Nordsol has developed the patented “flash-to-sweep” concept, which boosts the performance of the membranes by sweeping (flushing) the low-pressure permeate side of the membrane with flash gas from the liquefaction unit.
Sweeping the permeate side of membranes is very common in applications like gas drying. However, the use of flash gas as sweep gas is an innovative approach, which allows for the removal of both CO₂, water and even hydrogen down to very low concentrations in a simple single step, or so-called “unit operation”.
This energy-efficient gas treatment method is unique within the LNG industry. All LNG facilities in the world, both large and very small scale, apply the combination of amine CO₂ absorption, followed by (in series) gas dehydration by molecular sieves (Temperature Swing Adsorption – TSA).
LNG peak shaving facilities and some bio-LNG plants which are equipped with CO₂ bulk removal by membranes, use molecular sieves for combined CO₂ and water trace removal (polishing). Although limited to few % CO₂, this has many advantages compared to amine gas treating. In case of combined CO₂ and water removal by molecular sieves, the regeneration gas is returned to the biogas compressor or injected into the gas grid (LNG peak shavers). Thereby the need for amine scrubbing required for deep CO₂ removal is eliminated. However, the combined CO₂-water removal by molecular sieves requires frequent mode switching (cycles) and is not efficient because of a relatively high regeneration gas flow (recycle). Alternatives like regeneration with hot air, have significant downsides.
Moreover, technology and equipment suppliers do generally not integrate different, potentially competing technologies into the process. Gas treating and liquefaction technologies are commercially kept entirely separate to allow different combinations of technologies and only a few companies can design and offer both technologies, nicely integrated in one plant.
As an LNG technology integrator, Nordsol seized this opportunity, patented its iLNG process lineup, and tested the “flash to sweep” concept with various standard, off-the-shelve 4-port membranes. The flash-to-sweep concept reduces the CO₂ concentration in the retentate of the membrane by a factor of more than 200; e.g. from 4 mol% to 200 ppm. Without the sweep gas, the reduction of CO₂ concentration in the retentate would be only factor 10. The resulting integrated process is simple to operate, produces on-spec gas instantly after start-up, is continuous, waste-free, and energy-efficient.
Because of the continuous nature of the membrane gas separation process, the biogas feed flow to the facility is extremely constant with deviations of <1% around the capacity (biogas flow) setpoint. This is a unique and critical feature since a biogas buffer between digester and bio-LNG plant is usually not present or very limited. Unwanted variations of biogas flow to the bio-LNG unit by for example a mode-switching TSA unit, can easily result in frequent flaring of some biogas