May-2006

Production of elemental sulphur from boiler flue gas utilising the Labsorb regenerative SO2 scrubbing system and the RSR technology

Environmental agencies have been requiring companies to comply with ever increasing regulations In the oil industry the major air pollutants being addressed are particulates, SOx and NOx.

Nicholas Confuorto Belco Technologies Corp.
Mahin Rameshni WorleyParsons

Article Summary

Reducing these emissions has become a routine task for companies such as Belco Technologies Corporation (Belco) and others. However, in the many cases where local water discharge regulations do not allow disposal of the typical scrubber effluent, the task of air emission reduction becomes anything but routine.

Fortunately, systems such as the Labsorb regenerative scrubbing system avoid water discharge issues by regenerating the scrubbing buffer. Regeneration allows the same buffer to be used over and over thus avoiding the disposal issue. The product of the buffer regeneration process is a concentrated SO2 stream that is typically converted to elemental sulphur in a Sulphur Recovery Unit (SRU). The stream can also be converted to sulphuric acid if a Sulphuric Acid Plant (SAP) is available. The economics of each alternative are usually very specific to a particular plant.

For plants where a SAP or SRU are available the solution is fairly easy. However, plants that do not have either process available options are very limited and are based on economics and, in the case of SRU, are also based on the availability of H2S. As the reader may already know, an SRU operation requires a 2/3 to 1/3 ration of H2S to SO2 to generate elemental Sulphur. Until now no H2S meant no SRU. This is the case for many power plants and certain upstream applications. This article addresses that condition and provides an elegant solution by combining Labsorb with the patent pending Rameshni SO2 Reduction (RSR) process.

The Labsorb Regenerative Scrubbing System
Labsorb is the trade name used by Belco Technologies Corporation (Belco) for their regenerative wet scrubbing system. Belco initially acquired the exclusive rights to the Elsorb (later renamed Labsorb) technology based on a expectation that in the near future the high cost of conventional once-through reagents (such as caustic) would make regenerative scrubbers more economically viable for refinery applications. Today’s high prices for caustic proved that expectation and, for flue-gas sources with a high level of SO2, Labsorb may prove to be the more economic alternative. However, more so than the economics, Belco found that the Labsorb inherent avoidance of waste products (be it solids or liquids), made it more appealing than conventional scrubbing at refineries that have an issue with disposal of waste products. This regenerative process nearly eliminates all waste streams and it is commercially proven to provide low emissions and reliable operation.

 The system is now successfully installed at 3 refineries to reduce particulates and SOx to below the permitted levels. Each of these installations utilise the refinery’s existing SRU to convert the recovered SO2 to elemental sulphur. The SO2 from Labsorb is injected directly in the SRU displacing some of the incineration of H2S that is normally required in such systems to produce a balance of H2S to SO2. In short, since Labsorb provides the SRU with a stream of SO2, the SRU will not have to generate its own SO2by incinerating H2S. This in turn reduces the combustion air and allows more capacity through the same SRU.

There are, however, plants (such as power plants or in SAGD applications) that do not have H2S and therefore cannot operate an SRU. Other plants may have an SRU but may be limited on the amount of H2S available for the reaction with the additional SO2 from the Labsorb system. For those plants the combination of Labsorb and RSR will provide a solution. RSR (Rameshni SO2 Reduction) is a patent pending process marketed by WorleyParsons that converts a portion of the SO2 to H2S and then converts the combination to elemental sulphur. The process will be described further later in this article.

A general overview of the Labsorb process.
Figure 1 shows a simplified flow diagram of the Labsorb process. The main process units of the Labsorb system are:
(a) The pre-scrubber utilising re-circulating low pH water for removal of particulates and SO3.
(b) The absorption tower for SO2 capture.
(c) The evaporator system for the release of absorbed SO2 from the rich solvent in the regeneration of the buffer
(d) The process unit for removing accumulating sodium sulfate.

The important tasks of the pre-scrubber is to cool the incoming gas down to the adiabatic saturation temperature, and to remove gaseous and particulate contaminants. The pre-scrubber utilises only water and therefore it is a low pH operation to avoid scrubbing SO2.
SO2 scrubbing is purposely left as a function for the absorber section utilising the regenerative Labsorb buffer.

The Labsorb process can be applied to EDV Scrubbers, packed bed scrubbers, plate type scrubbers or spray tower type scrubbers. Standard absorption tower with packing or plates can be used on gas with low dust content. Special spray towers, such as Belco’s EDV Wet Scrubbing System, are recommended for treatment of gas flows high in particulate or where upsets with large particulate carry-over are possible. FCCU applications are one example of such high particulate applications that are well suited for the EDV scrubbing system offered by Belco Technologies Corporation.

Countercurrent operation in the absorber tower is achieved by dividing the absorber in several sections (called “stages”), operated in counter-current mode, each stage with its own liquid re-circulation system. Figure 2 depicts a tower with 3 EDV scrubbing stages and an EDV pre-scrubber section built into the bottom of the absorber.

The regeneration plant evaporators are of the standard design and, in cases where space is an issue, can be located in a remote location away from the scrubber. They are provided with external heat exchangers that utilise steam as the heating medium, and are operated with forced circulation.

Labsorb Process chemistry.
The main reactions taking place are:

SO2, (g) = SO2, (l)                              (1)

SO2, (l)+Na2HPO4, (l) +H2O(l) = NaHSO3, (l)+NaH2PO4, (l)                    (2)