Vacuum-distillation

This chapter will describe briefly what a vacuum is, how it is produced, and how it can be controlled to produce a quality separation. 

According to Aristotle some 2,000 years ago, That in which the pressure of body, though not actual, is possible [is] void. A void was a place with nothing in it, but into which something could be put. The Random House Dictionary defines vacuum as an enclosed space from which matter, especially air, has been partially removed so that the matter or gas remaining in the space exerts less pressure than the atmosphere.  

The international unit of pressure, the Pascal, Pa, is seldom used in this country at this time, mainly because it has inconvenient numbers that are not related to the common measuring devices. A person can visualize 10 mm of mercury, but not 1330 Pascals. Normally, mm Hg is used with coarse gauges, such as the U-tube manometer. For pressures obtained with mechanical pumps, the term torr is common (1 torr = 1 mm Hg), and the term micron x 10 atmosphere) is used with diffusion pumps. We must eventually switch to Pascals, so the pressure in Pascals or kilo Pascals (kPa) will be included, where convenient, in parentheses. A person s lungs produce a vacuum of about 300 torr, and the tentacles of an octopus can attain 100 torr. 

An example of how the boiling point decreases with a decrease in pressure is shown in Table 6-1, p. 58, for OT-nitrotoluene. By lowering the pressure to 1 torr (0.13 kPa), which you will find is very easy to do, the boiling point was lowered 162 C. As a general rule, the boiling point will deerease about 15 C each time the pressure is reduced by one-half. 

Place a ruler so it lies across columnp at 298 C and column C at 20 torr (2.66 kPa). Read the. urtmluthn Ai vidiich iiabout at 173-174 °C at21 tons 

The calculations necessary for vacuum distillation are carried out along lines corresponding to those for distillations at atmospheric pressure. The following additional points shoidd be observed. 

Calculation of the equilibrium curve for ideal and non-ideal mixtures at a given pressure (section 4.6.1, Table 10). 

Basic principles, calculation of tube diameters and testing for vacuuui tightness (section 5.4.1). 

Vacuum science started already in the seventeenth century (Otto von Guericke). With the invention of the diffusion pump (Wolfgang Gaede, 1916) the production of deeper vacuum became technically feasible and economical and, among others, the vacuum distillation technology could develop. 

The boiling temperature can be reduced by applying vacuum. For this purpose a vacuum pump is connected at the vent of the condenser. When reducing the pressure by 1 order of magnitude (e.g. from 100 to 10 mbar) the boiling temperature of most liquid mixtures decreases by more than 25 K. 

The applicable vacuum is mainly given by the available cooling medium for condensing of the vapours an evaporation plant for waste water, for example cannot be operated at below 50 mbar because of the usually available cooling water temperature. Materials with lower vapour pressure can be distilled at lower pressure. 

Pot stills and rectification columns can be operated down to a pressure of a few millibars. Lower operating pressures in the evaporator will not be observed even by use of a very powerful vacuum pump. This is due to the pressure drop of 

The head loss of the vapour flow can be eliminated by simply removing the vapour duct. Evaporation and condensation take place In the same chamber. Due to the short path of the vapours this technique is called short path distillation. The principle applies for distillation tasks at 1 mbar. 

Although accurate estimates of the effect of pressure upon the boiling point of a liquid may be made by use of charts or a nomograph (Fig. 2.41), two useful approximations of the effect of lowered pressure on boiling points are  

Reduction from atmospheric pressure to 25 torr lowers the boiling point of a compound boiling at 250-300 °C at atmospheric pressure by about 100-125 °C. 

Below 25 torr, the boiling point is lowered by about 10 °C each time the pressure is reduced by one-half. 

Reduced pressures may be obtained by connecting a water aspirator pump or a mechanical vacuum pump to a vacuum adapter that is fitted between the condenser and the receiving flask. The vacuum produced by a water aspirator is limited by the vapor pressure of the water at the ambient temperature and the condition of the aspirator pump. Pressures as low as 8-10 torr may be obtained from a water aspirator with cold water, but pressures in the range of 15-25 torr are more common. A good mechanical vacuum pump can evacuate the apparatus to less than 0.01 torr it is important to clean the oil periodically and maintain tight coimections in the distillation apparatus to achieve the lowest possible pressures for a particular pump. Some 

Pressure-temperature alignment nomograph (dashed lines added for illustrative purposes). How to use the nomograph Assume a reported boiling point of 120 °C at 2 torr. 

Source fleld Brent Bonny Lt Green Canyon Ratawi 

Atmospheric distillation of the best crudes yields about 60% naphtha plus middle distillates (kerosene and gas oil), but the average is closer to 40%. In contrast. Table 9 shows that during 1991-2003, the United States consumed, on average, 70% of its petroleum as gasoline and middle distillates. This 

The residue from an atmospheric distillation tower can be sent to a vacuum distillation tower, which recovers additional liquid at 0.7 to 1.5 psia (4.8 to 10.3 kPa). The vacuum, which is created by a vacuum pump or steam ejector, is pulled from the top of the tower. Relative to atmospheric columns, vacuum columns have larger diameters and their internals are simpler. Often, instead of trays, random packing and demister pads are used. 

The overhead stream - light vacuum gas oil - can be used as a lube base stock, heavy fuel oil, or as feed to a conversion imit. Heavy vacuinn gas oil is pulled from a side draw. The vacuum residue can be use to make asphalt, or it can be sent to a coker or visbreaker imit for further processing. 

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Commercial equipment is available which automatically switches from atmospheric distillation to vacuum distillation and calculates the distillation curve as temperatures under atmospheric pressure conditions as a function of weight or volume per cent recovery. 

Details are given in Table 3.3. As with all correlations, one should beware of using them if thp mea su ments taken are outside the region of the correlation that estabTishedthem. This method is commonly called ndM and is used mainly with vacuum distillates and lubricating oils. 

Mass spectrometry allows analysis by hydrocarbon family for a variety of petroleum cuts as deep as vacuum distillates since we have seen that the molecules must be vaporized. The study of vacuum residues can be conducted by a method of direct introduction which we will address only briefly because the quantitative aspects are ek r metiy difficult to master. Table 3.6 gives some examples the matrices used differ according to the distillation cut and the chemical content such as the presence or absence of olefins or sulfur. 

SARA (Saturates, Aromatics, Resins, Asphaltenes) analysis is widely practiced on heavy fractions such as vacuum and atmospheric residues and vacuum distillates for two purposes ... 

Feedstock Paraffinic crude Naphthenic crude Vacuum distillate Vacuum residue Deasphalted atmospheric residue... 

In the 1970 s, heavy fuel came mainly from atmospheric distillation residue. Nowadays a very large proportion of this product is vacuum distilled and the distillate obtained is fed to conversion units such as catalytic cracking, visbreaking and cokers. These produce lighter products —gas and gasoline— but also very heavy components, that are viscous and have high contaminant levels, that are subsequently incorporated in the fuels. 

During the production of mineral oils from vacuum distillates, one of the process steps, dewaxing , removes the high melting point materials in order to improve the oil s pour point. Dewaixing produces paraffins and waxes, the first coming from light distillates, and the second from medium or heavy distillates. 

The distillation of crudes chosen for their yield in heavy fractions is the most common means. Bitumen is extracted from the residue from a vacuum distillation column (a few dozen mm of mercury), the latter being fed by atmospheric distillation residue. Unlike the practice of a decade ago, it is now possible to obtain all categories of bitumen, including the hard grades. 

Once the distillation intervais of cuts coming from atmospheric distillation and vacuum distillation are specified, the preceding curves give the properties of the selected cuts. 

Vacuum distillation of the atmospheric residue complements primary distillation, enabli r.ecoyery of heavy distillate cuts from atmospheric residue that will un r o further conversion or will serve as lube oil bases. The vacuum residue containing most of the crude contaminants (metals, salts, sediments, sulfur, nitrogen, asphaltenes, Conradson carbon, etc.) is used in asphalt manufacture, for heavy fuel-oil, or for feed for others conversion processes. 

Properly speaking, steam cracking is not a refining process. A key petrochemical process, it has the purpose of producing ethylene, propylene, butadiene, butenes and aromatics (BTX) mainly from light fractions of crude oil (LPG, naphthas), but also from heavy fractions hydrotreated or not (paraffinic vacuum distillates, residue from hydrocracking HOC). 

Feedstocks for this very flexible process are usually vacuum distillates, deasphalted oils, residues (hydrotreated or not), as well as by-products from other processes such as extracts, paraffinic slack waxes, distillates from visbreaking and coking, residues from hydrocracking, converted in mixtures with the main feedstock. 

Figure 10.7 presents the case of an FCC feedstock comprising a mixture of vacuum distillate and light atmospheric residue, and the case of an FCC feedstock composed of vacuum distillate and DAO, as well as the constraints of such configurations. 

Figure 10.8 presents a variant of the FCC process, the RCC (Residue Catalytic Cracking) capable of processing heavier feedstocks (atmospheric residue or a mixture of atmospheric residue and vacuum distillate) provided that certain restrictions be taken into account (Heinrich et al., 1993). 

Feedstocks are light vacuum distillates and/or heavy ends from crude distillation or heavy vacuum distillates from other conversion processes visbreaking, coking, hydroconversion of atmospheric and vacuum residues, as well as deasphalted oils. 

Feeds vacuum distillates, deasphalted oil (from C, or process)... 

The conversion products, other than gas and hydrogen sulfide (H2S), are essentially a gasoline fraction that, after pretreatment, will be converted by catalytic reforming an average quality distillate fraction to be sent to the gas oil pool and an atmospheric residue or vacuum distillate and vacuum residue whose properties and impurity levels (S, N, Conr. 

Vacuum residue case Gasoline Gas oil Atm. residue Vacuum distillate Vacuum residue... 

It occurs chiefly as cinnabar, the red sulphide HgS, from which it is readily extracted either by roasting (to give the metal and sulphur dioxide) or by heating with calcium oxide the metal distils off and can be purified by vacuum distillation. 

It has already been pointed out that a liquid even when subjected to simple atmospheric distillation may become superheated and then bump violently in consequence this danger is greatly increased during distillation under reduced pressure and therefore a specially designed flask, known as a Claisen flask, is used to decrease the risk of superheating. In Fig. i2(a) a Claisen flask D is shown, fitted up as part of one of the simplest types of vacuum-distillation apparatus. ... 

For all vacuum distillations, a trap (cf. K, Fig. 14, p, 31) should be fitted near the pump, in ease the water sucks back . 

Fractional Distillation under Reduced Pressure. One great disadvantage of the simple vacuum-distillation apparatus shown in Fig. 12(a) is that, if more than one fraction distils, the whole process has to be stopped after collecting each consecutive fraction, in order to change the receiver F. This may be overcome by replacing the simple receiving flask F by a pig (Fig. 13) which collects consecutive... 

When a more delicate fractional vacuum-distillation is required, the flask and column shown in Fig. ii(b), p. 26, may be used, the side-arm of the column being fitted directly into receiver C (Fig. 14). A rubber stopper must then be used to fit the flask on to the fractionating column, and it should also carry a capillary tube leading to the bottom of the flask, to provide the usual fine stream of bubbles to prevent bumping. 

If a vacuum-distillation apparatus is not available for the above preparation, the crude product may be distilled at atmospheric pressure and the acetoacetate collected as the fraction boiling at i75 -i85 . A pure preparation cannot be obtained in this way, however, because the ester decomposes slightly when distilled at atmospheric pressure. 

Safety goggles should always be worn over the eyes when carrying out potentially dangerous operations, e,g. vacuum distillations, distillation of large volumes of inflammable liquids and experiments employing large quantities of metallic sodium. 

The dotted line in Fig. 11,17, 2 refers to the modification of the apparatus for use in fractional distillation under reduced pressure a Perkin triangle or equivalent receiver device ( vacuum distilling adapter ) is generally employed for collection of the various fractions. 

Attach the flask containing the mixture, with capillary in position, to the column make sure that the Perkin triangle (or vacuum distilling adapter ) and receiver are in place. See that the oil pump is functioning... 

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