Protected supersonic separator performance against variable CO2 content on natural gas processing: Energy and sustainability analyses

Abstract

In offshore oil-and-gas fields with high CO2 content, oil-and-gas processing entails recycling CO2 to the field for enhanced oil recovery. Despite blocking fossil CO2 ingress into the biosphere and improving oil production, this entails increasing %CO2 in oil-and-gas with time, challenging the processing plant. Supersonic separator is a compact gas processing solution that can simultaneously adjust water and hydrocarbon dew-points. But, supersonic nozzles can only work if sonic-throat flow occurs. In this work a supersonic nozzle sized to a high-pressure 25%mol CO2 feed is submitted to varying %CO2 from 5%mol to 80%mol under constant feed temperature/pressure and molar flow rate. Varying %CO2 affects nozzle performance with three possible outcomes: (i) feed is insufficient to reach sonic-throat flow, entailing no supersonic processing; (ii) feed is at design condition (25%mol CO2) and nozzle works tightly; and (iii) sonic-throat flow is achieved with a fraction of the feed; i.e., there is no room to process the rest. These apparent shortcomings of supersonic gas processing are examined and a control-scheme based on manipulating feed-pressure is proposed to circumvent feed deficit/excess. With the proposed feed-pressure adjustment to protect supersonic separator operation, energy and sustainability assessments are conducted to evaluate environmental and gas-fired power generation impacts from the supersonic processing of feeds with varying %CO2. Feeds with 10%mol/15%mol/20%mol CO2 have highest sustainability-degrees.