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Corrosion of the WSA condensers at the OMV refinery in Schwechat made them less economical in the long-term due to constant maintenance. A new condenser design engineered by OMV and INWA AG using PFA sheets reduced the maintenance costs in half while increasing both maintenance intervals and overall safety. A total of three condensers have already been rebuilt and lined by INWA AG.

As the largest industrial company in Austria, OMV refines fuel products and petrochemicals at its site in Schwechat, Austria. In the refining process, crude oil and natural gas are processed into high-quality fuels, oils and other special products such as paraffin, heating oil, bitumen, sulfur and sulfuric acid. The aim of all these processing steps is to achieve a high added value from the raw materials to work economically as a company while operating to sustainably protect the dwindling resources and the environment. In the end, all the residual products from the processes are being fed into a combined heat power plant to produce steam, electricity, district heating and other products through combustion energy. A downstream flue gas cleaning system (SNOx) cleans the flue gases and allows the burning of high-sulfur fuels in the power plant. Due to the high sulfur content in the fuels SO2 is produced during combustion, which is then used for the production of concentrated sulfuric acid. Sulfuric acid is the most commonly used acid in the world, and there is a global interest in this product. In conclusion, a contaminated flue gas is turned into clean exhaust air and a sellable industrial product, which is used in a variety of applications.

In the flue gas cleaning system dust is filtered via electrostatic precipitators in the first step. Afterwards the NOx is removed in a catalytic process in a second step. The remaining SO2 gases are then oxidized via converters to SO3. In the last step of the converter, the majority of the SO3 reacts with H2O to form H2SO4 vapor. The vapor is transported to the heat exchanger, in which the gaseous sulfuric acid is condensed to liquid H2SO4 with a concentration >94%. The condensing droplets of sulfuric acid are formed on the surfaces of the heat exchangers (acid condenser) and the surrounding equipment. The dew point of the flue gases is reached, if the gas temperature drops below approx. 240 °C - 260 °C, which depends on the exact combustion gas composition. The condensation of the acid droplets on metal surfaces would lead to the so called “dew point” corrosion, in which regular steels and stainless steels are destroyed quickly. In order to achieve sufficient resistance to the highly aggressive application conditions, a multi-layer structure is often used, which consists of the following materials: chemical stone lining, foam glass layer, non-welded PTFE plates, chemical protection layer and a carbon steel tank.

This lining system was also used at OMV; however, due to the significant increase in maintenance work and the associated operational downtimes and repair costs incurred, the economic efficiency of the SNOx plant decreased a lot over time. During the repair work, it became apparent that the multi-layer structure did not create a long-term leak-proof lining system, which caused corrosion of the carbon steel tank by the condensing sulfuric acid. As a result, it was necessary to remove the lining system, which was already soaked with acid, at great safety expense and to refurbish the steel structure and lining in a further step.

Due to the apparent downtimes and the increasing repair costs, OMV decided to fundamentally redesign the condenser area, whereby the objectives were defined as follows:

• At least an equal production quantity and product quality compared to the current system
• Increase in reliability or rather availability and safety
• Extension of maintenance intervals
• Reduction of maintenance costs due to easier possibilities of repair

In the course of the design work, detailed solutions were worked out, that make it possible to shut-off a certain part of the SNOx plant, so that the plant can also be operated under partial load during its maintenance. In addition, a two-layer structure already patented by OMV was developed so, that the process can be permanently checked for leaks via a monitoring system. The main focus of the development, however, was the selection of a lining system that will withstand the harsh operating conditions and meet the specified project objectives. In the selection process, eight different lining systems were evaluated and analyzed. Due to experience in the field of PFA linings for sulfuric acid condensers INWA AG was already able to rely on a combination of their experience and on their long-term references that demonstrate the functionality of this lining system. In addition AGRU commissioned a Swedish testing institute to carry out an exposure test, in which AGRU PFA sheets were exposed for one year at temperatures and concentrations, that exceeded the operation conditions (260 °C, 98% sulfuric acid) without any significant changes to the product properties. Based on the available long-term references and the investigation carried out, the PFA fixed-point lining system was specified as the optimal lining system. INWA AG was commissioned to start with the first refurbishment of four plant sections in 2018. Figure 1 shows the new tank design, which consists of a double layered steel tank, both layers lined with PFA.

Compared to the old design, the chemical protection layer now consists of two PFA sheets located on top of each other, whereby the PFA sheet at the bottom was simply installed for additional operational safety. The steel tank at the bottom, which is not in contact with the medium during normal operation, was additionally lined with a PFA sheet in order to achieve a comprehensive corrosion protection even in the event of leakages.

The PFA fixed-point lining offers simple repair options since the sheet can be welded again after proper preparation in the case of damages. In the future time-consuming special repair work with dissimilar materials will no longer be necessary reducing downtime.

Figure 2 shows the installation and welding work of the AGRU PFA sheets in the sump area. Figure 3 shows the final lined condenser, before put into operation.

PFA Fixed Point Lining
With the PFA fixed point lining, a PFA sheet is fixed to a steel structure by a mechanical fastening using bolts or screws. This system is mainly used in flue gas applications and desulfurization plants up to 260 °C, see Figure 4.

In order to be able to seal the multi-layer PFA sheet structure, a thermoformed PFA covering cap was developed in cooperation between INWA AG and AGRU, see Figure 5. It was successfully used for all refurbishment steps.

Due to the great success of the project, three plant sections were already relined through 2021. As a system supplier, AGRU was able to produce all the required PFA products and thus made an important contribution to the realization of this project. Since the PFA products are used under very aggressive operation conditions,
the PFA production was inspected and controlled by INWA AG and OMV through regular audits. Due to the good experience during the past 3 years, also last part of the SNOx condensers will be renovated using PFA fix point lining. It is currently in the planning stage and the installation
is scheduled for spring and summer 2022.

Conclusion
In the course of a routine inspection carried out in 2021, no damages or abnormalities were detected on the PFA lining installed in 2018. As the person responsible for the project on the OMV side, Mr. Ronald Hoffer, DI (FH) Dipl.-Ing., concluded after the inspection of the plant area: “Despite the highly aggressive application conditions, the PFA lining shows no noteworthy damage or changes after an operating period of three years. It seems that sheet has just been installed”.

Thanks to the successful development and the performed reconstruction, OMV and INWA AG were able to apply for a patent for this lining system. In addition, the specified project goals were achieved:

• Increase in reliability, availability and safety
• Halving of maintenance costs
• Reduction of downtimes by 75%
• Patented leakage monitoring system enables permanent control of the plant components
• Increased operational safety due to an additionally installed tight lining system

This article was originally published in MTI CONNECT 2022, Issue 2. Download the complete article with figures.