Process Safety Process Automation
Partial oxidation is a process that requires careful control and monitoring so as to ensure that accidental temperature runaways are avoided, especially during transient conditions such as start-up and shutdown. Process licensors have developed sophisticated control systems together with integrated automated start-up and shutdown procedures, which play an important role in safe operation.
Butzert described the most important features of such systems in 1976. Each licensor includes details specific to his own process and experience. The fundamentals described by Butzert are still valid today, although the radical change in instrumentation and control hardware since then has allowed the incorporation of many additional functions. For instance, de Graaf and colleagues (1998; 2000) report on advanced features of the system in Pernis incorporating reactor preheat, panel restart after a spurious trip, and gas transfer to the turbines controlled so that flaring of sulfur-containing gas could be eliminated. Further refinements are reported by Weigner and colleagues (2002). Plants in a predominately power production application include a load following function to comply with the demands of the grid.
Another safety aspect typical of all partial oxidation processes is the necessity of controlling and monitoring the reactor shell temperature in case of damage to the refractory lining. Early plants had to rely on a multiplicity of point thermocouples around the reactor, usually chosen to correspond with potential weak points in the refractory brickwork. This was often complemented by the use of thermosensitive paint and regular thermography. Eater, special coaxial cables with a temperature — sensitive resistance between core and sheath became available. With these coils a continuity of coverage became possible far in advance of the discrete point measurements previously used. On the other hand, location of the hot spot from these measurements was only possible with a grid of such coils that noted which two were showing the high temperature.
Fiber-optic systems are now available that provide a combination of continuity and localization. With appropriate software this can be converted to a screen visualization identifying hot spots on the control panel screen (see Figure 5-35) (Nicholls 2001). This fiber-optic system has been used on oxygen-fired secondary reformers, and recently two Texaco gasifiers have been equipped in this manner.
|
Figure 5-35. Screen Shot of Fiber-Optic Reactor Shell Temperature Monitor (Source: N і cholIs 2001) |