TECHNICAL FIELD

The present invention relates to a process for extracting palm oil from oil palm fresh fruit bunches and more particularly to a process for separating and clarifying palm oil from a liquor consisting of a mixture of palm oil, free water, bound water, suspended solids, dissolved solids and gums originating from prior processing steps in the palm oil extraction process in a manner that no highly polluting liquid effluent is produced as a byproduct that must be treated prior to discharge to minimize its impact on the environment.

BACKGROUND ART

The conventional palm oil extraction process is shown in Figure 1. Fresh fruit bunches (FFB) 1 are cooked during sterilization 2 using steam and then, as sterilized fruit bunches 3, stripped 4 to separate the sterilized fruits from the empty fruit bunches. The sterilized fruits 5 are then reheated and agitated in steam-heated vessels known as digesters 6 to loosen the mesocarp from the nuts in preparation for pressing 7. The screw press expels a liquor 8 consisting mainly of oil, water and solids and a press cake of fibre and nuts. The solids in press liquor 8 consist of coarse and fine fibrous solids, dissolved solids and sand. The water in press liquor exists in one of two states, either as free water or as bound water. Free water has the properties of bulk water, and is quite easily removed, either by mechanical dewatering or by the application of heat. Bound water is bound to the solid particles, either physically or chemically, and consequendy requires more energy for it to be removed. The press liquor also contains a high level of phosphorus compounds, commonly referred to as phospholipids or gums.

The oil in press liquor 8 has to be separated from the water and solids and this takes place during clarification. In the conventional clarification process, the primary separation is achieved in settling tanks using gravity. For optimum separation, it is first necessary to dilute the press liquor 8 with hot water 21 to reduce its viscosity. The diluted press liquor is then screened 9 to remove the coarse fibrous solids 20 that are subsequendy returned to the digesters 6. The screened and diluted press liquor 10 is then heated and pumped into the clarification tank 11 where it separates into two phases, i.e. oil 12 and sludge 16.

The oil layer 12 in the clarification tank is skimmed off and passed to centrifugal purifier 13 which reduces the dirt content to 0.01 percent or less. The centrifuged oil is then dried in vacuum drier 14 to give a product of crude palm oil 15 with a moisture content of approximately 0.1 percent.

Sludge 16 from the clarification tank has approximately 4 to 10 percent oil, the bulk of which can be recovered using centrifugal separator 18 after desander 17 leaving substantially de-oiled sludge 19. The oil recovered by the sludge centrifuge 18 contains some water and dirt and is therefore returned to the clarification tank 1 for further treatment.

The process generates about 0.6 tons of palm oil mill effluent (POME) for every ton of FFB processed. It is widely acknowledged that a more effective method of treating the POME than the widely used anaerobic/aerobic ponding system is needed. One solution to the problem is to use an evaporator system for removing water from POME, thereby reducing the amount of POME discharged. The high moisture content and large quantity of raw POME means that the load on the evaporator system will be high. Concentration beyond 20 to 30 percent solids is also not possible because the product becomes highly viscous. The use of a decanter to n inimize the suspended solids concentration before evaporation and a drier to increase the solids concentration beyond 20 to 30 percent after evaporation adds to the cost. The need to burn empty fruit bunches to meet the very high energy demands of the evaporator and drier systems also add to the overall cost. An alternative solution to the effluent problem is to modify processes in the mill so that the amount of POME discharged is reduced significantly to facilitate its treatment cost-effectively using zero-discharge technologies such as composting. In Malaysian Patent Application No. PI20071265, Sivasothy Kandiah et. al. disclose a new process, illustrated by Figure 2, for extracting palm oil from a feed liquor where the water content of the feed liquor is first reduced using an evaporator. Oil/sludge separation after evaporation is achieved using either a decanter or a settling tank. To minimize the carryover of oil with the sludge phase from the oil/ sludge separation means and to facilitate its further treatment using oil recovery means such as centrifuges, not all of the free water is removed by evaporation. Hence, although the amount of liquid effluent is significandy reduced by using this process, the liquid effluent must still be treated before it can be discharged.

In Malaysian Patent Application No. PI20090863, Sivasothy Kandiah et. al. disclose another process, illustrated by Figure 3, for carrying out the clarification process based on evaporation. In this process, a significant portion of the suspended solids in the feed liquor is first removed by a two-phase separation means, such as a decanter or a filter, to facilitate both the moisture removal by evaporation and the subsequent oil/ sludge separation and oil recovery steps without the addition of water. To facilitate oil/sludge separation and oil recovery after evaporation using equipment similar to that used in a conventional palm oil mill, only about 50 percent of the water in the feed liquor is removed by evaporation. As in the previous process, the reduced quantity of liquid effluent will still require treatment before it can be discharged. In view of the large quantity of POME discharged from the conventional palm oil extraction process, and the failure of existing effluent treatment methods to provide a satisfactory solution to this problem, a means is required for separating and clarifying palm oil using a process that generates no liquid effluent, thus obviating the need for treating the effluent using an effluent treatment plant. DISCLOSURE OF INVENTION

It is an objective of the present invention to provide a simple and cost-effective process for clarifying palm oil in a manner that produces no liquid effluent as a byproduct, thereby obviating the need for treating the effluent biologically using the traditional anaerobic/aerobic ponding system.

It is a further objective of the present invention to provide a process for clarifying palm oil that maximizes the extraction of palm oil from the mcoming feed by resolving the emulsion problem brought about by the presence of gums. It is a further objective of the present invention to provide a process for clarifying palm oil that facilitates, at least, partial recycling of process water and steam condensate, thus reducing the water footprint and water treatment costs.

It is a further objective of the present invention to provide a process for clarifying palm oil under conditions that minimizes the extent of oil quality deterioration and requires significandy less processing time than the conventional process. The shorter processing time will lead to improvements in oil quality.

It is a further objective of the present invention to provide a process for clarifying palm oil that facilitates the utilization of the by-products of the milling process.

The above objectives are achieved in the present invention by providing a multi- step clarification process for extracting palm oil from a feed liquor consisting of a mixture of palm oil, free water, bound water, suspended solids, dissolved solids and gums originating from prior processing steps in the palm oil extraction process, wherein the free water in the feed liquor is removed in an energy-efficient manner by evaporation to convert the waste discharged from the clarification process from a liquid form to a more easily disposable solid form by making use of the large amount of oil in the feed liquor as a fluidizing agent. The crude palm oil extracted from palm fruits by screw pressing contains a high level of phosphorus compounds commonly called phospholipids, or phosphatides or gums. It is vital to remove the phospholipids from crude palm oil because their presence will impart undesirable flavour and color, and shorten the shelf life of the oil. The gums are mosdy water-soluble and end up in the effluent discharged from the clarification process. Any remaining gums in the oil are removed during the refining process by a degumrning step.

The gums are well-known for their emulsifying action. By lowering the interfacial tension between the oil and sludge phases, they promote the formation of stable oil droplets in the sludge phase, and lead to higher oil content in the underflow from the settling tank. The gums are also responsible for the formation of a stable oil- in-water layer at the interface between the oil and sludge layers. This third layer can significandy slow down and reduce the overall effectiveness of oil/sludge separation in the settling tank.

The presence of gums in the effluent discharged from the conventional clarification process makes the use of methods based on filtration inappropriate for effluent treatment due to susceptibility to plugging up of pores in the filter media by the gel-like gummy substance.

Evaporation provides a simple method for resolving the problems brought about by the presence of gums. Removing most of the free water breaks the oil/water emulsions and leads to the gums becoming insoluble and forming a gel-like substance in the evaporated liquor. An effective method must, however, be found for removing the large quantity of gums in a manner that minimizes the carry-over of oil with the gums. The use of a disk-stack centrifuge with intermittent solids discharge will be inappropriate for this application since they are generally only applied for separation tasks involving relatively small concentrations of solids. The use of methods based on filtration will also not be appropriate due to the plugging problem. The gums can, however, be removed by using a decanting centrifuge by ensuring that the feed to the decanting centrifuge contains sufficient suspended solids to act as a carrier for conveying the gums out of the decanting centrifuge. The new extraction process, therefore, incorporates the following process steps as part of the overall clarification process: (a) removal of most or all of the free water by an evaporation means, using oil in the feed to the evaporation means as a fluidizing medium, to convert the effluent discharged from a liquid form to a solid form and to break oil/water emulsions and form an evaporated liquor in which the emulsifying gums are mosdy insoluble and form a gel-like substance; and

(b) removal of gums and solids by a decanting centrifuge means by ensuring that there is sufficient suspended solids in the feed to the decanting centrifuge means to act as a carrier to facilitate conveying the gel-like gums out of the decanting centrifuge means.

Various alternative clarification process configurations are possible based on coupling of the abovementioned process steps with other process steps. A number of these clarification process configurations are based on carrying out evaporation prior to separation using a decanting centrifuge. The removal of most of the free water will lead to an increase in viscosity, in spite of the higher oil content, since the evaporated liquor will contain both fibrous suspended solids and the gel-like gums. An obvious disadvantage of these clarification process configurations is that the viscous nature of the evaporated liquor will increase the likelihood of fouling of the evaporator. Separation of the solids and gums from the evaporated liquor is possible by using a decanting centrifuge, in spite of the higher viscosity, with the suspended solids in the evaporated liquor acting as a carrier to facilitate conveying of the gel-like gums out of the decanting centrifuge. Oil loss with the solids discharged from the decanting centrifuge will be high due to the viscous nature of the evaporated liquor. If the oil discharged from the decanting centrifuge is fairly clean, then the carry-over of oil with the solids will be even higher.

To miriimize the carry-over of oil with the solids, the dirt content of the oil must be increased. Alternatively, the amount of water removed by evaporation must be reduced. In both of these cases, a small amount of recycling will be necessary. A significant advantage of the clarification plant configurations based on carrying out evaporation prior to separation using a decanting centrifuge is that little or no recycling is required. The main disadvantages are the increased likelihood of evaporator fouling and the likelihood of higher oil loss with the solids discharged from the decanter. If the solids leaving the decanter still contain a large amount of recoverable oil, it may require further treatment to minimize this oil loss. Due to its solid form, the recovery of some of this residual oil can, most probably, only be achieved by solvent extraction.

A number of clarification process configurations are also possible based on carrying out separation using a decanting centrifuge prior to evaporation. A significant advantage offered by these clarification process configurations is that they facilitate removal of suspended solids and gums before evaporation to minimize fouling of the evaporator. Removal of sufficient water by evaporation will produce a liquor in which the emulsifying gums are mostly insoluble and form a gel-like substance that readily precipitates due to the lower viscosity of the evaporated liquor. A method is required for removing the large quantity of gums from the evaporated liquor since it contains insufficient suspended solids to act as a carrier to facilitate removal of the gel-like gums by a decanting centrifuge. Studies have shown that if the evaporated liquor is either recycled completely, or recycled after separating most of the oil by using a settling tank, and mixed with the feed liquor, the suspended solids in the feed liquor can act as a carrier to facilitate removal of gums by the decanting centrifuge. Recycling also makes use of the free water in the feed liquor to n inimize the carry-over of oil with the solids discharged from the decanter.

The main disadvantages of carrying out separation using a decanting centrifuge prior to evaporation is the tendency for the overall clarification process configuration to be more complicated and the large amount of recycling required.

Evaporation of the free moisture from the feed liquor in the present invention provides a number of advantages compared to evaporation of moisture from POME as proposed in a prior art process. The presence of a significant quantity of oil in the feed to the evaporation means ensures that the evaporated liquor does not become viscous. If the evaporation means used is a forced circulation evaporator, fouling of evaporator tubes will be significantly reduced, even if a large percentage of the moisture in the feed liquor is removed. Also, the quantity of moisture to be removed by evaporation from the feed liquor is significantly less than the quantity of moisture to be removed from raw POME in a conventional mill.

The use of a multiple-effect evaporator system makes possible the removal of moisture using a fraction of the energy required by a drier to remove an equal amount of moisture from POME, especially if heated air is used to supply the energy for drying. The significant energy savings can be explained by using the following example. If we consider the heat balance of a single-effect evaporator, we find that the heat content (enthalpy) of the evaporated vapour is approximately equal to the heat input on the heating side. About 1 kg/hr of vapour will be produced by 1 kg/hr of live steam, as the specific evaporation heat values on the heating and product sides are about the same. If the vapour produced is used as heating steam in a second effect, the energy consumption of the overall system is reduced by 50 percent. This principle can be continued over further effects to save even more energy. The theoretical steam consumption of a triple-effect evaporator system is therefore one-third of the steam consumption of a single-effect evaporator system for an equivalent evaporation load.

The removal of free water in the feed liquor by evaporation to convert the effluent leaving the clarification process from a liquid form to a more easily disposable solid form according to the present invention provides a number of advantages over the methods disclosed in Malaysian Patent Application No. PI20071265 and Malaysian Patent Application No. PI20090863. Since there is no liquid effluent discharged, no effluent treatment system is required. The solids leaving the decanter may be disposed or sold off without undergoing any further treatment in the palm oil mill. To enhance its value as a food product or a fertiliser, its moisture content may be further reduced by drying. If the solids leaving the decanter still contain a large amount of recoverable oil, it may require further treatment to minimize this oil loss. BRIEF DESCRIPTION OF DRAWINGS

Figure 1 illustrates schematically the conventional palm oil extraction process using oil palm fresh fruit bunches as the feed.

Figure 2 illustrates schematically the prior art process for extracting palm oil using evaporation disclosed in Malaysian Patent Application No. PI20071265.

Figure 3 illustrates schematically the prior art process for extracting palm oil using evaporation disclosed in Malaysian Patent Application No. PI20090863.

Figures 4a and 4b illustrate schematically one preferred embodiment of the present invention, wherein evaporation is carried out prior to separation using a two- phase decanting centrifuge and evaporation achieves substantially complete removal of the free water.

Figures 5a and 5b illustrate schematically another preferred embodiment of the present invention, wherein evaporation is carried out prior to separation using a two- phase decanting centrifuge and evaporation does not achieve substantially complete removal of the free water.

Figures 6a and 6b illustrate schematically another preferred embodiment of the present invention, wherein evaporation is carried out prior to separation using a three- phase decanting centrifuge and evaporation does not achieve substantially complete removal of the free water.

Figures 7a and 7b illustrate schematically another preferred embodiment of the present invention, wherein separation using a two-phase decanting centrifuge is carried out prior to evaporation and the evaporated liquor is fed to a settling tank and the underflow from the settling tank recycled and mixed with the feed liquor. Figures 8a and 8b illustrate schematically another preferred embodiment of the present invention, wherein separation using a two-phase decanting centrifuge is carried out prior to evaporation and the evaporated liquor is recycled and mixed with the feed liquor and fed to the decanting centrifuge.

Figures 9a and 9b illustrate schematically another preferred embodiment of the present invention, wherein separation using a three-phase decanting centrifuge is carried out prior to evaporation and the evaporated liquor is recycled and mixed with the feed liquor and fed to the decanting centrifuge.

In describing the preferred embodiments of the invention, which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

MODE(S) FOR CARRYING OUT THE INVENTION

Referring generally to Figures 4a to 9b, the feed liquor to the new process emanates from prior processing steps used for extracting palm oil from FFB and consists of a mixture of oil, bound water, free water, coarse and fine fibrous solids, dissolved solids, particles of sand and gums. The feed liquor may either be a mixture comprising the liquor drained from digesters and the liquor expelled from presses or a mixture comprising the liquor drained from digesters, the liquor expelled from presses and all or a portion of the condensate discharged from the sterilization process. No dilution water is added to the feed liquor to facilitate oil/ sludge separation in the new clarification process. The undiluted feed liquor may advantageously be subjected to pre-treatment using a screening means to remove coarse fibrous solids (that may subsequently be recycled) and a de-sanding means to remove sand. The most commonly used screening means and de- sanding means in a palm oil mill are multi-deck vibrating screens and multi-stage de- sanding hydro-cyclones, respectively. According to one mode for carrying out the invention, as illustrated in Figures 4a and 4b, the feed liquor 200, with or without pre-treatment, is optionally mixed with condensate discharged from the sterilization process and water used for cleaning-in-place (CIP) of pipes, vessels, and process equipment 209 and fed to evaporation means 212 for substantially complete removal of free water, generating process condensate 218 and evaporated Uquor 214. The substantially complete removal of free water breaks oil/water emulsions and leads to the gums becoming mostly insoluble and forming a gel-like substance in the evaporated liquor 214. The evaporated liquor 214 is reheated to approximately 90°C using heat exchanger 221 and then fed to two-phase decanting centrifuge means 223 to separate solids and gums in heated evaporated liquor 222 using centrifugal force to generate easily disposable solid phase 224 and oil phase 225. Oil phase 225 leaving the process may still contain some moisture and impurities and may therefore require further treatment. The substantially complete removal of free water by evaporation means 212 will result in a high carry-over of oil in solid phase 224.

According to another mode for carrying out the invention, as illustrated in Figures 5a and 5b, the feed liquor 300, with or without pre-treatment, is mixed with underflow 330, and is optionally mixed with condensate discharged from the sterilization process and water used for CIP of pipes, vessels, and process equipment 309, to form mixed liquor 311. Mixed liquor 311 is fed to evaporation means 312 for removal of most, but not all, of the free water. The removal of free water breaks oil/ water emulsions and leads to the gums becoming insoluble and forming a gel-like substance in evaporated liquor 314. Evaporated Uquor 314 is reheated using heat exchanger 321 to approximately 90°C and then fed to two-phase decanting centrifuge means 323 to remove soUds and gums in heated evaporated Uquor 322 to generate easily disposable soUd phase 324 and Hquid phase 325. liquid phase 325 consists mainly of oil, but will also contain water and impurities such as soUds and gums, and wiU therefore require further treatment to clarify the oil. The further treatment consists of using settling means 328 to remove the impurities to produce oil 329. The settling means may advantageously be a simple holding tank, in which case the underflow from the settling means 330, consisting of a mixture oil, water and impurities not removed by decanting centrifuge means 323, is recycled as described above. The oil 329 leaving the process may still contain some moisture and impurities and may therefore require further treatment. The removal of most, but not all, of the free water by evaporation means 312 will result in a low carry-over of oil in solid phase 324. Another mode for carrying out the invention is illustrated in Figures 6a and 6b.

This mode is quite similar to the mode illustrated in Figures 5a and 5b, except for the use of a three-phase decanting centrifuge means instead of a two-phase decanting centrifuge means. In this mode for carrying out the invention, the feed liquor 400, with or without pre-treatment, is mixed with heavy phase 425, and is optionally mixed with condensate discharged from the sterilization process and water used for CIP of pipes, vessels, and process equipment 409, to form mixed liquor 411. Mixed liquor 411 is fed to evaporation means 412 for removal of most, but not all, of the free water. Evaporated liquor 414 is reheated using heat exchanger 421 to approximately 90°C and then fed to three-phase decanting centrifuge means 423. The use of a three-phase decanting centrifuge means simplifies the clarification process by making it unnecessary to use a settling means after the decanting centrifuge means for clarifying the oil. Three- phase decanting centrifuge means 423 facilitates separation of heated evaporated liquor 422 into three phases, to generate easily disposable solid phase 424, heavy phase 425 and oil phase 426. Heavy phase 425 is recycled as described above. Oil phase 426 leaving the process may still contain some moisture and impurities and may therefore require further treatment.

According to yet another mode for carrying out the invention, illustrated in Figures 7a and 7b, feed liquor 500, with or without pre-treatment, is mixed with underflow 527 to generate mixed liquor 510 and heated using heat exchanger 511 to approximately 90°C and then fed to two-phase decanting centrifuge means 513 to remove solids and gums in heated mixed liquor 512 to generate easily disposable solid phase 514 and liquid phase 515 having a much lower viscosity than mixed liquor 510 due to the removal of solids and gums. Liquid phase 515 is optionally mixed with the condensate discharged from the sterilization process and the water used for CIP of pipes, vessels, and process equipment 516 and fed to evaporation means 519 for removal of most, but not all, of the free water. The removal of free water by evaporation means 519 will break oil/water emulsions and lead to the gums becoming insoluble and forming a gel-like substance that precipitates quite easily due to the low viscosity of evaporated liquor 521. Evaporated liquor 521 is fed to settling means 526 to clarify the oil to produce oil 528. The underflow 527, consisting of a mixture oil, water, solids and gums, is recycled as described above. Oil 528 leaving the process may still contain some moisture and impurities and may therefore require further treatment.

According to yet another mode for carrying out the invention, illustrated in Figures 8a and 8b, feed liquor 600, with or without pre-treatment, is mixed with evaporated liquor 630 to generate mixed liquor 610 and heated using heat exchanger 611 to approximately 90°C and then fed to two-phase decanting centrifuge means 613 to remove solids and gums in heated mixed liquor 612 to generate easily disposable solid phase 614 and liquid phase 615 having a much lower viscosity than mixed liquor 610 due to the removal of solids and gums. Liquid phase 615 is fed to settling means 618 to clarify the oil to produce oil 620. The underflow 619, consisting mainly of water, solids and gums, is optionally mixed with oil 624 to increase the oil content, and optionally mixed with condensate discharged from the sterilization process and water used for CIP of pipes, vessels, and process equipment 625, and fed to evaporation means 628 for removal of most, but not all, of the free water. The removal of free water by evaporation means 628 will break oil/ water emulsions and lead to the gums becoming insoluble and forming a gel-like substance that precipitates quite easily due to the low viscosity of evaporated liquor 630. Evaporated liquor 630 is recycled as described above. Oil 623 leaving the process may still contain some moisture and impurities and may therefore require further treatment.

Another mode for carrying out the invention is illustrated in Figures 9a and 9b. This mode is quite similar to the mode illustrated in Figures 8a and 8b, except for the use of a three-phase decanting centrifuge means instead of a two-phase decanting centrifuge means. In this mode for carrying out the invention, feed liquor 700, with or without pre- treatment, is mixed with evaporated liquor 728 to generate mixed liquor 710 and heated using heat exchanger 711 to approximately 90°C and then fed to three-phase decanting centrifuge means 713. The use of a three-phase decanting centrifuge means simplifies the clarification process by making it unnecessary to use a settling means after the decanting centrifuge means for clarifying the oil. Three-phase decanting centrifuge means 713 facilitates separation of heated mixed liquor 712 into three phases, to generate easily disposable solid phase 714, heavy phase 715 having a much lower viscosity than mixed liquor 711 due to the removal of solids and gums, and oil phase 716. Heavy phase 715, consisting mainly of water, solids and gums, is optionally mixed with oil 720 to increase the oil content, and optionally mixed with condensate discharged from the sterilization process and water used for CIP of pipes, vessels, and process equipment 723, and fed to evaporation means 726 for removal of most, but not all, of the free water. The removal of free water by evaporation means 726 will break oil/ water emulsions and lead to the gums becoming insoluble and forming a gel-like substance that precipitates quite easily due to the low viscosity of evaporated liquor 728. Evaporated liquor 728 is recycled as described above. Oil phase 719 leaving the process may still contain some moisture and impurities and may therefore require further treatment.

The embodiments of the invention described herein are only meant to facilitate understanding of the invention and should not be construed as limiting the invention to those embodiments only. Those skilled in the art will appreciate that the embodiments of the invention described herein are susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within the scope of the inventive concept thereof.

INDUSTRIAL APPLICABILITY

The present invention finds ready industrial applicability in the palm oil industry as it is a novel process for separating and clarifying palm oil from a liquor consisting of a mixture of oil, water, solids and gums originating from prior processing steps in the palm oil extraction process while achieving environmental compliance and reducing the carbon and water footprints by using evaporation technology. The process provides a number of advantages compared to the conventional process.