Design Calculation of Two-Section Columns

Purity of the products is the set (specified) parameter at designing, and number of trays in each section rir and Hs and reflux number L/D are the parameters that have to be determined. 

The first four items of this algorithm are of general nature and do not depend on the split. But the efficiency of the choice of the initial point and of the direction of calculation by method tray by tray depends to a great extent on the accepted split. In some cases, it is easy to calculate the whole column in one direction (the direct and the indirect sphts). It is considerably more comphcated to perform calculation at intermediate splits and at sphts with one distributed component. It is shown in the next section that for these most general splits the calculation of each section trajectory should be performed from the end of the column. We examine aU the listed cases. 

Direct and Indirect Splits of Mixtures with Any Number of Components 

Fxpi tori = 3,4... n. Components are arranged in the order of decreasing phase equihbrium coefficients. For split 1 23,4, it is important that component 1 is the lightest one in all the points of both section trajectories (both trajectories are 

To determine x°f at which (n -I- n ) is minimum, it is necessary to perform several calculations of the column at different points x/ at segment [x , x ]. This algorithm was introduced in the work (Julka, 1993). The similar algorithm can also be used at indirect separation, but calculation should be executed top-down from point xd- 

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We state below a rigorous method of design calculation of Petlyuk columns and of columns with side sections based on the design calculation of two-section columns described in Section 7.3. 

Calculation of tray numbers of three two-section colunms beginning with the first column by algorithms of design calculation of two-section columns at fixed parameters L/V and fixed purity of pseudoproducts and B[ of the first column (see Section 7.3). 

Design calculation of Petlyuk columns is carried out on the basis of the algorithm of design calculation of two-section columns with optimization by distribution coefficient of pseudoproduct flow rates of the prefractionator, by excess factors of reflux and by distribution of trays among sections in each two-section column. 

The general approach to design calculation of extractive distillation columns is similar to the approach applied for two-section columns. We use our notions about the structure of intermediate section trajectory bundles (see Sections 6.4 6.6), about possible compositions at the trays adjacent to the feed cross-section from above and below (see Section 7.2), and about possible directions of calculation... 

We propose a method of design calculation based on decomposition of the column with side strippings into a system of two-section columns, on rigorous design calculation of each two-section column and on simultaneous design of distillation columns and of system of heat exchange. 

The task of designing of extractive distillation columns, besides calculation of section trajectories, includes a number of subtasks. These are the same subtasks as for two-section columns and additional subtasks of determination of minimum entrainer flow rate and of choice of design entrainer flow rate. Optimal designing of extractive or autoextractive distillation includes optimization by two parameters - by entrainer flow rate and by reflux number. Figure 7.14 shows influence of entrainer flow rate on section trajectories at fixed value of parameter a = LfV)mlK j (as is shown in Section 6.4 (L/y) = K j). 

Figure 7.15. Calculation of Petlyuk columns (a) specifications (in brackets), (b) calculation of minimum reflux and reboU ratios and product distribution in two-section columns, and (c) calculation of design variables.

Because minimum values of these parameters were determined before at the stage of calculation of the mode of minimum reflux (see Section 6.8), design their values should be chosen reasoning from economic considerations taking into account energy and capital expenditures. This choice is similar to that of optimal reflux excess coefficient for two-section columns. Along with that, the equaUty of vapor flow rates in the second and third columns in the cross-section of output of side product is taken into consideration. 

The main purpose of design calculation is to determine necessary tray numbers for all sections at fixed values of mode parameters. At design calculation, one takes into consideration the equality of compositions at the tray of output of the side product obtained at the calculation of the second and third columns. Each two-section column entering into a Petlyuk column is calculated with the help of algorithms described before for two-section columns. The algorithm of calculation for splits with a distributed component is used for the first column, the algorithms for the direct and the indirect sphts are used for the second, and the third columns at separation of a three-component mixture, respectively. At separation of multicomponent mixtures, the algorithms for intermediate separation are used. 

Operation of a column at total reflux is important in two ways it is a convenient startup condition that enables a column to be lined out at steady state before feed is processed, and in experimental work it is a simple and yet effective means for obtaining mass transfer information. The number of stages at total reflux is also important in design calculations in that it represents a lower limit to the required stages and it also represents a parameter used in short cut estimates of stage requirements (to be discussed in Section 5.3-7). 

Click Next and run the simulation and look at the Report (View Report check block). Note that the two sections have different diameters. If the column was designed with a single section it would all have to be the larger diameter. [Note the methods in Section 10.4 might allow design of a column with constant diameter between the two values calculated.]... 

The procedure for design is one of trial and error. For example, as a first trial, heat losses might be neglected and trays calculated with fixed difference points, and after the size of the resulting column has been determined, the first estimate of heat losses for the two column sections can be made by the usual methods of heat-transfer calculations. The heat losses can then be apportioned among the trays and the number of trays redetermined with the appropriate difference points. This leads to a second approximation of the heat loss, and so forth. 

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