Continuous-contact operations distillation

In the following, we divide continuous-contact operations into two distinct categories. The first deals with classical packed-column operations in countercurrent flow. We revisit the packed-gas scrubber we first saw in Chapter 2 and provide a general survey of packed-gas absorption operations. Packed-column distillation is addressed next and, in a somewhat unusual departure from the norm, we reexamine coffee decaffeination by supercritical extraction. 

As the potentialities of liquid extraction as a separation method were developed, the need for efficient, continuously operated, multistage equipment became apparent. It was natural therefore to turn to devices which had been so successful in other similar fluid-contacting operations, such as the bubble-tray tower and the packed tower of distillation. These devices have proved to be disappointing in liquid-extraction service, however for example, bubble-tray towers provide tray efficiencies in liquid-extraction operations of less than 5% (S7), and conventional packed towers show heights of transfer units of 10 to 20 ft. or more (T3). 

Systems that exhibit behavior of the type illustrated in Fig. 4 cannot be purified in a single crystallization stage. They represent situations in which multiple stages or continuous-contacting devices may be useful. The principles of such operations are analogous to those of other countercurrent contacting operations—for example, distillation, absorption, and extraction. 

Contacting Columns. A tall cylindrical column or tower can be filled with packing for continuous contact of two or more phases or fabricated with a number of trays at fixed distances apart for stagewise contact operation. Such columns, either alone or in series, are commonly specified for separations-in gas absorption, distillation, extraction, and humidification. 

D8. We plan to batch distill a mixture of methanol and water in a batch distillation system at 1.0 atm pressure. The distillation system consists of a large still pot that is an equilibrium contact, a distillation column that acts as two equilibrium contacts (total 3 equilibrium contacts), and a total condenser. The feed to the system is F = 10.0 kmol and Xp = 0.4 (mole fraction methanol). We operate with a constant Xq = 0.8 as we increase L/D. The batch operation is continued until L/D = 4.0. CMO is valid. Equilibrium data are in Table 2-7. Find ... 

In 1964, White published contradictory results on the water wettability of gold, and Fowkes offered the suggestion that, since dispersion forces alone act from noble metal surfaces, the observation that water would spontaneously spread on them meant they were covered by surface oxide or some hydrophilic coating. In 1965 Erb, working with a closed distillation system in which a pure stream of water was continuously cycled over a pure gold surface, reported a steady state value of the contact angle of between 55° and 85° after several thousand hours of continuous still operation. 

Figure 8.1.32. (a) Packed tower continuous contact distillation column, (b) McCabe-Thiele plot of operating lines in a packed tower... 

Most distillations conducted commercially operate continuously, with a more volatile fraction recovered as distillate and a less volatile fraction recovered as bottoms or residue. If a portion of the distillate is condensed and returned to the process to enrich the vapors, the Hquid is called reflux. The apparatus in which the enrichment occurs is usually a vertical, cylindrical vessel called a stiU or distillation column. This apparatus normally contains internal devices for effecting vapor—Hquid contact the devices may be categorized as plates or packings. 

The transfer of mass from one phase to another is involved in the operations of distillation, absorption, extraction, humidification, adsorption, drying, and crystallization. The principal function of the equipment used for these operations is to permit efficient contact between the phases. Many special types of equipment have been developed that are particularly applicable for use with a given operation, but finite-stage contactors and continuous contactors are the types most commonly encountered. A major part of this chapter, therefore, is devoted to the design aspects and costs of stagewise plate contactors and continuous packed contactors. 

The separation operation called distillation utilizes vapor and liquid phases at essentially the same temperature and pressure for the coexisting zones. Various kinds of devices such as random or structured packings and plates or trays are used to bring the two phases into intimate contact. Trays are stacked one above the other and enclosed in a cylindrical shell to form a column. Packings are also generally contained in a cylindrical shell between hold-down and support plates. The column may be operated continuously or in batch mode depending on a number of factors such as scale and flexibility of operations and solids content of feed. A typical tray-type continuous distillation column plus major external accessories is shown schematically in Fig. 13-1. 

Solvent extraction. The press cake emerging from a screw press still retains 3 to 15 percent of residual oil. More complete extraction is done by solvent extraction of the residues obtained from mechanical pressing. The greater efficiency obtained in the solvent extraction process encouraged the industry for direct application to oilseeds. In the United States and Europe, continuous extractor units are used in which fresh seed flakes are added continuously and are subjected to a counterflow of solvent by which intimate contact is achieved between the seeds and solvent. The common solvent for edible oil is commercial hexane or heptane, commonly known as petroleum ethers, boiling in the range of 146 to 156°F (63.3 to 68.9°C). After extraction, maximum solvent recovery is necessary for economical operation. The solvent is recovered by distillation and is reused. The extraction oil is mixed with prepress oil for refining. The extracted meals contain less than 1 percent of residual oil. 

Vapor-liquid mass-transfer operations, such as absorption, stripping and distillation, are carried out in packed and plate columns. The key difference is that counterflowing vapor and liquid are contacted continuously with packings, and discretely with plates. The equilibrium and operating lines of packed and plate columns are identical under the same operating conditions—feed and product flowrates and compositions, temperature and pressure. Models for the design and analysis of packed columns are based on their close analogy to plate devices. 

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