An experimental adsorbent screening study for CO₂ removal from N₂

Abstract

The selection of a suitable adsorbent for CO₂ removal from flue gas (mixture of CO₂ and N₂) has been carried out. The limiting heats of adsorption, and Henry’s Law constants for CO₂ with a N₂ carrier, were determined for a group of 13 zeolite based adsorbents, including 5A, 13X, NaY, NaY-10, H-Y-5, H-Y-30, H-Y-80, HiSiv 1000, H-ZSM-5-30, H-ZSM-5-50, H-ZSM-5-80, H-ZSM-5-280, and HiSiv 3000. The CO₂ pure component adsorption isotherms and expected working capacity curves for pressure swing adsorption (PSA) application were determined for a selected promising subgroup of these adsorbents. The results show that the most promising adsorbent characteristics are a near linear CO₂ isotherm and a low SiO₂/Al₂O₃ ratio with cations in the zeolite structure which exhibit strong electrostatic interactions with carbon dioxide.

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, K_p, 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 (K_p, mol/kg/atm). The K_p values are proportional to the initial slope of the adsorption isotherm as a large K_p value corresponds to a steep initial slope of the adsorption isotherm. When K_p 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).

$$K_{\mathrm{p}}=K_0\exp \left[\frac{-\mathrm{\Delta }H}{\mathrm{RT}}\right]$$

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 K_p 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).