7 replies to this topic

[catia]

Hi all!

 

I'm simulating a heat exchanger in aspen hysys for a company and I have most of the data I need but the results aren't really good. I'm not allowed to attach the file, but I'll put a screenshot here.

 

This heat exchanger is a reflux condenser that uses brine as a utility. In this case, brine is a mixture of water and brine (30%) and it condenses the mixture in the tube side. All the geometry data is also in the simulation and I'm assuming is correct.

 

All the inlet conditions that I have are in the simulation. In addition to this, I also have some outlet conditions that need to be met:

 

Tube side out temp: 10ºC

Tube side non condensables: 574,87 kg/h

 

I don't have the brine mass flow and the brine outlet temperature. Nonetheless, I know that the brine solution is not supposed to leave the condenser at 23ºC, but that is the result I'm getting in the simulation. It is supposed to leave the condenser at a negative temperature, at least. The brine mass flow I adjusted by hand to make the tube side out temperature equal to 10ºC.

 

I'm trying to figure out what I'm doing wrong, but I really don't see it. The mass fractions of the inlet of the tube side are design fractions, while the conditions I'm using are real process conditions. Maybe the error can be there? I don't have a way to obtain the real process conditions....

 

Any ideas as to how I can get the brine the leave the condenser at a much lower temperature?

 

Thanks in advance.

 

 

 

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Edited by catia, 11 July 2024 - 09:52 AM.

[Pilesar]

The exchanger duty on the hot side must be the same as the exchanger duty on the cold side. For the cold side brine with no phase change, the duty equals the mass flow times the heat capacity times the temperature difference. If you want the brine temperature difference to be smaller while keeping the duty the same, you can increase the mass flow of the brine. The pressure drop you show on the brine side is very low which also suggests you should have much more brine flow.

[Pilesar]

 Confirm the flow rates you provided. Are these flow rates off by orders of magnitude? The duty is small. So you do not know the cold side outlet temperature or the cold side flow rate. You don't know the hot side composition or the hot side flowrate but have to match the vapor flow portion of a partially condensed process stream at a measured temperature of 10 C. There are a lot of unknown parameters. Consider possible error in the process-side temperature measurement. Small equipment may have more significant heat transfer with the environment. What do you hope to accomplish with your heat exchanger simulation? Are the cold side shell inlet and outlet nozzles really located as depicted in your diagram? Are you certain you are partially condensing the process inside the tubes of a horizontal multipass exchanger while near atmospheric pressure? This seems a very unusual exchanger configuration -- perhaps the process side is highly corrosive. Confirm the process stream pressure. Adjust the process composition as needed. Allow some variability in the hotside outlet temperature. The hotside duty is in the condensing so changing the composition or temperature a little might have a large effect on the duty. Adjust the brine flow as needed to match the duty with the imposed outlet temperature.

[catia]

 Confirm the flow rates you provided. Are these flow rates off by orders of magnitude? The duty is small. So you do not know the cold side outlet temperature or the cold side flow rate. You don't know the hot side composition or the hot side flowrate but have to match the vapor flow portion of a partially condensed process stream at a measured temperature of 10 C. There are a lot of unknown parameters. Consider possible error in the process-side temperature measurement. Small equipment may have more significant heat transfer with the environment. What do you hope to accomplish with your heat exchanger simulation? Are the cold side shell inlet and outlet nozzles really located as depicted in your diagram? Are you certain you are partially condensing the process inside the tubes of a horizontal multipass exchanger while near atmospheric pressure? This seems a very unusual exchanger configuration -- perhaps the process side is highly corrosive. Confirm the process stream pressure. Adjust the process composition as needed. Allow some variability in the hotside outlet temperature. The hotside duty is in the condensing so changing the composition or temperature a little might have a large effect on the duty. Adjust the brine flow as needed to match the duty with the imposed outlet temperature.

Thank you for the response. The heat exchanger is actually a reflux condenser, so it is vertical. That is considered in the simulation, I just didn't rotate the icon. This simulation is meant to be a starting point for a process optimisation, so, I'm verifying that I can trust my simulation by making sure it matches the process conditions and then suggest improvements. But I must say I'm having some difficulties trusting my simulation when the process results are bad. I have confirmed and re-confirmed all the conditions, I've been at this for a longer time than I should... I will try to adjust the compositions and see if I can get the results I want and allow some error in the results I get.

Thanks again!

[shvet1]

Any ideas as to how I can get the brine the leave the condenser at a much lower temperature?

 

Increase the brine flowrate

[catia]

 

Any ideas as to how I can get the brine the leave the condenser at a much lower temperature?

 

Increase the brine flowrate

 

Increasing the brine flowrate will make it exit the condenser at a lower temperature but the process side temperature will also decrease and won't meet the specifications

[Pilesar]

catia,

The condensing side of the exchanger has a duty. You can calculate that duty independently based on the inlet conditions and the outlet conditions of the hot side of the exchanger if you have sufficient information. Can you do this? If not, why not? If you cannot calculate the duty on the hot side of the exchanger, then the hotside duty is unknown! What about the coldside duty? Do you have a flowrate and heat capacity and temperature difference of the cold side? If so, you can calculate the coldside duty! The coldside duty and the hotside duty are considered equal (with opposite signs) since the heat transfer with the environment is usually considered insignificant. Note that these two duty calculations can be performed without knowing anything about the exchanger configuration! Write out your heat transfer equations for the hot side and cold side. Since you know the relationship between the hot side duty and the cold side duty, you can then solve the equations simultaneously to fill in the missing variables. If your calculations result in the hotside duty not equal to the coldside duty, then look more closely at the inputs where there is more likelihood of error. From this information found, you can rate the exchanger using rigorous methods to determine effective heat transfer coefficients.

[shvet1]

@catia

I do not have my PC on hand to provide a definite reply, but still recommend you to increase cold side flowrate. You have specified this HE incorrectly. Software gives you what you had specified, not you wanted/intended.

Edited by shvet1, 15 July 2024 - 11:15 PM.