Hydrocarbon conversion, deactivation

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

In an automobile s catalytic converter, CO and hydrocarbons present in the exhaust gases are oxidized. Unfortunately the effectiveness of these units decreases with use. The phenomenon was studied by Summers and Hegedus in /. Catalysis, 51, 185 (1978) by means of an accelerated aging test on a palladium impregnated porous pellet packed bed converter. From the reported data on hydrocarbon conversion shown below, develop an expression to represent the deactivation rate of this catalyst. 

Pfefferle carried out prereforming of JP-4 fuel, feeding a mixture of fuel, H2, and steam with Pt and Rh supported on alumina containing 6% silica at a temperature below 700°C. The presence of H2 in the feed allows prereforming to occur at low temperatures and, subsequently, a S/C ratio as low as 1 can be used without substantial catalyst deactivation. More than 90% hydrocarbon conversion to methane, H2, CO, and CO2 was observed in prereformer without any catalyst deactivation. 

The model includes fundamental hydrocarbon conversion kinetics developed on fresh catalysts (referred to as start-of-cycle kinetics) and also the fundamental relationships that modify the fresh-catalyst kinetics to account for the complex effects of catalyst aging (deactivation kinetics). The successful development of this model was accomplished by reducing the problem complexity. The key was to properly define lumped chemical species and a minimum number of chemical reaction pathways between these lumps. A thorough understanding of the chemistry, thermodynamics, and catalyst... 

Although the reaction classes discussed earlier are sufficient to describe the hydrocarbon conversion kinetics, an understanding of the elementary reaction sequence is needed to describe catalyst deactivation. Several of the overall reactions require formation of olefinic intermediates in their elementary reaction sequence. Ultimately, these olefinic intermediates lead to coke formation and subsequent catalyst deactivation. For example, the ring closure reaction... 

The general mathematical description for determining reforming kinetics is shown below for hydrocarbon conversion and deactivation rates, respectively,... 

Following the general form of Eq. (3), hydrocarbon conversion with deactivation is given by... 

While the 13 hydrocarbon lumps accurately represent the hydrocarbon conversion kinetics, they must be delumped for the deactivation kinetics. In addition, delumping is necessary to estimate many of the product properties and process conditions important to an effective reformer process model. These include H2 consumption, recycle gas H2 purity, and key reformate properties such as octane number and vapor pressure. The following three lump types had to be delumped the C5- kinetic lump into Cl to C5 light gas components, the paraffin kinetic lumps into isoparaffin and n-paraffin components, and the Cg+ kinetic lumps into Cg, C9, C10, and Cn components by molecular type. 

Complete reforming kinetics have been developed for several commercial catalysts, including those used in Mobil reformers. Since KINPTR affects Mobil s business strategy, the complete reforming kinetics are proprietary. However, as an example, KINPTR C6 kinetics will be presented for UOP s R16H platinum-rhenium-alumina catalyst. Both the hydrocarbon conversion and the deactivation equations [Eqs. (36), (40)] can be directly applied to the C6 system. For the C6 hydrocarbon conversion, Eq. (40) becomes... 

In hydrocarbon conversion over zeolite catalysts, the formation and retention of heavy products (carbonaceous compounds often called coke ) is the main cause of catalyst deactivation. 5X 77 XI1 These carbonaceous compounds may poison or block the access of reactant molecules to the active sites. Moreover, their removal, carried out through oxidation treatment at high temperature, often causes a decrease in the number of accessible acid sites due to, e.g., zeolite dealumination or sintering of supported metals. 

Several authors found in hydrocarbon conversions at different catalysts the same reaction order for catalyst deactivation and also no dependence of the rate law on the concentrations of reactants 3,4,5]. 

Heterogeneous catalysts for hydrocarbon conversion may require metal sites for hydrogenation-dehydrogenation and acidic sites for isomerisation-cyclisation and these reactions may be more or less susceptible to the effect of carbonaceous overlayers depending on the size of ensembles of surface atoms necessary for the reaction. In reality we must expect species to be transferred and spilled-over between the various types of sites and if this transfer is sufficiently fast then it may affect the overall rate and selectivity observed. If there is spillover of a carbonaceous species [4] then there may be a common coke precursor for the carbonaceous overlayer on the two types of site. Nevertheless, the rate of deactivation of a metal site or an acidic site in isolation may be very different from the situation in which both types of site are present at a microscopic level on the same catalyst surface. The rate at which metal and acid sites deactivate with carbonaceous material may of course not be identical. Indeed metal sites may promote the re-oxidation of a carbonaceous species in TFO at a lower temperature than the acid sites would allow on their own and this may allow differentiation of the carbonaceous species held on the two types of site. 

Study of Deactivation of Metal and Acid Sites in Hydrocarbon Conversion... 

Thus it is clear that both metal and acid sites can be poisoned and deactivated rather quickly in hydrocarbon conversion reactions and at very mild experimental conditions. 

With the advent of more sophisticated testing methods and a better understanding of testing phenomena, we can expect to see more interest for the basic understanding of the underlying processes involved, such as Hydrocarbon Conversion Kinetics, Catalyst Deactivation Mechanisms, Multiphase Mass Transfer and Diffusion,etc. 

Coking, widely experienced in the catalysis of hydrocarbon conversion (7), can deactivate both metallic and acid catalytic sites for hydrocarbon reactions (2). Accumulation of such carbonaceous deposits affects selectivity in hydrocarbon conversion (5). Adsorbed ethene even inhibits facile o-p-Hj conversion over Ni or Pt (4 ), the surface of which it appears is very nearly covered at lower temperatures in such deposits. H spillover may enhance hydrocarbonaceous residue formation (6). Accumulated carbonaceous residues can be removed by temperature programmed oxidation, reduction and hydrogenation TPO, TPR, TPH, etc (7) as part of catalyst regeneration. 

Boock, L. T., Petti, T. F., and Rudesill, J. A., International Symposium on Deactivation and Testing of Hydrocarbon Conversion Catalysts, ACS Annual Meeting, Chicago, IL, August 1995)... 

Sie, S.T. Paper TI, Int. Symp. on the Deactivation and Testing of Hydrocarbon Conversion Catalysts, August 20-25,1995, Chicago. 

The objective of this book is to serve as a practical reference work on testing for the main hydrocarbon-conversion processes applied in oil refineries catalytic cracking, hydroprocessing, and reforming. These fields were combined because of the clear analogies and congruence between the areas, such as deactivation of active sites by coke, mass-transfer phenomena of hydrocarbons into solid catalysts, hydrocarbon chemistry and reaction kinetics, and downscaling of commercial conditions to realistic small-scale tests. 

Supported metal catalysis are employed in a variety of commercially important hydrocarbon conversion processes. Such catalysts consist, in general, of small metal crystallites (0.S to 5 nm diameter) dispersed on non-metallic oxide supports. One of the major ways in which a catalyst becomes deactivated is due to accumulation of carbonaceous deposits on its surface. Catalyst regeneration, or decoking, is normally achieved by gasification of the deposit in air at about 500°C. However, during this process a further problem is frequently encountered, which contributes to catalyst deactivation, namely particle sintering. Other factors which can contribute to catalyst deactivation include the influence of poisons such as sulfur, phosphorus, arsenic and... 

Carbonaceous deposits are the principle cause of zeolite deactivation in processes involving hydrocarbon conversions. Firstly they can poison active sites or block their access. Secondly, their removal, which is carried out through oxidative treatment at high temjjeratures, has detrimental effects i.e., dealumination and degradation of the zeolite and sintering of supported metals. 

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