Distilasyon Agent

It can be seen from the above that the higher the valence of metal ion is, the more obvious the salt effect is. That is to say, the order of salt effect is: AICI3 > CaCh > NaCl2; A1(N03)3 > Cu(N03)2 > KNO3. Besides, the effects of acidic roots are different, in an order of

(2) Separation of isopropanol and water

Azeotropie distillation with benzene as the entraîner is also a conventional process for the isopropanol-water separation. The Ishikawajima-Harima Heavy industries (IHI) company in Japan has developed a process for isopropanol production from aqueous solution using the solid salt, calcium chloride, as the azeotrope-breaking agent. In a 7300 tonnes per year plant, IHI reported a capital cost of only 56%, and an energy requirement of only 45% of those for the benzene process [44, 45],

(3) Separation of nitric acid and water

A process in North America using extractive distillation by salt effect is the production of nitric acid from aqueous solution using a solid salt, magnesium nitrate as the separating agent. Hercules Inc. has been operating such plants since 1957, and reported lower capital costs and lower overall operating costs than for the conventional extractive distillation process which uses a liquid solvent, sulfuric acid, as the separating agent [44, 45],

3.1.4. The advantages and disadvantages of extractive distillation with solid salt

In the systems where solubility considerations permit their use, solid salt as the separating agents have major advantages. The ions of a solid salt are typically capable of causing much large effects than the molecules of a liquid agent, both in the strength of attractive forces they can exert on feed component molecules, and in the degree of selectivity exerted. This means that the salt is of good separation ability. In the extractive distillation process, the solvent ratio is much smaller than that of the liquid solvent (mentioned afterwards), which leads to high production capacity and low energy consumption. Moreover, since solid salt isn't volatile, it can't be entrained into the product. So no salt vapor is inhaled by operators. From this viewpoint, it is environment-friendly.

However, it is a pity that when solid salt is used in industrial operation, dissolution, reuse and transport of salt is quite a problem. The concurrent jam and erosion limit the industrial value of extractive distillation with solid salt. That is why this technique, extractive distillation with solid salt, isn't widely used in industry.

3.2. Extractive distillation with liquid solvent

3.2.1. Definition of extractive distillation with liquid solvent

Like the extractive distillation with solid salt, in certain systems where solubility permits, it is feasible to use a liquid solvent dissolved into the liquid phase, rather than salt, as the separating agent for extractive distillation. Therefore, the extractive distillation process in which the liquid solvent is used as the separating agent is called extractive distillation with liquid solvent. Note that, herein, the ionic liquids aren't included in the liquid solvents although they are in the liquid phase from room temperature to a higher temperature. The contents about ionic liquid as a special separating agent will be discussed afterwards.

3.2.2. The process of extractive distillation with liquid solvent

See Fig. 4 (the process of extractive distillation with liquid solvent).

3.2.3. Case studies

(1) Reactive extractive distillation

There are many examples for extractive distillation with a single liquid solvent as the separating agent, as shown in Table 1. Herein, this example is concerned with the separation of acetic acid and water by reactive extractive distillation in which chemical reaction is involved.

Acetic acid is an important raw material in the chemical industries. But in the production of acetic acid, it often exists with much water. Because a high-purity of acetic acid is needed in industry, the problem of separating acetic acid and water is an urgent thing. By now, there are three methods commonly used for this separation, i.e. ordinary distillation, azetropic distillation and extractive distillation [49-54], Although ordinary distillation is simple and easy to be operated, its energy consumption is large and a lot of trays are required. The number of trays for azeotropic distillation is fewer than that for ordinary distillation. But the amount of azeotropic agent is large, which leads to much energy consumption because the azeotropic agents must be vaporized in the column. However, in the extractive distillation process the separating agents aren't vaporized and thus the energy consumption is relatively decreased. Therefore, extractive distillation is an attractive method for separating acetic acid and water, and has been studied by Berg [49, 50],

The reported separating agents in extractive distillation are sulfolane, adiponitrile, pelargonic acid, heptanoic acid, isophorone, neodecanoic acid, acetophenone, nitrobenzene and so on. It is evident that the interaction between acetic acid or water and these separating agents is mainly physical force including the van der Waals bonding and hydrogen bonding.

Recently, a new term, reactive extractive distillation, has been put forward [149-150], and a single solvent, tributylamine, is selected as the separating agent.

If we select the solvent tributylamine (b.p. 213.5°C) as the separating agent, then the following reversible chemical reaction may take place:

where HAc, R3N and R3NH • OOCCH3 represent acetic acid, tributylamine and the salt formed by reaction, respectively. This reaction may be reversible because weak acid (acetic acid) and weak base (tributylamine) are used as reactants. That is to say, for the extractive distillation process the forward reaction occurs in the extractive distillation column and the reverse reaction occurs in the solvent recovery column. Therefore, this new separation method is different from traditional extractive distillation with liquid solvent, and based on the reversible chemical interaction between weak acid (acetic acid) and weak base (separating agent). So we call it reactive extractive distillation.

A new substance, R3NH+ • OOCCH3. is produced in this reaction, which can be verified by infrared spectra (IR) technique. It can be seen from the IR diagrams that a new characteristic peak in the range of 1550 cm"1 to 1600 cm" appears in the mixture of acetic acid and tributylamine, and is assigned to the carboxylic-salt function group, —COO". This indicates that chemical reaction between HAc and R3N indeed takes place.

On the other hand, this chemical reaction is reversible, which can be verified by mass chromatogram (MS) technique. Two peaks denoting acetic acid and tributylamine respectively can be found for the mixture of acetic acid 10 %wt and tributylamine 90 %wt.

So it indicates that HAc, R3N and the product produced by them all can be detected by the combination of IR and MS techniques. In general, the reaction rate between weak acid and weak base is very quick. So the chemical reaction between HAc and R3N may be reversible. The further proof of reversible reaction is supported by investigating the chemical equilibrium constant.

In order to verify the effect of tributylamine as the separating agent, the vapor liquid equilibrium (VLE) was measured by experiment. Fig. 19 shows the equilibrium data for the ternary system of water (1) + acetic acid (2) + tributylamine (3), plotted on a tributylamin free basis. It may be observed that the solvent, tributylamin, enhances the relative volatility of water to acetic acid in such a way that the composition of the more volatile component (water) is higher in the liquid than in the vapor phase. The reason may be that the interaction forces between acetic acid and tributylamin molecules are stronger than those between water and tributylamin molecules because in the former reversible chemical reaction takes place. As a result, water would be obtained as the overhead product in the extractive distillation column, being acetic acid and the solvent, tributylamin, the bottom product.

By analyzing the reaction system, it is found that the new group -NH is formed and the old group -OH is disappeared during the reaction. This reaction is exothermic and the heat generated can be obtained from the reference [54], i.e. -2.17 kJ mol"1. In addition, the ionization constant PK^ of acetic acid and tributylamine at 25 °C can be found from the reference [55], i.e. 4.76 and 10.87 respectively. Therefore, the chemical equilibrium constant

K at 25 °C can be deduced, and thus the relationship of chemical equilibrium constant with temperature is given by:

where T is the absolute temperature (K), and C is the mole concentration (kmol m"3).

It is found that chemical equilibrium constant is small, about 0.02 under the operation condition of the extractive distillation column. This means that this reaction is reversible, and the chemical interaction between acetic acid and tributylamine is weak.

In terms of the mechanism of reactive extractive distillation, the following criteria should be satisfied in order to make ensure that this method can be implemented.

(a) The chemical reaction is reversible. The reaction product (here as R3NH+ • OOCCH3) is used as carrier to carry the separated materials back and forth.

(b) One of the reactants (here as acetic acid) is a low boiling-point component. It can be easily removed by distillation so that the separating agent (here as tributylamine) can be regenerated and recycled.

(c) There are no other side reactions between the separating agent and the component to be separated. Otherwise, the separation process will be complicated and some extra equipment may be added, which results in no economy of this technique.

Fig. 19. VLE curves on the solvent free basts for the tertiary system of water (!) + acetic acid (2) + tributylamine (3) at 101.33 kPa. ♦ — solvent/feed volume ratio 2:1; • — solvent/feed volume ratio I: I; A — no solvent,

it is obvious that the system consisting of water, acetic acid and tributylamine meets these requirements. So it seems to be advisable lo separate water and acctic acid by reactive extractive distillation. However, it should be cautious that one solvent with a tri-amine R3N. not di-amine R2NH or mono-amine RNH2, can be selected as the separating agent bccause the reaction between solvent with two or single amine groups and acctic acid is irreversible, and some amides will be produced.

(2) The mixture of liquid solvents

Compared with the single solvent, the mixture of the liquid solvents as the separating agents is complicated and interesting. However, in most cases the number of the liquid solvents in the mixture is just two. According to the aims of adding one solvent to another, it can be divided into two categories: increasing separation ability and decreasing the boiling point of the mixture, (a) Increasing separation ability

As wc know, selection of (he most suitable solvent plays an important role in the economical design of extractive distillation. Moreover, the most important factor in selecting the solvents is relative volatility. Increasing separation ability of the solvent means increasing the relative volatility of the components to be separated. That is to say, when one basic solvent is given, it is better to find the additive to make into a mixture to improve the relative volatility, and thus to decrease the solvent ratio and liquid load of the extractive distillation column. It has been verified by many examples that adding a little of extra solvent (additive) to one basic solvent will increase the separation ability greatly.

For example, adding small amounts of water has improved the separation ability of acetonitrile (ACN) in separating C4 mixture [56, 57], Some results are listed in Table 4, where the subscript (1 - 13) represents n-butane, isobutane, isobutene, 1-butene, trans-2-butene, propadiene, cis-2-butene, 1,3-butadiene, 1,2-butadiene, methyl acetylene, butyne-1, butyne-2 and vinylacetylene (VAC) respectively, and 100%, 80% and 70% are the solvent weight fraction in the mixture of ACN and C4.

Table 4

The relative volatility of C4 mixture to 1,3-butadiene at 50°C

Table 4

The relative volatility of C4 mixture to 1,3-butadiene at 50°C

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