Process and compositions for the recovery of ascorbic acid

The invention provides an extraction process for the recovery of ascorbic id from an aqueous feed solution containing the acid at a concentration of less than 0.7 mol/kg.

The present invention relates to a process for the production of ascorbic 
acid. More particularly, the present invention relates to the recovery of 
ascorbic acid from aqueous solutions containing the same in dilute 
concentrations. 
As described, e.g., in Kirk-Othmer's Encyclopedia of Chemical Technology, 
Third Edition, ascorbic acid (L-ascorbic acid, L-xylo-ascorbic acid, 
L-threo-hex-2-enonic acid .tau.-lactone) is the name recognized by the 
IU-IUB Commission on Biochemical Nomenclature for vitamin C. The name 
implies the vitamin's antiscorbutic properties, namely, the prevention and 
treatment of scurvy. L-ascorbic acid is widely distributed in plants and 
animals. The pure vitamin (C.sub.6 H.sub.8 O.sub.6, mol. wt. 176.13) is a 
white crystalline substance derived from L-gulonic acid, a sugar acid, and 
synthesized both biologically and chemically from D-glucose. 
##STR1## 
Although natural and synthetic vitamin C are chemically and biologically 
identical, in recent years a limited amount of commercial isolation from 
vegetable sources, e.g., rose hips, persimmon, citrus fruit, etc., has 
been carried out to meet the preference of some persons for vitamin C from 
natural sources. L-ascorbic acid was the first vitamin to be produced in 
commercial quantities, and manufacture is based on the well-known 
Reichstein and Grussner synthesis, which involves the steps of 
hydrogenation of D-glucose to D-sorbitol; fermentation (oxidation) to 
L-sorbose; acetonation to bis-isopropylidene-.alpha.-L-sorbofuranose; 
oxidation to bis-isopropylidene-2-oxo-L-gulonic acid, and hydrolysis, 
rearrangement and purification to L-ascorbic acid. 
A direct fermentation of glucose to ascorbic acid would be very attractive, 
saving on operations and on expensive reagents, in addition to its being 
derived from a natural fermentation process, as opposed to a synthesis 
involving chemical steps. There are indications that such direct 
fermentation to ascorbic acid is feasible. Yet industrial production of 
ascorbic acid through direct fermentation seems impractical, in view of 
the low product concentration in the fermentation liquor, which normally 
is in the range of less than 0.7 mol/kg. Purifying the ascorbic acid by 
conventional methods would result in a purified product of concentrations 
similar to those in the fermentation liquor. Due to its high solubility in 
water, the cost of ascorbic acid crystallization by water evaporation 
would be prohibitive. 
Several methods were proposed for combining purification of carboxylic 
acids with their concentration. In the case of citric acid, it is achieved 
by the addition of lime to crystallize calcium citrate, which has very low 
solubility in water. This salt is separated, washed and acidulated with 
sulfuric acid. Purified and concentrated citric acid is obtained. This 
method is not applicable for ascorbic acid, as its alkali and alkali earth 
salts are highly soluble. 
A process was proposed in which carboxylic acids were extracted and then 
displaced from the extractant by a solution of concentrated mineral acids. 
Both liquid (long chain amines) and solid (resins carrying amine groups) 
anion exchangers could be considered for this purpose. The purity of the 
displaced carboxylic acid depends on the preference of the extractant to 
the mineral acid. Such a process might be applicable for ascorbic acid 
separation and concentration, provided that the extractant is strong 
enough to reach high extraction yield, that it shows high preference to 
the displacing acid, and that the ascorbic acid is stable at the high 
acidity of the displacing solution. 
The regeneration of the anion exchanger would require neutralization by a 
base. Using HCl as the displacing acid and distilling it of the extractant 
was proposed, but the high temperatures required and the extractant's 
decomposition at these conditions are prohibitive. If the anion exchanger 
is represented by B, the ascorbic acid in the fermentation liquor and in 
the pure form are AA.sub.F and AA.sub.P, respectively, the displacing acid 
is HCl, and the neutralizing base is NaOH, the equations of the process 
stages and of the overall reaction are as follows: 
EQU B+AA.sub.F .fwdarw.B.AA 
EQU B.AA+HCl.fwdarw.B.HCl+AA.sub.P 
EQU B.HCl+NaOH.fwdarw.B+NaCl+H.sub.2 O 
EQU AA.sub.F +HCl+NaOH.fwdarw.AA.sub.P +NaCl+H.sub.2 O 
Reagents are consumed, and a by-product salt of no (or negative) value is 
produced. 
Thus, despite the widely felt need for a more attractive process to meet 
the exceedingly high demand for ascorbic acid, to date no such process has 
been proposed or commercialized. 
In 1976, there issued British Patent 1,426,018 and in 1981 there issued the 
corresponding U.S. Pat. No. 4,275,234, directed to the recovery of acids 
from aqueous solutions. In said patents, there are exemplified the 
recovery of citric acid, lactic acid, oxalic acid, and phosphoric acid 
from an aqueous solution of the same acid; in fact, said U.S. Patent is 
specifically limited in its claims to the recovery of one of said four 
acids. 
While the process of the present invention as defined herein formally falls 
within the scope of said aforementioned British patent, the relevant 
teachings of which are incorporated herein by reference, and in this sense 
constitutes a selection therefrom, as will be explained further below, not 
only do said patents neither teach, suggest, nor exemplify the 
applicability of said process to the recovery of ascorbic acid, but in 
fact, from a careful analysis of said patents, one would not expect said 
process to be feasible for the recovery of ascorbic acid, as is also 
evidenced by the fact that nineteen years have passed from the publication 
of said British patent without any person skilled in the art either 
suggesting or applying said process to ascorbic acid recovery. 
Referring now to said patents and the teachings thereof, one finds that the 
process taught therein utilizes the effect of temperature on phosphoric 
and carboxylic acid extraction by amine-based extractants. The term 
"amine" as used herein means water-immiscible amine, with a total of at 
least 20 carbon atoms on its chains. Said patents teach that such 
amine-based extractants (ABE) lose much of their extraction efficiency 
upon temperature elevation. This loss of efficiency is referred to as 
"temperature sensitivity of extraction" (TS). The magnitude of this TS can 
be represented by the ratio of the distribution coefficient at the lower 
temperature (D.sub.T1) and the distribution coefficient at the higher 
temperature (D.sub.T2 ). High TS provides for the purification and the 
concentration of carboxylic acids through altering the temperature between 
extraction and back-extraction. The acid is extracted from the 
fermentation liquor by an ABE at low temperature, and is then 
back-extracted with water at an elevated temperature. The aqueous solution 
obtained from that back-extraction is, in many cases, more concentrated 
than in the fermentation liquor. This process is referred to herein as the 
"temperature swing process" (TSP). The attraction of such processes is in 
the fact that the sole energy consumption is that of sensible heat, which 
saves a lot of the latent heat of water evaporation in the final 
concentration. 
As explained in U.S. Pat. No. 4,275,234: 
"The concepts of "lower temperature" and "higher temperature" are not 
understood in absolute terms. What matters . . . is the temperature 
differential. This will have to be at least 20 degrees (centigrade), both 
for operation convenience and in order to make both the extraction and the 
back-extraction as complete as possible. The extraction may be carried out 
at temperatures as low as near the freezing point of the aqueous acid 
solution and the temperature of the back-extraction may be at or near the 
boiling point of the extract or the water at atmospheric pressure, or if 
the back-extraction is carried out under elevated pressure, at an even 
higher temperature, always on condition that the temperature and pressure 
are so chosen that the amine remains in the organic phase. In many cases 
the extraction can be carried out at or near room temperature, and the 
stripping operation at a temperature of about 20 to 40 degrees 
(Centigrade) above room temperature. As a rule, the stripping operation is 
the more effective, the higher the stripping temperature, but the 
extraction and stripping temperatures will be selected in individual cases 
in accordance with practical factors, such as corrosion-resistance and the 
costs of the equipment, costs of heating and cooling of the streams of the 
acid solution, the extract and the extractant, the required concentration 
of stripped acid, etc. 
"If the aqueous liquid used for stripping the extract is water, the 
back-extract is an aqueous solution of the free acid. If desired, the 
back-extracting operation may be so conducted that the back-extract is an 
aqueous solution of a salt of the extracted acid. For example, 
back-extraction with an aqueous alkali metal (in this context "alkali 
metal" includes ammonium) hydroxide solution yields an aqueous solution of 
the corresponding alkali metal salt of the extracted acid. Or the aqueous 
back-extracting liquid may be, for example, an alkali metal chloride 
solution. In this case, too, the back-extract contains the corresponding 
alkali metal salt of the extracted acid while the amine in the extractant 
is converted into its hydrochloride. This will thus have to be decomposed, 
e.g. by treatment with calcium hydroxide, for reconstituting the 
extractant. Sometimes it is advantageous to perform first a 
back-extraction with water in order to recover the major part of the acid 
in the free state. The residue of acid remaining in the solvent extract 
can then be back-extracted with an alkali metal hydroxide or salt 
solution. 
"The most favourable selection of the temperature of the extracting 
operation and of the compositions of the extractant, as regards both the 
amine and the solvent, will also be determined according to the given 
condition of particular cases, e.g., the kind of acid, its concentration 
in the original aqueous solution, the impurities present in that solution. 
The major aim in both the extracting and stripping operations will be to 
achieve as favourable a distribution coefficent as possible for the 
distribution of the acid between the aqueous and organic phases. In the 
extraction operation, this has to be in favour of the extractant; in the 
stripping operation, in favour of the aqueous phase." 
As stated above, the characterizing feature of said patents is that 
back-extraction is performed at a temperature higher than that of the 
extraction. For certain acids, there is shown efficient extraction at 
about room temperature. Back-extraction at about 100.degree. C. provides 
for a back extract, the concentration of which is similar to, or even 
higher than, that of the feed. In fact, a major part of citric acid 
production in the world is based on this process, using tridodecyl amine 
as the primary extractant and 1-octanol as the enhancer [Kirk-Othmer, 
Encyclopedia of Chemical Technology, 4th Ed., Vol. 6, p. 364]. 
The degree of product concentration in the TSP (the uphill pumping effect) 
depends strongly on the magnitude of the TS. The thermodynamic explanation 
for the TS is not clear enough. One could suggest that as the extraction 
process is exothermic, equilibrium is shifted backwards on temperature 
elevation. That would, however, be too simplistic. Thus, the most 
exothermic extraction is that of strong mineral acids, but no TS is found 
for their extraction. To the best of our knowledge, this complex 
phenomenon was not fully explained in said patents, and no tools were 
provided for predicting the magnitude of TS from the structure of the 
extracted acid. 
The magnitude of the TS for extraction of various carboxylic acids by an 
extractant composed of 0.5 mol/kg trilauryl amine (Henkels Alamine 304) 
and 10% octanol in a kerosenic diluent have now been tested. The results 
are presented below in Table 1: 
TABLE 1 
______________________________________ 
The temperature sensitivity of carboxylic acid extraction by 
0.5 mol/kg Alamine 304 + 10% octanol in kerosene. 
The temperature sensitivity (TS) is presented as the 
distribution coefficient at 30.degree. C., divided by that at 75.degree. 
C., 
at various equilibrium aqueous phase concentrations. 
TS in Equilibrium with 
Aqueous Solutions of (mol/kg) 
Acid pKa 0.05 0.2 0.3 0.475 
______________________________________ 
Maleic.sup.2 
1.93 1.1 1.0 1.0 1.0 
Oxoglutaric.sup.2 2.57 2.4 1.5 1.3 1.1 
Malonic.sup.2 2.83 3.6 1.5 1.3 1.1 
Tartaric.sup.2 3.01 3.4 3.2 2.7 2.4 
Citric.sup.3 3.13 6.0 3.1 2.6 2.2 
Malic.sup.2 3.22 4.0 4.3 4.0 4.0 
Gluconic.sup.2 3.75 2.1 2.3 2.4 2.6 
Lactic.sup.1 3.86 2.5 2.4 2.4 2.2 
Succinic.sup.2 4.2 4.3 4.0 4.0 4.1 
Glutaric.sup.2 4.4 3.9 4.5 4.5 4.4 
Acetic.sup.1 4.76 2.3 2.4 2.4 2.4 
Butyric.sup.1 4.81 2.1 2.0 2.0 1.8 
Isobutyric.sup.1 4.84 1.9 1.5 1.4 1.1 
Propionic.sup.1 4.87 1.7 1.5 1.3 1.1 
______________________________________ 
.sup.1 Monocarboxylic acid 
.sup.2 Dicarboxylic acid 
.sup.3 Tricarboxylic acid 
One can see that the TS may depend on the equilibrium concentration of the 
acid in the aqueous phase and that it varies significantly from one acid 
to the other. No linear correlation is found, however, between the TS and 
the strength of the acid or another defined characteristic thereof. The 
strongest TS was found for citric acid at the low concentration of 0.05 
mol/kg; some dicarboxylic acids show a higher TS than their monocarboxylic 
analogues. That might indicate a tendency of TS to increase with an 
increase in the number of carboxylic groups. Isolating this parameter from 
the others is difficult. 
Extraction of strong mineral acids by ABE is very efficient, reaching 
stoichiometric levels already at equilibrium with dilute aqueous 
solutions. That is true even for the weakest straight chain aliphatic 
amines, the tertiary ones reaching the stoichiometric extraction of 1 mol 
of HCl per mol of amine in equilibrium with aqueous solutions of about 
0.5%. High efficiency is also found in extracting strong carboxylic acids 
having a pKa less than 2.5. The efficiency is, however, much lower on 
extracting weaker carboxylic acids by tertiary amines in a kerosenic 
diluent. Said low efficiency is particularly pronounced in the low 
concentration range. In order to avoid low yields of extraction, 
extraction enhancers are introduced into the extractant. 
It is well-known that polar and protic compounds provide for enhancement of 
acid extraction by amines. These compounds may act as acid extractants by 
themselves, but are much weaker extractants than the amines. Extractants 
comprising amines and enhancers show synergistic effects in most cases, 
i.e., acid extraction by such extractants is much higher than the added 
contribution of the components. 
In the description of the invention herein, and to avoid confusion, the 
term "primary extractant" will be used for long-chain amines used for 
extractions, and the term "enhancer" will be used for polar and protic 
extractant components, the extraction power of which is smaller than that 
of the primary extractant. Suitable enhancers are polar, and preferably 
protic compounds, including alkanols, ketones, aldehydes, esters and 
ethers of various molecular weights. 
Desired extractants should provide high efficiency in extraction 
(relatively low extractant volumes, a small number of extractant stages 
and high yields), high selectivity, low water miscibility, low toxicity 
(particularly for food grade products), and efficient stripping of the 
extracted acid from the extract. The acid can be removed from the extract 
through interaction with an aqueous solution of a base to form its salt. 
In most cases, however, the acid is the required product rather than the 
salt, and acid recovery from the extract is performed by back-extraction 
with water or by distillation, where feasible. 
As is known, high efficiency in extraction from the feed and high 
efficiency in stripping are conflicting requirements. Back-extraction of 
the extracted acid from a strong extractant requires high volumes of water 
and results in a very dilute aqueous solution of the acid (back-extract). 
The high cost of product concentration may make the whole process 
impractical. Distillation from a strong extractant requires high 
temperatures and may result in the decomposition of the acid and/or the 
extractant. 
Extraction enhancers are polar and, preferably, protic compounds that have 
very low extraction capacity on their own, but significantly improve the 
extraction efficiency of ABE. The enhancement is explained by 
stabilization through salvation of the amine-acid ion pair. Octanol is 
used as an enhancer in the industrial TSP for production of citric acid. 
Extraction enhancers have, however, an adverse effect on TSP, as the 
temperature sensitivity decreases with an increase in enhancer content. 
Such an effect is shown below in Table 2: 
TABLE 2 
______________________________________ 
The dependence of the temperature sensitivity of citric acid 
extraction by amine-based extractant on amine concentration, 
enhancer (octanol) concentration, and on equilibrium aqueous 
phase concentration. 
The temperature sensitivity is presented as the ratio of 
distribution coefficient at 30.degree. C. and 75.degree. C.). 
Amine Octanol D30/D75 at Aqueous Concentration 
mol/kg mol/kg 0.02 0.5 1.5 
______________________________________ 
0.2 0.31 30.0 6.4 2.1 
0.2 0.62 10.8 2.0 1.3 
0.2 2.0 4.9 1.3 1.1 
0.5 0.31 31.3 3.7 1.4 
0.5 0.62 4.6 1.5 1.1 
0.5 2.0 2.1 1.1 1.05 
1.0 0.31 10.5 1.2 1.07 
1.0 0.62 4.9 1.1 1.01 
1.0 2.0 1.8 1.08 1.03 
______________________________________ 
There is, therefore, a trade-off between extraction efficiency and the 
magnitude of the TS. Thus, aiming at a higher degree of product 
concentration in the process leads to lower efficiency, particularly at 
the low concentration end, resulting in lower recovery yields, i.e., 
higher product losses. The absolute losses, expressed, for example, by the 
product concentration in the raffinate, depend on the shape of the 
distribution curve at the low concentration end. The proportional loss is 
mainly determined by the concentration of the acid in the fermentation 
liquor. 
The TSP was implemented for citric acid recovery from fermentation liquors 
due to the unique, favorable combination of very high temperature 
sensitivity (the highest reported so far) and the relatively very high 
concentration of citric acid in the fermentation liquor, typically 16-18%. 
Even at these unique conditions, the enhancer level should be reduced to a 
minimum. R. Wennerstern [J. Chem. Tech. Biotec., No. 33B, pp. 85-94 
(1983)] studied the effect of the various extractant parameters and 
concluded that hydrocarbons are the preferred diluents, as polar diluents 
reduce the temperature effect. Cooling below ambient temperature or 
preconcentration of the fermentation liquor [U.S. Pat. No. 4,994,609] are 
required to avoid major product losses. 
The above limitations brought Bauer, et al. to conclude, in 1989, that a 
TSP is not even economic for citric acid, and that displacement of the 
extracted acid by another acid (acetic) is preferable [Bauer, et al., Ber. 
Bunsenges. Phys. Chem., Vol. 93, pp. 980-984 (1989)]. 
It is important to note at this juncture that ascorbic acid does not carry 
a carboxyl group and therefore it is not a carboxylic acid, nor is it a 
mineral acid. Consequently, patents and disclosures which are directed to 
processes for treating or recovering carboxylic and/or mineral acids do 
not include ascorbic acid within their scope. 
According to its pKa, ascorbic acid is quite weak, being more than an order 
of magnitude weaker than citric acid. Its low acidity and high 
hydrophilicity (since it carries 4 hydroxyl groups) reduce its extraction 
efficiency. 
Extraction efficiency is determined by the distribution coefficient 
dependance on the aqueous phase concentration (the shape of the 
distribution curve). The distribution coefficient at the high 
concentration end determines the maximal loading of the extractant, and 
thereby, the volume of the recycled extractant. The distribution 
coefficient at the low concentration end determines the ability to 
approach complete extraction, and thereby, the extraction yield. For 
extraction of a component from a dilute feed, the yield of extraction is 
very important. Reaching high yields in extracting from a dilute feed a 
relatively weak and highly hydrophilic acid, such as ascorbic acid, would 
require high enhancer levels. 
Test results in Table 1 above show that the strongest temperature 
sensitivity so far is found for citric acid, and that this temperature 
sensitivity drops with a decreasing number of carboxyl groups. Nothing in 
these results, or in those found in the literature, indicates that 
ascorbic acid would show a higher temperature sensitivity than citric 
acid. 
Even if ascorbic acid extraction had the temperature sensitivity of citric 
acid extraction, one would not consider its recovery from dilute solutions 
in the TSP, due to the fact that at low enhancer levels, the losses would 
be extremely high. On the other hand, at high enhancer levels, the 
temperature sensitivity decreases. Thus, the major advantage of the 
process, i.e., recovering the product at a concentration substantially 
higher than that of the fermentation liquor, would be lost. 
In light of the above, it was extremely surprising to discover that the 
temperature sensitivity of ascorbic acid extraction by amine-based 
extractants is very high and is maintained, even at high enhancer levels. 
Based on this discovery, there is now provided, according to the present 
invention, a process for the recovery of ascorbic acid from an aqueous 
feed solution containing said acid at a concentration of less than 0.7 
mol/kg, comprising extracting said ascorbic acid with a water-immiscible 
organic extractant composition comprising (a) at least one secondary or 
tertiary alkyl amine in which the aggregate number of carbon atoms is at 
least 20, as a primary extractant, and (b) a polar extraction enhancer 
compound; wherein said extractant composition comprises at least 2 moles 
of said polar extraction enhancer compound per one mole of primary 
extractant; separating said ascorbic acid-containing organic extractant 
composition from residual aqueous solution, and subjecting said ascorbic 
acid-containing organic extractant composition to a stripping operation 
with aqueous solution at a temperature of at least 20.degree. C. higher 
than the temperature at which said extraction is carried out; whereby 
there is obtained an aqueous solution of ascorbic acid in which the 
concentration of ascorbic acid is higher than its concentration in said 
aqueous feed solution. 
The process of the present invention is so effective that in preferred 
embodiments thereof as described hereinafter, said ascorbic acid can be 
recovered from an aqueous feed solution containing said acid at a 
concentration of less than 0.5 mol/kg. 
Extractants comprising relatively strong amines as the primary extractant, 
show nearly no temperature sensitivity on the efficiency of extracting 
strong mineral acids. It was, however, found that relatively weak amines 
do show such effect. An example of such weak amines is the 
sterically-hindered, branched chain amines with branching on a carbon 
close to the nitrogen atom [Eyal, et. al., Solvent Extraction and Ion 
Exchange, Vol. 9, pp. 195-236 (1991)]. These amines are weaker by more 
than two orders of magnitude than straight chain amines, and weaker than 
branched chain amines with branching far from the nitrogen atom. Such 
amines are too weak to extract most weak acids and are not suitable for 
use as primary extractants in the present invention. For simplicity of 
language, the term "branched chain amines" will be used here just for 
sterically hindered, relatively weak amines with branching close to the 
nitrogen atom. 
Branched chain amines are too weak to extract many of the carboxylic acids, 
particularly hydroxycarboxylic acids. Straight chain amines are much more 
efficient, but complete extraction without resorting to high cooling costs 
requires the use of extraction enhancer. This is particularly true for 
extraction from dilute feed solutions. Yet, the stronger is the enhancer 
and the higher its contents, the lower is the sensitivity of extraction 
efficiency to temperature. Thus, amine-based extractants, comprising 
relatively strong enhancers at high proportions of enhancers, show high 
efficiency in extraction, but lose most of the advantage in 
back-extraction at higher temperature, according to U.S. Pat. No. 
4,275,234. 
According to the known practice, there have been suggested four main 
options, as well as variations and combinations thereof: 
a) Use of a weak enhancer or a strong enhancer, at a minimal concentration 
required for extraction completion (non-optimal extractant composition in 
extraction, high extractant volume, many stages in extraction and 
relatively high losses). This option was chosen for the citric acid 
production. 
b) Increase the temperature span between extraction and back-extraction 
(expensive cooling and high viscocity in extraction, and expensive heating 
and thermal degradation in back-extraction). 
c) Distill at least part of the enhancer from the extract prior to 
back-extraction (high energy cost, limitation to volatile enhancers that 
in most cases have relatively high solubility in the aqueous streams, 
requiring additional recovery operations). 
d) Add to the extract an a-polar solvent that acts as extraction 
suppressor, and removal of this solvent prior to the use of the 
regenerated extractant (low efficiency, high energy cost). 
In contradistinction to the above options, a further preferred aspect of 
the present invention is based on the discovery that polar organic 
compounds with steric hinderance of the polar group have, at about ambient 
temperature, an enhancement effect similar to that of similar non-hindered 
compounds, but lower enhancement effect at elevated temperature. As a 
result, efficient extraction is achievable using amine-based extractants 
at about ambient temperature, in combination with convenient amounts of 
enhancer, while efficient back-extraction is achieved at elevated 
temperature, without resorting to unduly high temperatures in 
back-extraction and/or high energy-consuming removal of extractant 
components, either prior to back-extraction or after it. 
Furthermore, it is well known that enhancer-containing extractants provide 
for more efficient extraction, but at the cost of reduced temperature 
sensitivity of the extracting power. The advantage of enhancer application 
in the extraction may be out-balanced by the reduced temperature 
sensitivity. Thus, for extraction of an acid from an aqueous feed of a 
relatively high acidity, particularly if incomplete extraction can be 
tolerated, non-enhanced (or slightly enhanced) extractants are preferred. 
On the other hand, in extraction from dilute aqueous solutions of acids, 
and particularly in extraction from aqueous solutions of relatively high 
pH, an enhanced extractant is essential for efficient extraction 
(alternatively, a non-enhanced, very strong amine can be used as a primary 
extractant, but stripping is impractical for such extractants). 
In light of the above, there is now provided, according to preferred 
embodiments of the present invention, a process for the recovery of 
ascorbic acid from an aqueous feed solution containing said acid at a 
concentration of less than 0.7 mol/kg, comprising extracting said ascorbic 
acid with a water-immiscible organic extractant composition comprising (a) 
at least one secondary or tertiary alkyl amine in which the aggregate 
number of carbon atoms is at least 20, as a primary extractant, and (b) a 
sterically hindered, polar, organic, extraction enhancer compound having 
at least 5 carbon atoms, a basicity weaker than that of said primary 
extractant, and temperature-sensitive, extraction-enhancing properties; 
wherein said extractant composition comprises at least 2 moles of said 
extraction enhancer compound per one mole of primary extractant; 
separating said ascorbic acid-containing organic extractant composition 
from residual aqueous solution, and subjecting said ascorbic 
acid-containing organic extractant composition to a stripping operation 
with aqueous solution at a temperature of at least 20.degree. C. higher 
than the temperature at which said extraction is carried out; wherein said 
extraction enhancer compound both enhances the extracting power of said 
primary extractant composition and facilitates said temperature-sensitive 
stripping operation, and whereby there is obtained an aqueous solution of 
ascorbic acid in which the concentration of ascorbic acid is higher than 
its concentration in said aqueous feed solution. 
In said preferred embodiments of the present invention, said sterically 
hindered, polar, organic extraction enhancer compound is preferably 
selected from the group consisting of alkanols, carboxylic acids, tertiary 
amines, or trialkylphosphates, having a sterically hindering substituent 
attached to the carbon carrying said polar group, or to a carbon which is 
alpha, beta, or gamma to said carbon. 
Polar, and particularly protic, organic compounds act as enhancers of acid 
extraction by amines, due to their ability to solvate the amine acid ion 
pair formed on such extraction. Organic compounds suitable for use as 
enhancers in the present invention have at least one such polar or protic 
group, the solvating properties of which are hindered by the structure of 
the molecule. The polar group is preferably a hydroxyl, an ester, an 
aldehyde, a carboxyl, a ketone, or an amine, or said polar group can 
comprise a halogen, sulfur, nitrogen or phosphate atom. The hindrance can 
be achieved through substitution of a hydrogen atom in the alkyl chain by 
an aliphatic group, i.e., branching on the carbon atom carrying the polar 
group, or on a carbon which is alpha, beta, or gamma to said carbon. 
The enhancer should be a weaker base than the amine used as the primary 
extractant in the extractant composite. On equilibrating it with a 0.1M 
aqueous HCl solution in a proportion that provides for enhancer to HCl 
molar ratio of 2, the aqueous phase pH will remain below 2. on a similar 
equilibration, with the amine acting by itself as the non-enhanced 
extractant, the pH of the aqueous phase increases to about 2.5 or higher. 
In addition to the primary extractant and the sterically-hindered, polar, 
organic enhancer compound, the extractant may comprise a water-immiscible, 
polar or non-polar solvent, for example, aliphatic or aromatic 
hydrocarbon, hydrocarbons carrying nitro or halo substituents, and 
alcohols. 
In preferred embodiments of the present invention, said sterically 
hindered, polar, extraction-enhancing compound is selected from the group 
consisting of secondary or tertiary alkanols, tris-2-ethylhexyl amine, and 
tris-2-ethylhexyl phosphate. 
The present invention also provides an extractant composition for use in a 
process for the recovery of ascorbic acid from an aqueous feed solution 
containing said acid or a salt thereof, said composition comprising (a) at 
least one secondary or tertiary alkyl amine, in which the aggregate number 
of carbon atoms is at least 20, as a primary extractant; and (b) a 
sterically-hindered, polar, organic extraction enhancer compound having at 
least 5 carbon atoms, a basicity weaker than that of said primary 
extractant, and temperature-sensitive, extraction-enhancing properties. 
In preferred embodiments of the present invention, said extraction 
composition comprises at least 3 moles of said polar extraction enhancer 
compound per one mole of primary extractant. 
In especially preferred embodiments of the present invention, said 
stripping action effects the back-extraction of at least 80% of the 
ascorbic acid contained in said organic extractant composition. 
As will be described and exemplified hereinafter, one of the major 
advantages of the process of the present invention for the recovery of 
ascorbic acid is that, after said stripping operation, the remaining 
organic extractant composition can be recycled, and further extraction 
carried out with said recycled organic extractant composition provides 
yields of at least 90%, and preferably at least 95%, ascorbic acid. 
The invention will now be described in connection with certain preferred 
embodiments with reference to the attached figures, so that it may be more 
fully understood. 
With specific reference now to the examples and distribution curves shown 
in the attached figures in detail, it is stressed that the particulars 
described and shown are by way of example and for purposes of illustrative 
discussion of the preferred embodiments of the present invention only, and 
are presented in the cause of providing what is believed to be the most 
useful and readily understood description of the principles and conceptual 
aspects of the invention. In this regard, no attempt is made to provide 
details of the invention more than is necessary for a fundamental 
understanding of the invention, the description taken with the drawings 
making apparent to those skilled in the art how the several forms of the 
invention may be embodied in practice.

Referring to FIG. 1, wherein Z is the acid/amine molar ratio in the organic 
phase, it is seen that the extraction is enhanced by octanol, and the 
effect is particularly strong at the low concentration end. 
FIG. 2 shows distribution curves for extraction by an extractant composed 
of 1.2 mol/kg tricaprylyl amine and 2.4 mol/kg octanol in kerosene. 
Extraction of ascorbic acid at 25.degree. C. from an 0.2 mol/kg solution 
can reach extractant loading of about 0.1 mol/kg. At 80.degree. C., 
however, by extrapolating the bottom curve, this extractant loading of 
about 0.1 mol/kg is equivalent to 0.8 mol/kg ascorbic acid in the aqueous 
phase. 
The result indicates that in using this extractant over the temperature 
gradient of 25-80.degree. C., the uphill concentration factor for ascorbic 
acid is about 4. For citric acid and for succinic acid at these 
conditions, the factor is about 2. At this extractant composition, the TS 
for ascorbic acid is higher than those for citric acid and for succinic 
acid. Comparison with succinic acid was included herein in case one were 
to think that pKa is a factor in the results of the present invention, the 
pKa of succinic acid being the same as that of ascorbic acid. 
As can be seen, however, the extraction for ascorbic acid is not yet 
sufficiently efficient and higher enhancer levels are preferred as 
described hereinafter with regard to FIG. 3. 
FIG. 3 illustrates distribution curves for extraction by an extractant 
composed of 1.2 mol/kg tricaprylyl amine (50%) and 3.8 mol/kg octanol 
(50%). The loading of the extractant in contact with 0.2 mol/kg ascorbic 
acid containing aqueous solution is about 0.5 mol/kg. Thus, increasing the 
content of the enhancer and avoiding the kerosene strongly enhanced the 
extraction, as compared to that shown in FIG. 2. The effect is even more 
impressive at the low concentrations end. The effect of the high enhancer 
level on the temperature sensitivity is surprisingly small. A 
concentration factor of about 4 can be reached on extraction at 25.degree. 
C. and back-extraction at 96.degree. C. Practically no temperature 
sensitivity is found for citric acid extraction at these conditions. 
Referring to FIG. 4, two extractants were tested. In both, the amine was 
tricaprylyl amine (Henkel's Alamine 336) and its concentration was 50 w/w 
%. In one of the extractant compositions, the enhancer was an octanol; in 
the other extractant composition, it was 3-ethyl-3-pentanol. In both 
cases, the enhancer content was 50% with no diluent having been used. 
Distribution of ascorbic acid between water and these extractants was 
tested at ambient temperature and at 75.degree. C. The results are shown 
in FIG. 4. As can be seen, the extraction at ambient temperature was 
similar for both extractants, or even slightly higher in the use of 
3-ethyl-3-pentanol. At the elevated temperature, however, the extractant 
comprising 3-ethyl-3-pentanol was less efficient. 
From the results of the test exemplified in FIG. 4, it can be realized that 
using a sterically hindered polar organic compound having at least 5 
carbon atoms, a basicity weaker than that of the primary extractant, and 
temperature-sensitive, extraction-modifying properties as the extraction 
enhancer compound of the present invention, is indeed preferred. 
Referring once again to the teachings of U.S. Pat. No. 4,275,234, it will 
be noted that several difficulties are indicated in the examples of said 
patent: 
In most examples, no enhancer was used in the extractant, or it is used in 
a limited proportion of up to 5%. In Example 7, the extractant composition 
is 50% tri-tridecylamine and 50% nitrobenzene. Being a polar component, 
nitrobenzene is quite efficient as an enhancer. An extract containing 9.3% 
citric acid was back-extracted with water (100 g per 100 g of extract) at 
60.degree. C. (35.degree. C. higher than the extraction temperature). Only 
13% of the initial citric acid was back-extracted, forming a dilute 
solution of 13% citric acid. Adding 150 g hydrocarbon to dilute the amine 
and the enhancer was needed to improve the back-extraction. This example 
concluded that "the extract could not readily be back-extracted unless a 
hydrocarbon fraction was added to it." Addition of the hydrocarbon at the 
extraction step would have reduced its efficiency, as non-polar solvents 
act contrary to the enhancers and could be referred to as extraction 
inhibitors. 
Example 16 of said patent describes the back-extraction of oxalic acid from 
an extractant composed of 25% w/w dilaurylbenzyl amine, 69% w/w n-octane 
and 6% 1-n-octanol. For efficient back-extraction, 50 g of n-octane were 
added to about 37 g of oxalic acid-containing extract. Thus, even at 
relatively low initial enhancer levels, substantial dilution by an 
extractant inhibitor was required. Only about 79% of the extracted acid is 
back-extracted at 80.degree. C. Temperatures of 120-160.degree. C. are 
recommended (Example 18). 
The yield of lactic acid recovery from an initial solution comprising 1.1 
mol/kg acid was 95% (Example 13). Enhancer-free extractant was used. The 
yield for H.sub.3 PO.sub.4 recovery from an initial solution of 0.8 mol/kg 
was 88% (Example 14). Here again, no enhancer was used. The extraction 
yield for citric acid in Example 5 was 95%, using an extractant comprising 
5% enhancer (octanol). 
In said patent, there also appears in Example 12 a description of the 
extraction of dilute lactic acid in which high amounts of enhancer are 
ostensibly used with good results. According to the principles and theory 
of the present invention, the results obtained in Example 12 of U.S. Pat. 
No. 4,275,234 did not appear to be possible or correct. In order to 
clarify this point, the extraction of lactic acid from a 2% (0.22 mol/kg) 
solution and its stripping from the extractant were repeated as in Example 
12. The extractant was composed of 50% w/w tridodecylamine and 50% w/w of 
1-n-octanol. The extraction was conducted at 25.degree. C. and the 
stripping at about 96.degree. C. 
Extraction as in Example 12 (100 g aqueous, 40 g extractant, 3 
countercurrent stages) results in practically complete extraction of the 
acid to form an extract (loaded extractant) comprising 5% w/w lactic acid. 
Stripping as in Example 12 (40 g extract, 40 g water, 5 countercurrent 
stages) results in an aqueous solution comprising 0.7 g lactic acid in 
concentration of 1.8%. About two-thirds of the extracted lactic acid stays 
in the organic phase. Re-use of this organic phase in extraction from 2% 
lactic acid solutions results in low yields; not more than 20% of the acid 
is extracted. Increasing the number of stages in extraction has only a 
small effect. Near complete stripping and thus high yield in re-use of the 
organic phase requires about 150 g water per 40 g of extract, and 6-7 
countercurrent stages. The lactic acid in this case is obtained in a 
dilute solution of about 0.5% w/w. 
Thus, using an extractant comprising about 4 moles of enhancer per mole 
amine provides for nearly complete extraction of lactic acid from a dilute 
solution of 0.22 mol/kg, but on stripping, a high proportion of water is 
required and the acid is diluted 4 times, compared to its concentration in 
the feed. The cost of concentrating this solution is enormous. 
Using the same extractant for extracting ascorbic acid from 0.22 mol/kg 
solution, 65 g of extractant per 100 g aqueous solution and 5-6 
countercurrent stages, are required to reach an extraction yield of at 
least 95% at 25.degree. C. 
Stripping the extract at 96.degree. C. with 35 g water results in an 
aqueous solution comprising 0.6 mol/kg ascorbic acid and an organic phase 
practically free of ascorbic acid. Re-use of this organic phase in 
extraction provides an extraction yield of at least 95% at the above 
conditions. 
Thus, while in the case of lactic acid, practically complete extraction 
with recycled extractant results in a lactic acid product diluted 4 times 
compared with the feed, in the case of ascorbic acid at the same 
conditions and with similar extractant, practically complete extraction 
with recycled extractant results in ascorbic acid product solution 
concentrated 3 times compared with the feed. 
Therefore, it is clear that one following the teachings of U.S. Pat. No. 
4,275,234 and repeating the examples contained therein would come to the 
inescapable conclusion that the process taught therein is not suitable for 
the commercial production of ascorbic acid. Furthermore, said patent 
certainly does not teach or suggest the use of a stearically-hindered, 
polar, organic, extraction enhancer compound as described and claimed 
herein. 
It will be evident to those skilled in the art that the invention is not 
limited to the details of the foregoing illustrative examples and that the 
present invention may be embodied in other specific forms without 
departing from the essential attributes thereof, and it is therefore 
desired that the present embodiments and examples be considered in all 
respects as illustrative and not restrictive, reference being made to the 
appended claims, rather than to the foregoing description, and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.