Automatic Filtering and Reodorization of Adsorbed Natural Gas Storage Systems
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Automatic Filtering and Reodorization of Adsorbed Natural Gas Storage Systems ´ ´ ISABEL A.A.C. ESTEVES, MARTA S. LOPES, PEDRO M. NUNES, MARIO F. EUSEBIO AND JOSE´ P.B. MOTA∗ Requimte/CQFB, Departamento de Quimica, Faculdade de Ciˆencias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal [email protected]
Abstract. The presence of high molecular weight hydrocarbons and sulphur-based odorants in natural gas has a negative impact on the storage of this fuel using adsorption technology. The reason for this is the deterioration of the adsorbent capacity on extended cyclic operation. Although a good adsorbent is key to the success of ANG, its potential will be limited if it is not integrated into a well-designed system which compensates for the weakness inherent in the adsorption process, deleterious poisoning and heat effects. Although there appears to be a consensus on the use of a especially tailored carbon-based filter or guard-bed to prevent the contamination of the storage tank, this system has not yet been studied or evaluated in real depth. In this work, it is demonstrated for the fist time that it is indeed possible to filter and reodorize the delivered natural gas using a properly operated carbon-based filter. Keywords: natural gas, adsorption storage, activated carbon, guard bed
1.
Introduction
The decrease in storage capacity of the adsorbent with the number of cycles is undoubtedly one of the most important problems in vehicular and large-scale applications of adsorbed natural gas (ANG) storage. If the impurities are not prevented from entering the storage tank during filling, they tend to accumulate from cycle to cycle until adsorption/desorption equilibrium is reached (Mota, 1999). This can lead to a significant decrease in adsorbent storage capacity. Furthermore, in the case of large-scale stationary storage safety standards require that the gas be properly odorized when delivered to the consumer. The use of a well-designed guard bed, which itself is an adsorbent system, with a carefully selected adsorbent for rapid preferential adsorption of the higher molecular weight components, is effective at maintaining the storage capacity of an ANG storage system. Detailed information, which relates directly to the de∗ To
whom correspondence should be addressed.
sign, effectiveness and operation of carbon guard beds in ANG, is somewhat scarce (Cook et al., 1999). The desirable characteristics of active carbons for guardbed applications, the design of the guard bed itself, e.g., fixed or mobile, and its method of operation, i.e., heated or non-heated, have not yet been studied or evaluated in real depth. The proposed filtering system was developed within an European project,1 its method of operation is sketched in Fig. 1. The discharged stream from the main storage tank is heated dynamically before being fed to the guard bed so that the odorants are released at a prescribed concentration. The control scheme applies a pre-computed optimum temperature ramping to the heated gas, while simult
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