Vacuum rectification

The principles and main units for vacuum rectification resemble those for atmospheric rectification. The major exceptions are that larger-diameter towers are used to maintain comparable vapor velocities at reduced operating pressures. A vacuum of 50 to 100 mm of Hg absolute is produced by a vacuum pump or steam ejector. The capacity of modern vacuum rectification units is about 3.5 million tons per annum. 

Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 

Residue from the rectification under the atmosphere pressure, 2. Light oil, 3-6. Distillate, 7-9. Vacuum residue 

In the two-step rectification unit, the feed 1 in the first tower is distilled to obtain the following fractions light oil 2, middle oil 3, and partly distilled vacuum residue 6. The vacuum residue from the first tower passes through oven 01 to rectification tower T2, where it is fractionated to narrower fractions. In comparison with the one-step scheme, the two-step scheme requires more energy for production, but the quality of the oil fractions is much higher. 

The residue remaining after vacuum rectification is called Goudron . This may be used for blending to produce road asphalt or residual fuel oil, or it may be used as a feedstock for thermal cracking or coking units. Vacuum rectification units are an essential part of the many processing units required for the production of lubricants. 

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Ibid, Operating Method for Vacuum Rectification—Part II, Chem. Eng. Feb. 27 (1967) p. 149. 

Process vapours from the esterification reactors and EG from the EG-vapour jet, as well as from the vacuum stages of the spray condensers, are purified in the distillation unit. The distillation unit commonly consists of two or three columns and is designed for continuous operation. The purified EG is condensed at the top of the third vacuum rectification column and returned to the process via a buffer tank. Gaseous acetaldehyde and other non-condensables are vented or burned and high-boiling residues from the bottom of the third column are discharged or also burned. 

Organochlorosilanes can be also rectified in horizontal rectification towers (Fig. 6). These towers are peculiar in that they use rotating impellers 2, which spray liquid through vapour (in vertical towers, on the contrary, pressurised vapour passes through liquid) and thus mix liquid and vapour. Besides, horizontal towers use new constructions of water drain devices and breaker blades. Due to the impellers, horizontal rectification towers are more convenient to operate than vertical ones. Horizontal towers do not have to be veiy long, because there is no liquid loss in them, like in vertical apparatuses. Moreover, horizontal towers allow for vacuum rectification with veiy slight pressure differences, because there is no necessity of an increased pressure for mass exchange. 

After that, the tank and tower are cooled to 20-30 °C for the transition to the vacuum rectification stage. At 40-65 GPa in vacuum (residual pressure) one begins to separate intermediate fraction II into receptacle 14. The temperature in the tank is 50-70 °C, and in the higher part of the tower it is 40-50 °C. This fraction is then sent for repeated rectification. 

For 1 hour vacuum rectification tower 21 operates in the self-serving mode, and then starts separating benzene, which is collected in collector 24 (from there it can be sent to the synthesis again into batch box 3). After the distillation of benzene residual pressure of 107 GPa is created in the rectification system after the constant mode is established, the intermediate fraction is separated into receptacle 25. If the methylphenyldichlorosilane content in the intermediate fraction exceeds 5%, this fraction can be sent for repeated rectification in tank 20. After the intermediate fraction, the main fraction, methylphenyldichlorosilane, is separated into receptacle 26. The fraction with the density of 1.1750-1.1815 g/cm3 and chlorine content of 36.9-37.8% is separated. The separation is conducted as long as reflux is extracted. From receptacle 26, technical methylphenyldichlorosilane flows into collector 27. 

The process comprises two main stages (Fig. 19) continuous chlorination of dimethyldichlorosilane (with simultaneous distillation of unreacted substance) vacuum rectification of chlorination products. 

Fraction IV, a mixture of methyl(chloromethyl)dichlorosilane and methyl(dichloromethyl)dichlorosilane is separated in a constantly increasing vacuum (the residual pressure of 350-300 GPa) in the 102-108 °C temperature range. The separation continues until the distillate density reaches 1.40 g/cm3. As soon as fraction IV accumulates, it can be returned for vacuum rectification. 

The pre-distillates of the vacuum distillation from collector 9 and of vacuum rectification from collector 14 are distilled in a vacuum rectification system similar to the one described above. It is done in order to extract the unreacted allylamine and triethoxysilane, as well as tetraethoxysilane and the target product. 

Synthesis and vacuum rectification of 2-oxydiethylsulfide. As stated above, 2-oxydiethylsulfide is obtained by the interaction of ethylene oxide with ethyl mercaptan. The synthesis is carried out in vertical bubble tower 1 consisting of three ring sections. Each ring section has a jacket, the lower two also have coils. 

Materials. Trans-stilbene (Fluka AG) and (+)-a-pinene (Aldrich Chemical Company) were used as received C -stilbene and perisobutyric acid (PIBAC) were prepared as described in [19], (-)-Caryophyllene (>99%) and eugenol (95%) were isolated from the oil of Eugenia caryopyllata by vacuum rectification. (+)-3-Carene (95%) and dipentene [( )-limonene, 95%] were prepared by rectification of the Pinus sylvestris turpentine. 

Energy consumption of the rectification process is reduced via integrated atmospheric and vacuum rectification as well as optimal utilization and operation of heat flows. MIDER claims to save some 50,000 tons of fuel oil per annum compared with a traditional distillation process. The process is characterized by the use of five instead of the usual two distillation columns. The process development was based on the objective of avoiding unnecessary overheating of the light components. Additionally, it avoids degrading the thermal levels associated with the drawing off of heavy fractions. 

In modem refineries, the sections for petroleum desalting and drying are combined with atmospheric and vacuum rectification. 

The one-step vacuum rectification unit does not allow for production of oil fractions with desired market quality. This is why one-step vacuum rectification units can be found nowadays only in small refineries. The scheme that allows for the production of oil fractions with a higher quality is the two-step vacuum rectification. An illustration of the two-step vacuum rectification plant is shown in Figure 5.18. 

In modern refineries, atmospheric and vacuum rectification processes are rarely carried out in separate units. Usually, combined atmospheric-vacuum rectification units are used for these processes. An illustration of a scheme for this combined unit used in refineries in Russia is shown in Figure 5.19. 

It is seen that in the illustration (Fig. 5.19), a two-step atmospheric rectification unit is combined with a one-step vacuum rectification unit. In modem refineries, however, the combined rectification unit consists of a two-step atmospheric unit and a two-step vacuum rectification unit. At the MIDER refinery in Germany (the most modern refinery in Europe), a combination of a three-step atmospheric and a one-step vacuum rectification unit is used. A scheme for this unit is shown in Figure 5.20. 

The bottom product from the T4 is sent to the vacuum rectification tower T5 for further rectification. The bottom product from vacuum tower T5 is the middle distillate. 

Delayed coking is a thermal cracking process used in refineries to upgrade and convert crude oil residue known as vacuum tower bottom product (i.e. the bottoms fraction from a vacuum rectification tower) into liquid and gas product streams leaving behind a solid concentrated carbon material, coke. The vacuum towers referred to are generally used to further fractionate virgin atmospheric-... 

Bottom product of vacuum rectification. Often called bitumen or goudron. 

Fig. 2-32. Vacuum rectification unit with condenser/evaporator, steam jet aspirator, and recirculation of the condensate.

Figure 2-32 shows a simplified setup of a vacuum rectification process including a condenser/evaporator, a steam jet aspirator, and a condensate recycle. 

The difference between the top and bottom temperature is sometimes large, for example, in vacuum rectification with a high number of separation stages. The economical inclusion of heat pumps is not possible in this case but, if necessary, energy may be saved using a heat transformer. 

TVay type, sizes (mm) and operating variables Vacuum rectification Normal pressure operation Overpressure rectification, absorption... 

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