(583fv) Phase Transfer Catalysis in Micro Channels, Milli Channels and Fine Droplets Column: Effective Interfacial Area

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[1][2]. 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 [3]. In the present study phase transfer catalysis (PTC) technique was used for the reactions in micro and mill-channels and also in fine droplet small diameter column system. PTC helps in the enhancement of the reaction between two immiscible phases using a small amount of phase transfer catalyst and operated under mild conditions and environmentally friendly way [4]. Usually PTC processes are mass transfer limited and need mass transfer rate enhancement techniques [5]. In this study the effectiveness of micro, milli-reactor and fine droplet 10mm diameter column reactors were evaluated for liquid-liquid system under PTC condition. Alkaline hydrolysis of n-butyl acetate with phase transfer catalysis was used as a model system.  Hydrolysis of n-butyl acetate was not very slow reaction and its progress is affected by mass transfer of reactants. In addition a conventional titration can be used to follow the progress of the reaction. 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 [6][4] and its lipophilicity which makes the reaction to occur only in organic phase [4]. The catalyst has almost nil solubility in water and this helps for the reusability of organic phase to save both catalyst and solvent cost. In performance evaluation of different reactors under PTC condition, it is required to know reaction rate constant of the reaction system and equilibrium concentration of active catalytic intermediate at the interface between the two phases [7]. No literature was found reporting the reaction rate constant of this system under dilute (solvent) condition in the organic phase. So experiments were conducted to find reaction rate constant under homogeneous conditions [8] and active catalytic intermediate was determined by the procedure reported in [9]. In the study three different reactor categories were involved. The first was micro reactor system containing tubes of 500mm, 750mm and 1000mm 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 form of fine droplets to produce more interfacial area. The needle internal diameters are 0.26, 0.337 and 0.603mm. These diameters are far less than nozzle diameters that have been reported in the literature and no application was found with such diameters for similar applications in fluid-fluid contact systems. The performances of these reactors were studied under different operating conditions such as reactants and catalyst concentrations and flow rates. Then after, the performances of these continuous reactors were evaluated in terms of conversion, volumetric rate of extraction and interfacial area for mass transfer [10]. The hydrolysis reaction using NaOH conducted homogeneous conditions followed second order kinetics [11]. The reaction rate constant evaluated by batch kinetic study was found to be 0.02 l/gmol s at 30°C. The reaction in continuous system was conducted under heterogeneous condition in the 10mm ID column, milli and micro reactors.  The OH ion of NaOH was transferred by PTC into the organic phase for intrinsic reaction. Based on the Hatta number and criterion given by Doraiswamy and Sharma, the reaction was observed to fall under slow reaction regime. This indicates that the reaction occurs in the bulk of the organic phase [12].

 Conversion, volumetric rate of 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 use of PTC doubled the conversion. Fine droplet in 10mm diameter column system gave 20% conversion and that of 2mm reactor achieved 35 % conversion for the same residence time. The effective interfacial area (a, m²/m³) generated and volumetric mass transfer coefficient (k_La, 1/s) obtained from fine droplet column system found to be comparable to that of 2.33/2.1 mm diameter tubes under the same operating conditions. But due to buoyancy force in fine droplet column system, droplets have less contact time and resulted in lesser conversion. 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. It was also observed that with increase flow rate, volumetric rate of extraction increased and conversion decreased. These can be explained by increase in turbulence and decrease in residence time at a higher flow rate respectively. 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 with the report of (14).   

References

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