1. The importance of pH measurement The pH value is the negative logarithm of the concentration of hydrogen ions in the solution (pH = -lg[H]), which represents the pH of the solution. FGD (FlueGas Desulfurization) The pH value of the slurry in the absorption tower is a key detection and control parameter, affecting the amount of absorbent and desulfurization efficiency in the absorption tower. The higher the pH value, the faster the diffusion rate of SO2 to the liquid film body, and the greater the mass transfer coefficient, which promotes the absorption of SO2, but it is not conducive to the dissolution of limestone, which can easily cause precipitation of the equipment system and plug the system; the lower the pH, there is The dissolution of limestone is favored, but the stronger the acidity is, the less likely the SO2 acid gas is absorbed, resulting in a decrease in the desulfurization rate. When the pH value is less than 4, the SO2 absorption of the system can hardly proceed, and the acidity of the absorbent slurry liquid is enhanced at this time, and the equipment is seriously corroded.
The proper pH value can make the desulfurization agent in the whole desulfurization system crystallize and form a massive gypsum through the processing of vacuum belt conveyor and filter press to facilitate recycling. In order to take both into account, for the forced oxidation of the absorption tower, according to engineering experience, the pH is generally controlled within the range of 5.0 to 6.0.
2. Arrangement and analysis of pH meter in engineering application The pH meter probes commonly used in wet desulfurization are composite sensors, ie sensors with measuring electrodes, reference electrodes and temperature measuring elements. During the measurement, the electrode is immersed in the liquid to be measured, the concentration of H in the solution is converted into an mV voltage signal, the signal is amplified and converted to a pH value by logarithm.
In the FGD slurry system, at least 2 pH meters are installed at each pH measurement point. In normal operation, the average value of their measured values ​​is generally taken and they are involved in the closed-loop control of the limestone slurry feed in the absorber tower. There are three general arrangements for pH sensors: immersed, flow-through, and plug-in. The characteristics of its layout, combined with the layout and application of the project, were separately analyzed as follows.
2.1. Immersion Arrangement The pH measuring slurry is collected at the outlet branch of the absorption tower discharge pump, and the measuring slurry is introduced into a measuring tank or tank equipped with a pH electrode. The slurry in the tank is continuously flowed. The tested slurry enters the waste slurry tank. Or return to the measurement box and install the pH meter probe vertically or obliquely on the measurement box to make contact with the liquid to be measured.
This method is easy to put in and take out the electrode, can reduce the erosion of the gypsum slurry on the measuring electrode, and it is convenient for on-site calibration with a portable pH meter. However, when the pressure inside the container is positive, the seal may not be good enough to cause the leakage of the slurry. Different manufacturers have different functional configurations, and their prices are different. Therefore, because the cost of this method is generally too high, it is often less used by environmental engineering contractors.
2.2. Circulation-type arrangement The flow-through type sensor is generally installed in the measuring pipe of the process medium. There is an isolation valve upstream and downstream of the meter, and the meter is also equipped with a regular automatic flushing valve downstream, as shown in Fig. 1.
In this method, the pH electrode is basically washed and worn less, and the on-line electrode can also be rinsed cleanly. However, the pressure of the flushing water must not be too large, otherwise the pH electrode may be easily broken. At the same time, it should be ensured that the bypass pipe is full of liquid, and it is not possible to select the point where the hysteresis is measured to install the measurement point. The disadvantage is that the sampling tube is easy to be blocked and the measurement piping system is more complicated.
2.3. Plug-in Arrangement The plug-in sensor is a variation of the flow-through sensor, but does not require a measuring branch. It is inserted directly into the fluid through the sealing material and an isolating valve, as shown in Fig. 2.
The plug-in sensor can be inserted obliquely in a vertical tube or inserted vertically in a horizontal tube. During installation, care should be taken not to allow air bubbles at the measuring electrode to avoid affecting the authenticity of the measurement. When installing on a horizontal pipe, deviate from the side of the pipe by 15 to 20 degrees in the vertical direction. Actual operation indicates that the inserted probe should not be placed at a high flow rate in the center of the tube. Fluid erosion tends to abrade the probe and the value drifts. The probe should be placed at the edge of the fluid and always be in contact with the fluid. In this way, the pH electrode is directly inserted into the main line, and the electrode is easily worn and affects its service life.
Among the above three arrangements, the arrangement of any sensor has its advantages and disadvantages. The immersed sensor is easy to maintain but leaks easily when the container is under positive pressure. The measurement branch of the flow-through sensor is easy to be blocked, the piping system is more complicated, and the plug-in sensor piping is simple, but the wear is serious.
3. Optimized arrangement in the desulfurization slurry system In practice, most of the pH meters are installed in circulating slurry lines or gypsum slurry discharge lines. In order to overcome the deficiencies of its three conventional arrangements described above, it may be considered to install the pH meter directly in the measurement pipeline of the absorption tower wall, as shown in FIG. 3 .
A straight pipe section 1 is drawn from the lower part of the absorption tower wall and is installed at an angle of 15° to the vertical center line of the tower. Another straight pipe section 2 is led out at the upper part of the tower wall and is installed at an angle of 15° to the horizontal line. The two straight pipes intersect with each other. A tee is formed at the top and a top cover is provided at the top of the tee. The pH meter probe is inserted into the tee down from the top cover. Valves are provided between the upper and lower tower wall straight pipe connections and the tee. In the straight pipe section 1 pipeline, a flushing interface for connecting the flushing pipe is provided.
During normal operation, under the action of the gravitational force of the absorption tower slurry, the new slurry enters the three-way pipe through the straight pipe section 1 and the slurry is separated. The gypsum in the slurry is precipitated as crystals and flows into the absorption tower through the straight pipe section 1. The solution from which part of the gypsum is separated is returned to the absorption tower via the straight pipe section 2 so that a natural circulation of the slurry is formed, and the pH meter probe continuously collects the liquid in the tower and simultaneously updates the pH of the displayed slurry. When the pH meter needs to be flushed, flush water flushes the gypsum crystals accumulated on the pH meter probe through the flushing interface to avoid abrasion and contamination.
This arrangement method realizes that the electrode can be rinsed cleanly online, and the real-time property of the pH measurement value is ensured, the wear of the pH electrode by the slurry is slowed, thereby prolonging the service life of the electrode, and during the entire measurement process of the pH meter, The self-weight of the slurry in the tower forms a natural circulation of the slurry, and the pH of the slurry can be obtained without any additional power equipment. Therefore, this arrangement is simple, reliable, easy to maintain, and can guarantee the real-time nature of the slurry pH.
The proper pH value can make the desulfurization agent in the whole desulfurization system crystallize and form a massive gypsum through the processing of vacuum belt conveyor and filter press to facilitate recycling. In order to take both into account, for the forced oxidation of the absorption tower, according to engineering experience, the pH is generally controlled within the range of 5.0 to 6.0.
2. Arrangement and analysis of pH meter in engineering application The pH meter probes commonly used in wet desulfurization are composite sensors, ie sensors with measuring electrodes, reference electrodes and temperature measuring elements. During the measurement, the electrode is immersed in the liquid to be measured, the concentration of H in the solution is converted into an mV voltage signal, the signal is amplified and converted to a pH value by logarithm.
In the FGD slurry system, at least 2 pH meters are installed at each pH measurement point. In normal operation, the average value of their measured values ​​is generally taken and they are involved in the closed-loop control of the limestone slurry feed in the absorber tower. There are three general arrangements for pH sensors: immersed, flow-through, and plug-in. The characteristics of its layout, combined with the layout and application of the project, were separately analyzed as follows.
2.1. Immersion Arrangement The pH measuring slurry is collected at the outlet branch of the absorption tower discharge pump, and the measuring slurry is introduced into a measuring tank or tank equipped with a pH electrode. The slurry in the tank is continuously flowed. The tested slurry enters the waste slurry tank. Or return to the measurement box and install the pH meter probe vertically or obliquely on the measurement box to make contact with the liquid to be measured.
This method is easy to put in and take out the electrode, can reduce the erosion of the gypsum slurry on the measuring electrode, and it is convenient for on-site calibration with a portable pH meter. However, when the pressure inside the container is positive, the seal may not be good enough to cause the leakage of the slurry. Different manufacturers have different functional configurations, and their prices are different. Therefore, because the cost of this method is generally too high, it is often less used by environmental engineering contractors.
2.2. Circulation-type arrangement The flow-through type sensor is generally installed in the measuring pipe of the process medium. There is an isolation valve upstream and downstream of the meter, and the meter is also equipped with a regular automatic flushing valve downstream, as shown in Fig. 1.
In this method, the pH electrode is basically washed and worn less, and the on-line electrode can also be rinsed cleanly. However, the pressure of the flushing water must not be too large, otherwise the pH electrode may be easily broken. At the same time, it should be ensured that the bypass pipe is full of liquid, and it is not possible to select the point where the hysteresis is measured to install the measurement point. The disadvantage is that the sampling tube is easy to be blocked and the measurement piping system is more complicated.
2.3. Plug-in Arrangement The plug-in sensor is a variation of the flow-through sensor, but does not require a measuring branch. It is inserted directly into the fluid through the sealing material and an isolating valve, as shown in Fig. 2.
The plug-in sensor can be inserted obliquely in a vertical tube or inserted vertically in a horizontal tube. During installation, care should be taken not to allow air bubbles at the measuring electrode to avoid affecting the authenticity of the measurement. When installing on a horizontal pipe, deviate from the side of the pipe by 15 to 20 degrees in the vertical direction. Actual operation indicates that the inserted probe should not be placed at a high flow rate in the center of the tube. Fluid erosion tends to abrade the probe and the value drifts. The probe should be placed at the edge of the fluid and always be in contact with the fluid. In this way, the pH electrode is directly inserted into the main line, and the electrode is easily worn and affects its service life.
Among the above three arrangements, the arrangement of any sensor has its advantages and disadvantages. The immersed sensor is easy to maintain but leaks easily when the container is under positive pressure. The measurement branch of the flow-through sensor is easy to be blocked, the piping system is more complicated, and the plug-in sensor piping is simple, but the wear is serious.
3. Optimized arrangement in the desulfurization slurry system In practice, most of the pH meters are installed in circulating slurry lines or gypsum slurry discharge lines. In order to overcome the deficiencies of its three conventional arrangements described above, it may be considered to install the pH meter directly in the measurement pipeline of the absorption tower wall, as shown in FIG. 3 .
A straight pipe section 1 is drawn from the lower part of the absorption tower wall and is installed at an angle of 15° to the vertical center line of the tower. Another straight pipe section 2 is led out at the upper part of the tower wall and is installed at an angle of 15° to the horizontal line. The two straight pipes intersect with each other. A tee is formed at the top and a top cover is provided at the top of the tee. The pH meter probe is inserted into the tee down from the top cover. Valves are provided between the upper and lower tower wall straight pipe connections and the tee. In the straight pipe section 1 pipeline, a flushing interface for connecting the flushing pipe is provided.
During normal operation, under the action of the gravitational force of the absorption tower slurry, the new slurry enters the three-way pipe through the straight pipe section 1 and the slurry is separated. The gypsum in the slurry is precipitated as crystals and flows into the absorption tower through the straight pipe section 1. The solution from which part of the gypsum is separated is returned to the absorption tower via the straight pipe section 2 so that a natural circulation of the slurry is formed, and the pH meter probe continuously collects the liquid in the tower and simultaneously updates the pH of the displayed slurry. When the pH meter needs to be flushed, flush water flushes the gypsum crystals accumulated on the pH meter probe through the flushing interface to avoid abrasion and contamination.
This arrangement method realizes that the electrode can be rinsed cleanly online, and the real-time property of the pH measurement value is ensured, the wear of the pH electrode by the slurry is slowed, thereby prolonging the service life of the electrode, and during the entire measurement process of the pH meter, The self-weight of the slurry in the tower forms a natural circulation of the slurry, and the pH of the slurry can be obtained without any additional power equipment. Therefore, this arrangement is simple, reliable, easy to maintain, and can guarantee the real-time nature of the slurry pH.
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