Coke composition determination methods

The composition of coke was determined at different coke contents through the method develop in our laboratory [4], i.e., dissolution of the zeolite in a hydrofluoric acid solution then recovery and GC/MS analysis of the part of coke soluble in methylene chloride. In this case, all the coke components are soluble in methylene chloride. It should be emphasized that practically no coke could be recovered in this solvent through a direct... 

While the two first conclusions could be obtained from the measurement of the total H/C ratio of coke deposits or from their in situ investigation by spectroscopic methods (IR, CP MAS NMR, UV VIS, EPR) this is not the case for the other conclusions. Indeed these methods can lead only to information concerning the coke content and the chemical identity of its components, which is not sufficient for specifying the mode of coke formation, the location of coke molecules and the cause of their retention in or on the zeolite. All this can be obtained from the coke composition. To determine this composition requires the recovery of coke from the zeolite followed by its analysis through GC, HNMR, SM... However great care must be taken not to modify coke components during their recovery. It has been shown that this is the case when coke is recovered from the coked samples through dissolution of the zeolite in a hydrofluoric acid solution. 

Ash, as determined by the standard test method (ASTM D-3174), is the residue remaining after burning the coal and coke and differs in composition from the original inorganic constituents present in the coal. Incineration causes an expulsion of all water, the loss of carbon dioxide from carbonates, the conversion of iron pyrites into ferric oxide, and other chemical reactions. In addition, the ash, as determined by this test method, will differ in amount from ash produced in furnace operations and other firing systems because incineration conditions influence the chemistry and amount of the ash. 

The classical method of investigation of effects of diffusion on reactions is typically to run a reaction with catalyst particles of various sizes. For zeolites, the resistance of intracrystalline diffusion is normally much larger than that characteristic of molecular diffusion or Knudsen diffusion that could occur in the spaces between the zeolite crystals in a catalyst particle. Thus, the crystal size of the zeolite has to be varied instead of the particle size to determine the effects of diffusion on zeolite-catalyzed reactions. Kinetics of the MTO reaction has been measured with SAPO-34 crystals with identical compositions and sizes of 0.25 and 2.5 pm 89). The methanol conversion was measured as a function of the coke content of the two SAPO-34 crystals in the TEOM reactor. 

Determination of the physical composition can be achieved by any of the test methods for determining the toluene-insoluble constituents of tar and pitch (ASTM D-4072, ASTM D-4312). Furthermore, a variety of samples can be used to give a gradation of soluble and insoluble fractions. The coke, of course, remains in the extraction thimble (Soxhlet apparatus), and the extracts are freed from the solvent and weighed to give percent by weight yield(s). 

The composition of a mixture of four representative coke-oven tars with a specific gravity at 15.5° C. of 1.186 and a benzene insoluble of 10.6 per cent is shown in Table XXXIX, as determined by Weiss and Downs.1 The amounts given should not be taken as commercially recoverable, since practical methods of refining would not permit the isolation of all of any one constituent in the tar. 

The major difficulty in the determination of the mode(s) of coking and of deactivation is to establish the composition of coke. Indeed coke is a complex mixture which is very difficult to separate from the zeolite. A critical review of the methods used for characterizing coke will constitute the first part of this paper whereas the second part will show how the composition of coke as function of various parameters varies. 

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