WO2007090768A1 - Process for obtaining vinyl compounds by evaporation - Google Patents

Abstract

A process for obtaining vinyl compounds from a viscous solution comprising vinyl compounds, by feeding the solution to a preheater and heating it, then decompressing it through a downstream pressure-retaining device and feeding the resulting biphasic mixture to a helical tube evaporator and reducing its content of vinyl compounds there by evaporating the liquid phase and drawing off a gaseous product stream with an elevated concentration of vinyl compounds.

Description

Process for recovering vinyl compounds by evaporation

description

The present invention relates to a process for recovering vinyl compounds from a viscous solution containing vinyl compounds.

Vinyl compounds, such as vinyl hydroxybutyl ether, vinyl imidazole, vinyl caprolactam, are important industrial chemicals and are used in the manufacture of numerous intermediates and consumables. Among vinyl compounds here are mainly intermediate products, which have a great importance as monomers for polymers, to understand. The vinyl group undergoes the usual addition reactions of aliphatic double bonds, such as addition of HCl, HCN and halogens. General preparation methods for vinyl compounds are the reaction of acetylene with compounds containing active hydrogen.

XH + HC≡CH ^■ * XCH = CH ₂

X can be, for example, Cl, RO, RS, RCOO, R ₂ N-.

The preparation of vinyl compounds is described, for example, in Ullmann's Encyclopedia of Industrial Chemistry [Fifth Edition, Volume A27, 435-442]. In the production of e.g. Vinyl ethers acetylene is reacted with alcohols in the presence of strongly basic alkalis (REPPE vinylation reaction) in the liquid phase. In a reaction tower, the KOH-added alcohol is supplied from above and the gas mixture consisting of acetylene and nitrogen is fed from below. The unused in the reaction acetylene is returned to the reaction tower (cycle gas) and supplemented by pure acetylene. The withdrawn from the reaction tower reaction output, in addition to the desired product vinyl ether (40 - 98 wt .-%), the unreacted alcohol (1 - 50 wt .-%) and secondary components formed (1 - 20 wt .-%) , Is freed of dissolved acetylene in a degasser.

Subsequently, the reaction discharge is usually carried out in continuously operated evaporators, e.g. Thin-film evaporators separated. The liquid withdrawn residue containing the high boilers and still residues of desired product is usually disposed of thermally. The distillate, which contains low boilers and the main amount of the desired product is further processed in a further distillation device.

The workup of the reaction effluent may be due to solids formation and / or polymerization and the high viscosity of the residue (1000 to 100000 mPa s, viscosity determination, for example, with capillary or falling ball viscometer) of the separated the residue can be carried out usually only by the addition of flow aids such as white oils in evaporators such as thin-film evaporators.

However, the method proves to be disadvantageous, since the use of thin-film evaporators procedurally elaborate. Furthermore, it is associated with very high investment costs and high operating costs due to the necessary maintenance and repair work. The addition of flow aids necessary in this process causes further, ongoing operating costs.

The object of the present invention was therefore to find a process for the evaporation of viscous vinyl compounds, which the vinyl compounds while avoiding the o.g. Disadvantages of equal or improved quality in terms of color, color stability, odor and purity provides. In addition, losses of value product due to residual contents in the sump discharge had to be minimized.

Accordingly, a process for the recovery of vinyl compounds from a viscous solution containing vinyl compounds was found, which is characterized in that the solution is fed to a preheater and heated, then expanded via a downstream pressure-holding device and fed the resulting two-phase mixture to a helical tube evaporator and there by evaporation reduces the liquid phase in its content of vinyl compounds and dissipates a gaseous product stream having an increased concentration of vinyl compounds.

It has surprisingly been found that the inventive method allows the use of a technically relatively simple helical tube evaporator for separating the vinyl compounds from the viscous residue, wherein moreover the use of additional flow aids can be avoided. This would not have been expected by the person skilled in the present invention with regard to the substance system and the product conditions associated therewith, since in addition to its high viscosity of the residue (depending on the vinyl compound between 1000 and 100,000 mPa s), the system also tends to polymerize. A helical tube evaporator - certainly not without the use of flow aids - would not have been considered by the expert here.

Helical tube evaporators are well known, they are described for example in US 3,550,669. Here, an evaporator is described in which the mechanical force to keep the heat exchange surface is not effected by rotating internals, but by flow forces. This evaporator consists of only a single, coiled tube, which is heated from the outside. This monotube evaporator is now operated so that the solution or suspension is superheated under pressure fed into the apparatus, so that already At the beginning of the apparatus evaporate a part of the volatile components of the solution. This steam takes over the function of transporting the viscous solution or suspension through the apparatus and ensures that the heat exchanger surface is kept clear.

The method according to the invention will be described in more detail below by way of example with reference to the FIGURE.

The solution to be worked up ("reaction effluent") with a concentration of vinyl compounds of 50-98% by weight is fed and heated via line (1) to a preheater (2) operated with heat transfer oil Pressure usually between 80 ° C to 250 ° C. The heated solution is removed from the preheater via line (3) .The pressure in the preheater is adjusted by a downstream pressure-maintaining valve (4) so that no evaporation of the solution takes place at any point in the preheater Conventional types of apparatus such as tube bundle apparatus, plate heat exchangers, spiral heat exchangers, etc. are used as preheaters The mixture overheated to pressure in the downstream system components is expanded by the pressure-maintaining valve. 5) by appropriate choice of geometry , the total quantity flow and the gas portion after the expansion, a wavy film flow is set in the tube.

At low gas velocity, the viscous liquid flows in the form of a stream on the tubesheet. Through the tube helix, the liquid accumulates and the dynamic pressure generated thereby eventually causes the pent-up mass rolls with increased flow velocity due to the evaporation in the form of a mass wave on the base layer, the tips tear off, temporarily entrained in the gas and then on the tube walls are deposited, whereby a thin film in the region of the pipe wall is formed (ring flow). During this movement, gas is continuously incorporated into the viscous liquid, so that the product continues to foam up over the run length. With increasing speed, ie with increasing vapor content as a result of the evaporation, the liquid is distributed more evenly over the circumference of the pipe. Due to the high vapor velocities, large rolling waves and small ripples form at the phase interface, which cause strong turbulence and mixing in the film. This is an intense heat and Stoffau jam seh created that allows the good separation performance of the apparatus. The transport values are ten times higher than in a smooth laminar film flow and come in the order of magnitude of the mechanical film apparatus. By adding external vapor or inert gas, the partial pressure of the vaporizable component can be lowered and the gas velocity can be increased.

Furthermore, high shear stresses exist in the area of the wall, so that the build-up of solids on the heated walls is effectively avoided. The product heating, depending on the vinyl compound and pressure typically between 100 ° C to 270 ⁰ C.

The to be reached evaporation rate - and thus the concentration of the desired product in the bottom product - is determined by the choice of the heating temperature and the pressure in the separator and can be determined for example by experiments. By evaporation rate is meant the ratio of distillate quantity and feed quantity.

The heating of the helical tube evaporator may e.g. by condensing steam or by means of a tempered oil circuit. An electric heating is possible.

In a downstream vapor separator (6), liquid and gas are separated from each other. The vapor stream may be in conventional capacitors e.g. Rohrbündelapparaten or Quenchkondensatoren - condensed. The resulting condensates, which in addition to low boilers contain the desired product (vinyl compounds) can be worked up in conventional distillation equipment or recycled directly. The concentration of the desired product (vinyl compound) in the condensate is usually between 50 and 99.9% by weight.

The bottom stream from the Brüdenabscheider contains substantially the high boilers formed in the reaction and the content of product value is here depending on the driving at less than 70 wt .-% of vinyl compounds, preferably less than 20 wt .-%, more preferably less than 15 wt .-% vinyl compounds.

The pressure in the vapor space within the vapor separator is set at 1 to 10 ⁴ mbar, preferably 1 to 10 ³ mbar, particularly preferably 1 to 100 mbar. In a preferred embodiment, parts of the bottom product are passed from the vapor separator back into the preheater. This is done to re-evaporate the product of value that has not yet evaporated. The return of the bottom product from the Brüdenabscheider to the preheater is 1 to 80 wt .-%, preferably 10 to 50 wt .-% recycled.

By means of line (7) can be added behind the expansion valve, a stripping gas such as nitrogen. This can be recommended in order to achieve the desired flow form in the helical tube evaporator or to increase the residence time of the solution in the helical tube. Adjust tube evaporator or stripping to remove light residues from the solution. The Strippgasmenge to Wendelrohrverdampfer is 0 to 50 wt .-% based on the total feed stream (product + stripping gas), preferably 0 to 20 wt .-% fed.

In general, the residence time can be specified by the flow velocity or the geometry of the helical tube evaporator (diameter and length). The residence time in the helical tube evaporator and the associated piping system is - depending on the polymerization tendency of the mixture to be worked up - advantageously limited to 0.5 to 10 minutes, preferably 0.5 to 2 minutes. As a result, the formation of polymers and / or solid failure is reduced.

The process is usually carried out continuously, but in principle it is also possible to carry out the separation in batches.

By appropriate choice of the operating point of the helical tube evaporator very high area-specific performance with low residence times are achieved. Due to the low residence time of the solution at higher temperatures, the formation of polymers due to excessive thermal stress is effectively prevented, so that the concentrated solutions, contrary to previous assumptions for a relatively long time homogeneous, i. without the formation of solid phases, remain and even without the addition of flow aids are still pumpable.

In addition to the operation in a single pass, it is also possible to mix parts of the bottom product of the feed stream to the helical tube evaporator. In the case of particularly high requirements, additional flow auxiliaries, such as white oils, can be added to the reaction effluent.

It may be particularly advisable to tap the spiral tube inside and / or outside. This refers to the attachment of ribs on the inside or outside of the helical tube, this has the advantage that thereby the performance of the helical tube is improved. This improvement is achieved both by providing a larger heat transfer area and by generating additional turbulence. Furthermore, one can equip the spiral tube inside completely or partially with wire mesh. This is understood as the introduction of wire mesh in the helical tube and it has the advantage that thereby the heat and mass transfer is improved.

The process according to the invention offers a process-technically simple solution for the production of vinyl compounds. Here, the equipment cost is greatly reduced and the inventive method has long service lives and low operating costs. Furthermore, the use of flow aids can be avoided. Examples

Example 1 (not according to the invention):

Concentration of a solution containing 97 wt .-% vinylimidazole (desired product), in a thin-film evaporator. The plant was equipped with an evaporator tube 51 ^* 350 mm (heat transfer area about 0.05 m ² ). The pressure in the vapor chamber was set to 8 mbar. The feed quantity was 6 kg / h of product and 0.5 kg / h of white oil, which was used as flow aid. The heating temperature was about 150 ° C. The evaporation rate achieved was about 86%, so that there was a lump loss of value product in the bottom of 14% (based on the amount of desired product in the feed). The solution could be worked up without problems in the thin-film evaporator.

Example 2 (not according to the invention):

Concentration of a solution containing 97 wt .-% vinylimidazole (desired product), in a thin-film evaporator. The plant was equipped with an evaporator tube 51 ^* 350 mm (heat transfer area about 0.05 m ² ). The pressure in the vapor chamber was set to 8 mbar. The feed quantity was 6 kg / h of product. The amount of flow aid (white oil) was reduced from 0.5 kg / h to 0.13 kg / h compared to Example 1.

The reduction in the amount of flow aid - with otherwise unchanged operating conditions - resulted in rapid and irreversible occupation of the heating surfaces due to the onset of solid state polymerization and / or polymerization. This led to damming of the liquid in the thin-film evaporator and to a no longer pumpable sump discharge, so that the system could not be operated stationary.

Example 3 (according to the invention):

Concentration of a solution containing about 97 wt .-% vinylimidazole (desired product), in a helical tube evaporator system according to Figure 1. The system was equipped with a glass coil (I = 6 m, inner diameter 7 mm, heat transfer surface about 0.19 m ² ). The pressure in the vapor chamber was set to 8 mbar. The feed amount was 6 kg / h of product, no flow aid was added. The heating temperature in the preheater was 170 ° C, in the spiral tube 176 ° C. The evaporation rate achieved was 86%, which resulted in a lump loss of value product in the bottom of 14% (based on the amount of desired product in the feed). The concentrated solution could be pumped out of the sump space without difficulty. A solid formation or polymerization of the desired product was not observed.

Claims

claims 1. A process for the production of vinyl compounds from a vinyl compounds containing viscous solution, characterized in that one feeds the solution to a preheater and heated, then via a downstream Relaxed pressure device and the resulting two-phase mixture fed to a helical tube evaporator where it reduces the liquid phase in its content of vinyl compounds by evaporation and dissipates a gaseous product stream having an increased concentration of vinyl compounds. 2. The method according to claim 1, characterized in that reduced in a single pass through the helical tube evaporator, the proportion of vinyl compounds in the withdrawn liquid phase to a content less than 20 wt .-%. 3. The method according to claim 1, characterized in that reduced in a single pass through the helical tube evaporator, the proportion of vinyl compounds in the withdrawn liquid phase to a content less than 15 wt .-%. 4. Process according to Claims 1 to 3, characterized in that a part of the helical tube evaporator removed, liquid stream again zuleitet the helical tube evaporator for further purification. 5. The method according to claims 1 to 4, characterized in that one uses in the helical tube evaporator valves or throttles for more intensive mixing of the liquid phase. 6. Process according to Claims 1 to 5, characterized in that a stripping gas is added behind the expansion device. 7. Process according to Claims 1 to 6, characterized in that an internally and / or externally ribbed tube is used in the helical tube evaporator. 8. Process according to Claims 1 to 7, characterized in that the spiral tube evaporator is equipped internally with wire mesh. 9. Process according to Claims 1 to 8, characterized in that the Inlet stream stabilizers or flow aids such as white oils added. 10. Process according to Claims 1 to 9, characterized in that the gaseous stream discharged from the helical tube evaporator is fed to a downstream vapor separator and the vapor separator is operated at a pressure of 1 to 100 mbar. 1 1. A process according to claims 1 to 9, characterized in that the condensed out of the helical tube evaporator gaseous stream in a quench condensed partially or completely.