Distillation columns bottom level response

Recognize the response pattern of an integrating process variable, such as distillation column bottom level or reflux drum level, when a step change in setpoint is introduced to the controller with a proportional gain of 1.25 and an integral time constant of 30 min... 

The manipulated bottoms flow rate scheme is not used very frequently because of problems with the column base level control using steam. When a thermosiphon reboller Is used and the steam flow Is Increased, there Is usually a reversal in the column base level response. The column level first rises and then falls. The usual case for using this scheme is when the feed concentration is 95% light key or more. In other words, most of the feed is distilled overhead from a few percent heavies or tars. The MRT point is usually in the reboiler. 

The magnitudes of various flowrates also come into consideration. For example, temperature (or bottoms product purity) in a distillation column is typically controlled by manipulating steam flow to the reboiler (column boilup) and base level is controlled with bottoms product flowrate. However, in columns with a large boilup ratio and small bottoms flowrate, these loops should be reversed because boilup has a larger effect on base level than bottoms flow (Richardson rule). However, inverse response problems in some columns may occur when base level is controlled by heat input. High reflux ratios at the top of a column require similar analysis in selecting reflux or distillate to control overhead product purity. 

Trying to control the liquid level at the bottom of the column with the reflux flow or distillate flow rate involves very long time responses because the action of the manipulated variable must travel the whole length of the distillation column before it is felt by the controlled variable. Therefore, control-loop configurations 4, 5,10,12,14,16,17,18, 20, 21, 23, and 24 are ruled out. 

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. 

An important example of a physical process that shows inverse response is the base of a distillation column with the reaction of bottoms composition and base level to a change in vapor boilup. In a binary distillation column, we know that an increase in vapor boilup V must drive more low-boiling material up the column and therefore decrease the mole If action of light component in the bottoms xg. However, the tray hydraulics can produce some unexpected results. When the vapor rate through a tray is increased, it tends to (1) back up more liquid in the downcomer to overcome the increase in pressure drop through the tray and (2) reduce the density of the liquid and vapor Ifoth on the active part of the tray. The first effect momentarily reduces the liquid flow rates through the column while the liquid holdup in the downcomer is... 

Figure 16.5 Open-loop response of bottom level to an 8 percent steam rate increase, featuring inverse response. (From "Inverse Response in Distillation Column," P. S. Buckley, RJC. Cox, and Dh. Rollins, Chemical Engineering Progress, vol. 71, no. 6, p. 83 (June 1975). Reproduced by permission of the American Institute of Chemical Engineers)...

The process dynamics of a few process variables can be characterized as an integrating process response. One example is the response of the liquid level in the bottom of a distillation column when liquid is being pumped out with a centrifugal pump and the liquid flow rate is restricted by an automatic valve after the pump. The column base liquid level may be steady with an initial automatic valve position, and opening the valve to a new position will increase the liquid flow rate. The difference between the two flow rates will be integrated over time as seen by the response in liquid level going down. Eventually, the column base liquid level will go to zero... 

For most distillation columns, the bottom level is controlled by the bottom draw-off. Sometimes, the bottom draw-off is used for quality control. In that case, the bottom level can be controlled, by adjusting the load of the reboiler. Problems can originate when there is an inverse response. This happens when X > 0.5. 

Consider Figure 19.2 where top-product flow is set by flow control, reflux flow is set by condensate receiver level control, boilup is fixed by flow control of steam or other heating medium, and bottom-product flow is determined by column-base level control. As shown by the dotted line, we wish eventually to control column top composition by manipulating distillate flow. Let us assume that feed rate, feed composition, feed enthalpy, and boilup are fixed and that we wish to find the changes (i.e., gains ) of top and bottom compositions in response to a change in D, the top-product rate. 

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