Column section simple columns

First consider thermal coupling of the simple sequences from Fig. 5.1. Figure 5.14a shows a thermally coupled direct sequence. The reboiler of the first column is replaced by a thermal coupling. Liquid from the bottom of the first column is transferred to the second as before, but now the vapor required by the first column is supplied by the second column instead of by a reboiler on the first column. The four column sections are marked as 1, 2, 3, and 4 in Fig. 5.14a. In... 

This remixing that occurs in both sequences of simple distillation columns is a source of inefficiency in the separation. By contrast, consider the prefractionator arrangement shown in Figure 11.9. In the prefractionator, a crude split is performed so that Component B is distributed between the top and bottom of the column. The upper section of the prefractionator separates AB from C, whilst the lower section separates BC from A. Thus, both sections remove only one component from the product of that column section and this is also true for all four sections of the main column. In this way, the remixing effects that are a feature of both simple column sequences are avoided4. 

The slowest part of the construction of this table is the evaluation of the entries in the first column. The simple trapezoid rule, as given by Eq, (65), is applied with successive sectioning of the slices. It can be seen that by descending the column a limiting value can, in principle, be obtained. However, the convergence is very slow. With the use of the recursion relation... 

The primary difficulty with the spatial approach is the need to section and analyze column segments. In particular, saturated zeolite materials are difficult to extrude from a column. In our work, therefore, we have used a scooping procedure, in which a stainless steel spoon is used to remove material from one end of the column. This simple approach has yielded smooth contaminant mass profdes, with good reproducibility between replicate columns. A similar procedure was reported by Fuhrmann et al. (1995). 

A simple column can be regarded as a two-section complex column. Section 1 is the rectifying section, while section 2 is the stripping section, Table 2,6 draws an analogy between the simple and the complex... 

Section 3.1.1 states that in a process design, a separation is specified in terms of purities and product flows. For a simple column, two specifications are made and at least one must be a purity. Section 3.1.1 also states that the purity specification can be substituted by a physical property which is a function of the purity or composition, while a product flow can be substituted by a recovery specification. 

The intermediate hopper acts as a storage balancing the inventory change. Evidently there is essentially no inlet and outlet effect, inasmuch as the voidage curves taper off into their respective asymptotic values for both the top and bottom sections of the fast column. This simple boundary condition facilitates flow modeling and the solution of the hydrodynamic equations. It should also be noted that the acceleration zone for the materials tested is too short to call for consideration. 

To improve the stage distribution for each section in the crude tower, the tower is decomposed into a sequence of simple columns as a first step. After that, the ideal number of stages and the optimum feed stage for each simple column are found. Finally, the simple columns are merged back to the complex column with new numbers of stages for each section. 

To redistribute the stages in the remaining sections, a shortcut simulation is used to find out the required number of trays, the feed tray location and the minimum reflux ratio for each column in the sequence. To make use of the existing column with the same number of trays (24 trays) iterations are required to adjust the sum of the rectifying sections in each column equal to 24 (number of trays in the main column). Finally, the sequence of the simple columns is merged into a complex column. The main column is not changed, but the side strippers and pump arounds need to be relocated or adjusted. 

A simple absorber is itself a column section. However, in the case of binary systems, it is not practical to have a column section as a unit operation on its own it must be part of a total column. The reason for this is that, in order to operate such a column section, one would need binary liquid and vapor feeds with disparate compositions. Generating such streams would have required some separation process in the first place. 

As a simple example, an absorber is itself one column section with zero degrees... 

By definition, the column section is limited in practical application to modeling simple absorbers, strippers, and liquid-liquid extractors—processes with one feed and one product at each end and with no heat addition or removal. Given a reliable method for solving the column section, however, it could be used as a module to build more complex configurations. 

To begin the calculations the column variables must be first initialized to some estimated values. Simple methods can be used for this purpose, based on the column specifications and possibly supplemented by shortcut methods. The column temperature profile may be assumed linear, interpolated between estimated condenser and reboiler temperatures. The values for Lj and Vj may be based on estimated reflux ratio and product rates, assisted by the assumption of constant internal flows within each column section. The compositions Xj- and T, may be assumed uniform throughout the column, set equal to the compositions of the liquid and vapor obtained by flashing the combined feeds at average column temperature and pressure. The other variables to be initialized are Rf,Rj, and Sj, which are calculated from their defining equations. The values for Qj may either be fixed at given values (zero on most stages) or estimated. 

Consider a section of packing of height Az within the simple column. Liquid and vapor streams enter this section at flows of L and V, and compositions of x and y, respectively. With the mass transfer that occurs between the two phases, it is assumed the total flows do not change, but the respective compositions do. The compositional change is as indicated in Figure 2.11. 

A simple column (i.e., one with a single feed and two products) will thus have two CSs, one below and one above the feed, as shown in Figure 3.1a. These particular CSs are more commonly referred to as the rectifying section (1) and stripping section (2). Figure 3.1b extends the idea of CS breakdowns to an arbitrarily chosen distillation configuration. 

FKiURE 3.1 A CS breakdown of (a) a simple column into the rectifying [1] and stripping sections [2], and (b) an arbitrary column into a sequence of column sections. 

It is worth noting that the DPE is based on the pioneering work by Doherty and coworkers [2 6] for conventional (nongeneralized) rectifying and stripping sections, as found in simple columns. It will be shown in Chapter 5 how the generalized DPE can be applied to these conventional distillation sections, and in fact produce the same result. 

Xa in CSs with Condensers/Reboilers When considering the individual CSs of a simple column in Figure 3.9, it can easily be shown by mass balance that for a rectifying section terminated by a condenser, the difference point is... 

The general definition given in Equation 3.20 can be written specifically for a simple column rectifying section using Equation 3.11 to give the familiar definition of reflux... 

This superimposition is quite a remarkable result. It implies that at some operating conditions and initial compositions, a CS will have its hottest temperature at the top of the CS, thereby creating an inverse temperature profile (remember, in conventional simple columns the coldest temperature is at the top). While an inverse temperature profile is not necessarily desired in simple columns, it may very well be a feasible mode of operation in complex columns that have CSs that are neither rectifying nor stripping CSs found in simple columns. This result of an inverse temperature profile in a CS has further been validated experimentally, and the reader is referred to Section 4.5.3 for the details. 

Figure 3.20a shows the pinch point loci for Xas in the MET, also shown in detail in Figure 3.19. In Section 3.5.2, it was discussed that CSs terminated by a reboUer or condenser which produce product streams will have the composition of the product equal to Xa for the CS. In other words, for these CSs, the design is limited to Xas in the MET, and the associated pinch point loci (Figure 3.20a). Thus, the behavior of the pinch points in simple columns is constrained to only these flow patterns and their associated topological behavior.

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