Olefin content

Despite numerous efforts using various techniques, precise knowledge of olefin content remains an unresolved problem. That is why it is worthwhile to mention two methods commonly employed which provide an answer to the problem. 

Regarding product characteristics, European specifications were established in 1992. They concern mainly the motor octane number (MON) that limits the olefin content and which should be higher than 89, and the vapor pressure, tied to the C3/C4 ratio which should be less than 1550 mbar at 40°C (ISO 4256). On the other hand, to ensure easy vehicle start-ups, a minimum vapor pressure for winter has been set which is different for each country and depends on climatic conditions. Four classes. A, B, C, and D, are thus defined in Europe with a minimum vapor pressure of 250 mbar, respectively, at -10°C (A), -5 C (B), 0°C (C) and -t-10°C (Z)). France has chosen class A. 

These are carbon monoxide, CO, unburned hydrocarbons (HC), and the nitrogen oxides, NO. In the U.S.A., a program called Auto/Oil (Burns et al., 1992), conducted by automotive manufacturers and petroleum companies, examined the effect of overall parameters of fuel composition on evaporative emissions and in the exhaust gases. The variables examined were the aromatics content between 20 and 45%, the olefins content between 5 and 20%, the MTBE content between 0 and 15% and finally the distillation end point between 138 and 182°C (more exactly, the 95% distilled point). 

The tendency for a solvent to form deposits by polymerization of impurities such as olefins is measured by the test for potential gums . Olefin content can also be represented by the bromine index , which is a measure of the degree of unsaturation (see paragraph 3.4.1). 

Hydrotreating processes are applied to finished products to improve their characteristics sulfur content, cetane number, smoke point and the aromatics and olefins contents. 

Characterization. In many cases, ftir is a timely and cost-effective method to identify and quantify certain functionaHties in a resin molecule. Based on developed correlations, ftir is routinely used as an efficient method for the analysis of resin aromaticity, olefinic content, and other key functional properties. Near infrared spectroscopy is also quickly becoming a useful tool for on-line process and property control. 

Eor most hydrocarbon resins where numerous stmcturaHy different monomers are polymerized, nmr is typically used as a general tool to quantify the aromatic and/or olefinic content of a resin. In conjunction with gpc and ftir, nmr measurements are used to identify and quantify particular functionaHties or monomers present in hydrocarbon resins. 

A lower a-olefin content is required in a uniformly branched ethylene copolymer to decrease its crystallinity and density to a given level. 

Chain Structure. LLDPE resins are copolymers of ethylene and a-olefins with low a-olefin contents. Molecular chains of LLDPE contain units derived both from ethylene, —CH2—CH2—, and from the a-olefin, —CH2—CHR—, where R is C2H for ethylene—1-butene copolymers, for... 

Physical Properties. LLDPE is a sernicrystaUine plastic whose chains contain long blocks of ethylene units that crystallize in the same fashion as paraffin waxes or HDPE. The degree of LLDPE crystallinity depends primarily on the a-olefin content in the copolymer (the branching degree of a resin) and is usually below 40—45%. The principal crystalline form of LLDPE is orthorhombic (the same as in HDPE) the cell parameters of nonbranched PE are a = 0.740 nm, b = 0.493 nm, and c (the direction of polymer chains) = 0.2534 nm. Introduction of branching into PE molecules expands the cell slightly thus a increases to 0.77 nm and b to around 0.50 nm. 

Content of Ot-Olefin. An increase in the a-olefin content of a copolymer results in a decrease of both crystallinity and density, accompanied by a significant reduction of the polymer mechanical modulus (stiffness). Eor example, the modulus values of ethylene—1-butene copolymers with a nonuniform compositional distribution decrease as shown in Table 2 (6). A similar dependence exists for ethylene—1-octene copolymers with uniform branching distribution (7), even though all such materials are, in general, much more elastic (see Table 2). An increase in the a-olefin content in the copolymers also results in a decrease of their tensile strength but a small increase in the elongation at break (8). These two dependencies, however, are not as pronounced as that for the resin modulus. 

Table 2. Effect of a-Olefin Content in LLDPE on Modulus...

Amylenes. Amylenes (C monoolefins) produce alkylates with a research octane in the range of 90—93. In the past, amylenes have not been used widely as an industrial alkylation charge, although in specific instances, alkylation with amylenes has been practiced (23). In the future, alkylation with amylenes will become more important as limits are placed on the vapor pressure and light olefin content of gasolines. 

Products from catalytic cracking units are also more stable due to a lower olefin content in the liquid products. This reflects a higher hydrogen transfer activity, which leads to more saturated hydrocarbons than in thermally cracked products from delayed coking units, for example. 

Olefins are not the preferred feedstocks to an FCC unit. This is not because olefins are inherently bad, but because olefins in the FCC feed indicate thermally produced oil. They often polymerize to form undesirable products, such as slurry and coke. The typical olefin content of FCC feed is less than 5 wt%, unless unhydrotreated coker gas oils are being charged. 

Alternatively, the bromine index is the number of milligrams of bromine that will react with 100 grams of the sample, and is most often used by the chemical industry for stocks that have very low olefin contents. 

In 1990, U.S. gasoline contained about 10 vol% olefins, the majority of which emanate from FCC gasoUne. FCC gasoline has 25 vol% to 35 vol% olefins. Of these olefins, Cg-C olefins account for about 85% of the total pool. For non-RFG gasoline, as with sulfur, the regulation allows the maximum olefin content to be 125% of the 1990 baseline values. 

Low Octane Low Olefin Content High CokeMake High H/C Ratio Low Conversion... 

On cobalt, the typical shape of the curve of olefin content vs. carbon number fractions in Figure 9.16 shows a low value at Nc = 2, indicating the high reactivity of ethene for hydrogenation and growth. The olefin content declines from Nc = 3... 

FIGURE 9.16 Olefin contents in carbon number fractions primary and secondary olefin selectivity. 

FIGURE 9.18 Influence of pressure on reaction rate, olefin content in the C3 fraction, and methane selectivity with cobalt as the catalyst for FT synthesis. Catalyst 100Co-18Th02-100 Si02 (Kieselguhr), H2/CO = 1.8, 175°C. 

Octgain A hydrofinishing process which reduces the sulfur and olefin content of gasoline without reducing its octane number. A zeolite catalyst is used. Developed by Mobil in 1994. Chem. Eng. (N.Y.), 1994,101(7), 25. 

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