Scholarly article on topic 'The Role of Water in Adsorption-based CO2 Capture Systems'

The Role of Water in Adsorption-based CO2 Capture Systems Academic research paper on "Chemical engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Energy Procedia
OECD Field of science
Keywords
{"CO2 capture" / "Activated carbon" / "Adsorption isotherms"}

Abstract of research paper on Chemical engineering, author of scientific article — Dorian Marx, Lisa Joss, Max Hefti, Ronny Pini, Marco Mazzotti

Abstract Cyclic adsorption processes such as Pressure, Temperature, and Vacuum Swing Adsorption have received increased attention as potential techniques for the capture of CO2 from gas mixtures in industrial processes. A key challenge in the design of these processes is to gain a thorough understanding of the interactions between the numerous gas phase components and the sorbent material, as an effective separation requires a selective adsorption between carbon dioxide and the other gases in the mixture. In all cases of interest moisture is present and interferes with the mechanisms involved in the adsorption of CO2 and other gases; it is therefore important to study the effect it has on the separation process. This impact can be assessed by measuring the equilibrium adsorption of water vapor on AP3- 60 activated carbon (Chemviron Carbon, Germany), as well as investigating its dynamics.

Academic research paper on topic "The Role of Water in Adsorption-based CO2 Capture Systems"

Available online at www.sciencedirect.com

SciVerse ScienceDirect

Energy Procedia 37 (2013) 107 - 114

GHGT-11

The Role of Water in Adsorption-based CO2 Capture Systems

Dorian Marxa, Lisa Jossa, Max Heftia, Ronny Pinib, Marco Mazzottia*

aETH Zurich, Institute of Process Engineering, Sonneggstrasse 3, CH-8092 Zurich, Switzerland _bStanford University, Department of Energy Resources Engineering, Stanford, USA_

Abstract

Cyclic adsorption processes such as Pressure, Temperature, and Vacuum Swing Adsorption have received increased attention as potential techniques for the capture of CO2 from gas mixtures in industrial processes. A key challenge in the design of these processes is to gain a thorough understanding of the interactions between the numerous gas phase components and the sorbent material, as an effective separation requires a selective adsorption between carbon dioxide and the other gases in the mixture. In all cases of interest moisture is present and interferes with the mechanisms involved in the adsorption of CO2 and other gases; it is therefore important to study the effect it has on the separation process. This impact can be assessed by measuring the equilibrium adsorption of water vapor on AP3-60 activated carbon (Chemviron Carbon, Germany), as well as investigating its dynamics.

© 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT

CO2 capture; activated carbon; Adsorption isotherms

1. Introduction

Adsorption-based processes continue attracting interest for their potential use in carbon capture systems for both pre-combustion and post-combustion capture. In particularly, Pressure Swing Adsorption (PSA) shows promise for application in pre-combustion capture in IGCC plants, where a stream at elevated pressure (35 to 45 bar) consisting mostly of CO2 and hydrogen (H2) has to be separated [1], while temperature swing adsorption (TSA) is appealing for post-combustion capture, where the feed

* Corresponding author. Tel.: +41 44 632 2456; fax: +41 44 632 1141. E-mail address: marco.mazzotti@ipe.mavt.ethz.ch.

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi: 10.1016/j.egypro.2013.05.090

stream is at a lower pressure, but heat for regeneration of the sorbent is often available at low cost. This has led to an increasing amount of research focused on the interactions of carbon dioxide with various sorbents. One aspect that has to be considered in these processes is that in all applications of interest the feed stream contains water vapor, which interferes with the mechanisms involved in the adsorption of CO2 and other gases.

In this work a number of methods for the characterization of the adsorption of water vapor onto AP3-60 activated carbon (AC) are investigated with regards to its use as a commercial sorbent for the capture of carbon dioxide (CO2) from industrial point sources such as power plants.

1.1. Previous work

Several studies have addressed measurement and description of the adsorption of water on a variety of adsorbents. Because of capillary condensation, the mechanisms of water adsorption can differ greatly from one adsorbent to the other depending mainly on surface properties and on the pore size distribution. Moreover, the strong deviation of water adsorption from ideal behavior is reflected in binary adsorption with other gases such as CO2. In fact, binary CO2-water static adsorption data available in the literature show very different interactions; for dilute systems the presence of a small amount of moisture enhances CO2 sorption at low partial pressures [2,3,4], but in the presence of higher CO2 and water concentrations, water was shown to inhibit CO2 adsorption [5].

Studies on cyclic adsorption processes that consider humid feeds are mostly restricted to air purification purposes, i.e. ppm levels of CO2. The rising interest in gaining a better understanding of the role of water in CO2 capture systems is reflected in an increase of studies dealing with high humidity feeds containing bulk CO2 concentrations [5-8]. Ribeiro et al. investigated a PSA process with AC beads for the purification of hydrogen from a high humidity H2/CO2/CH4/CO/N2 mixture. Dynamic adsorption studies from breakthrough experiments performed under both dry and humid conditions revealed that the presence of water in the feed does not significantly affect the breakthrough behavior of the other species due to the high water capacity of AC beads leading to a very slow progression of the water front. Nonetheless, the cyclic PSA experiments performed with the humid feed did not reach cyclic steady state because of water accumulation in the fixed-bed. Also in the case of zeolite 13X, which has a high capacity for water, it has been reported by several authors [5, 7] that the presence of water does not significantly affect the breakthrough behavior of CO2. Nevertheless, when operating in a cyclic mode, the presence of water in the bed is inevitable. It is therefore crucial to accurately describe the adsorption of water in order to design optimal regeneration strategies which can cope with the presence of water vapor.

Very few studies have attempted to deal with the presence of a hysteresis loop, as it is frequently observed in the desorption of water, and its effect on adsorber dynamics. While modeling studies have been done, and some static measurements are available [9-12], the incorporation into dynamic models is far from straightforward. In particular, scenarios that involve alternating increases and decreases in humidity within the hysteresis loop are difficult to predict, as these result in adsorption or desorption along an intermediate scanning curve rather than the primary branches of the isotherms. Stepanek et al. have developed a framework to predict these scanning curves, and have implemented it into an isothermal and isobaric fixed bed adsorption column model [13].

While research in this field has intensified, there is still a need for a comprehensive analysis of the different aspects involved, such as competitive adsorption and hysteresis. It is therefore crucial to also consider their implications regarding both the thermodynamics and kinetics and how these translate to

adsorption separations in fixed-bed columns.

2. Experimental methods

2.1. Adsorption of water

In order to measure the adsorption of water vapor, an existing setup used for the measurement of adsorption of dry gases was modified. It centers on a magnetic suspension balance (MSB) from Rubotherm (Bochum, Germany). To extend the capabilities of this setup to include the measurement of water vapor adsorption, a humidity sensor and a reservoir with liquid water were added, as is shown in Figure 1.

This measurement procedure is initially the same as it is for dry gases. It is described in detail in a previous work [14, 15]; however in the following those aspects important to measurements with water will be reiterated. About 2 g of adsorbent are placed in the basket of the MSB and regenerated under vacuum at a temperature of 150°C. At these conditions the starting mass of the metal parts and the sample can be directly measured.

dry C02

Figure 1: Schematic of the setup used to measure adsorption of gases and water vapor.

Once the balance and sample have reached the temperature of the measurement, the attached vacuum pump is first used to evacuate the piping of the setup, and to degas the water in the reservoir. As the liquid water replenishes the gas phase with water vapor, dosing valves on the water tank and the vacuum pump are used to maintain the desired level of pressure in the balance chamber. The valve leading to the MSB chamber is then closed, and the sorbent is left to equilibrate with the humidity level. The attached humidity sensor from Michell Instruments (Ely, UK) is used to monitor the level of humidity. It can be used at temperatures from -30°C up to 200°C, and can withstand pressures up to 30 bar. To increase the humidity in the balance the procedure above is repeated. Great care is taken to avoid a decrease of humidity during the adsorption measurements to ensure that only the adsorption branch is measured.

Measuring desorption is straightforward, as it is done by using the vacuum pump to gradually reduce the pressure, and therefore the humidity inside the balance, stopping at intervals to let the system equilibrate. Both ad- and desorption need to be performed, as hysteresis is observed.

2.2. Dynamic experiments

While the MSB provides valuable data about the equilibrium adsorption of water and other gases in the presence of humidity, the modeling of cyclic adsorption processes requires knowledge of the kinetics of the system such as heat and mass transfer. To study these, breakthrough experiments are performed using a column packed with AP3-60 AC. The column has a length of 20 cm and a diameter of 0.5 cm, and the sorbent particles are between 0.2 and 0.5 mm in diameter. A schematic of the setup used can be seen in Figure 2. The column used in this setup is interchangeable to enable experiments with different sorbents and for different residence times. This column is placed in a furnace from Memmert (Schwabach,

Figure 2: Schematic of the fixed bed setup used for dynamic experiments [16].

Germany) that can maintain temperatures up to 300°C for the regeneration of the sorbent material. To record the temperature of the bed, a thermocouple is installed at the center of the column. The feed gas flow is controlled by a mass flow controller (MFC) (Bronkhorst, Rheinach, Switzerland) operating in the range of 2-250 Nml/min. The pressure is maintained by a back pressure regulator (BPR) from Equilibar (USA), downstream of the column. For experiments with water, a pressurized liquid water source and a liquid flow meter (LFM) (Bronkhorst) can be used to feed up to 140 mg/h of water, which is evaporated and mixed with the gas stream to reach the desired feed composition. Any part of the tubing that transports water vapor can be heated electrically up to 150°C to avoid the formation of liquid water. The product gas stream is analyzed using a mass spectrometer (Pfeiffer Vacuum, Switzerland). Further details of the setup and characterization of the effect the downstream piping has on breakthrough measurements are described in a previous work [16].

3. Results

3.1. Adsorption of water vapor

The experimental adsorption isotherm of water vapor on AC at 45°C against the relative humidity is shown in Figure 3. Values measured during adsorption are shown as the filled circles, while those measured during desorption are shown as empty symbols. The shape of the isotherm can be classified as Type V.

Figure 3: Results of the adsorption isotherm measurement of water vapor on AC at a temperature of 45°C. Filled circles were measured during adsorption, while empty circles were measured during desorption.

The hydrophobic nature of this AC leads to a relatively low uptake at low levels of humidity. However, above 50% relative humidity capillary condensation begins to take place, which manifests itself in a strong increase of water uptake. This leads to a very high capacity of about 22 mol/kg at conditions of high humidity. During desorption a clear hysteresis is observed.

3.2. Desorption fixed-bed experiment

As adsorbed water will have to be removed in between steps to regenerate the sorbent bed, the desorption behavior of water from the sorbent is of equal importance as the adsorption behavior. Figure 4 shows the exit profile of a desorption step. The column, having previously been equilibrated with a humidified feed, was dried using helium at 45°C with a flow rate of 1 cm3/s. The initial state of the column was not fully saturated, but to the left of the inflection point seen in the isotherm in Figure 3, in the part of the isotherm that can be classified as unfavorable. As it would be expected for such an isotherm, the desorption shows a sudden decrease in the water content of the product stream, called a shock.

Figure 4: Fixed bed experiment carried out on AC at 1 bar and 45°C. This plot shows the exit profile measured at the outlet of the column during the drying of the column.

4. Research challenges

Future work should address equilibrium measurements of the water-AC system in a variety of conditions. The temperature dependence of the equilibrium behavior should be established as the high heat of adsorption of water onto AC may give rise to a strong temperature wave within the column. Preliminary results suggest that the temperature dependency of water vapor adsorption can be accounted for through the vapor pressure, such that the adsorption against the reduced pressure (i.e. relative humidity) is independent of temperature [17]. A series of breakthrough experiments should be performed in order to gain a more thorough understanding of the kinetics of the water-AC system in both adsorption and desorption mode. Furthermore, the effect of hysteresis on breakthrough times and profiles should be investigated, as significant differences between predictions neglecting hysteresis and the actual case may occur.

It is worth noting that the adsorption of water vapor does not only play a role in the processes discussed here, but also in other adsorption-based processes with potential for carbon capture and storage. As such, research should encompass not only AC and other sorbents used for gas separations (e.g. zeolites), but also those for other applications such as direct CO2 capture from air, or CO2 storage in coal seams through enhanced coal bed methane recovery.

5. Conclusion

In this work two methods are presented for the investigation of the effect water has on adsorption-based CO2 capture systems, one static and one dynamic. Both are needed to be able to comprehensively describe cyclic adsorption processes. The static approach measures adsorption equilibria gravimetrically, using a humidity sensor to monitor the water vapor concentration in the gas phase. Adsorption-desorption equilibrium of water vapor on AC was measured at 45°C. It was classified as a type V isotherm according to the IUPAC system, with a total capacity for water of around 22 mol/kg. A hysteresis loop was observed upon desorption of the adsorbed water phase. Such a hysteresis loop needs to be considered in the regeneration step of a cyclic sorption process.

Desorption of water from an AC bed was investigated by a breakthrough experiment, where a pre-saturated column packed with AC was dried with a helium stream. The breakthrough profile is consistent with the measured isotherm, as a shock was observed after about 6 minutes.

By measuring the adsorption of water vapor over a range of temperature, its temperature dependency as well as the heat of adsorption of water on AC can be established. These are important to consider in the simulation of dynamic experiments as well as full cyclic processes. In addition to investigating the adsorption of pure water vapor, it is crucial to also study the effect it has on the adsorption of other gases. The presented equipment allows for the characterization of the adsorption of CO2 and other gases in the presence of water vapor. By assessing the impact water has on the adsorption on different sorbents (e.g. zeolites), their potential for the use in cyclic adsorption processes can be established. Incorporating these findings into simulation tools then allows for the detailed design of adsorption-based separation processes.

Acknowledgements

Partial support of the Swiss National Science Foundation through grant NF 200021_130186 is gratefully acknowledged.

References

[1] Casas N, Schell J, Pini R, Mazzotti M. Fixed bed adsorption of CO2/H2 mixtures on activated carbon: experiments and modeling. Adsorption 2012;18:143-161.

[2] Rege S, Yang R, A novel FTIR method for studying mixed gas adsorption at low concentrations: H2O and CO2 on NaX zeolite and y-alumina. Chem. Eng.Sci. 2001; 56: 3781-3796.

[3] Brandani F, Ruthven D, The Effect of Water on the Adsorption of CO2 and C3H8 on Type X Zeolites. Ind.Eng. Chem. Res.2004; 43 (26),8339-8344.

[4] Wang Y, LeVan M, Adsorption Equilibrium of Binary Mixtures of Carbon Dioxide and Water Vapor on Zeolites 5a and 13X. J. Chem. Eng. Data. 2010; 55: 3189-3195.

[5] Li G, Xiao P, Webley P, Zhang J, Singh R, Marschall M. Capture of CO2 from high humidity flue gas by vacuum swing adsorption with zeolite 13X, Adsorption 2008; 14 (2-3): 415-422.

[6] Ribeiro A, Grande C, Lopes F, Loureiro M, Rodrigues A. Four Beds Pressure Swing Adsorption for Hydrogen Purification : Case of Humid Feed and Activated Carbon Beds, AIChE 2009; 55 (9): 12-14.

[7] Ribeiro R, Grande C, Rodrigues A. Adsorption of Water Vapor on Carbon Molecular Sieve: Thermal and Electrothermal Regeneration Study, Ind. Eng. Chem. Res. 2011; 50 (4): 2144-2156.

[8] Ferreira D, Magalh R, Taveira P. Effective Adsorption Equilibrium Isotherms and Breakthroughs of Water Vapor and Carbon Dioxide on Different Adsorbents, Ind. Eng. Chem. Res. 2011; 50: 10201-10210.

[9] Müller E, Gubbins K. Molecular Simulation Study of hydrophilic and hydrophobic Behavior of activated Carbon Surfaces. Carbon 1998; 36 (10): 1433-1438.

[10] Kaneko K, Hanzawa Y, Iiyama T, Kanda T, Suzuki T. Cluster-mediated Water Adsorption on Carbon Nanopores. Adsorption 1999; 5: 7-13

[11] Horikawa T, Do D, Nicholson D. Capillary Condensation of Adsorbates in porous Materials. Advances in Colloid and Interface Science 2011; 169: 40-58.

[12] Monson P. Understanding Adsorption/Desorption Hysteresis for Fluids in mesoporous Materials using simple molecular Models and classical Density Functional Theory. Microporous and Mesoporous Materials 2012; 160: 47-66.

[13] Stöpänek F, KubiCek M, Marek M, Soöä M, Rajniak P, Yang R, On the modeling of PSA cycles with hysteresis-dependent isotherms, Chem. Eng. Sci. 2000; 55 (2): 431^40.

[14] Ottiger S, Pini R., Storti G., Mazzotti M., Bencini R., Quattrocchi F., et al. Adsorption of pure carbon dioxide and methane on dry coal from the sulcis coal province (SW Sardinia, Italy), Environ. Prog. 2006; 25: 355-64.

[15] Pini R., Ottiger S, Rajendran A., Storti G, Mazzotti M. Near-critical adsorption of CO2 on 13X zeolite and N2O on silica gel: lack of evidence of critical phenomena, Adsorption 2008; 14: 133-41.

[16] Joss L, Mazzotti M. Modeling the extra-column volume in a small column setup for bulk gas adsorption. Adsorption 2012; doi: 10.1007/s10450-012-9417-z.

[17] Leppäjärvi T, Malinen I, Kangas J, Tanskanen J. Utilization of P;sat temperature-dependency in modelling adsorption on zeolites. Chem. Eng. Sci. 2012; 69: 503-513.