Control of impurity levels

The concentration of impurities (present in the feed) in the process evolves over a very slow horizon (days or, possibly, weeks). Moreover, the presence of impurities in the feed stream, together with significant material recycling, can lead to the accumulation of impurities in the recycle loop, with detrimental effects on the operation of the process and on its economics (Baldea et al. 2006). Therefore, as was shown in Chapter 4, the control of the impurity levels in the process is an important operational objective, and, according to the analysis presented above, it should be addressed in the slow time scale, using the flow rate of the purge stream, up, as a manipulated input. 

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Finally, we analyzed the control implications of the presence of impurities in a process, concluding that the control of impurity levels must be addressed over an extended time horizon using the flow rate of the purge stream as a manipulated input. To close the impurity-levels loop, one should resort either to an appropriately tuned linear controller (e.g., a PI controller with long reset time) or to a (nonlinear) model-based controller that uses (an inverse of) the reduced-order model of the slow dynamics - as developed in this chapter - to compute the necessary control action. 

Alloying additions are made to improve the performance of an anode material. Of equal importance is the control of the levels of impurity in the final anode, since impurities (notably iron and copper) can adversely affect anode performance. Thus careful quality control of the raw materials used and the manufacturing process adopted is essential to sound anode production. This too is discussed below. 

In case of reduced availability of an impurity a possible approach is to prepare a spiked sample , i.e. a known amount of impurity is added to the CRS and may serve in a system suitability test as well as for the control of the level of this impurity. An example is given in the monograph for chlorprothixene hydrochloride (Monograph 0815 1999) where the content of the E-isomer is controlled to a level of not more than 2 per cent, Figure 5.5. 

The quality of the EDC sent to pyrolysis must fulfil strict purity specifications, but too low an impurity level implies high energy consumption. The concentrations of T and I2 in the bottom of S2 must not exceed 100 and 600 ppm, respectively, while the concentration of I3 must be kept around its optimal value at 2000 ppm. It is worth mentioning that these contradictory requirements cannot be fulfilled by any standalone design of S2. The effective control of impurities becomes possible only by exploiting the positive-feedback effects of the recycle loops that are balanced by the negative-feedback effects of chemical conversion and exit streams. 

When the accumnlation of impurities can be monitored and maintenance staff can be alerted of an impending failure in a GIS, damage to the overall substation may be prevented by controlled shutdown of turbines. Changes in the concentration of degradation products formed in SFg provided such a measure of impurity levels... 

Some of the hydrocydone overflow is purged to balance the impurities, that is, to set the impurity concentration in the process to a constant level. This is executed by collecting the solution in an intermediate tank that is divided into two parts. One compartment is entirely filled and overflows into the other part, from where the solution is returned to the receiver tank B. From the filled compartment, the purge liquor is taken off back to the solar ponds, controlling the impurity level in the crystallizer stage 4. 

The control objective is to maintain the purity of the retentate ethanol product stream leaving the last stage of the pervaporation unit. The desired purity is 99.42 mol% ethanol (99.77 wt%). Process control wisdom says that it is more effective to control the impurity level than the purity level in a high-purity stream. Therefore, we choose to control the water impurity at 0.58 mol% water. A 5 min deadtime is assumed for the composition measurement... 

However, the development of secondary Mg alloys requires a completely different concept compared with standard alloys, which obtain their corrosion resistance by reducing the levels of impurities below certain alloy-specific and process-dependent limits. Although control of impurities is technically possible, this would make the secondary alloys too expensive. A cheaper... 

V-alloy development efforts have been made for apphcation to fusion reactors in the condition of maintaining the low activation properties. These requirements involved limitation of the alloy element selection and necessity of precise control of impurities to ppm level which can have a long-life radioactivity, such as Mb, Mo,... 

Chemical Conversion. Except for control of nitrogen impurity levels, the same chemical conversion methods used for nitrogen purification at low flow rates can also be used for argon purification. Although used less commonly for argon purification than for nitrogen purification, these chemical conversion methods are appHed in point-of-use purifiers located close to where the gas is consumed. 

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