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Complex Gas Separation Plant Dynamics Case Study

 

The Challenge

Following an incident at a gas processing plant, an examination and analysis were needed to determine what factors may have led to the occurrence. The process was so complex that even careful study by many experts was unable to clearly establish the cause-and-effect. The incident centered around the loss of liquid level and breakthrough of a high-pressure two-phase mixture into low pressure equipment (HP/LP Interface), leading to cold temperatures and subsequent brittle failure of a vessel. The company needed help identifying root causes.

Our Approach

The ioMosaic team created a computer simulation using Process Safety Office® SuperChems™, a detailed dynamic simulator, to elucidate the connections between the events and actions taken and the final incident. The events and actions to be simulated included:

  • Feed rate changes
  • Pump outages and restarts
  • Controller set point changes

Feed rate changes are shown in Figure 1. The model outputs (thick lines) are compared to the actual measured flows (thin lines). To match the measured flows, controllers were set using the process simulators event scheduler.

Figure 1. Flows to the Absorbers
Figure 1. Flows to the Absorbers

The process consisted of two parallel reboiled absorber columns with an absorber solvent recovery system. The solvent recovery system consisted of three columns, two reboiled strippers and one fractionation column. The process was highly energy integrated and had six majors recycles loops. To ensure the validity of the model, the first step was to construct models of each of the individual columns. The steady-state operation of four of the five columns had been studied. The data from the studies were used to validate the operation of each column in a steady-state mode before even attempting the dynamic simulation. When the individual steady-state column models had been validated, a dynamic model was constructed using HYSYS®.

Because of the extensive heat recovery, reboilers on two of the columns had to be split out as separate unit operations. They could not be included in a “column” simulation and solved directly using the normal techniques. That made the initial convergence to a starting (steady-state) solution a little difficult. But once convergence of the initial solution was achieved, the conversion to a dynamic simulation continued in a straightforward manner. Figure 2 shows the dynamic response of one of the reboilers. Note that the simulation was started from a steady-state condition at about 11:00 pm. The dynamic and steady-state simulators are exactly the same and some time (about two hours in this case) was required to obtain a “stable” dynamic solution after the dynamic simulation was initiated.

Figure 2. Dynamic Performance of a Reboiler
Figure 2. Dynamic Performance of a Reboiler

One event of particular interest was overfilling in the sumps of the parallel absorbers. The sumps had to be included as separate unit operations to be able to closely monitor them during the dynamic simulation. The temperature, pressure and level in the absorber sumps were part of the crucial output from the dynamic model. Figure 3 to Figure 5 show the carryover from the sumps of each absorber and the effects on the feed tray (the tray just above the sump). The impact of the carryover shows up as large temperature and level excursions in Figure 5.

Figure 3. Amount of Carryover from Absorber Sump
Figure 3. Amount of Carryover from Absorber Sump

Figure 4. Impact of Carryover on the Tray Above the Sump for Absorber 101
Figure 4. Impact of Carryover on the Tray Above the Sump for Absorber 101

Figure 5. Impact of Carryover on the Tray Above the Sump for Absorber 102
Figure 5. Impact of Carryover on the Tray Above the Sump for Absorber 102

The Benefits

Identifying root causes is an important part of any incident investigation. The validated dynamic model enabled the case team to reliably and quickly confirm or rule out specific scenarios and/or combinations of operator actions that could have led to the incident. The company now had data that gave invaluable insight into the incident, allowing them to make more informed safety decisions.

Learn More

Our mission is to help you protect your people, your plant, your stakeholder value and our planet. To learn more about how we can help you manage risk, call us at 1.844.ioMosaic.