Distillation columns contacting devices

TABLE 5.7-1 Criteria for the Selection of Distillation Column Contacting Devices... 

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 separation operation called distillation utihzes vapor and hquid phases at essentially the same temperature and pressure for the coexisting zones. Various lands of devices such as r andom or sti uctui ed packings and plates or tr ays are used to bring the two phases into intimate contact. Trays are stacked one above the other and enclosed in a cyhndrical shell to form a column. Pacldngs are also generally contained in a cyhndrical shell between hold-down and support plates. A typical tray-type distillation column plus major external accessories is shown schematically in Fig. 13-1. 

Packed column. A distillation column filled with an inert material to enhance vapor/liquid contact. The packing can be beads, pellets, Raschig rings, metal chains, or specifically shaped devices such as saddles, helices, or rings. 

A stage in a separation process could, for instance, be a tray in a distillation column, a section in a packed absorption column, or a partial condenser or reboiler. An equilibrium stage is a contacting device—a vessel—in which two or more phases can mix thoroughly and interact with each other for a sufficient length of time until the number of moles of any component moving from one phase to another equals the number of moles of that component moving in the opposite direction, that is, until no more net mass transfer takes place between the phases. The composition of each phase is then the equilibrium composition. 

The case of total reflux also represents a practical condition for operating a distillation system during startup or for determining the efficiency of various contacting devices for a distillation column. Performance data for such devices, shown later herein, are likely obtained under total reflux conditions. 

The typical distillation column has dimensions sufficient for handling the requited flows of vapor and liquid and for making the desired separation. The vessel contains internal devices that ate designed to promote inornate contacting of the vapor and liquid. The amounts and properties of the streams usually follow from the equilibrium stage or transfer unit calculations (Sections 5.3 and 5.6). 

The contacting devices, or internals, of the distillation column may be classified as shown in Table... 

The devices of contact between liquid and vapour in the distillation and absorption columns which we meet most generally are trays with sieves and with valves and, in a more or less large extent, of trays with bubble caps. The capacity of retention of these trays is mostly limited by the phenomena of entrainment or flooding. In all the distillation columns of trays type, liquid entrainment and flooding can be present. 

The cascade shown in Figure 3-5 can be further simplified by building the entire system in a column instead of as a series of individual stages. The intermediate heat exchange can be done very efficiendy with the liquid and vapor in direct contact on each stage. The result is a much sinpler and cheaper device. A schematic of such a distillation column is shown in Figure 3-6. 

An enormous amount of work has gone into the study of contacting devices, and the literature on their performance characteristics is extensive. Most of the work dealing with laiger equipment has been based on plant observations, but a significant portion of it has been based on controlled experiments in commercial-scale equipment by Fractionation Research, Inc. (FRI). The methods for analysis and design of distillation columns, present in this section, are based on a combination of fundamental research papers, results released by FRI, and many repotted plant tests. 

An absorber column is a device used to remove selected components from a gas stream by contacting the stream with a gas or liquid. A typical gas absorber is a plate distillation column or packed distillation column that ensures intimate contact between raw natural gas and an absorption medium. Absorption can roughly be compared to fractionation, although absorption columns work differently than typical fractionators because during the process the vapor and liquid do not vaporize to any degree. 

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