Evaporative crystallization
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The sulfuric acid AN post-treatment system for the ammonia-based desulfurization unit in our company uses single-effect evaporation crystallization. Due to a break in the oxidation air duct inside the tower, the oxidation rate of the sulfuric acid AN solution in that tower can only be maintained at around 65%. As production demands require it, it is not possible to shut down the operation to repair the oxidation air duct for the time being. During operation, it was observed that after a few hours of running, as time progressed the flow rate of the heating steam gradually decreased, the temperature inside the crystallizer increased steadily, the current drawn by the axial flow pump rose rapidly, while the negative pressure at the outlet of the crystallizer remained unchanged. We tried to analyze the reasons: 1. The flow of condensate in the secondary steam cooler was poor; some of the condensate covered certain sections of the cooler tubes, resulting in a reduced heat exchange area ； 2. There are too many fine particles in the sulfuric acid an crystals; these fine particles have high viscosity and poor flowability, leading to adhesion inside the heater tubes. We also tried to adjust the operations: 1. We opened the low-pressure discharge of the secondary steam cooler to increase the drainage volume of the condensate from the cooler, but this did not yield good results ； 2. Soak the heater tubes in clean water for a period of time (while supplying heating steam); after injecting fresh sulfuric acid solution, the steam flow increases, and the temperature inside the crystallizer is lower than that before soaking in clean water, resulting in enhanced evaporation. The second reason cannot be explained at all using theoretical knowledge. Could all the sea friends please help analyze which other reasons we might have failed to consider?
Based on your description, in addition to the failure to drain the condensate in a timely manner, it is also possible that there is an excessive amount of non-condensable gas inside the evaporator, which fails to be removed from the system. This leads to an excess of non-condensable gas in the evaporator, reducing its efficiency and thus decreasing the amount of vaporized fluid. Article 2 concerns the situation where the heat exchange tubes get blocked, which results in poor heat exchange efficiency and a decrease in evaporation rate. After clearing the blockages using your method, the evaporation rate improves to some extent. As the evaporation rate increases, the liquid level needs to be replenished. Since the newly added sulfuric acid AN material has not been preheated to the temperature inside the crystallizer, its temperature drops slightly (but not too much; it remains at or above the evaporation temperature) after entering the crystallizer. This, in turn, increases the temperature difference for heat exchange, thereby improving the efficiency of the heat exchanger and leading to an increase in evaporation rate. This is one possible explanation. Another possibility is related to the location of the temperature sensor’s measuring point: within a short period of time, the material inside the crystallizer is not evenly mixed, so the temperature inside the crystallizer is not uniform. Since the temperature sensor is located right there, this leads to the observed drop in temperature. There’s no need to worry too much about it: lol