[Abstract] Aiming at the problem of medical oxygen-enriched air supply, this article describes three common oxygen-enriched air supply sources: medical liquid oxygen storage tanks, medical molecular sieve oxygen generators, and medical oxygen-enriched air cylinder manifolds, and focuses on the working principle, design process, and main equipment components of medical molecular sieve oxygen generators. The molecular sieve oxygen generator is based on the principle of variable pressure adsorption, using zeolite molecular sieve as the adsorbent, and separating the oxygen-enriched air from the air through the process of adsorption under pressure and desorption under pressure.
During the design process, the oxygen-enriched air demand is firstly calculated according to the number of beds in the medical institution, then the oxygen concentrator with corresponding air production capacity is configured, and finally the design scheme is completed and the area of the room is taken into consideration.
In the equipment introduction part, the detailed description of the air compressor, refrigerated dryer, all levels of filters, molecular sieve adsorption tower (main machine), oxygen-enriched air tank, oxygen-enriched air booster, oxygen-enriched air purity monitor, oxygen-enriched air flow meter, intelligent control system, as well as the role of the components of the back-up oxygen source and the work requirements. The importance of using low-pressure, oil-free scroll compressors to ensure the cleanliness of the air and the intelligent control system to monitor and control the operation of the equipment are particularly emphasised.
In addition, the paper also gives a specific design process example, according to the relevant national norms of the medical molecular sieve oxygen plant station layout and safety protection put forward clear requirements, including fire resistance level, door and window materials, firewall settings, and standby power supply equipment and so on.
In general, medical molecular sieve oxygen concentrator plays an important role in solving the problem of oxygen-enriched air supply in medical institutions due to its convenient local oxygen production capacity and adaptability to remote areas, especially for medical institutions far away from large-scale oxygen-enriched air production units, it is a kind of ideal solution for oxygen-enriched air supply.
Key Words: Medical oxygen concentrator; Pressure swing adsorption; Design; Analysis
1. Introduction
There are three common oxygen-enriched air supply sources for medical and health institutions: medical liquid oxygen storage tank supply sources, medical molecular sieve oxygen concentrator supply sources, and medical oxygen-enriched air cylinder manifold supply sources.
Early centralized oxygen supply mostly used cylinder manifolds. In the late 1980s, with the development of cryogenic oxygen production and various domestic and foreign capital investments in the construction of a large number of professional gas production equipment, and the rapid increase in the number of beds in a single hospital, liquid oxygen vaporization oxygen supply with a larger supply capacity gradually became a major oxygen supply method for hospitals. For some hospitals in marginal provinces, because they are far away from oxygen-enriched air production units and transportation is inconvenient, it is suitable to choose medical molecular sieve oxygen generator oxygen supply.
The molecular sieve oxygen generator takes the air in the earth’s surface-atmosphere as the air source and makes use of the principle of variable pressure adsorption to produce oxygen-enriched air, which is convenient to produce locally.
2. Principles of Operation
The molecular sieve oxygen generator is an automatic equipment that takes zeolite molecular sieve as adsorbent and uses the principle of adsorption under pressure and desorption under pressure to adsorb and release nitrogen from the air, so as to separate the oxygen-enriched air.
The zeolite molecular sieve is a kind of spherical granular adsorbent with surface and inner covered with micropores processed by special pore processing technology. Through adsorption, the oxygen-enriched air molecules are mainly enriched out from the adsorption tower. Variable pressure adsorption molecular sieve oxygen generator is the use of zeolite molecular sieve selective adsorption characteristics, more than two (or even more) molecular sieve containers, through the programmable control of the opening and closing of the pneumatic valve, so that the A container is in the adsorption process at the same time, the B container is in the process of desorption, and vice versa. The compressed air enters the adsorption tower alternately to realize air separation, thus continuously producing oxygen-enriched air with high oxygen content.
The molecular sieve adsorption tower (main machine) is the core of the entire oxygen production system. The phenomenon that the molecules of a certain substance concentrate on a porous solid surface is called adsorption. The adsorbed substance is called “adsorbate”, and the substance with a porous solid surface is called “adsorbent”. When the temperature remains unchanged, the adsorption capacity of the adsorbent for the adsorbate changes with the change in pressure. When the pressure increases, the adsorption amount increases, which is called pressurized adsorption; when the pressure decreases, the adsorption amount decreases, which is called decompression desorption. This process is called pressure swing adsorption process.
3. Design Process
Firstly, according to the number of beds in medical and health institutions, calculate the demand for oxygen-enriched air; secondly, according to the total amount of gas used to configure the molecular sieve oxygen generator with the corresponding gas production capacity; finally, make a design plan, including design drawings, project cost, and requirements for the area of the room.
For example, a hospital has 786 beds in general wards, 10 operating theatres, 45 beds in ICU and KCU, 3 beds in wake-up rooms and 15 beds in resuscitation rooms. According to the air point oxygen-enriched air flow in Table 1.
| No. | Oxygen inlet | Calculation basis (L/min) | Maximum oxygen consumption coefficient | Normal oxygen consumption coefficient |
| N1 | General ward oxygen inlet | 4 | 15% | 10% |
| N2 | Operating room oxygen inlet | 10 | 100% | 50% |
| N3 | ICU, KCU oxygen inlet | 10 | 100% | 50% |
| N4 | Rescue room oxygen inlet | 10 | 100% | 30% |
| N5 | Hyperbaric oxygen chamber oxygen inlet | 10 | 100% | 30% |
| N6 | Delivery room, delivery room oxygen inlet | 10 | 100% | 30% |
Table 1
Calculation formula:
Q={4*n1*10%+10*(n2+n3)*50%+10*(n4+n5+n6)*30%}*Ψ/1000×60(Nm3/h) where Ψ is the selection coefficient, the coefficient of one type of hospitals with oxygen can be taken as 0.7~0.8; the coefficient of the second type of hospitals with oxygen can be taken as 0.5~0.6; the coefficient of the third type of hospitals with oxygen can be taken as 0.3~0.4. It is taken that The coefficient of the second type of hospital 0.5
Calculation result:
Q={4*786*10%+10*(10+45+3)*50%+10*15*30%}*0.5/1000×60(Nm3/h)
Q=19.5Nm3/h
Therefore, the hospital can be equipped with 20m3/h molecular sieve oxygen generator.
4. Introduction of Main Equipment
Molecular sieve oxygen generator consists of air compressor, air buffer tank, refrigerated dryer, multi-stage filter, molecular sieve adsorption tower, oxygen buffer tank, oxygen filter, oxygen-enriched air booster, oxygen-enriched air storage tanks, oxygen-enriched air purity monitor, oxygen-enriched gas flowmeter, intelligent control system, piping valves, standby oxygen source, etc. (Figure 1)
Figure 1
4.1 Air Compressor
Air compressor for molecular sieve adsorption tower to provide air source, The best working condition for zeolite molecular sieve is that the incoming air is clean and dry and the pressure is appropriate. The appropriate pressure range is between 0.45 and 0.65 MPa. Therefore, it is recommended that the air compressor use a compressor of 0.7 to 1.0 MPa, and the output compressed air is 100% oil-free, thus ensuring the cleanliness of the air. In addition, the main cost of the molecular sieve oxygen generator in the later stage is electricity, that is, the air compressor energy consumption, it is recommended to select energy-saving motor, such as the use of frequency conversion technology. The air volume should be increased in thin air areas.
4.2 Air Dryer
When ambient air enters the air compressor, the compressed air temperature will rise. If the high-temperature air directly enters the molecular sieve adsorption tower (host), it will affect the adsorption of the molecular sieve. Therefore, a refrigerated dryer is needed. On the one hand, it can cool the high-temperature air; on the other hand, it can dry the air.
4.3 Filter
The air passes through the primary filter, activated carbon filter and high efficiency precision filter to remove dust and dirt, then the clean compressed air enters the molecular sieve adsorption tower (main machine). Oxygen-enriched air then passes through the bacterial filter to remove bacteria, dust particles and microorganisms in the oxygen-enriched air, thus obtaining clean oxygen-enriched air.
4.4 Molecular Sieve Adsorption Tower
Medical molecular sieve oxygen generator uses the principle of pressure swing adsorption to produce oxygen-enriched air. Purified compressed air into two (or even more) filled with zeolite molecular sieve adsorption tower, the tower in the molecular sieve work alternately, the nitrogen adsorption in the air, not adsorbed oxygen-rich air out of the adsorption tower into the oxygen-enriched air buffer tanks, produced by oxygen-enriched air after deodorization, sterilization, can be sent to the hospital area for patients to use.
It is recommended to adopt the international advanced double-tower process, original equalisation procedure and reflux buffer structure. The oxygen production efficiency is high, the oxygen purity is high, the oxygen pressure is high, and the stability of equipment operation is good. Figure 2 is the principle flow of double-tower oxygen production process.
4.5 Intelligent Control System
The intelligent control system (control cabinet) is the control of oxygen equipment, and display center, the core of its control is a programmable logic controller, oxygen-enriched air concentration, flow, pressure real-time display, concentration of historical data storage display, can be adjusted through the button or touch screen equipment parameters. In the appropriate change of alarm conditions, all alarm sensors should be proved its role.
Control cabinet with motor protection unit with motor overload, phase failure, and phase sequence protection function; parameters abnormal local sound and light alarm function, sound stronger than 85dB (A). It is recommended to design networked remote data interface to achieve the monitoring of the oxygen generator system equipment and on-site data collection, processing, and command output.
4.6 Station Requirements
Medical molecular sieve oxygen stations should comply with relevant national standards, should be arranged as a separate single-story building, fire resistance level should not be less than two, building maintenance structure on the doors and windows should be opened to the outside, and shall not be wood, plastic and steel and other combustible materials production. With other buildings adjacent to the wall adjacent to the fire-resistant limit of not less than 3.0h and no doors, windows, holes in the firewall, the station should be set up at least one door directly to the outdoors.
In addition, there is a need to pay attention to the place, oxygen storage tanks and machines between the fire-resistant limit of the partition wall should not be less than 1.5h, and machines between the contact door should be used a fire door. According to the standard requirements, the medical molecular sieve oxygen generator group supply source, must be set up for emergency standby power.
5. Ending
Through the above discussion on the design of the medical molecular sieve oxygen generator, a detailed description of the functional principle of the oxygen generator, structural components, oxygen-enriched air concentration specification requirements of 93% (V/V), although not as high as the concentration of medical oxygen, but the same applies to the medical treatment, in particular, to solve the remote areas, away from the production of medical oxygen production units far away from the inconvenience of transporting the medical institutions of the difficulty in the supply of oxygen.