Dehydration of water soluble monomers with liquid carbon dioxide
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 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 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 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 2  to function in the above systems will depend upon the solubility of CO 2  with other substances. &#34;Ternary Systems of Liquid Carbon Dioxide&#34; 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 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 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° 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° 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° C. Temperatures below this may cause crystals to form. However, during the crystallizing step, temperatures of about 0° 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 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 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° 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 2 . 
     It is also possible to first evaporate part of the CO 2  and then cool the remaining solution to crystallize and separate the carbon dioxide hydrate. There should be about 0.31 g CO 2  /gm of water remaining to form the hydrate. Some additional CO 2  should be present to prevent the product from crystallizing during the extraction. 
     The weight ratio of CO 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 2  at about 25° C. and 840 psig. The weight ratio of CO 2  : acrylic acid solution was 1.97. After equilibration, the lower aqueous phase was withdrawn, and the CO 2  phase was cooled to 0° C., where white crystals were formed. The crystals were separated from the liquid, and CO 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 2 . 
     EXAMPLE 2 
     A 34 wt. percent solution of acrylic acid in water was extracted with liquid CO 2  in a countercurrent extraction unit having six theoretical stages of contacting. Conditions were 25° 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 2  removal--was 92.5% acrylic acid, 7.5% water. Cooling the extract to 0° 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.