Are you confident with your process control?
You modeled a steady state mechanically refrigerated gas processing unit, and then you created P&ID by adding all the control logics. Are you confident your closed refrigeration loop is going to work as you expect?
Dynamic Simulation is the solution.
The best way to verify your control logic is to build a dynamic simulation. This particular model is built in VMGSim Dynamic.
As you see, natural gas first goes through the gas / gas exchanger, then gas / liquid exchanger, and then chilled down to -23.3 F in the chiller. The liquid NGL is extracted at the bottom of the cold separator and it is sent out to a tank after cold recovered in the gas / liquid exchanger. While the gas is sent to a pipeline after heat recovered in the gas / gas exchanger.
The refrigerant in the chiller is vaporized and then sent to the first stage of the compressor. After combining the vapor from the economizer, it is further compressed in the 2nd stage. After the refrigerant is condensed in the condenser, it is sent to the receiver. Before entering the economizer, the liquid is flashed into vapor and liquid. The vapor portion feeds the 2nd stage compressor, while the liquid is further throttled to feed the chiller.
Reasonable-looking Control
Since we have a compressor, do you think we need to control the compressor discharge pressure? No. The discharge pressure is actually determined by the condenser and the air temperature.
Since we have a receiver, do you think we need to have a level controller on the receiver? No. we actually control the liquid level in the chiller and the economizer. The receiver liquid level will be whatever it is, since the total mass in the refrigeration loop remains the same.
Reasonable Control
For this particular project, we need to have a total of five controllers.
- we need to have a level controller to control the cold separator liquid level by adjusting the valve at the cold separator liquid outlet.
- we need to have a level controller to control the chiller liquid level by adjusting the valve before entering the chiller.
- we need to have a controller to control the cold separator temperature by adjusting either the speed, IGV or slide valve of the compressor
- we need to have a pressure controller on the economizer.
- we need to have a level controller on the economizer.
Currently the five controllers are in the automatic mode and they are very stable.
Control Optimization in Dynamic Simulation
Can I save some energy on the compressor? Currently the power consumption is about 594 HP.
Let me try to set the economizer pressure to 70 psi instead. It seems I did not save the power consumption, instead, I increased it to 604 HP. It’s a bad idea.
The current cold separator temperature is -23.3 F. What's the lowest possible cold separator temperature? Right now the compressor is running at 65%. Let us increase to 100% and see what's going to happen. The temperature dropped to -26.4 F and the power consumption increased to 670 HP.
Benefits of VMGSim Dynamic Simulation
With dynamic simulation, you can test out all your control ideas before they fail on site and cost a lot of time and money. What's more, you can even virtually optimize your plant operation and save some operating cost while still achieve the same products, thanks to VMGSim Dynamic Simulation software.
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