PARAMETERS AFFECTING EFFICIENCY FOR NITROGEN GAS GENERATION BY PRESSURE SWING ADSORPTION (PSA)

I. INTRODUCTION

The pressure swing adsorption method (PSA) is based on the concept that gases tend to be trapped, or "adsorbed," on solid surfaces under high pressure. The more gas that is adsorbed, the higher the pressure. The gas is expelled or desorbed when the pressure is reduced. PSA can be used to separate gases in a mixture because certain gases are adsorbed more or less forcibly onto a specific solid surface. When air is driventhrough a zeolite adsorbent layer that binds nitrogen more strongly than oxygen in a vessel, only a small amount of nitrogen remains in the bed, and the gas leaving the vessel contains more oxygen than the mixture entering. Pressure swing adsorption (PSA) has been at the cutting-edge technology for gas separation since its inception. PSA technology has grown significantly in size, adaptability, and complexity since its commercial launch in the late 1950s. Modern PSA systems, which are widely employed in the gas separation sector, can range in size from two adsorbent beds separating air to sixteen adsorbent beds producing pure hydrogen at rates exceeding 100,000 Nm3/hr. Despite its rapid expansion and technical advancements, however, the PSA design process is heavily reliant on experimental procedures to test and assess the influence of various design and operational decisions on real-time performance. Despite their numerous applications in chemical engineering, in the late 1980s, they became extensively accepted in commercial processes.

PSA Process 
The first bed's carbon molecular filter adsorbs oxygen and other contaminants, enabling high-purity nitrogen to flow through. The sieve adsorbs oxygen and other pollutants at the bottom of the second bed, enabling highpurity nitrogen to flow through. This bed enters a regeneration phase when it achieves saturation, while the second bed begins the separation process automatically. Regeneration is performed by lowering the pressure and letting the released gases to escape through the pelletized bed material. The non-nitrogen components have been released from the bed after a few minutes, and it is ready to separate air once more. This method ensures continuous production of high purity nitrogen. PSA systems are generally utilised to produce gaseous nitrogen at rates of 30-3000 Nm3/h. Adsorption pressure is 8 atm, regeneration pressure is 1 atm, and half cycle time is 1-2 minutes in a nitrogen separation system [5]. PSA performance is at a certain nitrogen purity level, assessed in terms of the specific product and yield. The yield is the ratio of the net flow rate of nitrogen produced to the flowrate of nitrogen input to the system, while the specific product is defined as the product flow rate per unit adsorbent volume. The feed flow rate per unit of adsorbent volume is known as the specific feed.
A drying system is typically used to prevent decline in quality of the carbon molecular sieve due to water adsorption. The drying system for the vessels can be external or internal. A chiller is used to reduce moisture in the first case. In the second case, a drying agent is used to dry out the gas internally, eliminating the need for large external dryers. The materials used as drying agent, on the other hand, are all polar and, Have a preferential adsorption of nitrogen over oxygen in the case of zeolites. Nitrogen purities from commercial pressure swing adsorption processes were previously less than 99.95 percent. An additional step of reducing oxygen impurities was required to achieve higher purities. The poor quality nitrogen product is fed into a deoxygenation unit with a fuel, such as hydrogen, and the oxygen impurities are transformed over a catalyst. Palladium or platinum impregnated alumina is most commonly utilised. Although purities of up to 99.9999 percent are possible, humidity and extra fuel are introduced into the nitrogen output. Although this may not be a problem in some applications, such as atmospheric reduction, it does necessitate the availability of the fuel. The use of vacuum during the regeneration of the adsorbent beds is an alternative method for achieving higher nitrogen purities. As the rate of difference between adsorption and desorption pressures increases, so do productivity and yield. However, the disadvantage of using an additional rotating unit is not only in terms of capital cost, but also in terms of maintenance. Over the last few decades, researchers at various institutions have developed oxygen selective sorbents. These metal complexes act as biological oxygen carriers, selectively removing oxygen from the air. Materials based on cobalt, iron, and manganese have been developed with oxygen capacities ranging from 1 to 10% by weight. The most serious disadvantage is their chemical instability as a result of autoxidation reactions. Either the ligand or the central atom is oxidised, or peroxo-bridged dimers form and lose their oxygen adsorption capacity.

Nitrogen PSA performance has meaningly improved over the last 20 years. Since 1980, power consumption has been rock bottom in half and productivity has more than doubled, resulting in smaller air compressors and smaller units. These advancements, which have enabled the commercialization of pure pressure swing adsorption systems supplying nitrogen with purities greater than 99.999 percent, are the result of two distinct areas of research: molecular sieve developments and process improvements.

Carbon has most likely resulted in the most important breakthroughs. There have been large gains owing to improved sieve utilisation as a consequence of PSA process adjustments, but there have also been significant improvements due to molecular sieve advancements. The addition of new phases to the basic cycle was one of the first modifications. During the pressure equalisation stage, both beds are linked to transport gas from the high pressure bed to the low pressure bed, saving both air input and compression energy. During this stage, the gas near the product end of the bed is forced back, serving as a purge and therefore enhancing purity and recovery. The pressure equalisation step is critical in determining the structure of the solid mass transfer zone, and it is especially important in cycles with short feed and desorption times. The most significant advances have most likely resulted from improvements in carbon molecular sieves, but there have also been significant improvements due to better sieve use as a result of PSA process modifications. Early enhancements include the addition of new stages to the basic cycle. During the pressure equalisation process, both beds are coupled to transport gas from the high pressure bed to the low pressure bed, saving both air input and compression energy. During this stage, the gas near the product end of the bed is forced back, serving as a purge and therefore enhancing both purity and recovery. The pressure equalisation step is crucial in determining the shape of the solid mass transfer zone, and it is especially important in cycles with short feed and desorption times. Purging the beds with product gas during the desorption step and/or using vacuum improves bed regeneration. Although the purge aids in cleaning the bed and achieving higher purity, it can have a negative impact on recovery. When the adsorbent beds are repressurized by feed gas at a slow rate at first, followed by a rapid feed second stage, the performance improves. Before going into production, a combination of product and feed repressurization of the bed can also improve process performance. Another critical variable in the process is the total cycle time. As the cycle time increases, the performance reaches a peak. Because non-productive phases in the cycle have a substantial effect at low cycle periods, both the yield and the specific product are zero. Long cycle times, on the other hand, cause the bed to become saturated, and no separation is feasible. For fast cycles, the system behaviour is determined by kinetic selectivity, whereas for slow cycles, it is determined by equilibrium selectivity. Other measures that can help increase production include the use of a dense packing approach for bed filling and the use of layers with variable quality sieve. Molecular sieves made of zeolite can also be used to separate nitrogen from air. Although the kinetic selectivity of carbon molecular sieves is identical, the equilibrium selectivity favours oxygen. Shin and Knaebel discovered that purity and yield had a trade-off, which is typical of diffusion-induced separations. Have a huge impact on the cycle. Long cycle times,

on the other hand, cause the bed to become saturated, making separation impossible. The kinetic selectivity determines the system behaviour for fast cycles, while the equilibrium selectivity determines the system behaviour for slow cycles. The use of a dense packing strategy for filling the beds, as well as the usage of layers with variable quality sieve, are two more approaches that can aid increase productivity. Zeolite molecular sieves can also be used to separate nitrogen from air. Although the kinetic selectivity of carbon molecular sieves is identical, the equilibrium selectivity favours oxygen. Shin and Knaebel discovered that purity and yield had a trade-off, which is typical of diffusion-induced separations. PSA's flow sheet is shown in Figure 1.

Figure 1

A number of authors have also investigated and analysed the temperature and concentration profiles in the bed during a PSA cycle. Raghavan and Ruthven performed a numerical simulation of nitrogen pressure swing adsorption over a carbon molecular sieve. They utilised a linear isotherm and a linear driving force model to analyse the evolution of the gas and solid phase composition profiles after running a basic Skarstrom type cycle with no equalisation step. Although the mass transfer zones are shaped like an S, there is no indication of maximum or minimum inside the bed. The roll-up effect generated by the displacement of oxygen by nitrogen has been demonstrated by more accurate modelling studies as well as actual observations, as well as the reduced working capacity between adsorption and pressurisation caused by the displacement of oxygen by nitrogen.

Factors affecting Efficiency of PSA 
1. Nature of Adsorbent.

The type of the adsorbent influence’s gas adsorption. A gas can be absorbed in varying amounts on various absorbent surfaces. Under certain conditions, hydrogen is moderately adsorbed on the alumina surface but substantially adsorbed on the nickel surface. Although CMS is made of diverse polyimides, it has an effect on the efficiency of PSA. Carbon Molecular Sieve (CMS) has played an important role in the nitrogen separation from air pressure swing adsorption(PSA) technique. The key differences between them and activated carbon are pore size distribution and surface area. The diameter of activated carbon ranges from 20 armstrom to 100 armstrom. Coal and coconut shell are the primary sources of CMS for commercial processes.
Typical steps for preparation of CMS from coal is shown in figure 2. The main process are carbonization and oxidation [2].

Figure 2
Recently, carbon-based molecular sieves containing inorganic oxides and supporting metals have been produced. The overarching goal is to develop a composite structure that combines the molecular sieving properties of carbon with the surface chemical and physical properties of inorganic oxides. Sharma and Seshan reported on copper-modified CMS for selective oxygen removal at temperatures below 200°C. To summarise, CMS has a significant impact on the effectiveness of PSA for nitrogen separation from air.

2. Surface Area
Activated carbon is made in a number of ways, the majority of which differ in content. The final product's nature is determined by both the starting material and the activation procedure in terms of pore size distribution and surface polarity.. A very high pore size is necessary for liquid-phase adsorption, and such materials can be produced from a wide range of carbonaceous starting materials using both thermal and chemical activation processes. The pores of activated carbon employed in gas adsorption are typically significantly smaller, with a significant portion of the overall porosity in the micropore range. Although higharea small-pore carbons can be manufactured from sources such as coconut shells, the product is often too 
weak for PSA applications. As the surface area of the adsorbent rises, so does the adsorption of gases. This is because the number of adsorbing sites grows as surface area increases. As a result, finely divided materials and some porous compounds work well as adsorbents.
3. Nature of the gas.
Pure gas permeation and equilibrium sorption measurements were used to characterise CMS dense films. In general, a gas will be more easily absorbed if it is more liquefiable. For example, liquid gases such as NH3, HCl, Cl2, CO2 are more readily adsorbed on the solids surface than permanent gases such as O2, H2, and so on.
4. Exothermic nature.
Exothermic adsorption occurs when energy is freed during a process when a gas is adsorbed on a solid surface. Furthermore, the residual pressures on the adsorbent's surface decrease during this process, resulting in a reduction in surface energy. As a result, the heat of adsorption is defined as the energy liberated when 1 g mol of a gas is adsorbed on a solid surface. As the temperature rises, the kinetic energy of the gas molecules increases, resulting in more collisions between the molecules and the surface. As a result, the exothermic nature of the adsorbent influences PSA efficiency.
5. Pressure.
When a gas is adsorbed on an adsorbent's surface, the volume of the adsorbent decreases. If the temperature is held constant, the amount of gas absorbed by metal adsorbent increases with the increase in pressure, according to Le Chatleier's principle. Over a narrow range of pressures, a gas's adsorption is directly proportional to its pressure. Figure 3 shows the extent of adsorption over an increasing pressure [3].

Figure 3
Thus, to conclude, On the solid surface, there are a fixed number of adsorption sites where gas molecules can be adsorbed. When the pressure is increased, the rate of adsorption increases at first because the number of gas molecules striking on the surface increases. As a result, increasing the pressure linearly increases the rate of adsorption. However, after a while, the pressure will have no influence on the rate of adsorption since the number of adsorption sites is set and no more adsorption may occur in those locations. As a result, the extent of adsorption will be independent of the pressure at that site.

6. Effect of particle size.
Every day, molecular sieve is used to separate, dehydrate, and generate desired product streams for a variety of applications. Different particle sizes of molecular sieve, whether beads or pellets, may be required for these diverse activities, depending on the system conditions and intended outcomes. Although each type of molecular sieve, such as Type 4A, will accomplish the same activities regardless of size, the size is what determines how efficient the process is, how long it takes to complete a cycle, and general mass transfer characteristics [4].

II. CONCLUSION

Pressure swing adsorption Plays a crucial role in Industries. In this Review paper, we are going to discuss some of the parameters affecting the efficiency of Pressure swing adsorption. Mainly Pressure swing adsorption Efficiency depends on Nature of adsorbent and Pressure; and operational parameters like Surface area, Nature of Feed gas, Exothermic nature and Particle size. Since, Air separation by PSA is a very effective, inherently safe, and extremely flexible teaching and learning tool for the Unit Operations Laboratory. Different Researchers, Raghavan and Ruthven, Lemcoff ef al and many others carried out studies of parameters affecting efficiency on PSA. Therefore we, found that efficiency could be increased by selecting CMS (formed from activated carbon) and maintaining the temperature of the adsorption column; and effect of practical size on PSA. Thus, assuming the above parameters (discussed in review paper) we can increase efficiency of nitrogen above 99.99.

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Source: International Research Journal of Modernization in Engineering Technology and Science