**1. Introduction**

In the literature, many studies on molecular sieve adsorption materials, the pressure swing adsorption process, and nitrogen gas generators have been published adopting diverse perspectives. Currently, many manufacturers all over the world create nitrogen gas generators to supply the open market. However, the results of simulations and experimental investigations into the technological parameters of a single fixed-bed device are very limited. Thus, most of these parameters are still known only by manufacturers due to the copyright of molecular sieve adsorption materials and equipment, without clarification via experiment [1].

In [2], a single-bed N2 gas generator using pressure swing adsorption (PSA) was simulated and investigated to present the material equilibrium equation describing the change in concentration of substance adsorbed in the gas phase and solid phase over time upon changing the height of the bed. The thermal equilibrium equation described the change in temperature over time upon changing the

height of the bed, as well as the capacity of heat adsorption, desorption, and transfer through the column wall. The Langmuir adsorption isotherm equation was predicted, whereas the momentum equation described the changing pressure and drop pressure according to the height of the bed and its head and boundary conditions. These equations could be solved using analytical, finite element, and numerical methods or commercial software such as Pascal, Fortran, Matlab, or Visual Basic when determining parameters such as porosity (ε), axial diffusion coefficient (*DL*), and velocity (*u*) in the column. However, verifying these models is very difficult because parameters such as concentration and temperature vary smoothly according to the height of the column, and their determination needs a high-precision measuring device. This article was not intended to solve these problems.

In the study conducted by Ashkan et al. [2], the development model was applied to N2PSA systems, generating good results when compared to experimental and simulated models. The effects of flow, loading rate, cycle time, and column length on purity and product recall were investigated. The results showed that the N2 purity decreased as the period of the cycle and the feed flow increased. However, upon increasing the feeding rate and bed length, the purity of the product also increased. All of the above conclusions were inversely proportional to the amount of recovery. It was observed that, during a defined cycle time, the effects of feed and discharge flow on purity and recovery were larger than the other parameters. The effects of drop pressure and non-isothermal conditions did not show a significant change.

Furthermore, in [2], there was no equation regarding time-varying pressure upon changing the height of the bed, which can be transformed via the material equilibrium equation due to a change in concentration in terms of it being regarded as an ideal gas. At the same time, the experiments determined some influencing factors such as feeding rate, purging rate, time, and height of the bed with regard to the purity of N2. However, they did not study the effects of process parameters such as pressure, flow, and concentration of the bed or the influence of pressure, flow, drop pressure, pressurization time, adsorption time, and amount of O2 gas absorbed at different pressures to find the optimal working regime of the bed. While the findings of other studies [1–14] proposed different mathematical models to simulate and examine the work cycle of the equipment, the use of a single bed as the basis for research cycles and equipment was not mentioned [15–19].

In Vietnam, many companies are willing to use this equipment for small- and medium-scale production, where it is not convenient to transport liquefied N2. Therefore, the Institute of Technology, General Department of Defense Industry, Vietnam researched, designed, and manufactured an N2 gas generator from open air using a CMS-240 carbon molecular sieve, implementing a pressure swing adsorption cycle to investigate, simulate, and optimize the industrial scale-up.

In this work, we solve the problems of building a single fixed-bed experimental model to study the unresolved problems in Reference [2], whereby we develop a model of pressure change over time according to the bed height, thus making it easier to determine the parameters according to the height of the bed with selected high-accuracy pressure sensors. This work investigates influencing factors such as the amount of O2 gas adsorbed and the optimal working mode of the bed. This study is important to determine the mutual influence of technological parameters and to optimize the best working mode. Hence, the ultimate goal is to optimize the N2 gas generator, allowing an industrial scale-up of the used equipment. The results of this study were used to verify other published theoretical and simulation studies using this equipment. Simulation and experimental studies of a single fixed-bed model to produce N2 gas using a CMS-240 adsorption material and the PSA cycle were carried out. In particular, the authors built and simulated pressure as a function of the height of the column over time to find the optimal working mode of the bed in case of instability.

This paper is divided into five main sections. A brief introduction is given in Section 1. The model and theoretical basis are presented in Section 2. Calculations and simulations are presented in Section 3. Experimental results and discussions are given in Section 4. Finally, Section 5 concludes some of the remarkable results of this work.

#### **2. The Model and Theoretical Basis**

In this section, there are many symbols used to represent the mathematical, physical and chemical quantities, all their meanings can be seen detail in the Abbreviations.
