CS270611B1 - A method for separating and purifying methanol and apparatus for carrying out the method - Google Patents

Abstract

The method of separating and purifying methanol, released during the preparation of higher methacrylic acid esters, consists in enriching the vapors emerging from the reaction mixture with methanol and, after condensation and cooling, bringing them into contact with water, while the lighter, separated hydrocarbon phase is continuously returned to the column and the heavier methanol phase is discharged periodically after the end of the batch operation and purified in three continuously operating rectification stages.

Description

The invention relates to a process for the separation and purification of methanol liberated from a reaction mixture in the preparation of higher methacrylic acid esters by batch reesterification of methyl methacrylate with mono- or polyhydric aliphatic, cycloaliphatic or aromatic alcohols having at least 2 carbon atoms in the molecule. t i

The process for the preparation of higher methacrylic acid esters by reesterification of methyl methacrylate is well known and its principles have been described, for example, in a review by J.K. Haken: Syntheiss of Acrylic Esters by Transesterification (Noyes Development Corp. USA). ·.

Upon re-esterification of methyl methacrylate with higher alcohols, methanol is released into the reaction mixture. Distillation of methanol affects the equilibrium composition of the reaction mixture so that almost all of the higher alcohol gradually reacts. The problem with reesterification is the efficient removal of methanol and its economic use, while eliminating aqueous and organic 'unusable wastes, the disposal of which would adversely affect the process and its ecology. . .

Most often, the reesterification is carried out with a larger excess of methyl methacrylate (molar ratio to the starting alcohol 2: 1 to 3: 1), which forms an azeotropic mixture with methanol containing up to 85% by weight of methanol. However, the boiling points of binary mixtures of methanol and methyl methacrylate are only slightly dependent on the composition at higher methanol concentrations, so for efficient removal of methanol from the reaction mixture highly efficient rectification columns of 15 to 50 theoretical plates are recommended, operating with a gradually increasing reflection ratio controlled by complex algorithms using highly sensitive sensors. The distillate obtained, which preferably contains at least 30% by weight of methyl methacrylate, is used either as fuel or, if the dislocation of the production plant allows, is returned to the extraction part of the methyl methacrylate production. Combustion, which requires a special furnace design and preferably the use of catalysts, or transport over longer distances, places an increased burden on the process.

The use of solvents in reesterification, which form heteroazeotropes with methanol, is also well known and patented. For example, the azeotropes of n-hexane and cyclohexane with methanol are separated into two layers after condensation, the upper one, containing only 5 to 8% methanol, can be returned directly to the reactor or to the rectification column as reflux, thus reducing solvent consumption. Even in this case, however, an efficient rectification column is necessary for the reesterification, because when the methanol concentration falls below a certain level, the composition of the distillate falls outside the heterogeneous range. The phase separation conditions can be improved by cooling the distillate to a temperature of 10 to 15 ° C. However, the energy consumption increases and at lower temperatures of the cooling medium, in some parts of the plant and especially at the end of the reesterification, when the methanol content in the distillate is low, the hydrocarbon solvent may solidify and clog the pipes.

Under suitable conditions, the lower phase of the withdrawal from the distillate separator may contain, for example, 92% by weight of methanol, 7.2% by weight of hexane and 0.8% by weight of methyl methacrylate. This mixture cannot be returned to the production of methyl methacrylate and only its incineration or a suitable combination of physico-chemical separation methods can be considered for its disposal. It is not possible to separate the individual components by simple fractional distillation. However, a simple method suitable for the industrial processing of this mixture has not yet been described in the literature.

An improvement of the process is the use of water for washing methanol from the distillate according to DE 2 317 226. The methanol is distilled off together with cyclohexane through a rectification column with an efficiency of 10 theoretical plates at a reflux ratio of 1: 1 and the distillate is placed at the bottom of a 50 cm high separator. layer of water. The methanol-free cyclohexane is returned to the top of the column. The methanol is introduced into the water until it reaches a concentration of more than 70% by weight and then

CS 270611 B1 is replaced by .clean water. However, the patent does not disclose a process for regenerating methanol from said aqueous solution, which in addition contains cyclohexane and methyl methacrylate as secondary components. It is an object of the present invention to simplify and streamline the process for separating and purifying methanol released from the reaction mixture in the preparation of higher methacrylic acid esters by batch reesterification of methyl methacrylate with mono- or polyhydric aliphatic, cycloaliphatic or aromatic alcohols having at least two carbon atoms in the molecule. is higher than 120 ° C, which does not form an azeotrope with any of the reactants, in the presence of hydrocarbon solvents of the η-, iso-, cyclo- and alkylcycloalkane type and mixtures thereof having 5 to 8 carbon atoms in the molecule, and to propose an economical method of its regeneration as and other components of the distillate, which would eliminate the formation of unusable waste. The essence of the process according to the invention is that the vapors emerging from the boiling mixture during the reesterification of methyl methacrylate are enriched with methanol in a rectification column with at least four theoretical stages and contacted with water after condensation and cooling, the lighter separated hydrocarbon phase returning continuously as reflux at the top of the column, while the heavier aqueous methanolic phase is discharged periodically, usually after the batch operation, and purified in three continuously operating rectification stages, the first stage separating the hydrocarbon solvent and methyl methacrylate in the distillate, the second stage separating the water containing less than 0.1% by weight, preferably less than 0.1% by weight, of organic impurities, and in the third stage methanol of a purity of more than 99.5% by weight is obtained. '

The distillate from the first rectification stage and at least part of the water from the bottom outlet of the second rectification stage are recycled and brought into contact with the condensed vapors in the subsequent reesterification operation.

The total amount of water which is contacted with the condensed methanol-rich vapors during the reesterification is chosen in a molar ratio to the equivalent of the starting higher alcohol of 0.45 to 1.8: 1, preferably 1 to 1.2: 1. Larger amounts of water increase the investment and operating costs of methanol regeneration. With a smaller amount of water, the solubility of methanol in the returned solvent increases and the efficiency of its separation from condensed vapors deteriorates.

The device for the separation and purification of methanol according to the invention is schematically drawn in Fig. 1. It consists of a cylindrical extraction vessel 7, a separation tank 2, a mixing tank 3 and three rectification packed or tray columns 4, 5 and 1, with condensers and reflux dividers. , the vessel 1 being connected to the reservoir 2 and to the feed tray at 8-10 / 10 of the column height £, the bottom of the column £ to the hopper 2 ^{and the} feed tray at 2 to 5/10 of the column height 5, the column head 5 to the feed tray of 2 to 6/10 of the height of column £ and the foot of column '£ with tank 2 ·

Cylindrical, extraction vessel 2> 4 diameter to height ratio 1: 5 to 1:20, its volume is filled from the bottom with 10 to 70% built-in, which helps to mix liquid streams, eg Raschig rings, perforated or screen partitions or circular-type partitions etc., the methanol-rich condensate inlet is located at the bottom of the vessel and the lighter phase outlet at the top of the vessel. ·

The rectification column E has efficiencies of up to 15 theoretical plates at a reflux ratio of up to 5: 1, column E to 20 theoretical plates at a reflux ratio of up to 3: 1 and a column of up to 25 theoretical plates at a reflux ratio of 5: 1. 1.

CS 270611 Bl

Any alcohols that do not distill with methanol and solvent can be used for the esterification. These can be aliphatic primary and secondary alcohols having n- and iso-alcohols having at least 5 carbon atoms in the molecule, cycloaliphatic and alkylcycloaliphatic alcohols, substituted arylalkonols or phenols. It is also possible to reesterify ethylene, propylene and butyl glycols, their oligomers and ethers, trimethylolpropane, pentaerythritol with other polyhydric alcohols, amino alcohols such as dimethyl or diethylaminoethanol, etc. It is not suitable to use ethanol in the reesterification and purification of waste methanol by the process butanols, which, due to their relatively high volatility and ability to form azeotropic mixtures with some solvents, contaminate waste methanol and prevent its regeneration. ·

Suitable solvents are, for example, dimethylpentanes, hexane, heptane, cyclohexane, methyl and dimethylcyclopentanes, petroleum ether and the like.

In the process for the preparation of higher methacrylic acid esters The high esterification column is not required for the esterification of methyl methacrylate according to the invention. It has been shown experimentally and numerically that a column with an efficiency of 4 maximum of eight theoretical plates will suffice. The cause is the composition of the reflux into the column, which is derived not from the top of the column but from the separator and contains a minimum amount of methanol and water. It has been shown that a column of the above efficiency is sufficient to prevent water from entering the reactor, which was extremely undesirable because it would irreversibly deactivate the catalyst, even at a concentration of up to 1%, which is more than 100 times its solubility. Therefore, water should not enter the reactor even during emergency modes. Thus, a reflux and distillate separator and the introduction of reflux into the column before its extraction is not necessary, as is suggested according to DE 2 317 226. The distillate does not need to be subcooled. Phase separation is reliable. The system has self-regulatory effects. Thus, the difficulty of measuring small changes in vapor temperatures at the top of the column and the demanding progressive optimal control of the reflux ratio are eliminated. Depending on the amount of distilled methanol or the density of its aqueous solution, the reesterification process can be easily monitored and the end of the reaction can be correctly estimated without additional analysis.

It has surprisingly been found that a very low layer of water, less than 5 cm in the laboratory and less than 30 cm on a model scale with a batch size of 150 kg, is sufficient for efficient methanol extraction when using a built-in phase separator. It is advantageous to recycle the distillate from the first rectification stage to the separator-extractor. The components of the distillate, methanol, methyl methacrylate and partly the hydrocarbon solvent are dissolved in the present water and do not strain the reesterification process.

Water is circulated as described between extractor 1 and the methanol recovery device. The amount of circulating water usually increases only slightly at the expense of the moisture content of the input and recycled raw materials. According to CS AD 259 978, this moisture should not exceed 0.3% by weight. Therefore, in the least favorable case, the increase in water should not exceed 5% per operation. If all the recovered water is returned to the extractor, it may contain up to several percent methanol. However, if part of the water is drained from the system, the methanol content must be as low as possible so that it can be degraded, for example, in biological treatment plants.

Equipment for methanol regeneration can also be used for regeneration of components from wastewater of other nodes of the technological process, eg water from catalyst separation and absorption from gases. The main effect of the invention is that the waste distillate during the reesterification of methyl methacrylate yields methanol, as required in high purity, suitable for any industrial use, and the other components, solvent, methyl methacrylate and water are returned to the process. If water is discharged from the process, the amount of impurities contained in it can be reduced to less than 0.01% by weight, so that biological treatment of these waters does not cause problems.

CS 270611 Bl

Other effects include simplifying the equipment and reesterification process and increasing its reliability. '

The invention is further elucidated on the basis of an exemplary embodiment, which, however, does not limit or exhaust its scope: i »Ϊ

A reesterification plant in which a batch of 15.0 kg of alpha 1620 (trade name for the industrial mixture of cetyl and stearyl alcohol manufactured by Condea, Haburg), 6.8 kg of methyl methacrylate, 12.0 kg of cyclohexane and a corresponding amount of basic catalyst was introduced. connected via a rectification column 10 cm in diameter and 200 cm high, packed with 10 mm diameter Raschig rings, with a condenser and an extraction vessel 7. The extraction vessel was 150 cm high, 100 mm in diameter. At a height of 100 mm from the bottom outlet of the extraction vessel, the false bottom was perforated and the layer of 200 mm Raschig rings sprinkled on it was also diameter. ru 10 mm. The condensed distillate was placed under the false bottom of the extraction vessel. The upper part of the extraction vessel was piped to the upper part of the rectification column. Before the start of the reesterification, 2.0 l of water were poured into the extraction vessel and the extraction vessel was filled up to the overflow with cyclohexane.

During the reesterification, the temperature at the top of the column slowly rose from the original 71 to 78 ° C. The reflux of cyclohexane from the extraction vessel to the column was approximately 33 l / h. At extraction time X, the aqueous methanolic solution was gradually concentrated. After 1 h, the level of the aqueous phase increased by 2.1, after 2 h to 1.5 l, after 4 h to 1 l. The reesterification was terminated after a total reaction time of 4 h 15 min. in the extraction vessel is no longer noticeable. Analysis of the aqueous phase revealed 65% by weight of methanol, 4% by weight of methyl methacrylate and 2% by weight of cyclohexane. '

The aqueous phase from the extraction vessel was lowered into the tank 2, in this case the PE bottle, and from there it was continuously injected by a metering pump at a rate of 1450 l / h onto the column head of the first rectification stage. The glass column had a diameter of 40 mm, a height of 1,100 mm and was provided with a tempered jacket which ensured its adiabatic function. The coke boiler was heated with a constant input. The function of the column was controlled by the feed rate, which was regulated so that the temperature at the head was in the range of 60 to 63 ° C. A distillate was obtained with the composition: 53.6% by weight, methanol, 6.2% by weight of water, 27.2% by weight. methyl methacrylate and 13% by weight of cyclohexane.

The bottom effluent from column χ was collected in tank 2- It contained, in addition to water and methanol, less than 0.005% by weight. methyl methacrylate and undetectable amounts of cyclohexane. It was continuously injected by pump into the next column 2, also made of glass, 40 mm in diameter and 1,300 mm high. The feed was introduced at a height of 500 mm from the bottom edge of the column. The column was operated at a constant reflux ratio of 2: 1. The distillate contained more than 90% by weight. and a heel draw of less than 0.005% by weight. . of methanol and less than 0.01 wt. methyl methacrylate. The distillate was immediately introduced into column 6 at a height of 780 mm from the bottom. This column also had a diameter of 400 mm. It was 2,000 mm high and operated at a constant reflux ratio of 2: 1. The heat input to the reboiler was controlled so that the temperature at the bottom of the column was kept between 70 and 75 ° C. The heel draw-off then contained less than 80% by weight of methanol. Distilled pure methanol without detectable amounts of methyl methacrylate and cyclohexane with a water content of up to 0.1 wt.

The columns were filled with 4 mm diameter crutch saddles. The distillate from column 4 and the water from the bottom of column 5 were poured into extraction vessel 2, 1.75 l of fresh water were added and the separator was made up to volume with recycled cyclohexane. After adding fresh and recycled raw materials to the reaction plant, another reesterification operation was started.

Claims (6)

SUMMARY OF THE INVENTION 1. A process for the separation and purification of methanol liberated from a reaction mixture in the preparation of higher methacrylic acid esters by batch reesterification of methyl methacrylate with alcohols boiling above 120 ° C in the presence of hydrocarbons. η-, iso-, cyclo- and alkylcycloalkane type solvents and mixtures thereof having 5 to 8 carbon atoms in the molecule and extracted. water, characterized in that the vapor mixture leaving the boiling mixture in the reesterification of methyl methacrylate enriched in the rectification of the column with methanol is contacted with water after condensation and cooling, the molar ratio of water to equivalent higher alcohol being chosen to be 0.45 to 1.8: 1, wherein the lighter separated hydrocarbon phase is continuously returned as reflux to the top of the rectification column, while the heavier aqueous metaholic phase is periodically discharged, preferably after the batch operation, and purified in three continuously operating atmospheric rectification stages, of which in In the first stage, a hydrocarbon solvent and methyl methacrylate are separated in the distillate, in the second stage, water containing less than 0.1% by weight, preferably less than 0.01% by weight of organic impurities is separated in the bottoms, and in the third stage. obtains methanol of purity higher than 99.5% by weight. 2. A process for the separation and purification of methanol according to claim 1, characterized in that the distillate from the first distillation stage and at least part of the bottom effluent water from the second distillation stage are recycled and contacted with condensed vapors during the subsequent reesterification operation. ' 3. The process for separating and purifying methanol according to items 1 and 2, characterized in that the total amount of water which is contacted with condensed methanol-rich vapors during reesterification is selected in a molar ratio to the equivalent of higher starting alcohol, preferably 1 to 1 , 2: 1. 4. Apparatus for the separation and purification of methanol according to items 1 to 3, consisting of a cylindrical extraction vessel, a separation tank, a mixing tank and three rectification packed or tray columns with condensers and reflux dividers, characterized in that the cylindrical extraction vessel (1) is connected to separating tank (2) and with feed tray at 8 to 10/10 column height (4), column foot (4) with mixing tank (3) and feed tray at 2 to 5/10 column height (5), column head (5) ) with an injection tray at a height of 2 to 6/10 of the height of the column (6) and the foot of the column (6) with the mixing tank (3). 5. Apparatus for the separation and purification of methanol according to claim 1, characterized in that the cylindrical extraction vessel (1) has a diameter to height ratio of 1: 5 to 1:20, its volume being filled from the bottom by 10 to 70% with a built-in to aid mixing liquids. currents, eg Raschig rings, perforated or mesh partitions of the ring-annulus type, etc., the inflow of methanol-rich condensate is located at the bottom of the vessel and the outflow of the lighter phase at the top of the vessel. 6. Apparatus for the separation and purification of methanol according to claim 4, characterized in that it consists of a column (4) with an efficiency of up to 15 theoretical plates, a column (5) of 20 theoretical plates and a column (6) of up to 25 theoretical plates.