Smallest catalyst volume

With hot cycle CO emissions, conversion is essentially 100% after the catalyst has warmed up, even at the smallest catalyst volume. Most probably, the small CO emissions during the hot cycles of the FTP are not affected by kinetic effects, but they are related to the transient nature of the vehicle operation during the FTP. An 02 deficiency could occur if the exhaust gas flows and air pump output were not properly matched. Small CO spikes were observed during vehicle accelerations that were superimposed on a very low, nearly zero, background CO emission level during the hot cycles. There also might be localized 02 deficiencies if the exhaust gases and air were not well mixed before they entered the converter. 

The performance of the platinum catalysts was distinctly different. CO conversions remained quite high, even at the smallest catalyst volume. HC emissions increased at the smallest converter size, perhaps because of methane oxidation limitations. CO conversions may have been limited by 02 availability, as indicated by CO spikes during vehicle accelerations. 

Fig. 6A, B and C show operating lines for the three types of reactors in methanol synthesis (confer Table 4, case 2). The situation is the same here. The internally cooled reactor gives the best approach to the optimum operating line and may as a consequence be designed for the smallest catalyst volume, whereas the quench cooled reactor requires the largest volume. It is not, however, possible to base a choice between the reactor types solely on the required catalyst volume. As indicated in Table 1, a number of other considerations must be taken into account.

Purely adiabatic fixed-bed reactors are used mainly for reactions with a small heat of reaction. Such reactions are primarily involved in gas purification, in which small amounts of noxious components are converted. The chambers used to remove NO, from power station flue gases, with a catalyst volume of more than 1000 m3, are the largest industrial adiabatic reactors, and the exhaust catalyst for internal combustion engines, with a catalyst volume of ca. 1 L, the smallest. Typical applications in the chemical industry include the methanation of traces of CO and CO2 in NH3 synthesis gas, as well as the hydrogenation of small amounts of unsaturated compounds in hydrocarbon streams. The latter case requires accurate monitoring and regulation when hydrogen is in excess, in order to prevent complete methanation due to an uncontrolled temperature runaway. 

The activities of fresh, supported platinum and base metal oxidation catalysts are evaluated in vehicle tests. Two catalysts of each type were tested by the 1975 FTP in four 600-4300 cm3 catalytic converters installed on a vehicle equipped with exhaust manifold air injection. As converter size decreased, base metal conversions of HC and CO decreased monotonically. In contrast, the platinum catalysts maintained very high 1975 FTP CO conversions (> 90% ) at all converter sizes HC conversions remained constant 70% ) at volumes down to 1300 cm3. Performance of the base metal catalysts with the 4300-cm3 converter nearly equalled that of the platinum catalysts. However, platinum catalysts have a reserve activity with very high conversions attained at the smallest converter volumes, which makes them more tolerant of thermal and contaminant degradation. 

The effect of a fourfold change in catalyst volume on catalyst deterioration is depicted in Figure 4. The smallest converter (1000 cm3) had a rapid initial loss in activity followed by a milder loss rate. The milder loss rate paralleled the rate of activity loss of the larger converters. The larger converter was clearly better for poison tolerance. 

It is seen that the operating lines for the internally cooled reactor is closest to the optimum reaction path and that the operating line for the quench reactor is the poorest approach to the optimum line- As a consequence, the required catalyst volume is normally smallest for the internally cooled reactor and largest for the quench cooled reactor. 

Increasing photocatalyst particle concentration is predicted to cause progressively sharper radial diminution of photon flux the competing trade-off between increased catalyst surface and decreased photon distribution gives rise to a predicted and experimentally observed maximum in the product yield versus catalyst solid fraction. The maximum predicted at 25x10" volume fraction or approximately 1.08x10" g/cm corresponds reasonably to the smallest solid volume fraction (0.1 wt%) [108] found to give complete absorbance in a reactor of comparable radial dimensions. This study is the... 

Important physical properties of catalysts include the particle size and shape, surface area, pore volume, pore size distribution, and strength to resist cmshing and abrasion. Measurements of catalyst physical properties (43) are routine and often automated. Pores with diameters <2.0 nm are called micropores those with diameters between 2.0 and 5.0 nm are called mesopores and those with diameters >5.0 nm are called macropores. Pore volumes and pore size distributions are measured by mercury penetration and by N2 adsorption. Mercury is forced into the pores under pressure entry into a pore is opposed by surface tension. For example, a pressure of about 71 MPa (700 atm) is required to fill a pore with a diameter of 10 nm. The amount of uptake as a function of pressure determines the pore size distribution of the larger pores (44). In complementary experiments, the sizes of the smallest pores (those 1 to 20 nm in diameter) are deterrnined by measurements characterizing desorption of N2 from the catalyst. The basis for the measurement is the capillary condensation that occurs in small pores at pressures less than the vapor pressure of the adsorbed nitrogen. The smaller the diameter of the pore, the greater the lowering of the vapor pressure of the Hquid in it. 

The smallest pores that can be observed using this approach depend on the highest pressure to which the mercury can be subjected in a particular piece of equipment. Volumes corresponding to pore radii as small as 100 to 200 A can be measured with commercially available equipment. Beyond this point the pressures required to fill up the capillaries with smaller radii become impractical for routine use. Unfortunately, there are many catalysts of industrial significance where these very small capillaries contribute substantially to the specific surface area. Special research grade mercury porosi-meters capable of measurements down to 15 A radii have been developed but, for routine measurements, the desorption approach described below is more suitable. 

It is well established that the smallest crystals are the most effective as catalysts as long as the catalytic reaction proceeds in the intercrystalline void volume [1,8]. Increased crystal size will result in an increase in pore length and thus in the Thiele modulus. This will result in a reduced effectiveness factor, viz. a reduced actual rate of reaction. 

The silica-magnesia catalysts, DA-5 and Nalco, in the virgin state, along with Davison silica gel have practically their entire area and pore volume contributed by the very smallest of pores that are encountered in catalyst structures that is, pores in the 10 to 15 A. radius range. It is apparent in Fig. 2 that for these materials there is no appreciable adsorption at the high relative pressures. This indicates the absence of large pores. One and one-half monolayers according to the BET theory effectively fill the pore volume of the DA-5 and the Davison silica gel, and only two monolayers are required for Nalco. Very little hysteresis is observed for any of these three materials. 

Nam, Eldridge and Kittrell studied the pore size distribution for vanadia/alumina catalysts for the removal of NOx by reaction with ammonia. The pore size distributions are found to change dramatically as sulfur poisons the de-NOx reaction. The smallest pores (<10 nm in radius) are found (by porosimetry) to be filled first. As a result the surface decreases by up to 90% with 12% sulfur content, although the pore volume decreased by only 20%. The associated de-NOx activity decreased substantially. It was proposed that ammonium sulfate, bisulfate, or aluminum sulfate formed on the surface to deactivate the catalyst. 

FTP Emissions. The overall system performance during the 1975 FTP tests for HC and CO emissions as a function of catalytic converter volume is plotted in Figures 1 and 2 respectively. Emissions are expressed in g/mile of vehicle operation with the cold start, hot stabilized, and hot cycle emissions weighted as prescribed (3). There was a distinct difference in the performance of the base metal and platinum catalysts with decreasing converter volume. Both HC and CO emissions with base metal catalysts increased monotonically as converter volume was decreased. In contrast, when platinum catalysts were used, both HC and CO emissions decreased to a minimum at 1300 cm3 and then increased at the smallest volume. 

The conversion of exhaust HC over platinum catalysts increased slowly (if at all) with decreasing converter volume, and then decreased rapidly at the smallest volume. The sharp increase in HC emissions at... 

The DHC deactivation is accompanied by coke production. The conversion degree depends more strongly on the coke content of the catalyst than BET surface area and pore volume do (Fig.2). The average pore radius even increases at low coke values (which could be interpreted by coke filling the smallest pores). It should be mentioned that these results could hardly be explained assuming that the decease of conversion is caused by increasing mass transport limitations. 

Figure S shows the influence of different carriers on polymer contents, lonexchange capacity and stability at low DVB content and at low dilution ratios. Stable catalysts can only be prepared with carriers of low and medium pore volume and with narrow or medium pores. Supports with narrow pores have limited polymer contents at fewer polymerization steps than those with wider pores. High polymer contents lead not alv< ys to high lonexchange capacities. Activatton by sulfonatlon may become masstransport limited. Only the core of the polymer contains sulfur. This was confirmed by microprobe measurements In a REM and with staining experiments using methyl-red as acid indicator. This effect Is most pronounced for the carrier with the smallest pores.

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