Model application

InTech, Feb 2004 by Muravyev, Aleksandr I, Kelahan, Robert C, Kowallis, Paul C, Torgesen, Greg L

Model development

Model developers used the general purpose Hysys process simulator. The custom components for modeling specific features of the pressure-relief devices used spreadsheet calculations and Hysys User Variables (Visual Basic programming). Developers built the model in two versions, basic and extended. The basic version is a model for the existing plant configuration in the normal operational mode, with the thermal oxidizer unit connected, which you can easily turn off to give the current process layout. The extended version contains, in addition, a vent system model, including the existing seals and new ones planned for installation.

Main features of the model

Due to complexity of the plant, the model is a multilayer dynamic flow sheet, containing up to four nested levels of sub-flow-sheets. The main flow sheet contains unit operations for the common header (piping, valves, condensate traps), a simplified model of the kiln, and sub-flow-sheets for the furnaces/processing units and thermal oxidizer. The furnace/processing unit flow sheet includes, in turn, sub-flow-sheets for its component and pressure-relief devices (seals).

This multilayer solution provides simplicity in reading and navigating through the model and flexibility in its development and modification. Particularly, the seal models can easily reposition along the process or switch on/off to analyze alternative structures of the pressure-relief system.

The plant piping system has a fundamental impact on the process dynamics. Because the simulator currently does not support dynamic unit operations for the pipe segment, it used a combination of a separator, simulating the pipe volume, and valves for the pipe resistance. The valve sizing used plant steady state data for one(gas) or two-phase (liquid-gas) flow. To correctly calculate the pipe resistance in transition from one- to two-phase flow, the manufacturer used the variable valve opening and additional dynamic element, the lag transfer function.

The model also includes a part of the plant control system. The controllers and valves used simulator unit operations. The valve and actuator characteristics came according to plant data (design and experimental), and the tuning parameters of controllers were set equal to the real plant ones. The developers programmed the main process interlocks using the simulator Event Scheduler. The Event Scheduler was also a model for plant events, such as pressure excursions, load changes, and process equipment going down in emergency situations. A combination of these features, complemented with custom models of the plant seals, provides a highly realistic environment for studying plant behavior and testing the new control strategies and process design options.

Custom models

The existing plant seals are primarily of two types: gravity weighted and fixed cap. In the first type, the seal is broken (opened) when the lifting force, provided by the pressure under the moving bell, becomes higher than the bell weight. In the second type, the bell is not moving, and the seal is opened when the pressure under the bell is high enough to push out all liquid from the space under the bell (inner space of the seal) into the space between the outer surface of the bell and the seal tub (outer space of the seal). The new seal is a fixed-cap type with improved capability of resealing and liquid-gas separation.