Clear liquor overflow

Clear liquor overflow (CLO), also known as double draw-off (DDO), increases the residence time of crystals in the growth zone and increases the magma density. This mode of operation also allows excess fine crystals to be removed from the system. Most crystallizers suffer from excessive nucleation, and to produce reasonably large crystals the excess fines must be removed. In terms of mass, the small crystals may appear insignificant the cumulative masses of crystals in a mixed suspension varies as the fourth power of their size so, for... 

In general terms, clear liquor overflow (CLO) tends to reduce cycling behaviour while classified product removal (CPR) increases it. A common form of unintentional CPR is the use of a badly designed product outlet which causes segregated magma removal. Fines destruction (FD) can be used to control the population density of nuclei in the crystallizer. Much attention is being paid at the present time to ways in which on-line CSD measurement can be reliably used to assist the transmission of information to control systems to allow rapid adjustments to be made in temperature, feed rate, FD rate, etc. to stabilize the system. 

Example 9.6 Continuous cooling with clear liquor overflow... 

Using the data and product specification given in Example 9.4, estimate the working volume of an MSMPR crystallizer operating with 2 m h clear liquor overflow. 

Thickener The lime reactor products flow by gravity to a thickener where the calcium solids settle. The thickener overflow, which is a clear liquor, is separated and is piunped to a clarifier to remove any dissolved calcium sulfate. The thickener underflow is pumped to a rotary vacuiun filter to further concentrate the calcium sulfite solids. 

Clarifier In the clarifier, the thickener overflow is treated to precipitate any dissolved calciiun sulfate. The solids from the clarifier are returned to the thickener where it is settled and discharged with the thickener underflow. The clear liquor from the clarifier is returned to the EDV as make-up for SO2 and particulates removal. 

A clear-liquor advance operation is one in which the overflow liquid is continuously removed from the tank. The overflow is not actually clear but contains small crystals that have not settled in the overflow section. This method is a simple way of controlling the CSD, because the residence times of the clear mother-liquor overflow and crystal are different. The flows are separated and the excess nuclei are removed with the overflow. The nuclei in the overflow can be used, for example, in the following crystallization unit. The larger the clear-liquor flow for a given feed flow, the longer the residence time of the crystals in the product stream. 

The operation of the fines-destruction process is similar to that of clear-liquor advance as shown in Figure 64.4, the difference is that when the clear liquor is used, the small particles return back to the product flow, thus increasing the proportion of small crystals. For the fines-destruction operation, very small fines can be withdrawn, and the suspension density of the overflow is very small. The number of crystals in the crystallizer decreases as a result of eliminating the small-sized particles. This operation is very useful in increasing the size of crystals in a system that has a high rate of nucleation. This method does, however, lead to a wide particle size distribution. 

By allowing some clear liquor to overflow from the crystallizer Figure 9.7b) the crystal residence time can be increased and rendered independent of the liquor residence time, i.e. WjP If required, the liquor overflow can be... 

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