Olefin primary fractionator

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An olefin plant that uses liquid feeds requires an additional pyrolysis furnace, an effluent quench exchanger, and a primary fractionator for fuel oil separation. 

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. 

Modification of the zeolite appears to have affected the selectivity of Ru in these hydrogenation reactions. Exchange of K cations for Na cations in Y zeolite increases the basicity of the support (ref. 9). In Fischer-Tropsch reactions over similar catalysts, Ru/Y catalysts so modified yielded significant increases in the olefinic product fraction at the expense of paraffins. Olefins are believed to be primary products in F-T synthesis, with paraffins being produced from olefins in secondary hydrogenation reactions. In an analogous fashion, the Ru/KY catalyst used in the present study might also be expected to... 

One of the issues that concern liquid feedstock cracking operations is a higher rate of fouling. This is not only a consequence of heavier coke forming precursors, but also as a consequence of long lived free radicals which act as agents for the formation of a polymer (often referred to as pop-corn polymer) in the primary fractionator and downstream units. For instance, free radicals based on styrene or indene have sufficiently long half-lives to pass from the pyrolysis section into the primary fractionator. These can concentrate in this unit and produce polymer (free radical polymerisation) when sufficient amounts of suitable olefins are present, in particular styrene itself and di-olefins such as cyclo-pentadiene or butadiene. 

The cracked gas from an olefin furnace is cooled to between 360" and 420°F by heat exchangers, followed by a direct oil quench, before entering the primary fractionator. The function of this column is to cool the cracked gas by direct contact with quench oil and reflux. Simulation of this column is complex and uses at least 22 characteristic components, ranging from hydrogen to gas oil in boiling points. In addition to cooling the cracked gas, only C-8 and lighter compounds are of value in the overhead naphtha product, which requires some rectification of the vapor stream. 

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

Isoalkanes can also be synthesized by using two-component catalyst systems composed of a Fischer-Tropsch catalyst and an acidic catalyst. Ruthenium-exchanged alkali zeolites288 289 and a hybrid catalyst290 (a mixture of RuNaY zeolite and sulfated zirconia) allow enhanced isoalkane production. On the latter catalyst 91% isobutane in the C4 fraction and 83% isopentane in the C5 fraction were produced. The shift of selectivity toward the formation of isoalkanes is attributed to the secondary, acid-catalyzed transformations on the acidic catalyst component of primary olefinic (Fischer-Tropsch) products. 

The principal disadvantage of this procedure resides in its application to terminal olefins. Since the hydroboration step produces ca. 94% primary boron-bound alkyl groups, the maximum purity of primary carbinol is obviously limited to ca. 94%. Isolation of primary alcohol free of the contaminant secondary alcohol requires a tedious, yield-lowering fractionation procedure. This difficulty may be circumvented by employing a more selective hydroborating reagent, disiamylborane, as illustrated in the synthesis of 1-octanol. 

Olefinicity of the product is pictured in the diagrams of olefin content in carbon number hydrocarbon fractions and 1-olefin content in n-olefin carbon number fractions (Fig. 4). Horizontal lines (no influence of carbon number) represent primary olefin selectivity. Then these diagrams show that with both synthesis gases, from the beginning of FT-product formation, the olefins are almost exclusively of primary nature (almost solely a-olefins). This primary olefinicity increases with time, e.g. in Fig. 4. left, from about 35 to about 80 per cent. 

Linear a-olefins together with linear paraffins are the main primary products. On Fe the olefin content in the fraction of linear hydrocarbons for small carbon numbers was found to be about 80% (Fig. 4), which is very close to their primary selectivity [6]. This can be due to the high potassium loading, which suppresses the secondary reactions of the olefins. With increasing CO2 content a slight increase of the olefin content is observed. This can be due to the increasing amount of water formed from the reaction with CO2 instead of CO. The effect of added water on the olefin selectivity for a potassium promoted fused iron catalyst has been reported earlier by Satterfield [7]. With increasing CO2 concentration in the reaction gas on Co no more olefins were present in the products. 

Hydrocarbon product distributions on the zeolite-supported catalyst are shown in Table 1. The product distribution did not change greatly after deposition at 400 C, but the olefin fraction obviously increased. The shift in selectivity of paraffins to olefins is attributed to be due to the reduced capacity of the deactivated catalyst 10 hydrogenate primary olefins to the con-espondiiig paraffins. Similar results were also obtained with the alumina-supported catalysts. 

The same year, Gerlach described a synthesis of optically active 1 from (/ )- ,3-butanediol (7) (Scheme 1.2). The diastereomeric esters produced from (-) camphorsulfonyl chloride and racemic 1,3-butanediol were fractionally recrystallized and then hydrolized to afford enantiomerically pure 7. Tosylation of the primary alcohol, displacement with sodium iodide, and conversion to the phosphonium salt 8 proceeded in 58% yield. Methyl-8-oxo-octanoate (10), the ozonolysis product of the enol ether of cyclooctanone (9), was subjected to Wittig condensation with the dilithio anion of 8 to give 11 as a mixture of olefin isomers in 32% yield. The ratio, initially 68 32 (E-.Z), was easily enriched further to 83 17 (E Z) by photolysis in the presence of diphenyl disulfide. The synthesis was then completed by hydrolysis of the ester to the seco acid, conversion to the 2-thiopyridyl ester, and silver-mediated ring closure to afford 1 (70%). Gerlach s synthesis, while producing the optically active natural product, still did not address the problem posed by the olefin geometry. 

Although DME and methanol are the primary feedstocks for the ZSM-5 catalyzed MTG or MTO process (ref. 7)r higher alcohols that can be produced over methanol catalysts by reactions (5), or esters by reactions (6), have been demonstrated to be suitable feedstocks for conversion to aromatic gasoline or olefins over the ZSM class of acid catalysts (ref. 8). Hence, crude methanol containing from < IX to a large fraction of higher oxygenates may be used in the MTG and MTO processes. 

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