Solar heat for the decarbonization of chemical industry: dehydrogenation of ethylbenzene to styrene driven by a concentrating solar power plant with molten salts as heat transfer fluids

The dehydrogenation of ethylbenzene to styrene was used as a model of an energy intensive endothermic process to assess the economic sustainability of the utilization of solar heat from a concentrating solar power (CSP) plant to decarbonize an industrial chemical processes. To this purpose a process configuration compatible with the hybridization with a CSP plant using a binary mixture NaNO3/KNO3, 60/40 w/w as heat transfer fluid (HTF) was selected. The adopted chemical reactor is a shell and tube bundle converter with 30000 tubes of 6 meter length and 0.025 m inside diameter that approaches isothermal regime with a productivity of 103 kT/year of styrene if a flowrate of 200 kg/s of molten salt at 560°C are fed to the shell. The residual enthalpy of the HTF leaving the dehydrogenation reactor was further injected in the process by vaporizing and pre-heating ethylbenzene and dilution water. A cash flow analysis of the hybridized plant was performed considering solar field of increasing size so that the required solar power of 45 MW can be supplied for longer period of the year. We found that a CSP plant of 70 collectors can decrease CO2 emissions of about 50% with a rate of return on investment (ROROI) of 9.1% for the solar field of the hybridized plant and can grant 410 k€/year of economic benefit arising from the methane and the lower emissions of CO2. This study demonstrates that solar heat can be used to decarbonize energy intensive endothermic chemical processes without economic penalty for the plant profitability.