5 replies to this topic

[elmasojuaso]

Hello everyone,

 

I currently have the task of simulating a dual mixed refrigerant liquefaction process for natural gas on Aspen Hysys. I'm not used to the software so it has been a pain trying to do it alone. I only have the design specifications i have attached and the process diagram, but i know i also need a lot of temperatures and pressures to simulate the process because it is really complex. I have tried to use temperature and pressure values from some research papers buy i get a lot of overspecified or insensible errors when running the simulation. I would appreciate if anyone has a step by step guide to simulate this kind of process.

 

Thanks

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[Pilesar]

That's a complicated simulation! Although it is highly integrated, I suggest you try solving it a unit at a time. If you don't have converged units, it will be difficult to proceed. Back up and try again until you get convergence. Once you get a couple of the units converged individually, then try to connect the tear streams. To make things easier, use a larger cold stream flow in the simulation than you really have. If you get that converged, then back off the cold stream flow gradually. Observe how the heat curves behave in your exchangers. One of the key parameters is Minimum Internal Temperature Approach. This must be positive. Although your final target is 3 degrees, you will want to keep this much higher initially to improve ease of convergence. If you are optimizing the mixed refrigerant composition, then start with a light mix and gradually add heavier components so that the hot stream and cold stream heating curves approach each other. You will want to eventually minimize the space between them for final optimal results -- as close to parallel lines as possible. Be patient and persistent and make gradual changes. This is not an easy simulation. As you observe the heat curves, you should eventually get a feel for how changing parameters affect them.

[elmasojuaso]

Do I need to use recycles for the closed refrigerant loops?  In that case, do you have a guide on how to use them?

[Bobby Strain]

You have a steep learning curve. You need to start with the HYSYS manuals. Otherwise you are wasting your time. This also is not the kind of exercise that should be assigned to a student. You should be focused on learning fundamental theory, not wasting a lot of time using a process simulator. Your professor/instructor must be too lazy to actually teach the fundamentals.

 

Good luck.

Bobby

[Pilesar]

Why are you concerned about closed refrigerant loops? Have you even started modelling this yet? Do you have any converged process unit ops? If you are forced to model this process for your class, then you should concentrate on getting some results that show you tried.

  For the refrigerant loops themselves, you just specify completely the conditions of your stream 5 and stream 14 -- no loops in the model. The refrigeration compressors and condensers will be at the tail end of the refrigeration portion of your model and you just specify those units so that you match what you know forced stream 5 and stream 14 to be. Designate the outlet tail streams '5X' and '14X'. When your model is complete, stream 5 and stream 5X will be identical. Stream 14 and stream 14X will be identical. You don't need to use recycles in the simulation model.

  You will need the flow rate of some stream, so make one up for the natural gas if it was not given to you. You also need a starting point for your refrigerant streams. Make composition estimates for your natural gas and refrigerant streams if they are not supplied. So beginning the model, you have complete definitions for streams 5, 14, 22. With these, you can solve HX-1. See my previous comments about keeping a large temperature approach with excess refrigerant flow. With HX-1 solved, you try to add HX-2 and HX-3. Save your work often. I expect this simulation to be very frustrating for you. How many months do you have to complete it?

  I will tell you my approach to this model if I only had the info you provided. I would start with stream 5 miniscule flow! This leaves you with one mixed refrigerant loop! That is still not easy to model, but simplifies things. After solving for a single mixed refrigerant, then you can go back and add flow to your propane refrigerant loop if you want to improve overall efficiency. The advantage to this procedure is that you may get a converged model and an 'answer' even if it is not completely optimized for efficiency.

[elmasojuaso]

Why are you concerned about closed refrigerant loops? Have you even started modelling this yet? Do you have any converged process unit ops? If you are forced to model this process for your class, then you should concentrate on getting some results that show you tried.

  For the refrigerant loops themselves, you just specify completely the conditions of your stream 5 and stream 14 -- no loops in the model. The refrigeration compressors and condensers will be at the tail end of the refrigeration portion of your model and you just specify those units so that you match what you know forced stream 5 and stream 14 to be. Designate the outlet tail streams '5X' and '14X'. When your model is complete, stream 5 and stream 5X will be identical. Stream 14 and stream 14X will be identical. You don't need to use recycles in the simulation model.

  You will need the flow rate of some stream, so make one up for the natural gas if it was not given to you. You also need a starting point for your refrigerant streams. Make composition estimates for your natural gas and refrigerant streams if they are not supplied. So beginning the model, you have complete definitions for streams 5, 14, 22. With these, you can solve HX-1. See my previous comments about keeping a large temperature approach with excess refrigerant flow. With HX-1 solved, you try to add HX-2 and HX-3. Save your work often. I expect this simulation to be very frustrating for you. How many months do you have to complete it?

  I will tell you my approach to this model if I only had the info you provided. I would start with stream 5 miniscule flow! This leaves you with one mixed refrigerant loop! That is still not easy to model, but simplifies things. After solving for a single mixed refrigerant, then you can go back and add flow to your propane refrigerant loop if you want to improve overall efficiency. The advantage to this procedure is that you may get a converged model and an 'answer' even if it is not completely optimized for efficiency.

Hi, i have around 3 months to finish this, thanks for your reply it was really helpful