Question

How do you perform adiabatic flash calculations?

Answer 1

The adiabatic flash calculations are divided into three parts to find the necessary equations:

1. Mass balance
2. Energy balance
3. Vapour-Liquid Equilibrium Calculations

It is always good to represent the system with a flow diagram to represent the data clearly, thus a flow diagram showing general data required for flash calculations is shown below for components denoted as A and B. For simplicity, the mixture has been taken as a binary mixture.

Where,

F = Total number of moles of Feed

Z_A = Mole fraction of A in Feed

Z_B = Mole fraction of B in Feed

P = Pressure of Feed

T = Temperature of Feed

V = Number of moles of Vapour

y_A = Mole fraction of A in Vapour

y_B = Mole fraction of B in Vapour

L = Number of moles of Liquid

x_A = Mole fraction of A in Liquid

x_B = Mole fraction of B in Liquid

P_out = Pressure at the outlet

T_out = Temperature at the outlet

Mass Balance:

Overall Material Balance: F = V + L

Component Material Balance: Z_A (F) = y_A (V) + x_A (L)

Vapor-Liquid Equilibrium:

where

= Vapour pressure of A

= Vapour Pressure of B

Vapour pressures can be found using the Antoine's equations for the corresponding components:

Energy Balance:

Total enthalpy entering the system = Total Enthalpy Leaving the system

The equations obtained can be solved using Polymath or MATLAB to get the values of the unknowns.