19 replies to this topic

[processengineerx]

How to realistically model the mixing of two different streams (lets say water) of equal mass flowrate at different temperature and pressures in Aspen PLUS and Aspen HYSYS?

Suppose we have two streams consisting of water. One at 4.5 kg/cm2G & 65.50C, and the other at 62.23 kg/cm2G & 183.39C. After I installed a mixer for these two streams in Aspen PLUS/HYSYS, then the P and T of the outlet stream calculated by Aspen PLUS/HYSYS is 4.5 kg/cm2G & 82.76 C.
As we can see although the outlet T is the actual resultant T (achieved by Energy Balance) but the outlet P is not the actual resultant P (the outlet P is instead the lowest inlet P).

In HYSYS we have only 2 options in a mixer,
1.) either to equalize the inlet Pressures (which makes the outlet P=inlet P)
2.) or to make the outlet P=lowest inlet P

In Aspen PLUS
1.) either we can give any value for the outlet P of the mixer in mixer specifications.
2.) or we can put 0 kg/cm2 (which is the default value for any stream mixer). If we use 0 kg/cm2 as Mixer P spec. then the P of the outlet is selected to be equal to the lowest inlet P (which in our case is 4.5 kg/cm2G)

My question is how can we or [if not possible then why we cannot ] model the actual realistic resultant P after mixing 2 streams at different Pressures in Aspen PLUS/HYSYS?

Also no flashing occurs after the mixing (but in actual process the water at 62.23 kg/cm2 is flashed (vaporized) when it mixes with low pressure water at 4.5 kg/cm2.

In real scenario at our plant, the mass flowrate of water at 62.23 kg/cm2 is 10 M.Ton/hr and the mass flow rate of water at 4.5 kg/cm2 is 50 M.Ton/hr.

By putting actual mass flow rates although we get nearly actual temperature that is achieved after mixing of the 2 streams. But we don't get the actual P (which is greater than 10 kg/cm2) instead we get the outlet P=4.5 kg/cm2 (which is the lowest P of the inlet streams). Also flashing/vaporization occurs after the mixing of the streams in reality, but in Aspen PLUS/HYSYS no vaporization takes place at the mixer outlet.

Is there any mean in Aspen PLUS/HYSYS by which we can get the actual P at the mixer outlet?

Please help me in realistically model the mixing process.

Thanks in advance.

I have attached the respective Aspen PLUS PFD for understanding of the problem.

Attached Files

•  MIXER EXMAPLE.pdf   6.05KB   254 downloads

Edited by processengineerx, 22 January 2011 - 12:27 PM.

[processengineerx]

I am new on this forum. So, I don't know how long will it take for the public response of my query. I hope most of the simulation engineers might have come across this problem before. Therefore, I'm expecting a sound response from the professionals out there. I my self is also a practicing process engineer, and these days working on Steady State/Dynamic Simulations.

Attached Files

•  MIXER EXMAPLE.pdf   6.05KB   88 downloads

Edited by processengineerx, 22 January 2011 - 12:25 PM.

[processengineerx]

I'm expecting a reply especially from

Steve McGahey

a great simulation expert and knowledgeable person whose posts helped me and others a lot.

[processengineerx]

The following persons are also a great source f knowledge, and I would warmly appreciate their help and support in teh form of useful opinions which they provide in this forum.

Zauberberg,


Art Montemayor
Thanks once again..

[Zauberberg]

Process,

Thank you for your kind words and let me try to answer your question in a few simple words:

- The point of mixing of two streams (P1, P2) must be equal to the low-pressure stream (P1). In any other case, assuming that the resultant pressure would be higher than P1, the low-pressure stream could not flow into the mixer as this would mean flowing from lower pressure region to higher pressure region - and that is not going to happen in this Universe.

- You are correct regarding the temperature calculation: it is a simple energy conservation example. What comes into the mixer, must leave the mixer. Adiabatic flow, with no heat losses, addition of heat to the system, nor extraction of any kind of mechanical work is being considered.

[processengineerx]

Guys more than 30 views.. and still no replies.. please help...

[processengineerx]

@

Zauberberg

thanks so much.... your observation really heped..

it is true that a stream at lower P cannot enter a mixer operating at higher P.

But please consider a real problem we are facing at our plant.

The mixing point here is inside a vessel (deaerator) operating at 0.2 kg/cm2.

Water at 4.5 kg/cm2 and water at 62.23 kg/cm2 both are entering in to the deaerator (vessel). So, the operating P (and Temperature) of deaerator increases beyond 0.2 kg/cm2 (and 105 C) when the flow rate of water at 62.23 kg/cm2 increases from 7~8 M.Ton/hr.

Therefore the resultant P here in this case could not be equal to 4.5 kg/cm2 (actually in reality it is more than 0.2 kg/cm2).

Also the pipe connecting the water at 66 kg/cm2 to the deaerator is 4 inches (and flashing/vaporization occurs in this pipe and we are not able to increase the mass flow rate of water beyond 7~8 M.Ton/hr through this line because of the increase in volumetric flow rate due to the vaporization )
I hope now you have more understanding of the real problem.

[processengineerx]

I want to model the actual flashing/vaporization process occurring. So, please help me in that.

[processengineerx]

I have installed a flash separator now in the simulation (instead of mixer), and I have specified the P=0 kg/cm2 (for having delta P=0 in deaerator and perform flashing under vacuum), and Heat Duty=0 cal/sec (for making the process adiabatic and hence achieving realistic outlet temperature )

I also made the two pipes adiabatic.

Equivalent Length of 6-INCH pipe=30 m. Pipe rise=22 m (deaerator height= about 20m)
Equivalent Length of 4-INCH pipe=22 m. Pipe rise=22 m

But I don't see any flashing/vaporization occurring in 4-INCH line in Aspen PLUS, as in actual process the flashing/vaporization does occur in the 4-INCH line.

Please help me in that..

Attached Files

•  DEAERATOR-PROBLEM.pdf   8.75KB   133 downloads
•  DEAERATOR-PROBLEM-(including pipes blocks).pdf   16.73KB   117 downloads

[processengineerx]

@all

Please help me in this regard sooner... Thanks......

[processengineerx]

In this simulation attached, I have used the Gauge units of the Pressure (previously it was in absolute ) so that you might get a more clearer picture..

Attached Files

•  DEAERATOR-PROBLEM-PRESSURE IN kg per cm2 G.pdf   16.57KB   104 downloads

[processengineerx]

@all

I have attached the HYSYS simulation PFD also. In HYSYS the flash separator has a very detailed model. But still at the outlet the flash separator has specified the P= P of the inlet stream having the lowest P.

Please help..

Attached Files

•  DEAERATOR-PROBLEM-HYSYS.pdf   7.07KB   127 downloads
•  DEAERATOR-PROBLEM-HYSYS.pdf   7.07KB   79 downloads
•  DEAERATOR-PROBLEM-HYSYS.pdf   7.07KB   80 downloads

[Zauberberg]

Process,

Please take the time before you compile the subject you want to write about in this thread. Having multiple, consecutive posts, doesn't make the picture clearer - actually it does the opposite and it only confuses members of this forum. I'd suggest you to:

- Post a clear PFD of the system you are analyzing, involving all the equipment contained within the system (not posting the process model);
- Fill in all available process data in the PFD (the data being measured, not guessed);
- Explain the system behavior, based on field observations.

As said, take your time and compile a comprehensive set of data, and then upload it in this thread. Upon reviewing it, we can come up with our own analysis, or ask you for supplying additional information if judged as required.

Have a good time there,

[processengineerx]

@Zauberberg

Dear the final case is attached in my last 2 comments (in aspen PLUS and HYSYS). Further more I have also mentioned the pipe's length and elevation. The only equipment involved in this scenario is the Deaerator. But as there is no option/model available in Aspen PLUS/HYSYS for deaerator, so I just used the Flash Separator.

The process I want to model is the flashing of HP water in the 4-INCH pipe (which occurs in real case at our plant).

Hope this clears your lines of thought.

[processengineerx]

@Zauberberg

waiting for your reply..

[processengineerx]

anybody there for answering my Q........ ???

[Zauberberg]

Process,

As said before, I cannot comment on the flowsheet from a process simulator model. What I need from you, is a simple scheme (or P&ID) showing all equipment - including valves, orifices etc. I am sure that your model does not have all the sufficient data, and it does not represent 100% actual configuration in the field.

[Erwin APRIANDI]

Dear Process Engineer,

A simple sketch or P&ID of the system and description on how it work can be very useful (as per what zauberberg posted). and when you say 0.2 kg/cm2 at de-aerator operating pressure, how the pressure is controlled is there a PCV there which control the pressure of the de-aerator or it is simple connected
to the atmosphere.

And you also have to remember that what you are trying to use for simulation to use is hysys steady state, means it is a representative of your plant at steady condition. so if you what to compare the results of simulation with the real situation you may switch to dynamics so you can compare it on time basis.

And I've tried to simple model hysys as per what you have simulated, and I find that the vapor pressure for water at 4.5 kg/cm2 is at 154.9 oC, so if your mixture temperature is less than that temperature you would not get any flashing, and I also see that the vapor pressure for water at 0.2 kg/cm2 is at 105.0 oC

Edited by erwin.apriandi, 24 January 2011 - 06:51 AM.

[processengineerx]

@Erwin Apriandi

Thanks so much dear for your reply..

[fatemeh-m]

Hi.I'm new too... which errors can occur on Mixer?!? I have searched for the answer but I could not found it... plz help me!!!!