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COMMENTARIES Green Chemical Processing Using CO2 Whereas the birth of the modern environmental movement is often traced to the publication of Rachel Carson’s Silent Spring in 1962, one might trace the beginnings of the modern green chemistry movement to the publication of Molina and Rowland’s work on the link between chlorofluorocarbons (CFCs) and the depletion of the stratospheric ozone layer in 1974.1 Mounting evidence of the impact of CFCs on the degradation of the ozone layer led ultimately to the negotiated phaseout of CFCs in the Montreal Protocols of 1986. Ironically, CFCs had previously been considered green solvents, as they exhibit low toxicity, are nonflammable, and are somewhat “combinatorial” in character (various combinations of C, H, Cl, and F provide for an interesting range of physical properties). With the proposed elimination of CFCs, solvent replacements were urgently needed, particularly for refrigeration and the production of foamed polymers (thermoplastics and polyurethanes). As noted in an earlier Ind. Eng. Chem. Res. commentary by C. A. Eckert,2 interest in the use of carbon dioxide as a green solvent accelerated rapidly after a series of sessions at the AIChE meeting in New Orleans in 1980. Thus, a strong societal driving force for the replacement of chlorinated solvents appeared at the same time as scientific interest in CO2 was expanding rapidly, producing a concerted “push” for investigation of industrial opportunities for “green” CO2. Now, 20 years after the first wave of intense research investment in CO2 technologies, where has CO2 made significant inroads as a green process solvent, where have we seen more disappointment than success, and where should we focus our efforts in the future? In summary, the first commercial target for CO2 processing, extraction from natural products, has proven to be the most resilient. The commercial success of largescale extraction processes in the food industry (coffee and tea decaffeination, hops and spice extraction) has served to maintain scientific interest in CO2’s potential as a green solvent as other potential applications have come and gone. The number of CO2-based food processing plants continues to grow, and new applications (such as Praxair’s orange juice pasteurization scheme) continue to appear. Whereas food processing with CO2 is arguably the most successful process that uses CO2 as a green solvent, the use of CO2 in extractions from natural products is often perceived by academics as more art than science, owing to our inability to truly predict the phase behavior of complex molecules and mixtures in CO2. Likewise, the use of CO2 in enhanced oil recovery (probably the largest use of CO2 at present) also suffers from our lack of knowledge of the effect of molecular structure on phase behavior. The use of CO2 in oil recovery is preferable to the use of water as the flooding agent, as use of the latter requires energy-intensive remediation of the water employed. However, the low
viscosity of CO2 promotes fingering of the fluid through the petroleum rather than the desired sweeping; as a result, as much as 85% of the oil in the formation is bypassed. Recent results3 on the development of viscosity-enhancing additives for CO2 are promising, but an economical and effective additive is not yet within reach, partially because we do not know how to design, a priori, complex molecules that are highly CO2-soluble. Water and CO2 are the “greenest” solvents in our arsenal of sustainable solutions, yet we know surprisingly little about CO2’s behavior as a solvent when compared to water. Green Processing with CO2: Quo Vadis? Although CO2 itself is often described as green, for a CO2-based application to be truly green, one should be able to clearly identify quantifiable reductions in waste or energy input versus the conventional process. It must also be noted that, although CO2 is strongly implicated in global climate change, in the applications discussed here, CO2 is not being generated, but simply captured and employed. Below, we examine some of the important current trends in the use of CO2 as a green solvent, identifying commercial successes and failures, as well as technical obstacles to future development. Polymer Processing Using CO2. It could easily be argued that, during the preceding decade, CO2 research was driven by successes in materials synthesis and processing. For example, the adoption of the Montreal Protocols in 1986 created a pressing need among foamed polymer manufacturers to replace CFCs with sustainable alternatives. Although it had been determined in the late 1950s that urethane precursors (polyols, isocyanates) would solubilize sufficient CO2 at moderate pressure to create low-density foam, adaptation of this laboratory development to a continuous process proved difficult. During the early 1990s, Crain Industries4 created a “gate-bar” assembly that allowed for a controlled release of the pressure from a mixture of urethane constituents and liquid CO2, permitting construction of a purely CO2-blown, continuous flexible urethane process. By the end of the decade, over a dozen plants were in operation. During the same time frame, Dow5 created an all-CO2-blown polystyrene process, eliminating substantial pentane emissions. Although CO2-based foaming is now a commercial process, there are a number of unresolved technical issues surrounding the use of CO2 as the sole blowing agent in foamed thermoplastics, such as the following: (i) Can one use additives to enhance CO2 solubility in the melt without raising pressure significantly? (ii) Can one create allCO2-blown foam with low thermal conductivity? (iii) Can one inhibit foam collapse resulting from the rapid diffusion of CO2 through the polymer? Technical challenges (and thus research opportunities) remain, despite initial commercial success.
10.1021/ie0300530 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/12/2003
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By the end of the 1980s, interest in the use of CO2 as a green solvent had begun to wane, as it had become very clear that CO2 is a feeble solvent. Then, in 1992, DeSimone and co-workers6 published the first report of a truly “CO2-philic” material: a perfluoroalkyl acrylate polymer that is completely miscible with CO2 at pressures less than 150 bar despite containing over 2500 repeat units. This discovery ultimately led to Dupont’s construction of a semiworks facility to generate fluoropolymers, where CO2 replaced either fluorinated solvents or fluorinated surfactants (the PFOS materials currently under scrutiny by the U.S. EPA) and water. Unlike most solvents, CO2 does not support chain transfer, and hence, Dupont was able to produce polymers more sustainably and with better physical property control. Also during the early 1990s, Johnston’s group7 created polymer particles using CO2 as a nonsolvent; this work has spawned a host of process configurations (each with its own acronym) designed to generate fine (less than 10-µm) particles. The pharmaceutical industry is clearly interested in such processes, as evidenced by the flurry of acquisitions of companies with high-pressure expertise during 2000-2002. However, these acquisitions also point out the fundamental weakness in this technology: currently, each individual process (i.e., each acronym) is presumed to be in some way fundamentally different from the others, requiring acquisition of the skill base to continue development. Research groups such as DeBenedetti’s at Princeton and Randolph’s at Colorado are endeavoring8 to provide a fundamental engineering basis for particle creation, hopefully to one day create an actual unit operation (with the usual defining equations) from what is now an alphabet soup of acronyms. The definition of the fundamental processes that govern particle formation will ultimately help to create control algorithms for their use, improve the economics of such processes, and hence bring them into the commercial sphere. Creation of polymer particles using CO2 as a nonsolvent might not actually be greener than competing processes (spray drying, grinding) owing to solvent use and recycle issues, but the CO2 process might create a better product (always a useful driving force!). At present, the generation of very small (
