Technology (1961). 16) Toei, R., M. Okazaki and M. Asaeda: ibid., 8, 277 (1975). Kawamura, Y., H. Makihara and H. Shinagawa: Kagaku 17) Ueda, H. and Y. Takashima: ibid., 9, 121 (1976). Kogaku, 36, 1307 (1972). 18) idem: ibid., 10, 6 (1977). Mitani, T. and Y. Takashima : Preprint of the 13th Autumn 19) Yamakawa, N. and S. Ohtani: Kagaku Kogaku, 36, 197 Meeting of The Soc. of Chem. Engrs., Japan, at Nagoya (1972). p. 684 (1979). 20) White, J. E. and C. J. Cremers: Paper ofAIAA, (74-746)i, Mott-Smith, H. M.: Phys. Review, 82, 885 (1951). 1 (1974). Satou, K.: "Bussei Jyousu Suisanho", p. 129, Maruzen, Tokyo (1954). (A part of this paper was presented at the 6th International Sherwood, T. K. and C. Johannes: AIChEJ., 8, 590 (1962). Congress of Chemical Engineering, Chemical Equipment Design Toei, R. and M. Okazaki: /. Chem. Eng. Japan, 1, 125 and Automation, G3.3, Praha, Czechoslovakia, August 21-25, (1968). 1978.)
OVERALL RATE OF OZONE OXIDATION OF CYANIDE IN BUBBLE COLUMN
Masaaki TERAMOTO,Yoshihiro SUGIMOTO, Yasuhiro FUKUI and Hiroshi TERANISHI Department of Industrial Chemistry, Faculty of Industrial Arts, Kyoto Institute of Technology, Matsugasaki, Kyoto 606
Oxidation of cyanide with ozone was carried out using a semi-continuous bubble columnand a continuous bubble column. It was found that the overall reaction rate was strongly controlled by liquid-film mass transfer resistance. The observed rate was satisfactorily explained by the theory of gas absorption with chemical reactions if the intrinsic rate expressions of self-decom- position of ozone and the ozone oxidation of cyanide which had been obtained by the present authors were applied to the theoretical equations. A method for evaluating capacity coefficient of liquid-phase mass transfer from the absorption rate of ozone into water of high pHis also presented.
Cyanide is one of the toxic chemicals which can Introducti on react with ozone very rapidly, and ozone treatment is Ozoneis a powerful oxidizing agent second only to a promising method for the treatment of CN~ in fluorine, and it has been recognized that ozone is industrial wastewater. Ozonetreatment is, however, effective for disinfection, decolorization, deodoriza- carried out in a gas-liquid heterogeneous system where tion and the destruction of toxic chemicals in waste- the overall reaction rate as well as the degree of water treatment3}. Another advantage of ozone treat- utilization of ozone is influenced by mass transfer ment is that it does not cause secondary pollution, resistance, and this complication has mademanyin- such as the formation of chlorinated compounds vestigators draw wrongconclusions concerning the encountered in oxidation by chlorine, because of the reaction kinetics. self-decomposition of ozone in water. However,one The purpose of this paper is to clarify the charac- of the disadvantages of ozone treatment is that ozone teristics of ozone absorption in aqueous cyanide solu- is expensive compared to chlorine. Therefore it is tions using a semi-continuous bubble column as well important to apply ozone in decomposingvery reac- as a continuous bubble column, and to explain the tive substances so that it maybe effectively utilized. experimental data on the basis of the theory of gas absorption with chemical reactions. Amethod for Received July 25, 1980. Correspondence concerning this article should be estimating capacity coefficient of liquid-phase mass addressed to M. Teramoto. Y. Sugimoto is now with Prefectural Industrial transfer of the bubble column from the absorption Research Institute, Nara 630. Y. Fukui is now with Kyoeisha Yushi Chemi- cals, Nara 630. rate of ozone into water of high pH is also presented.
VOL 14 NO. 2 1981 111 a reciprocal pump and the liquid level was controlled by a level controller. The gas flow rate was 0.3 N- dm3/niin. Buffer solutions were prepared from solu- tions of0.2 mol/dm3 Na2B4O7and 0.1 mol/dm3 NaOH. Liquid-phase capacity coefficient was measured by absorption runs of ozone into cyanide free buffer solution as will be explained later. The temperature was 298 K. 2. Physical Properties, Mass Transfer Coefficient and Reaction Rate Constants Solubility of ozone was measured by analyzing Fig. 1 Experimental apparatus ozone concentration in 0.1 N H2SO4solution saturated with a mixture of ozone and oxygen of knownozone concentration by means of iodometric titration. Henry's constant thus obtained was 82 atmà"dm3/mol at 298 K, which is in good agreement with the value by Briner et al.1]. Diffusivity of ozone in water was estimated to be 2.0x l0-9 m2/s by the Wilke-Chang equation13} assuming that the molar volumeof ozone is 1.5 times that of oxygen. The diffusivity of CN~ was obtained from its mobility at infinite dilution10} (Ab=2.08xlO-9m2/s). It was found that most of the bubbles in both columns had diameters greater than 0.3 cm although precise measurementwas not carried out. Here liquid-phase mass transfer coef- ficient was estimated from the equation of Calderbank Fig. 2 Absorption of ozone in water of high pH et al.2) for "large bubbles" greater than 0.25 cm in diameter (kL=4A x 10~4 m/s). The gas-phase mass 1. Experimental transfer resistance was neglected because of the rela- The schematic diagram of the experimental ap- tively high value of Henry's constant. paratus is shown in Fig. 1. A bubble column 6cm The rate of ozone decomposition in water of pH in diameter and 40cm in height was used for semi- greater than 9 at 298 K was measured by the present continuous operation, and another bubble column authors with a stopped-flow technique11}. It is ex- 2.4cm in diameter and 30 cm in height was used for pressed as follows. continuous operation. Ball filters (Kinoshita Rika- r^-dmidt^UO.l ^1=0.857[OH-]°-88[s-1] (1) kogyo Co.) of spherical geometry made of sintered The reaction of ozone with CN~is expressed by glass were used as gas distributors. In the case of semi-continuous experiments, 1 dm3of aqueous solu- CN- +O3=CNCr +O2 (i) tion of potassium cyanide which was adjusted at and its reaction rate is given byU) desired pH was charged into the column, and a mix- r2--4CN-]/^=-4O3]/^-^[O3]°-8[CN-]0-55 (2) ture of ozone and oxygen from an ozonator (Nippon fe=0.22 m^mol-0-3^-1, 9.4
112 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN a0-å the film theory. As shown in Eq. (1), ozone decom- position follows first-order kinetics. Gas absorption rate accompanied by an irreversible first-order reac- tion isA^^Mlrepresented-a/cosh n) by12)(4) The diffusion rate of ozone from the liquid film to the bulk liquid is expressed as follows12K NAl =kLAMVoosh ri-a) (5)
Y1==Vk1DA/kL9 i81=7'1/tanh 7'1, a=A0/Ai (6) Assumingwherethat the liquid phase is completely mixed and the flow pattern of the gas phase is plug flow in the bubble column, the following material balance Fig. 3 Effect of cyanide concentration on Eo equationsGMdy= -NAadVare derived.(7) \ VLNAladV=kiAo VL (8) The boundary conditions Joof Eq. (7) are y=yf at V=0 (9) y=yQ at V=VL (10) From Eqs. (3)-(10), E is derived as follows. E=(l - "f cosh n{ri2kL/aDA+(ll-exp- 1/cosh2 (-fA)ri)}l (12) L +{l-exp(-?/31)}/coshri J Fig. 4 Effect of ozone concentration on EQ where £=kLaVLP/GMH and ao=A,j{PyfIH). These are simplified forms of the equations which increases remarkably with decreasing pH, and the were derived by taking both gas- and liquid-phase rate is limited by both liquid-film resistance and the mass transfer resistances into account8}. reaction rate in the bulk liquid. Because ozone decomposition is first order, it is If f1kL/aDA=k1/kLa>100, a0 can be taken as zero. anticipated that E does not vary with yf. According Then Eq. (ll) reduces to Eq. (13). to the experimental results, E was not influenced by £=l-exp (-fi81) (13) yf if j/>0.01. However, E increased slightly with WhenpH is ll.2, the experimentally observed value decreasing yf when yf was much lower than 0.01. ofEis 0.40 and k, calculated by Eq. (1) is 1.28s"1. The reason has not yet been clarified. Then jx is 0.126 and iS1=1.005. Substitution of these 3. 2 Ozone oxidation of CN~with semi-continuous values into Eqs. (ll) and (12) gives the value of kLa bubble column as 0.016 s"1. Using this value, E is calculated from Figure 3 shows the effect of [CN~] on E09 the initial these equations at each pH and is shown in Fig. 2. value of E. It is seen that EQincreases with [CN~], Agreement between calculated and experimental re- The relation between Eo and yf9 the feed ozone con- centration, is shown in Fig. 4, which indicates that EQ sults is good. Because the maximumvalue of jx is increases slightly with decreasing yf. To explain at most 0.4 in this experiment, ozone decomposition these results, the overall reaction rate was analyzed occurs mainly in the bulk liquid. In Fig. 2 is also based on the film theory. The basic equation is shown a0 calculated from Eq. (12), which indicates written as that a0 is very small in the region of high pH. In the D^A/dx'^k.A+hA0-^0-"5 (14) range of pH between 10 and 12, E does not vary with the boundary conditions significantly with pH. This indicates that absorp- x=0; A=AU B^Bt-Bo, dB/dx=0 (15) tion proceeds in the region of maximumphysical x=xL; A=A0~0, B=Bo (16) absorption rate where Aois muchlower than At and ^i is almost unity independently of pH, and absorp- Here it is assumed that Ao~0 and Bi~B0. Applying Hikita and Asai's approximation method5} to linearize tion rate is completely controlled by liquid-film mass Eq. (14), the following equation is obtained for the transfer resistance. In the range of lower pH, Ao VOL. 14 NO. 2 1981 ment with the experimental data. Figure 5 shows the time-course of [CN~] and E during the ozone oxidation. In this experiment an- other ball filter was used as a gas distributor although it was the same type used in the previous experiment. The value of kLa was measured by the same procedure described in 3.1, and found to be 0.012 s"1, which is lower than the previous value by 25%. This differ- ence maybe due to the slight difference in the porous structure of the sintered glass. The experiment was carried out at pH=12where rl9 the rate of self-de- composition, cannot be neglected compared to r2, the rate of reaction with CN~. For example, if [CN-]=10ppm and ^=2.6x10"4mol/dm3 which corresponds to the inter facial concentration when y= Fig. 5 Time-course of cyanide concentration and 0.021, rjr2 is 0.3. However, it is well-known that E during ozone oxidation of cyanide hydroxyl radical is produced by the OH" catalyzed ozone decomposition and this radical is very reac- tive^. Therefore, it is possible that CN~ is oxidized by -OH quantitatively. To confirm this, the stoi- chiometric coefficient of this reaction was calculated using Fig. 5. The number of moles of ozone absorbed until 90% of CN"was consumed was found to be approximately equal to that of CN~reacted. It was also confirmed that the reaction rate of ozone with the reaction product CNO~,expressed by Eq. (22)n), could also be ignored when [CN~]>1 ppm. r8= -rf[O8]/*= 1.60[O8]1-8[CNO-]0 -M (22) Fig. 6 Effect of mean residence time of liquid on effluent cyanide concentration and E Thus it is concluded that all ozone absorbed is effec- tive for the decomposition of CN~unless [CN~] is ozone absorption rate. too low. Material balance ofCN~is written by NA=p2kLAt (17) - VLdBQ/dt= GMyfE (23) /32-r2/tanh r2 (18) where Eis the function ofBo. IfBo is given, yQ can ^ Wl^WT^Mr *^^ (1 9) be otbained by integrating Eq. (21). Then E is calcu- If £o=lOO ppm=3.84x 10~3 mol/dm3, ^=0.008 and lated from Eq. (3), and Eq. (23) can be integrated pH=10.6, then Ai at the bottom=9.9x 10"5 mol/dm3, from the initial value of Bo. The computed results Aa=0.38 s"1 and (2/l.S)k2AiQ-2B0 agree well with the experimental data as shown in 0-55=97s"1. There- Fig.5. fore, ozone reacts exclusively with CN~ and the rate of self-decomposition can be ignored. In this case, 3. 3 Ozone oxidation of CN~with continuous bubble 7-2=l.O8 and f2kL/DAa=6100>100. This indicates column that Ao can be taken as zero, as discussed in 3.1. The To examine whether the analysis in 3.2 is applicable enhancement factor corresponding to instantaneous to continuous operation, a series of experiments was irreversible reaction, expressed by Eq. (20)4), is calcu- conducted where the mean residence time of liquid lated to be 41 for this condition. was changed. As shown in Fig. 6, CN~ is almost completely decomposed ifr>15 min. kLa was found P^ l + tDB/DJiBo/At) (20) to be 0.0492s"1 by the same procedure. This value Thus, the relation ^2 114 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN [m3] X = total liquid volume less than r2 if[CN~]>0.1 ppm. E and [CN~] at each =distance [m] value of r were calculated from Eqs. (21) and (24) by XL = liquid film thickness [m] a trial-and-error procedure. As shown in Fig. 6 the y = mole fraction of ozone in gas [-] prediction is satisfactory. Jo = y in effluent gas [-] It should be noted that the overall reaction rate of ozone with CN~as well as the reaction order is in- [-] [-] fluenced by liquid-phase mass transfer resistance and A ol(PyfIH) quantities defined by Eqs. (6) and (18) [-] that the rate equations7'9) which have been obtained reaction factor for infinitely fast in gas-liquid heterogeneous systems are probably ap- second-order irreversible reaction [-] parent rate expressions disguised by mass transfer quantities defined by Eqs. (6) and (19) [-] effect. kLaP VL/GMH [-] mean residence time of liquid [s] Conclusion D = concentration of CN~ [m2/s] Tanaka and H. Imagawa: Nippon Kagaku Kaishi, 602 E =diffusivity absorption efficiency defined by Eq. (3) [-] (1975). EQ initial value of E [-] 10) Robinson, R.A. and R. H. Stokes: "Electrolyte Solu- G volumetric gas flow rate [m3/s] tions", p. 463, Butterworths, London (1959). GM molar gas flow rate [mol/s] ll) Teramoto, M. and S. Imamura: Special Project Research H Henry's constant [atm - m3/mol] on Detection and Control of Environmental Pollution, kL liquid-film mass transfer coefficient [m/s] Vol. 3, Chemical Engineering For Water Pollution Con- kx reaction rate constant [1/s] trol, Japan, p. 142 (1979). k2 reaction rate constant [m1-05^©!0-35 à" s] 12) van Krevelen, D. W. and P. J. Hoftijzer: Rec. tray, chem., N [mol/m2 à"s] 67, 563 (1948). molar flux [atm] P total pressure [mol/m3 à"s] 13) Wilke, C. R. and P. Chang: AIChEJ., 1, 264 (1955). [s] r reaction rate [m3] t time (Presented in part at the llth Autumn Meeting of The Soc. V liquid volume of Chem. Engrs., Japan, at Tokyo, Oct. 6, 1977.) VOL. 14 NO. 2 1981 115