Why is the reaction temperature decreasing at each stage of sulfur recovery?

A reactor bed should not have a drop of 50°; the upper layer with newly installed catalyst has better activity, so it’s possible that the temperatures in the middle and lower layers show no increase or even a slight decrease. By comparing the reactor outlet temperature (as shown on-site and in the DCS) with the temperature at the bottom of the bed, if the outlet temperature is high, there may be a problem with the thermocouple. Let the experts analyze it

The high temperature at stage 1 is required to decompose organic sulfur compounds such as COS and CS2 at higher temperatures (300–340°C)； Level 2 (240–260°C) and Level 3 (210–230°C) are such because lower temperatures result in an increased equilibrium conversion rate; moreover, as the level increases, the H2S content decreases, leading to less reaction heat being generated, which is also a key reason for the gradual decrease in temperature.

It is best to use the concept of temperature difference instead of temperature to evaluate the reaction efficiency of the reactor! This is more intuitive!

The Claus reaction that takes place in the reactor is an exothermic reaction; the lower the temperature, the higher the equilibrium conversion rate. Use the lowest possible reactor inlet temperature in order to achieve a higher conversion rate. However, the inlet temperature of the reactor should generally be maintained at least 30°C above the dew point temperature of sulfur; otherwise, the catalyst bed is prone to sulfur accumulation and blockage. Since the partial pressure of gaseous sulfur in the process gas at the reverse inlet is the highest, its sulfur dew point is also high; therefore, the inlet temperature needs to be controlled at a higher level accordingly.

The high temperature at stage 1 is required to decompose organic sulfur compounds such as COS and CS2 at higher temperatures (300–340°C)； Level 2 (240–260°C) and Level 3 (210–230°C) are such because lower temperatures result in a higher equilibrium conversion rate; moreover, as the level increases, the H2S content decreases, and the amount of heat generated in the reaction also decreases, which is another reason for the gradual drop in temperature. hrbj999 has essentially provided the correct answer! I would like to add something here regarding the function of the three-stage condenser. The three-stage condenser releases steam at 0.4 Mpa; the temperature of saturated steam at 0.4 Mpa is around 150 degrees Celsius. In other words, as the process gas passes through each stage of the condenser, and the heat generated by the reactions inside the reactor is not sufficient to compensate for the heat taken away through heat exchange, the temperature of the process gas decreases step by step. The final process gas temperature will be close to 150 degrees Celsius.

It seems to have nothing to do with the low reaction temperature, as the cooled gas is heated by a heater; it’s mainly a matter of equilibrium. At this temperature, the conversion rate of H2S is highest. If the temperature rises, the equilibrium shifts in the reverse direction, and if the temperature drops, sulfur vapor will condense on the catalyst, blocking it

Organic sulfur compounds (such as CS2, COS, thiolates, etc.) require temperatures above 300°C to decompose effectively; therefore, the temperature of the primary bed layer needs to be kept at a relatively high level. If organic sulfur is not effectively converted in the primary reactor, it cannot be adequately treated in the subsequent amine scrubbing unit, which has a significant impact on the final emissions.

It is an exothermic reaction; lower temperatures facilitate the reaction and increase the conversion rate.

Organic sulfur does indeed need to be converted completely at 300°C, which is why some titanium-based organic sulfur catalysts are loaded in a single reactor. Contrary to what might be expected due to the high hydrogen sulfide concentration in the process gas, which results in large heat release (with the bed temperature rising by around 80°C), the outlet temperature of the reactor is usually above 300°C; however, this is not related to the question posed by the original poster.