Process simulation coupled with LCA for the evaluation of liquid - liquid extraction processes of phenol from aqueous streams

The steel industry is currently a major water pollution source, releasing high quantities of chemicals. One of the pollutants is phenol, known for its toxicity even when it is present in low concentrations. Liquid-liquid extraction employing various aromatics and cycloalkanes (i.e., benzene, toluene, cyclohexane, ethylbenzene) and ketones (i.e., methyl isobutyl ketone, cyclohexanone and mesityl oxide) as solvents, is studied in the present paper. A comparison among the seven solvents is performed based on process modelling simulation tools and Life Cycle Assessment (LCA). The simulations of the seven cases under study are conducted at the same wastewater flowrate of 100 tons/h, with a phenol content of 0.2 wt.% and compared using various parameters (i.e., quantity of solvent, steam and power used, quantity of solvent present in the output phenol streams). The simulation results show that the lowest quantity of solvent is registered in the phenol removal which uses cyclohexanone as solvent (e.g., 34,212.40 kg/h), followed by the case which uses mesityl oxide for the liquid-liquid extraction (e.g., 34,759.80 kg/h) and by the case involving cyclohexanone (e.g., 37,490.60 kg/h). The lowest steam consumption is registered also in the case of cyclohexanone usage (e.g., 47.56 GJ/h) while the lowest power consumption corresponds to mesityl oxide usage (e.g., 11.20 MJ/h). The simulation results are then used to perform an environmental analysis, quantified in terms of environmental key performance indicators, embedding several solvents production methods as well as various fuels. Our life cycle assessment leads to the conclusions that the most environmentally friendly design is phenol removal using cyclohexanone as a solvent, whose provision comes from cyclohexane, which in turn is produced from benzene in conjunction with steam production from natural gas. For instance, the lowest global warming potential indicator score is about 342 kg CO2 equivalents per kg of phenol, while the same indicator for the worst solvent, i.e., toluene produced using reforming technology and steam being produced using hard coal as fuel, is almost double (e.g., 341.94 kg CO2 equivalents per kg of phenol vs. 575.30 CO2 equivalents per kg of phenol). Lower values for other impact indicators are also obtained in the phenol removal using cyclohexanone as a solvent with steam being generated form natural gas (e.g., acidification potential indicator is 0.42 kg SO2 equivalents per kg of phenol, eutrophication potential is 4.21 × 10−2 kg PO43- equivalents per kg phenol, ozone depletion potential is 1.13 × 10-9 kg R11 equivalents per kg phenol).