Application: Test work to obtain operational data and basis for design of industrial-sized open, continuous extractors.
Capacity: The typical feed capacity is 5-20 kg/h and the minimum feed requirement is about 100 kg.
Specification: The material to be extracted is supplied by a screw feeder or a vibratory feeder and transported by two helical screws counter-currently to the water (which is supplied by a Mono pump).
The liquid enters at the top-end and flows by inclination-controlled gravity down as a submerged stream in the bottom half of the cross-sectional area through the slowly rotating solid in true, well-controlled counter-current extraction with close to ideal plug-flow in both phases. The speed of the screws can be adjusted to give the necessary residence time for the solid. The flow of liquid is adjusted independently to give an optimum concentration of the extract.
The water can be heated up to a maximum temperature of 95°C and the extraction chamber can be kept hot by means of circulating hot water in the jacketed walls.
The extract leaves at the lower end through special self-cleaning filters and a level-control reservoir. The extracted solid is slightly squeezed at the top end and continuously discharged.
Adjustment and automatic control is very simple, and scale-up from pilot plant tests is well proven.
Extraction is diffusion- and equilibrium-controlled. The solid is only submerged in the liquid half of the time, but the rate-limiting diffusion takes place all the time. The counter-flow ensures the best possible equilibrium limit. Therefore, a high yield and a high extract concentration can be obtained simultaneously. The overall efficiency equals 3-5 ideal counter-current equilibrium/separation-stages and the yield is typically adjusted to an overall optimum of around 90% of total extractables.
The equipment has been successfully used for many beverages, pharmaceuticals and generally all kinds of difficult vegetable materials such as leaves, stems and roots. The solid must be cut, milled, rolled or shredded as finely as possible to maximize extraction speed, however with a limit of typically maximum 10% smaller than 1 mm due to the built-in filter and the resistance to the gravity-controlled percolation of the liquid – which could be water or a suitable organic liquid.
The pilot plant has a working volume of 27 litres (0.027 m
3). Commercial units with up to 2700 litres(2.7 m
3) and capacities up to the 500-1000 kg/h raw material are available, with solids residence times in the ½ to 2 hour range.