Staged columns entrainment

The fatty acids that emerge from the top of the column contain entrained water, partially hydroly2ed fat, and the Zn—soap catalyst. This product stream is passed into a vacuum dryer stage where the water is removed through vapori2ation and the fatty acid cooled as a result of this vapori2ation process. The dried product stream is then passed to a distillation system. 

Fig. 3.46 Computer simulation output for a five-stage mixer-settler cascade with entrainment. 3.3.1.10 Staged Extraction Columns...

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

A more detailed discussion from an economic and operability viewpoint can be found in Doherty and Malone [8]. Despite the apparent advantage, the two-column sequence seems not to be the most economical because of the large entrainer recycle and large number of stages. On the contrary, the two-column sequence plus enrichment column, in total three units, offers the best compromise between investment and solvent-recycle costs. 

A pressure of 14 bar gives a good compromise between the above aspects. The RD column is simulated as reboiled stripper with reactive stages. Although the highly exothermic reaction should make unnecessary the use of a heat source, we consider just a small reboiler to prevent residual propylene entrained in the bottom. For this reason, few reactive stages below the low feed of propylene are useful. 

The above solution makes sense from the economic viewpoint, because the methyl ester is by far the most demanded, while 2-ethyl-hexanol is a cheap alcohol, in general a waste. The flowsheet in Figure 8.2 is still valid, in which the heavy alcohol is fed on the top stage, as an entrainer. Figure 8.15 shows the liquid concentration, reaction rate and temperature profiles. It can be seen that 2-ethyl-hexanol indeed plays its double role. The concentration of methanol in the top can be maintained at the minimum, such that there is no need for additional columns for methanol recovery and recycle. Note also that the operation takes place at low pressure (1.5 bar), in contrast with 6-12 bar and vacuum with individual light and heavy alcohols. 

The vapor velocity in a finite-stage contactor column can be limited by the liquid handling capacity of the downcomers or by entrainment of liquid droplets in the rising gases. In most cases, however, downcomer limitations do not set the allowable vapor velocity instead, the common design basis for choosing allowable vapor velocities is a function of the amount of gas entrainment which can result in improper operation or flooding of the column. 

The solvent, being a low trailer, must be edded high in the columa to ensure volatility enhancement throughout die column. Additional stages are used above the solvent feed point to remove any entrained or volatilized solvent from the vapors leading to the distillate product. 

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