Distillation columns pressure response, control

The flowsheet is completed with P-type controllers for level in the reflux drum and bottoms, and PI controller for pressure. These controllers ensure the basic Inventory control, but are not sufficient for quality control. Therefore, we are interested by distillate flow rate and purity faced with disturbances in the feed. Fig. 4.7 presents the open loop response to feed variation of +/- 10%. Increasing the feed to 110 kmol/hr gives an increase in purity over 99%, but a decrease of the distillate rate to less than 47.5 kmol/hr. After reset to initial conditions, the feed is reduced to 90 kmol/h. This time the distillate rate increases at 52.5 kmol/hr, but the purity drops dramatically to 86%. This behaviour seems somewhat strange, so the reader is encouraged to find a physical explanation. The need for quality control in a distillation column is obvious. This issue will be treated in the Examplel2.2. 

It is often said that derivative action should only be used in temperature controllers. It is true that temperatures, such as those on the outlet of fired heater and on distillation column trays, will often exhibit significantly more deadtime than measurements such as flow, level and pressure. However this is not universally the case, as illustrated in Figure 3.7. Manipulating the bypass of the stream on which we wish to install a temperature controller, in this case around the tube side of the exchanger, will provide an almost immediate response. Indeed, if accurate control of temperature is a priority, this would be preferred to the alternative configuration of bypassing the shell side. 

The phenomenon of overshoot or inverse response results from the zero in the above example and will not occur for an overdamped second-order transfer function containing two poles but no zero. These features arise from competing dynamic effects that operate on two different time scales (ti and T2 in Example 6.2). For example, an inverse response can occur in a distillation column when the steam pressure to the reboiler is suddenly changed. An increase in steam pressure ultimately will decrease the reboiler level (in the absence of level control) by boihng off more of the liquid. However, the initial effect usually is to increase the amount of frothing on the trays immediately above the reboiler, causing a rapid spillover of liquid from these trays into the reboiler below. This initial increase in reboiler liquid level, is later overwhelmed by a decrease due to the increased vapor boil-up. See Buckley et al. (1985) for a detailed analysis of this phenomenon. 

The vapor pressxire controller has a far greater sensitivity to composition changes than a temperature controller (68, 332, 362), gives fast response, provides an accurate measurement in binary systems, and is relatively inexpensive. This technique is popular in services where pressure compensation of the control temperature is needed and is difficult to achieve, such as low-pressure (particularly vacuum) distillation and close separations. A typical example is ethanol-water columns (361). The technique is also useful where a good sensitivity of control temperature to composition and good correlation between product composition and control temperature are difficult to meet simultaneously (Sec. 18.2). 

Figure 6.13 shows the response of the system to a 5% step increase in the setpoint of the feed flow controller at time equals 0.2 h. The solid lines are for the case with the control structure discussed above and shown in Figure 6.12. Both product flowrates (Bj and B2) and the distillate flowrates (Dj and D2) increase. The temperature on Stage 14 in the low-pressure column drops quickly from 79°C to about 73°C, but the controller eventually...

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