Available online at http://ijcpe.uobaghdad.edu.iq and www.iasj.net Iraqi Journal of Chemical and Petroleum Engineering Vol.23 No.3 (September 2022) 25 – 34 EISSN: 2618-0707, PISSN: 1997-4884 Corresponding Authors: Name: Mohammed Sattar Jabbar , Email: m.jabbar1207@coeng.uobaghdad.edu.iq, Name: Rana Th. Abd Alrubaye, Email: rana.thabet@coeng.uobaghdad.edu.iq IJCPE is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License. Adsorption Isotherms and Isosteric Heat of Adsorption of Metal Organic Frameworks as Gas Storage for Liquefied Petroleum Gas Vehicle in Iraq Mohammed Sattar Jabbar and Rana Th. Abd Alrubaye Chemical Engineering Department, College of Engineering - University of Baghdad, Baghdad, Iraq. Abstract This research provides a novel technique for using metal organic frameworks (HKUST-1) as a gas storage system for liquefied petroleum gas (LPG) in Iraqi vehicles to avoid the drawbacks of the currently employed method of LPG gas storage. A low-cost adsorbent called HKUST-1 was prepared and characterized in this research to investigate its ability for propane storage at different temperatures (25, 30, 35, and 40 o C) and pressures of (1-7) bar. HKUST-1 was made using a hydrothermal method and characterized using powder X-ray diffraction, BET surface area, scanning electron microscopic (SEM), and Fourier Transforms infrared spectroscopy (FTIR). The HKUST-1 was produced using a hydrothermal technique and possesses a high crystallinity of up to 97%, surface area 3400 m 2 /g, and pore volume 0.7 cm 3 /g. The prepared adsorbent (HKUST-1) tested using volumetric method, the maximum adsorption capacity of propane was (10.499 mmol/g) at a temperature of 298K and a pressure of 7 bar. Furthermore, adsorption isotherm study was conducted to understand the system equilibrium (i.e., the fitting with one of the known models Langmuir, Freundlich, and Temkin isotherm models). It was observed that the Freundlich isotherm model fitted well the experimental data. The Clausius-Clapeyron equation was used to determine the heat of adsorption, and the results revealed that the heat of adsorption increased as the propane adsorption capacity increased. The prepared HKUST-1, which has a large surface area and a high adsorption capacity, can be used as a major solution for gas storage for liquefied petroleum gas (LPG) in Iraqi vehicles. Keywords: metal organic frameworks, liquefied petroleum gas, adsorbed natural gas, Isosteric heat of Adsorption Received on 21/03/2022, Accepted on 11/07/2022, published on 30/09/2022 https://doi.org/10.31699/IJCPE.2022.3.4 1- Introduction Liquefied petroleum gas (LPG) (also called as "Propane Autogas") is mostly composed of propane (C3 H8) with percentage (100% or 60% or 35% )and butane (C4 H10) with percentage ( 40% or 65%) depending on the country and region, with some unsaturated components propene (C3H6) and butene (C4H8). LPG is produced by "wet" natural gas or refining petroleum (crude oil). The LPG component are gases at ambient temperature and pressure, but is liquefied at high pressures (more than 20 bars) [1]. Currently, LPG is a preferred internal-combustion engine fuel because it emits less pollution and leaves little solid residue, does not dilute lubricants, and has a high octane rating , therefore, it is considered as a clean alternative for gasoline [2]. The Iraqi government is aiming to urge citizens to switch to clean, domestically produced fuel. In Iraq, LPG (Propane Autogas) is being used as a clean vehicle fuel gas due to its low carbon emissions, low contamination risk, and environmentally friendly [3]. Although LPG burns cleaner than other fuel oils, it does have some drawbacks, such as higher fuel consumption due to its lower volumetric energy density than gasoline, and LPG pressure vessels are only filled to 80% of their overall capacity to allow for thermal expansion of the contained liquid, necessitating a larger storage tank [4]. Furthermore, because LPG has a low boiling point of about -50°C, it must be stored in pressurized steel tanks with a pressure of 20 bars or higher in order to convert the gases to their liquid state at room temperature [1]. The LPG drawbacks lead to the need of a heavier and larger metal tank with high-pressure. However, passenger car tanks should be light and constructed of carbon fibers, which are related to safety, cost, and space problems requirements, since the LPG tanks with these specifications are dangerous and expensive .As result, it was essential to find alternative method to store these gases for storing large amounts of gas. Different techniques have been used to store natural gas, such as compressed natural gas (CNG) at 200-300 bar or liquefied natural gas (LNG) at -161.5 °C [5]. The principal disadvantage of CNG is the need for the large and heavy tanks, as well as expensive multi-stage compression [6] http://ijcpe.uobaghdad.edu.iq/ http://www.iasj.net/ mailto:m.jabbar1207@coeng.uobaghdad.edu.iq mailto:rana.thabet@coeng.uobaghdad.edu.iq http://creativecommons.org/licenses/by-nc/4.0/ https://doi.org/10.31699/IJCPE.2022.3.4 M. S.Jabbar and R. Th. A. Alrubaye / Iraqi Journal of Chemical and Petroleum Engineering 23,3 (2022) 25 - 34 26 Whereas, the disadvantages of LNG are the energy and cost associated with liquefaction at -161.5 o C[7]. Presently, a new gas storage technique (Adsorbed natural gas (ANG)) is being employed as an alternative technology for storing natural gas under suitable conditions for mobile usage. This new technology (ANG) stores the hydrocarbon gasses in porous materials placed in the tanks. Porous materials act like a sponge, catching gas molecules [8]. This method allows for the same amount of gas to be stored as CNG at a significantly lower pressure (40-50 bars), lowering operating expenses. Furthermore, increasing the driving distance of a natural gas vehicle (NGV) by employing adsorbed natural gas technology with higher pressure [9]. Many types of adsorbents, including zeolites, activated carbons [10], and Metal Organic Frameworks (MOFs) [5] [11] [12], were developed and evaluated for adsorbed natural gas (ANG). MOFs have been shown to have a higher methane storage capacity than other adsorbents [13] [14] [15]. MOFs are a new class of crystalline porous materials that have drawn a lot of attention in the last two decades due to their large surface area (up to 5000 cm 2 /g), pore volume (up to 2 cm 3 /g), high thermal and chemical stabilities, and low densities (from 0.21 to 1 g/cm 3 ) [16], as well as their potential uses in gas storage, molecular separation, and other applications [17]. MOFs have features over activated carbon and zeolite because of their simple tunable and adaptable structures, and high crystallinity [18]. Additionally, it allows for the optimization of pore dimension and surface chemistry within metal-organic frameworks, which was previously impossible in zeolite materials [17]. MOFs are the preeminent forum for producing unique multifunctional products when it comes to gas storage applications [19]. The current study employs an adsorption technique for LPG storage to solve the drawbacks of the LPG storage method presently used using a novel adsorbent. The adsorption technique involves filling tanks with porous storage materials to store LPG. Previous studies on propane adsorption on activated carbons [20], zeolites 5A [21], and MOFs were done in the separation field [22]. Abedini et al. (2020) showed that MOFs (HKUST-1) have a greater adsorption capacity for propane than activated carbon and zeolites 5A when utilized to separate the mixture of propylene/propane [23]. HKUST-1 has been utilized for propane separation until recently, but there has been no research for employing it as gas storage for propane. The HKUST-1 (Hong Kong University of Science and Technology) is considered as one of the most important MOFs due to its high pore volume, good water adsorption/desorption stability, large surface area, and chemical stability [10]. Taking all of the facts into account, a new safe method of storing LPG gases that are used as fuel in vehicles is required to replace the greater cost and lower capacity of the current LPG vessel. As a result, HKUST-1 was prepared, characterized, and investigated in this study in order to reduce the cost of current LPG vessels in vehicles. HKUST-1 has been selected for this task because of its high surface area and micro pore volume that enable for effective adsorption- desorption for storing and delivering high amounts of LPG. 2- Experimental Work 2.1. Materials HKUST-1 was produced using copper (II) nitrate trihydrate [Cu (NO3)2 .3H2O, 99 percent] and benzene 1, 3, 5 tricarboxylic acid (trimesic acid) (BTC 95 percent) from Sigma Aldrich. Absolute ethanol (C2H5OH) was from Fisher Scientific. The Propane autogas (adsorbed gas) was supplied from a Gulf gas company in Baghdad (89.97 Vol. %). 2.2. Preparation of HKUST-1 HKUST-1 was produced using the hydrothermal process reported by Hill et al. [24]. Trimesic acid (0.42 g) was accurately measured and dissolved in 24 ml of a 1:1 solvent ratio (C2H5OH: H2O). Following that, the solution was mixed for ten minutes. After that, the copper nitrate trihydrate (0.875 g) was added to the solution and well stirred for 10 minutes. The resulting blue solution was transferred to a 150 ml stainless steel Teflon lined autoclave and heated to 100 o C for 30 hours in a furnace for crystallization once it was completely dissolved in the solvent. The reactor was then cooled to room temperature, resulting in the formation of a green crystalline powder, as illustrated in Fig. 1. The powder was filtered and thoroughly washed in a 60 ml water-ethanol solution (1:1 Vol percent). Finally, the green powder was activated under vacuum at 100°C for 16 hours using a rotary dryer evaporator and stored in a closed vessel. Fig. 1. Synthesized HKUST-1 M. S.Jabbar and R. Th. A. Alrubaye / Iraqi Journal of Chemical and Petroleum Engineering 23,3 (2022) 25 - 34 27 2.3. Characterization of HKUST-1 After degassing the sample to remove moisture content and other gases that induce blockage and lower surface area, the Surface area analyzer (Micrometrics ASAP2020, USA) was applied to measure the surface area and pore volume by nitrogen physical adsorption at 77 K, According to ISO 9277-2010. A large surface area value indicates that the adsorbent's activity has increased as the activity site has increased [25]. An X-ray diffract meter (Shimadzu SRD 6000) with a 3- 70° scan range, 40 kV tube voltage, 30 mA tube current, and 40 kV tube voltage was used to evaluate the samples. The structural and chemical bonds between MOF molecules were studied using an FTIR 8400S (600-4000) cm -1 Shimadzu. The HKUST-1 morphology was described using scanning electron microscopic analysis. 2.4. Propane Isotherms Adsorption An experimental setup based on the volumetric method was used to estimate the equilibrium adsorption capacity of synthesized adsorbents, as shown in Fig.2. The fittings are all connected to the copper pipes (1/4 inch). Stainless steel cylinders measuring around 25 cm 3 used as reservoirs and adsorption cells. The gas pressure was recorded with a gauge (Helicoid gauge) that ranged from (1-10 bar) and had a sensitivity of 0.02 percent and an accuracy of 1%. Both the reservoir and the adsorption chamber are maintained at the same temperature using a water bath. Before each experiment, a vacuum pump was used to evacuate the adsorption equilibrium measurement equipment. A cylinder connected to a pressure regulator supplies the feeding gas. The gas exit flow rate is controlled by a rotameter. Temperatures of (25, 30, 35, 40) o C and pressures of (1, 3, 5, 7) bar would be used to estimate propane isotherms. Nitrogen gas was used to determine the volume of the overall unit. The initial outgassing process was performed overnight under vacuum at 100°C. The degassed sample was then inserted in the adsorption chamber in a quantity of 0.2 g. All the valves were closed at the end of the degassing process to prepare the system for the experiments run. In experiment run, V3 and V4 valves opened, the gas was pumped into the reservoir chamber until it reached to equilibrium. V3 was closed when the pressure reached the equilibrium; V4 was remained open, and the pressure value was recorded. The V5 was then opened, allowing the gas to enter the adsorption chamber and achieving equilibrium. The amount of adsorbed gas was determined at equilibrium using the equations below [26]: q = ( Ci−Cf)Vr−Cf ɛ Va W (1) Where: (q) is amount of gas adsorbed (mmol/g), (C) is concentration (mmol/L) , (V) is volume of the vessel (L) , (ɛ) void fraction, (W) weight of adsorbent (g) , subscripts i,f,r,a refer to initial ,final, reservoir and adsorber respectively. Ci and Cf where calculate from equation of real gases [27]: P V= Z n R T (2) Where: C=n/V Ci = Pi Zi R T (3) Cf= Pf Zf R T (4) Z compressibility factor of feed and out gases may be estimated according to the generalized equation [27]: Z = 1 + B P R T (5) The coefficient of B can be determined as follows [27]: B = R TC PC (B o + ω B') (6) The coefficient of B o and B' may be calculated as follows [27]: B o = 0.083 - 0.422 Tr1.6 (7) B' = 0.139 - 0172 Tr4.2 (8) Where: Pc is critical pressure of propane (42.48 bar), Tc is critical temperature of propane (369.8 K), R gas constant (8.314 L.Kpa/ mol. K) and ω is centric factor (0.152). 1 2 3 � � I-3 I-4 � 4 5 9 10 6 7 8 11 12 13 14 15 16 1 Gas cylinder 2 Pressure regulator 3,4,5,12,13 valve 6,7,16 Pressure gauge 8 Thermometer 9 Reservoir chamber 10 adsoprption chamber 11 Water bath 15 Vacuum pump 14 Rotameter I-6I-7I-8 Fig. 2. Schematic diagram of apparatus used for adsorption equilibrium measurement M. S.Jabbar and R. Th. A. Alrubaye / Iraqi Journal of Chemical and Petroleum Engineering 23,3 (2022) 25 - 34 28 2.5. Adsorption Isotherms The adsorption behavior of gas (or vapor) is investigated by varying the applied gas (or vapor) pressure; the relationship between the amount of adsorbed and pressure is referred to as an adsorption isotherm ("iso" and "therm" denote equal and temperature, respectively) [28]. The adsorption isotherms were studied using the Langmuir, Freundlich, and Temkin isotherms, Eqs. ( 9, 10, and 11) respectively. Langmuir equation is [29]: 1 qe = ( 1 qm KL ) 1 Ce + 1 qm (9) Freundlich equation is [30]: Log (qe ) = Log (Kf) + 1 n Log (Ce) (10) Temkin equation is [31]: qe= β lnA + β lnCe (11) Where: qe [mmol g -1 ] is the amiount of gas uptake on adsorbent at equilibrium, Ce [mmol L -1 ] is the equilibrium concentration of gas, qm [mmol g -1 ] is the adsorption capacity constant (the equilibrium adsorption amount for a full monolayer), Kf is Freundlich adsorption equilibrium constant, KL [L mmol -1 ] is Langmuir adsorption equilibrium constant, 𝛽 related to the heat of adsorption and A(KT ) is equilibrium binding constant [J mmol -1 ]. 2.6. Isosteric Heat of Adsorption The isosteric heat of adsorption (Qst) (isosteric indicating constant loading) is also known as the heat of adsorption or enthalpy of adsorption. When developing adsorption processes, the isosteric heat of adsorption, which measures the change in adsorbent temperature during adsorption, is an important thermodynamic property to consider. Adsorption heat is proportional to the binding energy between adsorbed molecules and the adsorbent, as well as interactions between adsorbates [32]. The two techniques of determining Qst are experiment- based calculations and molecular simulations. The latter's simulated adsorption enthalpies are largely dependent on isotherms derived from grand canonical Monte Carlo simulations (GCMS) utilizing the ensemble fluctuation approach [33]. Direct and indirect calculations are two types of experiment-based calculations. In the direct technique, which employs a calorimetric-volumetric system, it is feasible to measure directly the emitted heat during adsorption using a calorimeter. Because these systems are extremely complex and expensive, only a few studies use the direct approach to calculate Qst [34]. The indirect method of calculating the isosteric enthalpy of adsorption as a function of the amount adsorbed (loading) using adsorption isotherms is by far the most general. In most cases, the adsorbate-adsorbent interaction energy is computed indirectly utilizing at least two adsorption isotherms (T1, T2) measured at close but different temperatures (ΔT ≈ 10-20 K). The most common method for obtaining adsorption isotherms is volumetric gas adsorption measurements [35].The isosteric heat of adsorption (Qst) can be estimated by the Clausius- Clapeyron equation as[22]: Qst = - RT 2 ( ∂ ln p ∂T )q (12) The integration leads to: Ln p = Qst R T + C (13) Where: Qst (kJ/mol) is isosteric heat of adsorption, T is temperature (K), P is the pressure (KPa), R is the gas constant and q is the adsorption amount (mmol/g). 3- Result and Discussion 3.1. Characterization of HKUST-1 The XRD analysis showed a typical characterization peaks of HKUST-1 with high purity phase, and a crystallinity of 97%, which is close to Al-Yassiry's work, which had 100% crystallinity under the similar conditions [36]. The HKUST-1 BET analysis has a pore volume of (0.7 cm 3 /g) and a surface area of 3480 m 2 /g, which was similar to Kareem's results (3635 m 2 /g) under the same operating conditions [18]. All of the IR analysis bands for HKUST-1 were in good agreement with the IR analysis [37]. The presence of the –COOH groups in the organic ligand reacting with metal ions is indicated by a strong stretching vibration of carboxylate anions at (1608.63)1/cm as seen in Fig. 3. A band painted at (3600–2800) 1/cm demonstrated the presence of water and –OH groups in the material's structure. The absence of CuO and Cu2O in the synthesis product during the production of HKUST-1 is indicated by peaks at (410, 500, 610, and 615) 1/cm. Increased and changed carboxyl absorption from (3,100) to (3,600) 1/cm in HKUST-1 revealed the loss of water molecules [38]. Fig. 4 demonstrates that the IR analysis for HKUST-1 does not change after the propane adsorption process, indicating that the adsorbent's composition did not change after desorption. SEM images of the prepared HKUST-1 at a magnification of 2062 x are shown in Fig. 5. As seen in this figure, the synthesized metal-organic framework has an octahedral shape. This result is similar to the one found by Lin, et al. [39]. M. S.Jabbar and R. Th. A. Alrubaye / Iraqi Journal of Chemical and Petroleum Engineering 23,3 (2022) 25 - 34 29 Fig. 3. FTIR of prepared HKUST-1before adsorption Fig. 4. FTIR of prepared HKUST-1 after adsorption Fig. 5. SEM image of the synthesized HKUST-1 3.2 Propane Adsorption on HKUST-1 a. Temperature Effect on Propane Adsorption Capacity The relationship between the propane adsorption capacity q (mmol/g) and temperature is seen in Fig. 6. At a pressure of 7 bar and a temperature of 298 K, the highest adsorption capacity was found to be 10.49 mmol/g. As can be seen in Fig. 6, increasing the temperature causes the amount of adsorbed propane on the adsorbent to decrease, particularly at 7 bars. Fig. 6 illustrates that increasing the temperature at constant pressure decreases the total quantity of adsorbed propane on the adsorbent, indicating that the process is exothermic. This can be explained by the fact that when the temperature increased, the propane adsorbed on the HKUST-1 surface became unstable, leading in desorption of more propane molecules. Lamia, et al. [40] found similar behavior of decreasing adsorption capacity when temperature is increased. Additionally, the adsorption capacities in this study (7.44 mmol/g at 1 bar and 303K) were close to those predicted by Rubes, et al [41] (at similar condition employing HKUST-1 as an adsorbent for adsorption of propane and propylene), which were (7.3) mmol/g. Fig. 6. Adsorption capacity as a function of temperature b. Pressure Effect on Propane Adsorption Capacity The influence of pressure on propane adsorption capacity was studied at a constant temperature in the range of 1 to 7 bars (25, 30, 35 and 40 oC). When pressure is increased, the propane adsorption capacity increases, as seen in Fig. 7. This is because increased pressure increases momentum, which reduces the resistance to mass transfer between the adsorbent and the adsorbate, resulting in a greater number of gas molecules on the adsorption site. This figure also shows a relatively high adsorption capacity at 1 bar (7.61 mmol/g) due to the significant interaction between large numbers of carbon atoms in the structure of propane and the HKUST- 1 surface. Ponraj et al. [42] used a simulated isotherm of HKUST-1 as an adsorbent to separate methane from ethane and propane at 298 K with pressures ranging from 0.001 to 100 bar, and found the similar behavior of greater adsorption capacity at low pressure. Fig. 7. Adsorption capacity as a function of pressure M. S.Jabbar and R. Th. A. Alrubaye / Iraqi Journal of Chemical and Petroleum Engineering 23,3 (2022) 25 - 34 30 3.3. Adsorption Isotherms The Langmuir, Freundlich, and Temkin isotherms were used to investigate the adsorption isotherms. These isotherms link propane intake per unit mass of adsorbent (qe) to the concentration of adsorbate in the gas phase at equilibrium (Ce). a. Langmuir Isotherm Equilibrium data for propane adsorption on the HKUST-1 were fitted using the linearized Langmuir equation described in eq. 9. Fig. 8 shows a linear plot of equilibrium concentration (1/Ce) versus equilibrium adsorption (1/qe). The Langmuir constants (KL and qm) were calculated using the slope (1/qm KL) and the intercept of the plot (1/qm) ,as shown in Table 1. The observed correlation coefficients (R 2 ) at 298, 303, 308, and 313 K were 0.8063, 0.8047, 0.8278, and 0.8392, respectively, according to the results obtained and listed in Table 1; the isotherm data does not fit the Langmuir equation. b. Temkin Isotherm Equilibrium data for propane adsorption on the HKUST-1 were fitted using the linearized Temkin equation described in eq. 11. Fig. 9 shows a linear plot of equilibrium concentration (ln Ce) vs. adsorption capacity (qe). The Temkin constants (KT and β) were calculated using the slope (β) and intercept of the plot (β ln KT), as shown in Table 1. The observed correlation coefficients (R 2 ) at 298, 303, 308, and 313 K were 0.9076, 0.9135, 0.9346, and 0.9465, respectively, according to the results obtained and listed in Table 1; the isotherm values fit the Temkin equation well. c. Freundlich Isotherm The propane adsorption equilibrium data were carefully investigated using the Freundlich isotherm. To fit the equilibrium data for propane adsorption on the HKUST-1, the linearized Freundlich equation described by eq. 10 was employed. Fig. 10 shows a linear plot of (log Ce) vs (log qe). The Freundlich constants (Kf and n) were calculated using the slope (1/n) and the plot intercept (log Kf) and listed in Table 1. The observed correlation coefficients at 298, 303, 308, and 313 K were 0.9345, 0.9369, 0.9531, and 0.9613, respectively, according to the results obtained and listed in Table 1; the isotherm values fit the Freundlich equation well. The results show that adsorption experimental data is suitable for Temkin and Freundlich models; however, when the R 2 values for the Freundlich and Temkin models are compared, the R 2 values for the Freundlich model are greater than those for the Temkin model. These results indicate that the Freundlich model is more suitable with propane adsorption on HKUST-1. In this research, the values of n at equilibrium were more than unity, indicating that the slope (1/n) was closer to zero, indicating a more heterogeneous adsorption process. Table 1. Isotherm parameters for propane adsorption on HKUST- 1 Isotherm Parameters Temperature (298 K) Temperature (303 K) Temperature (308 K) Temperature (313 K) Langmuir (qm) 9.8232 9.3809 9.0171 8.6655 Langmuir (KL) 0.3467 0.3826 0.4053 0.4561 Langmuir (R 2 ) 0.8063 0.8047 0.8278 0.8392 Freundlich (KF) 5.9375 5.9020 5.8237 5.8398 Freundlich (n) 9.7561 10.5708 11.1982 12.3762 Freundlich (R 2 ) 0.9345 0.9369 0.9531 0.9613 Temkin (β) 0.9056 0.8041 0.7316 0.6404 Temkin (KT) 383.1521 878.5400 1699.2534 5708.72027 Temkin (R 2 ) 0.9076 0.9135 0.9346 0.9465 Fig. 8. Langmuir isotherm Fig. 9. Temkin isotherm Fig. 10. Freundlich isotherm M. S.Jabbar and R. Th. A. Alrubaye / Iraqi Journal of Chemical and Petroleum Engineering 23,3 (2022) 25 - 34 31 3.4. Isosteric Heat خf Adsorption For the Clausius-Clapeyron technique, the two adsorption isotherms must be fitted with the same continuous function, such as a Langmuir, dual-site Langmuir, Toth, Sips, Jovanovic, Dubinin-Radushkevic, Freundlich-Langmuir, or other fit [43]. a. Clausius-Clapeyron Approach for Isosteric Heat of Adsorption Calculation The same model must be used for the two isotherms at two temperatures (298 and 313 K). Most MOFs have a Freundlich-Langmuir isotherm, which can be fitted using the equation below [35]. q = 𝑎∗𝑏∗(𝑃)^𝑐 1+𝑏∗(𝑃)^𝑐 (14) Where: q is the amount adsorbed (the loading) in mmol g – 1 , a is the maximal loading in mmol g –1 , P the pressure in kPa, c the heterogeneity exponent and b the affinity constant (1/kPa c ). By using Origin program the curve in Fig.7 is fitting to Fig. 11. Fig. 11. Adsorption isotherms after fitting by Freundlich- Langmuir isotherm using Origin Program Rearrange equation (14) and substitute the fitting parameters (a,b, and c) to determine the pressure at any temperature and the same loading. Equation (14) become: P(q) = √ 𝑞 𝑎∗𝑏−𝑞∗𝑏 𝑐 (15) Now, plot ln(P) versus T at the same loading as shown in Fig. 12 and find slop of every line which result and calculate Qst (Qst = slope * R) were R is universal gas constant and plot Qst versus amount adsorbed(q) as shown in Fig. 13. Fig. 12. ln P v.s 1/T Fig. 13. Isosteric heat of adsorption as a function of the amount adsorbed The isosteric heat of adsorption increased as the adsorption capacity increased, as seen in Fig. 13. Significant lateral interactions between adsorbates at HKUST-1 sites cause this increase. The propane molecules preferentially bind at cage center sites, followed by adsorption at cage window sites, as explained by Rubes [36]. The adsorption enthalpies rise as propane loading increases because the adsorbates at the cage center and cage window sites have strong lateral interactions. When the isosteric heat of adsorption increased with coverage (due to increasing the lateral interaction between adsorbate-adsorbate interactions while decreasing the binding energy between adsorbate molecules and the adsorbent surface), this resulted in a decrease in the amount of propane molecules that could be adsorbed and a decrease in the amount of gas delivered at desorption due to more gas molecules remaining in adsorbent sites due to the increased lateral interaction. 4- Conclusions In this work, The HKUST-1 was successfully prepared and used as an effective adsorbent for propane gas. The HKUST-1 was produced using a hydrothermal technique and possesses a high crystallinity of up to 97%, surface area 3400 m 2 /g, and pore volume 0.7 cm 3 /g. Adsorption was utilized to store propane using the prepared HKUST- 1. M. 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S.Jabbar and R. Th. A. Alrubaye / Iraqi Journal of Chemical and Petroleum Engineering 23,3 (2022) 25 - 34 34 وحرارة االمتزاز لألطر العضوية المعدنية المستخدمة لتخزين الغاز األيزوثيرمي االمتزاز البترولي المسال في المركبات العراقية محمد ستار جبار و رنا ثابت عبد الربيعي جامعة بغداد, كلية الهندسة, قسم الهندسة الكيمياوية الخالصة ( كنظام تخزين للغاز البترولي HKUST-1المعدنية )يقدم هذا البحث تقنية جديدة الستخدام األطر العضوية ( في المركبات العراقية لتجنب عيوب الطريقة المستخدمة حالًيا لتخزين غاز البترولي المسال. تم LPGالمسال ) وتم فحصها في هذا البحث للتحقق من قدرتها على HKUST-1تحضير مادة مازة منخفضة التكلفة تسمى (. تم تصنيع 7-1) bar( وضغوط 25، 30، 35، 40 ) ℃ات حرارة مختلفة تخزين البروبان في درج HKUST-1 باستخدام طريقة حرارية مائية وتم وصف خصائصه باستخدام حيود األشعة السينية ، ومساحة أظهرت نتائج األشعة (.FTIR( ، و األشعة تحت الحمراء )SEM، والمسح المجهري األلكتروني ) BETسطح بينما أظهرت نتائج تحليل المساحة السطحية ,%97المحضر نسبة تبلور جيدة HKUST-1السينية لـ تم اختبار المادة الماصة (.0.7cm3/gوحجم مسامي ) (3480m2/gمساحة السطحية مقدارها ) الحصول على 10.499) هي( باستخدام الطريقة الحجمية ، وكانت السعة القصوى المتزاز البروبان HKUST-1المحضرة ) mmol/g298 درجة حرارة ( عندK و ضغطbar 7 لفهم توازن النظام)أي .تم فحص نتائج االمتزاز األيزوثيرمي ( واظهرت نتائج االمتزاز Temkinو Langmuirو Freundlichالمالئمة مع أحد النماذج المعروفة ) أظهرت النتائج التي تم الحصول عليها من حسابات .Freundlichموديل بشكل جيد تناسب انها األيزوثيرمي أن حرارة االمتزاز تزداد مع زيادة سعة االمتزاز. Clausius- Clapeyronحرارة األمتزاز بواسطة معادلة المحضر والذي يتميز بمساحة كبيرة وقدرة امتزاز عالية كحل رئيسي لتخزين الغاز HKUST-1يمكن استخدام ( في المركبات العراقية.LPGالبترولي المسال ) العضوية المعدنية, الغاز البترولي المسال, الغاز الطبيعي الممتز, حرارة األمتزاز. الكلمات الدالة: األطر