Chemical plant design shutdown

Normal Operation. The designer of a chemical plant must provide an adequate interface between the process and the operating employees. This is usually accompHshed by providing instmments to sense pressures, temperatures, flows, etc, and automatic or remote-operated valves to control the process and utility streams. Alarms and interlock systems provide warnings of process upsets and automatic shutdown for excessive deviations from the desired ranges of control, respectively. Periodic intermption of operations is necessary to ensure that instmments are properly caUbrated and that emergency devices would operate if needed (see Flow measurement Temperaturemeasurement). 

This report examines the severe accident sequences and radionuclide source terms at the Sizewell pressurised water reactor with a piestressed concrete containment, the Konvoi pressurized water reactor with a steel primary contaimnent, the European Pressurised water Reactor (EPR) and a boiling water reactor with a Mark 2 containment. The report concludes that the key accident sequences for European plant designs are transient events and small loss-of-coolant accidents, loss of cooling during shutdown, and containment bypass sequences. The most important chemical and transport phenomena are found to be revaporisation of volatile radionuclides from the reactor coolant system, iodine chemistry, and release paths through the plant. Additional research is recommended on release of fission products from the fuel, release of fission products from the reactor coolant system, ehemistry of iodine, and transport of radionuclide through plants. 

Naka Y., Batres R., Adriani A. Operational Design for Startup and Shutdown of Chemical Plants Based on a Topological Approach, AlChE Spring National Meeting, Houston, USA, 1995. 

Green, D. L., and A. M. Dowell (1996). Cookbook Safety Shutdown System Design. i996 Process Plant Safety Symposium, Volume 1, April 1-2,1996, Houston, TX, ed. H. Cullingford, 552-565. Houston, TX South Texas Section of the American Institute of Chemical Engineers. 

Industrial H2 production plants operate at steady state. They were not designed either for numerous start-ups and shutdowns or for the cycling in load. The catalyst and other reformer materials can be chemically and physically damaged. 

Unsteady-state or dynamic simulation accounts for process transients, from an initial state to a final state. Dynamic models for complex chemical processes typically consist of large systems of ordinary differential equations and algebraic equations. Therefore, dynamic process simulation is computationally intensive. Dynamic simulators typically contain three units (i) thermodynamic and physical properties packages, (ii) unit operation models, (hi) numerical solvers. Dynamic simulation is used for batch process design and development, control strategy development, control system check-out, the optimization of plant operations, process reliability/availability/safety studies, process improvement, process start-up and shutdown. There are countless dynamic process simulators available on the market. One of them has the commercial name Hysis [2.3]. 

Chemical companies usually do not attempt the mechanical design of their own process equipment. Their engineers perform the calculations and conduct the experiments required to fill out a vendor s design data sheet such as shown in Fig. 2.1 for a vapor-liquid contactor. The vendor, who very frequently is not told what chemical is to be processed, submits a quotation and a recommended mechanical design. This must then be evaluated by the chemical companies engineers because, even though there may be performance guarantees, plant shutdowns for modification and replacement are costly. 

If shutdown is the chosen option it must be designed so as to be accomplished safely. While this may be a minor problem for some systems (e.g. those which are primarily electronic or electromechanical) shutdown of, say, a chemical process plant may take days and require a complex and carefiiUy controlled sequence of operations. Furthermore, a shutdown will be followed by a re-start and experience suggests that for many systems, start-up is the most dangerous part of operation. 

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