Catalyst beds making

Coke profiles exist in the 150 mm long catalyst bed, making coke a function of the axial position inside the reactor and creating the possibility of channelling,... 

Heat Release and Reactor Stability. Highly exothermic reactions, such as with phthaHc anhydride manufacture or Fischer-Tropsch synthesis, compounded with the low thermal conductivity of catalyst peUets, make fixed-bed reactors vulnerable to temperature excursions and mnaways. The larger fixed-bed reactors are more difficult to control and thus may limit the reactions to jacketed bundles of tubes with diameters under - 5 cm. The concerns may even be sufficiently large to favor the more complex but back-mixed slurry reactors. 

The size of the catalyst bed depends mainly on the degree of VOC reduction requited (14). VOC destmction efficiencies up to 95% can usually be attained using reasonable space velocities (14). However, the low GHSVs, and subsequently high catalyst volumes requited to achieve extremely high (eg, 99%) conversions, can sometimes make catalytic oxidation uneconomical. Conventional bed geometries may be found in the Hterature (14). 

Catalyst Deactivation. Catalyst deactivation (45) by halogen degradation is a very difficult problem particularly for platinum (PGM) catalysts, which make up about 75% of the catalysts used for VOC destmction (10). The problem may weU He with the catalyst carrier or washcoat. Alumina, for example, a common washcoat, can react with a chlorinated hydrocarbon in a gas stream to form aluminum chloride which can then interact with the metal. Fluid-bed reactors have been used to offset catalyst deactivation but these are large and cosdy (45). 

A schematic flow diagram of this process is presented in Figure 12 which depicts three catalyst beds operating in series. Fresh make-up gas is mixed with total recycle and fed to the first bed. The effluent from the first bed is partially cooled so that when it is combined with another portion of cold make-up, the mixture is at or above the initiating reaction temperature. The mixture is fed to the second bed. This procedure is continued to the third and subsequent beds. 

In the case of a catalytic membrane reactor (CMR), the membrane is (made) intrinsically catalytically active. This can be done by using the intrinsic catalytic properties of the zeolite or by making the membrane catalytically active. When an active phase is deposited on top of a membrane layer, this is also called a CMR because this becomes part of the composite membrane. In addition to the catalytic activity of the membrane, a catalyst bed can be present (PBCMR). The advantages of a CMR are as follows ... 

Fixed Bed Reactors. In its most basic form, a fixed bed reactor consists of a cylindrical tube filled with catalyst pellets. Reactants flow through the catalyst bed and are converted into products. Fixed bed reactors are often referred to as packed bed reactors. They may be regarded as the workhorse of the chemical industry with respect to the number of reactors employed and the economic value of the materials produced. Ammonia synthesis, sulfuric acid production (by oxidation of S02 to S03), and nitric acid production (by ammonia oxidation) are only a few of the extremely high tonnage processes that make extensive use of various forms of packed bed reactors. 

The packing itself may consist of spherical, cylindrical, or randomly shaped pellets, wire screens or gauzes, crushed particles, or a variety of other physical configurations. The particles usually are 0.25 to 1.0 cm in diameter. The structure of the catalyst pellets is such that the internal surface area far exceeds the superficial (external) surface area, so that the contact area is, in principle, independent of pellet size. To make effective use of the internal surface area, one must use a pellet size that minimizes diffusional resistance within the catalyst pellet but that also gives rise to an appropriate pressure drop across the catalyst bed. Some considerations which are important in the handling and use of catalysts for fixed bed operation in industrial situations are discussed in the Catalyst Handbook (1). 

CATOFIN [CATalytic OleFIN] A version of the Houdry process for converting mixtures of C3 - C5 saturated hydrocarbons into olefins by catalytic dehydrogenation. The catalyst is chromia on alumina in a fixed bed. Developed by Air Products Chemicals owned by United Catalysts, which makes the catalyst, and licensed through ABB Lummus Crest. Nineteen plants were operating worldwide in 1991. In 1994, seven units were used for converting isobutane to isobutylene for making methyl /-butyl ether for use as a gasoline additive. 

From the top of the column, the propane that always accompanies the propylene feed emerges, talcing with it some of the benzene. In a flash tank, the propane is vented, and liquid benzene is recycled. From the bottom of the catalytic distillation column come the cumene, the PIPB, and some miscellaneous heavies chat are separated in a fractionator to make cumene, of 99.9% purity. The PIPB is separated in another column and fed to a second reactor with another zeolyte catalyst bed. In there the PIPB reacts catalyti-... 

Figure 9—2 shows the plant with its three reactors. The pyrolysis furnace is in the middle. At the top of the figure, the basic feeds, to the plant are shown—ethylene, chlorine, and oxygen. Ethylene and chlorine alone are sufficient to make EDC via the route on the left. The operation, call it Reaction One like Figure 9-1 does, takes place in the vapor phase in a reactor with a fixed catalyst bed of ferric (iron) chloride at only 100—125°F. A cleanup column fractionates out the small amount of by-products that get formed, leaving an EDC stream of 96—98% purity. 

The above reaction can be carried out in the presence of a variety of catalysts including Ni, Cu/Zn, Cu/SiO, Pd/SiO, and Pd/ZnO. In the case of coal, it is first pulverized and cleaned, then fed to a gasifier bed where it is reacted with oxygen and steam to produce the syngas. A 2 1 mole ratio of hydrogen to carbon monoxide is fed to a fixed-catalyst bed reactor for methanol production. Also, the technology for making methanol from natural gas is already in place and in wide use. Ciurent natural gas feedstocks are so inexpensive that even with tax incentives renewable methanol has not been able to compete economically. 

The equations describing the concentration and temperature within the catalyst particles and the reactor are usually non-linear coupled ordinary differential equations and have to be solved numerically. However, it is unusual for experimental data to be of sufficient precision and extent to justify the application of such sophisticated reactor models. Uncertainties in the knowledge of effective thermal conductivities and heat transfer between gas and solid make the calculation of temperature distribution in the catalyst bed susceptible to inaccuracies, particularly in view of the pronounced effect of temperature on reaction rate. A useful approach to the preliminary design of a non-isothermal fixed bed catalytic reactor is to assume that all the resistance to heat transfer is in a thin layer of gas near the tube wall. This is a fair approximation because radial temperature profiles in packed beds are parabolic with most of the resistance to heat transfer near the tube wall. With this assumption, a one-dimensional model, which becomes quite accurate for small diameter tubes, is satisfactory for the preliminary design of reactors. Provided the ratio of the catlayst particle radius to tube length is small, dispersion of mass in the longitudinal direction may also be neglected. Finally, if heat transfer between solid cmd gas phases is accounted for implicitly by the catalyst effectiveness factor, the mass and heat conservation equations for the reactor reduce to [eqn. (62)]... 

In FBRs, particles are significantly larger than in slurry (1 to 10 mm) and packed in a fixed bed. A slowly moving L wholly wets the catalyst bed, giving excellent temperature stability and a close to perfect piston flow, whereas small gas bubbles ascend through the bed. The low gas flow makes FBRs not quite adapted for hydrogenation... 

Expanded-bed reactors operate in such a way that the catalyst remains loosely packed and is less susceptible to plugging and they are therefore more suitable for the heavier feedstocks as well as for feedstocks that may contain considerable amounts of suspended solid material. Because of the nature of the catalyst bed, such suspended material will pass through the bed without causing frequent plugging problems. Furthermore, the expanded state of motion of the catalyst allows frequent withdrawal from, or addition to, the catalyst bed during operation of the reactor without the necessity of shutdown of the unit for catalyst replacement. This property alone makes the ebullated reactor ideally suited for the high-metal feedstocks (i.e., residua and heavy oils) which rapidly poison a catalyst with the ever-present catalyst replacement issues (Figure 5-8). 

The standard k-e model simulates the turbulence in the reactor. For flow within the porous catalyst bed, however, we suppress the turbulence. We enter the appropriate physical properties of the system, and employ standard boundary conditions at the impermeable walls and the reactor outlet. To represent the turbulence of the feed stream at the inlet, we treat it as pipe-flow turbulence. These model equations can then be solved for instance, via the well-known Simple algorithm [3]. To facilitate fast convergence, it is useful to make a reasonable initial guess of the pressure drop across the catalyst bed. 

In principle, any catalyst bed used for reactive distillation or trickle bed operation can also be applied in reactive stripping. The performance will depend mainly on the optimal ratio between catalyst hold-up, the gas-liquid and the liquid-solid interface. However, recycling of the strip gas flow makes a low pressure drop (and therefore a high voidage) especially beneficial. In countercurrent operation, flooding - a well-known problem - must be avoided. The present studies have focused on structured catalyst supports, developed for either reactive distillation or reactive stripping, with a particular emphasis being placed on the use of so-called film-flow monoliths. 

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