State-of-the-Art Development of Offshore Water Deaeration Processes Utilizing High-Purity Nitrogen for Chemical Free Oxygen Removal

Abstract

Conventional deaeration of seawater for water injection use expensive chemical scavengers and heavy vacuum towers, which occupy valuable space on offshore installations. The scope of this study is to investigate, compare, and further advance the development of two state-of-the-art deaeration technologies which solve the aforementioned issues. The first method, compressorless deaeration with the use of stripping gas is a newly developed modification of an already efficient, light, compact, and worldwide implemented technology. This technology uses pure nitrogen in a regeneration loop, together with static mixers which allow oxygen mass-transfer from seawater into the gas phase. The second method utilize the same proven nitrogen regeneration loop in combination with novel membrane deaeration technology. Advanced dynamic process simulations, combined with field data, mechanical design and process calculations are utilized to quantify process parameters and design requirements for the two technologies. The results are presented and used for discussing advantages, disadvantages, possibilities and further development needs of both the stripping gas- and membrane technology. Results show that it is possible to further increase the robustness of the stripping gas-technology by eliminating the compressor. A clever ejector system with turn-down capabilities ensures the technology's advantage of positive operating pressure and utilizes energy from the seawater flow to compress the nitrogen. Power calculations, turn-down possibilities and oxygen removal efficiency at various operating conditions are presented. A suggested design for membrane deaeration is also presented, along with calculations and comparisons to the stripping gas technology, especially regarding flow rate capacity and the economies of scale. The main advantage of the proposed membrane technique compared to existing membrane concepts is the high-purity nitrogen regeneration loop, which offers improved mass-transfer capabilities across the membrane. The novelty of this work the feasibility of successfully operating a compressor-less, efficient, compact deaeration system on positive pressure. Additionally, the stripping gas technology requires no chemical scavengers in order to obtain an oxygen concentration lower than 10 ppb. The membrane deaeration process can also achieve low oxygen concentrations without chemical scavengers and it is further found that the technology might be economically viable compared to stripping gas for low flow rates.