Comparison Between Molecular Sieves and Activated Alumina for Gas Dehydration in an Ultra-Deepwater Production Unit

Abstract

Abstract Activated alumina and molecular sieve performances are compared regarding key parameters such as adsorbed water load, outlet moisture content, pressure drop and adsorption capacity using different regeneration conditions. The comparison was done by operating one out of three vessels of a Brazilian pre-salt FPSO gas dehydration unit loaded with activated alumina, whereas the other two vessels were loaded with molecular sieves with a silica gel top layer. During the early operation of the alumina, tests were performed by increasing adsorption time until the moisture breakthrough, conducted in two operational setups as follows: 1) a parallel-flow arrangement in which each vessel is loaded with one of the adsorbent materials, named dual bed phase; 2) a single stream flow to the vessel containing activated alumina, named single bed phase. Different regeneration temperatures were tested to verify the impact on the alumina capacity caused by low temperature regeneration. The results were compared to historical data collected using both type 4A and chabazite zeolites. During preliminary capacity tests performed in laboratory and in pilot scale, the activated alumina delivered gas with moisture content generally between 1 and 10 ppm. However, during the industrial scale test reported here, the activated alumina was able to consistently deliver gas with moisture under 1 ppm after its first regeneration, operating both in single bed and dual bed arrangement. This result indicates that activated alumina was able to achieve gas specification once considered possible only by the use of molecular sieve. The initial dynamic capacity calculated for the alumina was comparable to the maximum dynamic capacities observed for the molecular sieves. In this test, the larger particle size of the activated alumina compared to the zeolites resulted in lower pressure drop through the alumina bed, culminating into an uneven flow distribution between both beds when in parallel-flow arrangement. Recommended regeneration temperatures are lower for alumina in comparison with the zeolites, resulting in lower energy consumption and associated CO2 emissions when using electrical heaters. The novelty of this work lies in the use of activated alumina as the main adsorbent material in an ultra-deepwater gas dehydration unit with gas processing capacity over 6 MMSCMD achieving dry gas moisture as low as obtained by using molecular sieves. The use of activated alumina shows potential to improve efficiency and cost reduction that has yet to be confirmed by a longer test phase to check its stability and long-term performance.