DE1154455B - Process for distilling styrene - Google Patents

Description

Process for distilling styrene For the production of styrene is Ethylbenzene thermally dehydrated on special contacts in the presence of steam. A mixture of styrene and unreacted ethylbenzene as well as small amounts are formed Amounts of by-products. The main components, ethylbenzene and styrene, their boiling points about 9 C apart, and the other substances contained in the reaction mixturec are generally in a vacuum in distillation plants with a high theoretical Number of plates separated.

During the distillation, the hydrocarbon mixture is usually Added stabilizers. As such, z. B. known: hydroquinone, catechol, Sulfur and many others.

Such additives make the spontaneous polymerization of the polymerizable Shares largely pushed back.

However, the stabilizers mentioned cannot prevent themselves Form shoot polymers on the walls of the distillation plant. The amount of Adhering polymeric, swollen material increases significantly with longer operating times too, so that bottoms are deformed by the swelling pressure that sets in, column fillings crushed and even pipelines can be torn. Separating parts form germs for new deposits in other parts of the system; they clog Filters, pipes and pumps. Ultimately, this allows production to go to Come to a standstill. The problem with the formation of the shoot polymers is not so much a problem here associated loss of material. which is generally less than () .I ° / o of the enforced Amount, rather than the interruption of the workflow.

From previous studies it was known that shoot polymers are very strongly cross-linked, i.e. in the usual solvents such as benzene, toluene, Ethylbenzene, dichlorobenzene, are insoluble and only in these solvents sources. It is therefore not possible to carry out a shoot polymer once it has formed To remove leaching from the distillation plant. The columns must therefore be disassembled for cleaning, which requires a lot of work and leads to a considerable loss of production. Also it cannot be avoided that traces of sprout polymers remain in the column after restarting of the distillation plants act as seeds for further shoot polymer formation. Once sprout polymers have formed in distillation plants, so it is required to clean and overhaul them at regular intervals.

The cause of the formation of shoot polymers is not precise known. Possibly traces of divinylbenzene cause dehydration of ethylbenzene arise, or small amounts of leaks in the column of air entering the column in a still unknown way its formation. For the latter assumption speaks to the finding that the insoluble polymers preferably on the seals between the column sections or between columns and form distillation pot.

So far it has been suggested the formation of shoot polymers when distilling vinyl toluene, which contains small amounts of divinylbenzene. by adding o-nitrophenol or o-nitrothiophene. But it has been proven these compounds prove to be ineffective as an additive in the distillation of styrene.

It has now been found that when distilling styrene or of styrene-containing reaction products from the thermal dehydrogenation of ethylbenzene can prevent the shoot polymerization if the distillation is in the presence small amounts, e.g. B.

0.002 to 5 percent by weight, preferably 0.02 to 1 percent by weight p-quinone monoxime.

It was found that during a distillation already at least the proportions of shoot polymers formed after the addition of p-quinone monoxime can be partially dismantled again.

In addition to the shoot polymerization, p-quinone monoxime prevents, as stated also the spontaneous polymerization of styrene. However, this effect is not Subject of the invention. It can also be achieved with other known compounds will.

Of course, in addition to p-quinone monoxime, other Stabilizers are also used without the excellent effect of p-quinone monoxime, the formation of shoot polymers on the walls of the distillation system prevent being impaired.

The distillation can be carried out up to an absolute pressure of about 1 ata be carried out, but preferably at pressures of 20 to 200 torr. It can can also be distilled batchwise or continuously. Here it is possible To use packed columns or tray columns.

The following examples illustrate the principle of the invention but not a limitation on their application.

Example 1 In the following series of measurements, a styrene-divinylbenzene mixture was used which consisted of about 99.5 percent by weight of styrene and 0.5 percent by weight of divinylbenzene. Various stabilizers were added to this mixture in a glass flask equipped with a stirrer and the glass flask was placed in a boiling water bath. The time it took for the mixture to gel was determined. Time Amount by Stabilizer in gelled Weight percent minutes tert-butyl catechol. 0.025 20 Hydroquinone. 0.025 60 Sulfur ..... 0025 30 o-nitrophenol. . ¢ 01025 30 p-quinone monoxime 0.025 330 Without stabilizer. S 5 EXAMPLE 2 Mixtures of 99.5 percent by weight styrene and 0.1 percent by weight divinylbenzene to which various stabilizers were added were distilled in a glass distillation column (nm length, 5 cm diameter, filled with 5 mm Raschig rings), the mixtures being distilled in an amount of 1 kg per hour were metered in continuously. Nine tenths of the added amount was distilled off overhead, and one xehateí of the amount was withdrawn from the bubble; the return ratio was 5: 1. The temperature at the top of the column was 45 ° C .; it was distilled at a pressure of 33 torr. In each batch, the column was operated until the formation of insoluble polymer prevented further distillation or, if no insoluble polymer was formed, until the effectiveness of the stabilizer had been sufficiently demonstrated. Amount of time to Stabilizer in constipation Weight of the column percent in hours Hydroquinone. 0.005 120 Catechol. 0.005 137 Sulfur ..... . 0.005 72 tert-butylcatechol 0.005 40 o-nitrophenol 0.005 100 wquinone monoxime. 0.005 after 500 hours still no education of insoluble Polymer Without stabilizer. . ~ 50 Example 3 A continuously operating distillation apparatus was equipped with a column with 45 bubble-cap trays 2 m in diameter. In this device, a mixture of reaction products, such as those formed in the thermal dehydrogenation of ethylbenzene, with a styrene content of 40 percent by weight was continuously broken down into a top product with 0.5 percent by weight of styrene and a bottom product with 88 percent by weight of styrene. The feed had been stabilized with 0.03 percent by weight of hydroquinone and was added continuously at an amount of 3 ms per hour to the eleventh tray of the column. A pressure of 200 Torr prevailed in the still, the temperature was 1040 C. A reflux ratio of 1: 4 was used. After an operating time of 3 months, a 1 m long block of sprout polymers with a height and width of about 30 cm and four other polymer deposits of smaller dimensions had formed on the flange connecting the column to the still. Now, instead of the hydroquinone, pXhinone monoxime was used for stabilization. After a further period of operation of 10 months, the shoot polymer deposits had largely been broken down.

Claims (2)

PATENT CLAIMS: 1. Process for the distillation of styrene or of the styrene-containing reaction products of the thermal dehydrogenation of ethylbenzene, characterized in that to prevent the formation of shoot polymer Performs distillation in the presence of small amounts of p-quinone monoxime. 2. The method according to claim 1, characterized in that in addition to p-quinone monoxime other, already known stabilizers were also added to the substances to be distilled References Contemplated: U.S. Patent No. 2,413,255.