Study on Influencing Factors of Molecular Sieve Oxygen-Production System

Abstract

Molecular sieve oxygen-production technology, as a kind of air separation–oxygen production, is receiving more and more attention from every oxygen industry. The two-bed molecular sieve oxygen-production system studied in this paper can generally produce enriched gas with an oxygen concentration of more than 90%, which has the characteristics of strong applicability, high reliability, low cost and high efficiency. However, the gas oxygen concentration of a production system is greatly affected by internal and external factors such as the molecular sieve materials, atmospheric pressure and temperature environment. Through the continuous research of the molecular sieve oxygen-production system, it has been found that the oxygen cycle of the molecular sieve bed and the diameter of the washing and sizing hole also have an effect on the gas oxygen concentration of the production system. Therefore, in this paper, the two-bed molecular sieve oxygen-production device is the research object, with different oxygen-production cycles (including pressure time) and different washing and sizing hole diameter simulation experiments used to explore the molecular sieve oxygen-production system’s optimal oxygen-production cycle (including pressure time) and the rinsing sizing hole’s optimal aperture, to find the structure of the oxygen-production system and the control parameters for the oxygen-production efficiency of the law. The results show that the optimal oxygen-production cycle of the molecular sieve system is 8.0 s (pressure equalization time is 1.3 s), and the optimal diameter of the washing and sizing hole is 0.8 mm.

1. Introduction

Oxygen is the most important source of energy for maintaining life, playing a key role in the discharge of toxic substances and the intake of the proteins and carbohydrates necessary for human life. With the development of society and the continuous improvement of people’s living standards, people’s awareness of oxygen supplementation has gradually increased. Daily oxygen inhalation can eliminate fatigue, enhance memory, resist aging and improve body immunity. In addition, oxygen is also widely used in steelmaking, welding and other production types. Different oxygen-preparation technologies have emerged in response to the oxygen needs of different environments and fields [1].

Molecular sieve oxygen-production technology appeared in the 1960s. In the early 1980s, the United States and Japan successively realized industrialization. In the early 1990s, Praxair, Inc of the United States developed a lithium molecular sieve oxygen-production adsorbent and the VPSA (vacuum pressure swing adsorption) oxygen-production process according to the characteristics of a lithium molecular sieve, which allowed the pressure swing adsorption oxygen-production technology to develop rapidly, laying a solid foundation for its widespread use. This technology introduces the gas into an adsorption tower through pressure and uses the change in circulation conditions, temperature and pressure to cause a change in the adsorption capacity of the adsorbent, which realizes gas separation. VPSA has become a very important technology in gas separation and even liquid separation, by virtue of its advantages such as a large pressure regulation range, easy realization, short time consumption, low energy-consumption demand, and low equipment investment [2,3]. The core of molecular sieve oxygen-production technology is pressure swing adsorption.

Generally, the adsorption processes we know include physical adsorption and chemical adsorption. Physical adsorption widely exists among all molecules, usually in the form of single-layer adsorption, but, when the pressure increases, multi-layer adsorption will also occur. Since physical adsorption is combined between molecules by van der Waals force, the binding ability between molecules is weak, and the molecular heat generated by intermolecular binding is small. The adsorbed adsorbate is easily desorbed from the adsorbent, which is reversible. Chemisorption is achieved through the chemical reaction between molecules, so it has a strong action ability. This adsorption process is monolayer adsorption, which has selectivity and irreversible characteristics [4,5,6]. PSA (pressure swing adsorption) is a process of physical adsorption.

With the continuous development of separation technology, in the early 1990s, the Praxair company of the United States put their Li-X type molecular sieve into use, which marked a new development period for pressure swing adsorption oxygen-production technology. Currently, oxygen-production systems using an Li-X type molecular sieve occupy a leading position in the market. The Nitroxy SXSDM (SXS type) molecular sieve produced by the CECA company of France has good oxygen-production performance and has been used in the field of medical and healthcare oxygen production [7]. From the above research, it can be seen that the research and development of molecular sieve materials plays a key role in molecular sieve oxygen-production technology. With the gradual improvement of molecular sieve research and the wide application of molecular sieve oxygen-production technology, people are also gradually studying molecular sieve oxygen-production systems [8,9].

A molecular sieve oxygen-production system is a system that uses the adsorption characteristics of the molecular sieve to separate nitrogen from the air to produce oxygen under a normal temperature [10,11,12]. This process adsorbs nitrogen in the source air through a molecular sieve, so that the partial pressure of the oxygen in the gas increases. Scientific research has proven that the air contains 0.94% argon, 21% oxygen and 78% nitrogen. Analysis and calculations show that the partial pressure of oxygen in the gas after adsorption is 95% (21%/22% ≈ 95%). Theoretically, it can be concluded that molecular sieve oxygen-production technology can obtain an oxygen-rich gas with an oxygen concentration of 95%. However, due to the influence of many factors during the actual process of molecular sieve oxygen production, the actual oxygen produced is about 90~95%, so research on the influencing factors of molecular sieve oxygen production becomes particularly important [13,14,15].

In this paper, the influence of the oxygen-making cycle (including different pressure-equalizing times) and the diameter of the washing and sizing hole on the oxygen-making concentration of a molecular sieve oxygen making system is studied. Taking the two-bed molecular sieve oxygen-generation system as the research object, the air as the gas source and an air compressor as the air source, the optimal oxygen-generation cycle (including different pressure equalization time) and the optimal diameter of the washing and sizing hole for the oxygen-generation system were verified through experiments with different oxygen-generation cycles (including different pressure equalization times) and different washing and sizing hole diameters.

2. Operating Principle of Molecular Sieve Oxygen Generation

The molecular sieve oxygen-production system is developed on the basis of the Skarstrom cycle [16], and the traditional molecular sieve oxygen-production system is characterized by different pressure adsorption and pressure-reduction washing regeneration. The two-bed molecular sieve oxygen-production system studied in this paper uses the principle of the positive pressure adsorption of the molecular sieve and normal pressure desorption to separate oxygen from the air to produce oxygen. The system consists of an intake unit, an air compressor, a filter, a pressure reducer, an electromagnetic distribution valve, a control unit, a washing unit, a molecular sieve bed, a check valve, a gas storage tank, an output distribution device (including an outlet filter) and other functional components. The working principle of the system is shown in Figure 1.

The working principle of the two-bed molecular sieve oxygen-producing system is that the air compressor introduces compressed air from the surrounding environment through the filter of the oxygen-producing system into the pressure reducer, and the compressed low-pressure gas is distributed into a molecular sieve bed through the distribution valve. The molecular sieve in the molecular sieve bed adsorbs the nitrogen in the compressed gas, and the oxygen-rich gas flows out of the bed. The produced oxygen-rich production gas is divided into two ways, one of which flows into the gas storage tank, and the bypass enters the other sieve bed through the flushing sizing hole in reverse to carry out reverse washing desorption. After the first screen bed works for a period of time, the system stops supplying gas to the first screen bed through the control valve, and the screen bed entrance is connected with the exhaust channel for decompression, desorption and nitrogen discharge [17,18,19]. At the same time that the first sieve bed stops the air intake, the compressed air is fed into the second sieve bed, and the second sieve bed is the same as the first sieve bed regarding the adsorption, washing and desorption process. The two sieve beds work continuously in this way to produce oxygen-rich gas.

The pressure swing adsorption oxygen-production cycle is a complex dynamic process involving mass, heat and momentum transfer. Due to the complexity of the thermodynamic and kinetic processes between the gas and solid phases, it is often impractical to measure the changes of bed temperature, pressure, component concentration and flow rate during the cycle in practical applications. By converting the mass, heat and momentum transfer in the process into partial differential equations of space and time, the boundary conditions and process data were established to solve the mathematical model, to realize the gas–solid two-phase mass transfer, heat transfer and momentum transfer process and to obtain the changes of the bed parameters and the relationship between them, thus exploring the influence of the various process conditions on the cycle process.

3. Composition Analysis of Molecular Sieve Oxygen-Generation System

The main parts of the two-bed type molecular sieve oxygen system composition studied in this paper are the inlet unit, the air compressor, the molecular sieve material and the molecular sieve bed structure.

3.1. Oxygen System Air-Intake Unit

In this study, the air is used as the air source. In addition to its constituent gases, the air is also mixed with impurities such as particulate matter (such as dust), water vapor and oil droplets, which may be deposited during the process of flowing into the gas pipeline, thus, clogging the pipeline, and may also wear the easily worn structure of the air compressor (such as piston) [20,21,22]. Therefore, filtration of these doping impurities is particularly important. As part of the process of studying the removal of impurities, the relevant requirements in China Machinery Industry Standard JB/T5967-2020 for the corresponding air medium quality-grade regulations are shown in

Table 1. Air media quality class for pneumatic components and systems.

Pneumatic Components	Air Medium Quality Grade
	Particulate Matter	Steam	Oil
Air compressor	≤4	≤3	≤5
Pneumatic reversing valve	3—2	3—2	4—3

. Therefore, impurities should also be filtered according to the quality-grade requirements in the process of removal.

As shown in Table 1, the air’s particulate matter and water vapor quality level should be less than 3, and the oil level should be less than 4. Upon understanding the requirements of the impurities in the air source to remove them, air source debris removal is generally divided into two methods: the oil film method of debris removal and the fabric mesh method of debris removal. The oil film method of debris removal is through the oil film adhesion of solid particles in the air, but it should be noted that when there is more dust, the surface of the oil film easily adheres to saturation; the fabric mesh method of debris removal goes through fabric mesh to block solid particles, so the fabric mesh can not only intercept the dust according to the size of the pore but also remove a certain amount of water and oil. According to the characteristics of the two debris removal methods, the system filter studied in this paper should be selected to pretreat the air source using the fabric mesh method.

3.2. Oxygen-Production System Air Compressor

The air compressor is an important component of the molecular sieve oxygen system, which is also guarantees the power in the system, and its performance directly affects the oxygen-production efficiency of the oxygen system. The air compressor contains a number of circulation systems such as an air circuit circulation system and a distribution system; through the work of these circulation systems, the filtered and compressed gas in the gas source goes to the system air circuit.

This paper studies the molecular sieve oxygen system from its own oxygen-production process, which does not require the participation of other impurities, and the special choice of water as a piston air compressor promotes the mechanical movement of the piston [23,24,25]. The principle of a piston air compressor is to push the piston through the liquid to produce mechanical movement to increase the pressure of the gas in the cylinder, to achieve the purpose of compressing air with a high compression efficiency, a wide pressure range, adaptability, wear resistance and other advantages. This paper studies the demands of air compressors for molecular sieve oxygen-production systems as follows.

(1)

In general, the exhaust pressure of an air compressor in the process of oxygen production should be controlled between 0.5–4 bar. According to the amount of their exhaust pressure, air compressors can be divided into 0.05–2.0 MPa low-pressure air compressors, 20–100 bar medium-pressure air compressors, 100–1000 bar high-pressure air compressors and 1000 bar ultra-high-pressure air compressors. The exhaust pressure of the air compressor used in the molecular sieve oxygen system studied in this paper should be controlled between 0.8–1.7 bar, so the low-pressure air compressor was selected.

(2)

Due to the characteristics of air compressors, many of them work with an oil medium as the piston, with oil attached to the gas, so if there is oil gas in the oxygen-production process that goes into the oxygen system, causing the oil gas and a high concentration of oxygen to be mixed with an open fire, it can very easily explode, which is a great safety hazard; in addition, the oil gas will contaminate the oxygen, so that it can not meet the requirements of breathing oxygen. The most serious concern is that the oil gas can contaminate the molecular sieve, which may make the molecular sieve fail. In order to make the gas oxygen concentration high enough, the molecular sieve should be used normally, so it is necessary to avoid the interference of other impurities in the process of oxygen production and choose an oil-free air compressor.

(3)

The exhaust volume of the air compressor is generally around 3.5 m³/h. The molecular sieve oxygen system’s theoretical demand for oxygen flow has a limit value of 5 L/min, due to the molecular sieve bed cycle’s reciprocal work; the actual state of the air compressor is neither a continuous working state nor a completely continuous state. From the previous experimental research, it is known that the production gas recovery rate of the general oxygen system is generally about 25%; by taking a recovery rate of 25%, the required exhaust volume can be calculated as shown in Formula (1).

$$Q=\frac{Qm/r}{21\%}$$

(1)

where Q refers to the final exhaust volume of the system (m³/min); Qm refers to the maximum production gas flow rate of the system (m³/min); and r refers to the production gas recovery rate of the system. According to Formula (1), the calculation of the required air compressor exhaust volume has a maximum of 0.056 m³/min.

(4)

Turbulent noise and structural noise control. As the studied molecular sieve oxygen system is mostly used for highland travel or home oxygen health care, the system noise level generally needs to be kept under 55 dB. The main sources of noise are the gas turbulence and structural vibrations in the air compressor, so the noise generated by the air compressor in the process of gas introduction and the working vibrations should be strictly controlled.

Considering the above requirements, the air compressor selected for the molecular sieve oxygen system studied in this paper is the ZGK-500P small-piston-type low-pressure oil-free air compressor (Sinc Med, Suqian, China). The working voltage of the ZGK-500P air compressor is 220 V, the exhaust volume is 55 L/min, the exhaust pressure is 0.15 MPa, the working rated power is 60 W, and the working noise is less than 55 dB.

3.3. Molecular Sieve of Oxygen-Generation System

A zeolite molecular sieve is selected as the adsorbent for the molecular sieve oxygen-production system. Its properties, including its nitrogen oxygen selection coefficient, nitrogen-adsorption capacity and oxygen-production rate, will directly affect the oxygen-production effect. At present, the molecular sieves used for oxygen production mainly include 5A, 13X and Li molecular sieves, which have improved performance, on the basis of which the commonly used molecular sieves are the French SXSDM and the American OXYSIV MDX. In this study, several commonly used molecular sieves are taken as examples to compare performance indexes, and the results are shown in the table [26,27].

Particle size, wear-resisting strength and nitrogen and oxygen separation performance are important indicators to judge the performance of a zeolite molecular sieve. By comparing the molecular sieve codes 1, 2, 3 and 4 in

Table 2. Comparison of several commonly used molecular sieves.

Molecular Sieve Code	Particle Size/mm	Compressive Strength	Density of Bulk/(kg·L−1)	N2 Adsorption Capacity (1 bar/25 °C)/(NL·kg−1)	Oxygen Nitrogen Selection Coefficient (1 bar/25 °C)	Rate of Oxygen Production/(NL·kg−1·h−1)
1	1.6–2.5	>30 N	0.62	>8.3	>3.5	60
2	1.6 ± 0.1	>25 N	0.63	>19.5	>6.2	75
3	0.63 ± 0.07	>1.7 kg	0.61	>8.0	3.1 ± 0.2	120
4	0.4–0.6	>2.8 MPa	0.62	>19	>6	200

, it can be seen that the particle size of the molecular sieve has a great influence on the oxygen-production rate. According to the adsorption capacity of N², the selectivity coefficient of nitrogen and oxygen and the rate of oxygen production and other indicators, the code 4 molecular sieve (SXSDM type) has better performance and was selected as the adsorbent of the molecular sieve oxygen-production system in this study.

The theoretical loading amount of the molecular sieve is calculated according to Equation (2):

G = Q × 93%K

(2)

where G is the theoretical loading amount of molecular sieve, in kg; Q is the total oxygen production of the oxygen-production experiment platform, in NL/h; and K is the oxygen yield of molecular sieve, in NL/(kg·h). The oxygen yield of the molecular sieve oxygen-producing system in this study is 20 NL/min, and the theoretical loading capacity of the molecular sieve is 5.58 kg, as calculated by Equation (2).

The SXSDM molecular sieve is a commercial molecular sieve. Its uniform aperture is about 5 × 10⁻⁷ mm, its bulk density is 620 kg/m³, its specific surface area is 750~800 m²/g, its porosity is 47%, and its mechanical strength is greater than 95%.

3.4. Molecular Sieve Bed of Oxygen-Generation System

In this study, the oxygen production of the molecular sieve oxygen-production system is 20 NL/min, and the adsorption tower is divided into two groups, that is, the oxygen production of each group of adsorption tower is 10 NL/min. The two-tower, five-step PSA oxygen-production process is adopted, and the oxygen recovery rate is 33%. The required intake flow rate is calculated according to Equations (2) and (3):

Q_i = Q₀ × 93%η × C₀

(3)

Q_i is the adsorption tower into the air, in NL/min; Q₀ is the adsorption tower production volume, in NL/h; η is for the oxygen yield, and C₀ is for the oxygen concentration in the air, which is 20.95%. According to Equations (2) and (3), the required intake gas is 269 NL/h.

Firstly, the critical fluidization velocity of gas in molecular sieve is calculated:

u_max = dp₂ × (p_p − p) × g1650 × μ

(4)

u_max is the critical fluidization velocity of the gas in the molecular sieve, in m/s; dp is the molecular sieve’s particle size, in m; p_p is the molecular sieve’s bulk density, in kg/m³; p is the air density, in kg/m³; g is the acceleration of gravity, which is 9.8 m/s². μ fetches the gas dynamic viscosity, which is 5–1.81 × 10 pa, s. The gas velocity is usually calculated as 70% of the critical fluidization velocity, i.e.,

u = u_max × 70%

(5)

The minimum diameter of adsorption tower d_min is as follows:

d_min = √Q_i × 4u × π

(6)

By type (4), (5) type (6) calculated d_min is 80 mm, so the adsorption tower is greater than 80 mm in diameter; when combined with the existing materials, it picks up an 87 mm diameter aluminum alloy tube for the adsorption tower tanks. The effective height of the adsorption tower can be calculated as 380 mm, according to the loading amount of the molecular sieve, the density of the molecular sieve’s particle size and the diameter of the adsorption tower. The height to diameter ratio of this adsorption tower is 4.37.

In addition to the above internal research on the molecular sieve bed, an unreasonable design of the molecular sieve bed structure will directly lead to molecular sieve pulverization, uneven gas flow, the occurrence of molecular sieve necrosis space, premature “holes” in the adsorption layer and other phenomena, thus reducing the gas separation efficiency, which results in the final gas oxygen concentration of the system not meeting the actual requirements.

In this study of the molecular sieve oxygen-production system, the oxygen production is relatively small, so a three-dimensional structure is selected. The three-dimensional structure has limited influence on the oxygen-production effect. The oxygen-production effect can be improved by optimizing the weight and volume of the molecular sieve bed.

In order to make the adsorption performance of the molecular sieve have little influence on the overall oxygen-production effect, this study improved and designed a vertical molecular sieve bed on the basis of the original structure of the molecular sieve bed. The structure of the molecular sieve bed is shown in Figure 2. The material of the bed is aluminum, which is composed of two fixed end covers (stamping parts) and a middle aluminum cylinder (profile parts).

In order to further study the static structure of the molecular sieve bed, a three-dimensional drawing of the molecular sieve bed was carried out using a three-dimensional drawing software according to the bed structure designed in Figure 2. Figure 3 shows the three-dimensional structure of the molecular sieve bed. The aluminum cylinder, with a diameter of 67.6 mm and a length of 330 mm, is suitable for the two-bed molecular sieve oxygen-generation system in this study.

ANSYS software was used to analyze the static stress of the designed molecular sieve bed, fix the end covers at both ends, and then apply 0.8 bar and 1.7 bar pressure to the inner wall of the aluminum cylinder of the molecular sieve bed. The total deformation was obtained as shown in Figure 4a,b.

Under the pressures of 0.8 bar and 1.7 bar, the molecular sieve bed shows different deformation, and the specific deformation is shown in the broken-line diagram in Figure 5.

According to the analysis in Figure 5, it can be concluded that under the pressures of 0.8 bar and 1.7 bar, the total deformation range of the molecular sieve bed is within the bearable range of aluminum, so the molecular sieve bed does not have deformation protrusions.

A comprehensive structural analysis shows that the molecular sieve bed designed in this study can withstand the pressure from gas when the input pressure is 0.8–1.7 bar, and the structure is stable without deformation.

4. Experiment on Influence of Cycle Period and Calibrating Hole

4.1. Oxygen Cycle Selection

In order to study the effect of flushing sizing hole diameter on oxygen production, the oxygen cycle of the oxygen-generation device was selected. After previous research experiments, it is known that the oxygen concentration is the best when the oxygen cycle is roughly 6.0~8.5 s. In the case of an output gas flow rate of 5 L/min, with different input pressures, the effect of different working cycles on the oxygen concentration of the production gas is shown in Figure 6.

As shown by the data in Figure 6, the oxygen concentration of the gas produced by the molecular sieve oxygen system varies with the different working cycles under the conditions of constant input pressure and output flow rate, and the oxygen-production performance is relatively good when the different working cycles are 7.5 s, 8.0 s and 8.5 s. In order to make the oxygen-production performance of the oxygen system better meet the various requirements, it is necessary to match the relationship between the different working cycles and the working pressure, molecular sieve filling quantity, sieve bed structure, gas channel, etc., to further optimize the different working cycles.

4.2. Effect of Different Sizing Hole Diameters on Oxygen Concentration

Through the above data comparison, the cycle periods of 7.5 s, 8.0 s and 8.5 s can be selected as the cycle periods of the experiment, and the average pressure time is generally set to 1.3 s; the two can be matched to obtain the combined cycle of 7.5 s + 1.3 s, 8.0 s + 1.3 s and 8.5 s + 1.3 s. It is known from previous research experiments that the flushing sizing hole selected for the molecular sieve oxygen system is generally between 0.8 mm and 1.1 mm, and this experiment also uses this range as the system research range.

The experiment was carried out with a two-bed type molecular sieve oxygen system. The inlet unit, air compressor and molecular sieve were selected as the main parts, and the rest of the components were used in the market, such as a pressure reducer, safety valve, solenoid distribution valve, microcontroller control device, flushing device, molecular sieve bed, check valve, gas storage tank, output distribution device (including outlet filter), etc., to conduct this experiment. The gas oxygen concentration value that was produced was measured and read by a civilian oxygen-concentration flow meter, and the data were collected and analyzed by experimental collation, as shown in Figure 7.

From the analysis of the data in Figure 7, it is easy to see that the oxygen concentration of the gas produced by the molecular sieve oxygen system decreases with the increase in the flushing sizing hole diameter, and the oxygen concentration value at a diameter of 0.8 mm is close to the ideal value of 95%. Therefore, it is easy to conclude that the optimal flushing diameter of the molecular sieve oxygen system is 0.8 mm, and the ideal oxygen cycle time (including the equalization time) is 8.0 s + 1.3 s.

5. Conclusions

• In this paper, the two-bed type molecular sieve oxygen system is used as the experimental object, and key units of the system are selected. When the cycle is stable, the relationship between the outlet oxygen concentration of the two-bed-type molecular sieve oxygen system and the cycle period and the flushing sizing hole is obtained. With the change in the flushing cycle, the outlet oxygen concentration range changes, and an oxygen cycle of 8.0 s is more significant compared to 7.0 s and 8.5 s.
• Since the Li-X type lithium molecular sieve material has good adsorption performance under 0.08–0.17 MPa pressure, it is selected as the molecular sieve in the molecular sieve bed for the molecular sieve oxygen-production system studied in this paper. From the experimental data, it can be obtained that when the system lead gas is in the range of 0.08–0.17 MPa, the larger the inlet pressure is, the larger the oxygen concentration of the gas obtained from the oxygen production system is.
• The size of the flushing sizing pore diameter directly affects the desorption effect of the gas passing through the molecular sieve bed, thus affecting the oxygen concentration at the oxygen outlet of the system. Other conditions are certain, under different cycle configurations, and, according to the data analysis, the larger the oxygen concentration is, the smaller the corresponding optimal sizing hole diameter is; the best flushing sizing hole diameter of the molecular sieve system is 0.8 mm.
• The optimal oxygen production and flushing cycle configuration exist with each condition of the system being constant. From Figure 7, it can be seen that the oxygen concentration at the system outlet is optimal under a cycle time configuration of 8.0 s + 1.3 s, so 8.0 s + 1.3 s is the optimal oxygen production and pressure-equalization cycle configuration.