Boilup ratio

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. 

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... 

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. 

Consider a propane concentration of 88 percent in the feed. The McCabe-Thiele diagram, based on the Fig. 7.12a interpretation of the test data, predicts a pinch just below the feed (Fig. 7.12e). Due to the pinch, the concentration of propane in the tower bottom will be 17 percent, i.e,. much higher than the 2 percent propane in the test data. In practice, this pinch will probably be eliminated by increasing the boilup ratio (i.e., reducing the slope of the operating line). However, increasing the boilup ratio means more liquid and vapor traffic, a greater heat load on the reboiler, and possibly, a premature capacity bottleneck. 

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. 

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. 

RR-BR Reflux ratio controls distillate composition and boilup ratio controls bottoms composition. 

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. 

After these choices, we must now decide about level control in the recycle column. Contrary to the other columns, here the boilup ratio is large since the bottoms diphenyl flow is quite small compared with the toluene recycle rate. For this case, we choose to control base level with the steam flow because it has a much larger effect. 

The column may be operated to meet various performance specifications within certain ranges. The variables that can be specified in multi-component separation include all the component compositions, rates, or recoveries in the two products as well as the product rates, properties, and temperatures, the reflux and boilup ratios, the condenser and reboiler duties, and the tray temperatures and liquid and vapor rates. 

Solution. The nomenclatnre nsed here refers to a particular configuration as (C1C2), where Cj is assumed to be the controller output that is used to control the overhead composition, and C2 is the controller output that is used to control the bottoms composition. If we limit ourselves to controlling the overhead composition with L, D, or UD (the reflnx ratio) and the bottoms composition with V, B, or V/B (the boilup ratio), there are a total of nine possible confignrations. Here we limit the discussion to the following confignrations (L, B), (L, V), (UD, V/B), and (D, V). 

CLASS 3. SPECIFICATION OF THE REFLUX RATIO LJD AND THE BOILUP RATIO VN/B... 

The second set of specifications differs from the first in that the boilup ratio Vn/B is specified instead of the reboiler duty. 

The boilup ratio F31/L31 for the sidestripper No. 1 which has a reboiler (or alternatively, the reboiler duty QR3l for this stripper could be specified)... 

The last subscript refers to tjhe number of the column. The boilup ratio (VNl /By)n of column 1 is to be adjusted from one system trial to the next until a solution has been found such that Qcl = QRi. 

Instead of regarding all of the reflux and boilup ratios as fixed, let one of them be varied, say VNi/Bu as required to satisfy the condition that QRl = QC2. The new variable VNi/Bx is added to the set of variables and the variable QC1 is removed from the set of variables and functions by replacing QC2 by QRi. Thus, to solve this problem, the variables are taken to be... 

On the basis of an assumed value for VNl /Bl, one complete trial is made on column 1. The set of bottom flow rates bi% j so obtained is used as the feed to column 2. The reboiler duty QRl found for column 1 is taken to be the assumed value QC2 for making the trial on column 2. Then the capital 0 method is applied and a new value of Vsi/Bi is obtained. This value of the boilup ratio is used in making the second trial on column 1. Also, in the application of the IN Newton-Raphson method to column 1, the assumed values of Qcl and QRl are set equal to the most recent set of values found by use of the capital 0 method. In the application of the IN Newton-Raphson method to columns 1 and 2, the independent variables are taken to be Qcl, QRl, 7, Lj/Vj, for column I and Qc2 > Qr2 > Tfh Lj/Vj for column 2. 

When the 9 method is used to solve the equations for each column where the reflux ratio and boilup ratio are specified rather than the reflux rate and the distillate rate, the appropriate equations needed to apply the 6 method are developed in Sec. 7-4. 

Column l. N = 20 and /= 11. The column has a partial condenser, and is to be operated at a reflux ratio Lul/Di = 4.0 at a pressure of 250 lb/in2 abs. The pressure drop across each plate is negligible. The feed enters the column as a liquid at its bubble point (551.56°R) at the column pressure. The boilup ratio of column 1 is to be selected such that the reboiler duty QRl of column 1 is equal to the condenser duty Qc2 of column 2. Use the vapor-liquid equilibrium and enthalpy data given in Tables B-l and B-2. Since the K values in Table B-l are at the base pressure of 300 lb in2 abs, approximate the K values at 250 lb/in2 abs as follows... 

The specifications for this example are the same as those given for Example 7-1 except that the bottoms B2 of column 2 is to be returned to stage j = 15 of column 1 and the boilup ratio VNl/Bi is 5.6948. Also the reboiler of column 1 is to be operated independently of the condenser of column 2. For the heat exchanger (unit 3 of Fig. 7-3), the overall-heat-transfer coefficient U is 50 Btu/h ft2 and the area is 10 square feet. 

The system shown in Fig. 7-2 may be formed from the one shown in Fig. 7-1 by returning the bottoms B2 from column 2 to column 1. The addition of the recycle mass transfer stream does not change the general form of the equations for the capital 0 method where the reflux ratios and boilup ratios are specified. The heat balance enclosing column 1 must be modified, however, in order to account for the recycle stream B2. The resulting expression for the enthalpy function r10 is... 

In the specifications given by set 2 of Table 7-2, the reflux ratios and boilup ratios are fixed and the capital 0 method again consists of 14 functions in 14 independent variables. If the reflux rates Ll u Lx 2 and the total flow rates Dx and B2 are specified instead of the reflux ratios and boilup ratios, the capital 0 method reduces to two functions in two independent variables 0t and 02 see set 3 of Table 7-2. In this case the g functions for the capital 0 method are given by... 

In the same manner that the capital 0 method was applied above to solve problems involving specifications other than the total flow rates, the 0 method for single columns may be applied to solve problems involving specifications other than the total-flow rates. To demonstrate this application of the 0 method, it is applied to conventional distillation columns for which the reflux ratio L /D and the boilup ratio Vs/B are specified instead of the reflux rate Lj and the distillate rate D. The remaining specifications for the column are the same as those enumerated in Chap. 2 in the application of the 0 method of convergence to conventional distillation columns. 

Example 7-4 Instead of specifying Lx and D for Example 2-7, modify this example by taking the two additional specifications to be the reflux ratio Ll/D = 2.0 and the boilup ratio VN/B= 1.80585. When these particular values for the reflux ratio and the boilup ratio are selected, the corresponding final solution is the same as the one shown in Tables 2-3 through 2-5. For this pair of additional specifications, nine iterations were required and 1.26 seconds of computer time (AMDAHL 470 V/6 computer, FORTRAN H EXTENDED). The convergence characteristics exhibited by this example for this version of the 0 method are shown in Table 7-12. 

For the case where the reflux ratio L /D and the boilup ratio Vs/B are specified in lieu of the reflux rate Lt and the distillat rate D for a conventional distillation column having a partial condenser, formulate the g funct ns of the 0 method where the variables x are taken to be... 

Total condenser, P = 1 atm, boiling point liquid feed, reflux ratio, Ll/D = 10, boilup ratio, Vn/B = 2.588. The holdup is taken as 1.0 liter for the reboiler and 0.3 liter for each plate and the condenser. The molar densities of the liquids acetic acid, ethyl alcohol water, and ethyl acetate are 17.470, 17.129, 55.49, and 10.22 moles per liter, respectively. The mole fraction average of these was used as the molar density of the mixture. 

This formulation of the Newton-Raphson method for columns with infinitely many stages is analogous to the 2N Newton-Raphson method for a column with a finite number of stages. First the procedure is developed for a conventional distillation column with infinitely many stages for which the condenser duty Qc (or the reflux ratio Lx/D) and the reboiler duty QR (or the boilup ratio VN/B) are specified and it is required to find the product distribution. Then the procedure is modified as required to find the minimum reflux ratio required to effect the specified separation of two key components. 

After a solution has been obtained, the desired values for the reflux ratio and the boilup ratio are computed in the rather obvious manner... 

A batch distillation column with three theoretical stages (the first stage is the still pot) is charged with 100 kmol of a 20 mol% n-hexane in n-octane mixture. At a constant reflux ratio R - 1.0, how many moles of the charge must be distilled if an average product composition of 70 mol% n-hexane is required If the boilup ratio is 10 kmol/h, calculate the distillation time. The equilibrium distribution curve at column pressure is given in Figure 6.27. 

VB boilup ratio moles of boilup/moles of residue. 

---