Entrainment separator, discussion

Practical separation techniques for hquid particles in gases are discussed. Since gas-borne particulates include both hquid and sohd particles, many devices used for dry-dust collection (discussed in Sec. 17 under Gas-Sohds Separation ) can be adapted to liquid-particle separation. Also, the basic subject of particle mechanics is covered in Sec. 6. Separation of liquid particulates is frequently desirable in chemical processes such as in countercurrent-stage contacting because hquid entrainment with the gas partially reduces true countercurrency. Separation before entering another process step may be needed to prevent corrosion, to prevent yield loss, or to prevent equipment damage or malfunc tion. Separation before the atmospheric release of gases may be necessaiy to prevent environmental problems and for regula-toiy compliance. 

Fog formation. In the condensation of a vapour from a non-condensable gas, if the bulk temperature of the gas falls below the dew point of the vapour, liquid can condense out directly as a mist or fog. This condition is undesirable, as liquid droplets may be carried out of the condenser. Fog formation in cooler-condensers is discussed by Colburn and Edison (1941) and Lo Pinto (1982). Steinmeyer (1972) gives criteria for the prediction of fog formation. Demisting pads can be used to separate entrained liquid droplets. 

Thus, distillation line and residue curve maps are excellent tools to evaluate feasibility of azeotropic separations, with just one exception, namely, the use of high-boiling entrainers for separation. In such cases, the equi-volatility curves discussed in this chapter are a better way of determining separation feasibility. 

A fluidized-bed reactor consists of three main sections (Figure 23.1) (1) the fluidizing gas entry or distributor section at the bottom, essentially a perforated metal plate that allows entry of the gas through a number of holes (2) the fluidized-bed itself, which, unless the operation is adiabatic, includes heat transfer surface to control T (3) the freeboard section above the bed, essentially empty space to allow disengagement of entrained solid particles from the rising exit gas stream this section may be provided internally (at the top) or externally with cyclones to aid in the gas-solid separation. A reactor model, as discussed here, is concerned primarily with the bed itself, in order to determine, for example, the required holdup of solid particles for a specified rate of production. The solid may be a catalyst or a reactant, but we assume the former for the purpose of the development. 

As discussed in Section 15.5.2, the separation of two or more sublimable substances by fractional sublimation is theoretically possible if the substances form true solid solutions. Gillot and Goldberger(10°) have reported the development of a laboratory-scale process known as thin-hlm fractional sublimation which has been applied successfully to the separation of volatile solid mixtures such as hafnium and zirconium tetrachlorides, 1,4-dibromobenzene and l-bromo-4-chlorobenzene, and anthracene and carbazole. A stream of inert, non-volatile solids fed to the top of a vertical fractionation column falls counter-currently to the rising supersaturated vapour which is mixed with an entrainer gas. The temperature of the incoming solids is maintained well below the snow-point temperature of the vapour, and thus the solids become coated with a thin film (10. im) of sublimate which acts as a reflux for the enriching section of the column above the feed entry point. 

The VLB was also measured for binary and ternary systems of [ethanol + [C2Cilm][C2S04] and [ethanol + ethyl ferf-butyl ether + [C2Cilm][C2S04] at 101.3 kPa [151]. This ternary system does not exhibit a ternary azeotrope. The possibility of [C2Cilm][C2S04] use as a solvenf in liquid-liquid extraction or as an entrainer in extractive distillation for fhe separation of the mixture ethanol/ethyl fcrf-butyl ether was discussed [151]. 

Table I shows that, as the boiling point of the hydrocarbon used as the entrainer increases so does that of the azeotrope with water and the percent of water therein. A high percentage of water in the azeotrope is desired for the heat required for the distillation, which is mainly that of the latent heat of the water plus that of the entrainer. Sufficient entrainer should be available in the azeotrope for reflux to the column although this requirement is not large. Also, the solubility or dilution effect is better with lower-boiling hydrocarbons. Thus there are several factors to be balanced in choosing the azeotrope. The effect of relative boiling points, vapor pressures, and amounts of different entrainers in their azeotropes with water has been discussed as affecting the choice of entrainers for separating water from acetic acid (5). However, that represents a much more difficult selection because there the quantity of reflux is important and also the solvent characteristics of the entrainer for the acetic acid also control the choice.

Figure 6 Entrainer-based reactive distillation with entrainer/alcohol separation 2.3. Results and discussion...

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