Fig. 3. The efficiency of acetic acid oxidation depending on duration of the SCWO process, carried out in a batch mode at T = 673K; P = 25MPa.

Continuously-operated process of acetic acid oxidation has been carried out at T = 673K, 698K, 748K and Р = 25 MPa (volumetric ratio of acetic acid to hydrogen peroxide is 50:1).

The values of COD and pH as well as the efficiency () of the acetic acid oxidation process are presented below for the reaction product samples obtained with the use of continuously-operated plant (Fig. 4-6).

Fig. 4. COD values of the reaction product of acetic acid oxidation depending on duration of the process: 1 – T=673K; 2 – T=698K; 3 – T=748K. (P=25 MPa)

Fig. 5. рН values of the reaction product of acetic acid oxidation depending on duration of the process: 1 – T=673K; 2 – T=698K; 3 – T=748K. (P=25 MPa)

Fig. 6. The efficiency of acetic acid oxidation process depending on duration of the SCWO process carried out in a continuously-operated mode: 1 – T=673K; 2 – T=698K; 3 – T=748K. (P=25 MPa)

The figures above show that with an increase in temperature and duration of the process, the COD value decreases, that indicates an increase in the efficiency of the oxidation process. Changes in pH of the samples with the increase in temperature and duration of the reaction indicate that the reaction product becomes less acidic and more neutral.

The investigation of oleic acid oxidation reaction with hydrogen peroxide has been carried out on a batch-type plant with volume ratios of water - oleic acid - hydrogen peroxide of 6:1:1. The results of these studies carried out at T = 673-723K and P = 30MPa are presented in Table 2 and Figure 7:

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Table 2. COD (mgO2/l) of the reaction product of oleic acid oxidation in supercritical water

Parameters

Reaction duration t, min

T, K / P, MPa

0

10

20

30

298/30

44615

-

-

-

673/30

-

23924

20924

12088

698/30

-

14562

14032

7670

723/30

-

7846

6927

5196

Fig. 7. The efficiency of oleic acid oxidation process depending on duration of the SCWO process:

1 – T=673K; 2 – T=698K; 3 – T=723K. (P=30 MPa)

The results presented in Table 2 show that with an increase in duration and temperature of the oxidation process the COD value of oleic acid is significantly reduced, the smallest COD value has been obtained at T = 723K with reaction duration of 30 minutes. As it can be seen from Fig. 7, for the water-oleic acid-hydrogen peroxide ratio of 6:1:1, the efficiency of the process at T=723K is maximal.

Conclusion

1. Analyzing the results of the experiments carried out, a systematic decrease in the values of COD index of both acetic acid and oleic acid is observed. According to the fact that supercritical state of water is characterized by low strength of hydrogen bonds, the solubility of organic compounds increases. As a result of the investigation on SCWO process of acetic acid aqueous solution (10%), a decrease in the concentration of organic substances has been established, that is evidenced by a decrease in the initial COD from the value of 11590 mgO2 / L, common to all experiments with acetic acid, to the following values:

    1485 mgО2/l, atT = 748 K; 3750 mgО2/l, atT = 698 K; 3030 mgО2/l, atT = 673 K.

The results of the experiment carried out on the batch-type plant with 10% aqueous solution of acetic acid (Table 1 and Figure 3) show that the maximal effect of purification according to COD is observed at a reaction time of 30 minutes with value of 6120 mgO2/l. For the experiment on oxidation of oleic acid with hydrogen peroxide (Table 2, Figure 7), the maximum effect according to CODis characterized by the value of 5196 mgO2/l, at T = 723K and reaction time of 30 minutes.

2. The experiments carried out in the frames of the current research, show the possibility of implementation of SCWO process in a continuous mode for disposal of industrial effluents containing organic acids, as well as for obtaining water suitable for secondary use in production.

3. The efficiency of the supercritical water oxidation process implemented in a flow mode is higher than the same for the batch process.

Acknowledgement

The research has been carried out in Kazan National Research Technological University with the financial support of government represented by the Ministry of Education and Science of Russian Federation. in the frames of the government assignment No. 13.5112.2017/BCh.

References

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