15 replies to this topic

[trilok]

Dear Friends,

 

I want to simulate a small powerplant which oparates with high pressure(say 130 atm). so i am using a compression loop. That means the fluid is sent in to the same compressor( compression ratio of ~1.8) multiple number of times until 130 atm is reached. So in the below figure an fluid is input at 1 atm pressure and 35^oC (at material stream 1). now it is cooled in the gas chiller and sent to compressor. so at the outlet of the compressor (material stream 3)pressure of the fluid will be roughly 1.8 atm. But this pressure is not enough so i need to send it through the gas chiller and compressor again and this process repeats until my requirement of 130 atm(at material stream 3) is met. Once the pressure at the outlet of the compressor is greater than or eqal to 130 atm, it has to be sent to watlow heaters(material stream 4) to get heated. After heating the fluid in watlow heaters the gas should be expanded in a turbine to generate power and after the power is generated it should be sent again into the gaschiller and compressor. Now this cycle repeats.

 

So basically in the below figure what i want is a switch after the material stream 3 such that if pressure is greater than my specified value the fluid should go to material stream 4 else it should go to material stream 1 until my condition is met.

 

I dont know programming extensions for hysys neither do my friends or faculty. So please help me with this problem. I have been at this problem since 2 months with no progress. I hope someone will help me here.

 

Thanking You.

Trilok Chandu.

Attached Files

•  Sim Screenshot.JPG   23.61KB   6 downloads

[Padmakar Katre]

Hello,

It is really hard to understand your exact doubt. Will you please rephrase your post. Are you mixing steady-state and dynamic simulation together?

[thorium90]

I assume you mean a multistage compressor with interstage cooling. (I take it you don't mean compressing the same gas in a single stage compressor again and again in the same stage because that is inexplicably ridiculous!!!) Your scheme of producing power by compressing it, then heating it up and then expanding it will definitely yield a net power input, not a net power output.

 

Perhaps you can come up with a feasible process flow for the power plant first? Where will your energy input come from? Power plants typically use an energy source like natural gas, coal, nuclear or solar etc etc. After you have formulated a proper process flow diagram then we can talk more specifics like simulation. Your current flowsheet is very vague and pretty empty...

Edited by thorium90, 18 June 2013 - 05:39 AM.

[trilok]

I assume you mean a multistage compressor with interstage cooling. (I take it you don't mean compressing the same gas in a single stage compressor again and again in the same stage because that is inexplicably ridiculous!!!) Your scheme of producing power by compressing it, then heating it up and then expanding it will definitely yield a net power input, not a net power output.

 

Perhaps you can come up with a feasible process flow for the power plant first? Where will your energy input come from? Power plants typically use an energy source like natural gas, coal, nuclear or solar etc etc. After you have formulated a proper process flow diagram then we can talk more specifics like simulation. Your current flowsheet is very vague and pretty empty...

 

 

Hello,

It is really hard to understand your exact doubt. Will you please rephrase your post. Are you mixing steady-state and dynamic simulation together?

Dear Sirs,

 

This is the process i want to simulate(attached figure). If u have more doubts plase ask. This is completely steady state process. If u see the diagram u can notice that there is a recirculation valve where we can control whether the flow is to the heater or back to the compressor. So i want to model that switch.

 

Sir im sending the gas through the same single stage compressor multiple number of times because this is a small budget plant. The input pressure to the compressor is 1atm and the output should be 130 atm. so the compression ratio is nearly 130. So instead of using 73 compressors each of 1.8 pressure ratio each, I will compress the fluid in the same compressor until it reaches 130 atm pressure. after the pressure is 130 atm i will send it to the heater and turbine to generate my power.  1.JPG   34.41KB   2 downloads  2.JPG   80.04KB   2 downloads

Edited by trilok, 18 June 2013 - 11:16 AM.

[thorium90]

FIrst off. it is not 130/1.8~73. It would have been first stage 1.8, next stage 3.24, then 5.83, then 18.9, then 34, then 61, then 110. A ratio of 1.8 will get you about 8+ stages not 73...

 

Secondly, you cant put the same gas thorough the same stage to compress so many times. Every stage is different. Like you have said, a single stage can't do 1 to 130bar in 1 stage, therefore a compressor with 9 stages is needed.

 

The reason padmakar asked if it was dynamic is because your description is clearly one that implies process conditions that change with time, ie: dynamic. By saying when a certain pressure is reached, a certain event will occur, this is a clear statement that the process is not steady state. Steady state means the process conditions like temperature and pressure do not change (significantly) with time.

 

Next, your coloured diagram is a commendable effort, but its a tad too small, cant really see the values you have input.

Edited by thorium90, 19 June 2013 - 03:36 AM.

[trilok]

FIrst off. it is not 130/1.8~73. It would have been first stage 1.8, next stage 3.24, then 5.83, then 18.9, then 34, then 61, then 110. A ratio of 1.8 will get you about 8+ stages not 73...

 

Secondly, you cant put the same gas thorough the same stage to compress so many times. Every stage is different. Like you have said, a single stage can't do 1 to 130bar in 1 stage, therefore a compressor with 9 stages is needed.

 

The reason padmakar asked if it was dynamic is because your description is clearly one that implies process conditions that change with time, ie: dynamic. By saying when a certain pressure is reached, a certain event will occur, this is a clear statement that the process is not steady state. Steady state means the process conditions like temperature and pressure do not change (significantly) with time.

 

Next, your coloured diagram is a commendable effort, but its a tad too small, cant really see the values you have input.

Dear Sir,

 

I am really sorry. But all i want to know is how many times im recirculating the fluid in the same single satge compressor until my required pressure is reached. I will be very happy if you can help me out with this problem. Sorry for my incomplete knowledge.

Sir, the figure(previous reply handdrawn figure)  the input is given to the gas chiller at 101.3 kpa and 35^oC. now the fluid is made to follow the loop A until the pressure is abone 13MPa. Once the fluid pressure is above 13 MPa it takes the route of loop B.

[thorium90]

Hi trilok,

It wouldn't work in steady state. What you are describing is as follows.

 

A certain fixed mass of gas gets compressed up to 130bar in a single stage compressor whose power is derived not from the turbine but from an electric motor (because the gas needed to expand in the turbine is currently in the compressor). This gas has its heat removed due to the compression in the chiller.

This gas which is now at 130bar exits the loop on the right and then enters the left loop. The gas is then heated up and this fixed mass of gas expands in the turbine to produce power, in the process spinning the turbine up to a certain rpm and when all the gas has expanded to a low enough pressure, the turbine coasts back down to 0 rpm.

This fixed mass of gas then re-enters the right loop to enter the compression and cooling phase again. This cycle where the gas alternately enters either the left or the right loop is the process you are describing right?

 

Do you agree that due to the efficiency of the compressor and the turbine, the turbine alone cannot compress the gas back to 130bar, and that this process cannot be steady state but is cyclic?

 

Do you therefore also agree that the compressor and turbine are not running all the time and cannot be running at the same time? You cant be compressing some of the gas and expanding some of it at the same time either isn't it.

 

To summarise, it is not about how many times you will need to recirculate the fluid to reach 130bar. In the first place, your process is not feasible and not realistic!!!

Edited by thorium90, 19 June 2013 - 12:41 PM.

[trilok]

Hi trilok,

It wouldn't work in steady state. What you are describing is as follows.

 

A certain fixed mass of gas gets compressed up to 130bar in a single stage compressor whose power is derived not from the turbine but from an electric motor (because the gas needed to expand in the turbine is currently in the compressor). This gas has its heat removed due to the compression in the chiller.

This gas which is now at 130bar exits the loop on the right and then enters the left loop. The gas is then heated up and this fixed mass of gas expands in the turbine to produce power, in the process spinning the turbine up to a certain rpm and when all the gas has expanded to a low enough pressure, the turbine coasts back down to 0 rpm.

This fixed mass of gas then re-enters the right loop to enter the compression and cooling phase again. This cycle where the gas alternately enters either the left or the right loop is the process you are describing right?

 

Do you agree that due to the efficiency of the compressor and the turbine, the turbine alone cannot compress the gas back to 130bar, and that this process cannot be steady state but is cyclic?

 

Do you therefore also agree that the compressor and turbine are not running all the time and cannot be running at the same time? You cant be compressing some of the gas and expanding some of it at the same time either isn't it.

 

To summarise, it is not about how many times you will need to recirculate the fluid to reach 130bar. In the first place, your process is not feasible and not realistic!!!

Dear Sir,

 

Actually a fixed amount of gas is taken and it is compressed multiple number of times until 13MPa is reached. Now after exiting loopA it is sent to loopB whereit is heated and expanded. Now the gas in the turbine is expanded from 13 MPa to a pressure which is not 101Kpa but to the pressure somewhere around (13/1.8 + .02) MPa. Here the .02 is for pressure drop across chiller. So once the gas enters loopB there is no turning back to loopA.

 

Also Sir there is no connection between the turbine and compressor,. The compressor is electric powered and we dont care how much energy it takes while performing operations of loopA.

 

Sir, if its not too much can I ask for your phone number so I can explain my problem clearly, please?

 

Thank you very much.

Edited by trilok, 19 June 2013 - 01:58 PM.

[shan]

There is no such "Switch" operation in Hysys steady state simulation. You have to build a multiple stages compressor model to simulate Loop A recycle process.

[trilok]

There is no such "Switch" operation in Hysys steady state simulation. You have to build a multiple stages compressor model to simulate Loop A recycle process.

Sir,

 

The is no switch function by default but by combination of 'set', 'adjust' and 'recycle' we may be able to do it. I am very new to Hysys so I dont know. Also we can write a VB extension code?

[thorium90]

Hi trilok,

I think we are in different countries so a phone isnt really an option.

 

When the process is in steady state, the process conditions do not vary. The suction pressure and discharge pressure do not change with time. By putting the gas through the compressor again and again until it reaches 130bar, the suction and discharge pressure changes with time and this is known as an unsteady state process. Hysys can model steady state and unsteady state processes, but what you have described is clearly not steady state.

 

Next, after that specific mass of gas has expanded through the turbine, it is now like 7bar you say? What then? It stays in loop B where your heater and turbine is? It is only 7bar now? Clearly your turbine will stop spinning once the energy is used up and no more power will be generated by the turbine.

 

You need to try and imagine how your unit will run in reality, not just on paper. Visualise it in your mind and you may understand it.

[trilok]

Hi trilok,

I think we are in different countries so a phone isnt really an option.

 

When the process is in steady state, the process conditions do not vary. The suction pressure and discharge pressure do not change with time. By putting the gas through the compressor again and again until it reaches 130bar, the suction and discharge pressure changes with time and this is known as an unsteady state process. Hysys can model steady state and unsteady state processes, but what you have described is clearly not steady state.

 

Next, after that specific mass of gas has expanded through the turbine, it is now like 7bar you say? What then? It stays in loop B where your heater and turbine is? It is only 7bar now? Clearly your turbine will stop spinning once the energy is used up and no more power will be generated by the turbine.

 

You need to try and imagine how your unit will run in reality, not just on paper. Visualise it in your mind and you may understand it.

Sir,

 

If you see the loopB after the fluid exits the turbine it is sent to gas chiller and again compressor where the pressure becomes 130 bar again. Now heat is added to it and the fluid is expanded in turbine and the cycle goes on.

 

If not phone can i atleast ask for ur mail id?

 

Thank you

[thorium90]

Hi trilok,

 

wouldn't the email have the same stuff as the forum posts?

 

So the fluid will leave loop B after expanding in the turbine and re-enter loop A to be compressed. During the time when the fluid is in loop A, the turbine will not be turning and during the time when the fluid is in loop B, the compressor will not be pressurizing anything isn't it?

 

So what does this cycle of yours do? It appears to simply take a gas and repeatedly compress and expand it, utilizing energy in the process with no net useful output?

[trilok]

Hi trilok,

 

wouldn't the email have the same stuff as the forum posts?

 

So the fluid will leave loop B after expanding in the turbine and re-enter loop A to be compressed. During the time when the fluid is in loop A, the turbine will not be turning and during the time when the fluid is in loop B, the compressor will not be pressurizing anything isn't it?

 

So what does this cycle of yours do? It appears to simply take a gas and repeatedly compress and expand it, utilizing energy in the process with no net useful output?

Sir,

 

I f you see the compressor is both a part of loopA and loopB. In loopA it compresses the fluid multiple number of times until 130 bar is reached. But in loopB it works like a normal compressor where the fluid is pressurised from 70 bar to 130 bar and then it can be used.

 

Sir, basically this bench scale plant is to test the working of the turbomachinery when supercritical CO₂ is used as the working fluid.

[thorium90]

Perhaps someone else might be able to offer some better help because I'm getting more confused myself. I thought you said this was a power plant, but now you say it is a small benchtop experiment.

 

If what you are saying is that the fluid flows through both loops all the time and it is at steady state, then you do not need any kind of switch to channel the streams into the different loops. You just need the TEE unit op to split the stream and define the fraction going to each side.

 

Perhaps you can redo and upload your PFD with the stream parameters to clarify what you have done. Upload enough relevant information like what the OP has done in the following thread.

http://www.cheresour...mment-on-hysys/

Edited by thorium90, 21 June 2013 - 06:30 AM.

[shan]

Your LoopA is not feasible. If you are able to increase gas pressure to whatever value by recycle of a single stage compressor, why people spend money to install multiple stage compressor? You may try to liquefy the CO2 stream first and then use a CO2 pump to reach the higher pressure you desire.