(301d) Modeling Offgas Systems for the Hanford Waste Treatment Plant

To augment design calculations, dynamic models of the melter offgas systems in the Waste Treatment Plant now under construction at the Hanford Site have been developed using Aspen Custom Modeler?. The models consider the two major species in the offgas stream (air and steam or water vapor) and perform material and energy balance calculations that show the dynamic behavior of gas pressure, temperature, composition and flow throughout the offgas systems. The models are structured to perform accurate pressure drop calculations across the various unit operations using both standard engineering calculations and empirical data based correlations for specific pieces of equipment. The models include process controllers, exhaust fans and the offgas treatment equipment. The latest version of the model for the High Level Waste (HLW) system is comprised of the offgas trains from both HLW melters coupled to the Process Vessel Ventilation (PVV) system.

To provide a realistic simulation of process operation, the HLW model includes transient changes in process vessel volumes and includes the operation of Pulse Jet Mixers (PJMs) that are present in some of the vessels. To obtain reasonable computer run times, the simulation of long term plant operation with simultaneous filling and emptying of the process vessels was somewhat compressed into a 20 hour time period. This compression means that some gas flows out of the vessels during tank filling and into the vessels when the tanks are emptied may be higher than actual plant flows introducing a slight conservatism into the calculation. Even with this time compression, the full model required from 24 to 36 hours of run time on a 3 GHz workstation to complete a full simulation. This long run time is a result of the fast transients that continually occur throughout the simulation since both PJM and melter feed cycling take place in times that are on the order of 20 to 30 seconds.

Among other applications, the model has been used to:

? Evaluate different strategies to control PVV pressure.

? Evaluate different strategies to operate Pulse Jet Mixers in the process vessels.

? Assess the impact of steam surges in an HLW melter.

? Assess the impact of air sparging in an HLW Feed Preparation (HFP) vessel.

? Assess the impact of overblow of Pulse Jet Mixers in a condensate collection tank.

Some specific conclusions reached by the modeling studies include:

? An integrated pressure control strategy using a single pressure controller and the average of pressures measured just upstream of the two PVV control valves produced the best results of the various control configurations tested. The strategy maintained good pressure control in the vessels while at the same time minimizing PVV gas flow and balancing the gas flow between the two melter offgas systems.

? Operating one PJM at a time in each process vessel and doubling the PJM drive time maintained melter plenum pressures within about ±0.5 inch WG of the values obtained without PJM operation.

? During fast 7X and maximum credible 27X steam surges in an HLW melter, the standby valve connecting the melter plenum to the Submerged Bed Scrubber inlet opened which prevented the melter from losing vacuum Once the standby valve opened, no other safety limits were tripped.

? Air sparging at 360 CFM in a single HFP vessel with the offgas system operating in its normal configuration caused the vessel to pressurize and the flapper valve on the overflow line to open. The PVV system exhausts about 40% of the sparge air with the remainder released into the cell through the overflow line.

? Overblow of all four PJMs in a condensate collection tank for one second creates a large enough pressure surge in the melter plenum to trip the standby valve.

The structure of the HLW system model and some aspects of its application to the problems listed above will be discussed in the presentation.