Petrochemicals group Sasol will finalise the conceptual design of a 60 Mℓ anaerobic membrane bioreactor (AnMBR) at the end of this year, which is expected to generate 40 MW of electricity by treating a gas-to-liquids (GTL) plant’s industrial effluent to produce methane-rich biogas.
Sasol environmental technology manager Dr Sarushen Pillay tells Engineering News that the conceptual design will provide the group with an engineering package that is likely to be rolled out at the GTL plant Sasol is looking to develop in Louisiana, in the US.
If the plant is constructed, it will be capable of treating about 2 000 m3/h of water, which will be reused for cooling purposes. Further, the plant, which is expected to produce about 96 000 bbl of GTL a day, will reduce its carbon dioxide emissions by about 100 000 t/y.
Pillay notes that traditional anaerobic digesters use solid waste as the biomass feed source, which needs to be heated to generate biogas. However, because Sasol is using the effluent generated from its industrial processes, which is sufficiently heated, the biomass medium does not require any heat to generate the biogas fuel source.
Further, the system also includes membrane technology, as the relevant medium is a liquid rather than solid. The membrane technology is developed by US technology multinational company GE and is designed to retain biomass in the system.
“The membrane technology acts as a barrier, keeping the biomass inside the bioreactor. Water passes through the bioreactor, enabling the biomass to break down the organic components in the effluent stream. “The membranes prevent the biomass from being lost once the clean water passes through the system,” explains Pillay.
He adds that the inclusion of the membrane technology also enables the plant to run with a significantly higher concentration of biomass in the reactor, which enables the reactor to be smaller and makes the process more efficient, as it is able to generate more biogas.
“AnMBR technology will offer Sasol significantly more economic benefits than traditional wastewater treatment, particularly, since traditional wastewater treatment operates using aerobic processes, which converts contaminants into carbon dioxide. However, anaerobic processes convert contaminants into methane, which can be used to generate electricity,” explains Pillay.
Developing the Bioreactor Technology
The development of the AnMBR technology started in 2004, following Sasol’s sponsorship of a chemical engineering master’s student at the University of Cape Town (UCT), who was commissioned to investigate effluent treatment options available to the company.
The four-year-long research culminated in the development of a 20 ℓ laboratory-scale reactor at UCT.
The student went on to achieve his PhD in chemical engineering and was then employed by Sasol.
Pillay notes that the student’s research findings highlighted the potential gains of AnMBR technology for Sasol, but required GE’s expertise in membrane technology to upscale the project.
In 2009, Sasol partnered with GE, which resulted in the construction of a 50 ℓ bench-scale model of the AnMBR, which was completed in Sasolburg in 2010. The project was again upscaled with the construction of a 20 000 ℓ pilot-scale model in 2013, also in Sasolburg.
Pillay notes that the pilot-scale model took about 12 months to complete from design to construction. The plant took another eight months to grow the biomass necessary to create the biogas.
He points out that dealing with living organisms can be challenging, particularly, since biomass comprises individual bacteria that combine to form a single organism.
“Sasol is known for working with chemical processes that require a rise in temperature or pressure to increase process speed. “However, bioprocesses require more patience than chemical processes, as the bacteria needs to grow, and any change in temperature or a slight variation in the feed or flow composition could upset the process,” asserts Pillay.
He adds that, as a defence mechanism, when disturbed, biomass creates a film layer, which leads to the creation of foam that can damage the AnMBR system, as was the case when the pilot model’s compressor was damaged when it sucked in a high concentration of the foam.
Subsequently, Sasol installed foam-break tanks and antifoam spray systems to break down the foam, preventing any further damage to the system.