3P INSTRUMENTS offers two general ways to obtain information about mixed gas adsorption processes; first with the measurement of the dynamic sorption process with the mixSorb L (3P INSTRUMENTS) and second by use of the 3P sim software of the mixSorb L to calculate mixed gas data using pure gas isotherms. The mixSorb L offers the tools not only to measure the mixed gas isotherms in practice, but also to compare the results with various theories to include specific effects of the adsorption processes on the porous materials
investigated. The 3P sim software provides the ability to predict mixture equilibrium data by well-established models (Table 1). The 3P sim simulation software also provides the tools to estimate parameters for mixed gas separation processes such as the saturation capacity, the affinity constant, and the Sips exponent as well as the temperature dependencies of each.
Table 1. Models to Predict Mixture Equilibrium Data from Pure Component Isotherms Provided by 3Psim
Isotherm modelMaximal number of components provided by 3PsimRemarksIAST with Langmuir 4Solved by an iterative method for root determinationIAST with Toth 4Solved by Newton’s methodMulti component Langmuir (Markham and Benton) 4Simple, analytical solutionMulti-Component Sips 4Simplified equation, Simple, analytical
For this study, gas adsorption isotherm data from other sources were used in combination with isotherms measured at different temperatures with a volumetric analyzer shown in Figure 2 for methane and Figure 3 for carbon dioxide.
The isotherm parameters listed in Table 2 were determined from the equilibrium data by use of the 3P-Sim software package.
By applying these isotherm parameters and calculations for non-isothermal conditions, the breakthrough behavior can be described with the 3P sim software package as seen in Figure 4.
From the fitting of the measured breakthrough curves, a linear driving force constant (LDF-value ) was determined to be 12 per minute for methane and 9 per minute for carbon dioxide for the activated carbon A under experimental conditions. Due to the similar LDF values for methane and carbon dioxide, it can be concluded that the measured separation effect is based on the different sorption capacities of both adsorptives and that there are no relevant steric kinetic separation effects on this material.
Table 2 SIPS-parameter calculated from isotherm (evaluation according to documentation in mixSorb L manual)
Isotherm parameterValues for CH4Values for CO2Saturation capacity at 20 °C8.561 mmol g-118.353 mmol g-1Affinity constant at 20 °C0.09386 bar-10,0862 bar-1SIPS exponent at 20 °C0.796140.7874Temperature dependeny of affinity constant10.7 kJ/mol15.7 kJ molTemperature dependeny of saturation capacity1.1811.228Temperature dependeny of exponent0.48240.0642
Separation effects can be determined by measurement of the breakthrough curves of gas mixtures in a carrier gas using the mixSorb L. With the combination of the pure gas isotherms and their corresponding breakthrough curves along with data fitting using a dynamic model, LDF-constants can be estimated for further parametric studies to predict the source of the separation effect. It is possible to investigate adsorbents and their separation performance under real-world conditions using the dynamic method under lab conditions. Similar LDF-values from the 3P sim calculation shows that a separation effect is based on the difference in sorption capacities of both adsorptives and not on steric-kinetic separation effects.