Cocurrent solids flow

Cocurrent gas flow. The gas phase and solids particles both flow in the same direction (Fig. 12-33). 

FIG. 12-33 Cocurrent gas-solids flow in a vertical-lift dilute-phase pneumatic conveyor. 

Gas flow and solids flow are usually cocurrent, one exception being a countercurrent-flow spray diyer. The method of gas-solids contact-... 

Figure 26.1 Various contacting patterns in fluid-solid reactors a-d) countercurrent, crosscurrent, and cocurrent plug flow d) intermediate gas flow, mixed solid flow (e) semibatch operations.

The following section presents a review of the reported studies concerning countercurrently and cocurrently downwards-flow conditions in fixed-bed gas-liquid-solid reactors operating at elevated pressure. 

The TBR consist of a column that may be very high (above 10-30 m) equipped with a fixed bed of solid catalyst, through which a gas-liquid cocurrent downward flow occurs. In this cocurrent downflow configuration, the liquid flows mainly through the catalyst particles in the form of rivulets, films and droplets. 

The riser, gas-solid separator, downcomer, and solids flow control device are the fdur integral parts of a CFB loop. The riser is the main component of the system. In the riser, gas and solids commonly flow cocurrently upward, although they can also flow cocurrently downward. This chapter covers only the cocurrent gas and solid upward operation. In this operation, as shown in Fig. 10.1, the fluidizing gas is introduced at the bottom of the riser, where solid particles from the downcomer are fed via a control device and carried upward in the riser. Particles exit at the top of the riser into gas-solid separators. Separated particles then flow to the downcomer and return to the riser. 

Various methods of contacting fluids with particulate solids are shown in Figure 5.13. These contacting methods include countercurrent, crosscurrent, and cocurrent plug flow as well as mixed solids flow-intermediate gas flow, and semi-batch operations. Consideration of the residence time distribution for each type of fluid-solid contact is necessary to understand its effect on the conversion. As a result of given residence time distribution, E t), the average conversion of B, Xb, is given by... 

Gas flow and solids flow are usually cocurrent, one exception being a countercurrent flow spray dryer. The method of gas-solids contacting is best described as through-circulation however, in the dilute condition, solids particles are so widely dispersed in the gas that they exhibit apparently no effect upon one another, and they offer essentially no resistance to the passage of gas among them. 

Patterns of gas-solid interaction in dryers (a) gas flow across a static bed of solids (6) gas passing through a bed of preformed solids (c) showering action in a rotary dryer d) fluidized solids bed (e) cocurrent gas-solid flow in a pneumatic-conveyor flash dryer. 

The liquid-solid mass transfer coefficient may be obtained, in first approximation, from the correlations for single-phase flow, mentioned in Chapters 3 and 11, although the gas phase exerts a certain influence, as shown by Mochizuki and Matsui for cocurrent upwards flow [34]. Specific results for the situation considered here have been derived by Van Krevelen and Krekels [35], who correlated their data as follows ... 

This is the result of the lifting action of the flights and the slope of the dryer. The advance of a particle per cascade is equal to (sin 0/tm a) assuming that the descent path of the particle is vertical when there is no gas flow. With cocurrent gas flow there is increased advance of the particle due to the drag on the cascading solids, while the reverse action occurs with countercurrent flow. 

Finkelstein et al. (1971) found that, in both counter-current and cocurrent downward flow, both species of binary-sized solids moved at the same rate, provided that the solids residence time was short (< 1 minute) and the size ratio did not exceed 3. For more common situations, however, in which N solids species have left their initial acceleration zone, Richardson and Shabi (1960) have suggested that the relative or slip velocity, C/ri, of each particle species (e.g., each narrow cut of sieved solids of fixed density) depends on the total surrounding voidage in a manner similar to that of the same speeies acting monodispersely in the same liquid, viz.. 

A cocurrent evaporator train with its controls is illustrated in Fig. 8-54. The control system applies equally well to countercurrent or mixed-feed evaporators, the princip difference being the tuning of the dynamic compensator/(t), which must be done in the field to minimize the short-term effects of changes in feed flow on product quality. Solid concentration in the product is usually measured as density feedback trim is applied by the AC adjusting slope m of the density function, which is the only term related to x. This recahbrates the system whenever x must move to a new set point. 

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