Membrane-Based Pressure Gradient Separation

Capture Mechanism

Membrane

Furthest Progress*

TRL 2

Highest Risks

Energy
Cost
Environment
Storage

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Method Overview

Definition:

Polymeric membrane separation in which the CO2 is separated from other gases via differences in the molecular ability to pass through a polymer membrane (i.e., permeability); multiple capture stages, in which several membrane units are stacked together, may be required to achieve a concentrated CO2 product.*

Example:

CO2 in air permeates through a series of membranes, becoming increasingly concentrated until it can be utilized or stored.*

Advantages:

Membrane-based separation is a well-developed method of CO2 capture from concentrated industrial waste streams.
This method does not require heat for CO2 release and sorbent regeneration, although some process varients include it. Typically, a pressure differential drives the separation of CO2 from air using permeable membranes.
Equipment used in this approach (i.e. membrane layers and vacuum pumps) is simple and modular. This approach avoids more complex equipment such as moving adsobent beds, calciners, slackers, etc. that are involved in other DAC methods.

Disadvantage:

This method faces a substantial thermodynamic challenge for DAC. The concentration of CO2 in the atmosphere (0.04%) is too low to generate a sufficient driving force to permeate CO2 efficiently, so DAC systems would require an applied vacuum on the permeate side. Creating a sufficient pressure differential to move CO2 through the membrane would require substantial energy.
CO2 purity is strongly dependent on membrane selectivity, which is often oppositional to permeability. DAC systems that are permeable enough to be productive struggle to produce high-purity CO2 sufficient for geological storage. Higher purity can be achieved by placing tens of membranes in series, however this leads to substantial pressure drop and thus higher energy demand. Low purity CO2 (<40 %) is sufficient for greenhouse applications.

* Reproduced from The Applied Innovation Roadmap for CDR (2023) by RMI.

Company Overview

Plot of estimated funding vs. deployment status of companies utilizing this approach. Select data points to view company details. Only companies for which funding information is publicly available are included. Companies without funding information are tabulated with related details where relevant.

Summary of Deployments

View DAC deployments within this approach that have achieved or surpassed prototype scale. Planned deployments are included. Sort DAC deployments by company, scale, start of operations, and more. Because DAC is a rapidly evolving industry, this list may not be exhaustive.*

* Due to uncertain funding, plans for most DOE-funded DAC Hubs are not included in this analysis.

Risk Assessment

RMI's Applied Innovation Roadmap for CDR (2023) categorizes the risks associated with different classes of DAC technologies. Their assessment of this method is reproduced here and contextualized with current self-reported company data, where available.

Energy Requirements

Very high risk: Membrane separation is extremely energy intensive.

Energy needs meet our criteria for “very high risk” (>10 GJ/tCO2). Membrane separation alone requires approximately 44-62 GJ/tCO₂. More energy will be needed to further purify the CO2 product stream (currently at 40-60% CO2). The full pathway will require additional energy (e.g., compression, transport, and storage).

Material Requirements

Low risk: No rare materials are required.

Apart from basic construction materials (e.g., steel, cement), this solution requires chemicals (e.g., organic polymer membranes) that need to be synthesized and purchased.

Cost Requirements

Very high risk: Cost data is incomplete.

FOAK plant is expected at <10 ktCO2; however, plant data is not available. Low CO2 purities (40-60%) contribute to cost uncertainty. The net FOAK capture cost is expected to be at >$600/tCO2 (excluding compression, transport, and storage).

Land Requirements

Low risk: Significant land areas are suitable for DAC siting.

No information is publicly available, but it is expected that plant footprints will be very competitive with other DAC approaches. The footprint will be larger if land use for energy production is considered.

Water Requirements

Low risk: No water is required.

No water is required to perform this DAC approach.

Environmental Impact

High risk: No/incomplete LCA data is available.

No LCA is currently available. Apart from impacts associated with energy and material requirements, minimal to no other impacts are foreseen.

Co-Location of DAC and Storage

Medium risk: Co-location is challenging due to the low purity DAC output.

Co-location with storage is challenging due to the currently low CO2 purities obtained from the DAC unit.

Secure Storage for 100+ Years

High risk: Scalable, permanent options for low purity DAC are underdeveloped.

Geological sequestration or in-situ mineralization (primary storage options) are considered permanent storage approaches. However, high CO2 purity DAC output needs to be achieved first, and there is low certainty as to whether durable or permanent storage is feasible for low purity DAC outputs.

Path to MRV at Scale

Low risk: Measurement is straightforward.

DAC+S measurement and monitoring to inform MRV is straightforward. Capture occurs in a controlled environment and geological injection has established monitoring standards.

Key Innovations

While membrane-based separations are widely used to capture CO2 from more concentrated sources, their use in DAC must overcome the thermodynamic challenges associated with driving such a small percentage (0.04%) of CO2 in air through the system. Innovations in material design that improve membrane slectivity (i.e. captured CO2 purity) without sacrificing permeability must be accomplished before membrane-based pressure gradient separations can be considered a plausible DAC approach for geological storage. However, utilization in greenhouses is suitable for lower-purity CO2 streams. Process and equipment design are secondary concerns as both are relatively simple, compared to leading DAC approaches.

To the best of our knowledge, Carbon Xtract is the sole DAC technology producer in this field. Selected characteristics of the company are outlined below alongside related research efforts that seek to devel0p this DAC approach.

Material Design:

Selectivity and permeance - To be competitive with other forms of DAC (i.e., comparable energy efficiency and CO2 purity), membrane selectivity to CO2:N2 must be improved. Meanwhile, CO2 permeance must be improved. Approaches such as including Lewis acidic monomers, post-synthetically modifying copolymers, and adding porous filler materials like MOFs are being researched.
Durability - Porous membranes are susceptible to aging, wherein the pores of the material collapse. Likewise, CO2 flux at high pressures can cause movement of the polymer chains that disrupts the selectivity of the material for CO2.

Process Design:

Pre-treatment steps to remove H2O and/or O2 from inlet streams may be needed, depending on the membrane design. Water is a condensable gas than can affect the pore sizes of membrane and disrupt selectivity. O2 can oxidize amine-based membrane systems and reduce their performance.
Membrane based DAC can be designed around applications accomidating lower-purity CO2, such as greenhouses for agriculture and algae cultivation. Carbon Xtract follows this approach.
Hybrid systems using membrane DAC to pre-concentrate CO2 from ambient air for other more selective DAC methods have been proposed but not thoroughly investigated.

Equipment Design:

Contactor - Optimizing air contactor design is an important focus for membrane DAC. Thin membanes (<1 µm) are required for high gas permeance, making contactor designs such as spiral wound membranes or hollow fiber membranes attractive options to maximize exposed membrane surface area.