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Chemical 2
- Thread starter JWengtips
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Here's a link to a paper on the topic. It gives some background and references at the end.
Here's another one. Hope these help!
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UmeshMathur
Chemical
Hi, joanwang:
I am quoting from, and expanding upon, another post I made recently in a related forum. The short answer to your question is that binary interaction parameters are used in the equations of phase equilibria to help calibrate the extent of non-ideality of a given binary mixture. Here, non-ideality refers to deviations from Raoult's law in the liquid phase and Dalton's law for the vapor phase. Once you have these parameters for all possible binary mixtures in a given system, you can predict the multicomponent phase equilibria with fairly good reliability. This is because modern phase equilibrium models are readily extended to multicomponent cases once the constituent binaries are well defined. For a mixture with N components, there are N*(N-1)/2 binaries. So for 5 components, you'll have 5*4/2 = 10 binaries. You need to characterize the behavior of each of these binaries before you can predict mixture phase equilibria in all regions of composition, temperature, and pressure.
CHEMICAL SYSTEMS (ACTIVITY COEFFICIENTS)
Here, we refer to those systems where the liquid phase is modeled using an activity coefficient correction to Raoult's law, while the vapor phase is modeled using a fugacity coefficient correction to Dalton's law.
Picking the right activity coefficient model is important. I recommend the UNIQUAC model that, unlike Wilson or NRTL, can handle both VLE and LLE data with two interaction parameters per binary. Wilson cannot handle LLE and NRTL requires 3 interaction parameters per binary.
The best reference is "Computer Calculations for Multicomponent Vapor-Liquid and Liquid-Liquid Equilibria" by Prausnitz, Anderson, Eckert, Grens, Hsieh, and O'Connell (Prentice-Hall, 1980). This provides extensive computer programs, which are also available from Professor Prausnitz (U. of California, Berkeley) for a nominal fee, I believe.
You need to learn the fundamentals from this book. For the actual binary parameter estimation, I suggest that you use the activity coefficient regression package that comes with your process simulator (so that you're sure the physical properties of the pure components are consistent with what you'll end up using when you do simulations).
If you have a VLLE system, be careful as you then need to regress VLE and LLE data simultaneously. Code for that option is discussed in the book but is not printed there. However, it was available from Professor Anderson (U. of Connecticut) a few years ago.
For VLE of gases (i.e., components above their critical temperature) in solvents, you must use the "unsymmetric" activity coefficient convention. This is also discussed in the book I have cited.
If you have binaries in your mixture for which no VLE data is available, you will have to use the UNIFAC group contribution method to estimate their VLE or LLE. This is a whole separate subject. Generally, the UNIFAC method is also available with the major commercial simulation packages. It should be used only as a last resort for estimating VLE or LLE when no measured binary VLE or LLE data is available.
HYDROCARBON / PETROLEUM SYSTEMS (EQUATIONS OF STATE)
These systems are generally modeled using the same equation for both phases (e.g., the Soave-Redlich-Kwong or SRK equation of state), and a fugacity coefficient - i.e., deviation from Dalton's law - is calculated for each phase. When a vapor and liquid phase coexist, the cubic SRK equation provides a root for each of the phases and the fugacity coefficients are calculated at each of the roots. A good reference for such systems is Thomas E. Daubert "Chemical Engineering Thermodynamics" (McGraw-Hill, 1985).
For petroleum mixtures, The API Technical Data Book recommends using the SRK equation of state with zero interaction parameters. For light hydrocarbons with large H2 / CO2 / H2S / CO concentrations or at very high pressures or very low temperatures, zero interaction parameters cannot safely be used in general.
GENERAL REMARKS
The interaction parameters listed in the DECHEMA books are good only if you also use DECHEMA’s listed physical properties, especially vapor pressure when using activity coefficients, and the critical constants and acentric factor when using equations of state. Sometimes, these do not quite agree with what your simulator is using. (NOTE: DECHEMA is a German society, much like AIChE, that has published an extensive set of volumes that capture most of the published VLE data in the world's literature. These data have been analyzed for accuracy and the binary interaction parameters have been fitted to various commonly used models, both for activity coefficients and equations of state).
Finally, use the regression package that comes with your simulator carefully and do not extrapolate physical properties beyond their range of guaranteed accuracy. Check your results by doing some multicomponent bubble and dew point calculations to make sure you get good results before starting serious simulation work.
In general, this work requires some expertise and experience and should not be assigned to novices in thermodynamics, especially if you're responsible for plant design or something similar. I would recommend an apprenticeship with an experienced colleague who can provide hands on training on how to do such work efficiently.
Have fun.
I am quoting from, and expanding upon, another post I made recently in a related forum. The short answer to your question is that binary interaction parameters are used in the equations of phase equilibria to help calibrate the extent of non-ideality of a given binary mixture. Here, non-ideality refers to deviations from Raoult's law in the liquid phase and Dalton's law for the vapor phase. Once you have these parameters for all possible binary mixtures in a given system, you can predict the multicomponent phase equilibria with fairly good reliability. This is because modern phase equilibrium models are readily extended to multicomponent cases once the constituent binaries are well defined. For a mixture with N components, there are N*(N-1)/2 binaries. So for 5 components, you'll have 5*4/2 = 10 binaries. You need to characterize the behavior of each of these binaries before you can predict mixture phase equilibria in all regions of composition, temperature, and pressure.
CHEMICAL SYSTEMS (ACTIVITY COEFFICIENTS)
Here, we refer to those systems where the liquid phase is modeled using an activity coefficient correction to Raoult's law, while the vapor phase is modeled using a fugacity coefficient correction to Dalton's law.
Picking the right activity coefficient model is important. I recommend the UNIQUAC model that, unlike Wilson or NRTL, can handle both VLE and LLE data with two interaction parameters per binary. Wilson cannot handle LLE and NRTL requires 3 interaction parameters per binary.
The best reference is "Computer Calculations for Multicomponent Vapor-Liquid and Liquid-Liquid Equilibria" by Prausnitz, Anderson, Eckert, Grens, Hsieh, and O'Connell (Prentice-Hall, 1980). This provides extensive computer programs, which are also available from Professor Prausnitz (U. of California, Berkeley) for a nominal fee, I believe.
You need to learn the fundamentals from this book. For the actual binary parameter estimation, I suggest that you use the activity coefficient regression package that comes with your process simulator (so that you're sure the physical properties of the pure components are consistent with what you'll end up using when you do simulations).
If you have a VLLE system, be careful as you then need to regress VLE and LLE data simultaneously. Code for that option is discussed in the book but is not printed there. However, it was available from Professor Anderson (U. of Connecticut) a few years ago.
For VLE of gases (i.e., components above their critical temperature) in solvents, you must use the "unsymmetric" activity coefficient convention. This is also discussed in the book I have cited.
If you have binaries in your mixture for which no VLE data is available, you will have to use the UNIFAC group contribution method to estimate their VLE or LLE. This is a whole separate subject. Generally, the UNIFAC method is also available with the major commercial simulation packages. It should be used only as a last resort for estimating VLE or LLE when no measured binary VLE or LLE data is available.
HYDROCARBON / PETROLEUM SYSTEMS (EQUATIONS OF STATE)
These systems are generally modeled using the same equation for both phases (e.g., the Soave-Redlich-Kwong or SRK equation of state), and a fugacity coefficient - i.e., deviation from Dalton's law - is calculated for each phase. When a vapor and liquid phase coexist, the cubic SRK equation provides a root for each of the phases and the fugacity coefficients are calculated at each of the roots. A good reference for such systems is Thomas E. Daubert "Chemical Engineering Thermodynamics" (McGraw-Hill, 1985).
For petroleum mixtures, The API Technical Data Book recommends using the SRK equation of state with zero interaction parameters. For light hydrocarbons with large H2 / CO2 / H2S / CO concentrations or at very high pressures or very low temperatures, zero interaction parameters cannot safely be used in general.
GENERAL REMARKS
The interaction parameters listed in the DECHEMA books are good only if you also use DECHEMA’s listed physical properties, especially vapor pressure when using activity coefficients, and the critical constants and acentric factor when using equations of state. Sometimes, these do not quite agree with what your simulator is using. (NOTE: DECHEMA is a German society, much like AIChE, that has published an extensive set of volumes that capture most of the published VLE data in the world's literature. These data have been analyzed for accuracy and the binary interaction parameters have been fitted to various commonly used models, both for activity coefficients and equations of state).
Finally, use the regression package that comes with your simulator carefully and do not extrapolate physical properties beyond their range of guaranteed accuracy. Check your results by doing some multicomponent bubble and dew point calculations to make sure you get good results before starting serious simulation work.
In general, this work requires some expertise and experience and should not be assigned to novices in thermodynamics, especially if you're responsible for plant design or something similar. I would recommend an apprenticeship with an experienced colleague who can provide hands on training on how to do such work efficiently.
Have fun.
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