Single-Temperature Control Strategy

CSS Using Inventory Strategy 2 in the previous subsection, with temperamre at Stage 6 controlled by manipulating aqueous reflux flow, while the other two manipulated variables, reboiler duty and entrainer makeup, are ratioed to the feed flowrate. 

ACETIC ACID-WATER (ISOBUTYL ACETATE AS THE ENTRAINER) 

Compared to the more complex two temperature control structure CS2QA, the singletemperature control structure CS3 performs very satisfactorily. Both product compositions are maintained at light specihcalions even though the temperature control point is far away from the column bottom. However, control structure CS4 is not able to keep the acetic acid loss out of the column top close to the desired level. 

3 INDUSTRIAL COLUMN WITH PRELIMINARY DYNAMIC SIMULATIONS 

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One example of a single variable control strategy is applied to a stripper in a hydrocracking unit. The main purpose of the stripper is to remove H2S and noncondensable components from the bottom product. One of the key indicators identified is the water dew point at the top of the stripper column. As a matter of fact, the dew point is a function of column overhead vapor composition and the amount of water. There was no monitoring capability available for the dew point temperature. If the column top temperature is lower than the dew point, the hydrogen sulfide will dissolve in the condensed water and cause corrosion to the column overhead system. 

The idea of the simpler single-temperamre loop-control strategy is to use an aqueous reflux flowrate to hold some tray temperature inside the column and to ratio the other two manipulated variables (reboiler duty and entrainer makeup) to the feed flowrate. From the earlier sensitivity analysis in Figure 9.9a, the most sensitive control point inside the column is Stage 6, which is close to the top of the column. Since the main acetic acid product is drawn out from the bottom of the column, an alternative control point is to select the second most sensitive control point, which is closer to the bottom of the column. This alternative control point will be Stage 16. Thus two single-loop-control stmctuies will be evaluated using closed-loop dynamic tests. They are ... 

Due to the above-mentioned difficulties with manual and temperature-based feedback controllers, the control systems based on product moisture content have been investigated and applied in many graindrying installations in recent years. Forbes et al. compared different control strategies based on the product moisture content for commercial corn-drying units [49]. Three control schemes were studied (1) a model-based feedforward controller for which the corn-drying process is represented by an exponential-decay-type model with the corn-drying characteristics lumped into a single parameter (2) a feedback... 

In general, as extruded products have become more complex with more demanding polymers, multilayers, and overall thinner walls, control has become much more important. Each type of extrusion process has its own requirements, but there are several common control strategies. With extruders the single most important objective is to deliver a steady flow of material to the die. Temperature control and a consistent pressure are critical to avoid surging which results in a varying product and often reduced output. Therefore, most extruders will have ... 

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