Dehydration of water soluble monomers with liquid carbon dioxide

Separation of certain water soluble organic monomers such as acrylic acid from an aqueous solution can be accomplished by extraction with carbon dioxide, crystallizing the extract to remove carbon dioxide hydrate, and flashing off the remaining carbon dioxide solvent from the extract.

BACKGROUND OF THE INVENTION 
It is well known in the art that carbon dioxide can be used for extraction 
of various petroleum fractions. Liquid carbon dioxide is advantageous 
because of its low cost, non-corrosiveness, non-toxicity and ease of 
recovery from the extract of raffinate phases. U.S. Pat. No. 2,029,120 
discloses the separation of unsaturated hydrocarbons from gas mixtures 
using liquid carbon dioxide. 
U.S. Pat. No. 2,631,966 shows the separation of various lubricating oils 
with carbon dioxide. As noted therein, liquid CO.sub.2 has certain unusual 
miscibility relations with hydrocarbons. For example, it dissolves 
aliphatic and monocyclic aromatic hydrocarbons in preference to 
polycyclics of the same boiling range. 
Typically, CO.sub.2 has not been used alone, but as an aid to other 
solvents for extraction purposes. These solvents include liquid sulfur 
dioxide (U.S. Pat. No. 2,034,495) acetone (U.S. Pat. No. 2,246,227) and 
furfural or phenol (U.S. Pat. No. 2,281,865). A broad description of 
various solvents that can be used with carbon dioxide can be found in U.S. 
Pat. No. 2,631,966. 
Liquid CO.sub.2 has also been utilized to remove the solvents themselves 
from the extract phase, as can be found in U.S. Pat. No. 2,646,387. 
The ability of CO.sub.2 to function in the above systems will depend upon 
the solubility of CO.sub.2 with other substances. "Ternary Systems of 
Liquid Carbon Dioxide" by Alfred W. Frances, Journal of Physical Chemistry 
58, 1099 (1954) discloses the solubilities of 261 substances with carbon 
dioxide. 
None of the processes above, however, disclose the ability or process for 
the dehydration of an aqueous solution by using CO.sub.2. 
There are a large number of processes for the production of certain organic 
chemicals that produce water as a by-product or use steam as a diluent 
during the reaction. Among these processes is the production of acrylic 
acid. The aqueous solution resulting from the reaction normally contains 
from 30-60% acrylic acid with the remainder being mostly water. Various 
methods have been proposed for removing this water, such as the addition 
of certain drying agents like calcium chloride as found in U.S. Pat. No. 
2,922,815. 
Other processes, such as for the production of acrylonitrile, use water as 
an aid in distilling acrylonitrile from the reactor effluent. This 
produces an aqueous solution of acrylonitrile that must be dehydrated. 
The present invention provides a process for separating water from such 
aqueous solutions of organic chemicals that can achieve purities upwards 
of 99%.

DESCRIPTION OF THE INVENTION 
The invention can be considered a process for the separation of water from 
an aqueous solution containing organic chemicals miscible with CO.sub.2 by 
the steps of: 
(a) contacting the aqueous solution with liquid carbon dioxide to form an 
extract phase of the organic chemical, carbon dioxide and some water, and 
a raffinate phase containing water; 
(b) separating the extract phase from the raffinate phase; 
(c) crystallizing by cooling and separating carbon dioxide hydrate from the 
extract phase; and 
(d) evaporating the carbon dioxide. 
It is also possible to reverse steps c) and d) above such that the carbon 
dioxide solvent is partly evaporated prior to the crystallization and 
separation of the carbon dioxide hydrate. 
The above process is especially suited for recovery of heat sensitive 
materials, such as acrylic acid, methacrylic acid, acrolein, methacrolein 
acrylonitrile or methacrylonitrile. However, it may be applied to any 
material having a favorable distribution coefficient between carbon 
dioxide and water. This distribution coefficient is the ratio of 
concentration of organics in carbon dioxide and water. If this ratio is 
greater than 1, than the co-efficient can be considered favorable. 
A major advantage of the present invention is that operations are conducted 
at temperatures below 30.degree. C., limiting thermal degradation or 
polymerization of the heat sensitive materials. This temperature 
limitation is because the critical temperature of carbon dioxide is 
31.1.degree. C. 
The temperatures employed can vary depending on the step in the process. 
For example, one would not want to contact an aqueous solution containing 
acrylic acid at a temperature much below 10.degree. C. Temperatures below 
this may cause crystals to form. However, during the crystallizing step, 
temperatures of about 0.degree. C. can be used. The exact temperature will 
depend on factors such as the pressure used and the specific organic 
chemical to be dehydrated. 
Since carbon dioxide is being utilized in a liquid state, pressures in the 
range of 370 to 1060 psig are required. The exact pressure will depend on 
the temperature, the specific organic chemical, the amount of organics 
being separated, and the CO.sub.2 /aqueous solution weight ratio. For 
example, in the separation of acrylic acid from water, pressures of from 
450 to 850 psig are normally used. 
The liquid CO.sub.2 can be contacted with the aqueous solution either by 
batch operation or by continuous extraction. It is preferred to use 
continuous countercurrent extraction to perform the contacting and 
separation of the extract and raffinate phases. Such extraction processes 
are well known in the art. 
Carbon dioxide hydrate is crystallized by cooling the extract phase to a 
temperature where the white crystals are formed and precipitate. For 
acrylic acid systems, this temperature is about 0.degree. C. 
After the crystals have been separated, which can be accomplished in a 
normal manner such as by filtration, the remaining carbon dioxide solvent 
is removed by simple reduction in pressure. This flashes off or evaporates 
the CO.sub.2. 
It is also possible to first evaporate part of the CO.sub.2 and then cool 
the remaining solution to crystallize and separate the carbon dioxide 
hydrate. There should be about 0.31 g CO.sub.2 /gm of water remaining to 
form the hydrate. Some additional CO.sub.2 should be present to prevent 
the product from crystallizing during the extraction. 
The weight ratio of CO.sub.2 to the aqueous solution will vary, dependent 
upon the amount of organics to be separated. Typically, this ratio is 2:1 
but can be as low as 0.5:1 and high as 10 or 20:1. Preferred is the ratio 
between 1:1 and 3:1. 
The carbon dioxide can be recovered, liquified and reused in the process. 
EXAMPLE 1 
A 31.2 wt. percent solution of acrylic acid in water was contacted with 
liquid CO.sub.2 at about 25.degree. C. and 840 psig. The weight ratio of 
CO.sub.2 : acrylic acid solution was 1.97. After equilibration, the lower 
aqueous phase was withdrawn, and the CO.sub.2 phase was cooled to 
0.degree. C., where white crystals were formed. The crystals were 
separated from the liquid, and CO.sub.2 evaporated from the liquid by 
gradual reduction of pressure. The remaining non-volatile liquid was found 
to be acrylic acid containing 0.9% water and a trace of dissolved 
CO.sub.2. 
EXAMPLE 2 
A 34 wt. percent solution of acrylic acid in water was extracted with 
liquid CO.sub.2 in a countercurrent extraction unit having six theoretical 
stages of contacting. Conditions were 25.degree. C., 850 psig, 
solvent/feed (WT.)=3. Analysis of the extract and raffinate showed that 
95.3% of the acrylic acid fed was recovered in the extract. The extract 
composition --after CO.sub.2 removal--was 92.5% acrylic acid, 7.5% water. 
Cooling the extract to 0.degree. C. and removing the crystals so formed 
yielded a product of composition 99% acrylic acid, 1% water (solvent free 
basis). 
As can be seen above, extremely high purities of the organic chemicals can 
be achieved through the use of the present invention.