System for continuous solid/liquid extraction

A system for the solid/liquid extraction of in particular vegetable raw materials, such as oilseeds and oil-yielding plants, with low-boiling solvents, such as gasoline and the like, in continuous co-current manner. The equipment, which is also to be regarded as the actual extraction unit, is formed by the combination of a conveyor screw (1) having a screw flight pitch (3) which widens in the direction of the transport of material, and a screen-like or perforated bottom portion (4) provided at a short distance upstream of the discharge of the extracted material (6). The equipment is closed on all sides. Several individual sets of equipment of this type are combined in the installation and interconnected by a logical circulation system for the miscella in such a way that the extraction time is shortened, the extraction yield is improved and the residual solvent content in the fully extracted material (groats) is reduced. The installation is built up in accordance with the modular principle by arranging, in particular, four extraction units (sets of equipment) in series.

BACKGROUND OF THE INVENTION 
The invention relates to equipment and to a process for solid/liquid 
extraction, as used, for example, with vegetable raw materials for the 
production of fats and oils, flavoring substances, active ingredients of 
drugs, natural products, sugar solutions and the like. In particular, the 
invention can be employed advantageously, without thereby restricting its 
range of applications--in the solvent extraction of oilseeds and 
oil-yielding plants, the glyceride constituents (oils and fats) extracted 
from the predominantly solid raw material passing into the liquid phase, 
the so-called miscella. 
The extracting agents used for oilseeds and oil-yielding plants in 
industrial operation are almost exclusively gasoline, hexane, heptane, 
octane or mixtures thereof having boiling ranges of 60.degree.-100.degree. 
C. These relatively low-boiling extracting agents pose stringent 
requirements on the constructional expense on both the equipment and the 
processes. The expense relates to the safety of the maintenance and 
operating personnel coming into contact with the solvents and to optimum 
operational control, so that the extraction remains within economically 
acceptable limits. 
Equipment and processes for continuously operating extraction processes in 
one or more stages are widely known and are discussed in detail in the 
relevant specialist literature. Examples of suitable equipment are 
vertically arranged extraction towers with or without stirrer elements, 
horizontally aligned belt frame extractors, pot extractors, screw 
extractors, bucket extractors or basket extractors. Equipment of this type 
is described, for example, in French Pat. No. 1,020,991, British Pat. No. 
1,161,945, U.S. Pat. No. 2,587,556 or in German Pat. Nos. 1,617,004 and 
1,149,232. 
In particular in the case of the extraction of vegetable raw materials, 
such as oilseed and oil-yielding plants, to which particular reference is 
made below by way of example, most types of the known equipment work in 
counter-current manner, that is to say the fresh extracting agent is used 
for a final wash of the material already largely extracted. This end phase 
of the extraction can be preceded by further extraction stages, so that 
finally a miscella (liquid phase) is obtained which, depending on the type 
of extractor, contains about 15-35% of oil and 85-65% of solvent. 
In general, the same quantity by weight of fresh solvent as that of the 
solid material employed is added to the raw material. 
This quantitative ratio of solvent to solid raw material of about 1:1 
applies, for example, whenever, say, in the direct extraction of soya 
beans or in the extraction of pressed cakes of other oilseeds and 
oil-yielding plants, the so-called percolation process is operated in belt 
frame extractors, bucket extractors or basket extractors. In this 
connection, reference may be made to French Pat. No. 1,020,991, British 
Pat. No. 1,161,945, U.S. Pat. No. 2,587,556 or German Pat. No. 1,617,004. 
In the case of several extraction stages, it is necessary that the miscella 
obtained is circulated in each individual stage, since the solvent 
introduced into the extractor at the start is not by itself sufficient to 
insure optimum wetting of the material to be extracted. 
The reason for this is that the percolation capacity of the raw materials 
to be extracted is in many cases considerably greater than would 
correspond to the action of the relatively small quantity of fresh solvent 
at the start of the extraction. In fact, at the feed point of the fresh 
solvent, uniform and complete wetting takes place only to a limited 
extent; rather, channels soon form, through which the solvent percolates 
without being utilized 
As already mentioned above, there are predominantly economic reasons for 
the fact that the quantity of solvent is limited to a ratio of about 1:1 
relative to the raw material, although substantially more solvent would be 
required for good wetting. With a higher proportion of solvent, the costs 
and the equipment required for the subsequent distillation of the 
miscella, that is to say for the separation of solvent and extracted 
material, for example, oil, would rise considerably, and the profitability 
of the overall process would be put in question. 
In order to insure, nevertheless, uniform and adequate wetting of the raw 
material, which is to be extracted, in the extraction phase or in the 
successive further stages of a multi-stage extraction unit, considerable 
quantities of miscella, of an order of magnitude of about three to five 
times the fresh solvent quantity charged, are continuously circulated. The 
aim here is flooding of the introduced material by circulated miscella, 
and this is in fact realized in many cases. 
It is this flooding alone which insures that uniform wetting and hence 
percolation can be expected within one extraction zone or chamber. 
The measures discussed here are described, for example, by W. Kehse in 
Chemiker-Zeitung/Chemische Apparatur/Verfahrenstechnik, 94 (1970), Pages 
56-62. By circulating the miscella only within the region of individual 
extraction chambers, the latter are always kept at a defined liquid level, 
while the miscella is passed on from chamber to chamber only by overflow. 
As illustrated by FIGS. 1 and 8 (loc. cit.), the solvent enters the last 
chamber and flows from stage to stage in counter-current relationship to 
the material to be extracted. An optimum extract content (oil content) in 
the miscella is obtained only in the last stage or chamber. The use of 
pumps which, in a manner of speaking, are marking time and which in many 
cases take substantially more power than would be sufficient for the 
quantity of miscella to be delivered, is associated with a considerable 
cost for energy and equipment. 
It is therefore a first object of the invention to avoid the expense on the 
involved, time-intensive and energy-intensive as well as oversized 
equipment for the circulation pumps, which serve solely to wet the 
extraction material, within one extraction unit. 
As is known, the diffusion on which the extraction is based follows a 
thermodynamic equilibrium process which proceeds without producing work, 
and isothermally. 
The diffusion can be described by Fick's 1st and 2nd laws which, for the 
case that the concentration (c) depends only on a position coordinate (x), 
states the following: 
EQU m=D(dc/dx) (Fick's 1st law) 
with 
m=density of material flow (kg/m.sup.2 /h) 
c=concentration 
D=diffusion coefficient, 
whereas if D is independent of concentration, the time (t) must be 
included: 
EQU c/t=D(d.sup.2 c/dx.sup.2) (Fick's 2nd 
law). 
Accordingly, the rate of diffusion is thus proportional to the 
concentration gradient of an extraction time unit, that is to say the less 
the extracting agent (solvent) is enriched in extract (miscella) the 
higher is the diffusion coefficient and hence the rate of extraction. In 
other words, the degree of extraction 
##EQU1## 
approaches a limit value, the time interval of which is determined by the 
diffusion power of the solvent, the diffusion power being inversely 
proportional to the concentration of extract (for example oil) in the 
solvent. 
The requirements of Fick's laws of course presuppose complete wetting of 
the extraction material in industrial operation, but this cannot be 
attained by pure solvent alone, because of the unfavorable 
solvent/extraction material ratio (actual solvent/extraction material 
ratio about 1:1). 
It is therefore indispensable to increase the liquid fraction in the 
solvent/extraction material mixture by adding already enriched miscella 
(enriched solvent) to the fresh solvent and additionally, (+) as described 
above, (+) to insure adequate wetting by circulation within one extraction 
unit. 
However, such measures interfere, (+) as derived above, (+) with fulfilling 
Fick's law and are therefore not very suitable for obtaining an optimum 
operational efficiency of the extraction within an acceptable time 
interval. 
It is therefore a further object of the invention to achieve uniform and 
complete wetting of the extraction material, especially in a multi-stage 
extraction, with the miscella of only the preceding stage and without 
mixing with miscella of a subsequent stage, and without circulation of the 
miscella within one individual stage. 
This leads to a considerable shortening of the extraction period. Most of 
the extractors which have hitherto been disclosed and which are very 
expensive in equipment technology, have the disadvantage that they can be 
operated economically only with large volumes, that they require high 
investment in maintenance, that they hardly give scope for varying the 
capacity and that the extracted material, that is to say the groats in the 
special case of vegetable raw materials, still contains considerable 
proportions of solvent, predominantly 30% and more. 
These disadvantages affect not only the large-space carousel extractors, 
basket belt extractors, belt frame extractors and bucket extractors, but 
also the basket extractors or drum extractors, which operate at lower 
capacities. Although the latter also permit the throughput of small 
quantities of extraction material and solvent, they cause, instead, 
considerable technical problems in discontinuous operation, because of the 
involved opening, emptying, filling, closing and the like, all of which 
must be carried out batchwise. 
As already said, a factor which adversely influences the mass balance and 
energy balance of large-volume extractors is in particular the 
disadvantageous ratio of solvent in the miscella to solvent in the 
extracted groats. Many cases have become known, in which the "fully 
extracted groats" contain as much as 40% of solvent, which is not only 
missing for the dilution of the miscella (wetting of the extraction 
material) but must also be recovered from the groats in a downstream 
process with a considerable energy consumption. 
A reduction of the solvent content in the groats therefore has a 
considerable influence on the capacity of the downstream units 
(desolventizer, toaster and the like). Moreover, it improves the heat 
balance of the entire unit, since the saving of steam in the gasoline 
removal (expelling the solvent from the groats) runs parallel to the 
reduction of solvent in the groats. 
A further object of the invention is therefore the avoidance of the 
disadvantages, characteristic of large-volume extractors, with respect to 
the solvent content in the groats and the recovery of the solvent. 
SUMMARY OF THE INVENTION 
The subject of the invention is equipment for the continuous extraction of, 
in particular, vegetable raw materials with organic solvents. The novel 
equipment (extractor) operates in accordance with the principle of a 
transport screw with co-current relationship of extraction material and 
extracting agent (solvent). However, the novel extractor differs from a 
simple screw system by important constructional elements. It is 
constructed in such a way that the screw flights, starting at the feed 
point of the extraction material, widen in the direction of the discharge. 
This widening of the screw flights (flight pitch) can here be continuous, 
that is to say uniformly from screw flight to screw flight, but it can 
also be group-wise if, for example, a certain percentage of the total 
length of the screw is formed in each case by a certain number of equal 
flight pitches. In all cases, the flight pitch reaches its greatest value 
at the discharge of the material. 
The screw moves in a closed trough or a closed tub which, shortly upstream 
of the discharge of material, have a bottom portion in the form of a 
screen or with perforations. In the case of circular troughs or tubs, this 
screen-like or perforated bottom portion can amount to 60-75% of the 
circumference of the trough. 
Even though this is not obligatory, it has proved to be advantageous when 
additional stirrer elements are distributed over the length of the screw 
to insure additional thorough mixing of the contents of the extractor. 
The screw itself is at a distance of a few millimeters up to the order of 
magnitude of one centimeter from the screw bottom; by contrast, the 
distance from the upper portion of the trough or tub can be many times 
greater. 
Several pipe connections for introducing solvent and/or miscella lead into 
the trough or tub, predominantly in a vertical alignment and, in some 
cases, also in an oblique alignment. These pipe connections can be 
distributed over the entire length of the screw, but they are arranged 
more closely behind one another in the first third of the screw, that is 
to say in the region where the extraction material is fed in. 
Moreover, the invention relates to an installation having several, but at 
least two, extraction units which are arranged one after the other or one 
below the other and hence interact directly. The arrangement of several 
units below one another has the advantage that the extraction material is 
conveyed from unit to unit solely by gravity, and the outflow of miscella 
also takes place without conveying elements. 
In an installation with several extraction units according to the 
invention, the outlet of extraction material from a preceding unit leads 
into the feed of extraction material to the subsequent unit and so on, so 
that a continuous flow of the extraction material through the entire 
installation results. The miscella which is obtained in the individual 
extraction units and which in fact, (+) due to the special construction of 
the next extractor, or its screen-like or perforated bottom portion, (+) 
has already been separated from the extraction material in the extractor, 
is collected, without being mixed with miscellae of other extraction 
units, in a container having several compartments or in separate receivers 
and is circulated in each case into the preceding extraction unit. 
Product miscella, that is to say extract for further processing 
(oil/solvent separation) is in principle withdrawn from the first 
extraction unit of a multi-unit installation, that is to say from that 
unit to which fresh extraction material is charged. Fresh solvent, 
however, is charged to the last extraction unit, from which also the 
extracted material is withdrawn. 
The most important requirement for smooth interaction of several individual 
extractors within the combined system of a multi-unit installation is that 
fresh solvent is in principle fed into the last unit and is thus contacted 
with already depleted extraction material. The concentration of the weakly 
enriched miscella (extract) from the last extraction unit is further 
increased in the penultimate unit by contact with less extensively 
depleted extraction material, and so on. 
In all these steps, the extraction takes place in co-current manner, that 
is to say fresh solvent (in the last unit) or stepwise enriched solvent 
(miscella) are passed in the same direction as the extraction material, 
that is to say in co-current manner through the entire installation, 
independently of the number of extraction units within the installation. 
It is to be understood that, within such a multi-unit installation, the 
individual extraction units can have different capacities or different 
volumes with different speed of rotation (conveying speed) of the screw, 
or different flight pitches of the screw, and the like. For example, this 
can be important especially for the first and last unit of an 
installation. 
In installations having several extraction units, four such units have 
proved to be particularly suitable in operational practice. 
The invention also relates to a process for the continuous extraction of, 
in particular, oil-containing vegetable raw materials with organic 
solvents. In this process, the extraction material and the solvent or 
miscella are passed in co-current manner with one another through a 
conveyor screw arranged in a container or trough. In this case, the screw 
has screw flight pitches which widen in the direction of the transport of 
material and hence determine the contact time between the extraction 
material and the solvent or miscella. Complete wetting of the extraction 
material with solvent and/or miscella therefore already takes place in a 
very short time, even in the case of extraction material/solvent and/or 
miscella ratios of 1:1 or &gt;1:1. The new process requires a separation of 
miscella and extraction material already in the screw unit, and this is 
made possible by a screen-like or perforated end portion of the bottom of 
the screw container or trough. 
The process can also be operated in a combined system of several extraction 
units. In this case, fresh solvent is charged to a last extraction unit, 
the miscella taken from the last extraction unit is recycled to the 
penultimate unit, and so on, and the miscella of the first extraction unit 
is withdrawn for further processing or separation (oil/solvent). This 
process has proved particularly suitable for four extraction units in a 
combined system. 
As already stated in the introduction, a salient point of the new process 
is the fact that the extract, for example oil, can pass in each extraction 
unit from the extraction material into a solvent phase and/or miscella 
phase of lower concentration. If, for example, the extraction material 
contains 20% of oil and if the extraction material/fresh solvent batch 
ratio is about 1:1, the concentration of the miscella, in an installation 
having several extraction units, increases by a factor of 2.5 to 3 in each 
case from unit to unit. Since, according to the invention, the miscella of 
a preceding extraction stage is not mixed with the miscella of the 
subsequent extraction unit, before there is contact with the extraction 
material, the concentration gradient of the charged miscella relative to 
the concentration of the extract fractions still present in the extraction 
material is particularly steep, and this leads to a considerable 
shortening of the extraction period. In particular in the case of 
extraction units and installations for, for example, soya material with an 
oil content of about 20%, which require extraction periods of 40 to 60 
minutes for throughputs of extraction material of up to 3,500 tons/24 
hours, use of the equipment and installation of the invention can shorten 
the extraction period by about 50%. 
The design of the extraction units according to the invention also has 
considerable advantages in, in particular, equipment technology. Large 
extractors of previous design, as mentioned at the outset, can no longer 
be constructed as transportable units if the output is greater than 400 to 
600 tons/24 hours; rather, they can be transported only as part elements 
to the place of operation and must be assembled there to form the complete 
unit. By contrast, the relatively compact extractors with few awkward 
shapes according to the present invention are fabricated completely in the 
equipment factory and can be used directly as such at the place of 
operation, it being possible in a very simple manner to couple several 
units to form a multi-unit installation. 
It has proved to be particularly advantageous to design the screw of the 
extraction unit as a twin-screw with intermeshing screw flights. This 
leads to particularly good and rapid contacting between the extraction 
material and the solvent and/or miscella. 
The length and capacity of the extremely compact extractor according to the 
invention can be varied as desired. The extractor can therefore be 
fabricated in any desired standard sizes in the factory. Moreover, it can 
be adapted to the particular conditions of the extraction material 
(origin, oil content, structure of plant, and the like). 
The optimum contact between the extraction material and the solvent and/or 
miscella, which contact is achieved by the very specific construction of 
the equipment (extractor) and which leads to an early homogenization of 
the materials introduced in the narrower flight pitch range of the screw 
and from there passes the material forward to a gradually calmer zone with 
widened flight pitches of the screw, has the result of a measurably 
improved extraction effect which manifests itself not only in the 
shortened period mentioned above, but also yields a fully extracted 
material, the solvent content of which is considerably reduced. 
The miscella flows away continuously from the discharge of the extraction 
material, and this makes it possible to dispense with additional filter 
units before the removal of gasoline from the extraction material residue 
(groats). The groats are almost dry and the expense on gasoline removal 
(expense on equipment, personnel and energy) is substantially reduced. 
Moreover, it has been found that the new extractor is also capable of 
extracting those materials which are difficult to percolate, since the 
extraction takes place preferentially in the region of the screw with 
small flight pitch, where the extraction material is in true suspension in 
the solvent. In the region of the perforated trough bottom of the 
extraction unit, the supernatant solvent or miscella over the extraction 
material can flow off laterally to a considerable extent through the 
screen openings or perforations. 
Finally, it should be noted that the engineering design of the equipment 
(extractor) also provides improved safety for the operating personnel. 
Apart from the screw, there are no moving, mechanically interacting 
components, such as rollers, bearings, chains and the like in the 
extractor or in its gas space. The mutual interaction of metallic parts, 
which was hitherto observed in known large-volume extractors and could not 
be prevented and which can lead to the formation of sparks and hence to 
explosions and fires, does not occur in the equipment according to the 
present invention. 
It should also be noted that there is no lubrication of moving mechanical 
parts within the extraction space. Contamination of the extract with 
lubricants is therefore impossible.

DETAILED DESCRIPTION 
In FIG. 1, the novel extractor is shown diagrammatically in its most simple 
embodiment. It consists essentially of the screw 1 with the screw flights 
3 which have their lowest flight pitch or spacing at the feed point of the 
raw material (branch 9). This flight pitch widens continuously or 
group-wise and reaches its greatest value at the opposite end at the place 
of the groats discharge 6. 
The screw 1 is guided in a trough or tub 2, the bottom of which is only at 
a small distance, of say a few millimeters, from the crowns of the screw 
flights. By contrast, the distance of the upper portion of the trough 2 
from the crowns of the screw flights 3 is many times greater. 
A short distance upstream of the groats discharge 6, the bottom of the 
trough or tub has a screen-like or perforated portion 4, the openings of 
which are selected such that the miscella can run out, but the groats 
cannot pass through. The perforated portion 4 can be fitted 
interchangeably, so that different opening cross-sections (holes, slots 
and the like) can be provided, depending on the grain size of the groats. 
In FIG. 1, this portion is shown, for example, as a bottom with slot-like 
openings. 
The groats are pushed by the screw over the portion 4 and leave the 
installation with a very low solvent content via the discharge 6 which 
leads to the solvent recovery T. 
The miscella M collects underneath the screen portion 4 and flows 
continuously to further processing (filtration, clarification, 
evaporation) or is circulated for renewed feeding into the extractor. 
The extraction material R is introduced via the connection 9 at the start 
of the screw. It is to be understood that control equipment, metering 
equipment and measuring equipment, such as are usual in process 
engineering, are not mentioned here individually. 
In the running direction of the screw, a multiplicity of connection 
branches 10 for solvent and/or miscella are provided on the upper portion 
of the trough 2. These branches can be arranged vertically and thus guide 
the liquid in each case vertically onto the raw material, but they can 
also be fitted laterally at opposite points in the upper third of the 
trough, so that better wetting of the raw material is obtained. 
The branches 10 are connected via suitable control instruments to both 
fresh solvent and to tanks for recycled miscella. A suitable pipe 
connection system, not shown here, allows miscella or fresh solvent to be 
fed at any point of the extraction region. 
The trough or tub 2 can be fitted with a steam jacket 8 or another suitable 
heat source for indirect heating. 
The installation is completed by vent connections 11, which can, if 
necessary, lead to a dephlegmator for vaporized solvent, lines for feeding 
inert gas, not shown, and additional pipe connections 12 for feeding 
solvent for the final wash of the groats on the screen bottom 4. 
It is advantageous when the screw ends in a removable widened portion (box) 
7, which encloses the two discharge openings for groats 6 and miscella 5. 
FIG. 2 shows a section along II--II of the extractor according to FIG. 1. 
Finally, FIG. 3 also shows a particularly advantageous construction with 
two intermeshing screws 1,3 which are arranged in the common trough 2 with 
a perforated portion 4 at the end of the trough. Of course, these screws 
also have the change factor, significant for the invention, of the pitch 
of the screw flights. 
In the system in FIG. 4, as an example of an installation with four 
extraction units e1, e2, e3 and e4, the units are arranged one above the 
other in this special case. In principle, the extraction units e1, e2, e3 
and e4 are identical with the equipment according to FIGS. 1-3. 
The discharge of the extraction material from each extraction unit, with 
the exception of the fourth and last unit, is directly connected to the 
extraction material inlet of the subsequent extraction unit, that is to 
say the branch 6 of each unit is coupled to the branch 9 of a subsequent 
unit. 
This results in a continuously, completely closed flow of material through 
all the extraction units up to the discharge T from the fourth and last 
unit. This flow of material takes place solely by gravity, assisted by the 
screws running in the extraction units. 
According to FIG. 4, the dried and comminuted fresh extraction material, 
for example soya bean material (soya flakes) in introduced continuously 
via R into the first extraction unit e1, which in the present special case 
is at the top, and is wetted by and mixed with the miscella m3' from the 
subsequent, second extraction unit e2 located below. This miscella m3' 
flows out continuously through the screen bottom or the perforated portion 
4 of e2, compare FIG. 1, and passes via line m3 into a compartment or part 
receiver of the collecting vessel 20. From there, it is introduced, 
without being mixed with or diluted by miscella from other extraction 
units, or fresh solvent, via the pump p3 into the extraction unit e1 where 
it is contacted with fresh extraction material R, completely wets the 
latter and initiates the oil depletion process from the fresh extraction 
material. The extraction process or depletion process takes place in 
co-current manner. 
The miscella flowing out of the third extraction unit e3 passes via line m2 
in the same way into a compartment or part receiver of the vessel 20. It 
is then taken up by the pump p2 and is charged via line m2' to the second 
extraction unit e2. In the latter, the same process as in e1 takes place, 
but with slightly depleted extraction material and enriched miscella as 
the reaction component. The extraction process again takes place in 
co-current in a completely closed system. 
In the same way, the miscella from the fourth and, in this case, last 
extraction stage e4 passes via line m1 into a compartment or part receiver 
of the vessel 20, and is transported by pump p1 and line m1' into the 
third extraction unit e3. 
The circulation of miscella is completed by a fourth compartment of the 
vessel 20, into which the concentrated miscella from e1 flows continuously 
via line m4, and this miscella is then fed to a downstream treatment 
system M for separating oil and solvent. 
The oil-free or oil-depleted extraction material (groats) leaves the 
installation through the groats discharge 6, see FIG. 1, of the last, 
fourth extraction unit e4 and then passes to a treatment stage T (gasoline 
removal, toaster, and the like). 
Fresh solvent enters the process in the last and fourth extraction unit e4 
via all or one of the branches 10, see FIG. 1, and starts the circulation 
in the direction of e3, e2 and so on. By means of this procedure, 
illustrated by the example of a four-stage installation, the solvent is 
enriched stepwise, but continuously and without dilution by overflowing 
miscella from other stages. The enrichment of the solvent with extract 
(oil) takes place in the order e4-e1, while the depletion of oil from the 
extraction material proceeds in the converse order e1-e4. 
It has already been stated that the residence times of the extraction 
material and of the miscella in the individual extraction units and hence 
within the overall installation can be varied as desired by the 
controllable speed of rotation of the screw, the flight pitch and the 
nature of the flight pitch (continuous or group-wise widening) and by 
closing elements (slides, valves and the like, not shown) on the branches 
5, 6, 9 and 10. 
However, this also has the consequence that the novel extractor also makes 
it possible to use solvents having low boiling ranges and low molecular 
weights, which solvents, as is known, diffuse more intensively through the 
cell membranes of the raw materials. This provides for the first time a 
feasible method for the extraction, which has been desirable for a long 
time, by means of very low-boiling solvents. It is to be understood that 
the novel equipment is described here only by reference to its most 
important constructional features. Auxiliary elements, such as temperature 
sensors, pressure controllers, pipe systems, drive mechanisms and the 
like, which are known to any average expert, need not be described, since 
they are a matter of course. 
Moreover, the geometrical shapes of the trough 2, of the superstructure 
(widened portion) 7, of the screw flights 3 and the like as shown, are 
only illustrative possibilities and not fixed outlines which would limit 
the scope of the invention.