Specification boilup ratio

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. 

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 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. 

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. 

The inlet stream is supplied as a saturated liquid at 22.8 bar pressure the mole fraction of C3 in the top product is specified as 0.97. The column has 32 real trays with a total eondensor and a kettle reboiler. The operating specifications are reflux ratio and boilup ratio with the amount of 2.64 and 4, respectively, temperature and pressure of the main streams of the process are shown in table 1, for the case of the conventional column. Table 1. Conditions of the main streams of the conventional distillation process... 

The TAC of the second colunm ( 353,000) is only half of that of the reactive distillation column. This is a simple distillation with light impurities. The reflux ratio is around 2, and the boilup ratio is a little higher than 3. Thus, the top composition achieves the specification (95% D, LK component) with the two reactants as the impurities. The bottoms composition is 97.5% D with 2.5% C as the only impurity. This column has 62 trays, and the feed is introduced into tray 11. Figure 17.13fi provides the composition profiles in the distillation column. The two unreacted (light) reactants end up on the top of the column. 

Equality constraints h(D°, D°) = 0 may include, for example, a ratio between the amounts of two products, etc. Inequality constraints g(u, D°) < 0 for the overall operation include Equations 7.14-7.18 (the first two of which are easily eliminated when m and H are specified) and possibly bounds on total batch time for individual mixtures, energy utilisation, etc. Any variables of D° and D° which are fixed are simply dropped from the decision variable list. Here, Strategy II was adopted for the multiple duty specification, requiring B0 to be fixed a priori. Similar considerations hold for V, the vapour boilup rate. The batch time is inversely proportional to V for a specified amount of distillate. Also alternatively, for a given batch time, the amount of product is directly proportional to V. This can be further explained through Equations 7.24-7.26) ... 

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. 

By setting the product specifications at 98%, light key product in the top, and intermediate key product in the bottoms, the reflux ratio and the boilup rate are manipulated to control product purities. Distillate and bottoms flows are manipulated to control the inventory of reflux-drum and column base. 

Figures 11.7 and 11.8 gives responses to positive and negative 20% changes in vapor boilup, the throughput handle in this control structure. These disturbances are handled well by the two-temperature control structure. Stable base-level regulatory control is achieved. The increase in Vs results in increases in both fresh feeds, and the distillate and bottoms streams increase. Product purities xd(q and xb(d> are maintained fairly close to their desired values. Product purities drop slightly below their specifications for the increase in V5 but rise above specifications for the decrease in throughput. Reflux increases because of the reflux ratio control stmcture.

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