Literature Cited Asboth, K., Austrian Patent 172 931 (Oct 25, 1952). Bachmann, L., Cremer, E., 2.Anorg. Allg. Chem., 309, 65 (1961). Bischoff, F., Monatsh., 81, 606 (1950a). Bischoff, F.. 2.Anorg. Chem., 262, 288 (1950b). Bischoff, F., Claus, D., Lehmann, H., Tonind.-Ztg. Keram. Rundschau, 83, 293
(1959). Britton, H. T. S.,Gregg, S. J., Winsor, G. W., Trans. Faraday Soc., 48, 70
(1952). Clark, L. M., Rec. Trav. Chim., 68, 969 (1949). Cremer, E., Nitsch, W., Tonind.-Ztg. Keram. Rundschau, 83, 579 (1959). Cremer, E., Nitsch. W., Sci. Ceram., I , 295 (1962a). Cremer, E., Nitsch. W., 2.Nektrochem., 66, 697 (1962b). Curran, G. P., Fink, C. E., Gorin, E., Adv. Chem. Ser., No. 69, 141 (1967). Curran, G. P., Fink, C. E., Gorin, E., "Bench-Scale Research on CSG Process, Phase II. Operation Of The Bench-Scale Continuous Gasification Unit, Dec 1, 1965 To July 1, 1968.R and D Report No. 16,Interim Report No. 3,Book 3,"Government Printing Office, 1970. Curran, G. P., Rice, C. H.,Gorin, E., Am. Chem. Soc., Div. Fuel Chem., Prepr., 8 (l),128 (1 964). Davtyan, 0. K., Ovchinnikova, E. N., Soboleva, N. M., Nauch. Ezhegodnik, Odessk. Gosudarst. Univ., Khim. Fak., No. 2, 128 (1961). Dedman. A. J.. Owen. A. J.. Trans. FaradavSoc.. 58. 2027 11962). Dobner. S , Ph D Dissertation The City Cdllege of The City University of New York (Ch E ), 1976 Dobner, S.,Kan, G., Graff, R. A., Squires, A.M., Thermochim. Acta, 16, 251
(1976). Fischer, H. C., J. Am. Ceram. SOC., 38, 264 (1955). Galwey, A. K., "Chemistry of Solids", Chapter 5, Chapman and Hall, London,
1967. Garner, W. E., "Chemistry of the Solid State", Chapter 8, Academic Press, New York. N.Y., 1955. Glasson, D. R., J. Appl. Chem., 8, 793 (1958). Glasson, D. R., J. Appl. Chem., IO, 42 (1960). Glasson. D. R., J. Appl. Chem., 11, 201 (1961). Gluud, W., Keiler, K., Klempt, W., Bestehorn, R., Ber. Ges. Kohlentechnik, 3,
211 (1930). Hashimoto, H.,Kogyo Kagaku Zasshi, 64, 250 (1961). Haul, R. A. W., Schoning, F. R. L.. 2. Anorg. Chem.. 269, 120 (1952). Hedin, R., Svenska Forskningsinst. Cement Betong Vid Kgl. Tek. Hogskol. Stockholm Sartryck, 16, 661 (1961). Hedin, R., Tek, Tidskr., 92, 101 (1962). Hyatt, E. P., Cutler, I. B., Wadsworth, M. E., J. Am. Ceram. SOC., 41, 70
( 1970).
Kovalenko. E. N., Mat. Sci. Res., 3, 485 (1966). Kriek, H. J. S., Ford, W. F., White, J., Trans. Br. Ceram. SOC., 58, 1 (1959). Kruel, M., Juntgen. H., Chem. Ing. Tech., 39, 607 (1967). MacCallum, J. R., Tanner, J., Eur. Polym. J., 6, 1033 (1970). Maclntire, W. H., Stansel, T. B., Ind. Eng. Chem., 45, 1548 (1953). Nitsch, W., Z. Nektrochem., 66, 703 (1962). NOH, W., Ang. Chem., 62, 567 (1950). Ohno, Y., Sekko To Sekkai, 1, 1366 (1957). Ohno, Y., Fujiyama, S.,Sekko To Sekkai, 1, 1469 (1957). Pampuch, R., Silic. Ind., 23, 119 (1958). Pannetier, R., Souchay. P., "Chemical Kinetics", p 393,Elsevier, Amsterdam,
1967. Pell, M., Ph.D. Dissertation, The City College of The City University of New York (Ch.E.), 1971. Peterson, R. 0.. Cutler, I. E.,J. Am. Ceram. Soc., 51 (l),21 (1968). Proks, I., Jaskova, V., Silikaty, 11 (3),201 (1967). Proks. I., Siska, V.. Silikaty, 12 (l),13 (1968). Richer, A., Compt. Rend., 238, 339 (1954). Richer, A., Vallet, P., Compt. Rend., 252, 1780 (1961). Ruth, L., P h D Dissertation. The City College of The City University of New York (Ch.E.), 1972. Schwab, G. M., Taylor, J. S.,Spence. R., "Catalysis from the Standpoint of Chemical Kinetics", p 16,D. Van Nostrand, New York, N.Y., 1937. Schwob, Y., Rev. Mater. Constr. Trav. Publics, Ed. C. 411, 409 (1949). Schwob, Y., Rev. Mater. Constr. Trav. Publics, Ed. C, 413, 33 (1950a). Schwob, Y., Rev. Mater. Constr. Trav. Publics, Ed. C, 413, 85 (1950b). Sestak, J., Satava, V., Wendlandt, W. W.. Thermochim. Acta, 7 (5), 447
(1973). Shushinov, V. A., Fedyakova, K. G., Uchenye Zapiski Gor'kovst. Gosudarst. Univ. im. N.I. Lobachevskogo, Ser. Khim., No. 32, 13(1958). Siske, V., Proks, I., Chem. Zvesti, 12, 201 (1958a). Siske, V., Proks, I., Chem. Zvesti, 12, 275 (1958b). Tagawa, H., Sudo, F., Kogyo Kagaku Zasshi, 61, 949 (1959). Yanev, I. P., Angelov, B. D., Radenkova, M. Z., Dokl. Bolg. Akad. Nauk, 23 (lo),
1219 (1970). Young, D. A., "Decomposition of Solids", Chapter 3,Pergamon Press, New York. N.Y., 1966. Zawadski, J., Bretsnajder, S.,Z. Phys. Chem., 822, 60 (1933a). Zawadski, J., Bretsnajder, S.,Z. Phys. Chem., 822, 79 (1933b). Zawadski, J., Bretsnajder, S.,Z. Phys. Chem., 840, 158 (1935). Zawadski, J., Bretsnajder, S.,Trans. faraday SOC.,34, 951 (1938).
Received for revieul July 22, 1976 Accepted May 6,1977
(1958). Ketov, A. N., Pechkovskii, V. V., Larikov, V. V., Obsch. Prikl. Khim., No. 3, 48
Absorption of Nitrogen Dioxide in Sodium Sulfite Solution from Air as a Diluent Hiroshi Takeuchi,' Katsuroku Takahashi, and Nobuo Kizawa Department of Chemical Engineering, Nagoya University, Nagoya, 464, Japan
A study was made of the absorption of nitrogen dioxide with air as a diluent into aqueous solution of sodium SUIfite at 25 O C and atmospheric pressure. An agitated vessel with a plane interface was used to make sure of the effect of additive as an antioxidant of sulfite. For the sulfite solution with no additive a significant difference in the rate of NO2absorption was found between the nitrogen and air diluents. Examining some organic compounds as the additive, it was found that hydroquinone and monoethanolamine have a considerable effect on the inhibition in the sulfite oxidation. Furthermore, in order to discuss the applicability of the kinetics of the reaction between NO2 and SOS2-, which was proposed in a previous paper, the absorption of NO2 diluted with air was carried out in both a bubble cap- and a sieve-tray column. The overall capacity coefficients calculated from the equation based on the dual film theory and gas absorption accompanied by a pseudo mth order reaction were in reasonable agreement with the observations for the bubble cap tray.
Introduction In the previous paper (Takeuchi et al., 1977), the authors discussed the mechanism of NOz absorption in aqueous sodium sulfite and/or bisulfite solutions according to gas absorption accompanied by a fast, pseudo mth-order reaction, and proposed the mechanism with competitive reactions involving the hydrolysis of NO2 and the reaction between NO2 486
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 4, 1977
and sulfite or bisulfite ion. Nevertheless, using nitrogen as a diluent it was not necessary to consider an oxidation of sulfite. From a viewpoint of the processes for removing NO, from stack gases by sulfite solution, however, it is considered that the sulfite oxidation becomes significant from the presence of 0 2 remaining in the gases. The reaction between dissolved oxygen and sulfite ion is quite complex and not well under-
N,-gas
Details of a Bubbie C a p T r a y T r a y width
245cm
Tray length
430cm 270cm
Troy sacing
Cops per tray 9 Slots per cap I8 Area per slot 0.75~1~ Ratio of Iota1 slot ore0 to bubbling area : 16.1 % Slot height : 2.5crn
I X
Vent
no additiv;(N,/
,/"
I
I
1
/
Orifice meter
Annular area per cap.707cm' Riser ore0 per tray 102crnz Downcomer area : 6 6 . 5 ~ r n ' ~ Bubbling area 760cd Distance from inlet to outlet weir : 31.0cm '
J
Figure 1. Schematic diagram of the experimental apparatus for a tray column and details of a bubble cap tray.
8 Acetic acid, C,
-
stood in spite of extensive investigations. The reaction is extremely sensitive to both catalysis and inhibition. The catalysts most often employed in the sulfite oxidation are metals that is, cobaltous, manganous, and ferrous ions. Although it is well known that small traces of organic compounds such as phenols reduce the rate of reaction, there is little literature on the inhibition of the sulfite oxidation. It is the purpose of this paper to examine the effect of an additive which inhibits the sulfite oxidation but has no influence on the absorption of NO2 into sodium sulfite solution. Furthermore, in order to discuss the applicability of the absorption mechanism proposed in the previous work, the absorption of NO2 from air as a diluent was carried out in a tray column as a conventional gas-liquid contacting device.
Experimental Section For making sure of the effect of additive as an antioxidant of sulfite, the absorption of NO2 was carried out a t 25 f 0.1 "C and atmospheric pressure by using the same agitated vessel with a plane interface as in the previous work. To increase the gas side mass transfer coefficient in the absorption vessel, however, the clearance between the gas side blades and liquid surface in the vessel was changed to 6 mm, and air was used as a diluent of NOz. Absorbents employed were aqueous solutions of 0.1 and 0.01 M Na2SOs with several kinds of organic compounds, including hydroquinone(HQ), phenol, ethanolamines(MEA, DEA and TEA), ethylene glycol monoethyl ether (ethyle cellosolve), glycine, ethylenediaminetetraacetic acid (EDTA), and acetic acid, as the additive. As it is desirable that the concentration of inhibitor is as low as possible, the inhibition was examined at additive concentrations below IO-* M. On the other hand, for the experiments of gas absorption in both a bubble cap- and a sieve-tray column, the schematic diagram is shown in Figure 1. The column had a rectangular cross section (43.0 X 24.5 cm), and consisted of three stainless steel sections and a transparent acrylic resin section. The pertinent details of the bubble cap tray are given in the lefthand side of Figure 1. The sieve plate had a hole diameter of 3 mm and a percentage perforated area of 12.8%.The heights of both the inlet and the exit weirs were 5.0 cm. The gas-liquid contacting zone was on the middle tray, and the upper and lower trays were inserted as gas distributors. Air flow rates were measured upstream of the NO2 or SO2 injection by an orifice meter. Pure nitrogen dioxide or sulfur dioxide from a cylinder was injected sufficiently far upstream to ensure a uniform composition. Samples of the inlet and the outlet gases were taken from the spaces above and below the
2xl0-5
Cellosolve,
C O S O ~ ,-I ~ ~
I o4 P a r t i a l p r e s s u r e o f NO,
ICY (atm)
Figure 2. Effects of additives on the absorption rate of NO2 in 0.1 M
NaZS03 solution.
gas-liquid contacting tray, respectively, being analyzed by means of the NO, or SO2 analyzer. The concentrations of NO2 or SO2 in the inlet gas were as low as 300 or 2000 ppm, respectively. During the experiments on absorption in the tray column no attempt was made to achieve a steady-state condition except for the SO2-air-water system. An aqueous solution of 0.5 M NaOH or Na2S03 was placed in the reservoir a t the beginning of each run and the solution was recirculated through the system by a pump. The temperatures of the gas and liquid were in the range of 25-28 OC.
Results and Discussion Absorption of NO:! in Sodium Sulfite Solutions with Additive. During the absorption of NO2 from air as a diluent into the sulfite solution, if the oxygen is allowed to oxidize a considerable fraction of the sulfite, a reduction in the absorption rate might be observable. Furthermore, in the case of the sulfite solution containing a catalyst for the oxidation, it seems that the effect of oxygen absorption may be more remarkable. Figure 2 shows the experimental results for 0.1 M Na2S03 solution in the plane interface contactor as plots of the rate of NO2 absorption N A vs. the bulk partial pressure of NO2, P N O ~ .For no additive, a significant difference in the absorption rates is found between the two diluents; that is, the rate for NO2 diluted with air gives result about 40% lower than that with nitrogen. However, when water was used as an absorbent, the rates of absorption of NO2 from the air were in good agreement with the results from the nitrogen in the previous work. As for the reaction of oxygen with sulfite, the rate constant is well known to be small; for example, Yagi and Inoue (1962) have reported about 0.1 s-l as the pseudo-first-order rate constant for 0.07 M Na2S03 solution a t 20 "C. Considering only the magnitude of the rate constant, the considerable reduction in the absorption rate of NO2 diluted with air would be unexplainable. It is perhaps due to a rapid decline in the sulfite concentration as a result of radical progress in the oxidation, that is, a chain reaction mechanism of sulfite in the Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 4, 1977
487
A d d i t i v e . Conc. H O , 0.0IM " , 0.001
G ( m'/min~
or
L
L/min i
Figure 4. Effects of gas and liquid flow rates on kca, k L , and a on the bubble cap tray. I
1
I
,
I0 4 P o r t i o pressure
lo3
of NO2 Iotm I
Figure 3. Effects of inhibitors as the antioxidant of sulfite on the absorption rate. The solid line for Nz diluent indicates the results obtained in the previous work.
presence of NO2 or NOz-. Such behavior would be of important significance for the processes which remove NO, and SO2 simultaneously from stack gases by alkaline or sulfite solutions. In addition, the presence of Co2+as a catalyst shows a dramatic effect on the absorption of NO2 from the air, as can be seen in Figure 2. In the absence of oxygen, however, cobaltous ion was of no effect on the absorption of NO2 in the sulfite solution as well as in water. The inhibition by an additive on such sulfite oxidation can be checked out by comparing the absorption rates of NO2 into the sulfite solutions with and without additive. Thus phenols, amines, carboxylic acid, and ethers were tested as additives and the results obtained are also shown in Figure 2. From the figure, it is found that both of the HQ and MEA act as an inhibitor of the sulfite oxidation, and that an increase in the additive concentration has an observable effect, while organic compounds other than the HQ and MEA give no improvement in the absorption of NO:! even a t the concentration of M. In addition, a t trace concentrations of the additive, there is no inhibition even for the HQ, unlike the case of an oxidation of sulfite by air in the absence of N02. Chappell (1972) has reported that the HQ acts as an oxidation inhibitor and undergoes slow degradation due to reaction with NO*. Although the mechanism of the inhibition is not clear, the HQ also acts as a reducing agent in alkaline solution, and hence it may react with NO2 as well as 0 2 . Figure 3 shows the results obtained for the absorption of NO:! into M NaOH solutions with the HQ. It can be seen from Figure 3 that the absorption rate increases with increasing HQ concentration in spite of the presence of oxygen in the gas phase. However, it is not practical to use the alkaline HQ solution as an absorbent in the NO,-removal processes for stack gases. By visual observation in the absorption vessel during the simultaneous absorption of NO2 and 0 2 , the light brown solution changed to deep brown, indicative of fast degradation due to reactions of the HQ with NO2 and 0 2 . Similarly, the ethanolamines would also undergo a reaction with NO2 as an acid gas, as has been reported in the Japanese Patent (1975). However, the sulfite solution with HQ is colorless and degrades very slowly, and aqueous solution of HQ only has no effect on the absorption of NO2 from the air, as can be seen from Figure 3. In order to make sure of the effects of the HQ and ethanolamines as the antioxidant, further experiments were carried 488
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 4, 1977
out a t a lower sulfite concentration. The concentrations of the HQ and ethanolamines were M in 0.01 M NazSOa solution. The results obtained are also shown in Figure 3, which indicates that the observed orders in the inhibition are: hydroquinone > MEA > DEA > TEA. Such behavior might be of some commercial interest in the case of the NO, removal from stack gases.
Absorption of NO2 in Tray Column Interfacial Area on the Bubble Cap Tray. Interfacial area per unit volume of dispersion on the tray, a , was found by absorbing 0 2 from air into 0.5 M Na2SO:I solution containing Co2+as a catalyst. In a series of runs, the rate of oxygen absorption was determined from a conversion of the sulfite a t a fixed liquid circulation rate, since it depends only upon the gas-liquid contacting area. During the absorption of 0 2 , the absorbent liquid of 50-100 L was circulated for 30 min through the system, and the sulfite concentration and the pH of the liquid in the reservoir were measured a t intervals of 10 min, together with the froth and the seal heights. All these runs were made a t a constant ratio of the liquid and gas flow rates, that is, L/G = 1 (L/m3). The values of interfacial area on the bubble cap tray are shown in Figure 4 as a plot of a vs. G. Some determinations have previously been made of the area of the gas-liquid dispersion on bubble cap trays. Sharma et al. (1969) have obtained values of a as functions of superficial gas velocity u and submergence S by the chemical method and the calculated values from their correlation (a u o S-O l i ) are represented by a dotted straight line in Figure 4.Porter et al. (1966) have reported that the value of a for a large-scale bubble cap tray is independent of u and is 150 m2/m3for gas velocity of 0.5 to 1.2 m/s, corresponding to the gas flow rate from 2.3 to 5.5 m3/min in Figure 4. Although there are merely four data values in the present work, the interfacial area a t the gas flow rate of 5 m:'/min may be rather too high in comparison with the values of a cited in the literature, considering the experimental accuracy. Asano and Fujita (1969) have observed the bubbles on a model bubble cap tray by the photographic observation, and have proposed a dimensionless correlation for the area which varies as the 0.34 power of gas flow rate. Thus considering such a dependency of a on the gas flow rate, the lower solid line in Figure 4 was drawn with a slope of and it can be seen that it represents the data within about 20% except for the point a t G = 5 m3/min, including the literature values of Sharma et al. (1969). Individual Mass Transfer Coefficients on the Bubble Cap Tray. Individual film coefficients h ~ and a h ~ for a the gas-liquid dispersion on the bubble cap tray were obtained
by conducting the absorption runs of dilute SO2 into sodium hydroxide solution and water, respectively. For the absorption of SOr a t concentrations of 600-2000 ppm into 0.5 M NaOH solution, the major resistance to mass transfer lies within the gas film. The value of k e a can then be determined from the absorption rate per unit dispersion volume at the logarithmic average of SO2 partial pressures of the entering and the leaving gases. The vales of k e a obtained in the tray are also plotted in Figure 4 against the gas flow rate where the liquid to gas ratio L/G (L/m3) was varied from 1 to 3. From the figure it is found that the slope of the upper solid line is 0.76, which is in accord with what one is led to expect for a case of gas-film controlling, and being independent of the liquid flow rate. Sharma et al. (1969) have also reported that in the model 5 where D e is the diffusivity bubble cap tray k e a a ~ 0 . 7 DcO.~, of solute in the gas phase. As shown by a broken line in Figure 4, their values of kea are about 20% higher than those obtained in this work. However, in view of the complexity of the hydrodynamic conditions on bubble cap trays, it should be noted that agreement is quite good. On the other hand, as will be discussed later in this paper, liquid side mass transfer coefficient k~ is of little significance in the absorption process of NO2 in the sulfite solution. However, to test the performance of the bubble cap tray, values of k LU were evaluated according to the dual-film theory with the overall capacity coefficient Kea obtained for the absorption of SO2 into water (refer to Figure 5 ) and the k e a mentioned above. The solubility of SO2 in water was estimated a from an equation by Fujita (1963). The values of k ~ varied as the 0.5 power of the gas flow rate (that is, k ~ 0:a This dependence is also in conformity with the information available in the literature (for example, AIChE Research Report, 1960). Further, the values of k~ were calculated by dividing a the smoothed values of interfacial area these values of k ~ by in Figure 4, being also plotted in the same figure against the liquid flow rate. At first sight, though scattered in the results since they depend on G, there is a trend toward higher k L with increase of L . Then the h~ data for a constant gas rate of 1 m:'/min are best fitted by a straight line with a slope of 0.2. In the present work, however, no further conclusion for the effect of liquid rate on kL, will be drawn since the values of k~ depend on the interfacial area which is itself subject to uncertainty. From consideration of the individual film coefficients described above, it might be concluded that the interfacial areas on the bubble cap tray can be substantiated with the smoothed line in Figure 4 within about 20%. Absorption of NO2 in Sodium Sulfite Solution. In the absorption runs of NO2, the sulfite solution of 0.5 M as the absorbent was recirculated and used to the extent of 0.2 M. However, for the sulfite solution without the inhibitor, a rapid decline in the absorption rate of NO2 was observed as soon as the liquid was returned to the reservoir after contacting the gas phase on the tray. This behavior also is responsible for the chain mechanism in sulfite oxidation in the presence of NOZ, as described earlier in this paper. Therefore the absorption runs were conducted for the sulfite solution of an HQ concentration of lop2M. Removal efficiences for NO2 were observed a t concentrations ranging from 70 to 300 ppm in the inlet gas, being about 25% for the bubble cap tray and about 35% for the sieve tray. However, there was a trend toward lower efficiencies in both trays with increasing the gas flow rate. The values of Kea for the bubble cap tray are shown in Figure 5 against the gas rate, along with the results obtained in the sieve tray. The difference in the Kca for both trays may correspond to that of the gas-liquid contacting areas, as can be expected from the results for absorption of SO2 in NaOH solution. On the other hand, the value of Kea for the absorption of
I 3 5 IO G a s f l o w r a t e , G (m'/minI
Figure 5. Effects of gas flow rate on the overall capacity coefficients for the SOTNaOH, NOrNa2S03, and SOrwater systems in the tray column. The dotted line shows the values of KGa calculated by eq 2.
NO2 in the sulfite solution can be evaluated from the equation
which is based on the dual-film theory and eq 8 in the previous paper. In eq 1 and 2, a is a correction factor of H a for the sulfite solution. That is, in the previous work the reaction rate constants of NO2 with water and sos2-,hhyd and k l , have been obtained by assuming that the Henry's law constant H is of 4.1 X m o l L atm and the liquid diffusivity D A of 2.15 X 10-5 cm2/s for the N02-water system. However, at higher sulfite concentrations the reductions in H and D A due to the salting-out and the viscosity effects were strongly reflected in the absorption rate, as was noted in the previous work. In such a case, the value of ct for the sulfite solution of a given concentration can be evaluated from Figure 6 in the paper, for the 0.5 M solution being found to be 0.63. When using the known values of k c , a, and (Y along with k 1 of 6.6 X lo5 L/mol s, the values of K e a for the absorption of NO2 into 0.5 M Na2S03 solution in the bubble cap tray were calculated by eq 2. As shown by a dotted line in Figure 5 , the calculated values of K e a are in good agreement with the experimental results at higher gas flow rates. However, at lower rates the Kea data are seen to be somewhat less than the calculated value. A plausible explanation is that at lower gas flow rate a decline in the sulfite concentration of the liquid on the tray could be brought about from the lower liquid rate corresponding to the liquid/gas ratio. Then the average deviation of the data points from the line is 16.3%.Omitting the data at the gas rate of 1 m3/min, which is for a gas velocity of 0.22 mls and hence is unusual for gas-liquid contacting in tray columns, the deviation is no more than 7.7%. Further, in the case of the same solution, since the difference of the values of KGU calculated by eq 1 and 2 is merely a few percent, the contribution of the hydrolysis of NO2 to overall absorption rate could be neglected. Thus, from the agreement between the experimental and the calculated values of Kca on the bubble cap tray, it is suggested that the mechanism of NO2 absorption in sulfite solution, which is accompanied by the competitive reactions, Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 4, 1977
489
would be applicable to the design of a large-scale absorber for removing NO2 by sulfite solutions.
Conclusion In an agitated vessel with a plane interface, the absorption of NO2 in aqueous sodium sulfite solution from air as a diluent was carried out to make sure of the inhibition in the sulfite oxidation. For the sulfite solution with no inhibitor, it is considered that the sulfite oxidation in the presence of NO2 is due to a chain reaction mechanism. In such a case, hydroquinone and ethanolamines have a considerable effect on inhibition in the oxidation. Further experiments of NO2 absorption were done in both bubble cap- and sieve-tray columns. The overall capacity coefficients obtained in the bubble-cap tray are in reasonable agreement with those calculated from eq 2.
Literature Cited A.1.Ch.E. Research Committee, "Tray Efficiencies in Distillation Columns", University of Michigan, 1960. Asano, K., Fujita, S., KagakuKogaku, 30, 403 (1966). Chappell, G. A., EPA-R2-72-051 (1972). Fujita, S., KagakuKogaku, 27, 112 (1963). Japanese Patent, (Sumitomo Chemical Co., Ltd.), 50-17387 (1975). Porter, K. E., King, M. B., Varshney, K. C., Trans Inst. Chem. Eng., 44, T274 (1966). Sharma, M. M., Mashelkar, R. A,, Mehta, V. D., Brit. Chem. Eng., 14, 70 (1969). Takeuchi, H., Ando, M., Kizawa. N., lnd. Eng. Chem., Process Des. Dev., 16, 303 (1977). Yagi, S., Inoue, H., Chem. Eng. Sci., 17, 411 (1966).
Received for review August 3, 1976 Accepted May 28,1977 Grateful acknowledgment is made to the Foundation on Development of Technology for Preventing NO, Emission from Iron Steel Facilities for providing financial assistance for this investigation.
Model Studies for a Vinyl Chloride Tubular Reactor. 1. Steady-State Behavior Ronald S. H. Lee and John B. Agnew' Department of Chemical Engineering, Monash University, Clayton, Victoria, Australia 3 168
Heterogeneous one- and two-dimensional models are examined for a packed tubular reactor in which the catalytic hydrochlorination of acetylene is taking place on a pelletized catalyst, by comparing predicted and measured axial temperature profiles. Good agreement is obtained with a one-dimensional cell model in which the catalyst particle is assumed to be isothermal and intraparticle diffusion is accounted for by means of a differential dispersion model. Under less severe conditions, intraparticle diffusion is shown to be adequately accounted for by an effectiveness factor correction.
Introduction For a nonisothermal packed tubular reactor the conversion of reactants and variation in temperature along the tube are influenced by the complex interaction on the catalyst surface with the following physical transport processes: (i) intraparticle diffusion of heat and mass; (ii) interphase diffusion of heat and mass through the boundary layer surrounding the catalyst pellet; (iii) interparticle dispersion of heat and mass; (iv) heat transfer across the tube wall by a complex convection/conduction/radiation mechanism. The highly nonlinear nature of this physicochemical system demands that realistic models must be solved numerically by digital computer. Invariably the model is simplified as far as possible to minimize computation time, but oversimplification may lead to erroneous predictions. Pseudo-homogeneous models have been widely used because of their comparative simplicity of solution. They assume that gas-phase and solid-phase conditions are the same a t a particular "point" in the system. However, they are normally limited in application to mild operating conditions which result in small changes in temperature and concentration. In an earlier study, Lee (1974) found that plug-flow and one-dimensional-cell pseudo-homogeneous models of a vinyl chloride reactor gave reasonable agreement with experimental measurements when the maximum temperature rise along the tube axis was less than 10 "C. A two-dimensional cell model 490
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 4, 1977
was found to be satisfactory for up to 40 "C temperature rise. Heterogeneous models consider the gas and solid phases separately. Priestley and Agnew (1975) examined a simple heterogeneous version of the two-dimensional cell model which accounted for interphase but not intraparticle resistances, but only fair agreement was obtained with experimental temperature profiles. The plug-flow model is the simplest differential balance model to solve as it does not include axial and radial dispersional terms. The one-dimensional cell model incorporates axial fluid mixing, while the two-dimensional cell model of Deans and Lapidus (1960) additionally allows for radial mixing. Cell models are preferable to the analogous differential balance models because of the simpler computation involved. This becomes of considerable importance when considering the extension of the model to allow for dynamic behavior. The object of the present study is to determine the simplest heterogeneous model which adequately represents observed steady-state behavior for a reactor under conditions at which intraparticle mass diffusion is an important rate-limiting factor.
The Reaction System The reaction selected for study was the catalytic hydrochlorination of acetylene to yield vinyl chloride monomer. The
