Extension to Composition Control

Because of the steady-state offsets when using temperature control, it may be necessary to use offset-free composition control in acetic acid esterification. Because of the nature of the 

In the previous section, two temperature control trays are selected from the NRG analysis. Because the bottoms product composition is one of the controlled variables, the other controlled variable is the temperature further away from the acetate withdrawal ends. The fourth column of Table 13.5 gives the steady-state gain matrices of these five 2x2 systems. One temperature and one composition are controlled with two manipulated variables heat input and feed ratio. 

Once the controller structure is determined, sequential relay-feedback tests are performed to find the PI controller settings shown in Table 13.5. Similar to the settings for temperature control (Table 13.2), we have large reset times associated with the feed ratio and much smaller reset times for the heat input. Again, the control systems are designed in a systematic manner with minimal complexity in the design steps, identification, tuning, and controller types. 

Good disturbance rejection can be achieved for a feedrate increase. For the EtAc and IPAc systems (type II flowsheet), little difference in the speed of response, peak error, and steady-state offsets are observed (Figs, 3.6b, 13.6c, 3. 2b, and 13.12c). Thus, there is little incentive to seek composition control for the type II flowsheet. For the BuAc system, good control performance is possible using composition control. The 

In summary, one-temperature, one-composition control is recommended for the BuAc system, in which steady-state error can be eliminated without sacrificing closed-loop dynamics. For the AmAc system, we observed limited operability (Fig. 13.13c) when composition control is applied. For the MeAc system, composition control leads to a large peak error and oscillatory response. Therefore, a tradeoff has to be made between the off-spec acetate composition and poor control performance. An alternative is to use a temperature/composition cascade to solve this problem (e.g., CS3 in Figs. 12.58 and 12.84). As for the EtAc and IPAc systems, composition control provides little improvement in terms of the steady-state error. 

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Adiabatic Reactors. Like isothermal reactors, adiabatic reactors with a flat velocity profile will have no radial gradients in temperature or composition. There are axial gradients, and the axial dispersion model, including its extension to temperature in Section 9.4, can account for axial mixing. As a practical matter, it is difficult to build a small adiabatic reactor. Wall temperatures must be controlled to simulate the adiabatic temperature profile in the reactor, and guard heaters may be needed at the inlet and outlet to avoid losses by radiation. Even so, it is hkely that uncertainties in the temperature profile will mask the relatively small effects of axial dispersion. 

In conclusion, the release of exudates appears to be controlled mainly by the total primary production (photosynthetic activity), and although very little is known about the detailed chemical composition of algal exudates, existing studies have shown that carbohydrates, organic acids, and dissolved and free amino acids may be significant constituents. However, to be able to fully understand the role of algal sources of DOM, there is clearly a need for more studies on the release and chemical composition of algal exudates, particularly as the availability of various fractions of the released DOM to bacterial utilization seems to vary extensively (Norrman et al., 1995). 

An alternative to an extensive inorganic structure is an extensive organic one. Candidates for this could include any number of polymers. One approach is to make self-assembled capsules from complementary polymers that form layer by layer (LbL) vesicle-like structures [26], This has been achieved by templating the co-assembly of polymers around a removable core. The alternation of polymers with opposite charges allows the composition and thickness of the artificial cell walls to be controlled. The size of the core determines if the resulting capsule is a model for a cell or a smaller capsule like an organelle within a cell. The porous nature of the polymer allows chemical species to enter and leave the capsule but the potential for capsule growth and division, even with the presence of polymers in the external solution is very limited. 

We started this section on inferential composition control by using a single tray temperature. Then we used two temperatures to control a temperature difference. Then we used multiple temperatures for average temperature control. The logical extension of this approach is to use whatever measurements are available to estimate product compositions. There are several types of composition estimators. Steady-state... 

The analysis of metal artifacts has been used extensively to differentiate materials by sources. X-ray fluorescence and neutron activation analysis have both proved valuable in determining elemental concentrations. Native metals, such as gold, contained impurities that could, in some cases, be used to characterize their sources. However, the smelting of ores to recover the metals often changed the concentrations of impurities. Later, as alloys (e.g., bronze and brass) were produced, the compositions were intentionally altered and controlled. In some cases, the re-use of materials or the lack of quality control made the alloy composition quite variable (especially in terms of the trace components). 

The development of a control structure for this complex system turned out to be more difficult than for the stripper flowsheet. The initial control scheme evaluated is shown in Figure 10.21. It is a logical extension of the control structure used for the vapor sidestream column in which the vapor sidestream is ratioed to the reboiler heat input. As we will demonstrate below, this structure worked well for some disturbances, but it could not handle decreases in the composition of MeOH in the feed, resulting in a shutdown of the unit. 

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