Nordsol’s technology explained: Flash-2-Sweep

With Nordsol’s technology, bio-LNG is produced by liquefaction of biomethane extracted from biogas. At atmospheric pressure, biomethane becomes liquid at -162°C. At these low temperatures, other components in the gas, such as CO₂ and water, will easily freeze and cause blockages and potential damage to the equipment. Therefore, it is crucial that the biomethane is thoroughly purified before it is cooled and condensed. This happens in the first step of the process.

Nordsol only uses membranes to efficiently produce liquefaction-ready biomethane. In this article, we explain this unique technology and compare it to alternative technologies.

The second step in bio-LNG production is the biomethane liquefaction process. This step is described in another article in this series. It is important to understand, though, that the integration between these two steps results in significant economic and operational benefits.

Traditional gas treatment technologies used in the LNG industry typically apply the combination of CO₂ absorption with amines, followed by gas dehydration with molecular sieves (Temperature Swing Adsorption). Some bio-LNG plants that are equipped with membranes for CO₂ bulk removal also use molecular sieves for trace removal of CO₂ and water.

The use of membranes in combination with molecular sieves has advantages over gas treatment with amines. For example, the regeneration gas can be returned to the biogas compressor or be injected into the gas grid. However, the adsorption process also presents several challenges:

1. Adsorption processes with molecular sieves are not continuous. Frequent mode switching between adsorption and regeneration modes causes fluctuations in gas flow and temperature which makes the process more difficult to control and can easily result in flaring of some biogas.
2. The relatively high regeneration gas flow routed back to the biogas compressor requires a larger compressor that takes more energy. Alternatives like regeneration with hot air also costs energy and can result in methane emissions to atmosphere.
3. Adsorption processes require a lot of of equipment but also chemicals.

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.

Nordsol seized this opportunity. Membrane gas separation is a continuous and energy-efficient process. It’s the standard for biogas upgrading to biomethane that does not require heat or chemicals, and it doesn’t produce waste streams. However, the downside of standard membrane gas separation is that these cannot efficiently perform the trace removal of impurities down to the very low concentrations necessary for the liquefaction of biomethane. This limitation has been resolved in Nordsol’s patented Flash-2-Sweep technology.

Flash-2-Sweep boosts the performance of the membranes by sweeping (flushing) the low-pressure permeate side of the membrane with flash gas and boil-off gas (BOG) from the liquefaction unit. The permeate is routed back to the biogas compressor, preventing any loss of biomethane.

Sweeping the permeate side of membranes is very common in applications like gas drying. However, the use of flash gas and BOG as sweep gas is an innovative approach. This approach provides another synergy: the biogas compressor now also functions as a flash gas and BOG compressor.

The Flash-2-Sweep process was tested with various standard off-the-shelf 4-port membranes from various suppliers. The process 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.