Ktc

Feed

Discharge

Discharge

Feed

Discharge

Double drum dryer with center feed and bottom discharge of atomization used. An example of this is the tower configuration designed to accommodate the inverted jet of the two-fluid nozzle whereas the cylinder and cone of the more usual configuration are designed for the spray pattern produced by a disk-type atomizer (Fig. 6).

The product collection systems incorporated in spray-drying installations are many and varied and can constitute a substantial proportion of the total capital investment. In some cases, this can be as high as 20-25% of the installed plant cost. It also must be remembered that to be suitable for spray drying, the feed must be in a pumpable condition. Therefore, consideration must be given to the upstream process, ie, whether there is any need to reslurry in order to make the feed suitable for spray drying.

It generally is accepted that mechanical dewatering is less costly than thermal methods and while the spray dryer exhibits quite high thermal efficiencies, it often is at a disadvantage relative to other drying systems because of the greater absolute weight of water to be evaporated owing to the nature of the feed. It is, however, interesting to consider this point further. A classic case for comparison is provided by a thixotropic material that may be handled either in a spray dryer using a mechanical disperser doing work on a filtered cake to make it amenable to spray drying or alternatively, drying the same feedstock on a continuous-band dryer. In the latter instance, the cake is fed to an extruder and suitably preformed prior to being deposited on the conveyor band.

The operating costs presented in Table 2 are based on requirements for thermal and electrical energy only. No consideration is given to labor costs for either type of plant since these are likely to be approximately the same. Probably the most obvious figure emerging from this comparison is the 20% price differential in favor of the continuous-band dryer. Furthermore, while energy costs favor spray drying, they are not significantly lower.

It is, of course, impossible to generalize since the economic viability of a drying process ultimately depends on the cost per pound of the dried product and, as mentioned previously, the spray dryer usually has a greater amount of water to remove by thermal methods than other types. In the particular case illustrated, the spray dryer would have an approximate diameter of 21 ft for the evaporation of 6000 lb/h. If, however, the feed solids were reduced to 30% by dilution, the hourly evaporation rate would increase to 14,000 lb and the chamber diameter would be about 30 ft with corresponding increases in thermal input and air volume. The system would, as a result, also require larger fans and product collection systems. The overall thermal efficiency would remain substantially constant at 76% with reducing feed solids. Installed plant costs, however, increase proportionally with increasing dryer size necessary for the higher evaporation involved in producing a dried output equivalent to that shown in Table 2.

Spray drying does have many advantages, particularly with regard to the final product form. This is especially so where pressing grade materials are required, ie, in the production of ceramics and dust-free products such as dye-stuffs. It is certain that with the introduction of new ge-

Feed

Air inlet

Air inlet

Nozzle atomizer

Air outlet

Nozzle atomizer

Air outlet

Air inlet

Feed

Centrifugal ' atomizer

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