MULTISTAGE FILTRATION/EXPRESSION OF FOOD SUSPENSION WITH HIGHLY COMPRESSIBLE FILTER CAKE

摘要

Usually in a batch wise variable-pressure / variable-rate expression process, the expression is increased to a predetermined maximum pressure and then held constant until final equilibrium compression is obtained. For cocoa liquor, the variable pressure phase (constant rate) takes place between a constant low pressure feeding (below 1 Mpa) of the expression chamber and a final constant high-pressure compression (over 80 Mpa). The industrial process is performed at 100°C in hydraulic presses, which are a horizontal pile of pots (12 or 14). Each pot is a cylindrical chamber with two vertical filtering surfaces. Cocoa liquor is obtained by grinding of nibs (suspension of particles of average diameter 10 μm) and the objective is to reduce the butter content from 52% (corresponding to a porosity of 0.65) to 12% (porosity of 0.19). rnBased on the Darcy’s law, we have determined the cake specific resistance for the constant pressure feeding phase and for the constant pressure expression phase. Feeding was performed at 0.05, 0.1, 0.2, 0.3, 0.4, 0.6 and 0.8 Mpa. Tested final pressure was 35, 43, 54, 66 and 82 Mpa. rnAt low pressure, from 0.2 to 5 Mpa (end of filtration), the cake specific resistance remains constant around 10~14 m~-2. After the filtration phase, the resistance increases slowly until the pressure raises 15 Mpa. At the beginning of the higher final compression, the calculated specific resistance approaches 10~16 m~-2 and continues to increase during this constant pressure phase up to 10~18 m~-2.