Extractive distillation with solid salt

3.1. J. Definition of extractive distillation with solid salt

In certain systems where solubility permits, it is feasible to use a solid salt dissolved into the liquid phase, rather than a liquid additive, as the separating agent for extractive distillation. The extractive distillation process in which the solid salt is used as the separating agent is called extractive distillation with solid salt. However, herein, the ionic liquids aren't included in the solid salts although they are a kind of salts. Ionic liquid as a special separating agent will be discussed separately.

The so-called "salt effect in vapor-liquid equilibrium (VLE)" refers to the ability of a solid salt which has been dissolved into a liquid phase consisting of two or more volatile components to alter the composition of the equilibrium vapor without itself being present in the vapor. The feed component in which the equilibrium vapor is enhanced is thought to have been "salting-out" by the salt; otherwise, if the equilibrium vapor is declined, then "salting-in". This phenomenon can be described by the following famous Setschenow equation which expresses the solubility of a non-electrolyte in a solid salt solution with low salt concentration [42]:

The notation meaning of the above equation refers to Eq. (113) in chapter 1.

3.1.2. The process of extractive distillation with salt

The process of extractive distillation with salt is somewhat different from the process shown in Fig. 4, in that the salt isn't recovered by means of distillation. Anyway, any extractive distillation process can be taken on as consisting of one extractive distillation column, and one solvent recovery equipment but sometimes maybe not distillation column [43-45],

Fig. 17 demonstrates a typical flowsheet for salt-effect extractive distillation. The solid salt, which must be soluble to some extent in both feed components, is fed at the top of the column by dissolving it at a steady state into the boiling reflux just prior to entering the column. The solid salt, being nonvolatile, flows entirely downward in the column, residing solely in the liquid phase. Therefore, no scrubbing section is required above the situation of feeding separating agent to strip agent from the overhead product. Recovery of the salt from the bottom product for recycle is by either full or partial drying, rather than by the subsequent distillation operation for recovering the liquid separating agents.

Several variations on the Fig. 17 process are possible. For example, an azeotrope-containing system could be separated by first taking it almost to the azeotrope by ordinary distillation without adding any separating agent, then across the former azeotrope point by extractive distillation with solid salt present, usually containing a little components to be separated, and then to final purity by further distillation without solid salt. The solid salt is generally recovered by evaporation.

In this case, the flowsheet of extractive distillation with salt is shown in Fig. 18. One advantage of this process is that the salt isn't difficult to be recovered, only by evaporation. Thus the process is convenient to operate.

3.1.3. Case studies

(1) Separation of ethanol and water

Batch Distillation Flowsheet
Fig. 17. One flowsheet of extractive distillation with salt. 1- feed stream; 2- extractive distillation column; 3- salt recovery equipment; 4- bottom product; 5- the salt recovered; 6- reflux tank; 7-overhead product.
Reflux Tank
Fig. 18. One flowsheet of extractive distillation with salt. 1- feed stream; 2- extractive distillation column; 3- evaporation tank; 4- bottom product; 5- salt solution; 6- reflux tank; 7- overhead product.

Ethanol is a basic chemical material and solvent used in the production of many chemicals and intermediates. Especially in the recent year, ethanol has received increasing attention because it is an excellent alternative fuel and has a virtually limitless potential for growth. However, ethanol are usually diluted and must be separated from water. It is known that ethanol forms azeotrope with water, and can't be extracted to a high concentration from the aqueous solutions by ordinary distillation methods.

A process known as HI AG (Holz Industrie Acetien Geselleschaft), licensed by DEGUSSA and based on patents registered by Adolph Gorhan, employed extractive distillation using a 70/30 mixture of potassium and sodium acetates as the separating agent, and produced above 99.8% ethanol completely free of separating agent directly from the top of the column [44, 45].

The separation of ethanol and water is the most important application of extractive distillation with solid salt. The influence of various salts on the relative volatility of ethanol and water was investigated by Duan et al. [46-48], and the results are listed in Table 3, where the volume ratio of the azeotropic ethanol-water mixture to the separating agent is 1.0, and the concentration of salt in the separating agent is 0.2 g (salt)/ml (solvent).

Table 3

The influence of various solid salts and liquid solvents on the relative volatility of ethanol and water

Table 3

The influence of various solid salts and liquid solvents on the relative volatility of ethanol and water

No.

Separating agent

Relative volatility

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