Natural gas dehydration by molecular sieve in offshore plants: Impact of increasing carbon dioxide content

Abstract

Dehydration is a critical operation in natural gas conditioning as it reduces the potential for corrosion, hydrate formation and freezing in process equipment and transportation pipelines. Water dew point adjustment is particularly challenging in the remote ultra-deepwater natural gas reserves of the Brazilian Pre-Salt fields due to their very high carbon dioxide contents – from 30% up to 90% in raw natural gas – which is a consequence from the carbonaceous rock of the reservoir structure and long term elevation of carbon dioxide content due to its injection for early enhanced oil recovery. Under this scenario, the study evaluates the impact of the carbon dioxide content of the natural gas on the performance of water dew point via water adsorption on 4Å Zeolite molecular sieve beds. Process simulation with adsorption simulator Adsim (Aspen Technology, Inc), at varying operation pressures and carbon dioxide contents in raw natural gas, indicated that, although adsorption meets water removal specification in a condensation free operation, the high fugacity of carbon dioxide penalizes the dehydration performance due to probably two facts: (i) higher carbon dioxide fugacity in the humid natural gas imply higher saturation water content in the gas phase, which increases the service of dehydration units; and (ii) higher carbon dioxide fugacity in the humid natural gas establishes a discreet adsorption competition with water resulting in 6.5% increase of adsorbent bed volume for operating pressures of 35bar or higher.