Issue Date: April 16, 2012
A solid sorbent that selectively binds hydrocarbons may help the petrochemical industry reduce costs and energy consumption by increasing the efficiency of—or eliminating—energy-intensive separation processes, according to a study published in Science (DOI: 10.1126/science.1217544).
The investigation was conducted in part by researchers at the University of California, Berkeley, and the National Institute of Standards & Technology, Gaithersburg, Md. It demonstrates that metal organic framework (MOF) compounds can be used at relatively high temperatures to separate and purify gas mixtures containing ethylene, propylene, and other feedstocks in the C1 to C3 range that are used on a global scale.
Because of similarities in the sizes and volatilities of lightweight olefins and paraffins, separation of their mixtures requires low temperatures and high pressures. Those conditions impose a steep “energy penalty” and high cost on manufacturers because such mixtures are produced at elevated temperatures (~500 °C) by cracking long-chain hydrocarbons. The lightweight gas mixtures are then typically chilled well below 0 °C for separation.
“Cryogenic distillation is among the most energy-intensive separations carried out at large scale in the chemical industry,” notes Jeffrey R. Long, the UC Berkeley chemistry professor who led the study. As he explains, that costly separation is necessary to produce pure feedstocks of ethylene and propylene, which are required to make major industrial polymers such as polypropylene, a component of many consumer products.
Materials that can help separate low-molecular-weight hydrocarbons at atmospheric pressure and at temperatures higher than currently used in distillation would be a financial boon for the chemical industry. By reducing energy needs, such a material could also provide environmental benefits.
The research team, which also includes UC Berkeley’s Eric D. Bloch and NIST’s Wendy L. Queen, reports that the MOF compound it designed and tested appears to offer those benefits. MOFs are crystalline materials composed of metal ions or clusters that are connected by organic linkers. The compound at the center of the present study, Fe-MOF-74, features iron atoms, which function as hydrocarbon coordination sites, linked by functionalized benzenedicarboxylate units.
To evaluate the sorbent’s usefulness as a separation medium, the group conducted numerous gas adsorption and separation tests. For example, in experiments carried out at 45 °C, the team flowed equimolar mixtures of ethane and ethylene, and separately, propane and propylene, over a column loaded with the MOF. On the basis of gas chromatography analysis, the output of the C3 mixture was pure propane. Until the MOF was saturated, the column retained 100% of the propylene, within the limits of the instrument’s detection sensitivity. Similarly, the MOF separated ethane-ethylene mixtures into their component gases with purities exceeding 99%, the team reports.
Drawing on the experimental results, the team conducted several gas separation simulations. One analysis predicts that the iron-based MOF could readily fractionate mixtures of methane, ethane, ethylene, and acetylene, as needed to purify natural gas, and produce pure streams of each compound more energy efficiently than can be done with current methods.
Another simulation predicts that by using the solid sorbent, the 1% acetylene impurity typically found in ethylene produced by cracking naphtha could be reduced to 10 ppm, as required for ethylene polymerization. The energy savings would be substantial; today, the ethylene purification process is based on treating the olefin with liquid N,N-dimethylformamide to absorb acetylene.
The results “exemplify the potential of MOF-based materials relative to olefin/paraffin separations,” says Peter Nickias, a fellow at Dow Chemical. U.S. production of shale-derived natural gas, which can be a source of propylene and ethylene, is increasing, he points out. Optimizing the use of that new supply of olefin feedstock requires improvement in the energy efficiency of separation.
Researchers are examining numerous methods to cut energy use during refining and purification of olefins, Nickias points out. “Significant energy savings could be achieved if a nondistillation separation could be implemented,” he says. He adds that MOFs, such as the one investigated in this study, offer “new and exciting opportunities” for energy savings in gas separations.
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