In fixed bed reactors

Vanadium phosphoms oxide-based catalysts ate unstable in that they tend to lose phosphoms over time at reaction temperatures. Hot spots in fixed-bed reactors tend to accelerate this loss of phosphoms. This loss of phosphoms also produces a decrease in selectivity (70,136). Many steps have been taken, however, to aHeviate these problems and create an environment where the catalyst can operate at lower temperatures. For example, volatile organophosphoms compounds are fed to the reactor to mitigate the problem of phosphoms loss by the catalyst (137). The phosphoms feed also has the effect of controlling catalyst activity and thus improving catalyst selectivity in the reactor. The catalyst pack in the reactor may be stratified with an inert material (138,139). Stratification has the effect of reducing the extent of reaction pet unit volume and thus reducing the observed catalyst temperature (hot... 

Hydrogenations can be carried out in batch reactors, in continuous slurry reactors, or in fixed-bed reactors. The material of constmetion is usually 316 L stainless steel because of its better corrosion resistance to fatty acids. The hydrogenation reaction is exothermic and provisions must be made for the effective removal or control of the heat a reduction of one IV per g of C g fatty acid releases 7.1 J (1.7 cal), which raises the temperature 1.58°C. This heat of hydrogenation is used to raise the temperature of the fatty acid to the desired reaction temperature and is maintained with cooling water to control the reaction. 

Oxychlorination catalysts are prepared by impregnation methods, though the solutions are very corrosive and special attention must be paid to the materials of constmction. Potassium chloride is used as a catalyst component to increase catalyst life by reducing losses of copper chloride by volatilisation. The catalysts used in fixed-bed reactors are typically 5 mm diameter rings or spheres, whereas a 20—100 micrometer powder is used in fluid-bed operations. 

Scale-up in fixed-bed reactors is limited by the maximum size of the matrix that can be manufactured as a monolith. Hence, this system is appHcable for small- to medium-scale production of antibodies and other proteins, usually for the diagnostic market. This system has been described in greater detail ia the Hterature (22). 

Commercially, sulfonic acid ion-exchange resins are used in fixed-bed reactors to make these tertiary alkyl ethers (14). Since the reaction is very selective to tertiary olefins and also reversible, a two-step procedure is also used to recover commercially pure tertiary olefins from mixed olefin process streams. The corresponding tertiary alkyl ether is produced in the olefin mixture and then easily separated from the unreacted olefins by simple fractionation. The reaction is then reversed in a second step to make a commercially pure tertiary olefin, usually isobutylene or isoamylene. 

Types ofSCT Catalysts. The catalysts used in the SCR were initially formed into spherical shapes that were placed either in fixed-bed reactors for clean gas apphcations or moving-bed reactors where dust was present. The moving-bed reactors added complexity to the design and in some appHcations resulted in unacceptable catalyst abrasion. As of 1993 most SCR catalysts are either supported on a ceramic or metallic honeycomb or are direcdy extmded as a honeycomb (1). A typical honeycomb block has face dimensions of 150 by 150 mm and can be as long as one meter. The number of cells per block varies from 20 by 20 up to 45 by 45 (39). 

In many important cases of reactions involving gas, hquid, and solid phases, the solid phase is a porous catalyst. It may be in a fixed bed or it may be suspended in the fluid mixture. In general, the reaction occurs either in the liquid phase or at the liquid/solid interface. In fixed-bed reactors the particles have diameters of about 3 mm (0.12 in) and occupy about 50 percent of the vessel volume. Diameters of suspended particles are hmited to O.I to 0.2 mm (0.004 to 0.008 in) minimum by requirements of filterability and occupy I to 10 percent of the volume in stirred vessels. 

As mentioned in Section 2.2 (Fixed-Bed Reactors) and in the Micro activity test example, even fluid-bed catalysts are tested in fixed-bed reactors when working on a small scale. The reason is that the experimental conditions in laboratory fluidized-bed reactors can not even approach that in production units. Even catalyst particle size must be much smaller to get proper fluidization. The reactors of ARCO (Wachtel, et al, 1972) and that of Kraemer and deLasa (1988) are such attempts. 

Raghaven, K. V. (1992). Temperature Runaway in Fixed Bed Reactors Online and Offline Checks for Intrinsic Safety. Journal of Loss Prevention in the Process Industries 5, 3,153-59. 

This reaction is responsible for the deposition of carbon in the reactor tubes in fixed-bed reactors and reducing heat transfer efficiency. 

Sandelin, F., Salmi, T., and Murzin, D. (2006) Dynamic modelling of catalyst deactivation in fixed bed reactors skeletal isomerization of 1-pentene on ferrierite. Ind. Eng. Chem. Res., 45, 558-566. 

As for PAHs, attempts have been made to increase bioavailability by use of surfactants, and a complex picture has again developed (Fava and Di Gioia 1998). Triton-100 exerted both positive and negative effects in soil slurries even though it was not metabolized by the soil microflora, it adversely affected the degradation of chlorobenzoate intermediates, whereas in fixed-bed reactors, depletion of PCBs was enhanced. 

As a second process, the hydrogenation of a-methylstyrene is a standard process for elucidating mass transfer effects in catalyst pellets and in fixed-bed reactors... 

X. H. Ren, S. Stapf, H. Kuhn, D. E. Demco, B. Bliimich 2003, (Molecular mobility in fixed bed reactors investigated by multiscale NMR techniques), Magn. Reson. Imag. 21, 261. 

M. D. Mantle, A. J. Sederman, L. F. Gladden 2001, (Single- and two-phase flow in fixed-bed reactors MRI flow visualisation and lattice-Boltzmann simulations), Chem. Eng. Sci. 56, 523. 

J. N. Papageorgiou, G. F. Froment 1995, (Simulation models accounting for radial voidage profiles in fixed-bed reactors), Chem. Eng. Sci. 50, 3043. 

In Situ Reaction Imaging in Fixed-bed Reactors Using MRI... 

The maintenance of uniform flow distribution in fixed bed reactors is frequently a problem. Maldistribution leads to an excessive spread in the distribution of residence times with adverse effects on the reactor performance, particularly when consecutive reactions are involved. It may aggravate problems of hot-spot formation and lead to regions of the reactor where undesired reactions predominate. Disintegration or attrition of the catalyst may lead to or may aggravate flow distribution problems. 

When the velocity uz varies with radial position, equation 12.7.28 must be solved by a stepwise numerical procedure. Experimental evidence indicates that the axial velocity does indeed vary with radial position in fixed bed reactors. The velocity profile is relatively flat in the center of the tube. As one moves radially outward, the velocity increases gradually until a maximum is reached at a point about one pellet diameter from the tube wall. It then falls rapidly, until it reaches zero at the wall. If the ratio of the tube diameter to the pellet diameter... 

Hence, for a technical particle size in the range of several millimeters (Lpore = 6.75-10 4 m -+ dpartlcleiCyl = Lpore-4 = 2.7-10 3 m) and a typical reaction temperature of 523 K (X = 5% at p = 2 MPa), only 13% of the average pore is really used for synthesis, which has to be accepted to limit the pressure loss in fixed bed reactors. 

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