Supercritical adsorption also referred to as the adsorption of supercritical fluids, is the adsorption at above-critical temperatures. There are different tacit understandings of supercritical fluids. For example, “a fluid is considered to be ‘supercritical’ when its temperature and pressure exceed the temperature and pressure at the critical point”. In the studies of supercritical extraction, however, “supercritical fluid” is applied for a narrow temperature region of 1-1.2 T c {\displaystyle T_{c}} or T c {\displaystyle T_{c}} to T c {\displaystyle T_{c}} +10 K, which is called the supercritical region. ( T c {\displaystyle T_{c}} is the critical temperature)
History Observations of supercritical adsorption reported before 1930 was covered in studies by McBain and Britton. All of the important articles on this subject published between 1930 and 1966 have been reviewed by Menon. During the last 20 years, a growing interest in supercritical adsorption research under the impetus of the quest for clean alternative fuels has been observed. Considerable progress has been made in both adsorption measurement techniques and molecular simulation of adsorption on computers, rendering new insights into the nature of supercritical adsorption.
Properties According to the adsorption behavior, the adsorption of gases on solids can be classified into three temperature ranges relative to T c {\displaystyle T_{c}} : 1.Subcritical region (T< T c {\displaystyle T_{c}} ) 2.Near-critical region ( T c {\displaystyle T_{c}} <T< T c {\displaystyle T_{c}} +10) 3. The region T> T c {\displaystyle T_{c}} +10 Isotherms in the first region will show the feature of subcritical adsorption. Isotherms in the second region will show the feature of mechanism transition. Isotherms in the third region will show the feature of supercritical adsorption. The transition will take a continuous way if the isotherms in both sides of the critical temperature belong to the same type, such as adsorption on microporous activated carbon. However, discontinuous transition could be observed on isotherms in the second region if there is a transformation of isotherm types, such as adsorption on mesoporous silica gel. The decisive factor in such a classification of adsorption is merely temperature, irrespective of pressure. This is because a fluid cannot undergo a transition to a liquid phase at above-critical temperature, regardless of the pressure applied. This fundamental law determines the different adsorption mechanism for the subcritical and supercritical regions. For the subcritical region, the highest equilibrium pressure of adsorption is the saturation pressure P s {\displaystyle P_{s}} of adsorbate. Beyond P s {\displaystyle P_{s}} condensation happens. Adsorbate in the adsorbed phase is largely in liquid state, based on which different adsorption and thermodynamic theories as well as their applications were developed. For supercritical region, condensation cannot happen, no matter how great the pressure is.
Acquisition of supercritical adsorption isotherms An adsorption isotherm depicts the relation between the quantity adsorbate and the bulk phase pressure (or density) at equilibrium for a constant temperature. It is a dataset of specified adsorption equilibrium. Such equilibrium data are required for optimal design of process relying on adsorption and are considered fundamental information for theoretical studies.
Measurement of gas-solid adsorption equilibria
Volumetric method
Volumetric method was used in the early days of adsorption studies by Langmuir, Dubinin and others. It basically comprises a gas expansion process from a storage vessel (reference cell) to an adsorption chamber including adsorbent (adsorption cell) through a controlling valve C, as schematically shown in Figure 1. The reference cell with volume V r e f {\displaystyle V_{ref}} is kept at a constant temperature T r e f {\displaystyle T_{ref}} . The value of V r e f {\displaystyle V_{ref}} includes the volume of the tube between the reference cell and valve C. The adsorption cell is kept at the specified equilibrium temperature T a d {\displaystyle T_{ad}} . The volume of the connecting tube between the adsorption cell and valve is divided into two parts: one part with volume V t {\displaystyle V_{t}} with same temperature as the reference cell. The other part is buried in an atmosphere of temperature T a d {\displaystyle T_{ad}} . Its volume is added to the volume of adsorption cell V a d {\displaystyle V_{ad}} .
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