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Figure 6. Oslo cooling crystallizer.
Figure 6. Oslo cooling crystallizer.
One of the more important features of the Oslo type crystallizer is that the container for crystal growth has certain elements of design similar to all modes of operation (evaporative, vacuum cooling, cooling). In the crystal growth container a supersaturated solution of uniform temperature and concentration is conducted upward through a dense fluidized bed of crystals. The crystals are kept fluidized by this upward flow of liquor. This results in a classifying action in the crystal growth container, which keeps the large crystals suspended in the bottom layer of the suspension and the smallest crystals in the top layer, with the intermediate sizes suspended between. If the process dictates the need for crystals being present throughout the system, the fluidized bed may be expanded to allow a portion of the crystals to overflow the crystal growth container into the circulation loop.
A properly designed crystallizer should result in reasonably long periods between clean outs, uniform crystal growth, and minimal flashing in the vaporization container to reduce entrainment. These objectives are attained by keeping supersaturation well below the upper limit of the metastable region in all parts of the crystallizer, and by maintaining a large fluidized suspension of crystals in the crystal growth container to provide sufficient surface for desupersaturation.
In the Oslo design this is accomplished by continuously mixing the feed liquor with a large amount of circulating mother liquor. The mixture is passed through a heat exchanger, where the heat required by the process is added by raising the temperature of the circulating mixture to a few degrees (3-6°F) above the operating temperature of the crystallizer.
The heated solution is passed into the vaporization container where the temperature is lowered to the operating temperature by vaporization of an equivalent amount ofthe solvent. The supersaturated solution thus produced, flows down a central pipe and upward through the crystal growth container. As the supersaturated liquor passes the fluidized crystals, the supersaturation is released to the surface of the crystals, allowing for uniform growth.
The now saturated mother liquor is passed out of the crystal growth container into the circulation loop where it is again mixed with fresh feed liquor and the cycle repeated.
In the crystal growth container a sufficient quantity of crystals is maintained in a fluidized bed to achieve almost complete release of supersaturation. The individual crystals must be kept in constant motion, as they are by the fluidization, to prevent their growing together, but the motion must not be so violent as to cause excessive secondary nucleation. The amount of crystals required is a function of the crystal species, the solution and its impurities, the operating parameters, and the desired crystal size.
The heat added to the system must be done in such a manner that no boiling occurs in the heat exchanger tubes. Boiling in the tubes would cause scaling, and hence, result in frequent shutdowns for clean out.
The elements of design for a vacuum cooling crystallizer are the same as for the evaporative crystallizer except a heat exchanger is not required. The operating features are also similar. In this case, the heat for evaporation is supplied by the sensible heat of the feed and the heat of crystallization.
If it is desired to operate at a temperature which results in the solution having a vapor pressure below the vapor pressure of the available coolant, a steam-jet booster may be used in the vacuum system.
The crystal growth container is similar to the other type crystallizers outlined above, but the supersaturated solution is produced differently. A vertically arranged shell-and-tube heat exchanger is used to remove the sensible heat of the feed and the heat of crystallization.
By eliminating the evaporation, the vaporization chamber is eliminated and the vessel is now designed to operate at atmospheric pressure.
To keep the supersaturation of the solution in the metastable region, the temperature drop through the heat exchanger must be comparatively small. To prevent scaling of the heat exchanger surface, the temperature difference between the mother liquor and the coolant must be kept small.
Both batch and continuous operation are used in industry. The final choice between a batch and continuous process will be made in favor of the one which gives the most favorable evaluated cost.
In some cases, where the final solution has a very low concentration of the product, or the final solution has a high viscosity, or there is a large quantity of impurities, the batch crystallizer will be chosen because it can produce a crystal quality not achievable by a continuous crystallizer.
The basic design criteria used for a continuous crystallizer also apply to a batch crystallizer. These criteria are to:
1. Maintain the solution in the metastable region of supersaturation
2. Provide a large fluidized bed of crystal to allow effective, efficient release of supersaturation
The batch crystallizer is filled with hot feed solution and then cooled, either by evaporation of solvent by lowering of the operating pressure (vacuum cooling) or by using a heat exchanger and a coolant fluid. As the feed is cooled, a supersaturated solution is produced. From this supersaturated solution, the crystalline nuclei are formed. The crystals are grown to their final size as further cooling continues to produce supersaturation as the driving force. At the end of the batch cycle, the magma is removed from the crystallizer and sent to the dewatering equipment to recover the crystals.
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