I. The Principle of Producing Nitrogen using CMS
Carbon Molecular Sieve is a type of carbon sorbent. It is the carbon skeletal framework, formed by disordered accumulation of carbon. The capability of air separation is based on its skeletal framework. Dynamically CMS has different adsorptive rate for oxygen and nitrogen. Under balanced condition, the adsorption rates of oxygen and nitrogen are very close. However the speed of oxygen molecule passing through the framework is faster than nitrogen molecule. Thus oxygen and other impurities are adsorbed by the sieve, and unabsorbed nitrogen is allowed to pass through the sieve bed.
II. Nitrogen Generator using CMS
The apparatus is often called Air Separation Nitrogen Plants. The process uses the principle of Pressure Swing Adsorption (PSA). PSA nitrogen generation is achieved through the features of CMS, that are selective adsorption rate and the different adsorption capacity under different pressure. Oxygen molecules are withheld by CMS since its spreading speed is much greater than nitrogen molecules in the minute hole structure when the pressure is increasing. While the nitrogen is released, the carbon molecular sieve recovers, to form a circulatory operation and continuous nitrogen generation.
The PSA process includes Pressure increasing, Nitrogen producing, Pressure balancing, Pressure reducing, Nitrogen releasing. This process repeats and nitrogen is generated continuously. According to the style of re-generation, the process can be categorized into re-generation in vacuum process (VPSA) or re-generation in normal pressure process.
The nitrogen generation plant based on PSA process may consist of the air compressor and purification module, pressure swing and adsorption module, programmable valve controller module (the VPSA process needs the vacuum pump), and the Nitrogen supply module.
III. The Principle in Controlling Nitrogen Generation using CMS
1. Air compressor and purification module
It is very necessary to use purified air to feed into the adsorption tower. Impurities in the air will jam the skeletal framework, thus the performance of the sieve will be lowered gradually.
To purify air: a. keep the inlet of air compressor far away from dust, oil fog as well as organic gases; b. the air should firstly get through filter and cooler. The purifying module may filled with sorbent, so the air will go through for further processing.
2. Nitrogen Purity and Production Capacity
When generating nitrogen using CMS, the nitrogen concentration and flow rate can be adjusted according to individual users. In certain time and under certain pressure, when reducing the flow rate, the purity of nitrogen will increase. Otherwise the purity will drop. Users can adjust according to their needs.
3. The Time Period for Pressure Balancing
In the process of nitrogen generation, after one tower finishes adsorption, the compressed air can be fed both from top and bottom into the other adsorption tower which has been re-generated. Then the two towers reach the same pressure. This process is called the pressure balancing. If a proper time of pressure balancing is chosen, it will not only reclaim the energy, but also reduce the impact on the sieve particles. Thus the life span of the sieve is increased. The typical time for pressure balancing should be between 1 to 3 seconds according to the speed of the switching valve.
4. Nitrogen Producing Time Period
The CMS can reach saturation in adsorption of oxygen. On the other hand relatively short time period of nitrogen generation also increase the frequency of valve switching, which demands high performance valves. The normal time period of adsorption is between 30 to 120 seconds. Small-sized high purity nitrogen generators are recommended to have shorter period of generation time, while larger size low purity generator have longer period.
5. Operational Pressure
The adsorption rate is greater when the air pressure is higher, but this demands higher model of air compressor. The adsorption pressure also varies between normal pressure re-generation and vacuum re-generation. Considering all aspects, the suggested adsorption pressure should be set to 5 ~ 8 kg/cm2 for normal pressure re-generation, and 3~ 5 kg/cm2 for vacuum re-generation.
As a sorbent, lower temperature is favored by the CMS. Higher temperature will lower the performance of CMS.