Using Flash Calculator

The Flash Calculator (Self-Contained), FLSC.EXE, is a program for performing typical chemical engineering "flash" calculations or other vapor/liquid equilibrium calculations using the same physical property system as Flowsheet Toolkit (FT) and Distillation Guide (DGSC.EXE), former Digital Analytics offerings. Equilibrium calculations can be based on the Peng-Robinson EOS, the Wilson equation, or the UNIFAC methodology.

Currently there are two versions, the demo version, which can be downloaded without restriction, and the full version, which can be downloaded for limited free trial with the restriction that you must register it to make it work. The demo version only allows 3 components in the mixture, and the full version allows up to 34. The demo performs the single analyses (a) through (g) listed below, whereas the full version does these as well as graphical studies to give plots 1, 2, and 3 described below.

In most cases the user specifies the vector of feed mole fractions (weighted sum for both liquid and vapor if present). The user also specifies the final outlet pressure and/or temperature. FLSC can perform the following types of single analyses:

(a) If the outlet temperature and pressure are fixed at user-specified values, the program determines whether the outlet material is all liquid, all vapor, two phases, or a supercritical fluid (for 2 phases the program determines the split between liquid and vapor and liquid and vapor properties and compositions).

(b) If the total enthalpy per mole of the outlet material is fixed at a user-specified value, and the pressure is specified, the program determines the outlet material state as in (a).

(c) The user can specify that the feed is a saturated liquid (at its bubble point) which is flashed isenthalpically to a lower pressure. The program then determines the outlet state as in (a). It is also possible to add to or subtract from the saturated feed enthalpy by a user-specified Q per mole.

(d) Given a particular pressure, the program can compute the bubble point temperature of the feed and composition of the corresponding equilibrium vapor.

(e) Given a particular temperature, the program can compute the bubble point pressure of the feed and composition of the corresponding equilibrium vapor.

(f) Given a particular pressure, the program can compute the dew point temperature of the feed and composition of the corresponding equilibrium liquid.

(g) Given a particular temperature, the program can compute the dew point pressure of the feed and composition of the corresponding equilibrium liquid.

Certain of these calculations do not work for certain cases. No calculation can work in a small region near the critical point, and UNIFAC/Wilson computations are generally not for light gases or systems with wide range of volatilities. For mixtures with very wide range of volatilities, (d) and (f) may not work.

FLSC (registered version only) can also generate the following graphs:

Plot type 1 ---- At a specified temperature and feed composition, plot K's for each compound as a function of flash pressure. This is essentially repeating analysis (a) above for many pressures.

Plot type 2 ---- For binary mixtures only, plot equilibrium pressure vs. X's and Y's with temperature as a parameter. This calculation does not use the flash feed user input; instead it varies composition over almost the maximum possible range. (See Peng-Robinson example 1 -- moderate range of volatilities, Peng-Robinson example 2 -- wide range of volatilities, and UNIFAC example 3 -- with azeotrope).

Plot type 3 ---- For the user-specified (feed) composition, plot the locus of bubble and dew points on a pressure vs. temperature graph. (See example.)

Note that in types 1 and 3 a plot line is only generated if there are two phases present in a flash problem with the given feed. Type 2 automatically varies mole fraction of compound 1 from 0.01 to 0.99 and so does not use the user-specified feed composition. In Type 2 the lightest compound should be compound 1. If compound 1 is much less volatile than compound 2, the calculations may fail.

Starting with V2.14 (4/2011) the EDTECH program is incorporated into FLSC for simplified graph generation. Previously the user had to download and install EDTECH separately and establish the connection between the two applications.

Documentation for all versions of Flash Calculator is an html file with a contents topic which is linked to all other topics.