The feed flow rate, solvent flow rate and feed composition are given. The desired raffinate composition is specified, and the extract composition and the number of stages to obtain the desired raffinate composition are determined by mass balances. First, the mixing point composition is calculated and located on the ternary phase diagram. The mixing point corresponds to the composition that would be obtained if the feed and solvent flows were mixed together. The mass balance is:
is the feed flow rate (kg/h);
is the solvent flow rate (kg/h);
is the combined feed and solvent flow rate (kg/h);
refer to the mass fractions of the solute, solvent and carrier in
, where the superscripts
refer to the three kinds of stream: feed, solvent and mixed. The coordinates of the mixing point
on the ternary diagram are (
The mixing point can also be located on the phase diagram using the lever rule:
is the length of the line segment from the mixing point to the feed composition
is the length of the line segment from the mixing point
to the solvent composition S.
A line is drawn from the desired raffinate composition
through the mixing point until it intersects the phase boundary, giving the extract composition leaving stage 1,
The operating point
is located at the intersection of a line drawn through points
and a line drawn through points
, because the overall mass balance for the system is:
This equation is rearranged to define the operating point:
A tie line from
to the right side of the phase boundary yields the raffinate composition leaving stage 1,
; this line represents the first equilibrium stage.
A mass balance on stage 1 is:
Thus, the extract composition leaving stage 2,
, is found by drawing a straight line from
from the previous equation). Where this line intersects the left side of the phase boundary is the composition
This procedure is repeated for additional stages until the raffinate composition is nearly equal to the desired raffinate composition
. The number of orange lines drawn is the number of equilibrium stages.
See [1–4] for screencasts that describe the Hunter–Nash method and present examples.
This Demonstration is a Mathematica version of a browser-based simulation .