Applying flow and pressure regulation for direct capture of CO2 from seawater
CO2 extraction from the atmosphere vs the ocean
Direct air capture systems currently work by taking CO2 directly out of the air and removing it directly from the atmosphere where it contributes to global warming. While atmospheric direct air capture has a positive effect on the environment, it is also a relatively energy intensive process due to the requirements to adjust pressure and temperature conditions in adsorbent chambers.
In a new cyclic flow process developed by MIT researchers, CO2 can now be captured from saltwater located in the oceans by using electrochemical cells that firstly acidify and then alkalize the treated water before it is returned to the oceans. As the water is acidified, bicarbonates in the water are changed into molecules of CO2 which can be removed under vacuum before the water is alkalized.
Because CO2 is 100 times more concentrated in ocean water relative to air and MIT’s seawater process does not require either heating or pressurizing adsorbents as in direct air capture, MIT estimates the process cost to be just $56 per ton of CO2, which is much cheaper than atmospheric direct air capture for the same amount of CO2. Because the oceans act as a large carbon sink, removing CO2 from the oceans will allow for additional CO2 to be absorbed by the oceans from the air, ensuring continuation of the process until CO2 is removed from the atmosphere sufficiently to reduce or eliminate the effects of global warming.
How MIT’s CO2 capture from seawater works
MIT’s CO2 capture from seawater process works via a chloride-mediated electrochemical pH swing using separate electrochemical cells that firstly acidify the oceanwater and then alkalinize it. Before the treated water returns to the alkalinization electrochemical cell, the CO2 is removed as a pure gas by stripping under vacuum in a hollow fiber membrane contactor.
Each of the electrochemical cells works cyclically such that one cell is operating until its active electrode is depleted of protons while the other cell is regenerated as its active electrode is replenished. At the end of a cycle, when the active electrode in the first electrochemical cell is depleted, the flow direction of each cell is switched as are the polarities of the applied voltages. Repeating the process using cyclical flow allows for highly efficient removal of the CO2 from the seawater.
A membrane contactor is used in the MIT setup to promote CO2 release from water with a flowing nitrogen sweep stream, which is used to emulate the drawing of a vacuum on the unit. CO2 sensors determines the concentration of CO2 which is captured in the experiment. MIT notes that the CO2 can be separated from the water using additional methodologies.
Adding liquid and gas flow control to MIT’s CO2 capture from seawater process
Liquid flow control
In MIT’s research experiment setup, simulated oceanwater containing 0.5 M of NaCl and 2.5 mM of NaHCO3 is pumped into the electrochemical cell system at a flow rate of 1.04 mL/min via a peristaltic pump. As mentioned previously, after each cycle the flow direction is reversed. For this process, a single Alicat liquid flow controller and a pump can be connected to each of the two electrochemical cells such that flow can be automated bidirectionally.
Alicat’s CODA controller provides highly accurate flow control for seawater into and out of the system and can be automated using command scripts or connected to other sensors in the system via either serial, analog, or industrial protocols on PLC’s and computers. Within research settings, Alicat devices are used to optimize the operating conditions for electrochemical research and increase the validity of similar test results for electrochemical cells used in electrolysis and hydrogen fuel cells.
CODA controller features and specs:
- 40 g/h full scale to 100 kg/h full scale with a turndown of 2% – 100% of full scale
- NIST-traceable accuracy up to ±0.2% of reading or ±0.05% of full scale, whichever is greater
- Repeatability ± ±0.05% of reading or ±0.025% of full scale, whichever is greater
- Control response times as fast as 500 ms
Gas flow control
In MIT’s research process, nitrogen gas is flowed at 5mL per min to promote the CO2 release from the acidified seawater after it exits from the first electrochemical cell.
For this purpose, either Alicat’s MC-Series or CODA controller are both excellent choices due to wide control range, extensive communication options, and totalizing and batching features. As a result, high quality, accurate CO2 concentration can be measured by CO2 sensors due to control stability seamlessly sent to a PLC or computer for continuous data collection and automation.
MC-Series features and specs:
- 0.5 SCCM full scale to 5,000 SLPM full scale with a measurement range of 0.01% – 100% of full scale
- Repeatability up to 0.1% of reading and 0.02% of full scale
- Measurement response times as fast as 10 ms
Pressure regulation in CO2 capture from seawater
Although MIT’s research configuration used nitrogen to sparge out the CO2, the research paper alluded to a commercial process that would use vacuum separation.
Alicat pressure controllers are widely used with vacuum pumps in various industrial industries, allowing for precise and accurate chemical processing such as in chemical vapor deposition in synthetic diamond manufacturing. As like Alicat mass flow controllers, Alicat pressure regulators offer a wide variety of communication options, including analog, serial, and industrial protocols.
PC-Series features and specs:
- Full-scale ranges from max 0–3000 PSIA; min 0–15 PSIA and gauge ranges of max 0–3000 PSIG; min 0–0.07 PSIG (2 inH₂O)*
- Steady state control ranges from 0.01% – 100% of full scale
- Standard accuracy calibration, NIST-traceable up to ±0.25% of full scale
- Optional high-accuracy calibration, NIST-traceable up to ±0.125% of full scale
- Repeatability up to 0.08% of full scale
- Typical control response times as fast as 30 ms
- Warm-up times less than 1 s