Unusual Control Structure

In the last three chapters, we have developed a number of conventional control structures dual-composition, single-end with RR, single-end with rellux-to-feed, tray temperature control, and so on. Structures with steam-to-feed ratios have also been demonstrated to reduce transient disturbances. Four out of the six control degrees of freedom (six available valves) are used to control the four variables of throughput, pressure, reflux-drum level, and base level. Throughput is normally controlled by the feed valve. In on-demand control structures, throughput is set by the flow rate of one of the product streams. Pressure is typically controlled by condenser heat removal. Base liquid level is normally controlled by bottoms flow rate. 

The structure of the reflux-drum level loop depends on the reflux ratio. Distillation wisdom recommends that reflux-drum level should be controlled by manipulating reflux flow rate in high RR columns (RR 3). So reflux-drum level is sometimes controlled by manipulating distillate flow rate and sometimes controlled by manipulating reflux flow rate. In the latter structure, the flow rate of the distillate must be varied in some way. It cannot be fixed because this violates the first law of distillation control. 

The flow rate of a product stream cannot be fixed and still control a composition or temperature) in the column because the overall material balance has a dominant effect on compositions. 

Only in a purge column (a very small stream is removed to get rid of an inert component) can a product stream be fixed (or ratioed to feed flow rate). The distillate stream can be manipulated to control a composition (or temperature), or it could be manipulated to maintain a RR. In this structure, the reflux flow rate is measured, the flow signal is send to a multiplier, and the output signal of the multiplier is the set point of a distillate flow controller. 

The other two control degrees of freedoms are typically reboiler duty and reflux flow rate (or distillate flow rate in high RR columns). ReboUer heat input is an effective 

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UNUSUAL CONTROL STRUCTURE 245 Block B1 Temperature profile... 

Proposed Unusual Control Structure. Figure 8.48 shows the control structure proposed for achieving the objective of using the R/F structure in a high reflux ratio column. The details of the individual loops are hsted below... 

In this section, comparisons of the proposed unusual control structure with three more conventional control structures are presented. 

With regard to elastomers of controlled structure, those having unusual distributions of network chain lengths have been of particular interest [88,89]. The most novel elastomer of this type consists of a binary combination of unusually short network chains (molecular weights of a few hundred) and the much longer chains typically associated with elastomeric behavior (molecular weights of ten or twenty thousand). Such a network is sketched in Figure 6. 

Inorganic nanoparticles themselves can be assembled into mesoscopic structures. Dinsmore et al. proposed an approach for the fabrication of solid capsules from colloidal particles with precise control of size, permeability, mechanical strength, and compatibility (Fig. 2.9).44 This unusual mesoscopic structure is called colloidosome and is prepared through emulsion droplets at a water-oil interface. Following the locking together of the particles to form elastic shells, the emulsion droplets were transferred to a fresh continuous-phase fluid identical to that contained inside the droplets. The resultant structures are hollow, elastic shells whose permeability and elasticity can be precisely controlled. 

In order to prepare metastable states or possibly new phases of nano-scale metal particles, low temperature, kinetic growth methods should be used.(4J And atoms should be used, rather than salts or oxides since in the former case the high temperature reduction step can be avoided. In actuality, in recent years we have witnessed the development of several methods for the low temperature kinetically controlled growth of clusters from free atoms. Perhaps the most dramatic development has been the "cluster beam" approach where evaporated metal atoms are allowed to cluster in low temperature gaseous helium or argon streams.(5-2(9) Unusual cluster structures and reactivities have been realized. 

As often happens in chemistry, there are exceptions to this rule. There is a small class of ionic compounds that can allow charges to move through their crystals. The lattices of these compounds have an unusually open structure, so certain ions can move past others, jumping from one site to another. One of these salts, zirconium oxide, is used in a device that controls emissions from the exhaust of automobiles. 

Mechanisms for control of walking Walking is among the most highly automated movements of humans. The bipedal locomotion of humans is an unusually stable structure, although the mechanical structure is far from the favorable. All basic details of normal walking may be found in each and every able-bodied subject. Individual differences between subjects neither depend on the differences in the structure of walking, nor on the assembly of elements encountered, but only in the rhythms and amplitudes of the ratios between these elements. 

This last loop is the unusual element in the control structure. The tuning of most conventional level loops is simple because we use a proportional controller with a gain of 2. The level loop in the proposed stmcture is not conventional. In this application, we want fairly tight level control because the distillate/temperature loop depends on the vapor/level loop. In addition, the dynamics of the vapor/level loop contain some dynamic lags because the pressure loop is involved, that is, increasing reboiler heat input increases pressure, and the pressure controller increases condenser heat removal, which affects reflux-drum level. 

These clusters probably arise from closing of shells and subshells of the MacKay icosohedra indeed the ammonia adsorption data support this idea. However, for clusters with a more open structure, more than the predicted amount of ammonia was adsorbed. Evidence for such open shell structures was found more prevalently for Fe than Ni . It seems that t/-electron-rich metal atom clusters more readily form close-packed structures (Ni) but r/-electron-poor clusters growth is more controlled by kinetic factors, and many unusual, metastable structures appear to be possible even at as high a room temperature. 

Figure 11.18 gives temperature and composition profiles for both columns. The shape of the temperature profile in the extractive column is quite unusual. It is not obvious what stage to use for temperature control. We return to this issue in the development of a control structure for this system later in this chapter. 

The phenomenon of leaf and fruit drop is known as abscission and has been suspected for some time to be under hormonal control. Dr. Addicott s description of the structure and properties of Abscisin II, the abscission hormone isolated from cotton, climaxes over 12 years of his own investigations on the subject and represents perhaps the greatest advance in plant hormone research in the past decade. It also provides an unusual and unsuspected structural model to guide... 

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