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[vnilla_gorilla]

Hi, I searched the forums for this topic but did not find any relevant information so maybe someone can help. I realize more info is better so here is an excerpt from the assignment, Our progress and questions follows.

The objective is to design the optimum conventional distillation column to separate
2x10^8 pounds per year of a mixture of 70% mole propylene and 30% mole propane into
a distillate that is 99.7% propylene and recovers 99.8% of the fed propylene as distillate.
Notice that the question doesn’t concern whether to build the distillation column; the
question is what the optimum design is. The constraints are that the utilities available in
CapCost are the ones available at the refinery where this will go.

When you did the propylene/propane alpha plots before the start of this project, you
should have seen that increased temperature (caused by increased pressure) gave a
decreased alpha which, in turn, gives an increase in minimum number of theoretical
stages for the separation or an increased reflux. However, if you can operate the overhead
at 45° C you can use 35° C cooling water ($0.354/GJ) instead of the least expensive
refrigeration, 20° C cooling at $3.44/GJ. You also have refrigeration available at 5° C,
-20° C, and -50° C. These suggest that you should look at towers that have distillate
temperatures at 15° C, -10° C, and -40° C. You can change the distillate temperature by
adjusting the distillate pressure.

What we have done so far:
Used aspen to determine the column pressures needed to produce overhead condenser temperatures of approximately 10 degrees above the available cooling utilities in Capcost. We have produced 3 aspen models for each of these column pressures: 130 stages, 140 and 150 (theoretical). We have costed these in capcost and got NPV values for each. So far, the best NPV value has been at around 65 psi which is on the low end of our selected column pressures.

We have also determined the relative volatility as it relates to increased temperature and in effect pressure values. We have found that decreasing pressures return higher volatility which should give an easier separation.

My question is this:
So far, the column pressure of around 65 psi has returned the most favorable NPV values. So should we now settle on that pressure for our column and proceed to find the best amount of theoretical stages? What is keeping us from going even lower for the pressure (which gives increased volatility)?

Any insight, recommendations, tips or general comments would be greatly appreciated.
Our calculation spreadsheet is attached as well.

Attached Files

•  Master_10-18-11.xlsx   178.04KB   21 downloads

[pavanayi]

vnilla_gorilla,
Nice profile name


As you would have understood, the most important parameter of a distillation column is it operating pressure, which is selected based on available condensing utility.
Propylene-propane splitters are otherwise named super-splitters because of the difficulty in separating these two due to their very similar volatilities.

As you increase the operating pressure, fixed costs go up and operating costs go down.
Also, difficulty in separation also increases (decrease in relative volatility), which you have already reflected upon.
The rest of the project is just to reinforce this idea along with a basic economic analysis.

I have had a brief look at your excel file and more than the procedure, I see several inconsistencies.

1. In the statement above, you have stated the cost of cooling water as $0.354/Gj and that of 20°C utility as $3.44/Gj, but they don't match with values in the excel file which read $0.133 and $2 per Gj respectively?

2. First sheet of your excel file has mentioned reflux. Is this the theoretical reflux or actual reflux?

3. You have said the column pressure of around 65 psi (g or a??) has returned most favourable NPV, and on observation, I find it is the least negative NPV? I cannot find the reference for how you have arrived at these values, but what you have said is that In any case, the investor will loose money!!! It is just an option of how fast he looses it!!!

4. It will take some time to go through the said excel files, and these calculations will depend on the utility costs, but on a realistic real world situation, I would naturally expect the option where condensing by cooling water (operating pressure around 16-17 barg) to be the cheapest.