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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
Determination of Coke on Catalyst Surface Edward Furimsky Energy Research Laboratories, Canada Centre for Mineral and Energy Technology, Department of Energy, Mines and Resources, Ottawa, Canada, KLA OGI
The removal of soluble fractions must precede any estimate of the coke deposited on a catalyst surface. This is accomplished by an extraction which establishes the borderline between removable and nonremovable (coke) portions of deposits. The remaining coke is then determined by a burn-off method. The need for a standard method
of coke determination is acute to enable comparisons of results from different laboratories.
Carbonaceous deposits which decrease catalyst activity are always formed on the catalyst surface during hydrotreatment. Deposits form particularly rapidly in the initial stages of the operation (Beuther and Schmidt, 1964), blocking and plugging active sites and pores. In the scientific literature there is some confusion about the composition of these deposits. The chemical structure and molecular weight of the material are not well understood, although aromatic and S-, N-, and 0-containing heterocyclic compounds are believed to be the main precursors of coke deposits (Furimsky, 1978). The mechanism of coke formation is unknown. To further complicate the problem, no standard analytical method exists to determine coke deposit levels. Researchers usually make no reference to analytical procedures although the amount of coke determined is affected by the method applied. The amount of coke is shown in this note to be markedly affected by the extent of removal of liquid fractions left over on the catalyst surface after reaction. Thus, unless some standard analytical method is used, any real comparison among different studies of coke deposition becomes impossible. The catalysts investigated are those from bench scale hydrotreatment of either Athabasca bitumen or heavy gas oil derived from the bitumen. Testing conditions, the reactor, and the method of catalyst preparation have been described elsewhere (Ternan et al., 1978). Feedstock properties are summarized in Table I. The methods for removing adhering soluble hydrocarbon fractions from catalyst surface were as follows: Method A, heating of pellets for 1 h under high vacuum (0.7 mmHg) at a temperature of 290 "C which corresponds to 530 "C at 1 atm; Method B, Soxhlet extraction of pellets with benzene until the solvent was clear; Method C, Soxhlet extraction with pyridine as in B; to remove solvents from the catalyst surface after extraction (B and C), the pellets were subjected to a high vacuum at 100 "C overnight; Method D, heating the pellets to 530 " C in a stream of hydrogen at atmospheric pressure. A detailed description of this method was published elsewhere (Ternan et al., 1978). After these treatments, catalyst pellets were roasted at 600 "C in a furnace. The decrease in the pellets' weight was used to calculate the amount of coke deposited after a correction had been made to change the oxidic form of the catalyst back to its sulfided form. Hydrocarbon materials adsorbed on the surface of catalyst represent a wide range of fractions, continuously changing in properties such as boiling point. They include the coke deposited as well as the soluble hydrocarbon mixtures from the feed and products. The amounts of feed and product fractions carried over with catalyst particles 0019-7890/79/1218-0206$01 .OO/O
Table I. Properties of Feedstocks heavy Athabasca gas oil bitumen boiling range, C pitch ( + 5 2 4 "C), wt % Conradson carbon residue, wt % pentane insolubles, wt % benzene insolubles, w t %
343524 nil 0.97 0.3 nil
52 12.6 15.8
0.9
Table 11. Effect of De-oiling Procedures on the Amount of Coke Determined amount of coke, w t % method
heavy gas oil
bitumen
A B C D
18.4 9.6 5.5 3.2
15.9 8.7 12.5
I
are markedly affected by conditions to which they are subjected before and during draining of the reactor and may account for a significant part of the material adsorbed. It is essential, therefore, that the "carryovers" be removed from the surface before any determination of coke level is undertaken. In addition, liquid-phase resinous and asphaltenous species adsorbed on the catalyst surface differ markedly in structure from the coke. Because a straightforward definition of the coke does not exist, drawing a clear borderline between the coke and removable fractions is not possible. Determined values of coke depend, therefore, on the procedure applied during the removal of the fractions (de-oiling). Results obtained after applying four different de-oiling procedures are summarized in Table 11. Method A removed only 10.8 out of 29.2 wt % (determined by roasting of used pellets) of organic material adsorbed on the surface when the gas oil was used as feed. This fraction falls into the boiling range of the feed and may represent "carryovers". The remaining 18.4 wt % are less volatile and were formed during the upgrading process in which the gas oil feed contacted the catalyst surface. The Soxhlet extractions resulted in 19.6 and 23.4 wt 90 removal when benzene and pyridine were applied, respectively. Pyridine is sometimes considered to dissolve all hydrogenable material (Sternberg et al., 1975). By this token, the species remaining on the surface after the pyridine extractions should be inert to hydrogenation. In the case of the gas oil feed, however, the amount of coke determined after Method D was smaller than that after Method C. This is attributed to cracking and favorable conditions for hydrogenation, i.e., the continuous supply of fresh hydrogen and removal of hydrogenation products.
Published 1979 by the American Chemical Society
Ind. Eng. Chern. Prod. Res. Dev., Vol. 18, No. 3, 1979 207
2t 0
2
4
6
8
1
0
1
2
Mo 0, pt. $6 Figure 1. The amount of coke deposited vs. concentration of MOO, on the alumina support.
Surprisingly, the amount of coke on catalysts used in the bitumen hydrotreatment was larger after Method D than after Method C, apparently because additional coke formed during the temperature increase up to 530 "C from species which can be extracted by pyridine. Such coke formation is less evident on catalysts used in the heavy gas oil hydrotreatment, because the feed, as well as its products, is more volatile; i.e., they will distill off during the temperature increase rather than being converted to coke. Although Method D gives, for the gas oil feed, a lower value of coke than Method C, the structure of coke will inevitably change during de-oiling by Method D because of the conditions applied. After de-oiling of used catalysts pellets, deposits left on the surface are quantitatively determined. Pellets are usually roasted to burn off all carbonaceous material. During this step a catalyst is converted to an oxidic form form the operating sulfided form. The difference in weight before and after burning cannot, therefore, be taken as a measure of coke level, unless a correction is made to account for this change. The value obtained may be affected by the correction method applied. In case of supported cobalt molybdate catalyst such as used in the present work, a number of assumptions had to be made while selecting the correction method. They include replacement of two oxygens of Moos by sulfur (Masoth, 1977), the change of COOto CsS, and no change of A1203during the operation, and all steps following. There are, however, some other factors which are not well understood, such as diffusion of active ingredients into the support, complete sulfiding of Moo3, and formation of molybdenum aluminates. Determination of a true value for coke is therefore difficult. Some information on the amount of coke on the surface can be obtained from determination of carbon. This was done with a Perkin-Elmer 240 analyzer after benzene extraction (Method B) of a catalyst used in the bitumen hydrotreatment. The amount determined was 11.8 and 0.9 wt % for C and H, respectively, compared to 15.9 w t % of coke when the burning-off procedure was supplied. The difference is attributed to accumulation of heterocyclic S-, N-, and 0-containing compounds in the coke where they may account for almost 15% of its weight (Furimsky, 1978). The carbon determination method will not account for these heteroatoms although their presence in the coke is crucial from the catalyst deactivation point of view. For catalysts used in treating light feedstocks, containing no
metals or heterocyclic compounds, careful determination of C and H may provide a true value for carbonaceous deposits on the surface. Metals, V and Ni in particular, when present in feed, will most likely accumulate in the coke. No method discussed above will directly account for their presence in the coke. During burn-off, metals either change to their oxides or associate with catalyst material in an unknown manner. This adds to difficulties in applying the above-mentioned corrections. Two simultaneous analyses must be performed, i.e., one to determine the amount of carbonaceous deposits and the other to determine the amount of metal deposits, to obtain a total amount of deposits which will be only an approximate value. Because the amount of coke deposited on catalyst surface depends on the analytical procedure applied, the interpretations of results obtained may thus be affected. This can be recognized from Figure 1, in which the compositions of the supported molybdate catalysts are related to the amount of coke formed during catalytic hydrotreatment of the heavy gas oil. The shapes of curves obtained by Method C and D are different. The curve obtained by Method C shows a minimum of Moo3 concentration of about 6 wt %, while the curve obtained by Method D showed little effect when the amount of MOO, was higher than about 3 wt 70. The level of coke determined is also, in some cases, almost three times higher for Method C than for Method D. This is attributed to polar compounds which are adsorbed on the surface much more strongly than pyridine and cannot be desorbed from the surface by the latter. On the other hand, the species can be removed by hydrogenation under conditions applied in Method D. The results indicate that selection of a proper analytical method for coke determination is of crucial importance. In conclusion, the analytical procedure for determining the amount of coke deposited on a catalyst surface should include two essential steps. At first, soluble fractions carried over with catalyst particles must be removed. For this step Soxhlet extraction is recommended. After such extraction only fractions heavier than asphaltenes remain on the surface. One should refer only to this portion of deposits as coke. The amount of the coke is, then, estimated from weight change during roasting of the catalyst particles. A correction must be applied to account for chemical change in catalyst material, as discussed above. Results shown in Figure 1 (Method C ) were obtained by this procedure, with repeatibility of better than 5% of the mean. In the case of catalysts used in treating petroleum feedstocks containing no S, N, and 0, the amount of coke can be estimated from the determination of C and H. Here no correction needs to be applied. The proposed procedure can serve as a base for developing a standard method for the determination. Such a method is needed for comparing results obtained by different workers. Literature Cited Beuther, H., Schmidt, B. K., "Proceedings, Sixth World Petroleum Congress", Sect. 111, p 297, 1964. Furimsky, E., Ind. Eng. Chem. Prod'. Res. Dev., 17, 329 (1978). Masoth, F. J., Catalysis, 47, 300 (1977). Sternberg, H. W., Raymond, R., Akhtar, S., ACS Syrnp. Ser., No.20, 111 (1975). Ternan, M., Furimsky, E.,Parsons, B. I., fuelprocess. Techno/.,2(1). 45 (1979).
Received f o r reuiew October 11, 1978 Accepted April 12, 1979
