Scaling-up for construction of heat-and mass-exchange vessels

Abstract

It is noted that the creation of a uniform hydrodynamic field within the space of a vessel in the making is a critical component in the solution of the scale-up problem. The method of comparing the configurations of vessel designs with respect to their effect on the uniformity of their internal hydrodynamic field is examined. The method is based on assessment of irregularities in the velocity field of the potential flow of an incompressible liquid in a vessel. The method is illustrated by comparing several gas distributors used in fluidization equipment.The term "scaling-up" has been known for approximately half a century, while currently, the subject also remains one of the most urgent in the field of the building of heat-and mass-exchange vessels.The transition from laboratory to experimental, and from experimental to industrial vessels is accompanied, as a rule, by degradation of indicators of chemical-engineering processes (the extent to which the raw material is transformed, the composition of the final products, specific output, energy outlays, etc.).The scale-up factor (SUF), which has often been more than ten, is used to assess characteristics of a newly constructed device as compared with characteristics of prototype devices. In the petroleum industry [1], for example, plate-type extraction towers with a diameter of 3 m and height of 30 m have had 5-6 theoretical stages of separation, i.e., their height, which is equivalent to the height of the theoretical degree of separation (HTDS), has amounted to 5-6 m, whereas on laboratory towers, the HTDS has been about 0.5 m (SUF of approximately 10).The scale-up problem also arises under conditions where there is no heat and mass exchange in a form characteristic of the majority of devices employed in the chemical industry. For rational sublimation drying of a thermally unstable material in the foodstuff branch of industry, the rate of the process is determined by the density of the radiation flux impinging on the material being dried. Moreover, the power of the emitter is regulated so that the temperature on the surface of the material does not exceed a certain allowable value. In the laboratory, the drying of a product 10 mm thick may have been completed in 45-50 min, but 10-12 h is required in an industrial unit [2]. In that case, the SUF has amounted to about 15.The modern definition of scale-up is the "development of vessels with dimensions and outputs exceeding those of previously investigated prototypes" [3]. If attention is turned to the fact that the laboratory device is the first prototype, building of the majority of new vessels is inevitably associated with scaling-up.