Single-end control

In single-end control structures, only one composition or one temperature is controlled. The remaining control degree of freedom is selected to provide the least amount of product quality variability. For example, a constant reflux ratio RR can be maintained or the reflux-to-feed ratio R/F can be fixed. The control engineer must find out whether this more simple approach will provide effective control of the compositions of both product streams. One approach to this problem is to use steady-state simulations to see how much the reflux ratio and the reflux flow rate must change to maintain the specified impurity levels in both product streams (heavy-key impurity in the distillate X/ (hk) and light-key impurity in the bottoms Xb(lk)) when changes in feed composition occur. The procedure is call feed composition sensitivity analysis.  

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R-V Reflux flow controls distillate composition. Heat input controls bottoms composition. By default, the inventory controls use distillate flowrate to hold reflux drum level and bottoms flowrate to control base level. This control structure (in its single-end control version) is probably the most widely used. The liquid and vapor flowrates in the column are what really affect product compositions, so direct manipulation of these variables makes sense. One of the strengths of this system is that it usually handles feed composition changes quite well. It also permits the two products to be sent to downstream processes on proportional-only level control so that plantwide flow smoothing can be achieved. 

In addition, it is often possible to achieve very effective control without using direct composition measurements and without controlling both products. Single-end control stmctures are widely used because of their simplicity and effectiveness. 

If there are significant changes in both of these ratios, single-end control will probably be ineffective. Because the flow ratios have to change, the control structure must be capable of changing both manipulated variables (reflux and reboUer duty). This implies that two-end control is required. The structure could control two compositions, two temperatures or one composition and one temperature. This decision depends on the shape of the temperamre profile, which we explore in Section 6.3. 

If there are only small changes in one of the ratios, a single-end control structure with this ratio fixed may provide effective disturbance rejection in the face of both feed composition and feed flow rate disturbances. 

Table 6.1 gives results of the feed sensitivity analysis. They clearly show that in this system the required changes in reflux are very small. Therefore, a single-end control structure with a reflux-to-feed ratio has a good chance of providing effective control of both product purities. 

The job of the column is to achieve the desired product purities in the face of disturbances. Recall that the design specifications are 1 mol% zC4 in the distillate and 0.5 mol% C3 in the bottoms. We are using single-end control with a temperature on a tray in the column controlled by manipulating reboiler heat input. The other degree of freedom is held constant and depends on the control structure. In CSl, it is the reflux-to-feed ratio, and in CS3, it is the RR. In control structure CS2, the other degree of freedom is a fixed condenser heat removal. 

There are a host of conventional control structures, and the best choice depends on a number of factors, some of which we have already discussed the shape of the temperature profile and the sensitivity of the several flow ratios to feed composition. Economics obviously have an impact, mostly in terms of energy consumption versus control structure complexity. The control structure that minimizes energy is dual-composition control. But it is more complex than a simple single-end control structure. Therefore, we must evaluate how much money is lost by using a more simple structure. In areas of the world where energy is inexpensive, both the optimum design and the appropriate control structure are different than in areas with expensive energy. 

An unconventional control structure has been shown to provide effective control in the situation where reflux ratio is large and a reflux-to-feed ratio is preferred in a single-end control structure. The reflux-drum level is controlled by reboiler heat input. The condenser heat-removal loop controlling pressure must be on automatic for this structure to work because reflux-drum level is not directly affected by reboiler heat input. 

There are two remaining control degrees of freedom. The ideal dual-composition control structure would control C5 impurity in the distillate by manipulating reflux flow rate and C4 impurity in the bottoms by manipulating reboiler duty. However, this ideal control structure is the exception not the rule in industrial applications. We usually try to find a more simple control structure in which a single-end control scheme provides adequate regulatory control using a suitable tray temperature. 

Then the feed composition is changed to 0.6 mole fraction nC4 and 0.4 mole fraction C5 with the product specification held constant. The required reflux ratio and reflux-to-feed ratio are 1.075 and 0.6471, respectively. Next the feed composition is changed to 0.4 mole fraction C4 and 0.6 mole fraction nC5 with the product specification held constant. The required reflux ratio and reflux-to-feed ratio change to 1.665 and 0.6626, respectively. The third and flfth columns in Table 16.1 show the percent changes in the two variables from the design values. Since the changes in the reflux-to-feed ratio are quite small, a single-end control structure may be able to handle feed composition disturbances fairly well. 

Small loads are commonly processed in a box furnace. The product is placed on the furnace hearth through a door. Box furnaces may be single-ended or double-ended. A single-ended box furnace is usually used in an air atmosphere appHcation where the product can be removed hot from the furnace for cooling. A double-ended box furnace is usually used in a controlled atmosphere appHcation. In this case a water cooler is attached to one end. The product can be placed on the hearth (in the heat chamber) through the front door, then after the product reaches temperature, it is manually transferred into the water cooler for cooling before it is manually removed out the exit door on the other end of the water cooler. 

Selecting the SMPS controller IC. The important factors within this application that affect the choice of switching power supply controller IC are MOSFET driver needed (totem-pole driver), single-ended output, 50 percent duty cycle limit desired, and current-mode control desired. The popular industry choice that meets these needs is the UC3845B. 

The sample environment was filled with He gas to prevent the argon X-ray emission from air. Beam scanning, data acquisition, evaluation and the generation of elemental maps were controlled by a computer. Micro-PIXE measurements were performed with a scanning 2.5MeVH+ microbeam accelerated by the 3 MV single-end accelerator. The beam diameter was 1-2 pm, so that individual particles could be analysed. The beam current was < 100 pA and the irradiation time was about 3(M0 min. 

Figure 10.11. Single mistake during a chain-end controlled isotactic polymerization and schematic 13C NMR spectrum of the methyl region (assuming that chain continues in m mode)...

The schematic of the temperature control loop and the geometric mean circuitry of the single-ended mixed-signal architecture is shown in Fig. 5.7. 

Fig. 5.10. Performance of the digital PID temperatirre controller of the single-ended mixed-signal architecture in the stabilization mode...

The singled-ended analog architecture comprises a temperature sensor on the bulk chip, three single-ended analog proportional microhotplate temperature controllers... 

A micrograph of the single-ended hotplate-based microsystem is shown in Fig. 6.2 and features a die size of 5.0 x 2.9 mm. This system is a minimal implementation of a temperature-controlled microhotplate system. Temperature modulation is facilitated by an direct access to the input voltage A modulation of the input voltage is translated into a modulation of the microhotplate temperature. Another interesting application of the system includes its use as a microcalorimeter or as a material research platform [145]. The schematic of the temperature-control loop is shown in Fig. 6.3. 

Fig. 6.3. Temperature-control loop of the single-ended analog architecture...

The dominant pole of this temperature control system is determined by the thermal time constant of the microhotplate, which is approximately 20 ms. The open-loop gain of the single-ended analog architecture (Aol seaa) i given by Eq. (6.1) ... 

The performance of the single-ended analog proportional temperature controller in the tracking mode is shown in Fig. 6.4. The measurement was done at room temperature, and the control voltage of microhotplate 1 was increased in steps of 100 mV. For example, a control voltage of 1.60 V produced a microhotplate temperature of approximately 355 °C. Microhotplate 2 was kept at a constant temperature of 200 °C and microhotplate 3 was kept at 350 °C. 

Fig. 6.4. Performance of the single-ended analog proportional temperatm-e controller in the tracking mode...

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