Pentair goes one step further by recovering the CO2 gas after the membrane separation unit. This system recovers up to 100% of the CH4 by producing 100% pure CO2, instead of only venting the CO2-rich stream coming from the membrane system, which needs additional equipment for burning the CH4 slip to reduce emissions.
The heart of the system is the membrane separation unit. Membrane technology for upgrading biogas is a relatively new technology but already has an extensive proven record. The benefits of membranes against other types of separation systems such as water scrubbing, amine washing, and pressure swing systems are recognized by the gas upgrading market.
Membranes for gas separation
To make an efficient CO2/ CH4 separation possible, the impurities must first be removed. One of the most important techniques for doing this is the use of activated carbon that absorbs undesirable substances. The membranes separate the two major gas components: CH4 and CO2. The separation is accomplished through the pressure difference over the membrane surface.
The high-pressure biogas is fed into the membrane and the CO2 passes through the membrane surface to the permeate - the low-pressure side - much quicker than CH4. The membranes are mainly pressure-driven, so the higher the pressure the more CO2 will pass through the membrane surface.
The discharge from the membrane (the retentate) contains mainly CH4 as the CO2 has been pushed through the membrane surface. The CO2-rich gas will leave the membrane on the low-pressure side of the membrane (permeate).
Using a multi-membrane system 99 % of the CO2 can be removed from the biogas, which provides a cost-efficient biomethane production system. Pentair can also provide a two-stage membrane system together with a cryogenic system that operates with no methane slip. This system provides two valuable products: biomethane and 100 % liquid CO2.