Solvent recovery

Hydrogen chloride evolution is reduced in the distillation of chloroform from its admixture with an amide solvent when in contact with stainless steel, by incorporation of certain tertiary aliphatic amines.

DESCRIPTION OF THE PRIOR ART 
Aliphatic amide solvents are employed in a variety of industrial processes. 
In certain of these processes the effluent stream consists of the amide 
solvent and water which must be removed to permit recycling of the amide 
solvent. Conventional distillation can achieve the separation but this 
involves high energy consumption since the water has a high heat of 
vaporization and is first to distill off from the mixture. 
To mitigate the effect of high energy consumption, it has been proposed to 
combine the amide solvent/water stream with chloroform which 
preferentially dissolves the amide solvent and permits separation from the 
water by decanting. This procedure yields an amide solvent/chloroform 
solution from which the chloroform must be distilled if the amide solvent 
is to be recycled. This involves energy saving since the chloroform is 
removed from the amide with expenditure of less energy than for the 
removal of water. Attempts to distill off the chloroform in stainless 
steel apparatus have presented yet another problem, namely corrosion of 
the apparatus. This is believed caused by hydrogen chloride evolved due to 
degradation of the amide-chloroform mixture. The present invention 
addresses the problem. 
SUMMARY OF THE INVENTION 
This invention provides a novel composition comprising a solution of 
chloroform and an aliphatic amide solvent and an amount of a tertiary 
aliphatic amine selected from the group consisting of triethyl amine, 
tripropyl amine, tributyl amine, trioctyl amine and diisopropylethyl amine 
in an amount sufficient to suppress formation of hydrogen chloride and 
reduce corrosion when the solution is distilled d in contact with 
stainless steel for removal of chloroform and recovery of the amide 
solvent. 
Also provided is a process for the recovery of an aliphatic amide solvent 
from a solution of chloroform and the amide solvent while the solution is 
in contact with stainless steel which comprises, adding a tertiary 
aliphatic amine selected from the group consisting of triethyl amine, 
tripropyl amine, tributyl amine, trioctyl amine and diisopropylethyl 
amine, to the solution of chloroform and amide to suppress formation of 
hydrogen chloride, distilling the solution to drive off the chloroform and 
recover the amide solvent. 
DETAILED DESCRIPTION OF THE INVENTION 
In certain chemical operations as in the spinning of poly(meta-phenylene 
isophthalamide) filaments from solutions containing dimethylacetamide 
(DMAc), calcium chloride and water, and subsequent coagulation of the spun 
filaments with an aqueous bath as described in U.S. Pat. No. 3,756,908, 
solutions of DMAc and water are obtained from which the DMAc must be 
retrieved for recycling in the process. In other instances, it is desired 
to separate and recover N-methylpyrrolidone (NMP) from water. One way 
which has been proposed to effect separation of the amide solvent from the 
water has been to treat the mixture with chloroform to preferentially 
dissolve the amide solvent and then to decant the water layer. To recover 
the amide solvent from its solution with the chloroform, distillation of 
the chloroform from the solution in a stainless steel apparatus is the 
method of choice. It has been found that the combination of the amide 
solvent-chloroform mixture in contact with stainless steel during the 
distillation process promotes evolution of hydrogen chloride which in turn 
causes substantial corrosion of the stainless steel apparatus. Applicant 
has found that incorporation of an inhibiting amount of certain tertiary 
aliphatic amines, suppresses the evolution of hydrogen chloride and 
substantially reduces the corrosion. 
The invention is applicable to aliphatic amide solvent-chloroform solutions 
of varying proportions. As a practical matter one desiring to separate and 
recover aliphatic amide solvent from its solution in water would add a 
sufficient amount of chloroform to preferentially take up substantially 
all of the amide solvent to permit separation of the water layer from the 
amide solvent-chloroform layer as by decanting. The addition of an excess 
amount of chloroform beyond that needed to take up the amide solvent would 
be wasteful since the chloroform must subsequently be separated by 
distillation from the amide solvent. On the other hand the addition of an 
amount of chloroform inadequate to take up the amide solvent present in 
the aqueous solution of amide solvent would reduce the recovery of amide 
solvent. 
When first separated from the water layer, the chloroform may constitute 
from 20 to 30% by weight of the solution of amide solvent plus chloroform. 
During the distillation of the solution, the concentration of chloroform 
decreases as the distillation proceeds and chloroform is driven off. The 
corrosion rate appears to be greater at those sections of the distillation 
column where these less concentrated solutions are present. 
The amide solvent-chloroform solution, as separated from the water layer 
may contain inorganic salts as well as small amounts of water. The 
presence of these ingredients is not seen to interfere with the advantages 
obtained in the present invention. 
As mentioned previously, the selected tertiary aliphatic amine should be 
added to the amide solvent-chloroform mixture that is to be distilled or 
is being distilled and it should be added in an amount that is sufficient 
to reduce corrosion of the stainless steel apparatus. Suitable tertiary 
aliphatic amines include triethyl amine, tripropyl amine, tributyl amine, 
trioctyl amine and diisopropylethyl amine. A small amount, generally at 
least 0.5% by weight based on the weight of the chloroform in the solution 
to be distilled has been found satisfactory for the purpose. Amounts 
greater than 10% would appear to unduly increase costs without concomitant 
advantage.

The following examples are illustrative of the invention (except for the 
controls) and are not to be construed as limiting. 
EXAMPLES 1-8 
A Soxhlet extractor is set up using a 500 ml or a 1000 ml round bottomed 
flask. The apparatus is dried overnight in an oven. Stainless steel 
coupons (SS304) are cleaned and dried. The experimental apparatus is 
set-up and a dried coupon of SS304 is added to the round bottomed flask. 
The apparatus is blanketed with nitrogen. Addition of DMAc and chloroform 
and trialkyl amine to the system is under nitrogen via airless syringes. 
The solution in the round bottomed flask is refluxed for 16 hours. (In 
practice the chloroform that is distilled off would be removed and the 
amide solvent recovered.) The solution is then cooled to room temperature, 
the metal coupon is dried and weighed and the % wt. loss reported. 
The table below identifies the trialkyl amine and reports the relative 
proportions of the solution components and the % wt. loss of the coupon. 
TABLE 
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Chloroform Amine Amide 
Ex. Wt. % Amine Wt. % Wt. % % Wt. Loss 
______________________________________ 
1 100 None 0 0 0 
2 1 None 0 99 5.96 
3 1 TBA* 0.76 97.44 0.05 
4 1.05 TBA* 1.56 97.39 0.0112 
5 0.97 TBA* 0.47 98.56 0.02 
6 0.87 TOA** 0.37 98.79 1.61 
7 0.85 D1EA*** 0.29 98.86 0.66 
8 0.85 TPA**** 0.76 98.39 0.41 
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*tributyl amine 
**trioctyl amine 
***diisopropylethyl amine 
****tripropyl amine 
EXAMPLE 9 
A 1 liter 3-necked flask is dried in an oven until free of moisture and 
fItted wIth a chilled water condenser. A Teflon coated magnetic stirring 
bar is employed to stir the solution. The temperature of the solution in 
the flask is measured by a thermometer and maintained via a heating mantle 
which is controlled by a rheostat. 
To the flask as described above is added 700 ml of a solution consisting of 
8.2% chloroform, 0.5% water, 0.06% tributyl amine (0.73% by weight base d 
on chloroform content) and 91.24% dimethyl acetamide and coupon of 304 
stainless steel. This solution is stirred and heated at 148.degree. C. for 
16 hours. At the end of this time period, the corrosion rate, as 
determined based on the weight loss of the metal coupon, was 0 mils/year. 
In a control experiment, there is added to a similarly prepared flask, a 
coupon of 304 stainless steel, and 700 ml of a solution consisting of 7.9% 
chloroform, 0.5% water and 91.60% dimethyl acetamide. The solution is 
stirred and heated at 149.degree. C. for 16 hours. At the end of this time 
period the corrosion rate is measured. Based on the weight loss of the 
metal coupon, it was calculated to be 9.7 mils/year.