Sidestream Drawoff Controls

In many multicomponent distillations, a single column yields three or more product streams (Fig. 19.6). Besides the top and bottom products, each additional product is usually withdrawn as a side stream from an intermediate location in the column. As a general rule of thumb, a side product from the rectifying section is taken as a liquid, in order to 

Side products complicate column control. Each side product adds two control variables (side product lights and heavies concentrations), but only one manipulated stream (side product flow) to the column (Sec. 16.2). Only one of these two compositions can be controlled by the additional manipulated stream the other is either allowed to vary, or rides on the nearest end-product composition. Alternatively, both lights and heavies content of the side product can be controlled, at the expense of letting the composition of one of the end products vary. Side drawoffs also escalate the potential for interaction among control loops. 

For these reasons, it is often difficult to devise a satisfactory MB control system for the column. Frequently, one or more of the products is flow-controlled (or kept at a fixed ratio to the feed or to another product), and the flow (or ratio) is manually adjusted by the operator to achieve the desired purity. 

Control scheme selection for side-drawoff columns is governed by the relative quantity and economic value of each product stream. Generalization is difficult and will not be attempted here. Some common considerations are described below further discussion is available elsewhere (64, 68, 260, 263, 287, 332, 362). Luyben s articles (260, 263) are particularly illuminating, and were extensively used as a basis for the following discussion. 

In many side-drawoff columns, the side draw is very small compared to the other product streams. This is a common situation when the side stream serves to remove an intermediate impurity (Sec. 13.7). In many other side-drawoff columns, distillate is withdrawn as a side product a few trays below the top of the column, leaving an upper pasteurizing section for separating light ends as a small vent stream. Common examples are an ethylene plant Cg splitter, where small quantities of methane are pasteurized out of the ethylene product, and an alcohol still, where heads are pasteurized out of the alcohol product. The reverse situation has the bottom product drawn as a vapor side product a few trays above the bottom, leaving a small heavy end stream to exit from the bottom. 

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Similar problems can occur with vapor sidestreams, but the solution is not as easy because we cannot provide vapor holdup in the system. One approach is to use an internal vapor controller. The flowrate of the vapor sidestream and the flowrate of the steam to the reboiler are measured. The net flowrate of vapor up the column above the vapor sidestream drawoff tray is calculated. This flow is then controlled by manipulating the vapor sidestream drawoff rate. 

Suppose, however, that the customer insists on minimum application of feedback controls with no feedforward compensation or overrides. V at column design philosophy should be followed Ha Tng had considerable adverse experience with columns with primitive controls, particularly sidestream drawoff columns, and columns with heat recovery schemes, we suggest the following ... 

Basic control scheme for column with sidestream drawoff... 

To gain one more degree of fr dom for composition control, Doukas and Luyberr came up with the idea of changing the location of the sidestream drawoff tray. 

The concentration of the lightest component in the sidestream product is controlled by the location of the sidestream drawoff tray. As shown in Figure 7.6B, this easily can be implemented by conventional analog hardware. The fixed-gain relays are calibrated so that only two of the drawoff valves can be partially open simultaneously. Thus as the signal from the XSl composition controller increases, the valves for trays higher in the column open as valves for trays lower in the column close. 

The concentration of the heaviest component in the sidestream product is controlled by the sidestream drawoff rate. 

A) In the control system finally chosen, the toluene impurity content in the distiilate product is controiied by the reflux ratio. (B) The five alternative sidestream tray positions and their controls, which regulate the benzene and Iene Impurities in the sidestream drawoff, are shown in this blowup. 

Figure 7.7 has been designated by Doukas and Luyben as the L scheme. Sidestream drawoff location is used to control the concentration of the lightest component in the sidestream product. The D scheme of Figure 7.8 provides manipulation of the distillate product flow from the prefractionator to control sidestream composition. 

It is impractical to present here all possible schemes for controlling sidestream drawoff columns. A number of these are discussed briefly in a paper by Luyben. ... 

For columns with a sidestream liquid drawoff, it is necessary to maintain a minimum liquid flow down the column below the drawoff point. As shown by Figure 9.24, this is accomplished by subtracting the sidestream drawoff flow from the estimated internal reflux to calculate the net liquid flow down the column. If this flow is insufficient, the override controller pinches back on the drawoff valve tmtil the downflow becomes adequate. 

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