Equipment and process for physical refining and/or deodorisation of edible oils and fats

Abstract: Equipment for use in the physical refining and/or deodorisation of edible oils and fats, comprising a shell (1) containing a number of superimposed trays (3,8,10,12,13,28,29) for degassing, heating, cooling and/or deodorisation purposes and containing a scrubber (4), characterised in that the scrubber (4) is located at the lowest part inside the deodoriser shell (1). The process makes use of such equipment.

Description: The invention relates to an equipment and process used for physical refining and/or desodorisation of edible oils and fats. More particularly, the invention relates to a process for vacuum stripping triglyceride oils and fats. More specifically, the invention is also concerned with novel equipment used in this process. The invention is especially important for edible oils and fats to be sold for consumption or to be used in food products, since these outlets demand that these edible oils and/or fats have a bland taste. In addition, the invention is applicable to all kinds of edible oils and fats such as vegetable oils, animal fats and marine oils, their blends as well as to hydrogenated oils, fractionated oils, interesterification products and their blends. It will in general constitute the last step in edible oil processing before the product treated according to the invention is either packaged, or further processed into products such as for instance margarine. In edible oil refining, two different routes are being employed. The oldest route employs a chemical neutralisation of the free fatty acids in the crude oil as obtained by expelling and/or solvent extraction. This route commonly uses caustic soda to convert these free fatty acids into sodium soaps which are then removed by a centrifugal separator and water washing. Instead of removing residual soaps by water washing, they can also be removed by adsorption by for example silica hydrogel. Subsequently, the neutral oil is bleached by an adsorptive treatment involving the use of bleaching earth and/or activated carbon and then the malodorous compounds still present in the oil are removed by a vacuum stripping process called deodorisation. In the other route, the crude oil is first of all degummed by using an acid degumming or acid refining process to a low residual phosphorus content < 10 ppm P) without removing the free fatty acids. If such a degummed oils is then bleached it can be physically refined in a vacuum stripping process that removes both the free fatty acids and the malodorous compounds so that a bland tasting product of good keepability results. Physical refining therefore combines the neutralisation and the deodorisation of the oil. The process according to the invention can be usefully applied in both the deodorisation process and the physical refining process. In the deodorisation and physical refining processes already known, it is common to subject the oil to be processed to temperatures as high as 250°C or even higher, a pressure as low as 5 mbar or even lower and to the action of a stripping medium. The commonly used stripping medium is steam but other inert gases such as for example nitrogen are also being used as described in EP-A-0.580.896 to Cheng et al. In this context, the role of the stripping medium is to dilute the volatile compounds to be removed from the oil so that they can evaporate at higher pressures than if no stripping medium were present. For deodorisation and physical refining to be effective and to minimise the stripping medium usage, the concentration of the volatile compounds to be removed from the oil should be as close as possible to the physical equilibrium concentration as determined by the vapour pressures of the pure volatile compounds at the prevailing temperature and their concentrations in the oil. To this end, intimate contact between the stripping medium and the oil to be vacuum stripped is imperative. Such contact is achieved by introducing the stripping medium under the surface of a pool of liquid oil to be deodorised through small nozzles or by spreading this oil over a large surface that is in contact with the stripping medium. The latter method as described for instance in WO 98/00484 to Hillström et al. commonly employs packed columns for this purpose. However, when a small bubble of stripping medium is released under the surface of a pool of oil, the pressure at the point of release is equal to the sum of the system pressure and the pressure exerted by the oil column above the release point. When the bubble rises through the oil, the height of this column decreases so that the pressure inside the bubble also decreases as a result of which the bubble will expand. At the surface, the bubble will break and the gas contained in the bubble will be removed by the vacuum system. However, in doing so, the bubble will also entrain liquid oil and if this oil were to reach the vacuum system, it would be irretrievably lost and thus would constitute a yield loss. This loss is often referred to as the "neutral oil loss" or NOL. Accordingly, most industrial deodorisers have been provided with baffles that aim at retaining the oil that is being entrained by the emerging stripping medium and thus at reducing the NOL. However, these baffles often constitute a resistance to the vapour flow and thus cause the pressure at the point where the bubble leaves the oil surface to be considerably higher than the vacuum attained by the vacuum system itself. Packed columns suffer less from NOL since in these columns the stripping medium strikes over the oil surface but then, these columns themselves present a resistance to the vapour flow so that again, the pressure "seen" by the oil can be quite a bit higher than the pressure attained by the vacuum system. As mentioned before, oil to be deodorised or physically refined is brought to elevated temperatures since the vapour pressures of the volatile compounds to be removed by stripping increase with an increase in temperature so that their volatility increases. In order to save energy, the incoming oil is often pre-heated by the outgoing oil by an oil-to-oil heat recovery process so that external energy is only needed to raise the oil temperature from the level attained by heat exchange to the deodorisation temperature. The heat exchange system and/or the final heating system can located outside the deodorisation vessel proper but most installations in use incorporate these systems within the deodorisation shell. The deaeration stage on the other hand, tends to be outside this shell. Accordingly, such a shell usually contains a number of superimposed trays from which the oil to be deodorised flows downwards by gravity. From top to bottom these trays may serve the following functions: deaeration if this is contained within the shell, heating by heat exchange with outgoing oil, heating to final deodorisation temperature, deodorisation proper, cooling by heat exchange with incoming oil and finally further cooling to oil discharge temperature. In general, all trays are sparged with stripping medium which not only strips volatiles from the oil being processed, but also ensures agitation of the oil and thus promotes heat transfer. Deodorisation proper need not be confined to a single tray but may be carried out in a number of superimposed trays. Similarly, the oils can be heated or cooled after having been subjected to the first deodorisation treatment before being subjected to a subsequent deodorisation treatment. If the trays within the deodoriser shell are fixed to the deodoriser shell wall and surround a central chimney which collects the vapour emerging from the trays and mechanically supports these trays, the deodoriser is generally referred to as a single shell deodoriser. Instead of a central chimney, the gases can also be removed from each tray by individual vacuum connections connected to a common duct. If an annular gap between the trays and the outer shell acts as the vapour duct, the system is referred to as a double shell deodoriser. Flow diagrams of the various types, double shell, single shell, continuous and semi-continuous can be found in the Practical Handbook of Soybean Processing and Utilization, edited by D.R. Erickson, AOCS Press and United Soybean Board, 1995 on pages 251 and following. In general, a single shell deodoriser is less expensive to construct than a double shell deodoriser. In order to maintain a low pressure within the deodorisation shell, it has to be connected to a vacuum system. This system removes the stripping medium that is introduced into the shell, the volatiles entrained by this medium and any gas leaking into the shell. These volatiles have to be removed from the gas leaving the deodorisation shell and to this end, the gas is commonly passed through a scrubber. In this scrubber, the hot gas is contacted with cooled distillate into which the volatiles will condense. Accordingly, the vacuum system will not have to transport these volatiles and can thus have a smaller capacity which is especially important in the case of physical refining. This scrubber can be located in the top of the deodoriser shell and below the connection to the vacuum system. Liquified distillate is collected at the bottom of this scrubber and then it is partially recycled to the top of the scrubber via a cooler; what is not recycled is collected in an intermediate distillate storage vessel. However, this set-up implies that the low temperature scrubber section is immediately above a hot oil tray and this puts high demands upon the construction and construction material of the deodoriser which will have to withstand considerable thermal and mechanical stress in addition to remaining vacuum tight. Consequently, the scrubber can also be located outside the deodorisation shell in the duct that connects the deodoriser to the vacuum system. If this duct is located at the top of the deodoriser shell, the scrubber is usually nearby to minimise the pressure drop in the vacuum duct. If the vapours are collected from each individual tray into a common duct, the scrubber will be located close to this common duct but that can be at the top of this duct, at the bottom of this duct or somewhere in between. Examples of deodorising plants comprising a scrubber are disclosed by A. Athanassiadis in "Elimination of Air and Water Pollution by Double-stage scrubber, JAOCS, vol.59, No.12 (1982), pages 554-557. For the generation of the vacuum, steam ejectors and condensers are commonly used. Since these require barometric legs, the vacuum system requires at least 10 m height. Consequently, commonly used deodorisers display a connection to the vacuum system at the top of the vacuum shell for instance just above the scrubber if this is located within the deodoriser shell. Since gas needs a pressure difference to flow, this means that the system pressure in the lowest part inside the deodoriser shell is somewhat higher than at the top. Accordingly, the oil being deodorised is exposed to a higher pressure than the oil being heated or degassed since the heating and deaeration trays are situated above the deodorisation trays. This is a drawback of this type of construction since preferably, deodorisation should be carried out at the lowest pressure the vacuum system can generate. It is an object of the invention to provide equipment for the physical refining and/or deodorisation of edible oils that is less subject to thermal and especially mechanical strain than currently used equipment. It is also an object of the invention to reduce the neutral oil loss caused by entrainment. It is a further object of the invention to ensure that oil being deodorised is exposed to as low as possible pressure and thereby reduce stripping medium usage. It is yet another object of the invention to simplify the construction of the deodorisation equipment and especially the mounting of its auxiliary vessels and equipment. It is also an object of the invention to provide equipment for both continuous and semi-continuous operation. It is a further object of the invention to provide a process that can profitably be used in both single shell and double shell deodorisers. According to the invention the equipment for use in the physical refining and/or deodorisation of triglyceride oils comprises a shell containing a number of superimposed trays for degassing, heating, cooling and/or deodorisation purposes and a scrubber, whereby the scrubber is located at the lowest part of the deodorisation shell. Further embodiments of the equipment according to the invention are given in the accompanying claims 2 to 6. The invention also relates to a process for physical refining and/or deodorisation of edible oils and fats, characterised in that an equipment according to the invention is used. The invention thus claims a process according to which the tray above the scrubber is used for deaerating the oil entering the deodorisation shell, and from this tray the degassed oil is then pumped to the top tray from where it flows down to the trays underneath by gravity. Other embodiments of the process are given in the accompanying claims 9 to 16. To more clearly show the characteristics of the invention, preferred embodiments of an equipment and a process according to the invention are described in the following, as an example without any limiting character and with reference to the accompanying drawings, wherein: Figure 1 shows a flow diagram of deodorisation equipment according to the invention. Figure 2 represents a cross section of the bottom part of the deodoriser showing the path followed by the gases leaving the deodoriser shell. Figure 3 shows the flow diagram of a high capacity dual temperature, dual condensation deodoriser. As indicated in Figure 1, refined and bleached oil is continuously introduced into the deodoriser shell 1 by pump 2 into the deaeration tray 3 located above the scrubber 4. Pump 2 forces the oil through spraying heads 5 to ensure rapid degassing of the oil. It is also possible to provide stripping steam via inlets 6 to the deaeration tray 3 to assist the degassing process. At this stage, the oil temperature is for instance 90-100°C, that is to say the temperature oil normally has after having been bleached and filtered. From tray 3, the oil is fed by means of degassed oil transfer pump 7 to the final heating/pre-stripping top tray 8 within the deodoriser shell 1 via heat exchange coils 9 located in heat recovery tray 10. In the top tray 8, the oil is heated to deodorisation temperature, for instance 230°C, by means of high pressure steam generated in boiler 11 or by means of another heat exchange medium. The final heating top tray 8 is provided with sparging steam inlets 6 to agitate the oil and promote heat transfer. From top tray 8, the oil flows by gravity to the deodorisation tray 12 underneath, where it is deodorised while being sparged with steam via inlets 6. From tray 12, the hot oil flows by gravity to the heat recovery tray 10 where it is cooled by degassed oil and finally, the oil is cooled down to oil delivery temperature in final cooling tray 13 from where it is extracted by pump 14 as fully refined, bleached and deodorised oil. Trays 10 and 13 are also sparged via stripping medium inlets 6 to facilitate heat transfer and also to remove malodorous components from the oil. The vapours emerging from the various trays 3, 8, 10, 12 and 13 are collected in the central chimney 15 through regularly spaced windows 16 in the chimney wall. From this chimney 15, they are pumped out of the deodoriser shell 1 via vacuum connection 17 after having passed fatty acid scrubber 4 and demister 18. Some of the neutral oil entrained by the stripping medium bursting through the oil surfaces will enter the central chimney 15 which therefore has been provided with baffles 19 on its wall. These baffles 19 cause the gas stream to adopt a spiral motion so that the oil droplets are flung to this wall by centrifugal force. The baffles 19 on the chimney wall 15 also cause the oil running down this wall to stream around its windows 16 so that this oil will collect in the bottom of chimney 15 from where it flows to the degassing tray 3 to be reprocessed. Vapours leave the central chimney through window 20 to enter the scrubber section 4. In scrubber 4, cooled distillate is sprayed into the vapour stream through nozzles 21 causing its volatiles to condense. Pump 22 collects this distillate from the bottom pool of deodoriser shell 1 and pumps it via a cooler 23 through the spray nozzles 21 located in the vapour stream. Distillate collected in demister 18 also trickles down into the bottom pool. Vacuum connection 17 of shell 1 is connected to one or more booster pumps and atmospheric condensers ensuring the vacuum in the system. These boosters are preferably mounted on the side of deodoriser shell 1. In a preferred embodiment of the invention, the diameter of the cylindrical vapour enclosure 24 in the scrubber section 4 is larger than the diameter of the central chimney 15. This reduces the velocity of the vapour when it reaches the scrubber 4 and promotes condensation of the volatiles in the vapour stream by increasing residence time. This in turn reduces the carry-over of fatty matter into the vacuum system. Another way by which this carry-over is reduced even further is illustrated in Figure 2. This figure shows the path the vapour stream has to follow to reach the vacuum system. This vapour leaves vapour enclosure 24 through a window 25 that is located opposite the vacuum connection 17. Consequently, the vapour has to travel around half the vapour enclosure 24 before reaching vacuum connection 17. This again causes a circular motion and separation of condensate droplets by centrifugal force. These droplets collect on the wall of deodorisation shell 1 and finally in its bottom pool. Because the entire cross section of the deodoriser shell is available for this path, its tortuous route will hardly cause any pressure drop so that the capacity of the vacuum generating system is fully utilised. For simplicity's sake only a single deodorisation tray 12 has been shown in Figure 1. However, equipment with a multiple of superimposed deodorisation trays also falls within the scope of the invention. Similarly, Figure 1 depicts a single shell deodoriser where the trays are fastened to the deodoriser wall 1, but double shell deodorisers where the trays are free from the wall and the vapours escape by passing through the annular space between these trays and the wall instead of through a central chimney, also fall within the scope of the invention. According to the invention, additional heating and/or cooling trays can be introduced into the deodoriser shell 1 to allow the oil to be deodorised at different temperatures as shown in Figure 3. In both physical refining and deodorisation, it may be found to be advantageous to treat the oil first at a fairly low temperature and to collect the bulk of the free fatty acids and volatile malodorous aldehydes and ketones, and then to heat the oil to a higher temperature to remove the residual fatty acids and, if required, also strip tocopherols and sterols and other minor constituents from the oil. The condensate resulting from this high temperature stripping treatment will then have a relatively high content of tocopherols and sterols and other minor constituents and thus command a higher price per unit of weight than when their content was low. Besides, it will be fairly devoid of malodorous components since most of them will already have been removed during the low temperature stripping stage. Separate collection of low and high temperature distillates requires two separate scrubber systems. According to the invention, one of these 4 should be located at the lowest part inside the deodoriser shell but, as illustrated by Figure 3, the other scrubber 26 can be located near the top in which case this top is connected to the vacuum system by an external vapour duct 27. For constructional reasons, this second scrubber 26 is preferably located within the deodoriser shell 1 but a location outside this shell also falls within the scope of the invention. Figure 3, showing a flow diagram of this dual temperature and dual condensation system, refers to a high capacity deodoriser. Accordingly, it illustrates that both the low temperature treatment and the high temperature treatment occupy three trays 28 forming low temperature deodorising sections and three trays 29 forming high temperature deodorising sections, respectively and that two trays 10 are foreseen for the oil-to-oil heat recovery. Since the first deodorisation takes place at a lower temperature than the second deodorisation, the flow diagram also incorporates an intermediate high temperature heating section 30. The flow diagram of Figure 3 illustrates a system whereby the vapours emerging from the various trays are treated in either of two scrubbers (top vapour scrubber 26 or bottom vapour scrubber 4), but a system whereby all vapours are treated in two scrubbers in series also falls within the scope of the invention, provided one of these scrubbers is located in the lowest part inside the deodoriser shell 1. The first of these two scrubbers can be operated at a rather high temperature so that it will condense vapours with a high boiling point such as partial glycerides. The second of these scrubbers should be operated at a much lower temperature to really free the vapours from condensibles. This system has particular advantages for the physical refining and/or deodorisation of lauric oils, especially when the condensate from the high temperature scrubber is recycled so that loss of partial glycerides is minimised. A combination of the dual temperature, dual condensation system, whereby one or even both of the condensation systems consist of two scrubbers in series constitutes an embodiment of the invention provided one of the scrubbers of this system is located in the lowest part inside of the deodoriser shell. Figures 1 and 3 refer to a fully continuous deodorisation process but by providing the various trays with appropriate outlet valves, a semi-continuous operation according to the invention is also possible. In that case, returning splash oil to the deaeration tray may have to be suspended to avoid cross contamination between subsequent charges. Although it is common to use trays with more or less shallow pools of oil in physical refining and/or deodorisation, packed columns are also used industrially for these processes. When such columns are used, the oil is allowed to trickle down the packing while the stripping medium is passed counter-currently along the oil film on the packing. Such columns can also be incorporated into the equipment according to the invention by replacing one or more deodorisation trays. In that case, the vapours leaving the top of the packed column are to be guided to an annular space between the column and the deodoriser shell and from there into the central chimney if the deodoriser comprises only a single scrubber. In case of a dual condensation system, the vapours exiting the top of the column can for example be condensed in the scrubber 26 located at the top of the deodoriser shell. According to the invention it is not necessary but only preferable to incorporate the deaeration tray inside the deodoriser shell. The flow diagram of Figure 1 shows a situation whereby the degassing stage is performed by spraying the oil to de deodorised into vacuum inside the deodorising shell but this can also be done in a separate vessel outside the shell. Then the tray above the scrubber will be used for heat exchange purposes. However, from a constructional point of view there are advantages in having the coldest section scrubbing at the lowest part of the deodoriser, having a section at a somewhat higher temperature degassing above the coldest section, having a section with a still higher temperature oil-to-oil heat recovery superimposed upon the degassing section and only then allow of a further increase of the tray temperature as needed by deodorisation. In summary, the advantages of the equipment and process according to the invention over the present state of the art can be listed as follows, whereby it should be noted that these advantages far outweigh the cost of the additional pump 7 lifting the degassed oil to the top tray 8 according to the invention: Reduced neutral oil loss since splash oil is effectively separated from the gas stream in the central chimney and/or the vapour path inside the scrubber and thus recuperated automatically within the system; Improved vapour scrubbing and condensation leading to reduced fouling of the hot wells in the vacuum system; Reduced pressure drop over the scrubber section; Proximity of the final deodorisation tray to the scrubber so that this tray is exposed to the lowest possible system pressure; Minimisation of thermal and thus mechanical strains in the equipment by the separation of high and low temperature trays by intermediate temperature trays; Self supporting structure that is easy and fast to erect, that does not require to be housed inside a building and requires little floor space; Adaptability in that existing deodorisers according to the invention can be easily upgraded by adding further trays; Near absence of auxiliary vessels.

Claims: Equipment for use in the physical refining and/or deodorisation of edible oils and fats, comprising a shell (1) containing a number of superimposed trays (3,8,10,12,13,28,29) for degassing, heating, cooling and/or deodorisation purposes and containing a scrubber (4), characterised in that the scrubber (4) is located at the lowest part inside the deodoriser shell (1). Equipment according to claim 1, characterised in that contains a deodoriser of the single shell type having a central chimney (15) provided with windows (16) for extraction of gases and provided with baffles (19) that prevent oil from leaving the chimney through these windows (16) and provide the gas stream with a spiral motion before this gas stream enters the scrubber (4) through a window opening (20) into the scrubber (4). Equipment according to claim 1, characterised in that it contains a deodoriser of the double shell type. Equipment according to any one of the precding claims, characterised in that a deodoriser tray has been replaced by a counter-current packed column. Equipment according to any one of the preceding claims, characterised in that it incorporates two or more deodorisation sections operating at different temperatures. Equipment according to any one of the preceding claims, characterised in that valves have been provided to permit semi-continuous operation. Process for physical refining and/or deodorisation of edible oils and fats, characterised in that use is made of an equipment according to any of the preceding claims. Process according to claim 7, characterised in chat the oils and fats are degassed in the tray (3) above the scrubber (4) and the oil being processed is pumped from this tray (3) to the top tray (8) within the physical refining and/or deodoriser shell (1). Process according to claim 7 or 8, Characterised in that gases are extracted through windows (16) in a central chimney (15) in a single shell type deodoriser and sent to the scrubber (4). Process according to claim 9, characterised in that the neutral oil is collected in the bottom part of the central chimney (15) and is returned to the degassing tray (3). Process according to claim 7, characterised in that deodoration is performed in which in a counter-current packed column replacing the top degassing tray (12) and in that the vapour emerging from this packed column is at least partially collected by a scrubber (26) located at the top of the deodoriser, which scrubber may also be outside the deodoriser shell. Process according to claim 9 or 10, characterised in that a deodorisation is performed in a counter-current packed column and in that the vapours emerging from this column are guided towards the central chimney (15). Process according to any one of claims 7 to 12, characterised in that the gas stream follows a circular movement after having been scrubbed in the lowest part of the deodoriser shell (1) and before being collected by a vacuum system (17). Process according to any one of claims 7 to 13, characterised in that an equipment is used incorporating two or more deodorisation sections operating at different temperatures and the oil to be processed is treated first at a high temperature and then at a lower temperature, and cooled between these stages. Process according to claim 14, characterised in that the oil to be processed is heated after having already been treated before being treated again. Process according to any one of claims 7 to 15, characterised in that it comprises multi-stage scrubbing, in which the scrubbing temperature is lowered as and when the vapours move downstream.

Inventors: HARPER TONY, KELLENS MARC, HARPER, TONY, KELLENS, MARC
Applicants: SMET ENGINEERING N V DE, DE SMET ENGINEERING N.V.
Application year: 2001
Main classification: C11B3
IPC codes: B01D3, C11B3, B01D1, B01D19