Catalysts external area

Phj = hydrogen pressure H = Henry s law constant (atm/ft /mol) k = liquid film mass transfer coefficient a = interfacial area between gas and liquid fls = catalyst external area k = catalyst film mass transfer coefficient... 

Catalyst external area (n Catalyst effectiveness fact Gas-llquld Interfaclal area 3... 

Finally, one must know the effect of catalyst particle size on Kw. For a pore diffusion-controlled reaction, activity should be inversely proportional to catalyst particle diameter, that is directly proportional to external catalyst surface area. 

In the catalyst preparation area where the fire occurred, aluminum alkyl and isopentane are mixed in a batch blending operation in three 8000-gallon kettles. The flow rates of components are regulated by an operator at the control room. Temperature, pressure, and liquid level within the kettles are monitored by the control room operator. The formulated catalyst is stored in four 12,000-gallon vertical storage tanks within this process unit. Aluminum alkyl is a pyrophoric material and isopentane is extremely flammable. Each vessel was insulated and equipped with a relief valve sized for external fire. 

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... 

In addition, the reduction of NOj is a very fast reaction and is controlled by external and internal diffusion [27, 30]. In contrast, the oxidation of SO2 is very slow and is controlled by the chemical kinetics [31]. Accordingly, the SCR activity is increased by increasing the catalyst external surface area (i.e. the cell density) to favor gas-solid mass transfer while the activity in the oxidation of SO2 is reduced by decreasing the volume of the catalyst (i.e. the wall thickness) this does not affect negatively the activity in NO removal because significant ammonia concentrations are confined near the external geometric surface of the catalyst. 

If peUets are packed in a cubic configuration, then there are (2.5 x 10 ) catalyst peUets per liter. The external area of packed spheres in 1 liter is 8 x 10 m. The internal area of these porous pellets is 1.6 x 10 — 1.6 x lO km for 1 liter... 

The above rate is expressed per unit of external surface. To express the rate per gramme of catalyst the flux has to be multiplied by the catalyst specific area (m surf g J). 

The true intrinsic kinetic measurements require (1) negligible heat and mass transfer resistances by the fluids external to the catalyst (2) negligible intraparticle heat and mass transfer resistances and (3) that all catalyst surface be exposed to the reacting species. The choice of the reactor among the ones described in this section depends upon the nature of the reaction system and the type of the required kinetic data. Generally, the best way to determine the conditions where the reaction is controlled by the intrinsic kinetics is to obtain rate per unit catalyst surface area as a function of the stirrer speed. When the reaction is kinetically controlled, the rate will be independent of the stirrer speed. The intraparticle diffusional effects and flow uniformity (item 3, above) are determined by measuring the rates for various particle sizes and the catalyst volume, respectively. If the reaction rate per unit surface area is independent of stirrer speed, particle size, and catalyst volume, the measurements can be considered to be controlled by intrinsic kinetics. It is possible... 

In packed bubble columns the gas-liquid interfacial area also can be related to the external catalyst surface area, as in trickle flow reactors. However, in packed bubble columns channeling can occur with strongly reduced gas-liquid interfacial areas [11]. 

In this transport-limited region, the conversion increases at a slower rate than the increase of amoimt of catalyst, and thereby, the overall rate decreases instead of remaining constant as in the kinetic regime. In other words, in this region conversion is influenced by different factors (a) the additional catalyst surface area provided does not come in full contact with the pollutant due to external mass transfer resistance, and/or... 

Based on experiments with pure hydrocarbons and synthetic silica-alumina catalyst, it has been estimated that the cracking-rate constant at 932°F. should decrease by a factor of to when particle diameter is increased from about 0.5 mm. to 4 mm. (74). The influence of particle size on effective activity is especially pronounced at very high cracking temperatures (49). This behavior is in line with predictions because, with increasing temperature, reaction rate on the catalyst surface increases more rapidly than the rate of diffusion of reactants into the pores. Cracking of unsymmetrical diarylethanes is an exceptional case in which the reaction appears to depend entirely upon the number of collisions of the hydrocarbon with the external area of the catalyst particles (208). 

Zinco-aluminosilicates having different pore volume and external surface area were synthesized and their stability in aromatization was investigated. Table 2 shows some properties of catalysts. External surface area was measured by benzene-filled pore method described by Inomata et al. [5]. Average crystal size was measured geometrically by SEM images. Stability of those catalysts was examined under the same condition as stated above, and the results are shown in Fig, 2. 

In Eq. (7-1) is the usual mass-transfer coefficient based on a unit of transfer surface, i.e., a unit of external area of the catalyst particle. In order to express the rate per unit mass of catalyst, we multiply k by the external area per unit mass, a. In Eq. (7-2) k is the reaction-rate constant per unit surface. Since a positive concentration difference between bulk gas and solid surface is necessary to transport A to the catalyst, the surface concentration Cj will be less than the bulk-gas concentration Q. Hence Eq. (7-2) shows that the rate is less than it would be for = Q. Here the effect of the mass-transfer resistance is to reduce the rate. Figure 7-1 shows schematically how the concentration varies between bulk gas and catalyst surface. 

The external surface area of even very fine particles has been shown (Example 8-1) to be small with respect to the internal surface of the pores. Hence, in a catalyst pellet the surface resides predominantly in the small pores within the particles. The external surface of the particles, and of course the external area of the pellets, is negligible. 

Example 10-1 Experimental, global rates are given in Table 10-2 for two levels of conversion of SOj to SO3. Evaluate the concentration difference for SO2 between bulk gas and pellet surface and comment on the significance of external diffusion. Neglect possible temperature differences. The reactor consists of a fixed bed of x -in. cylindrical pellets through which the gases passed at a superficial mass velocity of 147 lb/(hr)(ft ) and at a pressure of 790 mm Hg. The temperature of the catalyst pellets was 480°C, and the bulk mixture contained 6.42 mole % SOj and 93.58 mole % air. To simplify the calculations compute physical properties on the basis of the reaction mixture being air. The external area of the catalyst pellets is 5.12 ft /lb material. The platinum covers only the external surface and a very small section of the pores of the alumina carrier, so that internal diffusion need not be considered. 

The reactor consisted of a fixed bed of x --in. cylindrical pellets. The pressure was 790 mm Hg. The external area of catalyst particles was 5.12 ft /lb, and the platinum did not penetrate into the interior of the alumina particles. Calculate the partial-pressure difference between the bulk-gas phase and the surface of the catalyst for SOj at each mass velocity. What conclusions may be stated with regard to the importance of external diffusion Neglect temperature differences. 

A similar approach to that used in equation (1-13) can be adopted by refemng the rate of model pollutant photoconversion to A, ., the external area of iiradiated catalyst,... 

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