MEMBRANE-BASED WASHING AND DEACIDIFICATION OF OILS

Abstract

Membrane-based method of washing and deacidification of oils, wherein a stream of oil is conveyed from an oil reservoir along one side of porous hydrophobic membrane, and washing aqueous solution is conveyed along another side of this membrane. The membranes form hollow fibers, and their total surface area and porosity are large enough for efficient removal of fatty acids, water, ions and hydrophilic organic impurities from oil. Membrane pore size is small enough, so that hydrodynamic mixing of oil and aqueous solution does not take place. Additional stabilization of oil/water meniscus in the pores is achieved by transmembrane pressure difference.

Claims

1. A membrane-based oil washing process to remove an excess of emulsified water, metal ions, water-soluble colored and smelly impurities, and fatty acids, comprising: a hollow fiber membrane module through which a flow of an oil solution and a flow of an aqueous solution are separated by one or more membranes of the hollow fiber membrane module.

2. The membrane-based oil washing process of claim 1, wherein the oil solution flows outside of one or more fibers of the hollow fiber membrane module and the aqueous solution flows inside the one or more fibers.

3. The membrane-based oil washing process of claim 1, wherein the oil solution and the aqueous solution are pumped in a recirculation mode from respective source vessels through the hollow fiber membrane module and then back to the respective source vessels.

4. The membrane-based oil washing process of claim 1, wherein both the oil solution and the aqueous solution flow in the same direction in a horizontal module, thus minimizing changes of transmembrane pressure difference in the hollow fiber membrane module.

5. The membrane-based oil washing process of claim 1, wherein the membrane is porous and is made of hydrophobic polymer material, so that the membrane pores of the one or more membranes are filled with oil.

6. The membrane of claim 5, wherein the effective pore size of the membrane pores is small enough such that hydrodynamic mixing of the oil solution and the aqueous solution does not take place.

7. The membrane-based oil washing process of claim 1, wherein additional stabilization of an oil/water meniscus in the membrane pores of the one or more membranes is achieved by applying a transmembrane pressure difference with a higher pressure in the aqueous solution.

8. The membrane-based oil washing process of claim 1, wherein the aqueous solution has a pH of greater than 10 to remove all fatty acids.

9. The membrane-based oil washing process of claim 1, wherein aqueous solution has a pH less than 5 to selectively remove shorter fatty acids.

10. The membrane-based oil washing process of claim 1, wherein to fractionate the fatty acids, one or more modules with different pH of washing solution are used in series, starting with a more acidic washing solution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The embodiments of this invention are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:

[0019] FIG. 1 illustrates kinetics of pH change due to transfer of dissolved in octane fatty acids with different alkyl chains through a flat membrane.

[0020] FIG. 2 illustrates fatty acid removal kinetics from more viscous mineral oil and less viscous octane.

[0021] FIG. 3. Gas chromatography of initial fatty acid mixture in octane, 1-octane, 2-octanoic acid, 3-pelargonic acid, 4-capric (n-decanoic) acid).

[0022] FIG. 4. Gas chromatography of the same solution after treatment in hollow fiber membrane module.

[0023] FIG. 5 Kinetics of pH change in alkaline aqueous solution due to transfer of mixture of fatty acids from octane through a hollow fiber membrane module.

[0024] FIG. 6. Schematics of the plant for fatty acid removal.

[0025] Other features of the present embodiments will be apparent from the accompanying detailed description that follows.

DETAILED DESCRIPTION

EXAMPLES

[0026] The process depends on several different physicochemical factors, and the examples below illustrate these dependences.

Example 1

[0027] This example illustrates effect of fatty acid structure, and it shows membrane-based extraction of different fatty acids from octane through flat porous membrane with surface area 10 cm.sup.2 into aqueous phase (FIG. 1). Both donor phases are stirred. Fatty acids with different alkyl chain length are washed out from octane into aqueous solution, which becomes more acidified. For the same molar acid concentration the initial rates are inversely proportional to octane/water distribution coefficients of the fatty acid, and they change by 15-20 times from ethylhexanoic to decanoic acid. Important for olive oil and other plant oils oleic acid (C.sub.18H.sub.34O.sub.2) practically is not removed from octane, as indicated in FIG. 1. At pH 4 the method may be used to selectively remove shorter fatty acids and acid-soluble colored components.

Example 2

[0028] This example illustrates initial fatty acid removal kinetics per unit area of the membrane from more viscous mineral oil and less viscous octane. FIG. 2 illustrates kinetics of 1M fatty acid transfer from different organic solvents into an aqueous buffer with initial pH 4.0. 1-pelargonic acid (C.sub.9H.sub.18O.sub.2) from mineral oil; 2-ethylhexanoic acid from mineral oil; 3-ethylhexanoic acid from octane. Similar to the first example, shorter ethylhexanoic acid is removed from mineral oil faster than longer pelargonic acid. Removal of ethylhexanoic acid is much faster from less viscous octane.

Example 3

[0029] Rate of fatty acids removal (measured in moles per unit area per unit of time) is increased by increasing acceptor pH. Thus, initial rate of octanoic acid removal at pH 11 is 10 times faster than at pH 4. For oleic acid at pH 12 initial rate is 10,000 times faster than at pH 4. As the result, at pH 12 the selectivity is practically lost, and all acids are removed simultaneously and faster. If the purpose is to remove all acids, it is better to use higher pH.

Example 4

[0030] Industrial hollow fiber membrane modules have surface area in the range from 1 to 200 m.sup.2, thus making the whole washing process much faster, which makes it possible to use membrane-based washing in practice. This is illustrated in FIG. 3 and FIG. 4. Gas chromatogram of the initial model solution in octane is presented in FIG. 3 in comparison to the results after treatment (FIG. 4). After treatment in a hollow fiber membrane module of a solution, which had a mixture of three fatty acids (0.56 M octanoic (C.sub.8) acid, 0.39 M pelargonic (C.sub.9) acid, and 0.07 M decanoic (n-capric) acid) their gas chromatography peaks practically disappeared.

Example 5

[0031] FIG. 5 demonstrates kinetics of pH decrease in a washing phase when a mixture of these three acids in octane (volume 0.5 L) was treated by 1 L of alkaline solution of K.sub.2CO.sub.3, pH 12.0 in a hollow fiber membrane module with total area 1.4 m.sup.2. After 2 hrs of washing the process practically stopped, and octane practically did not have dissolved fatty acids.

Example 6

[0032] Similar process can be used to remove fatty acid from glycerol esters (biodiesel). Water emulsified in oil and present in water alkaline metal ions, used as catalysts in biodiesel production, are removed simultaneously with fatty acids. Finally, water distribution is determined by its distribution coefficient, and for biodiesel/aqueous solutions water content is less than 0.05%.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] FIG. 6 illustrates the preferred embodiment. The process is conducted in a hollow fiber membrane module 100 with large membrane area per module volume, which may be above several thousand m.sup.2/m.sup.3. Membranes may have porosity 20-60%, and membrane thickness 15-75 microns. The membrane module has two entry ports and two exit ports. The inlet and outlet of a shell side are connected with the vessel 110 with oil, and the inlet and outlet of a tube side are connected with the vessel 120 with washing aqueous solution.

[0034] Both solutions are pumped in recirculation mode with pumps 130 and 140. Flow rate depends on the size of membrane module, and it can be varied. For example, flow rate of aqueous phase in example 4 was 150 ml/min in tube side; and flow rate of oil (in shell side) was 30 ml/min.

[0035] If the membrane is hydrophobic, oil fills the pores. To prevent oil from penetrating into aqueous solution, and to have meniscus in the pore, the pressure in tube side should be higher than that in the shell side. This pressure difference often is less than 0.1 atm, and may be regulated by simple increasing the flow rate in a tube side. Temperature range 4-120° C. (39-248° F.). Oil viscosity at 25° C. is in the range 0.5-100 mPa×s. Pressure, pH of aqueous solution and flow rates may be additionally monitored. Module can be cleaned, regenerated and used again for several months or even years.

[0036] We disclose a rapid, simple, energy—efficient and low cost process to deacidify organic oils and lipid solutions without direct mixing of organic and alkaline phases. Simultaneously it is possible to remove water and different low molecular weight impurities, including catalysts used to synthesize biodiesel. Method is based on spontaneous extraction of fatty acids and other impurities from oils into alkali solution, separated from oil by a porous membrane. Instead of a flat membrane a membrane module with membrane-based hollow fibers is used. Oil flows outside the fibers and aqueous alkali solution—inside. They are not mixed, but fatty acids, water and other low molecular weight impurities are extracted through the oil/water interface formed in membrane pores.

[0037] The method is carried out at room temperatures, which provides many benefits. First, it provides significant energy savings in comparison to distillation at high temperatures. Second, low temperature and the ability to conduct separation under anaerobic conditions means that the lipids and vitamins are not oxidized and remain stable. The benefit is a far better quality product including a clearer color, which is important for food oils, including palm and coconut oils, vegetable oils, soya and sunflower oils, etc.

Free fatty acids in alkaline solutions are converted into salts. These salts may form solid phase, which depends on temperature, and pH. Solids may be separated by traditional methods and used as a raw material, for example, in soap production or returned and reused in the biodiesel synthesis.

[0038] Advantages of the new technology:

[0039] 1. ability to remove both fatty and naphthenic acids;

[0040] 2. water is not added into oil but it is even removed from it;

[0041] 3. no water-in-oil emulsion formation, i.e., no need for demulsification or high temperature (95° C.) treatment to destabilize the emulsion;

[0042] 4. no centrifugation or mixer/settler is necessary;

[0043] 5. low loses of neutral components, low hydrolysis and saponification of lipids;

[0044] 6. ability to use alkaline solutions with ammonia, Ca(OH).sub.2, Mg(OH).sub.2;

[0045] 7. less bleaching earth is necessary;

[0046] 8. ability to use water/isopropanol mixtures as a strip phase;

[0047] 9. ability to treat miscella in hexane or octane as a feed phase;

[0048] 10. ability to improve biodiesel properties, i.e., to remove methanol, catalyst, soaps, water and glycerol (Standard ASTM D6751-02);

[0049] 11. less metal corrosion (NACE Standard TM0172-2001);

[0050] 12. process is continuous, easily scalable, energy efficient and environmentally friendly;

[0051] 13. purified oil often is ready to be used and no additional processing steps are necessary.