Debottlenecking sulfur units
In a typical modified Claus sulfur unit (Figure 12) H2S (hydrogen sulfide) is converted to SO2 (sulfur dioxide) and H2O (water) in a reaction furnace by the following reaction:
2H2S + 3O2 • 2SO2 + 2H2O
In the reaction furnace and in subsequent catalytic converters, the SO2 is reduced to S2 (elemental sulfur gas) as follows:
2SO2 + 4H2S • 3S2 + 4H2O
The gaseous sulfur is condensed into liquid in special condensers that follow the reaction furnace and the converters. Liquid sulfur mist is removed from the effluent gas by high-temperature mist eliminators, typically mounted diagonally in the condenser heads as suggested in Figure 12.
If the charge rate of H2S-rich gas from the amine treaters is increased beyond design capacity, liquid sulfur becomes entrained in the gas emerging from the condensers. The reheaters may not entirely evaporate the sulfur mist, so liquid sulfur may be deposited in the catalyst beds of the converters. This plugging deactivates catalyst, so that conversion of SO2 to S2 decreases. Plant capacity is reduced, and the SO2 concentration increases in tail gas sent to the incinerator. That gas also carries sulfur mist from the last condenser. Both effects result in more SOX in the stack gas.
In many cases, the main problem is that the diagonal mesh-type mist eliminators in the condenser heads flood with liquid and may even become partly plugged with solidified sulfur, greatly reducing mist elimination efficiency. The solution is as shown in Figure 13 for one of the condensers.
The diagonal mesh pad is removed, and a vane-mesh combination is mounted horizontally in a housing provided in the condenser head. In this special case, the vane unit is mounted upstream of the mesh, rather than downstream as described before for a mesh agglomerator. The vane removes most of the liquid sulfur as larger droplets, and the mesh serves to scrub the remaining finer mist. This pad is a special high-capacity, high-efficiency variety. The result is more condenser capacity, longer catalyst runs, and minimal plugging of the mesh mist eliminator.
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