Catalytic coke determination

The catalytic coke produced by the activity of the catalyst and simultaneous reactions of cracking, isomerization, hydrogen transfer, polymerization, and condensation of complex aromatic structures of high molecular weight. This type of coke is more abundant and constitutes around 35-65% of the total deposited coke on the catalyst surface. This coke determines the shape of temperature programmed oxidation (TPO) spectra. The higher the catalyst activity the higher will be the production of such coke [1],... 

A comprehensive study on coke deposition in trickle-bed reactors during severe hydroprocessing of vacuum gas oil has been carried out. On the basis of results obtained with different catalysts on the one hand, and analytical and catalytic characterisation of the coke deposits on the other, it is argued that coke is formed via two parallel routes, viz. (i) thermal condensation reactions of aromatic moieties and (ii) catalytic dehydrogenation reactions. The catalyst composition has a large impact on the amount of catalytic coke whilst physical effects (vapour-liquid equilibria, VLE) predominate in determining the extent of thermal coke formation. The effect of VLE is related to the concentration of heavy coke precursors in the liquid phase under conditions which promote oil evaporation such as elevated temperatures. A quantitative model which describes inter alinea the distinct maximum of coke deposited as a function of temperature is presented. 

Overall, evaluation of catalysts on resid feedstocks requires sophisticated and well integrated catalyst deactivation, catalyst stripping and cracking systems. It is important to determine not only the coke yield, but each of its components (Catalytic coke, contaminant coke, CCR coke and stripper (soft) coke). This paper provides details on how each of the components of the coke yield may be experimentally determined using catalyst metallation by cyclic deactivation, catalyst strippability measurements and modified catalytic cracking techniques. 

The catalysts (50 mg) were heated up to 673 K in He at a rate of 6 K/min and pretreated at this temperature in situ either in He or in a mixture of 5% HgS in H2 for 1 h. Thiophene HDS test reactions were carried out at 673 K and atmospheric pressure in a flow reactor system (30 cm min flow of 3% thiophene in Hg). Thiophene and the products were detected by GC. The conversion is the fraction of thiophene converted to coke and gaseous products, the yield is the fraction of thiophene converted to gaseous products. The catalytic properties were characterized by activities in C-S hydrogenolysis without C-C bond breaking (yield - (Ci+Cj+Cg products)), cracking (yield - C4 products), and coking (conversion - yield). The catalytic conversions determined after 5 minutes time on stream are discussed here, because all samples deactivated fast due to coke formation. 

An improved method for determining the AIT of solids has been described, and the effect of catalytically active inorganics on the reactivity and ignition temperature of solid fuels has been studied. Sodium carbonate markedly lowers the ignition temperatures of coal and coke [7], The volume of the vessel (traditionally a 200 ml flask) used to determine AIT has a significant effect on the results. For volumes of... 

The Pt-Re system has been studied extensively since the 1970s because adding Re to AhOs-supported platinum catalysts increases the resistance to deactivation of the catalysts used in naphtha reforming by preventing coke deposition. By using carbonyl precursors, well-defined bimetalhc species have been prepared. A proper characterization of these species allowed a relationship to be established between their structure and their catalytic behavior. Table 8.3 shows several Pt-Re bimetaUic catalytic systems prepared using different carbonyl species in which Pt-Re interactions determine the catalytic behavior. 

It is noted that Mo/DM is the best performing catalyst with the highest steady state activity and lowest deactivation rate. The deactivation rate is the lowest even under the influence of intense acid-catalyzed side reactions known to produce coke, i.e. oligomerization of styrene and cracking of ethylbenzene. Obviously, the high surface area and high connectivity of the support have played a determining role in the catalytic reaction. The effects they exert can be looked at in two ways ... 

The aim of this study is to convert as much as the hydrogen in the fuel into hydrogen gas while decreasing CO and CH4 formation. Process parameters of fuel preparation steps have been determined considering the limitations set by the catalysts and hydrocarbons involved. Lower S/C (steam to carbon) ratios favor soot and coke formation, which is not desired in catalytic steam and autothermal reforming processes. A considerably wide S/C ratio range has been selected to see the effect on hydrogen yield and CO formation. 

We used thermal analysis to determine the thermogravimetric analysis (TGA) and the differential thermogravimetric analysis (DTG) of SCT pitches to obtain information on volatility and coke yield at various temperatures up to 1000°C, DTG was found very useful in defining process modifications to reduce volatiles in the pitch and increase pitch coke yield. Figure 3 gives the DTG (in nitrogen) of several SCT pitches prepared by distillation, thermal and catalytic process, in comparison with petroleum and coal tar pitches,... 

Several factors determine the best feeds for catalytic crackers. Heavy feeds are preferred thus, the lower boiling point is about 650°F. The feed should not be so heavy that it contains an undue amount of metal-bearing compounds or carbon-forming material. Deposition of metals and coke can quickly deactivate the catalyst. 

Subsequent investigations, including IINS, were carried out to characterize the various resistances of such cokes to controlled after-treatments, such as oxidation or hydrogasification processes, as a basis for determining the feasibility of catalyst reactivation. The presence of metallic contaminants (iron, cobalt, and nickel) was of relevance, not only to the deposition of cokes and the catalytic transformation of the carbon structure, but also to the dynamic processes in the controlled decomposition of the material in catalyst regeneration procedures 50). 

Conversion and coke formation during catalytic ethene oligomerization catalyzed by HZSM-5 have been investigated in the TEOM and in a conventional gravimetric microbalance under similar conditions (2). The results show that the TEOM is a powerful tool for determination of the kinetics of deactivation of catalysts, with a design that makes determination of the true space velocity (or space time) easy. The TEOM combines the advantages of the conventional microbalance with those of a fixed-bed reactor, and the same criteria can be used to check for plug flow and differential operation. 

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