Merits and Demerits of Carbon Dioxide in Separation Processes

1. Introduction

In 2020~2021, there were many frequently cited articles published in Separations. Our Editorial Board noticed that several research articles were based on carbon dioxide, and there is an increasing trend of such articles in our journal. Carbon dioxide plays an important role in chemical separation, mineralization, energy production, etc. Carbon dioxide can promote the chemical separation of some substances; however, the separation of carbon dioxide itself from other substances can cause problems.

Carbon dioxide is stable. Its density is 1.53 times that of air, its critical temperature is 304.2K, and its critical pressure is 7.38Mpa. It can be liquefied under pressure at room temperature [1]. When the temperature and pressure are higher than the critical temperature, carbon dioxide enters the supercritical state. A supercritical carbon dioxide (SCO₂) chemical reaction occurs when carbon dioxide reaches a supercritical state, and has potential advantages in chemical separation and energy utilization [2,3,4,5]. SCO₂ extraction technology is a separation technology that uses SCO₂ fluid above the critical temperature and pressure as an extractant [6,7,8]. By adjusting the temperature or pressure, the solubility of SCO₂ can be changed [9]. When the solubility of SCO₂ increases 100~1000 times, SCO₂ can extract more kinds of substances [10,11,12]. Furthermore, SCO₂ extraction is also used in the energy sector. For example, using SCO₂ as a working fluid in the Brayton cycle confers the advantages of high thermal efficiency and compactness, showing great application prospects in the field of power generation and heat transfer [13,14].

SCO₂ could be used as an efficient solvent in the solvent extraction process due to having a lower viscosity, higher solubility capacity and faster diffusivity than organic solvents [15]. However, the temperature needed to carry out the supercritical extraction process is higher than room temperature, which makes it unsuitable for substances with requirements for thermal stability. Similar to the supercritical extraction technique, CO₂-expanded liquids (CXLs) technology also uses carbon dioxide for the extraction of chemical products [16]. CXLs refer to the dense liquid phase containing dissolved CO₂, which is formed when compressed CO₂ is added to an organic solvent and they reach the CO₂–liquid equilibrium (GLE). CXLs are environmentally friendly solvents, and the addition of high-pressure carbon dioxide greatly reduces the use of organic solvents.

Carbon peaking and carbon neutrality are critical for human development. The extensive use of fossil fuels has resulted in excessive emissions of carbon dioxide. As excessive carbon dioxide emissions cause significant amounts of greenhouse gases, countries have already attached great importance to reducing CO₂ emissions. Carbon capture, utilization and storage (CCUS) technology is considered one of the most effective ways to reduce carbon dioxide emissions [17,18,19,20]. CCUS technology mainly involves recycling captured carbon dioxide and realizing its reproduction. Methods to capture CO₂ include pre-combustion capture, post-combustion capture, pure oxygen combustion capture and chemical chain capture [21]. It is easy to capture high-concentration carbon dioxide from chemical engineering industry emissions. However, it is difficult to capture carbon dioxide in low-concentration and dispersed emissions from the metallurgy, cement and electricity industries [12,22]. An appropriate transportation mode could solve the problem of different capture sites and utilization sites of carbon dioxide. Long-distance, large-scale transportation of gaseous and liquid CO₂ and SCO₂ is feasible. Carbon dioxide storage methods mainly include geological storage, marine storage and mineral carbonation [23,24,25]. Geological structures with good gas tightness, such as abandoned oil fields, gas fields, coal seams or deep geological salt water layers, can be used to store captured carbon dioxide [26]. Deep salt water has the largest storage space. The oceans, which account for more than 70% of the Earth’s surface area, can absorb a significant amount of carbon dioxide. This has inspired the concept of ocean storage, which refers to transporting captured carbon dioxide to the seabed at a depth of more than 1000 m for natural dissolution, or transporting it to the seabed at a depth of more than 3000 m. This can be realized by converting carbon dioxide from gas into a high-density solid or liquid and then storing it in high-pressure and low-temperature seawater [27,28]. Mineral carbonation uses alkali metal elements (mainly calcium and magnesium elements) in natural minerals or industrial waste residues to convert carbon dioxide into a more stable form of carbonate for storage. Generally, mineralized products have a certain economic value [29,30,31,32]. Therefore, compared with geological storage and marine storage, mineral carbonation has certain advantages in environmental protection, safety, stability and cost. It has gradually become the main method of carbon dioxide storage and the subject of extensive attention and research. Of course, the efficient use of traditional fossil energy is the premise for the effective implementation of CCUS technology.

SCO₂ extraction, CXL extraction and CCUS are three issues of concern in the field of carbon dioxide research. Four research articles concerning these topics were published in Separations in 2020~2021. Separations also published an article discussing the geochemistry in this period, in which the role of CO₂ in the mineralization process was discussed. This Editorial aims to increase the visibility of these publications and encourage further carbon-dioxide-related publications in Separations.

2. Summary of Published Articles

The processing and utilization of rice is hugely related to human survival and development. Besides white rice, rice bran and rice husk by-products are also produced in rice milling industries. Rice bran contains not only protein, vitamins and minerals but also unsaturated fatty acid. The preparation of rice bran oil from rice bran is one of the most important ways to improve the economic value of rice bran [33]. Tânia I. Pinto proposed using SCO₂ technology to extract rice bran oil from rice bran [34]. The results showed that increasing the pressure from 20 to 40 Mpa was beneficial in enhancing the extraction rate, while temperatures ranging from 40 to 80 °C had no significant effect on this supercritical extraction process. Although different rice varieties were used, the results showed that the supercritical CO₂ extraction technique was most effective in extracting rice bran oil from rice bran. Additionally, based on SCO₂ extraction technology, Sadia Qamar studied the process of extracting cannabinoids in a research paper published in Separations [35].

Natural pigments refer to the dyes obtained from plants, animals or mineral resources without artificial synthesis and little or no chemical processing [36]. Crocin is a natural yellow pigment from plants. Traditional methods of extracting crocin from gardenia fruits include heat extraction and solid–liquid extraction. However, high temperatures limit the application of the hydrothermal method, and common organic extractants may be harmful to the environment or humans. Therefore, Kenji Mishima proposed using CXLs to extract natural pigments from Gardenia Jasminoides J.Ellis fruit pulp [37]. They set up an experimental device to explore the effect of CXL extraction with and without direct sonication. Different modifiers, including pure ethanol (CXE) or water (CXW), 50% of an aqueous ethanol solution (CXE50%) and CXE80%, affect the extraction of natural pigments. The research showed using ultrasound promoted the surface contact between the extractant and extract, and obviously enhanced their mass transfer in the CXE80% system. CXE80% was proved to be an optimal system for natural pigments from Gardenia Jasminoides J.Ellis fruit pulp.

Coal, oil and natural gas are the most important fossil energy sources in today’s society. The effective separation of coal is related to the clean and efficient utilization of coal, which is an important part of the carbon dioxide reduction process. Traditional wet coal separation technology can solve the problem of coal cleanliness, but it consumes vast amounts of water and causes water pollution. Under water-free conditions, dry coal separation technology takes advantage of the differences in the physicochemical properties of coal and ore [38,39]. Gas–solid fluidized dry separation technology, e.g., gas–solid separation fluidized beds, is an effective means of separating coal, taking advantage of the fluid-like nature of the contact between the agglomerate and the rising gas [40]. Fan proposed using a micron-sized particle-dense medium to estimate the bed expansion and separation density of gas–solid separation fluidized beds [41]. With an accuracy of 85%~92%, the bed expansion prediction model provides not only a theoretical basis for understanding the distribution characteristics of the bed and predicting the bed density, but also paves the way for its industrial promotion.

Exploring the mechanisms of formation and evolution of the continental crust is one of the most critical study areas in the field of geochemistry. The Cambrian period was the most important period of continental crustal growth, contributing 75% of the continental crust [42]. Carbon dioxide affects the mineralization mechanism and final composition of minerals. Under certain reaction conditions, metallic elements in minerals may react with carbon dioxide to form carbonate precipitates. Alexander Kravchenko proposed using a statistical analysis method to research the distribution of chemical elements in pre-Cambrian rocks of the Siberian Craton [43]. The formation of metal elements is closely related to the redox reaction. The chemical properties of carbon dioxide and precious metals determine their distribution in rocks. The geochemical characteristics, and even analysis, of the elemental distribution pattern of some areas can be determined using statistical methods.

3. Conclusions

Articles related to the theme of carbon dioxide published in Separations over the last two years covered SCO₂ extraction, CO₂-expanded liquids, efficient utilization of fossil energy and geochemistry. This research indicates SCO₂ extraction technology has the advantages of high density, low viscosity and high diffusivity, and could be effective in chemical pharmaceutical extraction and grain processing. CXLs formed by adding high-pressure CO₂ to a small amount of organic solvent could also be used in the green extraction of chemicals. Studies on CO₂ geochemistry have shown that CO₂ affects the mineralization mechanism and final composition of minerals. Under certain reaction conditions, metallic elements in minerals may react with carbon dioxide to form carbonate precipitates.