The Separation Section

The separation unit shown in Figure 5 can be described as a packed bed pressurized wash column with mechanically forced bed transport. The piston type consists of a cylinder, a moving piston with a porous head, and a rotating scraper for ice removal. The slurry from the recrys-tallizer enters at the bottom of the wash column through a central inlet in the porous head. The liquid concentrate can leave through the head while the ice crystals remain to form a packed bed of crystals. The round crystals pack, leaving a certain void space that remains filled with liquid concentrate. The moving piston forces the ice bed into the scraper at the top of the cylinder. Ice is scraped off and enters the melting circuit, where it is circulated through and melted in a small heat exchanger. Controlling the pressure in the melting circuit allows the possibility to force the melted water back through the ice bed, which washes the incoming crystals. The area where wash water meets concentrate in the ice bed is called the wash front and is generally very well defined. This stable displacement also depends on the recrystallization process shown in Figure 6.

The water (freezing at 0°C) meets crystals coming in with the slurry at the equilibrium temperature in the recrystallizer, say -8°C. This water will recrystallize onto the surface of the existing crystals and force the concentrate away from the crystal surface and toward the porous piston head. The wash water, now frozen, is carried toward the top of the column along with the washed ice bed to be melted and discharged. Washing carried out in this manner prevents

Melter ")

Melter

Water discharge

Melter ")

Melter

Fill with slurry

Compression and washing

Fill with slurry

Compression and washing

Figure 5 Packed bed wash column (piston type), (a) The column is filled with slurry, (b) The piston compresses the slurry. The upper portion of ice bed is removed by a rotating ice scraper. The crystals are melted by circulation of ice and water through the melter. Washing the crystal bed is completed by increasing the pressure in the melting circuit.

mixing, or recycling, of the wash water with concentrate, which provides for an efficient separation.

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