Characterizing Adsorbents for Gas Separations

Reprinted with permission from Chemical Engineering Progress (CEP), March 2018. Copyright © 2018 American Institute of Chemical Engineers (AIChE). Reactions and Separations Characterizing Adsorbents for Gas Separations Darren Broom Pressure-swing adsorption (PSA) and Hiden Isochema Ltd. temperature-swing adsorption (TSA) separate a specific gas species from a mixture of gases. This article explains how to evaluate the performance of an adsorbent for a given separation. eparations account for a significant proportion of required for gas separations and the laboratory techniques worldwide energy consumption (1). Energy-intensive used to obtain this information. The article also discusses Sdistillation dominates the chemical process industries methods for assessing multicomponent adsorption and iden- (CPI), but more-efficient alternatives, such as membrane tifies future challenges in the field. technology and adsorption by porous materials, are also in widespread use. Pressure-swing adsorption (PSA) and Working capacity and isotherm shape temperature-swing adsorption (TSA) are two common The performance of an adsorbent for a particular separa- gas separation processes. In a PSA process, adsorbents are tion depends on several factors. One of the most important regenerated by reducing pressure; in a TSA process, they are is its working capacity. For PSA, it is the difference between regenerated by applying heat (2–4). the uptake at the feed pressure and the uptake at the regen- Adsorbent performance as a function of temperature, eration pressure. For TSA, it is the difference between the pressure, and gas composition is a crucial aspect of PSA and uptakes at the feed temperature and the regeneration tem- TSA. To determine whether an adsorbent will be appropri- perature at the working pressure (Figure 1). ate for a specific application, you must assess its working For any given adsorbent, working capacity depends capacity, isotherm shape, selectivity, heat of adsorption, and partly on isotherm shape. For PSA, for example, the steeper sorption kinetics. Such data can be obtained using laboratory the isotherm in the operating pressure range at the process gas adsorption measurement techniques, prior to testing and temperature, the greater the working capacity (for an adsor- optimizing process designs in a pilot plant. bent with a given saturation uptake). Isotherms measured at Two previous articles, “Adsorption Basics: Part 1” (5) different temperatures, however, are also required to accu- and “Adsorption Basics: Part 2” (6) introduced the funda- rately and realistically simulate a process (4). mentals of adsorption, including equilibrium and mass- At the temperatures and pressures typically used for gas transfer considerations, the mass-transfer zone concept, separation, isotherms for most nanoporous adsorbents follow and column sizing, and discussed the most widely used the trend shown by the Langmuir equation. These are known adsorbents. as favorable isotherms and are concave to the pressure axis This article discusses how to characterize adsorbents (5), as shown in Figure 1. Loading as a function of pressure, in the lab. It covers the performance parameters and data however, often increases more gradually than described 30 www.aiche.org/cep March 2018 CEP by the Langmuir equation (the Langmuir model assumes be required for adsorbent regeneration. adsorption occurs on a homogeneous surface, which is Heats of adsorption can be defined in different ways (8), physically unrealistic for most adsorbents). When this is the but the most practically useful is the isosteric heat, qi (4). To case, other models, such as the Tóth and Sips (or Langmuir- calculate isosteric heat, measure isotherms at two or more Freundlich) equations, which describe heterogeneous temperatures (that are close in value) and use the following adsorption, are more appropriate (4). (Clausius-Clapeyron-type) relationship: Selectivity Working capacity and isotherm shape are basic proper- ties that define a material’s ability to adsorb gases. For sepa- where R is the universal gas constant, T is temperature, P is ration, however, selectivity is more important. Selectivity pressure, and ni is the uptake. can be defined in different ways depending on the separation To calculate qi, plot the natural logs of the pressures for mechanism, of which there are two general types — equilib- isotherm points at constant uptake against 1/T. A straight- rium and kinetic. Most commercial separation processes are line fit through these data has a slope of qi/R. Using this equilibrium-based, although the recovery of nitrogen from process, qi can be calculated for different values of ni. air using carbon molecular sieves, for example, is a kinetics- For pure gases, qi typically decreases with loading, as the based separation (4, 7). most strongly interacting sites or pores are occupied first. Equilibrium selectivity, Seq, is usually defined as: Increases at higher loading can occur due to adsorbate- adsorbate interactions, which become more significant at higher pressures and gas densities. (At lower loadings, adsorbate-adsorbent interactions dominate because the gas where n1 and n2 are the molar loadings of Species 1 and 2 at molecules are mostly interacting with the surface, rather partial pressures of p1 and p2, respectively, under the process than each other.) However, it is worth noting that calculated conditions. values of qi can be quite sensitive to the method used, so Kinetic selectivity, Skin, meanwhile, can be expressed as: some observed changes may simply be a result of the fitting process (9). Sorption kinetics where K1 and K2 are the Henry’s law constants and D1 and Equilibrium uptakes determined from isotherms are D2 are the diffusivities of Species 1 and 2, respectively (7). required to define working capacities and isotherm shape, Equilibrium selectivity therefore depends on the relative and to calculate equilibrium selectivities and heats of equilibrium quantities of each component adsorbed under adsorption; however, the kinetics are also crucial. Process the process conditions, whereas kinetic selectivity depends simulations require kinetic data, regardless of the separation on differences in diffusion rates. Both types of selectivity can be calculated from data obtained using the lab tech- n T niques described later in this article. ads ads Heat of adsorption e Swing Temperature Swing The heat of adsorption, usually expressed in kJ/mol, indi- Pressur cates the strength of interaction between the adsorbate and the adsorbent. Purely physical adsorption, or physisorption, T ndes des has a low heat of adsorption, whereas chemical adsorption, or chemisorption, has a higher heat of adsorption (2). The heat of adsorption defines the temperature and Amount of Gas Adsorbed, moles pressure at which adsorption occurs, but a higher value also means that more heat will be generated during the adsorp- P P des Pressure ads tion process. Particularly large temperature increases in adsorbent beds are possible, which can lower the transient p Figure 1. An adsorbent’s working capacity depends on the shape of or dynamic bed capacity, because the amount of adsorbate its isotherm. This illustration shows the pressure-swing and temperature- swing cycles in PSA and TSA (2, 3), where n is the equilibrium amount adsorbed decreases with increasing temperature. And, adsorbed and P is pressure, and the working capacity in each case is given higher heats of adsorption can adversely affect process by nads–ndes. Lowering Pdes in PSA increases nads–ndes, as does increasing efficiency, particularly for TSA, because more energy will Tdes in TSA. CEP March 2018 www.aiche.org/cep 31 Reactions and Separations mechanism. Kinetic data are also needed to calculate and the chemistry of the adsorbent (2). For example, CO2 kinetic selectivities. typically adsorbs more strongly than other gases such as H2, Although simple in principle, detailed analysis of sorp- N2, and CH4, because it has a larger quadrupole moment, tion kinetics and diffusion is a complex topic (2, 9, 10). whereas He interacts more weakly than H2, N2, and CH4, However, for engineering purposes, the kinetics can be because it has no dipole or quadrupole moment and it has approximated because the need for mathematical simplicity a relatively low polarizability. Polar surfaces also interact is often more important than having an accurate description more strongly with polar adsorbates. of the microscopic processes occurring in a full column. For Pore size also affects the kinetics. Particularly narrow simulations, kinetics are most commonly described using pores can inhibit diffusion, whereas wider pores permit the linear driving force (LDF) model (2, 4, 11). This simple more rapid mass transfer, although diffusion rates may approach is often sufficient for process simulation, because decrease if the adsorbate binds strongly to the pore walls. the differences in the kinetic curves for different models that In larger pores, surface diffusion — the hopping of adsor- describe diffusion at a microscopic level can be lost during bate molecules from one adsorption site to another — can the integration processes required to describe the perfor- occur alongside Knudsen diffusion (i.e., the diffusion that mance of a full column (11). occurs in the free molecule regime, where collisions with the pore walls are more likely, or occur more frequently, than The importance of adsorbent intermolecular collisions or interactions) at lower pressures. and

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