LNG vaporisers benefit from reverse osmosis technology
13 Jul 2007
The vaporisers are constructed using Process Combustion’s proven submerged combustion technology, which has inherently high heat transfer efficiency. Hot combustion gases are fed directly into a water bath containing about 150m3 of hot water, which, in turn, is used to heat a stainless steel gas heat exchanger within the vaporiser.
The quality of water in the bath has a major impact on the performance of the system. Acid gases – predominantly carbon dioxide – dissolve in the water lowering its pH and making it corrosive.
The problem is corrected by adding caustic soda, which reacts with carbon dioxide to produce sodium carbonate. This reacts with any hardness in the water to precipitate calcium carbonate scale on the heat exchange surfaces, which is expensive to remove and means extensive downtime.
Another problem concerns the potential release of nitrogen oxides (NOx) into the atmosphere. To suppress NOx emissions, water is sprayed directly into the burner to reduce the flame temperature. This water evaporates leaving behind any dissolved salts, particularly hardness salts, which deposit in the burner and need to be removed regularly.
To resolve these issues, the company decided to install a side stream reverse osmosis (RO) plant onto the water bath, as recommended by water treatment company ELGA Process Water.
The RO unit removes all the salts that accumulate in the water and virtually eliminates deposits in the burner, according to Alec Little of Process Combustion. The system continuously bleeds water from the bath, treats it by means of a membrane that removes most of the dissolved salts, and then recycles the purified water to the NOx suppression spray.
"On vaporisers using mains water we have to clean the burner every two months – sometimes more frequently – but the installation of side stream RO should extend this period to around two years. Reduced maintenance and downtime for cleaning has more than paid for the RO unit," explained Little.
