Monolithic performance

For each of the inlet-velocity cases, plot the centerline mole fractions of CH4 and C02. Discuss how the monolith performance depends on flow velocity, for example, in terms of fuel slip. 

Operating Conditions Affecting the Imprinted Monolith Performance... 

Incorporation of the catalyst into the monolith during manufacture. This method always leads to poor utilization of the catalytic material some is buried within the crystalline matrix and is not accessible to reactants. As the catalytic material modifies the monolith s physical properties, either a limit has to be placed on the amount of catalytic material incorporated or some deterioration in monolith performance has to be accepted. This method of manufacture is reserved for the preparation of base-metal catalysts, particularly nickel and manganese catalysts. Care has to be taken to avoid the formation of spinels, etc., although in some cases the formation of a complex oxide structure is actively sought. 

Cera.micA.bla.tors, Several types of subliming or melting ceramic ablators have been used or considered for use in dielectric appHcations particularly with quartz or boron nitride [10043-11 -5] fiber reinforcements to form a nonconductive char. Fused siHca is available in both nonporous (optically transparent) and porous (sHp cast) forms. Ford Aerospace manufactures a 3D siHca-fiber-reinforced composite densified with coUoidal siHca (37). The material, designated AS-3DX, demonstrates improved mechanical toughness compared to monolithic ceramics. Other dielectric ceramic composites have been used with performance improvements over monolithic ceramics (see COMPOSITE MATERIALS, CERAMIC MATRIX). 

In the second procedure, calcium nitrate was replaced by calcium alkoxide (60). Calcium and sificon alkoxides have very different rates of hydrolysis. To avoid the production of inhomogeneities, a slow and controlled hydrolysis of a mixture of sificon, calcium, and phosphorous alkoxide was performed. The resulting materials were highly homogenous, and monolithic pieces could be produced. The bioactivity of the gel-derived materials is equivalent or greater than melt-derived glasses. 

Since NO production depends on the flame temperature and quantity of excess air, achieving required limits may not be possible through burner design alone. Therefore, many new designs incorporate DENOX units that employ catalytic methods to reduce the NO limit. Platinum-containing monolithic catalysts are used (36). Each catalyst performs optimally for a specific temperature range, and most of them work properly around 400°C. 

Sulfur oxides resulting from fuel sulfur combustion often inhibit catalyst performance in Regions II, III, and a portion of Region IV (see Fig. 7) depending on the precious metals employed in the catalyst and on the air/fuel ratio. Monolithic catalysts generally recover performance when lower sulfur gasoline is used so the inhibition is temporary. Pd is more susceptible than Rh or Pt. The last is the most resistant. Pd-containing catalysts located in hotter exhaust stream locations, ie, close to the exhaust manifold, function with Httie sulfur inhibition (72—74). 

ActivatedL yer Loss. Loss of the catalytic layer is the third method of deactivation. Attrition, erosion, or loss of adhesion and exfoHation of the active catalytic layer aU. result in loss of catalyst performance. The monolithic honeycomb catalyst is designed to be resistant to aU. of these mechanisms. There is some erosion of the inlet edge of the cells at the entrance to the monolithic honeycomb, but this loss is minor. The peUetted catalyst is more susceptible to attrition losses because the pellets in the catalytic bed mb against each other. Improvements in the design of the peUetted converter, the surface hardness of the peUets, and the depth of the active layer of the peUets also minimise loss of catalyst performance from attrition in that converter. 

Some OCs were of the monolithic honeycomb type, but all General Motors cars used peUetted OCs. For a period in the late 1970s and throughout the 1980s, both TWC and OC were used in a dual-bed catalyst. Oxygen needed for OC performance was provided by an engine driven air pump or a reed... 

Fig. 6. Catalyst inhibition mechanisms where ( ) are active catalyst sites the catalyst carrier and the catalytic support (a) masking of catalyst (b) poisoning of catalyst (c) thermal aging of catalyst and (d) attrition of ceramic oxide metal substrate monolith system, which causes the loss of active catalytic material resulting in less catalyst in the reactor unit and eventual loss in performance.

Another important constraint comes from the pressure drop across the catalytic bed, which must be kept to a minimum to avoid a loss in engine power and performance. This requirement is satisfied by a very shallow pellet bed of no more than ten pellets deep, a monolithic structure with many open parallel channels, or a pile of metallic screens and saddles. 

A sophisticated quantitative analysis of experimental data was performed by Voltz et al. (96). Their experiment was performed over commercially available platinum catalysts on pellets and monoliths, with temperatures and gaseous compositions simulating exhaust gases. They found that carbon monoxide, propylene, and nitric oxide all exhibit strong poisoning effects on all kinetic rates. Their data can be fitted by equations of the form ... 

There is a general trend toward structured packings and monoliths, particularly in demanding applications such as automotive catalytic converters. In principle, the steady-state performance of such reactors can be modeled using Equations (9.1) and (9.3). However, the parameter estimates in Figures 9.1 and 9.2 and Equations (9.6)-(9.7) were developed for random packings, and even the boundary condition of Equation (9.4) may be inappropriate for monoliths or structured packings. Also, at least for automotive catalytic converters. 

Hydrocortisone microspheres (108,109) and films (110) based on poly(lactic acid) have been investigated. A cage implant technique was used to study the performance of monolithic poly (DL-lactide) films loaded with hydrocortisone acetate (110). Films 1.5 x 0.6 cm were inserted into titanium wire-mesh cages 3.5 x 1.0 cm. The cages were implanted in the backs of rats and the inflammatory exudate was sampled periodically. The white cell concentration in the samples was lower than that of controls at all times during the 21-day test. 

Autocatalysts, based on monoliths, are probably the most extensively used catalytic reactors around a hundred million have been installed and are performing well in car exhaust systems [10-12]. Reduction of volatile organic carbon (VOC) emissions [13] and removal of NOj, from stationary sources [14, 15] are also... 

Metal monoliths show good thermal characteristics. A typical support with herringbone channels made from Fecralloy performed satisfactory in automotive applications [27]. Modeling showed that overall heat transfer was about 2 times higher than for conventional pellets [28,29]. Hence, there is potential for structured catalysts for gas-phase catalytic processes in multitubular reactors. 

Firstly, there are technical reasons concerning catalyst and reactor requirements. In the chemical industry, catalyst performance is critical. Compared to conventional catalysts, they are relatively expensive and catalyst production and standardization lag behind. In practice, a robust, proven catalyst is needed. For a specific application, an extended catalyst and washcoat development program is unavoidable, and in particular, for the fine chemistry in-house development is a burden. For coated systems, catalyst loading is low, making them unsuited for reactions occurring in the kinetic regime, which is particularly important for bulk chemistry and refineries. In that case, incorporated monolithic catalysts are the logical choice. Catalyst stability is crucial. It determines the amount of catalyst required for a batch process, the number of times the catalyst can be reused, and for a continuous process, the run time. 

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