Evaluation of a fluidized bed heat exchanger as a heat source in steam cracking process for olefins production
Examensarbete för masterexamen
Sustainable energy systems (MPSES), MSc
One of the main problems today is global warming and the need for more efficient energy use. Industry in general holds almost a third of global emissions and energy demand, and in turn the petrochemical industry holds a share of more than 30% of the industrial emissions. In specific, the production of olefins by steam cracking holds a significant share of the petrochemical CO2 emissions and energy demand. Steam cracking involves highly endothermic reactions that require high temperatures and superheated steam to crack hydrocarbons (ex: ethane, naphtha…) into a combination of products including ethylene and propylene. The reactions occur in reactor tubes that hang inside a furnace and the required energy is provided by combustion of fossil fuels inside the furnace. In this paper, a fluidized bed heat exchanger (FBHE) using metal oxides as active bed material (also referred to as oxygen carrier aided combustion or OCAC) was investigated as an alternative to the furnace used in today’s steam cracking plants. The key advantage of a FBHE is its good heat transfer capability which allows for many potential benefits including lower temperature levels and less fuel consumption in the furnace. The advantages with active bed material include improved oxygen and fuel mixing in the dense bed (allowing more combustion to occur in the dense bed itself), use of very low excess air ratios, and achieving more uniform temperature distribution in the bed. A base case reflecting a typical ethane steam cracking process was chosen for reference and comparison with the FBHE alternative and the case was modelled in Aspen Plus. The focus of the model was the reactor tube, and a simplified reaction network and kinetics based on common and verified models from literature was used. However, the complete steam cracking plant layout and equipment were included in the Aspen model. Then an extensive literature review was conducted to select the FBHE dimensions, bed material, operating conditions, and heat transfer properties. The temperature required in the bed was then calculated and verified against the heat flux requirements in the reactor tube. The resulting temperature level, fuel consumption, and flue gas composition were noted to evaluate the benefits of using the FBHE. Finally, a sensitivity analysis on the heat transfer coefficient in the FBHE and on the reactor tube was conducted. The study showed that with the fluidized bed heat exchanger with oxygen carrier aided combustion an excess air ratio of 1.03 and an outside heat transfer coefficient (hout) of 600 W/m2.K could be used. As a result, the temperature in the furnace could be reduced by 1940C (down to 10640C) and fuel consumption by 48.5% while maintaining the same operating conditions as the base case. Sensitivity analysis showed that increasing values of the outside heat transfer coefficient have a decreasing impact on the temperature and fuel consumption. For the reactor tube, sensitivity analysis showed that the coil outlet temperature (COT) has the biggest effect on ethylene yield. In addition, there is an optimum range of operation for the steam to hydrocarbon ratio and COT at which the ethylene yield is maximized.
Energi , Hållbar utveckling , Energiteknik , Energy , Sustainable Development , Energy Engineering