CHEM. RES. CHINESE UNIVERSITIES 2012, 28(1), 129—132

——————————— *Corresponding author. E-mail: zhaiyc@smm.neu.edu.cn Received January 4, 2011; accepted September 22, 2011. Supported by the National Natural Science Foundation of China(No.51074205) and the Fund of Corporate Research Centre

for Greenhouse Gas Technology, Australia.

Separation of H2O/CO2 Mixtures with Layered Adsorption Method for Greenhouse Gas Control

XU Dong1,2, ZHANG Jun2, LI Gang2, XIAO Penny2 and ZHAI Yu-chun1* 1. School of Material and Metallurgy, Northeastern University, Shenyang 110004, P. R. China;

2. Department of Chemical Engineering, Monash University, Victoria 3800, Australia

Abstract Multiple-layered vacuum swing adsorption technique was used and investigated in order to effectively keep the feed gas that flows into zeolite 13X zone being dry and keep the CAPEX down(not adding pre-treatment equipment). Activated carbon fiber(ACF) and alumina CDX were laid at the lower parts of the column as pre-layers to selectively adsorb moisture. Zeolite 13X was laid on the top of those two adsorbents as the main layer to capture CO2. Systematic cyclic experiments show that water vapor was successfully contained within the ACF and CDX layers at cyclic steady states. It was also found that ultimate vacuum pressure played a decisive factor for stabilizing the water front, and achieving good CO2 purity and recovery. The findings also reveal the pathway for large-scale CO2 capture process. Keywords Separation; Carbon dioxide; Water vapor; Vacuum swing adsorption Article ID 1005-9040(2012)-01-129-04

1 Introduction

The fast increasing concentrations of CO2 in atmosphere are requiring human beings to find flexible techniques to con-trol the emissions of greenhouse gas to atmosphere[1,2]. There are a variety of approaches to separate CO2 from flue gas and the most likely options include: chemical absorption, physical adsorption, low-temperature distillation, and membranes[3,4]. Among the four approaches, CO2 capture by pressure swing adsorption(PSA) is a promising option in terms of its relatively low operating and capital costs. Particularly, this method shows a great potential in CO2 capture from coal fired power stations because of its promise in low energy consumption and system simplicity[5].

Before CO2 is sequestered, it must be concentrated, since the concentration of CO2 in the flue gas is typically only 10%―15%[6]. According to the literature[7―9], activated carbon and zeolite 13X are two classical excellent adsorbents in CO2 pressure swing adsorption at a lower carbon dioxide concentra-tion. For the capture of CO2 from dry flue gases, the simulation researches show that CO2 can be enriched from 17% to 99.997% at a recovery of 68.4% via activated carbon[10] and from 15% to 99% at a recovery of 53%[7]. A good technical performance(CO2 purity>95% and recovery>80%) was achieved by a three bed pilot-scale vacuum swing adsorption process with 13X as adsorbent from dry gas in our group[11]. Comparison of activated carbon and zeolite 13X shows despite high-temperature excursions, zeolite 13X is a better adsorbent than activated carbon in non-isothermal, adiabatic PSA process due to equilibrium selectivity[7].

However, real flue gases include water vapour(8%―12%) and trace amounts of SOx and NOx

[12,13]. Studies on CO2 sepa-ration from real flue gas show that the working capacity of zeolite 13X dropped greatly on processing flue gas with high absolute humidity level[14]. Conventional approaches using a pre-treatment/drying apparatus to remove moisture from post-combustion flue gas would considerably increase the overall capture cost[15].

In order to keep zeolite 13X always active, a triple-layered vacuum swing adsorption process was proposed in this study. Multiple adsorbent layering in an adsorption bed is a well es-tablished industry technique used in hydrogen purification, air separation and natural gas separation plants[16,17]. As for the desiccant selection, Activated carbon fibre(ACF) and activated alumina CDX were considered because of their great water adsorption capacity. In this study, a shallow ACF layer was loaded at the bottom of the bed, because ACF can adsorb con-siderable moisture at relatively high pressure and water desorp-tion is easier on ACF under lower vacuum conditions. The reason of a thin layer of ACF placement in stead of whole pre-layer package is that too much ACF can cause serious pressure drop in the bed and the pressure drop is detrimental to CO2 capture performance. The proprietary alumina-based ad-sorbent alumina CDX was laid at the middle of the bed to mainly adsorb/desorb water and manage the water front movement and protect the major layer, which was comprised of zeolite 13X. Activated alumina on the other hand is a common adsorbent applied in industries for removing moisture in PSA and other separation processes[18]. As there is around 10%― 15% CO2 in real flue gas, the feed gas CO2 volume fraction

130 CHEM. RES. CHINESE UNIVERSITIES Vol.28

was fixed at 10.5% or 15.5% respectively in this study.

2 Experimental

2.1 Adsorbents and Isotherm Measurement

The adsorption isotherm data for CO2 and N2 on zeolite 13X , CDX and ACF were taken on an ASAP 2010 Gas Ad-sorption Analyzer(Micromeritics, USA) at different tempera-tures over a pressure range of 0―118 kPa. The adsorption iso-therm data for water vapour on those three materials at p/p0 from 0 to 1 were obtained on an IGA-002 Intelligent Gravime-tric Analyzer system(Hiden Isochema, Ltd., UK). The physical properties of the three materials are listed in Table 1. Table 1 Physical properties of zeolite 13X, CDX and ACF Adsorbent 13X CDX ACF Chemical description NaX Alumina Carbon Shape of adsorbent Spherical Spherical Cloth Diameter/mm 2.0 2.0 ― Total pore vol./(cm3·g–1) 0.27 0.48 0.62 BET surface area/(m2·g–1) 445.5 441.5 1200

2.2 Experimental Set-up

The adsorption bed was made of a stainless steel column with an effective working length of 560 mm, an ID of 49 mm and a wall thickness of 3 mm. Along the length of the bed，ten T-type thermocouples were inserted into the column at the po-sitions of 60, 100, 140, 180, 220, 260, 300, 387, 474, and 560 mm from the bottom of the bed to measure the temperature profile during the cycles. Thermocouples were also installed at locations in the pipeline to monitor the process fluids entering and leaving the bed. Dry compressed air was humidified in a water bubbler, and the humidity level was simply controlled by air pressure and water temperature. Vacuum was achieved through an oil rotary vane vacuum pump. Various instruments for measuring humidity, pressure, and CO2 purity were equipped. Process control and data acquisition were realized by Advantech Data Acquisition and Control System.

2.3 PSA Cycle Design

In this experiment, the temperatures for the feed gas and column were all controlled at 30 °C. A simple single column and three-step PSA cycle that includes only adsorption step, desorption step and repressurization step(Fig.1) was designed

Fig.1 Pressure swing adsorption cycle design and process steps

for investigating the adsorption/desorption capacities of water vapour and CO2 component.

We emphasize that this is not a cycle that would be used in commercial CO2 capture plants since the latter would involve additional steps such as pressure equalization, product purge, etc.

3 Results and Discussion

3.1 Equilibrium Isotherms

Working capacity and selectivity are two major criteria for adsorbent selection[19]. As is shown in Fig.2(A), the isotherms of CO2 on zeolite 13X at different temperatures are of type I. Even at room temperature, the pure CO2 adsorption amount can be at around 4 mol/kg, indicating the great working capacity. The quite low N2 isotherm curves reveal a high selectivity be-tween CO2 and N2. However, it should be noticed from Fig.3(A) that water isotherm on zeolite 13X is of Type II and still retains a substantial working capacity even under vacuum conditions. What is even worse is that water desorption on zeolite 13X is quite hard even at relatively low pressure due to the strong interaction between Na+ cations and polar water molecules[20,21].

Fig.2(B) depicts CO2 and N2 isotherms onto ACF and CDX. Both the materials can still adsorb a certain amount of CO2 and less N2 although it is not as good as zeolite 13X for CO2 capture. Nevertheless, what is significantly focused de-pends on its isotherm about moisture, as is seen in Fig.3(B). The adsorption isotherm of water on alumina CDX is of the“BET” shape, a typical type II isotherm. This suggests that

Fig.2 Adsorption isotherms of CO2 and N2 on

zeolite 13X(A), ACF and CDX(B) at dif-ferent temperatures

(A) ■ CO2, 20 °C; * CO2, 40 °C; △ CO2, 90 °C; ▼ N2, 20 °C; ◇ N2, 40 °C; × N2, 90 °C. (B) ■ CO2, 0 °C, ACF; ● CO2, 40 °C, ACF; ★ CO2, 0 °C, CDX; ▼ CO2, 40 °C, CDX; ◇ N2, 0 °C, ACF; ◁ N2, 40 °C, ACF; ☆ N2, 0 °C, CDX.

No.1 XU Dong et al. 131

Fig.3 Adsorption isotherms of H2O on zeolite 13X(A), ACF and CDX(B) at different temperatures (A) ▼ Adsorption, △ desorption, 25 °C, p0=3.166 kPa; (B) ■ H2O on ACF; □ H2O on CDX.

there is a steady and quick increase of water loading at high humidity pressure. Meanwhile, a moderate hysteresis loop was also observed, indicating the existence of mesopores in alumina CDX, which has been confirmed by the results of liquid nitro-gen adsorption. Moreover, water isotherm on ACF shows a considerable water loading of more than 28 mol/kg at a water partial pressure of over 1.5 kPa. However, that capacity is even lower than that of CDX in the pressure range less than 1.0 kPa. Meanwhile, pressure gets down to a quite low value during evacuation step. Consequently, ACF may perform a better de-siccant to adsorb more moisture compared with CDX. However, ACF can cause significant pressure drop if there is too much ACF packaged inside the column. So we used triple-layered PSA for CO2 capture in this study.

3.2 Temperature Migration

The temperature profiles were observed at three important positions along the entire bed, which were exactly at the posi-tions of the loading zone of ACF, CDX and zeolite 13X respec-tively, as is shown in Fig.4. The positions were accordingly given by thermocouples T2, T4 and T6(at Z=100, 180 and 260 mm from the feed part). It is clear that ACF zone indicated the greatest temperature excursion due to its strong water adsorp-tion capacity. On the other hand, CDX zone showed an inter-mediate temperature change, but zeolite 13X zone illustrated the least wave fluctuation. That is because water adsorption heat is much greater than that of carbon dioxide[15]. There was not huge temperature excursion at zeolite 13X zone, which indicated water vapor was successfully kept inside the pre-layers.

The thermal profile contour plot of cycle time vs. whole

Fig.4 Temperature profiles within one cycle at cyclic steady state at positions of ACF(a), CDX(b) and zeolite 13X(c) loading zone, respectively

dimensionless column length is shown in Fig.5 after the appa-ratus running for over 15 h. Because of the endothermic de-sorption from the last cycle, the temperatures were relatively low throughout the column at the first part of feeding step. However, temperatures rose quickly to the highest level at the end of feed time that was caused by the CO2 adsorption at main-layer zone and H2O adsorption at pre-layer zone, respec-tively. Clearly, a high temperature spot can be observed at the bottom of the bed, which is obviously corresponding to the water front in the adsorption bed. Then it is concluded that water vapor has been stopped at the pre-layer zone. As the process continued to be evacuation step(45―150 s), tempera-ture dropped gradually to the lowest point caused by the CO2 and H2O desorption. It is worthy of mentioning that the most dropped temperature happened in the ACF pre-layer zone, which means water in ACF can be mostly desorbed.

Fig.5 3D contour plot of cycle time and whole

dimensionless column length

3.3 Water Evolution

When the cycle reaches steady state, water movement history at the outside bottom of the column is described in Fig.6. At the first 45 s of adsorption step, water increased sharply to around 5.4% and then the value dropped drastically to about 1.0% at the following evacuation step. Such a trend lasted until the end of evacuation step. During the last 3-second repressurization step, the water concentration climbed up to the same value as that of the previous feeding step, then a whole cycle finished. In the next cycle, there would be the same water evolution trend as shown in Fig.6. Fig.6 shows moisture from flue gas could be easily moved from the adsorption bed.

132 CHEM. RES. CHINESE UNIVERSITIES Vol.28

Fig.6 Water concentration swing at the bottom of adsorption bed during a cycle at cyclic steady state

3.4 Comparison of Dry to Wet Flue Gases

In order to understand the influence of moisture on CO2

capture performance, CO2 purity and recovery from the dry and wet gases were compared in Fig.7. Whatever the feed CO2 was 10.5% or 15.5%, the capture performance did not drop serious-ly. It is certainly that the CO2 purity was higher in 15.5% CO2 feed than that in 10.5% CO2 feed. So the double ACF and CDX layers have stopped the water front further moving.

Fig.7 Comparison of dry(a,c) and wet(b,d) flue gases as feed with 10.5% CO2(a,b) or 15.5% CO2(c,d)

3.5 Effect of Evacuation Pressure

The selection of evacuation pressure is significant because that can directly impact the CO2 purity, recovery and most im-portantly, system energy consumption. If the vacuum pressure is too low, the specific power consumption would be extremely high. However, if the evacuation pressure is too high, the CO2 purity and recovery will be low. These trends can be seen in Fig.8. As the increase of evacuation pressure, both CO2 purities

Fig.8 Effect of evacuation pressure on CO2 capture performance as feed with 10.5%(a,b) or 15.5% CO2(c,d), respectively

and recoveries decreased gradually. And even worse, when feed CO2 was at 15.5%, CO2 purity dropped greatly from 80% at 3.0 kPa to 73% at 4.2 kPa. Actually, the relationship between re-covery, purity and vacuum level is strongly dependent on the shape of the adsorption isotherm. Fig.2 shows that the slope of the adsorption isotherm on zeolite 13X is very large, then a lower vacuum level is required if we want to get good CO2 purity and recovery. On the other point, lower vacuum pressure guarantees that water vapor is mostly desorbed and kept inside the pre-layer.

Experimental studies were conducted to examine the fea-sibility of triple-layered vacuum swing adsorption in CO2 cap-ture and water vapor removal from flue gas simultaneously. Results show ACF, even only with a trace amount, together with alumina CDX performed well to keep moisture away from the main zeolite 13X layer. In addition, Evacuation pressure was significant to govern the system performance. The best CO2 purity and recovery were 74.2% and 65.7% for 10.5% CO2 feed, 80.15% and 74.1% for 15.5% CO2 feed, respectively.

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