CS223666B1 - A method of rectifying the separation of formaldehyde from the reaction solution from the production of pentaerythritol and the apparatus for carrying out this process - Google Patents

Abstract

Unreacted formaldehyde, both free and bound in labile compounds, is removed from the reaction solution from the production of pentaerythritol by rectification in a tray column equipped with a two-stage saturated steam generator with a pressure of 0.15 MPa. Rectification is carried out using a combined feed of direct steam under the lowest tray of the column and into a reflux system with circulation in a ratio of 10 to 25:1 to the discharged reaction solution freed from formaldehyde. The formaldehyde content after rectification is a maximum of 0.1% by weight. Rectification at 140 °C yields a distillate with a content of 18% by weight of formaldehyde. The two-stage steam generator system enables continuous production of low-pressure steam suitable for further use in the operation.

Description

223668 3

The present invention provides a method and apparatus for the efficient and economical separation of unreacted formaldehyde from a reaction solution resulting from the production of pentaerythritol.

Pentaerythritol is produced by reacting a formaldehyde with acetaldehyde at an alkaline environment. Typically, a considerable excess of formaldehyde is used over the stoichiometric consumption which has a beneficial effect on the increase in the yield of pentaerythritol based on the acetaldehyde feed. By the secondary reactions between the starting materials and the starting materials and their complexes, a diverse mixture of substances is also produced, depending on the technological conditions and the structure of the reaction system, of which the pentaerythritol and the higher pentaerythritol, the other forms of pentaerythritol and / or dipenta, are most prominent -erythritol and other substances formed by the most condensation reactions of formaldehyde by the valkalic medium.

In the early termination of the reaction by neutralization, the reaction mixture also contains the amount of unreacted formaldehyde which must be removed from the reaction mixture to a tiny residue, especially since formaldehyde already interferes very slightly with the crystallization of pentaerythritol at low concentrations and thus greatly reduces the yield of pentaerythritol in the separation process. The formaldehyde content of the reaction mixture, which is to be treated further, should not exceed 0.01% by weight, so that the crystallization takes place smoothly and with minimal peneterythritol losses. Another reason for removing the formaldehyde from the reaction mixture is to maximize its efficiency by regeneration, especially when using pentaerythritol synthesis technology requiring a high excess of formaldehyde compared to the stoichiometric consumption, while at the same time achieving the highest yields of pentaerythritol based on the acetaldehyde feed.

In order to remove unreacted formaldehyde from the reaction solution, various processes are used, the so-called thermal destruction by reaction at elevated temperature to the sulphurous substances and, on the other hand, the different rectification or distillation processes from the reaction solution by which the unreacted formaldehyde is recovered and At the same time, the favorable conditions modify the properties of the reaction solution in the direction of favorably reaching the maximum yields of pentaerythritol in subsequent separation operations.

In the known processes for the preparation of pentaerythritol, formaldehyde removal is carried out in the ordinary rectification of the reaction mixture at higher pressures than at atmospheric pressure. For example, according to U.S. Pat. No. 2,562,102, unreacted excess formaldehyde is removed from the mildly acidic reaction mixture by distillation with aqueous steam at ordinary or elevated pressure.

The formaldehyde content of the reaction mixture leaving the rectification is 0.1%. Similarly, the removal of formaldehyde by the acidified reaction mixture by distillation at a pressure of 1.7 to 3.1 at. U.S. Pat. No. 951,095 and U.S. Pat. at a pressure of 2.0 to 2.5 at. ° C, at a distillation temperature of about 130 ° C. Various modifications of the two-stage formaldehyde separation by two rectification columns operating at different pressures are described in U.S. Pat. 3 410 915, in V. Brit.pat. 1,240,254 and U.S. Pat. 1,910,057. pat. the procedure according to which a solution of pentaerythritol and calcium formate is concentrated in a vacuum of 20 torr and thereafter under pressure of 3 at. unreacted formaldehyde is distilled off.

Podlá frainc. pat. 1 349 534, unreacted formaldehyde is removed in the column by direct water vapor flashing at a temperature between 150 ° C and 200 ° C. The direct steam feed pressure is 15 at. To obtain a higher yield of pentaerythritol, it is important to adjust the pH of the reaction solution to a value of 5.5 to 4. Increasing the yield of pentaerythritol is attributed to the thermal decomposition of the so-called hemiformals. The formaldehyde removal of the reaction solution in the rectification column is solved and described in the following patents: V. Brit. pat. No. 824,442, NSR Pat. 1,158,950, V. Brit. pat. 936,083 and 989,243. A process for the preparation of di- to polyhydroxy compounds using the rectification or distillation of residual unreacted formaldehyde at 120-150 ° C from a reaction solution having a pH of less than 9, preferably less than 6, is described in MS. All the previously used methods for removing unreacted formaldehyde from a reaction mixture containing pentaerythritol and other substances resulting from the reaction of formaldehyde and acetaldehyde in an aqueous dilute alkaline medium have several disadvantages of low efficiency. and, in particular, high operating costs resulting from high steam consumption. Some of the proposed methods also have an adverse effect on the yield of ipentaerythritol and the low efficiency of formaldehyde re-generation due to the non-complete decomposition of labile pentaerythritol-containing formaldehyde-containing compounds, possibly due to other formaldehyde reactions under the prevailing conditions. in the separation devices used.

The disadvantages and drawbacks of the hitherto known methods of removing and regenerating unreacted formaldehyde result primarily from the non-ideal behavior of the water-formaldehyde system and the relatively high sensitivity of this process to adhering to the appropriate technological conditions 223666 5 with regard to structural parameters and constructional peculiarities of the zaria -dení. As a result, the reaction solution, which still contains a relatively high amount of formaldehyde of from 0.1 to 1.0% by weight, is obtained by known methods, thus increasing the solubility of pentaerythritol and making it difficult to isolate by crystallization or reducing the yield of pentaerythritol as a result of a non-thermal exposure of the reaction solution to high temperature, or a reduction in formaldehyde regeneration efficiency at a current high steam consumption of 1.5 MPa, even higher if the recrystallization is carried out at up to 200 ° C C.

These drawbacks are eliminated by a process of rectifying the removal of formaldehyde from the reaction solution from the production of pentaerythritol by the fraction of formaldehyde with acetaldehyde in the alkaline medium, which is carried out after neutralization of the reaction solution according to the invention so that for rectification, it is fed at a pressure of 0.4 to 0.61 MPa to the lower part of the rectification column, preferably below the lowest level of the rectification column, in a proportion of 0.5 to 15% of the amount fed to the column. injecting the reaction solution after neutralization, wherein a steam mixture is removed from the rectification column at a temperature between 125 ° C and 160 ° C, preferably between 135 ° C and 150 ° C, into a two-stage heat exchange system, wherein the first stage of the system is vaporized pressure in the range of 0.11 to 0.35 MPa, preferably 0.125 to 0.25 MPa, and in the second stage the temperature of the condensate mixture from the rectifier is reduced The column is brought to a temperature of 90 to 110 ° C, preferably 95 to 102 degrees Celsius, and after this cooling, it returns to the rectification system, the remainder being fed for further processing and the reaction solution free of substantial amounts of formaldehyde from the warning system. -hydu.

In order to carry out this process, a device consisting of a rectification column, heat exchangers and an armature is suitable, wherein the floor rectification column 2 provided by the feed line 1 is provided with a direct steam supply pipe and evaporation apparatus under the lowest tray 4 3, the inter-tubular space of which is connected to the heating steam supply line 17, the upper part of the stage-adjustment column 2 being connected by the distillate pipe 7 to the first heat exchanger 8, preferably of the tubular type, in which the second pipe is connected in series by the connecting pipe 11 of the tubular spaces. the heat exchanger 9 and the connecting pipe 10 in series connected casing spaces of the exchangers, the outlet 13 of the second heat exchanger being branched to the reflux conduit 14 connected to the stage rectification column 2 and to the exhaust duct 1S of the recovered water flow of formald In the case of the intermediate shell of the second heat exchanger 9, a supply line 12 of demineralized water and a low-pressure steam line 16 are connected to the intermediate space of the first exchanger 8.

The advancement of the novel process for separating formaldehyde from the reaction solution into the production of pentaerythritol and the new apparatus for its realization consists in the high utilization of heat in the process of rectifying the removal of unreacted formaldehyde while at the same time achieving high yields of peneterythritol calculated on the acetone feed. aldehyde and formaldehyde mainly due to the high efficiency of formaldehyde recovery. An important element is the achievement of a reasonable exposure time for the liquid and gaseous mixtures to be treated at high temperatures, sufficient for the necessary decomposition of the homogeneous material, without de-degradation of the desired products on the overheating walls of the exchangers. Another important element is the supply of direct steam to the bottom of the column and the correct volumetrometer to feed the feed with respect to the excess formaldehyde used against stoichiometric consumption, which has a significant positive impact, especially when the content needs to be reduced formaldehyde in the leaving reaction solution up to the desired value of 0.01% by weight, or more. Said requirement of deep removal of unreacted formaldehyde gives rise to additional peculiarities during the process, which appears to be due to the different behavior of the non-ideal system in the area of very low concentrations of one component. The present invention solves the problem of removing unreacted formaldehyde to the desired very low value by temporarily increasing the cost of manufacturing operation. The following examples illustrate both the process and the device in more detail, but do not limit the scope of the invention. Example 1

The rectification separation of unreacted formaldehyde from the reaction solution in the production of pentaerythritol is carried out schematically in the accompanying drawing. Auxiliary devices, pumps, fittings and control devices are not shown in the diagram.

The rectification separation of formaldehyde is carried out in a device consisting of a tray rectification column 2 equipped with valve trays with double load flasks provided with a feed inlet 1, a blasting device 3, a drain pipe 6 of a formaldehyde-free solution and a feed line 5 of direct steam under the bottom tray 4 of the tray.

The column 3 consists of two boilers equipped with a heating line 17 for heating steam to the interspace of the boilers. The formaldehyde-enriched steam is discharged from the head of the rectification column 2 by a conduit 7 of a distillate of the first heat exchanger 8 to produce high pressure steam and discharges the conduit 11 passes into the second heat exchanger 9, the condensate leaving the outlet 13 of the second heat exchanger, which branching 14 to the reflux flow and to the conduit 15 of the recovered aqueous formaldehyde solution which is exhausted use. This dewatered water is supplied to the second heat exchanger 9 via the de-mineralized water supply line 12 and is fed to the first heat exchanger 8 through the connecting pipe 10. use.

Especially the constructional parameters of the multi-stage column are as follows:

Column inside diameter: 1.2 m

Number of floors: 43

Overflow compartment length: 0.682 m Overflow compartment height (adjustable): 0.05 m

Overflow area: 0.06 m2

Number of flaps on deck: 110

Large Valve Diameter: 60 mm

Small Valve Diameter: 40 mm The vast majority of the heat required to be recycled is supplied in vertical cookers solved with a natural circulation circulating ratio of 10: 1.

The first heat exchanger 8 is a horizontal tubular heat exchanger, flooded to a maximum of 2/3 of the diameter, with a pipe pitch of 1.5 times as the outer diameter of the pipes. The second heat exchanger 9 is of similar construction but complete flooded. The column feed is a neutralized reaction solution in the production of pentaerythritol, adjusted to a pH of 4 to 4.5 in a quantity of 16,000 kg. h-1. It is fed to the column at 143 ° C, obtained by the previous heat exchange, with the withdrawal stream from the bottom of the column and heated in the steam preheater (to the diagram below). The weight fraction of formaldehyde in the feed is 0.076. The amount of vapors leaving the head of the column is on average 13,933 kg.h_1 The vapors from the bottom rectification column have a temperature of 140 DEG C. and contain 18% by weight of formaldehyde. The heat of condensation, when passing through the first heat exchanger 8 at a pressure of 392.3 kPa in the tube, results in an overall internal cross section of 0.0997 m 2.

The 130 ° C leaving condensate is transferred to a second heat exchanger 9 in which it is cooled to 100 ° C while the counter-flowing demineralized water is heated to 111 ° C. A condensate containing 12 wt. of formaldehyde, cooled to 100 ° C, is partially removed as a distillate in an average amount of 6667 kg. h_1 more use and second part in about 7333 kg. h_1 returns to rectification column as reflux. From the bottom, the solution in question comes out in practically the same amount as the feed feed, i.e. 16,000 kg. h1 with an average form aldehyde content of 0.015% by weight. From the first heat exchanger 8, the produced saturated water vapor leaves a pressure of 147.1 kPa. The average amount of steam produced is 1200 kg., H_1. Example 2

The rectification of formaldehyde from the reaction solution in the production of pentaerythritol is carried out under the same pressure and temperature conditions and on the same equipment as in Example 1, except that the direct steam supply to the bottom of the rectification column is completely stopped. As a result, the heat required for the rectification is supplied by steam boilers. In this process, the saturated steam generator operates in the same manner and practically as in Example 1, leaving a more concentrated aqueous solution of pentaerythritol at a total of 9333 kg. h-1, wherein the concentration of formaldehyde in this solution is greater than that of Example 1 and is on average 0.1714% by weight. The increased formaldehyde concentration in the effluent solution has a noticeable negative effect on the yield of pentaerythritol by further treatment of the solution by crystallization. Example 3

The rectification of formaldehyde from the reaction solution in the production of pentaerythritol is carried out under the same pressure and temperature conditions and on the same equipment as in Example 1, except that the supply to the boiling system and all the heat needed for rectification are complete. it is supplied by direct steam supply in an average amount of 13 933 kg. li1. In this process, the efficiency of the saturated vapor generator is practically the same as in the Example, the separation efficiency of the column is increased. The effective removal of free unreacted formaldehyde and formaldehyde, possibly resulting from the decomposition of the side compounds, has a positive effect on the increase in yield. pentaerythritol heavy extracts by further crystallization of the solution. However, the disadvantage of this method is the relatively

Claims (3)

223666, not a solution leaving the bottom of the column, which disproportionately impairs the subsequent separation operations. EXAMPLE 4 To compare the technical effect of the formalization of formaldehyde from the reaction solution in the production of pentaerythritol in a similar manner to that of Example 1, with the difference that the vapors leaving the top of the column have a temperature of 165 ° C. The recovery efficiency of the formaldehyde was reduced by an average of 8 to 12 percent. EXAMPLE 5 The rectification of formaldehyde from the reaction solution in the production of pentaerythritol is carried out under the same pressure and temperature conditions as in Example 1, wherein the apparatus is effected by draining only the first heat exchanger 6 to operate. . In such a modified device, there is a "ejection" of the boilers by the vapors generated, the boiling of the liquid only irregularly and the pressure fluctuation practically makes it impossible to use the produced saturated steam. OBJECT CLAIMS 1. A process for the rectification separation of formaldehyde from a reaction solution of a pentaerythritol by reacting formaldehyde with an acetaldehyde in an alkaline medium after non-neutralization of the reaction solution, comprising distilling the water vapor at elevated pressure, characterized in that a portion of the steam required for rectification is carried out at a pressure of 0.4 to 0.61 MPa to the bottom of the rectification column, preferably below the lowest level of the rectification column, in an amount corresponding to 0.5 to 15% of the amount to the feed column. of the reaction solution after neutralization, wherein a steam mixture is withdrawn from the rectification column at a temperature in the range of from 125 to 160 ° C, preferably in the range of from 135 to 150 ° C, into a two-stage heat exchange system, wherein the first stage of the system is pressurized From 0.11 to 0.35 MPa, preferably from 0.125 to 0.25 MPa, and in the second stage, the condensate temperature of the steam mixture is reduced by the rectification column. and a value of 90-110 ° C, preferably 95-102 ° C, and after cooling, a portion is returned to the rectification system, the remainder being led to a further treatment, and the reaction solution devoid of a substantial portion removed from the boiling system formaldehyde. 2. A process as claimed in claim 1, wherein the reaction solution is discharged from the depot of the boiling system, wherein a circulating ratio of 10 to 25: 1 is used for the natural circulation of the discharging system, with respect to the amount of reaction solution discharged. Apparatus for carrying out the invention according to the invention, comprising a rectification column, a heat exchanger, an armature, characterized in that the tray rectification column (2) provided with the supply inlet (1) is under the lowest floor (4) characterized by a direct line inlet duct (5) and a blasting device (3), which inter-tubular space is connected to the heating steam supply line (17), wherein the upper section of the rectification column (2) is connected by a discharge duct ( 7) a pair of distillate to a first heat exchanger (8), preferably of the tubular type, in which a second heat exchanger (9) and a connecting pipe (10) are connected in series by a connecting pipe (11) of tubular spaces in series, the enclosure spaces of these exchangers, the outlet (13) of the second heat exchanger being branched to the reflux conduit (14), connected to the charging rectification column (2) and to the outlet a conduit (15) of regenerated aqueous formaldehyde solution, a de-mineralized water conduit (12) and a low-pressure steam conduit (16) are connected to the intermediate space of the second heat exchanger (9) via a de-mineralized water conduit (12) and a subcoat space of the first exchanger (8) . 1 sheet of drawings