Delay-loop reactor

For the application referred to, the interdgital micro mixers were used on their own, without tubing attached, as reactors. Especially at low flow rates, the internal flow-through chamber acts as delay loop for providing a sufficient residence time. 

P 68] No detailed experimental protocol was given [61, 62,142,143]. Two reactant streams, the solution of the reactant in hexane and concentrated sulfuric acid, were fed separately in a specially designed micro reactor by pumping action. There, a bilayer was formed initially, potentially decomposed to a dispersion, and led to rapid mass transfer between the phases. From this point, temperature was controlled by counter-flow heat exchange between the reaction channel and surrounding heat-transfer channel. The reaction was typically carried out at temperatures from 0 to 50 °C and using residence times of only a few seconds. If needed, a delay loop of... 

FIGURE 7.2 Didactic representation (upper) and flow diagram (lower) of a typical single-line flow system with merging zones. S = sample R — reagent C — carrier stream LS/ Lr = sampling loops IC = injector-commutator RD = delay coiled reactor Rc = main coiled reactor D = detector shaded area = alternative IC position solid arrows = sites where pumping is applied. 

A number of different ways were originally considered for removing the loops. The initial methods were to use the reactor refuelling machine or a heavily shielded facility constructed on the reactor cap. The loops (Figure 6) are so constructed that they could be drawn into such a facility and then size reduced ready for disposal. For various reasons, including the lack of a suitable repository for the cut tubes, a decision was taken to delay loop removal until after the RDM had been installed. As a consequence the loops must now be removed using remotely-deployed tooling mounted on the reactor internals. 

As stated in the previous section, the major reactant feed was chosen as the manipulated variable. In the trial this feed was subjected to a pseudo-random binary sequence (PRBS) signal in an open loop operation of the process. The results of the trial, plotted in Fig. 2, show a strong -- but delayed -- cross-correlation between the manipulated feed rate and the reactor temperature. Using techniques described by Box and Jenkins (2), a transfer function relating the manipulated variable to the control variable of interest can be developed. The general form of this transfer function is... 

Any control procedure for polymer reactors must recognize and deal with these measurement problems. This rules out the use of conventional continuous control ideas taught to engineering students at most universities. These latter ideas are based on continuous measurements, in the presence of little or no measurement noise, and are applicable only to systems with rather short time delays in the feedback loop. 

Due to the fuel control system the converter operates dsmamically. The delay time in the feedback loop of the control system leads to an oscillating exhaust composition around the stoichiometric point at a typical frequency of 1 Hz which is of the same order as the turnover frequency of the reactions. Therefore in reactor modeUing the dynamic behaviour of these reactions has to be considered meaning that detailed kinetic models based on elementary steps have to be used. Furthermore accumulation both in the gas phase and on the catalyst surface should be taken into account. 

The model used in this section neglects the time delays due to recycles and the capacity of the mixing vessels. Consequently, the model is obtained by the combination of the differential equations describing the dynamics of the reactor and the closed-loop separation. F and c are molar flow rate and reactant concentration, respectively. Dimensionless values are denoted by /=c/c and z=F/Fo with reference to process inlet. Subscripts follow the numbering explained in Fig. 13.18. When two reactants are involved, a second subscript is used. Because high purity product C4 = Z4 = 0. 

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... 

Intermediate heat exchangers. Horizontal tube-and-shell IHXs with three modules connected in series, made of U-shaped tubes are used in the BN-350. The IHX of each loop consists of two sections connected in parallel both for primary and secondary coolant flows. The IHX is located in a suction loop upstream of the primary coolant pump, while in the secondary circuit it is in a pressure loop downstream of the circulating pump. The IHX tube bundles can be removed if necessary and replaced with new ones. The most stressed units in the IHX are the fixing joints for the tube module covers and for the fi-ame which stiffens the flat walls of the IHX body. Measurements of temperatures and stresses in various items of the IHX were carried out during reactor plant operation. On this basis requirements were formulated to limit the rate of the IHX heating-up in steps of 10% specified power with delays of 5-10 h in each step. By 1995 the IHXs have operated more than 160000 h at various power levels without any disturbances and failures. 

In in-line analysis, the chemical analysis is done in situ, directly in the main process stream or reactor, using a chemically sensitive probe. A condition is that the equipment has to be placed in the plant (with consequences for maintenance and safety aspects). In this case, there still is physical contact between probe and sample. Consequently, an in-Une process-monitoring device must often deal with hostile industrial processing conditions elevated p, T, fluctuating conditions, chemically aggressive environments, electrical noise, dust, and vibrational problems. Sampling delays are very short, or non-existent for in-Une devices. The feedback and control loop can be optimised in real-time manner. However, an in-line apparatus may interfere with the... 

An example of control loops for operation in the load range is sketched in Fig. 8.16. Here the speed of the feed-water pump is controlled by the temperature of the superheated steam at turbine inlet, the mass flow of the HP steam extractions is controlled by the feed-water temperature, the reheat temperature is controlling the steam mass flow of the reheater, and the pressure at the reactor outlet is controlling the turbine governor valve. The thermal power of the reactor, and thus with some delay in the generator power, is controlled by the control rods of the reactor core. 

General guidelines for temperature control loops are difficult to state because of the wide variety of processes and equipment involving heat transfer and their different time scales. For example, the temperature control problems are quite different for heat exchangers, distillation columns, chemical reactors, and evaporators. The presence of time delays and/or multiple thermal capacitances will usually place a stability limit on the controller gain. PID controllers are commonly employed to provide quicker responses than can be obtained with PI controllers. 

A study of the temperature-dependent open-loop kinetics of the LMFR has been carried out by Fleck [4]. The effect of delayed neutrons and the delayed moderator temperature coefficient were neglected. Under these conditions. Fleck found that the reactor responded rapidly and with little overshoot in temperature when subjected to the largest permissible reactivity excursions. 

An in-pile, forced-circulation loop has been built at BNL and two others at Babcock Wilcox Research Laboratory to test the corrosion stability of LMFR materials under conditions to be expected in the reactor experiment. In this loop, Bi containiiif approximately 1500 ppm of 180 ppm Zr, and 850 ppm Mg will be pumped at a rate of 5 to 7 gpm. The bulk A2 will be approximately 75°C, with a maximum temperature of 500°C. There are three sample sections in the loop one, containing samples of 1 % Cr-1/2% Mo steel, 2 % Cr-1% Mo steel. Be, and graphite, will be at the center of the reactor and will be in a flux of approximately 3 X lO " thermal neutrons one within the shield will see delayed neutrons at a temperature of 500°C and the third section will be outside the reactor at a temperature of 425°C. This test is presently being assembled, and will be operating late in 1958. 

At either end of the channel, probably on a platform erected outside the reactor, is a complete replica of all the apparatus necessary to operate the channel, i.e. the pumps for pushing the coolant through, the Burst Slug Detection Gear, delay tanks, heat exchangers, etc. You will see, therefore, that a loop is a miniature reactor with only a single channel instead of many. ... 

---