Distillation columns liquid entrainment

For preliminary design, liquid entrainment is usually used as a reference. To prevent entrainment, the vapor velocity for tray columns is usually in the range 1.5 to 3.5 ms-1. However, the entrainment of liquid droplets can be predicted using Equation 8.3 to calculate the settling velocity. To apply Equation 8.3 requires the parameter KT to be specified. For distillation using tray columns, KT is correlated in terms of a liquid-vapor flow parameter FLV, defined by ... 

The overhead stream of the distillation column may be a low-boiling binary azeotrope of one of the keys with the entrainer or more often a ternary azeotrope containing both keys. The latter kind of operation is feasible only if condensation results in two liquid phases, one of which contains the bulk of one of the key components and the other contains virtually all of the entrainer which can be returned to the column. Figure 13.29(a) is of such a flow scheme. When the separation resulting from the phase split is... 

While the limiting phenomenon of upper limit flooding in a vertical pipe is similar to ultimate capacity in distillation, there is a distinct difference. Upper limit in a vertical pipe applies to a design where a conscious effort should be made to minimize gas-liquid contact. Carried to extremes, it would involve separate tubes for liquid flowing down and vapor going up. In contrast, ultimate capacity in a distillation column corresponds to the condition where effective mass transfer disappears due to high entrainment. One could force more vapor up through the contactor, but fractionation would be poor. 

This means both vapor and liquid loadB are raised and lowered simultaneously. Increasing vapor rate reduces efficiency, while increasing liquid rates raises efficiency. The two effects normally cancel each other, and efficiency is practically independent of load changes (assuming no excessive entrainment or weeping). Figure 7.106 shows a typical dependence of tray efficiency on vapor and liquid loads for a commercial-scale distillation column. Anderson et al. (97) show a similar dependence for several different valve trays. 

Description Hydrocarbon feed is preheated with hot circulating solvent and fed at a midpoint into the extractive distillation column (EDC). Lean solvent is fed at an upper point to selectively extract the aromatics into the column bottoms in a vapor/liquid distillation operation. Nonaromatic hydrocarbons exit the column top and pass through a condenser. A portion of the overhead stream is returned to the column top as reflux to wash out any entrained solvent. The balance of the overhead stream is the raffinate product, requiring no further treatment. 

The first three of these are solely VLE-based approaches, involving a series of simple distillation column operations and recycles. The final approach relies on distillation (VLE), but also exploits another physical phenomenon, hquid-liquid phase formation (phase splitting), to assist in entrainer recovery. This approach is the most powerful and versatile. Examples of industrial uses of azeotropic distillation grouped by method are given in Table 13-20. 

The added third component, sometimes called the entrainer, may form a ternary azeotrope with the two components being separated. However, it must be sufficiently volatile from the solution so that it is taken overhead with one of the two components in the distillation. If the entrainer and the component taken overhead separate into two liquid phases when the vapor overhead is condensed, the entrainer phase is refluxed back to the column. The other phase can be fractionated to remove the dissolved entrainer and the residual amount of the other component before it is discarded. Alternatively, this second liquid phase is recycled to some appropriate place in the main process scheme. 

Exploitation of Homogeneous Azeotropes Homogeneous azeotropic distillation refers to a flowsheet structure in which azeotrope formation is exploited or avoided in order to accomplish the desired separation in one or more distillation columns. The azeotropes in the system either do not exhibit two-liquid-phase behavior or the liquid-phase behavior is not or cannot be exploited in the separation sequence. The structure of a particular sequence will depend on the geometry of the residue curve map or distillation region diagram for the feed mixture-entrainer system. Two approaches are possible ... 

Entrainment Corrections. Above about 80% of flood, the recirculation of liquid as entrainment between trays undermines the countercurrent action of the tray column, and efficiency therefore suffers. This is a particular problem in vacuum distillation where it may be optimum to allow a certain amount of liquid entrainment in initial design. Figure 13.41 shows an approximate method for entrainment correction to column efficiency or Murphree efficiency. The abscissa scale is the same parameter used for flooding prediction (Figure 13.32(b)). The ordinate value is used to correct from a dry to a wet efficiency (with entrainment) ... 

A typical flowsheet for the separation is shown in Eigure 10.4c. The fresh feed is combined with the entrainer and then fed to the azeotropic column, where the two azeotropes are separated. Each azeotrope product is sent to a liquid-liquid extraction column to dissolve the entrainer in some suitable solvent. The combined entrainer-solvent solution from both extractors is sent to a distillation column to separate the entrainer and solvent. These are recycled to the azeotropic column and extractors with makeup added to each, as needed. 

The earliest fabricated evaporator designs incorporated horizontal tubes. A vertical tank-like cylinder housed a horizontal tube bundle in the lower portion of the vessel, and the vapor space above the tubes served to separate the entrained liquid from the rising vapors. A later design based on a horizontal body and a removable U-type bundle is illustrated in Fig. % i). Another modification, the kettle type re-boiler, is similar and is more often employed as a bottoms heater for a distillation column than as an evaporator. 

Maximum Vapor Velocity. The maximum velocity of the vapor leaving a free-liquid surface from which liquid particles may be entrained by force balance has been investigated by Sanders and Brown (1934) in connection with distillation columns and by York and Popele (1963) in connection with mesh separators. 

Fig. 7.9 summarizes the hardware design of the RD column. Despite the large column diameter required in the reactive section (5.7 m), it is interesting to note that only 25 % of the column volume is available for holding up the liquid and carrying out the reaction. The rest of the column volume is taken up by freeboard above the tray that is necessary to minimize liquid entrainment to the tray above that, causes flooding. Distillation tray columns are not efficient devices for carrying out slow liquid-phase reactions. 

A Overall Vapor Composition from Azeo-Column (yji,) o Liquid in EquiBbrium with Overhead Vapor from Azeo-Column A Distillate Composition from Entrainer Recovery Column (jq,)... 

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