Sulfurex: Gas Desulphurization

Description

Sulfurex® CR (Caustic Reuse) is a chemical scrubber in which H2S is absorbed into a liquid at high pH to enhance the absorption capacity of the solvent, and then further oxidized during the aerobic treatment of the effluent in a wastewater treatment plant (WWTP).

Sulfurex® BF (Biotrickling Filter) is a biological process in which the H2S is absorbed in an aqueous solution at low pH and then biologically oxidized to sulfur/sulfate in-situ.

Sulfurex® BR (Biological Regeneration) is the combination of Sulfurex® CR with a bioreactor for the biological regeneration of caustic. In the Sulfurex® BR, H2S is absorbed into a solvent under alkaline conditions, then further oxidized into sulfur in the bioreactor.

Since the Sulfurex® BR technology is a robust combination of different desulphurization technologies, we here further explain how the process works.

Sulfurex® BR

Biogas is produced by fermentation of biological waste products and is considered as one of the most important renewable energies as replacement of natural gas. Syngas is produced by gasification of a carbon containing fuel and used as intermediate in generating synthetic natural gas. Both gases can contain hydrogen sulphide which needs to be removed due to potential mechanical problems, environmental and safety reasons.

Sulfurex® BR is a desulfurization process that combines chemical desulfurization at medium to high pH with biological regeneration of the solvent (caustic). The system consists of a packed column, a biological reactor and a settler. Figure 1 shows a basic Process Flow Diagram of a Sulfurex® BR process.

Packed column

The biogas enters the scrubber at the bottom of the tower and flows upwards through a packed column of approximately 12 meters in height. A caustic solution is distributed on top of the column over the packing media and falls down through the packing material in counter-current direction of the gas. The packing material inside the column ensures good contact between hydrogen sulfide and the process liquid for a maximum efficiency.

While the biogas flows through the packed column, H2S is absorbed in the solvent. The biogas leaves the column free of hydrogen sulfide at the top. The saturated process liquid is collected in the sump at the bottom of the scrubber, and flows under gravity to the bioreactor.

Bioreactor

In the bioreactor, the hydrogen sulfide present in the liquid is biologically oxidized into elemental sulfur (S0) by Thiobacillus bacteria. The oxygen required is supplied by an aeration system installed at the bottom of the bioreactor.

During the oxidation, the caustic solution is regenerated before being reused for another washing step in the scrubber.

In the bioreactor, water, nutrients and caustic are automatically refreshed for cellular growth and guarantee good operation conditions. Whenever the measured reactor fluid level is higher than the set point, part of the liquid will be discharged. The bleed stream is harmless and can generally be discharged without any form of post-treatment.

Under optimal conditions, 98 % of the hydrogen sulfide is converted into elemental sulfur.

Settler

The elemental sulfur in separated from the process liquid in the settler, which can be integrated inside the bioreactor. The settler is fed with a small part of the effluent coming from the bioreactor. The overflowing process liquid (low TSS) flows from the settler to the bioreactor. Sulfur sludge is removed from the bottom of the settler with a high dry matter content of 5-10 % mass.

This sulfur sludge can then be used as a high quality fertilizer.

Advantages

Sulfurex® BR is a flexible desulfurization technology that achieves low hydrogen sulfide outlet concentrations with low operational expenses thanks to the regeneration step. Since the air injection takes place in the bioreactor, the technology is suitable for biogas upgrading. The technology is suitable for high loads of sulfur, since the reduction in OPEX compensates the higher initial investment cost (in opposition to purely chemical or biological processes).

See the product description on DTM’s website.