Column distillation boilup

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). 

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.

An energy balance scheme as in Fig. 16.7 usually requires continuous adjustment of a product rate (in Fig. 16.7, the distillate rate) by the operator, and a very slow level control action (in Fig. 16.7, accumulator level). Consider a rise in concentration of the light component in the feed. The bottom section temperature will drop, and the temperature controller will raise boilup. Column pressure will rise, and the pressure controller will increase the condensation rate. The accumulator level will rise, and the level controller will pour more reflux into the column. This in turn will reduce control tray temperature, and the temperature controller will raise boilup again. This will continue until reflux and boilup sufiiciently rise to keep the bottom section temperature up. In the meantime, the light component accumulates in the system, and this will cause further increase of reflux and boilup. 

In addition, there are four degrees of freedom that are adjustable during design and are also adjustable during operation of the column reflux flow rate (/ ), vapor boilup (V), sidestream flow rate (5), and the liquid split ratio (jSl = i-p/i-R)- The variable Lp is the liquid flow rate fed to the prefractionator side of the wall, and Lp is the total liquid leaving the bottom tray in the rectifying section. Of course, the rest of the liquid coming from the bottom of the rectification section is fed to the sidestream side of the column. Distillate and bottoms flow rates are used to maintain liquid levels in the reflux drum and column base, respectively. 

The feed flow rate is 2000 kmol/day. Feed is 48 mol% methanol and 52 mol% water and is a subcooled liquid. For every 4 moles of feed, 1 mole of vapor must condense inside the column. Distillate conposition is 92 mol% methanol. Reflux is a saturated liquid, and Lq/D = 1.0. Bottoms conposition is 8 mol% methanol. Boilup ratio is V/B = 0.5. Equilibrium data are given in Table 2-7. Assume that CMO is valid. Find the optimum feed plate location and the total number of equilibrium stages required. 

For the unconstrained degree of freedom the suggested control variables tested were one of the following boilup in the HP column, fixed boilup to feedrate ratio (Qb/F), the pressure in the HP column, reflux ratio in the HP column, fixed reflux to feedrate ratio, distillate flow in HP column, bottom flowrate in HP column, temperatures in the HP column, temperatures in the LP column, distillate composition in the HP column and bottom composition in HP column. 

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. 

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. 

The idea is best explained with an example. Suppose the base level in a distillation column is normally held by bottoms product withdrawal as shown in Fig. 8.4a. A temperature in the stripping section is held by steam to the rcboiler. Situations can arise where the base level continues to drop even with the bottoms flow at zero (vapor boilup is greater than the liquid rate from tray 1). if no corrective action is taken, the reboiler may boil dry (which could foul the tubes) and the bottoms pump could lose suction. 

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. 

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. 

This criterion is similar to using a reflux ratio of 1.2 times the minimum reflux ratio in a full distillation column. It provides a reasonable compromise between the number of trays and the vapor boilup required. The slope of the resulting operating line is the liquid-to-vapor ratio F/D ... 

Seader and Henley (1998) considered the separation of a ternary mixture in a batch distillation column with B0 = 100 moles, xB0 = = <0.33, 0.33, 0.34> molefraction, relative volatility a= <2.0, 1.5, 1.0>, theoretical plates N = 3, reflux ratio R = 10 and vapour boilup ratio V = 110 kmol/hr. The column operation was simulated using the short-cut model of Sundaram and Evans (1993a). The results in terms of reboiler holdup (Bj), reboiler composition profile (xBI), accumulated distillate composition profile (xa), minimum number of plates (Nmin) and minimum... 

A liquid binary mixture with Bo = 10 kmol and xbo = <0.6, 0.4> molefraction is subject to batch distillation using a continuous column shown in Figure 4.11. The relative volatility of the mixture over the operating temperature range is assumed constant with a value of (a =) 2. The total number of plates is, N = 20 and the feed is introduced at the bottom plate of the column. The vapour boilup rate is, V = 5.0 kmol/hr. The column is operated with a continuous feed rate, F = 10 kmol/hr, therefore the column runs for 1 hr to process the total amount of feed (B0). The... 

The boilup rate in actual columns is seldom kept constant but falls off as the distillation proceeds (as discussed in Chapter 3, also see Greaves et al., 2001 Greaves, 2003). Robinson (1969) assumed that the boilup rate is a linear function of the still composition (with boilup V0 at time t = 0 and kis a constant)... 

Two binary mixtures are being processed in a batch distillation column with 15 plates and vapour boilup rate of 250 moles/hr following the operation sequence given in Figure 7.7. The amount of distillate, batch time and profit of the operation are shown in Table 7.6 (base case). The optimal reflux ratio profiles are shown in Figure 7.8. It is desired to simultaneously optimise the design (number of plates) and operation (reflux ratio and batch time) for this multiple separation duties. The column operates with the same boil up rate as the base case and the sales values of different products are given in Table 7.6. 

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. 

The SR method can be applied to distillation columns, but the equations of the algorithm do not allow the solution of the condenser and the reboiler with the other stages in the column. Because only the energy balances are used as independent functions, reboiler and condenser duties, reflux ratio, and the boilup ratio have to be specified. This overspecifies the column and the solution cannot be found. The condenser and the reboiler can be solved as separate unit operations in a flowsheet as demonstrated by Fonyo et al. (39). The SR method is used in the ABSBR step of FLOWTRAN of Monsanto, St. Louis, Missouri, and also in both the public release version of ASPEN and in ASPENPlus of AspenTech, Cambridge, Massachusetts. 

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. 

Figure 6.96 shows a column that is separating a mixture with a low relative volatility, so the column has a large number of trays and operates with a high reflux ratio. This type of column is called a superfractionator. Because of the high reflux ratio, reflux should be used to control reflux drum level. For the same reason, vapor boilup should be used to control base level. Therefore the two manipulators left to control composition are distillate and bottoms flowrates. Obviously these two... 

So from a plantwide control perspective, setting distillate flowrate to control reflux ratio is a better strategy than using distillate to control composition. Of course similar arguments can be made about bottoms flowrate in the case of a column with a high boilup ratio. 

Step 5. The distillate stream from the product column is salable benzene. Benzene quality can be affected primarily by two components, methane and toluene. Any methane that leaves in the bottoms of the stabilizer column contaminates the benzene product. The easy separation in the stabilizer column allows us to prevent this by using a temperature to set column steam rate (boilup). Toluene in the overhead of the product column also affects benzene quality. In this column the separation between benzene and toluene is also fairly easy. As a result, we can control product column boilup by using a tray temperature. To achieve on-aim product quality control, we most likely would use an on-line overhead composition analyzer to adjust the setpoint of this temperature controller,... 

Use Tyreus-Luyben settings for most loops, particularly distillation columns where large swings in vapor boilup or reflux are undesirable. These settings are much more conservative (robust) than Ziegler-Nichols ... 

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