Dense bed

A hypothetical moving-bed system and a Hquid-phase composition profile are shown in Figure 7. The adsorbent circulates continuously as a dense bed in a closed cycle and moves up the adsorbent chamber from bottom to top. Liquid streams flow down through the bed countercurrently to the soHd. The feed is assumed to be a binary mixture of A and B, with component A being adsorbed selectively. Feed is introduced to the bed as shown. 

The theoretical performance of the commercial simulated moving-bed operation is practically identical to that of a system ia which soHds dow continuously as a dense bed countercurrent to Hquid. A model ia which the dows of soHd and Hquid are continuous, as shown ia Figure 7, is therefore adequate. 

Bed-to-Surface Heat Transfer. Bed-to-surface heat-transfer coefficients in fluidized beds are high. In a fast-fluidized bed combustor containing mostly Group B limestone particles, the dense bed-to-boiling water heat-transfer coefficient is on the order of 250 W/(m -K). For an FCC catalyst cooler (Group A particles), this heat-transfer coefficient is around 600 W/(600 -K). 

The heat-transfer coefficient of most interest is that between the bed and a wall or tube. This heat-transfer coefficient, is made up of three components. To obtain the overall dense bed-to-boiling water heat-transfer coefficient, the additional resistances of the tube wall and inside-tube-waH-to-boiling-water must be added. Generally, the conductive heat transfer from particles to the surface, the convective heat transfer... 

Static This is a dense bed of solids in which each particle rests upon another at essentially the settled bulk density of the solids phase. Specifically, there is no relative motion among solids particles (Fig. 12-26). 

Most of the analytical treatments of center-fed columns describe the purification mechanism in an adiabatic oscillating spiral column (Fig. 22-9). However, the analyses by Moyers (op. cit.) and Griffin (op. cit.) are for a nonadiabatic dense-bed column. Differential treatment of the horizontal-purifier (Fig. 22-8) performance has not been reported however, overall material and enthalpy balances have been described by Brodie (op. cit.) and apply equally well to other designs. 

A dense-bed center-fed column (Fig. 22-li) having provision for internal crystal formation and variable reflux was tested by Moyers et al. (op. cit.). In the theoretical development (ibid.) a nonadiabatic, plug-flow axial-dispersion model was employed to describe the performance of the entire column. Terms describing interphase transport of impurity between adhering and free liquid are not considered. 

A comparison of the axial-dispersion coefficients obtained in oscil-lating-spiral and dense-bed crystalhzers is given in Table 22-5. The dense-bed column approaches axial-dispersion coefficients similar to those of densely packed ice-washing cohimns. 

There are two regions in the regenerator the dense phase and the dilute phase. At the velocities common in the regenerator, 2-4 ft/sec, the bulk of catalyst particles are located in the dense bed immediately above the air distributor. The dilute phase is the region above... 

The cracked products pass out through two stages of cyclones which collect entrained catalyst and return it to the dense bed. Velocities at the outlet of the dense bed are normally 2.0-3.0 ft./sec. Upon leaving the cyclones, the vapors go to the primary fractionator which separates the heavy products from the gasoline and lighter components. The light products go on to the light ends recovery unit. The heavy material is separated and either recycled to the reactor or withdrawn from the system. 

As flue gas leaves the dense phase of the regenerator, it entrains catalyst particles. The amount of entrainment largely depends on the flue gas superficial velocity. The larger catalyst particles, 50p-90p, fall back into the dense bed. The smaller particles, O 0.-5O i, are suspended in the dilute phase and carried into the cyclones. 

It is important that combustion of the coke in the spent catalyst occur in the dense bed of catalyst. Without the catalyst bed to absorb this heat of combustion, the dilute phase and flue gas temperatures increase rapidly. This phenomenon is called afterburning. It is critical that spent catalyst and combustion/lift air are being introduced into the regenerator as evenly as possible across the catalyst bed. It is also important to note that vertical mixing is much faster than lateral mixing. 

Optical probes were used to measure the bubble size, frequency and velocity within the dense bed. The bubble velocity for an actively bubbling bed was found to closely agree with the drift flux form proposed by Davidson and Harrison (1963). In contrast, the volumetric flow rate of the bubbles was found to be far less than that predicted by the two-phase hypothesis (Fig. 40). 

It is seen that for Geldart types A and B particles, fast fluidization requires superficial gas velocities approximately an order of magnitude greater than that for bubbling dense beds. In many applications of fast fluidization, the particles exiting top of the bed are captured by cyclones and recirculated for makeup injection at the bottom of the bed, hence this regime is also denoted as circulating fluidization, CFB. 

At higher gas velocities the BFB transforms into a TB—no distinct bubbles, much churning, and violent solid movement. The surface of the dense bed fades and solids are found increasingly in the lean region above the dense bed. 

Catalyst losses due to plugged diplegs and/or stuck trickle/counterweight valves may sometimes be reduced by adjusting the dense bed level or with a sudden bump in pressure. During the turnaround, the valve clearances should be verified. 

The regenerator dense bed level should be reduced within nnit constraints that is, avoid more of an increase in afterburn (i.e., difference between the temperature of the regenerator dense bed and dilute phase). 

In order to minimize vapor entrainment down the cyclone dipleg, ensuring the primary cyclone diplegs are sufficiently submerged in the dense bed and maintaining the dipleg valves on the secondary cyclone diplegs are essential. 

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