Column Sections

The concepts developed for a column section are then applied to the solution of a total column, and a graphical technique is implemented on the Y-X diagram. 

The simplifying assumptions that are necessary for the Y-X solution method are then relaxed to include enthalpy balances. A graphical solution of this more rigorous model, which requires enthalpy-composition data, is presented. 

Solution steps are described for cases with different sets of column specifications. A detailed analysis of column performance viewed through example applications will be covered in the next chapter. 

For the purpose of discussion in this chapter, a column section is defined as any part 

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. 

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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... 

Fig. 5.146, the four column sections are rearranged to form a side-rectifier arrangement. ... 

The spiral should preferably be ground into the inner walls of the long neck—compare the all.glaas Dufton column. Section 11.15. 

The effect of a sidestream withdrawal point is illustrated by Fig. 13-29. The material-balance equation for the column section below the sidestream is... 

Equations such as (13-21) through (13-24) when plotted on the y-x diagram furnish a set of operating lines. A point on an operating hne represents two pa.ssing. streams, and the operating line itself is the locus of all possible pairs of passing streams within the column section to which the hne apphes. 

Pressure drops in the column will he neglected, and the K values will he read at 827 kPa (120 psia) in both column sections from the DePriester nomograph in Fig. 13-14. When constant molar overflow is assumed in each section, the rates in pound-moles per hour in the upper and lower sections are as follows ... 

Operating lines can be found in the ual way from material balances. The slope of each such line is AC/AC = L/U, where L is the downflow rate in the particular column section and C is now the concentration in the descending stream. 

Most lakes affected by eutrophication will also have significant amounts of phosphorus in their sediments, which can be recycled into the water column (Section 4). The control of this source can be achieved by treating the sediments with iron salts or calcite to bind the phosphorus more tightly into the sediments. These methods have been used to some effect, but consideration has to be given to the quality of the materials used and whether or not the lake can become de-oxygenated in the summer. In the latter case this can be overcome by artificial de-stratification. 

Final design of packed columns should be performed by experts, but the layman is often required to provide preliminary designs for studies. This packed column section provides the information necessary for such estimates. 

For packed columns see the rules of thumb in Chapter 3, Fractionators, in the Packed Columns section. 

Tower Operations. The tower operator can quickly determine which type of flooding will tend to be the limiting one for a particular system. If a rigorous computer run is available for the anticipated or actual operation, the operator can quickly calculate the expected limiting column section. The operator can then provide DP cell recording for the entire column and limiting section(s). As mentioned previously, a DP cell is the best measure of internal traffic and flooding tendency. 

Flooding across a column section reflects itself in an increase in pressure drop and a decrease in temperature difference across the affected section. Product quality is also impaired, but it is hoped that the other indicators will allow correction of the situation before major change in product quality. When a column floods, the levels in the accumulator and bottom often change. It can occur that the accumulator fills with liquid carried over while the reboiler runs dry. 

By virtue of its chemical and thermal resistances, borosilicate glass has superior resistance to thermal stresses and shocks, and is used in the manufacture of a variety of items for process plants. Examples are pipe up to 60 cm in diameter and 300 cm long with wall tliicknesses of 2-10 mm, pipe fittings, valves, distillation column sections, spherical and cylindrical vessels up 400-liter capacity, centrifugal pumps with capacities up to 20,000 liters/hr, tubular heat exchangers with heat transfer areas up to 8 m, maximum working pressure up to 275 kN/m, and heat transfer coefficients of 270 kcal/hz/m C [48,49]. 

The packed column section contains a stripper pre-column (column 1), which separates the Cg+ fraction by back-flushing all compounds above -pentane in one peak. HjS, CO2, C2, O2, N2 and Cj are trapped in columns 3 and 4, while C3-C5 hydrocarbons elute from column 2 to the TCD. The remaining components are... 

Procedure. Prepare an anion exchange column (Section 7.8) using about 40g of Duolite A113 (chloride form). The ion exchange tube may be 16 cm long and about 12 mm internal diameter. Wash the column with 0.6M sodium nitrate until the effluent contains no chloride ion (silver nitrate test) and then wash with 50 mL of 0.3 M sodium nitrate. 

Species The test species, whether animal or human, are identified in this column. Section 2.5, "Relevance to Public Health," covers the relevance of animal data to human toxicity and Section 2.3, "Toxicokinetics," contains any available information on comparative toxicokinetics. Although NOAELs and LOAELs are species specific, the levels are extrapolated to equivalent human doses to derive an MRL. 

A middle-vessel hdXch column consists of two column sections and one middle vessel. The feed mixture is introduced in the middle vessel and the products are simultaneously withdrawn from the top and the bottom of the column, with the result that the material in the vessel is increasingly purified. 

A matrix is defined as an ordered rectangular arrangement of scalars into horizontal rows and vertical columns (Section 9.3). On the one hand, one can consider a matrix X with n rows and p columns as an ordered array of p vectors of dimension n, each of the form ... 

However, to fully understand the design of the column, the material balance must be followed through the column. To simplify the analysis, it can be assumed that the molar vapor and liquid flowrates are constant in each column section, which is termed constant molar overflow. This is strictly only true if the component molar latent heats of vaporization are the same, there is no heat of mixing... 

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 side-rectifier and side-stripper arrangements have some important degrees of freedom for optimization. In these arrangements, there are four column sections. For the side-rectifier, the degrees of freedom to be optimized are ... 

All of these variables must be optimized simultaneously to obtain the best design. Some of the variables are continuous and some are discrete (the number of stages in each column section). Such optimizations are far from straightforward if carried out using detailed simulation. It is therefore convenient to carry out some optimization using shortcut methods before proceeding to detailed simulation where the optimization can be fine-tuned. 

The partitioned thermally coupled prefractionator in Figure 11.14c can be simulated using the arrangement in Figure 11.14b as the basis of the simulation. However, like side-rectifiers and side-strippers, fully thermally coupled columns have some important degrees of freedom for optimization. In the fully thermally coupled column, there are six column sections (above and below the partition, above and below the feed in the prefractionator and above and below the sidestream from the main column side of the partition). The degrees of freedom to be optimized in partitioned columns are ... 

The rectifying or stripping section of a column must operate somewhere between total reflux and minimum reflux conditions. The range of feasible operation of a column section can thus be defined for a given product composition. It can be seen in Figure 12.19 that these section profiles are bounded for a stage column by the distillation line and the pinch point curve. As noted previously, the pinch point curve provides a minimum reflux boundary for both staged and packed columns,... 

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