Forced circulation advantage

Advantages and disadvantages will in general correspond to the type of reboiler to which forced circulation is applied. The advantages and disadvantages shown are in addition. 

For some products it has been found advantageous to pressurise the plate heat exchanger, with an orifice or valve preventing boiling until the liquor enters the separator, in what is known as the APV Paraflash system. This is a special case of the forced circulation evaporator described earlier. 

The great advantage of forced circulation is that careful calculation of the pressure drop through the reboiler and associated piping is not critical. But, as we can see in Fig. 5.6, the operator now has two tower-bottom levels to control. Further, if the hot-side liquid level rises above... 

The common types of reboilers (Fig. 15.1) are vertical thermosiphon, horizontal thermosiphon, forced-circulation, kettle, and internal. The advantages and disadvantages of each type are compared in Table... 

Since vertical thermosiphon reboilers are sensitive to level variations in vacuum systems, and because of the high-viscosity liquids often processed, forced-circulation reboilers eire frequently advantageous in such systems (114,124, 358). 

Increasing the stirring power, optimizing the geometry, and introducing an external forced circulation led to further advantages and the development of the attritor... 

Several types of evaporators may be used for SPA production, but the most popular is a forced-circulation system (Figure 11.25) developed by the Swenson E apo-rator Co, In production of superphosphoric acid (69%-72%) P2O5) by concentrating wet-process add, most of tbe fIuo.rine is volatilized so that the acid contains only 0.2%-0.3% F. By adding reactive silica during evaporation to enhance fluorine volatilization, the fluorine content can be further decreased to about 0.1%. Such acid is suitable for the manufacture of animal-feed supplement products, such as dicalcium phosphate or ammonium phosphate, and is used for that purpose. Other advantages of superphosphoric acid are ... 

The steam-side condensing coefficient outside the tubes can be estimated using Eqs. (4.8-20H4 8-26). The resistance due to scale formation usually cannot be predicted. Increasing the velocity of the liquid in the tubes greatly decreases the rate of scale formation. This is one important advantage of forced-circulation evaporators. The scale can be salts, such as calcium sulfate and sodium sulfate, which decrease in solubility with an increase in temperature and hence tend to deposit on the hot tubes. 

If crystallization is only used for separating substances, it is sufficient to create crystallized masses that can be readily separated. In such cases, a normal elbow-type circulation pump can be used, as is the case in the forced circulation crystallizers (Figure 11.2a). If coarser crystals are to be produced, a circulation pump that rotates more slowly will have to be used and the pressure drop of the system reduced. This is the case in the so-called DTB crystallizers (Figure 11.2b) that are equipped with an internal draft tube propeller pump. It is not unusual for these pumps to have a diameter of >lm up to about 2.5 m, with significantly lower tip speeds in comparison to the external pumps in the FC crystallizers (approximately 8-12 m/s instead of 16-20 m/s). In the case of evaporative crystallization, this crystallizer design offers the above-mentioned advantage that the system resistance H for the internal circulation pump is reduced and therefore the required specific energy input is considerably lower. Fewer nuclei are produced and the crystals become coarser. 

---