Rectification under Vacuum

Distillation is and will remain the premier separation method in the chemical and petrochemical industries. It is the process of physically separating components from liquid mixtures by partial vaporisation and condensation, thereby taking advantage of the differences in component s volatilities. In most distillation columns, vaporisation and condensation are continuously and multiply performed to achieve the desired fractionation effect, and, by convention, this continuous separation method of multiple distillations in one column is referred to as rectification. 

Vacuum Technology in the Chemical Industry, First Edition. Edited by Wolfgang Jorisch. 

The multiple phase contact inside the column is promoted by internal mass transfer equipment. Three groups of mass transfer equipment are commonly differentiated, which are separation trays, random packings and structured packings. Besides mass transfer equipment, further column internals are required in rectification to ensure the proper operation of the mass transfer equipment. Such internals may include support and hold-down plates, liquid distributors and redistributors, vapour distributor devices, gas-liquid phase separators and liquid collectors that usually do not participate on mass transfer. 

Depending on the ease of separating the components and the specific separation requirements related to the desired purity of fractions, more or less separation stages are required to achieve the target fractionation, respectively, in practice. 

---

Reaction of 4-chlorophenylcarbonimidoyl dichloride with triethyl phosphite ( ). To 21 g (0.01 mole) 4-chlorophenylcarbonimidoyl dichloride 33.2 g (0.02 mole) triethyl phosphite is added dropwise and with stirring. The reaction proceeds, as indicated by the evolution of ethyl chloride, and the exothermic reaction is controlled by the rate of addition (maximum temperature approximately 40°C). Heating to 50-60 C until the generation of ethyl chloride ceases, and rectification under vacuum, yields 33.6 g (82 %) of the corresponding bis-phosphonate, b.p. 190-195° C/0.05 mm. 

Packed towers are used for rectification under vacuum conditions, for gas scrubbing and absorption, for vaporization and in liquid-liquid extraction. Applications are also direct heat transfer between gas and liquid phases and/or between gas and liquid in heterogeneous reaction processes (often with parallel phase flow). 

Fine braided metal packing by Kloss (Montz GmbH) Spirally wound thin wire (diameter 0.2-0.3 mm) gauze, arranged to packings up to ca. 0.3 m with spacers in between Rectification under vacuum up to ca. 0.5 mbar 400 1-5 to 10 2.5-5 0.2-1.2... 

Liquid trickle unit (Montz GmbH) Type A3-500 Braided metal packing where single packings are shifted by 90° against each other Rectification under vacuum up to ca. 1 mbar 500 0.6-2.5 to 15 4-7 0.03-1.0... 

The first German edition concentrates on the description of the fluid dynamics of columns with random and structured packings in the vacuum and normal pressure range of up to approx. 2 bar and for specific liquid loads of up to 100 m m h for gas-liquid systems. This range covers a majority of the applications and tasks relevant in the absorption and desorption of highly and/or moderately soluble gases as well as in rectification under vacuum and normal pressure. 

The fraction enriched with phenyltrichlorosilane is then subjected to rectification in vacuum (under the residual pressure of 50-345 GPa) and separate the following fractions ... 

Due to the increased difficulty of separating THF and ethylacetate in the presence s of water, it was necessary to dewater the THF/ ethylacetate-mixture before the rectification. The ethylacetate/ THF rectification column (119) was designed for 60 theoretical stages and a reflux ratio of 15. The column was operated at 0.5 bar since the VLE-data are slightly more favorable under vacuum.3 A further reduction of the rectification pressure was not possible since water was used for condensing the distillate. 

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

Stepwise distillation, used for fractionating crude oil into fractions. There are in existence rectification under atmosphere pressure and under vacuum. The first is used for producing fractions with boiling point up to 350°C. The second is used for producing fractions with boiling point up to 550°C. 

Type B1 Type Cl Packing made of 0.2 mm embossed metal sheet with special surface structure and round form flow channels Similar to type Bl, but made from IT FE or polypropylene Rectification under overpressure, normal pressure and vacuum Absorption 100-500 0.6-3.0 Bl-300 to 30 Bl-300 ca. 4 Bl-300 0.1-2.0 Bl-300... 

Table 16.1 illustrates several characteristic effects from running rectification columns under vacuum conditions. For example, naturally, the bottoms temperature is lowest at vacuum conditions and it increases with pressure, but for this application this effect is of no practical relevance as none of the species is corrosive or subject to thermal degradation at the applied temperature conditions. 

The hquid loads in vacuum rectification is usually much lower than those encountered in rectification under atmospheric pressure or above, and the self-wetting characteristic of woven fibre fabric makes operation at very low hquid volume flows possible. 

The rectification of mixtures under vacuum ensures that the product purities achieved at the top and at the bottom of the column are equally high at a lower reflux ratio, compared to normal pressure rectification, since the relative volatihty a of the mixtures tends to increase as the top pressure drops. 

The use of lower reflux ratios for the separation of mixtures under vacuum conditions results in a lower consumption of heating steam, compared to rectification under normal pressure. 

The influence of the system on the resistance coefiflcient i[f is sufiflciently described for practical appHcations by the kinematic viscosity, expressed in the modified Reynolds number. In addition, it is sufficient to determine the function i/r = / (Rey) based on simulation tests with air under ambient conditions, in order to then calculate the pressure drop Apo/H of the dry packing, using Eq. (3-8), accurately enough for rectification systems under vacuum and normal pressure and for the high pressure range, as well as for any type of absorption process. 

Processing of petroleum under the above-specified conditions results in a vacuum residue of 585°C cut point. The combined rectification yields the following cuts ... 

The preparation of polycarbonates is carried out in two stages. In the first stage a polycarbonate of lower MW is synthesised, at 150-200 °C, by distillation of EG resulting from reaction 8.35 under moderate vacuum (6.6-26.6 MPa). As for all polyesterification processes, a rectification column assures the elimination of EG and ethylene carbonate and the glycol used as the starter are returned to the reactor. In the second stage, the low MW polycarbonate is heated up to 250 °C under conditions of high vacuum (13-1333 Pa), and is condensed to a higher MW polycarbonate [11]. 

This equation is applicable to vacuum rectification and absorption, operated under low and/or moderate liquid loads up. 

For practical reasons, key importance has been attached to the turbulent flow range, ReL > 2, as this is the range mostly used for operating packed columns containing large packing elements in rectification processes under normal pressure and vacuum conditions and/or in pressure rectification and in absorption processes at moderate liquid loads. 

---