Composition naphtha

Figure 5.34 Predictive models for naphtha compositional analysis (PINA).

Table 1.11 Japanese Open Spec and Some Typical Naphtha Compositions...

An indication of naphtha composition may also be obtained from the determination of aniline point (ASTM D-1012, IP 2), freezing point (ASTM D-852, ASTM D-1015, ASTM D-1493) (Fig. 4.2), cloud point (ASTM D-2500) (Fig. 4.3), and solidification point (ASTM D-1493). And, although refinery treatment should ensure no alkalinity and acidity (ASTM D-847, ASTM D-1093, ASTM D-1613, ASTM D-2896, IP 1) and no olefins present, the relevant tests using bromine number (ASTM D-875, ASTM D-1159, IP 130), bromine index (ASTM D-2710), and flame ionization absorption (ASTM D-1319, IP 156) are necessary to ensure low levels (at the maximum) of hydrogen sulfide (ASTM D-853) as well as the sulfur compounds in general (ASTM D-130, ASTM D-849, ASTM D-1266, ASTM D-2324, ASTM D-3120, ASTM D-4045, ASTM D-6212, IP 107, IP 154) and especially corrosive sulfur compounds such as are determined by the Doctor test method (ASTM D-4952, IP 30). 

Existing methods use physical measurements and require suitable standards. Tests such as aniline point (ASTM D-611) and kauri-butanol number (ASTM D-1133) are of a somewhat empirical nature and can serve a useful function as control tests. Naphtha composition, however, is monitored mainly by gas chromatography, and although most of the methods may have been developed for gasoline (ASTM D-2427, ASTM D-6296), the applicability of the methods to naphtha is sound. 

TABLE IX Regularity of Naphtha Composition Adapted from Smith (1968) ... 

Naphtha feed is often characterized using PINA analysis that simply is the weight % of K-paraffin, Ao-paraffin, naphthene and aromatic compounds. If the typical commercial indexes (specific gravity, PINA analysis and TBP curves or ASTM D86) are used properly, it is possible to empirically derive detailed naphtha composition by referring to the four different hydrocarbon classes and only to a limited number of reference components within each class. In fact, the PINA information indicates the relative abundance of the four different classes directly. The specific gravity and boiling curve allow the specification of the initial and final cuts of the hydrocarbon mixture as well as the relative presence and distribution of the reference pseudo components inside each fraction. 

Quantitative analysis by NIR of petrochemicals dates back to the 1930s. Since then a range of NIR calibrations have been developed, for example, for octane number, methyl group analysis, and methanol content in petroleum. NIR has been used to monitor water, detergent solids, and glycerol in shampoo, and to analyze moisture and lubricant levels on polymer films. Process NIR spectrometers have been used to monitor naphtha composition and NIR instrumentation has been used to monitor ethylene polymerization. 

Compositional information from gas chromatographic analysis is more definitive than distillation data. A naphtha composition is given in Table 1. The composition is given after the naphtha has been hydrotreated to lower concentrations of olefins and contaminants. Some of the paraffin isomers are lumped together in this particular analysis. The compositional information is often categorized into the classes of paraffins, olefins, naphthenes, and aromatics, and is called a PONA. The ease of reforming feedstocks has been correlated to the sum... 

A complete analysis of the naphtha using gas chromatography has become possible in recent years. A number of typical components for normal and iso-paraffins and naphthenes were selected and their disappearance was described through first order kinetics. Even with this detailed breakdown some variation in the rate coefficient for a given component was observed between the three naphthas. The variation can be traced back to the interaction between reacting species, which cannot be accounted for through simple molecular disappearance kinetics. The chemical environment in which a given component is cracked evidently depends upon the naphtha composition. 

Composition of fraction / in the product Composition of fraction / in the feed Composition of middles distillates Composition of gases Composition of naphtha Composition of VGO Composition of VR... 

Molar fraction of A Middle distillates composition Gas composition Naphtha composition Vacuum residue composition VGO composition Axial position, m... 

Two types of specifications are written into supply contracts for naphthas they concern the composition and the level of contaminants. 

Table 8. Composition of Naphthas from Various Sources...

Gas oil fractions (204—565°C) from coal Hquefaction show even greater differences in composition compared to petroleum-derived counterparts than do the naphtha fractions (128). The coal-gas oils consist mostly of aromatics (60%), polar heteroaromatics (25%), asphaltenes (8—15%), and saturated... 

The composition of the cracked gas with methane and naphtha and the plant feed and energy requirements are given in Table 9. The overall yield of acetylene based on methane is about 24% (14). A single burner with methane produces 25 t/d and with naphtha or LPG produces 30 t/d. The acetylene is purified by means of /V-methy1pyrro1idinone. 

The cracked gas composition is shown ia Table 10 for the water queach operatioa (16). Oae thousand cubic meters of methane and 600 m of oxygen produce 1800 m of cracked gas. If a naphtha quench is used, additional yields are produced, consuming 130 kg of naphtha/1000 of methane... 

Analytical Approaches. Different analytical techniques have been appHed to each fraction to determine its molecular composition. As the molecular weight increases, complexity increasingly shifts the level of analytical detail from quantification of most individual species in the naphtha to average molecular descriptions in the vacuum residuum. For the naphtha, classical techniques allow the isolation and identification of individual compounds by physical properties. Gas chromatographic (gc) resolution allows almost every compound having less than eight carbon atoms to be measured separately. The combination of gc with mass spectrometry (gc/ms) can be used for quantitation purposes when compounds are not well-resolved by gc. 

The predominant feeds for reforming are straight-mn naphthas from cmde stills. Naphthas from catalyst crackers and naphthas from code stills are also used. Typical compositions are summarized in Table 5. Typical operating conditions for catalytic reforming are 1.135—3.548 MPa (150—500 psi),... 

Asphaltenes seem to be relatively constant in composition in residual asphalts, despite the source, as deterrnined by elemental analysis (6). Deterrnination of asphaltenes is relatively standard, and the fractions are termed / -pentane, / -hexane, / -heptane, or naphtha-insoluble, depending upon the precipitant used (5,6,49). After the asphaltenes are removed, resinous fractions are removed from the maltenes-petrolenes usually by adsorption on activated gels or clays. Recovery of the resin fraction by desorbtion is usually nearly quantitative. 

A few industrial catalysts have simple compositions, but the typical catalyst is a complex composite made up of several components, illustrated schematically in Figure 9 by a catalyst for ethylene oxidation. Often it consists largely of a porous support or carrier, with the catalyticaHy active components dispersed on the support surface. For example, petroleum refining catalysts used for reforming of naphtha have about 1 wt% Pt and Re on the surface of a transition alumina such as y-Al203 that has a surface area of several hundred square meters per gram. The expensive metal is dispersed as minute particles or clusters so that a large fraction of the atoms are exposed at the surface and accessible to reactants (see Catalysts, supported). 

A high-nickel alloy is used for increased strength at elevated temperature, and a chromium content in excess of 20% is desired for corrosion resistance. An optimum composition to satisfy the interaction of stress, temperature, and corrosion has not been developed. The rate of corrosion is directly related to alloy composition, stress level, and environment. The corrosive atmosphere contains chloride salts, vanadium, sulfides, and particulate matter. Other combustion products, such as NO, CO, CO2, also contribute to the corrosion mechanism. The atmosphere changes with the type of fuel used. Fuels, such as natural gas, diesel 2, naphtha, butane, propane, methane, and fossil fuels, will produce different combustion products that affect the corrosion mechanism in different ways. 

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