Global demand for natural gas continues to rise. According to the Energy Outlook study published by energy group BP, natural gas is set to become the second most important energy source this year, knocking coal into third place. In order to meet growing demand, energy companies are increasingly also extracting natural gas from reserves with high carbon dioxide (CO₂) content. Using conventional technology to process natural gas from CO₂-rich reserves requires multiple, energy-intensive process steps. Linde Engineering and specialty chemicals company Evonik Industries have joined forces to develop membranes based on Evonik's Sepuran® technology that enable even CO₂-rich natural gas to be processed in an energy-efficient way.
The joint product will be marketed by Linde as the “HISELECT™ powered by Evonik” high-performance membrane package unit. The membrane is based on a robust, highly selective polymer developed by Evonik. It consists of hollow fibres that efficiently separate the carbon dioxide in the natural gas stream from the main hydrocarbon, methane. The membrane material has to be particularly robust as natural gas contains a host of other substances that can influence the separation performance of membranes. This new technology has proven its value in the field – the first HISELECT modules have already been successfully deployed in natural gas processing plants.
Conditioning gas mixtures to pipeline grade
Before natural gas can be transported by pipeline, it has to go through a complex purification process. As a natural product, natural gas doesn’t just contain the main fuel methane. It also has unwanted components such as water, carbon dioxide, nitrogen and heavy hydrocarbons. These impurities must first be removed in pretreatment plants. This involves using thermal and adsorptive processes to remove water and heavy hydrocarbons. The carbon dioxide content also has to be reduced – which is where the new membrane technology comes into play. “The CO₂ content of natural gas varies from one reserve to another and can be as high as 30 per cent. However, many of these reserves have now been depleted and so companies are developing deposits with up to 70 percent CO₂ content,” says Alexander Brandl, sales manager of the Adsorption and Membrane Plants product line. “Before natural gas can be fed into a pipeline, the carbon dioxide share has to be brought down to between two and three percent. This would be a typical industry-standard value for gas as a source of energy.”
Diffusion-based separation process
Until now, chemical and physical processes were generally used to separate carbon dioxide from natural gas mixtures during pretreatment. Referred to as gas scrubbing, these processes are well established and well researched. However, scrubbing also has a lot of drawbacks, as Brandl explains: “Chemical scrubbing is relatively energy intensive as it requires large amounts of steam. And as the CO₂ content rises, so too does the amount of steam – and therefore energy – required. “What's more,” adds Brandl, “in higher concentrations, the chemicals used act like a biocide and can therefore damage the environment.” Membrane processes, however, are based on a completely different separation principle and therefore offer major benefits over the conventional approach: The unwanted carbon dioxide is removed via a purely physical process known as diffusion. “In principle, the membrane works like a semi-permeable barrier layer,” elaborates Brandl. The high-performance SEPURAN membrane developed specifically by Evonik for natural gas processing forms the heart of the HISELECT modules. The hollow-fibre membrane is highly robust and resistant to pressure and temperature. “The fact that we don’t need any additional energy for the membrane process is a key advantage,” notes Brandl. “We simply use the pressure at which the natural gas comes out of the earth. This can be between 40 and 120 bar depending on the source.”
Using membranes to efficiently remove CO₂
After pretreatment, the stripped gas mixture is fed into the cartridges that contain the integrated high-performance membranes. The CO₂ molecules diffuse very rapidly through the wafer-thin plastic barrier layer. The methane molecules diffuse at a much slower rate and have much more difficulty passing through the barrier. This means that the useful energy carrier methane is left behind together with small yet similarly desirable fractions of butane, ethane and propane. The gas mixture is now ready to be fed into a pipeline.
There are two further benefits to Evonik’s special membranes: They are ideal for natural gas reserves with a high CO₂ content and are also highly selective. This last factor means that the pores of the high-performance polymer have been designed specifically for the diffusion of CO₂. As a result, only very few methane molecules can follow the carbon dioxide through the barrier. Brandl estimates the rate of methane loss at just one to four percent. He also explains how this figure can be brought even lower: “Where necessary, we can reduce losses further by incorporating a second separating step, where the resulting mix of methane and carbon dioxide is fed through the membrane cartridges one more time.”
Proven performance in helium recovery
Gas scrubbing is still the industry standard in natural gas processing. However, membrane processes with high-performance polymers offer a great deal of potential. The collaboration with Evonik has already proven to be a success in the field of helium recovery. In June 2016, a plant in Mankota, Canada, started operations using a hybrid process combining membrane separation with pressure swing adsorption. At the time, this technology was completely new. At the facility in Canada, the membranes separate the noble gas helium from the natural gas mix.
When Evonik’s specialists then decided to develop a plastic for processing natural gas, they were able to tap into the expertise and experience that Linde Engineering had already gained. During tests at a plant in Argentina, Linde's engineers gathered valuable information on how the membrane behaves in day-to-day operations. The tests confirmed the importance of ensuring the membrane remains stable when dealing with heavy hydrocarbons as this allows the CO₂ molecules to pass through the barrier layer as intended. “While other membranes experience a significant drop in performance after just a short time, HISELECT membranes remain stable even under challenging conditions and with high hydrocarbon levels,” reports Brandl proudly. He believes that both companies benefit from the collaboration because of this. As a sales manager, Brandl plays a key role in the development of this partnership. “This is a very close and productive alliance.”
Suited to plants of all sizes
The new membrane separation process can be deployed in natural gas processing plants of all sizes. “The HISELECT cartridges make this a scalable process and we can tailor our solution to the individual requirements of our customers,” explains Brandl. Five facilities in North and South America are already working with the new membranes. The new technology has also allowed Linde to close a gap in its portfolio. “We can now cover the entire natural gas processing value chain, including energy-efficient and robust membrane-based separation processes. This gives our customers a one-stop service for all of their gas processing needs,” concludes Brandl.