Crystallizers absolute pressure control

The simple form of vacuum crystallizer has serious limitations from the standpoint of crystallization. Under the low pressure existing in the unit, the effect of static head on the boiling point is important e.g., water at 45 F has a vapor pressure of 0,30 in. Hg, which is a pressure easily obtainable by steam-jet boosters. A static head of 1 ft increases the absolute pressure to 1.18 in. Hg., where the boiling point of water is 84°F. Feed at this temperature would not flash if admitted at any level more than 1 ft below the surface of the magma. Admission of the feed at a point where it does not flash, as in Fig. 27.10, is advantageous in controlling nucleation. 

The temperature in a vacuum crystallizer is normally controlled by an absolute pressure recorder controller that purges air into the vacuum system or bleeds off vent gas from a condenser if the vessel is operated above atmospheric pressure. It should be capable of maintaining the temperature in the vessel to within 1 /2 C of the set point. Typically, this is done through an absolute differential pressure cell mounted on top of the vessel so that drainage can be back into the vapor space and the control signal is transmitted to a remote recorder controller. 

Temperature. The process temperature of an evaporative crystallizer may be controlled by the absolute pressure in the vessel. Direct flow control of steam to an ejector, while economizing on steam utilization, is subject to pressure fluctuations due to disturbances in the steam supply pressure. Applying cascaded flow control would decrease the response time of the control loop. Flow control of a bleed gas or exhausted gas into the suction of the vacuum source is the most responsive and precise control option. Critically damped tuning of the pressure control loop should be implemented to prevent rapid temperature changes and high supersaturation generation from fast swings in pressure. 

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