Upgrading of natural gas ultra-rich in carbon dioxide: Optimal arrangement of membrane skids and polishing with chemical absorption

Abstract

Water depths in oil and gas offshore production are moving from shallow to deep and ultra-deepwaters. Floating Production Storage and Offloading platforms are preferred in such frontier offshore enterprises. However, ultra-deepwaters natural gas processing imposes challenges in the design of floating units, limited in area and weight of processing equipment. Oil reserves with high gas/oil ratio (>250 sm3/m3) and high carbon dioxide content create additional challenges due to the impacts in the deck area of the gas plant. Among other natural gas processing operations, removal of carbon dioxide is required to meet sales gas specification, being skid-mounted membrane modules well suited for this purpose. To avoid emissions and to increase oil production, the separated acid gas is injected for enhanced oil recovery, creating a changing scenario due to increasing carbon dioxide content in the reservoir along production lifetime. In this context, this work optimizes arrangements of membranes modules and operational conditions via nonlinear programming formulations to optimize total membrane area for minimum footprint of membrane skids, with either carbon dioxide content in the treated natural gas less than 3%mol (Type 1 Constraint) or methane losses in the injection gas limited by imposing carbon dioxide content in the injection gas greater than 75%mol (Type 2 Constraint). To reduce computational effort, surface response models are employed, regressed from data simulated with a rigorous phenomenological model of spiral-wound membrane modules. Optimization results – total and stage area, carbon dioxide contents in retentates and permeates and natural gas production – are obtained for three feed scenarios: 10%mol, 30%mol and 50%mol carbon dioxide in raw natural gas. Type 1 Constraint leads to higher methane losses while Type 2 Constraint demands a polishing Chemical Absorption to comply with carbon dioxide specification of sales gas, configuring a hybrid process. Life cycle costs and total footprint area point to superiority of carbon dioxide separation design resulting from Type 2 Constraint, with optimal service distribution between bulk removal in Membrane Permeation and polishing operation in Chemical Absorption, considering time-varying composition of the raw natural gas.