RU2284048C2 - METHOD OF AUTOMATIC REGULATION OF pH VALUE OF AQUEOUS SOLUTIONS - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: method can be used for production of chlorine and caustic soda received by method of electrolysis of NaCl aqueous solution as well as at any other technological processes where PH value should be kept stable. Method of automatic regulation of Ph of aqueous solutions due to change in acid or alkali supply to reactors-mixers is based upon usage of inverse function of nonlinear dependence of pH on content of acid or alkali and value of presetting of linearized inverse function in the system of automatic regulation system (ARS). Method provides high static and dynamic stability and precision in keeping of preset value of pH to certain limit. For example, when using NaCl solution during electrolysis, voltage in bats decreases and current output increases. Duration of bath run between two maintenance procedures prolongs as well. During wastewater cleaning process, high precision of pH level leads to decrease in hazardous effluents into environment.

EFFECT: improved efficiency.

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Description

The invention relates to methods for the automatic control of technological processes (TP) and can be used in the production of chlorine and caustic soda obtained by electrolysis of an aqueous solution of NaCl (brine) supplied to electrolytic baths with a solid or liquid cathode (diaphragm or mercury electrolyzers), and maintaining the pH of any aqueous solution by changing the flow rate of acid or alkali.

It is known that the best indicators of the efficiency of the electrolysis process (such as voltage on the cell, current efficiency, etc.) in diaphragm and mercury electrolyzers are achieved by supplying acidified brine with pH values not higher than 3.5 units [V. Kubasov, Bannikov V.V. Electrochemical technology of inorganic substances. M., "Chemistry", 1989, 55 pp.].

During diaphragm electrolysis, brine purified from impurities and saturated with NaCl is fed directly to the electrolyzers. In this case, the cleaning of the brine from impurities is carried out at a pH of the brine in the range of 9-11 units, and the recommended pH value of the brine before being fed into the electrolytic cells should be 2.5-3.0 units.

In mercury electrolysis, a NaCl solution (anolyte) flowing out of electrolysis cells undergoes dechlorination, saturation of NaCl and filtration before returning it to electrolysis. In this case, the anolyte before dechlorination and the brine fed directly to the electrolysis cells should be acidified, and the anolyte is alkalinized before repletion. In particular, the anolyte with a pH of 3.5–4.0 must be acidified before dechlorination to pH = 1.0–3.0, then before the saturation and filtration, the pH of the anolyte should be 9–11 units. The pH value of the brine before being fed into the electrolytic cells should be 2.5-3.0 units.

Thus, in the production of chlorine and caustic soda, it is necessary to automatically maintain the pH of aqueous NaCl solutions (anolyte and brine) at the stages of its preparation and supply directly to electrolysis. The change in the pH of the brine is carried out by changing the flow of acid (alkali) supplied to the corresponding nodes TP. As a rule, the sites of acidification of the anolyte and brine and the site of alkalization of the anolyte, as well as the technological processes for neutralizing wastewater, are the same regulatory objects. Typical schemes of these nodes are shown in figures 1 and 2. The nodes include tanks - collectors of acid or alkali (item 1 in figures 1, 2), providing a continuous supply of reagents to the reactors mixers. As mixers can be used as special devices (item 2, figure 1), and, for example, pumping pumps (item 2, figure 2). Automatic control systems (ASR) of these nodes are also of the same type. The pH value measured directly at the object using a pH meter (item 3 in Figs. 1, 2) is used as adjustable variables, the adjustable value and the task are fed to the input of the pH value controller (item 4 in Figs. 1, 2), and regulatory effects are acid (Fc) or alkali (Fsc).

At some facilities, cascading ASRs are used with correction of the assignment to the regulator of the ratio of acid (alkali) and anolyte (brine) flows according to the pH value measured directly at the facility.

Similar to the above are schemes for automatic regulation of pH in the processes of neutralization of wastewater. According to the pH value measured after the mixer, the flow of acid or alkali is dosed, depending on whether the effluents are alkaline or acidic.

The issues of automatic control and regulation of the pH of brine and anolyte are described in [I. Lomakin and others. Automation of chlorine production. M., "Chemistry", 1967, p.126], [Lomakin I.L. and others. Automation of chlorine production. M., "Chemistry", 1975, p.155], and questions of automatic regulation of the pH of wastewater - see, for example, in [Golubyatnikov V.A. and others. Automation of production processes and process control systems in the chemical industry. M., "Chemistry", 1978, p. 293].

According to the conditions of the technological process, fairly stringent requirements are imposed on pH control systems for the accuracy of maintaining the pH value. Usually this value is ± (0.10 ÷ 0.15) pH units. However, existing automatic control systems cannot provide the required accuracy of maintaining the pH value and, moreover, often work in the mode of self-oscillations with an unacceptably large amplitude.

Due to the unsatisfactory quality of regulation at most industrial enterprises, not an acidic but an alkaline brine with pH = 11-12 units is fed directly to the electrolysis stage, since brine acidification can lead to an emergency.

The unsatisfactory quality of regulation of the pH value is due to the sharply nonlinear nature of the dependence of the pH value on the concentration of acid or alkali in solution. When the activity of hydrogen ions or hydroxyl ions equal to unity, this dependence is described by the expression:

where - C _HCl - concentration of acid in solution, mol / l

or

where - With _NaOH - alkali concentration in solution, mol / L.

This dependence is shown in figure 3, where in order to combine these two graphs in the same figure, the acid concentration is conditionally assumed to be negative.

It can be seen from the figure that the change in pH with a change in the concentration of acid - alkali has the form of an S-shaped curve with saturation symmetrical with respect to a neutral medium, corresponding to pH = 7.0 units.

Note that the entire range of pH = 0-14 corresponds to very small absolute values of acid / alkali concentrations given in Table 1 as an example for hydrochloric acid HCl and alkali - NaOH.

Table 1. Acid / Alkali Concentration Acidic environment Neutral solution Alkaline environment pH 0 one 2 3 four 5 6 7 8 9 10 eleven 12 13 fourteen g mol / l 1,0 10 ^-1 10 ^-2 10 ^-3 10 ^-4 10 ^-5 10 ^-6 0 10 ^-6 10 ^-5 10 ^-4 10 ^-3 10 ^-2 10 ^-1 1,0 g / l 36.5 36.5 36.5 36.5 36.5 36.5 36.5 0 40 40 40 40 40 40 40 * 10 ^-1 * 10 ^-2 * 10 ^-3 * 10 ^-4 * 10 ^-5 * 10 ^-6 * 10 ^-6 * 10 ^-5 * 10 ^-4 * 10 ^-3 * 10 ^-2 * 10 ^-1 % 3.65 3.65 3.65 3.65 3.65 3.65 3.65 0 four four four four four four 4.0 * 10 ^-1 * 10 ^-2 * 10 ^-3 * 10 ^-4 * 10 ^-5 * 10 ^-6 * 10 ^-6 * 10 ^-5 * 10 ^-4 * 10 ^-3 * 10 ^-2 * 10 ^-1

Therefore, technological requirements, for example, to maintain the pH of the brine equal to 2-3 units, corresponds to the requirement to maintain the concentration of acid in the solution in the range of ≈ (0.04-0.36) g / l, i.e. a very high absolute accuracy must be ensured for controlling low acid concentrations. The same dependences are characteristic of alkaline solutions.

Since the consumption of acid or alkali is several orders of magnitude lower than the consumption of brine or anolyte, the concentration of anolyte or alkali in the stream linearly depends on the control action - the consumption of acid or alkali. Consequently, the dependences of the pH value on the control action — acid or alkali consumption — are of the same nonlinear nature, which determines the low quality of pH regulation processes.

The objective of the invention is to provide a method for automatically controlling the pH value, providing high static and dynamic accuracy of maintaining a given pH value.

To ensure the goal in pH ACP, it is necessary to linearize the nonlinear dependence of pH on the concentration of acid or alkali, i.e. actually linearize the non-linear characteristic of the pH meter sensor.

The linearity of the system will be ensured if not the pH value is used as the controlled variable, but the converted value of this value, namely, the inverse function of the dependence of the pH value on the concentration of acid or alkali.

For acidic solutions, the conversion is carried out according to the expression

for alkaline - in expression

The form of this dependence is shown in figure 4, where also the concentration of acid is conventionally assumed to be negative.

The task to the regulator should be presented in the same units as the controlled variable. Therefore, the reference signal before applying to the controller must also be converted according to the above expressions.

If the ACP pH values are implemented using modern microprocessor controllers (IPC), the conversion of the controlled variable and the reference value according to expressions (3) and (4) or the curve shown in figure 4 can be carried out using piecewise linear algorithms approximations.

The following are examples of the implementation of the proposed method for automatically controlling the pH of aqueous solutions for various technological processes.

EXAMPLE 1

Figure 5 shows a diagram of the implementation of the method on the example of ACP acidification of the anolyte.

The pH value signal after the QR-3 pH meter is fed to the input of the QY-5 computing unit. where according to expression (3) is converted into the value of the concentration of hydrochloric acid in a solution of C _HCl . According to a similar expression in the computing unit QY-6, the task of pH _S is recalculated into a given value of the concentration of hydrochloric acid - C _S. These values are fed to the input of the QC-4 regulator, which, by changing the flow rate of hydrochloric acid to the input of the mixer, ensures the preservation of the preset pH value of the anolyte with high dynamic accuracy of ± 0.05 units. pH

The scheme is implemented using the IPC R-130. The dependence (3) was realized by using the piecewise linear approximation algorithm of this expression. The measuring range of the pH meter was 0-5 units. pH

Table 2 shows the abscissa - pH values (in pH units and in% of the range of pH changes) and ordinates - HCl - acid concentrations (in mol / L and% of the range of concentration changes) used to configure the piecewise linear approximation of expression (3).

Table 2. PH, scale 0-5 Acid concentration Units pH % mol / l % 0 0 one one hundred one twenty 0.1 10 2 40 0.01 one 3 60 0.001 0.1 four 80 0.0001 0.01 5 one hundred 0.00001 0 Table 3. PH 7-12 Alkali concentration Units pH % mol / l % 7 0 0 0 8 twenty 0.000001 0.01 9 40 0.00001 0.1 10 60 0.0001 one eleven 80 0.001 10 12 one hundred 0.01 one hundred

EXAMPLE 2

Figure 6 shows a diagram of the implementation of the method on the example of alkalization of the anolyte.

The pH value signal after the QR-3 pH meter is fed to the input of the QY-5 computing unit. where according to the expression (4) is recalculated into an adjustable value. According to the same expression, the reference signal is converted into the set value pH _S in the QY-6 computational unit. The converted pH and pH _s are fed to the input of the QC-4 regulator, which, by changing the alkali consumption at the pump inlet, which is also a mixing reactor, ensures the preservation of the target anolyte pH with a high accuracy of ± 0.05 units. pH The measuring range of the pH meter is 7-12 units. pH

Table 3 shows the data for adjusting the piecewise linear approximation algorithm calculated using expressions (4).

EXAMPLE 3

Figure 7 shows a diagram of the implementation of the method on the example of brine acidification. Filtration of the brine is carried out in an alkaline medium at pH = 9-11 units. However, before serving in electrolytic cells, it is acidified with hydrochloric acid to pH = 2.5-3.5 units. Since an overdose of hydrochloric acid into the brine on electrolyzers may cause an explosive situation, the pH brine control system is cascading. It provides for the control and stabilization of the flow of hydrochloric acid using the regulator of the flow of hydrochloric acid FRC-6, the task of which is adjusted by the pH-QC-4 regulator. The structure of the correcting controller is similar to that in Example 1. The setup of piecewise linear approximation algorithms corresponds to Table 2

The accuracy of maintaining the adjustable value reached ± 0.15 units. pH, which meets the requirements of technological regulations. Previously implemented by a known method, the ACP of this site did not provide the required accuracy. Therefore, due to the possibility of emergencies during brine acidification, this ASR was turned off, and alkaline brine was supplied to the electrolysis, which led to a deterioration in the technical and economic indicators of the process.

EXAMPLE 4

On Fig is a diagram of the implementation of the method on the example of TP neutralization of alkaline effluents.

The system operates under conditions of constantly acting disturbances in the flow rate of alkaline effluents. In order to compensate for disturbances in the flow rate of drains controlled by FR-8, the flow rate of hydrochloric acid is dosed into the mixer pos.2 using the FFRC-7 ratio regulator.

The predetermined ratio of the flows to the ratio regulator is adjusted using the QC-4 regulator of the pH value of the effluents at the outlet of the mixer.

The structure of the correcting controller is similar to that in Example 2. The adjustment of the piecewise-linear approximation algorithms for QY-5 computing units corresponds to Table 3. As a result, the accuracy of maintaining the adjustable value reached ± 0.2 units. pH, which meets the requirements of technological regulations. Previously implemented by a known method, the ASR of this TP worked in the mode of self-oscillations with an amplitude of 2-3 units. pH, which is unacceptable under the conditions of the process.

Claims (1)

A method for automatically adjusting the pH of aqueous solutions by changing the supply of acid or alkali to mixing reactors and using as a controlled value the linearized inverse function of the non-linear dependence of the pH on the concentration of acid or alkali, characterized in that, in order to increase the dynamic accuracy and stability, as adjustable magnitude and magnitude of the task in the structure of an automatic control system (ASR) linearized inverse functions of nonlinear dependencies of quantities are used pH is different from the concentration of acid or alkali.

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