Processes with Side Columns

In the basic a/c-path there exist two sections with twofold separations, see Fig. 11.2-9 (Brusis 2003). These twofold separations can be avoided by arranging the rectifying section of colitrrm C-2 at the top of colitrrm C-1 and by arranging the stripping section of C-2 at the bottom of colurrm C-1. Two heat exchangers of the basic process (Fig. 11.2-5) can be discarded. The internal concentration profiles of the process with side colurrm are depicted in Fig. 11.2-10. No double fractionation exists in the modified process. The feeds into the side column are taken at the concentration maxima of the intermediate boiling compound b. 

The same holds for low concentratiorts of the low boiler in the feed. Here, the energy derrtand of the a/c-path with side colrrrrm is iderrtical with the energy demand for removing the high boiler c alone from a ternary rrrixture, see Fig. 5.2-32 in Chap. 5. This is a very import fact that demonstrates the effectiveness of the 

Side columns are used, for instance, in the most important distillation processes worldwide, the fractionation of air (see Fig. 11.2-18) and the distillation of cmde oil (Meyers 1996). The atmospheric tower of oil refineries consists of a main column and four stripping side columns (Fig. 11.2-12). In this tower the crude oil is split into six fractions which are processed further in several subsequent columns. Oil refineries also have some other interesting features. Steam is fed into the bottom of the main column and most of the side columns. This causes a stripping effect and reduces the temperatures in the columns (steam distillation). The overhead fractions of all side columns are fed into the main colunrn thus increasing the vapor flow there. So-called pump arounds effect a partial condensation of the vapor in the main column and, in turn, a reduction of the vapor flow rates in the upper sections. 

In processes with side columns the gas load is reduced in that section of the main column where the side column is operated in parallel. Since column diameter is pri- 

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Fig. 11.2-8 Flow sheet of the a-path left) and the c-path (right) with side columns. The energy demand of both processes is the same. In the shaded concentration range this process is superior to the a/c-path with direct column coupling...

In special cases, the general process consisting of three distillation columns can be simplified to a process with two columns only. An example of such a process is the fractionation of hydrochloric acid with the entrainer sulfuric acid (Stichlmair and Fair 1998). The boundary distillation hne in Fig. 11.3-6 rans along the binary side of the triangular diagram at high concentration of sulfuric acid. Therefore, the... 

In the section below the feed, the more volatile components are stripped from the liquid and this is known as the stripping section. Above the feed, the concentration of the more volatile components is increased and this is called the enrichment, or more commonly, the rectifying section. Figure 11.1a shows a column producing two product streams, referred to as tops and bottoms, from a single feed. Columns are occasionally used with more than one feed, and with side streams withdrawn at points up the column, Figure ll.lt . This does not alter the basic operation, but complicates the analysis of the process, to some extent. 

Chlorination processes in bubble column reactors<9> are unusual in showing a significant gas-phase resistance to mass transfer. It will be seen from the low value of the Henry law constant 3 in the list of data for the example below, that the solubility of chlorine in toluene is much greater than the solubility of either the carbon dioxide or oxygen considered in the previous examples. This means that when the gas-phase mass transfer resistance is taken in combination with the liquid-phase resistance according to equation 4.19 which is derived in Volume 2, Chapter 12, then the gas side contribution to the resistance is much greater if 3 is small. 

Transportation, 788-822, 1459 automated, 156 definition of, 788 driver scheduling, 812-817 column-generation methodology, 814-815 definition of problem, 813 generation of schedules, 816 iterative process for optimizing, 815-816 set-partitioning formulation with side constraints, 813-814 and driver scheduling, 812-817... 

The matrix approach is easily adapted to partial condensers and to columns with side streams (see Problems 6.C2 and 6.C1). The approach will converge for normal distillation problems. Extension to more complex problems such as azeotropic and extractive distillation or very wide boiling feeds is beyond the scope of this book however, these problems will be solved with a process simulator. 

In Fig. 2-30, this rectification separation process, in two columns operated at two different pressure levels, is explained as a tv/o pressure process for a binary mixture. The binary mixture consists of components 1 and 2, with mole fraction Xp of the low-boiling component 1. In the first column, operated at a lower pressure Pqj, the binary mixture is separated into component 2 as the bottom product, and an azeotropic mixture of composition, as an overhead product. In the second column, operated at a pressure Pg2 > Pgi l he azeotropic mixture is separated into component 1 (at the bottom) and azeotropic mixture x 2 the top). The azeotropic mixture of the second column is then fed into the side of column 1 at an appropriate location. 

Crude tar leaves the coke oven with a water content of 2 to 10% it is stored in tanks to further remove some water before the distillation process. The electrostatic coalescence of water droplets and subsequent removal of water, which is common practice in crude oil refining, is not possible in tar distillation, because of the different density ratios. Since crude tar usually contains chloride, neutralization with soda or sodium hydroxide is required to avoid damage by corrosion. Various schemes are in operation for the distillation of crude tar. Irrespective of the scheme, the first stage of the distillation process is dewatering. The sensible heat of the distillates is used to pre-heat the crude tar. Of the numerous processes for tar distillation, vacuum rectification with bottom pump-around has shown particular merit. In this process (Figure 3.16) the dewatered tar is fed into a main col-unm with around 60 trays after heating up in a gas- or oil-fired pipe still, then distilled into 4 to 5 fractions, together with the pitch residue. Further concentration of the tar constituents, such as naphthalene and anthracene then takes place in side columns. 

Earlier models for continuous processes based on DMT [134, 135] study the influence of variables, such as the initial stoichiometric ratio, reactor temperatures, and average residence times for CSTRs in series cotmected with distillation columns, on process performance. They show it is possible to optimize conversion and minimize formation of side products. Even if kinetics and physical equilibria are now much better known, their qualitative conclusions should still hold. 

The distillate stream from a distillation column (flowrate 35 m /h, density of water) is recycled. The destination of the recycle stream is at a location 3 m below the fluid level in the reflux drum. First, the recycle passes through a pump to supply sufficient head to overcome frictional losses (20 kPa) and reach the pressure of the stream with which it is mixed (250 kPa above distillate stream). Then it is heated from 90°C to 120°C using low-pressure steam at 140°C. Assume that all resistance is on the process-stream side of the exchanger (tube side). 

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