China is committed to become carbon neutral by 2060. To support the country towards that goal, researchers at the School of Chemical Engineering and Technology at Tianjin University have been developing key chemical technologies and solutions to reduce carbon emissions.
The school has a history of innovation that can be traced back more than 100 years. Chemists here have made major breakthroughs in green technologies including carbon capture, catalysis, and conversion for efficient use of the greenhouse gas, hoping to close the carbon loop.
One of the leading scientists at the school is Zhi Wang, who has been focusing on the multi-layer membrane of carbon capture. “The biggest challenge in designing a high-performance membrane lies in locating the ideal material that is long-lasting and easy to scale up, with a high permeability and selectivity for carbon dioxide (CO2),” he says.
An industrial-scale testing machine for multi-layer membrane preparation developed by Zhi Wang’s team.Credit: Tianjin University
“Over more than 20 years we have researched bridging the technological gaps in the integrated carbon capture chain, preparing high-performance membranes for large-scale application.”
It began with a series of foundational research projects on the CO2 transport process in the membrane, because a membrane material has to achieve high separation performance for CO2 versus other gases such as nitrogen, hydrogen and methane.
His team zoomed in on a class of water-soluble polymers called polyvinylamine (PVAm). Compared to the traditional choice of amine solvents to dissolve and separate the gases, PVAm membranes come without solvent pollution, and are easier to scale up with less energy required. It also has a higher CO2 selectivity compared with the existing commercial membrane materials.
“The amino group can interact with CO2 preferentially, especially in the presence of water, which makes PVAm have a high CO2 separation performance,” explains Wang.
In 2021, in the Journal of Membrane Science1 they reported a novel industrial-scale coating machine and membrane preparation technology for the scale up of multilayer PVAm composite membrane. The prepared membrane showed a relatively higher separation performance for CO2 over nitrogen compared with other large-area membranes.
Wang’s team also had a goal of overcoming the difficulty of optimizing the overall flow channel structure of membrane modules, as well as developing multi-stage membrane separation processes.
“Our co-simulation experiments aim to pair our membrane with a spacer mesh that enables uniform distribution of exhaust flow with high filling density in the membrane module, but low pressure drop and low concentration polarization (rejected solutes that accumulate on membrane surface),” he says.
Based on these achievements, a pilot-scale plant was built in Nanjing to test the CO2 capture performance of PVAm membrane2. They fed the system gases comprising different concentrations of CO2 — 14%, 25%, and 35% to mimic industrial exhaust gases typically emitted from coal-fired power plants, steel mills or cement plants, and other high CO2-concentration operations. The results were encouraging: the test membrane captured more than half of the greenhouse gases emitted, with a CO2 purity of 78.5% or higher.
Together with process simulation and 3D modeling, their experimental results have been applied in the carbon capture demonstration plant in Nanjing, which started running in 2021 with a processing capacity of 50,000 Nm3 (normal cubic metre) per day. The results were further improved due to optimized membrane process.
“We are exploring further optimization of the membrane materials, for example by introducing porous materials into the polymer matrix to capture more greenhouse gases from the flue gas, and purify other gases such as natural gas, biogas, and syngas,” he says.
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