PL215482B1 - Method of removing formic acid from trioxane - Google Patents

Abstract

The method for removing formic acid from trioxane is characterized by the addition of a macroporous weakly basic anion exchanger with a polystyrene matrix or a microporous anion exchanger with an acrylic matrix in the hydroxyl form in an amount of 0.5 - 5 to the melted trioxane at a temperature of 50 - 70°C, 0% by weight relative to the weight of trioxane. Then it is stirred for up to 120 minutes, preferably up to 60 minutes in the case of an anion exchanger with a polystyrene matrix, and up to 10 minutes in the case of an anion exchanger with an acrylic matrix, and then the anion exchanger is filtered off from the purified trioxane.

Description

The subject of the invention is a method for removing formic acid from trioxane using an ion exchange process using anion exchangers.

Polyacetals belong to a group of construction materials widely used in many industries, especially in the automotive, electronics, electrical engineering, machinery industry and others. This widespread use of polyacetals is the result of very good mechanical, thermal, electrical and chemical properties. Polyacetals with a linear structure have chains composed of repeating -CH2O- units. hence their synonymous name - polyoxymethylenes (POM).

One of the basic raw materials for obtaining POM is the cyclic formaldehyde trimer 1,3,5-trioxane. The polymerization of trioxane proceeds according to the cationic mechanism in the presence of a catalyst. BF3 and its complexes are used on an industrial scale. The polymerization process is carried out in the molten monomer mass, continuously, in a screw extruder type reactor.

Trioxane is produced on an industrial scale by heating a concentrated aqueous solution of formaldehyde in the presence of a catalyst. This process also produces significant amounts of formic acid, which distills simultaneously with trioxane, water and formaldehyde, as well as volatile by-products of the trioxane synthesis reaction.

Trioxane, after initial concentration by distillation to a concentration of approximately 70% by weight, is separated from the distillate by crystallization and filtration or by extraction with organic solvents immiscible with water. However, the purity of the trioxane thus obtained, containing 97-98 wt. trioxane, does not allow it to be used for copolymerization on polyacetals. As stated in the Polish patent description 160,837, the sum of impurities in trioxane used for the production of POM cannot exceed 0.005% by weight, because only then is it possible to obtain a polymer with a molecular weight above 100,000. The discussed patent description presents a method of obtaining pure trioxane by distillation from using a system of two distillation columns.

From literature reports, J.Stasiński, P.Krzyżanowski, J.Cieślak; Polymers, 42, 1997, No. 5, 312-315, it follows that impurities active in the chain transfer reaction, such as: H2O, HCOOH, CH2OH, present in the environment of cationic polymerization of trioxane, terminate the material growth of the macromolecule, reduce the molecular weight of the polymer and cause the formation of thermally unstable hemiacetals. end groups.

The Polish patent description No. 133,188 mentions that the copolymerization is carried out using trioxane purified by distillation or otherwise, and the degree of purity of trioxane should be at least 99.5%, preferably above 99.95%, and the content of water or other substances with an active proton should be below 300 ppm, preferably below 50 ppm.

The copolymerization process is very sensitive to any fluctuations in both the purity of raw materials and technological parameters. Even slight changes in the process parameters cause serious disruptions in the polymerization process.

The quality requirements for pure trioxane intended for polymerization are as follows:

- H2O content max. 50 ppm,

- HCHO content max. 20 ppm,

- HCOOH content max. 10ppm.

There are known methods of separating trioxane and purifying it from impurities by extraction using organic solvents, distillation and rectification or combinations of these methods, as well as various variants of crystallization. An effective way to remove formic acid and formaldehyde from trioxane separated during the crystallization process is a chemical process involving the use of extraction with concentrated sodium lye. The following chemical reactions take place in this process:

HCOOH + NaOH HCOONa + H ₂ O

2HCHO + NaOH HCOONa + CH ₃ OH

There is a method known from the Polish patent description No. 147,690 for purifying crude trioxane, isolated by crystallization from formaldehyde polymerization distillates, by extraction with a concentrated aqueous solution of NaOH or KOH, before which the crude trioxane is distilled to

PL 215 482 B1 reducing the water content to 1-3%. This water content in trioxane does not dilute the lye solution, which can be returned to the process without concentration. The patent description in question does not mention the presence and removal of formic acid.

Patent description 199,904 presents a method of purifying trioxane from liquid multi-component mixtures from impurities such as formaldehyde, methanol, methylases, dioxolane and others, by vacuum crystallization. The solution consists in the fact that a solution containing at least 40% of trioxane is subjected to a vacuum crystallization process at a pressure of 10-89 hPa at a temperature of 15 - 55°C, after which the obtained crystal is filtered and the resulting post-crystalline liquor is optionally combined with the stream of the post-reaction mixture directed to crystallization.

In addition to disturbing the polymerization process and reducing the purity of the final product, formic acid causes corrosion of technological equipment and devices. Due to its strong corrosivity, it is advisable to remove HCOOH from raw trioxane before further purification by distillation, as this avoids corrosion damage to the equipment and especially to the distillation columns.

The essence of the invention is that a macroporous weakly basic anion exchanger with a polystyrene matrix or a microporous anion exchanger with an acrylic matrix in the hydroxyl form is added under static conditions to the melted trioxane at a temperature of 50 - 70°C in an amount of 0.5 - 5.0% by weight. relative to the mass of trioxane and stirred for up to 120 minutes, preferably in the case of an anion exchanger with a polystyrene matrix, up to 60 minutes and in the case of an anion exchanger with an acrylic matrix, up to 10 minutes, and then the anion exchanger is filtered off from the purified trioxane.

Surprisingly, it turned out that the molten trioxane environment containing 90 - 95% trioxane, 4 - 9% water and volatile organic pollutants in an amount of up to 1%, including formic acid in an amount of 0.01 - 0.3%, can be removed using weakly alkaline anion exchangers capture formic acid without changing the composition of the remaining compounds.

Table 1 shows the results of chromatographic analyzes of trioxane solutions before and after ion exchange. Ion exchange was carried out at 55°C because this temperature allowed trioxane to be kept in a liquid state and did not cause its crystallization. At lower temperatures, the trioxane solution initially became cloudy and then crystallized. Apart from the content of formic acid, no noticeable changes in the content of volatile organic pollutants were recorded.

The presence of volatile organic impurities in the molten trioxane does not affect the exchange capacity of ammonites.

In order to quantitatively determine the ability to purify trioxane from formic acid of the used ion exchangers, a value was introduced called the HCOOH removal rate a _so , calculated according to the equation:

a _so = · 100% ^c in which:

a _so - HCOOH removal rate [% m/m],

C0 - initial concentration of HCOOH in the trioxane solution [% m/m],

Cx - concentration of HCOOH in the solution after ion exchange [% m/m].

T able 1

The influence of the contact time of an anion exchanger with molten trioxane on the degree of HCOOH removal using a microporous anion exchanger with a polystyrene matrix (Purolite A103S)

Contact time TOX [%] CH2O [%] CH3OH [%] H2O [%] HCOOH [%] Degree of purification TOX ^so [%] 1 2 3 4 5 6 7 0 min 96.52 0.28 0.18 3.02 0.0205 0.0 1 min 96.63 0.29 0.17 2.91 0.0190 7.4 5 min 96.40 0.28 0.17 3.15 0.0149 27.3 10 min 96.12 0.30 0.18 3.40 0.0137 33.3 15 min 96.50 0.29 0.18 3.03 0.0119 42.1

PL 215 482 B1 cont. table 1

1 2 3 4 5 6 7 20 min 96.33 0.27 0.18 3.22 0.0101 50.9 30 minutes 96.25 0.28 0.18 3.29 0.0081 60.4 45 min 96.46 0.29 0.19 3.06 0.0050 75.7 60 min 96.26 0.29 0.19 3.26 0.0019 90.9 120 min 96.16 0.28 0.18 3.38 0.0000 100.0 180 min 96.44 0.29 0.19 3.08 0.0000 100.0 240 min 96.26 0.27 0.18 3.29 0.0000 100.0 360 min 96.49 0.27 0.18 3.06 0.0000 100.0

Example

50 g of molten, crude trioxane were placed in three 250 ml glass beakers equipped with a heating mantle and a magnetic stirrer. The set was maintained at 55°C. Then, 0.50 g of dry ion exchanger was added to the melted trioxane, which constitutes 1% by weight. and mixed with trioxane for a specified period of time. The ion exchanger was in the mixture with trioxane for: 1 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 45 min, 60 min, 120 min, 180 min, 240 min, 360 min. A new portion of trioxane was taken for each sample. After reaching the set mixing time, the ion exchanger was separated from the trioxane by filtration.

Samples were taken from the filtered solution for testing for the content of formic acid, as well as for chromatographic analyzes to control the concentration of trioxane and formaldehyde. The formic acid content was determined using an automatic titration kit.

Table 2 shows the results of trioxane purification using three weakly basic ammonites.

T able 2

The influence of phase contact time on the degree of purification of trioxane from HCOOH

Macroporous anion resin with a polystyrene matrix (Purolite A100) Macroporous anion resin with a polystyrene matrix (Purolite A103) Macroporous anion resin with acrylic matrix (Purolite A830) 1 2 3 4 5 6 7 contact time HCOOH α HCOOH α HCOOH α [%] [%] [%] [%] [%] [%] 0 min 0.0174 0.0 0.0205 0.0 0.0132 0 1 min 0.0165 5.0 0.0190 7.4 0.0037 72.1 5 min 0.0145 16.4 0.0149 27.3 0.0000 100.0 10 min 0.0133 23.5 0.0137 33.3 0.0000 100.0 15 min 0.0122 30.0 0.0119 42.1 0.0000 100.0 20 min 0.0109 37.1 0.0101 50.9 0.0000 100.0 30 minutes 0.0092 47.3 0.0081 60.4 0.0000 100.0 45 min 0.0064 63.2 0.0050 75.7 0.0000 100.0 60 min 0.0043 75.1 0.0019 90.9 0.0000 100.0 120 min 0.0017 90.3 0.0000 100.0 0.0000 100.0

PL 215 482 B1 cont. table 2

1 2 3 4 5 6 7 180 min 0.0000 100.0 0.0000 100.0 0.0000 100.0 240 min 0.0000 100.0 0.0000 100.0 0.0000 100.0 360 min 0.0000 100.0 0.0000 100.0 0.0000 100.0

Patent claim

Claims (1)

Patent claim A method of removing formic acid from trioxane formed by the reaction of a concentrated solution of formaldehyde in the presence of sulfuric acid as a catalyst, distilled from the reaction liquid and then separated in the crystallization process, characterized in that it is added to the molten trioxane at a temperature of 50-70 ° C under the conditions of static macroporous anion exchanger slightly basic with a polystyrene matrix or microporous anion exchanger with acrylic matrix in hydroxyl form in the amount of 0.5 - 5.0% by weight. with respect to the weight of trioxane and the mixture is stirred for up to 120 minutes, preferably up to 60 minutes in the case of an anion exchanger with a polystyrene matrix and up to 10 minutes in the case of an anion exchanger with an acrylic matrix, and then the anion exchanger is filtered off from the purified trioxane.