JPS6245213B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing isoprene that includes a process for efficiently recovering the heat contained in the reaction gas distilled from the reaction zone.

Previously, the present inventors produced isoprene by reacting isobutene and/or tertiary butanol (sometimes referred to as _C4 ) with a formaldehyde source (sometimes referred to as FA) in the liquid phase. We investigated a method for synthesizing C4 and FA by continuously or intermittently supplying _C4 and FA together with water into an acidic aqueous solution and keeping the ratio of the partial pressure of organic matter and water in the reaction system within a specific range. It has been found that isoprene can be obtained in a good yield by carrying out the reaction while distilling simultaneously produced isoprene and unreacted raw materials together with water out of the reaction zone (Japanese Patent Application Laid-open No. 70623/1983). However, in this method, it is necessary to increase the C ₄ /FA ratio, and therefore a large excess of C ₄ relative to the produced isoprene ends up distilling out of the reaction zone together with water. In addition, in order to maintain the ratio of the partial pressure of organic matter in the reaction system to the partial pressure of water within a specific range, it is necessary to increase the amount of water to be evaporated in response to the increase in the amount of _C4 supplied. Therefore, when the above method is carried out on an industrial scale, a large amount of heat is released along with the reaction gas distilled from the reaction zone.

The present inventors conducted intensive studies to effectively recover the heat contained in the reaction distillate gas, and as a result, they arrived at the present invention. That is, according to the present invention, isobutene and/or tertiary butanol, a formaldehyde source, and water are continuously or intermittently supplied into an acidic aqueous solution, and the produced isoprene is removed from the reaction zone together with water and other low-boiling components. Isoprene is produced by reacting while distilling,
At this time, mainly water in the reaction distillate gas is condensed to recover the heat of the water, and the heat of the uncondensed reaction gas is used as a heat source for vaporizing raw material isobutene and/or by condensing the reaction distillate gas. ,
By using it as a heat source when recovering unreacted isobutene by distillation from the organic layer obtained by phase separation, the amount of heat contained in the reaction distillate gas can be effectively and extremely efficiently recovered. By adopting the method of the present invention, isoprene can not only be produced in good yield, but also the amount of energy consumed per unit of isoprene production can be significantly reduced. There is something extremely valuable about it.

In the process of the invention, the reaction distillate gas is sequentially condensed under substantially the same pressure as the reaction zone, thereby effectively recovering the heat contained in the reaction distillate gas.
The reaction distillate gas is first partially condensed at a suitable temperature at which water in the reaction distillate gas condenses, and the latent heat of the water is recovered. At this time, the sensible heat of the reaction distillate gas is also recovered. Heat recovery methods include generating steam by heat-exchanging the reaction distillate gas with water, or directly using the reaction distillate gas during recovery of isobutene, recovery of tertiary butanol, or recovery or purification of isoprene. There is a method in which it is introduced into a reboiler of a distillation column used for this purpose and used as a heat source for the reboiler. The amount of water vapor that can be generated by heat exchange between the reaction distillate gas and water is approximately equal to the amount of water contained in the reaction distillate gas, and the generated water vapor can be used for recovery of isobutene, tertiary butanol, or isoprene. It can be used as a heat source during the recovery or purification of When exchanging heat between the reaction distillate gas and water, steam having different pressures can be generated depending on each application by subjecting the reaction distillate gas to multi-stage partial condensation.

Of the water obtained by partially condensing the reaction distillate gas, the required amount is usually recycled to the reactor, and the remaining water is sent to the unreacted material recovery step,
There, tertiary butanol and FA are recovered.

The calorific value of the uncondensed reaction gas is used as a heat source for vaporizing the raw material isobutene and/or as a heat source for condensing the reaction distillate gas and recovering unreacted isobutene from the organic layer obtained by phase separation by distillation. . The raw material isobutene can be vaporized by heat-exchanging the uncondensed reaction gas with isobutene. When raw material isobutene is vaporized and supplied to the reactor, the heat load on the reactor is reduced and isobutene is easily dispersed in the acidic aqueous solution. Also,
The method of supplying the necessary amount of heat to the reactor is usually to circulate the reaction liquid through a heat exchanger placed outside the reactor, but when vaporized isobutene is supplied to the reactor through the heat exchanger, can lower the evaporation temperature of the reaction liquid in the heat exchanger. The heat of condensation of the uncondensed reaction gas before vaporizing isobutene or the uncondensed reaction gas after vaporizing isobutene can be used as a heat source when isobutene is distilled and recovered from the organic layer containing unreacted isobutene. This organic layer containing unreacted isobutene is used as a heat source for vaporizing isobutene or as a heat source for recovering unreacted isobutene by distillation. It can be obtained by condensing the condensate and phase-separating the condensate. When obtaining the organic layer, an aqueous layer is obtained in addition to the organic layer, which is sent to an unreacted substance recovery step, where tertiary butanol and FA are recovered.

When carrying out the method of the present invention, C ₄ /FA is preferably at least 3 in order to obtain isoprene in a good yield. From the point of view of reaction yield
The larger C ₄ /FA is, the better it is, and there is no upper limit in the strict sense of the word, but even if these values are increased unnecessarily, the effect of improving the isoprene yield will be small, and the amount of heat used will increase, resulting in economical problems. In general, C ₄ /FA should not exceed 20, as this will be disadvantageous. Since unreacted C ₄ has a composition close to the equilibrium composition of isobutene and tertiary butanol under the reaction conditions, as long as unreacted C ₄ is recycled to the reaction,
Even if either isobutene or tertiary butanol is introduced into the reaction solution as a starting material, both isobutene and tertiary butanol will eventually be used as reaction raw materials.

When adopting a reaction method in which isoprene and low-boiling components are distilled out of the reaction zone together with water while supplying C ₄ , FA source, and water into a reactor containing an acidic aqueous solution, the pressure inside the reactor must be adjusted. The ratio of each component to water that evaporates from the reaction zone can be determined by the method; when the pressure is high, the proportion of water to the total of components other than water in the distillate decreases, and when the pressure is low, the proportion of water to the total of components other than water decreases. The opposite phenomenon occurs. In order to obtain isoprene in a good yield, the reaction pressure (however, if a low-boiling compound that is inactive under the reaction conditions is supplied with the raw material, the pressure after subtracting its partial pressure) is preferably the same as the vapor pressure at the reaction temperature of the acidic aqueous solution. 1.1~2.5
It is better to be within the double range. The vapor pressure of the acidic aqueous solution at the reaction temperature (hereinafter referred to as Pw)
is a physical constant uniquely determined by the type and concentration of the acidic substance contained in the acidic aqueous solution. When the pressure inside the reactor exceeds 2.5 times Pw, the yield of isoprene decreases significantly. The pressure inside the reactor is Pw 1.1
If the amount is less than double, there is no significant decrease in isoprene, but the conversion rate of FA decreases, and the ratio of water to isoprene in the distillate increases, increasing the amount of heat consumed in the reaction. .

The amount of water fed to the reactor is usually adjusted so that the amount of acidic aqueous solution in the reactor is kept constant. That is, this amount is determined by the amount of water distilled from the reaction zone and the amount of water gained or lost by the reaction. The ratio of the number of moles of water distilled from the reaction zone to the number of moles of raw materials and products distilled off is determined by the pressure within the reactor. Since the number of moles of raw material and product to be distilled is approximately equal to the number of moles of C ₄ supplied, the ratio of water distilled to C ₄ supplied is determined by the pressure inside the reactor. Become. Therefore, the amount of water fed depends on the pressure inside the reactor,
The amount may be determined by taking into account the amount of C ₄ supplied and the increase and decrease of water due to the reaction.

In the method of the present invention, most of the heat required to distill water out of the reaction zone can be recovered by partially condensing the reaction distillate gas, so the heat required to produce per unit of isoprene is It is determined by the ratio of the number of moles of C ₄ to be supplied and the number of moles when converting the FA source to FA (hereinafter referred to as C ₄ /FA). i.e. spent
The amount of heat required to separate and purify isobutene from the BB fraction, vaporize isobutene, and recover isobutene and tertiary butanol will be determined by the amount of C ₄ supplied to the reaction zone.

In order to effectively recover the heat of water in the reaction distillate gas, the molar ratio of water supplied to the reactor and C ₄ supplied (hereinafter referred to as H ₂ O/C ₄ ) should not be too large. (for example, 2.0 or lower). If H ₂ O/C ₄ is kept at a relatively low value below 2.0, the amount of heat required to produce a unit of isoprene is determined by C ₄ /FA.

The reaction temperature suitable for the method of the present invention is determined by considering the acid strength of the acidic aqueous solution, and is usually 150 to 220°C.
Selected from the range of °C. If the reaction temperature is less than 150°C, the yield of isoprene will decrease even if the concentration of the acidic aqueous solution is increased to maintain the reaction rate at a constant level. Even if the reaction temperature exceeds 220°C, the isoprene selectivity does not decrease significantly, but the FA conversion rate under conditions that give the optimum selectivity decreases. If reaction conditions are selected that increase the conversion rate of FA, the sequential reactions from isoprene will increase, resulting in a decrease in the selectivity of isoprene.

The preferred feed rate of the FA source to the reactor is determined by considering the acid strength of the acidic aqueous solution, the reaction temperature, and the reaction pressure. In order to increase the supply rate of the FA source, it is necessary to increase the acid strength of the acidic aqueous solution or to increase the reaction temperature, and in this case, the problem of corrosion of the reactor arises. Therefore, the feed rate of the FA source is preferably 3 mol or less per hour per 1 kg of the acidic aqueous solution when the FA source is converted to FA. There is no strict lower limit for the supply rate of the FA source, but if the supply rate is unnecessarily reduced, the reactor will become larger and this will be disadvantageous in terms of equipment.
The supply rate of the FA source is preferably 0.2 mol or more per hour per 1 kg of the acidic aqueous solution when calculated as the FA source.

The catalyst used in the method of the present invention is an inorganic acid,
Acidic substances such as organic acids and their salts, which are used in the reaction zone in the form of an aqueous solution. The acid strength of these acidic aqueous solutions varies depending on the type of acidic substance, the reaction temperature, the supply rate of the FA source, etc., but is usually selected from the pH range of 0.5 to 2.5. The acidic substance is preferably one that is low volatile or nonvolatile under the reaction conditions, and specifically includes inorganic acids such as phosphoric acid, sulfuric acid, and boric acid, heteropolyacids such as silicotungstic acid and phosphotungstic acid, and P
- Mention may be made of organic acids such as toluenesulfonic acid, benzenesulfonic acid, trifluoromethanesulfonic acid, oxalic acid, and acid salts such as sodium hydrogensulfate.

Formaldehyde sources used in the method of the present invention include formaldehyde aqueous solutions, formaldehyde gas, etc. In addition, trioxane, which decomposes under the reaction conditions to give formaldehyde,
Paraformaldehyde and the like can also be used. Methyral and other formals can also be used. Since water is supplied to the reactor and formaldehyde takes the form of an aqueous solution in the reaction zone, it is advantageous in terms of reaction operation to use an aqueous formaldehyde solution as the formaldehyde source.

The isobutene and tertiary butanol used in the process of the invention may contain other hydrocarbons, 3-methyl-1,3-butanediol, 3-methyl-2-
Buten-1-ol, 3-methyl-3-butene-
1-ol, 3-methyl-1-buten-3-ol, methyl isopropyl ketone, 2-methylbutanal, methyl tert-butyl formal, 4.4
-dimethyl-1,3-dioxane, 4-methyl-
5,6-dihydro-2H-pyran and the like may be included. It is also possible to use alkyl tertiary butyl ethers, such as methyl tertiary butyl ether, which gives isobutene and tertiary butanol under the reaction conditions.

Next, an example of an embodiment of the method of the present invention will be explained using FIG.

The reaction gas distilled from reactor 1 is introduced into heat exchangers 3 and 3' to generate steam. The required amount of water condensed in heat exchangers 3 and 3' is recycled to the reactor, and the remainder is recycled to tertiary butanol and FA.
sent to the collection process. The uncondensed reaction gas exiting the heat exchangers 3 and 3' is introduced into the heat exchanger 4, and a portion of the uncondensed reaction gas is condensed to vaporize isobutene used as a reaction raw material. The vaporized isobutene vapor is fed to a heat exchanger 2 located outside the reactor. The uncondensed reaction gas leaving the heat exchanger 4 is introduced into the heat exchanger 5. The condensate produced in the heat exchanger 4 is led to a decanter 6 and separated into an organic layer and an aqueous layer. After the organic layer is preheated in a heat exchanger 5, it is supplied to a distillation column 7, where unreacted isobutene is recovered. The aqueous layer contains tertiary butanol and
Sent to FA collection process. The condensate produced in the heat exchanger 5 is led to a decanter 6, and similarly to the condensate produced in the heat exchanger 4, it is separated into an organic layer and an aqueous layer.
The bottom liquid from distillation column 7 is sent to isoprene recovery and purification and tertiary butanol recovery steps.

The present invention will be specifically explained below with reference to Examples.

Example 1 An experiment was conducted according to the flow shown in FIG.

From a stainless steel reactor 1 with an internal volume of 400 equipped with a raw material introduction pipe, a water introduction pipe, a thermometer, a pressure gauge, a stirrer, and a gas distillation pipe, and an external heat exchanger 2 connected to a circulation pump and an isobutene vapor introduction pipe. A reactor was used. The reaction gas distillation tubes were successively connected to a heat exchanger. 188 kg of 3% by weight phosphoric acid aqueous solution in the reactor
was charged, and the reaction was carried out at 180°C under a pressure of 16.8 Kg/cm ² . The vapor pressure of a 3% by weight aqueous phosphoric acid solution at 180°C is 10.1 Kg/cm ² . The rotation speed of the stirrer is per minute
It was set to 500 rotations. 8.9 kg/hour of 48 weight% formalin water, 22.0 kg/hour of tertiary butanol,
Circulating water is continuously supplied at a rate of 22.7 kg/hour, and the recovered isobutene vapor is transferred to an external heat exchanger at a rate of 46.6 kg/hour.
It was fed at a rate of Kg/hr. Distillation reaction gas amount is 100
Kg/hour. At this time, the amount of heat consumed by the external heat exchanger 2 is 24×10 ³ Kcal/hour, and the medium pressure (15Kg/hour) is
cm ² ) The amount of steam used was 51 kg/hour. The distilled reaction gas was successively condensed while maintaining the pressure at approximately the same pressure as the pressure of the reaction system. First, the distilled reaction gas is introduced into heat exchangers 3 and 3' divided into two stages, at 170°C and 140°C, respectively.
It was degenerated at ℃. The amount of heat exchanged at this time is
6.5×10 ³ Kcal/hour, 8.5×10 ³ Kcal/hour, and the amount of steam generated is 4Kg/cm ² steam 13Kg/hour, 2.5
Kg/ ^cm2 steam was 16 Kg/hour. The amount of reaction gas condensed in the heat exchanger 3 was 31 kg/hour, which corresponded to almost the entire amount of distilled water. Then, the uncondensed gas is introduced into the heat exchanger 4 and partially condensed at 110°C.
Isobutene was evaporated. The evaporation of isobutene is
The reaction was carried out while maintaining the pressure at 18 Kg/cm ² which was slightly higher than that of the reactor. The amount of partial condensation heat of the reaction gas in heat exchanger 4 was 4×10 ³ Kcal/hour. The uncondensed gas leaving the heat exchanger 4 was 55 kg/hour, and was introduced into the heat exchanger 5 where it was completely condensed at 97°C. The total heat of condensation at this time was 5×10 ³ Kcal/hour. Using this amount of heat, the feed liquid to the distillation column 7, which recovers unreacted isobutene from the organic layer obtained by phase-separating the reaction liquid condensed in the heat exchangers 4 and 5, was preheated. If the pressure on the feed liquid side of heat exchanger 5 is maintained at 5.7Kg/ ^cm2 , the temperature will be
The temperature reached 74℃. In the heat exchanger 5, most of the isobutene content in the feed liquid was flashed, and this was supplied to the distillation column 7 as steam. In this way, the heat of condensation of the reaction gas coming out of the reactor can be used extremely effectively.
Almost 100% of the heat consumed in the reaction zone was recovered.
The bottoms from distillation column 7 were further subjected to high-boiling and low-boiling cuts, and finally purified isoprene was obtained at a rate of 6.7 kg/hour.

[Brief explanation of the drawing]

FIG. 1 is a flow diagram illustrating an embodiment of the method of the present invention. 1...Reactor, 2...External heat exchanger, 3,
3', 4 and 5... Heat exchanger, 6... Decanter, 7... Distillation column, A... Steam, B... Water, C
...Isobutene, D...Bent gas, ...Third
A recovery process for primary butanol and FA, a recovery and purification process for isoprene, and a process for recovering tertiary butanol.

Claims (1)

[Claims] 1. Continuously or intermittently feeding isobutene and/or tertiary butanol, a formaldehyde source, and water into an acidic aqueous solution and reacting while distilling the produced isoprene out of the reaction zone along with water and other low-boiling components. At that time, mainly water in the reaction distillate gas is condensed to recover the heat of the water, and the heat of the uncondensed reaction gas is used as a heat source and/or for the reaction when vaporizing the raw material isobutene. A method for producing isoprene, which is characterized in that it is used as a heat source when distilled gas is condensed and unreacted isobutene is recovered by distillation from an organic layer obtained by phase separation.