GB2191490A - Trialkyl phosphine oxides and their use in a process for recovery of organic acids from aqueous solutions - Google Patents

Abstract

A mixture of trialkyl phosphine oxides having the following formula: <IMAGE> wherein R1, R2 and R3 are the same or different alkyl groups of 5 to 9 carbon atoms and the total number of carbon atoms per molecule is from 15 to 27. The phosphine oxide mixture or a tri(C2-10)alkyl phosphine oxide may be used in a water immiscible solvent as extractants for recovering organic acids from fermentation broths.

Description

SPECIFICATION Trialkyl phosphine oxides and their use in a process for recovery of organic acids from aqueous solutions This invention relates to a mixture of trialkyl phosphine oxides and their use as an extractant for the recovery of organic acids from aqueous solutions.

Recovery of acid from aqueous solution is extensively practiced in industry. Organic acids, such as citric acid, malic acid, tartaric acid and oxalic acid, have been commercially produced by fermentation or by chemical synthesis. Because of impurities existing in fermentation broths, the acid is precipitated as a calcium salt and is liberated by adding sulfuric acid so as to obtain pure organic acid solution. Then, the final product of the acid is obtained after evaporation and crystallization. This procedure has the disadvantages of requiring expendable lime and sulfuric acid as well as disposal of calcium sulfate.

A liquid-liquid extraction process, using different extractants and techniques, is known as an alternative means of recovery of organic acid from aqueous solution.

Procedures which employ amines are disclosed in British Patent No 1,428.018 and U.S. Patent No 2,539,472. U.S. Patent No 3,944,606 discloses the use of amines to recover citrate.

Because of the lower selectivity of amines, contaminant acid, such as oxalic acid, from fermentation would be co-extracted into amine solvent during the extraction process, and the unknown toxicity of amines limits their use for edible purposes.

Phosphoryl-containing extractant is also used by the invention of U.S. Patent No 3,816,524 and Japanese Patent No 47-5715. However, expensive centrifugal equipment has to be used in treating the emulsion existing in the extraction of fermentation broth.

U.S. Patent No 4,571,671 discloses the use of amides to extract citric acid and European Patent No 0,049,426 discloses a mixture of amines and water immiscible organic acid as solvent.

The present invention provides a process for the recovery of acids, especially organic acids, from aqueous solution by extraction using a tri-(C2-C,O)alkyi phosphine oxide. Preferably, fermentation broth containing organic acid is pretreated by active carbon in order to prevent emulsion formation, then the pretreated aqueous solution is extracted in suitable equipment, e.g. a Mixture-settler and several columns. The extractant is preferably a mixture of trialkyl phosphine oxides (HTRPO) which is not disclosed by any prior art, because of its good extraction ability and selectivity as well as good ability and selectivity as well as good adaptability for various processes in the extraction and stripping.

Preferably the pretreated solution of the acid is extracted with a solution of the extractant in a suitable water immiscible diluent. The acid is preferably citric acid, malic acid, oxalic acid and tartaric acid.

A novel method for preparing the mixture of trialkyl phosphine oxides (HTRPO) is also provided in the invention.

The aqueous solution used as starting material in the present invention includes fermentation broths containing citric acid, malic acid, tartaric acid or oxalic acid. The concentration of the acid in the broth is preferably from 2 to 30 percent by weight.

It is found in the present invention that protein and other unknown substances existing in the fermentation broth are the main cause of a stable emulsion which harms the extraction process.

It is also disclosed that the removal of protein and other unknown substances can be carried out by means of adsorbing on active carbon or other chemical means.

The adsorption formula is as follows: X/M =KCo-592 Where X=amount absorbed M=amount of carbon used C=amount absorbed at equilibrium K=experimental constant The particle size of active carbon used in the invention is generally from 20 to 10 mesh, and preferably from 40 to 60 mesh. The specific surface of granular active carbon is generally from 1500 to 3000 M2/g, and preferably from 2000 tio 25000 M2/g. The pore size of granular active carbon is generally from 5 to 20A, and preferably from 10 to 15A. The adsorbing is carried out in a column or fluidized bed packed with said granular active carbon before extraction of the aqueous solution. The temperature of the adsorbed bed is desirably different from common absorption processes wherein the temperature should be held as low as possible.Whereas, the temperature range of the process of the present invention is desirably maintained at from about 40" to 800C and preferably from 50 to 70"C. The flow rate of broth is conveniently from 1 to 10 M3/M2 hr and preferably from 2 to 5 M3/M2 hr.

When the pretreated aqueous solution is contacted with the water immiscible solvent containing the extractant proposed in the present invention, no emulsion should exist in the interface between the solvent mixture and aqueous solution.

The phosphoryl-containing extractants disclosed in the present invention are selected from the group consisting of: (a) Trialkyl phosphine oxides,

wherin R1 R2 and R3 are the same or different alkyl groups of 2 to 10 carbon atoms.

A mixture of trialkyl phosphine oxides (HTRPO) is preferred, in which the total number of carbon atoms per molecule is from 15 to 27, and R1, R2 and R3 are the same or different alkyl groups of 5 to 9 carbon atoms.

(b) Trialkyl phosphates,

Wherein R4, R5 and R6 are the same or different alkyl groups of 2 to 8 carbon atoms, such as tributyl phosphate (TBP) (c) Dialkyl alkyl phosphonates,

Wherein R4, R5 and R6 are the same or different alkyl groups of 2 to 8 carbon atoms.

Accordingly, compounds (b) and (c) may be used as extractants in place of a trialkylphosphine oxide.

The solvents used in the present invention may be aliphatic hydrocarbons, aromatic hydrocarbons, kerosine, sulfonated kerosine or ethers. The proportion by volume of extractant : Diluent is preferably from 5:1 to 1:4.

The loaded water immiscible solvent mixture may be stripped with pure water or an aqueous solution of alkali. The temperature of stripping with pure water is generally from 50 to 90"C, and preferably from 80" to 90"C.

A method for preparation of the mixture of trialkyl phosphine oxides (HTRPO) is also provided by the present invention, which method comprises reacting a mixture of monohydroxy alcohols of 5 to 9 carbon atoms having a boiling point of 1600 to 1900C with iodine, and reacting the resultant product with red phosphorus and nitric acid, then collecting the fraction at 1600 to 260"C under a vacuum pressure of 640 to 650 mm Hg as the mixture of trialkyl phosphine oxides (HTRPO).

A second method involves the oxidation of trialkyl phosphine according to the following reaction:

wherein (R1R2R3)P is produced by the reaction of alkene and phosphine, PH3.

A third method involves th use of Grignard reagent. A mixture of monohydroxy alcohols with 5 to 9 carbon atoms is reacted with chlorine sulfoxide in the presence of DMF to from alkyl chlorides. Magnesium powder is added to the reaction mixture in the presence of ether. The Grignard reagent so formed is reacted with phosphorus oxytrichloride, POCI3. The resulting mixture of trialkyl phosphine oxides is then purified by vacuum distillation.

The key step in the third method is that the reaction involving the Grignard reagent must be completely free of alcohol and water, otherwise an intense explosion may result. The problem can be solved by using absorption techniques.

The process proposed by the present invention is especially useful for the extraction of the fermentation broth containing citric acid from the fermentation of dried sweet potato, molasses and starch to produce citric acid for edible and pharmaceutical purposes as well as trisodium citrate. The invention is described in further detail in the following examples.

EXAMPLE 1 A mixture of monohydric alcohols of 5 to 9 carbon atoms was distilled under atmospheric pressure. The collected fraction of 160 to 1800C was reacted with iodine at a temperature from 100" to 1300C. The resulting alkyl iodide was reacted with red phosphorus by adding nitric acid, then the fraction of 160 to 260"C under vacuum pressure of 640-650 mm Hg was collected as a mixture of trialkyl phosphine oxides (HTRPO). The data of infra-red spectra analysis, element analysis and average molecular weights showed that the product was a mixture of trialkyl phosphine oxides containing 15 to 27 carbon atoms.

EXAMPLE 2 A mixture of C5 to C4 alkene gas was reacted with phosphine, PH3. Nitric acid was added to the mixture under gentle stirring in a vessel under vacuum pressure. The products were collected at a vacuum pressure of 640-650 mm Hg. A mixture of trialkyl phosphine oxides (HTRPO) was produced. The results of infrared analysis, elemental analysis and average molecular weights showed that the product was a mixture of trialkyl phosphine oxides containing 15 to 27 carbon atoms.

EXAMPLE 3 A mixture of monohydric alcohols of 5 to 9 carbon atoms was distilled under atmospheric pressure. The fraction collected at 160"C-180"C was reacted with chlorine sulfoxide in the presence of DMF at a stoichiometric ratio of 1:1 to form alkyl chlorides.

Magnesium powder was added to the reactor in the presence of ether. The Grignard reagent so formed was reacted with phosphorus oxytrichloride The product was collected at a vacuum pressure of 640-650 mm Hg as a mixture of trialkyl phosphine oxides (HTRPO). The results of infrafed analysis, elemental analysis and average molecular weights showed that the product was a mixture of trialkyl phosphine oxides containing 15 to 27 carbon atoms.

EXAMPLE 4 Extraction of aqueous solution containing 9.0% by weight citric acid was carried out in a separating funnel at 30"C by the extractant HTRPO described in Example 1. The solvent was hexane, phase ratio (O/A) was 1, and contacting time was 2 minutes. The distribution coefficients of different proportion between the extractant and the diluent are shown as Table 1.

TABLE 1 proportion (vol%) distribution HTRPO Hexane coefficient 20 80 0.352 40 60 1.52 60 40 2.71 EXAMPLE 5 Crude broth from fermentation of dried sweet potato containing 116 g/l citric acid and 0.40% protein by weight was introduced into a column 50 mm in diameter packed with granular active carbon at 65"C. The flow rate of the broth was 50 ml/min.

Table 2 below shows that the physical and chemical properties of the fermentation broth were slightly changed after pretreatment, and that some of the proteinaceous and unknown substances were removed. This shows that the pretreatment is not a separation process, but is useful only to prevent emulsion.

TABLE 2 Before Treatment After Treatment Citric acid, g/l 87.5 91.2 density 1.045 1.040 protein ,ó by weight 0.3969 0.1810 dioptre index 1.4374 1.4365 viscosity, CP 1.466 1.433 surface tension, dyne/cm. 8.95 12.32 The adsorbed solution containing 110 gtl citric acid and 0.17% protein by weight was then contacted with a solvent mixture of the extractant and the diluent described above in a mixturesettler and a rotating disc contactor (RDC), in which no emulsion occured at the interface between the solvent mixture and the aqueous solution.

EXAMPLE 6 According to the flow sheet shown as Fig. 1 of the accompanying drawing, the fermentation broth described in Example 5 was introduced into pretreatment vessel (1). The obtained aqueous solution was extracted by a solvent which was composed of 40% by volume of HTRPO and 65% volume of sulfonated kerosine in extractor (2) at ambient temperature, and the flow rates of the aqueous solution and the solvent were 5 I/hr and 12.5 I/hr respectively. The loaded solvent mixture was then stripped with pure water in stripper (3) at 80"C and with a water flow rate of 4 I/hr. The aqueous phase containing 150 g/l of citric acid from stripper (3) passed through ion-exchanger (4), evaporator (5) and crystallizer (6) in turn, and the final product was citric acid monohydrate.

The solvent from stripper (3) was recycled into extractor (2). Both extractor (2) and stripper (3) used here were mixture-settlers.

EXAMPLE 7 According to the flow sheet shown as Fig. 1, the fermentation broth described in Example 5 was introduced into pretreatment vessel (1), the obtained aqueous solution was extracted with a liquid composed of 70% TBP by volume and 30% sulfonated kerosine by volume in extractor (2) at 10 C, and the loaded organic phase was stripped with pure water in stripper (3) at 60"C. The aqueous phase containing 120 gfl citric acid from stripper (3) entered into ion exchange (4), evaporator (5) and crystallizer (6) in turn, and the final product was citric acid monhydrate.

Both extractor (2) and stripper (3) used here were RDC (Rotating Disc Contactor).

EXAMPLE 8 Citric acid produced from Example 6 and Example 7 satisfied the requirements of the British Pharmaceutical Codex (1980); the data of chemical analysis are shown as Table 3.

Also no organic phosphorus residues were left in the crystals of citric acid.

TABLE 3

Demands Products Products Exam. Item of B.P. from Ex. from Ex.

(6) 7 Identification Yes Yes Yes Citric acid 99% 99.4 99.5% Oxalic salt clear clear clear Sulphate s 40.05 0.005 0.003 Calcium salt ; no white no no precipitation As ppm < 2 0.25 0.25 Fe ppm < 50 1 3 Pb ppm < 5 1 1 Ashes % < 0.10 0.025 0.031 Carbonizables < 0.90 0.32 0.37 (O.D) Based on the data shown as Table 3 as well as the solubility and toxicity of extractant etc, the HTRPO is considered as the best extractant among the extractants.

EXAMPLE 9 Exraction of pretreated aqueous solution containing 100 g/l oxalic acid was carried out in a separating funnel at 30"C and the phase ratio (O/A) was 1. The extractant and solvent were diheptyl ethyl phosphonate and sulfonated kerosine, respectively. The data of distribution coefficients of different proportions between the extractant and the diluent are shown in Table 4.

TABLE 4 Proportion (Vol.%) Diheptyl ethyl Sulfonated Distribution phosphonate kerosine coefficient 100 0 2.09 80 20 1.42 65 35 1.02 50 50 0.76 30 70 0.33 EXAMPLE 10 Extraction of 100ml aqueous solution containing 100 g/l tartaric acid was carried out in a separating funnel, the phase ratio (O/A) was 1, the contacting time was 5 minutes, and the extractant liquid was composed of 80% HTRPO by volume and 20% dibutyl ether by volume.

The data of distribution coefficients at different temperatures are shown as Table 5. The solution had been pretreated before the extraction.

TABLE 5 Temperature "C 16 30 50 65 80 distribution 3.07 2.77 1.81 1.80 1.20 coefficient EXAMPLE 11 Extraction of pretreated aqueous solution containing 100 g/l malic acid was carried out in a separating funnel at 30"C, the phase ratio (O/A) was 1, and the extractant and solvent were HTRPO and hexane respectively. The data of distribution coefficient of different proportion between the extractant and the diluent are shown as Table 6.

TABLE 6 Proportion (Vol%) distribution HTRPO Hexane coefficient 100 0 2.35 80 20 1.46 65 35 1.07 50 50 0.53 30 70 0.46 EXAMPLE 12 According to the flow sheet show as Fig. 1, the fermentation broth described in Example 5 was introduced into pretreatment (1) and extractor (2) in which the extractant mixture was as described in Example 4. Then 200ml of loaded organic phase containing 37.8 g/l citric acid was back-extracted with 47.3 g of 10% sodium hydroxide aqueous solution and all the citric acid was thus converted into tri-sodium citrate. 9 g of Na3C6H5O7.5H20 in a high purity was obtained after evaporation and crystallization.

Claims (15)

1. A mixture of trialkyl phosphine oxides having the following formula:

wherein R1, R2 and R3 are the same or different alkyl groups of 5 to 9 carbon atoms and the total number of carbon atoms per molecule is from 15 to 27.

2. A process for preparing a mixture of trialkyl phosphine oxides according to claim 1 comprising: (a) reacting a mixture of monohydroxy alcohols having 5 to 9 carbon atoms with boiling points in the range of 1600C to 1900C with iodine, (b) reacting the resultant product with red phosphorus and nitric acid; and (c) collecting the fraction as a mixture of trialkyl phosphine oxides at about 160"C to 260"C at an absolute pressure of about 640 to 650 mm Hg.

3. An extraction process for the recovery of acids from aqueous solutions wherein there is used as an extractant a trialkyl phosphine oxide having the following formula:

wherein each R1, R2 and R3 is the same or different alkyl group of 2 to 10 preferably 5 to 9 carbon atoms, and the total number of carbon atoms per molecule is preferably from 15 to 27.

4. A process as claimed in claim 3, wherein there is used a mixture of different trialkyl phosphine oxides.

5. A process as claimed in claim 3 or claim 4 wherein the acid which is recovered is an organic acid.

6. A process as claimed in claim 5, wherein the acid is citric acid, malic acid, tartaric acid or oxalic acid.

7. A process as claimed in any one of the claims 3 to 6, wherein the aqueous solution containing the acid to be recovered is brought into contact with a mixture of extractant and an organic solvent comprising an aliphatic hydrocarbon, an aromatic hydrocarbon kerosine, sulfonated kerosine or an ether.

8. A process as claimed in claim 7, wherein the proportion by volume of extractant to organic solvent is from 5:1 to 1:4.

9. A process for the recovery of citric acid, malic acid, tartaric acid or oxalic acid from a fermentation broth, wherein the concentration of the said acid is from about 2 to about 30 percent by weight, comprising the steps: (a) Pretreating the fermentation broth at a temperature in the range of about 40"C to 80"C with granular active carbon with a particle size of from 1500 to 3000 M2/g and a pore size from about 5A to 20A; (b) Extracting the pretreated broth at a temperature in the range of about 10"C to about 35"C with an extractant in a water immiscible solvent comprising an aliphatic hydrocarbon, an aromatic hydrocarbon, kerosine, sulfonated kerosine or an ether, the extractant having the formula::

and being a mixture of trialkyl phosphine oxides having a total of 15 to 27 carbon atoms and wherein each R1, R2 and R3 is the same or different alkyl group of 5 to 9 carbon atoms; and (c) Re-extracting the extract acid from the extractant with distilled water at a temperature in the range of from about 50"C to about 90"C.

10. A process as claimed in claim 9, wherein the carboxylic acid is citric acid and the waterimmiscible solvent is sulfonated kerosine.

11. A process as claimed in claim 9 or claim 10, wherein the temperature for pretreating the fermentation broth with granular active carbon is in the range of about 50"C to 70"C.

12. A modification of the process claimed in any one of the 3 to 11 wherein the extractant is a trialkyl phosphate having the formula

wherein each R4, R5 and R6 is the same or different is an alkyl groups of 2 to 8 carbon atoms.

13. A modification of the process as claimed in any one of claim 3 to 11, wherein the extractant is a dialkyl alkyl phosphonate having the formula

wherein R4, R5 and R6 is the same or different alkyl group of 2 to 8 carbon atoms.

14. A process of recovering an organic acid from an aqueous solution substantially as hereinbefore described in any one of the examples.

15. An organic acid when obtained by a process as claimed in any one of claims 3 to 14.