Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to the removal of acidic
or~anic compounds from aqueous solution and in particular to the
use of phosphine oxide mixtures of two or more such oxides without
the use of a diluent for the removal of lower carboxylic acids and
phenolic compounds from commercial effluent.
~ he treatment of aqueous effluent for removal of contami-
nants and also the recovery of valuable compounds ~rom solution is
a most essential part of modern chemical plants. A number of pro-
cedures are used such as steam-stripping and the somewhat more
complicated solvent extraction, the latter technique being largely
dependent upon the properties o~ the compounds to be recovered.
The choice of solvent is critical and solvent loss must be mini-
mized.
Some organic compounds such as acetic acid and phenol in
dilute aqueous solu-tions are particularly difficult to remove. It
is known to extract acetic acid using esters or ketones as solvent.
However, the e~uilibrium distribution coefficient, Kd (weight
fraction of solute in solvent phase/weight fraction in aqueous
phase, at e~uilibrium) for acetic acid with these solvents is about
1.0 or less. This low Kd necessitates relatively hi~h solvent flow
E~es .in the extraction process and recovery wi-th these solvents
.is not economically attractive when -there is less -than 3 -to 5 wt%
o~ a~.id in the aqueous solution.
Alternative, and somewhat improved solvent systems have
been obtained by the use of certain organophosphorous compounds and
in particular phosphine oxides in a diluent. These extractant/
diluent systems are disadvanta~eous, however, since the presence of
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a diluent (which is often necessary in order that hi~her melting
point extractants can be used) effectively reduces tlle concentra-
tion of extractant and also hinders subsequent stripping operations
by volatilizing concurrently with the compound which has been
removed from aqueous solution.
The use of 100% extractant as solvent withou~ -the use of
a diluent is therefore desirable but is limited by the melting
point of the extractant and the economic operating temperatures at
which removal is conducted. In particular -the use of neat tri-
L0 alkylphosphine oxides is known but their relatively high meltingpoints require that the removal operation be carried out at above
ambient temperatures thus incurring the risk of freeze up during
plant malfunction.
It has now been unexpectedly discovered that by use of
trialkylphosphine oxides mixtures, not only is the melting point
at a more acceptable level but that the ability of the mixture to
extract acidic organic compounds from dilute aqueous solutions i5
high. The mixtures provide unexpectedly high extraction coeffi-
cients for weakl~ extracted compounds such as acetic acid. The
~0 or~anic phase with removed compound can be stripped using several
me~hods such as distillation or stripping with an alkali solution.
Thus, according to the present invention, there is
prov.ided a process for removing an acidic organic compound selected
~rom the group consisting of a substituted or unsubstituted car-
boxylic acid having one to five carbon atoms and a substituted or
unsubstituted phenolic compound from a dilute aqueous solution
which comprises contacting said aqueous solution with a mixture of
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at least two phosphine oxides, where a first phosphine oxide has
the formula:
R
\
R2 P = O
R3
and a second phosphine oxide has the formula:
Rl \
/
R3 ~
wherein Rl R2 R3, Ri R2 and R3 are individually selected from the
group consisting of alkyl, cycloalkyl, aralkyl and substituted
aralkyl, each having C4 - C18, and the total number of carbon atoms
in said first phosphine oxide is at least 15, and the total number
of carbon atoms in said ~econd phosphine oxide is at least 17, the
difference in the total number of carbon atoms in the first and
second oxides being at least 2, at least one said phosphine oxide
being present in an amount of at least 5% by weight and not more
than 60~ by weight.
PreEerably, the total number of carbon atoms in said
second phosphine oxide is at least 19 and the diE:Eerence in the
total number of carbon atoms in the Eirst and second oxides i9 at
least 4.
More preferably at least one phosphine oxide is present
in amount of between about 25-45 wt% and said mixtures of phosphine
oxide has a melting point below about 50C.
While the process of the present invention is believed to
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be useful with a variety of valuable pollutants or impurities in
dilute aqueous streams, it is particularly useful for acidic
organic compounds such as carboxylic acids and phenolic compounds.
In particular the process is used for removing carboxylic acids
having one to five carbon atoms, preferably acetic, propionic,
butyric and valeric acids (commonly found in industrial effluents)
and also phenol. The carboxylic acids may be substituted by one
or more halogen, hydroxyl, cyano or alkoxyl groups. Other specific
acids which may be removed by the process of the present invention
are exemplified by hexanoic, heptanedoic, octanoic, nonanoic,
benæoi.c, succinic, oxalic, malic, lactic, cyanoacetic, glycolic,
~nd maleic acids. Phenolic compounds subjec~ to the instant
invention include those substituted by one or more alkyl groups.
Examples of phenolic compounds which can be removed from dilute
aqueous streams include p-cresol, resorcinol, l-naphthol, 2-naph-
thol, o-, m- and p-xylenol and unsubstituted or substituted
hydroquinone, phloroglucinol and pyrogallol~
The compound or compounds removed from dilute a~ueous
solution can be present in any low or moderately low amount in the
~0 dilute solution, although usually in an amount less than 5wt~ arld
more likely less than 2wt% or even lwt%.
The process of the present invention is particularly
useful for the recovery of carboxylic acids from paper mill and
synthetic fuel oil plants effluents. The process is also valuable
for the recovery of phenol from phenolic resin production effluent
and in coal gasification. It is believed that the recovery of
organic and inorganic compounds which are only normally weakly
extractable (e.g. Sb, As, Bi compounds) can be carried out by the
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process of the present invention.
In the phosphine oxides, when one or more R groups are
alkyl, preferred alkyls include about C4 to about C18 straight and
branched chain alkyls while preferred cycloalkyls include six
carbon to eight carbon substituted or unsubstituted cycloalkyls.
Examples of suitable phosphine oxides include, but are
not limited to, tri-n-hexylphosphine oxide (THPO), tri-n-octyl-
phosphine oxide (TOPO), tris(2,4,4-trimethylpentyl)-phosphine
oxide, ~ricyclohexylphosphine oxide, tri-n-dodecylphosphine oxide,
L0 tri-n-octadecylphosphine oxide, tris(2-ethylhexyl)phosphine oxide,
di-n-octylethylphosphine oxide, di-n-hexylisobutylphosphine oxide,
octyldiisobutylphosphine oxide, tribenzylphosphine oxide, di-n-
hexylbenzylphosphine oxide, di-n-octylbenzylphosphine oxide, 9-
octyl-9-phosphabicyclo[3.3.1]nonane-9-oxide, and the like. TOPO
and THPO are preferred oxides in a two part mix.
While one oxide should be present in an amount of at
least Swt~ and not more than 60wt%, a preferred amount is between
about 25-~5 wt% and more preferably about 35wt%. Although a
number of phosphine oxides can be used in the mixture, it is most
~0 conve~ient to use a two part mix. A particularly preferred two
p~rt mix is tri-n-octyl-phosphine oxide (TOPO) toge-ther with tri-n-
hexyl-phosphine oxide (THPO) the preferred ratio for the TOPO/THPO
mix-t~lre is 35/65 wt~. However a synergistic effect to give unex-
pectedly increased Kd values may be obtained with two or more part
mixes of the above named phosphine oxide.
The phosphine oxides used in the mixture are selected
so that the difference in the total number of carbon atoms in the
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first and second oxides is at least 2, and preferably at least 4
and more preferably 6 or 8.
Preferably the melting point of the mixture is below
about 60C which is the usual upper temperature a~ which commercial
e~fluent solutions are treated by liquid/liquid extraction pro-
cesses. However, it is desirable that the melting point be lower
in order to improve the e~ficiency and costs of the process. A
melting point of about 50C is preferable and thus a phosphine
oxide mix melting below about 50C, and more preferably 30C or
25C, will from a practical viewpoint be selected for use in the
process provided its ability to extract the acidic organic compound
is acceptable. The most preferred mixture of TOPO/THPO in a 35/65
wt% ratio melts at about 15C.
Apart from the energy savings obtainable by the use of a
low melting point phosphine oxide mixture, other advantages of low
m.p. mixtures include the avoidance of diluent to be used so that
the phosphine oxides can be used neat, and also the avoidance of
the possibility of freeze up during plant malfunction. The low
m.p. eutectic mixture of phosphine oxides also permits, by virtue
Of depressed m.p. phosphine oxides to be used which previously
could only be used at increased temperatures or together with a
~lluent, the latter use frequently complicatin~ subsequent strip
pin~ operations.
~ owever, the principal advantage of the phosphine oxide
mixtures used in the process of the present invention is the
unexpectedly increased extractability provided by such mixtures.
This extractability exceeds that of an equal amount of
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one phosphine oxide when used alone and thus provides for opera-
tional savings in amount of phosphine oxide required to extract a
desired solute from dilute aqueous solution.
The present invention will now be described in more
detail with reference to examples provided by way of illustration
only.
Example 1
Samples of solvent were tested for extractibility of
acetic acid and also phenol from dilute aqueous solution. Each
solvent sample was shaken and mixed with a fixed amount of aqueous
solution containing acetic acid. After several minutes the aqueous
phase and organic phase were allowed to separate and the aqueous
phase analysed for acetic acid presence. The procedure was
repeated with the aqueous phase until all acetic acid had been
recovered and passed into the organic phase. The amount, by
volume, or organic phase (solvent) required for 100% recovery is
indic~ted by the aqueous/organic (A/O) ra-tio.
Acetic Acid (Commercial Efluent~
A/O ~or
Sample Solvent 100% Recovery
1 150 gpl TOPO in DPA o.5
2 ~00 gpl TOPO in DPA 0.66
3 THPO/TOPO (65/35 wt% ratio) 2.0
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Phenol (Synthetic Solution)
A/O for
Sample Solvent 100% Recovery
1 100 gpl TOPO in Conoco 500 2
2 200 gpl TOPO in Conoco 500 3
3 325 gpl TOPO in Conoco 500 5
4 THPO/TOPO ( 65/35 wt% ratio) 10
Thus in samples 1 and 2, TOPO was dissolved in D.P.A., a
commercial diluent supplied by Conoco which is formed of diphenyl
~lkanes, and it can be seen that an increased amount of TOPO in the
solvent requires less organic phase to be used, i.e. the aqueous/
organic (A/O) ratio is increased. However, in solvent sample 3,
the phosphine oxide mixture (65 wt% : 35 w~% of THPO:TOPO) gives
substantially increased ability to extract the acetic acid. Also,
in the phenol extraction, sample 4 ~ives a substantially increased
A/O value thereby clearly indicating that less organic (solvent)
phase is necessary for 100% recovery o~ phenol from the aqueous
solution.
Example 2
An aqueous commercial waste effluent containin~ acetic
ana propionic acid in an amount of 6.15 and 1.50 gpl was extrac-ted
us.in~ a THPO/TOPO mixture. The equilibrium concentration for each
o~ ~he c~rboxylic acids was measured for different ~/O ratios.
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Table 1
Carboxylic Acid Extraction from Commercial
Waste Effluent which includes Acetic and Propionic Acid
Solvent (wt%) : 65 THPO, 35 TOPO
Temperature : 50C
Equilibrium Concentration (gpl)*
Acetic - Propionic
A/O Org. Aq. KD Org. ' Aq. KD
518.3 2.50 7.3 6.150.27 22.8
29.70 1.30 7.5 2.780.11 25.3
15.54 0.61 g.l 1.470.03 49.0
*Based on aqueous analysis and mass balance.
Isotherms for rema;n;ng acids in co-incidence with the Y axis.
Additionally, the same effluent was extracted using
solvent containing different concentrations of TOPO.
Table 2
Carboxylic Acid Extraction from Commercial Waste Effluent
Using 150 and 400 gpl TOPO Solvents
Temperature : 50C
Diluent : DPA
Equi;libr~ium ~Gon;cen;t~r;a;ti;on;;(gpl;)*
'A'c'e'-t'icPropionic
Solv~nt A/O Org.Aq. KD ~ ~ KD
150 ~pl TOPO 2 3.~0 4.20 0.9 l.g0 n.60 3.0
.in DP~ 1 3.452.70 1.3 1.150.35 3.3
400 ~pl TOPO 2 5.10 2.60 2.0 2.53 0.24 10.5
in DPA 1 4.701.45 3.2 1.400.10 14.0
*Based on aqueous analysis and mass balance.
Isotherms for other acids essentially co-incident with the Y axis
It is clearly demonstrated that Kd values for the THPO/
TOPO mixture are far in excess of the Kd values for the TOPO alone
in a diphenyl alkane (DPA) diluent, for corresponding A/O ratios.
Example 3
An aqueous solution containing 10 gpl phenol was extrac-
ted with solvents having different extractant concen~rations.
Table 3
Phenol Recovery
The Effect of Extractant Concentration
Solvents : (1) 100 gpl TOPO in Conoco 500
(2) 200 gpl TOPO in Conoco 500
(3) 325 gpl TOPO in Conoco 500
(4) 65 w/o THPO, 35 w/o TOPO
Aqueous : 10 gpl Phenol (nominal)
Solution
Temperature : 50C
Equi;librium Phenol Con~.(;gpl)
100 gpl TOPO 200 gpl TOPO 325 gpl TOPO THPO/TOPO
~/0 ~ Aq. KD Oxg. A~. KD Org. A~ KD Org. A~ KD
1 10.1 0.10 101 9.84 0.04 246 10.5 0.04 262 9.93 0.02 497
2 19.~ 0.47 41 19.6 0.10 196 20.9 0.08 260 19.8 0.03 660
5 3~.3 3.3~ 10 45.0 0.87 5~ - - - 49.4 0.07 706
The equilibrium phenol concentrations in the organic and
aqueous phases for various A/O values was determined and the equi-
librium distribution coefficient, Kd calculated. Again it can beseen that the Kd values for the phosphine oxide mixture is unexpec-
tedly higher than the single oxlde solvents, for all A/O values.
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