| Table 13gives an overview on the solid bed behaviour of different natural materials depending on pre-compaction. Pressure difference is obtained by measuring inlet and outlet pressure by the use of manometers. Small pressure differences can be measured via a pressure difference gauge. In a certain range of mass flow and corresponding pressure drop a definite value of permeability can be calculated according to Darcy’s law for laminar flow (eq. 1), where 2 CO M & is CO2 mass flow, h dynamical viscosity, B bed permeability and cyl A the cross-sectional area of the cylinder.
Table 13: Permeability of different materials depending on pre-compaction pressure
Generally speaking, it is possible to form gas proof plugs with extruded caraway as well as the two types of hop that were examined. The minimum compaction pressure required to reach this is higher for hop than for caraway. For cocoa it was not possible to obtain such a gas tight plug. Processing (extrusion) of cocoa proved to be very difficult anyway. This might be due to the relatively low water content of the original material.
Measurements with extracted material (results not shown) made clear that a certain moisture content is required to enable sufficient compression and - as permeability is strongly related to the degree of compaction - to obtain a gas proof lock. |