Available online at http://ijcpe.uobaghdad.edu.iq and www.iasj.net Iraqi Journal of Chemical and Petroleum Engineering Vol. 24 No.2 (June 2023) 81 – 87 EISSN: 2618-0707, PISSN: 1997-4884 *Corresponding Author: Name: Marwa Hussein Mohammed Ali, Email: marwa.ali1607m@coeng.uobaghdad.edu.iq IJCPE is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License. Isotherms and Kinetics Study for Adsorption of Nitrogen from Air using Zeolite Li-LSX to Produce Medical Oxygen Marwa Hussein Mohammed Ali a, *, Raghad F. Almilly a, and Riyadh Kamil Abid b a Chemical Engineering Department, College of Engineering, University of Baghdad, Baghdad, Iraq b Petroleum and Petrochemical Research Center, Ministry of Science and Technology (Most), Iraq Abstract This research investigates the adsorption isotherm and adsorption kinetics of nitrogen from air using packed bed of Li-LSX zeolite to get medical oxygen. Experiments were carried out to estimate the produced oxygen purity under different operating conditions: input pressure of 0.5 – 2.5 bar, feed flow rate of air of 2 – 10 L.min-1 and packing height of 9-16 cm. The adsorption isotherm was studied at the best conditions of input pressure of 2.5 bar, the height of packing 16 cm, and flow rate 6 Lmin-1 at ambient temperature, at these conditions the highest purity of oxygen by this system 73.15 vol % of outlet gas was produced. Langmuir isotherm was the best models representing the experimental data., and the model parameters were the maximum monolayer coverage (qm) 200 mg. g-1 and Kl 0.00234 L.mg -1. Also, from the Freundlich isotherm model, the sorption intensity (n) indicated favorable sorption of 1.435. The average free energy estimated from the DRK isotherm model was 0.02 KJ.mol-1, which proved the adsorption process to follow physical nature. The results got from experiments showed a coincidence to the pseudo-first-order kinetic model. Keywords: Zeolite Li-LSX, medical oxygen, adsorption isotherm, adsorption kinetic, nitrogen adsorption. Received on 04/08/2022, Received in Revised Form on 13/10/2022, Accepted on 15/10/2022, Published on 30/06/2023 https://doi.org/10.31699/IJCPE.2023.2.9 1- Introduction 1.1. Pressure – Vacuum Swing Adsorption Process COVID-19 outbreak stimulates researchers to investigate another source of medical oxygen as an alternative to the conventional method of the cryogenic process, which consumes a large amount of energy. That is because of the huge increase in the need for portable medical oxygen concentrators (MOCs) because of pulmonary failure caused by COVID-19 as well as chronic bronchitis, pneumonia and chronic obstructive pulmonary disease (COPD) to avoid problems caused by hypoxemia [1-3]. Adsorption, membranes, and cryogenic separation are the three main ways to separate the different parts of air [1]. Adsorption is the most promising method for its simplicity, appropriateness in terms of moderate conditions, and low energy consumption, moreover, the separation method by adsorption is used to make very pure oxygen [1]. A lot of equipment that produce medical oxygen are made to get pure oxygen [2]. Different adsorption methods were experimented with to adsorb nitrogen gas selectively from air to produce pure oxygen gas. Pressure swing as well as vacuum swing, or pressure/vacuum swing adsorption processes, which is symbolized by PVSA, were tried using N2-selective adsorbents [3]. Also, Temperature Swing Adsorption (TSA) process was experienced [6]. It was demonstrated that PSA was more feasible than TSA process [2]. That is because PSA process cycle has a time of between one to several seconds, unlike the TSA process cycle which has a time extends to hours [6]. PVSA process is the common way to separate gases since the adsorption step is carried out at above atmospheric pressure and the step of regeneration is conducted under a vacuum pressure [7]. There are three points that make PVSA excel than PSA. The first is that the O2/N2 recovery rate and purity of the PVSA process is better than those of the PSA process [3]. The gas exits from heavy-reflux and light-reflux streams is used and that is why PVSA yield is more than PSA. This leads to an increase in both PVSA process productivity and capacity of adsorbents relative to those of PSA. The second point is in terms of total energy use. PSA process uses more energy than PVSA process. This is because PVSA includes vacuum pump which uses less energy in comparison with a conventional pump. The third point is that the mass transfer zones for the light and heavy components often get in a common way in a formal dual- reflux PSA process with has an intermediate feed. This problem can be solved with these newly integrated PVSA processes. Zeolites are likely to be used to separate air into its components because N2 gas molecules' dipole and quadrupole moments interact with extra cations of the zeolites' frame. Lithium (Li) forms an attractive cation for adsorbing N2 molecule better than O2 molecule on LSX http://ijcpe.uobaghdad.edu.iq/ http://www.iasj.net/ mailto:marwa.ali1607m@coeng.uobaghdad.edu.iq http://creativecommons.org/licenses/by-nc/4.0/ https://doi.org/10.31699/IJCPE.2023.2.9 M. H. Mohammed Ali et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,2 (2023) 81 - 87 82 zeolite when it is mixed with air [8]. Talking about adsorbents' capacity and productivity leads to study the adsorption isotherms of the system used in this research since it gives information about the full benefit of the adsorbent. Also, the study of adsorption kinetics makes a clear image of the rate of adsorption and the mechanism it followed. 1.2. Adsorption Isotherm Models Adsorption isotherms can be defined as suitable mathematical models to describe the distribution of the adsorbed ions between the adsorbent material and the solution at equilibrium. Langmuir, Freundlich, Dubinin– Radushkevich, and others [8, 9] came up with models of adsorption isotherms. Langmuir model, which was made to represent gas/solid phase interaction, was used in comparing, and measuring the ability of different adsorbents to take up molecules [8]. Langmuir's isotherm takes surface coverage into account by balancing the relative rates of adsorption and desorption (dynamic equilibrium). While adsorption depends on how much of the adsorbent surface is open, desorption depends on how much of the surface is covered [6]. One of several forms of the Langmuir equation is Eq. 1 [6]: 1 qe = [ 1 qₘ.Kl ] 1 Ce + 1 qₘ (1) Where: qe = the amount of adsorbate adsorbed per gram of the adsorbent at equilibrium (mg. g-1), qm = maximum monolayer coverage capacity (mg. g-1), Kl = Langmuir isotherm constant (L.mg- 1), Ce = the equilibrium concentration of adsorbate (mg. L-1). Freundlich model assumed non-ideal adsorption; the adsorbent has a heterogeneous surface and the adsorption is not restricted to the formation of a monolayer. The model can be linearized as Eq. 2 [9, 10]: 𝑙𝑛 𝑞𝑒 = 𝑙𝑛 𝐾𝑓 + (1/𝑛) 𝑙𝑛 𝐶𝑒 (2) Where: qe = the amount of adsorbate adsorbed per gram of the adsorbent at equilibrium (mg. g-1), Kf = Freundlich isotherm constant (mg. g-1), n = adsorption intensity, Ce = the equilibrium concentration of adsorbate (mg. L-1). Generally, the Dubinin–Radushkevich isotherm is used to describe how a Gaussian energy distribution on a heterogeneous surface lead to adsorption [11]. The model has often done a good job of fitting data for high solute activities and the middle range of concentrations. The equation describes this model is Eq. 3 [11]: ln 𝑞𝑒 = ln 𝑞𝑚 − KDR 𝜀 2 (3) Where: qe = the amount of adsorbate adsorbed per gram of the adsorbent at equilibrium (mg. g-1), qm = theoretical isotherm saturation capacity (mg. g-1), KDR = Dubinin– Radushkevich isotherm constant (mol2. KJ 2),  = is the polyanion potential (KJ. mol-1). 𝜀 can be estimated by the following Eq. 4 [11]: 𝜀 = 𝑅𝑇 𝑙𝑛 (1 + 1/𝐶𝑒 ) (4) Where: 𝑅 is the gas constant (8.31 J mol−1 K−1), T is the absolute temperature and Ce is the equilibrium concentration of adsorbate (mg. L-1). The model is applied to indicate physical and chemical adsorption of metal ions as well as the free energy, E, which is defined per molecule of adsorbate (that is removing a molecule from its position in the sorption layer to the infinity) may be calculated by Eq. 5 [11]: 𝐸 = (1 /√2𝐾) (5) 𝐸 is the mean adsorption energy less than 8 KJ.mol-1 in physical adsorption but for the chemical adsorption the energy is between 8 to 16 KJ.mol-1 [12]. This study focused on the adsorption isotherm models of N2 gas from ambient air on Li-LSX zeolite to get medical oxygen as a necessary step in scaling – up the system. 1.3. Adsorption kinetics Kinetics studies the adsorption rates to explain the mechanism that dominates in a certain system. Studying adsorption kinetics means investigating the experimental conditions that affect the rate of adsorption and, in turn, finding the factors that affect reaching equilibrium. These kinds of studies tell us about the possible way adsorption works and the different steps that lead to the final adsorbate–adsorbent complex. They also help come up with the right mathematical models to describe how things work together. Once the rates and factors that affect them are clear, they can be used to make adsorbent materials for use in industry and to figure out how the dynamics of the adsorption process is complex [7]. Adsorption kinetics are very important for figuring out the equilibrium adsorption capacity and the rate constants. The most common types of kinetic models are pseudo- first order and pseudo-second order. [13]. In pseudo-first order model, the adsorption capacity is related to the rate of adsorption as follows in Eq. 8 [7]: 𝑙𝑛 (𝑞𝑒 − 𝑞𝑡) = 𝑙𝑛 𝑞𝑒 − 𝑘1 𝑡 (6) Where: qe= adsorption capacity at equilibrium, mg. g-1, qt= adsorption capacity at any time, mg. g-1, k1 = rate constant for pseudo 1st order adsorption process, min -1. This model is found to be fit for the initial 20 to 30 min of interaction between the adsorbate and the adsorbent and not fit the overall extent of contacting [14]. While pseudo-second order, which Ho and McKay established within 1998 [15] can be represented in the following Eq. 7: (dq /dt) = k2 (qe - qt) 2 (7) Where: k2 = the Ho-McKay equation's rate constant, g min.mg-1. M. H. Mohammed Ali et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,2 (2023) 81 - 87 83 The pseudo-second-order kinetic model can be solved to take the linearized form as in Eq. 8 [8]: 𝑡 𝑞𝑡 = 1 𝑘₂ 𝑞𝑒2 + t 𝑞𝑒 (8) The initial phases of the adsorption process are assumed to be described by the pseudo-first-order model but for the whole range of adsorption it is likely that the adsorption process follows a non-linear model that represents the complex mechanism of interaction between the absorbate and the adsorbent. For porous adsorbents, the diffusion of the adsorbate molecules into the pores needs to be considered when looking for a good kinetic model for the process. In many cases, the rate at which an adsorbate is taken in may be controlled by intra-particle diffusion. This is presented by the following well-known expression [16]. q𝑡 = 𝑘𝑖 ⋅t 0.5 (9) The crucial aspect of this formula is that the linear plot of q t vs t0.5 must pass through the origin (zero intercepts). Consequently, the intra-particle diffusion model is easily testable, demonstrating that the diffusion process dominates the kinetics. The slope of the graph can be utilized to calculate the rate coefficient ki (mg/ (g. min 0.5) [17]. 2- Experimental Work 2.1. Materials Zeolite Li-LSX was used in the experiments. The technical specification of it is listed in Table 1. Table 1. The Technical Specification of Li-LSX Zeolite (Commercial Name JLOX-101) Property Unit JLOX-101 Country Diameter mm 0.4 – 0.8 China N2 adsorption capacity ml.g -1 ≥ 22 N2/O2 Selectivity ⁓ ≥ 6.20 Crush strength N ⁓ Bulk density g.ml-1 0.63±0.03 Moisture content wt.% ≤ 0.5 Particle ratio % ≥ 95 2.2. Equipment The experimental setup is shown in Fig. 1. Fig. 2 shows photos of the experimental setup. All the equipment used is listed in Table 2. Table 2. Equipment used in the Research # Device Specification Range Country 1 Air compressor ingco industrial ,220-240V, ⁓50Hz, AC25508 0-8 bar China 2 Pressure gauge with filter unit Filter unit D =2.5 cm, L = 5 cm Pressure gauge 0-10 bar China 3 Inlet flow meter PMB CV.P. A10.LM. G2 1-10 L.min-1 China 4 Adsorption column glass type QVF, L= 17 cm, id= 4cm 5 Pressure gauge 0-3 bar China 6 O2 gas sensor GDX-O2, L = 15.5 cm, D = 2.8 cm 0-100% O2 U. S. 7 Purge flow meter Matheson U310 0.5 - 6 L min-1 China 8 Valves China 9 Drum L= 60 cm, D= 37 cm V=64480 cm3 Fig. 1. Schematic Diagram of Experimental Setup M. H. Mohammed Ali et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,2 (2023) 81 - 87 84 Fig. 2. View of the Experimental Setup 2.3. Procedure At the beginning, the adsorbent (zeolite Li-LSX) was heated for 45 minutes in an oven at 110 ℃ to eliminate moisture and other impurities. Then zeolite was packed randomly in the column. Helium gas was let to flow passing the packing for refreshing it and prepare it to adsorb N2. Air was compressed to a specific pressure through the drum to maintain the flow of air to be stable in the column during the experiments. Air was also passed through a filter filled with silica gel to get rid of moisture and impurities. The flow was set at a certain value of flow rate by a flow meter. Nitrogen gas was adsorbed on the zeolite and oxygen-rich gas was produced at the outlet. The oxygen produced was split into two streams: one to the sensor for measuring oxygen purity one to the storage cylinder and the remainder as a volume percentage of the generated gas. Nitrogen volume percent in the inlet and outlet was determined by subtracting the oxygen percent from total 100% complete air components. There was also a purge stream to manage any unusual increase in the outlet pressure. After each adsorption experiment, there was a desorption operation to regenerate the zeolite. The desorption operation was carried out under vacuum pressure of -0.9 bar for 2 minutes. The amount of adsorbed nitrogen was estimated by the difference between the inlet and outlet concentrations after converting the partial pressure to concentration considering nitrogen as an ideal gas at the experimental conditions. 2.4. Response Surface Methodology The response surface approach was investigated. to investigate the interactions between the variables that affected the purity of O2. Also, it was used for optimizing and scaling up the current laboratory setup. In this respect, experiments were designed using the Box- Behnken Design (BBD). Fifteen experiments were carried out with various combinations of the studied variables which were: inlet pressure, packing height, and flow rate to determine which factors and their interactions had the major influence on the purity of the generated oxygen [18]. 3- Results and Discussions 3.1. Isotherm Model The Langmuir, Dubinin – Radushkevich, and Freundlich isotherm models were used to examine the adsorption data. The adsorption isotherms were applied at the best operating conditions obtained in this study: 2.5 bar pressure, and 16 cm height of packing which gives the highest purity of oxygen (73.15 vol % of outlet gas) which was the basis for optimizing the results to reach the utmost purity required for medical purposes [18]. These models relate the quantity of nitrogen adsorbed on a solid surface Fig. 3 to Fig. 5 below explain the adsorption isotherm model (Freundlich, Dubinin-Radushkevich, and Langmuir) with the results in Table 3. Fig. 3. Langmuir Isotherm Model for Adsorption of Nitrogen at 2.5 bar Input Pressure and 16 cm Packing Height Fig. 4. Freundlich Isotherm Model for Adsorption of Nitrogen at 2.5 bar Input Pressure and 16 cm Packing Height The results in Table 3 show that all the models fit well the experimental data, but Langmuir model has the maximum value of the correlation coefficient (R2=0.917). This led to the conclusion that the adsorption process followed Langmuir hypotheses of monolayer of adsorbed molecules and that no forces of interaction existed between them [15]. Langmuir parameters were the maximum capacity of adsorption 200 mg. g-1 which M. H. Mohammed Ali et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,2 (2023) 81 - 87 85 indicated a large capacity for adsorption and Kl 0.00234 L.mg-1 which indicated a favorable adsorption [16]. Also, the apparent energy (E) was found to be +0.02 KJ.mol-1 from DRK isotherm model. The low positive value of energy agrees well with the heat of physical adsorption in the gas phase with n equal to 1.435 from the Freundlich model [10, 12]. Fig. 5. Dubinin-Radushkevich Isotherm Model for Adsorption of Nitrogen at 2.5 bar Input Pressure and 16 cm Packing Height Table 3. Langmuir, Freundlich, and Dubinin– Radushkevich Isotherm Constants for the Adsorption of Nitrogen at 2.5 bar Input Pressure and 16 cm Packing Height Isotherm model Model parameter Value 2R Langmuir , mg/gmq 200 0.917 Kl, L/mg 0.00234 Freundlich , mg/ gFK 1.552 0.89 n, - 1.435 Dubinin 2/kJ2mol D,K 1946.3 0.7219 3.2. Adsorption Kinetics The kinetic data for the adsorption of nitrogen on zeolite Li-LSX adsorbent was Three fundamental kinetic models were investigated: the pseudo-first-order model, the pseudo-second-order model, and the intra-particle kinetic model. Fig. 6 through Fig. 8 show the three models. Table 4 lists the three kinetic models' correlation coefficients and other characteristics. The three models showed acceptable values of correlation coefficient (R2) with the pseudo-first order model (0.9834) somewhat higher than the others. The equilibrium capacity of pseudo-first order 63.79 mg /g was close enough to the experimental value 58.5 mg/g to make the decision that the pseudo – first order model represented well the kinetics of this system. Therefore, the mechanism of which the adsorption followed as well as the speed of adsorption depicted nonlinear interaction between nitrogen gas molecules and Li-LSX zeolite. Table 4. Kinetics Models Constants for Adsorption of N2 on Zeolite Li-LSX at 2.5 bar Pressure and 16 cm Packing Height Kinetics Model Model constants Model constant value K1 (L/min) 2.746 Pseudo - first- order qe (mg/g) 63.79 R2 0.9834 K2 (g /mg. min) 0.048 Pseudo- second- order qe (mg/g) 217.391 R2 0.9772 Intra particle Ki (mg/g.min 1/2) 58 R2 0.9193 Fig. 6. The Pseudo-First-Order Kinetic Model of N2 Adsorption on Zeolite Li-LSX at 2.5 bar Pressure and 16 cm Packing Height Fig. 7. The Pseudo-Second-Order Kinetic Model of N2 Adsorption on Zeolite Li-LSX at 2.5 bar Pressure and 16 cm Packing Height Fig. 8. The Intra-Particle Diffusion Model of N2 Adsorption on Zeolite Li-LSX at 2.5 bar Pressure and 16 cm Packing Height M. H. Mohammed Ali et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,2 (2023) 81 - 87 86 4- Conclusions The equilibrium sorption was investigated in a packed bed of zeolite Li-LSX at pressure 2.5 bar, flow rate of 2- 10 L.min-1, and height of packing of 16 cm for the adsorption of N2 from air to produce medical oxygen. The adsorption isotherm models were studied to get the necessary information of the adsorbent material that it confirmed its feasibility in industrial application. The sorption data were applied into Langmuir, Freundlich, and Dubunin - Radushkevich isotherms. Langmuir adsorption model was the best correlation represented the experimental data. All the models proved that the adsorption was physical in nature and the concept of one layer was applicable. The experimental data correlated well with the pseudo -first-order kinetic model which indicated the nonlinear interaction between nitrogen molecules and Li-LSX zeolite. References [1] R. R. Vemula, M. D. 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Mohammed Ali et al. / Iraqi Journal of Chemical and Petroleum Engineering 24,2 (2023) 81 - 87 87 Li-دراسة متساوية الحرارة والحركية المتزاز النيتروجين من الهواء باستخدام الزيواليت LSXإلنتاج األكسجين الطبي 2 رياض كامل عبدو ،1رغد فريد الملي ، ، *1مروة حسين محمد علي العراق بغداد،قسم الهندسة الكيمياوية، كلية الهندسة، جامعة بغداد، 1 العراق ، وزارة العلوم والتكنولوجيا،مركز بحوث النفط والبتروكيمياويات 2 الخالصة ة من وحركية امتزاز النيتروجين من الهواء باستخدام طبقة معبأ حرارة االمتزاز هذا البحث متساوي درسي حت ظروف تشغيل تاألوكسجين الناتج أجريت تجارب لتقدير نقاء . إلنتاج األوكسجين الطبي Li-LSXالزيوليت وارتفاع التعبئة ( 1-دقيقة .لتر 01-2)، معدل تدفق الهواء الداخل (بار 2.5 - 0.5)ضغط اإلدخال : مختلفة اء لألوكسجين االمتزاز في أفضل الظروف التي أنتجت أعلى درجة نق حرارة تمت دراسة متساوي (. سم 9-16) بار 2.5ضغط إدخال هي:كانت هذه الظروف (. الغاز الخارج حجم من % 73.15 ( بواسطة هذا النظام رةحرا نماذج متساوي ان. عند درجة الحرارة المحيطة 1-دقيقة .لتر16سم ومعدل تدفق 16وارتفاع التعبئة كانت قيم(. DRK)متساوي حرارة النكماير وفريوندليش ودوبينين رادوشكيفيتش التي بحثت كانت:االمتزاز كيفيتشو ودوبينين رادوش 0.89، فريوندليش0.917نموذج متساوي الحرارة النكماير: كالتالي 2R االرتباطمعامل ان موير كاوي الحرارة النج، لكن متسمثياًل جيًدا للبيانات التجريبيةأظهرت جميع النماذج المدروسة ت. 0.7219 Kl 0.00234و 1-غم.مجم mq( 200( ، وكانت معلمات النموذج هي أقصى تغطية أحادية الطبقةاألفضل إلى امتصاص ( n)أيًضا ، من نموذج متساوي الحرارة فريوندليش ، أشارت شدة االمتصاص . 1-ملي غم.لتر ودوبينين رادوشكيفيتش الحرارة متساوي نموذج من الحرة الطاقةتم تقدير متوسط . 1.435إيجابي قدره لنتائج اأظهرت . الفيزيائية الطبيعةوالذي أثبت بوضوح أن تجربة االمتزاز اتبعت 1-كيلوجول . مول 0.02ليكون . التجريبية مطابقة جيدة للنموذج الحركي من الدرجة األولى الزائف .تزاز النتروجين، حركية االمتزاز، امكسجين الطبي، متساوي الحرارة لالمتصاص، األو Li-LSXالزيوليت :دالةالكلمات ال