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128 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 1.7. Process design using supercritical fluids: are CO2-based plants inherently uneconomical? The number of processing plants operating world- wide that employ supercritical CO2 is slightly above 100 and growing steadily [35]. Most of the current plants use CO2 to process food in some way (extrac- tion or fractionation), yet other types of plants have been or are being brought on stream (e.g. fluoropoly- mer synthesis by DuPont, hydrogenation by Thomas Swan, coatings by Union Carbide, polyurethane pro- cessing by Crain Industries). Despite this steady growth, there is a general sense (or unease) within both the academic and industrial communities that there are elements connected to the design and con- struction of CO2-based plants that effectively block greater use of the technology. Several authors have reviewed aspects of process design and costing of ‘supercritical’ plants [36]; these reviews typically focus on a specific industry. For ex- ample, Perrut reports that for the case of extraction, the relative cost of a supercritical plant scales as (V*Q)1/4, where V is the column volume and Q the flow rate. This is consistent with what we report in Section 1.7.1, where minimizing equipment size and flow rate will help to minimize process cost. Each of the authors who has reviewed process design using supercritical CO2 emphasizes that one needs access to the relevant fundamental parameters in order to complete and optimize the design. Such parameters include both the relevant thermodynamic model for the mixture(s) in question with the ap- propriate binary interaction parameters, reaction data (rate constants, heats of reaction, Ahrrenius constants) and transport constants (densities, diffusivities and viscosities). Note that these parameters are exactly the same as would be required to design a one-atmosphere process and hence there is nothing inherently ‘foreign’ about a CO2-based process that inhibits design and costing. Indeed, high pressure alone is not sufficient to explain the perceived difficulty of CO2-based pro- cess scale-up, given that hydroformylation operates at 200–300 bar at large scale, while low density polyethy- lene is produced at over 2000 bar. If one has access to the necessary basic information, one can employ software such as ASPEN to accomplish the process design and ICARUS to handle the costing (the author has carried this out successfully with colleagues). Hence, we must conclude that, if the inhibition in the scale-up of CO2-based processes is real rather than perceived, then it must be due to a lack of the fun- damental parameters needed for process design, plus other factors that would inhibit the commercialization of any ‘new’ technology. For example, it is relatively difficult at present to predict the effect of molecular structure on phase behavior in CO2 of molecules that exhibit any substantial degree of complexity. Carbon dioxide exhibits both non-polar tendencies (low di- electric constant) and ‘polar’ properties (Lewis acidity, strong quadrupole moment) and hence predictions of phase behavior are not straightforward (as in the case of alkanes or alkenes). Recent work [37] has shown that the statistical associating fluid theory (SAFT) can provide good descriptions of the phase behavior of complex mixtures including CO2, yet the complexity of this model and/or lack of suitable parameters may currently limit its use industrially. Group contribution models have been applied to CO2 solutions somewhat narrowly, generally targeting a single class of solutes [38]. What appears to be needed is a means to easily predict the properties of mixtures involving CO2, such that confident predictions of process requirements and costs can be made using conventional process software such as ASPEN. 1.7.1. Operating a process economically with CO2: heuristics While use of CO2 as a solvent is often considered to be ‘green’, operation of any process at high pressure typically involves higher costs than the analogous pro- cess operated at one atmosphere. If such a process is considered ‘green’, but cannot be created and operated economically, then the process will be of academic in- terest only and its potential green benefits unrealized. There are some simple ‘rules of thumb’ that one can use to render the cost of a CO2-based process as low as possible. 1.7.1.1. Operate at high concentration. One way in which to minimize the cost of a CO2-based process is to minimize the size of the equipment. Given that CO2 is typically proposed as a solvent (rather than a reactant), the most obvious means by which to min- imize equipment size is to minimize the amount of solvent (CO2) flowing through the process. Conse- quently, one should try to choose or design substratesPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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