# Frequently asked questions

**Q:**What is VLE - Calc .com for?

**A:**VLE - Calc .com is a free application providing phase equilibrium data, phase diagrams, and model calculations on different kinds of distillation, based on vapor-liquid and liquid-liquid equilibrium calculations. The program provides calculated (predicted) data and no stored experimental data.

**Q:**Is this program free of charge?

**A:**Yes, the use of VLE - Calc .com is completely free of charge.

**Q:**What's so special about this program compared to other on-line databases?

**A:**Although lots of free on-line databases are available containing a wealth of experimental data for thousands of substance pairs, in engineering practice it is

*not*trivial to find experimental data for exactly the substances and temperature/pressure conditions of interest. More over, in industrial practice the mixtures are rarely two-component, rather three-component or multi-component. For these mixtures it is much less likely to find useful data. This program uses professionally well-known calculation methods to replace missing experimental data sets and it is a free tool for process modeling.

**Q:**What sort of calculations are used? How does it work?

**A:**Finally the detailed documentation is done. A brief overview:

The program uses fundamentally two physico-chemical models to describe vapor-liquid equilibra:

- The Raoult's Law modified with the activity coefficient.

*or/and*

- An equation of state (EOS) suitable for describing real gases.

The latter is used in mixtures containing supercritical or near-critical components, with the PSRK model. For the former, the activity coefficient is calculated by the UNIFAC (modified, Dortmund) model.

**Q:**What about the reliability of the calculation results?

**A:**They are generally precise. The literature is abundant with articles discussing the accuracy and reliability, usefulnes of the models. So this program is generally useful for informational, educational/demonstrational use. But when applying very special materials some caution is recommended.

**Q:**Found an error/mistake.

**A:**However during developing the program the point was to achive maximum reliability, errors can still occur, either due the inaccuracy of the predictive models (UNIFAC, PSRK) either due the imperfectness of the algorithms applying them. Sorry for any false or strongly inaccurate result, but the author of this app can not take any responsibility for any damages or losses originating from such calculation results. Use the program at your own risk!

**Q:**How can I download the program?

**A:**No downloadable version yet, but it is already under construction.