Method of purifying water from hydrogen sulfide

Abstract

FIELD: purification of central water supply.SUBSTANCE: purification of central water supply, drinking and mineral water, and water treatment in the production of beverages and food products. Method which involves feeding purified air into the tank of a water purification plant and spraying water over the surface of the filter medium. The method is based on increasing the air flow rate at 1 m3 / h increments of an air flow rate of at least 3 m3 / h and a water flow rate of at least 10 m3 / h. At each increment, an audio recording is made for at least 5 seconds, and when recording noise with a frequency of 10-20 Hz and a sound power of at least 70 dB, the minimum water flow rate at which the noise is generated is recorded. The identified feature allows us to establish the required increment of the air flow Vair with an increment of the water flow Vwater to maintain the required (established) level of hydrogen sulfide evaporation, taking into account changes in the parametric characteristics of water spraying.EFFECT: increasing the efficiency of water purification from hydrogen sulfide.1 cl, 5 dwg, 1 tbl

Description

[0001] The invention can be used for the purposes of water purification of central water supply, drinking and mineral water, water treatment in the production of beverages and food products, etc.

[0002] The presence of volatile and semi-volatile organic compounds, as well as hydrogen sulfide, in water used for public and food-processing purposes can pose both a toxicological hazard and cause unsatisfactory organoleptic properties, such as taste, color, and odor. Frequently detected volatile compounds in toxicological and organoleptic water deterioration include hydrogen sulfide and sulfur-containing organic compounds, such as mercaptan. One of the most common methods for removing volatile compounds, including hydrogen sulfide, is by spraying water into a mobile air phase, which causes substances to evaporate from the surface of the resulting droplets.In this case, the volume of mass transfer becomes a function of the total surface area of ​​the droplets, and achieving its efficiency turns out to be a non-trivial task: if the air flow (consumption) is insufficient, the water purification from undesirable substances will be incomplete; if it is excessive, there will be an excessive consumption of purified air and equipment resources, and a change in a number of water parameters will be observed, for example, the pH level and oxidation-reduction potential, which are essential for subsequent water treatment processes and the reagent correction of which is highly undesirable.

[0003] In practice, determining the water-to-air flow ratio is typically done experimentally. This involves determining the required airflow at the minimum and maximum permitted water flows for specific water treatment equipment to achieve 91-99% removal of undesirable volatile organic compounds and dissolved gases, such as hydrogen sulfide. Parameters for intermediate values ​​are established using a linear relationship. However, this carries the risk of either insufficient flow or excessive resource consumption, which can disrupt essential parameters of the purified water (pH or oxidation-reduction potential).

[0004] In general, the problem of achieving the most efficient use of media can be reduced to the most accurate definition of the mass transfer surface, i.e., the total surface area of ​​atomized droplets. In theoretical terms, this issue is still not considered fully resolved. The main proposed descriptions of this process are based on the quasi-stationarity of the temperature and pressure fields in the vicinity of the droplet. N.A. Fuchs substantiates the validity of the quasi-stationary approach based on the implementation of mathematical models of non-stationary evaporation processes and subsequent numerical estimates of the evaporation rate of medium-sized droplets [1]. At the same time, according to N.A. Fuchs, droplet evaporation cannot be a stationary process, since the droplet radius, and, consequently, the evaporation rate, continuously decreases.

[0005] Furthermore, such models do not take into account the polydispersity of the droplet system sprayed in the flow path of various devices, as well as the movement of the droplet boundary, the inclusion of which leads to qualitatively different equations of droplet dynamics [2]. Due to the impossibility of interpreting the actual process occurring with a polydisperse system of particles, as a first stage, some researchers compare the experimentally established characteristics of a polydisperse mixture of moisture droplets sprayed by nozzles of various types [3-6]. In particular, L.N. Moskalov et al. and A.V. Klinov et al. experimentally obtained data in the form of graphs of the distribution function of moisture droplet densities by size for various types of nozzles [5, 6].This relationship, at least at relatively high spray pressures, is nonlinear due to the fact that, according to experimental data from the author and other researchers, when using nozzles used in water treatment equipment, average droplet sizes decrease by approximately 3-5% with a 10% increase in water pressure (flow) starting at a certain limit in the operating range of simple single-phase hydraulic nozzles. That is, starting at a certain limit, with increasing water flow, the mass transfer surface increases disproportionately.Droplet size distribution density curves follow typical log-normal distribution patterns, which seems entirely natural for processes where several independent factors influence the outcome of an event not additively, but in accordance with their magnitudes. This is observed in the case of droplet size distribution during spraying, where such factors include, for example, flow velocity and the effective nozzle size. A significant problem, however, is that the average droplet diameters, including the average linear diameter D, are not identical. 10 , average surface diameter D 21 , average volumetric diameter D 30 , average Sauter diameter D 32 , and in general, the parameters of their size distribution begin to change after reaching a certain threshold of injector performance.

[0006] Research conducted by O.N. Zhornova, N.S. Vasilevsky, and L.G. Galperin demonstrated that mass transfer parameters in droplet systems sprayed by nozzles of different types can vary significantly. At the same time, calculations based solely on theoretical models prove to be extremely complex, and models constructed using experimental data and obtained for specific installations prove to be the most suitable for practical application [4-7].

[0007] Thus, the problem solved by creating the claimed invention is to establish the required increase in air flow V air with an increase in water flow V water to maintain the required (established) level of hydrogen sulfide evaporation, taking into account changes in the parametric characteristics of water spraying, while the technical result achieved by solving such a problem consists in increasing the efficiency of water purification from hydrogen sulfide.

[0008] To achieve the desired result, an appropriate water purification method is proposed, which includes feeding purified air into the tank of the water purification plant and spraying water over the surface of the filter medium, while:

[0009] (1) Before starting the unit, determine the calibration coefficient K to calculate the required air flow in the unit with the existing water flow, for which:

[0010] (1.1) in a water purification plant, a microphone is installed 0.1-1.0 m above the loading level of the filter media, and with an air flow of at least 3 m 3 / h and a water flow of at least 10 m 3 / h increase the water flow in 1 m increments 3 / h, while at each step an audio recording is made lasting at least 5 s, and, when registering noise with a frequency of 10-20 Hz and a sound power of at least 70 dB, the minimum water flow at which the specified noise occurs is registered;

[0011] (1.2) determine the optimal air flow V' airat reference water flow V' water from the condition that with a constant flow of water V' water increase the air flow in 1 m increments 3 / h and measure the hydrogen sulfide content, and the minimum air flow level at which, at the next step of increasing the air flow, the efficiency of removing hydrogen sulfide increases by less than 0.5% is taken as the value of the optimal air flow V' air at water flow V' water ;

[0012] (1.3) determine the calibration coefficient K using the formula:

[0013] K=In(V' water / V' air )+(V' water / V o air ) 2 , where V o air - reference air flow equal to 100 m 3 / h;

[0014] (2) After the installation is put into operation, a gradual change in the observed water flow V is ensured water to the target water flow level V'' water , For what:

[0015] (2.1) inside the container, the working pressure and air flow are created from the condition:

[0016] (2.2) increase water flow V water by 5 m 3 / h;

[0017] (2.3) check for noise as described in (1.1);

[0018] (2.4) in case of no noise, air flow V air set according to the formula

[0019] V air =V water exp((V' water / V o air ) 2 -K), where V o air - reference air flow equal to 100 m 3 / h;

[0020] (2.5) in case of noise, air flow V air set according to the formula

[0021] V air =V water exp((V water / V° air ) 2 -K), where V o air - reference air flow equal to 100 m 3 / h;

[0022] (2.6) check whether the required target flow of purified water V'' is greater than water observed flux V water more than 5 m 3 / h;

[0023] (2.7) if the target flow of purified water is V water exceeds the observed flux V water more than 5 m 3 / h, then proceed to step (2.2);

[0024] (2.8) in case the target flow of purified water is V'' water does not exceed the observed flux V water more than 5 m 3 / h, then the transition to stage (2.9) is made;

[0025] (2.9) check whether the target flow of purified water is equal to V'' water observed flux V water ;

[0026] (2.10) in case the target flow of purified water is V water is equal to the observed flux V water then proceed to stage (2.6);

[0027] (2.11) if the target flow of purified water is V water is not equal to the observed flux V water then they check whether the target flow is greater or less than the observed one;

[0028] (2.12) in case the target flow is V water less than the observed flux V water , then the water flow is reduced by 1 m3 / h and the transition to stage (2.3) is made;

[0029] (2.13) in case the target flow is V water greater than the observed flux V water , then the water flow is increased by 1 m 3 / h and the transition to stage (2.3) is made.

[0030] The value of the reference (maximum possible) air flow V o air - see steps (1.3), (2.4), (2.5), corresponds to the air flow value at which the mass transfer of gas from the dissolved state to the gas phase becomes unconditionally thermodynamically favorable. It is established for the average value of water flow at which a specific noise occurs (usually about 40 m 3 / h, see example below) and corresponds to the stripping factor value S=1. S=(Q G / Q L ) / (H S,CP *RT), where H S,CP - Henry's dimensional constant from Sander's table [9], for hydrogen sulfide corresponding to H S,CP (298.15K)=1.0*10 -3 mol*m -3 *Pa -1 , Q G- gas (air) flow rate through the filter (column), m 3 / h, Q L - gas (air) flow rate through the filter (column), m 3 / h.

[0031] Thus for hydrogen sulfide S=(Q G / Q L ) / (1.0*10 -3 *8.314*298.15)=(Q G / Q L ) / 2.48=0.403 (Q G / Q L ) and the condition S=1 will be fulfilled in the case of a water flow of 40 m 3 / h at an air flow of 100 m 3 / h, set as the reference air flow, while the removal of hydrogen sulfide will be almost complete, which is not required for the purposes of applying the present invention, and the impact of negative factors (overconsumption of air, electricity, changes in oxidation-reduction potential, pH and a number of other physical and chemical indicators of water) will be excessive.

[0032] Fig. 1 shows a graph of the hydrogen sulfide removal efficiency (%), ordinate axis, in a spray water treatment plant depending on the air flow, abscissa axis, at different water flows;

[0033] Fig. 2 - frequency characteristics of noise on the surface of a water treatment plant;

[0034] in Fig. 3 - optimal ratios of water flows (along the abscissa axis) and air (along the ordinate axis) in a spray water purification unit;

[0035] Fig. 4 - basic design diagram of the installation for implementing the method;

[0036] Fig. 5 is a basic block diagram of the method implementation.

[0037] The ideology underlying the creation of the declared method and the achievement of the desired result is based on the following.

[0038] A distinctive feature of water treatment equipment used in industrial practice is that it has both upper and lower performance limits, determined by both the parameters of the pumping equipment used (which is more decisive for the upper performance limits) and the filtration parameters through bulk filters, which require a certain minimum flow, below which the necessary physical, mechanical, and biological processes of water purification simply will not occur. Thus, the problem of calculating the air flow rate in order to achieve the most efficient mass transfer from water to air is reduced to determining the dependence of the effective droplet size (average Sauter diameter, D) 32 ) from the parametric characteristics of the spraying process at a certain range of equipment performance

[0039] At the water treatment department of Aqua Star LLC (Kostroma), producer of Svyatoy Istochnik drinking water, hydrogen sulfide content was measured at various water and air flows in spray units for water purification from volatile and semi-volatile organic compounds (VOCs and PVOCs, respectively). The results showed that

[0040] hydrogen sulfide removal increases with increasing air flow, but upon reaching a certain value - about 92% removal, cells with such values ​​are highlighted in gray, further increase in air flow has little effect on the efficiency of hydrogen sulfide removal (less than 0.5% increase in efficiency with an increase in air flow by 1 m 3 / h) - see Table 1 and the graph in Fig. 1. As can be seen from Fig. 1, the dependence of the air flow on the water flow up to a certain threshold value of the water flow (in the example of Fig. 1 - about 40 m 3 / h) is non-linear, and after exceeding it, it becomes linear. The air flow at which the maximum hydrogen sulfide removal is achieved at a given water flow rate is recognized as optimal, since it achieves the highest removal efficiency with the lowest flow rate of the media. It was also noted that at approximately the same water flow rates at which the linear dependence of the optimal air flow on the water flow begins, a change in the noise characteristics recorded on the outer surface of the working tank of the water purification plant occurs. When studying the acoustic characteristics, it was discovered that at a flow above the specified threshold values, low-frequency noise of 10-15 Hz begins to be recorded, undetectable at lower flows or in similar plants without spraying water onto the surface of the sand substrate - Fig. 2, associated with the formation of smaller droplets, but possessing greater kinetic energy, upon reaching the specified threshold.

[0041] Based on the theoretical model of the dependence of the optimal air flow on the water flow, the following empirical expression for this dependence was proposed

[0042] - for nonlinear interval V air = V water exp((V' water / V o air ) 2 - TO);

[0043] - for linear interval V air = V water exp((V water / V o air ) 2 - K);

[0044] whereV air - reference (maximum possible) air flow, equal to 100 m 3 / h, K - calibration coefficient, V' water - a flow of water that produces a specific low-frequency noise.

[0045] The threshold separating the nonlinear interval of dependence from the linear one is the appearance on the surface of the working capacity of the water purification plant of low-frequency noise <20 Hz with a power of more than 70 dB on the full scale dBFS.

[0046] Calculations have shown - see graph in Fig. 3 - that the use of this model in application to specific installations shows a fairly high correlation coefficient (R 2 >0.92) with the actual data obtained. The observed values ​​(blue line) correspond quite accurately to the calculated ones (orange line) in accordance with the stated method. The graph shows the deviation of the observed values ​​above the water flow threshold (40 m 3 / h)

[0047] from the linear approximation (gray dotted line). There is also a deviation from the established nonlinear relationship at flow below the threshold (black dotted line), but it is less significant.

[0048] Fig. 4 shows a general diagram of a water purification plant for implementing the claimed method, in which hydrogen sulfide is removed in a closed container with a volume of 5 to 50 m 3Water is sprayed over the surface of the filter media (sand, gravel) using single-phase hydraulic nozzles. Purified air is then supplied to the tank, creating an internal pressure of 1.2 to 3 atmospheres and an air exchange rate of 3 to 30 m3. 3 / h. The air pressure inside the tank and the water flow are adjusted so that the water level is below the level of the filter medium (so-called "dry filtration"), due to which a kind of "air bubble" is created inside the tank.

[0049] Bibliography

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[0054] 5. Moskalov L.N., Moskalov I.N., Vilokhin S.A., Khalikov M.R. Study of droplet distribution during water flow from a centrifugal nozzle with an insert / / Bulletin of the Kazan Technological University. 2014. Vol. 17, No. 15. P. 205-206.

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[0058] 9. Sander, R. Compilation of Henry's law constants (version 5.0.0) for water as solvent. Atmospheric Chemistry and Physics 23 (2023): 10901-12440. https: / / doi.org / 10.5194 / acp-23-10901-202.

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