Sepuran has been on the market for the past eight years. During this time it has established itself—for example, in biogas plants, where it helps to purify methane. It’s also used to extract nitrogen from ambient air and to concentrate valuable gases such as helium and hydrogen. All of these applications have one thing in common: In all of them, the feed gas is channeled into the interior of the hollow fibers. The gas that diffuses through the membrane, which is known as the permeate, is separated from the other gases, which are known as the retentate. In other words, the separation process goes from the inside to the outside of the fibers.
For the new application, Evonik has reversed the direction of the separation and changed the design. “When the membrane is used to purify natural gas, it’s subjected to greater stress, because the pressure and the temperatures are higher,” Baumgarten explains. “That’s why we came up with a structure that involves wound fibers. That gives us the durability we need.”
The membrane is in fact so tough that Linde could not detect any loss of performance even after several months of continuous operation. “For cellulose acetate membranes, there’s already a noticeable drop in performance after one month, and after three months you need to replace some of the cartridges,” says Schiffmann. “With Sepuran, we had the same separation performance after six months as we had in the beginning.” For plant operators, this means they less frequently need to partially or fully stop the facility in order to change the membranes. A further benefit is that if liquid hydrocarbons temporarily form inside the plant, the Sepuran membranes regenerate themselves. “If that happens to a cellulose acetate membrane, the membrane is destroyed,” says Schiffmann.
ONE LESS SEPARATION STEP
In a test, the separation effect was so good that it affected the entire design of the natural gas processing plant. “You often need two separation stages,” says Schiffmann. “After the first stage, the permeate still contains so much methane that we have to compress it with a compressor and send it through a second membrane stage. That way we can separate out most of the methane.” This calls for an immense amount of energy and capital, he adds. “But with the hollow-fiber membranes, the retentate is already so pure after the first stage that we can sometimes switch these machines off or operate them at half power. All of this saves money and increases availability.”
Schiffmann’s team uses the operating data from the plant and from their own lab to plan new plants and retrofit existing ones. But that’s not all: “We also transmit our data, information, and knowledge back to Evonik so that the construction can be refined.” It’s a win-win situation for both partners.
According to Götz Baumgarten, even if one starts with pessimistic assumptions, the new technology has tremendous market potential. The higher the CO₂ content of a natural gas deposit, the more likely it is that membranes will be used rather than the still widespread chemical purification processes such as amine scrubbing. “In the market for natural gas sweetening, membranes have a market share of just eight percent, and until now 80 percent of these membranes have been made of cellulose acetate,” says Baumgarten. He estimates that a market worth several hundred million euros is waiting for the sector to catch on—and Evonik aims to secure a large part of that market for itself.
Photo: The Linde Group;
Publishing date 05 March 2019