Chapter 8, Problem 8.49P

Interpretation Introduction

(a)

Interpretation:

The rate of heat removal to produce $300\text{ kg/h}$ of dry ice is to be calculated.

Concept introduction:

According to the Hess’s law, the total enthalpy for a reaction is the sum of all the changes in the multiple steps of the reaction. According to this law, enthalpy is a state function.

Specific heat capacity $\left(C_p\right)$ of a substance is the amount of heat needed for a unit mass of substance to raise its temperature by $1^{\circ }\text{C}$. It is temperature dependent and varies with temperature of the substance. The formula to calculate $C_p$ is:

$C_p\left(T^{\circ }\text{C}\right)=a+bT+c{T}^2+d{T}^3$ .......(1)

Here, $a,b,c\text{ and }d$ are constants which is specific for a substance. The values of these constants are given in table B.2 of appendix B.

Change in specific enthalpy $\left(\Delta \hat{H}\right)$ of any substance for the temperature change from $T_1{}^{\circ }\text{C to }{T}_2{}^{\circ }\text{C}$ is given by:

$\Delta \hat{H}={\int }_{T_1}^{T_2}{C}_{p}dT$ .......(2)

The rate of heat transfer is calculated as:

$\dot{Q}=\dot{n}\Delta \hat{H}$ .......(3)

Interpretation Introduction

(b)

Interpretation:

An expression for the rate of heat removal is to be written in terms of the heat capacities and latent heats of ${\text{CO}}_{\text{2}}$ in different phases.

Concept introduction:

According to the Hess’s law, the total enthalpy for a reaction is the sum of all the changes in the multiple steps of the reaction. According to this law, enthalpy is a state function.

Specific heat capacity $\left(C_p\right)$ of a substance is the amount of heat needed for a unit mass of substance to raise its temperature by $1^{\circ }\text{C}$. It is temperature dependent and varies with temperature of the substance.

Change in specific enthalpy $\left(\Delta \hat{H}\right)$ of any substance for the temperature change from $T_1{}^{\circ }\text{C to }{T}_2{}^{\circ }\text{C}$ is given by:

$\Delta \hat{H}={\int }_{T_1}^{T_2}{C}_{p}dT$ .......(2)

The rate of heat transfer is calculated as:

$\dot{Q}=\dot{n}\Delta \hat{H}$ .......(3)