Effects on Surface Area. Intake Capacity and Regeneration of Monoethanolamine

Abstract 2

1. Introduction

2. Materials and Methodology
2.1. Materials
2.2. Methodology
2.2.1. AC samples characterization

3. Results and Discussion
3.1. Breakthrough time results
3.3. Dynamic adsorption beds capacity improvement
3.4 Unhindered MEA and hindered AMP stoichiometry
3.5. Adsorption isotherm of impregnated and non-impregnated 500 µm AC beds
3.6. Linear regression of Dubinin-Astakhov (D-A) equation
3.7. Thermal characterization of the AC beds
3.9. Effect of regeneration with high temperature on the performance of MEA- impregnated AC beds

Acknowledgment

References

Abstract

Granular palm shell activated carbon (AC) was impregnated separately with monoethanolamine (MEA) and 2-amino-2-methyl-1-propanol (AMP) to improve its natural capacity and selectivity for carbon dioxide (CO₂) adsorption. The total surface area, micropore volume, as well as the heterogeneity of the impregnated AC particles was considerably reduced due to impregnation. CO₂ intake of impregnated 500 µm AC particles improved significantly and adsorptive capacity of 500 µm MEA-impregnated AC particles improved by 172 % and 44 % comparing to non-impregnated and AMP-impregnated AC particles respectively. Solid state amine stoichiometric results indicated that adsorption capacity of unhindered amine (MEA) is higher than that of hindered amine (AMP) by 50 % contrary to liquid amines standard stoichiometry. Exhausted AMP-impregnated beds were regenerated by sweeping at room temperature with stream of pure nitrogen (N2) flowing at 60 ml/min for 4 hours. Heating up to 75 °C was required to regenerate exhausted MEA-impregnated beds. Increasing feed gas flow rate has adverse effect on breakthrough time more than increasing bed operating temperature. Breakthrough time was utilized to evaluate the performance of the different adsorption beds.

Keywords: Activated carbon; CO₂; Adsorption; Impregnation; MEA; AMP.

1. Introduction

The topics of climate change and global warming have been discussed and investigated thoroughly. While both of them are naturally occurring phenomena, climate change is the average long-term changes in climate. Global warming, which is one of the causes of climate change is the increase in the temperature of the lower atmosphere due to the effects of greenhouse gases. Global warming occurs naturally in cycles throughout periods of time due to changes in the profile of the Earth path around the sun known as Milankovitch cycles, but the current episode of global warming is believed mostly to be man-made [1] because of the unprecedented increase in the concentration of greenhouse gases above the earth surface particularly due to the anthropogenic emissions of the industrial revolution, which had added substantial quantities of greenhouse gases (GHGs) to the atmosphere. GHGs like water vapor, carbon dioxide, methane, nitrous oxide, and other minor atmospheric gases absorb some of the outgoing infrared radiation (heat) reflected by the earth surface. The retained heat would accumulate and raise the lower atmosphere temperature [2]. The most important anthropogenic GHG is CO₂, which is virtually considered responsible for causing the greenhouse impact IPCC, 2007 IPCC, Intergovernmental panel on climate change, WMO, UNEP. Climate change 2007 the physical science basis, Summary for policymakers. IPCC WGI Fourth Assessment Report. SPM2feb07 (2007). [3]. Harmful effects of changes in precipitation and temperature is partly believed to be due to the GHGs mainly as a result of fossil fuel consumption such as coal, oil and natural gas in daily life activities, which is providing more than 85% of global energy needs [4], but at the same time extensive relying on fossil fuels increased the average atmospheric temperature by 0.8 °C in the last 150 years [5]. To reduce CO₂ concentration in the atmosphere, various technical options, such as adsorption [6], absorption [7], membrane separation [8] and cryogenic [9] have been proposed and investigated for the capture and sequestration of CO₂. Because of low energy requirement, minimal effects on environment, and ease of applicability, adsorption is considered as one of the most capable techniques in the commercial and industrial applications [10]. Solid sorbents, like AC is one of the most auspicious options for post-combustion CO₂ adsorption [11]. Porous AC is produced from a wide range of carbon-rich raw materials and biomass resources, which is considered renewable [12]. AC texture is adaptable as the surface functional groups trait and density as well as the pore size distribution can be adjusted to suit the application [13]. Microporous AC is superior adsorbent due to its high internal surface area, where the most of the adsorption occurs, narrow pores, where the adsorption capacity is significantly improved, and the presence of chemically active sites, which are vital for the adsorption of specific groups [14]. It is well known that the ability of ACs to adsorb acidic gases will increase when there are nitrogen functional groups on its surface [15]. Modification of AC’s surface chemistry for gas and liquid adsorption by impregnation via slurry method are widely used, albeit surface area reduction and pore blockage are common occurring obstacles [16]. CO₂ capture capacity by modified ACs was found to be improved due to the introduction of alkaline nitrogen groups on their surface [17]. Calcium acetate solution blocked the micropores of AC particles when they were impregnated onto AC samples reducing their surface area but in the same time enhancing CO₂ adsorption by the basic sites formed on the AC surface [18]. Adsorption of CO₂ on impregnated palm shell AC particles is limited in literatures [19] and the topic is even more limited on the dynamic CO₂ adsorption onto palm shell AC particles. Industrial alkanolamine solutions used to absorb CO₂ are; monoehtanolamine (MEA), diethanolamine (DEA), N-mehyldiethanolamine (MDEA), piperazine (PZ) and 2-Amino-2-methyl-1-propanol (AMP). MEA is considered the most effective among the other mentioned alkanolamines [20] because of its high reaction rate. Primary amine MEA is considered frugal [21] and has a higher absorption rate than other secondary, tertiary and sterically hindered amines in the order: MEA > AMP > DEA ≫MDEA [20], but there are some shortcomings in using MEA and AMP as absorbents due to their corrosive behavior, moderate biodegrability and toxicity [21]. In the meantime CO₂ capture technologies available for carbon capture and sequestration (CCS) are pre-combustion, post-combustion and oxyfuel or oxy-combustion [22]. For the first and second systems as the name implies, CO₂ is removed before or after a fuel is burned respectively, while the third system is using pure oxygen instead of air for combustion. Post-combustion CO₂ capture is offering significant advantages over the other two systems due to its readiness to be used in many fossil fuel-fired power plants scattering around the world without the need to modify the combustion process and the other technologies involved, which is usually expensive and extensive [23].

In this work, fixed bed packed with granular AC particles, which are cost-effective adsorbent and its raw materials abundantly available in Southeast Asia particularly Malaysia [24], impregnated separately with primary (MEA) and sterically hindered (AMP) amines, was employed in a dynamic real time experiments to adsorb CO₂ from gas mixture. Water was used as ecologically friendly medium to facilitate the impregnation of the amines. Dynamic breakthrough time plays major rule in this research as a tool for examining and evaluating the beds adsorption quality.

2. Materials and Methodology

2.1. Materials

2.1.1.

Analytical grade monoethanolamine and 2-amino-2-methyl-1-propanol were used in this study, as shown in Table 1.

Table 1 Chemical formulas and molecular weight of MEA and AMP

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2.1.2. Palm shell AC

Commercial granular palm-shell AC (particle size, 710-500 µm), which was produced by physical activation with steam as the activating agent was purchased from Bravo Green SDN BHD (Sarawak, Malaysia). Total BET surface area was found to be 838 cm2/g. Energy Dispersive X-ray (EDX) elements microanalysis showed that the surface of the AC particles is comprised of 88.89 % carbon, 7.75 % oxygen, 2.81 % silica, 0.14 % aluminum and 0.41 % nitrogen by weight.

2.1.3. Gases

Gases used in this study were:

1. Mixture of 15% CO₂ and 85% N2.
2. Pure N2.

2.2. Methodology

2.2.1. AC samples characterization

AC particles were crushed and sieved. Sieves of the sizes of 850, 710 and 500 µm were employed to characterize the AC particles. Two samples were obtained, namely: 710 µm (particles passing 850 and stopping on 710 µm) and 500 µm (particles passing 710 and stopping on 500 µm).

2.2.2 MEA and AMP selection

MEA and AMP were selected among other amines because, MEA is considered as one of the most mature amine absorbents for CO₂ absorption [25], and AMP due to its less energy requirement for regeneration than other amines [26].

2.2.3. Impregnation of AC samples

Impregnation was carried out by stirring 2 g of each MEA and AMP with 5 g of AC for 1 hour, 10 g of deionized water was added as an environmentally friendly medium to facilitate the impregnation process. The final slurry was dried at 70 °C, under vacuum pressure (- 0.1 bar) for 6 hours. Loading efficiency was found to be 91% for both samples.₂

2.2.4. Adsorption of CO

CO₂ molecules were adsorbed onto a combination of solid state MEA and AMP impregnated separately on mostly micropore palm shell AC particles. Solid state amines employment was essential to minimize the harmful effects of liquid amines on environment and to curb their corrosive impacts and their degradation and the high energy required during regeneration [27].

2.2.5. Evaluation method of the adsorption beds performance

Breakthrough time was used to evaluate the adsorption beds performance in real time experiments. Breakthrough time is an expression in time unit standing for the adsorption bed capacity and quality. It can be defined as the elapsed time for adsorption bed to be fully or partially saturated with adsorbate molecules as in this study when CO₂ molecules break through the adsorption bed, which was monitored by Guardian Plus CO₂ monitor. Data Acquisition Logger was connected to the CO₂ monitor to measure the breakthrough time in minutes.

2.2.6. Experimental setup

Experimantal setup is shown in Figure 1. Details of experimental setup’s apparatuses and their specifications can be found in [28].

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Figure 1 Experimental Setup

3. Results and Discussion

3.1. Breakthrough time results

Results in Figure 2 are showing the differences in breakthrough time for three adsorption beds at room temperature. Adsorption of CO₂ molecules is considered concluded when the display of the CO₂ monitor, which is connected to the outlet of the adsorption column, is showing that CO₂ molecules start to exit the bed. These results were obtained using 5 g of each adsorbent, at room temperature and 10 ml/min feed gas flow rate.

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Figure 2 Breakthrough time at room temperature for 500 µm, non-impregnated, AMP impregnated and MEA-impregnated AC beds

Breakthrough time values in Figure 2, increased from 34 min for non-impregnated AC bed to 62 min and to 90 min for AMP-impregnated for MEA-impregnated AC bed respectively. The increase in breakthrough time for MEA and AMP-impregnated beds compared with non-impregnated AC bed was due to that amine molecules have formed many active sites for CO₂ adsorption, where chemisorption was dominant over physisorption leading to adsorb selectively more CO₂ molecules from the feed gas stream. Steric hindrance influence is responsible for breakthrough time reduction of AMP-impregnated AC beds in comparison with MEA-impregnated AC beds.

3.2 Steric hindrance effect

Breakthrough time results are showing that MEA-impregnated AC beds have extended breakthrough time comparing to AMP-impregnated AC beds because of the steric hindrance effect on large amine molecules [29], like AMP, which would hinder CO₂-N2 reaction causing more CO₂ molecules to leave the adsorption bed, hence reducing breakthrough time as shown in Table 2.

Table 2 Adsorbing beds and their breakthrough time

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3.3. Dynamic adsorption beds capacity improvement

The beds dynamic adsorption capacity, mg/g. (mg of CO₂ adsorbed / g of adsorption bed), was calculated using equation 1, for the period identified from the beginning of the adsorption experiment when CO₂ molecules first flow through the adsorption bed until their breakthrough out of the bed ending the adsorption experiment.

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Where:

Q = Volumetric feed gas flow rate (ml/min)

= CO₂ density at 25 °C (mg/ml)

C = CO₂ (%) in the feed gas

t = Breakthrough time (min)

W = Weight of amine-impregnated bed (g)

Results in Table 3, are showing the improvement of MEA-impregnated and AMP-impregnated bed comparing to non-impregnated bed.

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