Proceedings of the 11 th Brazilian Congress of Thermal Sciences and Engineering -- ENCIT 2006 Braz. Soc. of Mechanical Sciences and Engineering -- ABCM, Curitiba, Brazil,- Dec. 5-8, 2006 Paper CIT06-0633 GAS DISCHARGE CAPACITY INTENSIFICATION OF ADSORBED NATURAL GAS RESERVOIRS Luciano G. Lara Petrobras S/A – ENGENHARIA/IEABAST/EAB/ENPRO, Rio de Janeiro, RJ, 20031-004, Brazil lglara@petrobras.com.br Paulo Couto Federal University of Rio de Janeiro – Petroleum Engineering – DEI/POLI, Rio de Janeiro, RJ, 21941-972, Brazil pcouto@petroleo.ufrj.br Renato M. Cotta Federal University of Rio de Janeiro – PEM/COPPE, Rio de Janeiro, RJ, 21941-972, Brazil cotta@serv.com.ufrj.br Dayse M. A. Sophia Federal University of Rio de Janeiro – PEM/COPPE, Rio de Janeiro, RJ, 21941-972, Brazil dayse.sophia@gmail.com Abstract. The discharge capacity intensification of an adsorbed natural gas reservoir under a slow discharge process is discussed in this work. A theoretical two-dimensional transient model, developed by the authors, was used to analyse the influence of heat trans- fer parameters on the discharge capacity. This model is also able to analyse the influence of a thermal control device located axially at the centre of the reservoir. A heat pipe is considered as a thermal control devices (a heat pipe is a two-phase flow heat transfer device). The coupling of the one-dimensional transient heat pipe model and the two-dimensional transient model for natural gas slow discharge is presented and discussed. The following parameters are analysed: convection heat transfer at the reservoir exter- nal wall, effective thermal conductive of activated carbon bed, thermal capacity of the reservoir wall and discharge mass flow rate. Regarding the heat pipe, the temperature of the heat source at the heat pipe’s evaporator is analysed. The results of the analyses are shown and then discussed. Keywords. Natural Gas, Adsorption, Heat Pipe, and Gas Transportation. 1. Introduction Adsorption is the gas molecules uptake process in an interfacial layer (usually a highly porous media), whether by capillary condensation or by Van der Walls forces (Gregg and Sing, 1982). This new technology has been considered for natural gas storage and transportation in the last 10 years (Brady et al., 1996; Mota et al., 1997; MacDonald and Quinn, 1998; Vasiliev et al., 2000a and 2000b; Burchell and Rogers, 2000; Litzke and Wegrzyn, 2001; Biloé et al., 2001a,b and 2002) as an alternative to Compressed Natural Gas (CNG) and Liquefied Natural Gas (LNG). In the case of natural gas, the uptake process occurs at relatively low pressures, in the order of 35 to 50 atmospheres, which pro- vides three main benefits: (1) compression costs reduction, (2) increased safety of the reservoir, and (3) design flexibil- ity of the storage tanks. Despite of these benefits, the storage capacity of ANG reservoirs is lower than LNG and CNG, being of the order of 90 V/V (Brady et al., 1996) up to 164 V/V (Inomata et al., 2002). The U.S. Department of Energy storage target for ANG vessels was set at 150 V/V and it was later revised to 180 V/V. According to Biloé et al. (2002), the performance and viability of an ANG system depends closely on the micro-porous characteristics of the adsorbent as well as on the heat and mass transfer properties. The problems affecting ANG technology associated with the heat and mass transfer properties are be described below. The concentration of the adsorbed phase, say q, in a porous matrix follows closely a power law function of the pres- sure, P, and temperature, T, of the reservoir, and it increases with pressure and decreases with temperature. Usually, the concentration is expressed as a function only of pressure for a given temperature and it is known as adsorption iso- therms [q = f(P) T ]. Therefore, affinity must exist between the adsorbent bed and the gas to be adsorbed so that adsorp- tion will occur naturally with increasing pressure. For natural gas, activated carbon has been used as adsorbent beds. However, the adsorption phenomenon is an exothermic process. During the charging process of an ANG reservoir, the pressure increase will cause the natural gas to adsorb in the activated carbon bed. The gas, in the gas phase, must release energy to change from gas to adsorbed phase. This energy release is quantified by the latent heat of adsorption (∆H) and it causes a temperature increase in the adsorbent bed, which according to the adsorption isotherm, results in less stored methane capacity under dynamic conditions (see Fig. 1 – left). This process can be compared to a condensa- tion process. In the other hand, during discharge of an ANG reservoir, the pressure drop caused by the gas extraction, will cause the adsorbed phase to change to the gas phase. This phase change will require an amount of energy (∆H), which will be