How to model changes in flowrate in HYSYS

[Wazobia]

I'm a [junior] process engineer, learning how to use HYSYS. The tutorials that came with the software have been helpful, however, when i finished modelling the ACID-gas sweetening with DEA tutorial, there was an excercise which basically asked me to make changes to the model to accept a doubling of the input [sour gas] flowrate.

I hope someone can help me. Perhaps someone out there has worked through the same tutorial. Doubling the flowrate directly wouldnt work.

 

[joerd]

What doesn't work? Does the model no longer converge, don't you meet the specs?
I haven't done the tutorial, but it seems that the problem is stated to make you think about the changes to make in the model in addition to just simply doubling the flow rate. There are probably a number of parameters fixed in the simulation, which are related to the gas flow rate. I can think of e.g. amine circulation rate and regenerator steam rate, for example. Go through the model, see what inputs are fixed, and think about it if these inputs are related to the feed gas flowrate.
If you have a more specific problem, I'll be glad to help.

Cheers,
Joerd

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MortenA]

is this a dynamic simulation?

 

[Wazobia]

Thanks for the reply.

Sorry i didnt reply earlier, i've been away and havent even done anything on the above simulation.
Joerd, you are right, it isnt just about doubling the flowrate, there are other things involved in model in which i have to make changes. I'll definately be looking into that sometime this week and get back to you if i have any problems.

Morten, it is a steady-state process.

 

[Wazobia]

OK, i tried looking into it again and still am stumped. Basically you have a sour gas feed entering a seperator, which removes the water and sends the "dry" gas to the DEA contactor [Absorber operation]. The contactor receives another feed input, from a recycle DEA stream. The output here is the sweet gas at the top, and a stream rich in DEA at the bottom [which isen processed back to the recyle DEA stream]

My problem starts from the DEA contactor. I cannot figure out what parameters to change here...i've tried manipulating the number of trays but no avail. I wouldnt expect that to be a factor though, but i could be wrong.

Anyone out there who could help? For those who are able to make templates on hysys, any suggestions would be appreciated.

thanks

 

[MortenA]

Do you need to change any? Is your problem that your contactor wont converge?

ou havnt really told us what goes wrong after you changed the gas flow rate.

If the collumn is a pure absorber and i assume thats the case with a amine "contactor") then as far as i remember you dont have any free variables (or spec as they call them in HYSYS). The collum may still not converge - and this can be a little fustrating. But you may then have to adjust the amine flow rate or the number of trays and or P/T of the feeds etc.

Best regards

Morten

 

[Wazobia]

Oops, i feel a bit silly. Yes, the problem is that the contactor wouldnt converge. And of course, every other stream and operation after it.

When the flowrate is doubled, i get an "Fatal error - Zero diagonal in column matrix. There may be no solution with current specs" message.

You are correct, there are no specs to change as the column has a zero degree of freedom. I had just noticed that myself a while ago, and was contemplating changing the flowrate of the recycled DEA when isaw this message. i'll maniuplate that, and the trays / P / T as you suggested.

Thanks again

 

[UmeshMathur]

Lest anyone misconstrue my remarks, let me say at the very beginning that I think of Hysys as a truly exceptional simulation product which has pushed the state-of-the-art consistently for many years.

However, the real problem under discussion here is that Hysys does not offer a flowsheet scaling option, as do some other simulators. Ideally, after converging a simulation, the user ought to be able to re-scale the entire problem to match a specified flow for any stream in the flowsheet. In other simulators, this does not require a rerun of the flowsheet, but is handled instead as part of the comprehensive report generation (unit operations and stream properties). Unfortunately, such a report is not generated by Hysys at all.

You may wish to examine your problem and determine which stream flows will need to be scaled to the new flow. Then, try to edge the simulation in a number of discrete steps (I would recommend at least 5, starting first with the amine circulation rate) until you reach the the desired final condition.

An alternative (that I would favor over the one described above) would be to transfer the stream and unit operations summaries to Excel and do the scaling in Excel manually.

By the way, as you scale up, there should be no reason for you to adjust the number of theoretical stages at all, as that changes the problem definition.