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130 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 1.7.1.2. Operate at as low a pressure as possible. Operation of a process at high pressure is more expen- sive than at one atmosphere, owing to equipment de- sign and construction, as well as the additional safety features that are necessary. Further, the capital cost of a high-pressure process is not linear with pressure be- cause the pressure ratings of certain vital equipment (flanges, for example) are available in discrete steps (60 and 100 bar, for example). In addition, the number of companies with experience in high-pressure pro- cess design drops dramatically as the operating pres- sure rises above 200 bar. Clearly, these caveats strongly recommend oper- ating at the lowest pressure possible. One means by which to accomplish this is in the chemical design of reactants and/or substrates. It has been known for a number of years that certain functional groups are more ‘CO2 -philic’ (thermodynamically more CO2 -friendly) than others. Use of CO2 -philic func- tional groups in the design of substrates or catalysts can greatly lower the needed operating pressure, al- though it should be remembered that their use could easily raise raw material costs. Given that carbon dioxide is a relatively feeble solvent, a classic technique for lowering operating pressure (or raising operating concentration) is to employ co-solvents. Methanol and ethanol are most commonly used [1,39], but a wide range of organic solvents has been employed in this fashion, usually at concentrations <40%. Regarding whether the use of co-solvent/CO2 mixtures is green, one must make a determination on a case-by-case basis. For example, in a conventional chemical process, one must decide whether it is more efficient to use a low pressure process with 100% organic solvent or a high pressure process using only 5–10% organic solvent (for exam- ple) with the balance CO2. To date, the typical answer has been to opt for the low pressure, solvent-based process. However, if the solvent (owing to the nature of the process) is to be emitted to the atmosphere, there are examples where the choice has been to opt for the CO2/co-solvent route. In the UniCarb coatings process [40] developed by Union Carbide during the 1980s and 1990s, CO2 was employed to replace one component of a solvent mixture used in spray coating, creating a CO2/co-solvent based process. The foam- ing of thermoplastics such as polystyrene [41] is often conducted using a mixture of CO2 and an alkane, a more efficient route than employing either 100% alkane or 100% CO2. One can also employ relatively lower process pressures by operating in the two-phase regime (gas–liquid) rather than employing pressures high enough to maintain a single phase; more about this option will be described in a later section. Another somewhat obvious route to the lowering the operating pressure is by operating at sub-ambient temperatures. Here, however, one must balance the advantage gained by reducing the operating pressure with other impacts, such as the energy cost for cooling and any reduction in reaction rate owing to reduced temperature. Whereas dropping the temperature is an obvious mechanism to reduce the operating pressure, there are others that have received far less attention. For example, the identification of a minimum boil- ing azeotrope where CO2 is the majority component could provide a solvent that is both green and exhibits a vapor pressure far lower than that of pure CO2. Azeotropes are desirable in that process steps requir- ing flashing of the material (or small leaks) will not change the composition of the solvent. Azeotropes can be maximum boiling (where the vapor pressure of the mixture is higher than either of the pure component vapor pressures) or minimum boiling (the opposite, and here desired situation) [42]. Although addition of a second component might lessen the sustainability of the solvent, a solvent that is mostly CO2 is typically better than one than contains no CO2 and the reduc- tion of the pressure through use of a minimum boiling azeotrope might lower the operating pressure suffi- ciently to allow economical scale-up of the process. Some CO2-based azeotropes have been identified [43] as a result of research by CFC-producing companies in a search for alternative refrigerants. Consequently, most of the known CO2 azeotropes are mixtures with fluorocarbons (it is also known that ethane forms an azeotrope with CO2 ). Because azeotropes typically form between compounds whose boiling points are separated by 50 K or less, the number of potential azeotrope-forming cosolvents for CO2 is likely lim- ited, but this could provide an interesting route to solvents that are both green and versatile. 1.7.1.3. Recover products without high-pressure drops. It has been mentioned in the literature that use of CO2 as a solvent is advantageous because reduction of the pressure to one atmosphere resultsPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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