# Phase Equilibrium in Chemical Engineering

## Summary
Chemical Engineers are interested in the amount of stuff in a system, which is described by the various types of balance equations that we introduced earlier. However, they are also interested in the state of stuff in the system, e.g., is stuff a liquid, a gas, or a solid. 

Many exciting applications with societal or technological benefits have been developed by exploiting or manipulating the state of a working material in a system. For example, [power generation](https://en.wikipedia.org/wiki/Rankine_cycle) and [cooling processes](https://en.wikipedia.org/wiki/Vapor-compression_refrigeration) take advantage of the phase change (moving from a liquid to gas, and then back again) as a critical element of the process. [Flash separation](https://en.wikipedia.org/wiki/Flash_evaporation) takes advantage of the phase change (moving from a liquid to a mixture of liquid and gas) to separate chemical mixtures without any moving parts. Similarly, [crystallization](https://en.wikipedia.org/wiki/Crystallization), often a critical step in many chemical and pharmaceutical processes, involves moving from a liquid mixture to a mixture of solid and a liquid. Finally, many food products, e.g., salad dressings or consumer products such as cosmetics, are [emulsions](https://en.wikipedia.org/wiki/Emulsion); an [emulsion](https://en.wikipedia.org/wiki/Emulsion) is a mixture of two or more liquids that are usually immiscible on their own but can be mixed with the aid of an emulsifier, a chemical component which stabilizes the mixture. Thus, the state of the working fluid in the system is often a critical component of a process. 

Toward understanding the system state in this unit, we will introduce:
* [Theory of single and multiple component phase equilibrium](./single-component-phase-eq.md) 
* [Societal and technological applications of vapor-liquid equilibrium (VLE)](./vle.md)
* [Societal and technological applications of liquid-liquid equilibrium (LLE)](./lle.md)

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