An experimental adsorbent screening study for CO2 removal from N2
Introduction
With an increasing worldwide population and an increased per capita demand for energy, concern is now being raised over the CO2 emissions resulting from the gaseous by-products of combustion processes. In order to meet the present and future constraints placed on the allowable emissions of CO2, the solution lies with reduction and recovery. Pressure swing adsorption (PSA) is a very promising separation and recovery process for this type of application.
In the design of an adsorption based separation process, the choice of the adsorbent is the most crucial design consideration [20], [21], [25]. In this study, several adsorbent–adsorbate characteristics have been considered. These include the initial loading effects (Henry’s Law constant, initial heat of adsorption), pure component adsorption capacity and isotherm shape, and the expected working capacities for PSA applications.
For this study, an extensive literature review and experimental adsorbent screening with CO2 and N2 gases have been completed. Thirteen adsorbents, described in Table 1, have been examined and the most promising ones were selected for further study. Pure component CO2 adsorption isotherms were determined at 22 °C and pressures ranging from 1 to 1900 Torr.
The concentration pulse method (CPM) was used for determining the Henry’s Law constants and the associated heat of adsorption parameters [26], [24], [18], [1], [2], [11]. In this method, a column (dimensions are given in Table 2) was packed with the adsorbent to be studied. The adsorbent was regenerated under helium gas purge at 200 °C for 12 h to get rid of all the moisture and other gases that may be adsorbed by the adsorbent. The same regeneration procedure was used for all the adsorbents studied. After the regeneration, the column temperature was reduced to the experiment temperature. The helium gas was used as the carrier gas and a very small amount of adsorbate was injected into the carrier gas as an impulse. The response of the column was monitored by measuring the concentration of the adsorbate at the end of the column. From the first moment of the response peak, Henry’s Law constant, Kp, can be determined. More details of the experimental set-up can be found elsewhere [11], [12], [14]. The pure component isotherms were determined with a modified constant volume apparatus (Micrometetics Accusorb 2400).
The quickest and easiest method for the initial loading study is by using the concentration pulse method for determining Henry’s Law constants (Kp, mol/kg/atm). The Kp values are proportional to the initial slope of the adsorption isotherm as a large Kp value corresponds to a steep initial slope of the adsorption isotherm. When Kp values are determined at different temperatures (as P[CO2] → 0), a Van’t Hoff plot can be constructed and the limiting heat of adsorption is defined by Eq. (1).The heat of adsorption is a function of the strength of adsorption. This quantity is defined by the slope found by regression of the experimental data in dimensional form. The experimental conditions used with the concentration pulse method are given in Table 2.
When the Kp and −ΔH values are considered at the same time, an initial adsorbent screening can be performed. This is accomplished by removing all adsorbents that exhibit very high heats of adsorption, as these tend to require higher energy costs for the regeneration cycle. Since the adsorption process is exothermic, a high −ΔH value will produce large quantities of heat within the adsorbent bed during an adsorption process cycle. This will cause an increase in the local column temperature. This increase in the column temperature will have an adverse effect on the adsorption capacities of the components. The result is a loss of capacity, and thus decreased process throughput. Similarly, in desorption a large temperature drop may also occur. This drop may cause certain components to freeze out on the adsorbent (such as moisture).
Section snippets
Initial loading effects
There are several sources of initial loading data available for CO2 and N2 with several adsorbents (i.e., [27], [9], [7], [8]). However, the availability of Henry’s Law constants at the required design conditions is limited. A comparison of the available literature data was difficult since the studies were performed under varied thermodynamic conditions; temperature and pressure. Selected results of a literature survey based on the heat of adsorption of the type of adsorbents used in this study
Conclusions
This study enabled a detailed examination of how the temperature and isotherm shape can greatly affect the working capacity of the adsorbent. Also, since the adsorption process is exothermic, the heat released during adsorption will tend to increase the column temperature as the heat transient moves through the column. Another important property is that the net heat effects of adsorption decrease as temperature increases due to lower adsorption potentials. Therefore, the importance of the heat
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