16 replies to this topic

[A_D_M_MII]

Hi everyone,

 

I am doing my final student project using Aspen Hysys V.10. and a problem has shown up in my simulation.

 

• I have a Vessel which has 2 inlets and 2 outlets, one of the inlets comes from other part of the simulation, but the other one is the same current that left the vessel to a rigorous heat exchanger, the other outlet stream is a purge. So, there is a current that needs to be model with a recycle (the one which goes to the HX), the problem is that the recycle gives me a yellow message  because of the pressure drop of the current when it go across the HX. (The HX is a TEMA BXU which works like a water boiler).
• ¨Limit iteration without convergence¨ is the message from the recycle

INLET OF RECYCLE: 260,2ºC, 47,02 Kg/cm2_g

OUTLET OF THE RECYCLE: 260,2ºC, 47,03 KG/CM2_G

 

It could be nice if it was possible to indicate 0 pressure drop in shellside in my rigorous HX.

(The outlet of the HX a is bifasic current).

If i put the 47,02 kg/cm2_g before conect the recycle in the upper stream, it will drop to 47,01 and it is imposible for me to close that loop.

 

 

Thank you so much!

I could be great if you could give me a helping hand. 

 

ADM.

 

 

[Bobby Strain]

I don't understand what your problem is. Or, your question.  A screenshot of your flowsheet is much better.

 

Bobby

[Pilesar]

The inlet stream to the heat exchanger is not the same pressure as the source vessel. There is an elevation difference so that the heat exchanger inlet pressure is higher than the pressure of the vessel where the exchanger discharge returns. Did you account for the static pressure at the exchanger inlet caused by the elevation change?

[Pilesar]

Sometimes problems in simulations mimic the problems you would see in the field. You need more available pressure drop. A steam drum typically holds the liquid supply for the boiler as well as serves as a phase separator for the generated steam. The elevation difference between a steam generator and steam drum is because of the pressure drop needed to drive the circulation. The pressure drop on the feedwater side of the loop matches the pressure drop on the 2-phase side of the loop. This means that the steam drum elevation is usually higher for high pressure boilers because the density of the higher pressure vapor exerts more static head. In reality, only a portion of the feedwater in each pass to the steam generator is converted to steam because this is what results from the pressure balance. There is a similar situation for a thermosiphon reboiler where the static head in the exchanger feed means that the liquid entering the exchanger is subcooled. The subcooled feed should be accounted for in the exchanger design.

You don't give much detail, but if you are generating steam on the tube side of the exchanger, it is probably a poor design and you should reconsider. I think you will need to model the loop hydraulics (piping and elevations) to determine the feed and performance of the exchanger. It is an iterative design process.

[A_D_M_MII]

I don't understand what your problem is. Or, your question.  A screenshot of your flowsheet is much better.

 

Bobby

Hi, thank you for answer me, there are 3 screenshot of my flowsheet problem:

Attached Files

•  duda hysys 2.JPG   32.64KB   3 downloads
•  Duda hysys.JPG   24.78KB   1 downloads
•  temperature hysys.JPG   27.05KB   1 downloads

[A_D_M_MII]

 

A veces, los problemas en las simulaciones imitan los problemas que vería en el campo. Necesita más caída de presión disponible. Un tambor de vapor normalmente contiene el suministro de líquido para la caldera y también sirve como separador de fases para el vapor generado. La diferencia de elevación entre un generador de vapor y un tambor de vapor se debe a la caída de presión necesaria para impulsar la circulación. La caída de presión en el lado del agua de alimentación del circuito coincide con la caída de presión en el lado de 2 fases del circuito. Esto significa que la elevación del tambor de vapor suele ser mayor para las calderas de alta presión porque la densidad del vapor de mayor presión ejerce una carga estática mayor. En realidad, solo una parte del agua de alimentación en cada paso al generador de vapor se convierte en vapor porque esto es lo que resulta del equilibrio de presión. Existe una situación similar para un hervidor de termosifón donde la carga estática en la alimentación del intercambiador significa que el líquido que ingresa al intercambiador está subenfriado. La alimentación subenfriada debe tenerse en cuenta en el diseño del intercambiador.

No da muchos detalles, pero si está generando vapor en el lado del tubo del intercambiador, probablemente sea un diseño deficiente y debería reconsiderarlo. Creo que necesitará modelar el circuito hidráulico (tuberías y elevaciones) para determinar la alimentación y el rendimiento del intercambiador. Es un proceso de diseño iterativo.

 

Hi, thanks for your response!

 

In my simulation:

 

Steam is generated in the SHELL of my HX.

The design of my HX is fixed, it is a commercial HX and I need to use it.

I have posted three screenshots in my answer to bobby, so you can better see my simulation.

 

I don't understand when you said elevation change, I'm new to using Hysys so I don't know how to account for it in my simulation.

 

 

 

ADM

Edited by A_D_M_MII, 31 March 2021 - 04:32 PM.

[Bobby Strain]

I still need to see what is on the right. Show the complete process at least to the next operation. I still don't understand the process you are simulating. Maybe you can provide a sketch of the operations, including elevations. Is the process as Pilesar described? And I understand very little Spanish.

 

Bobby

Edited by Bobby Strain, 31 March 2021 - 04:19 PM.

[A_D_M_MII]

Todavía necesito ver lo que está a la derecha. Muestre el proceso completo al menos para la siguiente operación.

 

Poli

Hi, there is only one duplicate stream that I have put into a simple weighted HX that replicates the rigorous one but is not connected, so it is not important.

 

So on the right are two duplicate streams and the simple weighted HX I made to see what happens when there is no pressure drop on the shell side.

 

 

 

The screenshot I posted is a small simulation of the problem area (Vessel + HX) that I did to "experiment" instead of using the big simulation, but I cannot close that loop in any other way.

 

 

 

ADM.

Edited by A_D_M_MII, 31 March 2021 - 04:31 PM.

[Pilesar]

'Elevation change' means the fluid is not all on the same level. How does the liquid flow from your NH-V-01-2 vessel to the exchanger? There is no pump to move the fluid. Perhaps the liquid in NH-V-01-2 flows to the exchanger because of gravity? Gravity could only help if there is an elevation change. So either you add a pump to the circuit to move the fluid or you model the hydraulics of the piping using specified pipe segments where you input the elevation change along with the pipe diameter and length.

How did you decide how much flow you are sending to the heat exchanger? You must have pressure drop for flow to occur. The simulation is telling you that you cannot have a flow without pressure drop and so it does not converge.

[pmoneysh]

 To me, the problem is in the specification of 'splitter' that is causing the material imbalance. 

[A_D_M_MII]

'Elevation change' means the fluid is not all on the same level. How does the liquid flow from your NH-V-01-2 vessel to the exchanger? There is no pump to move the fluid. Perhaps the liquid in NH-V-01-2 flows to the exchanger because of gravity? Gravity could only help if there is an elevation change. So either you add a pump to the circuit to move the fluid or you model the hydraulics of the piping using specified pipe segments where you input the elevation change along with the pipe diameter and length.

How did you decide how much flow you are sending to the heat exchanger? You must have pressure drop for flow to occur. The simulation is telling you that you cannot have a flow without pressure drop and so it does not converge.

Hi,

Thank you, i know what you mean, i have tried adding a pump in order to solve that situation because i have not the elevation of the vessel (and i have no idea how to indicate that height in hysys, it would be better because the pump is a ficticius one and in the real plant there is no pump, it must be a difference of level which impulses the liquid to the HX)

BUT, i have attached here the situation,

how much pressure should i put in the pump in order to solve that situation?

I have tried 0,01 kg/cm2_g to have the same pressure of the right side, but it enter in a internal loop of pressure losing and give me a warning advertesing.

 

The flow is specified by the vendor, and it is also specified the purge, so i should almost have a close results in my simulation.

In the upper splitter next to the vessel, i have specified the flow of the problematic stream which goes to the HX, both streams, (before and after recycle) have the same flow in kg/h of water.

 

THANK YOU 

 

ADM

Attached Files

•  0104.JPG   25.5KB   1 downloads
•  1 april 21.JPG   53.29KB   0 downloads
•  warning.JPG   47.5KB   0 downloads

[A_D_M_MII]

 To me, the problem is in the specification of 'splitter' that is causing the material imbalance. 

Thank your for answer,

 

But, I have only specified de flow of the problematic current, which is the same of the specified flow at the right of the recycle, the two streams have the same parameters but is the pressure the problem, i have added a pump but it does not solve the problem.

On the other hand, if i do the HX in simple weighted, with no DP in shellside, it doesn' t show me any problem.

 

ADM 

[Pilesar]

Put enough pressure at the pump discharge so that you have higher pressure out of the exchanger than your NH-V-01-2 vessel. Pressure up more than you need and back it off later if you want. Without a converged run, you won't be able to adjust the model correctly. With the pressure drop of the exchanger, your simulation is in a downward pressure spiral and needs a boost somewhere. Setting NH-V-01-2 vessel to be a fixed outlet pressure might be enough as a workaround.

[Bobby Strain]

Increasing the stream pressure can be easily accomplished by using a valve with negative pressure loss. And, I would never simulate a rigorous heat exchanger in a recycle loop. Use a simple heater and set the rigorous exchanger as a separate operation once you are satisfied with the recycle operation.

 

Bobby

Edited by Bobby Strain, 01 April 2021 - 12:54 PM.

[pmoneysh]

Bobby is right.... use a simple heat-exchanger if it is the only problem. You can export the heat exchanger to aspen EDR to size your heat exchanger. If you want to go further, aspen EDR file can be exported to HTRI.

[A_D_M_MII]

Put enough pressure at the pump discharge so that you have higher pressure out of the exchanger than your NH-V-01-2 vessel. Pressure up more than you need and back it off later if you want. Without a converged run, you won't be able to adjust the model correctly. With the pressure drop of the exchanger, your simulation is in a downward pressure spiral and needs a boost somewhere. Setting NH-V-01-2 vessel to be a fixed outlet pressure might be enough as a workaround.

Thank you so much, Pilesar.

I did what you said, i put enought pressure at the pump and it allowed me to close de recycle and it works with the rigorous HX!

After recycle converged, i have reduced the pressure given by the pump with no problems.

 

ADM 

[A_D_M_MII]

Increasing the stream pressure can be easily accomplished by using a valve with negative pressure loss. And, I would never simulate a rigorous heat exchanger in a recycle loop. Use a simple heater and set the rigorous exchanger as a separate operation once you are satisfied with the recycle operation.

 

Bobby

Thank you Bobby, i have done what Pilesar proposed me in his post, AND, even if i use a rigorous HX, it works.

 

I have already solved my problem with your help.

 

Thank you from Seville, Spain.

 

ADM