Guaranteed safety during mercury decontamination procedure

Menno Springintveld, senior strategy & planning engineer at SABIC Geleen: “All important materials in a cracker, such as naphtha or gas condensate, contain a certain quantity of mercury. Mercury is found in different shapes in the earth’s crust and often emerges with the extraction of the ‘feedstock’ for the cracker installation. By heating this feedstock, metallic mercury is formed, which then deposits on the inside of the installation. If left unattended for too many years the mercury forms an amalgam with other non-ferrous metals, which damages the installation. The presence of mercury can also have a negative effect on the performance of the catalysts and complicates the maintenance of the installation. Our organization therefore decided to have a specialized company design a recirculation system that could run during the next planned turnaround in order to remove the mercury. A turnaround is normally planned every six years. It was really a good timing.”

“We started all necessary preparations mid-2012,” says Jan Zwaan, head of technical support at Mourik Global BV. “Although we are familiar with the removal of mercury, we never handled a project this size before. In close consultation with the SABIC engineers we therefore decided to divide this huge project into 28 smaller projects and a mercury removal area was defined. In this area of approximately 40 x 40 meters and with special safety measures in place, hundreds of meters of pipes had to be installed. The area also had to host a compressor installation made of several large compressors, a number of integrated cooling circuits and a complete absorption unit with filters. Another crucial task during the preparation phase was to align the process drawings with the reality of the plant. 28 different circuits had to be cleaned in seven days. From a logistics point of view the challenge was the different locations and various altitudes of each of the circuits. In addition, every circuit had its own diameter, volume and equipment.”

An essential success factor in the cleaning of the naphtha cracker was to be able to very accurately manage the flow, the pressure and the temperature in the system. Springintveld explains: “For that reason we chose to install a closed system. We filled up the part of the cracker that had to be cleaned with nitrogen in order to get the oxygen out of the installation. By doing this, we reduced the chance of an explosion to a minimum. Safety, both process and for the personnel, comes first.” “For every sub-project we were able to heat the nitrogen, cool it down again, purify, filter and recirculate it.” Zwaan adds: “It would have been much simpler just to flare the nitrogen after each use, but then the entire project would have cost an extra 1.2 million euros. The safety hazards for process and personnel would also have been higher.”

Geert Jansens, sales engineer at Atlas Copco Rental: “In the mercury removal area we installed six oil-free PTS 1600 mobile compressors for the recirculation system. Those were part of our rental fleet already, but were rebuild to NTS 1600 compressors to be able to compress nitrogen in a closed-loop system. We needed compressed nitrogen to be recirculated on the one hand and to heat it to a process temperature of 100°C on the other hand, a requirement imposed by SABIC.” In order to meet this demand Atlas Copco equipped all of its nitrogen compressors with an in-house developed temperature and flow meter. Zwaan explains: “Mourik had to build a special filter installation in order to protect the nitrogen compressors and the naphtha cracker. The applied dust filters had to collect dust particles of maximum P3 size. With every circuit that had to be cleaned, we monitored pressure, temperature and flow very closely, to keep complete control over the entire process at all times.” “Interesting to know is that the pipes in the mercury removal area were all insolated, so the temperate was manageable and the weather only had a minor effect on the process.” When asked why Jansen suggested to use oil-free compressors, he explains: “There were two main reasons. First because oil could have had a negative effect on the efficiency of the filter installation built by Mourik. Secondly because the temperature management system could not have been installed in combination with oil injected compressors."

As soon as the construction of the recirculation system was completed and the naphtha cracker filled with nitrogen, the actual cleaning of the installation could start. Jansens: “By adding energy to the Nitrogen in the format of heat, and applying the right flow for every system, turbulent gas flows originated within the naphtha cracker. Those gas flows took the vaporized mercury out. The mercury was then removed from the Nitrogen flow using custom designed active carbon filters. The purified nitrogen flow then went back into the compressors and the naphtha cracker, starting the process all over again until the system was completely cleaned. In total the operation took seven days. Meanwhile Intertek, an independent laboratory, measured, analyzed and recorded the relevant operational parameters and concentrations during the whole project.”

Zwaan: “We are proud to say that our team, after an intense preparation phase, succeeded in this project, with all its logistic difficulties, within the predefined time and with the desired result.” Springintveld tells us that SABIC had already decided to implement a continuous cleaning system in order to prevent mercury deposit in the future. Therefore a permanent filter installation has been placed inside of the naphtha cracker, continuously extracting the mercury from the gas flow. In this filter installation the mercury and the silver loading of the filter form a chemical compound. Lea Jaspar, communication manager at SABIC: “The importance of safety during the entire project gave the complete operation a whole other dimension. SABIC as well as the contractors invested immensely in both process and personal safety and we are proud to say it paid off. The complete project ran flawlessly.”