Process for cleaning up wastewaters from the preparation of alcohols

A process for cleaning up the wastewaters which originate from the preparation of alcohols. The alcohol preparation includes an aldolization reaction followed by hydrogenation and then alcohol distillation. The resulting wastewaters include 3 wastewater fractions from respective steps in the alcohol preparation. The process for cleaning up these wastewaters include the steps of (a) combining two of the three fractions and adjusting the pH of the combination, (b) bringing at least one of the fractions into contact with a coalescing filter, and (c) extracting a mixture of all three fractions.

FIELD OF THE INVENTION 
A process for cleaning up wastewaters which are produced in the 
aldolization of identical or different aldehydes or ketones, or in the 
mixed aldolization of aldehydes and ketones, which is followed by 
hydrogenation. 
STATE OF THE INVENTION 
The term aldolization is used to include aldol addition and aldol 
condensation. The term aldol addition describes the base- or 
acid-catalyzed addition of activated methylene groups to the carbonyl 
groups of aldehydes or ketones, with the formation of 
.beta.-hydroxy-carbonyl compounds. If the aldol addition is followed by 
elimination of water, which occurs readily and is usual when acid 
catalysts are used, this is called aldol condensation. The aldol 
condensation products are .alpha.,.beta.-unsaturated carbonyl compounds. 
The aldolization of two molecules of the same aldehyde or the same ketone 
is of particular importance. Reactions of this type are also utilized 
industrially. One example of the industrial application is the preparation 
of 2-ethylhexanol, the most important synthetic alcohol after the lower 
alcohols, methanol to butanol, by aldolization of n-butyraldehyde and 
subsequent hydrogenation. Phthalic esters of 2-ethylhexanol are very 
widely used as plasticizers for plastics. 
For the preparation of alcohols with the incorporation of an aldolization 
step, aldehydes, as starting material, in the presence of aqueous, basic 
solutions, are first reacted in an aldol condensation to form an 
.alpha.,.beta.-unsaturated aldehyde. In the preparation of the 
abovementioned 2-ethylhexanol, n-butyraldehyde is thus reacted to form 
2-ethylhexenal under the action of aqueous sodium hydroxide solution, for 
example. The organic phase comprising the .alpha.,.beta.-unsaturated 
aldehyde is then separated off from the catalyst-containing aqueous phase. 
After this, the organic phase is washed with water and the 
.alpha.,.beta.-unsaturated aldehyde is hydrogenated to give the desired 
alcohol, in the case of 2-ethylhexenal to give 2-ethylhexanol. The crude 
alcohol thus obtained is then purified by a distillation and the residue 
of this distillation is subjected to further washing with water. 
In the course of the aldolization and the subsequent hydrogenation, three 
different wastewater fractions are thus produced. These are 
(1) the catalyst-containing aqueous phase from the aldolization reaction, 
(2) the washwater from the purification of the .alpha.,.beta.-unsaturated 
aldehyde produced in the aldolization and 
(3) the washwater from the cleanup of the alcohol distillation residue. 
These wastewater fractions comprise both water-soluble and water-insoluble, 
and thus emulsified, by-products, which can be formed in the individual 
process stages, in addition to unreacted starting materials and very small 
amounts of the products produced. 
The unreacted starting materials are the starting aldehydes of the 
aldolization, such as n-butyraldehyde, and the products produced are the 
.alpha.,.beta.-unsaturated aldehydes of the aldolization and the alcohols 
formed therefrom after hydrogenation. These products are present only in 
extremely small amounts in relation to the aldolization and hydrogenation 
by-products in the total amount of wastewater. In the case of the 
aldolization of n-butyraldehyde, 2-ethylhexenal, for example, is present 
at at most 0.2% by weight, based on the total amount of all organic 
compounds present in the wastewater, i.e. the sum of unreacted starting 
materials, by-products from aldolization and hydrogenation and products of 
aldolization and hydrogenation. 
The by-products which are formed during the aldolization from the aldehyde 
used are primarily 
branched isomers of the aldehyde used, 
unbranched or branched alcohols having the same carbon number as the 
aldehyde, 
unbranched or branched alcohols having one more carbon atom than the 
aldehyde used, 
cyclic esters (lactones) having the same carbon number as the aldehyde, 
carboxylic acids, present as salts, particularly alkali metal salts, having 
the same carbon number as the aldehyde and the corresponding free 
carboxylic acids, 
aldehydes which are produced by hydrogenation of the 
.alpha.,.beta.-unsaturated aldehyde from the aldolization reaction, 
cyclic lactones and cyclic diols having 4 carbon atoms more than the 
desired alcohol from aldolization and following hydrogenation, 
carboxylic esters from the desired alcohol and the corresponding carboxylic 
acid and 
carboxylic esters from the desired alcohol and the carboxylic acid having 
the same number of carbon atoms as the starting aldehyde. 
In addition, the following may be present in a very small amount as 
by-products 
variously branched alcohols, aldehydes, ethers, lactones and acids having 
either one or two carbon atoms fewer or 1-4 carbon atoms more than the 
desired alcohol, from aldolization and following hydrogenation. In 
addition, the wastewater can also comprise hydrogenation products of 
by-products from the aldolization reaction and higher-boiling condensation 
products. 
If 2-ethylhexanol is prepared by aldolization of n-butyraldehyde in the 
presence of sodium hydroxide solution followed by hydrogenation, the 
overall wastewater comprises, for example: n-butanal, i-butanal, 
n-butanol, i-butanol, 2-methylbutanol, n-butyric acid, sodium butyrate, 
4-heptanol, 3-methyl-4-heptanone, 3-methyl-4-heptanol, 
2-ethyl-4-methylpentanal, 2-ethyl-4-methylpentanol, 2-ethylhexanal, 
2-ethylhexenal, 2-ethylhex-3-enol, 2-ethylhexanol, 2-ethylhexyl butyrate, 
2-ethylhexyl 2-ethylhexanoate, 2-ethylhexanoic acid, 
2-ethylhexane-1,3-diol, cyclic C.sub.12 -diol, cyclic C.sub.12 -lactone, 
cyclic saturated and unsaturated C.sub.12 ethers, C.sub.10 ethers, n-butyl 
2-ethylhexyl ether and trimeric n-butanal. 
The concentration of such organic compounds in aqueous media is customarily 
described by the COD value. The COD value (the abbreviation COD means 
chemical oxygen demand) is the amount of potassium dichromate, expressed 
as oxygen equivalent, which is consumed by the oxidizable constituents of 
one liter of water. The COD value is determined by a standardized 
procedure, which is described, for example, in Ullmanns Enzyklopadie der 
technischen Chemie [Ullmann's Encyclopedia of Industrial Chemistry], 4th 
edition (1981), volume 6, pages 376 ff. 
Before the wastewater can be introduced into conventional treatment plants, 
into river courses or other receiving water bodies, its content of organic 
impurities must be markedly decreased to comply with the strict 
requirements of the legislature with respect to the maximum pollutant 
concentrations in wastewaters. 
EP-A-0 631 988 discloses a process by which the concentration of organic 
impurities in the wastewaters from aldolizations and following 
hydrogenation reactions can be considerably lowered. In this process, the 
above-described three wastewater fractions are combined and adjusted to a 
pH of 0-6. An organic phase which separates out after this is removed, if 
appropriate, and the wastewater is then extracted with monohydric alcohols 
which contain 8 or more carbon atoms in the molecule and/or with 
hydrocarbons which contain more than 6 carbon atoms in the molecule. 
The process is simple to carry out industrially and makes it possible to 
remove at least 90% of the organic compounds present in the combined 
wastewater. The extraction medium present after the extraction and loaded 
with the organic compounds and by-products is, under practical conditions, 
subjected to a distillation for recovery. The resulting distillation 
residue and a fraction taken off at the top of the distillation column 
comprise a major part of the organic by-products and are supplied to 
thermal utilization. The extraction medium recovered via a side takeoff is 
recirculated to the extraction process, with supplementation by fresh 
extraction medium. 
An important aspect of the process of EP-A-0 631 988 is to adjust the 
generally alkaline wastewaters to a pH of 0-6, preferably 1-3. This 
protonates the carboxylic acids, for example, present as water-soluble 
alkali metal salts and thus converts them into the free carboxylic acids 
which free acids are water-insoluble substances. There is thus a 
development, to a greater or lesser extent, of an organic phase which is 
removed from the wastewater, which can already achieve a certain reduction 
in COD value. Setting said pH also causes the solubility of the extraction 
medium in water to be further reduced. This is because it has been found 
that the solubility of the extraction media in water is pH-dependent and 
is markedly lower in acidic media than in alkaline media. Therefore, in 
acidic solutions, they make only a small contribution to increasing the 
COD value. 
Despite the reduction in pH followed by removal of the resulting organic 
phase, the wastewater in the process of EP-A-0 631 988, in addition to the 
dissolved water-soluble organic compounds, still comprises a certain 
amount of water-insoluble organic compounds which are emulsified in the 
wastewater. These substances can thus not be removed from the wastewater 
until the extraction step following the pH adjustment. Since 
higher-molecular-weight compounds and high-boiling condensation products 
are also removed by the extraction, high temperatures must be used in the 
distillation of the extraction medium loaded with these impurities, which 
in turn undesirably leads to increased formation of high-boiling 
condensation products in the distillation bottom phase. So that the 
distillation temperature does not need to be increased continuously, under 
practical conditions relatively large amounts of the distillation bottom 
phase are continuously taken off from the distillation reactor, which also 
conjointly removes 2-ethylhexanol each time, however and this is thus lost 
from the recirculation. This means that the 2-ethylhexanol recirculated 
downstream of the distillation must be constantly replenished by fresh 
2-ethylhexanol. 
OBJECTS OF THE INVENTION 
The object of the invention is to provide an improved process for cleaning 
up of wastewaters from an aldolization and downstream hydrogenation which 
permits a more effective recovery of the extraction medium. 
These and other objects and advantages of the invention will become obvious 
from the following detailed description.

THE INVENTION 
The process of the invention for cleaning up wastewaters from the 
preparation of alcohols, which preparation comprises an aldolization 
reaction, a following hydrogenation and a concluding distillation of the 
alcohol, which wastewaters are comprised of three fractions, 
(1) the catalyst-containing aqueous phase from the aldolization reaction, 
(2) the washwater from the purification of the .alpha.,.beta.-unsaturated 
aldehyde produced in the aldolization and 
(3) the washwater from the cleanup of the alcohol distillation residue, 
and the combined wastewater fractions having been extracted with monohydric 
alcohols of 8-16 carbon atoms in the molecule and/or with hydrocarbons of 
6-12 carbon atoms in the molecule, comprising 
(a) upstream of the extraction, first combining at least two of the three 
wastewater fractions and adjusting them to a pH of 0 to 6 and optionally 
removing the organic phase forming during this and 
(b) in the course of the process, bringing at least one of the three 
wastewater fractions individually and/or collectively into contact with a 
coalescing filter. 
The wastewaters from the preparation of alcohols used in the process of the 
invention, which preparation comprises an aldolization reaction, a 
following hydrogenation and a concluding distillation of the alcohol, are 
the three wastewater fractions (1), (2) and (3), which can comprise the 
water-soluble and water-insoluble organic compounds which have already 
been specified in detail. 
The coalescing filters used in the process of the invention are 
liquid/liquid phase separators for very fine demisting, in which the 
physical action of coalescence is exploited in specially constructed 
cylindrical fiber bed elements. Coalescing filters of this type are known 
in the prior art (see, for example, Chemie-Technik, vol. 18 (1989) pp. 
14-21). They consist of, for example, plastic fibers of polypropylene or 
polytetrafluoroethylene, or of glass fibers or metal fibers. A driving 
differential pressure of approximately 0.1 bar is necessary for flow 
through the fiber bed. 
When flow passes through the fiber bed of the coalescing filter, the 
water-insoluble organic compounds which are finely dispersed in the 
wastewater combine, or they are retained by an organic separating layer 
already present on the fiber bed and form a wetting film. This wetting 
film migrates with the driving wastewater stream to the exit of the fiber 
bed, where large droplets of the organic phase then separate off at 
intervals and separate out due to gravity alone. 
This organic phase can in turn be separated off from the wastewater which 
results in the fact that the wastewater entering the following extraction 
has a lower COD value. Thus, only a smaller amount of organic compounds 
must be removed from the wastewater in the extraction. The extraction 
medium leaving the extraction is therefore only loaded with a smaller 
amount of organic compounds, and the temperatures required in the 
distillation reactor for recovering the extraction medium are 
correspondingly lower. This markedly decreases the formation of further 
high-boiling by-products in the work-up of the used extraction medium by 
distillation. Correspondingly, only small amounts of the distillation 
bottom phase need to be continuously removed, as a result of which the 
loss of the extraction medium during work-up is markedly lower than in the 
case of the process according to EP-A-631 988, when no coalescing filter 
is used. This is accompanied by the advantage that after the work-up of 
the extraction medium by distillation, only markedly smaller amounts of 
fresh extraction medium need to be added. 
The intermediate connection of a coalescing filter provides, in the process 
of the invention, a design for improving wastewater cleanup which is 
associated with many-sided beneficial effects. A further advantage of the 
process of the invention is based on the fact that the coalescing filters 
are suitable for a continuous mode of operation and the overall process 
for cleaning up wastewaters can also be carried out continuously 
subsequent to the aldolization and hydrogenation reactions. 
Various embodiments have proved particularly useful for the process of the 
invention. The embodiment Al comprises first combining the wastewater 
fractions (1) and (3) with one another according to step (a), adjusting 
them to a pH of 0 to 6, preferably 1 to 3, and optionally freeing them 
from the organic phase which forms and then combining the remaining 
wastewater with the wastewater fraction (2), conducting them collectively 
through a coalescing filter and then subjecting them to the extraction. 
An embodiment A2 comprises again first combining the wastewater fractions 
(1) and (3) with one another according to step (a), adjusting them to a pH 
of 0 to 6, preferably 1 to 3, and optionally freeing them from the organic 
phase which forms, and then combining the remaining wastewater with the 
wastewater fraction (2) which has already been conducted through a 
coalescing filter and feeding the mixture to the extraction. 
In the embodiment A3, the wastewater fractions (1) and (3) are first 
combined with one another according to step (a), adjusted to a pH of 0 to 
6, preferably 1 to 3, optionally freeing them from the organic phase which 
forms and passing them through a coalescing filter. The remaining 
wastewater is then combined with the wastewater fraction (2) which has 
likewise already been passed through a coalescing filter and the mixture 
is fed to the extraction. 
A further embodiment A4 is distinguished by the fact that all three 
wastewater fractions (1), (2) and (3) are first combined, adjusted to the 
pH of 0 to 6, preferably 1 to 3, and optionally freed from the organic 
phase which forms, and are then collectively conducted through a 
coalescing filter and subjected to the extraction. 
To adjust the pH of the wastewater, inorganic acids such as hydrochloric 
acid, sulfuric acid, nitric acid or phosphoric acid are used with 
preference being given to sulfuric acid. 
Since the deposition of solid particles in the fiber bed of the coalescing 
filter can lead to an impairment of the separation efficiency, it can be 
expedient to connect, upstream of the coalescing filter, a suitable filter 
stage for removing solid particles. 
As extraction medium in the process of the invention, use is made of 
alcohols of 8 to 16 carbon atoms in the molecule. They can be unbranched 
or branched, saturated or unsaturated and it is not necessary to use pure 
alcohols. Suitable extraction media are also mixtures of isomeric alcohols 
or of alcohols of different molecular size. Those which have proved to be 
useful are 2-ethylhexanol, 3,5,5-trimethylhexanol, isooctanol, nonanols, 
decanols and isodecanols, and the mixtures of the isomeric octanols, 
nonanols and decanols. 
In addition to the alcohols, hydrocarbons of 6 to 12 carbon atoms in the 
molecule can also be used as extraction medium. The hydrocarbons can also 
be unbranched or branched, saturated or unsaturated. Those which have 
proved particularly useful are mixtures of different hydrocarbons which 
are produced as low-boiling fractions in the distillation of petroleum, 
particularly light fuel oil. 
Apart from alcohol mixtures or mixtures of different hydrocarbons, mixtures 
of alcohols and hydrocarbons can also be used for extracting the organic 
compounds from the wastewater. The mixing ratio can extend over a broad 
range and is primarily restricted by the miscibility of the components. 
The alcohols and/or hydrocarbons used in the invention have proved to be 
excellent extraction media for the organic substances usually present in 
the specified wastewaters. It is also of importance that they are 
dissolved only to a very slight extent by the water phase. 
The organic compounds are extracted from the wastewaters by the process of 
the invention in apparatuses customary for solvent extraction. Those which 
have proved to be useful are extractors which are constructed as 
single-stage or multiple-stage as extraction batteries of a plurality of 
mixer-settler pairs connected one after the other. Likewise, use can be 
made of extraction columns having static internals, such as packed 
columns, or having moving internals, such as stirred columns, extraction 
medium and carrier liquid being conducted concurrently or, preferably, 
countercurrently. 
The extraction medium may be regenerated in a simple manner by distillation 
and can therefore be used repeatedly. The distillation residue and the 
overhead takeoff from the distillation column comprise the organic 
compounds to be removed and are fed to thermal utilization, while the 
extraction medium is recovered via a column side takeoff and recycled to 
the extraction process. 
The novel process is suitable for working up wastewaters of the 
aldolization reaction and downstream hydrogenation, independently of the 
aldolization process employed and the starting materials used. The 
aldolization can therefore have been carried out, for example, with alkali 
metal hydroxide, alkali metal carbonate or amines as catalysts and with 
aldehydes or ketones. 
Various modifications of the process of the invention may be made without 
departing from the spirit or scope thereof and it is to be understood that 
the invention is intended to be limited only as defined in the appended 
claims.