Application: Test work to obtain operational data and basis for design of
industrial-sized open, continuous extractors.
Capacity: The typical
feed capacity (solids) 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). Other
extraction liquids than water can be used for special applications.
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 yield and 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
m3). Commercial units with up to 2700 litres(2.7 m3) and
capacities up to the 500-1000 kg/h raw material are available, with solids
residence times in the ½ to 2 hour range.