Ind. Eng. Chem. Res. 2007, 46, 8687-8692
8687
Economic and Environmental Impact Analyses of Catalytic Olefin Hydroformylation in CO2-Expanded Liquid (CXL) Media Jing Fang,†,‡ Hong Jin,†,| Thomas Ruddy,§ Kent Pennybaker,§ Darryl Fahey,† and Bala Subramaniam*,†,‡ Center for EnVironmentally Beneficial Catalysis and Department of Chemical and Petroleum Engineering, UniVersity of Kansas, Lawrence, Kansas 66045-7609, and RiVer City Engineering, Lawrence, Kansas 66046
Recently, CEBC researchers reported a new hydroformylation process concept that uses CO2-expanded liquids (CXLs) as reaction media. The hydroformylation turnover frequencies (TOFs) were up to 4-fold higher in CXLs than those in neat organic solvent. The enhanced rates were achieved at milder conditions (30-60 °C and 4-12 MPa) compared to industrial processes (140-200 °C and 5-30 MPa). Preliminary economic and environmental analyses of the CXL process are presented here and benchmarked against a simulated conventional hydroformylation process for which nonproprietary data were obtained mostly from the Exxon process. The simulation results indicate that the CXL process has clear potential to be economically viable and environmentally favorable subject to nearly quantitative recovery and recycle of the rhodium-based catalysts. For the simulated conventional process, acetic acid discharged during the catalyst recovery steps is the dominant source of adverse environmental impact. These analyses have provided guidance in catalyst design and in choosing materials and operating conditions that favor process economics while lessening environmental footprint. 1. Introduction Industrial hydroformylation of higher olefins employs cobaltbased catalysts that require harsh operating conditions (140200 °C and 5-30 MPa).1 Large quantities of acid and alkaline solutions are involved in the catalyst recovery (demetallization) step for that portion of catalyst that cannot be directly recycled.2 Rhodium-based catalysts are more efficient than cobalt-based catalysts for higher olefin hydroformylation but are significantly more expensive. Hence rhodium-based technology must demonstrate near-quantitative catalyst recovery and durability in order to be economically viable. Recently, investigations at the Center for Environmentally Beneficial Catalysis (CEBC) laboratories and elsewhere3-8 have demonstrated how a relatively new class of solvents, CO2expanded liquids (CXLs), are promising alternative media for performing catalytic reactions. The improved transport properties of CXLs3 (compared to neat organic solvents) and the enhanced solubilities of permanent gases (i.e., O2,5,9 H210,11) in CXLs, which alleviate mass transfer limitations and increase the availability of gaseous reactants in the liquid phase, have been shown to intensify overall reaction rates in laboratory experiments. Other potential advantages of CXL usage include substantial replacement of volatile organic solvents with nontoxic CO2 and milder operating pressures (tens of bars) compared to reported supercritical CO2-based process concepts. The use of supercritical CO2, which represents one end of the CXL media spectrum, typically requires well in excess of 10 MPa. In addition, the ability to easily tune CXL polarity by changing the CO2 content provides an opportunity to separate and recycle polar homogeneous catalysts.5 * To whom correspondence should be addressed. Tel: +1-785-8642903. Fax: +1-785-864-6051. E-mail:
[email protected]. † Center for Environmentally Beneficial Catalysis, University of Kansas. ‡ Department of Chemical & Petroleum Engineering, University of Kansas. § River City Engineering. | Present address: ConocoPhillips, Bartlesville, OK 74003.
In the CXL-based hydroformylation process concept demonstrated by us,6,7 part of the liquid substrate and solvent is replaced with dense CO2 to generate a CO2-expanded reaction medium. For Rh(acac)(CO)2 catalysts modified with the triphenylphosphine (TPP) ligand, the turnover frequencies (TOFs) for higher olefin hydroformylation were up to 4-fold higher in CXLs than those in either neat organic solvent or neat CO2. The turnover frequency (TOF) is defined as the number of moles of 1-octene reacted per mole of catalyst per hour of batch run time. The enhanced rates were achieved at milder conditions (30-60 °C and
