Naphtha Quality

In order to assist buyers of naphtha, parcels are characterised in terms of this PNA (paraffin, naphthenes and aromatics) analysis. In addition some organisations use the UOP or Watson K-factor as a characteristic. 

The UOP K-factor attempts to judge the paraffinicity of a fraction. It can he used for any petroleum fraction as well as naphtha. It 

Where Tb is the molal average boiling point of the fraction in degrees Rankin and s is the specific gravity of the Iraction. The K-factor can be correlated with other physical parameters of the fraction API gravity and viscosity API gravity and flash point API gravity and aniline point flash point and refractive index. 

Parafflnic fractions have K-factors of about 12.5 naphthenes have K-factors of about 11.5, whereas aromatic fractions have K-factors of about 10. 

The properties in the table are representative and there is a range for each type. Straight-run usually contains lOOppm sulphur or less but there are some exceptions. Generally straight run has a high level of paraffins (P), few if any olefins (O) and a varying amount of naphthenes 

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Two undesirable aspects of FCC naphtha quality are that it may contain unacceptably high amounts of foul smelling mercaptans, and that its thermal stability may be too low. Mercaptans are usually found in the light FCC naphtha and may be removed or converted to sulfides and disulfides by a sweetening process such as Merox, developed by UOP. Thermal stability is improved in sweetening processes through removal of cresylic and naphthenic acids. It may be further improved by clay treating and by addition of oxidation inhibitors such as phenylene diamine. 

We cite isomerization of Cs-Ce paraffinic cuts, aliphatic alkylation making isoparaffinic gasoline from C3-C5 olefins and isobutane, and etherification of C4-C5 olefins with the C1-C2 alcohols. This type of refinery can need more hydrogen than is available from naphtha reforming. Flexibility is greatly improved over the simple conventional refinery. Nonetheless some products are not eliminated, for example, the heavy fuel of marginal quality, and the conversion product qualities may not be adequate, even after severe treatment, to meet certain specifications such as the gasoline octane number, diesel cetane number, and allowable levels of certain components. 

Types of fuels ineluded True distillates (naphtha, kerosene, no. 2 diesel, no. 2 fuel oil, JP-4, JP-5) High-quality etudes, slightly eontaminated distillates Navy distillate Residuals and low-grade etude (No. 5 fuel. No. 6 fuel. Bunker C)... 

Hydrofining has been applied to Varsols and various other solvents for the control of odor, sulfur, and corrosion characteristics. For example. Hydrofining of Iranian and Kuwait distillates demonstrated its effectiveness as a means of producing "White Spirit", a high-quality solvent naphtha distributed in the United Kingdom. 

Acidic isomerization of the C5-C6 naphtha and some heavy alcohols from the aqueous product refinery (not shown in Figure 18.5) produced a reasonable-quality olefinic motor gasoline (Table 18.10). The octane value varied depending on the carbon number distribution of the feed, which could result in a product with an octane number up to ten units higher. 

The data from the density (specific gravity) test method (ASTM D1298 IP 160) provides a means of identification of a grade of naphtha but is not a guarantee of composition and can only be used to indicate evaluate product composition or quality when used in conjunction with the data from other test methods. Density data are used primarily to convert naphtha volume to a weight basis, a requirement in many of the industries concerned. For the necessary temperature corrections and also for volume corrections, the appropriate sections of the petroleum measurement tables (ASTM D1250 IP 200) are used. 

Introduction of zeolites into catalytic cracking improved the quality of the product and the efficiency of the process. It was estimated that this modification in catalyst composition in the United States alone saved over 200 million barrels of crude oil in 1977. The use of bimetallic catalysts in reforming of naphthas, a basic process for the production of high-octane gasoline and petrochemicals, resulted in great improvement in the catalytic performance of the process, and in considerable extension of catalyst life. New catalytic approaches to the development of synthetic fuels are being unveiled. 

The selection of steam cracker feedstock is mainly driven by market demand as different feedstock qualities produce different olefins yields. One of the commonly used feed quality assessment methods in practice is the Bureau of Mines Correlation Index (BMCI) (Gonzalo et al., 2004). This index is a function of average boiling point and specific gravity of a particular feedstock. The steam cracker feed quality improves with a decrease in the BMCI value. For instance, vacuum gas oil (VGO) has a high value of BMCI and, therefore, is not an attractive steam cracker feed. The commonly used feedstocks in industry are naphtha and gas oil. 

The "tar" stream which is highly aromatic is made up of tar naphtha, creosote and pitch. The tar naphtha is hydrofined to remove N, S and 0 compounds. It consists mainly of toluenes and xylenes. The creosote can either be sold as such (eg for wood preservation) or be hydrofined to yield a product rich in higher aromatics. The tar pitch is used for impregnating cellulose fibre pipes or is converted to high quality coke (for the production of carbon electrodes). 

In 1992, researchers developed an engineering and costing design for a fixed unit that operated at a rate of 2 tons per hour. Costs were estimated to be 149 (Canadian) per metric ton of soil treated. This estimate was based on the following assumptions the unit used medium naphtha as a solvent operations were 24 hours per day, for 260 days per year utilization factor of the facility was 83% capital costs were 2,548 million (Canadian) and capital amortized over 10 years at 10%, two payments per year. The estimate stipulated that the recovered oil was of suitable quality to be sold to offset process costs. It was estimated that the largest component of process costs would be labor ( 56 per ton of waste treated). Other cost components listed were capitalization costs ( 38 per ton), utilities ( 29 per ton), insurance ( 9 per ton), trucking and maintenance (each 5 per ton), equipment rental and site excavation and restoration (each 3 per ton), and waste disposal was estimated to cost 1 per ton (D17896F, p. 8). 

Cycle Oil. Heavier, distillate range compounds formed during FCC processing can accumulate within the FCC fractionator. The primary fraction is called light cycle oil (LCO) and contains high percentages of monoaromatic and diaromatic compounds plus olefins and heavier branched paraffins. Unhydrotreated LCO is often quite unstable and has a very low cetane number. For this reason, it is blended into diesel fuel in controlled amounts. Heavy cycle oil and heavy naphtha are additional side cuts that can be produced. These streams can be pumped around to remove heat from the fractionator, used to supply heat to other refinery units, or used as low-quality blendstock component. 

Liquid products contain sulfur and nitrogen and must be hydroprocessed to improve quality. Separate hydroprocessing units for upgrading the naphtha, kerosene, and gas oil fractions can be used to optimize the overall process. Refined gas oil or diesel fuel is aromatic in character and contains more cycloparaffins than conventional crude oil. The resulting fuel is low in cetane number, high in density, and typically has very good low-temperature handling properties. 

Reforming Both thermal and catalytic processes are utilized to convert naphtha fractions into high-octane aromatic compounds. Thermal reforming is utilized to convert heavy naphthas into gasoline-quality aromatics. Catalytic reforming is utilized to convert straight-run naphtha fractions into aromatics. Catalysts utilized include oxides of aluminum, chromium, cobalt, and molybdenum as well as platinum-based catalysts. 

For commercial simulations, KINPTR s selectivity kinetics determine the reformate composition and overall yield at a target reformate octane. Reformer yield-octane behavior from pilot and commercial units are shown in Fig. 29a. The large variation in the reformate yields at a given octane, as much as 25%, results from the wide range of process conditions and naphtha feed quality used in Mobil reformers. As demonstrated in Fig. 29b, KINPTR accurately normalizes these reformate yields over a wide range of octanes, including those required for gasoline lead phaseout. 

Recent literature contains detailed correlations of the yield and quality of the gasoline product with operating conditions, quality of naphtha charge, and extent of outside additions of C8 and C4 fractions (7, H). 

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