Fractional distillation Separating chemicals

You ve probably simmered a liquid in a covered pot on the stove. And you ve probably noticed that when you remove the lid, water is on the inside of the lid. The heat has caused the water to evaporate from the liquid, and the vapors have condensed back into a liquid on the inside of the cooler lid. This is the most basic example of a process called distillation. 

The third fraction is composed of hydrocarbons of 12 to 16 carbon atoms in the boiling range of 150 to 275 degrees Celsius. This fraction is used as kerosene and jet fuel. In the next section, I tell you how this fraction is also used to make additional gasoline. 

1 The fourth fraction is composed of hydrocarbons in the 12 to 20 carbon-atom chains, with a boiling range of 250 to 400 degrees Celsius. This fraction is used for heating oil and diesel fuel Again, it can be used in the production of additional gasoline. 

The fifth fraction is composed of hydrocarbons in the 20 to 36 carbon-atom range, with boiling points of 350 to 550 degrees Celsius. They re used as greases, lubricating oils, and paraffin-based waxes. 

The sixth fraction is composed of the residue of semisolid and solid materials that has a boiling point well above 550 degrees Celcius. It s used as asphalt and tar 

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Petroleum is by far the largest source of the vast number of products broadly known as petrochemicals. Raw petroleum is a mixture of hydrocarbons containing up to 40 carbon atoms per molecule. These large molecules are not useful in their natural form, but they are broken into smaller molecules in petroleum refineries (Fig. 21.13 also see Fig. 18.7). Catalytic cracking essentially cracks the long carbon chains into shorter molecules of 5 to 10 carbon atoms. Fractional distillation separates hydrocarbons into fractions that boil at different temperatures. Alkanes of up to 4 or 5 carbon atoms per molecule may be obtained in pure form by this method. The boiling points of larger alkanes are too close for their complete separation, so chemical methods must be used to obtain pure products. 

Although isotopes have similar chemical properties, their slight difference in mass causes slight differences in physical properties. Use of this is made in isotopic separation pro cesses using techniques such as fractional distillation, exchange reactions, diffusion, electrolysis and electromagnetic methods. 

One of the problems with all the current phenol conversions is that a certain amount of other phenols, such as resorcinol and hydro-quinone, will be formed along with the catechol (don t ask). These species are very hard to separate from the catechol because they are all so similar. Aside of carefully monitored fractional distillation there are some vague strategies which can be found in the Chemical Abstract references 116-118. 

Classical methods for separation and purifica tion include fractional distillation of liquids and re crystallization of solids and these two methods are routinely included in the early portions of laboratory courses in organic chemistry Because they are capa ble of being adapted to work on a large scale frac tional distillation and recrystallization are the preferred methods for purifying organic substances in the pharmaceutical and chemical industries... 

Considering their heat sensitivity, the separation of fatty acids and rosin with minimal degradation by fractional distillation under vacuum and/or in the presence of steam is surprisingly good (3). Tad od rosin (TOR) contains about 2% fatty acid and smad amounts of neutrals. Tad od fatty acid (TOFA) contains as Htde as 1.2% rosin and 1.7% neutrals. In typical U.S. TOFA, 49% of the fatty acids is oleic, 45% linoleic, and 3% palmitic, stearic, and eicosatrienoic acid. TOR and TOFA are upgraded to resins and chemicals for the manufacture of inks (qv), adhesives (qv), coatings (qv), and lubricants (see Lubrication AND lubricants). 

The isomeric mixture is a colodess, mobile Hquid with a sweet, slightly irritating odor resembling that of chloroform. It decomposes slowly on exposure to light, air, and moisture. The mixture is soluble ia most hydrocarbons and only slightly soluble ia water. The cis—trans proportions ia a cmde mixture depend on the production conditions. The isomers have distinct physical and chemical properties and can be separated by fractional distillation. 

Distillation (qv) is the most widely used separation technique in the chemical and petroleum industries. Not aU. Hquid mixtures are amenable to ordinary fractional distillation, however. Close-boiling and low relative volatihty mixtures are difficult and often uneconomical to distill, and azeotropic mixtures are impossible to separate by ordinary distillation. Yet such mixtures are quite common (1) and many industrial processes depend on efficient methods for their separation (see also Separation systems synthesis). This article describes special distillation techniques for economically separating low relative volatihty and azeotropic mixtures. 

FIG. 13-41 Comparison of rigorous calcnlations with Gilliland correlation. [Henley and Seader, Eqnilihrinm-Stage Separation Operations in Chemical Engineering, Wiley, New York, 1981 data of Van Winkle and Todd, Chem. Eng., 78(21), 136 (Sept. 20, 1971) data of Gilliland, Elements of Fractional Distillation, 4th ed., McGraw-Hill, New York, 1950 data of Brown and Maiiin, Trans. Am. Inst. Chem. Eng., 35, 679 (1.93.9) ]... 

Introduction Reactive distillation is a unit operation in which chemical reaction and distiUative separation are carried out simultaneously within a fractional distillation apparatus. Reactive distillation may be advantageous for liqiiid-phase reaction systems when the reaction must be carried out with a large excess of one or more of the reactants, when a reaction can be driven to completion by removal of one or more of the products as they are formed, or when the product recoveiy or by-product recycle scheme is complicated or macfe infeasible by azeotrope formation. 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

Separations for removing undesirable by-products and impurities, and making suprapure fine chemicals constitute a major fraction of the production costs. There is an enormous variety of methods for product separation and purification and many books on the subject have been published. Here, we deal with the problem in a very general way and we refer the reader to advanced books for details. Conventional techniques for product isolation and purification, such as fractional distillation, extraction, and crystallization, still predominate. Some guidelines for scale-up of these techniques and producing experimental data for scale-up are given in Chapter 5. More information on specific separation and purification techniques applied to particular problems of fine chemicals manufacture the reader can find in Chapter 6. 

The finished product is centrifuged and purified via a number of processes, including filtration, fractional distillation, condensation, crystallization, and chromatographic separation techniques. The purified API is tested and then it is ready to be formulated into the finished dosage form, as discussed in Section 10.6. Exhibit 10.5 illustrates some of the typical reagents for API manufacture and Exhibit 10.6 presents selected chemical reactions as examples of the... 

The asphaltenes are nonvolatile and remain in the residue when the crude is subjected to distillation. The resins are partially volatile and therefore may be present in the lubricating oil fractions of higher boiling point as well as in the residue. Among the many methods employed for the separation of these materials from the oil fractions are distillation, adsorption, chemical treatment, and precipitation by special solvents. 

Coal-Tar Process. The largest quantities of naphthalene are obtained from the coal tar that is separated from the coke-oven gases. The coal tar first is processed through a tar-distillation step where ca the first 20 wt% of distillate, i.e., chemical oil, is removed. The chemical oil contains practically all the naphthalene present in the tar. It is processed to remove the tar acids by contacting with dilute sodium hydroxide and, in a few cases, is next treated to remove tar bases by washing with sulfunc acid. Principal U.S. producers obtain their crude naphthalene product by fractional distillation of the tar acid-free chemical oiL... 

The most common method for the separation and concentration of flavor chemicals before chromatography is solvent extraction. If the aroma active components in a sample are less than a microgram/liter then solvent extraction followed by fractional distillation can be used to concentrate the analytes above 1 4g/liter. This is done for two reasons (1) to remove the odorants from some of the interfering substances and nonvolatiles, and (2) to concentrate the sample for greater sensitivity. The choice of solvent(s) depends on a number of issues, but similar results can be obtained with many solvents. Table Gl.1.2 lists a number of solvents, their polarity, and physical properties. Pentane is the least polar and ethyl acetate the most. The sample must be an aqueous or dilute sample, dissolved or slurried into water to a final concentration of 80% to 90% water. Dilute aqueous samples will present the greatest polarity difference between the solvent and the sample, driving more volatiles into the extracting solvent. 

Alcohols are dried and sent to a distillation train where they are separated by conventional fractional distillation. Crude alcohols are separated into C2-C%, C6-C10, C12-Cu, C16-C18, and C20 + fractions. High purity, individual homologs are prepared by redistillation of the appropriate mixture. The product alcohols are marketed as ALFOL alcohols by Conoco Chemicals. 

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