Low space velocity

Molybdenum and tungsten are unique in that they are resistant to sulfur, and, in fact, are commonly sulfided before use. The Bureau of Mines tested a variety of molybdenum catalysts (32). They are moderately active but relatively high temperatures are required in order to achieve good conversion, even at low space velocities. Selectivity to methane was 79-94%. Activity is considerably less than that of nickel. Although they are active with sulfur-bearing synthesis gas, the molybdenum and tungsten catalysts are not sufficiently advanced to be considered candidates for commercial use. 

In Fig. 28, the abscissa kt is the product of the reaction rate constant and the reactor residence time, which is proportional to the reciprocal of the space velocity. The parameter k co is the product of the CO inhibition parameter and inlet concentration. Since k is approximately 5 at 600°F these three curves represent c = 1, 2, and 4%. The conversion for a first-order kinetics is independent of the inlet concentration, but the conversion for the kinetics of Eq. (48) is highly dependent on inlet concentration. As the space velocity increases, kt decreases in a reciprocal manner and the conversion for a first-order reaction gradually declines. For the kinetics of Eq. (48), the conversion is 100% at low space velocities, and does not vary as the space velocity is increased until a threshold is reached with precipitous conversion decline. The conversion for the same kinetics in a stirred tank reactor is shown in Fig. 29. For the kinetics of Eq. (48), multiple solutions may be encountered when the inlet concentration is sufficiently high. Given two reactors of the same volume, and given the same kinetics and inlet concentrations, the conversions are compared in Fig. 30. The piston flow reactor has an advantage over the stirred tank... 

This data also suggests that operating at 100% CO conversion could only provide transient data, and provide no information on the deactivation of the catalyst. As Figure 1 shows, the catalyst has a high enough activity at the low space velocity of... 

In this equation, B stands for adsorption coefficients and C for concentrations. The thermodynamic control imposes the use of very high pressures, low space velocities, and very active catalysts. For the specific case of aromatic saturation and in the presence of H2S or any other sulfur compound, NiW is the recommended catalyst [66], However, in those cases where a precious metal catalyst may be used then, it becomes the preferred choice [67],... 

Fig. 8. Stabilization of the value z for ethyl alcohol and acetaldehyde on Cu, by shortening the length l of the catalyst bed. Full curve, O, low-space velocity dashed curve, X, two high space velocities.

For low space velocities of synthesis gas, the products from nitrided catalysts in fixed-bed reactors are somewhat similar to those of the Synol process employing high space velocities and low temperatures. Possibly the yields of alcohols and other oxygenated molecules obtained from nitrided catalysts under the conditions employed in the Synol process would be even higher than those obtained from nitrides at low space velocities. 

Despite intensive works in this field, the most promising system, Cu-MFI, is subject to inhibition by water, very sensitive to poisoning by S02, efficient only at low space velocities, and the catalyst activity and selectivity are not satisfactory (49). 

In contrast to the above-mentioned variables, the dependence of the converter performance on the H2/N2 ratio shows a true maximum (Fig. 81). The optimum conversion at high space velocity [SV = m3 (STP) gas h l- nrf3 catalyst] lies close to an H2/N2 ratio of 2 and approaches 3 at low space velocities. The reason is that equilibrium plays a greater role at low space velocities and has a maximum at a ratio of 3, except for small corrections [33] with regard to the behavior of real gases. Usually, the ratio is adjusted to 3, because in most plants, conversions near equilibrium are attained. 

Axial Molecular Diffusion. Molecular diffusion can be an important cause of a spread in residence time, particularly at low velocities (low space velocities and short bed lengths) and with fluids of high diffusivity. Therefore, it is important in microreactors with gas flow. 

Summary The synthesis of trichlorosilane (TCS) from silicon and HCl produces considerable amounts of less desired chlorosilanes by side reactions, especially silicon tetrachloride (STC). The results of this paper support the view that the undesired STC is formed from TCS in a consecutive reaction, which is probably catalyzed by Si impurities and which is preferred at low space velocity and high temperatures. It seems that TCS selectivity losses are due to regions or spots in the industrial reactor with such conditions. XPS surface concentrations of Si impurities dramatically change with the proceeding synthesis reaction because of the mobility of the impurity species and do not correlate with results of Si bulk analysis. 

Naphthalene conversion and 2MN selectivity (S2MN) dependence on space velocity was studied at 360 °C, using H-ZSM-11. The conversion increases at low space velocity but the S2MN decreases (producing as by product 2,6 di-methyl naphthalene), see Fig. 6. 

Because all heat dissipated in the gas must eventually be conducted to the walls, the maintenance of a low temperature in the discharge requires a narrow gap (walls close together), low currents, and low space velocities (low specific rates of O3 production). These requirements are opposite to the conditions which would be suggested by the known sensitivity of ozone to wall catalysis and the enhanced rate of destruction of such carriers as 0, 0 , and at walls. All of these contribute to low efficiency O3 production. To the extent that heat is not successfully dissipated, the mean temperature rises and the thermal decomposition of O3 becomes increasingly important. 

During experiments at low space velocity (22,000 h-1), no deactivation was detected up to 12 h on stream. We only observed an inverse first order kinetic dependence on H2O concentration (0 to 10% in the feed) between 673 and 923 K. Indicating that H2O acted only as a competitor against the reactants. 

Yokoyama and Misono [78] concluded from their studies that Ce-exchanged ZSM-5 was more active than Cu-ZSM-5 at 573 K and at a low space velocity of 10000 h. ... 

NO2 reduction in gas phase has been studied in the absence of catalyst in a small test device equipped with on-line mass spectrometer (Fig. 5). The reduction occurs at low temperature but at very low space velocity. The maximum conversion rate is about 50%. At higher temperature ethylene is oxidised by O2 which accounts for the decrease of NO2 reduction rate. As NO2 reduction rate is 50 % at maximum at GHSV of 6 000 h, it must be very lower at 50 000 h and could not significantly contribute to the elimination of NO2 in the gas phase. 

Low space velocity High temperature High pressure... 

The hydrogenation of benzene to cyclohexane has received considerable attention in view of its importance as a raw material for the manufacture of caprolactam. Supported nickel and platinum catalysts are widely used for the hydrogenation of benzene. Ni/A Os or Ni/Si02 catalysts are often operated at a low temperature around 130 °C and at a very low space velocity of 0.2 LHSV whereas Pt/AUOs catalysts are operated at a higher temperature, 250 C and at a higher space velocity, 1.0- 1.5 LHSV in commercially operating plants. 

Interestingly, mass spectroscopic analyses on the bottoms product over a range of conversions in their pilot plant showed that low space velocities, and therefore low reactor temperatures, consistently favored the formation of two of the high VI components, the isoparaffins (VI approximately 155) and the monocycloparaffins (VI approximately 142) (Figure 7.20). 

These two curves are similar for all types of carbon monoxide hydrogenation catalysts. They are important for explaining the different views concerning the optimum operation conditions for the synthesis. If the price of the synthesis gas determines the suitability of the process, working at comparatively low space velocities will be inevitable. If, however, the capital costs of reactor and catalyst are the more important items, it will be more favorable to work at high space velocities, if the problem of the removal of the exothermic reaction heat can be solved,... 

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