Heavy fraction

There are little or no olefins in crude oil or straight run (direct from crude distillation) products but they are found in refining products, particularly in the fractions coming from conversion of heavy fractions whether or not these processes are thermal or catalytic. The first few compounds of this family are very important raw materials for the petrochemical Industry e.g., ethylene, propylene, and butenes. 

When it comes to the heaviest of petroieum fractions, modern analytical methods are not able to isolate and characterize the molecules completely. In the absence of something better, the analyst separates the heavy fractions into different categories, which leads merely to definitions that are workable but are no longer in terms of exact structure. 

As stated above for the TBP distillation, petroleum cannot be heated above 340°C without its molecules starting to crack. Because of this, analytical distillation of heavy fractions is done according to the ASTM D 1160 method for petroleum materials that can be partially or completely vaporized at a maximum temperature of 400°C at pressures from 50 to 1 mm of mercury (6.55 to 0.133 kPa). 

SARA (Saturates, Aromatics, Resins, Asphaltenes) analysis is widely practiced on heavy fractions such as vacuum and atmospheric residues and vacuum distillates for two purposes ... 

The liquid dynamic viscosities at 100°F and 210°F are used to characterize petroleum fractions, notably the heavy fractions. 

For heavy fractions whose boiling temperatures exceed 600 K, it is better to use the method published by Lee and Kesler in 1975 ... 

This method is reserved for heavy fractions. The distillation takes place at low pressure from 1 to 50 mmHg. The results are most often converted into equivalent atmospheric temperatures by using a standard relation that neglects the chemical nature of the components ... 

This method is applicable to heavy fractions whose boiling point is greater than 200°C. The average error is around 10% for pressures between 0.001 and 10 bar. ... 

The gasoline end point should not exceed a given value, currently established for Europe at 215°C. In fact the presence of too-heavy fractions leads to incomplete combustion and to a number of accompanying problems ... 

The distillation of crudes chosen for their yield in heavy fractions is the most common means. Bitumen is extracted from the residue from a vacuum distillation column (a few dozen mm of mercury), the latter being fed by atmospheric distillation residue. Unlike the practice of a decade ago, it is now possible to obtain all categories of bitumen, including the hard grades. 

The different cuts obtained are collected their initial and final distillation temperatures are recorded along with their weights and specific gravities. Other physical characteristics are measured for the light fractions octane number, vapor pressure, molecular weight, PONA, weight per cent sulfur, etc., and, for the heavy fractions, the aniline point, specific gravity, viscosity, sulfur content, and asphaltene content, etc. 

Properly speaking, steam cracking is not a refining process. A key petrochemical process, it has the purpose of producing ethylene, propylene, butadiene, butenes and aromatics (BTX) mainly from light fractions of crude oil (LPG, naphthas), but also from heavy fractions hydrotreated or not (paraffinic vacuum distillates, residue from hydrocracking HOC). 

Products of conversion from catalytic cracking are largely olefinic for light fractions and strongly aromatic for the heavy fractions. 

Steam reforming is, along with catalytic reforming, a process that can produce the additional hydrogen needed for upgrading and converting the heavy fractions of crude oil. 

This form of limited-conversion hydrocracking is a process that selectively prepares high quality residues for the special manufacture of base oils of high viscosity index or treating residues having low BMCl for the conversion of heavy fractions to ethylene, propylene, butadiene and aromatics. 

Its purpose is to partially convert heavy fractions highly contaminated by natural compounds such as sulfur, nitrogen, metals Ni, V, and asphaltenes and to prepare feedstocks for deeper conversion or to produce low-sulfur fuel-oil. 

Front-end hydrogenation is also possible. This approach uses a different type of catalyst, and the reactor is located upstream of the demethanizer. For this design, a deethanizer or depropanizer tower is located upstream of the demethanizer to remove heavy fractions. This approach has been utilized by some Hcensors with some success. 

Sulfur Compounds. All crude oils contain sulfur in one of several forms including elemental sulfur, hydrogen sulfide, carbonyl sulfide (COS), and in aliphatic and aromatic compounds. The amount of sulfur-containing compounds increases progressively with an increase in the boiling point of the fraction. A majority of these compounds have one sulfur atom per molecule, but certain aromatic and polynuclear aromatic molecules found in low concentrations in crude oil contain two and even three sulfur atoms. Identification of the individual sulfur compounds in the heavy fractions poses a considerable challenge to the analytical chemist. 

Solvent extraction may also be used to reduce asphaltenes and metals from heavy fractions and residues before using them in catalytic cracking. The organic solvent separates the resids into demetallized oil with lower metal and asphaltene content than the feed, and asphalt with high metal content. Figure 3-2 shows the IFP deasphalting process and Table 3-2 shows the analysis of feed before and after solvent treatment. Solvent extraction is used extensively in the petroleum refining industry. Each process uses its selective solvent, but, the basic principle is the same as above. 

The API method is a generalized method that predicts mole fraction of paraffinic, naphthenic, or aromatic compounds for an olefin-Ifee hydrocarbon. The development of the equations is based on dividing the hydrocarbon into two molecular ranges heavy fractions (200 < MW < 600) and light fractions (70 < MW <200). Appendix 7 contains API correlations applicable to the FCC feed. 

Feed residue coke is the small portion of the (non-residue) feed that is directly deposited on the catalyst. This coke comes from the very heavy fraction of the feed and its yield is predicted by the Conradson or Ramsbottom carbon tests. 

Gas not containing a heavy fraction easily condensable under normal separation and transport conditions. 

The gas has a high content of monomers (ethylene and propylene) and other useful hydrocarbons with only some 15% being methane. The feedstock is collected in two stages since the heavy fraction is a wax below about 60 °C. The heavy fraction is typically 60% by weight of the product with the light fraction being 40% by weight. 

The liquefied plastic fraction is heated to over 400 °C. This leads to cracking of the plastic into components of different chain lengths. Gases count for 20%-30% and oils for 60%-70% they are separated by distillation. Any naphtha produced is treated in a steam cracker, resulting in monomers like ethylene and propylene that are recovered. Such monomers can be used to produce plastics again. The heavy fractions can be processed into synthesis gas or conversion coke and then be transferred for further use. At most 5% of the input is converted into a mineral fraction. It is likely that this consists mainly of the inorganic additives in plastics. 

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