- Email: [email protected]
Some mechanisms of electric field influence on mass transfer processes in the gas-liquid system
Journal of
ELECTROSTATICS ELSEVIER
Journal of Electrostatics 40&41 (1997) 663-668
SOME MECHANISMS OF ELECTRIC FIELD INFLUENCE ON MASS TRANSFER PROCESSES IN THE GAS-LIQUID SYSTEM
E.P. Maximuk, M.K. Bologa
Institute of Applied Physics,5 Akademiei Str.,Kishinev, Moldova
One of the promissing methods for the intensification and control of mass transfer processes is the application of the electric field effect.However,in spite of the growing interest in this scientific subject,scrappy information about mechanisms of mass transfer intensification is not systematized yet. A lot of mechanisms of heat and mass transfer intensification by electric fields were suggested. However, the results of the latest investigations allow for expressing doubts in some of them. For example, M.K.Bologa and co-authors [1] studied the effect of electric fields on the heat transfer coefficient for condensation. According to these authores, the causes for heat transfer intensification are : the formation of a regular wave structure on the surface of liquid, the increase of the conden-sate film velocity, the change in the physical properties of heattransfer agent under the electric field effect, the alteration of the phase equilibrium in gas-liquid system by the electric field. The possibility of an electric field to change the phase equilibrium coefficient was disct/ssed elsewhere [2], but no experimental evidence was given. In order to obtain the evidence, special experimental investigations were carried out [3]. It was shown that electric field had no effect on phase equilibrium. The purpose of this article is not to suggest any new mechanisms of mass transfer intensification by electric fields. It is to point out real mechanisms amongst many suggested ones. This article also aims at showing that in fact there are not so many real mechanisms, that each of them has both specific conditions of realization and specific degree of the maximum mass transfer intensification. The results of our investigations show the effect of mass transfer intensification in the gas-liquid system by an electric field to be connected with changes in phases hydrodynamics. Different mechanisms may be the base for this effect. Which of them will take place in each case depends on the process conditions 0304-3886/97/$17.00 © Elsevier Science B.V. All fights reserved. P l l S0304-3886(97)00112-5
664
E.P. Maximuk, M.K. Bologa/Journal of Electrostatics 40&41 (1997) 663-668
and parameters, on the electric field intensity, and the design of mass transfer devices. It is possible to systematize these conditions in the following way: - electric discharge [4,5]; electrohydrodynamical (EHD) spraying of liquid [6]; - application of an electric field, when both an electric discharge and an EHD spraying of liquid are absent; gas barbotage through dielectric liquid, when an electric field influences a dispersoid [7]. The mechanism of mass transfer intensification for the case of an electric discharge in the gas phase may be described as below. An electric discharge in the gas phase of a mass transfer device is followed by an electric wind. There is a space charge in the outer region of a corona discharge. Ions move from a hightension electrode to an earthed one, collide with gas (vapour) molecules and transmit some energy to them. As a result, neutral gas molecules begin to move, and an additional gas flow, called an electric wind, is formed. The flow of the electric wind is directed from the high-tension electrode to the earthed one, and has a circle shape. This character of the electric wind is in accordance with the distribution of electric field and with the electric space charge density, which are ~ a x i m u m near the high-tension electrode and drop with moving away from it. The electric wind additionally turbulizes the gas phase,and as a result, the intensity of mass transfer increases especially for the processes limited by the gas phase resistance. The degree of the mass transfer intensification depends on the method used for the realization of mass transfer processes. For example, for the evaporation of stationary liquid, when the external gas (vapour) flow is absent, the minimum relative intensification may be about 40 and the maximum one may be about 100 times. In the case of the stationary liquid and external laminar air flow, the degree of mass transfer intensification is 3 to 4 times. And finally, in the case of the motion of both phases, the intensification of mass transfer by an electric wind is about 1.5 to 2 times. The decrease of the electric field effect is accounted for by the influence of an external gas (vapour) flow on the typical parameters of electric wind and especially on its velocity. This mechanism takes place in the mass transfer sections where the high voltage electrodes have little radius of curvature. It may be a wire electrode (Fig. 1a), a needle electrode or a thomy electrode. Another mechanism may cause mass transfer intensification in the gas-liquid system, with a liquid film destabilized by an electric field. This mechanism takes -
-
665
E.B Maximuk. M.K. Bologa/Journal o f Electrostatics 40&41 (1997) 6 6 3 - 6 6 8
°~ e~ ~\\\N.\\\~N.\\X~N.\N.'X\\\ ¢, ,o
.N.N.\N.N.~N.N.N.N.'xX_N.N.N.\'xN.~d
~0
~\\\\\\\\'~\\N.\\
\ ' x \ "~
666
E.P. Maximuk, M.K. Bologa/Journal of Electrostatics 40&41 (1997) 663-668
place in the mass transfer sections where the high voltage electrodes have big radius of curvature (Fig. lb). The investigations of the electric fields influence on the hydrodynamics of the film flow show that with the increase of the electric field intensity to some critical value the structure of the wave flow is changed.The interaction of an electric field with polarization and free charges in liquid is responsible for the increase of the wave frequency and amplitude. The rise of the electric field intensity above the critical value causes both the electrohydrodynamical spraying of liquid and the transfer of charge droplets to the opposite electrode. It is necessary to note that highly dispersed EHD spraying of liquid is not always possible. However, in a general case,we can distinguish two probable regimes-the regime of the film flow of liquid and the regime of EHD spraying. The influence of an electric field on mass transfer in the case of the film flow of liquid was studied [8]. In the experiments on mass transfer in the liquid phase for oxygen absorption and desorption the intensification was 40...50 %, and in the experiments on rectification the mass transfer coefficient was 2-2.5 times higher than under free conditions. The effect of an electric field decreased when the flow rate of the liquid increased. It was shown that with the rise of the electric field intensity the surfase of the liquid is changed: smooth film turos into a wavy one with a distinct structure of waves; the frequency and amplitude of these waves increase with theelectric field intensity. If originally the liquid surfase has a wave shape, then the electric field effect results in the increase of the frequency and amplitude of large waves,and some regulating of interphase structure takes place. When the flow rate of the liquid rises, relative changes in the frequency of large waves diminish. It was found out that in the case of the film flow of liquid the influence of an electric field on mass transfer manifests itself in the change of kinetic coefficients of the phase resistanse mainly in the liquid phase, and it results from the increase of the frequency and amplitude of large waves,and from the emergence of interphase instability. The influence of electric fields on the intensity of mass transfer for the regime of EHD spraying was studied for the rectification of conductive liquids [9]. The investigation showed that in the regime of EHD spraying of liquid,the effect of electric fields is due to the formation of film-drop structures and to the influence of electric fields upon drops. This causes the widening of interfacial area,the rise of velocity of the relative movement of phases,the additional turbulization of phases, and the intensification of mass transfer. The application of electric fields for the EHD spraying of conductive liquids intensifies mass transfer for rectification up to 20 times and is economically
E.P. Maximuk, M.K. Bologa/Journal of Electrostatics 40&41 (1997) 663-668
667
advantageous. An additional consumption of energy in this case is smaller than for an electric discharge. If mass transfer takes place under the condition of gas barbotage through a dielectric liquid (Fig lc),then the mechanism of an electric field influence on mass transfer is due to the polarization and deformation of bubbles. The influence of a nonuniform and variable electric field in this case may lead to the fluctuations of the bubbles shapes,the relative increase of instantaneous interfacial area being insignificant. The main factor of mass transfer intensification is the flows inside and around the bubbles, which appear as the result of the oscillation of the bubbles shapes .The intensity of mass transfer grows with the frequency of an electric field and reaches its maximum value at the resonance regime of the bubbles oscillation when the frequency of the electric field is equal to the frequency of oscillations of the bubbles shapes. The maximum relative intensification is about 3 times. When the intensity of an electric field is of a large value the crushing of bubbles takes place. In this case the increase of intensity of mass transfer is mainly due to the increase of the interface area. For the condition of bubbling it is characteristic for the degree of mass transfer intensification to decrease with the velocity of vapour. It takes place because the influence of the electric field becomes commensurable with the turbulizing effect of the gas phase. It is necessary to note that the methods of the direct electric field influence on mass transfer are discussed here. However, another approach to the problem is available. It is the following. Electric fields may be used for the suppresion of the phenomena reducing the efficiency of the traditional mass transfer apparatus. For example, such a phenomenon as the carryover of droplets in the gas stream. On the basis of the results of the work presented here the following conclusions may be drawn. - The intensification of mass transfer by an electric field in the gas-liquid system is mainly due to the influence of an electric field on the hydrodynamics of phases. - It is necessary to distinguish several mechanisms of the electric field influence on mass transfer processes in the gas-liquid system. Which of them will take place in each case depends on the process conditions. These conditions are in accordance with the method chosen for the realization of the mass transfer process, with the process parameters, with the electric field intensity, and with the design of mass transfer devices.
668
E.B Maximuk, M.K. Bologa/Journal of Electrostatics 40&41 (1997) 663-668
REFERENCES 1. M.K.Bologa, V.EKorovkin, I.K.Savin, Two-phase liquid-vapour systems in an electric field, Moldova, Kishinev, 1992. 2. L.S. Atroshenko, S.M. Voronina,Teoretical base of the chemical technology, 14 (1980) 285. 3. A.V. Zevakin, S.M. Makareev, Sh. Shtrougalova, V.V. Maximov, V.V. Butkov, V.A. Malusov, Teoretical base of the chemical technology, 22 (1988) 816. 4. Yu.N. Gordeev, E.P. Maximuk, M.K. Bologa, The method for realization of mass transfer processes, USSR Patent No.1637818 (1991). 5. Yu.N. Gordeev, E.P. Maximuk, L.M. Moldavski, M.K. Bologa, The method of fractional distillation, USSR Patent No. 1669470 (1991). 6. Yu.N. Gordeev, E.P. Maximuk, M.K. Bologa, The method for the mass transfer processes, USSR Patent No.1680247 (1991). 7. M.K. Bologa, EM. Sajin, O.V. Motorin, E.P. Maximuk, The method of gasliquid interaction and the device for it realization, USSR Patent No. 1620111 (1991). 8. V.V. Maximov, A.V. Zevakin, V.A. Malusov, Teoretical base of chemical technology, 27 (1993) 136. 9. E.P. Maximuk, M.K. Bologa, Yu.N. Gordeev,Joumal of Electrostatics, 30 (1993)
ELECTROSTATICS ELSEVIER
Journal of Electrostatics 40&41 (1997) 663-668
SOME MECHANISMS OF ELECTRIC FIELD INFLUENCE ON MASS TRANSFER PROCESSES IN THE GAS-LIQUID SYSTEM
E.P. Maximuk, M.K. Bologa
Institute of Applied Physics,5 Akademiei Str.,Kishinev, Moldova
One of the promissing methods for the intensification and control of mass transfer processes is the application of the electric field effect.However,in spite of the growing interest in this scientific subject,scrappy information about mechanisms of mass transfer intensification is not systematized yet. A lot of mechanisms of heat and mass transfer intensification by electric fields were suggested. However, the results of the latest investigations allow for expressing doubts in some of them. For example, M.K.Bologa and co-authors [1] studied the effect of electric fields on the heat transfer coefficient for condensation. According to these authores, the causes for heat transfer intensification are : the formation of a regular wave structure on the surface of liquid, the increase of the conden-sate film velocity, the change in the physical properties of heattransfer agent under the electric field effect, the alteration of the phase equilibrium in gas-liquid system by the electric field. The possibility of an electric field to change the phase equilibrium coefficient was disct/ssed elsewhere [2], but no experimental evidence was given. In order to obtain the evidence, special experimental investigations were carried out [3]. It was shown that electric field had no effect on phase equilibrium. The purpose of this article is not to suggest any new mechanisms of mass transfer intensification by electric fields. It is to point out real mechanisms amongst many suggested ones. This article also aims at showing that in fact there are not so many real mechanisms, that each of them has both specific conditions of realization and specific degree of the maximum mass transfer intensification. The results of our investigations show the effect of mass transfer intensification in the gas-liquid system by an electric field to be connected with changes in phases hydrodynamics. Different mechanisms may be the base for this effect. Which of them will take place in each case depends on the process conditions 0304-3886/97/$17.00 © Elsevier Science B.V. All fights reserved. P l l S0304-3886(97)00112-5
664
E.P. Maximuk, M.K. Bologa/Journal of Electrostatics 40&41 (1997) 663-668
and parameters, on the electric field intensity, and the design of mass transfer devices. It is possible to systematize these conditions in the following way: - electric discharge [4,5]; electrohydrodynamical (EHD) spraying of liquid [6]; - application of an electric field, when both an electric discharge and an EHD spraying of liquid are absent; gas barbotage through dielectric liquid, when an electric field influences a dispersoid [7]. The mechanism of mass transfer intensification for the case of an electric discharge in the gas phase may be described as below. An electric discharge in the gas phase of a mass transfer device is followed by an electric wind. There is a space charge in the outer region of a corona discharge. Ions move from a hightension electrode to an earthed one, collide with gas (vapour) molecules and transmit some energy to them. As a result, neutral gas molecules begin to move, and an additional gas flow, called an electric wind, is formed. The flow of the electric wind is directed from the high-tension electrode to the earthed one, and has a circle shape. This character of the electric wind is in accordance with the distribution of electric field and with the electric space charge density, which are ~ a x i m u m near the high-tension electrode and drop with moving away from it. The electric wind additionally turbulizes the gas phase,and as a result, the intensity of mass transfer increases especially for the processes limited by the gas phase resistance. The degree of the mass transfer intensification depends on the method used for the realization of mass transfer processes. For example, for the evaporation of stationary liquid, when the external gas (vapour) flow is absent, the minimum relative intensification may be about 40 and the maximum one may be about 100 times. In the case of the stationary liquid and external laminar air flow, the degree of mass transfer intensification is 3 to 4 times. And finally, in the case of the motion of both phases, the intensification of mass transfer by an electric wind is about 1.5 to 2 times. The decrease of the electric field effect is accounted for by the influence of an external gas (vapour) flow on the typical parameters of electric wind and especially on its velocity. This mechanism takes place in the mass transfer sections where the high voltage electrodes have little radius of curvature. It may be a wire electrode (Fig. 1a), a needle electrode or a thomy electrode. Another mechanism may cause mass transfer intensification in the gas-liquid system, with a liquid film destabilized by an electric field. This mechanism takes -
-
665
E.B Maximuk. M.K. Bologa/Journal o f Electrostatics 40&41 (1997) 6 6 3 - 6 6 8
°~ e~ ~\\\N.\\\~N.\\X~N.\N.'X\\\ ¢, ,o
.N.N.\N.N.~N.N.N.N.'xX_N.N.N.\'xN.~d
~0
~\\\\\\\\'~\\N.\\
\ ' x \ "~
666
E.P. Maximuk, M.K. Bologa/Journal of Electrostatics 40&41 (1997) 663-668
place in the mass transfer sections where the high voltage electrodes have big radius of curvature (Fig. lb). The investigations of the electric fields influence on the hydrodynamics of the film flow show that with the increase of the electric field intensity to some critical value the structure of the wave flow is changed.The interaction of an electric field with polarization and free charges in liquid is responsible for the increase of the wave frequency and amplitude. The rise of the electric field intensity above the critical value causes both the electrohydrodynamical spraying of liquid and the transfer of charge droplets to the opposite electrode. It is necessary to note that highly dispersed EHD spraying of liquid is not always possible. However, in a general case,we can distinguish two probable regimes-the regime of the film flow of liquid and the regime of EHD spraying. The influence of an electric field on mass transfer in the case of the film flow of liquid was studied [8]. In the experiments on mass transfer in the liquid phase for oxygen absorption and desorption the intensification was 40...50 %, and in the experiments on rectification the mass transfer coefficient was 2-2.5 times higher than under free conditions. The effect of an electric field decreased when the flow rate of the liquid increased. It was shown that with the rise of the electric field intensity the surfase of the liquid is changed: smooth film turos into a wavy one with a distinct structure of waves; the frequency and amplitude of these waves increase with theelectric field intensity. If originally the liquid surfase has a wave shape, then the electric field effect results in the increase of the frequency and amplitude of large waves,and some regulating of interphase structure takes place. When the flow rate of the liquid rises, relative changes in the frequency of large waves diminish. It was found out that in the case of the film flow of liquid the influence of an electric field on mass transfer manifests itself in the change of kinetic coefficients of the phase resistanse mainly in the liquid phase, and it results from the increase of the frequency and amplitude of large waves,and from the emergence of interphase instability. The influence of electric fields on the intensity of mass transfer for the regime of EHD spraying was studied for the rectification of conductive liquids [9]. The investigation showed that in the regime of EHD spraying of liquid,the effect of electric fields is due to the formation of film-drop structures and to the influence of electric fields upon drops. This causes the widening of interfacial area,the rise of velocity of the relative movement of phases,the additional turbulization of phases, and the intensification of mass transfer. The application of electric fields for the EHD spraying of conductive liquids intensifies mass transfer for rectification up to 20 times and is economically
E.P. Maximuk, M.K. Bologa/Journal of Electrostatics 40&41 (1997) 663-668
667
advantageous. An additional consumption of energy in this case is smaller than for an electric discharge. If mass transfer takes place under the condition of gas barbotage through a dielectric liquid (Fig lc),then the mechanism of an electric field influence on mass transfer is due to the polarization and deformation of bubbles. The influence of a nonuniform and variable electric field in this case may lead to the fluctuations of the bubbles shapes,the relative increase of instantaneous interfacial area being insignificant. The main factor of mass transfer intensification is the flows inside and around the bubbles, which appear as the result of the oscillation of the bubbles shapes .The intensity of mass transfer grows with the frequency of an electric field and reaches its maximum value at the resonance regime of the bubbles oscillation when the frequency of the electric field is equal to the frequency of oscillations of the bubbles shapes. The maximum relative intensification is about 3 times. When the intensity of an electric field is of a large value the crushing of bubbles takes place. In this case the increase of intensity of mass transfer is mainly due to the increase of the interface area. For the condition of bubbling it is characteristic for the degree of mass transfer intensification to decrease with the velocity of vapour. It takes place because the influence of the electric field becomes commensurable with the turbulizing effect of the gas phase. It is necessary to note that the methods of the direct electric field influence on mass transfer are discussed here. However, another approach to the problem is available. It is the following. Electric fields may be used for the suppresion of the phenomena reducing the efficiency of the traditional mass transfer apparatus. For example, such a phenomenon as the carryover of droplets in the gas stream. On the basis of the results of the work presented here the following conclusions may be drawn. - The intensification of mass transfer by an electric field in the gas-liquid system is mainly due to the influence of an electric field on the hydrodynamics of phases. - It is necessary to distinguish several mechanisms of the electric field influence on mass transfer processes in the gas-liquid system. Which of them will take place in each case depends on the process conditions. These conditions are in accordance with the method chosen for the realization of the mass transfer process, with the process parameters, with the electric field intensity, and with the design of mass transfer devices.
668
E.B Maximuk, M.K. Bologa/Journal of Electrostatics 40&41 (1997) 663-668
REFERENCES 1. M.K.Bologa, V.EKorovkin, I.K.Savin, Two-phase liquid-vapour systems in an electric field, Moldova, Kishinev, 1992. 2. L.S. Atroshenko, S.M. Voronina,Teoretical base of the chemical technology, 14 (1980) 285. 3. A.V. Zevakin, S.M. Makareev, Sh. Shtrougalova, V.V. Maximov, V.V. Butkov, V.A. Malusov, Teoretical base of the chemical technology, 22 (1988) 816. 4. Yu.N. Gordeev, E.P. Maximuk, M.K. Bologa, The method for realization of mass transfer processes, USSR Patent No.1637818 (1991). 5. Yu.N. Gordeev, E.P. Maximuk, L.M. Moldavski, M.K. Bologa, The method of fractional distillation, USSR Patent No. 1669470 (1991). 6. Yu.N. Gordeev, E.P. Maximuk, M.K. Bologa, The method for the mass transfer processes, USSR Patent No.1680247 (1991). 7. M.K. Bologa, EM. Sajin, O.V. Motorin, E.P. Maximuk, The method of gasliquid interaction and the device for it realization, USSR Patent No. 1620111 (1991). 8. V.V. Maximov, A.V. Zevakin, V.A. Malusov, Teoretical base of chemical technology, 27 (1993) 136. 9. E.P. Maximuk, M.K. Bologa, Yu.N. Gordeev,Joumal of Electrostatics, 30 (1993)