CA1066218A - Polymerization inhibitor for vinyl aromatic compounds - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:

A process for the distillation of a readily polymerizable vinyl aromatic compound. The process of the invention comprises subjecting the compound to distillation in a distillation zone containing from about 50 to about 100 distillation stages, at a reboiler temperature of from about 150°F to about 300°F and a reboiler pressure of from about 30 mm. to about 400 mm. Hg., and in the presence of a-polymerization inhibitor selected from the group consisting of NO, NO2, N2O3, NOCl and the adduct of NO(gas) and styrene, the inhibitor being present in an amount of from about 20 ppm to about 3000 ppm. The process results in higher recovery of a high purity unsaturated vinyl aromatic compound and concomitantly in the production of less undesirable by-products.

Description

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BACKGROUND OF THE INVENTION

The present invention relates to a process for the distillation of readily polymerizable vinyl aromatic compounds, and more especially, to a process for the vacuum distillation of styrene, substituted styrene, divinylbenzene and polyvinyl-benzenes wherein the amount of said materials polymerized dur-ing distillation is reduced over an extended period of time, wherein the material accummulating in the bottom or reboiler ~ ~ -area of the distillation apparatus is free of material con-taminated with sulfur and wherein the rate of throughput for a given distillation apparatus can be increased over the rate ~ -at which such apparatus may be operated in accordance with ~, ..... .
conventional methods.
It is well known that vinyl aromatic compounds such ~t; as monomeric styrene, lower alkylated styrene, e.g., alpha-t methyl styrene, and divinylbenzene polymerize readily, and ;~ ~
furthermore, that the rate of polymerization increases with ;
increasing temperature. In order to prevent polymerization during distillation of vinyl aromatic compounds, various types ; 20 of known polymerization inhibitors have been employed in con-nection with prior art distillation processes, e.g., 4-tert-.
butylcatechol (TBC) and hydroquinone. It is preferred, however, to purify vinyl aromatics by using vacuum distillation tech-niques, whereby these commonly employed inhibitors are rendered unsuitable in view of the fact that they are effective only in ~ the presence of oxygen. Sulfur is perhaps the polymerization b inhibitor most commonly employed to inhibit polymerization of t vinyl aromatic compounds during distillation, since sulfur does provide effective inhibition in the absence of oxygen; however, 30 its use results in one very significant disadvantage, namely there is formed in the reboiler bottoms of the distillation ; , , ~ .
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column a valueless waste ma-terial which is highly contaminated with sulfur and represents a significant pollution or waste removal problem.
A limited number of nitroso compounds have proven to , be effective for inhibiting polymerization of styrene monomer during distillation. For example, N-nitroso phenylhydroxyl-amine 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 United States Patent No. 3,816,265 has been demonstrated to be a particularly efEective ~;~
polymerization inhibitor under vacuum distillation conditions for both styrene and divinylbenzene, whereas N,N-nitroso-methylaniline, as disclosed in U.S. Paten-t No. 4,050,993, 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 sulfur and ; N-nitroso phenylhydroxylamine.
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. For example, in the process of distilling crude divi.nylbenzene (a mixture containing divinylbenzenes, diethylbenzenes and monovinylbenzenes) to obtain high purity ; divinylbenzenes, even when inhibited with sulfur and TBC, a divinylbenzene product is obtained which contains significant quantities of polymer which are difficult to separate from the product and are detrimental to the end use of such divinyl-benzenes.
Nitrogen oxides are known according to the prior art to be effective for inhibiting polymerization of certain '' .

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unsaturated compounds, and accordingly, they have been em-ployed as polymerization inhibitors in certain types of applications. However, the use of the normally-gaseous nitro gen oxides is predominantly confined to static eonditions, e.g., storage, since the use of a gaseous material is strongly suggested against under any conditions where the inhibitor could readily escape. Thus, the use of normally-gaseous in-hibitors,such as the nitrogen oxides, has found substantially - no applieation in distillation or similar purifieation pro- ~ -eesses involvin~ heat, and this is partieularly true in the ease of vaeuum distillation for the obvious reasons.
~ -SUMMARY OF THE IN~7ENTION

The following are therefore objeets of the present ~ I
invention:
to provide a new and improved proeess for the distil-lation of readily polymerizable vinyl aromatic compounds, which `~
proeess results in higher recovery of a high purity unsaturated vinyl aromatic compound and concomitantly in the produetion of less undesirable by-produets;
to provide a process for the distillation of vinyl aromatie eompounds whieh results in the produetion of sub-stantially less polymerized material in the distillation apparatus;
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to provide a process for the distillation of vinyl aromatie eompounds whieh avoids the produetion of a highly polluting, contaminated bottom or reboiler residue;
to provide a 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 effieieney; and .

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to provide a process for the dis-tillation of vinyl aromatic compounds which provides all of the foregoing-enumerated advantages in a vacuum distillation process.
In accordance with the present invention, there is thus ; provided a process for the distillation of a readily polymerizable vinyl aromatic compound, which comprises subjecting the vinyl aromatic compound to distillation in a distillation zone containing from about 50 to about 100 distillation stages,at a reboiler temperature of from about 150F to about 300F and a reboiler pressure of from about 30 mm. to about 400 mm. Hg., and in the presence of a polymerization inhibitor selected from the group consisting of NO, NO2, N2O3, NOCl and the adduct of NO(gas) and styrene, the inhibitor being present in an amount of from about 20 ppm to about 3000 ppm, whereby polymerization of the vinyl aromatic compound during distillation is substantially inhibited.
In one aspect of the process according to the invention, a normally gaseous inhibitor, selected from NO, NO2, N2O3 and NOCl, -~is simply introduced into the distillation system by injection into the reboiler area of the distillation apparatus, or alternatively, by injection into the incoming stream of vinyl aromatic compound to be purified. It is one salient feature of the invention that the mode of introducing and metering the amount of polymerization inhibitor is considerably simplified due to the ease of metering a normally gaseous material and due to the simplicity of the equipment necessary therefor.
In ano-ther aspect of the process according to the ~ .
~ invention, the NO-styrene adduct inhibi-tor is simply introduced i into the distillation system by adding it to the reboiler area . i .
of the distillation apparatus, or a:Lternatively, by incorporating i-t into the incoming stream of vinyl aromatic compound to be purified.

In contradistinction to the aforeno-ted gaseous inhibitors, the NO-styrene adduct, having an empirical formula C8H8N2O3, is a white crystalline powder having a melting point "3 ' , ' .
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of approximately 120C. At the temperatures typically en-countered in the distillation of vinyl aromatic compounds it has been observed that the inhibitor decomposes to liberate a gas, probably a nitrogen oxide or mixture thereof. Accordingly, the adduct provides a vehicle for introducing an effective gaseous inhibitor via injection of a solid particulate and, thus falls within the broad scope of the present invention.
The amount of inhibitor necessary to effectively re-tard polymerization of the vinyl aromatic compounds may vary 10 over a broad range depending upon various factors of the dis- -tillation process e.g., temperature, amount of reflux, if any, pressure residence time, etc. Generally, it is found that the ` amount of inhibitor present may vary broadly within the range i of from 20 ppm to 3000 ppm, depending upon the above-enumerated variables. Table I sets forth typical requirements for the amount of inhibitor found to be sufficient to substantially in-hibit polymerization of vinyl aromatic compounds under normal distillation conditions at 105C.
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TABLE I ~

20 InhibitorAmount to Inhibit Polymerization at 105C. (ppm) ~ ;

N2O~50 - 1000 Adduct: NO-styrene 200 - lO00 Through the use of the process according to the pre-sent invention, the amount of polymerization occurring within the distillation apparatus is significantly reduced in com-; parison to conventionally employed methods. In addition, the amount of desired distillation product is increased in propor-tion to the decrease in the amount of polymer formation. Also, the rate of operation of a given distillation apparatus can be increased over and above the rate of operation for the ; same apparatus utilizing conventional methods, since lower ~- - 6 ~,.
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vacuum levels and higher distillation temperatures are possible according to the present invention. Still further, the material accummulating in the bottom or reboiler area of the distilla-tion apparatus can be reused, e.g., for its fuel value or for reprocessing, which is a distinct advantage over conventional methods utillzing sulfur as a polymerization inhibitor which produce a highly polluting waste material in the reboiler area.
Furthermore, it has also been found that any polymeric material inadvertently formed during the process of the invention is of 10 a low molecular weight character and therefore presents fewer ~ ~
problems in connection with fouling of the distillation appa- -ratus. Finally, use of the inhibitor according to the inven-tion has proven to be surprisingly advantageous in preventing polymér build-up over the entire extent of the distillation apparatus, i.e., in the upper portions of the columns, whereas this result is not achieved in accordance with prior art methods.
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The distillation process of the present invention employs a nitrogen oxide, or derivative thereof, as a polymeri-zation inhibitor during the distillation process carried out under reduced pressure, e.g~, vacuum distillation, and one of the significant advantages of the invention is that the use of sulfur in the distillation system can be avoided.
The distillation technique of the process of the pre-~' sent invention is 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 _ 7 _ ~. . " i . , .;
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2~3 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 applica-tion of the present invention relates to the distillation of -crude divinylbenzene or crude styrene under vacuum distilla-tion conditions.
The amount of polymerization inhibitor added may vary over a wide range depending upon the conditions 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 within the ranges set forth in Table I (above), havel ..
generally provided suitable results, depending primarily upon -the temperature of the distillation mixture and the degree of inhibition desired.
During typical vacuum distillation of divinylbenzene- ~
20 containing mixtures and styrene-containing mixtures, the ~ ~ -temperature of the reboiler is preferably maintained from ;~
about 150E'. to about 250F. by controlling reboiler pxessure at from about 30 mm. to about 400 mm. of Hg; however, the temperature may vary up to about 300F. Viewing NO2 as ex-emplary of the various gaseous inhibitors, under such tempera-ture conditions as set forth above, in a distillation apparatus having a distillation zone containing from about 50 to about 100 distillation stages, inhibitor concentrations of from about 20 to about 3000 ppm are suitable, whereas concentrations of ~ -i 30 from about 50 to about 100 ppm are preferred in the case of styrene distillation and concentrations in the range of from about 100 to about 2000 ppm are preferred for distillation of ; -' ' .: ~ ' . ' ': . :' ' ', _ ~ ., . , .,, , . ' !

vinylbenzenes. Obviously, amounts of inhibitor greater than those specified hereinabove may be employed, although the advantages of adding the additional inhibitor are not signi-ficant and are outweighed by the corresponding increase in ~
cost. - ;
In addition, within the foregoing general ranges spe-cified for the inhibitor concentration, preferred ranges have been developed. Thus, NO2 is preferably employed in an amount of from about 100 to 1000 ppm in the distillation of styrene compounds and in an amount of from about 200 to 1000 ppm in the distillation of divinylbenzene at temperatures between about ~-150 and 300F., preferably between about 200 and 300F. for styrene and residence time between about 2 and 4 hours.
Obviously, in the lower portions of the temperature and resi-- .
~ dence time ranges, smaller amounts of inhibitor are required.
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Having considered NO2 for purposes of illustration, and understanding that the same general considerations prevail for the remaining inhibitors,including the adduct of NO (gas) and styrene, Table II sets forth the preferred and most pre-20 ferred ranges for the amount of inhibitor employed to substan- ;
tially completely preclude polymerization during distillation ; of styrene or divinylbenzene for residence times of from about 2 to about 4 hours, in each case.

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` ~(~66;Z:~8 The normally gaseous polymerization inhibitors 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. Typi-i cally and most advantageously, the re~uired amount of gaseous inhibitor is simply injected into the reboiler area of the -distillation column, although equivalent results may be obtain-ed by injecting the inhibitor into the incoming hot stream of vinyl aromatic compound. The gaseous nature of certain of the inhibitors according to the invention provides significant advantages in terms of the ease with which addition of the in-hibitor may be accomplished. Thus, the apparatus necessary for including the inhibitor may be greatly simplified over that ;
required to introduce conventional inhibitors, since simple injection valves and conventional metering systems may be readily adapted for this purpose. In this way, control of the ~ amount of inhibitor added is particularly simplified.
`~j The NO-styrene adduct polymerization inhibitor of the present invention may likewise be introduced into the distilla-tion apparatus in any convenient manner which permits efficient ; distribution of the inhibitor throughout the apparatus. Typi cally 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.
Since the inhibltor is gradually depleted during op-eration, 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 . ~ . .
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It has been found in accordance with the invention that the volatility of the inhibitor provides unexpected ad-vantages in terms of the degree of inhibition of polymer achieved during the subject distillation process,. The volatil-ity of the inhibitor of the invention causes it to be more 10 effectively distributed throughout the entire length of the distillation apparatus. As a result, polymerization is more effectively inhibited at points in the apparatus remote from the reboiler area than is the case with conventional distilla-tion processes using liquid or solid inhibitors which, unlike the NO-styrene adduct, do not exhibit the aforementioned de- -~`~i ~ composition to an effective gaseous inhibitor. The need for Z adding inhibitor at a plurality of points in the distillation column is therefore eliminated.
The distillation apparatus can operate at an increased 20 rate because the inhibitor of thepresent invention is a more efficient inhibitor than the conventional inhibitors, and will thus permit higher distillation temperatures and higher pres-sures. 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 dis-7 tillation procedures.
Upon recovery of the distillation product obtained from the process of the present invention, it is found that a higher percentage of thepure readily polymerizable vinyl aroma-30 tic compound is recovered in an unpolymerized state. Further-more, it has been noted that the polymeric products which are ' formed during the distillation process of the invention exhibit - 12 -. ' , ' ~: . , : : . ' ' , , ' ' ',, :, .

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~0~ 8 significantly lower molecular weight characteristics than poly- ~ ' - meric products formed in accordance with conventional distilla-tion techniques in the presence of the usual inhibitors. This -result provides the advantage that there is less fouling in the apparatus and accordingly less chance of plugging. More- ', over, 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, ,~ -10 the following Examples are presented which are intended to be ' -' merely illustrative and not in any sense limitative of the - ' invention. ~ ' j Example 1 ,~
A procedure is conducted whereby the three neck 't~i 100 ml. reaction flask is fitted with a magnetic stirrer and ~; three septums, and the flask is charged with 50 grams of sty-rene which is then flushed with nitrogen for 10 minutes utilizing a syringe needle at both inlet and outlet. The flask is then injected with the amount of gaseous inhibitor indicated 20 in,,the table below, the gas being injected below the surface of the liquid. The flask is then placed in a heated oil bath maintained at the temperature indicated in the table below and samples are taken at approximately 1/2 hour intervals and tested r by combining 1 ml. samples of styrene with 3 ml. of methanol ~ and examining the resulting mixture for turbidity to determine . .j ,,' if styrene polymer has formed.
, The styrene remaining from certain tests is trans-ferred to 250 ml. round bottom flask, and the contents are evaporated to dryness in a rotary evaporator. The weight of 30 the residue and corresponding percentage polymer in the styrene are calculated. In each instance the residue is slightly tacky and is apparently a low molecular weight, soluble polymer.
; The results are summarized in Table III.
TABLE III
Results Inhibitor Temp of Length of %Polymeron Type Amount(ppm) Bath(C. ? Time(hrs) Coagulation Evaporation NO2100 118 1 ~2- 2 No " 250 118 2 No " 500 118 6 No "1000 118 6+ No 1.95%
NO400* 115 1 V4 No "800* 125 1 No "1200* 130 1 No ~j "1600* 135 3/4 No "~ "2000* 145 1** No( TugrblditY) 118 2-2 ~4 No " 100 118 3 1/2 No "100+50/hr. 118 7 1/4 No 20N2O3900* 90 1 No "1800* 105 1 No "2700* 115 1 No "3600* 125 1 No "4000* 139 1/2** Begins NOCl400 115 1 No " 400 115 3 1/2 No ' " 400 115 4 1/2 No (Turbid)

3~1 " 200 115 3 No (Turbid) ¦ ~ 400 130 2 No (Turbid) *Gas bubbled into monomer over continuous test per,iod.
**With reflux.

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Example 2 30 ml. of divinylbenzene (containing ~1% meta-isomer, 11.2% para-isomer, 7.6% naphthenes and 0.2% meta-ethylvinyl-benzene) is withdrawn from storage and sealed in a 50 ml. flask ;~
with a septum closure. The divinylbenzene exhibits a faint cloud on testing with methanol, which indicates that some polymer has been formed upon storage. The flask is purged with -nitrogen to remove all oxygen, and then an amount of inhibitor, as specified in the table below, is injected into the flask.
The flask is placed in an agitated control temperature oil bath at 105C., and samples are taken periodically and tested with methanol for coagulation.
The test is discontinued and the divinylbenzene is ;, cooled to room temperature. The polymeric residue is racovered ; and its percentage measured. The residue obtained in each instance is resinous and appears to be a low molecular weight, soluble polymer, a result which is surprising for divinyl-, benzene.
The rezults are summarized in Table IV.

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Results Inhibitor Temp of Length of ~ Polymer on Type Amount(ppm) Bath(C.) Time(hrs): Coagulation Evaporation NO2 250 105 1 3/4 No " 250 105 2 3/4 No " 250 105 4 1/4 Yes 12.9 NO 250 105 1 No " 250 110 1 No " 250 115 3/4No (Turbidj 2O3 250 105 2No (Opaque) " 250 105 3 No " 250 105 5 Yes 11 , . , :: NOCl 400 105 1 No ..~;, " 400 110 1 No ~ " 400 115 3/4No (Turbid) .~ Example 3 '~t The procedure of Example 1 is repeated utilizing ~ -~`,, various known inhibitors and the inhibitors of the invention.
20 The samples of styrene containing these inhibitors are main- ~:
~ tained at approximately 105C. for a period of 4 and 1/4 hours, ~ :
., after which the samples are analyzed for polymer content. The ~ :
, results of these tests are set fort~l in Table V.

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TABLE V -.
Inhibitor Polymer ! Sample Inhibitor Concentration (By Wt.) Content % Wt.
' ' 1 None-Control 0 15.5 ~-2 Sulfur 500 ppm. 2.8 3 Sulfur 1250 ppm. 1.1 ; 4 Diphenylamine500 ppm. 17.1 N-nitroso-N-methylaniline300 ppm. 1.6 6 p-nitroso-N,N-dimethylaniline300 ppm. 2.7 ;- 7 N-nitroso diphenyl-amine 300 ppm~ 0.3 8 N-nitroso diphenyl-~, amine 200 ppm. 0.4 l~ 9 N-nitroso diphenyl-i~ j amine 150 ppm. 0.6 Nitrosophenol-~ sodium salt .05% 16.2 -. .
, 11 Nitrosophenyl hydroxylamine .05% 0.6 12 No2 50 ppm. 0.3 NO2 100 ppm. 0.3 , ~ .
14 No2 250 ppm. 0.4 NO 50 ppm. 0.3 16 NO 100 ppm. 0.2 17 NO 250 ppm. 0.2 :~ 1' 18 N2O3 100 ppm. 0.4 19 N2O3 250 ppm 0.3 , ~ ~
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Example 4 ~6~18 A 12 inch diameter distillation column is packed with Pro-pac* column packing (a commercially available stainless steel packing manufactured by Scientific Design Company). The column is charged with 22 gallons of monomeric styrene, and NO2 ; is charged to the column as polymerization inhibitor. The column is operated at inhibitor feed rates of 250 ppm per hour, 100 ppm per hour and about 50 ppm per hour, and the reboiler liquid is maintained at a temperature of about 220 F. After approximately 28 hours of operation, the viscosity of the reboiler liquid is about 8 cp. at 70F. as measured with a Brookfield viscometer. The percentage of polymer residue upon evaporation is approximately 17.3% after this period of time. The results of this run are summarized in the following Table.

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Example 5 The procedure of Example 4 is repeated, except NO is charged to the column as polymerization inhibitor at a rate of about 15 to 20 grams per hour initially. The steam jacket is maintained at a temperature of about 310F., whereas the re-boiler liquid is at a temperature of about 220F. The reflux ratio is set at 1:1 and the first distillate is recovered overhead after approximately 2 hours of operation under these conditions. The column is operated for a period of approxi-mately 23 hours while maintaining the inhibitor feed rate at approximately 100 ppm per hour based upon the steady state ;
volume of styrene in the column (approximately 400 ppm based ~ ~-on styrene fed into column), although the rate of inhibitor ~
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per hour. After approximately 12 hours of operation, the reboiler temperature is raised to approximately 235F. and ~-l l - -: : :-; the styrene feed rate is increased to decrease the residence time in the system to approximately 4 hours. After approxi-mately 23 hours of operation, the viscosity of the reboiler liquid isstill not capable of measurement with a Brookfield viscometer. Upon evaporation to isolate the residue in a re-boiler liquid at the end of this period, it is found that only 3.9% polymer is present. The results of this run are summarized '~ in the following Table. ~
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Example 6 100 ml. of styrene at room temperature (25C.) are ~ -placed in a 250 ml. flask. Through a tube located below the surface of the styrene NO gas is bubbled slowly through the styrene. A precipitate is observed to form il~mediately. Gas addition is continued for approximately 15 minutes until a considerable amount of precipitate has formed, and then the -gas is shut off and the styrene solution filtered to recover the precipitate. Upon drying the precipitate at 80-100C., a 10 white crystalline powder is obtained having a melting point -of 120C. The powder shows an elemental analysis of C8H8N2O3 and decomposPs at temperatures above its melting point to liberate a gaseous product.

Exam~le 7 50 grams of styrene are placed in a 100 ml. flask, and there is added thereto 0.025 grams (approximately 500 ppm) ~` of the product prepared in Example 6. The flask is sealed with ~,..................................................................... .
' three septums and is purged with nitrogen for 15 minutes using a hypodermic needle at both the inlet and the vent in order to move all dissolved oxygen. The flask and contents are placed in a heated oil bath which is thermostatically controlled at ~- 107C. 1 ml. samples of the styrene are periodically withdrawn from the flask and are mixed with 3 ml. of methanol and examined for cloudiness indicating the presence of polymerized styrene.
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Samples taken during the first 4 hours of the test show abso-lutely no clouding whatsoever in the styrene solution, thereby indicating that the inhibitor is extremely efficient. The ;' ~l~ sample taken at the end of 5 hours of elapsed time shows an ~ opaque reaction and coagulates.

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F.xample 8 The procedure of Example 7 is repeated except that divinylbenzene is employed in place of the styrene and the amount of inhibitor employed is .050 g. (approximately 1000 ppm). Results equivalent to Example 7 are obtained.

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Claims (25)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the distillation of a readily polymerizable vinyl aromatic compound, which comprises subjecting said compound to distillation in a distillation zone containing from about 50 to about 100 distillation stages, at a reboiler temperature of from about 150°F to about 300°F and a reboiler pressure of from about 30 mm. to about 400 mm. Hg., and in the presence of a polymerization inhibitor selected from the group consisting of NO, NO2, N2O3, NOCl and the adduct of NO(gas) and styrene, said inhibitor being present in an amount of from about 20 ppm to about 3000 ppm, whereby polymerization of the vinyl aromatic compound during distillation is substantially inhibited.

2. The process as defined by claim 1, wherein said polymerization inhibitor is continuously added to said vinyl aromatic compound.

3. The process as defined by claim 1, wherein said vinyl aromatic compound is styrene.

4. The process as defined by claim 1, wherein said vinyl aromatic compound is divinylbenzene.

5. The process as defined by claim 1, wherein said temperature is between about 200°F.and 300°F.

6. The process as defined by claim 3, wherein said inhibitor is NO and is present in an amount of from about 50 ppm to about 1000 ppm.

7. The process as defined by claim 6, wherein said inhibitor is present in an amount from about 50 ppm to about 500 ppm.

8. The process as defined by claim 3, wherein said inhibitor is NO2 and is present in an amount of from about 50 ppm to about 1000 ppm.

9. The process as defined by claim 8, wherein said inhibitor is present in an amount of from about 100 ppm to about 1000 ppm.

10. The process as defined by claim 3, wherein said inhibitor is N2O3 and is present in an amount of from about 50 ppm to about 1000 ppm.

11. The process as defined by claim 10, wherein said inhibitor is present in an amount of from about 50 ppm to about 500 ppm.

12. The process as defined by claim 3, wherein said inhibitor is NOCl and is present in an amount of from about 100 ppm to about 1000 ppm.

13. The process as defined by claim 12, wherein said inhibitor is present in an amount of from about 100 ppm to about 500 ppm.

14. The process as defined by claim 3, wherein said inhibitor is the adduct of NO(gas)-styrene and is present in an amount of from about 100 ppm to about 2000 ppm.

15. The process as defined by claim 14, wherein said inhibitor is present in an amount of from about 100 ppm to about 500 ppm.

16. The process as defined by claim 4, wherein said inhibitor is NO and is present in an amount of from about 100ppm to about 2000 ppm.

17. The process as defined by claim 16, wherein said inhibitor is present in an amount of from about 100 ppm to about 1000 ppm.

18. The process as defined by claim 4, wherein said inhibitor is NO2 and is present in an amount of from about 100 ppm to about 2000 ppm.

19. The process as defined by claim 18, wherein said inhibitor is present in an amount of from about 100 ppm to about 1000 ppm.

20. The process as defined by claim 4, wherein said inhibitor is N2O3 and is present in an amount of from about 100 ppm to about 2000 ppm.

21. The process as defined by claim 20, wherein said inhibitor is present in an amount of from about 100 ppm to about 1000 ppm.

22. The process as defined by claim 4, wherein said inhibitor is NOCl and is present in an amount of from about 100 ppm to about 2000 ppm.

23. The process as defined by claim 2?, wherein said inhibitor is present in an amount of from about 100 ppm to about 1000 ppm.

24. The process as defined by claim 4, wherein said inhibitor is the adduct of NO(gas)-styrene and is present in an amount of from about 100 ppm to about 2000 ppm.

25. The process as defined by claim 24, wherein said inhibitor is present in an amount of from about 200 ppm to about 1000 ppm.