CS256743B1 - Method of formaldehyde rectification regulation from reaction solution during pentaerythritol production - Google Patents

Abstract

A new way of managing rectification is being addressed of formaldehyde from the reaction solution in the production of pentaerythritol. With constant heating steam flow the rectifier column is changing reflux ratio so that when the content increases of formaldehyde in the solution leaving the bottom of the column above the 0.1% setpoint wt. the reflux ratio decreases and vice versa, while reducing the formaldehyde content of the distillate below the desired value increases. while formaldehyde concentration in the distillate it ranges from 8.0 to 20% by weight; pressure in the column by keeping it constant value by controlling the steam pressure produced in a column condenser, within the range of 0.20 to 0.25 0.40 MPa.

Description

The present invention relates to a novel process for regulating the rectification of formaldehyde from a reaction solution in the production of pentaerythritol, consisting mainly of water, dissolved pentaerythritol, calcium formate and formaldehyde, as well as minor amounts of methanol, and some other products of formaldehyde reaction with acetaldehyde in alkaline and alkaline. formic acid.

The formaldehyde content of the reaction solution is variable. It depends on the formaldehyde: acetaldehyde molar ratio used and how this ratio is stabilized. Typically, the concentration of formaldehyde ranges from 3.0 to 6.0% by weight. sprayed into the device for separation by rectification.

The separation of formaldehyde from the reaction solution can only be effected efficiently by rectification at elevated pressure over atmospheric, since then a vapor phase richer in formaldehyde than liquid phase is obtained. By rectifying at a pressure below atmospheric pressure, a vapor phase is obtained that is less poorer in formaldehyde than an equilibrium liquid phase. The formaldehyde-water equilibrium is generally very complex, it depends very much on the working pressure, it is complicated by the formation of pseudoazeotropic mixtures and also by the content of other substances, e.g. methanol has an effect; for the course of rectification.

For this reason, the separation of formaldehyde from the reaction solution in the formaldehyde production is difficult both to stabilize important technological conditions and secondly to select and cause their change to achieve the desired degree of separation. The basic requirement of separation is that the content of formaldéhyde in the reaction solution leaving the bottom of the rectification column is not more than 0.1% by weight.

The higher formaldehyde content causes problems in the crystallization of pentaerythritol and reduces its yield. For this reason, regulation is required which maximally prevents the excess of formaldehyde from entering the residues leaving the bottom of the rectification column. The rectification must be carried out in such a way that the reaction solution containing formaldehyde is subjected as little as possible to the adverse effect of the high temperature at which the rectification must be carried out. This reduces the formation of syrupy substances and increases the efficiency of formaldehyde recovery.

These problems have not yet been satisfactorily resolved. Usually, the contents of the cookers were reduced to a minimum while maintaining the necessary heating surface, direct steam stripping instead of indirect heating, or a combination of both. These processes are described in some of the older and newer patents, e.g. in Fran. pat. 1 349 534 and Czechoslovak. AQ number 223 666.

These solutions proved to be insignificant in terms of formaldehyde losses, respectively. influencing the formation of syrupy substances because they concerned that part of the rectification column where, in normal operation, there is practically no longer any formaldehyde from which the syrupy substances may still be produced in the environment.

It has been shown that the course of rectification of formaldehyde from the reaction solution in the production of pentaerythritol on a given rectification plant is influenced mainly by the system of stabilization of important technological parameters, their automatic regulation and unconventional control of parameters affecting the separating ability of the depleting part of the rectification column.

As an orientation when considering this rectification problem, it should be noted that when rectifying formaldehyde from an aqueous formaldehyde solution, there is only a slight temperature gradient and is highly dependent on the working pressure. Measured deliberate data from several authors is not consistent and anomalies in the concentration and temperature profile of the column were observed when monitoring the course of rectification. More detailed information on these problems can be found in J. F. Walker's monograph "Formaldehyde", II. ed., New York 1953 in the chapter on rectification of formaldehyde solutions. The liquid-vapor equilibrium of the formaldehyde-water mixture at elevated pressure is also dealt with in a recent paper in the article in Jurtal Prikladnoj Chimii 50 (1), 39, 1977.

Obviously, a good result in such circumstances can only be achieved by an unconventional way of controlling the rectification process and automatic regulation, especially when there are high requirements for deep removal of formaldehyde from the liquid mixture while minimizing the exposure of the formaldehyde solution at high temperatures.

It has been shown that by rectifying formaldehyde from the reaction solution to produce pentaerythritol, the desired formaldehyde concentration of 0.1% w / w can be achieved. at the same time minimizing the formation of syrupy substances by a new way of controlling rectification, which consists in reducing the reflux ratio at a constant, automatic regulation maintained by the supply of heating steam to the rectifier column, with an increased formaldehyde content in residues leaving the rectification column; and the column temperatures are maintained by automatic regulation of the vapor pressure produced at. rectification column capacitors.

In this method of regulating the rectification of formaldehyde, a surprising effect is achieved in improving the separation capacity of the bottom of the column while reducing the reflux ratio. Furthermore, this method of regulating rectification effectively prevents the penetration of formaldehyde into the downstream nodes of pentaerythritol production and the formation of si256743 β.

is less in comparison with other methods of regulating rectification. Explaining this positive effect is not unambiguous. It may be related to the preservation of the boiling pattern at the bottom of the column, but may also result from the abnormal behavior of aqueous formaldehyde solutions in rectification and from shortening the exposure time of the high temperature to a solution with higher formaldehyde concentration.

The following are examples that illustrate, but do not limit, the boundaries of a new way of managing rectification.

Example 1

The rectification of formaldehyde from the reaction solution in the preparation of pentaerythritol is carried out in the apparatus shown in FIG.

1. The schematic diagram of the apparatus is simplified and does not depict apparatus, interconnections, and control circuits not directly related to the subject matter of the application.

The nodal apparatus is a rectification column 1, equipped with a reboiler 7, a condenser 15 and a feed preheater 3. The neutralized reaction solution is fed via line 2 to the feed preheater 5 and screened via feed line 8 to the rectification column 1. The feed rate is kept constant. The feed preheater is provided with a steam supply 4. The cooker 7 is provided with a steam supply 9, the flow of which is maintained at a constant value corresponding to the feed to the column by the steam flow control circuit 8. The cooker 7 is of the circulating type connected to the exhaust duct 10. The formaldehyde-free pentaerythritol solution is discharged from the column for further processing through the exhaust duct 11.

The vapors from the column 1 pass through a steam conduit 13 to a condenser 15 provided with a condensate conduit 16, a distillate withdrawal conduit 19 and a steam withdrawal equipped with a pressure control circuit 17. The distillate flows into the intermediate buffer 20 from where regenerate is collected in an amount corresponding to the formaldehyde introduced. in the spray, leads to reuse.

Reflux is pumped through pump 12 via reflux line 14. The amount of reflux is maintained by a remotely controlled reflux flow control circuit 18, adjusted manually or automatically according to the formaldehyde content at the bottom of the column. Automatic adjustment of the reflux flow rate is not shown in the diagram.

In this example, a reaction solution neutralized with formic acid to an average pH of 5.68 is continuously fed to the rectification column 1. The reaction solution to be fed contains unreacted formaldehyde in addition to the reaction fumes. Its amount depends on the excess formaldehyde used versus the stoichiometric ratio of the reaction of formaldehyde with acetaldehyde to peritaerythritol. The average formaldehyde content of this example was 4.41% by weight. The reaction solution contained, on average, 7.6 wt. % pentaerythritol including diperitaerythritol, 1.5 wt. % methanol, 5.5 wt. % dissolved calcium formate, 0.35 wt. % calcium hydroxide and 0.20 wt. syrups. The balance was water. The reaction solution of the above composition was fed to the rectification column 1 after preheating in the feed preheater 5 to 85 ° C. The temperature in the digester 7 was 152 ° C, the temperature gradient over the column height was 2 ° C. The column pressure was controlled by keeping the vapor pressure in the condenser 15 at 0.32 MPa. The rectification of formaldehyde in this example was controlled so that at an automatically maintained constant feed rate of 15,500 kg. h ¹ and the automatic maintenance of a constant flow of steam at a pressure of 1.0 MPa · the reboiler 7, the change of the reflux ratio maintained the concentration of formaldehyde in the distillate at an average of 14.0% by weight. in the range of +1.0% / wt. abs. In this rectification control method, the reflux ratio varied between 1: 1 and 1.5: 1. The formaldehyde content of the solution leaving the bottom of the column was between 0.04 and 0.06% by weight. The required maximum formaldehyde content in this solution is 0.1% by weight.

In this method of rectification control, a stable run of the column was achieved while maintaining the required parameters. The formaldehyde content of the solution leaving the bottom of the column was always below the maximum permissible level and the syrup content was the same as in the feed. As a result, virtually complete recovery of unreacted formaldehyde was achieved. An important advantage was to maintain the concentration of formaldehyde at a stable value, thereby simplifying the dosing of the raw materials when reused in the reaction system.

Example 2

The apparatus and procedure are the same as in Example 1 except for the concentration of formaldehyde in the distillate, which in this example gradually increased to 21% after a 3-hour run, increasing the reflux ratio to 2.3: 1. Under the other same conditions, the average formaldehyde content of the solution leaving the bottom of the column increased to 0.105% by weight, which was above the permissible level of 0.1% by weight. For these reasons, the reflux ratio was reduced to 1.8, gradually reducing the concentration of formaldehyde in the distillate to 13-15% by weight. and within 2 h the formaldehyde concentration in the digester decreased to 0.08 wt.

Claims (1)

Management method. rectification of formaldehyde from. the reaction solution at _t peptaerytritolu production, .tým characterized in that, at a constant flow of steam vykurova.eej dq. The reflux ratio of the rectification column can be varied such that ppi increases in the content of farmidehyde in the solution coming from the column. the formaldehyde content of the distillate below the value given below. the reflux ratio increases with the concentration of formaldehyde in the distillate being in the range of 8.0 to 20.0% hpiPt ·, preferably in the range of 12.0 to 16.0% by weight. at a column pressure maintained constant by the vapor pressure produced in the column condenser at a constant value in the range of 0.20 to 0.40 MPa.