Abstract:
Disclosed is a process for the distillation of readily polymerizable vinyl aromatic compounds and a new polymerization inhibitor therefor. The process comprises subjecting a vinyl aromatic compound to elevated temperatures in a distillation system in the presence of a new polymerization inhibitor comprising m-nitro-p-cresol.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a process for the distillation of readily polymerizable vinyl aromatic compounds. More particularly, the present invention relates to a process wherein styrene, substituted styrene, divinylbenzene and polyvinylbenzenes are subjected to elevated temperatures such as in the distillation thereof, wherein the amount of said materials polymerized during distillation is reduced over an extended period of time. 
     It is well known that vinyl aromatic compounds such as monomeric styrene, lower alkylated styrene, e.g., alpha-methyl styrene, and divinylbenzene polymerize readily, and furthermore, that the rate of polymerization increases with increasing temperature. Inasmuch as styrene and divinylbenzene produced by common industrial methods contain impurities, these compounds must be subjected to separation and purification processes in order to be suitable for most types of further industrial use. Such separation and purification is generally accomplished by distillation. 
     In order to prevent polymerization at the conditions necessary to distillation of vinyl aromatic compounds, various types of known polymerization inhibitors have been employed in connection with prior art distillation processes. For example, common inhibitors useful for inhibiting the polymerization of vinyl aromatics under distillation conditions include 4-tert-butylcatechol (TBC) and hydroquinone. It is preferred, however, to purify vinyl aromatics by using vacuum distillation techniques, whereby these commonly employed inhibitors are rendered unsuitable in view of the fact that they are effective only in the presence of oxygen. The partial pressure of oxygen in a vacuum distillation column is accordingly too low for these conventional inhibitors to be effective. Sulphur is perhaps the polymerization inhibitor most commonly employed to inhibit polymerization of vinyl aromatic compounds during distillation, since sulphur does provide effective inhibition in the absence of oxygen. While sulphur provides a reasonably effective inhibitor, its use in distillation processes results in one very significant disadvantage, namely, there is formed in the reboiler bottoms of the distillation column a valueless waste material which is highly contaminated with sulphur. This waste material futhermore represents a significant pollution of waste removal problem. 
     Although many compounds are effective for inhibiting the polymerization of vinyl aromatic compounds under differing conditions, e.g., storage, other purification techniques, etc., for a number of reasons which are not entirely understood in view of the diverse and unpredictable results obtained, only extremely few of these compounds have proved to be of any utility for inhibiting vinyl aromatic polymerization under distillation conditions, particularly under vacuum distillation conditions. In addition, certain compounds which are useful for inhibiting polymerization of one type of vinyl aromatic compound, for example, styrene, have proved to be essentially ineffective for inhibiting polymerization of another species of vinyl aromatic compound, for example, divinylbenzene. A limited number of nitroso compounds have proven to be effective for inhibiting polymerization of styrene monomer during distillation. For example, N-Nitroso phenylhydroxylamine and p-nitroso-N,N-dimethylaniline are reasonably effective inhibitors for the distillation of styrene, although they are not particularly soluble in styrene monomer. On the other hand, N-nitroso diphenylamine disclosed in U.S. Pat. No. 3,816,265, assigned to the assignee of the present application has been demonstrated to be a particularly effective polymerization inhibitor under vacuum distillation conditions for both styrene and divinylbenzene, whereas, N-Nitroso-methylaniline as disclosed in U.S. patent application Ser. No. 288,138, also assigned to the assignee of the present application, has been found to be an excellent polymerization inhibitor for styrene under vacuum distillation conditions. One of the most effective inhibitor systems known for divinylbenzene comprises a mixture of sulphur and N-nitroso phenylhydroxylamine. 
     In a typical distillation process for vinyl aromatic compounds utilizing a polymerization inhibitor, the mixture of vinyl aromatic to be distilled is generally contacted with the chemical polymerization inhibitor prior to being subjected to distillation conditions in the distillation apparatus. It remains as a significant problem today that the amount of polymer formed in the distillation apparatus and in the high purity product recovered therefrom is substantially higher than desired, and occasionally, that complete polymerization occurs inside of the distillation apparatus. For example, in the process of distilling crude divinylbenzene (a mixture containing divinylbenzenes, diethylbenzenes and monovinylbenzenes) to obtain high purity divinylbenzenes, even when inhibited with sulphur and TBC, a divinylbenzene product is obtained which contains significant quantities of polymer which are difficult to separate from the product and detrimental to the end use of such divinylbenzenes. Furthermore, the material which is removed from the bottom or reboiler area of the distillation apparatus is a highly polluting sulphur-containing waste material which must be disposed of. 
     It is therefore desirable to provide new polymerization inhibitors which are useful for styrene and vinyl benzenes under elevated temperatures such as those used under distillation conditions, particularly vacuum distillation conditions, and which are not subject to the disadvantages outlined above. 
     It is therefore an object of the present invention to provide a new and improved process for the distillation of readily polymerizable vinyl aromatic compounds. 
     A further object of the invention is to provide a new and improved process for the distillation of readily polymerizable vinyl aromatic compounds, which process results in higher recovery of a high purity unsaturated vinyl aromatic compound and concomitantly in the production of less undersirable by-products. 
     A further object of the invention resides in the provision of a new and improved process for the distillation of vinyl aromatic compounds which results in the production of substantially less polymerized material in the distillation apparatus. 
     Yet another object of the invention resides in the provision of a new and improved process for the distillation of vinyl aromatic compounds which avoids the production of a highly polluting, contaminated bottom or reboiler residue. 
     It is also an object of the present invention to provide a new and improved process for the distillation of vinyl aromatic compounds which permits the distillation apparatus to be operated at an increased rate of throughput without a reduction in efficiency. 
     It is still a further object of the present invention to provide a new and improved process for the distillation of vinyl aromatic compounds which provides all of the foregoing enumerated advantages in a vacuum distillation process. 
     A specific object of the invention resides in the provision of a new and improved polymerization inhibitor system for use at the elevated temperatures required in the distillation of vinyl aromatic compounds. 
     SUMMARY OF THE INVENTION 
     In accomplishing the foregoing and other objects, there has been provided in accordance with the present invention a process useful for the distillation of a readily polymerizable vinyl aromatic compound comprising subjecting the vinyl aromatic compound to elevated temperatures required as part of the distillation conditions in a distillation system, in the presence of an inhibitor which is m-nitro-p-cresol. 
     In one aspect of the process according to the invention, the m-nitro-p-cresol inhibitor is simply introduced into the distillation system by adding it to the reboiler area of the distillation apparatus, or alternatively, by incorporating it into the incoming stream of vinyl aromatic compound to be purified. The amount of inhibitor necessary to effectively inhibit polymerization of the vinyl aromatic compounds may vary over a wide range depending upon various factors of the distillation process, e.g., temperatures, reflux ratio, pressure, residence time, etc. Typically, however, it has been found that an amount of the inhibitor between about 50 and about 3000 ppm is sufficient to inhibit polymerization of vinyl aromatic compounds under normal distillation conditions (105° C. and above). 
     In another aspect of the present invention, the m-nitro-p-cresol inhibitor is used in any situation in which the vinyl aromatic compound is subjected to elevated temperatures. For example, should a distillation operation have to be shut down on short notice without time for the vinyl aromatics to be brought back to ambient temperature conditions, then the present invention is particularly useful in preventing the vinyl aromatics within the distillation train from polymerizing. 
     Through the use of the process according to the present invention, the amount of polymerization occurring within the distillation apparatus is significantly reduced in comparison to conventionally employed methods. In addition, the amount of desired distillation product is increased in proportion to the decrease in the amount of polymer formation. Still further, the material accumulating in the bottom or reboiler area of the distillation apparatus can be reused, e.g., for its fuel value or for reprocessing, which is a distinct advantage over conventional methods utilizing sulphur as a polymerization inhibitor which produce a highly polluting waste material in the reboiler area. 
     Other objects, features and advantages of the invention will become apparent from the detailed description of preferred embodiments which follows: 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The distillation process of the present invention employs m-nitro-p-cresol as the polymerization inhibitor during the distillation of vinyl aromatic compounds. Typically, the distillation process is carried out under reduced pressure, e.g., vacuum distillation, and one of the significant advantages of the invention is that the use of sulphur in the distillation system can be avoided. 
     The distillation techniques of the process of the present invention are suitable for use in virtually any type of separation of a readily polymerizable vinyl aromatic compound from a mixture wherein the vinyl aromatic compound is subjected to temperatures above room temperature. Surprisingly, the process of the present invention has been found particularly useful in vacuum distillation techniques, the preferred method for separating unstable organic liquid mixtures. In its most useful application, the distillation process of the invention is applied to a distillation mixture containing one of the vinyl aromatic compounds selected from the group consisting of styrene, alpha-methylstyrene, vinyltoluene, vinylnaphthalene, divinylbenzenes and polyvinylbenzenes. The preferred application of the present invention relates to the distillation of crude divinylbenzene or crude styrene under vacuum distillation conditions. 
     The amount of polymerization inhibitor added may vary over a wide range depending upon the conditons of distillation. Generally, the degree of stabilization is proportional to the amount of inhibitor added. In accordance with the present invention, it has been found that inhibitor concentrations generally between about 50 ppm and about 3000 ppm by weight have generally provided suitable results, depending primarily upon the temperature of the distillation mixture and the degree of inhibition desired. More often, however, with the inhibitor of the present invention it is used in concentrations of 100 to 1000 ppm. 
     During vacuum distillation of the divinylbenzene-containing mixtures and sytrne-containing mixtures, the temperature of the reboiler is preferably maintained from about 150° to about 250° F. by controlling reboiler pressure at from about 30 to about 400 mm. of Hg. Under such conditions, in a distillation apparatus having a distillation zone containing from about 50 to about 100 distillation stages, inhibitor mixture concentrations of from about 100 to about 2000 ppm by weight are suitable, whereas concentrations of from about 100 to about 600 ppm by weight are preferably, 200 to 600 ppm by weight, in the case of styrene distillation and concentrations in the range of from about 200 to about 1000 ppm by weight are preferred for distillation of divinylbenzene. The foregoing ranges are based upon distillation temperatures of from 150° to 250° F. and residence times of between about 2  and 4 hours. Obviously, in the lower portions of the temperature and residence time ranges, smaller amounts of inhibitor may be utilized. Obviously, amounts of inhibitor greater than those specified hereinabove may be employed, although the advantages of adding the additional inhibitor are not significant and are outweighed by the corresponding increase in cost. 
     The polymerization inhibitor of the present invention may be introduced into the distillation apparatus in any convenient manner which permits efficient distribution of the inhibitor throughout the apparatus. Typically and most advantageously, the required amount of inhibitor is simply added to the reboiler area of the distillation column, although equivalent results may be obtained by incorporating the inhibitor into the incoming hot stream of vinyl aromatic compound. Also, the inhibitor may be added at both reboiler and directly into the distillation column. 
     Since the inhibitor is gradually depleted during distillation, it is generally necessary to maintain the appropriate amount of inhibitor in the distillation apparatus by adding inhibitor during the course of the distillation process. Such addition may be carried out either on a generally continuous basis or it may consist of intermittent charging of inhibitor into the distillation system. The means by which the maintenance of the necessary concentration of the inhibitor system is carried out is of no particular importance as long as the concentration of inhibitor is kept about the minimum required level. 
     Another factor enabling the distillation apparatus to operate at an increased rate in accordance with the present invention as opposed to conventional prior art processes is the fact that the inhibitor system of the present invention is a more efficient inhibitor at normal temperatures than the conventional inhibitors, and will thus permit higher distillation temperatures and higher pressures. In this way, the rate of distillation can be increased without increasing the amount of polymerization which has been deemed to be acceptable in accordance with conventional distillation procedures. 
     When the process of the present invention is utilized, the bottom material which accumulates during the distillation process can be drawn off and utilized for its heating value or for reprocessing. This represents another significant advantage in comparison to conventional processes for vacuum distillation of vinyl aromatic compounds which employ sulphur as the polymerization inhibitor, or sulphur in combination with other chemical polmerization inhibitors. In these conventional processes, a bottoms material is formed which is valueless for further use and constitutes a high polluting waste material which must be disposed of and which, in this regard, also presents a problem of disposal. 
     Upon recovery of the distillation product obtained from the process of the present invention, it is found that a higher percentage of the pure readily polymerizable vinyl aromatic compound is recovered in an unpolymerized state. Moreover, the concentrated distillation residues are more easily handled and removed from the apparatus, as by pumping or the like. 
     In order to more fully describe the present invention, the following examples are presented which are intended to be merely illustrative and not in any sense limitative of the invention. 
    
    
     EXAMPLE 1 
     50 grams of styrene free of tert-butyl catechol were placed in a 100 ml. flask fitted with a stirrer. The flask was also fitted with a reflux condenser open to the air. There was then added to the flask 0.020 grams (approximately 400 ppm) of m-nitro-p-cresol. The flask and contents were heated in an oil bath which is thermostatically controlled at 115° C. ± 2° C. 1 ml. samples of the styrene are periodically withdrawn from the flask and are mixed with 3 ml. of methanol to determine the qualitative extent of polymerization. At the end of 6 hours, there was still no significant precipitation of styrene polymer indicating m-nitro-p-cresol to be an effective retardant to polymerization during distillation of styrene. 
     EXAMPLE 2 
     The procedure of Example 1 was repeated except that only 200 ppm of m-nitro-p-cresol was employed. At the end of 4 hours the test was terminated and it was found that no significant polymerization of styrene occurred, the amount of polymeriation being approximately 4.2 wt. %. In a comparison run, 2-4-dinitrophenol was tested under the same conditions. The amount of polymer formed was 7.5 wt. %. Thus, the amount of polymer formed using the retardant of the present invention was substantially less than that formed with the 2,4-dinitrophenol, a known styrene inhibitor. 
     EXAMPLE 3 
     Example 1 was repeated in the concurrent tests using 400 ppm of m-nitro-p-cresol in one flask and 4,6-dinitro-o-cresol in another flask. After 6 hours, the tests were terminated. With the m-nitrop-cresol, the amount of polymer formed was about 3.4 wt.%, and with the 4,6-dinitro-o-cresol, the amount of polymer formed was 2.7 wt. %. Though more polymer was formed with applicant&#39;s retardant composition, the amount was not significantly more but most important, the present retardant composition is not known to be toxic, whereas the 4,6-dinitro-o-cresol is known to be a toxic material. 
     EXAMPLE 4 
     Example 3 was repeated but using only 200 ppm of m-nitro-p-cresol and 4,6-dinitro-o-cresol. The amount of polymer formed with the former was 6.1 wt. % and with the latter, 6.5 wt. %. 
     EXAMPLE 5 
     Example 1 is repeated substituting divinylbenzene for styrene. Substantially equivalent results are obtained.