Results of the spherical 1D static chemical model using the physical structure derived for B213 C2 and C6 (left panel), C10 and C12 (central vertical panel) and C16 (right panel). The column density profiles of CH₃OH and C¹⁸O (red and blue points) as well as their ratio (black points) have been computed at 10⁵ years (upperpanel), 10⁶ (central horizontal panel) and 10⁷ years (lower panel).