DE2061803C3 - Process for the production of isoprene - Google Patents

Description

The patent 2031921 relates to a process for the production of isoprene by dehydrating 3-methyl-3-buten-l-ol, in which the VMethyl-3-buten-l-ol at temperatures between 40 and 100 ° C with acids or with substances that form acids in an aqueous medium in the presence of water and Treated salts and, if appropriate, of diluents, the acids or the acids forming substances and the salts being chosen so that either the anion of the acid or the anion of the salt or both anions are halides.

After an expedient procedure of this process, the 3-methyl-3-butene-1-oil is added to the charged with the acidic aqueous salt solution and brought to the reaction temperature with stirring to the extent that isoprene formed is placed on the reaction vessel Fractionation column can be taken.

It has now been found that isoprene is obtained particularly advantageously by the process of the main patent if the reaction mixture is obtained at temperatures between the boiling point of isoprene and the boiling point of the azeotropic mixture escaping from the reaction mixture, a distillate is withdrawn, the distillate is optionally neutralized and the isoprene is isolated therefrom by fractional distillation.

According to this improved method, the isoprene is thus not isolated by fractional distillation from the reaction mixture, but instead obtained from a Distillate extracts by distillation the reaction mixture in the temperature range between 7 ", and T ₂ and the mixture is neutralized before isolation of isoprene. T ₁ means the boiling point of isoprene at the selected reaction pressure and T ₁ the boiling point - also at the selected pressure - of the azeotropic mixture that escapes from the reaction mixture and that consists of dimethylvinylcarbinol and water. Dimethylvinylcarbinol is the lowest-boiling isopentenol that is under the cataly -

static influence of the dehydration catalyst may arise as an intermediate stage.

When working under normal pressure, T ₁ = 34 ° C. and T ₂ = 86 ° C. The preferred temperature range for the distillate removal is between the Values T ₁ + 5 ° C and T ₂ - 10 ° C. The distillate is expediently removed through a dephlegmator and neutralized either in the gas phase or after previous condensation in the liquid phase before it is used to purify the isoprene

η is distilled.

Before the isoprene is purified by distillation, it is optionally neutralized, namely the gaseous one or liquid distillate. The neutralization in the gas phase has the advantage that - without additional Energy expenditure - the neutral gases directly from a continuously operating distillation column can supply, at the top of which pure isoprene and water and iso-pentenols are obtained as the bottom product. The latter return to the dehydration rer back. In this procedure, the temperature in the neutralization stage is at least as high as in Dephlegmator.

All bases are suitable for the neutralization with the acids that can be used for dehydration

form in salts. Examples include ammonia, caustic soda, sodium carbonate, calcium carbonate, calcium hydroxide, potassium hydroxide, basic ion exchangers and organic bases, such as primary, secondary and tertiary amines. When using the Neutralization gives better (about 1 to 5% higher) yields than without this measure.

The advantage of this particular embodiment of the process over the process of the main patent is that isoprene is made from the starting material with the reaction rate increased by a factor of 2 to 6. The reaction volume required to carry out the synthesis is correspondingly smaller and the process is more economical. The increasing factor of dehydration gc speed essentially depends on the choice th dephlegmator temperature. The more you increase this, the faster the dehydration occurs. At a reaction temperature of approx. 40 to 60 ° C, which is particularly recommended for the process, it is over about a factor of 2 to 5 higher than a procedure in which a rectifying column placed on the reaction vessel is used. Of course, the absolute reaction speed still depends of the parameters mentioned in the main patent, such as reaction temperature, catalyst concentration, Intensity of mixing and the type of catalyst chosen.

In the new embodiment of the method, depending on the selected Dephlegmatortempe temperature, small amounts of acid are lost. However, this does not aggravate the process, since one usually works with the cheap hydrochloric acid. At a Dephlegmator temperature of z. B. 45 ° C per 1000 parts of isoprene only about 1 to 2 parts of hydrochloric acid are discharged from the dehydration vessel via the dephlegmator. This loss can easily be replaced in the dehydration vessel.

Otherwise, the details of the main patent apply. example

A cylindrical reaction vessel provided with a stirrer and having a capacity of 3 parts by volume is filled with the acidic Salt solution obtained by mixing 400 parts of sodium chloride, 600 parts of water and 40 parts 35% hydrochloric acid is obtained, charged and heated to approx. 95 ° C heated. The reaction vessel is operated at 45 ° C with a reflux condenser as a dephlegmator and an attached descending cooler operated with coolant at -10 ° C. The isoprene formed and the isopentenols which have passed over with the vapor pressure condense therein (95% dimethylvinylcarbinol).

3-Methy! -3-buten-l-ol is poured into the stirred dehydration vessel kept at 95 ° C. in an amount of 0.45 parts per hour continuously freezes. This feed rate corresponds approximately to the dehydration rate of the system. If larger amounts of the starting compound are added every hour, so unreacted 3-methyl-3-buten-1-o accumulates! in the dehydration vessel. About 0.350 parts of condensate, composed of 0.33 parts, are obtained per hour Isoprene, 0.01 part of dimethylvinylcarbinol and 0.01 part of water. The aqueous phase is continuously returned to the dehydration vessel

ί leads. The organic phase is concentrated with aqueous sodium carbonate solution and neutralized on a continuously operated fractionation column with approx. 9 theuretic bases in pure isoprene (0.33 Parts) (content> 99.9%) and isopentenols separately.

ίο The isopentenols (0.01 parts) return to the dehydrator return.

At hourly intervals, enough aqueous phase is removed from the dehydrator that it was originally existing stand height is retained. The amount of common salt withdrawn from the system and Hydrochloric acid is supplemented accordingly. The dehydrator is also turned on every hour the amount of higher-boiling oligomers formed every hour (approx. 0.026 to 0.027 parts).

This test arrangement can be operated without problems over long periods of time.

Claims (1)

Claim: Process for the production of isoprene by dehydrating 3-methyl-3-buten-l-ol with Acids according to patent 2,031,921, in which the 3-methyl-3-butene-l-o! at temperatures between 40 and 100 ° C with the acids or with Substances which form acids in an aqueous medium are treated in the presence of water and salts and, if appropriate, of diluents, the acids or acids being formed Substances and the salts are chosen so that either the anion of the acid or the anion of the salt or Both anions are halides, characterized in that one from the reaction mixture at temperatures between the boiling point of isoprene and the boiling point of the the azeotropic mixture escaping from the reaction mixture, removes a distillate which If necessary, the distillate is neutralized and the isoprene is isolated therefrom by fractional distillation.