Downstream reactor , control conversion

Closed-loop response to process disturbances and step changes in setpoint is simulated with the model of Kiparissides extended to predict the behavior of downstream reactors. Additionally, a self-optimizing control loop is simulated for conversion control of downstream reactors when the first reactor of the train is operating under closed-loop control with dead-time compensation. 

These simulated results for the high emulsifier concentration operating condition demonstrate the utility of dead-time compensation to the control of conversion from the first reactor in a train. With implementation of this degree of control on the first reactor, control schemes for downstream reactors can be simplified as discussed in the next section. 

Operotion/surface tension The reactor should be operated in such a manner that large transients in surface tension are avoided. Conversion and surface tension oscillations will tend to contribute to wall polymer formation. Start-up polides, system design, and control procedures should he selected to insure steady, free emulsifier levels in the particle formation reactor. In some cases it may also be desirable to add more emulsifier to downstream reactors. 

Monomer conversion can be adjusted by manipulating the feed rate of initiator or catalyst. If on-line M WD is available, initiator flow rate or reactor temperature can be used to adjust MW [38]. In emulsion polymerization, initiator feed rate can be used to control monomer conversion, while bypassing part of the water and monomer around the first reactor in a train can be used to control PSD [39,40]. Direct control of surfactant feed rate, based on surface tension measurements also can be used. Polymer quality and end-use property control are hampered, as in batch polymerization, by infrequent, off-line measurements. In addition, on-line measurements may be severely delayed due to the constraints of the process flowsheet. For example, even if on-line viscometry (via melt index) is available every 1 to 5 minutes, the viscometer may be situated at the outlet of an extruder downstream of the polymerization reactor. The transportation delay between the reactor where the MW develops, and the viscometer where the MW is measured (or inferred) may be several hours. Thus, even with frequent sampling, the data is old. There are two approaches possible in this case. One is to do open-loop, steady-state control. In this approach, the measurement is compared to the desired output when the system is believed to be at steady state. A manual correction to the process is then made, based on the error. The corrected inputs are maintained until the process reaches a new steady state, at which time the process is repeated. This approach is especially valid if the dominant dynamics of the process are substantially faster than the sampling interval. Another approach is to connect the output to the appropriate process input(s) in a closed-loop scheme. In this case, the loop must be substantially detuned to compensate for the large measurement delay. The addition of a dead time compensator can... 

The reducing agent, NH3, is adsorbed onto the SCR and then consumed by NOx reduction reactions. The amount of ammonia fed to the reactor must be controlled depending on the operating conditions in order to maintain a high level of NH3 adsorption and NOx conversion of the SCR on the one hand. On the other hand, NH3 emissions from the aftertreatment system into the environment are not desired and have to be avoided. In order to reduce as much as possible the amount of released ammonia, the addition of another catalytic device downstream the SCR one can be an efficient and reliable solution. Furthermore, to maintain the system as compact as possible, one solution is to add an ammonia oxidation functionality directly in the rear part of an SCR monolith (Fig. 18.1a). In this way, an NH3 slip catalyst (ASC) is added after the SCR to oxidize NH3 leaving the SCR brick [2]. 

The temperature control of the oxidative conversion of methane can of course be achieved by diluting the reactants. This can be done by increasing the recycle ratio, which however will make the complicated separation system downstream the reactor very expensive. As an alternative, dilution can be accomplished by addition of steam (Leyshon et al., 1991), but this will spoil the process economy. These arguments lead to the selection of a fluidized bed. 

---