Sucrose integration into COCO

[Farhan]

What do you suggest about the models I selected, ideal gas for vapor and ideal solution for liquid. can I get more precise results by selecting a different set of models?

[jasper]

If you use ideal gas for the vapor phase you are surely making a small error there. Probably any of the other equations of state is better.

For the liquid phase, ideal activity implies that there is no interaction between the sugar and the water. Surely the activity of the water will be less than unity, presuming the sugar "wants" to be surrounded by water.

As this is a two-compound system only, and one of them is water, and you want to be really accurate, you should probably base the water data on the steam tables, and the addition of sugar to the liquid phase upon measurements. Just google "water activity sucrose" and you will find a number of sources. However, such a model will require some coding; you'd have to write the physical properties and flash calculator yourself. Now the phase distribution of sucrose is trivial. That of water is not; essentially you will want to find the mole fraction of water in the liquid phase such that the vapor fugacity is matched.

All of that can be done via CAPE-OPEN, and perhaps the open source steam table package of coco (http://cocosimulator.org/down.php?dl=wa ... source.zip) could serve as a starting point. However, as I said, coding is required, and I guess all of this may exceed the scope of your exercise.

[Farhan]

Kindly provide me some literature on interaction parameters and a link for the coding (explaining procedures) which you discussed in previous post. I need accuracy because the model is based on experimental results and I need to achieve those results. Flow rate of the concentrated solution is greater than the experimental results and I need to overcome this problem.

[Farhan]

I have little knowledge of selection of proper model for a specific solution, recommend me a link/study stuff so that I can overcome it. Please inform me if you having a model of evaporator modeled in excel unit operation because I am intended to create its model.

[jasper]

If you have experimental results you can use those directly. You can either program the phase equilibria yourself, in which case your experimental data can simply be adjusted to the mole fraction of water in the liquid phase (the mole fraction of water in the vapor phase is unity, the mole fraction of sucrose in the liquid phase is 1 minus the mole fraction of water in the liquid phase, so you are really only looking for one parameter). This relates directly to the liquid flows and vapor flows as

VF + LF * X_water = F_water
LF * (1 - X_water) = F_sucrose

and

(VF = vapor fraction, LF is liquid fraction, F is compound flow rate at the feed)

F_vapor = VF * (F_water + F_sucrose)
F_liquid = LF * (F_water + F_sucrose)

and of course (VF + LF) = 1.

Again this takes some programming. Rather than explaining procedures of CAPE-OPEN, I can point to the version 1.1 thermodynamic standard specification that explains all procedures. Have a look at the water example at the COCO simulator web site, or the ideal thermo example at the CO-LaN web site.

Alternatively you could use an activity model that is already present in TEA, such as the Margules model (which comes to mind for a 2 compound system, as this has only two parameters; http://en.wikipedia.org/wiki/Margules_activity_model) and fit the two parameters to your experimental data. Unfortunately an automated fit procedure at this moment, nor are the parameters exposed by the thermodynamics so that you can set up your own fitting automation. So the only way to get the fit done in this case is 'manually'; go into the package configuration and change the parameters. Of course in case you need to fit two parameters, you need to match at least two experimental data points.

[Farhan]

what is meant by overall enthalpy in the equilibrium calculation. Is it enthalpy of the feed stream?

[jasper]

Overall enthalpy is the enthalpy of the stream, in J/kg or J/mol; this is the sum of [the phase enthalpies multiplied by the phase fractions].

[Farhan]

Average activity of water for Sucrose is 0.90. Tell me how to enter the values of interaction parameters in Sucrose/Sucrose, Water/Water, Sucrose/Water, Water/Sucrose.

[jasper]

typically you cannot specify an average value. An average value would neither apply in the limit of pure water, nor in the limit of pure sucrose. Hence, the values are typically concentration dependent. The extent of concentration dependence is capture in the activity model.

For the Margules model you would need to know the water acitivities at two (or more) distinct compositions to back calculate A12 and A21 (see http://en.wikipedia.org/wiki/Margules_activity_model); the value of water activity = 1 for pure water does not help, this is always satisfied with the Margules model (for most activity models this condition is automatically satisfied).

[Farhan]

Does the COOLER unit operation count only for SENSIBLE heat or it also make use of LATENT heat ??