From Pilot Studies to Full Scale Plant

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Natural gas is widely used as a primary energy source and is one of the most efficient fuel source. For this reason, more and more gas is utilized exceeding existent production facilities. Oil and gas companies therefore expand there measures and step behind borders finding and exploit new resources.

One area of major interest is the Barents Sea, a remote area far from industry and civilisation. There climate conditions, deep water approach and a very sensitive ecological environment are challenging. In the Barents Sea the gas is exploited some hundred miles off the shore and delivered through subsea installed pipelines to an onshore liquefaction plant. But the natural gas does not float through the pipeline that easy. The crude is a mixture of mainly hydrocarbon gas-hydrocarbon condensate-water fluid, contaminated with formation water and corrosion products. The system pressure may lead to clogging of the pipeline due to the gas hydrate formation inside the pipeline. This is a major incident, which has to be prevented under any circumstances. Historically methanol is used to inhibit the clogging. Since the northern Atlantic region and the Barents Sea bear very sensitive ecological systems, mono ethylene glycol (MEG) is used instead as a 'green chemical' for inhibition at the wells. Despite of its price, MEG has the advantage of dissolving more salts in the solution than methanol does.

In this case, recycling is not that easy. In a first step solid, gaseous and liquid fraction has to be separated; this is done by the slug catcher. The gas is processed in the gas plant, the solids are discharged and the liquid, a MEG - water mixture processed further. In one step the remaining fine particles are removed, then the salt content is taken out followed by the distillation to a certain MEG - water ratio. At the end the purified MEG is stored and sent back to the wells in order to keep the production running.

In the MEG purification and recycling process the solid removal appeared as a still not sufficiently solved problem. The solids, corrosion products from the production production pipeline and scale, together with the adherent condensate cause scale formation at downstream heated surfaces. The heaters needed to be shut down for mechanical cleaning periodically. This standstill for cleaning had to be avoided in a new production facility, built at the polar cycle by STATOIL, near Hammerfest: the Snohvit project. New technologies were demanded. There are many filtration technologies available for purification of liquids. In this case EnviroChemie has chosen the membrane filtration with the main advantages of continuous operation and low to no solid contain of the filtrate. Opposite to gravity filtration the liquid in membrane filtration is routed perpendicular to the filtration surface. So the system is operated at higher solid concentration compared to other filtration techniques.

Pilot Test Laboratory
The first approach was done using the laboratory scale test plant. There a MEG-water sample from the slug catcher from an existing gas plant was shipped to EnviroChemie laboratory. Different plant settings were to be tried. Different materials of membranes and different pore sizes were applied. Ceramic membranes were used as stick membranes where the polymer membranes are of tubular construction. For plastic material polypropylene and PVDF were chosen for the filtration trails. In the pilot tests polypropylene membranes revealed the best performance regarding filtration rate per area and time. The filtration rate was stable over the whole filtration time of one day. Based on this result a small scale plant was designed to run field tests for a longer period of time on the gas production site.

Field Test
The field tests were scheduled at an onshore natural gas production plant. There special precautions have to be considered. Especially the safety terms has to be read twice to get the allowance to find entry to the plant, the Envopur8 MFI 8 of Enviro-Chemie. The wiring and switch cabinet were all executed according the ATEX - regulations and local country specific regulations. These inputs were adapted to the test plant, the plant was inspected at the work shop by the customers authorities and finally delivered to its designation.

The filtration of MEG began with the most promising membrane system, the polypropylene membrane assembly. At the test site a similar MEG, in terms of solids and water content which was expected to occur at the Barents Sea. was used. But first of all the solid concentration was adjusted by concentration of the solids. Then long term exposure of the membrane system was intended to do. Here the filtration rate against time and filtration rate against solid concentration in the feed volume was examined. The optimum filter length was determined as well. The filtration was started with the long arrangement.

As the filtration rate was less then already observed in the workshop testing, shorter membranes were implemented. The filter elements were shortened to find an efficient balance in terms of feed volume and efficient filtration rate. The most important to know was the filtration time in between the two cleaning events. Several filtration scenarios were done finding an optimum. The test plant therefore was designed to run completely automatically in order to operate overnight without any process uncertainty or safety violations.

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