ml/min and operated with S/F (solvent to feed flow rate) ratio =1 in diameter of different channels. The experimental results were analysed for acetic acid concentration. Ethyl acetate was colored and was identified in forming the slug. Volumetric mass transfer coefficient K L a for the transfer of acetic acid in micro channels was determined in a similar manner discussed above in the case of packed column by knowing the rate of extraction of acid from the aqueous phase into the organic phase. Results and Discussion A) Packed column Table 1 gives the K La and N tl for different S/F and acetic acid concentration in aqueous feed for the random packing. For a constant S/F ratio, with the increase in acetic acid concentration in the feed, the values of K L a, percentage extraction, N tl almost remained constant till 20 % acetic

4 acid concentration and there was very slight increase in the values after 20 % and again it remained constant till 30 % of acetic acid. For fixed feed concentration as the S/F ratio increased the values of K L a has shown an increased trend. This may be due to increase in drop surface. Table 1: Effect of acetic acid concentration in feed and S/F ratio in packed column with Raschig rings : h=20 cm, v p =192.5 cm 3, ε R =Pore volume/packed volume =0.363, Y 2 = 0. Acetic acid in feed V f V s S/F X 1 Y 1 X 2 R A K L a 10 4 % E N tl ml/min ml/mn ratio g/ml g/ml g/ml g/min (sec -1 ) 10 % % % % % From Table 2, it is evident that for a constant feed concentration, S/F ratio and volume of packing, the values of K L a, percentage extraction, N tl were higher for a structured sulzer packing than a random packing with Raschig rings. Time required for extraction decreases significantly for structured packing which in turn causes the reduction in the solvent consumption. For the fixed column diameter and the height of packing the porosity available for structured packing was where as for random packing was Thus due to decrease in porosity of structured packing, the values of K L a and percentage extraction and N tl increased. This may be due to the increase in the area available for mass transfer and hold-up of the dispersed phase. It is, therefore likely that the random packing surface renewal rate is less than that for the structured packing under otherwise, uniform conditions. Hence a structured packing is preferred over random packing. Table 2. Comparison of results in packed column with random and structured packings: 20% acetic acid, h=20 cm, v p =192.5cm 3, ε R = 0.363, ε S =0.197,V f =4ml/min,V s =16 ml/min,y 2 =0. Type of contactor V f V s S/F R A K L a 10 4 % E N tl t E min ml/min ml/min ratio g/min (sec -1 ) Random packing Structured packing B) Micro Channels The runs were conducted using different diameters of the micro channels, however maintaining the same residence time (t R ).It is to be noted that to achieve same t R for same volume different lengths of micro channels to be used as repoted earlier. Slug flow (Segmented flow) is most promising flow patterns for good mass transfer (large interfacial area and secondary velocity

5 profile). Slug flow, which occurs at a relatively low flow rates, is characterized by elongated bubble flowing in the axial direction. Due to the equal volumetric flow rates, the ratio of the organic and aqueous segment is supposed to be equal. The experiments were conducted using 20% acetic acid in feed and the data collected are presented in Table 3. K L a and percentage extraction increase because of increase in surface to volume ratio which reduces the mean distance of the specific fluid volume to the reactor walls or to the domain of the second fluid.as a consequence mass transfer performance either with the channel or with a second fluid is enhanced. Such a behaviour is attributed to increase in surface to volume ratio due to decrease in diameter though all the flows are in laminar regime only and the elements flows as laminar. Hence, in such cases as the distance for molecular diffusion is decreased, the interaction between molecules has increased which improved the percentage extraction. Extraction using slug flow is dependent on mass transfer by molecular diffusion and rate is enhanced by internal circulation. As the residence time was varied by varying the volume flow rates. Overall, mass transport is increases with increasing flow velocities, according to findings of Kashid et al. [11]. They assume that the recirculation within the slugs enhances surface renewal resulting in an increased mass transfer between organic and aqueous slugs. This behaviour has been tested with µ-piv measurements. Increasing the flow rates means shorter residence times in the micro channels and therefore could lead to poor extraction yields if equilibrium is not reached. In segmented flow systems high flow rates result in shorter aqueous and organic slugs with increased internal circulation, enhancing the driving force for mass transfer. Lower flow rates might on the other hand lead to a poorer extraction efficiency due to a decrease of the slugs internal circulation but due to residence time domination at lower total flow rates the % extraction increased significantly for all three channels. Table 3.Effect of micro channels on mass transfer characteristics: 20% acetic acid, V m = cm 3, C or,i = 0. Diameter Q aq Q or S/F C aq,i C aq,o C or,o t R K L a % E (mm) ml/min ml/min ratio g/ml g/ml g/ml (s) (sec -1 ) Table 3 shows the effect of residence time on % extraction. It appears that as residence time increases (total flow rate decreases) % extraction increases for all the three channels. This result confirms that the decrease in % extraction at higher flow rates was due to the limited residence time in the micro channels, which did not allow equilibrium to be reached. When increasing the extraction time by using lower flow rates full equilibrium was again obtained. Apparently, when the slugs are of comparable size, mass transfer occurs faster where as longer extraction times are required if small organic slugs are used to extract large sample slugs. This confirms that the

6 slugs relative dimensions play a very important role regarding the time required to obtain complete mass transfer. Here the residence time dominating factor than internal recerculations generated inside the slugs.hence higher percentage extraction can be achieved with increase in residence times.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 [12]. Multiphase-fluid dynamical responses are commonly successfully characterized by using some dimensionless numbers. Depending on wetting properties, flow velocities, fluid viscosity and geometrical features, the flow patterns in multiphase channel flow change between segmented flow and stratified flow. A segmented flow uses discrete volumes of fluids, whereas a stratified flow relies on fluids, which are continuously introduced into the system to form stable interfaces. From the experimental, data dimension less analysis was carried out and tabulated in Table 4. From this it is evident that Reynolds numbers obtained were less than 10 and hence laminar profile can be observed in these micro channels. For a particular channel, with the increase in Reynolds number, Weber number, Capillary number there is an increase in the value of K L a and decrease in the percentage extraction of acetic acid. For Channels with constant volume, a decrease in diameter gave higher Reynolds number, Capillary number, Weber number, which in turn gave higher K L a and also higher percentage extraction. Table 4.Dimensionless analysis: 20% acetic acid, V m = cm 3, C or,i = 0. D Q aq Q or S/F N Re N We N Ca N Re / N Ca N Re / N We K L a % E Mm ml/min ml/min ratio (sec -1 ) The increase in K L a with N Re is the consequence of increase in the surface renewal velocity and increase in the interfacial mass transfer area due to interface disturbance of the two immiscible phases and mass transfer distance becomes shorter.so that mass tranfer process is intensified. For a particular channel as Reynolds number increases the percentage extraction has decreased.this may be due to Reynolds number increase slug linear velocity increase, which in turn causes less residence time in micro channels. Due to lower residence time percentage extraction has shown a decreased trend. K L a increased with increase in the N We in the three different micro channels. Interfacial forces have more influence than the inertial forces.it can also be seen that this effect of interfacial forces is more in the small channel diameters than in the larger diameters.same reason can be assigned to this as above.for a particular channel as N We increases percentage extraction decreases where as micro channels with constant volume, a decrease in diameter gave higher N We,which in turn gave higher percentage extraction when compared to channels with higher diameters. This may due to interfacial forces have more influence than the inertial forces.it can also be seen that this effect of interfacial forces is more in

7 the small channel diameters than in the larger diameters.the shape of the interface between the immiscible liquids was controlled by a competition between the viscous forces and the local interfacial tension. For a perticular channel a decrease in diameter gave higher N Ca numbers which in turn gave higher K L a values. It is also observed that N Ca to for the same N Re variation of 1 to 8,implying that interfacial forces have more influence than viscous forces. It can also be seen that this effect of interfacial forces is more in the small channel diameters than in the larger diameters.that interfacial forces have more influence on flow, than viscous forces. Such behaviour is the consequence of increase in the surface renewal velocity and increase in the interfacial mass transfer area due to interface disturbance of the two immiscible phases and mass transfer distance becomes shorter.so the mass tranfer process can be intensified. For a channel with constant diameter and volume, with the increase in the ratio of N Re /N Ca which is the ratio between the interfacial tension and viscous forces of the liquid mixture in the micro channel, there was an increase in the percentage extraction of acetic acid and decrease in the K L a value. Table 5. Comparison of results in packed column and micro channels Type of contactor % Acetic acid in feed S/F ratio K L a ( sec -1 ) Percentage extraction Random packing 20% % Structured packing 20% % Micro channels 20% From above Table5 it can be seen that with solvent to feed flow rate (S/F) ratio=1, very high percentage extraction was obtained for micro channels when compared with the packed columns. To obtain the same kind of percentage extraction in packed columns almost S/F =4 must be employed and which gradually needs higher solvent and higher cost of operation. Hence to obtain maximum percentage extraction of acetic acid with less consumption of solvent i.e. ethyl acetate one should prefer performing liquid-liquid extraction in micro channels than macro columns. The order of magnitude of K L a values obtained in packed columns was about 10-4 and was about 10-2 in micro channels and hence 30 times increase in mass transfer coefficient from bringing down the operation to micro level from macro level. Higher K L a and hence higher mass transfer was achieved in micro channels, which is because of the higher surface/volume ratio in micro channels when compared to packed columns and hence more transfer. Conclusions From the results obtained in the macro (packed) columns it is observed that higher K L a and higher percentage extraction of acetic acid from water using ethyl acetate using a continuous countercurrent extraction was obtained with S/F ratio 4 and with a concentration greater than 20% of acetic acid in the feed. In the packed column, higher percentage extraction of acetic acid and higher K L a was obtained with structured packing when compared with random packing and time for extraction in structured packing was less than the random packing which entails reduction in solvent consumption. Higher K L a and percentage extraction can be achieved with channels of decreasing diameter with constant volume in micro channels due to increase in the surface to volume ratio. Interfacial tension, viscous forces, inertial forces are playing a major role inside the micro channel. For the S/F ratio=1, the K L a values obtained in micro channels

8 were almost 30 times greater compared to macro packed columns. With S/F ratio=1, high percentage extraction was obtained for micro channels when compared to the macro packed columns. To obtain the same kind of percentage extraction in packed columns almost S/F =4 must be employed and which needs more solvent and higher cost of operation. References 1. Benz K., K.P. Jackel, K.J. Regenauer, J. Schiewe, K. Drese, W. Ehrfeld, V. Hessel, H. Lowe,Utilization of micromixers for extraction processes, Chem. Eng. Technol., 24,11,17, Kashid M.N.,A. Renken, L. Kiwi-Minsker,Gas-liquid and liquid-liquid mass transfer in microstructured reactors,chem. Eng. Sci., 66, , Nobuak Aoki, Ryuichi Ando and Kazuhiro Mae, Gas-Liquid-Liquid Slug Flow for Improving Liquid Liquid Extraction in Miniatured Channels, Ind. Eng. Chem. Rrs.,50, , Lin W.Y., Y.J. Wang, S.T. Wang, H.R.Tseng, microfluidic reactors, Nano Today, 4, , Hudson S.D., Poiseuille flow and drop circulation in micro channels,rheol. Acta, 49, , Yuchao Zhao,Guangwen Chen and Quan Yuan, Liquid-Liquid Two Phase Mass Transfer in the T-Junction Microchannels, AIChE J., 53,12, , Kashid M.N., A. Renken, L. Kiwi-Minsker,Gas-liquid and liquid-liquid mass transfer in microstructured reactors,chem. Eng. Sci., 66, , Yoshihito Okubo,Taisuke Maki,Nobuaki Aoki, Teng Hong Khoo,Yoshikage Ohmukai, Kazuhiro Mae, Liqud liquid extraction synthesis and separation by utilizing micro spaces, Chemical Engineering Science 63, , Usha Rao H., Study of micro scale fluid dynamics an input for micro reactor modeling and design,ph.d.,thesis submitted to Osmania University, Treybal R.E., Liquid Extraction (Chemical Engineering Series) s edition McGraw-hill Co.Inc., New York Toronto, London, Kashid M.N., I. Gerlach, S. Goetz, J. Franzke, J.F. Acker, F. Platte, D.W. Agar, S. Turek, Internal circulation within the liquid slugs of a liquid-liquid slug-flow capillary micro reactor, Ind. Eng. Chem. Res., 44, , Yuanhai Su, Guangwen Chen,Yuchao Zhao and Quan Yuan,Intesification of Liquid- Liquid Two Phase Mass Transfer by Gas Agitation in a Microchannel, AIChE J.,55,8, ,2009.