Fig. 11. The process of the combination of extractive distillation and azcotropic distillation
In addition, the energy consumption of this process is greater than that of extractive distillation, but less than that of extractive distillation. So it is concluded that this process is especially suitable for the separation of requiring high purity of product. For instance, in the case of separating acetic acid and water, the concentration of acetic acid in water below 20ppm is required in industry.
(2) Combination of extractive distillation and liquid-liquid extraction
This method has been sucessfully used for the recovery of benzene and toluene from pyrolysis hydrogénation gasoline fraction [27, 28]. The mixture of benzene and toluene is firstly separated into two parts, i.e. one mainly containng benzene and the other mainly containing toluene. Then the rich-benzene mixture is dealt with by extractive distillation, while the rich-toluene mixture is dealt with by liquid-liquid extraction. The solvent used in these two processes is the same, sulfolane.
The reason why extractive distillation is selected for the separation of the rich-benzene mixture is that its boiling-point is lower than that of rich-toluene mixture, and thus the boiling-point difference between the rich-benzene mixture and the solvent is great enough, which facilitates the solvent recovery. However, the boiling-point difference between the rich-toluene mixture and the solvent is relatively small, which leads to some impurities accumulated in the solvent. Therefore, it is wise to select liquid-liquid extraction for the separation of the rich-toluene mixture.
The practical result from the Aromatics Plant of Yangzi Petrochemical Company with a capacity of 360kt a year showed that the the freezing point of benzene obtained by this method was 5.48 °C, the sulfur content in benzene was less than 0.5jo.g g~', and the non-aromatics content in toluene was less than 1000(j.g g' . The recovery yield of benzene and toluene was 99.9% and 99.1% respectively. This proves that the combination of extractive distillation and liquid-liquid extraction is effective for the recovery of benzene and toluene from pyrolysis hydrogénation gasoline fraction.
One prominent characteristics of extractive distillation is that the solvent ratio (namely the mass ratio of solvent and feed) is very high, generally 5-8, which restricts increasing capacity. The liquid load is very large in the extractive distillation column, but the vapor load is relatively small. So when we design the extractive distillation column, it should be paid more attention to the channels of passing liquid phase.
In most cases, either tray column or packing column can be adopted. However, if the extractive distillation column is operated under the middle or even high pressure such as the separation of C3, C4 and C5 mixtures, or if the components to be separated are easy to polymerize such as the separation of C5 mixture, then it is better to choose tray column. Herein, two types of plate trays used in the extractive distillation column are introduced [29-35],
Under the middle or even high pressure, the plate trays, especially double overflow trays, are generally used as internal fittings. Both double overflow valve trays and double overflow slant-hole trays have ever been adopted. However, it is reported that, if double overflow valve trays are replaced by double overflow slant-hole trays in a column for separating propane and propylene, the feeds to be treated are raised above 50% with a tray efficiency similar to or higher than that of the valve trays and an energy save of 10% by decreasing pressure drop to about 1/3 of the original.
Slant-hole trays, very excellent and extensively applied in the industry, has opposite stagger arrangement of slant-holes, which causes rational flowing of vapor and liquid phases, level blowing of vapor, permission of a large vapor speed, no mutual interference, steady liquid level and high tray efficiency. On the basis of studying and analyzing the columns with multiple downcomer (MD) trays, a new-type multi-overflow compound slant-hole trays were invented, which adopt the downcomer similar to MD trays. The number of downcomers used is not too many, but only two. The downcomer has the feature of simple structure, longer flowing distance of liquid, higher capacity of column and high efficiency of trays. The configuration of the double overflow slant-hole trays is diagrammed in Fig. 12. In terms of the vapor and liquid load of the extractive distillation column, the tray parameters can be determined often by a computer program for tray design. Of course, it should be mentioned that the designed values must be within the range of normal operation condition.
Under the condition that the components to be separated are easy to polymerize, the big-hole sieve trays are desirable. For instance, for the separation of the C5 mixture, the key components being pentene and isoprene, it is known that the polymerization reactions among unsaturated hydrocarbons take place and hinder the normal operation of the column while the traditional valve trays are employed. However, in this case the big-hole sieve trays with the hole diameter of 10-15 mm are effective because the holes with large diameter prevent the trays from jam due to the formation of polymer. In addition, this type of trays eliminates the gradient of liquid layer and causes the rational flowing of vapor and liquid phases on the tray, with the help of directed holes arranged for decreasing the radial mix and bubble promoter installed in the outlet of the downcomer.
i ig. 13. The gradient of liquid layer on (he traditional trays.
Fig. 14. The gradient of liquid layer on the big-hole trays.
Fig. 15. The flowing direction of liquid phases on the big-hole trays,
Figs. 13 and 14 illustrate the gradient of liquid layer on the traditional trays and the improved gradient of liquid layer on the big-hole sieve trays, respectively. Fig. 15 shows the flowing direction of liquid phases on the big-hole sieve trays. This manifests that the flowing of liquid phases on the big-hole sieve tray is indeed rational.
We know that the simplest extractive distillation process is made up of two columns, i.e. an extractive distillation column and a solvent recovery column. Herein, we call it double-column process. However, at few times, only one column^ which is either as extractive distillation column or as solvent recovery column, is employed in the extractive distillation process. Herein, we call it single column process.
(1) Double-column process
For extractive distillation, double-column process is often adopted. The double-column process can be operated either in the batch way or in the continuous way. In the continuous way, two columns are operated at the same time, while in the batch way, there is still one column left not run.
Frankly speaking, we would like the continuous way than the batch way because the batch operation is tedious and the production efficiency is very low. However, the reason why the batch way is carried out arises from the solvent recovery column. Due to the very high boiling point of the solvent, the solvent recovery column has to be operated at vacuum pressure in order to decrease the temperature of the bottom and thus prevent the solvent from decomposition. Moreover, at some time there may be a strict requirement for the purity of the recycled solvent. All these lead to the difficulty of the continuous operation in the solvent recovery column. A strategy to solve them is by decreasing the boiling point of the solvent and canceling the vacuum system (see the next section 3.2. Extractive distillation with liquid solvent).
However, in fine chemical engineering, the batch way is often used. Although it is complicated, it is flexible and the amount of treated feed is not so many.
(2) Single column process
For the single column process, there are two cases: batch mode and semi-continuous mode [36-41], In the batch mode, the solvent is charged in the reboiler with the feed mixture at the beginning of operation. Therefore, it limits the amount of feed mixture to be processed, as the reboiler has a limited capacity. This increases the number of batches to be processed in a campaign mode operation. In this mode, finding the optimum feed charge to solvent ratio is an important factor to maximize the productivity.
In the semi-continuous mode, the solvent is fed to the column in a semi-continuous fashion at some point of the column. There may be two strategies in this mode of operation while charging of the initial feed mixture is concerned:
(a) Full charge. In this strategy the feed mixture is charged in the reboiler to its maximum capacity at the beginning of operation. For a given condenser vapor load, if the reflux ratio and the solvent feed rate aren't carefully controlled, the column will possibly be flooded.
(b) Fractional charge. "Full charge" isn't a suitable option to achieve the required product specifications for many azeotropic mixtures in a given sized column, operating even at the maximum reflux ratio, because the amount of solvent required in such separations is more than those that the column can accommodate without flooding. Mujtaba  proposed a fractional feed charge strategy for this type of mixture. In this strategy the feed mixture is charged to a certain fraction of the maximum capacity. The column can operate at a reflux ratio greater than maximum reflux ratio for some period until the reboiler level reaches to its maximum capacity.
However, it can be deduced that comparing with batch mode and semi-continuous mode, the latter is more promising because this mode can deal with much more products in the same column. A complete batch extractive distillation of semi-continuous mode consists of the following steps:
(a) Operation under total reflux without solvent feeding.
(b) Operation under total reflux with solvent feeding. For decreasing the concentration of the less volatile component (LVC) in the distillate, this step isn't absolutely necessary because the first part of the distillate containing too much of the LVC can be recycled to the next charge.
(c) Operation under finite reflux ratio with solvent feeding (production of the more volatile component (MVC)).
(d) Operation under finite reflux ratio without solvent-feeding (separation of the LVC from the solvent). Before the start of the production of LVC an off-cut can be taken.
It should be noted that the holdup on the reboiler must not exceed the maximum capacity at any time within the entire operation period to avoid column flooding.
Many researchers focus on exploring the semi-continuous mode in such aspects as simulations, improving equipment with an external middle vessel and so on, among which batch extractive distillation with an external idle vessel is an interesting topic. The typical process and apparatus are illustrated in Fig. 16.
In one example , the packed column with a 38 mm diameter was composed of three parts, i.e. scrubbing section, rectifying section and stripping section. The scrubbing section was packed to a height of 150 mm with 2.5 X 2.5 Dixon rings, and the rectifying and stripping were, respectively, packed to a height of 500 mm with 2.5 X 2.5 Dixon rings. The external middle vessel is a 2000 ml agitated flat bottom flask. The materials to be separated were charged to the middle vessel. The liquid flow from the middle vessel to the stripping section was controlled by an electric-magnetic device. The column was operated under the following steps:
(a) Operation under total reflux without solvent feeding;
(b) Operation under total reflux with solvent feeding;
(c) Operation under finite reflux ratio and total reboiler ratio with solvent feeding (production of the more volatile component from the top of the column, while no production of solvent from the bottom);
(d) Operation under finite reflux ratio and finite reboiler ratio with solvent feeding (production of more volatile component from the top and solvent from the bottom of the column simultaneously);
(e) Operation under finite reflux ratio and finite reboiler ratio without solvent feeding (production of slop cut or less volatile component from the top while solvent from the bottom);
(f) When the less volatile component or solvent in the middle vessel reaches the required purity or there is only a little liquid in the middle vessel, the operation is stopped.
The operation and simulation of this process in a batch column with a middle vessel is very complex, but it has many advantages such as flexibility, multi-component separation in one column and so on. It is especially suitable for the batch extractive distillation with large solvent ratio. However, when the boiling point of solvent is very high while the boiling point of the components to be separated are low, it is inconvenient to use the column, since the reboiler needs a high temperature heat resource or vacuum system.
Fig. 16. Single column process with an external middle vessel.
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