Efficient New Processing System for Water Deoxygenation

Abstract

ABSTRACT A new concept for DEOXYGENATION has been developed based on the use of an inert gas, which is recirculated; a specially designed stripping chamber and a catalyst bed. The concept has been fully proven in a prototype unit handling 500 m3/hour sea water. This unit is located on the Norwegian east coast and interested parties are welcome to inspect the installation. Long term development work will be carried out with this unit to assess its suitability for other applications and also to optimise mechanical and energy requirements. The prototype has demonstrated the considerable advantages now available, when compared with conventional vacuum and gas stripping units. Some of these advantages are particularly attractive for offshore operators viz:Substantial savings of space and weight.Very low levels of residual dissolved oxygen.The need for chemical scavengers can be eliminated.No large volumes of stripping gas requiring flaring - a major advantage over conventional gas stripping.Units will operate with unfiltered sea water without a build up of foulants.Units can be sited in platform legs thus saving deck space.Modular construction provides flexibility in performance. The process comprises a stripping chamber in which nitrogen is used to remove dissolved oxygen from the liquid phase. Separated "gas" is thenpassed over a catalyst bed in the presence of a "reactant". Oxygen free gas is then returned to the stripping chamber for re-use. Details of the process, laboratory studies and prototype operations are presented in this paper. Introduction. Sea water injection is widely used for enhanced oil recovery in offshore fields. Removal of dissolved oxygen from sea water reduces the incidence of corrosion "pitting" of unprotected carbon steel equipment. The elimination of corrosion is necessary to avoid "fouling" of flow lines or a build up of deposits at the well-bore face. Current practice is to deoxygenate with large, packed towers, operating either under vacuum, or with natural gas as a stripping medium. Such systems can reduce dissolved oxygen to very low levels, particularly gas stripping units, but the ratio of gas to water volume is very high and many governments have restricted the volume of gas which can be flared during normal operations. Conventional units must therefore be regarded as a compromise in terms of:SIZEWEIGHTCONFIGURATION (location)PROCESS RESTRICTIONS andPERFORMANCE Residual dissolved oxygen levels of 50 to 100 ppb are obtained without theaddition of chemical scavenger. Further reduction to less than 5 ppb O2 is achieved by the addition of a solution of either ammonium hydrogen sulphite or sodium hydrogen sulphite. Large sea water injection systems consume substantial quantities of chemical oxygen scavenger. Such chemicals are not expensive in themselves but the logistics should be considered since bulk storage is often required both onshore and offshore, expensive travel tanks are required to satisfy IMCO standards (class I).