Patent Publication Number: US-2011067417-A1

Title: Method and apparatus for cooling liquids

Description:
This invention relates to a method and apparatus for cooling liquids. 
     It is frequently desired in industry to cool liquids rapidly. One example of such a requirement is when the liquid is to be converted into solid particles and rapid cooling promotes favourable microstructural features, or the absence of unfavourable microstructural features, in the resulting solidified particles. 
     It is known to carry out the rapid cooling by atomising the feed liquid and contacting the resulting atomised feed liquid with a cryogen such as liquid nitrogen. The necessary heat can be extracted from the atomised particles in only a fraction of the time that would be needed were the necessary cooling to be performed on an undivided body of the feed liquid. Further, if the feed liquid is supplied at or above ambient temperature, the large temperature difference between the atomised particles and the liquid feed facilitates rapid cooling. If the cryogen is a liquefied gas, its enthalpy of evaporation can also be contributed to the cooling. 
     EP-B-0 393 963 relates to the cryogenic crystallisation of molten fats. The molten fat is sprayed from an atomising nozzle facing vertically downwards. A divergent flow of atomised particles in the shape of a vertical cone is formed. The cone is arranged to be coaxial with a liquid nitrogen spraying which directs jets of droplets of liquid nitrogen radially inwards at the conical flow of the atomised particles of the molten fat. The jets are directed downwards at an angle of 45° so as to prevent impingement of the liquid nitrogen on the atomising nozzle itself. As a result of contact between the atomised particles of molten fat and the droplets of liquid nitrogen, the fat is almost instantaneously converted into solid particles having a favourable microcrystalline structure. 
     In commercial practice, the apparatus according to EP-B-0 393 963 is located at the top of a cylindrical chamber. The solid particles of the fat are extracted at the bottom of the chamber. We have found that recirculation patterns are created in the chamber which prevent effective operation of the apparatus at its maximum theoretical capacity. Accordingly, to meet a large demand for the crystallisation of fat several such chambers would be required. 
     The above-described problem is not confined to the cryogenic spray crystallisation of fats and oils. It applies in the cryogenic cooling of particles of any liquid. 
     According to the present invention there is provided a method of cooling a feed liquid comprising forming at least one sheet of flowing particles of the feed liquid and directing cryogen at the particles from both sides of the sheet. 
     The invention also provides apparatus for cooling a feed liquid, comprising at least one nozzle for forming at least one sheet of flowing particles of the feed liquid, at least one first cryogen discharge member having a plurality of cryogen discharge orifices arranged for directing cryogen at one side of the sheet, and at least one second cryogen discharge member having a plurality of cryogen discharge orifices arranged for directing cryogen at the other side of the sheet. 
     By arranging the flow of the particles of the liquid in a relatively flat space, i.e. a sheet, rather than in a cone, as in EP-B-0 393 963, it becomes possible to limit recirculation of the particles of the feed liquid. As a result, it becomes possible to operate the atomising nozzle at nearer its maximum theoretical flow rate than in the prior arrangement discussed above. Further, more efficient utilisation of the cryogen is made possible by the method and apparatus according to the invention. Another advantage of the method and apparatus according to the invention is that by arranging the nozzles in lines relatively large throughputs of liquid can be achieved in a single chamber of similar size to that which would be required were a single nozzle to be used. In addition, for even larger throughputs of feed liquid, a plurality of contiguous cuboidal or box-shaped chambers can be employed without adding greatly to the space occupied by the apparatus according to the invention. 
     The sheet of flowing particles of the feed liquid is typically formed by atomising the feed liquid. The feed liquid is preferably atomised by a compressed gas and the nozzle may therefore have an inlet for the atomising gas. Alternatively, mechanical atomisation may be employed. 
     The or each atomising nozzle preferably points vertically downwards so as typically to provide a flow of particles of the liquid in a vertical plane. 
     The or each sheet of particles of the liquid particles may be essentially planar. Such a sheet may be produced by a nozzle having a rectilinear elongate outlet. Alternatively, the nozzle may have a row of separate outlets which cooperate together to provide a flow of liquid particles in the form of a sheet. If a plurality of nozzles is employed, the nozzles may be arranged in one or more straight lines. Preferably some or all of the straight lines are parallel to one another. One or more parallel rows of nozzles may therefore be provided. The sheets formed by adjacent nozzles may be contiguous to one another, may merge into one another or may be spaced apart. In alternative arrangements the nozzles may be disposed in straight lines that define a geometric figure, for example, a triangle, a square or a polygon. An advantage of providing the nozzles in parallel rows or along the sides of a geometric figure is that several nozzles can readily be accommodated in a single chamber. 
     In an alternative geometric arrangement a curved sheet of particles of the feed liquid is produced. A nozzle having a curved or arcuate elongate outlet may be employed to provide such a curved sheet of particles. If desired, a plurality of such nozzles may be arranged circumferentially with the result that the sheets can define together a generally hollow cylindrical shape. Such an arrangement is another that offers the advantage of enabling several nozzles to be readily accommodated in a single chamber. 
     The orifices of the first and second cryogen discharge members are preferably disposed in geometric configurations complementary to that or those of the nozzle. For example, if there is a single line of nozzles, then that line is flanked on one side by a complementary line of cryogen discharge orifices in the first cryogen discharge member and on its other side by a complementary line of cryogen discharge orifices in the second cryogen discharge member. Alternatively, if the nozzles are arranged circumferentially, there is a complementary inner and a complementary outer ring of cryogen discharge orifices. 
     The orifices of the first and second discharge members are preferably disposed such that in use they are all equidistant from the sheet of particles at which they are directed. They are also all preferably orientated so as to direct cryogen at the sheet of particles near to its source. 
     In one preferred arrangement of the apparatus according to the invention the nozzles are disposed in the upper region of a single, generally cuboidal, chamber. In another arrangement the nozzles are disposed in the upper regions of a plurality of contiguous generally cuboidal chambers. Such an arrangement lends itself to a modular construction of the apparatus according to the invention. If, for example, it is determined that an apparatus with a single chamber of given dimensions with a specified number of atomising nozzles can cool a particular feed liquid at a certain feed liquid flow rate, then if, say, it is desired to cool the same liquid at four times the liquid flow rate, four identical chambers will be required. Preferably, if an arrangement of contiguous chambers is required the chambers are open to one another through their common sides. 
     Typically, the first and second cryogen discharge members are both spray headers. 
     The method according to the invention may be used to cool a large number of different feed liquids. The feed liquid may be a molten substance which is a solid at 15° C. and which needs to be heated to above 15° C. in order to be converted to a liquid. Alternatively, the feed liquid may naturally occur as a liquid at 15° C. For example, it may be an aqueous liquid. The method according to the invention is particularly suitable for solidifying a liquid. Various examples of liquids that may be so solidified include, molten fats, oils, and aqueous solutions, emulsions and dispersions. The resulting solid particles may have use as foodstuff or a pharmaceutical or may be used in the manufacture of other products. Alternatively, the feed liquid to be solidified (in the form of a powder) may be a molten metal or alloy. 
     The method according to the invention is particularly suited to the solidification and crystallisation of (edible) molten fats, oils, and other edible substances. Rapid cooling typically enables a desirable and stable microcrystalline structure to be obtained within the particles of the fat. By forming atomised particles of an average size of less than 50 μm, more preferably of less than 10 μm, and most preferably of less than 5 μm extremely rapid cooling rates can be achieved, for example a rate of at least 1000 K/s. Accordingly, the liquid can be essentially completely solidified at the exit from the chamber. 
     The cryogen is preferably a liquefied gas which preferably has a boiling point lower than −100° C., although liquefied carbon dioxide which has a triple point of −78° C. can be used instead, the liquefied carbon dioxide being converted into a mixture of gas and solid particles on passing through the cryogen discharge orifices. The preferred liquefied gas is liquid nitrogen, although liquid argon or liquid air may alternatively be used. 
     The nozzle(s) and the cryogen discharge members are typically housed in a chamber having an outlet for the chilled particles and the same or a different outlet for spent cryogen. The apparatus according to the invention may advantageously include a sensor for sensing the temperature of the spent cryogen, the sensor being operatively associated with at least one flow control valve for controlling the flow of cryogen to the cryogen discharge members. Such an arrangement enables the flow of cryogen to be adjusted automatically in concert with changes in the flow of the feed liquid so as to ensure that adequate cooling of the particles is obtained, for example, so as to achieve internal as well as external solidification of the particles, without the spent cryogen having an unnecessarily low exit temperature from the chamber. Alternatively, the flow rate of the cryogen may be adjusted manually on the basis of previous experiments determining the optimum cryogen flow rates for different feed liquid flow rates. 
     In the solidification of liquids there is a tendency of some fine solid particles to be carried out of the chamber entrained in the spent cryogen. The apparatus according to the invention therefore preferably includes a cyclone communicating with the outlet for gas from the chamber so as to disengage the fine particles from the spent cryogen. If desired, once the fine particles have been disengaged, the spent cryogen may be compressed in a compressor and used to atomise the feed liquid. Alternatively, a separate compressed gas such as air can be used to atomise the feed liquid. Using spent cryogen, if that cryogen is, say, nitrogen, is advantageous in the event of air adversely affecting, for example, oxidising, the feed liquid. If there is such recycle of the spent cryogen, the chamber may have another outlet for spent cryogen. The third outlet may communicate with a baghouse for disengaging fine particles from the spent cryogen. 
    
    
     
       The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic sectional side elevation of a cryogenic spray crystalliser having a single feed nozzle; 
         FIG. 2  is a schematic plan view of the crystalliser shown in  FIG. 1 ; 
         FIG. 3  is a schematic plan view of an alternative crystalliser employing a single line of feed nozzles; 
         FIG. 4  is a schematic plan view of a further alternative crystalliser employing two parallel lines of feed nozzles; 
         FIG. 5  is a schematic plan view of another alternative crystalliser employing two parallel lines of feed nozzles, one employed in a first chamber and the other in a second chamber; 
         FIG. 6  is a schematic plan view of yet another alternative crystalliser, employing an arrangement of feed nozzles around the sides of a square; 
         FIG. 7  is a schematic plan view of a final alternative crystalliser, employing a circumferential arrangement of feed nozzles; and 
         FIG. 8  is a general schematic view of an apparatus which may incorporate any of the forms of crystalliser illustrated in  FIGS. 1 to 7 . 
     
    
    
     The drawings are not to scale. In  FIGS. 2 to 7 , the views are with the top of the crystalliser chamber omitted and with the corrections of the atomising nozzles and the spray headers to, respectively, a molten fat supply line and a liquid nitrogen supply line, not shown. 
     With reference to  FIGS. 1 and 2 , a generally box-shaped (cuboidal) chamber  102  houses a single nozzle  104  for atomising molten liquid fat, a first cryogen discharge device in the form of a first spray header or tube  106  having a row of cryogen discharge orifices  108  formed in it and a second cryogen discharge device (not shown in  FIG. 1 ) in the form of a second spray header or tube  110  having a row of cryogen discharge orifices  112  formed in it. 
     The atomising nozzle  104  is mounted at the top of the chamber  102  and has an elongate, rectilinear outlet  114  facing vertically downwards into the chamber  102 . The nozzle  104  has a first inlet  116  for the molten fat communicating with a source (not shown) of the molten fat and a second inlet  118  for atomising gas communicating with a source (not shown) of the atomising gas. The molten fat is typically pumped to the nozzle at elevated pressure, for example in the range of 2 to 10 bar, though, if desired, higher pressures, say, up to 30 bar may be used. A peristaltic pump may be used for this purpose. The atomising gas is typically also provided under a pressure in the range of 2 to 10 bar and the internal configuration of the atomising nozzle  104  is such that the stream of molten fat supplied to it issues as particles in the form of very fine droplets. Such atomising nozzles are known and are commercially available. It is generally desirable that the molten fat (or other liquid to be atomised) be supplied at a higher pressure than the atomising gas. 
     In view of the shape of the outlet  114 , the outer contour of the downwardly directed spray of liquid fat particles issuing from the nozzle  104  is not in the shape of a cone that is entirely symmetrical about the axis of the nozzle  104 , but is instead more in the shape of a thin planar sheet  120 . Typically, particularly when the nozzle  104  is operated at close to its maximum throughput, the sheet  120  may fan out, particularly vertically, but in horizontal cross-section always has a large aspect ratio. The first spray header  106  is positioned on one side of the sheet  120  with its orifices  108  facing the sheet  120 . The orifices  108  in the spray header  106  are typically circular in shape and are of small diameter. The number and size of the orifices  106  may be chosen in accordance with the flow rate of the molten fat out of the atomising nozzle  104 . The orifices  108  are evenly spaced. The spray header  106  is positioned such that the cryogen impacts the particles of molten fat at points relatively close to the tip of the nozzle  104 , though not so close as to cause any solidification of the molten fat in the nozzle  104  itself. For this reason, the orifices  106  do not point horizontally at the sheet  120 , as in this event some cryogen will tend to issue with a component of momentum in the upward direction, but instead are pointed downwards at angle to the horizontal of up to 45°. The extent of the row of orifices  106  is sufficiently wide that the particles will encounter cryogen across the entire width of the sheet in the region of contact between the cryogen and the particles of the molten fat. 
     The cryogen is preferably liquid nitrogen, although, for example, liquid argon or liquid air could be used instead. All these cryogens have in liquid state a temperature well below −100° C. Accordingly, they are at a substantially lower temperature than the molten fat which is typically provided at a temperature above +50° C. They are accordingly effective coolants. They provide not only cooling by the extraction of sensible heat from the particles of the atomised fat, they also provide cooling by the extraction of heat necessary for the vaporisation of the liquefied gas. In practice, therefore, they are able to provide almost instantaneous solidification of the particles of molten fat particularly if the size of the latter is kept to below 10 microns. Since there is a tendency for some of the liquefied gas to vaporise as it flows from the spray header  106  into contact with the flowing particles within the contours of the sheet  120 , it is desirable that this distance of travel is kept to a minimum. Accordingly, the length of the path that the liquefied gas has to travel before encountering the particles of molten fat is preferably less than 50 mm. It is also desirable to cause the liquid nitrogen or other liquefied gas to be ejected from the orifices  108  in the form of droplets at high velocity. This result can be achieved by supplying the liquefied gas to the spray header  106  under an elevated pressure, typically in the range of 2 to 6 bar. The liquefied gas may be stored at a suitably elevated pressure and there is generally no need to use a mechanical pump to create a flow of pressurised liquefied gas. The liquefied gas is preferably conducted to the spray header  106  through a thermally-insulated pipeline (not shown). If the storage vessel (not shown) for the liquefied gas is remote from the spray crystalliser, it may be desirable to disengage vaporised gas from the liquid at a location close to the spray crystalliser. Devices for effecting such disengagement are well known in the art. 
     The second spray header  110  is essentially a mirror image of the first spray header  106  and therefore has a size, number and spacing of its cryogen discharge orifices  112  equal to those of the spray header  106 . The second spray header  110  (as shown in  FIG. 2 ) is located on the opposite side of the sheet  120  of particles from the first spray header  106 . The distance between, the row of orifices  112  and the sheet  120  is equal to that between the row of orifices  108  and the sheet  120 . Accordingly, there is no net lateral displacement of the particles of molten fat as they are impacted upon by the cryogen. Further, the confining of the particles to flow in a relatively thin sheet  120  facilitates the rapid contacting of them with the cryogen and therefore their rapid, essentially instantaneous solidification. 
     Although not shown in  FIGS. 1 and 2 , the chamber  102  is essentially open at its bottom, and the resulting solidified particles of fat may be collected as a free-flowing powder in a stationary or moving collection device. The collection device may, for example, be an auger. Notwithstanding that the known spray crystalliser described in the introductory paragraphs of this specification was similarly open at the bottom, we have found that considerably recirculation of particles of solidified fat tend to take place. In a comparable spray crystalliser according to the invention recirculation is considerably reduced. 
     Various edible fats and compositions containing fat may be solidified with advantage by the method and the apparatus according to the invention. For example, hydrogenated fats so crystallised are found to have superior rheological properties to those crystallised by conventional scraped heat exchanger surface technology. This is attributed to the formation of a particulate product with a multiplicity of microcrystals in a liquid oil phase within the body of each solidified particle. The almost instantaneous achievement of a maximum number of crystals per unit mass of solid fat in the product has the consequence that in certain food compositions the proportion of hydrogenated fat can be reduced without loss of qualitative properties such as texture, taste and general organoliptic attributes. Present day research into coronary and other diseases suggests that such a reduction would be beneficial to human beings consuming such compositions or foodstuffs prepared from them. Examples of hydrogenated fats that can be solidified and crystallised by the method and apparatus according to the invention include hydrogenated rape seed oil, hydrogenated soya bean oil, hydrogenated palm oil, and hydrogenated sunflower oil. 
     It is also possible to solidify edible oil-in-water emulsions of dairy or vegetable fats by the method and with the apparatus according to the invention without substantial destabilisation of the emulsion on remelting the resultant solid particles. An example of such an emulsion that can be effectively solidified or frozen in this way is whipping cream. 
     Referring again to  FIG. 1  of the drawings, the nozzle  104  is optionally surrounded by a heating element  130  which is operable to prevent any of the molten fat flowing therethrough from solidifying. The heating element  130  may be operated intermittently or continuously, or only at start up or when cleaning the apparatus shown in  FIG. 1 . A further electrical heating element (not shown) may be used to heat the walls of the chamber  102  at the end of operation to melt any solidified fat which has accumulated during operation on their inner surfaces. 
     All the parts of the apparatus shown in  FIGS. 1 and 2  may be made from materials whose use is acceptable in the food industry, for example, stainless steel. 
     Referring now to  FIG. 3 , there is illustrated an apparatus comprising a generally box-shaped chamber  202  housing a row of nozzles  204  for atomising liquid fat and producing a sheet or sheets  220  of atomised fat particles, a first cryogen discharge device or member in the form of a first spray header or tube  206  having a row of cryogen discharge orifices  208  formed in it and a second cryogen discharge device or member in the form of a second spray header or tube  210  having a row of cryogen discharge orifices  212  formed in it. The apparatus shown in  FIG. 3  has a configuration and operation analogous to that shown in  FIGS. 1 and 2  save that instead of a single atomising nozzle  104 , there is now a straight line row of at least two and typically three or more atomising nozzles  204 . These nozzles  204  are spaced so as to elongate the sheet or curtain of particles produced in operation relative to that shown in  FIGS. 1 and 2 . The individual “sheets”  200  produced by the individual nozzles are preferably contiguous, but could be spaced, or, less preferably merge into one another at a level above, at or below the region upon which the cryogen impinges. In consequence the spray headers  206  and  210  and the rows of orifices  208  and  212  need to be longer than in their counterparts in the apparatus shown in  FIGS. 1 and 2 . If, for example, the apparatus shown in  FIG. 3  has a row of three nozzles  204  it may produce spray crystallised fat particles at up to three times the rate of the apparatus shown in  FIGS. 1 and 2  without any comparable increase in the volume of the chamber and without creating the recirculation problems that are associated with operation of the known spray crystalliser which produces a conical distribution of particles. 
     Although not shown in  FIG. 3 , the three nozzles may be supplied with pressurised molten fat from a header located outside the chamber  202 . 
     Referring now to  FIG. 4  of the drawings, the apparatus illustrated therein comprises a generally box-shaped chamber housing two arrays of nozzles and spray headers, both wholly analogous in configuration and operation to the single arrange described above with reference to  FIG. 3 . The first arrange comprises a row of nozzles  304 ( a ) for atomising liquid or molten fat and producing a sheet  320 ( a ) of descending particles, a first cryogen discharge member in the form of a first spray header or tube  306 ( a ) having a row of cryogen discharge orifices  308 ( a ) formed in it and a second cryogen discharge device or member in the form of a second spray header or tube  310 ( a ) having a row of cryogen discharge orifices  312 ( a ) formed in it. The second array is analogous and its nozzles  304 , spray headers  306  and  310 , and orifices  308  and  312  are all shown in  FIG. 4  with the suffix (b). If there is a total of six nozzles  304  in the two rows, then the apparatus is operable at up to six times the production rate of that described above without there being recirculation problems but at the expense of a larger chamber. Indeed, if desired, the length of each row may be extended so as to accommodate additional atomising nozzles. It is also possible to place a third row of atomising nozzles  304  between the other two rows and arrange for the spray headers  310 ( a ) and  306 ( b ) to be provided each with a second set of orifices such that they direct cryogen at liquid issuing from the third row of atomising nozzles and effect its spray crystallisation. If desired, further rows of atomising nozzles and the requisite further cryogen spray headers may be accommodated. 
     The apparatus shown in  FIG. 5  of the drawings embodies a different approach to the design of a spray crystalliser capable of operating at a production rate many times in excess of the maximum achievable with the apparatus described above with reference to  FIGS. 1 and 2 . now there are two identical contiguous chambers  402 ( a ) and  402 ( b ), one array of nozzles  404 ( a ), and spray headers  406 ( a ) and  410 ( a ) with respective rows of orifices  408 ( a ) and  412 ( a ) being housed in the chamber  402 ( a ) and producing a sheet  420 ( a ) of particles and the corresponding parts identified by the suffix (b) being housed in the chamber  402 ( b ). Although the two chambers  402 ( a ) and  402 ( b ) may occupy a larger volume than the single chamber  302  of the corresponding apparatus described above with reference to  FIG. 4 , because they are of identical configuration, they make possible the execution of a modular approach to the design and construction of a spray crystalliser. Thus it is possible to build and operate a spray crystalliser comprising a row of such modular units. Further, in view of the large aspect ratio of the sheet or contiguous sheets of particles in each chamber, the aspect ratio of each chamber is similarly large. Thus, each time a module is attached, the aspect ratio may be reduced, with the increase in size of the apparatus taking place in one dimension only. 
     Referring now to  FIG. 6 , there is shown a yet further apparatus embodying another different design approach. Now there are four rows of atomising nozzles  504 ( a ),  504 ( b ),  504 ( c ) and  504 ( d ), arranged on the sides of a square in an upper region of a box-shaped chamber  502 . Each row of atomising nozzles  504  has two spray headers  406  and  510  associated with it, one located on one side of the-four sided figure defined by the “sheets”  520  of atomised molten fat particles, and the other located on the other side. The spray headers  506  and  510  are endless and square in configuration and are formed with rows  508  and  512 , respectively, of cryogen discharge orifices. If there are three nozzles  504  in each row, then the apparatus shown in  FIG. 6  is capable of operating at a production rate of up to 12 times higher than the apparatus shown in and described with reference to  FIGS. 1 and 2 . A disadvantage of the apparatus shown in  FIG. 6  is, however, that such is the number of spray headers and nozzles in a confined space that there may be difficulties in accommodating all the associated piping without making the chamber  502  unnecessarily large. (In general, it is preferred to minimise the cross-sectional area of the chamber such that it is not that much greater than, say, that required by the spray headers but sufficient to accommodate the large increase in volume undergone by a vaporising cryogen such as liquid nitrogen without any attendant rise in pressure much above atmospheric pressure.) The spray crystalliser shown in  FIG. 6  may, if desired, be scaled up in one of two different ways. First, the size of the square figure along the sides of which the atomising nozzles are disposed may be increased, thereby allowing more nozzles to be accommodated. Second, there may be added further concentric and alternating arrays of atomising nozzles and cryogenic spray orifices. 
     Yet another embodiment of spray crystalliser is shown in  FIG. 7 . This embodiment is different from those shown in  FIGS. 1 to 6  in that the housing  602  has a cylindrical shape, whereas in all the other embodiments the housing is box-shaped or cuboidal. Further, the atomising nozzles  604  are all circumferentially disposed and equally circumferentially spaced. In addition, rather than having rectilinear outlet apertures, as in the other illustrated embodiments, the nozzles  604  have elongate arcuate outlet apertures. As a result the atomised particles are ejected from each nozzle  504  as an arcuate sheet. The spacing of the nozzles  504  is typically that the sheets are contiguous or merge into one another to occupy most or all of the area of a cylindrical surface  620 . A further consequence is that the spray headers  606  and  610  instead of being rectilinear tubes are in the form of rings similar to the one illustrated in FIG. 2 of EP-B-393 963. Of course, now, one spray ring  606  is positioned inside the cylindrical “sheet”  620  and the other spray ring  610  is positioned outside the sheet  620 . In use of the inner spray ring  606  the cryogen is ejected generally radially outwards from a ring of orifices  608  and in use of the outer spray ring  610  the cryogen is ejected generally radially inwards from a ring of orifices  612 . In other respects, however, the operation of the spray crystalliser shown in  FIG. 7  is analogous to the operation of those described with reference to  FIGS. 1 and 2  and  FIGS. 3 to 6 , respectively. 
     The spray crystalliser shown in  FIG. 7  may, if desired, be scaled up in one of two different ways. First, the size of the circle on the circumference of which the atomising nozzles may be increased, thereby allowing more nozzles to be accommodated. Second there may be added further concentric rings of atomising nozzles and cryogenic spray orifices. 
     If desired, in any of the spray crystallisers shown in  FIGS. 1 to 7 , the position and/or orientation of the cryogen spray headers and their orifices relative to the atomising nozzle(s) may be adjustable and may be optimised for the particular product to be spray crystallised. In most cases in which a plurality of atomising nozzles is used the preferred arrangement is one in which the intersection of the cryogen with the oil (or other liquid to be spray crystallised) occurs upstream of (or above) any merger of oil sprays from separate atomising nozzles. 
     Referring now to  FIG. 8 , a chamber  702  is provided with an atomising nozzle  704  and liquid nitrogen spray headers  706  and  710  analogous to the comparable parts described with reference to and illustrated in  FIGS. 1 and 2 . The atomiser  704  is supplied with hot molten fat. The exact temperature of the molten fat is carefully controlled so as to ensure that delivery ducts and nozzles, including those of atomisers, are never at risk of blocking as a result of phase change (solidification). A pump  750  is employed to deliver the molten fat to the atomiser  704 . The pump  704  is capable of delivering the molten fat at the required pressure for atomisation and at a chosen and accurately controlled mass flow rate. The pump  750  may for example be of the peristaltic kind. The spray headers  706  and  710  are supplied with liquid nitrogen via a common main  752  communicating via a pipeline  755  with a source  754  of pressurised liquid nitrogen (which source  754  may take the form of a conventional storage vessel). 
     The chamber  702  is open at its bottom terminating into a chute  756  which is able to guide free-flowing solidified particles of crystalline fat into the inlet of an auger  758  and through which gas comprising spent vaporised liquid nitrogen and atomising gas also flows into the inlet of the auger  758 . Operation of the auger  758  urges the particles to a collection station  760  where they may be fed into suitable storage containers (not shown), for example into drums or sacks. The gas now largely but not completely free of entrained particles of solidified fat flows along a conduit  762  into a cyclone  764  in which residual entrained fine particles of solid fat are finally disengaged from the gas. The particles are discharged from the bottom of the cyclone  764 , if desired, through a rotary valve  766  and may be collected in suitable storage containers. The gas may be vented from the top of the cyclone  764  to the atmosphere, or, if desired, may, as shown in  FIG. 8 , be compressed in a recycle compressor  768  to an atomising pressure and passed via a conduit  770  to the nozzle  704  as an atomising gas. 
     If such recycle is performed, another means is provided for venting gas from the apparatus shown in  FIG. 8 . For example, the chamber may be provided with an outlet  772  for gas at its top. The outlet  772  communicates via a pipeline  774  with a baghouse  776  operable in a conventional manner to remove particulate material from the gas. The gas may be vented via outlet pipe  778  to the atmosphere. If desired, a blower  780  may be provided to assist the flow through the baghouse  776 . If desired a flow control valve  782  may be disposed in the pipeline  774  and associated via a programmable valve controller  784  of conventional kind with a pressure sensor  786  located in the chamber  702 , the arrangement being such that the pressure may be maintained at a chosen pressure, typically in the range 1 to 1.5 bar, by automatic adjustment of the position of the valve  782 . 
     If desired, the flow of liquid nitrogen (or other liquid cryogen) into the main  752  may be controlled by means of a flow control valve  790  in the pipeline  755 . The valve  790  is operatively associated with a temperature sensor  792  (which may take the form of a thermocouple) able to sense the temperature of the spent gas. The temperature sensor  792  may be located in the chute  756  and generate temperature signals that are relayed to a programmable valve controller  794  of a conventional kind. In one arrangement, the flow control valve is operated to maintain the sensed temperature of the spent gas at a chosen value (say, minus 10° C.) or in a chosen range. The chosen value or range may be determined empirically to be that at or in which adequate solidification of the fat takes place with minimal or approaching minimal consumption of liquid nitrogen. One of the advantages of the method and apparatus is that they make it possible to keep down recirculation of solidified particles of fat within the chamber. Thus, the chosen temperature and nitrogen flow rates can be optimised to achieve set criteria, which may include desired product characteristics and/or minimum cryogen consumption per unit production of processed fat.