Remedy nonperformance of membranes in meeting fuel gas CO2 specifications

A midstream operator in Colorado, USA, has a gas processing facility to treat approximately 25 MMcf/d [708,000 m³/d] of natural gas. The dry feed gas is first chilled for hydrocarbon dewpointing and later routed to a gas membrane system for CO₂ removal. The gas membrane system—a single-stage membrane skid comprising three banks operating at a pressure of 840 psi [5.8 MPa]—is designed to reduce CO₂ levels from 3% to 2.48% in the product gas. The majority of the product gas is delivered delivered to a third-party gas transmission pipeline and the remainder is used as fuel gas to drive compressors downstream of the membrane system.

The existing cellulose acetate (CA) spiral-wound membranes used at the facility were experiencing accelerated degradation. Because they were not performing as intended, the operator was unable to meet the required pipeline specifications and fuel gas CO₂ specifications to power the compressor engines. The entire membrane skid was underperforming at 65% of the design capacity, forcing the operator to either recycle off-spec gas or shutdown the plant entirely.

The operator was looking for a robust membrane technology that could meet the CO₂ removal specification so it could stop resorting to a blended gas stream to meet turbine requirements. The solution required membrane technology that could retain higher hydrocarbons in the product gas and lower the membrane replacement rate.

Replace existing membranes with Apura membranes

Schlumberger provided Apura membranes to help meet product gas CO₂specifications and minimize downtime and plant upsets. Apura membranes provide a multilayer composite membrane with higher CO₂ and hydrocarbon selectivity and longevity to manage variations in inlet feed gas conditions. Due to their composite nature and the inert substrate, these membranes have a longer operating life and recover more hydrocarbons. Apura membranes enable plug-and-play replacement of the incumbent membranes without any modifications to the existing skid.

The operator replaced one bank with Apura membranes to reduce 6.5 MMcf/d [184,000 m³/d] of gas from an inlet CO₂ of 3% down to 2.48% in the product fuel gas.

Increased feed capacity by treating at a higher flow rate

In the first three months of operation, the skid with the Apura membranes installed successfully treated a higher feed gas flow rate of 8 MMcf/d [227,000-m³/d] with an inlet CO₂ concentration of 3% down to 2.3% at the outlet. This was a 25% increase in the feed gas capacity when using the same membrane surface area. Not only did the Apura membranes perform at a higher flow rate, but they consistently treated at 20% better than the outlet requirements for the fuel gas.