GB2056481A - Feeding Nitrosophenols as Polymerization Inhibitors - Google Patents

Abstract

In a distillation process for separating and purifying a styrene from the reaction product in the production of that styrene, a nitrosophenol is fed, as a polymerization inhibitor, in the form of a solution thereof in a solvent (A) which is at least one solvent selected from alcohols, ketones, and aldehydes each being in liquid state and having a boiling point lower than 100 DEG C, or in a solvent (B) comprising a mixture of a solvent (A) and an aromatic compound.

Description

SPECIFICATION Method of Feeding Nitrosophenols as Polymerization Inhibitors This invention relates generally to methods of feeding polymerization inhibitors. More specifically, the invention relates to a method of feeding a polymerization inhibitor to a styrene when it is distilled from the reaction products in a process for producing it.

Styrenes are compounds of extremely high industrial usefulness as starting materials for products such as polystyrenes, synthetic rubbers, and polyester resins and are ordinarily obtained by the dehydrogenation reaction of ethylbenzene. The reaction products comprise mainly a mixture of styrene and ethylbenzene. Therefore, for industrial uses, it is necessary to subject this mixture to separation to obtain styrene of high purity. In general, separation and purification are carried out by distillation. Since styrenes such as styrene (C8H8), substituted styrenes, and divinyl benzene have the property of being readily polymerized, this distillation is ordinarily carried out in the presence of a polymerization inhibitor.

Various studies have been made from the past on the selection and the use of the polymerization inhibitor, but difficulties have been encountered. For example, when sulfur is used, there arises the problem of environmental pollution at the time of combustion of the distillation residue containing the sulfur. Polymerization inhibitors such as 4-tert-butylcatechol and 2,4-dinitrophenol do not exhibit enough inhibiting capability or performance in all cases at the distillation temperatures.

Nitroso compounds, particularly nitrosophenols, exhibit high inhibiting performance and are excellent polymerization inhibitors, but they have a drawback in that their solubility in aromatic compounds, such as styrene, is low.

In order to prevent polymerization within the distillation column, the polymerization inhibitor must have, in addition to high inhibiting performance, good solubility. The reason for this is that good solubility is greatly conducive to the manifestation of the activity of the polymerization inhibitor and, in addition, makes possible continuous and, moreover, smooth feeding of the inhibitor at a constant rate by dissolving the inhibitor in styrene, ethylbenzene, or the like.

Accordingly, there is an urgent demand for a method of improving the solubility of nitrosophenols thereby to feed the same continuously at a constant rate.

Summary of the Invention We have found that the above described problem can be solved by feeding a polymerization inhibitor comprising a nitrosophenol in the form of a solution thereof in a solvent.

According to this invention, briefly summarized, there is provided a method of feeding a nitrosophenol as a polymerization inhibitor in a distillation section for separating and purifying a styrene from the reaction product in the production of said styrene, which method comprises feeding the nitrosophenol in the form of a solution thereof in a solvent (A) which is at least one solvent selected from alcohols, ketones, and aldehydes, each being in liquid state and having a boiling point of or lower than 1000 C, or in a solvent (B) comprising a mixture of a solvent (A) and an aromatic compound.

By the practice of the method of this invention, a nitrosophenol can be continuously fed at a constant rate without lowering its inhibiting performance, and there is no occurrence of trouble such as clogging of the feeder which tends to occur in the feeding of an inhibitor in powder form.

The nature, utility, and further features of this invention will be more clearly apparent from the following detailed desciption concluding with reference examples and an example constituting a preferred embodiment of the invention.

Detailed Description of the Invention 1. Nitrosophenol Inhibitor A substance used as a polymerization inhibitor in this invention is a nitrosophenol. Nitrosophenols have a tautomeric relationship with respect to benzophenoneoximes. Accordingly, the term "nitrosophenols" are herein used includes also tautomers thereof.

Nitrosophenols suitable for use in this invention are, in addition to non-nuclear-substituted nitrosophenols, particularlyp-nitrosophenol, nuclear substituted nitrosophenols, particularly lower alkyl (of the order of C, to C4) -substituted, above all methyl-substituted nitrosophenols. Specific examples of methyl-substituted nitrosophenols are 2-methyl-p-nitrosophenol, 3-methyl-p-nitrosophenol, and 2,3,5-trimethyl-p-nitrosophenol.

2. Solvent Solvents suitable for use in this invention are alcohols, ketones, aldehydes, etc., which are liquids at room temperature and atmospheric pressure and have boiling points of 1 000C or lower.

Specific examples of suitable solvents are monohydric alcohols having 1 to approximately 3 carbon atoms, particularly methanol, ethanol, and propanol; monoketones having carbon atoms of a total of 3 to approximately 5, particularly acetone, methylethylketone and methylisopropylketone: and monoaldehydes having 3 to 4 carbon atoms, particularly propionaldehyde,.butylaldehyde, and the like.

The principal reasons for which these solvents are selected are that, in addition to the solubility of nitrosophenols therein, they can be readily separated by distillation from ethylbenzene, styrene, etc., and that, even when benzene, toluene, and the like are coexistent, these solvents have no adverse effect on an after-treatment process such as, for example, a dealkylation process.

Among the above enumerated solvents, methanol is particularly desirable.

Furthermore, for the aromatic compound to be used jointly with the above enumerated solvents, benzene, toluene, ethylbenzene, styrene, and mixtures of ethylbenzene and styrene, and others can be utilized.

If an excessive quantity of an aromatic hydrocarbon is used, the solubility of the nitrosophenol will decrease. For this reason, it is desirable that its quantity, in terms of volumetric ratio, be 20 times or less, particularly 1 0 times or less, that of the alcohol, ketone, aldehyde, or the like.

The nitrosophenol is used in the form of a solution thereof in the above described solvent in a concentration in the range of 5 to 30 percent, preferably 5 to 20 percent, at a temperature of from room temperature to 500C. Quantities herein expressed in percentages or parts per million (ppm) are by weight unless otherwise specified.

A nitrosophenol fed in this manner ordinarily exhibits its effectiveness as a polymerization inhibitor at a temperature within the distillation column of 90 to 1 200C in a concentration in the range of lotto 100ppm.

3. Styrenes and Stabilization Thereof Representative styrenes, in addition to non-substituted styrene (C8H8), are nuclear and/or side chain-substituted styrenes, particularly methyl-substituted styrenes such as vinyl toluene, for example.

A styrene of this character is typically produced by dehydrogenation of the corresponding ethylbenzene. In this case as mentioned hereinbefore, it is obtained as a mixture of the styrene and the ethylbenzene. This mixture is ordinarily passed for purification through a first column where lighter products such as benzene and toluene are distilled off, a second column where ethylbenzene when it has not been removed is distilled off, and a third column where styrene is obtained from the top of the column freed from heavier products.

This invention can be practised during the feeding of the inhibitor into any of the first second and third columns. Typically, it is convenient to add the inhibitor solution to the crude styrene when this crude styrene is fed into the first column or to feed the inhibitor solution into the first column according to this invention.

In order to indicate more fully the nature and utility of this invention, this invention will now be described with respect to reference examples and an example, but it is to be understood that this invention is not limited in scope to these examples.

Reference Example 1 The solubilities of p-nitrosophenol in ethylbenzene were found to be as follows.

0.02 percent at 300C 0.05 percent at 500C Reference Example 2 The solubilities of p-nitrosophenol at 300C respectively in the following solvents were as indicated below.

Solvent (volumetric ratio) methanol:ethylbenzene Solubility ( /0) 1:0 11.5 1:5 6.5 1:10 3.0 1:15 2.0 Reference Example 3 Into a 1 00-ml., three-necked, round-bottom flask, 40 ml. of purified styrene monomer (SM) (C8H8) and a p-nitrosophenol solution (5%) in a solvent mixture of methanol and ethylbenzene (volumetric ratio 1:5) were added to cause the p-nitrosophenol concentration to become 100 ppm.

This solution was heated to 1 00C, and the rate of polymerization was periodically measured. As a result, the polymerization rate was found to be 0.1% after one hour and to be 0.25% after two hours.

This measurement of polymerization rate was carried out by a calibration curve method by means of a gel permeation chromatography.

Example To a styrene-containing, reaction-product liquid of the composition set forth below from a dehydrogenation reaction vessel, in a styrene-plant, a p-nitrosophenol solution of 5-percent concentration in a methanol/ethylbenzene solvent mixture (volumetric ratio 1:5) was fed so as to bring the concentration of the p-nitrosophenol to 200 ppm/styrene.

Composition of Reaction Product Liquid (as Determined by Gas Chromatography) benzene 0.6% toluene 2.4% ethyibenzene 56.0% styrene 40.5% remainder 0.5% Then, by means of a three-column distillation apparatus, this process material was first divided into a fraction of benzene and toluene and a fraction of ethylbenzene and styrene. Next, the ethyl benzene and styrene was separated. Finally, styrene was distilled out from the column top.

The styrene yield was 98.2 percent.

The distillation conditions were as set forth below.

Distillation column inner diameter: 3 cm.

Feed rate of reaction-product liquid: 500 ml/hr.

Column bottom temperature: 1 070C No. of Reflux plates ratio Separation of benzene-toluene fraction and ethylbenzene styrene fraction 20 7.0 Separation of ethylbenzene and styrene 70 5.5 Separation of styrene and high boilers 20 1.0 In this operation, the p-nitrosophenol was fed in the form of a solution, and this feeding was accomplished in a very smooth manner.

Claims (7)

Claims

1. A method of feeding a nitrosophenol as a polymerization inhibitor in a distillation process for separating and purifying a styrene from the reaction product in the production of said styrene, which method comprises feeding the nitrosophenol in the form of a solution thereof in a solvent (A) which is at least one solvent selected from alcohols, ketones and aldehydes, each being in liquid state and having a boiling point lower than 1 000C, or in a solvent (B) comprising a mixture of a solvent (A) and an aromatic compound.

2. A method according to claim 1 in which the nitrosophenol is p-nitrosophenol, 2-methyl-p nitrosophenol, 3-methyl-p-nitrosophenol, or 2,3,5-trimethyl-p-nitrosophenol.

3. A method according to claim 1 or 2 in which the solvent (A) is methanol.

4. A method according to any one of the preceding claims 1 to 3 in which the aromatic compound is benzene, toluene, ethylbenzene, styrene, or a mixture thereof.

5. A method according to any one of the preceding claims 1 to 4 in which the solvent is solely the solvent (A).

6. A method according to any one of the preceding claims 1 to 4 in which the solvent is a mixture of the solvent (A) and an aromatic compound of a quantity which is twenty times by volume that of the solvent (A).

6. A method according to any one of the preceding claims in which the nitrosophenol is dissolved at a temperature of from room temperature to 500C in the solvent to a concentration of 5 to 30 percent by weight.

7. A method according to claim 1 substantially as herein described with reference to the specific example.