A study on CO 2 absorption using hybrid solvents in packed columns

Greenhouse gases emissions from large scale industries as well as gasoline based vehicles are mainly responsible for global warming since the 1980s. At present, it has triggered global efforts to reduce the level of GHG. The contribution of carbon dioxide (CO 2 ) in polluting the environment is at a peak due to the excessive use of coal in power plants. So, serious attention is required to reduce the level of CO 2 using advanced technologies. Carbon dioxide capture and storage may play an important role in this direction. In process industries, various carbon dioxide capture techniques can be used to reduce CO 2 emissions. However, postcombustion carbon dioxide capture is on top priority. Nowadays the researcher is focusing their work on CO 2 capture using hybrid solvent. This work highlights a review of carbon dioxide capture using various kind of hybrid solvent in a packed column. The various challenges for absorption efficiency enhancement and future direction are also discussed in the present work. It is concluded through the literature survey that hybrid solvent shows better efficiency in comparison to the aqueous solution used for CO 2 capture.


INTRODUCTION
Air pollution is a serious concern nowadays because of the fuel consumption in various process industries and exhausts through vehicles in the entire world. Carbon dioxide gas emission is mainly responsible for air pollution and climate change in the atmosphere in the majority. The unabated outflow of carbon dioxide into the atmosphere from different sources is indicating the alarming situation for the future. Its removal is highly required in large scale process industries during processing operations. The Paris Agreement in 2016 forces the entire world to search out effective policies and technologies regarding greenhouse gas (GHG) abatement. At present existing coal-fired power plants are working using biomass as fuel in the blended form with coal in a certain quantity. According to the available data, coal-fired power plants are emitting carbon dioxide (CO 2 ) at a rapid rate in the atmosphere and accounting for roughly 40% of total CO 2 emissions [1]. The alternative way is that coal-based power plant should use biomass with coal to reduce GHGs [2]. Proximate and ultimate analysis of coal and sawdust has been compared as shown in Figures 1 and 2, respectively [3]. So, there is a number of biomass options that exist and its use in process industries may reduce the level of GHGs in the environment. The use of biomass energy is also proposed in low emission scenarios along with carbon capture and storage [4]. The GHGs increase gradually to 78% from the years 2000-2010 and concentrated on property impacts and liquid phase viscosity [5]. Sector-wise global CO 2 emissions by various sector are shown in Figure 3 for the years 2000-2005 [6]. At present, most of the people are using gasoline-based vehicles in the majority for their journey. According to data available, it  can be predicted that fossil fuel derivatives are utilized to create about 67% of the world's power, where, coal, flammable gas and oil contribute about 41%, 21% and 5%, respectively. Developing nations are in the race to acquire advanced technology to compete with developed countries resulting in atmosphere destabilization caused by expanding air convergence of CO 2 discharged amid the ignition of fossil fuel derivatives. So, CO 2 capture and its storage are highly required for other useful purposes. Broad research is going on around the entire world to handle these issues. Nowadays, a number of researchers are focusing their research on the adoption of various techniques to capture CO 2 in various process industries. Several techniques are available to solve this issue, but the major limitation is concerned with the requirement of higher energy and cost required to employ these techniques in process industries. The implementation of these techniques is not only consumed more energy but also reduces the efficiency of the concerned plant. However, environmental pollution can be reduced using these technologies in the process industries in the present scenario. Human being health and environmental pollution have a strong relation, so a reduction in GHGs is highly required for a human being healthy life. A comparison of per-capita CO 2 emissions in different countries is shown in Figure 4.

CARBON DIOXIDE REMOVAL TECHNIQUES
Carbon dioxide capture and sequestration is a process that comprises the set of various technologies to stripped out CO 2 from various process industries like coal and gas-based power plants. There are many technologies that can be applicable nowadays like a liquid solvent, chemical looping. Three basic techniques used to capture carbon dioxide viz. Pre-combustion, Post-combustion and oxy-fuel combustion are shown in Figure 5. The pre-combustion technique is used to reduce the level of carbon dioxide from the fuel before its combustion stage (see Figure 6). In post-combustion, carbon dioxide is captured after the fuel combustion in a closed circuit where no new medium replenished (see Figure 7). In order to separate CO 2, the stripper is used and the absorber is used to absorb it. The process takes place in different columns. The main purpose of oxy-fuel combustion is to remove inert gases from the combustion of flue gases. In this method, complete combustion is done by firing fuel with only oxygen. This oxy-fuel combustion process is shown in Figure 8. Amongst these techniques, post-combustion CO 2 capture using chemical process is most efficient and cost effective in comparison to other techniques used [2]. Its efficiency is around 90%. This process has more capacity to absorb CO 2 at less time. The changes in design parameters and with different column structure, the capture ratio using different solvents can be increased. So, this study is mainly based on the CO 2 capture using hybrid solvent.

CARBON DIOXIDE CAPTURE SCENARIO
At present, environmental pollution is a big challenge for the entire world. CO 2 emissions in the atmosphere through process industries are increasing at a rapid rate. Nowadays population growth and its requirement to fulfill its daily needs is increasing and to satisfy these needs they used vehicles and household products and consumed electricity. According to the Paris summit, India committed that by 2020 they would reduce emission about 30-35% [13]. The CCS system (carbon capture and storage) is a rising technology to reduce greenhouse emissions. The various new approach and application are made to reduce GHGs according to Paris settlement on 4 November 2016 [14]. Due to this agreement, renewable energy resources are going to be promoted by various nation governments with the implementation of subsidy offered to customers. It will also help in to reduce emissions in the environment. The current report of the International Energy Agency stated that CO 2 emissions are gradually increasing in the present scenario and is approximately 32.3 Gt occurring from fossil fuel. The various decision making and optimization models were reviewed to capture CO 2 [15]. The new approach to capture CO 2 has also been developed. In the recent developments, different types of hybrid process to capture CO 2 , such as absorption type, adsorption type, membrane type, cryogenic type and hydrate chemical looping combustion,  has occurred [16]. A new system is evolved for the absorption of carbon dioxide using double absorption heat transformation, which increases the energy level to a certain limit to reach the required temperature. The Aspen Plus 11.0 simulation software is used to stimulate the temperature and flow of CO 2 [17]. The new technologies and the method of implementations are carried out using chemical absorption and scrubbing methods. Alternatively, power-intensive methods like membrane module for carbon dioxide separation in the latest years appears to be an aggressive replacement for standard chemical absorption [18]. In order to reduce the emissions, the project was made by the clean development mechanism to control these emissions. According to the Central Electricity Authority 2017 report, power generation capacity using various resources have been shown in Figure 9 to a capacity of 25 MW [19].
The optimal solution and important parameters like flow rate and gas rate etc are calculated by the Taguchi method to capture CO 2 from packed bed scrubber [20]. An experimental set-up was proposed for post-combustion CO 2 capture using a hybrid solvent [Monoethanolamine (MEA)-Methanol (MeOH)]. A comparison with MEA and MEA-MeOH solvent was compared and concluded that MEA-MeOH hybrid type solvent is better than MEA to capture CO 2 in process industries [21]. The regeneration performance of desorption of CO 2 from the hybrid solvent was proposed and a comparison was done with the packed columns [22]. The bio-inspired material (BMS) is used for carbon capture and storage (CCS) and is used in energy applications. A comparison with MEA and MeOH solvent was proposed and concluded that concentration and liquid flow rate have a large effect on system performance. The emission increases rapidly in the environment. The carbon dioxide capture process is used for solvent selected intensified in post-combustion [22]. A continuous bubble column scrubber is used to capture CO 2 gas by MEA and concluded that the gas flow rate and temperature are minimal [23]. A kinetic model was also used to capture CO 2 using post-combustion process. A comparison was done with MEA degradation in large scale [24]. The performance of potassium carbonate in the solvent absorption process was studied using Aspen simulation [25]. In this post-combustion process, regeneration heat was proposed by utilizing the chemical solvent and comparison was done with diethylenetriamine (DETA) and MEA [26]. The research has been done on the comparison of chipped retention and desorption with amine desorption using multiple fast screening methods [27]. The fluid stream rate was assumed to be greater with the wetted territory in CO 2 capture [28]. The carbon dioxide capture efficiency found to be greater than 85% with a low ammonia content up to 6 wt% [29]. The interfacial flows by the organized pressing of the segment using computational fluid dynamics have been analyzed to improve the numerical reproduction method [30]. A study also has been done to measure the impact of reaction parameters     on absorption efficiency, absorption process intensification along with simultaneous capture with another pollutant [31]. This study focused on the measurement of three characteristics of packing material contact area of liquid-gas, mass transfer and five different structured and random packing [32]. The CO 2 absorption in aqueous diethylene amine was also performed and compared with structured packing using artificial neural network [33]. The novel solvent was used to evaluate the performance of CO 2 absorption and compared it with the packed columns. It was observed that the gas stream rate has a greater impact on the absorption performance of CO 2 [34]. A study based on an efficiency penalty with carbon capture and storage system in coal-fired power plants was suggested. According to this study, it was concluded that the role of CO 2 scrubbing solver may play an important role and may increase the efficiency of power plants by 2% [35]. A study based on reactive absorption in a pilot plant using four solvent namely N-methyldiethanolamine, N-methyl propane di-amine, Amino-2-methyl-1-propanol and N-methyl-1,3-propane-amine was done and concluded that solvent 4 is promising one [36]. The activated carbons that are used as a separation medium to capture CO 2 seize are examined in term of absorption equilibrium and it avoids the regeneration process [37]. The development of technology from a few years are promising taking place in the application of the post-combustion process. Among all of them Ca-looping is one of the developed technology [38]. The BMS was used in various applications to protect the environment. It possesses excellent chemical and thermal stability [39]. The chemical looping is also going to be used by the researchers for carbon capture and it takes place with the gasification process [40]. Nowadays, CO 2 capture is also proposed with the new hybrid system integrated molten carbonate fuel cells using Aspen Plus software. The performance of the system is examined and compared with the coal power plant to capture CO 2 using MEA Figure 9. Power generation capacity using various resources [19]. solution [41]. Global warming is increased gradually so that new techniques are developed regularly to utilize CO 2 in the present scenario. For this purpose, CO 2 is captured and stored regularly in a certain tank [42]. The CO 2 capturing is achieved with three new technologies, mainly absorption, along with the time-based solvents, likely econamine and econamine FG+ and Ca-looping, and were evaluated with the power plants [43]. The solvent desorption was used to decrease the energy utilization in carbon dioxide capture using MEA solution to stimulate the process by HYSYS 3.2 software [44]. In this process, absorption takes place and divides the GHGs from emissions and numerical modeling provocating the post-combustion process [45]. There are several techniques to reduce the emission in the present scenario. Three new techniques evolved i.e. amine scrubber, chemical looping and oxy-fuel combustion [46]. The oxy fuel-fired plant performance was analyzed and the energy was stored. The solution is provided with a sensible heating temperature incorporated with ASU and a central processing unit for storage of energy [47]. The mainly occurring emissions are from coal. The power generation is around 60-80% from the coal [48]. In the pre-combustion process, the integrated gasification cycle is utilized to capture CO 2 . The rate of flow can be examined by oxygen transport membrane [49]. The performance of CO 2 absorbed in the absorption column and certain dixon rings were used with DETA solution at a temperature ranging from 301-303 k. The solution was examined with standard solution MEA. The volumetric typical mass transfer took place in the absorption column into aqueous DETA and the column is packed with random packing like Sulzer DX-type packing with certain temperature ranging from 30-50 • C [50]. CO 2 sustainable and conventional scenario to 2050 is shown in Figure 10  A new model (inexact carbon capture and storage model) to capture and store CO 2 has also been developed [51]. The most evolved technology for CO 2 capture is Amine scrubbing where purification of the solvents is highly required [52]. Investigated research on the ability of the carbon capture and storage (CCS) system implemented in power plant affect the cooling system and water utilization. The water utilization and demand for usage gradually increases from 2030 to 2050 and may create a problem in the future [53]. So, the new system is evolved in the capturing of CO 2 is the cryogenic process. This is installed in the underline of the power plant and examined with hybrid systems [54]. The study has also done on the cost of the post-combustion process in the supercritical power plant and natural gas power cycle power plant [55]. It was also concluded that the most reduced emission takes place in oxy-fuel combustion in comparison to postand pre-combustion [56]. A solvent-based post-combustion solar assisted carbon capture was proposed for a coal-based power plant [57]. The gasification process has been made for the reduction of emission in nature. So, the waste reduction algorithm was used for gasification among the integrated gasification to capture CO 2 . The new techniques evolved, like a gas-liquid solvent, chemical looping and iron-oxygen carriers. Among this chemical, looping was a promising one [58]. The experiment was examined for gases present in the atmosphere; these were absorbed in the absorption column with the packed structure to improve the capacity of absorption in the hybrid solution like MEA-MeOH [2]. A new technology (external cooling loop cryogenic carbon capture) was used to capture CO 2 for a coal fired with capacity 550 MW and captures nearly 85-90%. The stimulation was based on Aspen Plus software [59]. In the year 2050, the entire world should adopt one policy to decrease the emission coming out of various industries. It is the decarbonization of power storage and the different process is required to capture the carbon dioxide [60]. The hot potassium carbonate solvent was used in pre-combustion to achieve 90% of CO 2 capture [61]. Further, there is scope for amine based product to capture CO 2 [62].

CHALLENGES AND FUTURE SCOPE
It is already concluded through the literature that MEA makes a strong bond with CO 2 and requires a huge amount of energy to split it up. So, the process optimization is the primary goal to minimize the energy consumption and thereby a cost-effective process is highly required to enhance plant efficiency. The impact of the solvent used in the process on environment is also a big concern. In the near future, it is expected that the absorption process may be improved and challenges can be overcome for the experimental set-up in process industries to capture CO 2 . Also, the improvement in hybrid solvent based CO 2 capture systems will increase the efficiency of process industries. The absorption efficiency of the process may be optimized using controlled operating factors. At present, the development of closed processes for removal and capture of CO 2 is highly required in process industries.

CONCLUSIONS
This paper discusses the use of various hybrid solvents in a packed column for CO 2 capture. The various techniques using hybrid solvent are compared in this study. It is concluded that MEA-MeOH shows better efficiency in comparison to aqueous MEA solution. Regeneration temperature is also found to be lower than an aqueous MEA solution. The study also concludes that MEA-MeOH performance cannot be enhanced by adding glycerol with it. However, a number of hybrid solvents have been studied; still, more research is highly required to minimize methanol vaporization.