Startup reactors

Monomer concentration dynamics are presented in Figure 5. Additional observations for Run 5 are accurately correlated during the reactor startup and at final steady state. The observation at one residence time, Run 4, may be in error. The total cummu-lative, molar concentrations of macromolecules as a function of time are presented in Figure 6. The errors associated with this dependent variable are also evident during the steady state analysis of initiation... 

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. 

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 next two steps after the development of a mathematical process model and before its implementation to "real life" applications, are to handle the numerical solution of the model s ode s and to estimate some unknown parameters. The computer program which handles the numerical solution of the present model has been written in a very general way. After inputing concentrations, flowrate data and reaction operating conditions, the user has the options to select from a variety of different modes of reactor operation (batch, semi-batch, single continuous, continuous train, CSTR-tube) or reactor startup conditions (seeded, unseeded, full or half-full of water or emulsion recipe and empty). Then, IMSL subroutine DCEAR handles the numerical integration of the ode s. Parameter estimation of the only two unknown parameters e and Dw has been described and is further discussed in (32). 

Ohmic heating of catalyst is often used as a simple method of igniting the chemical reaction during reactor startup, for instance, in the oxidation of ammonia on platinum-rhodium gauze catalysts. Another application is the prevention of cold-start emissions from automotive catalysts responsible for much of the residual pollution still produced from this source (21). The startup times needed for the catalyst to attain its operating temperature can be cut by a factor of 5 or more by installing an electrically heated catalyst element with a metallic support upstream of the main catalyst unit. Direct electrical catalyst heating permits facile temperature control but requires a well-defined catalyst structure to function effectively. 

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. 

In this chapter, modeling of monolith reactors will be considered from a first-principles point of view, preceded by a discussion of the typical phenomena in monoliths that should be taken into account. General model equations will be presented and subsequently simplified, depending on the subprocesses that should be described by a model. A main lead will be the time scales at which these subprocesses occur. If they are all small, the process operates in the steady state, and all time-dependent behavior can be discarded. Unsteady-state behavior is to be considered if the model should include the time scale of reactor startup or if deactivation of the catalyst versus time-on-stream has to be addressed. A description of fully dynamic reactor operation, as met when cycling of the feed is applied, requires that all elementary steps of a kinetic model with their corresponding time scales are incorporated in the reactor model. 

In reactor startup it is often very important /tow temperature and concentrations approach their steady-state values. For example, a significant overshoot in temperature may cause a reactant or product to degrade, or the overshoot may be imacceptable for safe operation. If either case were to occur, we would say that the system exceeded its practical stability limit. Although we can solve the imsteady temperature-time and concentration-time equations numerically to see if such a limit is exceeded, it is often more insightful to study the approach to steady state by using the temperature-concentration phase plane. To illustrate these concepts we shall confine our analysis to a liquid-phase reaction carried out in a CSTR. 

Other factors such as temperature and the presence of toxins, which affect the growth rate of the MTBE degrading organism, are also expected to have an influence on reactor startup time. 

It has been reported that some BTEX compounds may stimulate the growth of MTBE degraders [83,88,90]. Growth on BTEX for microorganisms is expected to be much more favourable than with MTBE. The presence of BTEX in reactors may reduce reactor startup times for MTBE degradation if these compoimds increase the quantity of the MTBE degrading biomass. 

The initial biomass concentration and the presence of co-contaminants have been found to influence the startup of MTBE reactors. Higher initial seed concentrations generally lead to a faster reactor startup. MBRs or FBRs seeded with a high biomass concentration can be started within 10-30 days. Reactors seeded with only a low biomass concentration will generally take about 150-200 days to achieve startup. The organisms which oxidise cocontaminants will compete with the MTBE degrading biomass for dominance and occupation in reactors. Therefore, high concentrations of co-contaminants can increase the time required for reactor startup in some systems. 

Cometabohc cultiues in MTBE degrading reactors may have some positives. The cometabohc strains normally grow much faster than strains which utihse direct metabolism. Furthermore, the simple branched chain alkanes used as energy soiuce diuing cometabohsm reactions are normally present in MTBE plumes caused by gasoline leaks. The use of cometabohc strains can result in faster reactor startup. Knowledge of the apphcabihty and hm-itations of cometabohc strains in bioreactors is limited and needs further research. 

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-... 

Reactor startup consisted of setting the final system temperature and pressure with CO2 (or pure isobutane when CO2 was not used) and starting the olefin feed pump (defined as zero time). Prior to shutdown, the reactor was flushed with CO2 at reaction temperature and pressure until no hydrocarbons were observed in the effluent, following which the reactor was cooled and depressurized. 

One of the concerns with bulky reactors is the time taken to start up the reactor. Startup involves bringing the temperature of the essential components to the required level to kick off the reaction kinetics, supplying the required fuel to the reactors, and ensuring durable operation where all the reactors work seamlessly. In conventional systems, a large number of units need to be brought up in temperature before activating the reactions. In the case of a membrane reactor-based FPS, the catalytic partial oxidation (CPO) reformer, vaporizers, and membrane reactor need to be brought up in temperature. 

The models as presented and analyzed in the three previous sections are steady state and thus yield no information as to how the state was attained. Reactor startup and... 

Reactor startup with excessive step power change 50... 

Signals from the ex-vessel neutron detectors in conjunction with the in-reactor startup neutron detectors are utilized to derive neutron flux... 

The nuclear instrumentation must be operable prior to reactor startup. The automatic rod control during startup will not operate if more than one of the three ex-vessel wide-range channels is out of service. The Safety Protection Subsystem requires at least three of its four nuclear input channels operating. The power range neutron flux control will not operate automatically with more than two of the six input channels out of service. 

Three channels of linear power x te Instrumentation will reduce dependence on operator procedures during reactor startup. Each of the three channels will have five pairs of BTD s located in each quadrant of the reactor with one pair in the center of the... 

Ihese level trip limits can be fixed at or near equilibrium power levels for reactor startup and then adjusted to the proper percentage trips as the reactor approaches equilibrium. 

The control rod system provides for automatic control of the required reactor power level and its period reactor startup manual regulation of the power level and distribution to compensate for changes in reactivity due to burn-up and refuelling automatic regulation of the radial-azimuthal power distribution automatic rapid power reduction to predetermined levels when certain plant parameters exceed preset limits automatic and manual emergency shutdown under accident conditions. A special unit selects 24 uniformly distributed rods from the total available in the core as safety rods. These are the first rods to be withdrawn to their upper cut-off limit when the reactor is started up. In the event of a loss of power, the control rods are disconnected from their drives and fall into the core under gravity at a speed of about 0-4 m/s, regulated by water flow resistance. 

Following the reactor trip the operators monitor the progress of the RSSE from a display panel in the Central Control Room. They may intervene if any particular item of auxiliary plant fails to respond in its correct time sequence. Flowever, sufficient redundancy of essential plant is provided, making such intervention unnecessary on safety grounds. Finally, all discrepancies are noted and defects rectified and functionally tested before reactor startup. 

The computer can be programmed for reactor startup with various reactor boiler and turbine temperatures and pressures. The operator controls the... 

Boron dilution prevention during reactor startup, including hardware and procedure improvements ... 

Before reactor startup, the IVHMs are placed in their stored positions in the reactor. The instrument trees and eontrol rod shafts are restored to power-operation status, the adapter and floor valve are removed, and the fuel transfer port is sealed with its plug. 

The CLEM then picks up the CCP, containing the SNF assembly, and transfers it to the interim decay storage vessel. The cyclic process of moving fresh fuel to the reactor core and SNF back to the interim decay storage vessel continues until all the refueling operations have been completed. Before reactor startup, the IVHMs are placed in their stored positions in the reactor, the instrument trees and control rod shafts are restored to power-operation status, the adapters and floor valves are removed, and the fuel transfer ports are sealed with their plugs. 

The design of the reactor internals has not been addressed yet, but they likely will be made of graphite or carbon composites to accommodate the high-core outlet temperature required by the NGNP (1000°C). It is possible that carbon-insulated metallic alloy will be used for the core support structure, although this has not been evaluated yet. Control rods will be required to provide for reactor startup, normal operation, and shutdown. The munber and placement of control rods has not been evaluated yet, but the rods will be constructed from carbon composites for the drive shafts and absorber casing and boron carbide or other high-temperature absorber for the neutron absorber. The control rod drive mechanisms will be located above the reactor enclosure head. 

C. N. Shen and F. G. Haag, Applications of optimum control to nuclear reactor startup. lEE Trans. Nucl. Sci. 11, 2 (1964)... 

Dry electrochemical reprocessing of irradiated fuel on the NPP site after relatively short cooling, a factor of 3 smaller consumption of Pu (or enriched U) for reactor startup, and radical reduction of fresh and irradiated fuel transportation... 

Normal reactor startup from shutdown through criticality to power ... 

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