【Process Discussion】Precautions for Cooling Concentrated Sulfuric Acid

Changing the flow direction of the coolant from reverse to forward improves resistance to clogging. Couldn’t a heat exchange device be used with water at -18°C to transfer the cold energy to the refrigerant at 3°C?

For 98% concentrated sulfuric acid when it is dried, what parameters should be used to determine when it’s time to replace the sulfuric acid or make adjustments? In other words, to what concentration should the acid be diluted? It is also necessary to consider whether crystallization will occur at -18°C; if it is to be used, the heat exchange area needs to be recalculated. Additionally, the pipes at -18°C need to be insulated, and it is worth considering whether these pipes can be installed alongside the existing pipes at 3°C

The original poster also provided the freezing points of concentrated sulfuric acid at different concentrations; eighteen degrees below zero is indeed quite low, and problems will arise in the system. Even if the control temperature of concentrated sulfuric acid is achieved by adjusting the flow rate and heat exchange area, the system becomes very unstable once there are fluctuations.

This post was last edited by arpcd on 2017-4-8 at 17:51. There are no issues; it can be used. I carried out a similar modification for a factory years ago (using saline at -15°C, with actual operating temperatures sometimes reaching -20°C). The process diagram in the *version is incomplete – it doesn’t show where the concentrated acid is added, and the information regarding the crystallization temperature is also insufficient. Let’s first look at this crystalization temperature chart. The reason why it is said that you haven’t provided the complete information is that when using low-temperature chilled brine, it is important to keep the temperature entering the acid cooler strictly within the range of 92.5–95% (normally, 94% is the ideal value; however, note that this 94% figure needs to be considered in conjunction with your drying efficiency). In other words, the concentration of the dilute acid after water absorption must be tightly controlled – too much water or too little water are both problematic, as either situation can lead to freezing. Don’t assume that keeping the temperature at 15°C will suffice; for heat exchangers, since the wall temperature is closer to the side with a higher heat transfer coefficient, crystallization is still possible. But by switching to brine at -18°C, the temperature difference on the cold side of your original heat exchanger was 12°C, while now it is 30°C. It is recommended to reduce the brine circulation rate slightly, a bit lower than the 3°C value used before. If the temperature difference in the original chilled water system was 5°C (inlet at 3°C, outlet at 8°C), you can now keep it between 8–10°C (inlet at -18°C, outlet at -8°C), and try to increase the circulation rate of the acid pump to ensure that the acid side of the plate heat exchanger does not freeze. Pay attention to the solid composition in the table – what is the composition at the time of freezing? What should be done if the parameters are not controlled properly? The saltwater inlet valve can be closed briefly, and this will basically solve the problem of ice blockage. In practice, the modified device has hardly encountered any issues with ice blockage, making it quite easy to operate. Another thing to pay attention to is the location where the concentrated acid is added. This is a simple schematic diagram from the renovation process, and it’s easy to understand. The addition point for the concentrated acid has been modified – the packing material has been replaced with bubbles. In reality, this doesn’t offer much benefit; the only advantage is that it allows operators to be a bit less careful, as bubbles have a greater resistance to clogging compared to packing material. Furthermore, the operating conditions of the plate heat exchanger have changed; please have the manufacturer check this. Generally, there should be no issues, as the plates are made of low-alloy stainless steel, so such temperature changes should not pose a problem. Basically, just pay attention to that and it’ll be fine; as for things like heat preservation, you can think about them on your own. . . .

1. Just based on the table provided, with 98% concentrated sulfuric acid, the crystallization point is -0.7°C; using -18°C will cause freezing. Using frozen water at -18°C requires 95% concentrated sulfuric acid. 2. Concentrated sulfuric acid is used for drying, and its concentration decreases (PS: A decrease in concentration releases heat; does that have an impact?) (I don’t quite understand.) 3. The question states that a new set needs to be added; since the concentrated sulfuric acid in the old set absorbs water and gets diluted, what should be done? It would be better to lower it to 95% and continue using this new set.

When using 95% concentrated sulfuric acid, the water removal efficiency needs to be evaluated, right? ……

Haha, thanks again for your guidance, Brother Dao~ I think the key to this issue lies in what you said: when using low-temperature frozen brine, it’s important to keep the temperature entering the acid cooler strictly within the range of 92.5–95% (normally, 94% is the ideal value; however, keep in mind that this 94% figure needs to be considered in conjunction with the drying efficiency). Due to the requirements of the manufacturing process, very low levels of moisture in HCL are necessary. In the current first-phase drying system, two sets of concentrated sulfuric acid drying towers are used for drying – the concentration of sulfuric acid in the first tower is maintained at 94–96%, while that in the second tower is kept at 96–98%. Therefore, based on the requirements of the process, it’s not allowed for the concentration of sulfuric acid to drop below 94%. In other words, the concentration of concentrated sulfuric acid entering the cooler should be at least 96% at -12.6°C; using chilled water at -18°C seems inappropriate in this case.

1. From a process requirements perspective, in order to achieve complete drying, the concentration of concentrated sulfuric acid cannot be reduced to 95%. As I said upstairs, at least 96% concentration at -12°C. I don’t think using water at -18°C here will work; freezing is inevitable. 2. I didn’t understand your second question: concentrated sulfuric acid absorbs water, which reduces its concentration and results in heat release; therefore, circulation cooling is used at the bottom of the tower. 3. The old process produces 94% dilute acid, which we have other uses for, you know, in wastewater treatment~