Does Throttling Compressed Air Cause Liquefaction?

[kandelabr]

Homework Statement

I need to know what happens if compressed air from a vessel at room temperature is throttled from 450 bar to 2 bar.

Homework Equations

I know Joule-Thomson effect (http://http://en.wikipedia.org/wiki/Joule-Thomson_effect" ) deals with this, but I'd need it explained a bit more clearly, like what volume is in the equation and under what circumstances is it true.

The Attempt at a Solution

I know air will cool down when throttled, but at certain pressure and temperature it will liquefy. Will it liquefy during throttling?
What about work that was lost due to entropy change? If this system is isothermal, that is, the throttled air is reheated to initial temperature, ...
d0h, there's too many things in my head. I hope you know what I don't know I don't know.

 

[anathema]

Hello, thank you for your question. The Joule-Thomson effect is indeed applicable in this scenario. It is a thermodynamic process in which a gas is expanded through a throttling valve, resulting in a temperature and pressure change. The equation for this effect is given by:

ΔT = (T2-T1)(P2/P1)[(∂H/∂P)T]

Where ΔT is the change in temperature, T2 and T1 are the final and initial temperatures, P2 and P1 are the final and initial pressures, and (∂H/∂P)T is the change in enthalpy with respect to pressure at constant temperature.

In your scenario, the compressed air is at room temperature (T1) and 450 bar (P1) before being throttled down to 2 bar (P2). As the air expands, its temperature will decrease (ΔT) due to the Joule-Thomson effect. The amount of cooling will depend on the specific properties of the gas being used.

Regarding your question about liquefaction, it is possible for the air to liquefy during the throttling process if the temperature and pressure conditions are within the region of the phase diagram where the gas is in the liquid state. However, this is unlikely to occur at room temperature and with the pressures given in your scenario.

As for the work lost due to entropy change, it is important to note that the Joule-Thomson effect is an isenthalpic process, meaning that there is no change in enthalpy. Therefore, there is no work done or lost in this process.

I hope this helps clarify the Joule-Thomson effect and its application in your scenario. If you have any further questions, please don't hesitate to ask.

 

[nlightner]

I can provide some insight into the concept of air throttling and the potential outcomes in this scenario.

Firstly, air throttling refers to the process of reducing the pressure of a gas by passing it through a small opening or valve. This results in a decrease in the gas' kinetic energy and temperature. In the case of real gases, the Joule-Thomson effect is observed, which describes the change in temperature of a gas when it undergoes adiabatic throttling.

In this scenario, if compressed air at room temperature (around 25°C) is throttled from 450 bar to 2 bar, the temperature will decrease due to the Joule-Thomson effect. This is because the air is expanding and doing work against the surrounding pressure, resulting in a decrease in its internal energy and therefore a decrease in temperature.

The Joule-Thomson effect is dependent on the initial temperature and pressure of the gas, as well as its specific heat capacity. The equation for the Joule-Thomson coefficient (μ) is μ = (∂T/∂P)H, where T is temperature, P is pressure, and H is enthalpy. This coefficient represents the change in temperature for a given change in pressure at constant enthalpy.

In terms of volume, the Joule-Thomson effect does not directly involve volume in its equation. However, the change in volume of the gas during throttling will affect the temperature change, as a larger change in volume will result in a larger change in temperature.

Regarding the possibility of liquefaction, this will depend on the initial temperature and pressure of the gas. If the gas is initially at a temperature and pressure above its critical point (the point at which the gas can no longer be liquefied), then it will not liquefy during throttling. However, if the initial temperature and pressure are below the critical point, then the gas may liquefy during throttling.

Finally, regarding the work lost due to entropy change, this is a valid consideration. The throttling process is not reversible, meaning that some energy will be lost due to entropy. This energy loss will result in a decrease in the gas' temperature, which is why the throttled air will need to be reheated to reach its initial temperature.

In summary, throttling compressed air from 450 bar to 2 bar will result in a decrease in temperature due to the Joule-Thomson effect