1994 Korean J. Chem. Eng., 31(11), 1994-2002 (2014) DOI: 10.1007/s11814-014-0100-9 INVITED REVIEW PAPER pISSN: 0256-1115 eISSN: 1975-7220 INVITED REVIEW PAPER † To whom correspondence should be addressed. E-mail: shahsavand@um.ac.ir Copyright by The Korean Institute of Chemical Engineers. Reliable modeling of discharge process for adsorbed natural gas storage tanks Mahdi Khorashadizadeh*, Mahdi Niknam Shahrak**, and Akbar Shahsavand* ,† *Chemical Engineering Department, Faculty of Engineering Ferdowsi University of Mashhad, Mashhad, P. O. Box 91775-1111, Iran **Department of Chemical Engineering, Quchan University of Advanced Technology, Quchan, P. O. Box 84686-94717, Iran (Received 16 July 2013 • accepted 8 April 2014) Abstract -Natural gas consumption has doubled in the last fifteen years. Among all storage techniques, adsorbed natural gas (ANG) provides a reliable vehicle for safe utilization of natural gas. Despite all favorable characteristics of the ANG process, thermal adverse effects during charge and discharge processes are the most challenging issues facing adsorbed natural gas applications, especially for automotive usage. Mathematical modeling of an ANG tank can provide a reliable method to analyze and solve such problems. A robust and lumped model is presented to mimic the discharge process of an ANG tank storing pure component. The proposed model is very convenient compared to other available conventional models that require extensive computational efforts. Two experimental measurements and two simulation data sets (borrowed from literature) are recruited to validate the model predictions. The simulation results indicate proper agreement between the proposed model predictions and the validation data. Keywords: Natural Gas Storage, Adsorption, ANG, Lumped Modeling, Discharge Process INTRODUCTION Availability, clean burning and low price are some of the most important reasons to focus researches on using natural gas (NG) in various industries such as automotive industry in the last two de- cades [1-28]. Despite the benefits of NG, its storage, transportation, and energy density are the most serious problem in the way of spread of its application. Several storage techniques are developed to cope with this issue. Compressed natural gas (CNG) storage technique is presently the most popular method to use natural gas in on-board reservoirs for automotive applications. However, it suffers from a number of shortcomings: extremely high pressure requirement (200- 250 bar), high cost due to its high pressure, heavy vessels and re- stricted storage reservoir geometries. LNG is another available method to store and transport natural gas. LNG provides highest energy density (around 600 volumes of natural gas per volume of the ANG vessel (v/v)) amongst other forms of natural gas storage techniques. But it cannot be easily and di- rectly used in cars or other vehicles due to its extremely low tem- perature requirements ( -163 o C) [17]. Moreover, use of LNG needs appropriate technical equipment, so it is necessary to find a cheaper and safer method to store natural gas for vehicle usage. In the same manner, natural gas hydrate (NGH) storage can also be used for long distance transportation of large quantities of natural gases [29,30]. Adsorbed natural gas, which has gained more attention in the last couple of decades, is a promising alternative to CNG, LNG and NGH. In this application, maximization of gas storage density is the ulti- mate requirement, in order to store and deliver highest volume of gas per volume of ANG storage vessel (v/v). Depending on the ad- sorbent (confined in the vessel), the delivered v/v will be different and so will the energy density. In 2000, the U.S. Department of Energy (DOE) raised the material-based adsorbed methane storage target from 150 to 180 v/v at 298 K and 35 bar [18,23]. New materials are synthesized every day to increase the v/v amount of NG storage capacity. The overall performance and practicality of ANG vessels also depend on the characteristics of the adsorbent as well as the heat and mass properties of the system. Although ANG does not have some of CNG, NGH and LNG problems and can be considered as the most promising way in on- board natural gas storage technique among other scenarios, there are some challenges. Thermal effects in the bed during charge and discharge processes are the major problems facing ANG applica- tions. Since adsorption and desorption of stored gas in the vessel are exothermic and endothermic, respectively, the heat released or demanded in these processes reduces the storage capacity. Thus, a profound understanding of heat and mass transfer phenomena oc- curring in ANG tank during charge and discharge processes is essen- tial for better design of such reservoirs. Several experiments have been carried out [14,16,19] to address the thermal behavior of ANG storage technique during charge and discharge processes. The real response of such processes can be analyzed and studied by resorting to a suitable mathematical model. A brief review of such attempts for lumped [3,21,25] and distributed [1,2,5,8,18,20,26,28] model- ing of ANG storage vessels during charge and discharge processes is presented below. In a comprehensive pioneering work in 1996, Chang and Talu [1] studied the impact of heat of adsorption on ANG performance during discharge process under realistic condition of a vehicle appli- cation. The effect of various parameters on the ANG performances such as flow direction, demand rate and outside condition was also