2 replies to this topic

Hi,

I am a chemical engineering student from KUL University, Belgium.

We have to compare a separation process using classic distillation with other seperation techniques, which have to be non-mebrane. It concerns the separation of 5000 kg/h (T=353K, P=1atm) of 10 vol% water and 90 vol% acetic acid. We modelled this separation using distillation with Aspen (radfrac and dstwu). In order to estimate the costs related to this operation, we need to estimate a few things:

1) What are common pressures in a column? We now use an overal pressure of 1 atm, assuming no pressure drop. We don't know how much we can change pressure before things get unrealistic or highly unlikely. Also, we don't know if this affects the operational costs of the column substantially.

2) What are regular heights of a column? We don't know how many theoretical plates we are allowed to use and what HETP is a good rule-of-thumb.

3)How much is a rough estimate for the building, operational and maintenance costs of a column?

4)Energy costs are the most substantial operational cost we believe, but how much are they exactly (electricity is probably cheaper for a firm than for a consumer)?

5) There are no product purities specified, we assumed a purity of at least 97 mole% in distillate and botom stream. Is this a good value?

If you know an answer to any of these questions, a good book, website or article which covers this subject or any information about seperation of acetic acid and water using non-membrane techniques, please reply to this post. Thank you very much!

(My excuses for spelling mistakes or bad grammar, English is not my first language)

[Art Montemayor]

mailliw:

Welcome to our Forums.

Your English is exceptional. Our citizens here in the USA should speak and write as well as you do. I also understand that you attend an excellent university.

My responses to your questions are listed below:

1) What are common pressures in a column? (There are many “common pressures” in distillation columns – it all depends on what you are distilling and under what conditions.) We don't know how much we can change pressure before things get unrealistic or highly unlikely. (You have to analyze what your overhead product(s) and bottom conditions are. The vapor pressure existing on your top plate (or top section of your packed column) is what sets your column pressure. And you arrive at this pressure-temperature situation by setting your overheads composition.)

2) What are regular heights of a column? We don't know how many theoretical plates we are allowed to use and what HETP is a good rule-of-thumb. (You are never “allowed” to use theoretical plates. You calculate how many you need and apply them with the relationship of HETP – the Height Equivalent to a Theoretical Plate. It is that simple.)

3) How much is a rough estimate for the building, operational and maintenance costs of a column? (You find this in the normal ChE plant design text books and in Perry’s Handbook)

4)Energy costs are the most substantial operational cost we believe, but how much are they exactly (electricity is probably cheaper for a firm than for a consumer)? (Find out for yourself by raising the phone and calling your energy suppliers in Belgium. Use your feet and go visit them, if you have to. More importantly, use your ingenuity and resourcefulness to obtain what you need from them.)

5) There are no product purities specified, we assumed a purity of at least 97 mole% in distillate and botom stream. Is this a good value? (Why you would not be given a product purity by your instructor or professor is beyond me. That is definitely your problem and you must insist (or demand) that they give you a full definition of what is expected of you. We have no inkling or idea as to the required purity level of your product. That’s between you and your instructor.)

I suspect that you are not getting a clear message or understanding about your project and that it is imperative that you get together with your instructor and go over each of the above points in detail. I can assure you that he/she will most probably agree with all of my comments/responses. The degree of your confusion regarding what you have been assigned is very critical at this stage of your preparation and you must act NOW and QUICKLY to get a full, clear, and accurate understanding of what you are to do in order to pass this test or assignment with a good grade. If you have a weak understanding of what distillation is and how it is carried out, you must remedy that immediately and successfully. Otherwise, you can't go forward.

Await your comments or reply.

[FosterJ]

Students at the University of North Dakota (where I go) commonly use a 5 PSI pressure drop across all unit operations for a scoping study phase design (+-40% actual cost).

This is a good bit higher then the actual amount of pressure drop across a unit operation such as a column, and will account for small lengths of piping as well. You shouldn't know the exact amount of piping at this stage in the design anyways, and this adds some contingency.

I don't know if that'll fly with your professor, but it's what we use at my university.

By the way, your spelling and grammar were superb.