$5\mathrm{.}13.$ At $650C$ phosphine vapor decomposes as follows:

$4P{H}_3{P}_4(g)+6H_2,-r_{\text{p}\text{h}\text{o}\text{s}}=(10h{r}^{-1})C_{\text{p}\text{h}\text{a}\text{s}}$

What size of plug flow reactor operating at $649C$ and $11\mathrm{.}4$atm is needed for $75\%$ conversion of $10mo\frac{l}{h}r$ of phosphine in a $\frac{2}{3}$ phosphine$-\frac{1}{3}$ inert feed?