Abstract: Kenaf is a green engineering material that has a great potential to be used as CO2 adsorbent. To enhance the capability of kenaf core as CO2 adsorbent, it should be cleaned and modified via various kinds of treatments. Prior to treatment process, the sample should be cleaned to remove all the adherent extraneous matters. In this study, kenaf core sample has been cleaned by using different types of cleaning methods such as using distilled water at room and boiling water, NaOH solution and HCl solution. This study revealed that the cleaning by using distilled water at room temperature is the most efficient way as compared to other methods. Scanning Electron Microscope (SEM) carried out showed that the surface of kenaf core after cleaned by using distilled water at room temperature indicates an open cylindrical channel with similar size of honeycomb shape gaps that is beneficial to provide an active site for CO2 to be trapped or adsorbed. CO2 adsorption study conducted in Pressure Swing Adsorption (PSA) column showed that kenaf core sample after cleaning by using distilled water at room temperature could adsorb CO2 up to 84.45% and relatively higher than the other methods. This study also revealed that 5 min is sufficient for adsorption to take place. Further increase in the adsorption time does not indicate any significant change to the percentage of CO2 adsorption. The future works may involve a treatment and modification of kenaf core to enhance the CO2 adsorption process.
INTRODUCTION
Global warming is caused by the emission of anthropogenic Greenhouse Gases (GHG) to the atmosphere which contributed by the combustion of fossil fuels such as coal, natural gas or petroleum and industrial processing. Among these anthropogenic gases, Carbon Dioxide (CO2) is the most abundant gas which could give severe effects to the environment and human health. The removal of carbon dioxide is the crucial problem in gas purification operations such as the production of hydrogen gas, landfill and natural gas treatment and hydrocarbon purification process. To address these issues, several methods such as fossil fuel replacement with environmental friendly bio-fuel, improvement in energy efficiency and CO2 sequestration from its source (Bernal et al., 2004) should take place. Among these methods, the sequestration of CO2 from natural gas stream has received considerable attention (Lin and Freeman, 2005). In this process, CO2 is removed from natural gas in order to enhance the purity of the natural gas (Li et al., 2008). The technologies involve for CO2 removal are liquid absorption (Hook, 1997; Zhang et al., 2002; Akanksha et al., 2007; Samanta and Bandyopadhyay, 2009), solid adsorption (Anson et al., 2009; Zhao et al., 2010), cryogenic techniques (Zanganeh et al., 2009) and selective diffusion through polymer, ceramic or metallic membranes. All these technologies are the sub-set of the technology known as Carbon Capture and Storage (CCS).
Recently, the removal of CO2 by using amine-based liquid absorption has been used commercially (Samanta and Bandyopadhyay, 2009). But, this technology has several disadvantages such as needs a large equipment size, inefficient, involves high energy, solvent degradation and corrosion in the presence of water. Thus, solid adsorption system is seen as a potential alternative to replace this conventional method since it is the most promising option in terms of low energy consumption, low equipment cost and easy to apply (Mandal and Bandyopadhyay, 2009). But, the most important consideration to implement this system is the selection of the appropriate adsorbent. The adsorbent should have features like high selectivity or affinity towards CO2 at high temperature, good adsorption/desorption capability, adequate adsorption/desorption kinetics, good mechanical strength, stable adsorption capacity after cycle and selective at elevated temperature (Dantas et al., 2011; Yong et al., 2002). The adsorbents that have been proved to capture CO2 are carbon-based adsorbent, metal oxides sorbents, zeolites 13X, Hydrotalcite-like Compounds (HTIcs) and basic alumina (Yong et al., 2002). However, the selection of the green-based adsorbent is an attractive method in CCS technology due with its availability, inexpensive and environmental-friendly.
Kenaf (Hibiscus cannabinus L.) that belongs to the family of Malvaceae is the potential green-based adsorbent that could be used to replace the commercial adsorbent due to its high availability in Malaysia, easily growth plant which can achieve a height of 3.5-4.5 m within 4-5 months (Cuerda-Correa et al., 2008; Elsaid et al., 2011) and has porous structure. Due to its highly micro-pores structure and low density (0.09-0.11 g cm-3), kenaf core could become a superior adsorbent to capture CO2. The natural kenaf core used in this study is categorized into a macro-pore structure since the presence of pore size is in the range of 1.22-2.09 μm. Therefore, the natural kenaf core can be modified in order to enhance its capability for CO2 adsorption. Prior to modification process, kenaf core needs to be cleaned in order to remove all the adherent extraneous matters that trapped on its surface (Othman and Akil, 2008).
According to the previous study, kenaf inner core was cleaned with water for use as oil and heavy metal adsorbent (Othman et al., 2008). Apart from that, Aber et al. (2009) used double distilled water to clean the kenaf natural fiber prior to be carbonized and activated for adsorption of phenolic compounds. In removing lead (II) from waste water, kenaf fiber was first cleaned by using hot distilled water before turned into activated carbon (Chowdhury et al., 2011). The similar method has been selected by Mahmoud et al. (2012) as an initial procedure in preparing a portion of kenaf fiber char to remove methylene blue dye from aqueous solutions. Another portion of the kenaf fiber char was cleaned with 3 M HCl followed by hot distilled water until the achieved pH value is 6 (Mahmoud et al., 2012). Sajab et al. (2011) conducted the preliminary treatment for kenaf core fibers via 0.1 M NaOH before rinsed with de-ionized water until the pH dropped to ~7. In addition, Edeerozey et al. (2007) found that 6 v/v% of NaOH is the optimum concentration for cleaning kenaf fiber surface. Based on these studies, an experiment was conducted on the cleaning of kenaf core by using different approaches such as distilled water, NaOH and HCl solution and the performance of the adsorbent was evaluated based on the percentage of CO2 removal in Pressure Swing Adsorption (PSA) system.
The objectives of the present work are to study the effect of different cleaning methods on the kenaf core surface; thereby to the capability in capturing CO2 by using Pressure Swing Adsorption column (PSA). On the other hands, this study also developed in order to determine the sufficient adsorption time for CO2 adsorption process by using different cleaning approaches. To achieve these objectives, kenaf core adsorbent should be characterized via physical characterization analysis in order to provide a fundamental knowledge of gas adsorption characteristics for adsorption process. At the end of this work, the authors are able to find the most effective ways to clean kenaf core adsorbent and also the most sufficient adsorption time for CO2 adsorption process by using different kinds of cleaning methods. This study also revealed the capability of kenaf core as a green-based engineering adsorbent that is comparable to commercialize polymeric adsorbent.
MATERIALS AND METHODS
Preparation of cleaning process: The raw kenaf core was obtained from National Kenaf and Tobacco Board (LKTN), Kelantan in the chips form prior to be crushed into smaller size and prepare for cleaning process. Then, the kenaf core sample has been cleaned by using different cleaning approaches including distilled water, hot distilled water, sodium hydroxide (NaOH) solution and hydrochloric acid (HCl) solution and labeled as sample A, B, C and D, respectively. To prepare sample A, 2 g of kenaf sample was soaked using 80 mL distilled water for 1 h. The same amount of kenaf core sample also soaked into hot distilled water, 3 M NaOH and 3 M HCl prior to be filtered and dried in the oven for overnight. After that, these samples were analyzed to observe the surface morphology by using Scanning Electron Microscopy (SEM) with a gold metal layer coating.
CO2 adsorption process: The cleaned kenaf core powder was loaded into the Pressure Swing Adsorption (PSA) column for CO2 adsorption study at temperature of 27°C (300 K). The height dimension of the column is 15 cm; whereas 4 cm for the adsorbent height (Fig. 1). Apart from that, the molecular sieves and glass wool were inserted into the column in height dimension of 3.5 and 1 cm, respectively in order to adsorb the extra moisture in the gas stream and to prevent the kenaf core adsorbent from flowing inside the column. The adsorption study was conducted using high purity grade CO2 (99.999%) gas. Meanwhile, nitrogen with high purity of 99.999% was acted as the purge gas and these gases are supplied by Mega Mount Sdn. Bhd. The flow rate of CO2 to the adsorbent bed was controlled by using Gas Flow Controller (GFC). The gas flow rate was monitored until the pressure of the adsorbent bed was achieved to 1.5 bar.
| Fig. 1: | Image of PSA column |
Then, the gas flow rate was closed and time taken to adsorb the amounts of CO2 was measured. In this study, the adsorption time was manipulated for 5, 10 and 15 min, respectively to study the effect of the adsorption time to the CO2 adsorption process. The percentage of the CO2 adsorbed by the adsorbent was calculated according to the CO2 concentration released from the system by gas chromatography (Agilent Technologies 7820A GC System). Besides, non-adsorbing Helium (He) was acts as a carrier gas in the GC system.
RESULTS AND DISCUSSION
Scanning electron microscopy: In this study, the physical characterization analysis was carried out by using Scanning Electron Microscopy (SEM) images that provides the surface texture and morphology of the kenaf core after cleaning process. The image of raw kenaf core was also obtained for comparison. Figure 2 showed the SEM image of the cleaning kenaf core by using distilled water at room temperature, distilled water at 100°C, sodium hydroxide (NaOH) solution and hydrochloric acid (HCl) solution at magnification of 500X and these results were also compared to the previous study.
From Fig. 2, the channels appeared for the raw kenaf core sample seems to have a partially obstructed with a rough and irregular surface texture. This image was similar to the image obtained by Mahmoud et al. (2012) as shown in Fig. 2f for the Kenaf Fiber Char (KFC) before treatment process. However, the cylindrical channels of the kenaf core became more open after immersed in the distilled water at temperature of 27°C. Moreover, the formation of these cylindrical channels reveals that the pores within the adsorbent particles are exhibit honeycomb shape gaps with similar sizes than the parent kenaf core (Mahmoud et al., 2012; Macias-Garcia et al., 2012). But, when the temperature was raised up to the boiling temperature of 100°C, the SEM image exhibits irregular sharp ridges with a hairy-looked macro-pores hollow fiber. Sample soaked in NaOH solution has non-uniform cylindrical channels with a severe and broken structure. On the other hand, sample soaked in HCl solution appeared to have heterogeneous pores within the adsorbent particles with a presence of honeycomb shape gaps and equivalent to the result obtained by Mahmoud et al. (2012) for kenaf particle after HCl treatment as represented in Fig. 2g. In general, this result suggested that the most appropriate method for cleaning kenaf fiber without damaging the structure of the cylindrical channels is by using distilled water at room temperature.
CO2 adsorption process: The cleaning kenaf core samples with different cleaning methods were loaded into Pressure Swing Adsorption (PSA) column for CO2 adsorption study. The results of CO2 adsorption (in percentage) by cleaning kenaf core samples are presented in Fig. 3. This result is based on 5 minutes adsorption time and the raw kenaf core sample also used for comparison.
The percentage of CO2 adsorbed was calculated before and after CO2 passed through the column which containing kenaf core adsorbent. The result reveals that the amount of CO2 adsorbed was affected by the cleaning process. As shown in Fig. 3, cleaning method using distilled water shows the highest percentage of CO2 adsorption (84.45%) followed by hot distilled water (82.44%), sodium hydroxide (64.54%), hydrochloric acid (56.30%) and raw kenaf (51.53%). Based in the preliminary result, it is suggested to use distilled water at normal temperature to clean kenaf core sample due to the presence of parallel flow-through open cylindrical channels with a highly heterogeneous pores that allow these structure acts as a potential adsorbent to trap and adsorb CO2 (Mahmoud et al., 2012). In addition, Sajab et al. (2011) recommended that pH 7 exhibits maximum adsorption intended for kenaf core fiber. However, increase the temperature of the distilled water up to 100°C caused the kenaf core structure became irregular sharp ridges; thus reduce the amount of CO2 captured towards the surface.
| Fig. 2(a-g): | SEM image for cleaning process at magnification of 500X (a) Raw kenaf core (b) Distilled water at 27°C
(c) Distilled water at 100°C (d) NaOH solution (e) HCl solution (f) Raw kenaf core (Mahmoud et al.,
2012) and (g) HCl solution (Mahmoud et al., 2012) |
| Fig. 3: | CO2 adsorption for cleaning samples at 5 min adsorption time |
| Fig. 4: | Percentage of CO2 adsorption on kenaf samples using different cleaning method |
The sample of kenaf core soaked in the NaOH and HCl solution produced severe and broken cylindrical channels that reduce the surface area available for CO2 to adsorb. More importantly, it was observed that the cleaning kenaf core had higher CO2 adsorption capacities rather than the raw kenaf core itself. It can be supported by the existence of the heterogeneous pores within the adsorbent particles after conducting a cleaning process that provides a high possibility for CO2 to be trapped and adsorbed (Mahmoud et al. 2012). Moreover, it was noticed that there was no apparent relationship between the sample surface area and its CO2 adsorption capacity.
Adsorption time: Figure 4 illustrates the percentage of CO2 adsorbed by kenaf core adsorbent at different adsorption time (5, 10 and 15 min). In this study, the pressure in the adsorption column gradually decreased as the interaction between adsorbent and adsorbate occurred. During this process, CO2 gas passed through the bed of adsorbent, diffuses to the adsorbent surface and retained in the pore due to the weak attraction forces of intermolecular cohesion between adsorbent and adsorbate (CO2). The CO2 molecules are packed more tightly together as the pressure increased up to 1.5 bar and more molecules have a chance to “hit” the adsorption sites, thereby increase the number of CO2 molecules adsorbed. After 5 min adsorption time, the amount of CO2 not adsorbed was measured. As the adsorption time increased to 10 min, the amount of CO2 adsorbed does not change significantly. Subsequent increase in adsorption time of 15 min showed no change to the percentage of CO2 adsorption. Therefore, it can be inferred that 5 min adsorption time is the sufficient time for CO2 adsorption process in PSA column using kenaf core adsorbent.
CONCLUSION
This study revealed that the cleaning kenaf core lead to have a higher CO2 adsorption and cleaning by using distilled water at room temperature is the most suitable method as compared to hot distilled water, sodium hydroxide (NaOH) and hydrochloric acid (HCl). By cleaning using distilled water, the porous structure can be improved and CO2 molecules can be adsorbed up to 84.45%. This is due to the presence of open cylindrical channels with highly heterogeneous pores within the adsorbent particles that provides a high possibility for CO2 to be trapped and adsorbed. This study also found that 5 min is the sufficient adsorption time for CO2 adsorption process in PSA column. Generally, this study has proved that kenaf core has a capability to be used as CO2 adsorbent.
ACKNOWLEDGMENTS
The authors are grateful to the Ministry of Higher Education (MOHE) and Universiti Teknologi Malaysia (No.Vot 00H42) for their opportunity and financial support to conduct a research of ‘Kenaf Core as CO2 Adsorbent’. A full appreciation is dedicated to National Kenaf and TobaccoBoard (LKTN), Kelantan for their material supplied and guidance throughout this study.