CN1322859A - Paired electrolysis procss of preparing glyoxalic acid - Google Patents

Abstract

In the electrolysis process, an electrolyzer with fixed bed cathode and anode is adopted and current density is made ot change with the glyoxalic acid content in electrolyte. The fixed bed cathode and anode have thickness of 4-50 mm, and preferably 10-30 mm, and average apparent current density up to 4500 A/sq m, and optimally 1000-3000 A/sq m. Compared with plate electrode reactor, the present invention has several times higher production capacity of electrolyzer. The electrolysis process has a total current efficiency up to 170 % and anode and cathode selectivity higher than 95%.

Description

Method for preparing glyoxylic acid by paired electrolysis

The invention relates to a method for preparing glyoxylic acid, in particular to a method for preparing glyoxylic acid by electrolysis in a paired fixed bed reactor.

Glyoxylic acid is an important fine chemical raw material, and the structural formula of the glyoxylic acid is as follows: CHOCOOH can be used for preparing fine chemical products such as vanillin, allantoin, etc. Currently, glyoxylic acid is produced by an oxidation process and a reduction process. The oxidation method mainly comprises a glyoxal nitric acid oxidation method, a glyoxal hydrogen peroxide oxidation method and a glyoxal anode electrolysis oxidation method. These are reported in U.S. Pat. Nos. 4146731 and 4235684, etc. The reduction method is mainly an oxalic acid electrolytic reduction method. The reports of this aspect are us patent 3779875 and chinese published patent CN1281063A, etc. In addition, a paired electrode electrolysis method disclosed in Chinese patent publication No. CN1064111A is also disclosed.

U.S. Pat. No. 4146731 discloses a process for preparing glyoxylic acid by oxidizing glyoxal with nitric acid, which is difficult to control the reaction conditions strictly, thus difficult to inhibit side reactions and ensure selectivity of glyoxylic acid, and the amount of nitric acid used is large, and considerable amount of nitric acid remains in the product and is not easy to remove. Moreover, nitric acid has serious corrosion to equipment, and nitric oxide generated in the oxidation process easily causes serious environmental pollution.

Both U.S. Pat. Nos. 4235684 and 3779875 and the published Chinese patent CN1281063A report single-sided electrolysis. Wherein us patent 4235684 reports the production of glyoxylic acid by the anodic oxidation of glyoxal. The current density used by the process is small (less than or equal to 200 amperes/meter)²) The electrolysis period is long, and the current efficiency and the yield of the glyoxylic acid in the process are not high (67-85 percent and 66-82 percent respectively). The process has not been industrialized to date. Both U.S. Pat. No. 3779875 and Chinese published patent CN1281063A report the preparation of glyoxylic acid by the cathodic reduction of oxalic acid. Although the current density used by the process, the current efficiency of the electrolytic process and the chemical selectivity of the glyoxylic acid are high, the anode reaction is oxygen evolution in the cathodic reduction process of the oxalic acid, and the anode process cannot be effectively utilized, so the process cannot fully exert the electrolytic advantages.

Chinese published patent CN1064111A reports the electrolytic synthesis of glyoxylic acid using paired electrodes. Compared with single-side electrolysis, the total current efficiency of the method and the production capacity of the electrolytic cell are improved because the cathode and the anode generate the glyoxylic acid. However, the process also has the following defects:

① the flow rate of electrolyte is large (>1 m/s). to achieve such a high linear flow rate in industrial production, the power consumption of the power system is very surprising and difficult to bear, and the high flow rate puts higher demands on production equipment and pipelines.

② the current density in the electrolysis process is small (800-900A/m)²). Since the process must meet the low current density requirements of the anodic process, the overall cell capacity is not increased but instead the capacity of the cathodic process is reduced, thereby increasing the average production cost of the glyoxylic acid product.

③ the current efficiency of the cathode and anode electrolysis processes is low (cathode current efficiency is about 50%, anode current efficiency is less than 80%).

The object of the present invention is a process for the preparation of glyoxylic acid by means of variable current (density) in an electrolysis cell with a fixed-bed anode and a fixed-bed cathode in order to increase the capacity and the current efficiency of the electrolysis cell.

The idea ofthe invention is that:

to increase the capacity of the cell, the input current to the electrolysis process must be increased. According to the property that the large current density in the electrolytic process reduces the current efficiency and the chemical selectivity of the glyoxylic acid, the production capacity of the electrolytic cell is improved by increasing the input current, and the actual current density of the electrode is reduced by increasing the area of the electrode so as to ensure that the electrolytic process has higher current density and chemical selectivity of the glyoxylic acid.

Along with the progress of the electrolytic reaction, the concentration of the glyoxylic acid in the electrolyte is higher and higher. When the concentration of the glyoxylic acid is higher in the later period of electrolysis, the current efficiency and the glyoxylic acid chemical selectivity in the cathode and anode processes are greatly reduced by high-current density electrolysis. Therefore, the invention adopts a scheme of variable current density according to the change of the concentration of the glyoxylate in the electrolyte, so as to ensure that the electrolytic reaction still keeps higher current efficiency and glyoxylate chemical selectivity when the concentration of the glyoxylate is very high in the later period of electrolysis.

According to the above concept, the present invention proposes the following technical solutions:

the electrolyte temperature, cation exchange membrane and initial composition of the electrolyte used in the present invention are referred to in U.S. patent 4235684 and chinese published patent CN 1281063A. The fixed bed electrode electrolyzer was manufactured in accordance with the chinese patent publication CN 1083871A.

The process of the present invention includes two steps of paired electrolysis and separation and purification of primary glyoxylate product.

(1) Paired electrolysis process:

adding glyoxal, hydrochloric acid and deionized water into an anode mixing tank at one time to form an anode electrolyte; the composite additive and deionized water are added all at once, and oxalic acid particles are added all at once or added into a cathode dissolving tank periodically in batches to form a cathode electrolyte, so that the oxalic acid solution is always in a saturated state, and the possibility of hydrogen evolution is reduced; and respectively sending the anolyte and the catholyte into a fixed bed electrode electrolytic tank for paired electrolytic reaction by using an anolyte circulating pump and a catholyte circulating pump, wherein the reaction formula is as follows:

and (3) anode reaction:

and (3) cathode reaction:

the anolyte and catholyte containing the glyoxylic acid produced by the reaction are returned to the anodic mixing tank and the cathodic dissolution tank, respectively. And circulating for many times until the content of the glyoxylic acid in the electrolyte meets the requirements specified by the process. And the cathode electrolyte and the anode electrolyte are respectively sent to respective separation and purification devices for separation and purification.

In accordance with the foregoing considerations, the apparent current density of the electrode can be reduced by increasing the electrode area. If the increase of the electrolytic area of the electrode is achieved by increasing the length and width of the electrode, this method isnot economical and cannot fundamentally solve the problem. Compared with a flat electrode, the fixed bed electrode is a three-dimensional electrode and has the advantages of large surface area, large apparent current density, compact bed layer structure, high space-time rate of an electrolytic tank and the like. The fixed bed electrode electrolytic tank can improve the electrolytic area of the electrode without increasing the length and width of the electrolytic tank. Therefore, the low current density of the electrolysis process is maintained under the condition of increasing the electrolysis current, thereby ensuring that the whole electrolysis process has higher average current density and glyoxylate selectivity.

Research shows that in the anode chamber (during the process of synthesizing glyoxalic acid by oxidizing glyoxal), when the electrolysis reaches the later stage, namely when the content of glyoxalic acid is very high and the content of glyoxal is very low, the current efficiency and the chemical selectivity of glyoxalic acid in the process are greatly reduced by high-current density electrolysis. This is also the main reason why the current efficiency and glyoxylic acid chemical selectivity of the whole electrolysis process are not high under the condition of large current density. Therefore, the invention adopts a variable current (density) electrolysis mode, namely, the high-current electrolysis is adopted when the glyoxal content is very high and the glyoxylate content is not high in the early stage of electrolysis; along with the electrolysis, the content of glyoxylic acid is higher and higher, the content of glyoxal is continuously reduced, and when the electrolysis reaches a certain degree, the electrolysis is carried out by using a small current. Thus, the method can ensure that the whole process has higher average current (density) and also can ensure that the whole electrolysis process has higher current efficiency and glyoxylate chemical selectivity. Meanwhile, in a cathode chamber (in the process of synthesizing glyoxylic acid by reducing oxalic acid), the glyoxylic acid content in the cathode solution in the later period of electrolysis is very high, so that the large-current density electrolysis also causes the side reactions of reducing the glyoxylic acid into glycolic acid, glyoxal and the like. Therefore, for the cathodic oxalate reduction reaction, the low-current (density) electrolysis adopted at the later stage of the electrolysis is also beneficial to the improvement of the current efficiency and the chemical selectivity of the glyoxylic acid.

The current density range of the electrolysis process is 1000-5000 amperes/meter²If the current density in the initial stage of electrolysis is too high, although the electrolysis reaction rate is high, the current efficiency and the glyoxylate chemical selectivity in the initial stage of electrolysis are not high due to the occurrence of side reactions, so that the current efficiency and the glyoxylate chemical selectivity in the whole electrolysis process are not ideal, if the current density in the initial stage of electrolysis is too low, the production capacity of the whole electrolysis process is difficult to ensure, in order to ensure higher current efficiency and glyoxylate chemical selectivity in the later stage of electrolysis, the invention adopts as low an apparent current density as possible in the later stage of electrolysis, the electrolysis process is preferably carried out in a way that the ratio of the concentration of glyoxylate to the concentration of glyoxal in ① anolyte is 0-5: 1 current density: 1 × I ②, the ratio of the concentration of glyoxylate to the concentration of glyoxal in anolyte is 5: 1-40: 3 current density: α × I

In the formula: the concentration is mass percent, I is current density, ampere/meter²，

α = 0.3-40.9, preferablyThe selection range is 0.5-0.7. Thus the average current density of the whole electrolytic process can reach 4500 ampere/meter at most²。

The current efficiency and glyoxylate chemoselectivity referred to in the present invention are defined as:

current efficiency = theoretical amount of electricity consumed per mole of glyoxylic acid produced/amount of electricity actually consumed per mole of glyoxylic acid produced.

Glyoxylate selectivity = amount (moles) of glyoxylate produced/amount (moles) of glyoxal or oxalic acid consumed.

(2) Purification of the glyoxylic acid primary product:

the above-mentioned primary products of the cathode and anode electrolytes containing glyoxylic acid can be obtained into the product of 40% glyoxylic acid which meets the requirements only by adopting conventional reduced pressure evaporation and low-temperature filtration respectively (anode product: glyoxylic acid content is greater than or equal to 40.0%, glyoxal content is less than or equal to 3.0%, oxalic acid content is less than or equal to 3.0%, cathode product: glyoxylic acid content is greater than or equal to 40.0%, glycolic acid content is less than or equal to 3.0%, oxalic acid content is less than or equal to 3.0%), and the invention is not repeated herein.

FIG. 1 is a flow chart of a process for preparing glyoxylic acid by paired electrolysis.

1-cathode dissolving tank 2-catholyte circulating pump 3-catholyte heat exchanger

4-fixed bed electrode electrolytic tank 5-cation exchange membrane

6-fixed bed cathode 7-fixed bed anode 8-anolyte circulating pump

9-catholyte separation purification device 10-anode mixing tank

11-anolyte heat exchanger 12-anolyte separating and purifying device

As shown in fig. 1, oxalic acid particles, a composite additive and deionized water are put into a cathode dissolving tank 1 to form cathode solution; glyoxal, hydrochloric acid and deionized water are put into an anode mixing tank 10 to form an anolyte. The catholyte and the anolyte are simultaneously sent into a cathode chamber and an anode chamber of an electrolytic tank 4 by a catholyte circulating pump 2 and an anolyte circulating pump 8 for paired electrolytic reaction, the catholyte and the anolyte containing the glyoxylic acid generated by the reaction are respectively sent back into a cathode dissolving tank 1 and an anode mixing tank 10 after heat exchange by heat exchangers 3 and 11, and are simultaneously sent into the electrolytic tank 4 for paired electrolytic reaction after mixing. Repeating the steps, and finally respectively sending the mixture to separation and purification devices 9 and 12 for separation and purification. The whole electrolysis process is preferably carried out according to the current density change rule.

The present invention will be further illustrated by the following examples.

Example 1

All percentages in the examples are by mass.

The thickness of the cathode of the fixed bed was 4 mm (lead contained 99.99%) and the thickness of the anode of the fixed bed was 4 mm (DSA). The CM001 type cation exchange membrane is used as a diaphragm. The catholyte is deionized water, industrial first-grade oxalic acid and an additive with the total content of 0.5 percent, wherein the molar content ratio of tetraethylammonium bromide to heptyl tributyl ammonium chloride is 0.5. The initial content of glyoxal in the anolyte is 7%, and the initial content of hydrochloric acid is 5%. The electrolyte is circularly operated by a magnetic pump, the flow rate of the electrolyte is 0.14 m/s, the inlet temperature of the cathode of the fixed bed is 10 ℃, the inlet temperature of the anode of the fixed bed is 30 ℃, and the apparent current density of the electrode in the electrolysis process is 1600 amperes/m²(constant current density electrolysis). The electrolysis result is:

electrode chamber	Composition of preliminary product for electrolysis (%)	Current efficiency (%)	Glyoxylic acid chemical selectivity (%)
				Cathode chamber	Glyoxylic acid 6.82 Oxalic acid 5.40	80.4	86.1
Anode chamber	Glyoxylic acid 7.33 Glyoxal 0.51	65.8	74.0

Example 2

The thickness of the cathode of the fixed bed was 30 mm (lead contained 99.99%) and the thickness of the anode of the fixed bed was 30 mm (DSA). The CM001 type cation exchange membrane is used as a diaphragm. The catholyte is deionized water, industrial first-grade oxalic acid and an additive with the total content of 0.5 percent, wherein the molar content ratio of tetraethylammonium bromide to heptyl tributyl ammonium chloride is 0.5. The anolyte contains 7 percent of glyoxal and 5 percent of hydrochloric acid. The electrolyte is circularly operated by a magnetic pump, the electrode apparent flow rate of the electrolyte is 0.3 m/s, the inlet temperature of a cathode of a fixed bed is 15 ℃, the inlet temperature of an anode of the fixed bed is 30 ℃, and the change rule of the electrode apparent current density in the electrolysis process is as follows:

① the ratio of the concentration of glyoxalic acid to the concentration of glyoxal in the anolyte is 0-5: 1, and the current density is 3000A/m²

② the ratio of the concentration of glyoxalic acid to the concentration of glyoxal in the anode electrolyte is 5: 1-40: 3, and the current density is 1500 amperes/m²。

The average current density of the whole electrolysis process is 2640A/m². The electrolysis results were as follows:

electrode chamber	Composition of preliminary product for electrolysis (%)	Current efficiency (%)	Glyoxylic acid chemical selectivity (%)
				Cathode chamber	Glyoxylic acid 7.66 Oxalic acid 5.07	89.6	96.2
Anode chamber	Glyoxylic acid 8.61 Glyoxal 0.63	84.8	93.7

The above-mentioned primary products of the cathode and anode electrolytes containing glyoxylic acid can be obtained into the product of 40% glyoxylic acid which meets the requirements only by adopting conventional reduced pressure evaporation and low-temperature filtration respectively (anode product: glyoxylic acid content is greater than or equal to 40.0%, glyoxal content is less than or equal to 3.0%, oxalic acid content is less than or equal to 3.0%, cathode product: glyoxylic acid content is greater than or equal to 40.0%, glycolic acid content is less than or equal to 3.0%, oxalic acid content is less than or equal to 3.0%).

Claims (4)

1. The electrolytic process of synthesizing glyoxylic acid includes two steps of electrolytic synthesis of glyoxylic acid andseparation and purification of the initial product of glyoxylic acid, and features that the electrolytic process uses electrolytic bath with fixed bed cathode and anode electrodes.

2. The method of claim 1, wherein the fixed bed anode has a thickness of 4 to 50 mm and the fixed bed anode has a thickness of 4 to 50 mm.

3. The method of claim 1, wherein the electrolysis is carried out by a current density of 0-5: 1 xI of glyoxylic acid concentration in ① anolyte and a current density of α xI of ②: 5: 1-40: 3 of glyoxylic acid concentration in anolyte;

in the formula: the concentration is mass percent, I is current density, ampere/meter²，

α = 0.3-0.9, preferably 0.5-0.7.

4. A method according to claim 3, wherein the electrolysis process is carried out at a current density in the range 1000 to 5000A/m²。