Membrane vs. Molecular Sieve Nitrogen Generators for LC-MS - Nitrogen Separation Principle

Currently, laboratory nitrogen generation commonly uses two technical routes: one uses hollow fiber membranes to separate nitrogen, and the other uses pressure swing adsorption (PSA) technology with molecular sieves to separate nitrogen.

First, let's discuss the principle of membrane separation technology. The specific diagram is as follows:

1. Principle of membrane nitrogen separation

In membrane separation technology, air is pressurized to a certain pressure and then passed through a hollow fiber membrane bundle. Oxygen, carbon monoxide, carbon dioxide, hydrogen, and water vapor are squeezed and filtered out through the fiber membrane wall and discharged into the air. Nitrogen is retained in the hollow fiber membrane, thus continuously producing nitrogen.

2. Material and structure of the separation membrane

The core material of the nitrogen separation membrane is mainly a polymer, primarily polyimide. This material is then processed into a hollow fiber membrane using a special process. The outer diameter of the membrane is generally 0.2-0.4 mm, and the inner diameter is 0.1-0.2 mm. This structure can withstand relatively high pressure and has excellent chemical stability and mechanical strength (due to this characteristic, many manufacturers claim that the separation membrane can be used for "10 years"). The membrane wall of the hollow fiber membrane has nano-scale micropores,used to separate nitrogen and oxygen.

3. Effective service life

Under ideal conditions, the service life of a nitrogen separation membrane is typically 4-6 years (referring to the lifespan before significant purity degradation). However, the actual lifespan depends on the operating environment and maintenance level. Because of the nano-scale micropores in the hollow fiber membrane wall, they are easily blocked by impurities in the upstream air (impurities include trace amounts of oil mist, particulate matter, and water-soluble substances). Once the micropores are blocked, it is irreversible. Since membrane separation is equivalent to unidirectional filtration, the more the micropores are blocked, the lower the filtration efficiency becomes. Even if the membrane's lifespan is not reached, the decrease in filtration efficiency causes CO, CO2, and O2 impurities that should be squeezed out of the hollow fiber membrane by high pressure to not be effectively squeezed out, remaining in the hollow fiber membrane and mixing with the nitrogen to become the product gas flow to the downstream mass spectrometer. This is why the nitrogen purity of membrane separation generators decreases significantly after 1-2 years of use.

4. Membrane brands and types

Membrane separation manufacturers with higher quality requirements generally use membranes from UBE (Japan). To control costs, many companies now use membranes from South Korea, and some use domestic membranes. The price and theoretical service life of each membrane vary, and the separation efficiency also differs. This has led to considerable price confusion in the domestic membrane separation generator market.

5. Nitrogen purity and adsorption efficiency of membrane separation

The theoretical nitrogen purity of membrane separation can reach 99.5%. In actual mass spectrometry applications, the nitrogen purity of membrane separation is mostly around 95%. The adsorption efficiency is around 35-40%.

Next, let's discuss the principle of pressure swing adsorption (PSA) technology with molecular sieves. The specific diagram is as follows:

1. Principle of pressure swing adsorption (PSA) technology with molecular sieves:

Pressure swing adsorption (PSA) technology with molecular sieves is an advanced gas separation technology that is irreplaceable in on-site gas supply worldwide. Using carbon molecular sieves (CMS) as the adsorbent in the adsorber, it can effectively adsorb oxygen, carbon dioxide, and water vapor in the air under a certain pressure, while nitrogen is not easily adsorbed, thus producing nitrogen. Considering adsorption saturation and release, two adsorbers are generally needed for molecular sieve adsorption, working alternately. While one adsorber is working, the other adsorber releases pressure, discharging the oxygen, carbon monoxide, carbon dioxide, and water vapor adsorbed by the molecular sieve.

The two adsorbers alternately perform adsorption and regeneration to produce product nitrogen with stable flow rate and purity. The switching of the two adsorbers is automatically controlled by an intelligent control system.

2. Material and structure of the molecular sieve

The molecular sieve adsorption technology uses porous rod-shaped carbon particles, generally made from coconut shells through a special process. The diameter is generally 1.0-1.2 mm, the bulk density is 0.65-0.69 g/ml, and the particle strength is greater than 38N.

3. Effective service life

Under ideal conditions, the service life of a molecular sieve is typically 10-12 years, but the actual lifespan depends on the operating environment and maintenance level. Because carbon molecular sieves are columnar porous particles with many nano-scale micropores, these micropores are easily contaminated by liquid water or oil mist, leading to blockage or pulverization of the internal micropores of the molecular sieve, commonly known as molecular sieve poisoning, causing the molecular sieve to completely fail. Therefore, manufacturers using molecular sieve adsorption technology are very strict about the pretreatment of upstream compressed air.

4. Brands and types of molecular sieves

Molecular sieve technology is very mature. In industries such as semiconductors, pharmaceuticals, and heat treatment that require high-purity nitrogen, industrial-grade nitrogen systems all use molecular sieve adsorption technology. The molecular sieves used are mainly domestic brands and have essentially monopolized the international market.

5. Nitrogen purity and adsorption efficiency of molecular sieve technology

The theoretical nitrogen purity of molecular sieve technology can reach 99.9995%. In actual mass spectrometry applications, the nitrogen purity of molecular sieve separation is mostly around 98-99.5%. The adsorption efficiency is around 50%.

Based on the requirements of nitrogen in mass spectrometry applications and the analysis of the two technologies, we can compare the two technologies from multiple dimensions.

1. Size and weight

Membrane separation technology, due to its relatively simple structure with only one membrane, is smaller and lighter in volume. Molecular sieve technology, considering nitrogen purity and process requirements, has two adsorbers that work alternately and regenerate repeatedly. It also has an air storage tank, making it relatively larger and heavier in volume. However, some manufacturers offer models that can be placed under the laboratory bench or mass spectrometer table.

2. Noise

If the membrane components are separated, the hollow fiber membrane of membrane separation is noiseless, and the molecular sieve adsorption will have the sound of pneumatic valve switching. However, both membrane separation and molecular sieve adsorption technologies require a compressed air source, which is mainly provided by the compressor. In a generator, the compressor is the biggest noise source of the generator. Therefore, from this aspect, the noise of membrane separation and molecular sieve technology is the same, mainly depending on the effect of each manufacturer's noise reduction of the compressor.

3. Purity

The purity requirements of nitrogen in different instruments are indeed different. In LC-MS, nitrogen is mainly used as nebulizing gas and purge gas, and partly as collision gas. Usually, the mass spectrometry manufacturer requires the lower limit of nitrogen purity to be 95%. From this aspect, the normal nitrogen purity of membrane separation technology just reaches this starting line, while the nitrogen purity of molecular sieve technology can reach more than 97%, and the highest can reach more than 99.5%. Here, we need to talk about the national standard definition of nitrogen purity, which refers to the non-oxygen content, that is, the oxygen content in 95% purity nitrogen is 5%, while the oxygen content in 97% purity nitrogen is 3%, and the oxygen content in 99% purity is 1%. Therefore, for the same nitrogen, if it is 95% purity and 99% purity, the oxygen content is not a percentage relationship, but a multiple relationship.

4. Dew point, water content

The drying degree of nitrogen is determined by the water content inside. Generally, when the dew point is -25 degrees, the water content is 629ppm. When the dew point is -35 degrees, the water content is 220ppm. When the dew point is -45 degrees, the water content is 71ppm. Generally, because the hollow fiber membrane of membrane separation is a polymer, from the physical structure, it is not afraid of water, oil mist,or impurities. At most, the micropores of the pipe wall are blocked, and the nitrogen purity is reduced. Therefore, some manufacturers dare to advertise a service life of 10 years. For molecular sieve technology, due to the particularity of molecular sieve, the molecular sieve technology is almost harsh on the quality of the front-end gas. Therefore, there are usually heat exchange condensation systems, water and oil mist filtration systems, freezing or membrane separation deep dehydration systems, and other multi-level purification systems, making the molecular sieve inlet gas quality far superior to that of membrane separation. At the same time, because the molecular sieve also adsorbs a certain amount of water vapor, the dew point of nitrogen produced by molecular sieve technology is generally between -35 and -45. For specially treated molecular sieve technology, the nitrogen dew point can reach -65 or lower. Membrane separation is generally around -25.

Many membrane separation devices, once not maintained in time, will have liquid condensate in the air pipe and nitrogen pipe. Some manufacturers have also added so-called liquid drop alarms for this. (With pictures of membrane separation rear-end air pipe and nitrogen management with liquid water)

5. Compressor load

Due to the separation characteristics of membrane separation, the adsorption efficiency is about 40% at 95% purity, while the molecular sieve adsorption technology, due to its good adsorption depth and large adsorption capacity, has an adsorption efficiency of about 52% at 95% purity. The high and low adsorption efficiency directly determines the demand for the front-end intake. Therefore, under the same purity and flow rate, the amount of compressed air required for molecular sieve adsorption technology is less than that of membrane separation, and the compressor load is also smaller. It is precisely because of this technical characteristic that molecular sieve adsorption technology is basically used in many large-flow nitrogen generation systems, because the larger the flow rate, the more prominent the energy efficiency advantage.

6. Maintenance

Membrane separation technology does have few moving parts, or even no moving parts, and nitrogen separation can be completed by directly flowing compressed air through the hollow fiber membrane. Molecular sieve technology not only has two sets of adsorbers, but also control valves and control units. However, these control valves and control units do not belong to wearing parts and consumables, and do not need to be replaced within the normal service life. The front-end gas source and filtration system of membrane separation and molecular sieve need to replace filter elements, compressor seals, piston rings and other wearing parts and consumables. In the separation part, because membrane separation is unidirectional filtration, the purity decreases year by year, with an annual purity attenuation rate of about 15%. Therefore, if the membrane separation generator is used for 2-3 years, the purity will generally drop to below 90%, and even the purity is around 84%, the oxygen content is 16%, which is more than 3 times higher than the 5% required by mass spectrometry (under 95% purity). Therefore, under normal circumstances, if the membrane separation generator needs to maintain the basic purity, the separation membrane should be replaced every year, and this cost will be much higher than the cost of conventional gas source consumables. For molecular sieve adsorption technology, due to the dual adsorber design, the two adsorbers work alternately and reciprocate, so that the molecular sieve purity can be continuously guaranteed, and the normal purity attenuation rate is about 1%. Therefore, it is generally not necessary to replace the molecular sieve within the normal design life cycle, so the maintenance cost is lower than that of membrane separation. If membrane separation does not consider purity and does not replace the hollow fiber membrane every year, the maintenance cost is similar to that of molecular sieve.

7. Initial equipment purchase price

Because molecular sieve technology has a more complex internal structure and a certain degree of technical difficulty. Because there are two sets of adsorbers, a program control valve is needed to control the operation and cycle regeneration of the two adsorbers. At the same time, because the flow rate of the adsorber needs to be controlled,adsorption cycle, and release cycle need to be precisely controlled, a central control unit, digital analog processing module, solenoid valve, pneumatic valve and other components need to be configured. Membrane separation only needs a compressor, a filter component, and a separation membrane, which is particularly simple. In comparison, the cost of molecular sieve technology is much higher than that of membrane separation technology. Some membrane separation manufacturers also have molecular sieve generators. Under the same flow rate, the price of molecular sieve generators is generally about 50% higher than that of membrane separation. (With two schematic diagrams of the technical process)

In summary, molecular sieve adsorption technology has no advantages in volume, weight and purchase price, but it has advantages in nitrogen quality, purity, dew point content, service life, and maintenance cost.