Recovering CO2 and H2S from waste streams

Thanks to the development of innovative technologies, a European refinery is now able to recover CO2 and H2S from its waste gas streams.

Mahin Rameshni and Stephen Santo
Rameshni & Associates Technology & Engineering (RATE) USA

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Article Summary

US and European refineries must comply with some of the most stringent environmental regulations in the world. In the US, these include the Clean Air and Clean Water Acts and the Toxic Substances Control Act (TSCA) (EPA, 1970) (EPA, 1972) (EPA, 2016). Some of these regulations are progressive in that they incorporate mechanisms designed to reduce the level of allowable emissions over time, such as the European Emissions Trading Scheme for carbon dioxide (CO2) emissions and 'Best Available Techniques', or BAT, for reducing emissions of specified pollutants (European Commission, 2021), (Concawe, 2013).

In addition, the new marine emissions regulation, IMO 2020, mandates a maximum sulphur content of 0.5% in marine fuels globally. The driver of this change is the need to reduce air pollution created in the shipping industry by lowering the sulphur content of fuels for ships not fitted with scrubbers. The additional processing to produce these low  sulphur marine fuels results in increased amounts of waste streams containing SO2, NOx, and CO2 in refineries.

Under these environmental regulations, refinery residuals, gases, or materials that would otherwise be emitted to air or water or disposed of as waste are required to be recovered and, where possible, converted to useful products.

Conventional processes to restore molecules and energy from waste streams are designed by creating a chemical reaction that combines the carbon-based materials in the waste streams with air or oxygen, breaking them down into molecules, removing pollutants and impurities, and recovering the sulphur.

Waste recovery plants are designed differently to other types of units and, due to environmental regulations, investors are trying to convert such waste streams into useful products and accepting the challenges.

Sulphur recovery and CO2 capture
Recently, we have been working on a project developed for CO2 recovery and SO2 emissions control in Europe. The technologies developed are unique and the proprietary design is patented by RATE. They include:

-   The RATE CO2 liquefaction unit
 The RATE two-stage sour water stripper (with H2S absorber)
-   The RATE carbonyl sulphide (COS) hydrolysis reactor — an additional reactor after the hydrogenation reactor in the tail gas unit (TGU)
    RATE SETR technology, as alternate to caustic scrubbing, to capture SO2 before the stack
    The acid gas partial enrichment unit by using a tail gas treating absorber.

Recovered H2S is converted to sulphur through processing in the sulphur recovery unit (SRU) using oxygen enrichment technology. Recovered CO2 is compressed and sent to the RATE CO2 liquefaction unit to purify CO2 further. CO2 can then be reinjected or used in different applications (such as the transport medium for solid waste conveying, pressure medium for the lock hopper system, seal gas for feeding and withdrawing screw feeders, or as a stripping gas). In this project, CO2 was used as a stripping gas.

There were many challenges in this project regarding the design of the SRU and the acid gas removal unit (AGRU) due to the different feed compositions and impurities in these units.

The waste management plants require a quench system, COS /hydrogen cyanide (HCN) hydrolysis, AGRU, and CO2 removal, as well as wastewater treatment with a SWS, SRU, and another hydrolysis unit. Here a brief description of the project would help in understanding the impacts on the SRU design.

Figure 1 represents the configuration of the project for major units. In the COS/HCN hydrolysis section, CO and HCN will be catalytically converted to H2S, CO2, NH3, and H2O, which could be further removed from this plant. The conversion is described in the following hydrolysis reactions:

COS + H2O à H2S + CO2
HCN + H2O à NH3 + CO2

In the CO shift reactor, CO is converted to H2 according to the water gas reaction:

CO + H2O à H2 + CO2

The gas stream from the hydrolysis section flows to the AGRU where a physical solvent, such as Selexol or similar, is used. In the AGRU, the treated gas containing high CO2 is sent to the RATE CO2 liquefaction unit to purify CO2 further as the product. The acid gas stream from the AGRU is sent to the SRU.
The RATE process condensate stripper is proprietary as it uses CO2 as the stripping gas to strip H2S. This design is unique to this application. The SWS and condensate stripper comprises two separate stripper columns: the process condensate stripper and NH3 stripper.

The first treatment step is the removal of sour gases and volatile components in the RATE process condensate stripper. The liquid phase of the process condensate flash drum is preheated and fed to the stripper column in-between the upper and middle packing. The process condensate stripper column consists of two sections. In the upper section, CO2 strip gas is utilised to remove H2S and to minimise stripping of NH3. In the lower section, volatile components and CO2 are removed by means of uprising steam. Additionally, any dissolved carbonates are thermally decomposed. The stripped water is routed to the NH3 stripper where NH3 is removed and mixed with H2S from another stripper and then routed to the SRU.

As described above, the design comprises COS/HCN hydrolysis, however there is still some HCN and COS not fully hydrolysed, which flows to the AGRU and eventually reaches the SRU. HCN can be washed as well as combusted in the reaction furnace, and we provided an additional feature in the TGU to hydrolyse the remaining COS.

Oxygen enrichment technology
There are a number of units involved in this plant, including the SRU, which require oxygen enrichment. In gas plants and refineries, oxygen enrichment technologies are used to expand sulphur recovery capacity or reduce the number of trains. Oxygen enrichment raises the flame temperature by eliminating the diluent effect of nitrogen in the air. An economical source of oxygen is the key in this case.

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