Shutdown strategies

Startup and Shutdown Strategies. In addition to safe operation, the usual goal of a reactor startup is to minimize production of off-specification material. This can sometimes be accomplished perfectly. 

Neither method will achieve a bumpless startup for complex kinetic schemes such as fermentations. There is a general method, known as constant RTD control, that can minimize the amount of off-specification material produced during the startup of a complex reaction (e.g., a fermentation or polymerization) in a CSTR. It does not require a process model or even a realtime analyzer. We first analyze shutdown strategies, to which it is also applicable. 

Example 14.6 Explore the consequences of the following shutdown strategy for an isothermal, constant-density CSTR that has been operating at steady state. At time zero, the discharge flow rate is increased by a factor of 1 -b 5. Simultaneously, the inlet flow rate is made proportional to the fluid volume in the vessel. When does the vessel empty and what happens to the composition of the discharge stream during the shutdown interval ... 

These shutdown times are moderate 2.4Ffor5 = 0.1 and 3F for 5 = 0.05. Equation (14.2) governs the outlet concentration during the shutdown interval. For this shutdown strategy, it becomes... 

As mentioned earlier in this chapter, HTPEM fuel cells are subject to severe carbon corrosion during open circuit operation. Therefore a shutdown strategy is necessary to avoid this diuing system shutdown. 

It was nevertheless observed that drying method could possibly lead to performance drop at rated conditions (Pinton, 2009a Luo, 2010). Consequently, if the shutdown strategy procedure includes a drying stage, it will have to be chosen carefully using the following options ... 

Fire Protection FIVE and FRANC support quantitative and qualitative fire risk, assessmeiil. FRANC also supports the analysis of alternative safe shutdown paths during an Appendix R review. System train availability is exhibited on a system status panel, Using this a strategy can be defined to upgrade Thcrnm 1. if identify risk significant areas. 

These refer to the control of a chemical process and include planning for familiar tasks (e.g., change type of fuel-firing in a furnace) or planning for familiar but infrequent tasks (e.g., start-up or shutdown a furnace). Methods of task analysis and error analysis can be used to analyze well-established strategies that operators use to perform procedural tasks and identify the user s information needs. An implication for display design would be that all information needed... 

Identified hazards can either be reduced at the root (e.g., reduction of inventory) or by introduction of protective systems (e.g., automatic emergency shutdown). The increased understanding of the chemistry and the process that is obtained from hazard evaluation procedures provides guidance for many other elements of process safety management as well (e.g., procedural instructions, emergency strategies, personnel training, and preventive maintenance). 

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]. 

---