5 replies to this topic

[LiveFree]

This is entirely a "How to use a specific function of Aspen HYSYS" question.

 

I am working on a design for a customer that has a Depropanizer column with a large amount of C2- in the feed.  Due to this C2-, I am designing the tower condenser as a partial condenser.  No problem here, I've done this plenty of times.  

 

The customer wants the tower partial condenser to be specified:  vapor to liquid of product streams (note this is excluding the column reflux) is to be 50/50 ratio.  Since the reflux is also considered in the column unit op specification, I cannot simply set the vapor fraction to 0.5 and call it a day.  I need to add an adjust unit op the the tower to adjust the vapor fraction / vent rate / (many other specifications work here) of the tower condenser.  However, I cannot find the column specifications (vent rate, vapor fraction, etc.) when I try to set an adjust unit op.  Where are these located? 

 

What I've checked so far:  

 

(1) When I try to define the adjust unit op, I have tried looking under the "Main TS @Col#" category and there are no variables available to choose.  So I tried to find these variables using a spreadsheet unit op.  This worked, but I could not find the tower specifications.  

 

 

I simply want to adjust the vent rate of the column condenser until a 50/50 ratio of the overhead liquid to vapor is achieved.  

 

Any help here would be greatly appreciated.  Thanks!

 

(EDIT) Summary:  How would I add an adjust unit op to a column to adjust one of the column specifications to meet specific target?  Where can I find the column specifications when trying to setup the adjust unit op?

Edited by LiveFree, 26 November 2018 - 02:04 PM.

[pavanayi]

My first question is, why would you want a 50/50 vapour/liquid product? Where is the top product going? to another column? I cant really think of a reason why that particular combination is required. The composition of both product streams will be different? Is that what you want to achieve?

 

Nevertheless, to achieve what you have asked for, try this:

Define the column as a total reflux type, with the entire distillate product from the column taken out in vapour phase. The other solver specs you have defined will result in a particular top product draw rate. Add a coolerop on the vapour product off the column to achieve 50% vapour fraction, and split the 2 phase stream as a vapour product and a liquid product. You can do that (adding cooler op, splitting the resulting stream etc.) within the column sub-flowsheet or do it outside in the main flowsheet. That way you can control the column with its specs and not fiddle with the column solver, but still achieve the result you want. In reality, that might be the only way to achieve and control that particular combination of vapour and liquid, by adding a second partial condenser to get the split of 50/50 (even though I can't fathom why)

 

Would that get you your solution?

 

Edit: That thought process may not be valid in all circumstances, so if that is not what you had in mind, could you post a sketch of the column and the necessary details for us to understand what you are trying to model?

 

Regards, 

Pavanayi

Edited by pavanayi, 26 November 2018 - 05:24 PM.

[Bobby Strain]

I don't believe the suggested method will give you the correct vapor and liquid compositions. Look around at the specs available to you. I am sure that you will find what you need if you look carefully.

[Ahsan67]

It would be better if you can share PFD and composition of feed with required product parameters to assist. 

[Bobby Strain]

You can specify the vapor product in the column design. The simplest way to get a 50-50 split is to adjust the vapor draw to achieve the desired flows.

[imeagor]

i want to know whether the vapor distillate is considered when the reflux ratio is defined.