Catalyst mass

MgCl2-Supported Catalysts. Examination of polymerizations with TiCl catalysts has estabUshed that only a small percentage of titanium located on lateral faces, edges, and along crystal defects is active (52) (see Titanium and titanium alloys). This led to the recognition that much of the catalyst mass acted only as a support, promoting considerable activity aimed at finding a support for active titanium that would not be detrimental to polymer properties. 

The appHcations of supported metal sulfides are unique with respect to catalyst deactivation phenomena. The catalysts used for processing of petroleum residua accumulate massive amounts of deposits consisting of sulfides formed from the organometaHic constituents of the oil, principally nickel and vanadium (102). These, with coke, cover the catalyst surface and plug the pores. The catalysts are unusual in that they can function with masses of these deposits that are sometimes even more than the mass of the original fresh catalyst. Mass transport is important, as the deposits are typically formed... 

Although they are termed homogeneous, most industrial gas-phase reactions take place in contact with solids, either the vessel wall or particles as heat carriers or catalysts. With catalysts, mass diffusional resistances are present with inert solids, the only complication is with heat transfer. A few of the reactions in Table 23-1 are gas-phase type, mostly catalytic. Usually a system of industrial interest is liquefiea to take advantage of the higher rates of liquid reactions, or to utihze liquid homogeneous cat ysts, or simply to keep equipment size down. In this section, some important noncatalytic gas reactions are described. 

Over the next four years, Houdry, working closely with Sun s engineering team headed by Clarence Thayer, worked to build a commercial plant. The limitations imposed by a static catalyst bed design imposed a major obstacle, particularly in the formation of carbon deposits that fouled the catalyst mass and impeded a continuous system of production. 

Example 10.10 Suppose the reaction in Example 10.9 is first order. Determine the pseudohomogeneous rate constant, the rate constant based on catalyst mass, and the rate constant based on catalyst surface area. 

The reactant concentration per unit mass is used for the rate based on catalyst mass ... 

Hints First convince yourself that there is an optimal solution by considering the limiting cases of ij near zero, where a large hole can almost double the catalyst activity, and of ij near 1, where any hole hurts because it removes catalyst mass. Then convert Equation (10.33) to the form appropriate to an infinitely long cylinder. Brush up on your Bessel functions or trust your S5anbolic manipulator if you go for an anal5dical solution. Figuring out how to best display the results is part of the problem. 

Refers to reaction rate based on catalyst mass... 

Ethylene hydrogenation was carried out in a once-through flow reactor. The effluent gas mixture was analyzed with an online gas chromatograph (Hewlett-Packard HP 6890) equipped with an AI2O3 capillary column and a flame ionization detector. Testing conditions included Phydrogen = 200 Torr, Pethyiene = 40 Torr, catalyst mass of 10 to 20 mg and temperature varied from -50 to -25°C. 

The catalytic reaction was carried out at 270°C and 101.3 kPa in a stainless steel tubular fixed-bed reactor. The premixed reaction solution, with a molar ratio catechol. methanol water of 1 1 6, was fed into the reactor using a micro-feed pump. To change the residence time in the reactor, the catechol molar inlet flow (Fio) and the catalyst mass (met) were varied in the range 10 < Fio <10 mol-h and 2-10 < met < 310 kg. The products were condensed at the reactor outlet and collected for analysis. The products distribution was determined quantitatively by HPLC (column Nucleosil 5Ci8, flow rate, 1 ml-min, operating pressure, 18 MPa, mobile phase, CH3CN H2O =1 9 molar ratio). 

The ethylene selectivity (Fig. 5) and thus the ethylene yield depend strongly on the adsorbent mass (Fig. 5). For fixed catalyst mass, oxygen supply I/2F and methane conversion there is an optimal amount of adsorbent for maximizing ethylene selectivity and yield (Fig. 5). Excessive amounts of adsorbent cause quantitative trapping of ethane and thus a decrease in ethylene yield according to the above reaction network. This shows the important synergy between the catalytic and adsorbent units which significantly affects the product distribution and yield. 

E. Crezee, B.W. Hoffer, R.J. Berger, M. Makkee, F. Kapteijn and J.A. Motrlijrt, Three-phase hydrogerration of D-glucose over a carbon supported mtherrirrm catalyst - mass transfer and kinetics, Applied Catalysis A General 251 (2003) 1. 

We take the results of a series of experiments conducted by Morgan (1967, his Fig. 23) at pH 9, 9.3, and 9.5. He used an initial Mn11 concentration of 4.5 x 10-4 molar, a carbonate concentration of 1.6 x 10-3 molar, and an oxygen partial pressure of 1 atm. We can figure an approximate value for the rate constant l<+ from oxidation rate at the end of the experiment, when the mass of catalyst is known from the depletion of Mn11, then estimate the initial catalyst mass from that value and the oxidation rate at the onset of reaction. Running the simulation, we can refine the two numbers until prediction matches observation. 

Here, we take the initial catalyst mass as 2 mg, and set a rate constant of 3 x 1012 molal-4 s-1, which gives good results. As shown in Figure 28.2, the reaction starts slowly, increases in rate as the catalyst accumulates, then decreases to zero as the supply of reduced manganese is depleted. 

In this case, we find we can match the experimental results using a single value for the rate constant, but an initial catalyst mass that depends on pH, as shown in Figure 28.3. Here, we take the rate constant to be 1013 molal-4 s-1, and use initial catalyst masses of 6 mg at pH 9.5, 5 mg at pH 9.3, and 0.6 mg at pH 9. This form of the rate law seems more satisfactory than the previous form, since we might reasonably expect the amount of catalyst, perhaps an MnC03 colloid, initially present to be greatest in the experiments conducted at highest pH, but the rate constant to be invariant. 

Fig. 28.3. Concentration of Mn11 versus time in simulations of the autocatalytic oxidation of manganese at pH 9.0, 9.3, and 9.5, at 25 °C, compared to results of laboratory experiments (symbols) by Morgan (1967). Simulations made assuming rate law of a form carrying mMa++, rather than mMnn. Rate constant in the simulations is taken to be 1013 molal-4 s-1, and the initial catalyst mass is 0.6 mg (pH 9.0), 5 mg (9.3), and 6 mg (9.5).

The rates (and rate constants) can be expressed on the basis of catalyst mass (e.g., mol kg-1h 1), or of catalyst surface area (e.g., mol m-2 s-1), or as a turnover frequency (molecules site-1 s-1), if a method to count the sites exists. 

Care must be taken to specify properly the basis for the reaction rate. The most useful basis for design of an FBCR is the catalyst mass, that is, the rate is (—rA)m, in units of, say, mol (kg cat)-1 s-1. ( -rA)m is related to the rate per unit reactor (or bed) volume, (—rA)v through the bed density ... 

Heterogeneously catalyzed hydrogenation is a three-phase gas-liquid-solid reaction. Hydrogen from the gas phase dissolves in the liquid phase and reacts with the substrate on the external and internal surfaces of the solid catalyst Mass transfer can influence the observed reaction rate, depending on the rate of the surface reaction [15]. Three mass transfer resistances may be present in this system (Fig. 42.1) ... 

Fig. 1. Relationship between catalyst temperature and reaction time in methane partial oxidation catalyzed by Ni/Si02 (temperature of the gas phase (a) 1019 K, (b) 899 K, (c) 809 K, (d) 625 K). The reaction was carried out in a fixed-bed reactor (a quartz tube of 2 mm inside diameter) at atmospheric pressure. Before reaction, the feed gas was allowed to flow through the catalyst undergoing heating of the reactor from room temperature to 1073 K at a rate of 25 K min-1 to ignite the reaction, and then the reactant gas temperature was decreased to the selected value. Reaction conditions pressure, 1 atm catalyst mass, 0.04 g feed gas molar ratio, CH4/O2 = 2/1 GHSV, 90,000 mL (g catalyst)-1 h-1) (25).

Fig. 5. Methane conversion and oxygen flux during partial oxidation of methane in a ceramic membrane reactor. Reaction conditions pressure, 1 atm temperature, 1173 K, feed gas molar ratio, CH Ar = 80/20 feed flow rate, 20 mL min-1 (NTP) catalyst mass, 1.5 g membrane surface area, 8.4 cm2 (57).

A catalyst mass is made up of a mixture of porous spheres with a range of diameters. The average diameters of 10% cuts are shown. A second order reaction with k = 2.5 and C0 = 1.2 is performed in a PFR. The Thiele modulus... 

Sample Catalyst mass for two platelets (mg) % CeOz % Pt Rate at 260 °C based on Pt and assuming 0.58 dispersion... 

Fig. 3.4 Modification of starch by butadiene telomerization. Influence of the catalyst mass and temperature on the degree of substitution (DS).

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