Flash Separations: Screencasts

Presents the material balances, energy balance, and Raoult’s law for an adiabatic flash drum. The feed is an ideal binary liquid.

We suggest you list the important points in this screencast as a way to increase retention.

This screencast describes a flash distillation, derives the material balances and operating line, and shows how to use the operating line on an x-y diagram.

We suggest you list the important points in this screencast as a way to increase retention.

Important Equations:

Antoine equation for component $$i$$: $log_{10}(P_i ^{sat}) \,= \, A_i \, – \frac{B_i}{C_i+T}$ Where $$P_i^{sat}$$ is the saturation pressure, $$T$$ is the temperature (most often in $$^oC$$), and $$A_i, B_i,$$ and $$C_i$$ are constants for a given component, $$i$$.

Raoult’s Law: $x_iP_i^{sat} \, = y_iP$ for each component $$i,$$ where $$x_i$$ is the liquid-phase mole fraction and $$y_i$$ is the vapor phase mole fraction.

Raoult’s Law and mass balances combined $x_i\,=\,\frac{z_i}{1+\left(\frac{V}{F}\right)\left(K_i-1\right)}$

Where $$z_i$$ is the feed mole fraction of component i, $$K_i$$ is the K-factor, which is $$y_i/x_i$$ for component i, $$F$$ is the feed molar flow rate, and $$V$$ is the vapor molar flow rate out.

Overall mass balance: $F\,=\,V\,+\,L$ where $$F$$ is the feed flow rate, $$V$$ is the vapor flow rate, and $$L$$ is the liquid flow rate.

Component mass balance: $z_iF\,=\,y_iV\,+\,x_iL$ where $$z_i$$ is the mole fraction of component i in the feed, $$y_i$$ is the vapor mole fraction of component i, and $$x_i$$ is the liquid mole fraction of component i.

Mole fractions in a liquid phase: $\sum x_i\,=\,1$

Mole fractions in a vapor phase:$\sum y_i,=\,1$

Energy Balance for an adiabatic system: $F(H_F)\,=\,L(H_L)\, +\,V(H_V)$ where $$H_F$$ is the molar enthalpy of the feed (kJ/mol), $$H_L$$ is the molar enthalpy of the liquid effluent (kJ/mol), and $$H_V$$ is the molar enthalpy of the vapor effluent (kJ/mol).