Do

Compressor

Column

Overhead i product

Expansion'' valve r, T

Compressor

Bottom product

Bottom product

FIG. 13-2 Complex distillation operations with single columns. (a) Use of intermediate heat exchangers. (b) Coupling of intermediate heat exchangers with heat pump. (c) Heat pump with external refrigerant. (d) Heat pump with vapor compression. (e) Heat pump with bottoms flashing.

Sometimes, alternative single- or multiple-stage vapor-liquid separation operations, of the types shown in Fig. 13-7, may be more suitable than distillation for the specified task.

A single-stage flash, as shown in Fig. 13-7a, may be appropriate if (1) the relative volatility between the two components to be separated is very large; (2) the recovery of only one component, without regard to the separation of the other components, in one of the two product streams is to be achieved; or (3) only a partial separation is to be made. A common example is the separation of light gases such as hydrogen and methane from aromatics. The desired temperature and pressure of a flash may be established by the use of heat exchangers, a valve, a compressor, and/or a pump upstream of the vessel used to separate the product vapor and liquid phases. Depending on the original condition of the feed, it may be partially condensed or partially vaporized in a so-called flash operation.

If the recovery of only one component is required rather than a sharp separation between two components of adjacent volatility, absorption or stripping in a single section of stages may be sufficient. If the feed is vapor at separation conditions, absorption is used either with a liquid MSA absorbent of relatively low volatility as in Fig. 13-7£> or with reflux produced by an overhead partial condenser as in Fig. 13-7c. The choice usually depends on the ease of partially condensing the overhead vapor or of recovering and recycling the absorbent. If the feed is liquid at separation conditions, stripping is used, either with an externally supplied vapor stripping agent of relatively high volatility as shown in Fig. 13-7a or with boil-up produced by a partial reboiler as in Fig. 13-7c. The choice depends on the ease of partially reboiling the bottoms or of recovering and recycling the stripping agent.

If a relatively sharp separation is required between two components of adjacent volatility, but either an undesirably low temperature is required to produce reflux at the column-operating pressure or an undesirably high temperature is required to produce boil-up, then refluxed stripping as shown in Fig. 13-7g or reboiled absorption

FIG. 13-3 Complex distillation operations with two or more columns. (a) Multieffect distillation. (b) SRV distillation.

FIG. 13-4 Distillation sequences for the separation of three components. (a) Direct sequence. (b) Indirect sequence.

FIG. 13-5 One of 14 different sequences for the separation of a 5-component mixture by simple distillation.
(a)

Product 3

FIG. 13-6 Thermally coupled systems for separation into three products. (a) Fractionator with vapor sidestream and side-cut rectifier. (b) Petlyuk towers.

Product 3

FIG. 13-6 Thermally coupled systems for separation into three products. (a) Fractionator with vapor sidestream and side-cut rectifier. (b) Petlyuk towers.

as shown in Fig. 13-7f may be used. In either case, the choice of MSA follows the same consideration given for simple absorption and stripping.

When the volatility difference between the two components to be separated is so small that a very large number of stages would be required, then extractive distillation, as shown in Fig. 13-7h, should be considered. Here, an MSA is selected that increases the volatility difference sufficiently to reduce the stage requirement to a reasonable number. Usually, the MSA is a polar compound of low volatility that leaves in the bottoms, from which it is recovered and recycled. It is introduced in an appreciable amount near the top stage of the column so as to affect the volatility difference over most of the stages. Some reflux to the top stage is utilized to minimize the MSA content in the distillate. An alternative to extractive distillation is azeotropic distillation, which is shown in Fig. 13-7/ in just one of its many modes. In a common mode, an MSA that forms a heterogeneous minimum-

Liquid MSA

Feed

Feed vapor

Liquid MSA

Vapor MSA

Feed vapor

Feed liquid

Vapor MSA

Liquid MSA

Feed y y

- Liquid

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