The transesterification reaction is governed by the amount and type of alcohol participating in the reaction. Considering the type of the alcohol, the use of methanol is advantageous as it allows the simultaneous separation of glycerol. The same reaction using ethanol is more complicated as it requires a water-free alcohol, as well as an oil with a low water content, in order to obtain glycerol separation [162]. Methanol is the most commonly used reactant both in conventional and microwave assisted transesterification reactions. Ethanol is more sensitive to the presence of moisture content in the oil causing soap formation and has less dielectric constant compared to methanol. Ethanolysis proceeds at a slower rate than methanolysis because of the higher reactivity of the methoxide anion in comparison to ethoxide. As the length of the carbon chain of the alkoxide anion increases, a corresponding decrease in nucleophilicity occurs, resulting in a reduction in the reactivity of ethoxide in comparison to methoxide [163]. An example of this phenomenon is the transesterification (at 25C) of canola oil with a 1:1 mixture of ethanol and methanol (to provide an overall molar ratio of alcohol to oil of 6:1) that results in 50% more methyl than ethyl esters [164, 165]. Therefore, for microwave assisted reactions, it is more favorable to use methanol as a solvent. On the other hand, ethanol has environmental acceptance due to its environmental friendly production from biomass. Since the transesterification reaction is an equilibrium reaction, excess amounts of alcohols need to be added to drive the reaction to completion within reasonable time. Alcohol-oil ratios of wide ranges (30:1) have been tested by many researchers with most common ratio being 9:1.
Microwave-enhanced organic/inorganic synthesis is considered as green chemistry and a preferred method due to several advantages such as lower energy consumption, substantial reduction in reaction times and solvent requirements, enhanced selectivity, and improved conversions with less by-product formation. Many reactions that do not occur under classical methods of heating can be carried out with high yields under microwave irradiation. Microwaves have the potential for large scale applications specifically in biodiesel production due to their ability to interact with a variety of reagents. Laboratory scale results in both batch and continuous conditions are encouraging and few pilot scale studies need to be developed to test their ability and efficiency for large scale adaptability. The reactor design, configurations, flow patterns, reactor safety and operational logistics are yet to be developed. Understanding the effect of microwaves on biomass extraction and transesterification reactions can be beneficial in the reactor design. Similarly, understanding microwave effect on different catalysts and solvents is crucial to develop safe reactors. Specific areas of challenges that need critical attention prior to large scale development are: controlled heating since biodiesel process is sensitive to temperature variations, efficient transfer of microwave energy into work area with fewer losses to the reactor walls and environment, compatibility of the process with rest of the process pipeline which includes biodiesel product separation and purification. Other important areas are better fundamental understanding and modeling of microwave-material interactions, better preparation of reaction mixtures and compositions tailored specific to microwave processing, better process controls, electronic tuning and automation (smart processing). Finally, availability of low-cost equipment, supporting technologies and other processing support hardware is to be considered. Combining the microwave effect with other innovative heating methods can be beneficial. Ultrasonics and radiofrequency waves can complement the microwave effect to improve the overall reaction performance in hybrid reactors; especially use of ultrasonic technology seems promising. Research in this area is in its infancy; however if successfully demonstrated, combined effect of these two innovative technologies can be enormous.
Iron Speed Designer Product Key.epub
Aseptic packaging involves placing commercially sterilized food in a sterilized package which is then subsequently sealed in an aseptic environment [79]. Conventional aseptic packaging utilizes paper and plastic materials. Sterilization can be achieved either by heat treatment, by chemical treatment, or by attributing both of them [79]. Aseptic packaging is highly used to preserve juices, dairy products, tomato paste, and fruit slices [75]. It can increase the shelf life of food items to a large extent; as an example, UHT pasteurization process can extend the shelf life of liquid milk from 19 to 90 days, whereas combined UHT processing and aseptic packaging extend shelf life to six months or more. Packages used for aseptic processing are produced from plastics having relative softening temperature. Moreover, aseptic filling can accept a wide range of packaging materials including: (a) metal cans sterilized by superheated steam, (b) paper, foil, and plastic laminates sterilized by hot hydrogen peroxide, and (c) a variety of plastic and metal containers sterilized by high-pressure steam [80]. Wide variation of packages thus enhances proficiency of aseptic packaging and diminishes cost. 2ff7e9595c
Comments