Designing a simple volumetric apparatus for measuring gas adsorption equilibria and kinetics of sorption. Application and validation for CO₂, CH₄ and N₂ adsorption in binder-free beads of 4A zeolite

Highlights

• A volumetric unit was developed for measuring gas adsorption equilibria at mg scale.

• The unit measures the kinetics of sorption by a piezometric method.

• A simple LDF model has been developed to model uptake rate data.

• The statistical analysis of the adsorption equilibria data was performed by RSM.

Abstract

The screening of adsorbents (zeolites, MOFs, ACs, etc) requires the measurement of adsorption equilibria and kinetics at the milligram scale. In this regard, a volumetric apparatus (constant volume variable pressure – batch adsorber) has been developed for studying adsorption equilibria and kinetics of sorption. Its validation was accomplished by studying the adsorption equilibria of carbon dioxide (CO₂), methane (CH₄) and nitrogen (N₂) and the kinetics of sorption of CO₂ on commercial binder-free 4A zeolite. The data collected has an acceptable agreement with already published values by a gravimetric and breakthrough flow technique. The isotherms were modeled using Sips model from low pressure till 8 bar at 303, 343 and 373 K. The sorption kinetics of CO₂ was measured from the uptake rate experiments and fitted with a solid-film linear-driving-force model (LDF). It was observed that the LDF mass transfer coefficients increase with pressure and temperature. Finally, the statistical analysis of the data was performed by Response Surface Methodology (RSM) to determine the interactions among process variables such as temperature and pressure in the respective gas adsorption equilibria data.

Introduction

Nowadays, one of the world challenging issues is the global climate change and rising level of greenhouse gases (GHGs) in the atmosphere, originating from anthropogenic activities [1], [2]. This environmental concern deriving from the combustion of coal, petroleum and petrochemical industries [3], [4] is expected to be intensified as consequence of economic growth and industrial development [5], [6], [7]. In this context, carbon capture and storage technology has been introduced as a promising policy to reduce the GHGs emissions as well as decrease the mitigation costs of relevant industries [8]. On the other hand, due to the limitations of fossil fuels resources, attempts have been intensified to develop renewable sources of energy such as biogas [7], [8]. To this end, numerous researches are devoted to find out highly efficient processes for trapping and separation of GHGs. Among different considered technologies [1], adsorption onto porous solids was introduced as one of the attractive strategies [9]. In this way, gravimetric and volumetric techniques were introduced as main techniques, which have been mainly employed for gas storage and separation studies. In the gravimetric technique, as a clear and direct method, the loading capacity is calculated by measuring the mass of sample with a magnetic suspension balance before and after experimental tests [10]. In spite of it’s accuracy, this method has some drawbacks including: considering the buoyancy effect on the volume of system, lack of flexibility for multicomponent studies and high cost of the unit [11]. In return, in the volumetric technique, the uptake capacity of samples is simply calculated using the pressure drop. Checchetto et al. [10] reported that the accuracy of volumetric technique is higher than the gravimetric one since recording pressure changes is easier than measuring minor variations in the mass of sample.

The Development of the volumetric technique for measuring gas adsorption equilibria is date back to the early of 20th century, by Sieverts, which considered a glass volumetric apparatus for absorption and diffusion of gases [11]. This early volumetric apparatus passed it’s evolution and was improved by pressure measurement, temperature control and performance elevation for different gas adsorption processes [12], [13]. Conventional volumetric units are generally required gram-scale quantities of adsorbents [14], [15], which by considering the required time to synthesis, cost and loading capacity of novel samples (particularly in the cases of MOFs and COFs) is one of the main drawbacks of developed apparatus. On the other hand, available commercial instruments to measure isotherms volumetrically in the milligram scale have also some restrictions [16]. Generally, these type of units dońt have too much flexibility regarding studies in different adsorption systems including: 1) pressure and temperature ranges (e.g. for cryogenic studies), 2) type of adsorbents (powder, beads, pellets, 3D printed carbons, monoliths, etc.), 3) the size and the ratio of adsorption cell to the reference cell. At the same time, supplying commercial units require a high budget than home-made ones.

In the volumetric unit, the critical overview is the description of the setting up of the process and specifying the accurate volumes, including: reference volume, adsorption volume and dead volume, to acquire precise gas adsorption results [17], [18], [19]. Nevertheless, few studies have been devoted to accurate description of designing procedure and volumetric calibration as main aspects of assessment of adsorbents screening and isotherms evaluation [18], [19], [20]. In some cases, researchers [19], [20] utilized the liquid technique (toluene or water) for calibrating the reference cell. This technique encompasses some problems such as using liquid to fill the reference cell is not appropriate and precise; also, this technique is offline method, which limits the capacity of this method [17]. On the other hand, detailed analyses of developed volumetric units and the calculation procedure of the absolute mass of adsorbed gases onto the adsorbents were not also well documented in the literature, while they are key elements to acquire reproductive data [21], [22], [23].

In this study, based on scope of CCS technology, a volumetric apparatus was designed, constructed and calibrated to assess the gas adsorption equilibria measurements in the milligram scale from low pressure till high pressure in a broad temperature range, with the particularity of using a circulating gas in a closed loop, to extend the measurement of data for multicomponent systems. In this way, the basic principles related with its construction, calibration procedure and data acquiring are highlighted. The unit has been developed and validated by measuring adsorption equilibria isotherms of carbon dioxide (CO₂), methane (CH₄) and nitrogen (N₂) on 4A binder-free zeolite, while other gases including hydrogen (H₂) or different adsorbents such as metal organic frameworks (MOFs), activated carbons (ACs) and etc. can also be considered in future works. Finally, uptake rate measurements were also recorded and a solid-film linear-driving-force batch adsorber model developed to calculate mass transfer coefficients related with the kinetics of sorption, and the statistical analysis of the adsorption equilibria data performed by Response Surface Methodology (RSM) strategy.