Exploring exhaust gas recovery processes

I visited several sulfuric acid plants to see their methods for treating exhaust gases; in all cases, spray towers using NAOH as a desulfurization agent were used, and an investment of around 300,000 yuan was considered quite sufficient for such systems. Two plastic towers, two pumps, and a few plastic pipes – that’s all needed

Using sodium hydroxide as the absorption solution can only address the issue temporarily, such as during restarts after major repairs or in case of emergencies. If the exhaust gas absorption device is operated continuously without interruption, the cost of using sodium hydroxide becomes too high. That’s also not feasible.

The fan capacity must be fully considered during design; otherwise, it will affect achieving or exceeding production targets

Sodium hydroxide can be used for this purpose; sodium bisulfite itself is a raw material used in pharmaceutical manufacturing, and it is economically viable to use it if there is demand for it. I know of several companies that use this method to treat exhaust gases. Some facilities even extract the SO2 from the converter inlet to produce sodium bisulfite. There is also the traditional ammonia absorption method, but its effectiveness is limited when dealing with low-concentration exhaust gases, and there is significant ammonia loss. Currently, there is also the method of using activated coke/charcoal for SO2 removal through adsorption/desorption, which produces dilute sulfuric acid and SO2 gas that can be used in sulfuric acid production processes. However, the initial investment and operational costs are high. The most important factor regarding exhaust gas concentration is improving the conversion rate. Mengmoke and Topso offer vanadium catalysts and CS catalysts that achieve very high conversion rates: for 3+1 type exhaust gases, the concentration can be reduced to below 135 ppm, and for 3+2 type gases, it can be reduced to below 100 ppm. At such concentrations, it is possible to meet emission standards even without any exhaust gas treatment

Could someone explain the ammonium bisulfite exhaust gas recovery process? Thank you very much.

Reply to 6# yiyunjin: Ammonium bisulfite process? It involves using ammonia water or liquid ammonia to absorb SO2 from exhaust gases, in order to control the total salt and alkalinity levels and produce products that meet the required standards. Ammonia water is recommended for use; however, both ammonia water and liquid ammonia are hazardous chemicals, and the reaction temperatures are too high, making it difficult for operators to adjust the dosage. There is plenty of information available online on this topic – it’s a fairly mature processing technique. Options include three-tower absorption and two-tower absorption processes. The ammonium bisulfite produced can be further oxidized using air to yield ammonium bisulfate. But when using ammonia to absorb exhaust gases, if the concentration of those gases needs to be kept within acceptable limits, ammonia mist will be quite noticeable (it’s not clear whether there are any restrictions on the ammonia content in exhaust gases from an environmental perspective). Visually, white smoke can still be observed, and this also means that there is significant ammonia loss. Cost considerations must therefore be taken into account; thus, it’s important to control the process parameters carefully. If ammonia is to be used for treating exhaust gases, it’s necessary to ensure that the exhaust gases contain low levels of acid mist. Additionally, a demisting device may need to be installed at the end of the process (although it may not be necessary depending on specific conditions and local environmental regulations). If a demisting device is used, it will add approximately 500–700 mmH2O of resistance to the entire system (the value could be higher). This will increase the load on the main fan, so electricity consumption also needs to be considered. I downloaded some articles related to this topic in order to learn more, but found them to be of limited use. There are many such articles, so I won’t attach them here. If you need them, contact me at yanzhibin2001@126.com

The exhaust gas is absorbed using the ammonia method, but white smoke is still visible. What is the reason?

Reply to 8# txglyl: Personally, I think that in actual production, reducing the amount of ammonia solution used can help minimize white smoke. However, this will lead to an increase in SO2 emission levels. Our facility uses a two-tower, two-tank absorption process, with one tank used for producing the product and the other for process adjustments. To ensure product quality, only one tank is used for adjustments, which limits the range within which process parameters can be adjusted. Some facilities use a three-tower, three-tank system along with a demister; in such cases, the gradient of total salt and alkalinity might be more suitable, and it is said that no white smoke appears in such systems. Additionally, I believe that besides the total salt and alkalinity levels of the circulating liquid, factors such as the temperature of the exhaust gases and air humidity also play a role. When the gas temperature is low and the air humidity is high, white smoke is more noticeable; obviously, white smoke is more prominent on rainy days, while it is less visible on hot and sunny days. It is possible that this is related to the balanced decomposition of ammonium bisulfite in conjunction with the moisture present in the gases, but I have not verified this. Our facility uses a two-tower absorption system, so currently we can only reduce white smoke by operating at levels close to the upper limit for SO2 emissions

Give it a try – treating the exhaust gas with two bases works very well