(576d) Exergy Analysis of a Gas to Liquid Process

Interest in Gas-To-Liquid process (GTL) using Fischer-Tropsch reactors (F-T) has increased in recent years because of their potential to displace ordinary petroleum and produce sulphur free fuels. However, the efficiency of the process is still quite low and a large amount of the initial exergy of the gas is used to convert it into liquid fuel. In the present study, we analyze both carbon and thermodynamic efficiencies and point out the main sources of entropy production.

Our process design is based on autothermal reforming for production of syngas and cobalt catalyst for the slurry phase F-T reactor. The analysis focuses on the entropy production of the process core, namely the syngas production unit, the F-T reactor and the power plant. The model includes realistic feature such as reduced catalyst activity, limitation due to metal dusting, and criogenic air separation unit. The largest production of entropy occurs in the syngas unit and in the power plant, but it is also related to the limited activity of the catalyst. Since conversion of syngas into liquid fuels cannot be achieved in a single step, reactor tailgases have to be recycled through the reforming unit and this increases the size of the reforming unit as well as its entropy production.

The selectivity of the F-T catalyst plays a major role in the plant efficiency, meaning that an increased effort to improve the catalyst is important. Moreover, we outline other possible areas of the process where improvements may be achieved.

By changing the selectivity of the catalyst, we have shown that the efficiency of the process increases, and that overall CO2 emissions are reduced.