Reactor Startup Transients

The Infinite. outage startup transient curves Indicate that the maxi mum excess reactivity during the startup and operation Is the cold, xenon-free reactivity This differs from the present Hanford reactors where the turnaround level for an Infinite-outage startup Is hlipa r than the initial cold, clean reac-tlvlty level  

Sven If the reactor were soranned at turnaround , the Immediate reactivity effect associated with cooling down of the fuel and coolant would not result In an excess reactlvligr beyond that for the cold reactor  

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During the reactor startup transient, steam will be supplied by the oil-fired boiler until process generated steam is available. The boiler capacity will be sufficient to provide reactor flows of about half... 

The method of false transients converts a steady-state problem into a time-dependent problem. Equations (4.1) govern the steady-state performance of a CSTR. How does a reactor reach the steady state There must be a startup transient that eventually evolves into the steady state, and a simulation of... 

Example 14.3 The initial portion of a reactor startup is usually fed-batch. Determine the fed-batch startup transient for an isothermal, constant-density... 

The coke profiles in the reactor bed can be predicted excellently by the model as shown by the solid lines in Figure 1. Figure 2 shows good consistency is also obtained for the average coke content over the reactor bed versus time on stream. Note that within the time period of reactor startup plus one hour of operation, the average coke content of the reactor bed is already at about 5 wt%. The model cannot be applied to this startup and initial period with the rapid transients of temperature, activity "spike" and concentration. However, compensation for this interval can be made by a time translation of the model a model time of 36 hours is fixed at an experimental time of zero. A temperature difference of more than 20C between the center of the bed and outer wall of the reactor in the startup stage has been observed in our laboratory for some experiments. About three-fourths of this difference is across the catalyst bed itself. Startup of the reactor at reasonably lower temperatures in order to control the coke formation and to better maintain the catalyst activity is important, if not critical. 

The method of false transients converts a steady-state problem into a time-dependent problem. Equations 4.1 govern the steady-state performance of a CSTR. How does a reactor reach the steady state There must be a startup transient that eventually evolves into the steady state, and a simulation of that transient will also evolve to the steady state. The simulation need not be physically exact. Any startup trajectory that is mathematically convenient can be used even if it does not duplicate the actual startup. It is in this sense that the transient can be false. Suppose at time f = 0 the reactor is instantaneously filled with fluid of initial concentrations ao, bo, — The initial concentrations are usually set equal to the inlet concentrations, ai , , but other values can be used. The simulation begins with gin set to its steady-state value. For constant-density cases, gout is set to the same value, and V is constant. The variable-density case is treated in Section 4.3. 

The initial portion of a reactor startup is usually fed batch. Determine the fed-batch startup transient for an isothermal, constant-density stirred tank reactor. Suppose the tank is initially empty and is filled at a constant rate go with fluid having concentration Oin. A first-order reaction begins immediately. Find the concentration within the tank, a, as a function of time f < ifuu-... 

Typical reactor startup reactivity transients are shown In Figure 6 3 Itl for the green reactor and the reactor with an average fuel exposure of 700 MWD/T. The curves ai for an Infinite" outage The green infinite. outage startup transient should inpresent the most extensive case Insofar as excess reac. tlvlty Is concerned during a startup. 

The various temperature and exposure reactivity contributions have been combined to determine typical reactor startup and shut down reactivity transients. 

Typical reactivity shutdown transients are given in Figure C 12 for the reactor loaded with green fuel and fuel at an average exposure of 600 NND/T. The initial equilibrium reactivities are those ex peeted with 0.947 per cent enriched fuel - actually the reactivity carried in control rods at equllibrivn will be nearer one per cent, so the entire corves can be moved down about one per cent. To be consistent with the startup transient (Figure 5.1 in Volume 1) the reactivities are shown as they are expected to be with the full 0.947 per cent enrichment. 

A system represented by a set of equations describing a physical system is said to be stable around a steady state if the transients of the system are bounded when the system is subjected to small perturbations from the steady state. The system is said to be asymptotically stable if it is stable and it eventually returns to the steady state. If in addition, the steady state is approached exponentially, it is called exponentially asymptotically stable. If any magnitude of perturbation is allowed, it is said to be globally stable. The stability problem as related to reactor startup and control is concerned with the following questions (Luss 1977) ... 

Heat exchanger-like, multi-tube reactors are used for both exothermic and endothermic reactions. Some have as much as 10,000 tubes in a shell installed between tube sheets on both ends. The tubes are filled with catalyst. The larger reactors are sensitive to transient thermal stresses that can develop during startup, thermal runaways and emergency shut downs. 

We have thus far considered only steady-state operation of the CSTR and the PFTR. Thi, s is the situation some time after the process was started when all transients have died out, and no parameters vary with time. However, all continuous reactors must be started, an d parameters such as feed composition, flow rate, and temperature may vary because feed composition and conditions change with time. We therefore need to consider transier it operation of the CSTR and the PFTR. Transients are a major cause of problems in reactoir operation because they can cause poor performance. Even more important, problems during startup and shutdown are a major cause of accidents and explosions. 

Whenever multiple steady states in a reactor are possible, we must be very concerned that we are operating on the desired steady-state branch. This requires a proper startup procedure to attain the desired steady state and suitable operation limits to make sure that we never exhibit a sufficiently large transient to cause the system to fall off the desired conversion branch. We will consider transients in the CSTR in the next section. 

Detailed transient studies and plots such as these must be done whenever one is interested in designing a nonisothermal reactor to determine how startup and shutdown transients might cause unacceptable transient conditions in the reactor. 

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