Feed tray location, distillation

Multicomponent distillation, 393 absorption factor method, 398 azeotropic, 420-426 bubblepoint (BP) method, 406-409 computer program references. 404 concentration profiles, 394 distribution of non-kevs. 395 Edmister method, 398,399 extractive, 412, 417-422 feed tray location, 397 free variables, number of 395 Lewis-Matheson method 404 MESH eauations. 405-407 molecular, 425-427 nomenclature, 405 number of theoretical trays, 397 packed towers, 433-439 petroleum, 411-415 reflux, minimum, 397 reflux, operating, 397 SC (simultaneous correction) method, 408-411... 

Distillation towers feed-tray location for, 10 optimum reflux ratio for, 371-376 specifications for, 16 (See also Bubble-cap contactors, Packed towers. Sieve trays, and Valve trays) Distribution costs, 194, 196, 207, 211 Distribution in statistical analyses, 745-746 Dividends, tax exemptions for, 259 Documentation, 137-149,452-476 Double-entry bookkeeping, 143-144 Downcomers in tray columns, 684-686 Drives, cost of 532-533 Dryers, cost of 713-716... 

Federal environment regulations, 75-78 Feed-tray location in distillation towers, lo Fiberglass reinforced plastics (FRP), 436-437 Fibonacci search, 407 Fifo method for materials accounting, 148... 

Once one has proposed alternative configurations for systems of separation devices to effect a desired separation, one must then design these devices so the various alternatives may be compared. For a distillation column, the first set of design decisions is to choose the number of trays, the feed tray location, and the reflux ratio at which to operate it. For a binary separation, the McCabe-Thiele diagram (or the concepts behind it) is an indispensable aid in making these decisions. 

Earlier chapters use simplified and binary models to analyze in a very informative manner some fundamentals such as the effect of reflux ratio and feed tray location, and to delineate the differences between absorption/stripping and distillation. Following chapters concentrate on specific areas such as complex distillation, with detailed analyses of various features such as pumparounds and side-strippers, and when they should be used. Also discussed are azeotropic, extractive, and three-phase distillation operations, multi-component liquid-liquid and supercritical extraction, and reactive multistage separation. The applications are clearly explained with many practical examples. 

Parametric column simulations for the I POAc system were performed with different Damkohler numbers, reflux ratios, reboil ratios as well as total number of stages, (N-I-) and feed tray location, (/). The distillate and bottoms compositions obtained were recorded in transformed composition space. Fig. 6.9 compares the products obtained from column simulations with 30 stages and using different values of r and s at D = 0.25 and D = 0.75. The column feed specification is the same as that to the co-current flash cascade. The flash trajectories provide a good estimate of the product compositions from a continuous column. We also compared the product compositions from column simulations with the flash trajectories in mole fraction space. We found that product compositions from column simulations surrounded the flash trajectories, in agreement with the hypothesis that the flash trajectories lie in the feasible product regions for continuous RD. 

The design of a distillation column involves many parameters product compositions, product flow rates, operating pressure, total number of trays, feed-tray location, reflux ratio, reboiler heat input, condenser heat removal, column diameter, and column height. Not all of these variables are independent, so a degrees of freedom analysis is useful in pinning down exactly how many independent variables can (and must) be specified to completely define the system. 

The steady-state simulation of distillation columns in Aspen Plus discussed in previous sections took a rating approach to the problem. Specific values for the total number of trays and the feed-tray location were selected, and the required reflux ratio and reboiler duty were determined for this specific configuration, subjected to attaining the desired product specifications. Then, economics must be used to find what the optimum tray configuration is. 

Initially, the total feed is split equally between the two columns. This is achieved in the Splitter labeled Tl on the flowsheet shown in Figure 5.33. Two Design SpecA ary are set up in each column to adjust distillate flow rate and reflux ratio to attain the 99.9 mol% product purities of all foiu streams. The optimum feed tray location is determined by finding the feed stage that minimizes reboiler heat input. In column Cl, it is Stage 19. In column C2, it is Stage 18. 

In conventional distillation design, tray holdup has no effect on steady-state compositions. In reactive distillation, tray holdup (or amount of catalyst) has a profound effect on conversion, product compositions, and column composition profiles. Therefore, in addition to the normal design parameters of reflux ratio, number of trays, feed tray location, and pressure, reactive distillation columns have the additional design parameter of tray holdup. 

First, a steady state model was built. The three reactors are modelled as CSTR and PFR rectors while the reaction kinetics are modelled with the available standard Arrhenius kinetic expressions in HYSYS.PLANT with the kinetic data available in the literature (see Ref. [45-48]). The four separation columns are modelled and simulated, at steady state, as full rigorous distillation columns based on the specifications of the inlet streams, colunms pressure profiles, required number of trays and feed tray location. Moreover, two more specifications are required for each column with both reboiler and condenser. These specifications could be the duties, reflux flow rate, draw streams rates, composition fractions, column recovery, etc. 

Next, shortcut methods are applied to find the minimum number of trays (Fenske equation) for distillation columns, locate the feed tray location (Kirkbride equation), and size the... 

The design variables to be determined in the flowsheet include the solvent-to-feed ratio (FE/FF), total stages of the extractive distillation colmnn (A i), entrainer and fresh feed tray locations (NpE and Npp) total stages of the entrainer recovery column N-2), and feed tray location of entrainer recovery column Np. As can be seen in Knight and Doherty and also summarized in Chapter 5 of Doherty and Malone, the entrainer feed temperature can also been considered as another design variable, so a cooler is included in Figure 10.10. 

Improved response speed can be combined with accurate composition control by means of a cascade loop. It requires a temperature controller whose sensing element is located somewhere between the end of the tower and the feed tray, manipulating distillate flow. The set point of the temperature controller is then positioned in cascade by a composition controller sensing product quality. 

With all feed conditions and the column configuration specified (number of trays in each section, tray holdup in the reactive section, feed tray locations, pressure, and desired conversion), there is only one remaining degree of freedom. The reflux flowrate is selected. It is manipulated by a distillate composition controller to drive the distillate composition to 95 mol% C. The vapor boilup is manipulated to control the liquid level in the base. Note that the distillate and bottoms flowrates are known and fixed as the dynamic model is converged to the steady state that gives a distillate composition of 95 mol% C. The composition of the bottoms will be forced by the overall component balance to be 95 mol% D. 

Feed tray location is an important design parameter in reactive distillation, especially in the ternary decomposition system. As shown in the bottom graph in Figure 6.3, there... 

TABLE 16.2 Results for Reactive Distillation Columns with Optimized Feed Tray Location... 

EFFECTS OF FEED TRAY LOCATIONS ON DESIGN AND CONTROL OF REACTIVE DISTILLATION... 

We emphasize that the reactive section of a reactive distillation column can be viewed as a cascade-type two-phase reactor with the reactor temperature determined by the bubblepoint temperature of the tray liquid-phase composition. It is clear that the composition and temperature profiles will certainly affect the performance of the reactive zone, and the feed tray locations appear to be one of the most effective variables for these profiles redistribution. In this section, we are interested in how the composition profile will be affected by changing the feed tray location. Each individual feed tray location is changed while holding the other constant. 

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