Plate Heat Exchanger in Edible Oil Processing 2
4. Special Alkaline Refining Processes
Plate heat exchanger is widely used in the special Alkaline oil refining processes.
4.1. Neutralisation of crude oil
This method has become established mainly in the USA for refining soybean oil, the so called long mix process. The process is a combination of degumming and neutralization. For the conditioning of the nonhydratable phosphatides, a small quantity of phosphoric or citric acid is added to the crude non-degummed oil. In some cases, the acid is added in the feed tank with an agitator upstream of the installation; several hours of reaction time must be taken into consideration. However, a more effective method is to add the acid upstream of a centrifugal mixer. The intensive mixing enables the reaction time to be reduced to a few minutes. Diluted caustic soda with a concentration between 7 and 12% is then added in order to neutralize the free fatty acids. It is necessary to ensure that adequate water is present with the caustic in order to hydrate the phosphatides.
Following a reaction time of several minutes in special retention mixers, the oil is heated and sent directly to the first separator in order to separate the soapstock. The neutral oil is washed in order to further reduce the residual soap content. For this purpose, approximately 3 – 10% of hot water is added to the oil, mixed in a dynamic mixer and separated into wash water and oil in a separator. The residual humidity of the oil is further reduced in a vacuum drier.
4.2. Miscella refining
This method was originally developed for neutralizing cottonseed oil. It enables most of the gossypol to be removed with simultaneously low neutralization losses.
The miscella which discharges from the oil seed extraction stage is preconcentrated either by adding the press oil, or in the first stage of hexane evaporation. The miscella temperature is then briefly set below the boiling point of the solvent (n-hexane), and the necessary quantity of caustic soda is added in order to neutralize the free fatty acids. Mixing is followed by a reaction time in special retention mixers. The soapstock is separated in the separator. Because of the high specific density difference between miscella and soapstock, the separating efficiency is at its optimum level, and it is not necessary to wash the oil after hexane evaporation. If it is necessary to treat oils with a higher phosphatide content, it is recommended that an acid is added before the caustic.
Because of the risk of explosion posed by hexane, all installation parts, and in particular the centrifuge, are explosion-proof. For these special safety considerations, the centrifuge is even blanketed with inert gas.
Some vegetable oils, such as sunflower or corn oil, contain waxes (esters of long-chained fatty alcohols and fatty acid esters), which crystallise at low temperatures and result in turbidity in the oil. Wet winterisation in combination with neutralisation is suitable for removing these waxes.
The crude oil is initially neutralised (see under neutralisation). This means that, in order to condition the gums, an acid is initially added and the free fatty acids are then neutralised with caustic soda. After the soapstock is separated in the first separator, the oil is conveyed to the actual winterising stage. A small quantity of caustic soda is again added in order to set a specific residual soap content in the oil. This soap is subsequently required as a wetting agent during crystallisation in order to bind the waxes to water. After mixing of caustic and oil, the product is cooled to the crystallisation temperature. Wax crystals form in two to four crystallization tanks connected in series. Because of the soap in the oil, these wax crystals bind to the water added to the crystallisers. In order to reduce the viscosity, the oil is carefully heated and conveyed to a separator, which continuously removes the waxy water. The oil is washed again in order to further reduce the residual soap content. For this purpose, the oil is heated, the corresponding quantity of hot water is added; after intensive mixing, it is separated in a further separator. Vacuum drying then takes place.
4.4. Cold refining
This process is an alternative to winterisation preceded by neutralization. However, use of the process is confined to oils with a relatively low FFA content as the oil losses are otherwise too high. Sunflower oil is therefore an ideal product for this application.
A small amount of acid is added to the crude oil without prior heating. After thorough mixing and a short retention time the oil is cooled. The caustic quantity required for neutralising the free fatty acids is admixed with the oil. The oil then flows through crystallisers with a residence time of several hours. The oil is gently heated to reduce the viscosity before the soapstock is separated in a centrifuge along with the waxes. The dewaxed neutral oil has to be washed to reduce the soap content. It is first heated, the required wash water quantity is added and, after thorough mixing with the oil, it is separated again in a second centrifuge. Finally, the oil is dried in a vacuum dryer.
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