This invention relates to a method of separating a solute from one or more other solutes in a solution and particularly, but not exclusively, to the use of this method in the manufacture of N-acyl taurate derivatives, and especially in the manufacture of sodium N-cocoyl, N-methyl taurate (SCMT) . ^*

It is sometimes desired to separate one solute in a solution from other solutes in the solution. For example, one solute may be a desired reaction product and the other solute(s) may be undesired by-product(s) . Where the solutes are of different molecular size, membranes may be used to effect a separation. Thus, ultrafiltration, nanofi1tration and reverse osmosis membranes are all characterized by the maximum molecular weight solute which will pass therethrough.

Ultrafiltration membranes have a molecular weight cut-off (MWCO) which defines the maximum molecular weight solute which will pass through the membrane.

Ultrafiltration membranes typically have a molecular weight cut-off of between about 2000 and 200000 daltons.

Reverse osmosis is characterized typically by the 7. retention or rejection of a solute, e.g. sodium chloride , or calcium chloride. Reverse osmosis membranes generally have a sodium chloride retention from about 307. to 997..

Nanofi 1 trat ions membranes are intermediate between reverse osmosis membranes and ultrafiltration membranes. Nano i 1 tra ion membranes typically have a molecular weight

cut-off of 2000 daltons to a sodium chloride retention of 307..

However, when the molecular size of the solute to be separated from the other solutes in solution is very close to that of one of the other solutes, good separation is difficult to achieve, and there can be significant losses of the desired product through the membrane.

It has previously been thought that if the molecular sizes of the solutes in a solution are small and of such similar size that it is not possible to achieve good separation using an ultrafiltration or nanofi1tration membrane, then the only way to achieve good separation is by reverse osmosis. This is because a reverse osmosis membrane is very tight and it can distinguish between solutes of closely similar size. However, in reverse osmosis, it is necessary to use high pressures in order to drive the solute through the membrane. These conditions are generally undesirable.

We have now devised an effective method of separating a solute of small molecular size from other similar size solutes in a solution, without having to use reverse osmosis and high pressure.

According to the present invention, there is provided a method of separating a solute from one or more other solutes in a solution, which method comprises insolubi1ising the solute to be separated from the solution and passing the mixture of insolubi1ised solute and the remaining solution in contact with a membrane, the molecular weight cut-off of the membrane being such that the solute(s) remaining in solution pass through the membrane but the insolubi 1 ised solute does not.

By bringing the required solute out of solution, it is then possible to use a looser membrane, ie. one having a larger molecular weight cut-off, since the size of the insolubi 1 ised material is much greater than its size in solution. Thus, the method can be carried out at low

temperatures and pressures, making the process safer and less expensive to operate. Ultrafiltration is typically carried out using pressures of from about 0.6 to 1.0 MPa (6 to 10 bar) and nanofi1 tration typically uses pressures of about 4 to 5 MPa (40 to 50 bar); whereas reverse osmosis uses high pressures of over 6 MPa (> 60 bar).

It will be understood that the membrane used will be chosen according to the molecular weights of the solutes in the solution to be processed. Preferably, the membrane is a nanofiltration membrane or an ultrafiltration membrane. As stated above, by nanofi1tration membrane, we mean a membrane with a molecular weight cut-off of 2000 daltons to a sodium chloride retention of 307.; and by ultrafiltration membrane, we mean a membrane with a molecular weight cut-off of between about 2000 and 200000 daltons .

The solute which is to be separated from the solution is selectively insolubilised , eg. by a physical process. For example, in suitable cases, the solute may be selectively insolubilised by cooling the solution to a sub- ^' ambient temperature. The exact temperature to which the solution should be cooled prior to contact with the membrane will, of course, depend upon the temperature at which any particular solute crystallizes out of solution so that it is in the form of insoluble particles which cannot pass through the membrane. The temperature to which the solution will need to be cooled will also therefore depend upon the molecular weight cut-off of the membrane being used.

The solute could also be insolubilised, in suitable cases, by changing the ionic strength of the solution or by adding a non-aqueous solvent, for example.

The solute to be separated from the solution may also be insolubilised by a chemical process, for example by a chemical precipitation reaction.

The method of the present invention has been found to be particularly suitable for separating an N-acyl taurate

- -

derivative from other solute contaminants in a reaction mixture, wherein the N-acyl taurate derivative is insolubi1ized so that it is in the form of insoluble particles which cannot pass through the membrane.

In accordance with a further aspect of the present invention, there is provided a method of preparing an N-acyl taurate derivative by reacting an acid chloride with an aminoalkane sulphonic acid in aqueous medium in the presence of an acid neutralizer and removing a precipitate of the N- acyl taurate derivative directly from the reaction mixture by effecting filtration of the reaction mixture at a sub- ambient temperature.

Preferably, di-afi1 tration is used. Diafi1 tration is a membrane filtration technique whereby the process fluid is washed free of small species with a molecular weight less than the molecular weight cut-off of the membrane. Preferably, water is added continuously to the reaction mixture to maintain a constant volume during the filtration ' process.

Suitably, the solids contents and filtration temperatures are varied, so that the N-acyl taurate derivative is in the form of agglomerated insoluble particles which cannot pass through the membrane, and impurities remain in solution and pass through the membrane, thereby leaving behind the pure N-acyl taurate derivative.

The method of the present invention allows the preparation, for example, of N-acyl taurate derivatives based on lauric (C12), myristic (C14), palmitic (C16), stearic (C18) or oleic (C18 mono unsaturated) acids.

Most preferably, the invention allows the preparation of sodium N-cocoyl , N-methyl taurate (SCMT).

SCMT is an anionic surface active sulphonate that is used as a detergent in the personal care industry and, in particular, in gum-sensitive toothpastes because of the unique compatibili y of SCMT with strontium salts which are

used in these products.

Several processes have been described for the preparation of SCMT-type detergents, but the only process which is presently employed commercially is the Schotten- Baumann reaction, in which an acid chloride is reacted in aqueous medium with an aminoalkane sulphonic acid in the presence of an acid neutralizer such as sodium hydroxide. Thus, SCMT is prepared commercially by the reaction:

RC0C1 + HNMeCH ₂ CH ₂ Sθ3Na > RCONMeCH ₂ CH2Sθ3Na + HCl

The hydrogen chloride produced is immediately quenched with the aqueous base to form sodium chloride.

There are several disadvantages to this process. Firstly, the Schotten-Baumann reaction itself must be carried out in dilute aqueous solution because at concentrations above 357., the viscosity of the reaction mixture becomes limiting and the competing reaction of the alkaline hydrc- ^' ysis of the acid chloride to fatty acid becomes significant. The presence of free fatty acid is unwanted in the final product.

A second disadvantage of this process is that the reaction mixture contains approximately 157. of sodium chloride (dry weight) which must be separated from the product in order to produce pure SCMT (>957. active). Other minor contaminants such as sodium N-methyl taurate and free coconut fatty acid must also be reduced or removed at the same time.

The commercial process achieves this purification by removal of the water by evaporation/drying, followed by dissolving in solvent (such as methanol). filtration to remove the insoluble sodium chloride and recrysta11 isation . This purification stage has a further disadvantage in that the solvent-rich filter cake must be disposed of safely, and the mother liquor remaining from the recrystal 1 isation solution contains up to 507. of uncrysta11 ized SCMT. This

material again must be disposed of safely and. moreover, this also represents a significant loss of yield.

Attempts have been made to modify this process so as to filter off the precipitated SCMT directly from the Schotten-Baumann reaction slurry. It was anticipated that the lower solubility SCMT could be filtered out and the more soluble sodium chloride could be carried through in the filtrate.

However, it was found that the filtration of these slurries was extremely difficult because of the small particle' size of the SCMT, which had a great propensity to blind the filter, giving rise to very slow filtration rates and leaving the filter cake with a high water content. Consequently, there was still a significant amount of sodium chloride remaining in the saltcake. Despite many attempts with different configurations of conventional filter equipment, it was found to be impossible to obtain complete removal of the sodium chloride.

Attempts have also been made to purify the slurry by a conventional form of ultrafiltration, using a crossflow configuration where the liquid to be processed is made to flow tangentially to the membrane surface. This tangential flow inhibits the formation of deposits, and turbulence within the process fluid minimises fouling.

However, the molecular size of the SCMT is close enough to that of sodium chloride for a significant loss of activity to occur through the membrane when this technique was applied to remove the sodium chloride. The use of ultrafiltration was not, therefore, thought to be appropriate .

It was therefore previously believed that there was no satisfactory filtration technique which would allow the precipitated SCMT from the Schotten-Baumann reaction slurry to be removed directly from the reaction mixture, whilst achieving a very satisfactory reduction in the amount of sodium chloride remaining in the SCMT.

JP 4149169 describes the manufacture of N-acyl taurate derivatives, where water is added to the final reaction mixture in order to dissolve the N-acyl taurate derivative, and the liquid is subjected to reverse osmosis. However, such a process is necessarily operated at high pressures in order to drive the water and sodium chloride through the membrane. This process is not, therefore, a satisfactory process for obtaining SCMT commercially.

However, the method of the present invention provides an effective way of recovering the SCMT from the Schotten-Baumann reaction slurry. The method is also applicable to the manufacture of other N-acyl taurates.

Thus, in accordance with the present invention, an N-acyl taurate may be prepared by reacting an acid chloride with an aminoalkane sulphonic acid in aqueous medium in the presence of an acid neutralizer (eg. sodium hydroxide), and the reaction mixture then subjected to the membrane to effect separation, the N-acyl taurate derivative being in the form of insoluble particles which cannot pass through the membrane.

In accordance with the present invention, therefore, the above-mentioned problem of the porosity of the membrane to SCMT is overcome by the precipitation of the SCMT such that it is substantially only present in the slurry as agglomerated insoluble particles. In this way, the precipitate can be obtained substantially free of chloride (e.g. sodium chloride) and other contaminants.

Preferably, diafi1tration is used; most preferably with water being added continuously to the slurry mixture to maintain a constant volume.

Although it is possible to insolubilize the N-acyl taurate derivative in a variety of ways, as discussed above, the preferred way of insolubi 1 izing the N-acyl taurate derivative is by cooling the reaction mixture.

Preferably, the insolubilised solute and the remaining solution are passed in contact with the membrane

at a temperature below about 10 ^* C, more preferably, at a temperature below about 6 ^* C, and most preferably, below about 3 ^* C.

When SCMT is being prepared, the SCMT slurry is suitably adjusted to approximately 15 to 307., e.g. about 247., solids prior to contact with the membrane.

By the process of the present invention, it is possible to reduce the sodium chloride content remaining in SCMT, for example, to below 17. (on a dry weight basis), and also other contaminants such as free coconut fatty acid and sodium N-methyl taurate are substantially reduced to below 17. (on a dry weight basis).

As the aqueous solution passes through the membrane during ultrafiltration, more water is added to the circulating slurry mixture to maintain a constant volume. In this way, the process fluid is washed free of small species with a molecular weight less than the MWCO of the membrane.

Towards the end of the process, when all the contaminants have been removed, the addition of water may be stopped and the concentration of the slurry increased as the permeate continues to pass through the membrane. When the process is used to manufacture SCMT, for example, the process may be stopped at a concentration of up to 30-347.. Upon warming to 25 ^* C, the SCMT becomes fully water-soluble again up to about 347., and is therefore suitable for spray drying or any other alternative form of isolation of the pure 957. SCMT.

The advantages of the method of the present invention in the manufacture of SCMT are numerous. For example, firstly, in the laboratory, yields of greater than 907. have been achieved (based on active SCMT). Secondly, the use of methanol and the requirement to recycle this solvent has been avoided, and thirdly, the only by-product of the process is water containing sodium chloride and small amounts of other water-soluble contaminants, which may be

disposed of easily.

In order that the invention may be more fully understood, examples of the manufacture of sodium N-cocoyl, N-methyl taurate (SCMT) and sodium N-myristoyl, N-methyl taurate (SMMT) will now be described by way of illustration only. It will be understood, however, that the process may be adapted to make other N-acyl taurate derivatives, and also that the method of the present invention is generally applicable where it is desired to separate a solute of small molecular size from other similar size solutes in a solution, without having to use reverse osmosis and high pressure .

The accompanying single figure drawing illustrates the results .obtained for SCMT.

Example 1

Ultrafiltration of Adinol Slurry

The ultrafiltration trials have been carried out using a PCI Membrane Systems BUF unit with one 4ft (1.22m) module, having a filtration area of 0.9m^ . FP100 membranes have been used which are PVDF and have a nominal molecular weight cut-off of 100,000 daltons. The slurry is pumped using a single speed positive displacement pump so that the rate of passage of the slurry through each module in the ultrafiltration unit is about 22 litres/min. The actual volume displaced by the pump will be a function of the size of the pump. (On a full-size unit, a variable speed pump could be used) .

The SCMT slurry made by the Schotten-Baumann process is currently about 32-377. solids. This is difficult to process directly, due to possible problems associated with pumping and, more importantly, because of the pressure drop across the modules leading to significantly reduced flux. Air intake into the slurry can also be a problem, especially when the slurry is warm and/or high in solids, and some of the trial batches have been carried out without

agitation, due to air within the feed. A slow gentle agitator can be used to solve this problem.

During the ultrafiltration trials, the SCMT slurry has been diluted to approximately 247. solids (warm) before being naturally cooled, and then chilled to a temperature of 6 ^* C, in order to bring the SCMT out of solution.

During the ultrafiltration process constant volume (continuous) diafi1tration is used, and the retentate recycled from feed vessel to UF module to feed vessel, until the salt content is below the required limit. The salt, sodium N-methyl taurine and free fatty acid (all in water) pass through the membrane and are collected as permeate. The constant addition of diafi1tration water (2 volume replacements are necessary) results in the final desalted slurry having a solids content of about 157.. The desalted slurry is then concentrated up to about 247. solids, suitable for spray drying. Desalting of Adinol CT Slurry By Ultrafiltration.

SCMT slurry (23.17. solids) in a storage vessel was heated to 50 ^* C with agitation and 235.5 kg of the slurry was then weighed and charged into the feed vessel, and the contents were mixed for about a minute to produce a homogenous slurry. A sample of the slurry was then taken for analysis.

The slurry in the feed vessel was then allowed to cool naturally, without agitation, until the temperature had dropped to 20.6 ^* C. Glycol cooling was then applied to the feed vessel jacket and the heat exchanger. The plant was then configured for recycle, and the slurry pumped from the feed vessel, through the heat exchanger, to the UF module. Both retentate and permeate were returned to the feed vessel. When the feed temperature had reduced to 9 °C , the agitator on the feed vessel was started and recycle continued until the feed temperature reached 6"C.

The plant was then configured for filtration (retentate being returned to the feed vessel, and permeate

fed into a receiver). The inlet pressure on the UF module was increased to 6.0 bar by means of the pressure control valve, and the run started.

As permeate was removed, chilled diafi11ration water (<6 ^* C) was continuously added to the feed vessel. Readings of process temperature, inlet pressure, outlet pressure, retentate flowrate, flux, and diafi1tration volume added, were recorded at 30 minute intervals. Samples of the slurry in the feed vessel, and of the bulk permeate were taken after every half diafi1tration volume added (110.2kg).- However, on a full-scale plant, it would not be necessary to take samples every half volume.

The process was continued until 2 diafi 1tration volumes had been added. (However, on a full-scale plant, the continuous diafiltration ultrafiltration would continue until proposed sodium chloride monitoring equipment indicated that the salt levels were below the required limit. It is anticipated that each batch will require 2 diafiltration volumes, but the amount of water added back would not necessarily be set. If the salt levels were satisfactory after 1.5 volumes, the batch would be concentrated up.)

A sample of the slurry in the feed vessel was then analysed for salt content, and the slurry recycled through the UF module until the result was known. When the salt content was known to be below the required limit, the plant was again configured for filtration, and the slurry concentrated until a solids content of about 247. was reached .

A sample of slurry was then analysed for 7. solids, activity, sodium chloride content. N-methyl taurine content, and pH. The product was then filled off into kegs.

Results

Table 1 below gives the analytical results of both the initial and desalted slurry.

TA B LE 1

Mass Balance

Actives balance

initial mass in feed vessel:

235.5 kg © 16.757. active 39.45 kg actives final mass in feed vessel:

121.0 kg © 23.907. active 28.92 kg actives final permeate:

565.5 kg © 1.617. active 9.10 kg actives

Yield of actives in slurry = 73.37. Actives lost to permeate - 23.17. of initial actives Actives Unaccounted for 3.67. of initial actives (due to mass losses which uould not necessarily occur on large seale . )

Mass balance summary

experύrental error

95/30473

13

Examp 1 e 2

The method described in Example 1 was repeated except that the production unit was run on AN 620 membranes. These membranes are po1 yacryIon i tri le in nature and have a nominal molecular weight cut-off of 25.000 daltons.

Table 2 below gives the analytical results of both the initial and desalted slurry.

TABLE 2

Mass Balance

Actives balance

initial mass in feed vessel:

220 kg © 18.517. active = 40.72 kg a i es final mass in feed vessel:

148.0 kg © 24.297. active = 35.95 kg acti es final permeate :

510 kg © 0.877. active 4. L kg acti es

Yield of actives in slurry = 88.37. Actives lost to permeate = 10.97. of initi l acti es Actives unaccounted for 0.87. of initial actives (due to mass losses which would not necessari ly occur on large scale. )

Mass balance summary

* experimental error Example 3

The method described in Example 1 was repeated using a small laboratory unit with the FPlOO membranes except that sodium N-myristoyl. N-methyl taurate (SMMT) was used .

Table 3 below gives the analytical results of both the initial and desalted slurry.

TABLE 3

Mass Balance

Actives balance

initial mass in feed vessel :

14260 g © 14.37. active = 2039 £ ctives final mass in feed vessel :

9300 £ © 19.97. active = 1851 R actives fin l per ea te :

19980 ε © 0.87. .ictivc 160 y, act ive

Yield of actives in slurry = 90.87. Actives lost to permeate = 7.87. of initial actives Actives unaccounted for 1.47. of initial actives (due to mass losses which would not necessarily occur on a la rge scale) .

Mass balance summary

* Experimental error