Boilup rate change

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. 

Investigate the response of the column to changes in the boilup rate, V. 

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. 

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. 

At total reflux the reboiler duty is balanced by the condenser duty. If, at this point, a distillate product is drawn at a certain rate without changing the reflux rate, both reboiler and condenser duties must be increased to handle the higher rate of vaporization and condensation. In general, any set of reboiler and condenser duties determines the reflux and distillate rates. The reflux and product valve positions must be controlled to maintain reboiler and condenser liquid levels. The boilup rate is also determined from the other variables by material balance. Thus, of the five variables—reboiler duty, condenser duty, boilup rate, reflux rate, and product rate—two are independent and the system has two degrees of freedom. 

Changing distillate flow rate has no direct effect on temperatures in the column. Reflux and vapor boilup are the inputs that change temperatures (and compositions) inside the vessel. However, the distillate flow rate changes the level in the reflux drum, which results in a change in vapor rate when the level loop is on automatic. So the level loop is nested inside the temperature loop in this structure. The level loop must be on automatic for the temperature control loop to be effective. 

Once you have convergence and the final values of the reflux ratios, change the boilup rate in column 2 to obtain 0.995 furfural or slighdy higher and 0.990 water or slighdy higher in the bottoms to columns 1 and 2, respectively. 

The largest improvements occurred when the product from a distillation column was sold out, and another pound produced meant another pound sold. In one case, the distillation column was being operated at the maximum boilup rate recommended by the vendor of the distillation trays. However, the use of small incremental changes by the sequential optimization routine increased the boilup over a period of several weeks and resulted in the discovery that the trays could be operated at 136% of the maximum capacity recommended by the vendor before flooding occurred. This resulted In additional sales of 800,000 one year during a peak in demand for the product with no added capital. 

In another case, the product from a new world-scale plant was sold out, and the bottleneck for the plant was the size of the reboiler on a distillation column. The boilup rate for the column was being manipulated by a temperature controller. The capacity of the plant was increased by setting the reboiler to run at its maximum capacity, that is, at the constraint limit, all of the time, and the control strategy was changed to use a temperature control loop to manipulate the reflux flow rate. This resulted in an increase in production that year of 32 million. 

Now the absolute value of Ps does not change very much so boilup rate is mostly controlled by variation in exposed tube area. 

In the Eastman control stnictuEc, two PI temperature controllers are used to maintain two tray temperatures in the column by manipulating the two fresh feedstreams. Production rate changes are achieved by changing the vapor boilup. The base level is controlled by manipulating the bottoms flowrate. 

For the two-temperature control structures studied thus far, one of the fresh feeds is fixed, which provides a direct handle on the production rate. The ratio to the second fresh feed (feed ratio FR) is adjusted to control a tray temperature and subsequently the stoichiometric balance. A second tray temperature is controlled by manipulating the reboiler heat duty. This is called the Q-FR control structure (Fig. 13.8a). Roat et al. ° propose a different two-temperature control stmcture for the control of the MeAc system. Two tray temperatures in the column are maintained using two fresh feedstreams. Production rate changes are achieved by adjusting the vapor boilup (Fig. 13.8c). We call this an F-F control structure, which has been studied by Kaymak and Luyben. A third control stmcture is similar to the F-F control stmcture but, instead of manipulating both feedstreams, one fresh feedstream is used to maintain a tray temperamre and the ratio to the second fresh feed is adjusted to control a second tray temperamre, which we call the F-FR control stmcture shown in Figure 13.8b. The purpose here is to compare the effectiveness of these three alternative control stmctures. A comparison will be made for dynamic responses for production rate changes for the MeAc, BuAc, and AmAc systems. 

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. 

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... 

Control of the column material balemce may be difficult when the distillate stream is small relative to reflux flow. Unless large swings in distillate flow are acceptable, changes in the small distillate flow will have little impact on accumulator level, and schemes 16.4a, b, and e will not maintain a steady accumulator level. In most cases, the level in the accumulator will be allowed to drift and will be periodically adjusted manually by changing reflux, boilup, or condensation rate. However, this mode of control does little to maintain the column material balance, and over a period of time may cause accumulation or depletion of the light components. Scheme 16.4d does not suffer from the above problem and is often favored with small distillate flows (300). 

Tuning can be troublesome with the vapor inlet scheme if flow across the valve changes from noncritical to critical upon reboiler turndown (67, 68, 362). As boilup falls, so does the absolute pressure downstream of the valve. When the ratio of upstream to downstream pressure exceeds a critical value, critical flow is established through the valve, and the downstream pressure ceases to affect the vapor flow rate. The controller dynamics differ under critical and noncritical flow. A loop tuned for noncritical flow tends to be unstable when flow becomes critical, while a loop tuned for critical flow tends to be sluggish when flow becomes noncritical (67, 68). 

In a sidestream column, changing the flow rates of liquid streams above or at a product withdrawal location affects the composition of that product and all other products below it. Changing the flow rate of the sidestream has little effect on the products above it. Therefore, sidestream composition can be controlled by vapor boilup, reflux flow rate, liquid split, or sidestream flow rate. As discussed later, we want to control a composition near the top of the prefractionator, and the logical manipulated variable to achieve this control is the liquid split. If reflux is used for distillate control, we are left with using either sidestream flow rate or vapor boilup for the control of the sidestream composition. 

Many distillation column use reboiler heat input as a primary manipulated variable, usually to control a temperature on an appropriate tray. This means that reboiler heat input is not constrained during normal operation with normal feed flow rates. However, as the feed flow rate to the column is decreased, less vapor boilup is required to achieve the same separation. If the feed drops to the point where the low vapor-boilup limit is encountered, the control structure must change. Three alternative control structures for achieving stable operation at minimum vapor flow rates are discussed in the following. 

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