Process of extracting phenols from a phenol-containing waste water by means of a solvent mixture

Phenols are extracted from a phenol-containing waste water by means of two solvents A and B. The waste water is first of all passed through a first extraction zone, and then through a second extraction zone. To the first extraction zone a mixture of the solvents A and B is supplied, and into the second extraction zone at least one of the two solvents is introduced. From the first extraction zone, solvent-mixture loaded with phenols is withdrawn, phenols am separated therefrom, and the solvents are used again in at least one of the extraction zones. Solvent B is lower-boiling and has a lower water solubility at a temperature in the range from 10.degree. to 60.degree. C. than solvent A. Solvent B has a water solubility at 40.degree. C. of up to 2 wt.-% and a boiling point at 1 bar of 50.degree. to 100.degree. C., and it is miscible with solvent A. Solvent A has a boiling point at 1 bar of not more than 172.degree. C. and a water solubility at 40.degree. C. of not more than 5 wt.-%.

DESCRIPTION 
This invention relates to a process of extracting phenols from a 
phenol-containing waste water by means of a solvent mixture, where the 
waste water is first of all passed through a first extraction zone and 
then through a second extraction zone, and where the solvent mixture is 
supplied to the first extraction zone and at least one of the solvents of 
the mixture is introduced into the second extraction zone, solvent mixture 
loaded with phenols is withdrawn from the first extraction zone, phenols 
are separated therefrom, and the solvents are used again in at least one 
extraction zone. Such an extraction by means of two solvents should 
preferably be effected with as little steam as possible. 
Such process is known from "Chemical Engineering Progress" (May 1977), pp. 
67 to 73. The loaded solvent mixture is supplied to a first distillation 
zone, and the lower-boiling solvent is stripped from the top and 
recirculated to the second extraction zone. In a second distillation zone, 
the phenols are separated from the remaining solvent mixture, and the 
solvent withdrawn at the top is recirculated to the first extraction zone. 
In the known process, relatively high-pressure steam of preferably at 
least 10 bar is required for effecting the separation of the high-boiling 
solvent from the extracted phenol mixture. 
It is the object underlying the invention to save energy when extracting 
phenols by means of a solvent mixture, and to flexibly effect the 
distillative separation of the phenols and the separation of the solvent 
mixture prior to its reuse. In accordance with the invention, this is 
achieved in the above-stated process in that 
a waste water, which contains at least one component from the group of 
phenols, with a pH of 6 to 10 and usually 7 to 9 is supplied to the first 
extraction zone, that a mixture of the solvents A and B is passed through 
the first extraction zone at temperatures between 10.degree. and 
60.degree. C. and mostly 30.degree. to 50.degree. C., 
wherein solvent B is lower-boiling and has a lower water solubility at a 
temperature in the range from 10.degree. to 60.degree. C. than solvent A, 
solvent B has a water solubility at 40.degree. C. of up to 2 wt-% 
(preferably not more than 1 wt-%) and a boiling point at 1 bar of 
50.degree. to 100.degree. C. (and preferably 60.degree. to 75.degree. C.), 
and a distribution factor D for resorcinol of at least 0.5 and preferably 
1 to 2, and solvent B is miscible with solvent A, 
wherein solvent A has a boiling point at 1 bar of not more than 172.degree. 
C. and usually not more than 160.degree. C., a water solubility at 
40.degree. C. of not more than 5 wt-% (preferably not more than 2 wt-%) 
and a distribution factor D for resorcinol of at least 5 and preferably 7 
to 20. The distribution factor D is defined as C1/C2, where C1 is the 
equilibrium concentration of resorcinol in solvent B or A, and C2 is the 
equilibrium concentration of resorcinol in the waste water. In the process 
of the invention, solvent B is introduced into the second extraction zone 
at temperatures in the second zone of 10 to 60.degree. C. and mostly 
30.degree. to 50.degree. C., and solvent B leaving the second extraction 
zone, which has absorbed solvent A from the waste water, is supplied to 
the first extraction zone. From the second extraction zone, a treated 
low-phenol waste water is withdrawn, which has a content of not more than 
50 ppm monovalent phenols and a resorcinol content corresponding to a 
removal of at least 90% of the resorcinol contained in the supplied waste 
water. The solvent mixture loaded with phenols, which has been withdrawn 
from the first extraction zone, is separated in a first distillation zone, 
where from the top of the first distillation zone a phenol-free mixture of 
solvents B and A is withdrawn, and from the bottom of the first 
distillation zone a phenol-containing solvent mixture is withdrawn. From 
the phenol-free mixture of solvents B and A, which has been withdrawn from 
the first distillation zone, part of solvent B is separated in a second 
distillation zone, this part of solvent B is supplied to the second 
extraction zone, and the remaining solvent mixture is supplied to the 
first extraction zone. Resorcinol is one of the divalent phenols, which is 
usually contained in the waste water to be treated. Resorcinol is hard to 
extract, and is therefore utilized here as relevant substance. 
The waste water to be treated originates for instance from the gasification 
of coal, and in particular from the gasification of coal in a fixed bed, 
or from the hydrogenation or carbonization of coal. 
As solvent A there can for instance be used a ketone, an ester, a ketone 
mixture, an ester mixture or a ketone-ester mixture. Only to give an 
example for solvent A, methyl isobutyl ketone (MIBK) as well as n-butyl 
acetate or isobutyl acetate are mentioned. 
For solvent B, ether or ether mixtures should be used above all, and only 
to give an example the following are mentioned: diisopropyl ether (DIPE), 
ethyl-t-butyl ether (ETBE) or t-amyl-ethyl ether (TAME). 
The treated waste water withdrawn from the second extraction zone has a 
content of not more than 50 ppm and usually not more than 5 ppm monovalent 
phenols. It is achieved that the content of resorcinol in the treated 
waste water corresponds to a removal of at least 90% and preferably at 
least 99% of the resorcinol content in the untreated waste water. The 
treated waste water, which comes from the second extraction zone, is 
passed through at least one stripping zone and is in addition heated to 
temperatures of 100.degree. to 180.degree. C. and preferably at least 
130.degree. C. By means of stripping and heating it is possible to purify 
the waste water to such an extent that it can be directly supplied to a 
biological waster water purification. It is no longer necessary that 
before or after the biological waste water purification, the phenol-free 
waste water is passed through an adsorption filter, e.g. an 
activated-carbon filter. 
In known processes employing only one solvent which has very good 
extraction properties (e.g. MIBK or butyl acetate), the distillation of 
the solvent from phenol requires a relatively large amount of 
high-pressure steam (10 bar or more). In addition, an even larger amount 
of low-pressure steam is required for stripping solvent from the 
raffinate. The process described in "Chemical Engineering Progress" (see 
above), which employs two solvents, does not require steam for stripping 
solvent from the water, but an additional effort is required for effecting 
the condensation of the second solvent, as the same cannot be condensed 
with cooling water at a pressure of 1 bar. The known "Phenosolvan" process 
(Lurgi) is the energetically most favorable of the known processes, as 
both for the distillation of the solvent from phenol and for stripping the 
solvent from the raffinate, the waste heat of the bottom product of the 
NH.sub.3 stripping column is used. Only for stripping minor solvent 
quantities from crude phenol a small amount of steam is required. The 
disadvantage of this process lies in the fact that at solvent ratios 
required for a complete extraction of the polyvalent phenols the waste 
heat of the waste water is not sufficient for the distillation. 
The process in accordance with the invention combines the advantages and 
eliminates the disadvantages of the aforementioned processes in that it 
uses two solvents with suitable properties as regards boiling point, water 
solubility and distribution factor. 
In the inventive process, the utilization of the waste heat of the waste 
water from the NH.sub.3 stripping column can be effected as follows: At 
least in part through an indirect heat exchange, the heated waste water is 
supplied to one distillation zone or also to both distillation zones, 
where it acts as heating medium through an indirect heat exchange. One 
possibility of this indirect heat exchange consists in the fact that in 
the first distillation zone liquid is passed to the outside through an 
intermediate outlet, and the liquid is heated with heated waste water, 
before it is returned to the distillation zone. 
It is in addition recommended to preheat the solvent mixture loaded with 
phenols, which has been withdrawn from the first extraction zone, before 
it enters the first distillation zone. This preheating can be effected in 
different ways. One special possibility of preheating consists in partly 
evaporating the phenol-containing solvent mixture when the same is 
preheated. This can be done in one stage or in several stages. Preferably, 
vapours are introduced into a section of the distillation zone located at 
a higher level, and liquid is introduced into a lower section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1, the phenol-containing waste water to be treated is 
supplied to the first extraction zone (1) through line (2). A mixture of 
solvents A and B is supplied through line (3) to the first extraction 
zone. The waste water treated in the first extraction zone is delivered 
through line (4) to the second extraction zone (5) and is withdrawn via 
line (6) as virtually phenol-free waste water. Solvent B is supplied 
through line (7) to the second extraction zone, where it chiefly serves to 
remove solvent A from the waste water. A mixture of solvents A and B 
leaves the second extraction zone (5) through line (8) and together with a 
mixture of solvents A and B from line (9) is delivered through line (3) to 
the first extraction zone (1). 
Usually, each extraction zone consists of a plurality of series-connected 
extraction stages. When extraction stages of the mixer-settler type are 
used, see FIG. 2, the first extraction zone (1) comprises for instance 3 
to 10 stages, and the second extraction zone (5) comprises 1 to 6 stages, 
but the number of extraction stages can be varied as required. 
The extractor schematically represented in FIG. 2 comprises an inlet 
chamber (17) with an inlet (18) for waste water and a line (19) for the 
solvent mixture. From the chamber (17), which also serves as mixing 
chamber, the mixture is supplied by means of the pump (20) to a separating 
chamber (21), which has a gas outlet (22). In the chamber (21) the heavy 
water phase separates from the lighter solvent phase. The light phase is 
withdrawn through line (23), and the water phase flows through the opening 
(24) first of all to the collecting chamber (25) and then through line 
(26) to the next extractor. Extractors of the type described in FIG. 2 are 
commercially available. 
Solvent mixture, which is chiefly loaded with the absorbed phenols, is 
withdrawn from the first extraction zone (1) through line (29) and is 
heated through an indirect heat exchange in the preheater (30). The 
solvents evaporate partly or largely in the succeeding heater (13), before 
the mixture is delivered via line (43) to the first distillation zone 
(44). Other than represented in the drawing, line (43) can be divided into 
two lines, and vapours can be introduced into a higher section and liquid 
into a lower section of the distillation zone (44). The mixture of 
solvents A and B withdrawn from the top of the first distillation zone 
(44) through line (47) serves as heating medium in the preheater (30). The 
vaporous solvent mixture is withdrawn via line (31), is passed through a 
condenser (32), and a condensate is obtained which chiefly consists of 
solvent A. This condensate is delivered via line (33) to the second 
distillation zone (40). The vapour phase, which chiefly consists of 
solvent B, is withdrawn from the condenser (32) through line (34), and 
expediently slightly above the orifice of line (33) is likewise delivered 
to the second distillation zone (40). The separation in the second 
distillation zone (40) is effected in a manner known per se by means of an 
indirect heat exchanger (12), and from the top through line (35) with a 
first cooling (36), partial reflux through line (37) and second cooling 
(38) solvent B is withdrawn, which is recirculated via line (7) to the 
second extraction zone. Fresh solvent B is supplied, if necessary, through 
line (7a). The mixture of solvents A and B obtained at the bottom of the 
second distillation zone (40) is recirculated through the cooler (39) and 
then through line (9) to the first extraction zone (1). Fresh solvent A is 
added, if necessary, through line (9a). The mixture in line (9) contains 
the solvents A and B usually in a weight ratio of 1:10 to 3:1, the 
temperature of line (9) usually is about 40.degree. C. 
In the first distillation zone (44) part of the liquid present in zone (44) 
is constantly delivered through an intermediate outlet (45) to the outside 
to an indirect heat exchanger (11), in which part of the liquid is 
evaporated, before steam and liquid are recirculated through line (46) to 
the first distillation zone (44). The vaporous top product of the first 
distillation zone (44) is first of all delivered to the heat exchanger 
(30) through line (47), where condensate is recirculated via line (48). By 
means of the heater (49), which is usually heated with steam, a 
temperature of about 150.degree. C. is adjusted at the bottom of the 
distillation zone (44). 
Each of the two distillation zones (44) and (40) is realized in practice 
through one or more distillation columns, which in a manner known per se 
contain liquid- and vapour-permeable trays or packings. 
A liquid mixture whose main components are phenols and solvent residues 
leaves the first distillation zone (44) through line (52) and is supplied 
to the column (55), which here is also referred to as phenol column. It 
comprises a heating (56) of the bottom liquid, and for stripping purposes 
steam is supplied through line (57). The temperature at the bottom usually 
lies in the range from 130.degree. to 170.degree. C., and preferably 
140.degree. to 150.degree. C. From the bottom of column (55) crude phenol 
is withdrawn through line (58), is passed through the heat exchanger (53) 
for cooling purposes, and then flows through line (58a). A partial stream 
of the crude phenol is removed from the process as product through line 
(59). The remaining crude phenol is delivered through line (60) to a 
cooler (61) and then through line (62) to a column (63), which here is 
also referred to as absorber. Solvent-containing liquid withdrawn from the 
bottom of the column (63) is supplied through line (64) and the heat 
exchanger (53) to the phenol column (55) after having been preheated. The 
top product of the phenol column (55) is withdrawn via line (65), is 
cooled in the condenser (66), and the condensate produced is recirculated 
via line (67) to the first extraction zone (1). This condensate chiefly 
consists of water and solvent, and also contains residual phenols. 
The treated waste water withdrawn via line (6) from the second extraction 
zone (5) is first of all heated in the indirect heat exchanger (15) and 
then added to a stripping column (70). Through line (71) recycle gas is 
supplied to the stripping column as stripping gas, which wholly or partly 
consists of inert gas (e.g. nitrogen). The recycle gas loaded with 
solvent, which has been withdrawn from the stripping column (70), flows 
through line (72) to the absorber (63), where solvent is removed. Gas 
withdrawn from the top of the absorber (63) is recirculated to the 
stripping column (70) via the blower (74). If necessary, the gas of line 
(71) can also be passed through a not represented water washing for 
removing phenols. 
The waste water, which is withdrawn from the bottom of the stripping column 
(70) through line (75), is first of all delivered to the indirect heat 
exchanger (14) for being heated, and from there to a further column (77). 
By means of heating, disturbing gases are stripped off from this column, 
where the top product is partly recirculated via the condenser (78). The 
remaining partial stream of the exhaust gas, which chiefly consists of 
NH.sub.3, CO.sub.2, H.sub.2 S and steam, is withdrawn via line (79). 
The bottom liquid of the column (77) is heated by means of steam, which in 
the indirect heat exchanger (80) serves as heating medium. Via line (10) 
waste water is withdrawn from the column (77) at a temperature in the 
range from 100.degree. to 180.degree. C., and preferably at least 
130.degree. C. This waste water has now already been purified to such an 
extent that after a further cooling it can be directly delivered to a not 
represented waste water purification. It is important that in the above 
described process the heat of the waste water in line (10) is properly 
utilized. For this purpose it is possible, for instance, to pass the waste 
water as heating medium in succession through the heat exchangers (11), 
(12), (13), (14) and (15), where this order can also be varied. So as not 
to impair the clarity of FIG. 1, this circulation of the waste water is 
not represented. 
EXAMPLE 
To a plant as shown in FIG. 1 a phenol-containing waste water from a coal 
gasification plant is supplied through line (2), which waste water had 
largely been liberated from tar, oil and solids prior to the extraction 
treatment. Solvent A is methyl isobutyl ketone (MIBK), and solvent B is 
diisopropyl ether (DIPE). Per hour, 7 kg DIPE and 0.1 kg MIBK are supplied 
to the plant through line (7a) and (9a), respectively, so as to compensate 
losses. 900 kg/h steam serve as stripping medium in line (57). In Tables I 
and II, the quantities and the components of the liquid (in kg/h) as well 
as the temperature (in .degree. C.) and the pressure (in bar) are 
indicated for the various lines; the data have in part been calculated. 
TABLE I 
__________________________________________________________________________ 
Line 2 4 6 7 8 9 10 29 31 
__________________________________________________________________________ 
Quantity 
105594 
106914 
107396 
9863 
9380 
13467 
99593 
23445 
21935 
H.sub.2 O 
100000 
100656 
100891 
324 
89 -- 99579 
324 324 
NH.sub.3 
1500 
1500 
1500 
-- -- -- 10 -- -- 
CO.sub.2 
3500 
3500 
3500 
-- -- -- -- -- -- 
H.sub.2 S 
100 100 100 -- -- -- -- -- -- 
MIBK -- 363 5 0.01 
358 
9355 
-- 10183 
9354 
DIPE -- 790 1396 
9539 
8933 
4112 
-- 12263 
12257 
Phenol 
263 0.1 0.01 
0.002 
0.1 
-- 0.01 
423 0.002 
Cresols 
107 -- -- -- -- -- -- 133 -- 
Xylenol 
34 -- -- -- -- -- -- 34 -- 
Resorcinol 
82 4 4 -- -- -- 4 78 -- 
Pitch 8 -- -- -- -- -- -- 8 -- 
Temperature 
35 35 35 35 35 35 140 35 102 
Pressure 
1.5 1.5 1.5 1.2 
1.5 
1.4 1.4 1.5 1.3 
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TABLE II 
__________________________________________________________________________ 
Line 43 52 58 59 62 64 67 75 79 
__________________________________________________________________________ 
Quantity 
23445 
1510 
8679 
499 
8184 
8312 
2044 
106002 
6409 
H.sub.2 O 
324 -- 158 9 149 
149 
891 100891 
1312 
NH.sub.3 
-- -- -- -- -- 61 61 1500 
1490 
CO.sub.2 
-- -- -- -- -- 19 19 3500 
3500 
H.sub.2 S 
-- -- -- -- -- 47 47 100 100 
MIBK 10183 
829 
0.5 -- 5 5 833 0.1 0.1 
DIPE 12263 
6 0.1 -- 0.1 
2 8 7 7 
Phenol 423 423 
4538 
263 
4275 
4275 
160 0.01 
-- 
Cresols 
133 133 
1846 
107 
1739 
1739 
26 -- -- 
Xylenol 
34 34 587 34 553 
553 
-- -- -- 
Resorcinol 
78 78 1411 
78 1333 
1333 
-- 4 -- 
Pitch 8 8 138 8 130 
130 
-- -- -- 
Temperature 
98 153 
140 80 35 55 35 50 100 
Pressure 
1.5 1.5 
1.2 1.2 
1.2 
1.2 
1.2 1.2 1.8 
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