Comparison of microwave and conventional heating for CO2 desorption from zeolite 13X

https://doi.org/10.1016/j.ijggc.2021.103311Get rights and content

Highlights

  • Microwave energy was applied to temperature swing CO2 capture using zeolite 13X.

  • Microwave sorbent regeneration is at least 50 % faster than thermal regeneration.

  • Desorption kinetics are mass diffusion limited during microwave regeneration of 13X.

  • Apparent desorption activation energy is reduced using microwaves for regeneration.

  • Proof-of-concept testing demonstrates feasibility of microwave sorbent regeneration.

Abstract

This study investigates microwave irradiation as an alternative to conventional heating for temperature swing adsorption processes. The performance of microwave and conventional heating during sorbent regeneration was evaluated by measuring CO2 desorption from zeolite 13X at different temperatures. Experimentally, a fixed bed of zeolite 13X was saturated by a 150 sccm flow of 15 % CO2 at room temperature followed by sorbent regeneration under nitrogen at 55, 100, or 150 °C by applying either microwave irradiation or conventional heating. Microwaves reduced sorbent regeneration times by at least half compared to regeneration by conventional heating. Under conventional regeneration, desorption curves were resolved into two peaks representing physisorbed CO2 (mass diffusion limited) at low temperature and bicoordinated CO2 (thermally limited) with increasing temperature. Under microwave regeneration, only one desorption peak was observed suggesting that CO2 desorption was limited by mass diffusion through the porous zeolite 13X structure, rather than by temperature. Depending on microwave power, apparent activation energy of the microwave-assisted regeneration was 15.8–18.1 kJ/mol, compared to 41.5 kJ/mol for conventional regeneration. The reduction in apparent activation energy is mainly attributed to selective microwave heating of CO2 adsorption sites (Na+ sites) resulting in greater steady state temperatures of Na+ relative to framework atoms, suggesting greater heating efficiency due to microwaves compared to conventional heat transfer. Due to rapid cycling and efficient heat transfer to CO2 sites on zeolite 13X, microwave regeneration is found to increase adsorption/desorption cycling productivity and potentially reduce the energy penalty of temperature swing capture.

Introduction

Carbon dioxide emissions from fossil fuel-fired electric generation plants are typically removed from flue gas using amine-based aqueous solvents, e.g. monoethylamine (MEA). While they are commercially mature, amine-based aqueous solvents are corrosive, degrade over multiple cycles, and suffer from high energy penalties during sorbent regeneration due to the high heat capacity of water (Luis, 2016). The energy penalties associated with aqueous solvents can be prohibitively expensive and could be avoided by using non-aqueous solid sorbents, which require significantly less steam for regeneration and lower reboiler duties (Goto et al., 2013). Recently, solid sorbents with low heat capacity have been investigated for CO2 capture as alternatives to aqueous solvents in post combustion carbon capture processes (Samanta et al., 2011). The success of solid sorbents would benefit with high CO2 adsorption capacity, high CO2 selectivity, good stability over multiple adsorption/regeneration cycles, and rapid adsorption/regeneration kinetics (Chronopoulos et al., 2014). Studies on various solid sorbents including zeolites, carbonaceous materials, metal-organic frameworks, ordered mesoporous silica, and supported amines have demonstrated good CO2 adsorption capacity as well as regeneration ability (Dinda, 2013; Siriwardane et al., 2003; Zhang et al., 2016). Sorbent regeneration performance is important for evaluating the suitability of candidate sorbents for carbon capture systems as the energy requirement for CO2 desorption may limit commercial application (Hedin et al., 2013).

Carbon capture using solid sorbents can be achieved by several different cycling approaches, for which the most common methods include pressure and/or vacuum swing adsorption (PSA or VSA), temperature swing adsorption (TSA), and hybrid modes of temperature vacuum swing adsorption (TVSA) (Jiang et al., 2020; Dhoke et al., 2020). While PSA is effective for capture from high concentration sources, TSA has been found to be better suited for capture from dilute sources such as post combustion capture which has typical CO2 concentrations on the order of 12–15 % CO2. In a typical TSA process, CO2 is adsorbed onto the sorbent, and then regenerated by raising the sorbent temperature to desorb CO2. In a conceptualized process, whether batchwise with fixed beds or continuous with sorbent transport (e.g. moving bed), adsorption and regeneration would occur under different feed gas composition and temperature. The sorbent is cooled back to the adsorption temperature to repeat the cycle. In a conventional regeneration approach, the sorbent is regenerated by indirect means by a stripping gas, for example by passing a heated inert gas over the sorbent bed, which requires considerable energy both upstream to heat the gas via heat exchangers, as well as downstream during gas separation due to the low dew points of inert gases. In a TSA process, the sorbent bed may also be regenerated by heating the regenerator column by resistive heating elements resulting in energy losses to heating the regenerator column walls. In addition to the considerable inefficiencies of typical TSA, sorbent heating by conventional means can be slow in sorbents with low thermal conductivity leading to long regeneration times and limiting TSA cycling productivity (Cherbański and Molga, 2009; Verougstraete et al., 2020). On the other hand, by application of microwave irradiation, heat is generated rapidly within the sorbent bed, provided there is good coupling of the sorbent material with the electromagnetic field, resulting in increased regeneration rates compared to conventional heating.

The ability of microwaves to heat a material selectively and volumetrically by dielectric loss mechanisms (i.e. dipole rotation, ionic conduction, or interfacial polarization) without first heating the reactor walls leads to greater energy transfer efficiencies compared to conventional heat transfer mechanisms (i.e. conduction and convection), especially in materials with low thermal conductivity (Hamzehlouia et al., 2018). Rapid heating and cooling periods using microwaves could increase cycling productivity of TSA processes. In addition, microwaves can selectively irradiate adsorption sites while minimizing heating in the bulk solid, which could significantly reduce energy losses for a microwave-assisted TSA process. With variable frequency microwave reactors, it may be possible to tune the frequency to improve selective site irradiation with minimal microwave coupling with the bulk material. This concept could deliver microwave energy more efficiently for sorbent regeneration while keeping bulk temperatures low; however, further examination is needed. In addition to the need to improve regeneration efficiency of the TSA process, another technical challenge is reducing the large amount of adsorbent required at commercial scale (Hedin et al., 2013). The amount of adsorbent will be constrained by the working capacity of the material handling equipment and large sorbent volumes may be prohibitively expensive. The amount of sorbent needed per cycle could be reduced by achieving shorter cycling times. Compared to conventional TSA, microwave-assisted TSA has been shown to greatly reduce sorbent regeneration times, which could reduce the amount of adsorbent required, thus reducing production costs (Chronopoulos et al., 2014).

In the literature, microwave regeneration has been investigated for various desorption processes, mainly solvent-based technologies, which have been reviewed previously (Cherbański and Molga, 2009). Mao et al. (2015) compared microwave and conventional regeneration of toluene and acetone loaded on microporous activated carbon. They found desorption rates during continuous power microwave regeneration to be 20–40 times faster than conventional regeneration tests using a conductively heated fixed sorbent bed wrapped in insulated heating tape. In another study, Turner et al. (2000) investigated the selectivity of microwave irradiation for desorption of co-adsorbed cyclohexane (non-polar) and methanol (polar) from high silica and DAY zeolite sorbents. They found microwaves to change the sorption selectivity compared to conventional heating as the greater dielectric loss material (methanol) selectively desorbs. They also found system temperatures required for desorption to be lower under microwave compared to temperatures needed for the conventional desorption process, such as resistive heating or steam stripping.

Microwave-assisted regeneration has been previously demonstrated at a proof-of-concept level for CO2 desorption from aqueous MEA as well as various solid sorbents (Chronopoulos et al., 2014; McGurk et al., 2017; Atwater et al., 1997; Nigar et al., 2016; Webley and Zhang, 2014). In the case of MEA, microwave regeneration was found to desorb CO2 at lower temperatures and more rapidly compared to conventional regeneration techniques, potentially reducing the energy penalty of a TSA process using MEA. Literature on microwave-assisted CO2 desorption from solid sorbents have similarly shown significantly faster CO2 desorption rates compared to conventional desorption. Zeolites or activated carbons have been most commonly used in microwave regeneration studies because of their good microwave absorption and relatively high CO2 adsorption capacity. For regeneration of microporous activated carbon, microwaves resulted in four times shorter regeneration times under microwave compared to conventional regeneration (Chronopoulos et al., 2014). Microwave vacuum regeneration of zeolite 13X was tested by using microwaves to mitigate the vacuum levels required for the desorption process under dry and humid CO2 conditions (Webley and Zhang, 2014). It was found that it was possible to regenerate the sorbent under elevated vacuum levels using short microwave exposure times.

By application of microwaves to the regeneration process, this study aims to accelerate desorption kinetics to improve cycling productivity and reduce the energy required by temperature swing adsorption (TSA)/desorption processes. In this study CO2 desorption from zeolite 13X is investigated under both microwave and conventional heating to elucidate the effect of heating mode on sorbent regeneration in a TSA process. Microwave regeneration could be suitable for small scale TSA capture applications where steam or waste heat is not available for sorbent heating. Zeolite 13X was selected as a model sorbent as it has been widely studied as a post combustion carbon capture sorbent due to its relatively high adsorption capacity and high selectivity for CO2, provided low moisture flue gas (Siriwardane et al., 2001; Lee and Park, 2015). A detailed kinetic analysis of the regeneration process, including an estimation of apparent desorption activation energy, under microwave and thermal regeneration are presented. By comparison of CO2 desorption performance under microwave regeneration and thermal regeneration, new insights into microwave-specific CO2 desorption mechanisms are described.

Section snippets

Textural properties

To analyze textural properties of zeolite 13X, N2 adsorption isotherms were acquired using a Micromeritics ASAP 2020 (Micromeritics, Norcross, GA) and pressure ratios (P/P0) from 7.74 × 10−8 to 0.99 at 77 K. Specific surface area was determined by the Brunauer–Emmett–Teller (BET) method using five adsorption points from 0.005 to 0.03 P/P0. Pore size distribution including microporosity (< 2 nm) and mesoporosity were analyzed by means of the BJH model applied to the nitrogen desorption isotherms.

Textural properties

Textural properties from N2 adsorption/desorption isotherms of zeolite 13X are summarized in Table 1. BET surface area of fresh zeolite 13X was found to be 660 m2/g, which is within the scope of 534−743 m2/g previously reported in literature for commercial zeolite 13X pellets (Park et al., 2016; Yi et al., 2012; Chen et al., 2017). Significant adsorption at low pressures (<0.05 P/P0) suggests microporous structure, which is reflected in the micropore volume of 0.24 cm3/g as determined by t-plot

Conclusion

With increasing atmospheric CO2 it is important to develop CO2 capture technologies that reduce the energy penalty of TSA processes to make them commercially viable. This study investigated microwave-assisted TSA using zeolite 13X as a model sorbent. Compared to conventional regeneration, microwaves were found to greatly decrease CO2 desorption times by at least 50 % due to the rapid heat rates induced by microwave irradiation. Conventional regeneration results in two desorption peaks,

Disclaimer

This work was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with Leidos Research Support Team (LRST). Neither the United States Government nor any agency thereof, nor any of their employees, nor LRST, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus,

Author statement

Candice Ellison – Conceptualization, methodology, data curation, formal analysis, writing - original draft.

James Hoffman – Conceptualization, methodology, writing – review & editing.

Dushyant Shekhawat – Conceptualization, methodology, writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was performed in support of the US Department of Energy’s Fossil Energy Carbon Capture research program. The research was executed through the National Energy Technology Laboratory Research and Innovation Center’s Transformational Carbon Capture field work proposal. Research performed by Leidos Research Support Team (LRST) staff was conducted under the RSS contract 89243318CFE000003.

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