Thermal Swing Adsorption Process Research Articles

Electrical swing adsorption on 3D-printed activated carbon monoliths for CO2 capture from biogas

Direct, resistive or Joule heating of the adsorbent allows for the intensification of Thermal Swing Adsorption processes by reducing process cycle time and reducing heat losses. In this work, an electrically conductive activated carbon monolith was 3D-printed using a potassium silicate binder. The monolith was assessed for biogas upgrading, using Joule heating for regeneration of the monolith. The electrification of the 3D-printed monolith resulted in very rapid and homogenous heating, to 150 °C in less than 30 s at a potential of 8 V. Next, the monolith was subjected to cyclic adsorption-desorption experiments for CO2 capture from a synthetic biogas mixture (30 v% CO2 – 70 v% CH4). The main parameters in ESA, namely voltage, electrification time, purge conditions and an additional rinsing step were investigated by determining the impact on the heating, regeneration efficiency and purity. Increasing the voltage from 4 V to 8 V step is preferred for recovery (82–95%) as well as for energy consumption, as it increases the efficiency (27.1–28.4%). It was also found that using the same amount of purge gas, it is preferred to use a higher purge flow rate for a shorter time (100 Nml/min for 15 s) over a lower purge flow rate for a longer time (50 Nml/min for 30 s). This benefits both recovery (87.3% vs 84.0%) and purity (84.5% vs 83.4%). Lastly, the benefit of adding a rinse step to increase the purity was demonstrated, where having a rinse step prior to electrification allowed to augment the total purity from 58% to 81.2%, while the regeneration of the monolith was as efficient.

Self-regenerative rapid thermal swing adsorption process using structured fiber sorbents for energy-efficient H2O removal

Rapid thermal swing adsorption (RTSA) using hollow fiber sorbents (HFSs) is an emerging separation process that enables significant energy savings. Herein, we propose a self-regenerative RTSA process based on multi-modules of zeolite-13X-impregnated HFSs for energy-efficient H2O removal in various chemical processes. The proposed dehumidification process consumes almost 82% lower energy than the conventional adsorption counterpart for H2O removal because it uses a combination of high heat transfer efficiency and self-regeneration. The steam recovered from the desorption process can be combined with the thermal fluid for H2O desorption from the HFSs, which is referred to as the self-regenerative RTSA process. In addition, the residual sensible heat emitted from the desorption process can be internally integrated into the heating process, which reduces both the sensible heat of the HFSs and desorption heat of H2O. Consequently, the energy evaluation revealed that the newly proposed self-regenerative RTSA process required a total energy of ca. 0.68 GJ t-H2O-1 for H2O adsorption with a heat integration efficiency of 80%. The current work successfully demonstrates an extended application of the RTSA process for dehumidification.

Induction heating as an alternative electrified heating method for carbon capture process

• Two ferromagnetic sorbents were produced by using commercially available ingredients. • The ferromagnetic sorbents generated a maximum of 150 W heat per gram of Fe 3 O 4 . • The CO 2 desorption under induction heating was done in nearly two minutes. • The ferromagnetic sorbents desorb 34 mg/g min CO 2 . Electrification of chemical processes is one of the approaches designated to reach carbon neutrality in Europe by 2050. To take steps towards this goal, replacing traditional methods of heating with more advanced electrified methods is under investigation. In the present study, the fast thermal regeneration of composite sorbents was investigated under induction heating, as an alternative to traditional heating in Thermal Swing Adsorption processes. Extrudates of composite sorbents containing 13X zeolite and Fe 3 O 4 as the main ingredients were prepared and subjected to heating quantification, equilibrium measurement, and dynamic ad/desorption experiments. From the heating quantification, it was deduced that the electric current in the induction coil has a direct impact on the specific rate of heat absorption with the maximum heat-absorption rate of 150 W per unit gram of Fe 3 O 4 at 171 A. The equilibrium measurement of CO 2 and N 2 adsorption proved that the mass adsorption capacity of the hybrid material decreases as the content of Fe 3 O 4 increases; however, due to the dense nature of Fe 3 O 4 , the content of 13X per unit volume of the sorbent with 20 wt% Fe 3 O 4 (13X-IO20) is only 2% less than that of a sorbent with 10 wt% Fe 3 O 4 (13X-IO10). Under the same electric current in the coil, the net amount of energy that the 13X-IO20 can absorb is almost twice as big as the one of 13X-IO10, while the maximum desorption rate of the 13X-IO20 was almost three times bigger than that of the 13X-IO10. By comparing the measured desorption rates with reported data in literature, it was found that at high coil currents the desorption rate is one order higher than the reported values.

Carbon dioxide recovery from a simulated dry exhaust gas by an internally heated and cooled temperature swing adsorption packed with a typical hydrophobic adsorbent

• Internally heated / cooled temperature swing adsorption was tested for CO 2 capture. • Two typical hydrophobic adsorbents were employed for wet emission gas processing. • The separation performance of High Silica Zeolite was explored. • The unique separation behavior of Carbon Molecular Sieves was examined. This study focused on the carbon dioxide (CO 2 ) separation and recovery by an internally heated and cooled thermal swing adsorption process. As part of the preliminary investigation, the CO 2 capture and enrichment performance of carbon molecular sieve (CMS), a typical hydrophobic adsorbent, and high silica zeolite (HSZ), which indicates hydrophobic behavior among zeolites, were examined in a simulated dry exhaust gas. In addition, the hydroscopic adsorbent, CaA type zeolite, was employed for comparison. The effects of the regeneration temperature and the regeneration air flow rate on the separation performance were primarily investigated. In general, the CaA type of zeolite indicated a greater CO 2 concentration performance and recovery ratio due to its relatively larger adsorption capacity. Comparison between HSZ and CMS showed that the CO 2 separation and recovery performance of HSZ were greater than that of CMS. A CO 2 concentration four times higher than that of the feed gas was obtained at a regeneration temperature of 80 °C. Moreover, further enrichment of CO 2 at the desorption outlet was achieved by reducing the regeneration air flow rate to 1/40 of the feed gas of the adsorption process in exchange for lowering the CO 2 recovery ratio. In contrast, CMS could only produce three times greater the concentration of enriched CO 2 when compared to the feed gas. The enriched CO 2 concentration was almost independent of the regeneration air flow rate. This behavior was due to a lower CO 2 adsorption selectivity of CMS; this interpretation was supported by time profiles of desorption gas outlet volume and the CO 2 concentration.

Sorbent-based oxygen separation with YBC114 for energy storage systems

In this work, we aimed to design, build, and evaluate an oxygen separation system to provide an inert sweep gas with low oxygen partial pressure (pO2) to redox-active thermochemical energy conversion reactors for a range of applications, including two-step redox cycles for thermochemical energy storage, water splitting, and carbon-dioxide splitting. The separation is based on an oxygen-selective sorbent, YBaCo4O7+δ (YBC114), which has excellent oxygen sorption and desorption properties demonstrated in our previous work. The oxygen separation performance of YBC114 was comprehensively studied by thermogravimetric analysis, sorption breakthrough experiments, and temperature swing sorption - desorption cycles. The results reveal that YBC114 can produce inert sweep gas with an oxygen concentration of less than 100 ppmv for at least 20 min during the thermal swing adsorption (TSA) cycle with the current sorption bed configuration, and the performance is consistent from cycle to cycle. The optimal sorption and desorption temperatures for the TSA process with YBC114 are determined to be 300 °C and 500 °C, respectively. Although challenges remain for the current separation system (e.g., high sorption temperature and slow kinetics), this study demonstrates the potential to use the oxygen-selective sorbent to produce an inert sweep gas, the feasibility of the oxygen separation concept, and guides new sorbent material development to make this application economically practical. A simple procedure is described for designing the YBC114 oxygen separation process.

Rapid Cycling Thermal Swing Adsorption Apparatus: Commissioning and Data Analyses for Water Adsorption of Zeolites 4A and 13X Over 2000 Cycles

Evaluation of adsorbent integrity over thousands of cycles is necessary to establish the service time and sustainability of adsorbents employed in industrial dehydration. Herein, an adsorption apparatus for rapidly cycling multiple adsorbents through a thermal swing adsorption process is introduced with results for 2000 cycles. This apparatus has eight sample cells arranged in parallel, which are embedded in an aluminum block for rapid heating and cooling. At the outlet of each cell, the water content and temperatures are measured using capacitance relative humidity sensors, which incorporate resistance thermometers. The analysis of the breakthrough curves generated for each adsorbent gives inference into the change in water uptake capacity over continuous cycling. To handle the large sets of data generated by this instrument, an automated analysis program was implemented. To demonstrate the functionality of the instrument, zeolites 4A and 13X were cycled in a thermal swing process over 2000 cycles and the change in the uptake capacity was monitored by the analysis of the breakthrough plots for each cycle. Furthermore, the results of the breakthrough analyses were verified with the thermogravimetric analysis of the adsorbents. From these experiments, zeolites 4A and 13X were observed to lose 7 ± 3 and 19 ± 7% of the adsorption capacity, respectively.

CO2 Capture on an Adsorbent-Coated Finned Tube Heat Exchanger: Effect of the Adsorbent Thickness

The reduction of cycle time is crucial to obtain viable adsorptive CO2 capture processes. In thermal swing adsorption processes, heating and cooling of the adsorbent result in long cycle times. In ...

Suitability of Thermal Swing Adsorption for the Treatment of Siloxanes and Toluene in Biogas: An Experimental Study

The use of biogas has been proven to be an interesting alternative energy source. However, biogas usually contains trace compounds, which can lead to damage in the valorization installation. Consequently, their treatment is an important issue for the suitability of biogas valorization. Although thermal swing adsorption (TSA) has already been implemented, there is still a lack of knowledge about the deactivation of the adsorbent. This work examined under dynamic conditions, the treatment of three typical trace compounds from landfills (L2, D4, and toluene) via the TSA process using activated carbon cloth (ACC). The process was implemented according to temperature programs: one with a moderate desorption temperature (180 °C) and the other with a high desorption temperature (220 °C). Both experiments involved several cycles of adsorption and desorption (60 cycles). In the single-compound adsorption experiment, the adsorption capacities were 24%, 19%, and greater than 15% weight basis for toluene, L2, and D4, respectively. From TSA experiments, it was found that the adsorption capacities of the ACC decreased rapidly over the first four cycles for the two temperature programs. This decrease was lower at high temperatures: no significant decrease was found between the 4th cycle and the 50th cycle. On the contrary, a constant decrease was measured at moderate temperatures. Both the surface analysis (infrared measure) and the analysis of the gas from the desorption step indicate that siloxanes were deposited on the ACC and polymerized to a high-molecular-weight polymer. This reaction could cause ACC deactivation by pore clogging. Mass balances and gas analysis also suggested that toluene could be thermally degraded during the desorption step. The high temperatures promoted desorption of high-molecular-weight compounds, such as D5, L5, and D6, formed during adsorption/desorption processes. Thus, a high-temperature program was found to be more effective than a moderate-temperature program.

Rapid thermal swing adsorption process in multi-beds scale with sensible heat recovery for continuous energy-efficient CO2 capture

Abstract A rapid thermal swing adsorption (RTSA) process with heat integration is considered to be an energy-efficient method for large-scale CO2 capture. Herein, we propose a new heat-integrated RTSA process for post-combustion CO2 capture as a proof-of-concept based on rationally designed multi-fixed beds by incorporating the internal heat integration concept of the moving bed adsorption process. Our proposed RTSA process consists of multiple beds, each of which undergoes four sequential processes, including adsorption, heating, desorption, and cooling in a continuous cyclic mode. Also, the switching time of adsorption/desorption process is determined by CO2 adsorption/desorption kinetics while that of the heating/cooling process is determined by the heat transfer rate. Our unique RTSA design enables internal heat integration by recovering sensible heat from the cooling process. Furthermore, it can enhance the CO2 cyclic capacity of hollow fiber sorbents with high CO2 recovery by performing multiple adsorption processes simultaneously. Most importantly, our economic analysis based on experiments and rigorous mathematical modeling revealed that the energy demand of the optimized RTSA process is estimated to be approximately 272 kWh/t-CO2 with 58% sensible heat recovery. Such low energy penalty is comparable to that of the most mature amine-based absorption process (271 kWh/t-CO2) due to a combination of internal heat integration and enhanced CO2 cyclic capacity, supporting the feasibility of a RTSA process for post-combustion CO2 capture.

A new honeycomb carbon monolith for CO2 capture by rapid temperature swing adsorption using steam regeneration

A rapid process for CO2 capture is of key importance for the economic feasibility of the process in industry, consequently short adsorption/desorption cycles are crucial. With this aim in mind, a carbon based honeycomb monolith was evaluated for CO2 capture in a thermal swing adsorption process at short contact times. The effect of (1) regeneration time, (2) presence of water vapor during adsorption and desorption and (3) regeneration method (steam versus hot air) on CO2 adsorption was studied. The monolith was characterized in terms of porosity and CO2, N2, and H2O isotherms. Cyclic adsorption/desorption experiments were performed using different synthetic gas mixtures with concentrations of CO2 ranging between 6 and 15 vol%. The effect of water vapor in the synthetic gas mixture on adsorption capacity was limited but increases with relative humidity. Steam of 120 °C was used to heat the monolith and desorb CO2. Advantages of steam usage are the facile separation of steam and concentrated CO2 and the low (waste) heating energy cost of steam. It was demonstrated that the steam allows very fast heating and cooling of the monolith. However, the presence of residual condensation water after the cooling step reduces the cyclic adsorption capacity, requiring an additional drying step with hot or cold air.

Equilibrium theory analysis of thermal regeneration of a humid adsorption column: Selection of optimal hot purge gas temperature

A Thermal Swing Adsorption (TSA) process has been widely commercialised for its application to air drying. In designing an air-drying TSA, zeolite 13X is chosen as a desiccant when required to produce a very low dew point dry air due to its highly nonlinear, favourable isotherm. It is essential to make use of an external hot purge gas in producing such an extremely low dew point dry air, as the column has to be regenerated thoroughly for the next adsorption step. In case of the externally heated TSA, usage of a hot purge gas gives rise to huge energy consumption. Also the cycle time has to be made as short as possible to enhance the bed productivity. To optimise this energy-intensive process, it is crucial to find an optimal hot purge gas temperature at which the column can be regenerated as quickly as possible with less energy consumed. In this study, Equilibrium Theory approach was taken to analyse the thermal regeneration breakthrough of a zeolite 13X column saturated with a water vapour in which the equilibrium isotherm is estimated by Toth or Aranovich-Donohue isotherms. As a result of Equilibrium Theory analysis, it was found that the trailing front of a thermal regeneration breakthrough would exhibit a transition of the shape from simple to shock wave with increasing purge gas temperature. The purge gas temperature has to be chosen so that after thermal regeneration the targeted water vapour concentration remaining in the column lies within the concentration range occupied by the shock wave. A useful correlation that relates the targeted dew point of the remaining water vapour with the purge gas temperature was proposed. The presence of the optimal hot purge gas temperature estimated by Equilibrium Theory was validated by both full numerical simulation and ideal work consumption.

Dynamic modeling of a NG-fueled SOFC-PEMFC hybrid system coupled with TSA process for fuel cell vehicle

Abstract Fuel cell power technology has drawn extensive attentions due to its high efficiency, low emission and noise. Solid oxide fuel cell (SOFC) could generate the power by diverse fuels, such as natural gas (NG), while proton exchange membrane fuel cell (PEMFC) only feeds on pure H2. More and more attentions are paid on the combination of SOFC and PEMFC for high efficiency and convenient refueling in the practical applications. To obtain H2 fuel with high purity from SOFC as a reformer, the gas processing subsystem for H2 separation and purification should be applied between SOFC and PEMFC. In this present study, the gas processing subsystem, consisting of water gas shift (WGS) and thermal swing adsorption (TSA), is introduced into the SOFC-PEMFC hybrid system. Then, the SOFC-WGS-TSA-PEMFC hybrid system is modelled to investigate the transient behaviors under different operations. The simulation results show that the SOFC-WGS-TSA-PEMFC hybrid system has an improved energy conversion efficiency of approximately 64%, which is higher than the only-SOFC and the reform-PEMFC. The waste heat recovery for driving the TSA reaction accounts for the higher net electricity efficiency compared with the SOFC-PEMFC hybrid system based on the pressure swing adsorption (PSA) for H2 separation. Since the SOFC and PEMFC have completely different transient responses to the change of the loading, the influences of operating conditions of fuel cell vehicles on the transient behaviors of single SOFC and PEMFC and the overall performance of the SOFC-WGS-TSA-PEMFC hybrid system are further investigated. Through the analysis and discussion based on the dynamic modelling, the operation strategy is unveiled in this paper for the performance optimization of the hybrid system when installed in the fuel cell vehicles.

Separation and enrichment of CH4 and CO2 from a dry biogas using a thermally regenerative adsorbent-packed heat exchanger

Thermal swing adsorption (TSA) using a heat exchanger packed with an adsorbent material was examined as a means of removing CO2 from a simulated biogas. TSA driven by low-temperature waste heat or solar energy represents an environmentally-friendly gas separation process. In this process, the adsorbent material can be heated indirectly by circulating hot water through a heat exchanger and can operate with a smaller amount of regeneration air than in a conventional TSA system to significantly increase the CO2 concentration at the regeneration outlet. In the present work, carbon molecular sieves (CMS) and a high-silica zeolite (HSZ) were examined with regard to their CO2 adsorption from a simulated biogas containing 60% CH4 and 40% CO2. The effects of the hot water temperature supplied to the adsorber and the regeneration air flow rate on the separation performance were investigated. Increasing the regeneration temperature was found to improve the separation performance, and the HSZ was observed to be more selective for CO2 during the adsorption process. However, the CO2 concentration in the desorption outlet gas was not increased when the regeneration air flow was equal to that of the feed gas. Reducing the regeneration air flow rate to one-tenth the original value significantly increased the CO2 concentration at the desorption outlet while only slightly lowering the CH4 concentration in the product gas. A TSA process incorporating an adsorbent-packed heat exchanger is evidently an effective means of processing biogas, based on poor adsorption of CH4 and strong adsorption of CO2.

Simulation of the recovery of methane from low-concentration methane/nitrogen mixtures by concentration temperature swing adsorption

In this work, the concentration of methane from low concentration methane/nitrogen mixtures by a concentration thermal swing adsorption (CTSA) process with silicalite as adsorbent and steam as desorbent has been studied by simulation with a theoretical model based on conservation equations. The cycle consists of seven steps: (i) introduction of fresh feed, (ii) receipt of the high concentration mass transfer zone (MTZ), (iii) rinse with methane product, (iv) displacement of methane with steam, (v) displacement of steam with light product (nitrogen), (vi) bed cooling and (vii) receipt of the low concentration MTZ. The proposed CTSA process can concentrate methane from methane/nitrogen mixtures with concentrations of 5–20% methane up to +99% with +99% recovery with productivities of 7.5·10−5–2.7·10−4 mol kg−1 s−1 and thermal energy consumptions of 10–46 MJ/kg (assuming heat integration with other processes). It is found that the heat exchange rate through the bed wall is very important for this process, affecting the energy consumption significantly.

Experimental investigation on the CO2 separation performance from humid flue gas by TSA process

This work was aimed at removing CO2 from a fire power plant by using the thermal swing adsorption (TSA) process. It is well known that CO2 adsorption uptake is blocked by the coexistence of steam in the TSA process. In this paper, the influence of desorption temperature and relative humidity in the desorption process on the CO2 capture performance during the TSA process is kinetically investigated. Under conditions without water, the adsorption decreases by half; when the relative humidity is 1.1% at 393 K, adsorption decreases to 2.5 mg-CO2/ghoneycomb. The recovery ratio of the CO2 adsorption performance depends on the relative humidity, not the desorption temperature. Therefore, in the TSA process, it is expected that the influence of steam can be disregarded if a better steam desorption adsorbent is used. This has the added benefit of eliminating the need to remove steam during preprocessing.

Effect of vacuum regeneration of activated carbon on volatile organic compound adsorption

Vacuum swing adsorption (VSA) is a promising treatment method for volatile organic compounds (VOCs). This study focuses on a VSA process for regenerating activated carbon spent with VOCs, and then investigates its adsorption capacities. Toluene was selected as the test VOC molecule, and the VSA regeneration experiments results were compared to the thermal swing adsorption process. Cyclic adsorption-desorption experiments were performed using a lab-scale apparatus with commercial activated carbon (Samchully Co.). The VSA regeneration was performed in air (0.5 L/min) at 363.15 K and 13,332 Pa. The comparative results depicted that in terms of VSA regeneration, it was found that after the fifth regeneration, about a 90% regeneration ratio was maintained. These experiments thus confirm that the VSA regeneration process has good recovery while operating at low temperatures (363.15 K) and 13,332 Pa. Keywords: Activated carbon, Cyclic adsorption-desorption experiments, Recovery, Regeneration, Vacuum swing adsorption, Volatile organic compounds

Characterization of Oxygen Storage and Structural Properties of Oxygen-Loaded Hexagonal RMnO3+δ (R = Ho, Er, and Y)

Single-phase polycrystalline samples of stoichiometric RMnO3+δ (R = Er, Y, and Ho) were achieved in the hexagonal P63cm structure through solid state reaction at ∼1300 °C. Thermogravimetric measurements in oxygen atmospheres demonstrated that samples with the larger Ho and Y show rapid and reversible incorporation of large amounts of excess oxygen (0.3 > δ > 0) at an unusually low temperature range of ∼190–325 °C, indicating the industrial usefulness of RMnO3+δ materials for lower cost thermal swing adsorption processes for oxygen separation from air. Further increase of the excess oxygen intake to δ ∼ 0.38 was achieved for all the investigated materials when annealed under high pressures of oxygen. The formation of three oxygen stable phases with δ = 0, 0.28, and 0.38 was confirmed by thermogravimetric measurements, synchrotron X-rays, and neutron diffraction. In situ synchrotron diffraction proved the thermal stability of these single phases and the regions of their creation and coexistence, and demonst...

Theoretical and experimental studies of the recovery of volatile organic compounds from waste air streams in the thermal swing adsorption system with closed-loop regeneration of adsorbent

The cyclic thermal swing adsorption (TSA) process for volatile organic compounds (VOCs) recovery from the waste air is studied theoretically and experimentally. Toluene is chosen as the volatile organic compound. Activated carbon Sorbonorit 4 is used as an adsorbent. The TSA cycle is operated in three steps: an adsorption step with cold feed, a desorption step with hot purge gas and a cooling step with cold inert gas. The desorption and cooling are affected by nitrogen circulated through a heater, an adsorber and a condenser. A nonequilibrium, nonisothermal mathematical model is developed to simulate temperature and concentration breakthrough curves for both adsorption and desorption steps. The computer simulation results are compared with the experimental data. A bench scale fixed bed adsorption unit was used for the experimental study. It is shown that the theoretical model predicts the experimental results well. The computer simulation results are used to study the effect of the purge gas and condensation temperature on the process efficiency.

Thermal Swing Adsorption Process for Carbon Dioxide Capture and Recovery: Modeling, Simulation, Parameters Estimability, and Identification

In one of our previous studies, several experimental measurements were carried out on a thermal swing adsorption (TSA) process (adsorption and desorption steps) used for the capture of CO2 from a CO2/N2 mixture. The present study deals with a two-dimensional modeling and simulation of the aforementioned TSA process. The models are described by partial differential equations (PDEs) including conservation equations, models for equation of state, equilibrium, thermodynamic, and transport properties. The resulting models involve different unknown parameters to be estimated from the available experimental measurements. An estimability analysis was carried out in order to determine which parameters are able to be estimated from the available experimental results. The most estimable parameters are then identified, and the less estimable parameters are fixed from previous works or from literature. The resulting models are implemented and solved within Comsol Multiphysics software. The results show that the model ...

Discrete element simulation of the dynamics of adsorbents in a radial flow reactor used for gas prepurification

Radial flow reactors (RFR) are used in thermal swing adsorption (TSA) processes for gas prepurification. The aim of this work is to show the validity of the discrete element method (DEM) to simulate the effect of thermal expansion and contraction cycles occurring in such processes on the packed bed of RFR reactors. Both mono-layered and bi-layered packed beds of adsorbents are investigated. A DEM-based model of a full-scale size unit was developed, the parameters of which were calibrated by means of particle-scale experimental measurements and simple auxiliary DEM simulations. The DEM-based model used is isothermal and the thermal expansion and contraction phenomena are modelled through the displacement of the inner and outer walls of the computational domain. First, the accuracy of this model is assessed using analytical values of the static wall pressure (i.e. with no wall motion) as well as experimental measurements of the dynamic wall pressure (i.e. with wall motion) of a bi-layered bed. Next, simulation results for a few process cycles in the case of a bi-layered packed bed indicates that little mixing occurs at the interface between the two types of adsorbents. To our knowledge, this is the first time that simulation is used to investigate the behavior of the packed bed of a RFR in a TSA process. The results obtained with the proposed model show that the DEM is a valuable tool for the investigation of such slow dynamical processes, provided a careful calibration is done.