Source: Thermal Control Circle
The slurry used to absorb sulfur dioxide in the absorption tower needs to be introduced with oxidation air during operation, so that the slurry contains bubbles. During the chemical reaction of the slurry in the absorption tower, because the slurry contains chloride ions and dust, the slurry itself will bubble, which further aggravates the slurry bubble content. Due to the special structure of the absorption tower, the level measurement of the absorption tower cannot be carried out in the form of a top-mounted level gauge, and only a side-mounted pressure transmitter can be used to measure the liquid level according to the principle of different liquid levels and different bottom pressures.
h=P/ρg
ρ in the formula must be the density of the slurry containing the bubbles. In the case of bubbles in the absorption tower and the slurry contains bubbles, it is not reasonable to calculate the liquid level using ρ1 measured by the density measurement point of the absorption tower (we use ρ1 as the density of the absorption tower to measure the density of the absorption tower). Because the density measured at this measuring point is the density of the slurry after the bubbles are released, it is numerically greater than the density of the slurry containing the bubbles. The absorption tower level calculated with this ρ1 is lower than the actual level.
In fact, the number of bubbles in the slurry varies frequently depending on the operating conditions. If the slurry quality is not good, it will aggravate the degree of foaming, and the addition of defoamer will reduce or even disappear the bubbles. We have to measure the density of the slurry containing air bubbles in real time for level calculations, and the resulting level is accurate.
The bubbles contained throughout the tower are uneven, and in general, the bubbles in the tower change from less to more from bottom to top. For this reason, the formula h=P/ρg is used to calculate the liquid level, which is not rigorous and can only approximate the liquid level (the formula requires the liquid to be homogeneous, but the actual liquid is not uniform).
At present, there are two types of methods for measuring liquid level in the desulfurization absorption tower, one is the bottom differential pressure method, and the other is the middle and lower differential pressure method.
The bottom differential pressure method, which we call method 1, is to install a pressure transmitter 0.5 meters and 2.5 meters from the bottom of the absorption tower, and these two transmitters calculate the average density of the slurry between 0.5 meters and 2.5 meters, and then calculate the liquid level according to the pressure measured by the 0.5 meter pressure transmitter. Alternatively, a pressure transmitter can be installed at 0.5 m and the liquid level can be calculated based on ρ1 measured at the density measuring point of the absorption tower and the pressure measured by the pressure transmitter. These two methods are used as the bottom differential pressure method, and the density they use is the density after the bubbles are released or close to the density after the bubbles are released from the slurry, and the measurement error is large under the condition of slurry foaming. As a rule of thumb, the error can reach 2-3 meters.
The middle and lower differential pressure method, which we call method 2, is to install a pressure transmitter 0.5 meters and 10 meters from the bottom of the absorption tower, and these two transmitters calculate the average density of the slurry between 0.5 meters and 10 meters, and then calculate the liquid level according to the pressure measured by the 0.5 meter pressure transmitter. The density measured in this way reduces the deviation from the actual situation. As a rule of thumb, level measurement errors can be reduced to 1-2 meters. The reason for the error is that the average density of the slurry between 0.5 m and 10 m is much greater than the density of more than 10 m under the blistering condition, resulting in the calculated liquid level being lower than the actual liquid level.
If we install two pressure transmitters close to the liquid level, then the density calculated by the two pressure transmitters is the density close to the liquid level, and the pressure measured by the pressure transmitter is also close to the liquid level, such a working condition is the working condition (uniform liquid) required by the formula h=P/ρg, and the calculated liquid level near the liquid level is real-time and accurate. There is no need to consider the uneven density of the slurry near below the liquid level, no matter how complex the bubble condition is, we can simply add the elevation of the pressure transmitter to install it.
However, the liquid level is changing, and we cannot install the pressure transmitter in a position that changes with the liquid level. The next best thing we did was to install the two pressure transmitters below the minimum operating level, which was inlet from the overflow tube of the absorption tower.
This method is called method 3, the upper differential pressure method, that is, 0.5 meters and 1.5 meters below the inlet of the overflow pipe of the absorption tower, a pressure transmitter is installed respectively, the density calculated by these two transmitters can basically represent the density of the absorption tower above 1.5 meters below the inlet of the overflow pipe of the absorption tower, and the liquid level is calculated according to the pressure measured by the pressure transmitter 1.5 meters below the inlet of the overflow pipe of the absorption tower. This method measures the level of the absorption tower and has a measurement error of less than 0.2 meters within the normal operating range.
The following is a schematic illustration:
Slurry without bubbles:
The liquid level measured in method 1 is consistent with the actual liquid level:
The liquid level measured in method 2 is consistent with the actual liquid level:
The liquid level measured in method 3 is consistent with the actual liquid level:
The absorption tower is foaming, the slurry contains bubbles, the liquid level is higher than the highest point of the overflow pipe, and the working conditions of slurry overflow:
Method 1, the measured liquid level is far lower than the actual liquid level, and the operator is still reassuring self-absorption, but he does not know that the slurry has overflowed, and the absorption tower has become an island at sea.
Method 2, the measured liquid level shows that the liquid level of the absorption tower is high, and the operator is ready to take measures to lower the liquid level, but it is actually overflowing.
Method 3, the measured liquid level is consistent with the actual liquid level, and the operator finds that the liquid level is higher than the overflow liquid level, and informs the inspection to take measures to deal with the overflow slurry on the spot, and urgently lowers the liquid level.