Literature Cited Ayen, R. J., Ng, Y.-S., Int. J. Air WaterPoIIut., 10, l(1966). Ayen, R . J., Peters, M. S..Ind. Eng. Chem., Process Des. Dev., 1, 204 (1962). Baker. R. A., Doerr. R. C.. Ind. Eng. Chem., Process Des. Dev., 4, 188 (1965). Bauerle, G. L., Service, G. R., Nobe, K., Ind. Eng. Chem., Prod. Res. Dev., 11, 54 (1972). Bauerle, G. L.. Nobe, K., Ind. Eng. Chem., Prod. Res. Dev., 13, 185 (1974). Bauerle, G. L.. Sorensen, L. L. C., Nobe, K., Ind. Eng. Chem., Prod. Res. Dev., 13, 61 (1974). Chien, M. W., Pearson, I. M.. Nobe. K., I d . Eng. Chem., Rod. Res. Dev., 14, 131 (1975). Close, J. S.,White, J. M., J. Catal.. 38, 185 (1975). Dixon. W. J., Ed., "BMD Biomedical Computer Programs", University of California Press, Berkeley, Calif., 1973. Dolbear, G. E., Kim. G. W. R. Grace & Co. Report, "Variation of Selectivity with Support Chemistry in NO, Removal Catalyst", 1974. Jenkins, D. R., Voisey, M. A,, Atmos. Environ., 7, 187 (1973). Klimisch. R . L.. Taylor, K. C., Environ. Sci. Techno/.,7 , 127 (1973). Klimisch, R . L., Taylor, K. C., Ind. Eng. Chem., Prod. Res. Dev., 14, 26 (1975). Kobylinski, T. P., Hammel, J. J.. Swift, H. E.. Ind. Eng. Chem., Prod. Res. Dev., 14, 147 (1975). Kobylinski, T. P., Taylor, B. W., J. Catal., 33, 376 (1974). Lamb, A., Tollefson. E. L., Can. J. Chem. Eng., 51, 191 (1973). London, J. W., Bell, A. T., J. Catal., 31, 96 (1973). Nicholas, D. M.,Ph.D. Thesis, University of Pittsburgh, Pittsburgh, Pa., 1975.
Nicholas, D. M.. Shah, Y . T., Ind. Eng. Chem., Prod. Res. Dev., 15, 35 (1976). Nicholas, D. M., Shah, Y. T., Zlochower, I., Ind. Eng. Chem., Prod. Res. Dev., 15, 29 (1976). Otto, K., Shelef, M.. 2.Phys. Chem. (Frankfurt am Main), 85, 308 (1973). Shelef, M., Gandhi, H. S..Ind. Eng. Chem. Prod, Res. Dev., 11, 393 (1972). Shelef, M., Kummer. J. T., Chem. Eng. Prog. Sym. Ser. No. 175, 67, 74 (1971). Shelef, M., Otto, K., J. Catal., 10, 408 (1968). Taylor, K. C., Klimisch, R. L., J. Catal., 30, 478 (1973). Taylor, K. C., Sinkevitch, R. M., Klimisch, R. L., J. Catab. 35, 34 (1974).
Received for review February 17,1976 Accepted May 3,1976 Supplementary Material Available. Tables 1-111of experimental data (4 pages) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Business Office, Books and Journals Division, American Chemical Society, 1155 16th St., N.U'., Washington, D.C. 20036. Remit check or money order for $4.00 for photocopy or $2.50 for microfiche, referring to the code number PROD-76-172.
Carbon Monoxide-Oxygen Reaction in the Presence of Sulfur Dioxide Ajay Sood, C. W. Quinlan, and J. R. Kittrell' Department of Chemical Engineering, University of Massachusetts,Amherst, Massachusetts 0 1002
The activity of several commercial and experimental catalysts is compared for O2 reduction with CO, with and without SO2 present, using dry cylinder gases and actual flue gases. In cylinder gas tests only two catalysts, Pd-alumina and Pt-alumina, gave virtually complete O2removal in the presence of 2500 ppm of SO2 at 740 O F and 20 000 h-'. The Pd catalyst was slightly more active than Pt at higher space velocities and no decline in its activity was observed over a 110-h period. In actual flue gas tests, Pd-alumina pellets and a Pt-alumina monolith yielded essentially complete 0 2 removal with less severe reaction conditions than a Cu-alumina pelleted catalyst. The Pt-alumina monolith was operated successfully for 400 h of intermittent experimentation.
In recent years, a considerable amount of research effort has been concentrated on developing processes for the removal of air pollutants like sulfur dioxide and nitric oxide from stack gases. One such process, the catalytic reduction of these pollutants with carbon monoxide, has been studied by several investigators (Ryason and Harkins, 1967; Querido and Short, 1973; Quinian et al., 1973; Goetz et al., 1974). Stack gases normally contain 1to 3% oxygen. The oxygen may be removed from the stack gases by reduction with CO in a separate catalyst pre-bed, before the gases enter the main catalytic reactor, as suggested by Querido and Short (1973). A separate oxygen removal pre-bed is necessary for several reasons. (i) The presence of 0 2 is detrimental to the CO reduction of SO2 and NO (in the main reactor) as shown by several investigators (Roth and Doerr, 1961; Shelef et al., 1968; Haas et al., 1971). Thus, Haas et al. reported that an iron-alumina catalyst is poisoned for the CO-SO2 reaction by 0.5% 0 2 . (ii) Querido and Short (1973) state that the 0 2 reacts homogeneously with COS, a deleterious by-product of the CO-SO2 reaction in the main reactor, to produce SO2 and thus raise the effluent SO2 level. (iii) The 02-CO reaction is extremely exothermic and, 176
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 3, 1976
if carried out in the main reactor, could conceivably alter the catalyst activity and selectivity, besides causing serious temperature control problems. The objective of this study was to find a catalyst which would give virtually complete oxygen removal in the presence of about 2500 ppm of SO2 (a level typical of stack gases). Since SO2 is known to be a catalyst poison (Butt, 1972),considerable loss of activity could be expected in its presence. In the absence of S02, the CO-02 reaction has been extensively investigated from both an applied and fundamental point of view and many suitable catalysts suggested in literature. This literature is briefly reviewed below to provide guidance for selecting potentially active catalysts for the present study. In general, the CO-02 reaction is catalyzed by metal oxides and the catalytic activity is related to their electronic properties. Thomas and Thomas (1967) present a comprehensive overview of the different theories advanced to explain this reaction, citing the various electronic theories and substantiating calorimetric data. Dixon and Longfield (1960) point out that, from a qualitative viewpoint, the oxide catalysts can be arranged in the following order of activity: p-type metal
charged to each reactor weighed 3 g, with an average particle size of 20-/30+ mesh, and was supported on a 40 mesh stainless steel screen. Metal ion Electronic Operation The six titanium reactors were vertically mounted in a in oxide type temp, OF Activity Lindberg Heavi-Duty three-zone furnace and the temperature was automatically regulated by a Lindberg Heavi-Duty conco 2 + Below 300 High P troller. Catalyst temperature was measured by three chroBelow 300 High cu+ P mel-alumel thermocouples mounted on the outer wall of each Ni2+ Below 300 High P titanium reactor a t %in. intervals over the catalyst bed. AlMn4+ Below 300 High P though the settings of the three-zone furnace were adjusted cu2+ Intrinsic semiconductor 300-750 Medium to operate the catalyst bed isothermally within f 3 "F, due to n Fe3+ 300-750 Medium n Zn2+ the highly exothermic OyCO reaction, a temperature gradient 300-750 Low Cr3+ p-intrinsic 300-750 Low of 10 to 30 "F was observed across the catalyst bed. Upon the semiconductor addition of SO2 and subsequent loss of activity of all catalysts n 300-750 Low screened (except the Pt and P d catalysts), this large temperInsulator Low Above 750 ature gradient disappeared. The product gases from the titanium reactors were cooled to remove any sulfur formed by the CO reduction of SO2 and the composition was measured oxides are the most active and initiate the reaction at the by gas chromatography. Further details of the equipment used lowest temperature; n-type oxides are active at moderate in these cylinder gas tests can be found elsewhere (Sood, 1974; temperatures while insulators require fairly high temperatures Goetz et al., 1974). for operation. Table I summarizes the order of activity to be Severe screening test conditions, viz., a high catalyst temexpected for the various oxides according to Dixon and perature (740 O F ) and a relatively low space velocity (20 000 Longfield (1960). After testing a variety of catalysts, Klimisch h-l), were chosen for comparing the catalysts so that the loss (1968) also concluded that CuO, C0304, and MnO2 were suin activity upon SO2 addition could be observed. No preperior catalysts at 500 O F and a space velocity of 14000 treatment of the catalysts was done. The catalysts were exh-1. posed to the 02-cO-N~ mixture and the conversions checked Krylov (1970) applied statistical analysis techniques to CO several times over a period of 15-20 h to ensure attainment oxidation data reported by several authors and computed of steady-state conversions. After this, approximately 2500 average activities for various metal oxides at 300 "F. He ppm of SO2 was introduced in the feed and the steady-state showed that maximum activity was observed with oxides of conversions measured again. Most runs lasted 35 h or more. Mn, Co, Ni, Cu, and Cd while Fe203, Cr2O3, and V ~ 0 exhib5 Flue Gas Tests. Actual wet flue gas was obtained from a ited the least activity. The correlation of activity with the 1 gal/h oil fired boiler using No. 2 fuel oil. The flue gas conwidth of the forbidden zone was shown to be quite strong in tained approximately 2.5-6.0% 02,10-12% HzO, 12-14% CO2, these catalysts, thus supporting the earlier statement that with NZmaking up the balance. Some unburned carbonaceous electronic factors play an important role in CO oxidation. hydrocarbons and possibly ash were also present, but were Besides these metal oxides, several noble metal catalysts mostly filtered out before the catalyst bed. Pure SO2 and CO have also been studied for carbon monoxide oxidation. Schwab were blended with a slipstream of the combustion gases to get and Gossner (1958) investigated the kinetics of this reaction appropriate concentrations. The SO2 was added to a level over silver, palladium, and silver-palladium alloy catalysts between 2000 and 4000 ppm, while the CO levels were slightly at 480-985 O F . For the palladium catalyst, the rate of reaction greater than the stoichiometric requirements for the removal was shown to be inversely proportional to the carbon monof 0 2 , and hence of the order of 5 1 3 % CO depending upon 0 2 oxide concentration. Bond (1962) provides a summary of concentration (Quinlan, 1974). various investigations over platinum, palladium, gold, and The flue gas studies were carried out in a single, tubular flow silver catalysts. reactor. A 1-in. stainless steel tube approximately 15 in. in This paper reports the results of screening several comlength was utilized with catalyst supported on a stainless steel mercial and laboratory-prepared catalysts, including the metal screen. Temperature measurements were made by thermooxides listed in Table I, a platinum, and a palladium catalyst, couples on the outer wall of the reactor plus a single, movable in the absence and presence of SO2. The initial catalyst thermocouple in a thermowell running down the center of the screening was done with dry cylinder gases. Results of further reactor. The catalyst bed in these studies was operated esevaluation of three catalysts, viz., copper-alumina, pallasentially adiabatically. A single zone Lindberg Heavi-Duty dium-alumina and platinum-alumina (monolith) with flue furnace with Powerstat was utilized to pre-heat the bed. When gases generated in a laboratory burner are also reported. feed was introduced, the electrical input to the furnace was lowered significantly or completely turned off and the exoExperimental Section thermic heat of reaction sustained the catalyst temperature a t an operable level. Two separate experimental setups were used in this investigation: a six reactor unit for the dry cylinder gas screening Gas chromatography was employed for determination of of the catalysts and a single reactor for wet flue gas testing. stream composition. Analysis for 0 2 and CO was performed on a Molecular Sieve 5A column operated a t 95 "C while SO2 Cylinder Gas Tests. Pure, dry compressed gases were blended to yield a feed containing approximately 1%0 ~ ~ 3 . 2 % and H20 were observed on a Porapak T at the same temperature. Formation of such products as H2S and COS was CO, and the balance Nz. Approximately 2500 ppm of SO2 was monitored by a Porapak N column operated a t 62 OC (Quinadded to this feed for some experiments. Carbon monoxide lan, 1974). in excess of the stoichiometric requirements was used in all runs. The feed gas was split into seven streams, one being used Results a n d Discussion to sample the upstream gas composition while the others were fed to six titanium tubular reactors in parallel. The reactors Cylinder Gas Results. The oxygen conversions obtained were constructed from l/s-in. I.P.S. titanium pipe with an inwith and without the presence of SO2 are summarized in Table ternal diameter of Y4 in. No oxygen conversion was observed 11. In the absence of SOz, a comparison of Tables I and I1 with the empty titanium reactors. In all runs, the catalyst shows that all the metal oxides listed in the former, with the Table I. Expected Order of Activity of Metal Oxides for CO-02 Reaction
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 3, 1976
177
Table 11. Comparison of Catalyst Activities for Oxygen Removal with and without S02"
0
HARSHAW
Pd 0501
A
GIRDLER
0 43
conversion, % No With SO2 SOzc
0 2
Catalyst composition
Catalyst identification
0.1% Pt-alumina
100
0.3%Pd-alumina
Girdler G43 Harshaw Pd 0501 45% CoO-kieselguhr Harshaw Co 0101 13%Coo-alumina Harshaw Co 0501 Harshaw Co 23% CoO-alumina 0502 Harshaw Cu 10% CuO-alumina 0803 Harshaw Cu 8%Cu-alumina 0804 Harshaw Cu 79% CUO 1710 Experimental 20% CuO on Kaiser N-22 alumina Experimental 2% F on Cu 0803 N-15 Harshaw Ni 14% NiO-alumina 0301 Harshaw Ni 19%NiO-alumina 0302 Harshaw Ni 32% NiO-alumina 0304 Harshaw S i 3% NiO, 3% COO, 1601 3% FeZO3-alumina Girdler G49A 80% NiO-kieselguhr 19%MnO2-alumina Harshaw Mn 0201 Harshaw Mn 3% MnOz-silica 0501 Harshaw Zn 38% ZnO, 25% Crz030601 alumina 53% CuO, 38% Crz03 Girdler G13 41% CuO, 39% Cr203,12% Girdler G22 BaO Fez03,CrzO3 Girdler G3A 10%VzOs-alumina Harshaw V 0301 94% A1203 Kaiser KA 201
100
96 97
...
2
...
1
100
4
100
6
MFoO
30 ,ox)
40,003
SPACE V E L O C I T Y , hr-l
58
1
100
0
99
10
37
4
99
0
99
0
94
3
100
2
99
4
...
1
...
2
100
3
99 100
4 4
6
4
2 21
2
3
Test conditions: -740 "F, -20 000 h-l space velocity. Feed: Feed: 1%02,3.2% CO, 2500 ppm
1%02,3.2% CO, balance Nz. of SOz, balance N2.
exceptions of V205 and A1203,yield virtually complete oxygen removal, as would be expected from the work of Dixon and Longfield (1960). No difference in activity, as measured by 0 2 conversion, among the p-type semiconductors such as COO, CuO, NiO, and the mixed metal oxides such as ZnO-Cr203, CuO-CrzO3, CuO-Cr203-BaO was observed because of the high temperature (740 O F ) chosen for catalyst comparison. Manganese was not tested in the absence of S02. The temperature could have been dropped to obtain a better activity comparison among these catalysts. However, this was not the objective and hence was not done. A few comments are in order to explain the exceptions observed. The 8%Cu-alumina catalyst (Harshaw Cu 0804) gave a relatively low 0 2 conversion (58%)probably since it has both Cu+ and Cu2+ ions, thus giving it intrinsic semiconductor properties. Hence, according to Table I, it would be expected to possess medium activity. For the laboratory-prepared experimental catalyst N-15 (2%F on Cu 0803), the addition of fluoride may be responsible for lowering the activity since Cu 0803 gave complete oxygen removal. The ZnO-CrzOs, CuOCr2O3 and BaO promoted CuO-Cr203 catalysts gave total 178
,o,om
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 3, 1976
Figure 1. Dependence of oxygen conversion on space velocity for the Harshaw Pd 0501 and Girdler G43 (Pt) catalysts. Test conditions: feed contains 1%02,3.2%CO, 2500 ppm of Son, and balance Nz. Catalyst temperature 740 OF. oxygen conversion. However, the chromium promoted iron oxide catalyst, G3A, exhibited very low activity-this was in agreement with the findings of previous investigators as reported by Krylov (1970). When approximately 2500 ppm of SO2 was introduced in the CO-02-N2 feed, the oxygen conversions observed are noted in the last column of Table 11.The oxygen conversion with all the catalysts was extremely low (less than 6%in almost all cases), except for the 0.1% Pt-alumina (G43) and 0.3% Pd-alumina (Pd 0501) catalysts which gave virtually complete oxygen removal. An oxygen conversion of 10% was obtained with the experimental catalyst, N-22, containing 20% CuO on high surface area Kaiser alumina, KA 201. This catalyst was similar to the copper-alumina catalysts studied by Goetz et al. (1974) and found to possess high activity for the simultaneous reduction of SO2 and NO with CO. For the Pt and P d catalysts, the maximum SO2 conversion was less than 10% and no COS formation was observed; hence, the SO2 was probably being reduced to elemental sulfur over these noble metal catalysts. In short, all catalysts tested were poisoned by S02, with the exception of Pt and P d catalysts. In order to determine if this poisoning was readily reversible, SO2was removed from the feed stream for a few selected catalysts, viz., the Harshaw nickel catalysts. No significant increase in activity of these catalysts was observed over a period of 20 h. However, no further attempts were made to reactivate them. Although the Pt and P d catalysts gave the same oxygen conversion at a space velocity of 20 OOO h-l, doubling the space velocity showed that the Pd catalyst was slightly more active, as illustrated in Figure 1.Next, the activity of this Pd catalyst was studied over a period of 110 h a t a space velocity of 21 OOO h-l and a temperature of 740 O F . No decline in activity was observed and the oxygen conversion level stayed between 95 to 99% (Sood, 1974). Flue Gas Results. Of the catalysts screened with the dry cylinder gas system, three were tested with flue gas to assess their potential in conditions occurring in an actual application. The three catalysts were: 10% CuO-alumina (Harshaw Cu 0803, %-in.pellets), 0.3% Pd-alumina (Harshaw Pd 0501, %-in. pellets), and a 0.35% Pt-alumina monolith (Englehard "A"). Typical results of the flue gas tests in the absence and presence of SO2 are summarized in Table 111. The results are entirely consistent with the dry cylinder gas results reported earlier. Thus, regardless of SO2 level, the Cu catalyst was much lower in activity than the Pd or Pt catalysts. For example, in
Table 111. Flue Gas Results
Catalyst
Operating temp, OF
Space velocity, h-1
Cu 0803 (%-in. pellets) Pd 0501 (lh-in. pellets) Pt-alumina (monolith)
1100 1160 885 910 838 890
21 000 27 000 20 000 20 000 38 000 37 000
soz, PPm 0 1000-2000 0 2000 0
2500
Feeda %0 2
6.0 6.1 2.6 2.6 2.3 2.6
%
co
12.6 13.0 5.0 6.0 6.0 6.3
%0 2
conv. 80-90 85-90 95-100 90-95 75-85 90-95
Feed balance: 10-12% H20,12-14% COz and N2. Table IV. Stability of Pt-Alumina Monolith Hours operated (date)
Reaction initiation temp,OF
%02
Feeda %CO
5 574-662 2.6 6.0 (11-10-72) 400 554-697 7.0 14.0 (6-19-73) a Feed also contains: S 0 2 , H20, COz, and N2.
% 02
conv. 90-95 95-100
the presence of S02, the Cu catalyst required a high operating temperature of 1160 OF for 90% 0 2 removal, compared to 900 OF required by the Pt and Pd. Another indication of the superiority of Pt and Pd catalysts was the lower adiabatic temperature (700 O F ) a t which they could initiate the CO-02 reaction, as compared to that necessary for the Cu catalyst (850-900 OF). A comparison of P d and Pt activity based on these flue gas results is difficult because of the different operating conditions employed. Both appear to have comparable activity, in agreement with the dry gas results. However, the advantages of the monolithic configuration over pelleted catalysts, from the point of view of lower pressure drop and susceptibility to plugging by soot or fly-ash, make the overall performance of the Pt-alumina monolith superior to the Pd-alumina pellets. Another important criterion used for distinguishing between the three catalysts was the amount of SO2 conversion obtained. At the high temperatures necessary for 0 2 removal with the Cu catalyst, 60-100% SO2 conversion also occurred. The reaction products were COS and H2S, with the latter predominating. The formation of COS as a byproduct of CO reduction of SO2 by reaction of CO would be expected from the experimental results of Goetz et al. (1974). Moreover, the water vapor present in the flue gas could lead to the hydrolysis of COS to H2S. The production of H2S, of course, was not desirable. On the other hand, the Pt-alumina monolith, operating a t lower temperatures, was not effective for SO2 reduction. Even at 1010 O F , less than 10% SO2 conversion to COS and H2S was obtained a t a space velocity of 40 000 h-l. At lower operating temperatures, virtually no SO2 conversion occurred, while 0 2 conversion was in the 90-95% range. No measurements were made which could directly indicate degree of SO3 formation. The experimental studies conducted by Quinlan (1974) for SO2 and NO removal from flue gases satisfactorily utilized this Pt-alumina monolith as a pre-bed for oxygen removal.
The stability of the Pt-alumina monolith over 8 months of intermittent experimentation was investigated, involving a large number of start-ups and shut-downs and a wide range of concentrations of 02, H2, CO, H20, S02, NO, H&, and COS. During this 8-month period, the operating conditions of the catalyst varied significantly; however, no indication of catalyst deactivation was observed as shown by a comparison of two runs in Table IV. It must be pointed out that the 400 h of operation does not represent continuous on-stream time but the total number of operating hours over the 8-month period. Moreover, for several hundred additional hours during this period, the catalyst was subjected to oxygen free streams containing CO and SO2 and/or COS and NO.
Conclusions Of the base and noble metal catalysts screened with dry cylinder gases, only the Pt and P d catalysts gave virtually complete 0 2 removal in the presence of SO2. The P d catalyst was more active than Pt a t higher space velocities and showed stable activity over a 110-h run. The Pt-alumina monolithic catalyst was tested with actual flue gases and yielded virtually complete oxygen removal under conditions similar to the cylinder gas tests; its activity was stable over 400 h of intermittent experimentation. Thus, both Pt-alumina and Pdalumina catalysts could be potentially employed for oxygen removal from stack gases.
Literature Cited Bond, G. C., "Catalysis by Metals", pp 460-463, Academic Press, New York, N.Y., 1962. Butt, C., Adv. Cbem. Ser., No. 109, 259 (1972). Dixon, J. K., Longfield, J. E., "Catalysis", pp 347-368, Reinhold, New York, N.Y.. 1960. Goetz, V. N., S o d , A., Kittrell, J. R., Ind. Eng. Cbem., Prod. Res. Dev., 13, 110 (1974). Haas, L. A,, McCormick, T. M., Khalafalla, S.E.. U.S. Bur. Mines, Rept. Invest.. 7483 (1971). Klimisch, R. L., General Motors Research Publication GMR-842, 1968. Krylov, 0. V., "Catalysis by Non-Metals", pp 169-174, Academic Press, New York, N.Y., 1970. Querido. R.. Short, W. L., Ind. Eng. Cbem., Process Des. Dev., 12, 10 (1973). Quinlan, C. W., Ph.D. Thesis, University of Massachusetts, Amherst, Mass., 1974. Quinlan, C. W., Okay, V. C., Kittrell, J. R., Ind. Eng. Cbem., Process Des. Dev., 12, 359 (1973). Roth, J. F., Doerr, R. C., Ind. Eng. Cbem., 53 (4), 293 (1961). Ryason, P. R., Harkins, J., J. APCA, 17 (12), 796 (1967). Schwab, G. M., Gossner, K., Z.Pbys. Cbem. (Frankfurt am Main), 16, 39 (1958). Shelef, M.. Otto, K., Gandhi, H.. J. Cafal., 12, 361 (1968). Sood. Ajay, Ph.D. Thesis, University of Massachusetts, Amherst, Mass., 1974. Thomas, J. M., Thomas, W. J., "Introduction to the Principles of Heterogeneous Catalysis", pp 367-375, Academic Press, New York. N.Y., 1967.
Receiued for review April 24,1975 Accepted May 21,1976
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 3, 1976
179
