Pore size distribution catalyst

Catalyst circulation coke is a hydrogen-rich coke from the reactor-stripper. Efficiency of catalyst stripping and catalyst pore size distribution affect the amount of hydrocarbons carried over into the regenerator. 

Coke deposition alters catalyst pore size distributions significantly and is an effect to be followed in regard to catalyst aging. 

SEM and TEM images give detailed information about the porous structure of a supported heterogeneous catalyst (pore size distribution, typical sizes of the particles, etc.). The information from SEM and TEM images can be used in the reconstruction of porous catalytic medium. In the digitally reconstructed catalyst, transport (diffusion, permeation), adsorption, reaction, and combined reaction-diffusion processes can be simulated (Stepanek et al., 2001a). Parametric studies can be performed, and the resulting dependencies can serve as a feedback for the catalyst development. 

The sizes determined in this work are the apparent molecular sizes and not necessarily the sizes of the asphaltene and maltene molecules at process conditions. Association efforts for asphaltene molecules have been observed for both vapor-phase osmometry molecular weight and viscosity measurements (14, 15). The sizes reported here were measured at 0.1 wt % in tetrahydrofuran at room temperature. Other solvent systems (chloroform, 5% methanol-chloroform, and 10% trichlorobenzene-chloroform) gave similar size distributions. Under these conditions, association effects should be minimized but may still be present. At process conditions (650-850°F and 5-30% asphaltene concentration in a maltene solvent), the asphaltene sizes may be smaller. However, for this work the apparent sizes determined can be meaningfully correlated with catalyst pore size distributions to give reasonable explanations of the observed differences in asphaltene and maltene process-abilities (vide infra). In addition, the relative size distributions of the six residua are useful in explaining the different processing severities required for the various stocks. Therefore, the apparent sizes determined here have some physical significance and will be referred to just as sizes. 

Both asphaltene and maltene molecular size distributions were compared with the pore size distribution of a small pore desulfurization catalyst. Figure 4 shows the Kuwait maltene and asphaltene size distributions along with the catalyst pore size distribution. Most of the maltene molecules are small enough to diffuse into the catalytic pores. In contrast, the Kuwait asphaltenes have a... 

Size characterization measurements have provided useful information on the importance of the hydroprocessing catalyst pore size distribution and on the effects of visbreaking and hydroprocessing on the residua molecular size distributions. It is apparent that asphaltenes and maltenes are not unique entities, but instead have considerable overlap in their size distributions. A complete study of the effects of processing conditions would require consideration of all components of a residuum. 

Coke formation on catalyst pore size distribution and surface area... 

The textural characterization of the supports and catalysts pore size distribution, pore volume, and surface areas were determined by use of mercury intrusion porosimetry using a Micromeritics Poresizer 9320 and nitrogen gas adsorption/desorption isotherms carried out on a Micromeretics ASAP 2000 respectively. For the porosimetiy analysis a contact angle of 140° and surface tension of480mNm for mercury were assumed. 

Physical properties affecting catalyst perfoniiance include tlie surface area, pore volume and pore size distribution (section B1.26). These properties regulate tlie tradeoff between tlie rate of tlie catalytic reaction on tlie internal surface and tlie rate of transport (e.g., by diffusion) of tlie reactant molecules into tlie pores and tlie product molecules out of tlie pores tlie higher tlie internal area of tlie catalytic material per unit volume, tlie higher the rate of tlie reaction... 

The relation between the dusty gas model and the physical structure of a real porous medium is rather obscure. Since the dusty gas model does not even contain any explicit representation of the void fraction, it certainly cannot be adjusted to reflect features of the pore size distributions of different porous media. For example, porous catalysts often show a strongly bimodal pore size distribution, and their flux relations might be expected to reflect this, but the dusty gas model can respond only to changes in the... 

Catalyst performance depends on composition, the method of preparation, support, and calcination conditions. Other key properties include, in addition to chemical performance requkements, surface area, porosity, density, pore size distribution, hardness, strength, and resistance to mechanical attrition. 

Important physical properties of catalysts include the particle size and shape, surface area, pore volume, pore size distribution, and strength to resist cmshing and abrasion. Measurements of catalyst physical properties (43) are routine and often automated. Pores with diameters <2.0 nm are called micropores those with diameters between 2.0 and 5.0 nm are called mesopores and those with diameters >5.0 nm are called macropores. Pore volumes and pore size distributions are measured by mercury penetration and by N2 adsorption. Mercury is forced into the pores under pressure entry into a pore is opposed by surface tension. For example, a pressure of about 71 MPa (700 atm) is required to fill a pore with a diameter of 10 nm. The amount of uptake as a function of pressure determines the pore size distribution of the larger pores (44). In complementary experiments, the sizes of the smallest pores (those 1 to 20 nm in diameter) are deterrnined by measurements characterizing desorption of N2 from the catalyst. The basis for the measurement is the capillary condensation that occurs in small pores at pressures less than the vapor pressure of the adsorbed nitrogen. The smaller the diameter of the pore, the greater the lowering of the vapor pressure of the Hquid in it. 

Supports. The principal component of a typical catalyst is the porous support (49,50). Most supports are robust soHds that can be made with wide ranges of surface areas and pore size distributions. The most widely appHed supports are metal oxides others are carbon, kieselguhr, organic polymers, and zeoHtes. 

Transition aluminas are good catalyst supports because they are inexpensive and have good physical properties. They are mechanically stable, stable at relatively high temperatures even under hydrothermal conditions, ie, in the presence of steam, and easily formed in processes such as extmsion into shapes that have good physical strength such as cylinders. Transition aluminas can be prepared with a wide range of surface areas, pore volumes, and pore size distributions. 

The pore-size distribution and the nature of the pores in catalyst supports and hence the catalysts derived from them are important properties that significantly affect catalyst performance (16). In most cases, catalyst designers prefer an open-pore stmcture, that is, pores that have more than one opening, and a pore size as uniform as possible in order to obtain maximum utilization of the available pore volume. This can be achieved by careful choice of raw materials and processing conditions. 

When the catalyst is expensive, the inaccessible internal surface is a liabihty, and in every case it makes for a larger reactor size. A more or less uniform pore diameter is desirable, but this is practically reahz-able only with molecular sieves. Those pellets that are extrudates of compacted masses of smaller particles have bimodal pore size distributions, between the particles and inside them. Micropores have diameters of 10 to 100 A, macropores of 1,000 to 10,000 A. The macropores provide rapid mass transfer into the interstices that lead to the micropores where the reaction takes place. 

The nitrogen physisorption isotherm and pore size distributions for the synthesized catalysts are shown in Figs. 3 and 4. The Type IV isotherm, typical of mesoporous materials, for each sample exhibits a sharp inflection, characteristic of capillary condensation within the regular mesopores [5, 6], These features indicate that both TS-1/MCM-41-A and TS-l/MCM-41-B possess mesopores and a narrow pore size distribution. 

For most catalysts, mesopores are dominant, whereas for materials derived from zeolites or active carbons, micropores are the most important. Determination of the pore size distribution is indispensable in catalysis research. 

Fig. 3.3.3 (a) Hahn echo ]H intensity during heating cycle of cyclohexane filling the pores of catalyst pellets, (b) Pore size distribution obtained from (a) in comparison with BET measurements. 

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