Ionic liquid-amine blends and CO2BOLs: Prospective solvents for natural gas sweetening and CO2 capture technology—A review
Introduction
Global energy demand is increasing mainly because of increasing population and industrialization. Today, 85% global energy demand is met by fossil fuels such as oil, natural gas, coal, etc., and in near future these are anticipated to play a significant role in global energy economy mainly because of their abundance and availability (Kumar et al., 2011b). Oil, gas and coal are projected to be the most widely used fuels, and have potential to meet global energy demand, almost 80% of total energy consumption in 2040 (ExxonMobil, 2012). However, these are known to be a direct reason for greenhouse gas emissions, specifically CO2 emissions. Among these fossil fuels natural gas is the only who has less environmental impacts (Kumar et al., 2011b). Thus, natural gas is projected to surpass coal for the number-two position after oil and the demand of natural gas is projected to rise by more than 60% through 2040 (ExxonMobil, 2012). Natural gas (NG) at its geological conditions have some complex contaminates such as CO2, H2S, CO and mercaptans, which constitute great ecological threats when get to the atmosphere and also create problems in natural gas processing. Thus, it is recommendable to remove excess CO2 and other gases before its utilization.
Normally, NG is transported via pipelines from gas producing to consuming area. However, as transportation distance increases, pipelines become uneconomic, liquefied natural gas and gas to liquid are more viable routes (Kumar et al., 2011a). The liquefaction of natural gas takes place at -161 °C and as low as atmospheric pressure. Under these circumstances CO2 can freeze out on exchanger surface, plugging line and reduce plant efficiency. Further, in decades ahead, the world will need to increase energy supplies in a way that is safe, secure, affordable and eco-friendly. In this regard, harnessing of clean energy has become one of the major global issues of the 21st century. The level of the challenges is vast and need an integrated set of solutions. Therefore, removal of acid gases from NG before its liquefaction is a prime requirement, which is carried out not only to overwhelm the process bottleneck but also to meet the LNG product specifications, avoid corrosion of process equipment and environmental performance. There exist many acid gas removal techniques such as absorption process (chemical and physical absorption), adsorption processes on solid surfaces, cryogenics, physical separation (membrane) and hybrid solutions (mixed physical or chemical solvent), reported in Table 1 and presented in Fig. 1.
The chemical and physical solvents or the combination of both have been used extensively in existing base load LNG facilities (Kohl and Nielsen, 1985). Selection of an effective and efficient method out of the existing processes/techniques, for natural gas sweetening is an important issue. The major factors responsible for selecting an appropriate technology are: (I) nature and concentration of contaminants in feed gas, (II) the degree of elimination required, (III) hydrocarbon composition of gas and final specifications, (IV) amount of the gas to be processed and operating cost, and (V) circumstances at which the feed gas is existing for processing. The most common and widely used approach for natural gas processing is chemical absorption. Alkanolamines are the most commonly used chemical absorbents for this technology especially due to their low cost and high CO2 loading. Instead of chemical absorption with amine solutions, as was discussed above, physical absorption with physical solvents (e.g., Selexol™, IFPexol™, n-formylmorpholine (NFM)) is another option for acid gas removal. The primary advantage of physical solvents over amine solutions is lower energy requirement as CO2 absorption is accomplished through physical solubility interactions – not chemical reactions. In fact, unlike the energy-intensive regeneration stripping columns in amine-based chemical absorption processes, CO2 recovery via physical absorption processes uses a sequence of flash stages (i.e., successive pressure reductions) to desorb CO2 from the physical solvent. However, physical absorption processes also have several disadvantages:
- 1.
Low CO2 capacity: Physical solvents tend to have lower CO2 capacities than amine solvents. Thus, higher circulation rates and larger equipment are needed. On the other hand, CO2 absorption tends to increase significantly with increasing CO2 concentration or partial pressure.
- 2.
Pickup of hydrocarbons: Significant amounts of valuable hydrocarbons are absorbed by physical solvents. For natural gas processing applications, some of these hydrocarbons can be lost in CO2 waste streams.
- 3.
High circulation rates: Physical solvent processes may require twice the circulation rate as amine solutions. Higher circulation rates result in higher capital and operating expenses. Also, absorber columns using physical solvents typically have more stages of contact and are therefore much taller than those employing amine solutions.
- 4.
Solvent losses: Physical solvents can be entrained and lost to the treated gas. Refrigeration or water-washing may be used to minimize losses but this requires additional capital expense and increased operating cost.
Applications, advantages and disadvantages of these techniques have been reported by Olajire (2010) and D’Alessandro et al. (2010) in detail. Although each technology has its own advantages and disadvantages, still the chemical absorption technology is the most widely used technology and amines are the most commonly used solvents. Chemical absorption is desired for low to moderate CO2 partial pressure. The chemical absorption of CO2 from gaseous streams depends on acid–base neutralization. Due to some technical problems associated with the use of amine solutions, recently new kind of solvents (ionic liquids) and binding organic liquids (BOLs) have been searched out. Therefore, the main aim of this article is to review and compare the suitability of these solvents, their advantages over each other and future applicability of these solvents and their blend in order to find out an economical and efficient technique for natural gas processing and post combustion CO2 capture.
In this context, the application of amine solutions, ionic liquids, their blends and BOLs have been discussed in the proceeding sections. In addition, the impacts of amine and ionic liquids on aquatic and terrestrial organisms have been discussed and finally a set of robust conclusions have been provided with a focus on technologically latest updates of acid gas removal using aqueous amine solutions, ionic liquids and their blends, and their effects on human and environment. Moreover, the process economics is also reported and discussed.
Section snippets
Use of aqueous amine solutions for CO2 and H2S capture
The removal of acid gas impurities from sour gas streams and refinery gases is a significant operation in natural gas processing. Utilization of alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), methyldiethanolamine (MDEA), diglycolamine (DGA) for this task is a commercially available technology, which has been used in gas industry for 60 years and is still considered as the most established technology (Olajire, 2010). Their reactivity and availability
Use of ionic liquids (ILs) for CO2 and H2S capture
Due to various disadvantages such as the loss of amine during acid gas stripping, corrosion problem and high energy requirement for desorption, associated with the use of aqueous solutions of alkanolamines, recently, a new class of material (ionic liquids, ILs) having a set of exceptional physicochemical properties such as a wide range of liquid, thermal stability, negligible vapor pressure, tenability and reasonable CO2 solubility has been reported.
ILs are a category of compounds which are
Use of IL-amine blends for CO2 and H2S capture
To bring versatility and robustness to the CO2 capture technologies, a great deal of work using ILs for CO2 scrubbing is under progress. Due to their exceptional characteristics ILs are considered as viable alternates (Hasib-ur-Rahman et al., 2010). Numerous researches have been carried out related to the designing of task specific ILs (TSILs) for CO2 capture. However, these TSILs and their derivatives do not appear to be viable for industrial applications mainly because of their high viscosity
Use of binding organic liquids (BOLs, reversible nonpolar-to-polar solvents) for CO2 and H2S scrubbing
Most of the current CO2 capture and natural gas sweetening technologies are amine based and requirement of water as a solvent decreases the efficiency of the process as the solvent regeneration energy is largely defined by the amount of water in the process. Simultaneously, ILs are also hygroscopic in nature and the presence of water in natural gas may decrease the efficiency of the process. Therefore, it is required to find out a class of solvent that can work in the presence of moisture
Health and environmental impacts of aqueous amine, ILs and BOLs
A scrutiny of possible positive and negative effects of any developed material on health and environment is a must before its application on commercial level, regardless of its economic value and technical efficacy. Although the amine solutions and ILs are highly potential candidates for CO2 capture technologies, natural gas processing and petroleum refining, they have some environmental and health related issues. Their health and environmental issues and their remedies are discussed in this
Economics of the CO2 capture process
Scrubbing of CO2 using absorption/stripping process requires energy in the form of electricity and steam supplied by the power plant. Thus, the scrubbing process reduces the efficiency of power plant. Accordingly, it has become necessary to evaluate the economics of the CO2 capture process. In this regard, several studies have been carried out by many investigators (Dooley et al., 1999, Eckaus et al., 1996, Kim and Edmonds, 2000). They sought to integrate knowledge about the economics of CCS
Conclusions and future research directions
Today, the most widely used technology used for natural gas sweetening and pre and post combustion CO2 capture is the chemical absorption. Aqueous alkanol amines are the only chemical solvents which are used for CO2 and H2S capture on commercial level mainly because of their high gas loading capacity. However, many disadvantages such as solvent loss, high reboiler duty and corrosion have been observed while using them for natural gas processing and post combustion CO2 capture from different
Acknowledgement
This research was supported by a grant from the GAS Plant R&D Center funded by the Ministry of Land, Transportation and Maritime Affairs (MLTM) of the Korean government and also respectfully supported by BK 21 Program funded by the Ministry of Education (MOE) of Korea.
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