Answer
D
Work Step by Step
From the definition:
$s_2-s_1=c_p\ln{\dfrac{T_2}{T_1}}-R\ln{\dfrac{P_2}{P_1}}$
For an ideal gas, throttling is isothermal $T_2=T_1$
Given $R=0.287\ kJ/kg.K,\ P_2=200\ kPa,\ P_1=800\ kPa$
$s_2-s_1=0.40\ kJ/kg.K$
Hence $\dot{X}_{des}=T_0\dot{m}(s_2-s_1)$
Since $\dot{m}=0.5\ kg/s,\ T_0=298\ K$
$\dot{X}_{des}=59.3\ kW$
