Disturbance sensitivity

Annex V of the WFD lists the three main parameters of the macroinvertebrate community to be focused on for ecological assessment diversity, taxonomic composition and abundance, and the ratio of disturbance-sensitive taxa to insensitive taxa. Restoration from moderate to at least good status is required when these parameters differ from type-specific reference conditions to an extent coincident with the absence of major taxonomic groups. These parameters are not exclusive and others may be included to achieve the overall aim of an ecological status designation of the WFD as an expression of the quality of the structure and functioning of aquatic ecosystems (WFD, Article 2). 

The WFD explicitly provides definitions for high, good and moderate status in rivers (WFD Annex V, 1.2.1). For benthic macroinvertebrates, the factors that need to be taken into account when establishing monitoring and assessment method are taxonomic composition, abundance, the ratio of disturbance-sensitive taxa to insensitive taxa, and diversity. Only three mefiiods are shared between two or three countries, while Belgium and Spain apply different methods for different river types. Virtually all methods that are reported are multimetric indices, together covering all (or at least several) aspects that are required by tlie WFD. 

Mefloquine hydrochloride (lariam) is available for oral administration only. Tablets marketed in the U.S. contain 250 mg mefloquine hydrochloride, equivalent to 228 mg mefloquine base (this may vary in Canada and elsewhere). The dosing below is expressed in mg salt. Adults and children >45 kg body weight take 250 mg weekly starting 1-2 weeks before entering an endemic area and ending 4 weeks after leaving. Pediatric doses, taken by the same schedule, are 5 mg/kg for children up to 15 kg (may have to be prepared by a pharmacist) 62.5 mg (1/4 tablet) for 15-19 kg 125 mg (V2 tablet) for 20-30 kg 187.5 mg /k tablet) for 31 5 kg. Note Mefloquine is not recommended for children weighing <5 kg or individuals with a history of seizures, severe neuropsychiatric disturbances, sensitivity to quinoline antimalarials, or cardiac conduction abnormalities. 

III Suspected BSE Strongly pronounced behavior disturbances (nervous, anxious, easily startled), or Strongly pronounced disturbed sensitivity (over sensitive to manipulation, noise, light)... 

C and finally A. The output and manipulated variables were scaled for the disturbance sensitivity index, 6, such that a difference of 1°C in the hot stream output temperature is penalized equally as a 20 % difference in manipulated variable action. A disturbance of -i-l°C in the cold temperature inlet was considered. Table 3 shows for the various designs the disturbance sensitivity index and the average driving force. This Table shows that the controllability is closely correlated with the average driving force. An interesting observation is that this is independent of the flow configuration. 

The alternative designs generated for the two test cases (heat exchanger and binary distillation) on basis of this TCA principle prove to be remarkably well aligned with the results of a black-box, input-output controllability analysis using the steady-state disturbance sensitivity approach. 

For traditional cascaded plants, consisting mainly of process units connected in series, the controllability properties can easily be deduced from consideration of the individual units. The disturbance sensitivity is simply a product of the disturbance sensitivities of the units cormecting the disturbance and output under consideration, and any fundamental control limitation can be attributed to some individual unit. Since there exist a wealth of knowledge on how to improve the disturbance sensitivity, and avoid control limitations, through design of common units like reactors, distillation columns and heat-exchangers, design for controllability is fully feasible for cascaded plants. 

During the last decades, plants in the process industries have been steadily tighter optimized, both with respect to economic and environmental factors. One consequence of this optimization has been more complex plant structures, involving recycle flows of material and energy. For such plants, the dynamic properties, and hence the system controllability, is to a large extent determined by interactions between the process units. Previous studies have shown that the interactions due to recycling can affect stability [1], disturbance sensitivity and response time [2], oscillatory modes [3] and non-minimum phase behavior [4]. Recycling may also introduce... 

If the maximum attainable (Ob = min (OB, (OBu) is smaller than coj, then acceptable disturbance sensitivity can not be achieved using feedback control alone, and some modification of the process design is required in order to either increase the attainable (Ob and/or reduce cOd, i.e., the disturbance sensitivity of the process in the frequency range ft) [ft)B, ft)rf]. 

From the above results it is clear that we need to modify the process design in order to remove the non-minimum phase behavior and simultaneously increase the effect of the control input on the product composition. However, since the disturbance sensitivity requires a relatively high bandwidth of the control system, i.e., as > 0.2, it may be relevant to also modify the design with the aim of reducing the disturbance sensitivity at higher frequencies. In order to achieve these goals it is necessary to understand the source of the relevant behaviors, and for this purpose we shall in the next section consider decomposition of models for integrated process systems by means of tools from linear systems theory. 

Fig. 2. Scaled disturbance sensitivity of reactor-separator system. Effect of changes in reaction rate constant on distillate composition A magnitude larger than 1 at a particular frequency...

As stated above, it is usually difficult to predict whether a specific behavior in an integrated plant can be attributed to a single unit, or is caused by process unit interactions. For instance, for the reactor-separator example it is not obvious whether the non-minimum phase behavior is a property of the distillation column alone, or caused by interactions between the column and the reactor. Nor is it clear to what extent the relatively large disturbance sensitivity is due to unit interactions. 

The process sensitivity function for the modified reactor-separator problem is shown in Fig. 6. As seen from the figure, the recycle feedback increases the disturbance sensitivity by a factor 4,2 at low frequencies, while it in fact serves to slightly dampen the disturbance sensitivity, i.e., Sp < 1, at high frequencies. 

The fact that recycling tends to increase the disturbance sensitivity of a plant is well known, see e.g., [2]. However, as seen from the reactor-separator example above, the increase applies only at low frequencies, while the recycling in fact may have the opposite effect at higher frequencies. This can be explained using the well known Bode Sensitivity Integral [16]... 

The process sensitivity function Sp provides a direct measure of how the process unit interactions contribute to the overall disturbance sensitivity of the plant. From (17) it is clear that the effect of the interactions will depend strongly on the dynamic properties of the individual process units that are part of the recycle loop, i.e., on the loop transfer-function... 

If we assume that the loop-gain go(i ) < 1 for all frequencies, which corresponds to assuming that the recycling do not destabilize the process, we get a necessary and suffieient condition for when the process interactions will serve to increase the disturbance sensitivity... 

In summary, the phase-lag properties of the individual process units will play a critical role for the disturbance sensitivity of a plant with recycle flows and this fact should be utilized when designing integrated plants for controllability. 

Reactor-Separator problem revisited. In the previous section we modified the process design with the aim of removing the non-minimum phase behavior. This was achieved by a modification of the reactor design which reduced the individual disturbance sensitivity gR s) of the reactor. Since this disturbance sensitivity also affects the recycle loop gaing22( S)g/f( S)) this also resulted in a significant reduction in the disturbance sensitivity of the overall plant, i.e., from to yo- This can be seen from the lower plot in Fig. 5. At steady-state, a 20%... 

Fig. 8 shows the scaled disturbance sensitivity from Aq to yo after the design modification, and as can be seen we have achieved g = 0.05. Fig. 9 shows the effect of the delay tank on the process sensitivity function Sp. As can be seen, the delay reduces the sensitivity Sp at g from 2 to 0.8, implying that the interactions effectively serves to dampen disturbances at g after the design modification. The resonances that appear at higher frequencies are dampened out by the low-pass properties of the process units, and do therefore not pose a problem as can be seen from Fig. 8. 

Fig. 8. Scaled disturbance sensitivity of reactor-separator process before (dashed) and after (solid) addition of delay tank and mixed buffer. See also Figure 7.

We stress that design for controllability can either aim at reducing control bandwidth limitations, imposed by fundamental process properties, or at reducing the control requirements imposed by disturbance sensitivities. Based on results from linear systems theory we have presented simple model based tools, based on the decomposed models above, which can be used to improve stability, non-minimum phase behavior and disturbance sensitivities in plants with recycle. One important conclusion of the presented results is that the phase-lag properties of the individual process units play a crucial role for the disturbance sensitivity of an integrated plant. In particular, by a careful design of the recycle loop phase lag, it is possible to tailor the effect of process interactions such that they serve to effectively dampen the effect of disturbances in the most critical frequency region, that is, around the bandwidth of the control system. 

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