Catalyst contacting

Sulphur trioxide, SO3, m.p. 17 C, b.p. 49 C. Formed SO2 plus O2 over a catalyst (contact process - see sulphuric acid). The solid exists... 

Heat-flow calorimetry may be used also to detect the surface modifications which occur very frequently when a freshly prepared catalyst contacts the reaction mixture. Reduction of titanium oxide at 450°C by carbon monoxide for 15 hr, for instance, enhances the catalytic activity of the solid for the oxidation of carbon monoxide at 450°C (84) and creates very active sites with respect to oxygen. The differential heats of adsorption of oxygen at 450°C on the surface of reduced titanium dioxide (anatase) have been measured with a high-temperature Calvet calorimeter (67). The results of two separate experiments on different samples are presented on Fig. 34 in order to show the reproducibility of the determination of differential heats and of the sample preparation. 

The feed consists of isobutylene, fresh methanol, and recycled methanol. The isobutylene comes mixed with other C4 s (normal butylenes, iso-, and normal butane). As in Figure 13 1, the feed is charged to a fixed bed reactor and passes through the catalyst bed, indicated by the X. The solid catalyst, an acidic ion-exchange resin, sits loosely in the vessel to allow easy passage but intimate feed/catalyst contact. The combination of only moderate temperatures and the catalyst promotes the reaction between the methanol and the isobutylene. The reaction takes place at 120-200°F and 300 psi. It is slightly exothermic, and heat needs to be removed to keep the temperature below 210 F, or by-products will abound. About 90% of the isobutylene converts to MTBE in this reactor. 

In addition to using different catalyst flow patterns in packed and slurry reactors, the flow can be varied to attain different catalyst contacting patterns. As shown in Figure 7-27, many flow patterns such as radial flow and fluid recirculation can be used. These allow variation of the flow velocity u for a given reactor size and residence time x. These recirculation flow patterns approach the flow of recycle reactors so the reactor performance approaches that of a CSTR at high recirculation. 

Catalyst mass flowrates exceeding about 1600 Ib/ft -min (7800kg/m -min) results in poor steam/catalyst contacting, flooded trays, insufficient catalyst residence time, and increased steam entrainment to the spent catalyst standpipe. This is reflected by the stripper efficiency and catalyst density shown in Figure 7.10. The primary concern is hydrocarbon entrainment to the regenerator leading to loss of product, increased catalyst deactivation, increased delta coke and potential loss of conversion and total liquid yield, and feed rate limitation. A rapid decrease in stripper bed density is an indication that... 

Paraffin conversion to naphthenes is very unfavorable (last column of Table IV). For paraffins to be converted to naphthenes by ring closure, naphthenes must be at very low concentrations. If appreciable naphthenes exist, such as at short catalyst contact times, naphthene ring opening to paraffins can occur. Again, equilibria improve with carbon number. Eight-and nine-carbon paraffins behave quite similarly. 

This complex system would be difficult to solve directly. However, the problem is separable by taking advantage of the widely different time scales of conversion and deactivation. For example, typical catalyst contact times for the conversion processes are on the order of seconds, whereas the time on stream for deactivation is on the order of days. [Note Catalyst contact time is defined as the volume of catalyst divided by the total volumetric flow in the reactor at unit conditions, PV/FRT. Catalyst volume here includes the voids and is defined as WJpp — e)]. Therefore, in the scale of catalyst contact time, a is constant and Eq. (1) becomes an ordinary differential equation ... 

Equation (16) can then be rearranged to be linear in the catalyst contact time parameters ... 

An isothermal, plug flow, fixed bed reforming pilot plant (shown in Fig. 14) was used to generate the kinetic data. The reactor was U shaped and contained roughly 70 ml of catalyst. Five sample taps were spaced along the reactor length to determine compositions over a wide range of catalyst contact times. The reactor assembly was immersed in a fluidized sand bath to maintain isothermal conditions. 

With respect to catalyst contact time, the effects of temperature and pressure on the yields are shown in Figs. 18, 19, and 20. Activity (as measured by the C5- gas make) is a strong function of temperature, as shown in Figs. 18 and 19. Again, the higher-temperature operation favors benzene formation. KINPTR s prediction of activity as a function of pressure is shown in Fig. 20. Lower-pressure operation favors the yield of benzene. 

Based on Voorhies time-on-stream theory (4), catalytic coke is a function of catalyst contact time ... 

The coke deactivation exponent n, is typically estimated from riser pilot plant experiments at varying catalyst contact time for different catalyst types. A value of n of 0.2 was found for REY catalyst data base. For USY and RE-USY catalysts n was estimated to be 0.4. 

The first oil-catalyst contact is essential. The mixing temperature (about 530-600 °C) is very difficult to measure and is about 20-80 °C higher than the riser exit temperature (4, J). The superficial gas velocity at the inlet is several meters per second, much higher than the terminal velocity of the catalyst, which is about 0.20 m/s. The catalyst and the gas are transported upwards together but at a different superficial velocity, u (6). These two velocities are related by the slip velocity (sv), defined as follows ... 

A fixed bed reactor described by ASTM Method No. D3907 was employed for catalytic testing. A sour, imported heavy gas oil with properties described in Table II was used as the feedstock. Experiments were carried out at a reactor temperature of 800°K and catalyst residence time (9) of 30 seconds. Liquid and gaseous products were analyzed with gas chromatographs. Carbonaceous deposit on the catalyst was analyzed by Carbon Determinator WR-12 (Leco Corp., St. Joseph, MI). The Weight Hourly Space Velocity (WHSV) was varied at constant catalyst contact time to generate selectivity data of various products as a function of conversion. For certain experiments, conversion was also varied by varying the catalyst pretreatment conditions. 

Pilot plants are often used for studying the FCC process. In order for results to be meaningful, it is necessary that pilot plant operation be consistent with that of commercial units. When processing feedstocks containing residue, this becomes even more important. Failure to pay attention to details, such as feed/catalyst contacting, can lead to problems with data integrity and with coke buildup in the equipment. 

Reactants and catalyst must be contacted thus, a high external surface area of the catalyst (smaller particle size) is desirable to maximize reaction rate. The particle size of the catalyst must be optimized to permit filterability for ease of recycle while maintaining the high external surface area needed for maximum reactant-catalyst contacting. 

At typical catalyst temperatures of 800°C to 940°C, nitric oxide (NO) is thermodynamically unstable and slowly decomposes into nitrogen and oxygen. Decomposition losses are minimized by avoiding excessive catalyst contact time and by rapidly cooling the gases as they exit the converter. To achieve ammonia conversions of 93% to 98% the catalyst contact time must be as short as 0.0010 to 0.0001 seconds104. 

Figure 6. Variation of the normalized hydrocarbon product distribution as a function of catalyst contact time at 350 °C at a fixed [H2OMC2H2] of 0.6 and added He to vary the total VHSV.

Tracer methods proposed by Schwartz et al. (19) and Colombo et al. (21) were used to determine total and external catalyst contacting efficiency. These techniques have been described elsewhere (22). Total contacting efficiency, r)c defined as the fraction of total (external and internal) catalyst area contacted by liquid can be obtained by ... 

Dynamic tracer tests can be used to determine dynamic holdup and catalyst contacting which in trickle-flow regime can be correlated with Reynolds and Gallileo number. A simple reactor model for gas limiting reactant when matched to experimental results for one solvent and one catalyst activity predicts reactor performance well for different catalyst activities and in other solvents over a wide range of liquid velocities. 

The hydrodenitrogenation activity for both, mono- as well as bidispersed, decreased significantly within 100-150 hr of oil-catalyst contact time. Carbonaceous depositions seem to be the primary cause for catalyst activity decay. 

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