The second law optimal path of a four-bed SO_2 converter with five heat exchangers

摘要

The entropy production rate of a common industrial SO_2 converter was minimized, simultaneously varying the heights of four catalytic beds, the temperature differences across five intermediate heat exchangers, and the distribution of a fixed area available for heat exchange. The entropy production rate had contributions from the chemical reaction, the pressure drop and the heat exchange. The total area for heat exchange, inlet temperature, inlet pressure, inlet composition, outlet temperature, and outlet composition remained constant in the minimization. A new path of operation was found for the given constraints that saved 16.7% of the entropy production rate compared to an industrial SO_2 converter. There were changes in chemical, mechanical and thermal contributions to the entropy production. The savings can be taken out as higher quality heat output, lower quality heat input, or reduction in the total area for heat transfer. The optimum operating conditions also indicate that the same product can be obtained under milder thermal conditions than now. Furthermore, the requirement for catalyst, as measured by the total bed height, decreased with the area available for heat transfer, from 2.4 m at 2000 m~2 to 1.7 m at 6000 m~2. We conclude that there is a significant potential to improve the second law efficiency of the particular industrial SO_2 converter that is studied.