Flow profiles column distillation

The method starts with an assumption of the column temperature and flow profiles. The stage equations are then solved to determine the stage component compositions and the results used to revise the temperature profiles for subsequent trial calculations. Efficient convergence procedures have been developed for the Thiele-Geddes method. The so-called theta method , described by Lyster et al. (1959) and Holland (1963), is recommended. The Thiele-Geddes method can be used for the solution of complex distillation problems,... 

Another phenomenon of highly nonlinear systems is parametric sensitivity. We illustrated this behavior for the temperature profile in the plug-flow reactor. Nonideal distillation systems can also show this sensitivity. For example, in Fig. 6.5 a small change in the feed composition or organic reflux flow can dramatically change the composition ( and t emperature) profile in the column. Instead of a vinyl acetate-rich profile in the top section, a water-rich profile can be present. 

Figure 14.21. Calculated flow profiles in experimental extractive distillation column.

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... 

To a first approximation, the composition of the distillate and bottoms of a single-feed continuous distillation column lies on the same residue curve. Therefore, for systems having separatrices and multiple regions, distillation composition profiles are also constrained to lie in specific regions. The precise boundaries of these distillation regions are a function of reflux ratio, but they are closely approximated by the RCM separatrices. If a separatrix exists in a system, a corresponding distillation boundary also exists. Also, mass balance constraints require that the distillate composition, the bottoms composition, and the net feed composition plotted on an RCM for any feasible distillation be collinear and spaced in relation to distillate and bottoms flows according to the well-known lever rule. 

In his analysis of the open tube distillation column Westhaver (1942) goes into a detailed consideration of radial concentration gradients which is very similar to Taylor s approach. His final formula, however, is the same as if he had assumed a constant velocity profile and an effective diffusion coefficient (Dt + llU2r2l48Dt). This is just the diffusion coefficient that we have found for viscous flow in the presence of a film on the tube wall in which the solute concentration is infinitely greater than in the fluid. This is clearly the case for... 

The initial total flow rate and temperature profiles can make the difference between success and failure of a rigorous method. Usually for distillation columns, the condenser and reboiler temperatures are estimated and a calculation that assumes constant molal overflow Sec. 2.2.2) is used to initialize tbe internal vapor and liquid flow... 

Figure 3 shows the internal profile in the composition tetrahedron after simulation of the hybrid column in Hysys. A loss of about 0.18 % w/w of ethanol in methanol-rich distillate occurs. The side streams are located near the maximum in ethanol (ethanol-rich stream, 88.98 % w/w) and fusel (fusel-rich stream, 18.5 % w/w), respectively. The column has 35 equilibrium stages, a column diameter of 1.372 m, a section pressure drop of 11.8 kPa and a steam flow rate of 1800 kg/h. The vapour ethanol-rich stream is diverted to the first effect of the evaporation sector to provide heating while minimizing the steam demand of the plant. The condensed ethanol-rich stream is then fed to the pervaporation sector to remove the excess water. 

This method also considers the stage temperatures as the independent variables. The algorithm is applied to a single-feed, two-product column with a partial condenser and reboiler. As in the original Thiele-Geddes method, the problem definition is such that the feed component flow rates, are known and fixed. The column pressure profile is also fixed, as well as its configuration, which defines the number of stages and feed location. In addition, one product rate (the distillate) and one internal flow (such as the reflux rate, Lj) are specified. The solution method, outlined below, is described in detail by Holland (1975). 

Figure 4. Analysis of water extracts from coal. Pvunp Micro Feeder. Column SC-01, 50 cm X 0.22 mm i.d. Mobile phase acetonitrile-water, gradient profile as Indicated. Flow-rate 1.04 yl/min. Sample 2.5 ml of distilled water in contact with powdered coal. Precoliatm Develosil ODS-15/30, 10 X 0.2 mm i.d. Wavelength of UV detection 225 nm. Reproduced with the permission from Ref.7 Copyright 1983, Huethig.

The dichloromethane solution is concentrated to 2-3 mL by distillation using a 40-cm length Vigreux column, and finally to 200 pL under a nitrogen flow prior to GC/MS analysis. The GC/MS profile of free aroma compounds of a Muscat grape skin extract is shown in Fig. 4.6. 

To initiate the calculational procedure for the determination of the product distribution for specified reflux and distillate rates, a number of plates between the two pinches is selected. (As discussed in a subsequent section, too few plates but not too many plates may be selected.) Next L/V and temperature profiles for the plates between and including the two pinches as well as the distillate and bottoms temperatures are selected. Next the components of the feed are classified according to the above criteria. Since it is supposed that the complete definition of the feed, the reflux and distillate rates, as well as the column pressure and type of condenser are specified, the component-material balances can be solved for the component-flow rates throughout the column. The component-material balances may be simplified by taking advantage of the unique characteristics of the three classes of components, the distributed components, the separated lights, and the separated heavies. 

Visualisation of profiles inside complex equipment is a valuable instrument for understanding how the unit really works. A typical example is the plot of profiles insides a distillation column, in term of pressures, temperatures, and composition on stages, as well as of material and enthalpy flows. These profiles are highly recommended for debugging non-convergenee problems. Even in the case of successful runs internal profiles allow the evaluation of design elements. For example, the inspection of temperatures and composition profiles can easily identify incorrect feed loeation or superfluous stages in a distillation column. 

Tolliver and McCune (402, 403) proposed an improved procedure, based on the sensitivity of the column temperature profile to material balance variations. Accordingly, column temperature profiles are evaluated at different distillate-to-feed (J IF) ratios, while either reboiler duty or reflux flow is kept constant. The best temperature control point is where temperature variations are largest and most symmetrical (403). The analysis can be readily performed using steady-state computer simulation. 

This is different from the distillation profiles generated earlier in this book, where the CMO assumption was employed those profiles had only x associated with every point. The reflux was held constant, and did not change down the length of the CS. In this scenario, however, both compositions and flows change down the length of the MGS. Hence, both x and change along a membrane column profile. 

Fig. 9.12 a) Configuration of reactive distillation column for hydration of ethylene oxide to ethylene glycol used by Ciric and Miao (1994). b) Equilibrium model calculations for the ethylene glycol process showing column profiles for liquid phase mole fraction, temperature and vapor phase molar flow, c) Nonequilibrium model calculations for the ethylene glycol process for a column of diameter 1.7 m showing the corresponding column profiles. Details of the simulations are available in Baur et al. 

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