| For process feasibility it is important that the material plugs remain stable and are not “shot” out of the extruder because of the CO2 pressure in the extraction zone. In this the pressure distribution in the solid bed expressed by the radial to axial pressure ratio plays an important role. The radial to axial pressure ratio of, amongst others, caraway is measured in an apparatus, the working principle of which is shown in Figure 39. By applying a certain axial pressure using the piston and measuring the radial pressure at different axial locations of the fixed bed, the product of friction coefficient, m , and radial to axial pressure ratio, l , can be derived applying force balances.
 
Figure 33: Schematic drawing of Figure 40: Product friction coefficient and pressure ratio as a function of applied pressure experimental apparatus
Figure 40 shows l m × as a function of the applied axial pressure for the upper part of the bed for extruded caraway at room temperature. On the other hand pressure distribution behaviour in the compacted state may change to a more liquid-like behaviour (higher radial to axial pressure ratio) so that the product of the two factors does not change significantly in this case. However it is assumed that this depends on the material and on processing conditions like temperature and moisture as well. |