Dear Dr. Bernd,
In the code "spe11b_calculate_convection.py" line 84, I think that the conversion from mol fraction to weight fraction is not correct. Instead, it shall be
Xw_CO2 = (44 * Xm_CO2) / (44 Xm_CO2 + 18 (1-Xm_CO2))
where Xw is the weight fraction and Xm is the mol fraction.
Regards - AbdAllah