11 replies to this topic

[Anup87]

Dear All,

 

I am asking very basics of chemical engineering, which now a days confusing me a lot.  I am trying to get the equations which are used in HYSYS to calculate the vapor fraction at given temperature and pressure for specific composition.  Please help me to get these equations.

 

Thanks you all.

Anup B.

 

 

[MrShorty]

What you are describing is commonly referred to as a flash calculation. There are several different algorithms that can be used, and they will depend on the nature of the specific problem at hand.

 

A good introductory approach is to consider the case of fixed T and P where K is assumed to be independent of composition. The Rachford-Rice equation can be used in these situations to get x and y and fraction vaporized. Here's a quick article from wikipedia http://en.wikipedia....ash_evaporation I would also expect any good text in phase equilibrium thermodynamics would describe this and other flash algorithms.

[Art Montemayor]

You are trying to identify the equations used in HySys.  Good Luck.

 

A simulation program is not likely to explain to you how they come up with an answer (the algorithm).  That's how they make a living - by selling you the program.

 

Shorty is giving you the correct response.   Flash calculations should be a normal and well-known procedure for a chemical engineer and can be done manually or through a web page.  The answers are approximations - but then, so are the HySys answers.  It depends on who you trust best.  Not all HySys answers are correct or believable.

[Anup87]

Thanks to both of you...!

 

It will be really great help if some one can give me some good reference or explanation.

[breizh]

This xcelsheet could be a starting point .

 

Hope this helps

 

Breizh

[PaoloPemi]

perhaps the equations utilized for calculating gas and liquid fugacities are available in the operating manual or technical literature related to your software,

anyway you may find a detailed discussion for different EOS in The Properties of Gases and Liquids by Reid, Prausnitz, Poling and other textbooks which may be available at your university library,

about the isothermal flash itself, it can be solved in different ways,

as noted by Art developers prefer do not share the details,

however a simple method to solve a isothermal vapor-liquid equilibria is the Successive Substitution Method (SSM) with many variants discussed by Michelsen and other authors,  you may find examples (as EXCEL pages, FORTRAN codes etc.) with google,

things are more difficult in critical area (with strong composition's dependence) or multiphase equilibria,

in multiphase equilibria you can solve for a different number of phases and states (vapor, liquid, solid) at equilibria,

since this is a student's forum I wish to mention that there are free versions of tools which do that, for example Prode Properties which I utilize,

when coding your procedure the first step should be the methods for calculating fugacities of vapor and liquid phases,

there are Fortran codes by Michelsen and other authors which can help,

you can test the calculated values against those of  a software which exports fugacities and derivatives (see for example Prode Properties)

[ankur2061]

Anup B.

 

The Rachford-Rice equation for isothermal flash of a liquid mixture with an excel spreadsheet for isothermal flash operation can be found at:

 

http://www.cheresour...hase-separator/

 

Regards,

Ankur.

[Art Montemayor]

This thread has to be a prime example of why our Forums are such a popular and favorite choice of so many young graduate engineers and students.

 

It makes me glad and proud to see some of the top, experienced engineers in our Forums jumping into this topic and helping out with their valuable and detailed comments for what I consider one of the most basic and important operations in chemical engineering.

 

Thank you, and kudos to everyone great job of helping out.

[Anup87]

Thank you all for helping out.  I can say I am partly understood the subject.  However, anything extra will be really helpful.

 

Thanks...!

[thorium90]

Oh? Only partly? Which part? I kinda thought it was quite alot already. Looking through the excel file will even help you understand the calculation steps, how to actually apply the equations.

[César]

Dear Anup.

 

If you are interested in knowing details about the equations that are used into a process simulator, "the algorithm", please check http://dwsim.inforside.com.br/

 

DWSIM is an open source process simulator that offers users freedom for its using, analysis, distribution and/or modification for specific purposes. 

 

Because of this freedom you can study it's source code and understand the core of many calculations related to process simulation in chemical engineering. Also if you notice some improvement chance, you are very welcome to submit your suggestions.

 

Best regards!

 

César

[Anup87]

Thanks Cesar.....found link very useful.....