Use of Green Technology in Separation Processes
Green Technology is an encompassing term. It deals with using science and technology in order to protect the environment. A lot of techniques fall under this term such as the use of green chemistry, environmental monitoring, and more. All of these things have to deal with making sure that the environment remains protected. This technology is used to breathe life back from a damaged ecosystem. The main goal is to conserve nature, and to remedy the negative impact that humans have on it. Since the 1990’s a lot of focus is being put on green technology. Human beings need Earth to stay alive. This technology ensures that the Earth remains healthy for all life to continue existing.
Simple ideas such as recycling, purifying of water and air, conserving energy, pollution prevention, rejuvenating ecosystems, etc are all a part of Green Technology. All these terms somehow includes certain kind of separation processes. Separation processes are any set of operations that separate solutions of two or more components into two or more products that differ in composition. These may either remove a single component from a mixture or separate a solution into its almost pure components. This is achieved by exploiting chemical and physical property differences between the substances through the use of a separating agent (mass or energy). Separation processes are used for three primary functions: purification, concentration, and fractionation. Purification is the removal of undesired components in a feed mixture from the desired species. For example, acid gases, such as sulfur dioxide and nitrogen oxides, must be removed from power-plant combustion gas effluents before they are discharged into the atmosphere. Concentration is performed to obtain a higher concentration of desired components that are initially dilute in a feed stream. An example is the concentration of metals present in an electroplating process by removal of water. This separation allows metals to be recycled back to the electroplating process rather than discharged to the environment. Lastly, in fractionation, a feed stream of two or more components is segregated into product streams of different components, typically, relatively pure streams of each component. The separation of radioactive wastes with short half-lives from those having much longer half-lives facilitates proper handling and storage. As far as green technology is concerned, green solvents are always a good option for separation processes.
So what are “Green Solvents”? How do they form a better choice over other techniques?
THE NEED TO USE SOLVENTS
Solvents are ubiquitous auxiliary substances mainly used in paints, coatings, adhesives, cleaners, and cosmetics, but also in chemical and pharmaceutical processes. A worldwide solvent consumption of about 30 million metric tons per year has been recently estimated. From them, alcohols such as methanol, ethanol, n-butanol, and iso-propanol are the more typically used solvents, with an annual consumption of 6.5 million tons in accordance with a recent market study. Aromatics, ketones, esters, and ethers are also widely used. The global solvent consumption has continuously increased in the last years, and this trend seems likely to persist and even intensify in the future.
Chemical synthesis and separation processes are two main areas involving organic solvents. Thus, solvents allow carrying out countless chemical reactions, isolation and purification of target compounds, formation of azeotropes for separation, or temperature control, among other applications, with excellent performance. But there are issues related to the application of traditional solvents. This is a good reason for searching greener solutions. In this sense, the inception of green chemistry paved the way for a more careful and conscientious use of solvents. Regarding extraction and separation processes, several aspects should be considered to select a given solvent, including not only the environmental, health, and safety issues of considered solvents but, also importantly, metrological and economic aspects. Thus, the extraction and/or separation performance, energy demand, possibility of solvent recovery, and recycling, as well as solvent compatibility with analytical instrumentation should be carefully assessed.
GREEN SOLVENTS
In the last decades, scientists have paid increasing attention to the adverse effects of reagents and solvents used in chemical processes. The introduction of the 12 principles of green chemistry represented a turning point toward a reduction of environmental, safety, and health hazards associated to conventional chemical processes. The development and application of greener solvents is currently a hot topic in a variety of scientific and technological areas. Green solvents can be defined as those solvents that display reduced health, safety, and environmental issues and a reduced life cycle impact. Two main measures have been considered to make chemical processes greener when solventless approaches are not feasible. The more conservative one, yet challenging, involves a significant reduction of organic solvent consumption in a given chemical process. More desirably, the second measure involves a replacement of harmful solvents by greener alternatives.
SOLVENTS FOR ANALYTICAL SEPARATIONS
Chromatographic and electrophoretic separation techniques are widely used for separation and determination of target compounds in matrices of different complexity. A large number of reference methods of analysis involve separation techniques coupled with appropriate detectors. Non-chromatographic methods can sometimes be used for determination of relevant analytes with a negligible consumption of solvents. Nevertheless, analytical separations can be unavoidable or even more convenient than non-chromatographic methods in certain cases. An issue that cannot be omitted, however, is the application of solvents in separation techniques, especially in the case of liquid chromatography (LC). Separation techniques importantly contribute to the total solvent consumption in analytical laboratories and large amounts of liquid wastes are consequently produced. The magnitude of this activity is far from being negligible. In fact, it has been estimated that an amount of wastes of around 150,000 tons could be generated by LC systems on a yearly basis. While efforts have been mainly devoted to solvent reduction, reuse, and recycling, recently, much attention is given to apply greener solvents as mobile phases instead of conventional solvents such as acetonitrile or dichloromethane. It should be highlighted that apart from their ability to enable successful separations of a great variety of relevant compounds, the compatibility of alternative solvents with typical detection systems used in combination with analytical separation techniques is also of paramount importance and should therefore be demonstrated. Miniaturization of chromatographic systems, as well as reduction of packed columns dimensions or application of capillary columns in LC are efforts to the decreasing of mobile phases consumption. Alternative techniques to LC allow for separation of compounds without application of organic solvents. Supercritical fluid chromatography, gas chromatography, and capillary electrophoresis are considered to be green separation techniques, with their own advantages and disadvantages, but they all apply mobile phases that are more benign in their nature. The combination of these greener analytical separation techniques with solventless sample preparation approaches provides excellent possibilities for determination of target compounds at trace and ultra-trace levels while fulfilling the principles of green chemistry.
WATER AS THE FIRST CHOICE GREEN SOLVENT
Water can be considered as the greenest solvent from the wide list of substances traditionally used in the chemical industry. It is environmentally benign, nonflammable, and easily obtainable on a large scale with high purity. Remarkably, its physicochemical properties can be tuned by temperature and pressure. Also in addition to aqueous separations, water is also popular medium and reagent in synthesis/reactions, oxidation of waste materials and conversion of biomass into fuels and chemicals. Separations using water at elevated temperatures can be divided into different categories: (1) hot or high temperature (HT) water from ambient to 100 C, (2) pressurized hot (also subcritical or superheated) water (PHW) from 100C to 374C, and (3) supercritical water (SCW). This classification is not very strict since the term HT is also often used at temperatures above 100C. Temperature plays a dominant role in the determination of the solvent properties of water, and pressure has only a minor effect. However, because pressure affects greatly to water properties when the change of state from gas to liquid or vice versa occurs, the users should distinguish whether the water is in the state of liquid or steam. Water is conventional solvent in many separation techniques. Typically it is used in capillary electrophoretic (CE) separations. Capillary Electrophoresis is a Green Alternative Separation Technique. Example includes, CE have been applied to off-line analysis of amines and amino acids from Pressurized Hot Water (PHW) extracts and alkaloids from PHW extracts of Sophora flavescens Ait. Aqueous phase separations are also most common ones in fieldflow-fractionation for particles, polymers and macromolecules and many distillation (hydrodistillation and steam distillation) processes are done in water, for example, for the purification or isolation of compounds or oils from natural products and in some studies distillation is compared to extraction with PHW. PHW as steam state has also been successfully used for crude oil distillation where the parameters of the distillation system were optimized and modeled. Some noteworthy applications are
1. Today, hot or PHWE is widely explored options in biorefineries for the isolation of different chemicals or as biomass pretreatment techniques.
2. Hot-water extraction (HWE) and PHWE have extensively been studied with the purpose to get various bioactive compounds from different biomasses, such as plants, fruits, and bark of tree.
3. It has applications in Energy Technology as well. Different biomasses, after their treatment with PHW, usually produce energy dense bio-oil called biocrude. It can be further converted to hydrogen, liquid fuels, or different chemicals.
4. Since volatile oils from the plants are often isolated by the aqueous distillation techniques, e.g., by hydrodistillation, it is logical application also for PHWE. Hydrodistillation and PHWE were compared for the isolation of cinnamon bark volatile oils. Oil components are further separated by solid-phase extraction and gas chromatography.
5. Crude oil distillation with superheated water steam at 310C and 420 kPa was proposed and modeled by Samborskaya et al. to intensify the recoveries of light fractions and to reduce the energy consumption in industrial fractionation process.
6. Inorganic elements (Ba, Ca, Cu, Fe, Mg, Mn, Na, Pb, Sr, Zn) were successfully extracted by microwave-assisted PHWE and simultaneously determined by microwave-induced plasma spectrometry.
7. Separation of preservatives and sunscreens in skincare products have been the main applications in cosmetics.
8. Several green HT-LC studies with pure water as mobile phase are available and many of them deal with liquid sample matrices, such as milk, juices, beer and alcoholic beverages.
The exploitation of environmentally friendly water will certainly be increased in separations in the future. Extraction and pretreatment of various samples with PHW and SCW will be very valuable, not only in analytical scale separations but also in the case of biomass in the future bio-refineries. Green chromatography (HT-LC) either with pure water or with a few percent ethanol as a modifier will hopefully inspire more scientists to develop further methods and processes and new column materials that allow the use of HTs. These developments will certainly open up new horizons for better sustainable sample preparation/treatment systems.
BIO-BASED MOLECULAR SOLVENTS
Bio-based solvents offer an environmentally sustainable option for the replacement of Volatile organic compounds(VOC) or fossil fuel based solvents which are commonly used in separations processes such as solvent extraction. The search for environmentally sustainable solvents is becoming increasingly important in solvent extraction due to the increasing health and environmental concerns as well as economic pressures associated with VOCs. Bio-based solvents can offer favorable properties such as renewability, low toxicity, and biodegradability. Through understanding bio-based solvent applications and identifying existing markets where they could be employed, renewable solvents can determine entry points in industrial processing . For solvent extraction applications, the overall performance of these bio-based solvents has been reported to be comparable in terms of extraction yields and selectivity. Numerous methodologies, criteria, guides, and principles are now available to aid in “green” solvent selection. Some examples of Bio-Based Molecular Solvents are alcohols, Esters (Biodiesel, Ethyl Acetate), glycerol derivatives, Terpenes like α-Pinene, p-Cymene, d-Limonene, Ethers, ethyl lactate,etc.
SUPRAMOLECULAR SOLVENTS
Supramolecular solvents (SUPRASs) are nanostructured liquids produced in colloidal solutions of amphiphilic compounds by spontaneous, sequential phenomena of self-assembly, and coacervation. According to the IUPAC, coacervation is defined as the separation of colloidal systems into two liquid phases [4]; the phase more concentrated in colloid component is the coacervate and the other phase, containing a low colloid concentration, is the equilibrium solution. Because of the outstanding properties of SUPRASs for efficient solubilization of solutes in a wide polarity range, they have found extensive application in the analytical extraction of metals and organic compounds from environmental, food, and biological samples. They are especially suitable for multiresidue analysis. SUPRASs have been already proved as efficient extractants of contaminants from a variety of environmental, biological, and agrifood samples. Their adaption to different microextraction formats (e.g., single-drop microextraction, dispersive liquidliquid microextraction, hollow-fiber microextraction, etc.), compatibility with separation and detection techniques, suitability to develop generic sample treatments adaptable to the extraction of one/various analytes in very different types of samples with minor modifications, capability for multiresidue analysis, simplicity and quickness of the SUPRASbased procedures, low cost, etc. makes SUPRASs a powerful alternative to organic solvents in analytical extractions. These properties also render SUPRASs promising extractants of pollutants in wastewater treatment and high-added-value compounds from natural products.
IONIC LIQUIDS, SWITCHABLE SOLVENTS AND EUTETIC MIXTURES
Ionic Liquids have been used as efficient agents for separation and extraction processes, as well as vehicles for selective separations in chromatographic applications. Some selected recent concepts using ILs cover the removal of rare earth using ILs, the combination of ILs with membranes for efficient CO2 separations, or the removal of phenyl-ethanol from water using ILs as extractive phase. Herein, a remarkable field is the extraction of biological products using ILs, as recently has been comprehensively reviewed . The properties of the IL can be tailored to adapt them to the nature of the molecule to be extracted. This may lead to rather selective extractions of complex biological material. Subsequently, the addition of anti-solvents (like water, acetone, or methanol) enable the recovery of the desired molecule. When using ILs in extractive processes, typically water-immiscible ILs are used to form a two-phase system with the aqueous phase, from which the biological compounds are extracted in a selective way. In some cases, combinations of these concepts with membranes have also been shown, as well as some solid-liquid applications (e.g., cellulose dissolution) . Recent examples of this approach comprise the use of different IL families (e.g., common ones like phosphonium-based, imidazolium-based, or cholinium-based ILs, among others) for the extraction of alkaloids, antioxidants and proteins (e.g., keratin from feathers).
In the area of extractions and separations, DESs have shown promising potential both in liquid-liquid separations and in gas separations. Their inherent biodegradability and ease of production represent obviously further assets in this respect. With regard to extractions, processes with DES can be performed either by using solid biomass (e.g., to conduct a solidliquid separation) or by forming two-phase systems, in analogous way as shown for ILs . However, for that latter application it must be noted that DES are hydrophilic per nature — dissolving completely in aqueous solutions — and thus first of all the design of hydrophobic water-immiscible DES was needed. This has been recently achieved by using decanoic acid as HBD, and several quaternary ammonium salts with high hydrophobic profiles, as well as with menthol-based DES combined with carboxylic acids. This represents a step forward in the DES field, and it may be expected that novel water-DES two-phase applications will be reported in the coming years.
Switchable solvents have been assessed for different extractive and separation processes, given the promising features that they hold. Within biorefineries, they have been considered as selective extractors for the pretreatment of lignocellulose. Herein, depending on the switchable solvent and the processing conditions (pressure, temperature, type of biomass, etc.), different aims can be achieved: dissolving cellulose for its further processing; dissolving hemicellulose but not cellulose to separate them; or dissolving lignins, but not (hemi)celluloses to perform a delignification process. Given their ease of operation and potentialities, biorefineries are certainly a field for further research and development using switchable solvents.
GREEN MEMBRANE EXTRACTION
Membrane extraction is a one-stage process, which is characterized by concurrent extraction of target analytes from aqueous solution into membrane and re-extraction of the analytes from membrane into another aqueous solution. The membrane can be polymeric or liquid phase interface, which is not soluble in any of the aqueous phases. During membrane extraction, analyte(s) from the donor solution are transferred into the membrane and further from the membrane into the acceptor solution and, importantly, both transfers take place simultaneously. Green membrane extractions have a supported liquid membranes (SLMs). In SLM, the membrane is formed by an organic solvent, which is stabilized in porous polymeric support by immersing the support directly into the organic solvent and by filling the pores of the support with the solvent. Hydrophobic polymeric materials are usually used as supports for SLMs, which may be in form of planar or tubular membranes made of polypropylene (PP), polytetrafluoroethylene (PTFE, Teflon), or polyvinylidene difluoride (PVDF). In many applications, PP hollow fibers (HFs) are used as the most convenient support. Extraction across SLM is a three-phase extraction process, and it is apparent that composition of all three phases plays a key role in the extraction performance. Composition of membrane is the key parameter in membrane extractions. The most important aspect is the selection of organic solvent, which forms the liquid membrane, and of membrane modifiers, which are added to the organic solvent. Presence of the two components and their characteristics determine the membrane selectivity; in other words, they determine what type of compounds are transferred across and what type of compounds are retained by the membrane.
Green membrane are membranes that significantly reduce consumption of organic solvents compared to standard extraction techniques, such as LLE. This reduction is usually achieved due to the fact that analytes are simultaneously extracted from donor into membrane and from membrane into acceptor solution, the membrane contains minimal volume of organic solvent, and there is no need for additional use of solvents during extraction. On the other hand, one has to be aware of the fact that specific processes are required to produce materials, which are used for fabrication of green membranes. For example, organic solvents are frequently used to dissolve base polymers during membrane fabrication processes of porous as well as nonporous polymeric membranes. The main advantage of using membranes can be seen in the fact that they do not introduce, not even temporarily, foreign components to the extraction systems, and they can be used in both micro- and macroscale. It is also worth mentioning that on contrary to standard extraction techniques, which require large quantities of extracted samples as well as extracting solutions, separations by membrane extractions require significantly reduced sample and solvent volumes, and in many instances, membrane extractions are fully solvent-free. In addition, most membrane extractions are instrumentally very simple, require no sophisticated equipment, and are easily adaptable for portable formats. Various other types of membranes with varied functionality are porous, non-porous, single phase, multiphase, polymer inclusion, etc.
OTHER TYPES OF ADVANCED GREEN PROCESSES —
· Supercritical Fluids and Gas-Expanded Liquids for separation
· Microwaves for Greener Extraction
· Ultrasound-Assisted Extraction
· Environmentally Benign Supercritical Fluid Extraction
· Surfactant-Mediated Extraction Techniques
· Microextraction and Solventless Techniques
· Green Gas and Liquid Capillary Chromatography
· Supercritical Fluid Chromatography
· Capillary Electrophoresis
The versatility of green processes provide an ideal platform for the study of novel and green solvents for analytical separation. Today, environmental considerations have become an important factor when evaluating a separation process. Analytical separation will also become more important not least because the trend toward portable devices such as point-of-care applications and remotely operated micro total analysis systems (i.e., chip electrophoresis) is increasing. It is expected that the development of novel green solvents and use of electrokinetic separations will be a very promising avenue to overcome the trade-off between the greenness and analytical performance of a method.
RECENT RESEARCH IN THE FIELD
Development of magnetically recoverable catalyst supports containing silica, ceria, and titania
Magnetic separation can be considered a green technology because it is fast, efficient, consumes low energy, and minimizes the use of solvents and the generation of waste. It has been successfully used in laboratory scale to facilitate supported catalysts’ handling, separation, recovery, and recycling. Only few materials are intrinsically magnetic, hence the application of magnetic materials as catalyst supports has broaden the use of magnetic separation. Iron oxides, silica-coated iron oxides, and carbon-coated-cobalt are among the most studied catalyst supports; however, other metal oxide coatings, such as ceria and titania, are also very interesting for application in catalysis. In one of the report published, the preparation of magnetically recoverable magnetic supports containing silica, ceria, and titania was quoted. They found that the silica shell protects the iron oxide core and allows the crystalization of ceria and titania at high temperature without compromising the magnetic properties of the catalyst supports. Likewise many more researches are carried out in this field to make the chemical more greener and sustainable!
TAKE A RIDE ON THE GREEN SIDE…….
REFERENCES :
- The Application of Green Solvents in Separation Processes by Francisco Pena-Pereira and Marek Tobiszewski.
- “Separation Technology meets green chemistry : Development of magnetically recoverable catalyst supports containing silica, ceria, and titania”, Pure and Applied Chemistry | Volume 90: Issue 1 , Lucas L. R. Vono, Camila C. Damasceno, Jivaldo R. Matos, Renato F. Jardim, Richard Landers, Sueli H. Masunaga and Liane M. Ross, 11 Sep 2017
Thankyou for reading!