Delayed Strategies

The affected industries and the Chamber of Commerce returned to the familiar divert and delay strategy, challenging all of the regulations in appeals that were consolidated in the D.C. Circuit. Conservative think tanks argued that the regulations would force millions of farmers and other small emitters to obtain permits from EPA, force U.S. companies to relocate overseas, and have other devastating impacts on the nation s fragile economy. ... 

The Smith predictor is a model-based control strategy that involves a more complicated block diagram than that for a conventional feedback controller, although a PID controller is still central to the control strategy (see Fig. 8-37). The key concept is based on better coordination of the timing of manipulated variable action. The loop configuration takes into account the facd that the current controlled variable measurement is not a result of the current manipulated variable action, but the value taken 0 time units earlier. Time-delay compensation can yield excellent performance however, if the process model parameters change (especially the time delay), the Smith predictor performance will deteriorate and is not recommended unless other precautions are taken. 

Much of the experience and data from wastewater treatment has been gained from municipal treatment plants. Industrial liquid wastes are similar to wastewater but differ in significant ways. Thus, typical design parameters and standards developed for municipal wastewater operations must not be blindly utilized for industrial wastewater. It is best to run laboratory and small pilot tests with the specific industrial wastewater as part of the design process. It is most important to understand the temporal variations in industrial wastewater strength, flow, and waste components and their effect on the performance of various treatment processes. Industry personnel in an effort to reduce cost often neglect laboratory and pilot studies and depend on waste characteristics from similar plants. This strategy often results in failure, delay, and increased costs. Careful studies on the actual waste at a plant site cannot be overemphasized. 

STRATEGY We expect a positive entropy change because the thermal disorder in a system increases as the temperature is raised. We use Eq. 2, with the heat capacity at constant volume, Cv = nCV m. Find the amount (in moles) of gas molecules by using the ideal gas law, PV = nRT, and the initial conditions remember to express temperature in kelvins. Because the data are liters and kilopascals, use R expressed in those units. As always, avoid rounding errors by delaying the numerical calculation to the last possible stage. 

Figure 24.4 The decision-analytic model shows the three strategies that were examined by Arnold and researchers [22] to evaluate the financial implications of the direct thrombin inhibitor argatroban for early treatment (<48 hours after thrombocytopenia onset), compared with delayed treatment, of heparin-induced thrombocytopenia (HIT) with or without thrombosis.

The major disadvantages to endovascular strategies include the complexity of the procedure, the level of required technical expertise, delays in initiating endovascular treatment, and the additional risks and expense of an invasive procedure compared with IV rt-PA. 

Early detection and aggressive modification of risk factors is one of the primary strategies for delaying ischemic heart disease progression and preventing ischemic heart disease-related events including death. 

Sprague GL, Castles TR, Bickford AA. 1984. Assessment of the delayed neurotoxic potential of isopropyl triphenylphosphate using a nontraditional testing strategy. Toxicol Environ Health 14 773-788. 

The strategy depends on the situation and how we measure the concentration. If we can rely on pH or absorbance (UV, visible, or Infrared spectrometer), the sensor response time can be reasonably fast, and we can make our decision based on the actual process dynamics. Most likely we would be thinking along the lines of PI or PID controllers. If we can only use gas chromatography (GC) or other slow analytical methods to measure concentration, we must consider discrete data sampling control. Indeed, prevalent time delay makes chemical process control unique and, in a sense, more difficult than many mechanical or electrical systems. 

---