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Applying Ceramic Membranes to Molecular Separation Processes
Molecular Separation, Especially CO2,N2, and Liquids
NGK has developed various sub-nano ceramic membranes capable of separating specific gases from gas mixtures and specific liquids from liquid mixtures. These products are instrumental to establishing innovative separation processes that dramatically reduce both costs and energy consumption.
Types of Ceramic Membranes and Scope of Application
Sub-nano ceramic membranes are characterized by exceptionally small pore size, which enables molecular separation. As one example, these membranes can be used to isolate CO2 from a gas mixture, a function that contributes significantly to carbon neutrality.
Structure of a Sub-nano Ceramic Membrane
Molecular separation is achieved by forming a dense separation layer made of zeolite or other materials, which has fine pores of less than one nanometer (one billionth of a meter) in diameter, on the inner surfaces of approximately 1,600 cells penetrating lengthwise through a columnar porous ceramic base material measuring 180 mm in diameter and 1,000 mm in total length. By switching the separation layer type according to the target substance, we can control pore size to selectively separate desired molecules.
NGK’s sub-nano ceramic membranes separate specific molecules based on differences in molecular size and adsorption properties. Suppose we use NGK’s sub-nano ceramic membrane to separate a gas mixture containing molecules larger and smaller than the pores of the separation layer. Since only molecules smaller than the pores can pass, we can efficiently isolate a gas characterized by small molecules from a gas characterized by large molecules.
Features of Sub-nano Ceramic Membranes
Sub-nano ceramic membranes offer the following features:
(1) Capacity to perform molecular separation
These membranes achieve molecular separation of gases and liquids via a layer of zeolite or other materials, which has molecular-scale pores and covers the cell surface.
(2) Uniform pore size for high separation performance
These sub-nano ceramic membranes also allow selection of the separation layer type based on the target molecule, thereby enabling the separation of a wide range of molecules.
For example, DDR-type zeolite, composing the separation layer of DDR-type zeolite membranes, is a crystalline material characterized by oval pores with 0.36 nm × 0.44 nm. The minor axis of the pore (0.36 nm) is longer than a CO2 molecule (0.33 nm) but shorter than a CH4 molecule (0.38 nm). When a CO2-CH4 gas mixture is supplied to the DDR-type zeolite membrane, CO2 molecules will permeate preferentially to achieve high separation coefficient.
Size of zeolite pores and molecules
Molecule separation image using DDR-type zeolite membrane
Conventional membrane
Uniform pores based on crystal structure
Single gas permeance of DDR-type zeolite membrane
(3) Realizing more compact and energy-saving separation systems
Methods like amine absorption for CO2 separation, cryogenic separation for N2, and distillation for organic liquid dehydration offer high separation performance but require huge equipment and consume significant amounts of energy. Combining NGK’s sub-nano ceramic membranes with these methods should result in more compact and energy-saving systems.
(4) High heat resistance, pressure resistance, and durability
As they are made of ceramic materials, NGK’s sub-nano ceramic membranes offer excellent heat resistance and pressure resistance, making them ideal for use with various gases and liquids across a wide range of temperatures. Due to the high mechanical strength and rigidity of ceramics, membrane structures undergo minimal deformation even when exposed to pressure and heat, thus offer long service life.
(5) Reduced pressure loss and high permeability
The base material on which the separation layer is formed has a multilayer structure consisting of a surface layer and a support layer featuring arrangements of pores of different sizes. Compared to a single-layered structure with uniform pore size throughout the base material, the multilayer structure can greatly reduce pressure loss. We can achieve high permeability by forming a separation layer on such base materials with multilayer structure.
(6) Large membrane surface area for high processing capacity
The honeycomb structure created by the approximately 1,600 cells penetrating lengthwise through a columnar ceramic base material measuring 180 mm in diameter and 1,000 mm in total length results in high processing capacity. Unlike membrane tube products that need to be bundled together to create a module, NGK’s sub-nano ceramic membranes require fewer parts, resulting in more compact equipment and cost reduction.
Contributing to Carbon Neutrality with a Wide Range of Applications in Diverse Fields
Sub-nano ceramic membranes can separate various molecules such as CO2, H2, and H2O. Such separation technologies are in demand for applications like the ones introduced below.
(1) CO2-EOR (Carbon Dioxide Enhanced Oil Recovery)
Nearly 50 % of the injected CO2 remains fixed below ground. This technology is currently entering wide adoption as a key tool in Carbon Capture and Storage (CCS) processes.
(2) CO2 separation from industrial exhaust gas
- ※Separation processes using sub-nano ceramic membranes require a certain pressure difference between the feed side and permeate side.
(3) N2 separation from natural gas
Natural gas may contain N2 as an impurity. Since LNG and pipelines have limits on allowable N2 concentration, the N2 must be removed beforehand. Cryogenic separation, the most widely used method at this time, requires large-scale equipment and significant energy. Adopting NGK’s sub-nano ceramic membranes enables N2 removal with more compact equipment and less energy.
(4) H2O separation from hydrous organic liquids
Sub-nano ceramic membranes can also be applied to dehydration processes. For example, organic liquid dehydration is typically carried out by the distillation method, whereby the liquid is heated to separate water from organic components based on differences in boiling points. Incorporating NGK’s sub-nano ceramic membranes to this process is expected to reduce the energy needed to heat the organic liquid and allow more compact equipment design.
(5) Application of sub-nano ceramic membranes in membrane reactors
Fuel synthesis from CO2 and other processes that involve slower chemical reactions can be accelerated by extracting a portion of the reaction product from the reaction fields to keep the reaction from reaching chemical equilibrium. Since most chemical processes are performed at elevated pressure and temperature, expectations are high for the application of sub-nano ceramic membranes in membrane reactors, which use membranes to extract products from reaction fields.
Types and Separation Properties of Sub-nano Ceramic Membranes
Provided below is an introduction to the various types of NGK's sub-nano ceramic membranes and their separation properties.
Types and applications of sub-nano ceramic membrane
| Membrane type | Applications | Separation target | |
|---|---|---|---|
| Gas separation | ・DDR-type zeolite membrane |
・CO2 recovery from associated gas of CO2-EOR ・Natural gas purification ・Biogas purification |
CO2/CH4 |
| ・He recovery from natural gas | He/CH4 | ||
| ・Separation for chemical process ・H2 recovery from reaction gas |
CO2/H2 H2/Ethane H2/CH4 H2/Methylcyclohexane |
||
| ・Various zeolite membranes |
・Natural gas purification ・CH4 recovery from LNG boiled off gas |
N2/CH4 | |
| ・MFI-type zeolite membrane | ・CO2 recovery from industrial exhaust gas | CO2/N2 | |
| Dehydration | ・DDR-type zeolite membrane ・LTA-type zeolite membrane |
・Dehydration from organic liquid ・Dehydration in the ester synthesis process ・Recycling of used organic liquid ・Dehydration in membrane reactor |
H2O/Alcohol H2O/Ketone H2O/Ether H2O/Ester H2O/Aromatic |
Separation properties for a gas mixture using sub-nano ceramic membranes
| Membrane type | Composition of gas mixture [mol%] |
Test conditions | Performance | ||
|---|---|---|---|---|---|
| Pressure of feed gas [MPaG] |
Temperature of feed gas [℃] |
Composition of permeate gas [mol%] |
Separation coefficient※ [-] |
||
| DDR-type zeolite membrane | CO2 : CH4 = 50 : 50 | 0.3 | 25 | CO2 > 99 | >160 |
| H2 : CH4 = 60 : 40 | 0.4 | 25 | H2 > 99 | >100 | |
|
CO2 : H2 = 40 : 60 |
0.4 | 25 | CO2 > 87 | >10 | |
| Zeolite membrane for N2 separation | N2 : CH4 = 50 : 50 | 0.4 | 25 | N2 > 96 | >30 |
- ※Separation coefficient = Component ratio of permeate gas / Component ratio of feed gas
Dehydration performance from hydrous organic liquid using sub-nano ceramic membranes
| Membrane type | Organic liquid | Test conditions | Dehydration performance | |||||
|---|---|---|---|---|---|---|---|---|
| Organic liquid concentration of feed liquid [wt%] |
Temperature of feed liquid [℃] |
Vacuum pressure at permeate side [torr] |
Water permeation rate [kg/m2h] |
Organic liquid concentration of permeate liquid [wt%] |
Separation coefficient※ [-] |
|||
| DDR-type zeolite membrane | Organic acid | Acetic acid | 90 | 90 | 50 | 4.0 | <0.5 | >1,700 |
| Aromatic | Phenol | 50 | 90 | 50 | 30.0 | <0.01 | >10,000 | |
| Ester | Ethyl acetate | 97 | 70 | 50 | 1.5 | <0.5 | >6,400 | |
| Alcohol | Ethanol | 90 | 70 | 50 | 1.0 | <0.5 | >1,700 | |
| Isopropanol | 90 | 70 | 50 | 4.0 | <0.1 | >9,000 | ||
| n-Butanol | 90 | 70 | 50 | 5.5 | <0.1 | >9,000 | ||
| Ketone | Acetone | 90 | 50 | 10 | 1.5 | <0.1 | >9,000 | |
| Ether | Tetrahydrofuran | 90 | 50 | 10 | 3.5 | <0.1 | >9,000 | |
| LTA-type zeolite membrane | Alcohol | Ethanol | 50 | 60 | 50 | 2.7 | <0.01 | >10,000 |
- ※Separation coefficient = Component ratio of permeate liquid (Water/ Organic liquid) / Component ratio of feed liquid (Water/ Organic liquid)
Examples of Sub-nano Ceramic Membrane Systems
Provided below are examples of systems that use sub-nano ceramic membranes.
CO2 separation from natural gas (DDR-type zeolite membrane)
High-purity CH4 (methane) and CO2 can be obtained while suppressing methane loss by separating and removing the CO2 contained as an impurity in natural gas, through use of a DDR-type zeolite membrane in a one-stage membrane system.
N2 separation from natural gas (zeolite membrane for N2 separation)
N2 contained as an impurity in natural gas can be separated and removed using a zeolite membrane for N2 separation. A two-stage membrane system makes it possible to obtain high-purity methane while suppressing methane loss.
Biogas Purification (DDR-type zeolite membrane)
The biogas produced from biomass contains CH4 (methane) and CO2. The DDR-type zeolite membrane can also be used to separate and remove CO2 from biogas. A two-stage membrane system makes it possible to obtain high-purity methane and CO2.
Related Products
NGK offers a wide range of ceramic filter products with different pore sizes.
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