Answer
$s_{gen}=0.0475\ Btu/lbm.°R$
Work Step by Step
From the entropy balance:
$\dot{m}_1s_1-\dot{m}_2s_2+\dot{S}_{gen}=0$
$\dot{S}_{gen}=\dot{m}(s_2-s_1)$
$s_{gen}=s_2-s_1$
From tables A-11E to A-13E:
Inlet ($P_1=100\ psia, T_1=100°F$): $s_1=0.22902\ Btu/lbm.°R$
Outlet ($P_2=10\ psia, x_2=1$): $s_2=0.22949\ Btu/lbm.°R$
$s_{gen}=0.0475\ Btu/lbm.°R$
