Iraqi Journal of Chemical and Petroleum Engineering Vol.14 No.4 (December 2013) 1- 10 ISSN: 1997-4884 Mass Transfer Enhancement Using Extensions as Turbulence Promoters Q.J.M. Slaiman and Buraq S. Ahmed Chemical Engineering Department, College of Engineering, Al-Nahrain University Abstract Mass transfer was studied using a rotating cylinder electrode with different lengths of legs acting as turbulence promoters. Two types of rotating cylinder ,made of brass, were examined : an enhanced cylinder one, with four rectangular extensions 10 mm long, 10 mm wide, and 1mm thick, and an enhanced cylinder two with four longitudes 30 mm long,10 mm wide, and 1mm thick. The best performance was obtained for enhanced cylinder two at low rotation speeds while enhanced cylinder one was realized at high rotation speeds. The mass transfer enhancement as compared with a normal rotating cylinder electrode, devoid of promoters, is 53% or 58% higher. The enhancement percentage decreased as rotation speeds increased further, since, seemingly, full turbulence has been reached practically by means of rotation and turbulence promoters. Key Words: electrochemical reactors, mass transfer, rotating cylinder electrode, limit current, turbulence promoter. Introduction The legal limitations for environmental protection require the development of reliable and cost-effective processes for the treatment of effluents with small concentrations of dangerous species. The electrochemical treatment of effluents can be efficiently achieved by the use of rotating cylinder electrode. The advantages of the rotating cylinder electrode (RCE) may be summarized in terms of high mass transfer in turbulent flow at low rotation rates, an equipotential surface for potentiostatic control, good solution mixing in a relatively low volume cell, and versatility of design for continuous cascade reactor usage[1-3]. However, in order to further increase the space time yield the incorporation of turbulence promoters has been suggested. Therefore; Kappesser et al. [4] performed mass-transfer investigations at rotating cylinders with staggered diamond knurls machined on their surfaces. Sedahmed et al. [5] studied mass-transfer at rotating finned cylinders. The fins were made by cutting longitudinal rectangular grooves on the cylinder surface. Makanjuola and Gabe [6] reported mass transfer studies at V-grooved cylinders and the investigation was extended to pyramidal knurling and wires or meshes wound to the cylindrical rotating electrode [7]. Further mass-transfer works as a function of the roughness factor are discussed by Gabe et al. [8]. Nahle´ et Iraqi Journal of Chemical and Petroleum Engineering University of Baghdad College of Engineering Mass Transfer Enhancement Using Extensions as Turbulence Promoters 2 IJCPE Vol.14 No.4 (December 2013) -Available online at: www.iasj.net al. [9] studied Mass transport to rotating cylinder electrodes fabricated from reticulated vitreous carbon of different porosities. Using copper deposition as test reaction they reported that the Sherwood number was dependent upon the Reynolds number to the power 0.63, both defined in terms of the cylinder diameter. It was also concluded that the mass transfer coefficients are comparable to those at a smooth rotating disk electrode )RDE) and rotating cylinder electrode (RCE) of the same diameter. Thus, enhancements are mainly due to the large electro-active area of the three- dimensional matrix. The performance of electrochemical reactors with rotating cylinder electrodes of expanded metal was studied by Grau and Bisang [10, 11]. It was found that the mass transfer coefficients, using the reduction of ferricyanide, for rotating cylinder electrodes of woven- wire meshes are about three times higher than those obtained with smooth electrodes, because of the turbulence promoting action of the meshes. Furthermore, Reade, Ponce-de-León, and Walsh found that the introduction of baffles in the electrochemical cell has little effect on the behavior of a reticulated vitreous carbon rotating cylinder electrode, and a jet electrolyte flow towards the electrode can enhance the mass transfer rate by a factor ranging from 1.03 to 1.46, depending on the electrode type[12]. Grau and Bisang studied the mass transfer at a rotating cylinder electrode with different turbulence promoters using the reduction of ferricyanide as a test reaction. Four types of turbulence promoters were examined: expanded plastic meshes, Teflon structures, a plastic woven mesh and a plastic perforated net, which were rotated together with the electrode. They concluded that the best performance was obtained for the Teflon structures at low rotation speeds and for the plastic woven mesh at high rotation speeds. The mass-transfer enhancement factor related to a smooth rotating cylinder electrode was found twice as large [13]. The aim of present work is enhancement of mass transfer of dissolved oxygen using turbulent promoters in 0.1N NaCl solution at various temperatures under flow conditions and to compare the mass- transfer characteristics with rotating cylinder without extensions. Experimental Work The experimental rig which was used for performing the present work is shown in Fig 1. The experimental apparatus was composed of water bath to obtain different solution temperatures, mechanical agitator to obtain different rotational velocities, power supply to apply the current , variable resistance (rheostat) to control the current flow, digital ammeter to measure the current, digital voltmeter to measure the potential, graphite electrode as auxiliary electrode (anode) of an immersed area which was three times larger than the area of cathode (working electrode) to make sure that the limiting current density occurs on cathode. The reference electrode was a saturated calomel electrode (SCE) in order to measure the cathode potential using a Luggin capillary placed midway of the rotating electrode surface at a distance 1-2 mm from it. The electrical connection of cathode (working electrode) was achieved using brush. The working electrode (cathode) was a rotating cylinder 25 mm in diameter, 27 mm long with four extended rectangular legs 1 and 3 cm long, 1 cm wide , and 1 mm thick, made of brass to generate additional turbulence. The Q.J.M. Slaiman and Buraq S. Ahmed -Available online at: www.iasj.net IJCPE Vol.14 No.4 (December 2013) 3 lower and upper surfaces of the electrode were insulated. The experimental details are shown in figure 2 . Before each experimental run, the metal specimen was cleaned by emery paper and washed by tap water followed by distilled water, dried with clean tissue, degreased with annular ethanol, and dried with clean tissue. The specimens were then stored in a desiccator over highly active silica gel for overnight before use, and then directly exposed to the solution for cathodic scanning of limiting current [14].The test solution was 0.1M NaCl of PH = 6. When the constant temperature bath attained the required temperature, the working electrode was immersed and the electrical circuit was switched on. The power supply was set at 5 V (applied voltage). The specimen (working electrode) was cathodically polarized from a particular potentials (- 1.5 to -1.9 V) to the corrosion potential (where iapp. = 0) by changing the applied current using rheostat. The potential was recorded galvanostatically for step changes in current. One minute was allowed for steady state to be reached after each current increment. Then the polarization curve can be drawn and the limiting current can be obtained. In a set of experiments the above procedure was repeated for five values of rotation speeds 200,400,600,800, and 1000 rpm at different temperatures (35, 45 and 55 °C). Fig. 1, Experimental apparatus: 1) Power supply,2) Resistance box ,3)Water bath , 4)0.1M NaCl, 5) Luggin capillary tip, 6) Graphite electrode (anode) ,7) Working electrode ,8) Rotating shaft ,9)Reference Saturated Calomel Electrode (SCE), 10) Carbon brush ,11) Stirrer ,12) Stand ,13) Voltmeter ,14) Ammeter ,15) Electrical wires Mass Transfer Enhancement Using Extensions as Turbulence Promoters 4 IJCPE Vol.14 No.4 (December 2013) -Available online at: www.iasj.net (a) Enhanced (b) Enhanced cylinder 1 cylinder 2 (c) Electrolyte flow rate (d) Top view of produced by the electrode working electrode rotation Fig. 2, Schematic view of the working electrode Results and Discussion The limiting current density iL is obtained from polarization curves for smooth rotating cylinder under similar conditions. The limiting current plateau is not well defined, thus the method given by Gabe and Makanjoula [15] will be Adopted to find the limiting current density values as in Fig.3 … (1) Where i1 and i2 are the currents associated with E1 and E2 respectively. Fig. 3, Typical polarization curve of dissolved 02 Fig.4 shows the cathodic polarization curves in 0.1 M NaCl for smooth, enhanced one and two rotating cylinder at 200 rpm for 35, 45, and 55 ᵒC. (a) (b) (c) Fig. 4, Cathodic polarization curves in 0.1 M NaCl for smooth, enhanced one and two rotating cylinder at 200 rpm for (a) 35ᵒC, (b) 45ᵒC, and (c) 55 ᵒC Q.J.M. Slaiman and Buraq S. Ahmed -Available online at: www.iasj.net IJCPE Vol.14 No.4 (December 2013) 5 Also, the limiting current density was calculated for smooth cylinder of geometrical characteristics (25mm diameter, 27mm long), Enhanced cylinder one, and Enhanced two including areas of extensions as listed in table1. Table 1, Experimental limit current density as a function of velocity at different temperature T e m p e ra tu re Rotation rate 𝛚 (RPM) limiting current density, iL (mA/cm 2 ) of smooth cylinder limiting current density ,iL (mA/cm 2 ) of Enhanced cylinder one limiting current density ,iL (mA/cm 2 ) of Enhanced cylinder two 35 ᵒC 200 0.3134 0.4815 0.49523 400 0.5041 0.70845 0.7243 600 0.61715 0.8068 0.8367 800 0.8956 1.0855 1.138 1000 1.137 1.271 1.286 45 ᵒC 200 0.3543 0.53175 0.56535 400 0.5443 0.7454 0.7543 600 0.7194 0.894 0.9223 800 1.0012 1.14715 1.1455 1000 1.236 1.3645 1.3515 55 ᵒC 200 0.371 0.54765 0.566 400 0.66855 0.8509 0.8656 600 0.80865 0.9586 0.96835 800 1.146 1.308 1.302 1000 1.3965 1.5325 1.469 The mass transfer coefficient, k is calculated from the following equation, k = iL/neFCb …(2) Where ne =charge number of the electrode reaction, F= Faradays constant (96487 columb/equivalent), and Cb = bulk concentration(mole/m 3 ) , for system under mass transfer control Cb =Co2 which is the concentration of O2 in the solution bulk. Table 2, shows the oxygen solubility and concentration for different temperatures [19] T (ᵒC) Solubility (mg/l) 0.1NaCl Cb (mole/m 3 ) 35 5.9445 0.21718 45 4.894 0.18718 55 4.9445 0.154515 Figure 5 shows the mass transfer coefficient, k, as a function of Reynolds number for enhanced one, and enhanced two rotating cylinder electrodes at different temperatures. The experimental mass-transfer coefficients for a similar smooth rotating cylinder electrode are also reported in this figure, which provides a baseline for performance comparison. It can be seen that as Reynolds number increases k is increased. This can be attributed to the increase in oxygen supply from the bulk of the solution to the metal surface leading to higher iL [16] according to equation (2), thus k will be increased. This is because of leg extensions, which present a higher specific area and promote additional turbulence in the electrolyte flowing over the cylinder surface. The k values for enhanced one and two cylinders are higher than dictated by the additional cylinder area only as displayed in Fig.5. Figure 6 shows the mass transfer coefficient, k, as a function of the temperature for smooth, enhanced one, and two rotating cylinder electrodes at different Re numbers. It is clear that as the temperature increases k increases. This is due to the fact that increasing temperature accelerates the reaction rate as dictated by Arrhenius equation. Likewise, increasing temperature will increase the rate of oxygen diffusion to the metal surface and decrease the viscosity of water which will aid the oxygen diffusion. Moreover, as the temperature increases, the oxygen solubility decreases. The k values are still higher showing that the diffusion has a higher degree of effect than O2 solubility [18]. In order to determine the efficiency of enhancement due to extensions a Percentage (EP) can be defined as: …(3) Mass Transfer Enhancement Using Extensions as Turbulence Promoters 6 IJCPE Vol.14 No.4 (December 2013) -Available online at: www.iasj.net Fig. 5, Mass transfer coefficient k vs. Re for the three temperatures (a) 35ᵒC (b) 45ᵒC (c) 55ᵒC Q.J.M. Slaiman and Buraq S. Ahmed -Available online at: www.iasj.net IJCPE Vol.14 No.4 (December 2013) 7 Fig. 6, Mass transfer coefficient vs. Re (a) Smooth cylinder, (b) Enhanced cylinder1, and (c) Enhanced cylinder 2 Mass Transfer Enhancement Using Extensions as Turbulence Promoters 8 IJCPE Vol.14 No.4 (December 2013) -Available online at: www.iasj.net Fig.7 shows the enhancement percentage as function of the Re number for enhanced cylinder one, and two in terms of temperature. The enhancement percentage ranges from 58% to 5% depending mainly on rotation speed; the greater Re the smaller is the enhancement percentage for a given temperature. This may be attributed to the fact that the effect of extensions which act as turbulence promoters is practically limited, since full turbulence had already been achieved by means of rotation, i.e, Re. Also, Fig.7 compares enhancement percentages for enhanced cylinder one and two. As shown for the lower Re values enhanced cylinder two is more efficient than one. This indicates that the transition to turbulence occurs earlier due to promoters. However, the reverse, is, apparently indicated at higher Re. Moreover, it is to be noticed from Fig.7 that at high temperature the percentage of enhancement is lower. This can be explain as the temperature increase the mass transfer coefficient increase for smooth and enhanced cylinder electrode and the effect of extensions which acting as turbulent promoter will be diminish this lead to the values of K approximately close to each other and enhancement percentage reduce. Although, the effect of extensions length on the enhancement factor can be considered as few. Fig. 7, Comparison of mass transfer enhancement using rotating cylinder electrodes with extensions Conclusions 1. Mass transfer coefficients for enhanced rotating cylinder electrode are, in general, about 53% or 58% higher those obtained with smooth electrode without extensions. 2. The mass transfer enhancements are due in the main to leg extensions, which present a higher specific area and promote additional turbulence in the electrolyte flowing over the cylinder surface. 3. The enhancement percentage decreases as Re increased by the average 47.26%. This may be attributed to the fact that the effect of extensions which act as turbulence promoters is practically limited, since full turbulence had already been achieved by means of rotation, i.e, Re. Q.J.M. Slaiman and Buraq S. Ahmed -Available online at: www.iasj.net IJCPE Vol.14 No.4 (December 2013) 9 4. The enhancement percentage decreases as the temperature increased the average 33.727%. This can be explain as the temperature increase the mass transfer coefficient increase for smooth and enhancement cylinder electrode and the effect of extensions which acting as turbulent promoter will be diminish this lead to the values of K approximately close to each other and enhancement percentage reduce. References 1- D.R. Gabe, (1974),” The rotating cylinder electrode”, J.Appl.Electrochemistry, No.2, Vol.4, PP. 91. 2- D.R. Gabe and F. C. Walsh, (1983),” The rotating cylinder electrode: a review of development”, J. Appl. Electrochemistry, No.1, Vol.13, PP. 3. 3- Ralph E. White, (1984),” ElectrochemicalCell Design”, Springer-Verlag US, PP. 225-258. 4- R. Kappesser, I. Cornet and R. Greif, (1971),” Mass Transport to a Rough Rotating Cylinder”, J. Electrochemistry. Soc., No.10, Vol. 118, PP. 1957. 5- G.H. Sedahmed, A. Abdel Khalik, A.M. Abdallah and M.M. 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