Question

Speculate on why chlorine is stored as a liquid rather than being purchased and stored as a gas at the absorber temperature $\left(25^{\circ} \mathrm{C}\right),$ when storing it as a gas would eliminate the need for the chlorine vaporizer and possibly for the heat exchanger in the chlorine feed line. The chlorine fed to the tank dissolves in both the liquid phase and the polymer phase. The slurry inside the tank is well mixed, so that its composition is the same as the composition of the exiting stream. The head space above the slurry contains chlorine vapor in equilibrium with the dissolved chlorine in both condensed phases and water vapor in equilibrium with the liquid water in the aqueous phase. The condition inside the absorber is shown schematically in the following diagram: The following physical property data pertain to this system: $\cdot$ The solubilities of $\mathrm{Cl}_{2}$ in $\mathrm{PVC}$ and in water are correlated by a form of Henry's law: $$x_{\mathrm{p}}\left(\mathrm{kg} \mathrm{Cl}_{2} / \mathrm{kg} \mathrm{PVC} \text { phase }\right)=\alpha_{\mathrm{p}} p_{\mathrm{C}_{2}}, \quad x_{\mathrm{a}}\left(\mathrm{kg} \mathrm{Cl}_{2} / \mathrm{kg} \text { aqueous phase }\right)=\alpha_{\mathrm{a}} p_{\mathrm{Cl}_{2}}$$ where $p_{\mathrm{C}_{2}}$ is the partial pressure of chlorine in the head space. The coefficients $\alpha_{\mathrm{p}}$ and $\alpha_{\mathrm{a}}$ depend on the system temperature. $\cdot$ The solubility data shown below have been obtained for chlorine in $\mathrm{PVC}$ at $25^{\circ} \mathrm{C}$ : $$\begin{array}{|l|c|c|c|c|c|c|c|} \hline p_{\mathrm{Cl}_{2}}(\mathrm{atm}) & 0.75 & 1.18 & 1.23 & 2.95 & 3.03 & 3.87 & 4.82 \\ \hline \mathrm{wt} \% \mathrm{Cl}_{2} & 2.9 \% & 4.6 \% & 5.1 \% & 11.9 \% & 12.3 \% & 15.4 \% & 19.8 \% \\ \hline \end{array}$$ $\cdot$ At $25^{\circ} \mathrm{C},$ chlorine gas is 2.68 times more soluble in $\mathrm{PVC}$ than in water for any chlorine partial pressure. $\cdot$ Raoult's law provides a good correlation of the equilibrium between liquid water and water vapor in the system.

Name: Problem 7, Chapter 12: Elementary Principles of Chemical Processes
Uploaded: 2021-02-10T10:02:31-08:00
Duration: 3 min 38 s
Channel: Lottie Adams

   Speculate on why chlorine is stored as a liquid rather than being purchased and stored as a gas at the absorber temperature $\left(25^{\circ} \mathrm{C}\right),$ when storing it as a gas would eliminate the need for the chlorine vaporizer and possibly for the heat exchanger in the chlorine feed line.
The chlorine fed to the tank dissolves in both the liquid phase and the polymer phase. The slurry inside the tank is well mixed, so that its composition is the same as the composition of the exiting stream. The head space above the slurry contains chlorine vapor in equilibrium with the dissolved chlorine in both condensed phases and water vapor in equilibrium with the liquid water in the aqueous phase. The condition inside the absorber is shown schematically in the following diagram:
The following physical property data pertain to this system:
$\cdot$ The solubilities of $\mathrm{Cl}_{2}$ in $\mathrm{PVC}$ and in water are correlated by a form of Henry's law: 
$$x_{\mathrm{p}}\left(\mathrm{kg} \mathrm{Cl}_{2} / \mathrm{kg} \mathrm{PVC} \text { phase }\right)=\alpha_{\mathrm{p}} p_{\mathrm{C}_{2}}, \quad x_{\mathrm{a}}\left(\mathrm{kg} \mathrm{Cl}_{2} / \mathrm{kg} \text { aqueous phase }\right)=\alpha_{\mathrm{a}} p_{\mathrm{Cl}_{2}}$$
where $p_{\mathrm{C}_{2}}$ is the partial pressure of chlorine in the head space. The coefficients $\alpha_{\mathrm{p}}$ and $\alpha_{\mathrm{a}}$ depend on the system temperature.
$\cdot$ The solubility data shown below have been obtained for chlorine in $\mathrm{PVC}$ at $25^{\circ} \mathrm{C}$ :
$$\begin{array}{|l|c|c|c|c|c|c|c|}
\hline p_{\mathrm{Cl}_{2}}(\mathrm{atm}) & 0.75 & 1.18 & 1.23 & 2.95 & 3.03 & 3.87 & 4.82 \\
\hline \mathrm{wt} \% \mathrm{Cl}_{2} & 2.9 \% & 4.6 \% & 5.1 \% & 11.9 \% & 12.3 \% & 15.4 \% & 19.8 \% \\
\hline
\end{array}$$
$\cdot$ At $25^{\circ} \mathrm{C},$ chlorine gas is 2.68 times more soluble in $\mathrm{PVC}$ than in water for any chlorine partial pressure.
$\cdot$ Raoult's law provides a good correlation of the equilibrium between liquid water and water vapor in the system.

Elementary Principles of Chemical Processes

Richard M. Felder,… 3rd Edition

Chapter 12, Problem 7

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Solved by Expert Lottie Adams

02/10/2021

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Safety: Storing chlorine as a liquid is safer than storing it as a gas. In case of a leak, liquid chlorine would evaporate slowly, while gaseous chlorine would spread rapidly, posing a higher risk to people and the environment. Show more…

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Speculate on why chlorine is stored as a liquid rather than being purchased and stored as a gas at the absorber temperature $\left(25^{\circ} \mathrm{C}\right),$ when storing it as a gas would eliminate the need for the chlorine vaporizer and possibly for the heat exchanger in the chlorine feed line. The chlorine fed to the tank dissolves in both the liquid phase and the polymer phase. The slurry inside the tank is well mixed, so that its composition is the same as the composition of the exiting stream. The head space above the slurry contains chlorine vapor in equilibrium with the dissolved chlorine in both condensed phases and water vapor in equilibrium with the liquid water in the aqueous phase. The condition inside the absorber is shown schematically in the following diagram: The following physical property data pertain to this system: $\cdot$ The solubilities of $\mathrm{Cl}_{2}$ in $\mathrm{PVC}$ and in water are correlated by a form of Henry's law: $$x_{\mathrm{p}}\left(\mathrm{kg} \mathrm{Cl}_{2} / \mathrm{kg} \mathrm{PVC} \text { phase }\right)=\alpha_{\mathrm{p}} p_{\mathrm{C}_{2}}, \quad x_{\mathrm{a}}\left(\mathrm{kg} \mathrm{Cl}_{2} / \mathrm{kg} \text { aqueous phase }\right)=\alpha_{\mathrm{a}} p_{\mathrm{Cl}_{2}}$$ where $p_{\mathrm{C}_{2}}$ is the partial pressure of chlorine in the head space. The coefficients $\alpha_{\mathrm{p}}$ and $\alpha_{\mathrm{a}}$ depend on the system temperature. $\cdot$ The solubility data shown below have been obtained for chlorine in $\mathrm{PVC}$ at $25^{\circ} \mathrm{C}$ : $$\begin{array}{|l|c|c|c|c|c|c|c|} \hline p_{\mathrm{Cl}_{2}}(\mathrm{atm}) & 0.75 & 1.18 & 1.23 & 2.95 & 3.03 & 3.87 & 4.82 \\ \hline \mathrm{wt} \% \mathrm{Cl}_{2} & 2.9 \% & 4.6 \% & 5.1 \% & 11.9 \% & 12.3 \% & 15.4 \% & 19.8 \% \\ \hline \end{array}$$ $\cdot$ At $25^{\circ} \mathrm{C},$ chlorine gas is 2.68 times more soluble in $\mathrm{PVC}$ than in water for any chlorine partial pressure. $\cdot$ Raoult's law provides a good correlation of the equilibrium between liquid water and water vapor in the system.

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Key Concepts

Equipment and Process Design Trade-offs

The selection of storage mode impacts the overall process design, including the necessity for certain equipment such as vaporizers and heat exchangers. Although certain storage options might simplify some parts of the process by eliminating the need for phase change equipment, designers must balance these benefits against other considerations like the complexity of maintaining the required conditions for storage and handling.

Volumetric Efficiency in Liquid Storage

Storing a substance in its liquid state is often much more space?efficient than storing it as a gas, because liquids are significantly denser. This increased density allows for the storage of a greater mass of material in a smaller volume, reducing the size and potentially the cost of the required storage vessels and associated infrastructure.

Safety Considerations in Chemical Storage

Safety is a critical factor in chemical process design. The choice between gas and liquid storage can affect system pressure, the potential for leaks, and the overall reactivity of the stored material. Solid safety protocols often favor conditions that minimize hazards, such as lower operating pressures and reduced volume of reactive gases, making liquid storage a preferred option in many reactive chemical processes.

Phase Equilibrium and Vapor?Liquid Systems

This concept involves understanding how temperature and pressure govern the distribution of a substance between its gaseous and liquid states. Recognizing that a chemical may exist in both phases under equilibrium conditions is crucial for designing storage and handling systems, as the phase behavior determines required conditions and the necessary process equipment for phase transformations.

Henry's Law and Gas Solubility

Henry's law quantifies the relationship between the solubility of a gas in a liquid and its partial pressure above the liquid. In applications where a gas dissolves into liquids or polymers, this law explains how variations in pressure and temperature can affect the concentration in the condensed phase, influencing both process performance and design criteria.

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