Process for the separation of propylene glycol from fatty alcohols

A process for the separation of propylene glycol from a mixture of low-boiling fatty alcohols and propylene glycol which comprises extracting the mixture with water to produce a water-propylene glycol mixture and fractionating the water-propylene glycol mixture to produce propylene glycol that is substantially anhydrous and an apparatus for carrying out the process.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for the separation of propylene glycol 
from a mixture of low-boiling fatty alcohols and propylene glycol. The 
invention also relates to an apparatus for carrying out this process. 
2. Description of the Related Art 
The term "low-boiling fatty alcohols" applies to fatty alcohols of which 
the boiling points are close to the boiling point of propylene glycol. 
Because of this property, the propylene glycol cannot be separated by 
rectification. 
The problem addressed by the invention is to provide a process for the 
separation of propylene glycol from a mixture of low-boiling fatty 
alcohols and propylene glycol which is economical and provides for the 
substantially complete removal of propylene glycol from fatty alcohols. A 
solution to this problem was only found after extensive tests. 
SUMMARY OF THE INVENTION 
According to the invention, a process for separating propylene glycol from 
a mixture of low-boiling fatty alcohols and propylene glycol is provided 
comprising the steps of: (a) extracting the mixture with water to produce 
a water-propylene glycol mixture and a water-fatty alcohol acid mixture; 
(b) fractionating the water-propylene glycol mixture to produce a 
distillate that contains at least about 95% by weight propylene glycol. 
This process can be accomplished by means of an apparatus comprising (a) an 
extraction column having a first inlet means at the top portion of said 
column for receiving water; (b) a second inlet means at the bottom portion 
of said column for receiving a hydrogenation effluent first cut connected 
to the outlet of a rectification column; (c) an outlet means at the bottom 
portion of said extraction column for removing a water-propylene glycol 
mixture extract; (d) a heat exchanger means for heating said 
water-propylene glycol mixture extract having an inlet means for receiving 
said water-propylene glycol mixture extract connected to said outlet means 
at the bottom of said extracting column, and having an outlet means for 
removing heated water-propylene glycol mixture extract; (e) a 
fractionating column having an inlet means connected to said outlet means 
of said heat exchanger means and an outlet means at the bottom of said 
fractionating column for removing substantially anhydrous propylene glycol 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention provides a process for producing substantially 
anhydrous propylene glycol from a mixture of low-boiling fatty alcohols 
and propylene glycol which is obtained from the first cut from the 
fractionation of the product stream from the direct hydrogenation of 
glyceride oils. The low-boiling fatty alcohols and propylene glycol 
mixture is first extracted with water to produce a water-propylene glycol 
mixture and a water-fatty alcohol mixture. In accordance with the 
invention, substantially anhydrous propylene glycol is produced from the 
water-propylene glycol extract by rectification. By virtue of very 
favorable equilibrium data in the water-propylene glycol system, three to 
five theoretical stages or plates of a rectification column are sufficient 
to obtain from the extract a product which--starting from an extract of 35 
to 45% by weight propylene glycol and 55 to 65% by weight water--contains 
97 to 99% by weight propylene glycol. 
Accordingly, for the apparatus for the production of substantially 
anhydrous propylene glycol from a fatty alcohol/propylene glycol mixture, 
it is proposed in accordance with the invention that this apparatus 
comprise at least one extraction column and at least one following 
rectification column connected thereto. 
The process and the apparatus according to the invention are used in 
particular in the working up of the reaction mixture formed in the direct 
hydrogenation of glyceride oils. For purposes of this invention, the 
reaction mixture formed in the direct hydrogenation of glyceride oils is 
defined as the hydrogenation effluent. For hydrogenation conversions of 97 
to 99%, this reaction mixture predominantly contains fatty alcohols 
commensurate with the C-chain distribution of the glyceride oil used and 
propylene glycol commensurate with the proportion of bound glycerol and 
also low-boiling fractions. The object of a working-up process in this 
regard is to produce anhydrous propylene glycol and fatty alcohols free 
from propylene glycol. Tests have shown that it is of particular advantage 
in this case initially to fractionate the reaction mixture so that 
propylene glycol and low-boiling fatty alcohols are separated off as a 
first cut. 
The hydrogenation effluent is fractionated into the following boiling cuts: 
I. water/low-boiling fractions 
II. first cut: C.sub.6 /C.sub.10 fatty alcohols and propylene glycol 
III. main fraction C.sub.12 /C.sub.18 fatty alcohols. Analysis of a sample 
of the main fraction reveals C.sub.12 /C.sub.18 fatty alcohols according 
to specification which are eminently suitable, for example, for 
sulfatization. This does not apply to the first cut which has to be 
further worked up on account of the propylene glycol therein. The 
hydrogenation effluent first cut contains C.sub.6 /C.sub.10 fatty alcohols 
and propylene glycol. The extraction-based working up mentioned above is 
proposed for this purpose. Although extraction of the entire fatty alcohol 
after the separation of water/low-boiling fractions is possible in theory, 
it is disadvantageous on economic grounds. 
To ensure a high concentration of propylene glycol in the extract and, 
hence, to achieve low further processing costs, for example in the 
production of substantially anhydrous propylene glycol, it is proposed 
that extraction of the propylene glycol take place in apparatus comprising 
several theoretical separation stages. It is also of advantage to carry 
out extraction of the propylene glycol/water mixture continuously, 
particularly in countercurrent manner. 
Extraction is carried out particularly effectively in at least one 
sieve-plate extraction column. It is particularly preferred that the 
extraction column contain a plurality of reciprocating sieve plates. 
It is also of advantage to use at least one pulsed extraction column for 
extraction. A pulsed extraction column is also known as a reciprocating 
plate column. A reciprocating plate column provides for intimate mixing of 
the extractant and the liquid to be extracted by repeatedly forcing the 
two liquids together by mechanical agitation. 
To obtain propylene glycol of high quality in regard to color, odor and 
composition, the substantially anhydrous propylene glycol obtained from 
the rectification step can be further treated with active carbon or 
optionally distilled overhead in another column. In the latter case, a 
purity of 99.69%, an acid value AV below 0.01, a saponification value SV 
of 0.15 and a water content of 0.16% are obtained. 
In the apparatus according to the invention, the extraction column 
advantageously comprises several stages. 
In a particularly economical embodiment, the extraction column comprises 
fittings, more particularly sieve plates. 
In addition, the extraction column is advantageously a pulsed column. 
In one particularly advantageous embodiment, the apparatus according to the 
invention comprises at least one other rectification column which is 
connected at its first cut exit to the entrance of the extraction column. 
With this other rectification column, it is possible for example to 
fractionate the hydrogenation effluent into water and low-boiling 
fractions, the first cut and the main fraction so that only the first cut 
is further processed in the remaining part of the apparatus according to 
the invention. 
One example of embodiment of the invention is described in detail in the 
following with reference to FIG. 1. 
FIG. 1 is a flow chart of an apparatus according to the invention for 
working up the hydrogenation effluent from the direct hydrogenation of 
glyceride oils. 
Gas chromatographic analysis of a typical hydrogenation effluent produced 
the following values: 84% by weight fatty alcohols, 0.2% by weight 
hydrocarbons, 10.5% by weight propylene glycol, 1% by weight i- and 
n-propanol and 2.8% by weight water and traces of methanol and ethanol. 
The overall process for working up the hydrogenation effluent consists of 
three steps, namely: 
1. fractionation of the hydrogenation effluent, 
2. extraction of the propylene glycol from the first cut of fractionation 
in the extraction apparatus, 
3. rectification of the extract from the extraction apparatus and 
subsequent treatment of the product with active carbon. 
Referring to FIG. 1, the hydrogenation effluent (1) is fractionated into 
three boiling cuts in a rectification column (2). The proportions by 
weight of the three fractions are correspondingly 5%, 22% and 73%. 
Analysis of the main fraction (3) produces the following values: C.sub.10 
: 0.8%, C.sub.12 : 51%, C.sub.14 :21.7%, C.sub.16 : 11.2%, C.sub.18 : 
14.9%, C.sub.20 : 0.1%, hydrocarbons: 0.04%, water: 0.06%. Water and 
low-boiling fractions are separated off at (5) at the head of the column. 
The first cut (4) (fatty alcohols/propylene glycol) is extracted in a 
pulsating (or reciprocating) sieve-plate extraction column (6). The 
extraction column consists of 40 sieve plates (free cross-section 14%) at 
intervals of 100 mm. The nominal width of the column was 100 mm. The 
pulsation unit (7) operates with a pulsation (reciporication) stroke of 15 
mm and at a pulsation frequency of 1.5 to 1.6 l/s. 
The first cut (4) is delivered to the lower part and the extractant (8), 
water, to the upper part of the column (6). The throughputs of the two 
phases are as follows: first cut: 90 to 100 dm.sup.3 /h, water: 40 to 50 
dm.sup.3 /h, water being selected as the continuous phase on the basis of 
extensive preliminary tests. Colourless liquids are obtained as the 
raffinate (9) and extract (10). The raffinate (9) contains 95 to 97% by 
weight fatty alcohols, 3 to 4% by weight water and no propylene glycol. 
The extract (10) consists of 35 to 45% by weight 
propylene glycol, 55 to 65% by weight water and traces of the 
above-mentioned low-boiling fractions. 
The raffinate (9) is collected in the vessel (12). The extract from the 
vessel (11) is brought to the operating temperature (approx. 100.degree. 
C.) in the heat exchanger (13) and delivered to the rectification column 
(14). A packed rectification column (Kuhni Rombopack, height: 4 m, 
diameter: 316 mm) operated at normal pressure (1 bar) with a reflux ratio 
of 0.5 is used. The extract throughput is approximately 70 kg/h which--for 
the reflux ratio of 0.5 mentioned above--necessitated an energy input in 
the evaporator (19) of the column of approximately 40 to 47 kW. The 
throughputs of the two head and sump products amounts correspondingly to 
42 kg/h and 28 kg/h. The slight yellowish coloration of the sump product 
(16) (97 to 99% by weight propylene glycol) and its faint odor can be 
completely removed without difficulty by subsequent treatment (17) with 
active carbon (for example NORIT CA1 or BRILLONIT; active carbon 
consumption: approx. 1 to 2 g/100 g). The drawing also shows the column 
sump at (15) and a condenser at (18).