Catalyst void volume

The retention time of the non-adsorbing methane (ti) is the measure of the column void volume or holdup. Ethylene is adsorbed by the catalyst, hence it does not reach the detector until the available surface is saturated, at which point ethylene breaks through and is detected by the sensor (t2). The adsorbed volume of ethylene is given simply by ... 

This section discusses the techniques used to characterize the physical properties of solid catalysts. In industrial practice, the chemical engineer who anticipates the use of these catalysts in developing new or improved processes must effectively combine theoretical models, physical measurements, and empirical information on the behavior of catalysts manufactured in similar ways in order to be able to predict how these materials will behave. The complex models are beyond the scope of this text, but the principles involved are readily illustrated by the simplest model. This model requires the specific surface area, the void volume per gram, and the gross geometric properties of the catalyst pellet as input. 

Illustration 6.2 indicates how void volume and surface area measurements can be combined in order to evaluate the parameters involved in the simplest model of catalyst pore structure. 

To develop analytical models for processes employing porous catalysts it is necessary to make certain assumptions about the geometry of the catalyst pores. A variety of assumptions are possible, and Thomas and Thomas (15) have discussed some of these. The simplest model assumes that the pores are cylindrical and are not interconnected. Develop expressions for the average pore radius (r), the average pore length (L), and the number of pores per particle (np) in terms of parameters that can be measured in the laboratory [i.e., the apparent particle dimensions, the void volume per gram (Vg), and the surface area per gram (Sg). ... 

We will start by developing an expression for the average pore radius T. If we denote the mass of an individual catalyst particle by raP, simple geometric considerations indicate that the void volume per particle is given by... 

The reactor consists of a cylindrical tube 59 cm long packed with pumice stone. The catalyst charge was constant in all of the runs below and equal to W. The reactor void volume was approximately 100 cm3. The reaction is believed to be first-order in ethylene chloride under the conditions of this study and in all cases the feedstock was pure reactant. Both the reactants and the products are gases at the conditions involved. Ideal gases may be assumed. The following data were reported at 600°C and one atmosphere. 

Vq is the void volume per gram of catalyst Sq is the surface area per gram of catalyst Vp is the gross volume of the catalyst particle Sx is the gross exterior surface area of the particle ... 

Pseudo homogeneous models of fixed bed reactors are widely employed in reactor design calculations. Such models assume that the fluid within the volume element associated with a single catalyst pellet or group of pellets can be characterized by a given bulk temperature, pressure, and composition and that these quantities vary continuously with position in the reactor. In most industrial scale equipment, the reactor volume is so large compared to the volume of an individual pellet and the fraction of the void volume associated therewith that the assumption of continuity is reasonable. 

The ortho-para conversion of molecular hydrogen is catalyzed by NiO. A supported catalyst is available with a specific surface area of 305 m2/g and a void volume of 0.484 cm3/g. A spherical catalyst pellet has an apparent density of 1.33 g/cm3 and a diameter of 0.5 cm. If the system is not far from equilibrium, an apparent first-order rate constant (kr) can be defined in the following manner. 

A catalyst for cracking cumene is available commercially in the form of 0.35 cm diameter pellets. These pellets have a specific surface area of 420 m2/g and a void volume of 0.42 cm3/g. If the apparent first-order rate constant for this reaction is 1.49 cm3/sec-g catalyst at 412 °C, determine the effectiveness factor of the catalyst. 

V void volume per gram of catalyst y activity coefficient... 

Void volume of reactor 1 liter Weight of catalyst used 3 gm... 

There are two design options when considering an SCR unit for the FCCU upstream or downstream of an ESP or TSS. If the SCR unit is placed upstream of an ESP or TSS, then the refiner has to incorporate soot blowers for catalyst fines removal from the catalyst surface and use a wider pitch catalyst to handle the higher levels of catalyst particulates. The wider pitch catalyst contains more void volume and thus will directionally increase the catalyst bed dimensions since the NO reduction is based on the total amount of surface area, not just catalyst volume. 

Note Typically in reformer design, liquid hourly space velocity (LHSV) is defined as fresh liquid charge volumetric flow rate divided by catalyst volume. Catalyst volume includes the void fraction and is defined by WJpp( — e).]... 

In a reactor completely filled with liquid, the wetting efficiency is 100% or, in other words, the external wetting of the catalyst is complete (Burghardt et al., 1995). While it is true that when a fixed bed is completely filled with liquid wetting is complete (wetting efficiency is unity), the opposite is not true in a trickle bed, a portion of the bed voids will be always occupied by the gas phase. Thus, while in a well-operated trickle bed the wetting efficiency could be unity, its total liquid holdup based on the void volume is always lower than the bed voidage, i.e. the bed is never completely filled with liquid. 

Clearly by working with typical spatial resolutions of approximately 30-50 pm, individual pores within the material are not resolved. However, a wealth of information can be obtained even at this lower resolution (53,54,55). Typical data are shown in Fig. 20, which includes images or maps of spin density, nuclear spin-lattice relaxation time (Ti), and self-diffusivity of water within a porous catalyst support pellet. In-plane spatial resolution is 45 pm x 45 pm, and the image slice thickness is 0.3 mm. The spin-density map is a quantitative measure of the amount of water present within the porous pellet (i.e., it is a spatially resolved map of void volume). Estimates of overall pellet void volume obtained from the MR data agree to within 5% with those obtained by gravimetric analysis. 

There was yet another possibility that the enhancement could be due to the fact that some stable products formed on the catalyst wafer, upon desorption into the void volume, underwent further sequential reaction with propane. If so, the enhancement would not require the immediate adjacency of the catalyst wafer and the void volume and should be observable when the catalyst and the void volume were physically separated. Such separation, however, would quench any desorbed reactive intermediates. This was tested. The wafer was separated from the void volume, and the two were separately heated to the appropriate temperatures. The result was that only a small enhancement was observed in the separated mode. This confirmed that the enhancement was due not to sequential reaction of stable products but to desorption of reactive intermediates from the catalyst surface. The small enhancement could be attributed to the higher temperature throughout the separately heated void volume in the separated mode than in the other mode. 

Flow through granular and packed beds occurs in reactors with solid catalysts, adsorbers, ion exchangers, filters, and mass transfer equipment. The particles may be more or less rounded or may be shaped into rings, saddles, or other structures that provide a desirable ratio of surface and void volume. 

Two cases are considered an adiabatic reactor with no catalyst and an adiabatic reactor with catalyst. The void volume of the catalyst is 0.5, so the total volume of the reactor with catalyst must be twice as large as the volume of the reactor without catalyst. The reactor length is increased to 20 m in this case. The density of the solid catalyst is 2000 kg/m3, so the total amount of catalyst in the reactor is... 

Raman spectroscopy is characterized by lower sensitivity than IR spectroscopy, but in contrast to IR spectroscopy, Raman spectroscopy may be used to investigate catalysts under supercritical conditions of C02 or H20, because there are no strong absorptions by these molecules that interfere with the absorptions by the catalyst as is the case in IR spectroscopy. Griinwaldt et al. (2003) reviewed cell designs for spectroscopic experiments under supercritical conditions that either feature a window (lens) to focus the laser beam inside the cell, or fiber optics that are directly inserted into the cell (Howdle et al., 1994 Poliakoff et al., 1995). In some cases, several techniques may be combined (Addleman et al., 1998 Hoffmann et al., 2000). Such cells are designed with minimal void volume so that reliable kinetics and time-resolved analyses can be performed. 

Various methods for estimating KLs are described by Satterfield.150 The most conservative estimate of KLS is obtained as KI S = D/<5,, where D is the molecular diffusivity of the reactant in the liquid phase and <5L the average thickness of liquid film surrounding the particles. This estimation assumes no turbulence in the liquid film. The average thickness of the liquid film can be obtained from a knowledge of the dynamic liquid holdup and the outside area of catalyst particles per unit volume of the reactor, os. For example, if the dynamic liquid holdup is 50 percent of the void volume e, then <5L = e/2as. Various methods for estimating fcL and Ks under trickle-flow conditions are described in Chap. 6. 

Now consider a catalyst pellet with a random network of zig-zag pores. The surface of the pellet is composed of both solid material and pores. The flux equation derived earlier must be modified to account for the fact that the flux, N, is based only on the area of a pore. A parameter called the porosity of the pellet, or Bp, is defined as the ratio of void volume within the pellet to the total pellet volume (void -f solid). The flux can be expressed in moles of A diffusing per unit pellet surface area (containing both solids and pores) by using 8 as... 

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