Model application

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

Developmental approach to a dynamic plant gas system.

Dynamic modeling is a valuable tool for evaluating plant operating conditions and control strategies. High-fidelity dynamic simulators, developed for the process industries in the last decade, are a solid basis for accurately modeling the dynamic transitions.

Nevertheless, development of custom components (for specific features of the process and measurement system) often comes into play to provide a realistic plant model.

The following describes the development and application of a dynamic model for the plant off-gas system, characterized by the complex structure and strong interaction of the production units with fast dynamics and sharp unexpected changes in the process pressure. The model came about in several phases using a Hysys dynamic simulator.

Plant performance

The plant has three electric are furnaces, used for the production of elemental phosphorus. The reaction products coming off the furnaces are primarily phosphorus vapor and carbon monoxide gas. At the gas processing units, the phosphorus condenses and separates from the carbon monoxide in the condenser after particulate exits the stream in the electrostatic precipitator. Carbon monoxide goes to a common header and then to a rotary kiln, where it acts as the primary fuel in calcining phosphate ore in preparation for its use in the furnaces. Excess gas flares. In the near future the manufacturer will build a thermal oxidizer to process the excess carbon monoxide gas, thereby removing the flare most of the time.

The structure creates a highly interactive environment for pressure control in the system. Pressure control of the furnace off gas is critical to the safe and efficient operation of the plant. A surge of the furnace pressure propagates quickly to the common header, causing disturbances in the work of the kiln and parallel furnace units. On the other hand, the kiln load changes lead immediately to the header pressure disturbances, affecting the performance and pressure control of all parallel furnaces.

The dynamic modeling project came about to get a better understanding of this and several other plant control and performance issues. Due to the fast, interactive dynamics of the process, control system quality and speed of response are vital for plant performance. Using dynamic simulation is an easy and reliable way to test potential process control improvements (decoupling of the loops, gain scheduling, feedforwarding) before implementing them into the real plant. The other important control issue for the plant is a transition from the legacy system to the newer distributed control system (DCS). Dynamic modeling can help answer how the scan rate and speed of response of a new system will affect the quality of control.

The process redesign option for decreasing emissions is to install a thermal oxidizer, replacing the flare in the normal operational mode. Because of the strong interactions between furnaces, flares, thermal oxidizer, and kiln, design issues with the proposed thermal oxidizer and dynamics of the off-gas header are of great interest. Some of the issues to explore are how to maximize carbon monoxide use in the kiln, the maximum pressure in the system under a given set of upset conditions, and how quickly the thermal oxidizer must react to prevent flaring.

The main items for dynamic modeling in this part of the project are:

* create a dynamic model of the thermal oxidizer (TO) unit and develop its control system, providing stable TO performance under the changing input load;

* modify a control strategy for the common header pressure to take into account the TO as an additional interacting element;

* and evaluate the performance of the whole, modified process (with new strategies) under different scenarios of plant behavior, including abnormal and emergency situations.

Improving the pressure-relief system performance is a key plant safety issue. It relates to plant emissions. The pressure-relief devices of the existing system use steady state conditions, while the real values of the device-relieving pressure and capacity are dynamic in nature (depending, particularly, on the pressure impulse dynamics). Therefore, the first answer the manufacturer can get through dynamic simulation is to evaluate the real values of relieving pressure and relieving capacities for the devices and the overall system at various intensities and shapes of the pressure impulses. The subjects of interest for the vent system include what pressures would come about through the resulting deflagration if air is inadvertently introduced into the off-gas system, and how the pressure-relief devices interact during a pressure excursion. The next task is to use the dynamic model to modify the system. Particular items for modeling are (1) create the model of a new pressure-relief device and test its behavior before implementing it into the real plant; (2) using the simulation, evaluate different configurations of the vent system, consisting of new and existing devices; and (3) define an optimal relieving point for the new device.