Phase Transfer Catalysis in Micro Channels, Milli Channels and Fine Droplets Column: Effective Interfacial Area

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1 Phase Transfer Catalysis in Micro Channels, Milli Channels and Fine Droplets Column: Effective Interfacial Area Presentation ID#: Prof. Sankarshana Talapuru 1 and Alemayehu Chufamo Eromo 2 University College of Technology, Osmania University, Hyderabad, , India Abstract The move from conventional continuous process to miniaturized continuous process has number of advantages such as increased mass and heat transfer leading to increased conversion and product yield, improved safety and so on. But at the extreme end of miniaturization reaction systems suffer from some disadvantages like blockage of flow of materials, low capacity of production, difficulty of distribution of feed materials evenly in multichannel system and some operational difficulties. There are debates in the use of millimeter range reactors as a middle choice for improving capacity, efficiency and robustness for operation rather than explicit use of micro or conventional systems. In this study the effectiveness of micro[0.5, 0.75, 1.0mm], milli[2.1, 2.3mm] tubes and fine droplet 10mm diameter column reactors were evaluated for liquid-liquid system under phase transfer catalytic condition. Alkaline hydrolysis of n-butyl acetate with phase transfer catalysis was used as a model system. Conditions were identified in which each of these reactors will have better performance while considering robustness in operation. Conversion, volumetric rate of extraction and effective interfacial area were found to be inversely proportional to the diameters of micro, millichannels and the size of droplets in the column. The reaction was found to fall in the slow reaction regime. Keywords: Kinetics, Phase transfer catalysis, Milli-reactor, Micro-reactor, Hydrolysis, Volumetric rate of extraction. L 1. Introduction iquid-liquid heterogeneous reaction system is an important unit process having application in laboratory, industry and mainly for specialty chemicals. Reactions between two immiscible phases can also be enhanced using phase transfer catalysis [PTC] under mild operating condition and environmentally friendly way. Despite this advantage, most PTC processes are also mass transfer limited and need mass transfer rate enhancement techniques. Conventionally most of the PTC reactions are conducted in batch or continuous system. Usage of micro reactors for PTC system will enhance mass transfer between phases. Microreactor systems offer number of advantages against conventional systems such as fast mixing, improved heat transfer, increased conversion and less reaction time, less waste and positive environmental effect and increased safety. However, at the extreme end of miniaturization, reaction systems suffer from some disadvantages like blockage of flow of materials, low capacity of production at a given time, difficulty of distribution of feed materials evenly in multichannel system and operational difficulties 1. This study is intended to evaluate the effect of size in small scale continuous reactors and make comparative performance study among milli, micro and column reactors. PTC was used to speed up the reaction of hydrolysis of n-butyl acetate[buac] in solvent benzene by sodium hydroxide[naoh] in the aqueous phase as a case study in small scale reactors. The phase transfer catalyst, tricaprylmethyammonium chloride (aliquat 336), which is highly lipopholic, was used to enhance the rate of anion transfer from the aqueous phase to the organic phase and also increase the intrinsic reaction rate in the organic phase. Aliquat 336 was selected for its higher performance and its lipophilicity which makes the reaction to occur only in organic phase.

2 2. Experimental Experiments were conducted to find the reaction rate constant under homogeneous conditions and active catalytic intermediate was determined by the procedure reported in ref[2]. Mass transfer and mass transfer with chemical reaction studies were made in three different reactor categories were involved. The first was micro reactor system consisting of tubes of 500µm, 750µm and 1000µm diameter and different lengths required in the experimental plan. The second was milli-reactor system containing 2.1 mm and 2.33mm diameter tubes of 400 cm length. The third and the last was a column reactor of 10 mm internal diameter, 95 cm height, attached with T-junction which is equipped with a needle for jetting of one of the liquid into the other in the form of fine droplets. The needle internal diameters were 0.26, and 0.603mm Kinetics under homogeneous conditions For kinetic studies under homogeneous conditions, a 500 ml three-neck glass reactor, equipped with stirrer was used for the purpose and the three ports were used for agitation, temperature management and sampling. The reactor was kept in a water bath. Procedure: A known quantity of phase transfer catalyst, tri octyl methyl ammonium chloride (Quaternary ammonium chloride, QCl) was dissolved in benzene. This solution was mixed thoroughly with NaOH solution. The reaction between QCl and NaOH gives QOH(Quaternary ammonium hydroxide) in benzene and NaCl in aqueous phase. The mixture was taken in a separating funnel and allowed to settle. Then the two layers were separated and the benzene layer was titrated with methanoic- HCl for QOH concentration. A typical experimental procedure for batch kinetic run involves charging of a required quantity and known concentration of QOH solution as prepared above and required quantity of BuAc into the reactor and stirred mildly. At each interval of two minutes, sample solutions of required quantity was taken and arrested with excess methanoic-hcl solution and back titrated with methanoic-koh solution to determine the remaining QOH concentration. The reaction is as follows: Conditions: The temperature range to 313.5K, C BuAc range to kmol/m 3 and C QOH range to kmol/m Physical mass transfer Mass transfer studies were carried out in droplet column, micro tubes and milli tubes to determine the volumetric mass transfer coefficient. i. Droplet column reactor:the column was Pyrex glass having 0.95 m height and 0.01m i.d. A workshop fabricated stainless steel T-connector was fit to the bottom of the column as shown in Figure 1. One end of the T-connector was used for aqueous stream and the other end was used for organic. This end was fit with replaceable needles of diameters 0.26mm, 0.337mm and 0.603mm. Procedure: A typical column reactor experimental procedure involves filling of the aqueous solution containing NaOH of known concentration and organic solution with QCl of particular concentration into two 250ml bottles. The two feeds were pumped using peristaltic pumps at desired flow rates into the column. The organic stream was dispersed into the aqueous phase. The reaction occurs between NaOH and QCl with the formation of QOH at the interface. QOH diffuses from the interface in to the bulk. This set-up was used to find the volumetric mass transfer coefficient of QOH in the organic phase. Conditions: C NaOH =0.05kmol/m 3,T=303.15K, F T =0.134ml/s, AO=1, C QCl =0.0298, , and kmol/m 3, Needle 25G (0.26mm i.d.). ii. Milli and Microreactor experiment The set up consisted of milli tubes and micro tubes. Different i.d. of milli tubes and micro tubes were used. To one end of the tubes a suitable T-connector was fit to introduce the two streams using peristaltic pump. Figure 2 gives the schematic diagram of the set-up. Experiments were carried out in 2.33 and 2.1mm i.d. milli tubes and 1.0, 0.75 and 0.5mm i.d. for micro tubes. Here the experimental

3 procedure is similar to that detailed in the droplet column. Conditions: Conditions: C NaOH =0.05kmol/m 3, T=303.15K, C QCl = kmol/m 3, F T =0.015 to ml/s, AO= Mass transfer with chemical reaction Mass transfer with chemical reaction of OH - ion with BuAc was carried out in the three reactors: droplet column, milli and micro reactors, by adding known amount of BuAc to the organic phase. The experimental procedure is the same as described in section 2.2. The reaction steps can be expressed as follows. Conditions: The temperature was fixed 30 C. C BuAc range 0.1 to 0.4 kmol/m 3, C QCl range to kmol/m 3 and NaOH range 0.1 to kmol/m 3. In all cases the decrease in the NaOH concentrations was determined by titration using oxalic acid. 3. Results and discussion 3.1. Homogeneous kinetics The result of the run with different initial concentrations of QOH, C QOH are given in Table 1. The reaction between QOH and BuAc was an overall second order reaction and first order with respect to each of the reactants. The average value of the rate constant k 2 is 0.02 m 3 /kmol s at K. The average deviation of the individual values from the average value is observed to be 5 percent. Table 1: k with various initial C QCl concentrations at K. C QOHo (kmol/m 3 ) C BuAc (kmol/m 3 ) M=C BuAc /C QOH k (m 3 /kmol.s) ln((m-x)/(m(1-x)) Effect of n-butyl acetate concentration gmol/l gmol/l gmol/l gmol/l Time, s Figure 1: Effect of BuAc concentration on second order rate constant. Figure 1 gives the results of the reaction with different initial concentrations of BuAc, C BuAc. The reaction was observed to follow a straight line fit for vs time indicating overall second order. The average value of k 2 is m 3 /kmol s at 301.5K. -ln(k) 4.50 R² = /T x 10 3, K -1 Figure 2: Plot indicating temperature dependency of k for hydrolysis of n-butyl acetae.

4 The hydrolysis reaction was observed to follow the Arrhenius relation with temperature of , , and K. The results are plotted in Figure Physical mass transfer The catalyst, Aliquat 336 is insoluble in the aqueous phase containing NaOH. The phase transfer of OH - will occur through the reaction at aqueous-organic interface. The reaction is very fast. Hence the resistance to mass transfer of OH - in the form of QOH exits with in organic phase. The volumetric mass transfer coefficient of QOH, k L a is given by the expression [1] For the runs in the droplet column, and for different QCl concentration C QCl and AO=1, the k L a was found and the results are given in Figure 3. It can be observed that beyond C QCl =0.005kmol/m 3, k L a vs C QCl followed a linear relation. As expected, in the range studied, k L a increased with C QCl. Similarly, for AO=1, k L a in micro and millitubes were determined. The results are given in Figure 4. Upon comparison, the value of k L a in microchannels are relatively larger than in the milli channel and droplet column. The results of the droplet column are comparable to that of 2.0mm i.d. millichannel. k L a x 10 3, 1/s QCl x 10 2, kmol/m 3 Figure 3: Effect of C QCl on R A a in droplet column reactor for QOH transfer. Condition: C NaOH =0.05kmol/m 3,T=303K. k L a x 10 3, 1/s QOH transfer F T, ml/s 2.1mm 1.0mm 0.75mm 0.5mm Figure 4: Effect of tube diameter on R A a as a function of flow rate for QOH transfer. Condition: C NaOH =0.05kmol/m 3, C QCl =0.0225kmol/m 3, T=303K Reaction regime Based on the Hatta number and criterion given by Doraiswamy and Sharma, the reaction was observed to fall under regime 2. [Regime 1= very slow, 2=slow, 3=fast and 4=instantaneous]. This indicates that the reaction occurs in the bulk of the organic phase. Table 2: Reaction regime decision table for hydrolysis of BuAc in 1.0mm i.d. microchannel reactor F T, ml/s lkc F lkc Bo, 1/s lkc BO /k L a k L a/lkc BO k L a, 1/s T, ml/s Bo, k L a, 1/s lkc BO /k L a k L a/lkc BO 1/s As shown in Table 2, for 1.0 mm diameter tube k L a/lkc Bo ranges from 1.94 to 24 while that of lkc Bo /k L a values ranges from 0.04 to This shows that k L a > lkc Bo and the overall system has no significant mass transfer limitation. For the existing operating condition, the value of Hatta number varies from 0.98 to 1.9 and this also shows that is not far less than 3 or it doesn t satisfy the condition of to be very slow reaction regime. Therefore the overall system falls under regimes 2. Like the case of 1.0 mm tube diameter, the conditions in 2.3, 2.1, 0.75 and 0.5 mm tubes satisfy the condition under regime 2. Analogous trend was observed in the droplet column reactor. For this reactor system also even though k L a > lkc Bo, the value of which ranges from 2 to 4 is just around 3 and

5 doesn t satisfy both fast and very slow reaction regimes. As a whole, hydrolysis of BuAc with NaOH under PTC and solvent condition falls under regime Mass transfer with chemical reaction Catalytic contribution In order to determine percentage extraction corresponding to catalytic contributions, experiments were conducted both in absence and presence of catalyst, under otherwise similar experimental conditions in different reactors. It is clear from Figure 5 that the percentage extraction corresponding to catalytic contribution is significantly higher than the non-catalytic contribution. This shows that the catalyst is significantly contributing in enhancing the rate of reaction Effect of tube diameters for tubes of equal volume and equal residence time For a given flow rate to make residence time same for all tubes diameters, different length of tubes were used: 51.1cm for 2.1mm tube, cm for 1.0mm tube and 400cm for 0.75mm tube. This led to a condition that flow velocity was higher in smaller diameter tube than the larger diameter tube. As indicated in plots of Figures 6 a and b, for all concentrations of BuAc experimented, despite equality in the residence time, the smaller diameter tubes gave more percentage of extraction than the larger diameter tubes due to higher interfacial area produced and intensified internal circulation by the smaller diameter and a longer length tube and this shows the mass transfer limitation of the system. Figure 5: Comparison of percentage of extraction with catalytic and non catalytic condition. Reaction condition: C NaOH =0.05kmol/m 3, C QCl =0.0225kmol/m 3,C BuAc =0.2 kmol/m 3, T=303K. % Vol. Extraction Kmol/m 3 BuAc τ, s 2.1mm 1.0mm 0.75mm Figure 6: Comparison of percentage extraction for tubes of equal volume and residence time. Reaction condition: C NaOH =0.05kmol/m 3, C QCl =0.0225kmol/m 3, T=303K Effect of residence time in tubes of equal length Experiments were carried out at various BuAc concentration of 0.1, 0.2, 0.3 and 0.4 kmol/m 3 at a temperature of 30 C and with flow rate and with variable AO. The concentration of NaOH was kmol/m 3, catalyst QCl was kmol/m 3. Figure 7 gives the comparison of conversion achieved in different milli and micro tubes. For example, the same conversion is obtained in 0.5mm and 2.1mm tubes. This shows that milli tube can be preferred for the same output and conversion since it entails less operational and construction difficulties Effect of tube diameter for equal residence time and tube length This condition can be taken as an alternative experimental test method to selectively enhance mass transfer in small channels without affecting the kinetics. As observed from Figure 8, generally the volume of extraction increased with decrease in tube diameter. This is due to increase in the interfacial area and resulting in increase in mass transfer to give more extraction. This trend decreased with increase in BuAc concentration. This is due to reduction in mass transfer limitation at higher concentration of BuAc concentration. Hence the effect of tube diameter will decrease with increase in BuAc concentration for a system with the same length and equal residence time indicating in decrease in mass transfer limitation.

6 Conversion X, % mm 1.0mm 0.75mm 0.5mm τ, s Figure 7: NaOH conversion as a function of residence time in different tubes of equal length [400cm]. Reaction condition:c NaOH =0.05kmol/m 3, C QCl = kmol/m 3, C BuAc =0.02kmol/m 3 T=303K. Figure 8: Percentage volumetric rate of extraction against tube diameter for different concentrations of BuAc for tubes of equal length and equal residence time Interfacial area Three different interfacial area determinations methods have been used in order to include all the three categories of reactors in the process and to counter check the result of one method with the other Physical Method: For the physical method, the diameter of the droplets in the column reactor were determined by photographic technique and measured with reference to the scale which was imaged along with the droplets. Table 3: Interfacial area by physical method Needle 25G, F T =0.104 ml/s AO=1 Needle 23G, F T =0.104 ml/s AO=1 Needle 20G, FT=0.104 ml/s AO=1 Figure 9: Photograph of droplets taken from column reactor. droplet diameter d[mm] a [m 2 /m 3 ] Chemical Method: The volumetric rate of extraction for a given component A, R A a, can be defined using Danckwert s equation. [A=QOH and B=BuAc] With rearrangement [3] In the above equations, both R A a and are known and the plot of vs known as Danckwerts plot should give a straight line with slope of a 2 and intercept. Data followed straight line and both a and k L are calculated. a. Milli tubes Table 4 gives the interfacial area and the mass transfer coefficent calculated by Danckwerts plot in 1mm channel for different flow rates. The k L a thus calculted is compared with that obtained from physical mass transfer. b. Droplet column reactor Table 5 gives the interfacial area determined with physical and Danckwerts equation(chem. method) for runs in droplet column reactor. [2]

7 Table 4: Values of inetrfacial area and mass transfer coefficent for a 1.0 mm microchannel operating at different flow rates Flow rate, ml/s AO Summary of chemical method a, m 2 /m 3 k L, m/s k L a, 1/s Physical mass transfer k La, 1/s E E E E Table5: Comparison of interfacial area determined with the physical method and chemical method Chemical Physical AO ratio=1 method method Flow rate, ml/s a, m 2 /m 3 a, m 2 /m Hydrodynamic Method: From hydrodynamic study, all the experimental trials fall in the squeezing regime of slug formation with capillary number less than. Therefore it is possible to apply Garstecki et al. (2006) 4 linear scaling law mathematical model shown in equation 4. [4] Where, L=slug length, D=tube diameter for circular tube, F c =continuous phase flow rate, F d =dispersed phase flow rate, C 1 and C 2 = constants to be determined from the geometry of the tube. Using these values it was tried to determine slug length in different milli/micro-channels and then determined the interfacial area generated for each flow condition of the two phases. After knowing the slug length, the interfacial area can be approximated assuming the slug as a cylinder and with caps at both ends as shown below. As indicated in Figure 10, there are two assumptions in the gas-liquid or liquid-liquid flow of fluid in tubes: the first is the existence thin liquid film separating the tube wall and the dispersed phase and the second is pure slug flow. Kashid et al. 5 tried to evaluate and compare the models developed based on the above two assumptions with the experimentally measured values. They found that the model with the thin film estimates the interfacial area better than the model without film. In similar fashion, in this study it was tried to formulate an expression to calculate the interfacial area based on the geometry of the slug shown in the above figure. Similar expressions were used by Jovanovic 6 without considering the presence of liquid film. He was able to measure the cap height a by microscopic analysis and reported to be 65±10 µm for all his measurement of micro tubes. Finally the interfacial area can be calculated from the ratio of the above two expressions for the slug with film: [5] R L R-h R a x y Table 6:Comparison of interfacial area of hydrodynamic and chemical method for 1mm i.d. tube. Tube Diame ter, mm 1.0 FT, ml/s AO Slug length L d, mm a, m 2 /m 3 hyd. Method a, m 2 /m 3 chem. Method Figure 10: Dimensions of slug: x). without film and y). with film. As expected, the interfacial area increased with decrease tube diameter due to formation smaller droplets/slugs in smaller channels. However, in this study, there was no appreciable increase in the interfacial area in 0.75mm and 0.5mm tube diameters because of larger diameter of the T-junction. Despite this predicament, there was increase in the interfacial area with decrease in tube diameter.

8 Interestingly, the interfacial area obtained from droplet assisted column reactor by far is better than milli-meter range reactors and comparable to microreactors. This is mainly due to the formation of very fine micro-sized droplet in the droplet assisted column system resulting in higher interfacial area. Despite variations and uncertainties associated with each experimental measurement, the values of the interfacial area obtained in this study using different methods such as chemical, physical and hydrodynamic methods are in satisfactory agreement over the range of experimental conditions used in the study. From our unpublished result of experimental study of very slow reaction study, for very slow and between very slow and slow reaction systems, millichannel reactors with i.d. between 1 mm and 2mm are suitable candidate for their residence time advantage compared to column reactor and robustness for operation compared with microchannel reactors. In the case of very fast and instantaneous reaction system, fine droplet millimeter range column reactors may be used for their comparable and higher interfacial area advantage over millichannel reactors and robustness for operation compared to microchannel reactors. 4. Conclusions Conversion, percentage volumetric extraction and effective interfacial area determined, were inversely proportional to the diameter of micro and milli-tubes and the size of droplets in the column. The comparison of performances of micro and milli tubes is not straight forward and dependent on hydrodynamic factors and operating parameters. It was observed that at low concentration of organic phase reactant, micro-tubes gave higher conversion than milli-tubes under the same operating condition. This gap went decreasing as the concentration of organic phase reactant increases. This implies that micro-tubes are superior when there is mass transfer limitation; otherwise the conversions have been comparable with milli-tubes tested in the study. The hydrolysis of n-butyl acetate with sodium hydroxide under phase transfer catalytic and solvent condition falls in the slow reaction regime. Under similar operating conditions the interfacial area obtained, as expected, increased as we move from milli-tubes to micro-tubes. The numerical values of effective interfacial area calculated form physical and chemical method are in close agreement. When the reaction systems are intermediate and moderately fast both mass transfer and kinetics will affect the overall conversion. Under such conditions, microchannel reactors are the better choice in order to get both mass transfer and residence time advantage. References 1. M. Jonsson, B. Johnson, A. Laval, Millichannel Reactors: A Practical Middle Ground for Production, Chemical Engineering, 116 (2009) S. W. Park, H. B. Cho, D. W. Park, Kinetics of reaction of benzyl chloride with sodium acetate using Aliquat 336 as Phase-Transfer Catalyst, J. Ind. Eng. Chem., 9 (2003) L. K. Doraiswamy, M. M. Sharma MM, Heterogenous reactions, Analysis, Examples and Reactor Design, Vol II., John Wiley, New York, P. Garstecki, M. J. Fuerstman, H. A. Stone, G. M. Whitesides, Formation of droplets and bubbles in a microfluidic T-junction scaling and mechanism of break-up, Lab Chip, 6 (2006) M.N. Kashid, D.W. Agar, Hydrodynamics of liquid liquid slug flow capillary microreactor: flow regimes, slug size and pressure drop, Chem. Eng. J., 131(2007) J. Jovanovic, Liquid-liquid Microreactors for Phase Transfer Catalysis, PhD. Thesis, Eindhoven University of Technology, Netherlands, 2011.