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benzene and non-aromatics

NFM + an additive

However, one question arises, that is, the separation ability of the mixture of the solvents is strengthened or declined? The fact is that, at some time, the separation ability of the mixture of the solvents is improved as the example of Table 4, where the mixture of ACN and water not only improves the separation ability, but also decreases the boiling point. At other time, the separation ability of the mixture of the solvents indeed declines, but not obviously. The reason is that as the temperature decreases, there is a positive effect on the separation ability of the mixture of the solvents. The temperature effect on selectivity is given by d(]/T) 2.303 R

where L" is partial molar heat of solution, component i at infinite dilution in the solvent, T

is the absolute temperature (K), and R is gas constant (8.314 J mof' K)).

It is shown that lgS^ is proportional to the reciprocal absolute temperature. This means that although the separation ability of the additive is weaker than that of the basic solvent, the relatively low boiling point of the mixture of the solvents in some degrees offsets the negative influence of the separation ability of the additive, which leads to no apparent decrease of the separation ability of the mixture of the solvents. That is a reason why some researchers are willing to improve the solvent by decreasing the boiling point.

In addition, the mixture of the solvents can raise the yield ratio of the product. Zhu et al. [61] studied an extractive distillation process for recovering high purity of benzene from pyrolysis gasoline. The selected solvent is the mixture of N-formylmorpholine (NFM) and an additive. When the weight fraction of the additive is in the range of 5% - 20%, the yield ratio of benzene increases from 99.0% to 99.8% and the bottom temperature of the solvent recovery column is less than 190°C.

3.2.4. The advantages and disadvantages of extractive distillation with liquid solvent

In most cases, the solvent and feed mass ratio (i.e. solvent ratio) in the extractive distillation with liquid solvents is very high, up to 5-8. For example, for the separation of C4 hydrocarbon using N,N-dimethylformamide (DMF) or acetonitrile (ACN) as the solvents, the solvent ratio is 7-8 in industry, which leads to much consumption of energy. However, in the systems where solubility considerations permit their use, the separation ability of the solid salts is much greater than that of the liquid solvents, and thus the solvent ratio is generally low. The reason why extractive distillation with liquid solvent is more widely used in industry rather than extractive distillation with solid salt is that there is no problems of dissolution, reuse and transport for the liquid solvent because it is in the liquid phase under the operation condition of extractive distillation process. In summary, the advantages of extractive distillation with liquid solvent predominate over its disadvantages. On the contrary, the disadvantages of extractive distillation with solid salt predominate over its advantages.

3.3. Extractive distillation with the combination of liquid solvent and solid salt

3.3.1. Definition of extractive distillation with liquid solvent and solid salt

Like the extractive distillation with solid salt or liquid solvent, in certain systems where solubility permits, it is feasible to use a combination of liquid solvent and solid salt dissolved into the liquid phase, rather than only salt or liquid solvent, as the separating agent for extractive distillation. Therefore, the extractive distillation process in which the combination of liquid solvent and solid salt is used as the separating agent is called extractive distillation with the combination of liquid solvent and solid salt.

3.3.2. The process of extractive distillation with the combination of liquid solvent and solid salt

See Fig. 4 (the process of extractive distillation with liquid solvent). That is to say, the processes of extractive distillation with liquid solvent and with the combination of liquid solvent and solid salt, are identical.

3.3.3. Case studies

Extractive distillation with the combination of liquid solvent and solid salt as the separating agent is a new process for production of high-purity products. This process integrates the advantages of liquid solvent (easy operation) and solid salt (high separation ability). In industrial operation, when only salt is used, dissolution, reuse and transport of solid salt is quite a problem. The concurrent jam and erosion limits the industrial value of extractive distillation with solid salt only. However, the mixture of liquid solvent and solid salt can avoid the defects and realize continual production in industry.

Extractive distillation with the combination of liquid solvent and solid salt can be suitable either for the separation of polar systems or for the separation of non-polar systems, respectively.

Herein, we select aqueous alcohol solutions, ethanol/water and isopropanol/water, as the representatives of polar systems and C4 mixture as the representatives of non-polar systems. It is known that both anhydrous alcohol and 1,3-butadiene are basic chemical raw materials. Anhydrous alcohol is not only used as chemical reagent and organic solvent, but also used as the raw material of many important chemical products and intermediates; 1,3-Butadiene mainly comes from C4 mixture and is utilized for the synthesis of polymers on a large scale. It has been reported [13, 63-64] that the systems of aqueous alcohol and C4 mixture are able to be separated by extractive distillation. Since these two materials are very important in industry, the separation of them by extractive distillation is interesting. (1) Separation of the systems of ethanol /water and isopropanol/water

Firstly the equilibrium data of the ethanol (1) - water (2) system, which corresponded well with the reference data [65], were measured. It is verified that the experimental apparatus was reliable. Then the measurements were respectively made for the system ethanol (1) - water (2) - ethylene glycol (solvent/feed volume ratio is 1:1) and the system ethanol (1) - water (2) - ethylene glycol - CaCh (solvent/feed volume ratio is 1:1 and the concentration of salt is 0.1 g/ml solvent) at normal pressure.

On the other hand, measurements were also made for the system of isopropanol (1) / water (2) / ethylene glycol / glycollic potassium at normal pressure [13], Isopropanol (1) / water (2) and the mixture of ethylene glycol and glycollic potassium were blended with the feed/solvent volume ratio of 1:1. The mixture of ethylene glycol and glycollic potassium were prepared from ethylene glycol and potassium hydroxide with the weight ratio 5:1 and 4:1 respectively. During preparation, the mixture of ethylene glycol and potassium hydroxide were fed into a distillation column and the water produced was removed. Afterwards, the experimental VLE data were measured.

The experimental results from both systems show that under the same liquid composition the mole fraction of alcohol in the vapor phase with salt is higher than that without salt. It means that adding salt to ethylene glycol is advisable for improving the separation ability of the solvent, while alcohol as a light component and water as a heavy component.

(2) Separation of C4 mixture

When acetonitrile (ACN) is regarded as a basic solvent, organic solvents including water and salts will be added [64]. The aim is to explore the effect of them on a" . Among organic solvents, water and ethylenediamine are better additives than other solvents. However, it is found that a little of solid salt added to ACN can effectively improve the relative volatility and the effect of solid salt is close to water but stronger than ethylenediamine. However, formerly ACN was mixed with water to separate C4 mixture with the defect that ACN is prone to hydrolyze, which leads to equipment corrosion and operation difficulty.

On the other hand, N,N-dimethylformamide (DMF) is another solvent commonly used to separate C4 mixture. Due to the same reason as ACN, DMF is used as a single solvent. It is expected to modify it with solid salt. Many substances are strongly soluble in DMF including many kinds of solid salts. The influence of solid salts and organic additives on the separation ability of DMF is tested [63, 64]. The same phenomenon as ACN exists. Salts added to DMF also improve the relative volatilities of C4 to some extent, and at the same additive concentration the effect of the salts is more apparent than that of organic solvents. Moreover, if some factors such as relative volatilities, price, erosion, source and so on are considered, the salts NaSCN and KSCN are the best additives.

3.3.4. The advantages and disadvantages of extractive distillation with the combination of liquid solvent and solid salt

As mentioned above, extractive distillation with the combination of liquid solvent and solid salt as the separating agent integrates the advantages of liquid solvent (easy operation) and solid salt (high separation ability). From the above discussion, we know that whether it is the separation of polar or non-polar systems, extractive distillation with the combination of liquid solvent and solid salt may be feasible. So it is concluded that extractive distillation with the combination of liquid solvent and solid salt is a promising separation method. If we meet with the problems about extractive distillation in the future, it is wise to try to improve it by adding solid salt.

Unfortunately, many solid salts are corrosive to the equipment and easies to decompose at a high temperature. In some cases the kinds of solid salts that we can select are a few. An economic calculation must be made in determining the final salts. The benefit from adding salts in the production should exceed the price of salts and other charges.

On the other hand, since the amount of solid salts added to the liquid solvents is often small, the role of solid salts in improving the separation ability is doomed to be limited.

Besides, liquid solvents are volatile, which inevitably pollutes the top product of the extractive distillation column. Accordingly, new separating agents should be sought to avoid these problems brought on by liquid solvent and solid salt.

3.4. Extractive distillation with ionic liquid

3.4.1. Definition of extractive distillation with ionic liquid

Like the extractive distillation with solid salt or liquid solvent or the combination, in certain systems where solubility permits, it is feasible to use an ionic liquid dissolved into the liquid phase, rather than only salts or liquid solvents, as the separating agent for extractive distillation. Therefore, the extractive distillation process where ionic liquid is used as the separating agent is called extractive distillation with ionic liquid.

Then, what are ionic liquids? Ionic liquids are salts consisting entirely of ions, which exist in the liquid state at ambient temperature, i.e. they are salts that don't normally need to be melted by means of an external heat source.

Ionic liquids are often used in the chemical reaction [66-71], The cases of applications on the separation process are a few, especially rarely reported on the extractive distillation (only one patent found) [72],

Ionic liquids are called "green" solvents. When they are used in the extractive distillation, the "green" is brought out in the following aspects:

(1) Negligible vapor pressure, which means that ionic liquids don't pollute the product at the top of the column. However, when the liquid solvent or the combination of liquid solvent and solid salt is used as the separating agent, the solvent may be entrained more or less into the product. In the case of strict restriction for the impurity of the product, it is advisable to use ionic liquids as the separating agents.

(2) A wide liquid range of about 300°C with a melting point around room temperature. This temperature range in most cases corresponds with the operation condition of the extractive distillation.

(3) A wide range of materials including inorganic, organic and even polymeric materials are soluble in ionic liquids, which ensures that the ionic liquids have an enough solubility for the components to be separated, and can play a role in increasing the relative volatility in the liquid phase.

(4) Potential to be reused and recycled. Due to their non-volatility, ionic liquids are easy to be recovered from the components to be separated, and the simplest way is by evaporation in a tank.

(5) Many ionic liquids are high thermal and chemical stability with or without water under the operation temperature of extractive distillation. However, some excellent liquid solvents, which are commonly used until now, such as acetontrile (ACN), dimethylformamide (DMF), N-methyl-pyrrolidone (NMP), etc., are not thermal and chemical stability with or without water, and are easy to be decomposed.

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