Problems with Steady State Simulation
Steady state simulation is very good for designing a plant. However, when it comes to operation, there are so many questions a steady state simulation simply can't answer. Two typical questions are:
- Is my process control going to work?
- Does my cause and effect diagram really prevent me from any danger?
Dynamic Simulation Can Help
Steady state simulation can't answer these kinds of questions, but Dynamic Simulation surely can.
A dynamic simulation is developed based on a condensate stabilize with NGL recovery. The feed is split into two streams. The major stream first feeds the preheater E-120 and then feeds into the middle of the tower; while the minor stream feeds at the top of the tower. The liquid falls down the tower T-130 and then it is heated up in the reboiler RB-140 by the hot oil. The liquid which is not boiled off is sent to a tank after recovering the heat. The vapor rises in the tower and then partially condensed in an air cooler AX-150. The vapor off V-160 is sent to flare, while the liquid is pumped to an NGL tank.
The dynamic simulation is setup in VGMSim. As you see, the control logic is modeled as well. For example, FV-120 is used to control the total flow to the stabilizer with a set point of 120 GPM.
In this example, we are going to fill an NGL tank, however, due to an unknown reason, even after the tank is full, no action is being taken. In addition, based on the cause effect, if the liquid level in V-160 is higher than 85%, the pump P-170 will stop and all the valves will be closed to shut down this plant. Is this a safe shut down?
Test Your Process Control and Cause & Effect Chart
Let us start the simulation. At the beginning, the tank pressure, temperature, tank liquid level all rises up. At about 1000 seconds, the tank reaches the 100% liquid level. Tank pressure increases dramatically and the flow to the tank drops to 0 gpm. Let us take another look at another strip chart. As mentioned earlier, when the liquid level in V-160 increases to 85%, the plant will shut down. Right now FV-130 is still open. Let us continue to run the simulation. Now you see the FV-130 is closed with 0 mass flow. Even after we close all the inlet valves, the liquid level in V-160 still climbs to 100%. However, the pressure and the temperature is still in a manageable range, thus this is a safe shutdown.
With the dynamic simulation, you can test your process control and verify your cause and effect diagram, while a steady state simulation can never do that.
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