Cross-flow operations

Reactor type Multi-plate-stack, cross-flow Operating pressure -... 

TABLE 9.6. Performance Data of Batch Tray and Tray-Truck Dryers (a) Cross-Flow Operation... 

The fusel-rich stream leaving the decanter is fed to the fusel sector where the stream is washed with water to recover about 96 % of the incoming ethanol. The resulting water-rich stream is recycled to the hybrid column. To do this, an overall amount of 363 kg/h wash-water and seven separation steps are necessary. The conceptual design of a cross-flow operation is performed using DISTIL, while process simulation is done in Hysys. 

Fig. 16. Schematic drawing of dead-end and cross-flow operation.

The simplest design is a dead-end operation, as shown in Fig. 16A. As the feed is forced through the membrane, the concentration of rejected components in the feed increases and accumulates at the membrane interface, hence the permeate quality decreases with time. Therefore, for industrial applications, a cross-flow operation, as shown in Fig. 16B, is preferred for its lower fouling tendency comparing to the dead-end mode. 

In the cross-flow operation, the inlet feed stream entering the module at a certain composition and it flows parallel to the membrane surface. The composition of the stream changes along the module, and the stream is separated into two parts a permeate stream and a retentate stream. Flux decline is relatively smaller with cross-flow and can be controlled and adjusted by proper module configuration and cross-flow velocities. 

Fig. 17. Schematic drawing of some cross-flow operations (A) co-cnrrent (B) connter cnrrent (C) cross-flow with perfect permeate mixing (D) perfect mixing.

As cross-flow mode is preferred over a dead-end one, the proper choice of the module is the next crucial step. For a given module design and feed solution, the cross-flow velocity is the main parameter that determines mass transfer in the module. Four cross-flow operations shown in Fig. 17 are used ... 

Figure 13. Transmembrane pressure profiles (a) uniform transmembrane pressure operation, and (b) conventional cross-flow operation.

Principle of operation Continuous cross-flow operation... 

Fig. 1-64. Steady-state cross-flow operation of 3 stages.

For industrial applications, a cross-flow operation is preferred because of the lower fouling tendency relative to the dead-end mode (figure VIII - 14b). In the cross-flow operation, the feed flows parallel to the membrane surface with the inlet feed stream entering the membrane module at a certain composition. The feed composition inside the module changes as a function of distance in the module, while the feed stream is separated into two a permeate stream and a retentate stream. The consequences of fouling in dead-end systems are shown schematically in figure VUI - 15. In dead-end filtration, the cake grows with time and consequently the flux decreases with time.Hux decline is relatively smaller with cross-flow and can be controlled and adjusted by proper module choice and cross-flow velocities. 

As far as the cross-flow operations are concerned, counter-current flow gives the best results followed by cross-flow and co-current flow, respectively as can be demonstrated by process... 

Finally, they concluded that the technique is able to observe the deposition of particles as a cake inside the lumen of a hollow fiber membrane and also to detect deposition and fouling within the membrane structure and they suggested that it can also be applied to the evaluation of critical flux in cross-flow operation by observing the flux at which deposition begins. 

A column in cross-flow operation is unique because the gas flow is transverse to the vertically downward flow of liquid, so that the area for gas flow can be different than the area for liquid flow. If Ar is the ratio of the area for gas flow compared to the area for liquid flow, then when Ar is 1 the pressure drop through a cross-flow bed is the same as through a countercurrent bed. As the vsilue of Ar increases, the pressure drop through a bed in cross-flow operation becomes progressively less than for a bed having the same liquid flow area operated in countercurrent flow [22]. Thus, cross-flow operation will permit higher gas rates and the use of higher gas to liquid flow ratios than a countercurrently operated column. 

Influence of Packing Shape, Irrigated Packed Beds, High Liquid Rate Performance, Liquid Holdup in Packed Beds, Pressure Drop Calculation, Effects of Surface Tension and Foaming, Concurrent Flow Operation, Cross-Flow Operation, Example Problem, Notation, References... 

Fig. 12.25. The principal characteristics of such beds include cross flow of solid and drying gas, a solids residence time controllable from seconds to hours, and suitability for any gas temperature. It is necessary that the solids be free-flowing, of a size range 0.1 to 36 mm (59]. Since the mass flow rate of gas for thermal requirements is substantially less than that required for fluidization, the bed is most economically operated at the minimum velocity for fluidization. Multistage, cross-flow operation (fresh air for each stage) is a possibility [2], as is a two-stage countercurrent arrangement, as in Fig. 11.28 (58]. A tentative design procedure has been proposed (40].

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