tional pressure drop were in approximate agreement radius of gas passage, rw-b [cm] with the theoretical predictions for the case of no rw columnradius [cm] ScG gas-phase Schmidt number, /uG/PGDG [-] rippling liquid film. ShG average gas-phase Sherwood number, Experiments on the absorption of ammoniafrom a [-] turbulent air stream into aqueous sulfuric acid solu- u average gas velocity [cm/sec] tion were carried out under the conditions of rippling ut interfacial velocity [cm/s ec] on the surface of the liquid film. The agreement be- W mass flow rate of gas [g/s ec] X = dimensionless column height, Z/2r* [-] tween the measured values of ShGand the theoretical z = column height [cm] predictions for the case of no rippling liquid film was fairly good. r = mass flow rate of liquid per unit perimeter From these results, it may be concluded that rippling [g/cm - sec] on the surface of the falling liquid film does not affect APf = frictional pressure drop for gas stream through column [g/cm à" sec2] or [Kg-force/m2] appreciably either the frictional pressure drop or the fjLG9fiL = viscosities of gas and liquid [g/cm-sec] or [cP] rate of gas-phase mass transfer in the turbulent gas pG = density of gas [g/cm3] stream. Literature Cited

b Nomenclature 1) Emmert, R. E. and R. L. Pig ford: Chem. Eng. Progr., 50, = liquid film thickness [cm] 87 (1954). DG = gas-phase diffusivity of solute gas [cm2/sec] 2) Hikita, H.: Kagaku Kogaku, 23, 23 (1959). d = column diameter, 2rw [cm] 3) Hikita, H., K. Ishimi and H. Ikeki: /. Chem. Eng. Japan, fr = friction factor based on relative gas 10, 375 (1977). velocity, APfrilpG{um - uifZ [-] 4) Hikita, H., K. Ishimi, Y. Omotehara and T. Fukase: ibid., average gas-phase mass transfer coefficient ll, 96 (1978). [cm/s ec] 5) Kafesjian, R. L., C. A. Plank and E. R. Gerhard: AIChE gas-phase Reynolds number, A W]izdiiG [-] /., 7, 463 (1961). gas-phase Reynolds number based on 6) Kamei, S. and J. Oishi: Kagaku Kogaku, 18, 421 (1954). gas velocity relative to liquid surface, 7) Malyusov, V.A., S.K. Myasnikov and N.N. Kulov: 2riPG(um - Ui)/^G [-] Theor. Found. Chem. Eng., 1, 480 (1973). ReL = liquid-phase Reynolds number, 4r//jtL [-]

KINETICS OF ABSORPTION OF NITRIC OXIDE IN HYDROGEN PEROXIDE SOLUTIONS

K. K. BAVEJA, D. SUBBA RAO and M. K. SARKAR Department of Chemical Engineering, Indian Institute of Technology, Hauz Khas, NewDelhi-110029 India

Introduction cost and formation of only water and nitric acid in the absorption process. This last factor makes it Absorption is a useful technique for removal of specially attractive for nitric acid plant tail-gas ab- nitric oxide from a gas mixture in many applications. sorption, since the by-product from air pollution A number of absorbents have been proposed for the removal of NOfrom gases. Adrian et al.l) have abatement can go directly into the main product of reported on the development of a process for the the plant. There is practically no information in the literature reduction of NOcontent in flue gases using hydrogen on the kinetics of the reaction between H2O2 and peroxide solution as the absorbing liquid. Absorp- NOin the liquid phase. As a first step, an attempt tion with hydrogen peroxide solution has considerable is made to elucidate the kinetics of absorption of advantages, such as high absorption rate, low capital NOin hydrogen peroxide solution and to determine Received October 13, 1978. Correspondence concerning this article should beaddressed to M. K. Sarkar. the rate constant. The overall reaction with NOis

322 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN 2 NO+3 H2O2-+2 HNO3+2H2O preliminary runs on the physical absorption of pure CO2in water were carried out in the sameapparatus 1. Experimental at stirrer speeds of 61 and 100 rpm to determine kL9 Absorption experiments were carried out in a the liquid-side physical mass transfer coefficient. 10.15 cm I. D. glass stirred cell with a total volume 2. Theory of approximately 1 litre. The design of the stirred cell was similar to that used by Ladhabhoy and The theory of absorption accompanied by fast Sharma4). The top part was provided with one B-24 pseudo-m-th order reaction is applied to this system. and two B-l4joints. The stirrer shaft passed through For a reaction m-th order with respect to the gaseous a mercury seal through the central B-24 joint. The reactant A (NO) and n-th order with respect to the B-l4 joints were used to connect gas inlet and gas liquid reactant B (H2O2), the specific rate of gas absorption according to the film theory can be ex- outlet tubes. A glass stirrer with two sets of four pressed in terms of enhancement factor E by an flat blades spaced about 8 mmapart was used. During the experiment, the liquid level was maintained in the approximate solution2) : middle of the two sets of blades, so that one set causes stirring in the liquid phase and the other in |pf] /tanh^f^f] (1) the gas phase. The effective inter facial area in the where stirred cell was 81 cm2. E=RAIA*kL (2) Two stirrer speeds viz. 61 rpm and 100 rpm, were used in the experiments. At both these speeds the aqueous solution and of H2O2 having a concentration in the range of Ei=enhancement factor when the reaction is instan- 1.07X10"4 to 5.66X10"4 g-mol/cm3. The solute gas taneous, or rate of reaction is controlled entirely NO, suitably diluted with N2 and saturated with by diffusion water vapour, was fed into the absorber at a constant = 1 +V>bBo)/(zDaA*) (4) rate. The concentration of NO in the feed gas The above equations are applicable when the concen- ranged from 7 to 25vol%. The total pressure in tration of the gaseous component in the bulk liquid the system was 740mmof mercury, which is the (Ao) is zero. localBatchatmosphericexperimentspressure.were conducted by taking a When knownvolume of hydrogen peroxide solution (280 then E= *JW~A cm3) in the cell and purging the system first with and therefore nitrogen and then with the gas consisting of desired Ra=J concentration of NOin N2. The duration of the m+lDAkmnu(V) *y+l{Boy experimental run was 20 minutes. Hydrogen perox- ide concentration was measured iodometrically at The conditions given by expression (5) were satisfied the beginning and at the end of the experimental in all the results reported and are discussed later. run6). The change in solution concentration never These conditions also justify the assumption AQ=0. exceeded 8%of the initial value. The mean of the Further, if Eq. (7) is valid, the specific rate of absorp- two values was taken as the average H2O2 concen- tion should be independent of kL and hence of the tration. The concentration of NOin inlet and outlet speed of stirring. gas samples from the absorber was determined by gas chromatography using Molecular Sieve 13X 3. Results and Discussion as the columnpacking. The gas phase was assumed 3. 1 Evaluation of parameters to be completely back-mixed and the outlet gas con- The solubility and dirBsivity of nitric oxide in the centration wastaken as the representative gas con- reacting solution was taken to be same as that in centration in the cell. The amount of NOabsorbed water because of the low concentration of hydrogen during the experimental run was calculated from the peroxide used and of HNO3formed. The maximum acid formed in liquid and determined titrimetrically. concentration of HNO3formed in any experiment This was also checked in a few experiments by calcu- in 20minutes was 1.4x10"5 g-mol/cm3. The solu- lating the amount of NOabsorbed from the inlet bility data at 1 atmosphere pressure was taken from and outlet gas analysis, and the material balance was Perry5) and corrected for pressure. The diffusivity found to agree within 4%. data was taken from Hikita et a/.3). In view ofthelow Prior to the chemical absorption experiments, solubility of nitric oxide in the solution at the reaction

VOL 12 NO.4 1979 323 Fig. 1 Effect ofA* on specific rate of absorption Fig4. Effect of temperature on reaction rate RA, 30°C constant k2 experiments. 3. 2 Confirmation of regime Experiments on specific rate of absorption were carried out at stirrer speeds of 61 and 100rpm. The value of physical mass transfer coefficient at these stirrer speeds was independently found to be 1.7x lO~3 and 2.6X 10"3 cm/sec respectively. How- ever, the specific rate of absorption of NOin H2O2 was found to be independent of the speed of stirring. For the range of experiments conducted at 61 rpm, the value of E was found to vary between 30 and 65. For DA=DBand f^>l, Eq. (4) can be approxi- mated by Ei =B0/zA* (8) Fig. 2 Effect of Bo on specific rate of absorption RA, ,4*=4.0 X 10-7 g-mol/cm3, 30°C The values of B0/zA* varied from approximately 205 to 2,100. The ratio DA/DBis very likely to be close to unity. The values of E (and hence VMA)and Bo/zA* were evaluated individually for each ex- periment to ensure that conditions given by expression (5) are satisfied. 3. 3 Discussion of results The effect of A* on specific rate of absorption was studied for four different liquid concentrations, keep- ing stirrer speed at 61 rpm at 30°C. Figure1 is a plot of specific rate of absorption against A* with Bo as parameter. It can be seen that the rate of absorption is directly proportional to A*. Thus, ac- cording to Eq. (7) the order of reaction with respect to nitric oxide is one. Fig. 3 Effect of BQon specific rate of absorption Figure 2 is a cross plot from Fig. 1 between RAand RA, 22.5°C and 15°C £0 for a value of ^4*=4.0x10~7g-mol/cm3 on a temperature (less than 5x 10"7 g-mol/cm3), the con- log-log scale. The slope of the least-square fit line tribution due to the gas-side resistance would be ex- is 0.456, indicating the order of reaction with respect pected to be negligible. This has further been checked to H2O2 to be 0.91. by comparing overall gas-phase mass transfer co- Considering the order of reaction with respect to efficient and gas-side mass transfer coefficient. The both the reactants to be one (m=l, w=l), Eq. (7) gas-side resistance was estimated to be less than 1 % reduces to of the overall resistance to mass transfer in all the RA = (DAhrXA*)(Boy* (9) 324 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN A modified least square fit to the data of Fig. 2 yields concentration of A in the bulk of liquid[g-mol/cm3] solubility of NOin solution the equation (at A*=4.0 x 10~7 g-mol/cm3) (interfacial concentration of NO) [g-mol/cm3] l^=1.97x lO-fl(5o)1/2 reactant in the liquid phase (H2O2) B concentration of H2O2in bulk from which the value of the second-order rate constant diffusivity of NOin solution Bo diffusivity of H2O2in solution at 30°C is found to be enhancementfactor DA enhancementfactor whenthe reaction k2=SA2x 105 cm3/g-mol-sec DB is instantaneous or rate of reaction is controlled entirely by diffusion Figure 3 shows, on a log-log plot, the results of the E physical mass transfer coefficient Et (m -\-ri)-th order reaction rate constant experiments at 22.5 and 15°C in terms of rate of ab- [(cm3/g-mol)m + rl- 1 - measure of the ratio of A reacting in sorption as a function of hydrogen peroxide concen- the film to that going unreacted in the bulk B phase tration taken at constant gas concentration at each order of reaction with respect to NO order of reaction with respect to H2O2 temperature. From the plot, the mean values of rate specific rate of absorption [g-mol/cm2à"sec] constant at these two temperatures are found to be numbertemp oferatmolesureof H2O2reacting with [ ° K] 4.84x 105 and 2.52 x 105 cm3/g-mol à"sec respectively. one mole of NO(=1.5). [g-mol/cm3 ] Figure 4 shows an Arrhenius plot of k2 values ob- [cm2/sec] tained above. The value of activation energy is m [cm2/sec] found to be n £= 1 3,700 cal/g-mol Ra T The variation of k2 with temperature is given by the z [cm/sec] equation à"sec"1] ln ^2-36.4-6900/r Literature C ietd Conclusion 1) Adrian, J. C. and J. Verilhac: "Control of Gaseous Sulfur and Nitrogen Compounds", Vol. 2. Paper presented at The reaction between nitric oxide and aqueous the 2nd International Conference, University of Salford, hydrogen peroxide solution in the liquid phase is U.K., April 1976. relatively fast and is first-order with respect to both 2) Danckwerts, P. V.: "Gas-Liquid Reactions", McGraw- the reactants. The second-order reaction rate con- Hill Book Company, p. 122 (1970). 3) Hikita, H., S. Asai, H. Ishikawa and S. Hirano: /. Chem. stantat 30°C is found to be 8.42x 105 cm3/g-molà"sec Eng. Japan, 10, 120 (1977). with an energy of activation 13,700 cal/g-mol. 4) Ladhabhoy, M.E. and M. M. Sharma: /. Appl. Chem., 19, 267 (1969). Acknowledgement s 5) Perry, R. H. and C. H. Chilton Eds: "Chemical Engineers' The authors are thankful to Dr. A. K. Gupta, Chem. Eng. Handbook", 5th Ed. Table 3-121, McGraw Hill Kogaku- Dept., I.I.T., Delhi-1 10029 for very useful discussion. sha Ltd. (1973). 6) Vogel, A. I.: "A Textbook of Quantitative Inorganic Nomencl at ur e Analysis", 4th Ed., p. 363, Longmans Green and Company A = reactant in the gas phase (NO) (1947).

VOL 12 NO.4 1979 325