Fixed-bed pressure drop

L0 height of fixed solids bed before fluidization starts, cm Ly = u3pf/pg p, Lyashenko number, dimensionless m fixed-bed pressure-drop exponent, dimensionless M, = u(i/utl, terminal velocity... 

Fixed bed Pressure drop, heat transfer, kinetics... 

Koloini T, Farkas EJ. Fixed bed pressure drop and liquid fluidization in tapered or conical vessels. Can J Chem Eng 51 499-502, 1973. 

UOP. 199IB, Fixed-Bed Pressure Drop Calculations, Brochure F-4183b, reprinted 3/91. 

Figure 3.53 corresponds to an upflow operation, where the fluidized-bed pressure drop remains constant after the minimum fluidization velocity. On the contrary, if a fixed bed is operated in downflow mode, the pressure drop continues to increase by increasing the fluid velocity (dense line). This is the reason that fluidized beds may exhibit a lower pressure drop and thus the power cost is lower, for high fluid velocities. 

The minimum fluidization point, which marks the boundary between the fixed- and the fluidized-bed conditions, can be determined by measuring the pressure drop Ap across the bed as a function of volume flow rate V (Fig. 1). Measurements should always be performed with decreasing gas velocity, by starting in the fluidized condition. Only for very narrow particle-size distributions, however, docs a sharply defined minimum fluidization point occur. The broad size distributions commonly encountered in practice exhibit a blurred range conventionally, the minimum fluidization point is defined as the intersection of the extrapolated fixed-bed characteristic with the line of constant bed pressure drop typical of the fluidized bed (Fig. 1). 

FIG. 21 -26 Fluiciization measurement of permeabililty and fluidization behavior. Bed pressure drop AP/H for fixed and fluidized beds as a function of gas velocity U. After Rumpf, Particle Technology, Klutoer Academic, 1990.)... 

For a fixed-bed reactor, the size and shape of the catalyst not only relate to 77 but also to the bed pressure drop and to a certain extent, the production capacity of the process, thereby also relate to the energy consumption. 

A pebble heater is a fixed-bed device used to heat steam and/or other gases to temperatures higher than could be obtained in units solely fabricated of metal. For gases heated to 1037°C, changing pebble size from 0.0079 m to 0.0127 m increased the maximum allowable gas flow (kg/m sec) and the overall heat transfer coefficient (J/sec m K) but decreased the bed pressure drop. Comment on this occurrence. 

Gas mixing in laboratory internal recycle reactors used for gas-solid catalytic studies may be assessed from pressure drop (Berty, 1974 Berty, 1979), temperature drop measurements across the bed (Mahoney, 1984), or from mass transfer coefficient estimations (Caldwell, 1983). For a given impeller speed, the first method involves comparing the bed pressure drop of the recycle reactor v/iih pressure drop of a calibrated fixed catalyst bed conducted in a separate unit. Then knowing the fluid velocity versus pressure drop for the calibrated bed, the impeller speed versus fluid velocity can be drawn. The recycle rate can also be determined from thermodynamics based on the ratio of the adiabatic temperature change and the measured temperature difference. This method requires the measurements of temperatures across the bed and the mass flow rate. 

Most fixed-bed reactors are designed for downflow. I have in mind a typical multibed hydrotreating gas oil reactor or a hydrodesulferizer diesel oil reactor. The catalyst bed supports are designed to support the weight of the catalyst plus the downward force of the catalyst bed pressure drop. 

Pressure Drop. The prediction of pressure drop in fixed beds of adsorbent particles is important. When the pressure loss is too high, cosdy compression may be increased, adsorbent may be fluidized and subject to attrition, or the excessive force may cmsh the particles. As discussed previously, RPSA rehes on pressure drop for separation. Because of the cychc nature of adsorption processes, pressure drop must be calculated for each of the steps of the cycle. The most commonly used pressure drop equations for fixed beds of adsorbent are those of Ergun (143), Leva (144), and Brownell and co-workers (145). Each of these correlations uses a particle Reynolds number (Re = G///) and friction factor (f) to calculate the pressure drop (AP) per... 

For most large-scale processes, adsorbent particle size varies from 0.06 to 6 mm (0.0025 to 0.25 in), but the adsorbent packed in a fixed bed will have a fairly narrow particle size range. Pressure drop in adsorbers can be changed by changing the diameter to bed depth ratio and by changing the particle size (see Sec. 5). Adsorbent size also determines separation performance of adsorbent columns—increasing efficiency with decreasing particle size. In hquid-phase process-... 

More up-to-date data of this process are employed in a study by Rase (Fixed Bed Reactor Design and Diagnostics, Butterworths, 1990, pp. 275-286). In order to keep the pressure drop low, radial flow reactors are used, two units in series with reheating between them. Simultaneous formation of benzene, toluene, and minor products is taken into account. An economic comparison is made of two different catalysts under a variety of operating conditions. Some of the computer printouts are shown there. 

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