13 replies to this topic

[Mystified]

Hello,

 

I am working on multi-stream heat exchanger steady state simulation and got stuck at the start itself. Suitable advice would be of great help.

 

Before posting my query here, i searched forum, but couldn't find similar one.

 

I am using aspen hysys LNG exchanger for simulation. This particular heat exchanger has one stream that splits half way through the exchanger and one portion comes out at the middle of it and rest travels through it. 

 

I don't know how to configure this in the Hysys LNG exchanger. It only allows inlet and outlet streams. Please advise me.

 

Thanks!

 

[serra]

a limit with simulators is that you can solve only operations with predefined configurations,

however many simulators allow to write scripts or link with external tools to improve features.

 

Basically you need to solve a series of flash operations with specified P,H (or different specs) to simulate

the areas in your exchanger,

that is easy with tools as Excel, 

I have Prode Properties thermodynamic library and i have created pages in Excel to calculate the different types of

equipments such as exchangers, separators, compressors etc.

According my experience it is very easy to modify existing pages to add new features as those you mentioned.

Perhaps you can follow a similar approach.

[Zauberberg]

One solution that works is to use two exchangers in the flowsheet to represent one single exchanger in reality. The stream that leaves the 1st exchanger can be split by using tee operation, one fraction enters the 2nd exchanger and the other one goes wherever it has to. I have used the same approach in modeling MCHE exchanger in LNG plants.

[Mystified]

Thanks for the reply. I will try two exchanger option and let you know the result.

[Bobby Strain]

The two-exchanger model is guaranteed to work.

 

Bobby

[Mystified]

Hi,

 

I tried your suggestion of two exchanger and somehow ended up with error. Excel sheet with complete details is attached.

 

please advice me where i am going wrong. Thanks in advance for your valuable time.

Attached Files

•  Two exchanger option.xlsx   9.84KB   40 downloads

[Bobby Strain]

You need to change some specifications until you get a good result. It's quite difficult for one to follow what you are doing with your excel tables. If you post a screen shot showing temperatures in one and stream names in another, you might get some good advice.

 

Bobby

[Mystified]

Please see the attachment with screenshots. Please let me know if you find anything not clear.

Attached Files

•  Screenshots-Two Exchanger Option.docx   85.09KB   53 downloads

[Bobby Strain]

It looks like stream 1 should be fed to the coldest exchanger, not to the warm exchanger. And it would help if you post a screenshot showing temperatures. It makes it a lot easier to see what's going on.

 

Bobby

[Mystified]

Yes exactly. Stream 1 should have been added to cold exchanger. I did a silly mistake. Thanks for pointing out the mistake.

[Mystified]

One more question. In my case intermdediate temperature i.e. 7.5 C is known and assumed other stream temperature to be same as this because both are entering at similar conditions and calculated unknown third stream temperature.

 

What if i don't know the intermdediate temperature?

 

In some cases, there will be many side streams coming in and out from the exchanger, then following this method (splitting) would be tiresome. Is there any method that usually industry people use.

[Zauberberg]

Not knowing the intermediate temperature means that exchanger heat duty is unknown. Calculating this temperature is known as exchanger rating. You would have to know the exact geometry of the exchanger so that the simulator can try to estimate this value by calculating U-values for given flow patterns and inlet temperatures of the streams.

 

A second option is to assume fixed U-value, which may or may not be accurate - depending how far away the new flows/conditions are from the original set of conditions.

[daneshpour]

Hello everybody.

 

I have a question regarding to multi stream heat exchanger. I read all the above comments, but I need to simulate a multi stream exchanger with 7 inlet and 7 outlet streams for an air separation unit. it seems impossible to use many LNG exchanger, right? Or maybe I didn't get your ideas well. Would you please guide me to how simulate this equipment?

 

Thanks in advance.

[Bobby Strain]

If some streams enter and/or exit other that the ends, then you need to use multiple exchangers.

 

Bobby