BY GROUP SCREENING If calalysls are arranged in logical groups, and each group tested in a single run, the inactive materials can be weeded out and the total number of runs reduced W I L L I A M 0. HUNTER RElJl MEZAKI
38
INDUSTRIAL AND ENGINEERING CHEMISTRY
he conventional procedure for catalyst selectionToften a rather expensive process-is to test each catalyst individually. The purpose of this article is to point out that this is an area ideally suited for the application of group screening. I n this method, factors are initially tested in groups and then those groups which appear to contain the most effective factors are studied further. Experimental results from a screening study on catalysts for the oxidation of methane indicate that considerable savings are possible. I n this investigation fourteen commercially available catalysts were screened in essentially seven runs. Not only does this example illustrate the feasibility of group screening methods for catalyst selection but it also illustrates another point. This is simply that experimenters should make use of all available information when planning experiments. Although this is an obvious remark it nevertheless deserves some emphasis because it touches on an area of apparent misunderstanding between experimenters and statisticians. Adapting the general experimental design procedures of the statistician to specific engineering problems is to be encouraged rather than discouraged, even though some aspects of the general plan may be somewhat modified in the process. At the outset of this work the results, of course, could not be foreseen. But by making use of information from previous work, we were able to arrive a t certain conjectures regarding the relative prior probabilities of various outcomes. To a great extent, the actual results supported these initial conjectures, and consequently considerable additional savings were possible. To statisticians interested in the application of Bayesian methods to practical problems, this example may be of interest since it offers a rather concrete illustration of the use of subjective prior probabilities. An important problem that arises in chemical engineering is selection of the mwt suitable catalyst from a number of candidate catalysts for a given chemical reaction. This is a particular example of the general screen-
ing problem (3). Hypothetically, the “best” catalyst is that which possesses all the optimum properties, for example, the highest activity, the highest stability, and the lowest ignition temperature. Of course, the “best” catalyst, defined this way, may not exist because one catalyst may have the highest activity yet not the highest stability. Usually the experimenter is faced with the problem of finding the catalyst which has the most favorable combination of properties. Arriving at an appropriate compromise among the various criteria is often a difficult task and requires carcful judgment. Perhaps the most practical approach in general is to concentrate on a principal response and optimize it subject to certain restrictions (2). In the example discussed here activity was chosen as the principal response. As auxiliary responses, ignition temperature and stability were followed. I t was desirable to find a catalyst that had an ignition temperature lower than 350° C. and that remained active for at least thirty hours without replacement or regeneration. In industrial applications, of course, various costs associated with the different catalysts would also be taken into account. The conventional ‘experimental procedure is to test each catalyst individually. It is not uncommon to have as many as thirty catalysts (1, 8)or even more, and consequently the search for the best one can sometimes be tedious and expensive. The method of group screening can probably be employed with considerable savings in circumstances of this kind.
William G. Hunfcr is Assistant Professor in the Department of Sfatistics and the Engineering Expmimentntion Sfation, Uniunsify of Wiscomn, Madison Wis. W h m this article was written, Re+ Mezaki held fhe position of Research Assistant in fhe Chemical Engineering Deparhmf of the University of Wisconsin. He is presently employed by Sumitom0 Chmical Cwp., Osaka, Japan. Financial suppwf is a c h l e d g s d from the Wisconsin Alumni Research Foundation and the O&e of Naual Research, ContracfNONR-1202. AUTHOR
Watson’ (7) has given the name “group screening methcds” to those procedures which involve “the idea of putting the factors in groups, testing these groupfactors, and then testing the factors in the significant group-factors.” This technique has been successfully applied to a catalyst screening study for methane oxidation. Fourteen commercially available catalpts were screened in essentially seven runs. In this example, therefore, the amount of work was approximately half that which would have been required if the more traditional method of examining the catalysts one at a time had been used. As the name implies, the basic idea of group screening as applied to catalysts is to test them in groups; that is, instead of a single catalyst being charged to the reactor for a given run, a mixture of a number of catalysts is used, say, in equal parts by weight. After a number of these first-stage runs the experimenter would normally test further those groups which appeared to contain promising catalysts in order to select the best individual catalysts. In the example cited here these further second-stage tests were unnecessary since the best individual catalysts were singled out in the first stage. This was possible because of the fortunate way in which the catalysts were grouped, an arrangement which was suggested by our prior feelings about the catalysts. The success of such a scheme for catalyst screening rests mainly on two assumptions: (i) There are no interactions between catalysts. (ii) The presence of a catalyst adds to, but never subtracts from, the conversion. In other words, there are only main effects (4) and these are all non-negative. Because of the physical and chemical nature of catalysts (5) there is no reason to doubt the validity of these assumptions, and we are therefore provided with an ideal situation for the application of group screening. Experimental Syrkm
The problem was to find the best commercially available catalyst for the total oxidation of methane
CHI
+
COz
2 0 2
+ 2H2O
A Considerable amount of work had previously been done on catalysts for this reaction (1, 8). Platinum, palladium and the oxides of transition metals such as copper, cobalt, chromium, manganese, and vanadium were reported to have catalytic activity. For screening purposes the fourteen commercially available catalysts listed in Table I were obtained. The size of the catalyst pellets was reduced to -12 14 mesh (U. S. Standard Sieve) for the tests. The chemical compositions given in Table I were supplied by the manufacturers.
+
(Contimrcd on next pa@) VOL 5 6
NO. 3 M A R C H 1 9 6 4
39
TABLE I. CATALYSTS AND THEIR CHEMICAL COMPOSITIONS Catalyst Number
Catalyst
The catalysts were combined into the seven groups shown in Table 11. On the basis of previous investigations it ~ 7 a felt s that the best catalyst would probably be Chemical Compositions either number 5, 8, or 10. The arrangement shown in 20y0 C r 2 0 3on alumina Table 11, therefore, reflects the subjective prior probabilities of success for the various groups. Another CrpOa on y-alumina 18-20 70 0 . 3 % X i , 1 . 9 ~ o C o , 1 0 . 0 ~ o M o consideration was the chemical similarity within groups. on y-alumina Each group consisted of equal parts by weight of its constituent catalysts. The total weight charged for each 9 . 3 % MnOS and 3% C O O on alumina run was the same.
1
Chrome alumina-1
2
Chrome alumina-2
3
Cobalt
4
Cobalt molybdate
5
Cobalt oxide
Approx. 15co Co304 on alumina
6 7
Copper chromite-1
20% C u O Cr2O8 on alumina
Copper chromite-2
80 % CuO and 16% ' Cr203
8
Hopcalite
Mixture largely of MnOs and CUO
9
Manganese oxide
Si02 0.23c/'c, MnOa 1 3 . 0 % , COO 3.7070, N a 2 0 0.02870, Fe 0.0157,, Alsoa balance
10
Palladium
0 . 5y0 Pd on alumina
11
Vanadium oxide-I
10% V 2 0 son alumina
12
Vanadium oxide-2
10-12y0 V 2 0 son high activity alumina
13
Vanadium oxide-3
8 . 8 % VZOSand 1 4 . 2 % K2S04 on silica
14
Vanadium oxide-4
5y0 V 2 0 jand 5%
Moo3 on
a-alumina
TABLE I I . 1
Results
Catalyst 'Vumbers
A
3 4 9
B
11 12 13 14
C
6
D
E F G
Cobalt Cobalt molybdate Manganese oxide Vanadium Vanadium Vanadium Vanadium
A B C
D E F G
oxide-1 oxide-2 oxide-3 oxide-4
7
Copper chromite-1 Copper chromite-2
1 2
Chrome alumina-1 Chrome alumina-2
8
Hopcalite Palladium
5
TABLE 1 1 1 .
40
Catalysts in Group
10
Group
The tests were performed with approximately 1yo methane in air. The flow rate of each of these gases was controlled by a pressure regulating valve and a needle valve. The reactor, a one-inch I.D. quartz tube, 50 inches long, consisted of three sections: preheater, reaction zone, and quenching zone. The temperature in the preheater section, a 30-inch empty vertical tube, was controlled by a surrounding electric furnace. The reaction zone, heated by a second electric furnace, was comprised of a layer (approximately four inches deep) of solid catalyst pellets. The quenching zone consisted of a 1/4-inch layer of quartz chips and a 5-inch length of empty tube. The gases coming from the reactor were dried and purified to eliminate water vapor and carbon dioxide. A vapor fractometer was employed for the analysis of both reactant and product gases. For further details see reference (6).
T H E SEVEN GROUPS OF CATALYSTS
1 Group
Equipment
Cobalt oxide
EXPERIMENTAL RESULTS Activity (Per Cent Conversion at 350" C . )
Ignition Temperature, C.
0 0 0 0
400 430 400 380 310 210 320
10 20 3
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
The experimental results are summarized in Table 111. Groups A, B, C, and D did not give any appreciable conversion, consequentIy. they were eliminated from further consideration. Of the remaining three catalysts, cobalt oxide was comparatively inactive, yielding only a 37, conversion of methane whereas Hopcalite and palladium gave 10 and ZOyO, respectively. O n the basis of other work the standard deviations of these values were estimated to be less than 1. At this point cobalt oxide was also eliminated. Both Hopcalite and palladium satisfied the auxiliary requirement that the ignition temperature be lower than 350 C., the ignition temperature for palladium being 100" C. below that for Hopcalite. It was then decided to subject both of these catalysts to stability tests to check activity decay with time. -4lthough Hopcalite was found to be slightly superior in this respect, nevertheless, palladium was finally selected because of its marked superiority with regard to both activity and ignition temperature. LITERATURE CITED (1) Anderson, R. B., Stein, K. C., Feenan, J. J., Hofer, L. J. E., IND.ENC.CHEM. 53, 809 (1961). (2) Box, G. E. P., J . Basic Eng.82, 113 (1960). (3) Box, G. E. P., Tram. 77th Ann. Conv., Am. SOC.Quality Control (1958). (4) Box, G. E. P., Hunter, J. S., Technotnetrics 9, 311 (1961). (5) Hougen, 0. A., Watson, K. M.,"Chemical Process Principles, Part 111," Wiley, New York, 1950. (6) Mezaki, R., Ph.D. thesis, University of Wisconsin, Madison, Wis. (1963). (7) Watson, G. S., Technomelrics 3, 371 (1961). (8) Yant, W'. P., Hawk, C. O., J . A m . Chem. SOC.49, 1454 (1927).
