Control boilup

Figure 19.8a shows one control system frequently used for such services. The upper and lower temperature controllers maintain product lights and heavies content at the desired levels by manipulating the respective product streams. The side product is drawn using an IRC (in case of a vapor product, an IVC) system, similar to that described in Fig. 19.7. The bottom level manipulates boilup, and the accumulator level manipulates reflux. The bottom level and temperature controllers are sometimes interchanged, so that the level controls the bottom flow and that the lower temperature controls boilup. Similsuly, the top pressure and temperature controllers are interchanged when this can tighten pressure control (see Sec. 17.3 similar to the arrangements discussed in Fig. 17.8). A system similar to that in Fig. 19.8o has been proposed by Shinskey (362).

Distillate and bottoms were controlled by accumulator and sump levels, respectively, feed and reflux on flow control and boilup was temperature-controUed Tower pall rings were replaced by higher-capadfy rings (bottom) uid wire-mesh structured packing (top) to increase c )acity and reduce reflux. The column was sensitive to ambient disturbances (e.g., rainstorms). The reflux reductions escalated this sensitivity to an extent that annulled the revamp benefits. The temperature control was ineffective due to its narrow range of variation. Problems were solved by controlling boilup on sump level and bottom product on flow control. 

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. 

If the produc ts from a column are especially pure, even this configuration may produce excessive interaction between the composition loops. Then the composition of the less pure product should oe con-troUed by manipulating its own flow the composition of the remaining product should be controlled by manipulating reflux ratio if it is the distillate or boilup ratio if it is the bottom product. 

Solution Because vapor rate changes are reflected up and down the column much faster than liquid rate changes, the temperature difference controller w as disconnected and the tower was controlled instead by boilup. A temperature 10 trays from the bottom set reboiler heating medium and the reflux W as put on flow control. 

We will assume constant holdups in the reflux drum Aij> and in the column base Mg. Proportional-integral feedback controllers at both ends of the column will change the reflux flow rate and the vapor boilup V to control overhead composition and bottoms composition Xg at setpoint values of 0.98 and 0.02 respectively. 

To illustrate the disturbance rejection effect, consider the distillation column reboiler shown in Fig. 8.2a. Suppose the steam supply pressure increases. The pressure drop over the control valve will be larger, so the steam flow rale will increase. With the single-loop temperature controller, no correction will be made until the higher steam flow rate increases the vapor boilup and the higher vapor rate begins to raise the temperature on tray 5. Thus the whole system is disturbed by a supply-steam pressure change. 

If bottoms composition is to be controlled by vapor boilup, the control tray should be located as dose to the base of the column as possible in a binary system. In multicomponent systems with heavy components in the feed which have their highest concentration in the base of the column, the optimum control tray moves up in the column. 

Avoid saturation of a manipulated variable. A good example of saturation is the level control of a reflux drum in a distillation column that has a very high reflux ratio. Suppose the reflux ratio (R/D) is 20, as shown in Fig. 8.10. Scheme A uses distillate flow rate D to control reflux drum level. If the vapor boilup dropped ouly 5 percent, the distillate flow would go to zero. Any bigger drop in vapor boilup would cause the drum to run dry (unless a low-level override controller were used to pinch back on the reflux valve). Scheme B is preferable for this high reflux-ratio case. 

If the only disturbances were feed flow rate changes, we could simply ratio the reflux flow rate to the feed rate and control the composition of only one end of the column (or even one temperature in the column). However, changes in feed composition may require changes in reflux and vapor boilup for the same feed flow rate. 

There is a first-order dynamic lag of t minutes between a change in the signal to the steam valve and vapor boilup. The low base-level override controller pinches the reboiler steam valve over the lower 25 percent of the level transmitter span. 

Notice that the pairing assumes distillate composition jtp is controlled by reflux R and bottoms composition Xg is controlled by vapor boilup V. 

For example, in a distillation column the manipulated variables could be the flow rates of reflux and vapor boilup R — V) to control distillate and bottoms compositions. This choice gives one possible control stmcture. Alternatively we could have chosen to manipulate the flow rates of distillate and vapor boilup D V). This yields another control structure for the same basic distillation process. 

The essence of this particular formulation is the control of tray existence (governed by and the consequences for the continuous variables. In the rectifying section, all trays above the tray on which the reflux enters have no liquid flows, which eliminates any mass transfer on these trays where g = 0. The vapor composition does not change above this tray even though vapor flows remain constant. Similarly, in the stripping section, all trays below the tray on which the vapor boilup enters have no vapor flows and the liquid composition does not change below this tray even though liquid flows remain constant. The reflux and boilup constraints ensure that the reflux and boilup enter on only one tray. 

The purity of C in the bottoms product stream xFC is controlled by manipulating vapor boilup in the column V. 

Figure 3.39 gives the response of the system to a step 20% increase in the flowrate of the reactor effluent. The control structure provides good base-level regulatory control. The maximum deviation in reactor temperature is 0.6 K. Three cases are shown. In the first, the reflux flowrate is held constant. In the second, the reflux is ratioed to the feed. There is little difference in the responses of the reactor. But with a fixed reflux flowrate, the impurity of C in the distillate xDC increases from 0.0164 to about 0.025 mole fraction C. With the reflux-to-feed ratio, the impurity remains about the same. The change in the vapor boilup is larger with the reflux-to-feed structure. 

The improvement in system stability is clearly shown in Figure 3.43. Reactor temperature is well controlled in the face of the disturbance in F. The CB concentration drops to about 0.06 kmol/m3. The FB0 flowrate is constant. The FA0 flowrate increases initially because the reactor level drops, but it gradually returns to the initial value. Reflux, vapor boilup and recycle flows all increase because of the increase in F, but the product stream P returns to its initial level. 

As presented in the earlier chapters, the operating policy for a batch distillation column can be determined in terms of reflux ratio, product recoveries and vapour boilup rate as a function of time (open-loop control). Under nominal conditions, the optimal operating policy may be specified equivalently in terms of a set-point trajectory for controllers manipulating these inputs. In the presence of uncertainty, these specifications for the optimal operating policy are no longer equivalent and it is important to evaluate and compare their performance. 

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. 

Example 3.8 A benzene-toluene column normally operates as described in Example 2.1. The column is computer-controlled, using the Jafarey et al. algorithm. The algorithm manipulates boilup How to control toluene purity. If the... 

The computer control will therefore increase the boilup rate from 234 lb-mole/h (Example 2.1) to 384 lb-mole/h. 

The scheme for the reactor/stripper process uses a PI controller to hold product composition (xB) by manipulating vapor boilup in the stripper. The same analyzer deadtime is used. Proportional level controllers are used for the stripper base (manipulating bottoms flow ), the overhead receiver (manipulating recycle flow), and the reactor (manipulating reactor effluent flow) with gains of 2. 

Two control structures are shown in Fig. 2.11u and b. In both, the composition of component A in the product stream xSA is controlled by manipulating vapor boilup in the column. This prevents component A from leaving the system. Except for this small amount of A impurity in the product, all A that enters the system must be consumed in the reactor. This illustrates the point we made in Sec. 2.2 about the need to change conditions in the reactor so that the additional reactant is consumed and will not accumulate. 

The impurity of A xB-2 A) in the product stream B2 from the second column is controlled by vapor boilup in the second column. 

The impurity of B [x82,b) in the product stream B> from the second column is controlled by vapor boilup in the first column through a composition-composition cascade control system. Any B that goes overhead in the first column comes out the bottom of the second column. So the first column must be operated to prevent B from going overhead. The impurity of B in the first column distillate (xrn B) is controlled by a composition controller that manipulates the vapor boilup in the first column. The setpoint of this composition controller is changed by a second composition controller looking at the impurity of B in the product stream (x82M). 

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. 

Two of the control degrees of freedom must be consumed to control the two liquid levels in the process reflux drum level and base level. Reflux drum level can be held by changing the flowrate of the distillate, the reflux, the vapor boilup, the condenser cooling, or the feed (if the feed is partially vapor). Each of these flows has a direct impact on reflux drum level. The most common selection is to use distillate to control reflux drum level, except in high reflux-ratio columns (RR > 4) where Richardson s rule7 suggests that reflux should be used. 

Column base level (or reboiler level in a kettle reboiler) can be held by the flowrate of the bottoms, the vapor boilup, or the feed (if the feed is partially liquid and the stripping section does not contain too many trays). Since the typical hydraulic lag is 3 to 6 seconds per tray, a 20-tray stripping section introduces a deadtime of 1 to 2 minutes in the feed-to-base-level loop. Because of these hydraulic lags, reflux is only very7 rarely used to control base level. For this loop to work successfully, the column must be relatively short (less than 30 trays) and the holdup in the base must be large (more than 10 minutes). 

Feed split means the fraction of the feed that leaves in one product stream, e.g., the DIF ratio. Feed split can be set directly and explicitly by using either D or B to control one composition. Or it can be set indirectly and implicitly by using reflux or vapor boilup to control one composition and removing D or B to hold reflux drum or base level. 

Figure 6.8 Common control structures for distillation columns, (a < Reflux-boilup (6) distillate-boilup (el reflux ratio-boilup d) reflux-bottoms (e) reflux ratio-boilup ratio.

R-B When the boilup ratio is high bottoms flow should be used to control bottoms composition and heat input should control base level. However, in some columns potential inverse response may create problems in controlling base level with boilup. 

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