Distillation concentration profiles

Equimolar Counterdiffusion in Binary Cases. If the flux of A is balanced by an equal flux of B in the opposite direction (frequently encountered in binary distillation columns), there is no net flow through the film and like is directly given by Fick s law. In an ideal gas, where the diffusivity can be shown to be independent of concentration, integration of Fick s law leads to a linear concentration profile through the film and to the following expression where (P/RT)y is substituted for... 

Batch distillation with continuous control of distillate composition via the regulation of reflux ratio is illustrated in the simulation example BSTILL. In this an initial total reflux condition, required to establish the initial concentration profile with the column, is represented in the simulation by a high initial value of R, which then changes to the controller equation for conditions of distillate removal. 

The process is as described in Section 3.3.3.2 and consists of a distillation column containing seven theoretical plates, reboiler and reflux drum. Distillation is carried out initially at total reflux in order to first establish the column concentration profile. Distillate removal then commences at the required distillate composition under proportional control of reflux ratio. This model is based on that of Luyben (1973, 1990). 

Figure 13.12. Concentration profiles in two kinds of distillations, (a) Purifying column for fermentation alcohol small streams with high concentrations of impurities are withdrawn as sidestreams (Robinson and Gilliland, Elements of Fractional Distillation, McGraw-Hill, New York, 1939 edition), (b) Typical concentration profiles in separation of light hydrocarbon mixtures when no substantial inversions of relative volatilities occur (Van Winkle, Distillation, McGraw-Hill, New York, 1967).

Finally, nonlinear wave can also be used for nonlinear model reduction for applications in advanced, nonlinear model-based control. Successful applications were reported for nonreactive distillation processes with moderately nonideal mixtures [21]. For this class of mixtures the column dynamics is entirely governed by constant pattern waves, as explained above. The approach is based on a wave function which can be used for the approximation of the concentration profiles inside the column. The wave function can be derived from analytical solutions of the corresponding wave equations for some simple limiting cases. It is given by... 

At first sight, adsorption and reaction are well-matched functionalities for integrated chemical processes. Their compatibility extends over a wide temperature range, and their respective kinetics are usually rapid enough so as not to constrain either process, whereas the permeation rate in membrane reactors commonly lags behind that of the catalytic reaction [9]. The phase slippage observed in extractive processes [10], for example, is absent and the choice of the adsorbent offers a powerful degree of freedom in the selective manipulation of concentration profiles that lies at the heart of all multifunctional reactor operation [11]. Furthermore, in contrast to reactive distillation, the effective independence of concentration and temperature profiles... 

Whilst the enhancement of unwanted side reactions through excessive distortion of the concentration profiles is an effect that has been reported elsewhere (e.g., in reactive distillation [40] or the formation of acetylenes in membrane reactors for the dehydrogenation of alkanes to olefins [41]), the possible negative feedback of adsorption on catalytic activity through the reaction medium composition has attracted less attention. As with the chromatographic distortions introduced by the Claus catalyst, the underlying problem arises because the catalyst is being operated under unsteady-state conditions. One could modify the catalyst to compensate for this, but the optimal activity over the course of the whole cycle would be comprised as a consequence. 

The above analysis suggests a third alternative (Figure 3.17) with only two columns. The first split could deliver high-purity acetone, while the second split would give chloroform with acetone as impurity. The representation predicts that chloroform purity would not exceed 98% for a reasonable amount of entrainer. Again, computer simulation gives a much better solution. The concentration profile for the first column shows clearly that the distillation border is crossed at finite reflux, and high purity can be obtained in the second split. 

Figure 6.16 Temperature and concentration profiles in a catalytic distillation column.

Figure 8.17 (a) Concentration profiles of species in the entrainer-enhanced reactive distillation column ... 

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... 

Figure 8-1 Concentration profiles for crystallization by evaporation as a function of time of distillation or amount of solvent removed. A-B-C-E is the preferred pathway for favoring growth.

Figure 8-7 A concentration profile for a controlled crystallization by evaporation (revised procedure) with seeding and a seed age in Example 8-1. The seeding point was much closer to the solubihty curve, and sufficient seed aging was given to release the supersaturation before further distillation.

Note that the plate numbering in the program is slightly different to that shown in Fig. 1, owing to the use of the vector notation to include both the reflux drum and the column base. In the program, index number 1 is used to denote the reflux drum and product distillate, and index Nplate+1 is used to denote the reboiler and bottoms product. This is convenient in the subsequent plotting of the steady state composition profiles in the column. Both Nplate and the feed plate location Fplate are important parameters in the simulation of the resulting steady state concentration profiles and the resultant column optimisation. 

Use concentration profiles developed from either equilibrium or nonequilibrium reaction-separation to identify the reactive zone. The reflux ratio for reactive distillation is greater than for distillation. Use 1.2 to 1.4 X minimum. For catalytic structured packing, use liquid loadings up to 14 L/s-m and vapor capacities, F factor, of 2.5 m/s (kg/m ) (based on velocity and the root of vapor density). 

CALCULATION OF REQUIRED REFLUX RATIO AND CONCENTRATION PROFILES. The number of plates needed for a specified separation at a selected reflux ratio can be determined by a plate-by-plate calculation called the Lewis-Matheson method. The amount of all components in the products must be specified to start the calculation. From the composition of the distillate (which is the same as the vapor from the top if a total condenser is used), the temperature and liquid... 

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