LearnChemE

const arrows = document.getElementsByClassName("hfe-menu-toggle sub-arrow"); for(let i = 0; i < arrows.length; i++) { const arrow = arrows[i]; arrow.innerHTML = "▼"; arrow.style.fontSize = "0.6rem"; arrow.style.transform = "scaleY(0.8) translateY(3px)"; }

#### Throttling and Joule-Thomson Expansion: Summary

The answers to the ConcepTests are given below and will open in a separate window.
##### Key points from this module:
• Throttles are modeled as adiabatic.
• Energy balance for a throttle: enthalpy in = enthalpy out (for liquid or gas feed).
• Throttles are irreversible; the effluent entropy is higher than the inlet entropy.
• A high-pressure liquid fed to a throttle can partially vaporize at the lower exit pressure if the exit pressure is below the saturation pressure at the feed temperature. Because a throttle is adiabatic, the heat of vaporization is obtained by lowering the exit temperature. The exiting gas-liquid mixture is at saturation.
• An ideal gas does not exhibit a temperature change in a throttle because its enthalpy only depends on temperature, not pressure.
• For most gases, Joule-Thomson expansion through a throttle results in a temperature drop, but the temperature can increase. That is, the Joule-Thomson coefficient can be positive or negative; it depends on temperature, pressure, and the gas.
##### From studying this module, you should now be able to:
• Determine the outlet conditions for a throttle, given the inlet pressure and temperature, the outlet pressure, and an equation of state or a table of properties.

Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder