Paraffins normal

Normal paraffins in the C,o - C,5 range are recovered from petroleum fractions by adsorption-desorption using molecular sieves. Ammonia can be used to desorb the n-paraffins. By employing two beds of sieves, one on adsorption and one on desorption at all times, a continuous flow of the feed and ammonia is maintained. 

In the adsorption step, the hydrocarbon feed is vaporized in a furnace and is passed upward through the sieves which have been previously desorbed by ammonia. The n-paraffins in the feed are adsorbed by the sieve and displace the ammonia. The amount of ammonia removed depends largely on the molecular 

In the desorption step, ammonia is passed downflow through the bed which has completed the adsorption cycle. The ammonia is heated to approximately the same temperature as that of the feed in the adsorption step in order to maintain a nominally isothermal operation. The first portion of the desorbate, although rich in n-paraffms, contains impurities and is recycled to the second bed which is simultaneously operating on the adsorption cycle. The remaining product is condensed and separated from ammonia. The product is freed of dissolved ammonia by distillation. 

The sieves slowly lose capacity and are regenerated about by burning with dilute oxygen. The process is capable of recovering n-paraffms up to Cjo. 

High purity cyclohexane is manufactured by hydrogenating benzene at 400-500°F and 500 psig. Some cyclohexane was earlier produced by fractionating naphtha but its purity of 85-90% was too low to compete with 99-t- percent purity from benzene hydrogenation. A number of cyclohexane processes based on benzene hydrogenation are available. 

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They can be structured as straight chains as are the normal paraffins or... 

Isoparaffins have boiling points lower than normal paraffins witTilHe same number of carbon atoms. Table 1.1 presents some physical properties of selected paraffins... 

A correlation between retention times and boiling points is established by calibration with a known mixture of hydrocarbons, usually normal paraffins, whose boiling points are known (see Figure 2.2). From this information, the distribution of boiling points of the sample mixture is obtained. 

For physical processes, two examples are the elimination of normal paraffins from a mixture by their adsorption on 5 A molecular sieves or by their selective formation of solids with urea (clathrates)... 

Specific Analysis for Normal Paraffins hy Gas Phase Chromatography... 

This analysis is necessary because of the particular temperature behavior of these components. Normal paraffins are the first to crystallize as the temperature is reduced. 

Identification of normal paraffins by chromatography presents no special problems with the exception of biodegraded crudes, they are clearly distinguished. The problem encountered is to quantify, as shown in Figure 3.14, the normal paraffin peaks that are superimposed on a background representing other hydrocarbons. 

As a complementary process to reforming, isomerization converts normal paraffins to iso-paraffins, either to prepare streams for other conversions nCi —> /C4 destined for alkylation or to increase the motor and research octane numbers of iight components in the gasoiine pooi, i.e., the C5 or Cs-Ce fractions from primary distillation of the crude, or light gasoline from conversion processes, having low octane numbers. 

Gas hulk separations normal paraffins, isoparaffins, aromatics zeoHte... 

One version of the UOP IsoSiv process uses PSA to separate normal paraffins from branched and cycHc hydrocarbons in the to range. 

However, ia some cases, the answer is not clear. A variety of factors need to be taken iato consideration before a clear choice emerges. Eor example, UOP s Molex and IsoSiv processes are used to separate normal paraffins from non-normals and aromatics ia feedstocks containing C —C2Q hydrocarbons, and both processes use molecular sieve adsorbents. However, Molex operates ia simulated moving-bed mode ia Hquid phase, and IsoSiv operates ia gas phase, with temperature swiag desorption by a displacement fluid. The foUowiag comparison of UOP s Molex and IsoSiv processes iadicates some of the primary factors that are often used ia decision making ... 

Fig. 3. (a) Flame ionization detector (fid) response to an extract of commercially processed Valencia orange juice, (b) Gas chromatography—olfactometry (geo) chromatogram of the same extract. The abscissa in both chromatograms is a normal paraffin retention index scale ranging between hexane and octadecane (Kovats index). Dilution value in the geo is the -fold that the extract had to be diluted until odor was no longer detectable at each index. 

Table 8 shows that the naphthas produced by the EDS process have higher concentrations of cycloparaffins and phenols than do petroleum-derived naphthas, whereas the normal paraffins are present in much lower concentrations. The sulfur and nitrogen concentrations in coal naphthas are high compared to those in petroleum naphthas. 

The principal class of reactions in the FCC process converts high boiling, low octane normal paraffins to lower boiling, higher octane olefins, naphthenes (cycloparaffins), and aromatics. FCC naphtha is almost always fractionated into two or three streams. Typical properties are shown in Table 5. Properties of specific streams depend on the catalyst, design and operating conditions of the unit, and the cmde properties. 

Isomerization. Isomerization is a catalytic process which converts normal paraffins to isoparaffins. The feed is usually light virgin naphtha and the catalyst platinum on an alumina or zeoflte base. Octanes may be increased by over 30 numbers when normal pentane and normal hexane are isomerized. Another beneficial reaction that occurs is that any benzene in the feed is converted to cyclohexane. Although isomerization produces high quahty blendstocks, it is also used to produce feeds for alkylation and etherification processes. Normal butane, which is generally in excess in the refinery slate because of RVP concerns, can be isomerized and then converted to alkylate or to methyl tert-huty ether (MTBE) with a small increase in octane and a large decrease in RVP. 

The cetane number of a fuel depends on its hydrocarbon composition. In general, normal paraffins have high cetane numbers, isoparaffins and aromatics have low cetane numbers, and olefins and cycloparaffins fall somewhere in between. Diesel fuels marketed in the United States have cetane numbers ranging between 35 and 65. Most manufacturers specify a minimum cetane number of 40—45. 

Highly pure / -hexane can be produced by adsorption on molecular sieves (qv) (see Adsorption, liquid separation) (43). The pores admit normal paraffins but exclude isoparaffins, cycloparaffins, and aromatics. The normal paraffins are recovered by changing the temperature and/or pressure of the system or by elution with a Hquid that can be easily separated from / -hexane by distillation. Other than ben2ene, commercial hexanes also may contain small concentrations of olefins (qv) and compounds of sulfur, oxygen, and chlorine. These compounds caimot be tolerated in some chemical and solvent appHcations. In such cases, the commercial hexanes must be purified by hydrogenation. 

Separation of Norma/ and Isoparaffins. The recovery of normal paraffins from mixed refinery streams was one of the first commercial appHcations of molecular sieves. Using Type 5A molecular sieve, the / -paraffins can be adsorbed and the branched and cycHc hydrocarbons rejected. During the adsorption step, the effluent contains isoparaffins. During the desorption step, the / -paraffins are recovered. Isothermal operation is typical. 

The normal paraffins produced are raw materials for the manufacture of biodegradable detergents, plasticizers, alcohols, and synthetic proteins. Removal of the / -paraffins upgrades gasoline by improving the octane rating. 

Paraffin Isomerization. Another weU-estabhshed commercial process which employs zeoflte catalysts is the isomerization of normal paraffins into higher octane, branched isomers. The catalyst for the Hysomet process of the Shell Oil Co. is dual-functional, and consists of a highly acidic, latge-pote zeoflte loaded with a small amount of a noble-metal hydrogenation component. This catalyst possesses the same... 

Chemistry. Chemical separation is achieved by countercurrent Hquid— Hquid extraction and involves the mass transfer of solutes between an aqueous phase and an immiscible organic phase. In the PUREX process, the organic phase is typically a mixture of 30% by volume tri- -butyl phosphate (solvent) and a normal paraffin hydrocarbon (diluent). The latter is typically dodecane or a high grade kerosene (20). A number of other solvent or diluent systems have been investigated, but none has proved to be a substantial improvement (21). 

Fig. 4. Distribution of compound classes in cmde oils as a function of boiling point. Region A represents normal paraffins B, isoparaffins C, naphthenes ...

Catalytic dewaxiag (32) is a hydrocrackiag process operated at elevated temperatures (280—400°C) and pressures, 2,070—10,350 kPa (300—1500 psi). However, the conditions for a specific dewaxiag operatioa depead oa the aature of the feedstock and the product pour poiat required. The catalyst employed for the process is a mordenite-type catalyst that has the correct pore stmcture to be selective for normal paraffin cracking. Platinum on the catalyst serves to hydrogenate the reactive iatermediates so that further paraffin degradation is limited to the initial thermal reactions. 

Another approach is the simulated moving-bed system, which has large-volume appHcations in normal-paraffin separation andpara- s.yXen.e separation. Since its introduction in 1970, the simulated moving-bed system has largely displaced crystallisation ia xylene separations. The unique feature of the system is that, although the bed is fixed, the feed point shifts to simulate a moving bed (see Adsorption,liquid separation). 

Norma/andBranc/jedAlip/jatic Hydrocarbons. The urea-adduction method for separating normal and branched aHphatic hydrocarbons can be carried out in sulfolane (38,39). The process obviates the necessity of handling and washing the soHd urea—normal paraffin adduct formed when a solution of urea in sulfolane is contacted with the hydrocarbon mixture. OveraH recovery by this process is typicaHy 85% normal paraffin purity is 98%. 

Fig. 6. Boiling points of C —hydrocarbons. P, iso and normal paraffins C, C - and Cg-cycloparaffins and A, aromatics.

In the early 1960s the petroleum industry employing molecular sieve technology made available a low cost and plentihil supply of normal paraffin fractions of very high purity. This enabled chlorinated paraffin manufacturers to exploit new appHcations with a range of products specifically designed to meet the technical and commercial requirements. 

The principal feedstocks used today are the normal paraffin fractions CIO—C13, C12—C14, C14—C17, and C18—C20 together with paraffin wax fractions of C24—C30, precise compositions may vary depending on petroleum oil source. Chlorination extent generally varies from 30 to 70% by weight. The choice of paraffinic feedstock and chlorine content is dependent on the appHcation. 

Historically, citric acid was isolated by crystallization from lemon juice and later was recognized as a microbial metabohte. This work led to the development of commercial fermentation technology (13). The basic raw materials for making citric acid include com starch, molasses (sugar cane, beet sugar), and normal paraffin hydrocarbons. 

Many researchers have correlated the overall decomposition as an nxh. order reaction, with most paraffins following the first order and most olefins following a higher order. In general, isoparaffin rate constants are lower than normal paraffin rate constants. The rate constants are somewhat dependent on conversion due to inhibition effects that is, the rate constant often decreases with increasing conversion, and the order of conversion is not affected. This has been explained by considering the formation of aHyl radicals (38). To predict the product distribution, yields are often correlated as a function of conversion or other severity parameters (39). 

Dilute Binary Hydrocarbon Mixtures Hayduk-Minhas presented an accurate correlation for normal paraffin mixtures that was developed from 58 data points consisting or solutes from C5 to C32 and solvents from C5 to Cig. The average error was 3.4 percent for the 58 mixtures. 

The simplest form of ternary RCM, as exemplified for the ideal normal-paraffin system of pentane-hexane-heptane, is illustrated in Fig. 13-58 7, using a right-triangle diagram. Maps for all other non-azeotropic ternary mixtures are qiiahtatively similar. Each of the infinite number of possible residue curves originates at the pentane vertex, travels toward and then away from the hexane vertex, and terminates at the heptane vertex. 

It is important to note that simulated distillation does not always separate hydrocarbons in the order of their boiling point. For example, high-boihng multiple-ring-type compounds may be eluted earher than normal paraffins (used as the calibration standard) of the same boiling point. Gas chromatography is also used in the ASTM D 2427 test method to determine quantitatively ethane through pentane hydrocarbons. 

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