Distillation prefractionators

In addition, one other feature of the prefractionator arrangement is important in reducing mixing effects. Losses occur in distillation operations due to mismatches between the composition of the column feed and the composition on the feed tray. Because the prefractionator distributes component B top and bottom, this allows greater freedom to match the feed composition with one of the trays in the column to reduce mixing losses at the feed tray. 

Introduce complex distillation configurations. Introduce prefractionation arrangements (with or without thermal coupling), side-rectifiers, and side-strippers to the extent that operability can be... 

Extractive distillation, using similar solvents to those used in extraction, may be employed to recover aromatics from reformates which have been prefractionated to a narrow boiling range. Extractive distillation is also used to recover a mixed ben2ene—toluene stream from which high quaUty benzene can be produced by postfractionation in this case, the toluene product is less pure, but is stiU acceptable as a feedstock for dealkylation or gasoline blending. Extractive distillation processes for aromatics recovery include those Hsted in Table 4. 

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 elimination of mixing losses in the prefractionator arrangement means that it is inherently more efficient than an arrangement using simple columns. The same basic arguments apply to both distributed distillation and prefractionator arrangements, with the additional degree of... 

Consider now ways in which the best arrangement of a distillation sequence can be determined more systematically. Given the possibilities for changing the sequence of simple columns or the introduction of prefractionators, side-strippers, side-rectifiers and fully thermally coupled arrangements, the problem is complex with many structural options. The problem can be addressed using the optimization of a superstructure. As discussed in Chapter 1, this approach starts by setting up a grand flowsheet in which all structural features for an optimal solution are embedded. 

Fractionation is the basic refining process for separating crude petroleum into intermediate fractions of specified boihng point ranges. The various subprocesses include prefractionation and atmospheric fractionation, vacuum fractionation, and three-stage crude distillation. 

Prefractionation is an optional distillation process to separate economic quantities of very light distillates from the crude oil. Lower temperatures and higher pressures are used than in atmospheric distillation. Some process water can be carried over to the prefractionation tower from the desalting process. 

Nikolaides IP, Malone MF. Approximate design and optimization of a thermally coupled distillation with prefractionation. Ind Eng Chem Res 1988 27 811. 

Extractive distillation, that is, fractional distillation in the presence of a solvent, is used to recover aromatic hydrocarbons from, say, reformate fractions in the following manner. By means of preliminary distillation in a 65-tray prefractionator, a fraction containing a single aromatic can be separated from the reformate, and this aromatic concentrate is then pumped to an extraction distillation tower near the top, and aromatic concentrate enters near the bottom. A reboiler in the extractive distillation tower induces the aromatic concentrate to ascend the tower, where it contacts the descending solvent. 

Complex columns are distillation devices that can handle a mixture of minimum three components and deliver more than two products. A complex column consists of a main tower surrounded by additional columns, as prefractionator, side strippers and side rectifiers. As illustration, Figure 3.7 presents five alternatives for separating a ternary mixture ABC ... 

Figure 4.13. Prefractionation arrangements (a) removing light keys with absorber (b) removing heavy keys with stripper (c) heat pumping (d) vapor recompression (e) reboiler flashing. B bottom product, D distillate, V valve.

Description Raw pyrolysis gasoline is prefractionated into a heartcut C8 stream. The resulting styrene concentrate is fed to an extractive-distillation column and mixed with a selective solvent, which extracts styrene to the tower bottoms. The rich solvent mixture is routed to a solvent-recovery column, which recycles lean solvent to the extractive-distillation column and recovers the styrene overhead. A final purification step produces a 99.9% styrene product containing less than 50 ppm phenyl acetylene. 

Sidestream column with prefractionator. Figure 6.23c illustrates a complex configuration in which a prefractionator column is used to perform a preliminary separation of the ternary feed. The idea is to produce a distillate from the first column that contains very little of the heaviest component C. When this distillate is fed into the second column at a location above the sidestream drawoff, there will be only a small amount of C that must flow down past the sidestream tray. This permits the production of high-purity sidestream product. Similarly the prefractionator should let very little of the lightest component A drop out the bottom so that there is little A in the vapor stream flowing past the sidestream tray. This lets us achieve high sidestream purities. 

Simple Distillation. In this category, we include the separation of ideal or slightly non-ideal mixtures that do not form azeotropes, based on the differences in the relative volatilities of components. A simple column designates a device that separates one or several feeds in only two products top distillate and bottoms. Complex columns are can deliver more than two products. In this category we include columns with side-streams, columns equipped with auxiliary devices, as prefractionators, side-strippers and side-rectifiers, as well as thermally integrated columns. 

From the above discussion it can be observed that the initial diluted feed can be sent to the water-column and not to the ethanol recovery column (azeotropic distillation). In consequence, this alternative has only two columns, and has been occasionally presented as innovative. Doherty Malone (2001) discusses in detail all the alternatives discovered along the time using benzene as entrainer. The twO-column sequence is not the most economical because of large entrainer recycle and expensive columns. It seems that the two-column sequence with prefractionator, in total three units, offers the best compromise between investment and operation (solvent recycle) costs. 

Consider the separation of an equimolar mixture of pentane (A)/hexane (B) /heptane (C) with a total flow rate of 150 kmol/h. The desired purity for A, B and C is of 0.99, 0.98 and 0.99. Study the design of a heat-integrated distillation scheme with prefractionator. 

Bildea, C. S., A. C. Dimian, 1999, Interaction between design and control of a heat integrated distillation system with prefractionator, Trans. Inst. Chem. Engnrs., ChERD, vol. 77, Part A, 597-607... 

Crude allyl chloride from the bottom of the prefractionator is refined by taking top and bottom cuts in two continuous distillation steps. In the topping operation, light ends and traces of propylene are removed as vapors in the second distillation step, finished allyl chloride is taken overhead, and the bottom material is sent to storage. 

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