RU2177469C1 - Isoprene production process - Google Patents

Abstract

FIELD: industrial organic synthesis. SUBSTANCE: isoprene is prepared in two stage process involving liquid-phase condensation of formalin blend containing 20-30 wt % of formaldehyde with isobutylene in presence of acid catalyst at 85-105 C and pressure 18-20 atm. Intermediate fraction consisting largely of 4,4-dimethyl-1,3-dioxane and high-boiling by- products is subjected to decomposition in second reactor in presence of tert-butyl alcohol at 140-150 C and pressure 3-5 atm. Process is accompanied by continuous distillation of isoprene formed in vapor phase and recycle of liquid phase containing water, tert-butyl alcohol, and high-boiling products into decomposition stage. Vat product containing acid catalyst is evaporated, mixed with fresh formalin blend, and returned into condensation reactor. Condensation is carried out at isobutylene-to-formaldehyde molar ratio (0.8-1.1):1 and decomposition at tert-butyl alcohol-to-4,4-dimethyl-1,3-dioxane molar ratio (3- 4):1. EFFECT: reduced consumption of reactants and power. 4 ex

Description

 The invention relates to the field of basic organic and petrochemical synthesis, more specifically to methods for producing isoprene from isobutylene and formaldehyde.

 Isoprene is the initial product for the production of synthetic polyisoprene rubber, which is closest to the natural one in terms of the complex of physical and mechanical properties.

Numerous methods are known for producing isoprene from isobutylene and formaldehyde in the presence of acid catalysts, carried out at elevated temperature and pressure, ensuring that the reactants are in the liquid phase, with the formation of intermediate products containing 5 or 6 carbon atoms, which then decompose in one way or another under the influence of temperature and catalysts to form isoprene. The most widely used in industry was a method for the synthesis of isoprene through an intermediate stage of the formation of 4,4-dimethyl-1,3-dioxane (DMD), which includes the stage of liquid-phase condensation of isobutylene contained in the C ₄ hydrocarbon fraction with formaldehyde in the form of 20-40% aqueous solution, followed by isolation of the resulting DMD and its decomposition over a solid acid catalyst in the presence of water vapor [Ogorodnikov SK, Idlis GS Isoprene production. L .: Chemistry, 1973].

The use of this method provides the production of isoprene of monomeric purity, allows you to combine the production of DMD with the extraction of isobutylene from C ₄ fractions, provides high performance contact equipment. However, the principal disadvantage of this method is the increased energy costs associated with the consumption of superheated water vapor in the second stage (wt. Ratio of H ₂ O / DMD not less than 2.0) and with the strengthening of diluted formalin solutions formed at both stages of the process.

A known method of producing isoprene from isobutylene and formaldehyde, which consists in the fact that isobutylene, previously isolated from C ₄ fractions, in a mixture with compounds that easily form isobutylene, for example trimethylcarbinol (TMK), reacts with a dilute formaldehyde solution in a molar ratio of iso-C ₄ H ₈ / CH ₂ O greater than 2.5-3.0 and a temperature of ≥100 ^o C, and then the entire reaction mass is transferred to a second reactor, where the resulting intermediate products are isoprene precursors such as DMD, 3-methylbutanediol-1, 3 (MIA), isobutenylcarb inol (IBC, 3-methyl-3-buten-1-ol), dioxane alcohols (DS), etc., decompose at a temperature of ≥130-150 ^o C with the formation of isoprene, which is distilled from the reaction zone together with unconverted isobutylene and part water. This stream is again sent to the isoprene precursor synthesis reactor.

Since the technology does not include the stage of separation of intermediate products, this process is conventionally called the one-stage synthesis of isoprene from isobutylene and formaldehyde (OIF) [Pavlov S.Yu., Surovtsev A.A. Chem. industry, 1997, N 7, p. 466]. The method provides for the preliminary isolation of isobutylene from the C ₄ hydrocarbon fraction by hydrating it in a reactor filled with molded cation exchange resin, with the resulting TMC being fed into stage I or II reactors. The presence of an excess of isobutylene in the reaction mass in both reactors provides an almost complete conversion of formaldehyde, which eliminates the need for its recovery.

The application of this method also ensures the production of isoprene of monomeric purity, it can significantly reduce energy consumption compared to the two-stage process, however, the principal disadvantage of the UIF is the low productivity of the contact equipment associated with the use of a large excess of isobutylene (the molar ratio of iso-C ₄ H ₈ / CH ₂ O does not less than 2.5-3.0). The low conversion of isobutylene, not exceeding 33-40%, leads to significant costs for condensation and the return of the unconverted olefin to the reaction zone. In addition, the need for the use of large quantities of isobutylene causes an increased load on the hydrocarbon phase in the hydration reactor of this olefin at the stage of its separation from the C ₄ fraction.

There is also a known method for producing isoprene, comprising the liquid-phase interaction of formaldehyde with isobutylene and / or trimethylcarbinol in the presence of an aqueous solution of an acid catalyst at elevated temperature and pressure, carried out in two successive reaction zones with the selection of the second stage reaction products from the top of the reactor and the isolation of isoprene from them. Additionally, high boiling point by-products (WFP) of DMD synthesis are boiled into the first reaction stage, boiling above 140 ^° C., in an amount of from 1:10 to 3: 1 (by weight) with respect to the formaldehyde feed.

In this case, in the first reaction stage, the temperature is 30-110 ^o C, pressure 4-25 atm, in the second stage - temperature 110-200 ^o C and pressure 3-25 atm, after the first stage, the reaction mass is throttled at a pressure not lower than the pressure in the second stage , the vapor phase is separated, condensed and fed into the first reaction stage, trimethylcarbinol is fed in whole or in part to the second stage (RF patent N 2128637, class C 07 C, publ. 1999).

The runway fraction is a mixture of hydroxyl-containing derivatives of 1,3-dioxane and tetrahydropyran, which are partially hydrolyzed under conditions of stage I, and then split under conditions of stage II to form additional amounts of isoprene. The submission of the runway for the synthesis of DMD positively affects the yield of isoprene, but does not eliminate the fundamental drawback of the method — low productivity of the first stage reactor and the need for large costs for the return of unconverted isobutylene. The molar ratio of iso-C ₄ H ₈ / CH ₂ O in this method is 3-15: 1 and, accordingly, the olefin conversion does not exceed 6.7-33%.

The closest in technical essence to the present invention is a method for producing isoprene, which consists in the fact that the liquid-phase condensation of isobutylene and an aqueous solution of formaldehyde is carried out at a temperature of 30-110 ^o C and a pressure of 4-25 atm in the presence of acid, and then without the formation of the resulting DMD and other side and the intermediate products contained in the reaction mixture, the reaction mass is decomposed at 140-150 ^o C and a pressure of 3.5 atm in the presence of isobutylene and / or TMK with continuous distillation of the resulting isoprene .

In addition, an aqueous layer of the reaction mass of the DMD synthesis reactor in an amount of 1:10 to 3: 1 (by weight) with respect to the water supplied to the first stage of the UIF (RF patent N 2128636, is fed to the first stage of the UIF (in the isoprene precursor synthesis reactor) C. C 07 C, publ. 1999 - prototype). The aqueous layer of the DMD synthesis reactor contains formaldehyde (5-7%), an acid catalyst (≈3%) and organic compounds dissolved in water: DMD, MVD, IBC, runway, TMK). Under conditions of a stage I reactor (100-110 ^° C), DMD and runway are partially or completely hydrolyzed, MBD and IBC are dehydrated, and then, under conditions of a stage II reactor (110-200 ^° C), hydrolyzed products break down to form isoprene. An increase in the yield of isoprene is achieved, in particular, due to the involvement of formaldehyde in the reaction with isobutylene formed during the decomposition of dioxane rings DMD and runway. However, despite the positive effect achieved by increasing the yield of isoprene, as a result of supplying the aqueous layer to the stage I reactor, its productivity (as well as the installation as a whole) is further reduced and, accordingly, energy costs for processing additional water are increased. Since the molar ratio of iso-C ₄ H ₈ / CH ₂ O in this prototype method is also 3-15: 1, the method is characterized by high costs for the return of unconverted isobutylene and an increased load on the isobutylene hydration reactor.

 The objective of the invention is to increase the productivity of contact equipment, reducing the capacity of the isobutylene hydration reactor by increasing the yield of TMC at the stage of synthesis of isoprene precursors, simplifying the process technology, maximizing the use of the equipment of the DMD synthesis shop and increasing the yield of isoprene.

The problem is solved during the process of the proposed method for the synthesis of isoprene from isobutylene and formaldehyde, carried out in two successive reactors, in the first of which acid-catalyzed condensation of the starting reagents is carried out to obtain intermediate products - precursors of isoprene, mainly DMD, and in the second - decomposition isoprene precursors in the presence of TMK. The process is carried out as follows. Isobutylene or C ₄ fraction containing not less than 70% isobutylene is contacted in the presence of any acid of mineral or organic origin (preferably phosphoric or oxalic) at a temperature of 85-105 ^o C with a formalin charge containing 20-30 wt.% Formaldehyde, and the molar ratio of iso-C ₄ H ₈ / CH ₂ O at the inlet to the I reactor is 0.8-1.1, i.e. more than in the traditional two-stage process (0.60-0.65), but significantly less than in the prototype method (3-15).

At the outlet of the I reactor, the reaction mass is sent without pre-processing to the II reactor, which is a reaction distillation column in which the products contained in it are decomposed at a temperature of 140-150 ^o C and a pressure of 3-5 atm, in the presence of an acid catalyst, water and TMC coming from the isobutylene hydration reactor, with continuous distillation of the isoprene and isobutylene in the vapor phase at the top of the column and lateral extraction of 60-65% of the water entering the column in the liquid phase. The molar ratio of TMK / DMD is 3-4/1 at the inlet of the II reactor. The distilled water, together with the organic compounds contained in it (including TMK), is again returned to the cube of the reaction-distillation column (reactor II), and the bottled product is sent to the evaporator, mixed with a fresh formalin mixture containing 40-42 wt. % of formaldehyde, and returned to the reactor I. Dilution of isoprene with water and TMK avoids the thermopolymerization of this diene, and in order to avoid the accumulation of heavy products in the cube, the exfoliated part of the bottom liquid is periodically taken out for burning.

Due to an increase in the ratio of iso-C ₄ H ₈ / CH ₂ O at the inlet of the DMD synthesis reactor from 0.60-0.65 to 0.8-1.1 and an increase in the content of isobutylene in the initial C ₄ fraction from 40-50 to 70-100%, an increase in the yield of TMC is achieved, compared with the traditional dioxane process, from 10-12 to 20-24 kg / 100 kg of DMD. The formation of additional amounts of TMC at the stage of synthesis of isoprene precursors reduces the load on the isobutylene hydration reactor.

At the same time, an increase in the ratio of iso-C ₄ H ₈ / CH ₂ O leads to a sharp increase in the yield of the Ministry of Internal Affairs, compared with the "dioxane" method. If in the traditional method the yield of MDB on the converted formaldehyde does not exceed tenths of a percent, then in the proposed method the corresponding value is 6-10%, which significantly softens the conditions for the production of isoprene in II reactor. An increase in the olefin / aldehyde ratio also leads to a qualitative change in the chemical composition of the runway fraction boiling above DMD and formed during the synthesis of isoprene precursors. Firstly, the fraction of MBD in the composition of this fraction sharply increases, secondly, the yield of relatively easily decomposable ethers formed by TMC with other alcohols increases, and thirdly, the molecular weight of the so-called heavy residue decreases, i.e. compounds boiling above dioxane alcohols (DS). If in the traditional process under the conditions of "starvation" of an olefin, the main part of the heavy residue is made up of polyformals ДС - RO (CH ₂ O) _n OR, where n ≥ 4-6, then in the proposed method polyformals are formed, in the molecules of which n ≤ 3. These Compounds (the so-called light oxal) are relatively easily hydrolyzed under conditions of reactor II and not only intermediate products containing 5 or 6 carbon atoms (MDB, IBC, DMD, etc.) are involved in the formation of isoprene, but also compounds with the number of C atoms equal to 7 or more. Formaldehyde released during the decomposition of these compounds and DMD reacts with isobutylene formed during the dehydration of TMC, giving additional amounts of isoprene. All these factors make it possible to process the entire reaction mass of reactor I in reactor II without prior separation. It is also significant that under the conditions of isoprene production, a significant part of one of the most difficult to separate impurities to isoprene, methylal, decomposes in reactor II, which makes it possible to exclude the purification unit of isoprene-rectified oxygen-containing compounds from the scheme.

 The conversion of formaldehyde and isobutylene in a DMD synthesis reactor is 95-97 and 75-78%, respectively. In the reaction-distillation column, the formaldehyde conversion approaches 100%, and the conversion of TMC does not exceed 40-50%. Thus, the cost of recycling isobutylene is reduced, since the amount of return olefin is sharply reduced, and TMK recycling does not require large energy costs and also reduces the load on the olefin hydration reactor.

Salient features of the proposed method for the production of isoprene from isobutylene and formaldehyde are:
- synthesis of isoprene precursors (mainly DMD) at the iso-C ₄ H ₈ / CH ₂ O molar ratio of 0.8-1.1, the concentration of isobutylene in the C ₄ fraction of 70-100%, and the content of CH ₂ O in the formalin charge 20-25 wt.% at a temperature of 85-105 ^o C and a pressure of 18-20 atm;
- decomposition to isoprene in the reactor of the second stage of the reaction mass obtained in the reactor of the first stage, including TMK, DMD, MVD and runway, without prior separation upon contact with the TMK coming from the isobutylene hydration reactor, with a molar ratio of TMK / DMD equal to 3-4 at a temperature of 140-155 ^o C and a pressure of 3-10 ATM, with the distillation of the resulting isoprene and 60-65% of the water introduced into the reactor II.

The advantages of the proposed method, compared with the prototype, are:
- higher productivity of contact equipment, since a 20-30% formalin solution reacts with isobutylene, and the molar ratio of iso-C ₄ H ₈ / CH ₂ O does not exceed 1.1;
- reducing the load on the isobutylene hydration reactor, since at the stage of isoprene precursor decomposition, TMC is involved in the reaction, which is formed in significant quantities when isobutylene is contacted with formalin, and the conversion of TMC in the stage II reactor does not exceed 40%;
- simplification of the technology for the production of isoprene due to the lack of operations for the processing of the reaction mass of the first stage and the purification technology of the monomer by reducing the content of methyl in it.

 Industrial applicability of the proposed method are illustrated by the following examples.

 Example 1

In a 1 liter autoclave equipped with electric heating and an agitator with a sealed electric drive, 400 g of formalin mixture containing 30% CH ₂ O and 3.5% H ₃ PO ₄ and 192 g of isobutylene (99.85% purity) are charged. The mass ratio of ISO-C ₄ H ₈ / CH ₂ O is 1.6 (molar 0.86). The contents of the autoclave are heated to 95 ^° C and, upon reaching this temperature, a stirrer is turned on (the time point of the test) and the reaction mass is kept for 2.5 hours at 95 ^° C and P = 20 atm. Then stirring is stopped, the autoclave is cooled to 50 ^° C. and samples are taken for analysis from both phases of the reaction liquid. The analysis is carried out by GLC, and the concentration of acid and formaldehyde is determined by potentiometric titration.

 In accordance with the results of the analysis in the reactor I obtained (g): DMD - 182.9; Ministry of Internal Affairs - 37.8; Runway - 14.9; TMK - 45.7. The conversion of formaldehyde and isobutylene is respectively 91.5 and 77.6%.

 The reaction mass from reactor I is pumped into the cube of reactor II, which is a three-section reaction-distillation column filled with a nozzle (Rashig rings), the efficiency of which corresponds to about 50 tons, and the volume of the cube is 1.35 l. 363 g of TMK and 91 g of water were preliminarily filled into the cube.

 Thus, taking into account the TMK supplied with the feed from reactor I, the molar ratio TMK / DMD is 3.1.

The contents of the cube are heated to 150 ^o C, the pressure is set at 4-5 atm, and a stream containing mainly 392 hydrocarbons is taken from the top of the column at a temperature of 140 ^o C in the vapor phase for 2.5 hours through a throttle valve g, and 420 g of liquid are taken through a lateral outlet mounted slightly below the steam sampling point. The steam stream is cooled with water and condensed in a toluene trap placed in a bath filled with ice and salt.

 According to GLC analysis, the conversion of DMD, MVD and runway, respectively, is 96.8; 99.2 and 85.8%. The conversion of TMK is 30%. 260 g of isoprene and 102 g of isobutylene were collected in a trap. The residual amount of formaldehyde is 0.74 g. The content of methylal in isoprene is 0.007%.

 Thus, the yield of isoprene calculated on converted formaldehyde is 94% with a conversion of the latter of 98.8%.

 420 g of liquid, distilled as a side stream, are separated in a separatory funnel into the aqueous (198 g) and organic (222 g) phases. According to GLC analysis, the composition of the organic phase is as follows (% wt.): TMK - 89.7; DMD - 2.1; Runway - 0.9; Ministry of Internal Affairs - 0.2, water - the rest.

230 g of liquid discharged from the cube of the reaction-distillation column are mixed with 300 g of fresh formalin mixture containing 40 wt.% Formaldehyde and transferred to a cube of a vacuum laboratory column with an efficiency of 15-20 tons With a residual pressure of 100-120 mm Hg 130 g of distillate are distilled off at a cube temperature of 75-80 ^o C, and 400 g of a cubic product containing 3.5% acid and 30% CH ₂ O are reloaded into the autoclave and the experiment is repeated from the beginning according to the same procedure with the only difference that 222 g of the organic layer of liquid discharged in the form of a side stream containing 89.7% TMK are loaded into the cube of the reaction-distillation column, and accordingly, the amount of TMK loaded into the cube is reduced.

 Example 2

The process is carried out analogously to example 1, however, 400 g of formalin mixture containing 30 wt.% Formaldehyde and 3.5 wt.% Phosphoric acid and 297 g of C ₄ fraction containing 75% of isobutylene are loaded into the autoclave as raw materials. The molar ratio of iso-C ₄ H ₈ / CH ₂ O is 0.98. The temperature in the reactor I 85 ^o C, a pressure of 18 atm. The yield of isoprene on the converted formaldehyde is 92.8% with a conversion of the latter of 97.8%.

 Example 3

The process is carried out analogously to example 1, however, as a raw material in the autoclave load 420 g of formalin mixture containing 20 wt.% Formaldehyde and 4 wt. % phosphoric acid, and 192 g of a C ₄ fraction containing 90% isobutylene. The molar ratio of iso-C ₄ H ₈ / CH ₂ O is 1.1. The temperature in the reactor is 105 ^o C, pressure 22 atm. The yield of isoprene on converted formaldehyde is 91.9% with a conversion of the latter of 99.1%.

 Example 4

The process is carried out analogously to example 3, however, the molar ratio TMK / DMD in reactor II is 4 and the temperature in the cube of reactor II is 140 ^° C. and the top is 130 ^° C. The pressure in reactor II is maintained at 3-4 atm. The yield of isoprene on the converted formaldehyde is 90.5% with a conversion of the latter of 98.9%.

Claims (1)

The method of producing isoprene from isobutylene and formaldehyde by liquid phase condensation in the presence of an aqueous solution of an acid catalyst at a temperature of 85-105 ^o C and a pressure of 16-22 atm, followed by decomposition of the resulting reaction mass, consisting mainly of 4,4-dimethyldioxane-1,3 and high-boiling by-products of its synthesis, in the presence of trimethyl carbinol at the temperature of 140-150 ^o C and a pressure of 3-5 atm with continuous distillation of the desired product in the vapor phase is distilled off and recycled liquid phase containing at least water, trimethyl arbinol and high boiling point by-products, at the decomposition stage, and a bottoms product containing an acid catalyst, after evaporation and mixing with a fresh formalin charge in a condensation reactor, characterized in that a formalin charge containing 20-30 wt.% formaldehyde is used as a source of formaldehyde and condensation is carried out at an isobutylene / formaldehyde molar ratio of 0.8-1.1, and decomposition at a trimethylcarbinol / 4.4-dimethyldioxane-1.3 molar ratio of 3-4 / 1.