Distillation feedback control

Spreadsheet Applications. The types of appHcations handled with spreadsheets are a microcosm of the types of problems and situations handled with fuU-blown appHcation programs that are mn on microcomputers, minis, and mainframes and include engineering computations, process simulation, equipment design and rating, process optimization, reactor kinetics—design, cost estimation, feedback control, data analysis, and unsteady-state simulation (eg, batch distillation optimization). 

The use of high or low limits for process variables is another type of selective control, called an override. The feature of anti-reset windup in feedback controllers is a type of override. Another example is a distillation column with lower and upper limits on the heat input to the column reboiler. The minimum level ensures that liquid will remain... 

J.G. Segovia-Hernandez, S. Hernandez, V. Rico-Ramfrez, and A. Jimenez. A comparison of the feedback control behavior between thermally coupled and conventional distillation schemes. Comput. Chem. Eng., 28 811-819, 2004. 

Fig. 7.70. Combined feed-forward/feedback control of the overhead product stream of a distillation column. Feed composition is the only significant load variable...

Toijala (Waller), K. V. and Fagervik, K. C., "A Digital Simulation Study of Two Point Feedback Control of Distillation Columns," Kern. Teollisuus, 1972, 29, 5. 

Figure 5.9b A feedback control module, Nl a graphical representation of a subroutine that adjusts equipment parameters to force a desired value of a stream variable. In the figure the feedback control block compares the stream variable value with the specification. When the convergence tolerance is satisfied, the next unit in the calculation order is computed. Otherwise, the control block adjusts the reflux ratio so as to achieve a specified distillate mole fraction.

A pilot-scale distillation column located at the University of Sydney, Australia is used as the case study [60]. The 12-tray distillation column separates a 36% mixture of ethanol and water. The following process variables are monitored temperatures at trays 12, 10, 8, 6, 4, and the reflux stream, bottom and top levels (condenser), and the flow rates of bottoms, feed, steam, distillate and reflux streams. The column is operated at atmospheric pressure using feedback control. Three variables are controlled during the operation top product temperature, condenser level, and bottom level. Temperature at tray 8 is considered as the inferential variable for top product composition. To maintain a desired product composition, PI controllers cascaded on flow were used to manipulate the reflux, top product and bottom product streams. 

It is clear that our first reaction is to use a composition analyzer to measure the concentration of pentane in the distillate and then using feedback control to manipulate the reflux ratio, so that we can keep the distillate 95% in pentane. This control scheme is shown in Figure 2.2a. An alternative control system is to use a composition analyzer to monitor the concentration of pentane in the feed. Then in a feedforward arrangement we can change the reflux ratio to achieve our objective. This control... 

Figure 2.2 Three different systems for the distillate composition control of a simple distillation column (a) feedback (b) feedforward (c) inferential.

Luyben, W. L., "Feedback Control of Distillation Columns by Double Differential Temperature Control, Ind. Eng. Chem. Fund. 8(4), 1969, p. 739. 

Chapters 2, 3, and 4 deal with the distillation variables, and Chapter 5 covers distillation process control strategies. Chapter 6 describes some of the constraints on distillation variables and separation capabilities. Chapter 7 introduces the concepts that are critical to product quality and the measurements that evaluate performance criteria such as frequency of failure. Chapter 8 describes the concepts and nomenclature that are fundamental to PID control loops. Chapter 9 covers the concepts of tuning process controllers when they are operating in automatic output mode. Chapter 10 is about measuring the response of process variables when the controller is in manual output mode, that is, with no feedback from the process variable. 

The strategy of distillation control is open to many creative approaches because there are five or six main variables to manipulate and many possibilities for cascade, feed-forward ratios, and model-based computer control as well as conventional feedback control. 

Input variables are not state variables they are external to the system, but they affect the system or, in other words, work on the system. For example, the feed temperature and composition of the feed stream to a distillation tower or a chemical reactor or the feed temperature to a heat exchanger are the input variables. They affect the state of the system, but are not affected by the state of the system (except when there is a feedback control, and in this case, we distinguish between control variables and disturbances or input variables). 

The implementation of the output feedback control for distillation column can be configured such that only the liquid composition of the output flowrate is regulated (i.e., uncoupled one-point configuration control see second example by Alvarez-Ramirez et al., 1998). In such a configuration, the liquid compositions for the main product streams A, B and C (see Figure 1) were taken as the controlled variables whereas, respectively. 

Prior to extrusion the fused silica preform is usually treated with dilute hydrofluoric acid to remove any imperfections and deformations present on the inner and outer surfaces and then rinsed with distilled water and followed by annealing (55). In a cleanroom atmosphere, the preform is vertically drawn through a furnace maintained at approximately 2000°C. Guidance and careful control of the drawing process is achieved by focusing an infrared laser beam down the middle of the capillary in conjunction with feedback control electronic circuitry in order to maintain nniformity in the specifications of the inner and outer diameters in the final prodnct. 

Feedback from distillate composition is introduced through a divider in order to better compensate for the inverse relationship between distillate flow and composition, although exact correction is unnecessary. The principal factor is the maintenance of a D/F ratio, which in this case is adjusted as necessary by the output of the feedback controller. 

The second forward loop from feed composition to distillate flow is much more costly and less valuable. Feed composition cannot- change instantaneously without supply sources being switched. If feed stock comes from a single source, whether a tank or another processing unit, its composition can only change at a limited rate. It is entirely possible, in many columns, that the time response of the feed source is slower than that of the product-quality feedback loop. In these cases, feedback control is quite effective in coping with variations in feed composition. 

Feed-forward control takes quick action on any disturbance while feedback ensures that any persisting or increasing offset in the level is taken care of. Feed-forward control is often used in case of slow changing processes where the state variable reacts slowly on a distiubance. This happens, for example, in distillation columns with many trays, which are subject to large disturbances (for example feed changes from furnaces). Most process units, however, can be well controlled by using feedback control only. 

Interaction could exist in case both product qualities have to be controlled. Interaction does not pose a problem, if the product quality of only one component is controlled. If both quality control loops are configured as single loop feedback controllers, the manipulation of the one control loop will affect the response of the other control loop and vice versa. For example, manipulation of the reboiler load to control the bottom product quality, will affect the top product quahty. This interaction between the single loop controllers can lead to a considerable degradation in performance of the control system for the distillation column. Minimal interaction is required. 

Use of the Smith and other predictors recently has been shown by Seborg et al. to provide substantially better feedback control of distillation-column terminal compositions than do PI and PID controllers. Further work needs to be done to compare their performance with that achieved by PI or PID controllers with feedforward compensation. 

Maximum column capacity is increased since manipulated variables— reflux or distillate, boilup or bottom product—are not changed as much as if unaided feedback controllers were used. Similarly, the column is less likely to weep or dump. 

Lost composition control schemes for a binary column involve manipulating either reflux or distillate to control top composition, and either boilup or bottom produrt to control bottom composition. Changes in either of the two top manipulative variables will affect not only top composition, but bottom composition as well. Correspondingly, changes in either bottom-product flow or boilup will affect both bottom and top compositions. As a minimum, therefore, for feedback control purposes we are interested in two composition gains at each end of the column. 

ODlumn gains for feedback control. Here, for example, we need to know the change in top composition due to a change in distillate or reflux flow with other variables held constant. 

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