FIELD OF THE INVENTION

This invention relates to a continuous solids/fluid contacting process and to equipment in which appropriate solids are contacted with a fluid that may be either liquid or gaseous, or a combination thereof, under elevated pressure and optionally elevated or reduced temperature.

More particularly, but not exclusively, the invention relates to the contacting of solids and fluids under elevated pressure and optionally elevated or reduced temperature in order to conduct an extraction process, especially a continuous supercritical extraction process, in which one or more components of a subdivided solid are extracted using a fluid under pressure.

The invention is particularly useful in the food industry for the extraction of required values in the form of oils from seeds and other agricultural products.

BACKGROUND TO THE INVENTION

Virtually all "protein bars" and "natural" food bars; many infant formula products; thousands of vegetarian products (vegetarian burgers, vegetarian cheese, etc.) are made using soy protein. Soy protein is almost always described as safe and "natural" by the companies supplying it although much soy protein used in foods today has been shown to contain traces of hexane which is a toxic solvent. In many production processes soybeans are bathed in hexane as part of their processing by food manufacturers. There is, however, no requirement that food companies test their products for hexane residues (including those for use on soy-based infant formula). Hexane is a chemical by-product from an oil refining process and has been traditionally used as a solvent for extracting vegetable oil from oilseeds. Hexane solvent extraction is a process involving the injection of hexane into the oil bearing cells of seeds resulting in a miscella (mix) of oil and solvent. The hexane is subsequently evaporated from the resultant liquid phase to produce unrefined oil. The hexane is condensed and recycled and is thus used over and over again.

It has been reported that hexane residues of 21 ppm were discovered in soy meal commonly used to produce soy protein for infant formula, protein bars and vegetarian food products. These laboratory results appear to indicate that consumers who purchase common soy products might be exposing themselves and others to hexane residues. As a result, its use is prohibited when processing foods classified as organic foods.

It is evident that a replacement for the hexane extraction process should be found that is more environmentally friendly and "green".

One alternative would be to conduct a supercritical extraction process using only supercritical carbon dioxide as the extraction material in which instance there are no toxic residues at all. However, all supercritical carbon dioxide extraction systems of which applicant is aware are batch systems, and the required production rate can only be achieved by having a number of extraction chambers that are sequentially filled, extracted and emptied. In this way the process can only be termed semi-continuous at best and is still rather costly.

Applicant therefore believes that there is a need for a more continuous super critical fluid extraction system as well as a need for a solids/fluid contacting process and equipment that enables elevated pressures to be used. SUMMARY OF THE INVENTION

In accordance with this invention there is provided a continuous solids/fluid contacting process in which appropriate solids are contacted with a fluid that is selected from a liquid, a gas and a combination thereof, under elevated pressure and optionally elevated or reduced temperature, separating the solid phase from the fluid phase and processing the fluid phase or solid phase as may be necessary to yield a required component, the process being characterized in that the solids have properties that enable them to be compacted into a generally impervious plug and in that the contacting is carried out in a reaction zone defined between an inlet and an outlet with the inlet and outlet being by way of a barrel of an extruder having a longitudinally extending rotatable screw wherein flights of the screw are modified at least one position along the length of the extruder forming an inlet to the reaction zone and at least one position along the length of the extruder forming an outlet from the reaction zone with a high pressure extraction or reaction zone between the inlet and outlet positions and wherein the modified flights are arranged to form a generally solid plug of the solids in the region of said positions whilst allowing progressive movement of solids past the modified flights; wherein the solid phase is introduced into the barrel that is on the inlet side of the reaction zone and discharged on an outlet side of the reaction zone; and the fluid is introduced into the reaction zone and withdrawn from the reaction zone at selected positions.

Further features of the invention provide for the reaction zone to be a part of the length of the barrel of a single extruder between the inlet and outlet positions at which the flights are modified or, alternatively, for the reaction zone to be in the form of a reaction chamber having an inlet and an outlet formed by extruder barrels wherein the flights of the screw are modified to form generally solid plugs od solids as defined; for the process to be an extraction process in which instance the solids contain a extractable component and the fluid is an extraction fluid; for the type of modification of the flights of the screw to be selected according to the physical properties of the solids to be processed, such modification typically being selected from a short section over which the screw flights have a reduced pitch, a gap of suitable length in the screw flights, a short section of reverse hand of screw flights, a reduced cross-sectional flow path for solids that may be achieved by a flange secured to a screw shank or an increased diameter section of the shank, and a generally total block of the barrel by means of a flange in combination with a bypass through which the solid material is forced; for the fluid to be a supercritical fluid, especially a supercritical fluid selected from supercritical carbon dioxide, ethanol or water; and for the solids to be an agricultural product from which an oil is to be extracted.

The invention also provides equipment for the conduct of a process as defined above wherein an extruder has a barrel and associated screw for urging material towards a downstream end region from an upstream end region of the barrel and wherein the barrel communicates with a reaction zone and forms an inlet to the reaction zone and another extruder or a continuation of the same extruder form an outlet from the reaction zone, wherein the flights of the screws of the extruder or extruders are modified at a position corresponding to the inlet and a position corresponding to the outlet in a manner selected to form, in use, a generally solid plug of solids in the region of the modification whilst allowing progressive movement of solids past the modified flights towards a downstream end thereof; an inlet for introducing solid phase material into the barrel forming the inlet on the upstream side of the position at which the flights of the screw are modified, and at a position within the outlet from the reaction zone, a discharge outlet on the downstream side of the outlet; a fluid phase inlet into the reaction zone between said positions and a fluid phase outlet from the reaction zone between said positions.

Further features of the equipment provide for the reaction zone to be a part of the length of the barrel of a single extruder between the inlet and outlet positions at which the flights are modified or, alternatively, for the reaction zone to be in the form of a reaction chamber having an inlet and an outlet formed by extruder barrels wherein the flights of the screw are modified to form generally solid plugs of solids as defined; and for the modification of the flights of the screw to be selected from a short section over which the screw flights have a reduced pitch, a gap of suitable length in the screw flights, a short section of reverse hand of screw flights, a reduced cross-sectional flow path for solids that may be achieved by a flange secured to a screw shank or an increase in diameter of a section of the shank, and a generally total block of the barrel by means of a flange in combination with a bypass through which the solid material must be forced.

Still further features of the equipment will be quite apparent to those skilled in the art from the foregoing and from what follows.

The effect of different screw and barrel geometries in extruders on the pressure profile attained in the extruder requires matching with a particular solid material from which one or more constituents are to be extracted.

As is well known, a supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be "fine-tuned". Supercritical fluids are suitable as a substitute for organic solvents in a range of industrial and laboratory processes that are typically carried out in a batch procedure. Carbon dioxide and water are the most commonly used supercritical fluids, being used commercially for decaffeination and power generation, respectively.

Supercritical carbon dioxide behaves as a supercritical fluid above its critical temperature of about 304.25 ^Q K and critical pressure of about 7.39 Mpa and expands to fill its container like a gas but with a density of about 0.47 g/cm ³ like that of a liquid. Supercritical carbon dioxide is becoming an important commercial and industrial solvent due to its role in chemical extraction in addition to its low toxicity and environmental impact. The relatively low temperature of the process and the stability of carbon dioxide also allow most compounds to be extracted with little damage or denaturing. In addition, the solubility of many extracted compounds in carbon dioxide varies with pressure thereby permitting selective extractions.

Supercritical water exists at a temperature of about 647.1 ^Q K and pressure of about 22.1 MPa with a density of about 0.322 g/cm ³ .

Supercritical ethanol exists at a temperature of about 513.9 °K, a pressure of about 6.1 MPa with a density of about 0.276 g/cm ³ .

In order that the invention may be more fully understood one embodiment of the invention will now be described with reference to the accompanying drawings as well as various different modifications that can be made to the flights of the screw of the extruder.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:-

Figure 1 is a schematic sectional elevation of an embodiment of extruder modified to operate in accordance with the invention with the extruder being split approximately halfway along its length to facilitate illustration;

Figure 2 is a schematic sectional elevation of a part of the length of the extruder in which the flights of the screw have been modified by providing a short section of shorter pitch to cause the formation of a plug of solids material;

Figure 3 is the same as Figure 2 but showing the screw in full elevation;

Figure 4 is similar to Figure 2 but showing a modification in which a portion of the screw flights has been removed;

Figure 5 is similar to Figure 3 but shows a modification to the screw flights in which a short section of reverse hand is provided;

Figure 6 is similar to Figure 2 but showing a modification in which a reduced annular area for movement of solids is provided by a flange on the screw shank;

Figure 7 is similar to Figure 6 but shows a modification in which the diameter of the screw shank is increased to provide a reduced annular area for movement of solids;

Figure 8 is similar to Figure 6 but shows a modification in which movement of solids within the extruder barrel is substantially blocked by a flange and movement of solids past the flange is provided by an external passage that can be controlled, as may be required; and,

Figure 9 shows a variation of the invention in which a separate reaction chamber forms the reaction zone and has an inlet and outlet consisting of an extruder having modified flights of the screw as hearing described. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

In the embodiment of the invention schematically illustrated in Figure 1 , equipment for the conduct of a process according to the invention includes an extruder (1 ) having a barrel (2) and associated screw (3) having flights carried by an axially extending shank (4). The extruder has an inlet hopper (5) at an inlet end thereof and the opposite outlet end (6) is arranged to discharge residual solids after passage through the barrel. The screw is thus arranged to urge solid subdivided material from the hopper at the upstream inlet end region of the extruder towards the downstream outlet end region thereof.

In this embodiment of the invention, the barrel itself forms a reaction zone between a fluid phase inlet (7) and a fluid phase outlet (8) that are spaced apart along the length of the extruder between the inlet end region and outlet end region. Both of the fluid phase inlet and fluid phase outlet are located between two positions at which the flights of the screw are modified such that, in use, a generally solid plug (1 1 ) of compacted solids forms in the region of each of the modifications.

These solid plugs of solids nevertheless allow progressive movement of solids past the modified flights in the direction of the downstream end of the extruder. In this embodiment of the invention the fluid phase inlet and fluid phase outlet are arranged to provide counter current flow to the movement of solids along the length of the barrel between the two generally solid plugs (1 1 ). Nevertheless, if circumstances dictated it, co-current extraction could equally well be conducted. Indeed co-current movement of the solid materials and fluid phase may be desirable in conducting processes other than extraction, for example chemical reactions.

Clearly, the type of modification made to the flights of the screw will depend, at least to some extent, on the general nature of the subdivided solids and it must be remembered that not all solid materials will behave appropriately to forming a suitably solid plug of material whilst still providing for movement of solid materials through the positions at which the flights of the screw have been modified. Various types of possible modifications are illustrated in Figures 2 to 8.

In the arrangement illustrated in Figures 2 and 3, the modification made to the flights of the screw is merely to provide two successive convolutions (12) of the flight that have a significantly shorter pitch. Of course, more than two successive convolutions could be employed for this purpose and the number may well depend on the properties of the solids. The arrangement is such that the slowing down of movement of the feed of solid material along the barrel will cause an accumulation od solids and thus the formation of a plug as envisaged above.

In the arrangement illustrated in Figure 4 a plurality of successive convolutions of the flight are simply removed from the screw to leave a gap (13) in the screw flights. The effect of this it has been found is that due to the fact that solid materials fail to move in the region of the gap, an accumulation develops that forms a plug as envisaged above.

In the arrangement illustrated in Figure 5, two successive convolutions (14) of the flights of the screw are of reverse hand so that movement of solid materials to form a compacted plug is enhanced and the solid materials forming the plug are forced through the relatively short section of reverse hand by the substantially longer section of the screw feeding material towards the section of reverse hand.

In the arrangement illustrated in Figure 6, one convolution of the flights of the screw is removed and replaced by a flange (15) having a periphery that forms, with the inside of the barrel, an annular passage of reduced cross- sectional area through which the solid materials must be forced. This arrangement also serves to cause a plug to form upstream of the flange in a manner envisaged above.

In the arrangement illustrated in Figure 7, the shank (16) at a position between at least two successive convolutions of the flight is made to a larger diameter than the shank (4) of the rest of the screw to provide a reduced size of annular passage. This tends to form a plug of solid materials as envisaged above.

It is also not essential that the solid materials be restrained to passage through the barrel itself past the position where the flights of the screw have been modified. Thus, as shown in Figure 8, it is possible that a flange (17) can be installed on the screw that substantially blocks the passage along the barrel and solid materials need to be forced through a restricted bypass (18). The cross-sectional shape of the bypass can be made to be adjustable so that the nature of the plug that forms upstream of the flange can be controlled, as may be required.

Referring now to Figure 9, it is not necessary that the barrel itself formed the reaction zone and a separate reaction chamber (21 ) could have an inlet in the form of a first extruder (22) having a single modified zone (23) of the flights of the screw and a second extruder (24) forming the outlet from the reaction chamber and also having a single modified zone (25) of the flights of the screw. In this instance enhanced residence time of contact between the solids and fluid can be achieved.

Reverting to the process provided by the invention, a continuous solids/fluid contacting process can be conducted in the equipment described above with appropriate solids, for example, that contain an extractable component, being contacted with an extraction fluid that is selected from a liquid, a gas and a combination thereof, especially a supercritical fluid that may, for example, be selected from carbon dioxide, water and ethanol. The contacting may be conducted under elevated pressure and elevated temperature.

The solid materials are fed in by way of the hopper and the screw of the extruder is rotated in conventional manner. This causes the solid materials to be moved along the barrel and to form plugs at the positions described above so that the elevated temperature and pressure of the extraction or reaction process can be carried out continuously without interference. It will be understood that between the two plugs, the solid materials will be moved along the barrel very much in the usual way with a substantial amount of agitation and mixing taking place. Extraction may be enhanced consequent on the mechanical agitation as well as shear forces working on the solid material.

As indicated above, the extraction fluid may be a supercritical fluid, especially supercritical fluid carbon dioxide.

The outlet fluid phase that comprises the solvent, as well as the extracted fluid dissolved in the solvent fluid, may be removed from the barrel of the extruder, and, as may be necessary, conveyed to condensers in basically known manner.

The resultant subcritical fluid, for example carbon dioxide, may be recovered, and pumped back for recycling into the extraction process.

It will be understood that the process described above may be a continuous high pressure cooking process.

It is envisaged that a process provided by the invention may have many advantages as an extraction process as compared to traditional solvent based extraction. Firstly, a continuous extraction plant envisaged by the invention may be associated with lower capital cost than traditional plant of similar production capability. The technology is what is commonly termed green technology compared to traditional hexane extraction plants. Consequent on this there is little or no pollution risk and little or no health hazards compared to the use of hexane plants. There is furthermore very low to zero fire hazard. No "roasting" or heating of the product is required to evaporate the solvent with attendant environmental risks. It is considered that a process according to this invention may be safer as a result of smaller pressure vessel volume.

Numerous variations may be made to the embodiments and variations of the invention described above without departing from the scope hereof. In particular, the various modifications made to the flight of the screw of the extruder in order to cause plugs to develop can be varied widely and the examples given above are by no means exhaustive of the various possibilities.