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Applying Ceramic Membranes to Molecular Separation Processes

Molecular Separation, Especially CO2,N2, and Liquids

sub-nano ceramic membranes

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

NGK offers optimal filtration solutions based on a wide lineup of products, ranging from dust collection systems for industrial exhaust gas to low molecular separation systems for pharmaceutical manufacturing processes.
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.

Illustration showing NGK's filtration solutions based on a wide lineup of products.

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.

Appearance and cross-sectional structure of sub-nano ceramic membrane.
Illustration of cut model. Feed gas through the separation layer is permeated from the side slit.

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.

Separate specific molecules based on differences in molecular size.
Separate specific molecules based on differences in adsorption properties.

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

NGK’s sub-nano ceramic membranes use crystalline zeolite or other materials, which has uniform fine pores, for the separation layer, thereby achieving high separation coefficient compared to organic membranes and carbon membranes used in conventional membrane separation.
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

Size of zeolite pores and molecules.

Molecule separation image using DDR-type zeolite membrane

Conventional membrane

Ununiform pores

Conventional membrane composed of ununiform pores.

DDR-type zeolite membrane
Uniform pores based on crystal structure

Uniform pores based on crystal structure.

Single gas permeance of DDR-type zeolite membrane

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)

This technology for enhancing oil recovery rates involves injecting CO2 into the oil layer below the ground to lower the viscosity of the residual oil in the oil layer and increase its fluidity.
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.

Enhancing oil recovery rates involves injecting CO2 into the oil layer below the ground to lower the viscosity of the residual oil.

(2) CO2 separation from industrial exhaust gas

As the pursuit of carbon neutrality intensifies, the importance of technologies for collecting industrial exhaust gas from factories and other facilities will continue to grow. However, the gas pressure of industrial exhaust gas tends to be lower than gas pressures in other applications. The design of an optimal process characterized by reduced pressure loss and high permeability will be crucial to the development of such technologies. That is why NGK is also focused on developing CO2 separation membranes for industrial exhaust gas that can be used to separate CO2 contained in industrial exhaust gas from factories and other facilities.

  • Separation processes using sub-nano ceramic membranes require a certain pressure difference between the feed side and permeate side.
Developing CO2 separation membranes for industrial exhaust gas that can be used to separate CO2 contained in industrial exhaust gas.

(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.

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.

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.

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.

CO2 separation from natural gas.

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.

N2 separation from natural gas.

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.

Biogas Purification.

Related Products

NGK offers a wide range of ceramic filter products with different pore sizes.

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