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Labsorb: a regenerable wet scrubbing process for controlling SO2 emissions

The use of a regenerative type scrubbing system that utilises commonly available chemicals is an important evolutionary step in the continuing process of scrubbing system optimisation.

Scott T Eagleson and Nick Confuorto, Belco Technologies Corporation
Bjornar Pedersen ETM AS
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Article Summary
These systems must be highly reliable to eliminate equipment outages and must maintain the excellent emissions performance that has been associated with wet scrubbing systems. In addition, they must minimise operating costs by regenerating the chemical used to remove the acid gases from the emissions source while operating reliably and having the high availability that is necessary in today’s process facilities.

One regenerative scrubbing system, the Labsorb system, is examined with respect to the system design and operating history. The system design is discussed in detail. Also, the experiences of several years of operating history is examined, discussing the lessons learned with respect to maintaining the utmost reliability, maintainability, and performance.

This paper also discusses the application of Labsorb in today’s refining environment, particularly as it pertains to combined control of sulphur emissions from fluid catalytic cracking units and tail gas from Claus plants (SRU). The process technology is presented in detail and compared with traditional wet scrubbing as well as other regenerable technologies for SO2  control. Finally, justifications, both economic and technical, are provided for the use of Labsorb in a refinery.

Generally, the largest single source of air emissions in a refinery is the fluid catalytic cracking unit (FCCU). Due to the large amount of flue gas generated from the FCCU regenerator, the tons per day of pollutants usually far exceeds any other single source in the refinery. However, significant quantities of pollutants can be generated from other sources in the refinery. The SRU tail gas is one source. Additionally, if the crude distillation unit (CDU) heaters fire oil, they can generate significant emissions. Oil fired boilers will also produce significant emissions of pollutants. All of these sources noted will produce SO2 emissions, while the FCCU regenerator, CDUs, and boilers also will produce particulate emissions.

Traditionally, when significant SO2  and particulate emissions reductions have been required from these sources wet scrubbing systems have been employed for the control of these pollutants. Proven wet scrubbing systems on these processes have been very effective in reducing SO2 and particulate emissions. One example of this is a wet scrubbing system installed at Pennzoil in Shreveport, Louisiana. The wet scrubbing system, treating flue gas from the FCCU regenerator, reduces particulate emissions to less than 10 mg/Nm3 and reduces SO2 emissions by over 99.9%. Other sources in the refinery, such as the SRU, also have equally well proven systems for reducing SO2 emissions.

In a conventional scrubbing system where no regeneration of the reagent is performed, the reagent utilised will typically be caustic, magnesium oxide, or soda ash. In some cases, lime has been used but this is not normally done since the reliability of a lime based system does not match the need to continuously operate for three to five years.

When a global examination of the emissions from the refinery is made, it can be seen that control of SO2 emissions from all sources can result in high operating costs when conventional scrubbing technologies which do not reuse the scrubbing reagent are employed. The Labsorb process described in detail in this paper combines the demonstrated performance and reliability of the well proven EDV wet scrubbing system for the removal of SO2 and particulate, along with a unique regenerable Labsorb buffer for SO2 absorption, which dramatically reduces the system operating costs since the regenerative Labsorb system buffer is regenerated in the regeneration facility and returned to the scrubber for reuse.

Description of the Labsorb process
The Labsorb process is a regenerable process for the recovery of SO2 from flue and tail gases at utility and industrial plants, and represents a significant improvement over other flue gas desulphurisation (FGD) and SO2 recovery processes currently in use.

Unlike caustic/lime/limestone systems, the Labsorb process produces concentrated SO2 as a byproduct. This byproduct, through commercially available processes, is converted into valuable products such as liquid SO2, sulphuric acid or elemental sulphur. These products are widely used in the fertilizer, chemical, and pulp and paper industries.

The process is highly flexible and can be used in connection with flue gases generated from any kind of process. It is economically favoured when flue gases with high SO2 content are to be processed, and especially suited to meet all emission regulations even at high fluctuating SO2 loads in the flue gas. Gases containing from 1,000 ppmv to more than 3-5% (vol.) SO2 may easily be treated to meet stringent environmental standards. The flow diagram shown in Figure #1 outlines the process design and chemical basis for the Labsorb process.

The Labsorb process utilises a patented non-organic aqueous solution of sodium phosphate salts as buffer for the absorption of SO2. The second dissociation step of phosphoric acid represents the active buffer. The main reactions are:

SO2(g) = SO2(aq)

SO2(aq) + 2H2O=H3O+ + HSO3-

HPO42- + H3O+ =H2PO4- + H2O
The process operates in the pH region of 5.0- 6.5. The high buffer concentration and the high pH values throughout the process lead to high cyclic capacity and to high absorption efficiencies. Technical grade caustic and phosphoric acid are used for buffer production and makeup.

Typical system description
A Labsorb system normally consists of a quencher/pre-scrubber/absorber section, a regeneration unit, and a sulfate removal/buffer makeup unit. A simplified flow diagram of the process is shown in Figure 1. In the process shown, the regeneration is accomplished by single-effect evaporation. For the purpose of energy saving, double-effect evaporation may be used in larger installations. Low-pressure steam can be produced by cooling the incoming gas before the quencher/pre-scrubber in a waste heat boiler. This steam may be used to reduce the operating cost of the regeneration unit.
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