Two-temperature control

The second temperature controller TC2 manipulates the flow rate of reflux during those periods when steam cannot be used. The TC2 controller has a slightly higher set point than TCI, so during periods of normal operation, it will call for less reflux (since the 

this two-temperature control structures uses steam flow rate to control temperature during normal operation but switches to using reflux to control temperature when the minimum steam flow rate limitation is reached. 

The dynamic response between steam and a tray temperature is usually fairly fast. The dynamic response between reflux and a tray temperature depends on the number of trays between the top of the column where the reflux enters and the control tray. The further down the column, the slower the dynamics are. The example presented in a later section illustrates that the reflux-to-temperature dynamics are usually much slower than the steam-to-temperature dynamics. This means that tighter temperature control can be attained using steam than using reflux. 

So when the minimum steam limit is reached, there may be some deterioration in performance of the temperature loop. But the steam flow rate will never drop below its minimum. 

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Next, the two temperature controllers were activated and the sandwich was taken up to 90°C (194°F) for one hour to evaporate the solvents from the liquid Nation 117 catalyst coating. The temperature was then raised to 130°C (266°F) over the next 30 minutes. This is the PEM glass transition temperature. 

A fully automated system for performing detailed studies has been developed to improve the reproducibility and throughput (Fig. 12.2) [8]. It consists of two functional components a sample-deuteration device and a protein processing unit. The preparation operations (shown at the top of Fig. 12.2) are performed by two robotic arms equipped with low volume syringes and two temperature-controlled chambers, one held at 25 °C and the other held at 1 °C. To initiate the exchange experiment, a small amount of protein solution is mixed with a deuter-ated buffer and the mixture is then incubated for a programmed period of time in the temperature-controlled chamber. This on-exchanged sample is immediately transferred to the cold chamber where a quench solution is added to the mixture. 

Two temperature controllers are used. The first manipulates the flowrate of the A feed. A 45 min ramp in this reactor temperature controller is used with Kci = 0.5 and Tj2 = 20 min. Two 30-s lags are included in the loop. The span of the temperature transmitter is 50 K, and the maximum flowrate FA0 of the reactant A is 0.004 m3/s. The second temperature controller setting the flowrates of the hot and cold streams to the jacket is proportional-only with a 330 K setpoint and a gain of 0.05. The maximum cold water and hot water flowrates are 0.005 and 0.002 m3/s, respectively. 

All of these systems have some common control loops. The system pressure is controlled by manipulating the fresh feed of A (F0A). The concentration controller with ratio control is used to control reactor inlet gas composition by manipulating the fresh feed of B (F0B). Bypassing (Fhy) around the FEHE is used to control gas mixture temperature Tmix. Reactor inlet temperature (Tin or T ) is controlled by manipulating the furnace heat input QF. The setpoints of these two temperature controllers are the same, and the controller output signals are split-ranged so that bypassing and furnace heat input cannot occur simultaneously. 

The experimental device constructed to orient uniformly thick samples in simple shear is schematically represented in Fig. 3. It is basically a sliding-plate rheometer, the polymer sample being sheared between two temperature-controlled parallel plates. The upper plate is fixed whereas the lower plate can be displaced both horizontally and vertically with two pneumatic jacks. 

Clearly, the key control loops are the two temperature controllers. Temperature control by manipulating reboiler duty in the high-pressure column is conventional. Relay-feedback testing and lyreus-Luyben tuning give ATc = 0.157 and ti= 10.6 min for a temperature transmitter range of 250-350 °F and an output maximum of 94.8 x 10 Btu/h. The TC2 controller is reverse acting. 

In the low-pressure column, a direct acting temperature controller is used to manipulate feed to the column. Clearly, the two temperature controllers are interacting because vapor flow rates affect both columns, as do changes in feed flow rates. The mning strategy used was to tune the TC2 controller in the high-pressure column first with the TCI controller in the low-pressure column on manual (fixed feed flow rates to each column). Then the TCI controller was tuned with the TC2 controller on automatic. 

There is only one reactant that is fed to the column. The two products are removed out of the two ends of the column. Olefin metathesis is an example of this type of reactive distillation column. Figure 9.2 illustrates this system and gives an effective control scheme. A C5 olefin reacts to form a light C4 olefin, which is removed in the distillate, and a heavy C6 olefin, which is removed in the bottoms. The two temperature controllers are used to maintain conversion and product quality. The production rate is set by a feed flow controller. 

The key control structure issue is how to set the vapor sidestream flow rate. It certainly must change as disturbances enter the column. One effective way to achieve this is to ratio the sidestream flow rate to the reboiler heat input. This control structure is shown in Figure 10.11. The two temperature controllers (TC3 and TC50) have Imin dead times and lOOK... 

Figure 15.1 shows a control structure that uses two temperature controllers operating in parallel. The temperature controller TCI is the one that manipulates steam to the reboiler under normal operating conditions. The output signal from TCI is normally the set point signal to the steam flow controller. Note that reboUer duties (GJ/h) are shown in the figure instead of steam flow rates. 

Figure 15.7 (a) Two-temperature ramp results, (b) Two temperature control signals. 

Temperature is controlled by manipulating column feed during periods of low throughput with this stmcture. The TC2 controller has an integral time of 6.5 min, which is much smaller than that used in the two-temperature control stmcture (20 min). Temperature control should be tighter. 

Figure 15.10 gives a direct comparison of the performances of the three control structures for the same ramp disturbances. The dashed lines are for the two-temperature control structure. The dotted lines are for the VPC control structure. The solid lines are for the recycle control structure. 

The worst temperature control and the largest deviations in methanol product purity (xD) occur with the two-temperature control structure. The VPC control structure does not... 

Figure 15.11 gives results for the two-temperature control structure. When feed flow rate is dropped to 3000kmol/h, temperature rises up to 105 °C, and the TCI controller decreases reboiler heat input. At the same time, the R/F ratio controller produces an immediate drop in reflux flow rate. However, the rise in temperature causes the TC2 controller to increase reflux temporarily until the temperature is returned to the TCI set point of 101 °C. 

So the VPC control structure is better than the two-temperature control stmcture but has some performance issues. 

At present, four companies provide specialized impedance measurement instrumentation for chnical microbiology or food hygiene applications. BioMirieux manufactures the Bactometer, which can monitor conductance, capacitance, or impedance in up to two temperature-controlled incubators, each containing 256 samples. Malthus Instruments produces... 

Figure 5.8 shows the control structure developed for this system. Conventional PI controllers are used for all flows, pressures, and temperatures. Proportional controllers are used for all liquid levels. Relay-feedback tests are run on the two temperature controllers to determine ultimate gains and periods, and Tyreus-Luyben tuning is used. Each temperature controller has a 1-min deadtime in the loop. Reflux ratios are held constant in each column (2.84 in the low-pressure column and 3.11 in the high-pressure column). 

If the two columns are not heat integrated and have separate condensers and reboilers, a temperature in each column can be controlled by manipulating the corresponding reboiler heat input, and the reflux ratio in each column could also be controlled. This strucmre is shown in Figure 6.18. Each column has a pressure controller manipulating condenser heat removal. The output signals of the two temperature controllers are the two reboiler duties. 

Notice in the faceplates shown at the bottom of Figure 6.23 that the setpoints of the two temperature controllers are different in the partially heat-integrated case fiom those used in the fully heat-integrated system because the temperature profiles are slightly different. Of course the TCI temperature controller must be retuned since its output signal is a ratio. 

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