Application of Supercritical Fluid Technology in Chemical Pollution Control

The environmental problems caused by chemical products such as atmospheric pollution, ozone layer destruction, reduction of biological diversity, shortage of fresh water resources, and pollution have received increasing attention. The world currently produces 3.4 billion tons of hazardous waste every year. China's chemical industry discharges 22.5%, 7.82%, and 5.93% of its total industrial emissions, including waste water, waste gas, and solid waste. Many chemical products waste, such as plastics, toxic waste, The treatment of biological sludge and organic waste water using conventional technologies is not effective. It is inefficient, complicated and costly. Therefore, it is necessary to develop practical environmental protection technologies.

As a clean environment-based technology, supercritical fluid technology is increasingly receiving attention.

1 Characteristics of the supercritical fluid Fluid, SCF) refers to a fluid that is in excess of the critical temperature and the critical pressure of the material itself. SCF is an edge state where gas and liquid coexist. It has unique physicochemical properties. If there are similar liquid densities, similar gas viscosities and large diffusion coefficients, the mass transfer rate of the moving speed and the separation process is greatly increased. SCF is very compressible, and small changes in temperature or pressure can cause large changes in SCF density. It has a high density and therefore has a strong physical force and can be used as a solvent in physical separation. It can increase the speed of the chemical reaction, reduce the reaction temperature, the reaction can be carried out in the liquid phase, easy to separate the reactants and products. Wang Xiaodong, Institute of Chemical Engineering, University of Petroleum (Beijing), is studying and studying. Mailing address: School of Chemical Engineering, Beijing Changping Petroleum University, 102200 Control technology Selective environmental protection and high-responsiveness of oil and gas fields. It has been reported that supercritical solvents for treating sewage, sludge and contaminated soil include acetone, methanol, ethanol, isopropanol, dimethylamine, etc. (13) Supercritical carbon dioxide (SC-C02) and supercritical water (SC) -H20) is chemically stable and has the advantages of being non-toxic, odorless, non-combustible, and inexpensive. It is the most commonly used SCF. Application of 2SCF technology SCF technology in chemical pollution treatment includes reaction media or reactants. Substitute toxic and harmful organic solvents, so that the production process does not produce pollutants, to achieve a true sense of "green production"; SCF can be directly used for sewage treatment, contaminated soil recovery, waste treatment, industrial solid waste treatment and disposal, Resource reuse, etc.; SCF technology is also widely used in environmental pollution analysis.

2.1.1 Application of SC-C2 in polymer synthesis and processing SC-C02 has good dissolving and dispersing ability as a cleaning solvent, so it can replace traditional organic solvents (such as benzene and toluene) in the synthesis of polymer materials; With SC-C02 as the reaction medium, the phase transfer catalyst can also be dissolved in it. This kind of reaction usually requires more expensive solvent, which is harmful to the environment and is not easily separated from the product, but when the system uses SC-C02 as solvent At that time, the above problems can be solved very well. The application of supercritical CO2 technology in polymer synthesis and processing was first reported in 1992. The United States first reported using SC-Cft as a solvent, AIBN as an initiator, and 1,1 dihydroperfluorooctyl methacrylate, acrylic acid. The copolymerization of monomers such as butyl ester gave a polymer with a molecular weight of 27×10 4 . Since then, studies in the field of polymer synthesis and preparation have begun. The research involved homogeneous radical polymerization and telomerization of fluorinated monomers, dispersion polymerization of methyl methacrylate, precipitation polymerization of acrylic acid, and inverse emulsion polymerization of acrylamide. The supercritical Cft technology can also be used in blending systems. The principle is that supercritical CO2 swells a certain matrix polymer (such as high-density polyethylene), and monomers (such as styrene) and initiators dissolved in Cft also swell with it. The process infiltrated the polymer, heating to initiate polymerization and depressurizing to remove CO 2 , then a blend of two polymer molecules (PE/PS) was obtained.

In recent years, SC-C02 has been used as a medium for polymer synthesis and free radical polymerization and cationic polymerization. The polymerization methods include liquid phase polymerization, precipitation polymerization, dispersion polymerization and inverse emulsion polymerization.

2.1.2 Decomposition of polymer materials Plastics and their products have become indispensable materials for industrial production and daily life. The annual output of plastics in the world has reached 100 million tons, most of which are produced by cracking petroleum into ethylene and propylene and catalyzed polymerization. About 5% of this 100 million tons is used as waste after the year of use, such as bags, plastic film, lunch boxes, and automobile waste. China's promotion of plastic film covers an area of ​​466km2, the amount of plastic up to 300,000 tons, the treatment methods such as burial, incineration, thermal degradation, etc. to varying degrees, there is a large energy consumption, secondary pollution, carbonization of raw materials and complex processes and other issues.

The use of SCF technology for the treatment of waste plastics is carried out in a closed system. The product and energy are easy to collect; SCF can be recycled without pollution; the reaction speed is quick, the cost is low, and the efficiency is high. The plastic harmlessness in garbage can be completely realized. Recycling.

At present, SCF can put waste plastics such as polyolefins (such as polyethylene PE, polystyrene PS), halogen-containing compounds (such as PVC PVC, tetrakisphenol A, hexachlorobenzene), polycondensates (such as poly pairs Ethylene phthalate (PET, polyurethane, polycarbonate), resin matrix composites, etc. are decomposed into monomers or other useful components. For example, Fiber Reinforced Plastics (FRP) is used to make small ships due to its light weight, water resistance, and corrosion resistance. However, the waste plastics are difficult to handle and cause serious damage to the environment. However, in SC-H20, the temperature can be completely degraded in about 5 min at 380*C. According to another report, in January 2000, the Japan Telephone and Telecommunications Corporation announced the successful development of the recycling of waste plastics technology such as the recycling of recycled cable skins and telephones using supercritical water. The technology first peels off the coated plastics such as polyethylene and polyamide from the waste cable, and then decomposes them in 400*C, 40 MPa, 600*C, and 60 MPa supercritical water. The former can recover acetic acid and phenol, and the latter can recover chemical raw materials such as aniline and phenylenediamine respectively. This technology can not only turn waste into useful chemical raw materials, but also save land and costs, and fundamentally solves the problem of high efficiency of waste plastics.

Cellulose is also a polymer material that is widely found in nature. It is a renewable resource from which it can produce energy, chemicals, foods and drugs. The key step in the production of cellulose as a raw material is the hydrolysis of cellulose into glucose, but the use of acid as a catalyst to hydrolyze cellulose at high temperatures produces a large amount of acidic wastewater, which both erodes the equipment and causes environmental pollution, thus limiting its use. . If supercritical water is used as the reaction medium, at a temperature of 380C and a pressure of 25 MPa, cellulose is almost 100% converted in less than 15 s without any catalyst.

2.1.3 Pollution-free Supercritical C2 Fluid Dyeing Process The increasing depletion of global water resources and the increasingly serious environmental pollution have made traditional printing and dyeing processes that use water as a medium subject to severe challenges, and a non-polluting environment that does not use water as a medium. A new process of supercritical CFT dyeing has emerged. Supercritical CO2 fluid dyeing is mainly suitable for non-ionic, insoluble, disperse dyes. The dyed fibers include polyester, polyamide, and acetate fiber for oil and gas field environmental protection treatment technology. It has also been used for the dyeing of polypropylene and aramid fibers, and has recently been studied. Natural fibres such as wool and cotton are the most promising polyester dyes.

The use of supercritical CO2 dyeing is a major breakthrough in printing and dyeing technology. It solves the pollution problems that have been difficult to solve for many years and has the following advantages: 1 No water, no waste water pollution, is an environmentally friendly dyeing and finishing process; 2 Reduced pressure after dyeing, C02 rapid gas It does not require drying after dyeing, which not only shortens the technological process, but also saves the energy needed for drying; 3 Faster dyeing speed, good dyeing and dyeing performance, and excellent dyeing reproducibility; 4C2 itself Non-toxic, odorless, non-flammable, reusable; 5 dyes can be reused, dyeing without adding any dispersant, leveling agent, buffer and other additives, not only reduce production costs, improve dye utilization, but also Conducive to environmental protection, reduce pollution; 6 a wide range of applicable fiber, some difficult to dye synthetic fibers (such as polypropylene, aramid, etc.) can also be normal dyeing.

The Fluid Extraction (SFE) technology is a new separation technique using a supercritical fluid as a solvent. The principle is: in the supercritical state, the supercritical fluid is brought into contact with the material to be separated, the pressure and the temperature of the system are controlled to selectively extract one of the components, and then the supercritical fluid is reduced through temperature and pressure changes. The density of the extracted material is separated and the supercritical fluid can be recycled. SFE technology has the characteristics of rapid, efficient automation, no use or less use of organic solvents, and is superior to liquid-liquid extraction, Soxhlet extraction and other methods. It has played a special role in extracting organic pollutants in environmental samples such as soil, air, and water. SFE can also be combined with analytical instruments such as gas chromatographs, either offline or online, to further increase the level of automation in the analysis process and is thus widely used in environmental governance and environmental analysis. 2.2.1 Application of SFE technology in environmental governance Currently, SFE technology can be divided into one-step method and two-step method according to the specific process of waste disposal. One-step method is the direct contact of the supercritical fluid with the contaminants to remove harmful components. The one-step method is applied to the purification and solid contaminant treatment of high-grade fatty alcohols, aromatic compounds, esters, ethers, aldehydes and other streams. For example, polychlorinated biphenyls (PCBs), which are cooling media for transformers, often leak due to overheating, and occasionally form dioxins. The general treatment method is to freeze the contaminated soil containing the PCB with liquid nitrogen and transport it to a special factory for incineration. The treatment cost is quite high. The use of SC-C02 and a set of removable extraction devices to process the PCB in situ is a very inexpensive alternative. In the two-step method, the contaminants are first contacted with an intermediate medium (such as an adsorbent) to enrich the contaminants, and then the intermediate medium is extracted with a supercritical fluid at room temperature (or lower temperature). Out of the pollutants. The two-step method can be applied to improve the current wastewater treatment process, so the scale and cost of investment can be greatly reduced, which is conducive to the economic operation and industrialization of the process. For example, Epping studied the use of activated carbon to adsorb trace amounts of volatile pollutants such as gasoline, alcohol, and ketones in air streams, and used SCF to regenerate activated carbon. The results show that the regeneration efficiency and economic benefit of this process are very high, and the activated carbon can be completely regenerated through multiple cycles of SCF, and the adsorption capacity of the adsorbent after regeneration is almost the same as that of the new adsorbent.

2.2.2 Application of SFE technology in environmental analysis In the traditional environmental analysis technology, many samples are prepared by extraction methods. Most of the solvents used are toxic and they are expensive.

SFE uses a supercritical fluid as the mobile phase. It can not only analyze high-boiling, low-volatile, and heat-sensitive samples that are not suitable for gas chromatography, but also has faster analysis speed and higher time column efficiency than high-performance liquid chromatography. The characteristics of secondary pollution can also be combined with GD, GC/MS, TLC and HPLC instruments, which can further improve the analysis speed and accuracy of environmental samples, and realize the on-site inspection of environmental samples. SFE has become increasingly widely used in environmental analysis and has involved PAHs, PCBs, petroleum hydrocarbons, pesticides, herbicides, phenols, and heavy metal ions and other contaminants from mold oil, cholesterol, and aflatoxin B1 and fatty acids. In the field of drug analysis, SFE is used to separate caffeine, nicotine, and alkaloids from natural products and drugs. Also, if the concentration of pollutants such as halogenated hydrocarbons in the atmosphere is very low (1X10*3lXl*-6g/L), it will have a greater impact on the environment. Direct sampling is far from meeting analytical requirements. Solid adsorbents are often used to enrich airborne contaminants and desorb components from the adsorbent. Soxhlet extraction and ultrasonic desorption method take a long time, large solvent consumption, and low desorption rate. Thermal desorption method is affected by the thermal stability of the components and adsorbent. If SFE extraction is used, CO2 is a gas at normal temperature and pressure, thus avoiding the post-treatment process such as solvent distillation, which can greatly reduce the loss of components.

2.3 Application of SFC in Environmental Analysis Fluid Chromatography (SFC) is a chromatogram in which the supercritical fluid is the mobile phase. According to the characteristics of SFC, the dissolving power of the supercritical fluid can be changed by adjusting the temperature and pressure to change the density of the supercritical fluid. SFC combines high-speed, high-efficiency and high-selectivity of GC. In addition to control technology, environmental protection in oil and gas fields can be equipped with various detectors of GC and LC, and can also be coupled with mass spectrometry (MS) and Fourier transform infrared spectroscopy (FTIR). Used to greatly improve the detection sensitivity and resolution.

SFC is mainly used for the analysis of compounds with poor thermal stability, high molecular weight, strong polarity, and strong absorptivity. In recent years, SFC technology has become more and more widely used in the analysis of environmental organic pollutants such as polycyclic aromatic hydrocarbons, organic dyes and pigments, surfactants, pesticides, phenols, halogenated hydrocarbons, and polychlorinated biphenyls.

2.4 The application of SCWO technology in pollution control WaterOxidation, SCWO) technology is a new oxidation technology that can completely destroy the structure of organic pollutants. The SCWO technology has been developed and applied in environmental remediation, and its treatment targets include phenols, methanol, acetic acid, trichloroacetaldehyde, diaminodioxime, cyanamide and melamine, DDT, polychlorinated biphenyls, and dioxins. , printing and dyeing wastewater, papermaking wastewater, organic fermentation wastewater, nitrification wastewater and organophosphorus pesticides, chemical weapons BZ, Sarin nerve agents. Compared with other processes, SCWO's characteristics are: the removal rate of toxic substances can reach more than 99.99%, in line with the requirements of the closed treatment; due to the homogeneous reaction and residence time is short, the reactor structure is simple, small size; a wide range of applications, It can be used for a variety of toxic substances, wastewater waste treatment; product cleaning does not require further processing; at lower organic content, self-heating can be achieved and is considered to be the most promising waste treatment technology. For example, in Harlingen, Texas, USA, the first operation line for the treatment of municipal sewage sludge using the SCWO method was launched in 2001. When its 2nd line is put into use, the site will be able to handle 35,000 gallons/day of municipal sewage sludge containing 7% solids, which is two wastewater treatment plants in the Harlingen Waterworks System Plant and The total amount of sludge that is treated daily by industrial wastewater treatment plants.

3 Concluding Remarks As a result of the continuous efforts of the majority of scientific and technological personnel in China, supercritical fluid technology has made great progress. According to statistics, from the establishment of the first set of supercritical extraction devices in Beijing in 1993, more than 30 sets of devices with an extractor size of 100L or more have been built. The scope of application extends to the pharmaceutical, food, cosmetics, flavors and fragrances, biology, environmental protection and chemical industries.

However, compared with the world's advanced level, there are considerable gaps in the concentration and breadth of research. In order to achieve sustainable social development, the chemical industry must develop new processes and technologies for “clean chemical” and “green chemistry” by adjusting its own industrial structure and product structure. Supercritical fluid technology is a new technology that has been rapidly developed in the past 20 years. Therefore, China should further strengthen the research on the basis and application of supercritical technology so that it can be truly used in industrial production for the benefit of mankind and society.