Complex Gas Separation Plant Dynamics Case Study

 

The Challenge

A gas processing plant experienced an incident and reached out to ioMosaic to examine and analyze their processes to identify and help determine factors that may have led to the incident. To study the events that lead to an incident in a gas processing plant, a dynamic computer simulation was created by ioMosaic for an entire process. The process was so complex that careful study by many experts was unable to clearly identify the cause-and-effect of each of the events and actions. 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.

Our Approach

A computer simulation was created by ioMosaic to try 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, and 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 dynamics.

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 roots 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. Thereby, the company was equipped with data that provided invaluable insight about the incident to help 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.