Cracking catalysts area measurements

In the laboratory, the two deactivation mechanisms can be separated by steaming the cracking catalyst and Additive R separately. In this case, there can be no transfer of silica from the cracking catalyst to Additive R during the steaming. The Additive R only loses surface area. The steamed components are then blended together for SOx activity measurements. 

Wheeler [16] proposed that the mean radius, r, and length, L, of pores in a catalyst pellet (of, for that matter, a porous solid reactant) are determined in such a way that the sum of the surface areas of all the pores constituting the honeycomb of pores is equal to the BET (Brunauer, Emmett and Teller [17]) surface area and that the sum of the pore volume is equed to the experimental pore volume. If represents the external surface area of the porous particle (e.g. as determined for cracking catalysts be sedimentation [18]) and there are n pores per unit external area, the pore volume contained by nSx cylindrically shaped pores is nSx nr L. The total extent of the experimentally measured pore volume will be equal to the product of the pellet volume, Vp, the pellet density, Pp, and the specific pore volume, v. Equating the experimental pore volume to the pore volume of the model... 

The value of Do used to calculate the data in Table IV was obtained from direct measurement of the diffusion of hydrogen in the catalyst by the porous plug method (1, p. 189, method b). The value used was for the spherical beads of cogelled silica-alumina cracking catalyst used in the experiments to be reported here. The catalyst contained 10% AI2O3 by weight and had a surface area of 350 m.2/g. The value of the effective diffusivity of the catalyst particle for H2 at 27°C. (DHj) was found to be 7 X 10-3 cm.2/sec. The value of the effective diffusivity of the catalyst particle for cumene Dc, at reaction temperature was calculated from this measured hydrogen diffusivity by the equation... 

The compounds making up the catalyst sample can be clearly identified in the XRD pattern. Cupric oxide produces the peaks labeled C, zinc oxide the peaks labeled Z, and y-alumina the peaks labeled A in Fig. 12. Not only does the XRD pattern qualitatively identify the phases present in the catalyst, but the quantity of each phase can be determined by measuring the area under selected diffraction peaks relative to a standard. An example of quantitative analysis by XRD is found in the ASTM Standard Procedure D3906-80 for NaY zeolite in a cracking catalyst. 

Surface area is important in all applications where the process is surface-dependent like in mass and heat transfer, flow through packed beds or fluidization. Activity of drugs, setting time of cement and effectiveness of cracking catalysts are just three examples of direct dependence on specific surface. Some such materials, like fillers or catalysts, are often specified in units of specific surface rather than in particle size and its distribution. Specific surface also offers some practical advantages, in favourable cases, in the ease and speed of measurement and also in that it gives a... 

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. 

A number of physical measurements on cracking catalysts are employed to complement the direct activity tests. Ries (26) has reviewed the principal methods employed in determining the physical properties of porous solids, and no attempt will be made to do so again in detail here. The determinations of surface area, bulk density, particle density, and real density are practically routine procedures in studying all types of cracking catalysts. From these data, the pore volume of catalyst particles, intergranular free volume, and average pore diameter can be calculated (Emmett and DeWitt, 27). 

Contact catalysis, mechanism of, 2, 251 Contact catalysts, surface area measurements for studying, 1, 65 Cracked gases, polymerization from, of olefins, 8, 219 Cracking catalysts,... 

Stereochemistry, of unsaturated hydrocarbon hydrogenation, 16, 123 Structure, of cracking catalysts, 4, 87 Surface area measurements, for studying contact catalysts, 1, 66 Surface barrier effects, in adsorption, 7, 269... 

Physical properties, notably the specific surface areas, have been proposed by some authors as a measure for the activity of catalysts. This correlation is successful only when applied to catalysts which resemble one another in their composition and in their method of preparation. That surface area cannot be considered to be of exclusive importance to catalytic activity is demonstrated by the rather extreme examples given in Table VII. On the other hand, the fact that the capacity for quinoline chemisorption is quantitatively related to the activity of cracking catalysts is shown by Fig. 8 obtained with catalysts of various compositions, methods of preparation, and activities. The amount of quinoline chemisorbed thus measures a general property of this entire class of catalysts which is fundamentally related to their ability to act as catalysts. 

A commercial cumene cracking catalyst is in the form of pellets with a diameter of 0.35 cm which have a surface area. Am, of 420 m g and a void volume, Vm, of 0.42cm g. The pellet density is 1.14g cm. The measured l -order rate constant for this reaction at 685K was 1.49cm s g . Assume that Knudsen diffusion dominates and the path length is determined by the pore diameter, dp. An average pore radius can be estimated from the relationship fp = 2Vm/Am if the pores are modeled as noninterconnected cylinders (see equation 4.94). Assuming isothermal operation, calculate the Thiele modulus and determine the effectiveness factor, tti, vmder these conditions. 

Since the catalyst is so important to the cracking operation, its activity, selectivity, and other important properties should be measured. A variety of fixed or fluidized bed tests have been used, in which standard feedstocks are cracked over plant catalysts and the results compared with those for standard samples. Activity is expressed as conversion, yield of gasoline, or as relative activity. Selectivity is expressed in terms of carbon producing factor (CPF) and gas producing factor (GPF). These may be related to catalyst addition rates, surface area, and metals contamination from feedstocks. 

In an exploratory experiment, 13 different powder materials were tested in a FFB ACE unit. Most of the results were unremarkable except for three catalysts a low Z/M commercial maximum distillate catalyst (the same LZM catalyst used in the pilot riser experiment), a spray dried low surface area silica (inert) and the minimum aromatics breakthrough (MAB) catalyst. The inert material was included in the study to represent thermal cracking. The catalysts were steam deactivated in the fixed bed steamer prior to testing. Catalysts and the VGO-B feed properties are displayed in Tables 2.3 and 2.1, respectively. LCO aromatics were measured with 2D GC. Figures 2.7 through 2.9 illustrate the main results. 

The specific surface area was measured by nitrogen adsorption at -195 C. The cumene cracking reaction was conducted by pulse technique under the following conditions O.IO g catalyst, H, flow rate 75 ml/nin, pulse volume 1 ul. 

In a previous paper (7), we have illustrated that diffusion in FCC takes place in the non-steady regime and that this explains the failure of several attempts to relate laboratory measurements on FCC catalysts to theories on steady state diffusion. Apart from the diffusion aspects, Nace (13) has also indicated the limited accessibility of the zeolite portal surface area by comparing the cracking rates of various model compounds with an increasing number of naphthenic rings on zeolite and amorphous FCC catalysts, figure 2. 

Surface Area (SA, mf/g). The snrface area is the measure of the catalyst activity (as long as the same catalyst types are compared) and has a strong effect on the performance of an Flnidized Catalytic Cracking Unit (FCCU). High surface area also results in increased adsorption of hydrocarbons, and a higher steam rate in the stripper may be reqnired. The zeolite and matrix surface areas of a catalyst can be analysed separately. Matrix pores provide access of the hydrocarbons to the active zeolite sites and matrix surface area often correlates with the bottoms conversion activity of the catalyst or the Light Cycle Oil (LCO) yield at constant conversion. 

The effects of temperature and steam on the deterioration of catalysts employed in cracking processes are under continuous surveillance, and these effects may be studied in a rather straightforward manner by physical property measurements independent of the chemistry of the surface. For example, as will be shown later, if a significant increase in pore radius accompanies loss in area, steam deactivation is probably indicated. Moreover the pore structure as determined by adsorption techniques is undoubtedly related to the ease of admission of reactant molecules and the diffusion out of product molecules as well as to the regeneration properties when carbonaceous deposits must be removed. 

An alumina addition to silica produces proton-active catalysts for cracking purposes. The selective adsorption of gases with proton affinity can be used to measure the surface area covered with protons 34). The aluminum ions seem to form a unimolecular layer on the surface of the silica 36). 

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