Cyclic Adsorption Processes Research Articles

Dual mode roll-up effect in multicomponent non-isothermal adsorption processes with multilayered bed packing

Roll-up is a common phenomenon occurring in multicomponent adsorption processes when the concentrations of some components at the outlet of the adsorber exceed their inlet levels. Previous investigations attributed the roll-up effect typically to the displacement of one component by another because of the difference in adsorption affinities or diffusivities. In this work, we report the existence of a peculiar type of roll-up caused by thermal effect solely in our multicomponent non-isothermal breakthrough experiments where the light component CO 2 was swept off by a pure thermal wave developed from water vapour adsorption. Even though, the formation of a pure thermal wave is well known, it does not lead to the above roll-up without a multilayered bed packing. To our surprise, a secondary roll-up of CO 2 caused by H 2O displacement was also observed, since the front of the water concentration wave ran behind its thermal wave in our case. The parameters affecting the formation of this special dual mode roll-up were investigated and summarised. This work will give guidance to the design of multilayered cyclic adsorption processes, e.g. determination of layering ratios and duration of the feed step.

Suitability of Cu-BTC extrudates for propane–propylene separation by adsorption processes

In this work, we investigate the potential of a MOF porous material, Cu-BTC in combination with cyclic adsorption processes (vacuum swing adsorption, VSA, and simulated moving bed, SMB) for the separation of propylene/propane mixture. A gravimetric method has been used to measure the adsorption isotherms of propylene, propane and isobutane onto Cu-BTC extrudates over a temperature range from 323 to 373 K and pressures up to 500 kPa. These were complemented by experimental characterization of the Cu-BTC material using XRD and SEM-EDS techniques. Whereas the separation and its kinetics were studied by breakthrough experiments on a column packed with the Cu-BTC material in the extruded form at 373 K. The Cu-BTC extrudates are selective towards propylene, highlighting the potential of this material for the separation of propane and propylene mixtures by PSA process. Furthermore, in a range of temperatures and pressures, the affinity of Cu-BTC for isobutane was observed to be intermediate to that of propane and propylene which suggest that isobutane is a promising desorbent in SMB processes for propane/propylene separations.

Equilibrium and Fixed Bed Adsorption of 1‐Butene, Propylene and Propane Over 13X Zeolite Pellets

Propylene‐propane separation is one of the most difficult and demanding energetic operation currently practiced using cryogenic distillation. Extensive studies on various alternatives showed that cyclic adsorption processes, and particularly pressure swing adsorption (PSA), might be an option to replace the traditional distillation. In spite of the promising results of the PSA process, much attention is currently being paid to the simulated moving bed technology (SMB) for gas‐phase separations. The ingenious principle of this process is based on the choice of an adequate adsorbent‐desorbent couple. Thus, in the present work 1‐butene has been studied as an interesting desorbent to displace adsorbed propylene‐propane mixture on 13X zeolite. The measurements of pure 1‐butene adsorption isotherms over 13X zeolite were performed with a gravimetric experimental device for pressure ranging from 0 to 110 kPa and at temperature of 333, 353, 373, and 393 K. The experimental adsorption data were correlated using Toth model. The heat of adsorption at zero coverage and the maximum loading capacity of 1‐butene were found to be 54.4 kJ/mol and 2.10 mol/kg, respectively. The adsorption and desorption of 1‐butene on 13X zeolite packed on a fixed bed initially saturated either by a propane‐propylene mixture or a pure C3 hydrocarbon has been studied. The performance of 1‐butene has been compared with isobutane that was previously proposed to be a highly effective desorbent for C3H6/C3H8 separation. A model based on a double LDF approximation for the mass transfer combined to a heterogeneous energy balance taking into account a variable velocity of the gaseous bulk phase, has been used to describe the breakthrough curves obtained experimentally at 373 K and 110 kPa.

Propane/Propylene Separation by Simulated Moving Bed I. Adsorption of Propane, Propylene and Isobutane in Pellets of 13X Zeolite

The separation of propane‐propylene mixture is the most energy consuming operation in the petrochemical industry. Various studies have been investigated to relieve the cryogenic distillation ordinarily used for this separation, and the adsorption technology appeared to be a promising option. Considering the encouraging results obtained by cyclic adsorption processes and notably by pressure swing adsorption, the simulated moving bed (SMB) has been suggested as a new and competitive alternative. The keystone of a SMB for a gas mixture separation is the choice of an adequate and pertinent adsorbent‐desorbent couple. In this work, isobutane has been tested as a potential desorbent over 13X zeolite. A gravimetric method has been used to measure the adsorption equilibrium isotherms of propylene, propane, and isobutane on 13X zeolite pellets over a temperature range from 333 K to 393 K and pressure up to 160 kPa. Experimental adsorption equilibrium isotherms were correlated with the Toth model. The 13X zeolite shows an intermediate loading capacity for isobutane at low pressures. Equilibrium capacities for propylene, propane, and isobutane at 373 K and 110 kPa were 2.12, 1.61, and 1.53 mol/kg, respectively. The heats of adsorption at zero coverage for propylene, propane, and isobutane were found to be 42.4, 36.9, 41.6 kJ/mol, respectively. Breakthrough curves of pure components were measured at 373 K and 150 kPa with different initial conditions (adsorbent bed saturated with nitrogen or isobutane). Experimental breakthrough curves were well‐predicted by an exhaustive mathematical model taking into account the energy balance in the three phases (gas, solid, and wall column). Multi‐component fixed bed adsorption experiments allowed us to observe that isobutane could displace an adsorbed propane/propylene mixture from the 13X zeolite and itself was fairly easily displaced from the adsorbent by this same mixture. These results confirmed the assumption that isobutane is a good desorbent for the adsorptive separation of C3H6/C3H8 mixture by a simulated moving bed.

Equilibrium Analysis of Cyclic Adsorption Processes: CO2 Working Capacities with NaY

The most common application of adsorption is via pressure swing adsorption. In this type of design, the feed and regeneration temperatures are kept approximately equal, whereas the feed pressure is higher than the regeneration pressure. By exploiting the difference in the amount adsorbed at a higher pressure to the amount adsorbed at a lower pressure, a working capacity is realized. Therefore, by examining the expected (ideal) working capacity of an adsorbent, a performance characteristic can be analyzed for a pressure swing adsorption process (PSA). For this work, feed pressures up to 2.0 atm CO2 and feed temperatures from 20°C to 200°C were investigated. These limits were chosen due to the nature of the target process: CO2 removal from flue gas. Carbon dioxide adsorption isotherms were determined in a constant volume system at 23°C, 45°C, 65°C, 104°C, 146°C, and 198°C, for pressures between 0.001 and 2.5 atm CO2 with NaY zeolite. These data were fit with the temperature dependent form of the Toth isotherm. Henry's Law constants and the heat of adsorption at the limit of zero coverage were also determined using the concentration pulse method. Comparison of the Henry's Law constants derived from the Toth isotherm, and those obtained with the concentration pulse method provided excellent agreement. By using the Toth isotherm, expected working capacity contour plots were constructed for PSA (Pressure Swing Adsorption), TSA (Temperature Swing Adsorption), and PTSA (Pressure Temperature Swing Adsorption) cycles. The largest expected working capacities were obtained when the bed was operated under a high‐pressure gradient PSA cycle, or a high thermal and pressure gradient PTSA cycle. The results also showed that certain TSA and PSA cycle conditions would result with higher expected working capacities as the feed temperature increases.

Optimization of Pressure Swing Adsorption and Fractionated Vacuum Pressure Swing Adsorption Processes for CO2 Capture

This work focuses on the optimization of cyclic adsorption processes to improve the performance of CO2 capture from flue gas, consisting of nitrogen and carbon dioxide. The adopted processes are the PSA (pressure swing adsorption) process and the FVPSA (fractionated vacuum pressure swing adsorption) process, modified from the FVSA (fractionated vacuum swing adsorption) process developed by Air Products and Chemicals, Inc. The system models are currently bench scale and adopt zeolite13X as an adsorbent. The high-temperature PSA is better for high purity of product (CO2), and the high-temperature FVPSA is much better than the normal-temperature PSA processes. The main goal of this study is to improve the purity and recovery of carbon dioxide. The Langmuir isotherm parameters were calculated from experimental data taken at National Energy Technology Laboratory (Siriwardane, R.; NETL, DOE, 2004). Moreover, efficient optimization strategies are essential to compare these processes. To perform optimization work...

Pressure Drop in a Packed Bed under Nonadsorbing and Adsorbing Conditions

Simulation of cyclic adsorption processes for gas separations relies on accurate models of the bed pressure drop during dynamic pressurization, depressurization, and breakthrough steps. This is especially true for rapid pressure swing adsorption (RPSA), where large gas flows can cause significant changes in the axial pressure gradient through the bed, influencing process performance. Under these conditions, there is some question as to the validity of conventional steady-state models used to represent the dynamic, rapidly changing pressures. In this study, the ability of the steady-state Ergun equation to represent pressure drop under dynamic adsorbing and nonadsorbing conditions was tested. The Ergun equation accurately reproduced dynamic depressurization and breakthrough pressure profiles using values of κviscous and κkinetic (the two Ergun parameters) determined experimentally from the steady-state flow of a variety of gases through a packed bed of LiLSX pellets. The full momentum equation was also tested, and errors of <0.1% were observed between the Ergun equation and the full momentum balance under dynamic conditions for a nonadsorbing packed bed. These observations and simulations suggest the Ergun equation can be reliably used to reproduce experimental profiles under dynamic adsorbing conditions where high gas flows result.

Further Validation of the Quartic Concentration Profile Approximation for Describing Intraparticle Transport in Cyclic Adsorption Processes

Models developed previously by the authors that describe nonlinear adsorption, and simultaneous pore and surface diffusion in a single particle, that are based on intraparticle quartic and parabolic concentration profile approximations, and that utilize the summation of the gas and adsorbed phases approach in the material balance formulations, were further validated under more diverse, yet more realistic, cycling conditions. Periodic square, sinusoidal and triangular wave functions were used to more accurately represent the periodic boundary conditions that the external surface of an adsorbent particle may be exposed to during repeated adsorption and desorption cycles in a fixed bed adsorber. Analytical solutions that describe the periodic uptake and release of the adsorbate by the adsorbent were obtained for all three periodic wave functions, and for both the quartic and parabolic profile approximations. By comparing the predictions obtained from both models with the exact numerical solution, the superiority of the quartic model over the parabolic model was clearly demonstrated for all wave functions, and for a wide range of adsorbate-adsorbent systems and bulk concentrations. Excellent agreement between the quartic and exact models was obtained in most cases. In general, the predictions improved as the wave function changed more gradually with time (triangular more gradual than sinusoidal and sinusoidal more gradual than square), as the degree of mathematical linearity of the adsorbate-adsorbent system increased, and as the maximum external surface concentration decreased (an isotherm nonlinearity effect). Subtle differences in the predictive ability of the new approximate models, stemming from the use of the different wave functions, were exposed. Overall, these results exemplify the importance of comparing the predictive ability of new approximate models that describe intraparticle transport under more diverse cycling conditions than are typically utilized in the literature, which has been dominated by the square wave function.

A New Numerical Method for Accurate Simulation of Fast Cyclic Adsorption Processes

In the simulation of fast cyclic adsorption processes, to apply the Fickian diffusion model it is necessary to include an increasing number of numerical discretization points as the cycle time is reduced in comparison to the characteristic diffusional time constant. We propose a new numerical method based on the definition of two distinct regions within an adsorbent particle: an outer layer where the concentration varies significantly with large internal gradients leading to enhanced mass fluxes, and an internal region where the concentration profile is virtually flat. The proposed method leads to the automated generation of a numerical grid that has a constant number of elements independent of the process cycle time. The procedure is demonstrated on a model for the simulation of a heatless dryer pressure swing adsorption process.

Cyclic adsorption separation processes: analysis strategy and optimization procedure

An innovative analysis strategy and an optimization procedure have been developed with the purpose of design, evaluation and optimization of small- and large-scale units of cyclic adsorption processes using the classical Skarstrom's cycle: pressure swing adsorption (PSA) and vacuum swing adsorption (VSA). The system of partial differential equations of the dynamic simulator model was solved using a recent numerical technique developed within our group, based on an adaptive multiresolution approach, thus ensuring stability and accuracy. The simulator provides models for the multiple phenomena involved in fixed-bed adsorption: pressure drop, mass transfer resistance and energy balance. An extended parametric analysis is presented for the particular case of oxygen production from air by PSA and VSA: influence of the normalized purge flow rate, the high and low pressure values, dimensionless pressurization time, dimensionless production time, pressure drop and temperature in the bed.

Use of Concentration Pulse Chromatography for Determining Binary Isotherms: Comparison with Statically Determined Binary Isotherms

The characterization of adsorption properties of binary gas mixtures is an important factor in the design of cyclic periodic adsorption processes. Presently, there are a few methods for determining the binary adsorption behaviour. However, most of these methods are very time demanding (static approaches). One dynamic approach for determining the binary adsorption behaviour is by employing the concentration pulse method. Although this experimental method has been shown to reliably reproduce the data obtained with static approaches, the application has been limited to systems where the adsorption capacities are similar. In our previous work, a novel method (HT-CPM) was shown to be capable of handling systems where the adsorption capacity of the heavy component is much greater than the light component.

Multiobjective Optimization of Cyclic Adsorption Processes

This paper focuses on the development of a multiobjective optimization approach and the first application to highly nonlinear cyclic adsorption processes such as thermal swing adsorption (TSA) and rapid pressure swing adsorption (RPSA). The new method was modified from the conventional summation of a weighted objective function (SWOF) method, which is one of the most traditional and the simplest way of multiobjective optimization programming. The new modified SWOF technique eliminates the main drawbacks of the conventional SWOF and approximates the Pareto curve efficiently with several Pareto points as explained by the two examples (TSA and RPSA). The sensitivity of these processes was also analyzed.

A General Package for the Simulation of Cyclic Adsorption Processes

A general purpose package for simulation of fixed-bed and cyclic adsorption processes (PSA/VSA and TSA) has been developed. The package allows various models options depending on combinations of conservation equations: Equilibrium model, Macropore or Micropore model (LDF model), Bidisperse model (double LDF model). The fluid flow follows Ergun's equation locally and the operation of the column can be isothermal, adiabatic or non-isothermal, non-adiabatic. Two important industrial separation processes are considered: the propylene/propane and the n/iso paraffins systems. A three-step TSA and a four-step VSA are considered for propylene/propane mixture while a four-step PSA and two-step adsorption/purge-desorption processes are applied to the n/iso paraffins separation.

Adsorber dynamics and optimal design of layered beds for multicomponent gas adsorption

In order to minimize the bed size and to maximize the utilization of sorbents, layered beds containing different sorbents are being increasingly used for multicomponent separation by cyclic adsorption processes. Adsorber dynamics for multicomponent adsorption in layered beds is studied experimentally and also by numerical simulation. Adsorption in beds layered with activated carbon followed by 5A zeolite for hydrogen separation from a typical cracked gas mixture (H 2/CO 2/CH 4/CO) is used as the case study. While a single rollup in each breakthrough curve is common for the pure sorbent beds, double and multiple rollups are common with layered beds. Both equilibrium and kinetic rollups are observed and their origins discussed. Under the conditions of this study, combined thermal and concentration waves prevail for all components. The wave propagation velocity undergoes a step change upon crossing the interface between two sorbents, and such a change can be either increasing or decreasing. For this reason, transverse waves are seen in layered beds, All experimental features have been predicted by the model. It is demonstrated how the model can be used to provide optimal design of layered beds, based on the criterion that simultaneous breakthrough takes place in the layers, i.e. for the strong component (CO 2) in the weaker sorbent (carbon) and the weaker component (CO or CH 4) in the strong sorbent (zeolite). The optimal layering for a given gas–solid system depends on the feed composition and feed velocity.

Linear driving force approximation in cyclic adsorption processes: Simple results from system dynamics based on frequency response analysis

The linear driving force (LDF) approximation for cyclic adsorptive processes is discussed on the basis of model equivalence with the homogeneous diffusion model (HDM), the pore diffusion model (PDM) and the intraparticle diffusion and convection model (IDCM). Model equivalence is based on the frequency response of the adsorbent particle, namely on the equality of the amplitude ratio and the phase-lag functions. The analysis of the continuous stirred tank adsorber (CSTA) and of the plug flow adsorber (PFA) is addressed.

Application of new rate models to cyclic adsorption in adsorbents

In a recent paper, (Yao and Tien, Chinese J. Chem. Engng., 1998, 6, 1-11), we presented new models for evaluating the adsorption rate in an adsorbent pellet (which may be a slab, a cylinder or a sphere) and demonstrated their use in batch adsorption calculations. It was shown that both the modified shell-core (MSC) model and the general driving force (GDF) model are much more accurate than the linear driving force (LDF) model. In this paper, we apply the new rate models to a cyclic adsorption problem and thus illustrate their utility in modeling cyclic adsorption processes.

Modification of Resin-Type Adsorbents for Ethane/Ethylene Separation

Modified adsorbents, Ag+-exchanged resins, have been prepared and studied for ethane/ethylene separation by adsorption. On Ag+-exchanged Amberlyst 35 (36.5% exchange) at 25 °C and 1 atm, the equilibrium adsorbed amount for C2H4 is 1.48 mmol/g, and the equilibrium adsorption ratio for C2H4/C2H6 is 6.4. The adsorption capacity is completely restored at 100−105 °C, although small residual amounts exist after desorption at 25 °C and 75 °C. For the adsorption encompassing both physical adsorption and π-complexation with energy heterogeneity, the equilibrium data are correlated with an equilibrium isotherm equation employing two fitting parameters. The fitted results agree well with the experimental data. Furthermore, the isosteric heats of adsorption and the diffusion time constants are calculated from experimental data. Considering all adsorption characteristics, these adsorbents show potential for application employing cyclic adsorption processes.

Plasma Exhaust Purification by Thermal Swing Adsorption: Experimental Results and Modeling

For several years at the Joint Research Centre-Ispra laboratories, cyclic adsorption processes have been developed for the purification of the plasma exhaust stream of a deuterium-tritium fusion reactor. A purification process consisting of two coupled thermal swing adsorption systems seemed to be the most convenient process. In this context, a screening study was carried out to select the most suitable adsorbent materials and appropriate working temperatures. This was mainly done by experimental measurements of adsorption isotherms of the single components of the plasma exhaust stream and by a careful evaluation of the multicomponent adsorption equilibria. Experiments on adsorption dynamics were carried out in a pilot plant to demonstrate the feasibility and to evaluate the performance of the process. The experimental apparatus was designed to treat gas mixture flow rates up to 20 to 30 standard temperature and pressure l/h. A mathematical model was developed and tested against the experimental results to describe the adsorption process and, in particular, to evaluate and to optimize the process cycle time. 27 refs., 4 figs., 9 tabs.

New sorbents for olefin/paraffin separations by adsorption via π ‐complexation

AbstractNew adsorbents for olefin/paraffin separation are synthesized by effective dispersion of Ag(I) and Cu(I) cations on substrates with hydrocarbon‐phobic surfaces. These cations bind olefin molecules by a π‐complexation bond, a weak chemical bond. Ethane/ethylene and propane/propylene separations are considered. Cation exchange resins and CuCl/γ‐Al2O3 are effective substrates. On the Ag(I) resin at 25°C and 1 atm, the equilibrium adsorption ratio for C2H4/C2H6 = 9.2 and C2H4 capacity = 1.15 mmol/g; the corresponding values for C3H6/C3H8 = 10.4 and C3H6 capacity = 1.29 mmol/g. The CuCl/γ‐Al2O3 sorbent shows equally promising results. The sorption rates are pore‐diffusion‐controlled and rapid. The olefin selectivity; capacity, and rates are much higher than all previous attempts and are suitable for applications in cyclic adsorption processes.The equilibrium data are correlated with an isotherm equation that accounts for both physical adsorption and π‐complexation with energy heterogeneity, using only two true fitting parameters. Molecular orbital calculations using a C6H5SO substrate indicate that the π‐complexation bond is contributed mainly by the donation of olefin π‐bond electrons to the empty s‐orbital of the metal, while the d‐π* back donation contributes only 16%. Moreover, the relative order of the heats of adsorption is correctly predicted.

Sorbent materials for fusion reactor tritium processing

Abstract A fusion reactor (such as NET/ITER) which breeds its own tritium fuel requires tritium recovery, purification and separation from the other isotopes. Cyclic adsorption processes are strong candidates for several of the processes involved: amongst other advantages, they promise a low tritium inventory. A good adsorbent for such processes must have high adsorption capacity, high selectivity and very low tritium retention after each cycle. Pure zeolite powder is shown to have an excellent combination of these three properties. However, in practice problems can arise from tritium which is not removed by reactivation. In this paper we show that tritium retention in zeolites can be caused either by water retained in the zeolite structure, which can be removed by ore rigorous activation, or by water tapped on binders in commercial pellets.