Simulation model

The power failure scenarios are based on whether the motors that drive the pumps will lose electrical supply.

Since the feed or reflux flow is either at its normal flow or at zero flow depending on the relief scenario, we do not need to rigorously model the pump with its performance curve and flow control valve.

We model the pump, its driving motor, and its flow control valve as a single Valve with the FlowType parameter set to FlowRange. With this configuration, you can simulate a pump trip by setting the valve position to zero. In this case, the flow goes from normal flow to the minimum flow of zero.

As API 521 stipulates, we take no allowances for automatic control action unless it tends to increase relief loads.

While a column pressurizes, the temperature in the column also rises. With rising temperatures, the TIC controllers on the reboiler may reduce the steam flow to maintain the bottom temperature. However, this controller action may reduce the relief load and we should therefore ignore the controller action. To do this, we assume that the steam condensate out of the reboiler is constant by using the FlowRange approach in the Valve.

Sieve trays weep at low vapor rates, such as when the column is pressurizing. Tray weeping provides benzene to the column, which increases the reboiler temperature, driving force, and heat duty.

We model weeping in the sieve trays by setting the WeepVapFrac (vapor flow fraction at which liquid weeping begins) and DumpVapFrac (vapor flow fraction at which liquid dumping begins) variables in the Stage[i].TrayWeep submodel. See Figure 2 for the relationship between the weep vapor fraction, the dump vapor fraction, and the liquid weeping fraction.

If the column trays have valve trays, we can turn off weeping by updating the Stage[i].TrayWeep.WeepAreaFrac variable to 0%.

The relief valves are typical gas or vapor service reliefs valves.

Typical gas or vapor service relief valves pop open either completely or to some initial fraction. However, since our goal is to find the design relief load, we use a modulating relief valve. Modulating relief valves keep a constant process pressure to provide the required relief load for actual relief valve sizing.

Power failure trips the air cooler fans, but natural convection heat transfer remains.

We can configure natural convection heat transfer by updating the A1.AirSide.NatConv parameter with the minimum fractional air flow that we expect due to natural convection.

A fully-filled overhead receiver will flood the condenser.

We model the flooded condenser by adding the Flood flowsheet equation to the simulation. This equation calculates the heat transfer area fraction for the condenser as follows:

Flood: A1.Af = max(0.01, min(1, (1-V1.Level)/(1-0.95) ) )

The liquid tray holdup depends on the actual number of trays.

We model the column with 56 actual trays as 40 theoretical stages (70% efficiency) with the holdup adjusted to a value of 1.4.

PSV1.SP

The set pressure difference between the inlet pressure and the back pressure at which the pressure relief valve opens.

800 kPa

A1.Tai

The air-side inlet temperature in the air cooler.

24.85°C

A1.AirSide.NatConv1

The air-side natural convection fraction.

5%

T1.Stage[i].TrayWeep.WeepVapFrac2

The vapor flow fraction at which liquid weeping begins.

60%

T1.Stage[i].TrayWeep.DumpVapFrac2

The vapor flow fraction at which liquid dumping (100% weeping) begins.

20%