Temperature Control with Reflux Flow Rate

3 Temperature Control with Reflux Flow Rate 

In the reflux scheme, a column temperature controller manipulates a control valve in the reflux line. The reflux drum level controller manipulates a valve in the distillate line. The column base level controller manipulates a valve in the bottoms line. The feed and reboiler steam are each on flow rate control. In some cases, there is a controller for the pressure drop across the trays that manipulates the valve in the reboiler steam line. However, it is preferred to use a steam flow rate controller and simply monitor the tower pressure drop. With this scheme the separation power base is derived from the ratio of steam/feed. The distillate/feed material balance split is maintained by the MRT point controller. 

The reflux scheme can be difficult to start up because initially there may not be enough light components accumulated in the reflux drum, and the column temperature may be too hot so, the controller may want more reflux flow than is available. This can pump the reflux drum level down until the distillate flow stops and then proceed to pump the reflux drum empty. However, this situation can be handled with computer control by using a low-level constraint control that will constrain the reflux flow rate to maintain a low-level constraint setpoint until the column temperature is low enough, so the temperature controller calls for less reflux. This reflux scheme is recommended when the reflux/distillate ratio is less than 

The distillation column can be run very close to flooding because the boilup and vapor traffic can be run steady at maximum rate. This manipulated reflux scheme can also handle environmental disturbances well such as day and night temperature variations and rain storms. 

Another variation on the manipulated reflux scheme is to use a setpoint for the steam/feed ratio to establish the separation power base. Generally, the feed flow rate signal should be lagged with an 8 to 20 min capacitance lag (filter), so the steam flow is proportional to a trailing average of the feed rate. Sometimes, this falls into the category of model-based predictive control because the McCabe-Thiele model or a computer simulation model shows that the separation power base can be established by the steam/feed ratio as shown in Table 3.3. 

---

The bypassed vapor heats up the liquid there, thereby causing the pressure to rise. WTien the bypass is closed, the pressure falls. Sufficient heat transfer surface is provided to subcool the condensate, (f) Vapor bypass between the condenser and the accumulator, with the condenser near ground level for the ease of maintenance When the pressure in the tower falls, the bypass valve opens, and the subcooled liquid in the drum heats up and is forced by its vapor pressure back into the condenser. Because of the smaller surface now exposed to the vapor, the rate of condensation is decreased and consequently the tower pressure increases to the preset value. With normal subcooling, obtained with some excess surface, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control, (g) Cascade control The same system as case (a), but with addition of a TC (or composition controller) that resets the reflux flow rate, (h) Reflux rate on a differential temperature controller. Ensures constant internal reflux rate even when the performance of the condenser fluctuates, (i) Reflux is provided by a separate partial condenser on TC. It may be mounted on top of the column as shown or inside the column or installed with its own accumulator and reflux pump in the usual way. The overhead product is handled by an alter condenser which can be operated with refrigerant if required to handle low boiling components. 

In contrast, consider the response of schemes 16.4a,6, or e to a similar change. The drop in accumulator level will reduce distillate flow, while reflux flow rate will remain unaltered. The same quantity of reflux will enter the column, but at a lower temperature. It will be reheated upon entry by vapor condensation, ind this will increase liquid flow down the column. This is not the desired response. Eventually, the appropriate response will be established, but not until the control tray temperature drops and the temperature controller takes corrective action. In scheme 16.46, a subcooling disturbance disturbs at least the top part of the column. With the Fig. 16.4a and e schemes, it will distimb the entire column. 

It should be remembered that these are steady-state results and tell us nothing about dynamics. Temperatures on all trays in the column are quickly affected by changes in heat input, so pairing heat input with any tray temperature is dynamically feasible. However, a change in reflux flow rate takes a significant time to affect temperatures on trays near the bottom of the column because of liquid hydraulic lags (3-6 s per tray). Therefore, poor control can be expected when reflux is paired with a tray temperature significantly down from the top of the column. 

Figure 8.17b shows the controller faceplates. Notice that the flow controller on the vapor distillate (FCD) has a remote set point (on cascade ) coming from a multiplier ( ratio ) whose two inputs are the reflux flow rate and the CCxD controller output signal. The temperature controller is also on cascade with its set point coming from the CCxB controller. 

In the last three chapters, we have developed a number of conventional control structures dual-composition, single-end with RR, single-end with rellux-to-feed, tray temperature control, and so on. Structures with steam-to-feed ratios have also been demonstrated to reduce transient disturbances. Four out of the six control degrees of freedom (six available valves) are used to control the four variables of throughput, pressure, reflux-drum level, and base level. Throughput is normally controlled by the feed valve. In on-demand control structures, throughput is set by the flow rate of one of the product streams. Pressure is typically controlled by condenser heat removal. Base liquid level is normally controlled by bottoms flow rate. 

The reflux-dmm level controller is proportional with Kq = 2. The tuning of the temperature controller is essentially the same as in the previous structure. The reflux flow rate is... 

For example, suppose we set the sidestream flow rate at 3 kmol/h instead of the design 1.21kmol/h, then this reduces the concentration of the MeOH in the sidestream from 81.7 mol% to 33.3 mol% under design conditions where the feed composition is 1 mol% MeOH. Let us consider a control structure in which the temperature on Stage 17 is controlled by manipulating reboiler heat input and reflux flow rate is fixed. Now if the feed composition is increased to 2 mol% MeOH, the sidestream composition only changes to 42 mol% MeOH. This is not enough to remove all the additional MeOH in the feed, so the distillate purity is severely affected (increases to 1.55 mol% MeOH). Thus, a simple control structure with a fixed sidestream flow rate does not provide effective product quality control. The control structure must be able to adjust the sidestream flow rate in some manner so that MeOH cannot drop out of the bottom or go overhead. 

To test this, the control structure is modified to manipulate reflux to hold Stage 2 at 171.6 °C, as shown in Figure 11.30. The TC2 temperature controller has a deadtime of 1 min and is tuned in the usual way, yielding tuning constants Kc = 0.90 and Ti = 5.3 min (with a temperature transmitter range 150-250 °C and a maximum reflux flow rate of 70,000 kg/h). Note that this reset time is much smaller than that of the 95% boiling point controller, so faster closed-loop dynamics can be expected. 

To demonstrate this problem, steady-state mns are made at the 100% design level. Feed and reflux flow rate and reboiler duty are fixed at their 100% levels. There is no temperature control. Then very small changes are made in feed composition. Table 15.1 gives results when methanol feed composition is decreased from its design value of 81.53 mol% methanol with corresponding increases in water concentration. There is no temperature control. The effect on bottoms impurity is striking. 

Alatiqi presented (I EC Process Design Dev. 1986, Vol. 25, p. 762) the transfer functions for a 4 X 4 multivariable complex distillation column with sidestream stripper for separating a ternary mixture into three products. There are four controlled variables purities of the three product streams (jCj, x, and Xjij) and a temperature difference AT to rninirnize energy consumptiou There are four manipulated variables reflux R, heat input to the reboiler, heat input to the stripper reboiler Qg, and flow rate of feed to the stripper Lj. The 4x4 matrix of openloop transfer functions relating controlled and manipulated variables is ... 

As a minimum, a distillation assembly consists of a tower, reboiler, condenser, and overhead accumulator. The bottom of the tower serves as accumulator for the bottoms product. The assembly must be controlled as a whole. Almost invariably, the pressure at either the top or bottom is maintained constant at the top at such a value that the necessary reflux can be condensed with the available coolant at the bottom in order to keep the boiling temperature low enough to prevent product degradation or low enough for the available HTM, and definitely well below the critical pressure of the bottom composition. There still remain a relatively large number of variables so that care must be taken to avoid overspecifying the number and kinds of controls. For instance, it is not possihle to control the flow rates of the feed and the top and bottom products under perturbed conditions without upsetting holdup in the system. 

The control of the separation section is presented in Figure 10.11. Although the flowsheet seems complex, the control is rather simple. The separation must deliver recycle and product streams with the required purity acetic acid (from C-3), vinyl acetate (from C-5) and water (from C-6). Because the distillate streams are recycled within the separation section, their composition is less important. Therefore, columns C-3, C-5 and C-6 are operated at constant reflux, while boilup rates are used to control some temperatures in the lower sections of the column. For the absorption columns C-l and C-4, the flow rates of the absorbent (acetic acid) are kept constant The concentration of C02 in the recycle stream is controlled by changing the amount of gas sent to the C02 removal unit The additional level, temperature and pressure control loops are standard. 

Both modes usually are conducted with constant vaporization rate at an optimum value for the particular type of column construction. Figure 13.9 represents these modes on McCabe-Thiele diagrams. Small scale distillations often are controlled manually, but an automatic control scheme is shown in Figure 13.9(c). Constant overhead composition can be assured by control of temperature or directly of composition at the top of the column. Constant reflux is assured by flow control on that stream. Sometimes there is an advantage in operating at several different reflux rates at different times during the process, particularly with multicomponent mixtures as on Figure 13.10. 

---