Figure $1.$ A catalytic flow reactor having dimensions $WHL$ in the $x-,y-$ and z$-$directions such that $L>W$ and $W>H.$ Gaseous mixture of $CO$ and $H_{2}O$

enters the reactor through plane A and leaves through plane D$.$ Catalyst is coated on the bottom wall between planes B and $C$ which are at distances of $0\mathrm{.}5L$

and $0\mathrm{.}75L$ from the inlet. The reactor is pre$-$heated through the top and the bottom walls over the distance between planes A and $B.$

In Figure $1,$ if the reaction rate is very high, the CH$4$ species will get consumed as quickly as it reaches the surface. In this case, the boundary condition for the concentration of $[$CH$4]$ species at the bottom wall of the reactor between planes B and C is

$($i$)$ concentration $=0($ii$)$ concentration gradient $=0($iii$)$concentration gradient $=$ constant $($not equal to zero$)$

$($i$)$

$($ii$)$

$($iii$)$

boundary condition is not required