Contacting modes cocurrent

FIGURE 32.5 The main stages involved in the spray-drying process 1, nehulization of feed liqnid into small droplets (spray formation) 2, spray-air contact (mixing and flow), which can he made hy several modes cocurrent (the liquid is nebulized in the same direction as the airflow, as illustrated), countercurrent (liqnid droplets and hot air flowing in opposite directions), or mixed flow (the liqnid is sprayed upward and only remains in the hot zone for a short time) 3, droplet evaporation and 4, separation of dried product from the air. 

Direct heat rotary dryer. The direct heat units are generally the simplest and most economical in operation and construction, when the solids and gas can be permitted to be in contact. In design mode, the required gas flow rate can be obtained from a heat and mass balance. Bed cross-sectional area is found from a scoping design calculation (a typical gas velocity is 3 m/s for cocurrent and 2 m/s for countercurrent units). Length is normally between 5 and 10 times drum diameter (an L/D value of 8 can be used for initial estimation) or can be calculated by using an incremental model (see Examples 21 and 23). 

Process intensification is also possible by induced pulsing a liquid flow in trickle beds to improve liquid-solid contacting at low liquid mass velocities in the cocurrent downflow mode [64]. In a trickle bed reactor the liquid and gas phases flow... 

In choosing the type of drier for drying the flue gas gypsum it was found that of all the cocurrent driers available, such as rapid driers, flash driers, or contact driers, the directly heated cocurrent rapid drier was the most suitable. Its design and mode of operation are described in Fig. 5. 

Bubble columns (BCs) belong to a family of pneumatic bioreactors. These bioreactors do not have any mechanical or otherwise moving parts. Compressed air, which is used for mixing purposes, is injected into the base of a cylindrical vessel. This approach provides a cheap and simple method to contact and mix different phases (Diaz et al., 2008). The liquid phase is delivered in batch or continuous mode, which can be either countercurrent or cocurrent. The batch BC is the more common form, but the cocurrent version, shown in Figure 7.1, is also encountered. Countercurrent liquid flow is rarely used in industry as it provides minor, if any, advantages and multiple complications (Deckwer, 1992), with separation by evaporation being one of the few exceptions (Ribeiro Jr. and Lage, 2005). 

Similarly to partially overlapping channels, microchannels with mesh contactors (Figure 7.2h) are used to create the partial contact of fluids. The advantage of these contactors is that both modes of operation, cocurrent and countercurrent, can be apphed. Besides, the flow is stabilized because of the solid support between two fluids. The solid contactors are porous membrane [9, 10] and metal sheets with sieve-like structure [11]. Similarly to parallel flow, the mass transfer in both cases is only by diffusion and the flow is under laminar flow regime dominated by capillary forces. The membrane contactor has the advantage of being flexible with respect to the ratio of two fluids. In addition to flow velocities, the mass transfer is a function of membrane porosity and thickness. In another type of microextractor, two microchaimels are separated by a sieve-like wall architecture to achieve the separation of two continuous phases. However, the hydrodynamics in both types of contactors is more complex because of interfadal support and bursting of fluid... 

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