Atmospheric rectification

The petroleum prepared at the oil well comes to the petroleum refinery and the first process at modern refineries (excluding the refineries working only with non-conventional feed) is atmospheric rectification. The first refinery, which was opened in 1861, produced only kerosene and this was possible by using simple atmospheric distillation alone. The by-products of this refinery included tar and naphtha. For the next thirty years, kerosene still remained the main product that consumers wanted. Two significant events changed this situation  

Distillation is used to separate volatile products from non-volatile substances. The early experimentalists also employed distillation. Aristotle (384-322 BC) mentioned that pure water was made by evaporation of seawater. 

In the petroleum industry, the method of fractional distillation, differential distillation, or rectification is utilized for the primary separation of crude oil into fractions with regard to their boiling temperatures, because simple distillation is not efficient for separating liquids whose boiling points lie close to one another. 

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

Instead of the example shown, rectification towers are used for rectification in the petroleum industry. Rectification towers can be classified as follows  

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In the industrial method, fractionation is achieved by the method of rectification. Using this method, the fractions with boiling point up to 350°C are separated at atmospheric pressure. These are called the light fractions. Usually, during atmospheric rectification, the following individual fractions are obtained ... 

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. 

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. 

In the scheme (Fig. 6.5), the feed (heavy residue from atmospheric rectification) passes to the bottom part of tower T3 and to the upper part of the low-pressure evaporator T4. The feed in T4 mixes with the heavy gas oil vapors and then passes to T3. The feed from the bottom of T3 passes to oven Ol for the heavy feed. 

Figure 2 illustrates the three-step MIBK process employed by Hibernia Scholven (83). This process is designed to permit the intermediate recovery of refined diacetone alcohol and mesityl oxide. In the first step acetone and dilute sodium hydroxide are fed continuously to a reactor at low temperature and with a reactor residence time of approximately one hour. The product is then stabilized with phosphoric acid and stripped of unreacted acetone to yield a cmde diacetone alcohol stream. More phosphoric acid is then added, and the diacetone alcohol dehydrated to mesityl oxide in a distillation column. Mesityl oxide is recovered overhead in this column and fed to a further distillation column where residual acetone is removed and recycled to yield a tails stream containing 98—99% mesityl oxide. The mesityl oxide is then hydrogenated to MIBK in a reactive distillation conducted at atmospheric pressure and 110°C. Simultaneous hydrogenation and rectification are achieved in a column fitted with a palladium catalyst bed, and yields of mesityl oxide to MIBK exceeding 96% are obtained. 

SABAR [Strong acid by azeotropic rectification] A process for making nitric acid by the atmospheric oxidation of ammonia. The nitrous gases from the oxidation are absorbed in azeotropic nitric acid in the presence of oxygen under pressure ... 

After the separation of diphenyldichlorosilane, the tank heating is switched off and the pressure in the system becomes atmospheric. Then the tank is unloaded. If it is necessary, the tank residue is sent to the fourth rectification stage to extract triphenylchlorosilane. The rectification of triphenylchlorosilane is similar to the rectification of phenyltrichlorosilane and diphenyldichlorosilane. 

Fraction II, the mixture of dimethyldichlorosilane and methyl(chloromethyl)dichlorosilane, is separated into receptacle 12 72-98 °C at the tower top until the distillate density reaches 1.28 g/cm3. After the separation of fraction II at 98 °C at the top of the tower, a vacuum pump is switched on to create vacuum, which is followed by the separation of fraction III. The rectification of dimethyldichlorosilane chlorination products at atmospheric pressure causes considerable tarring in the tank the use of vacuum largely eliminates this phenomenon and facilitates the tank heating. 

The vapours of the products formed by the boiling liquid in tank 14 rise through tower 15. After the tower the vapours saturated with low-boiling products enter the built-in condenser. Part of the condensate from the top of the tower is sent back through vapour-heated (1 MPa) heater 16 to reflux the tower the other part is collected in receptacle 17. The rectification takes place under atmospheric pressure. It allows to separate two fractions HMDS (80-100 °C), concentrated PMS-lb (100-153 °C) and concentrated PMS-l,5b (153-194 °C). After the distillation the tank residue is cooled and loaded off into collector 18. 

III rectification stage. Concentrated PMS-lb is separated in the rectification apparatus similar to the one described above. It is distilled under atmospheric pressure into two fractions the head fraction (153-193 °C) and PMS-l,5b (193-195 °C). The head fraction is added to concentrated PMS-lb for further rectification, the tank residue is added to the tank residue of stage I. 

The vapours of the products rise up rectification tower 14 filled with Raschig rings, enter refluxer 15 cooled with salt solution (-15 °C), condensed and sent back into the tower. The uncondensed vapours are sent into the atmosphere through calcium chloride tower 16 and fire-resistant apparatus 17. The tower is heated and the reflux is completely returned, until the temperature on top remains constant for 1 hour. After that, the fractions are separated. 

The rectification of tetraethyltin is carried out in vacuum. Before rectification the whole system is pressurised with nitrogen (at atmospheric pressure) cooler 11 is filled with water, freezer 12 is filled with salt solution after that, the tank receives the solution of tetraethyltin from reactor 5. The vacuum pump is switched on when the residual pressure in the system is 50-70 GPa, benzene is distilled and collected in the freezer. After the distillation is over, the jacket of the tank is filled with vapour. Until the temperature in the vapours is constant, the tower operates without refluxing. After constant temperature has been established, the first fraction is separated into receptacle 13. This fraction (a mixture of tin ethyl bromides) is separated below 54 °C (at a residual pressure of 50-70 GPa) the target fraction, tetraethyltin, is separated above 54 °C into receptacle 14. 

When the synthesis ends, as shown by pressure fall and constant temperature in the reactor, the valve between reactor 1 and cooler 4 opens, and ethyl mercaptan is distilled from the reactor. It self-flows into collector 6 through coolers 4 and 5. Collector 6 serves as a kind of settling box for tank residue. From there, ethyl mercaptan enters apparatus 7 and is pumped with immersed batching pump 8 into rectification tower 10. After ethyl mercaptan has been distilled, the tank residue in reactor 1 is cooled down after that, it is pressurised with nitrogen under 3 atmospheres into... 

The SABAR (Strong Acid By Azeotropic Rectification) process makes nitric acid by the atmospheric oxidation of ammonia. Davy McKee developed the process and built plants based on this technology from 1974 to 1986. 

One mole of the phenol is dissolved in 400 cc. water containing 1 mole of sodium hydroxide and 80 grams of soda ash. To this solution is added 500 cc. 90 per cent alcohol (ethyl or methyl) and the whole is cooled to 10°. Then 1.75 moles of ethyl or methyl chloride is added and the mixture is heated, with stirring or rotating, in an autoclave at 100°C. and 4 to 5 atmospheres pressure for 8 hours. The alkylation is then completed. The mixture is poured into water, and the alkyl ether is separated, recovering the alcohol by rectification. The product, which is washed with small amounts of sodium hydroxide and water, should now contain no nitrophenol. 

In Davy McKee s Sabar process (Strong Acid By Azeotropic Rectification) the nitrous gases from the oxidation of nitrogen(ll) oxide are absorbed in azeotropic (ca. 68 to 69%) nitric acid in the presence of atmospheric oxygen (at 6 to 13 bar) and superazeotropic acid is formed ... 

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