Coked catalysts, physical properties

The catalysts used in this CCR commercial service must meet several stringent physical property requirements. A spherical particle is required so that the catalyst flows in a moving bed down through the process reactors and regenerator vessel. These spheres must be able to withstand the physical abuse of being educated and transferred by gas flow at high velocity. The catalyst particles must also have the proper physical properties, such as particle size, porosity, and poresize distribution, to achieve adequate coke combustion kinetics. 

Early workers viewed carriers or catalyst supports as inert substances that provided a means of spreading out an expensive material like platinum or else improved the mechanical strength of an inherently weak material. The primary factors in the early selection of catalyst supports were their physical properties and their cheapness hence pumice, ground brick, charcoal, coke, and similar substances were used. No attention was paid to the possible influence of the support on catalyst behavior differences in behavior were attributed to variations in the distribution of the catalyst itself. 

Physical Properties of Coked Catalysts. Surface areas for a series of Shell 244 (cobalt-molybdenum (Co-Mo) on alumina) catalysts varying from 0 to 22% coke were determined. The surface area is inversely proportional to coke deposition as shown in Figure 3. The catalysts with 10% coke deposit lose approximately 20% of their original surface areas. 

The coke deposits affect the physical properties of the catalysts, as described in the following section. 

In a previous study, we have evaluated physical properties and catalytic activites for a NiMo catalyst coked with anthracene (1). In the present study, we present similar data from a study in which the same catalyst Is coked with vacuum gas oil, and compare the results of the two sets of data. 

Table 1 lists the physical property data for the catalysts coked with VGO. The nitrogen content generally increases with... 

The laboratory evaluations of cracking catalysts may be divided into essentially two categories (1) testing of activity for the conversion of a standard gas-oil to gasoline, gas, and coke, and (2) measurement of physical properties such as particle size, density, surface area, and pore-size distribution. 

Finally, Carlos Querini (INCAPE, Santiago, Argentina) reviews the literature dealing with the characterization of coke. The difficulty in identifying the chemical and physical properties of coke on the working catalyst are well known. The author describes temperature programmed methods, spectroscopy, and extraction methods as alternatives to characterize the structure of coke. He provides specific examples of these methods in a way that will helpful to those working in the field. 

The most obvious differences between solid and liquid acids are in their physical properties. Solids can be heated, which enhances the rate of proton transfer reactions which are slow at room temperature, can be used in solid-liquid and solid-gas reactions and can readily be separated from reactants and products. One of their limitations, however, is that the catalyst can become covered in strongly adsorbed by-product, or at high temperatures by carbonaceous residue, coke , resulting in deactivation. In this case, the utility of the catalyst may ultimately be determined by how readily it can be regenerated. 

Physical Properties. The catalyst support material must be stable under process conditions and under the conditions used during start-up and shut-down of the plant. In particular, resistance to conditions during upsets may become critical. Degradation of catalyst may cause partial or total blockage of some tubes, resulting in the development of "hot spots , "hot bands" or totally hot tubes. Coking may cause similar problems. 

Early catalysts were produced from calcined ferric oxide, potassium carbonate, a binder when required, and usually chromium oxide. Subsequently a wide range of other oxides replaced the chromium oxide typical compositions are shown in Table 7.5. The paste was extruded or granulated to produce a suitable shape and then calcined at a high temperature in the range 900°-950°C. Solid solutions of a-hematite and chromium oxide (the active catalyst precursors) were formed and these also contained potassium carbonate to inhibit coke formation. Catalyst surface area and pore volume were controlled by calcination conditions. It has been confirmed by X-ray diffraction studies that a-hematite is reduced to magnetite and that there is some combination of potash and the chromium oxide stabilizer. There is little change in the physical properties of the catalyst during reduction and subsequent operation. 

An interesting problem, not much commented upon in the older literature, is the relationship between coke deposition and the physical properties of the catalyst. Pore blockage by coke deposition has been demonstrated in specific instances, but has been ignored in earlier analytical studies of coking (10>11). 

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

Soft coke The contribution of soft coke can be determined in the cyclic deactivation unit. The soft coke make depends both on physical and chemical properties, such as activity and catalyst accessibility. The initial soft coke is expected to increase with the zeolite content, stripping rates are higher for the very accessible catalysts. 

Four types of REY zeolite (Si/Al = 4.8) with different crystal sizes and acidic properties were used. The physical and chemical properties of the fresh zeolites are given in Table 6.4. Polyethylene plastics-derived heavy oil, shown in Table 6.2, was used as the feed oil. The cracking reaction was conducted in a tubular reactor filled with catalyst particles under the following conditions time factor W/F = 0.2-3.0 kg-catkg oil h and reaction temperature = 300-450°C. The lumping of reaction products were gas (carbon number 1-4), gasoline (5-11), heavy oil (above 12), and a carbonaceous residue referred to as coke. The index of the gasoline quality used was the research octane number (RON), which was calculated from Equation 6.1 [31]. 

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