he global environment is a major issue today, and global warming in particular is the focus of much attention. It is supposed that the T increasing concentration of carbon dioxide in the atmosphere i s a major contributor to this problem (Hansen et al., 1988). The sources of carbon dioxide are classified into two types. One i s the fixed source such as thermal power plants that emit large amount of flue gas. Another is the mobile source such as automobiles. A review of the past countermeasures taken by environmental administrations indicates that the reduction of CO, emissions at fixed sources is more effective than the reduction at mobile sources. The trend therefore favours the reduction of the consumption of fossil fuels by raising the efficiency of thermal power plants and using nuclear power plants more effectively. On the other hand, a variety of techniques have been studied to separate and recover CO, in large amount of gases emitted from the fixed sources (Shindo, 1992).Various methods such as the adsorption (Nishikawa, 1992), the absorption (Suda et al., 1992; Eliasson, 1994), the membrane separation (Osada et al ., 1999) and others have been investigated for the separation of CO, . In these methods, the pressure swing adsorption (PSA) presents such advantages as energy saving and relatively easy operation. In fact, there are many reports on the practical applications of this method to the separation of various mixed gases in many industrial processes. Therefore, PSA is also considered to be a useful method for the separation and recovery of CO, from the exhaust combustion gas (Steinberg, 1992; Kikkinides et al., 1993).Authors have proposed a method for the CO, separation and recovery system (referred to as 'the system' in the following) that combines pressure swing adsorption (PSA) with a super cold separator to separate and recover CO, from exhaust combustion gases (Saji et al., 1997). This system employed an equilibrium adsorption type PSA using Na-X zeolite as adsorbent. As the result of the test of the separation and recovery of CO, from simulated exhaust combustion gas on a dry base, it was confirmed that the target of the CO, recovery rate (90% or above) and the CO, concentration in the recovery gas (99% or above) could be achieved. However, the CO, recovery rate was only 70% by the super cold separator alone under the present operating conditions Although the super cold separator applied to the system for CO, recovery from flue gas can produce pure CO, liquid, the CO, recovery efficiency is low.Therefore, the addition of a PSA plant was considered for the secondary CO, recovery from the noncondensing gas to improve the efficiency. The PSA plant was operated for adsorption a t the same pressure as that of the super cold separator and for desorption a t the atmospheric pressure. From both the simulation and the experimental data, it was confirmed that CO, could be concentrated from 50% in the noncondensing gas to 70% in the recovery gas by the PSA plant and the CO, recovery efficiency of the plant ...
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