What will be the outcome when the molar ratio of H2S/SO2 in the exhaust gases from the Claus reaction deviates from 2:1?

According to the stoichiometric ratio of chemical reactions, the ratio of hydrogen sulfide to sulfur dioxide should be maintained at 2:1. It is not possible to achieve such strict control in actual production; either an imbalance in this ratio or too high a concentration will result in a reduced sulfur production rate, and acidic gases such as sulfur dioxide will enter the exhaust gas treatment system, increasing the workload of that system.

Agree with the view from the 2nd floor. >2:1; the load on the subsequent absorption section increases, affecting the efficiency of hydrogen sulfide absorption ； ＜2:1, the load on the exhaust hydrogenation unit increases, affecting the efficiency of hydrogenation and the lifespan of the catalyst.

First, it is necessary to understand why the molar ratio of 2S to SO2 should be 2:1? Because at this ratio, the driving force for the Claus reaction is greatest, the reaction is most complete, and the sulfur recovery rate in the Claus stage is highest. So we have the first answer: deviations, whether too large or too small, will affect the sulfur conversion rate, which in turn affects exhaust emissions. Next, it is necessary to discuss in detail the issues of values that are too high or too low. If the ratio is too high and there is an excess of hydrogen sulfide, as long as the absorption system has sufficient capacity to absorb all of this excess hydrogen sulfide, then there is no problem. Otherwise, the excess hydrogen sulfide will end up in the exhaust gas combustion section; this can lead to excessive emissions in mild cases, or even the formation of elemental sulfur within the exhaust gas combustion furnace, which can cause blockages in the heat exchangers and chimneys ； The ratio is too low; there is an excess of sulfur dioxide. As long as the hydrogenation reactor has sufficient capacity to convert all of this excess sulfur dioxide into hydrogen sulfide, then there isn’t much of a problem. Otherwise, the excess sulfur dioxide enters the quenching system, where it forms strong acids that accelerate equipment corrosion significantly, leading to catastrophic consequences ； Even sulfur dioxide, which cannot be completely converted through hydrogenation, and hydrogen sulfide produced by hydrogenation react in subsequent systems to form sulfur, thereby clogging the systems.

The analysis upstairs is thorough; that’s the right answer :)

I would like to add my personal opinion: this 2:1 ratio is a theoretical value. In our actual operations, since the feed components are not distributed in an ideal manner but contain various impurities, the actual ratio tends to be on the lower side. As a result, the emission levels from the chimney actually decrease in such cases! Converted to operational data, this means that the air distribution ratio should be slightly higher than the theoretical value.

First, it is necessary to understand why the molar ratio of H2S to SO2 should be 2:1? Because at this ratio, the driving force for the Claus reaction is greatest, the reaction is most complete, and the sulfur recovery rate in the Claus stage is highest. So we have the first answer: deviations, whether too large or too small, will affect the sulfur conversion rate, which in turn affects exhaust emissions. Secondly, it is necessary to specifically discuss the issues of being too large or too small; one needs to understand what the fundamental factor affecting the ratio is, and that is actually the air supply ratio, which makes things much clearer. An excessive amount of air supply reduces the conversion rate of H2S, causes the furnace temperature to rise, increases the temperature of the catalyst bed, and leads to catalyst deactivation. The low air supply volume reduces the conversion rate of H2S; the hydrocarbons in the acidic gases cannot burn completely, resulting in the formation of carbon black, which affects the quality of sulfur. This carbon black also covers the catalyst, impairing its activity and thus further reducing the H2S conversion rate. It also causes the temperature of the entire system to drop, making control difficult, and the H2S concentration in the exhaust gases exceeds the allowed levels. Therefore, it is better to increase the air supply volume rather than reducing it.