Intellectual control of the electrolysis process in the production of caustic soda

2Neural network controller

An artificial neural network is based on a set of connected units or nodes called artificial neurons.

Fig.2.
Neural networks learn (or are trained) by processing examples, each of which contains an "input" and an "output" and forming probabilistic associations between them, which are the network's is stored in a data structure stored in z. Training a neural network on a given example is usually done by distinguishing between the network's processed output (often a prediction) and the target output. The network then adjusts its weighted associations according to the learning rule and using this error value. After a sufficient number of these corrections, training can be stopped based on certain criteria.
Artificial neural networks are the most convenient for presenting information models and have a number of advantages. They do not require formalization of the problem and allow the properties of the neural network model to be adapted to problems of theoretically infinite size and complexity. The only requirement for neural networks is that the phenomena must be represented by continuous functions. Creating models using neural networks is significantly faster than creating models using traditional methods. In addition, the method of neural networks allows to maintain a new, promising and active competitiveness.
In this process, we take the electrolyzer as an object in the modeling by neural network.
Fig.3.
The process of electrolysis of salt solution is a complex process. The process of electrolysis of the saline solution is carried out in membrane electrolyzers pos. 4.XE01-06 type DN 350 from De Nora. The electrolyzer consists of 28 elementary membrane cells with a total membrane area of ​​98 m² (3.5 m² per elementary cell).
The unit cell consists of two compartments, anodic and cathodic, separated by a cation exchange membrane. The electrolyzer is assembled from 27 intermediate bipolar elements, which have anode and cathode sides, as well as one anode and one cathode final element and 28 membranes.
The elementary cells of the electrolyzer are electrically connected in series. The current is supplied by copper bars to the anode end element of the electrolyser and flows sequentially through all elementary cells, passing through the bipolar elements, and exits through the cathode end element. The electrolyzer is a filter-press structure held by a compression system. The filter-press structure is located on guide supports and is equipped with a circulation system consisting of manifold-distributors, which respectively ensure the supply of reagents to each elementary cell and the collection of waste products (liquid and gaseous).
The electrolyzers are connected in series into one electrical circuit and receive power from a transformer-rectifier. The current load is 13.8 kA, which corresponds to a current density of 3.94 kA/m², and determines the performance of chlorine and caustic soda.
A purified salt solution with a temperature of (65-70) °C and a mass concentration of NaCl (295-305) g/dm³ is supplied to the anode compartment of the electrolyzers.
FAHL 04101/04201/04301/04401/04501/04601 devices and locally installed FI 04101/04201/04301/04401/04501/04601 devices control the supply of salt solution to the electrolyzers.
A solution of sodium hydroxide with a mass fraction of sodium hydroxide (29-29.5)% and a temperature of (80-83) °C is supplied to the cathode compartment of the electrolyzers.
FAHL 04103 04603 devices and locally installed FI 04103 04603 devices monitor the flow of caustic soda solution into the electrolyzers. From the anode compartment with a temperature of no more than 90 ° C comes out: anod - a weak saline solution, with a mass concentration of sodium chloride g/dm³ and wet chlorine gas. From the cathode compartment with a temperature of no more than 90 °C comes out: catod - a solution of caustic soda with a mass fraction of sodium hydroxide (31-32)% and hydrogen gas.
Operating temperature of the membrane (85-90) °C.
The electrolysis temperature is maintained by regulating the temperature of the saline solution and caustic soda solution supplied to the electrolyzers.
The temperature of the caustic soda solution in the electrolyzer is controlled by TAN 04103 04603 devices. Devices TI 04101 04601 and TI 04102 04602, which control the temperature of the anolyte and catholyte, respectively, are located in the electrolysis room.
The pressure of the wet chlorine gas leaving the anode compartment of the electrolyzers is maintained within the range of (0.018-0.02) MPa [(0.18-0.2) kgf/cm²] and is controlled by PI 04101 04601 devices. The pressure of hydrogen leaving the cathode compartment is maintained within the range of (0.021-0.023) MPa [(0.21-0.23) kgf/cm²] and is controlled by the PI04102 04602 device. A pressure difference of 0.003 MPa (0.03 kgf/cm²) between the anode and cathode compartments ensures immobility of the membrane on the anode surface, reducing vibration and possible damage due to abrasive wear.
To create operating pressure in the anode and cathode compartments at the time of starting up and bringing the electrolyzers to operating mode, a nitrogen supply is provided to the anode and cathode compartments. Nitrogen pressure is controlled by devices РIС04005, РIС04006 and regulated by valves РV04005, РV04007.
To maintain the pressure of nitrogen supplied to the hydrogen collector 0.003 MPa (0.03 kgf/cm2) higher than in the nitrogen collector supplied to the chlorine collector, a correction is provided from the PIC04005 device to the PIC04006 device.
Hydraulic seals pos.4.VT02/03 are used to prevent the pressure in the nitrogen collectors from increasing above the set value. River water is supplied to the hydraulic seals as a barrier fluid.
The pressure difference between the anode and cathode compartments of each electrolyzer is controlled by PDIAHL 04103 04603 devices.
The pressure difference between the main chlorine and hydrogen collectors is controlled by the PDIAHLSHHLL 04104 device.
Hydrochloric acid is periodically injected into the pure brine supply line to maintain the oxygen content of the chlorine gas. The consumption of hydrochloric acid is controlled by devices FAH 04105 04605 and devices FI 04105 04605 installed locally. Analytical monitoring of wet chlorine gas from each electrolyzer for hydrogen content is carried out systematically twice a shift. The volume fraction of hydrogen in chlorine gas should not exceed 0.1%.
The main principle of safe operation of electrolysers is the timely identification and elimination of deviations from the normal technological regime.
The normal operation of a membrane electrolyzer depends on the quality of the feeding saline solution.
Decontamination of the anode and cathode due to the increased content of impurities in the saline solution, the low pH value of the anolyte (less than 2.0), as well as insufficient supply of feeding saline solution and caustic soda solution lead to an increase in voltage in the cells. The value of the group voltage of each electrolyzer is transmitted to the automated process control system.
Group voltage control of electrolyzer cells is carried out by devices:
Group I – EIANSNN 04151 EIANSNN 04651.
Group II – EIANSNN 04152 EIANSNN 04652.
In case of maximum voltage in one group of cells, when the lower limit value is reached, a warning alarm is triggered. When the upper value limit is reached, a warning alarm and blocking are triggered (the corresponding current circuit is opened).
The total voltage in the electrolyzer is controlled by EI04150 04650 devices. In the case of a maximum rate of voltage increase, depending on the current load in one group of cells, when the lower limit of the value is reached, a warning alarm is triggered.
When the upper value limit is reached, an alarm and blocking are triggered (the corresponding current circuit is opened).
The MELIS-test system allows you to identify defective membranes in electrolyzers without stopping or disassembling them.
During normal operation at high current densities, damage to one or more membranes has no effect on the cell voltage.
With significant damage to the membranes, the oxygen content in the chlorine gas increases and the current efficiency decreases.
The operating principle of the MELIS test system is based on various anodic reactions that occur during polarization at the anode, depending on the integrity of the membrane.
Using a special polarizing rectifier, a polarizing current (about 40 A/m²) is passed through the electrolyzer and the voltage of each individual elementary cell is recorded. A unit cell voltage of less than 2.3V indicates membrane damage.