Heavy Residues

Here again, this is not a refining process, properly speaking. Partial oxidation is one of the processes for the ultimate conversion of heavy residues, asphalts, coke and even coal. 

Heavy residues are not always converted. The use of low sulfur light crude and crudes having a reduced ultimate residue (higher ratio of gasoline + distillates/vacuum residue) as well as natural gas utilization has been intensified. 

Refining after year 2000 will be characterized by heavy residue conversion and the reduction in aromatics content. 

Heavy residue conversion is linked to the demand for high quality diesel motor fuel (aromatics content 10%, cetane number 55) as well as to the demand for production of light fuel-oil having very low sulfur, nitrogen and metal contents. 

The hquid remaining after the solvent has been recovered is a heavy residual fuel called solvent-refined coal, containing less than 0.8 wt % sulfur and 0.1 wt % ash. It melts at ca 177°C and has a heating value of ca 37 MJ/kg (16,000 Btu/lb), regardless of the quaUty of the coal feedstock. The activity of the solvent is apparently more important than the action of gaseous hydrogen ia this type of uncatalyzed hydrogenation. Research has been directed to the use of petroleum-derived aromatic oils as start-up solvents (118). 

Some unsaturated ketones derived from acetone can undergo base- or acid-catalyzed exothermic thermal decomposition at temperatures under 200°C. Experiments conducted under adiabatic conditions (2) indicate that mesityl oxide decomposes at 96°C in the presence of 5 wt % of aqueous sodium hydroxide (20%), and that phorone undergoes decomposition at 180°C in the presence of 1000 ppm iron. The decomposition products from these reactions are endothermic hydrolysis and cleavage back to acetone, and exothermic aldol reactions to heavy residues. 

Due to environmental considerations, many phenol plants are equipped with a special water treatment faciUty where acetone and phenol are recovered from the wastewater stream. Also, recovered heavy residue is considered a K-022 waste material by the U.S. EPA and must be properly disposed of by incineration or other means (12). 

Property 0-GT" Naphtha 1-GT" Light distillate 2-GT" Medium distillate 3-GT Heavy distillate 4-GT Heavy residual... 

The inlet tube for steam should reach to the bottom of the distillation flask. A i6-mm. bulb on the end of this tube with four 0.8-mm. openings helps to insure thorough mixmg of the heavy residue. If this is not well stirred the aldehyde distils very slowly. It is well to connect the flask to the condenser through a large Hopkins still head in order to prevent the entrainment of foam during the distillation. 

Fluid coking is very insensitive to poor gas-solids contacting, but has one problem not faced by cat cracking or hydroforming. If the heavy residual oil is fed too fast to the reactor, the coke particles will become wetted and stick together in large unfluidizable lumps. Correct control of feed rate is necessary to prevent this bogging. 

Visbreaking is a mild thermal cracking process that reduces the viscosity of heavy fuel oils and reduces the amount of low-viscosity blending stocks that must be added to the heavy residuals to meet viscosity specifications of the specific heavy fuel oil. The amount of heavy fuel oil production by a refinery is reduced by 20—30 percent if a visbreaker is used. The refinery profitability is improved with visbreaker operation, because heavy fuel oils are low value products. 

Vanadium and nickel are poisons to many catalysts and should be reduced to very low levels. Most of the vanadium and nickel compounds are concentrated in the heavy residues. Solvent extraction processes are used to reduce the concentration of heavy metals in petroleum residues. 

Upgrade lower-value materials such as heavy residues to more valuable products such as naphtha and LPG. Naphtha is mainly used to supplement the gasoline pool, while LPG is used as a fuel or as a petrochemical feedstock. 

Coking is a severe thermal cracking process designed to handle heavy residues with high asphaltene and metal contents. These residues cannot be fed to catalytic cracking units because their impurities deactivate and poison the catalysts. 

This formulation is designed to control combustion and slagging problems associated with vanadium, sodium, and sulfur in heavy residual fuel oils. 

A modern refinery is a complicated collection of conversion processes, each tailored to the properties of the feed it has to convert. The scheme shown in Fig. 9.1 summarizes the most important operations some reasons for these processes are given in Tab. 9.2, along with relevant catalysts. First the crude oil is distilled to separate it into fractions, varying from gases, liquids (naphtha, kerosene and gas oil), to the heavy residue (the so-called bottom of the barrel ) that remains after vacuum distillation. 

The most important undesired metallic impurities are nickel and vanadium, present in porphyrinic structures that originate from plants and are predominantly found in the heavy residues. In addition, iron may be present due to corrosion in storage tanks. These metals deposit on catalysts and give rise to enhanced carbon deposition (nickel in particular). Vanadium has a deleterious effect on the lattice structure of zeolites used in fluid catalytic cracking. A host of other elements may also be present. Hydrodemetallization is strictly speaking not a catalytic process, because the metallic elements remain in the form of sulfides on the catalyst. Decomposition of the porphyrinic structures is a relatively rapid reaction and as a result it occurs mainly in the front end of the catalyst bed, and at the outside of the catalyst particles. 

The material balance is consistent with the results obtained by OSA (S2+S4 in g/100 g). For oil A, the coke zone is very narrow and the coke content is very low (Table III). On the contrary, for all the other oils, the coke content reaches higher values such as 4.3 g/ 100 g (oil B), 2.3 g/ioo g (oil C), 2.5 g/ioo g (oil D), 2.4/100 g (oil E). These organic residues have been studied by infrared spectroscopy and elemental analysis to compare their compositions. The areas of the bands characteristic of C-H bands (3000-2720 cm-1), C=C bands (1820-1500 cm j have been measured. Examples of results are given in Fig. 4 and 5 for oils A and B. An increase of the temperature in the porous medium induces a decrease in the atomic H/C ratio, which is always lower than 1.1, whatever the oil (Table III). Similar values have been obtained in pyrolysis studies (4) Simultaneously to the H/C ratio decrease, the bands characteristics of CH and CH- groups progressively disappear. The absorbance of the aromatic C-n bands also decreases. This reflects the transformation by pyrolysis of the heavy residue into an aromatic product which becomes more and more condensed. Depending on the oxygen consumption at the combustion front, the atomic 0/C ratio may be comprised between 0.1 and 0.3 ... 

The Asphaltenic Bottom Cracking (ABC) process (Fig. 17) is a proprietary catalytic HDT process for heavy residual oils [118],... 

Higman, C.A.A., Methanol production by gasification of heavy residues, Presented at the IChemE Conference, London, November 22-23, 1995. 

Heavy residual fuel oils and asphalts are not amenable to gas chromatography and give similar infrared spectra. However, a differentiation can be made by comparing certain absorption intensities [52], Samples were extracted with chloroform, filtered, dried, and the solvent evaporated off at 100 °C for a few minutes using an infrared lamp. A rock salt smear was prepared from the residue in a little chloroform, and the final traces of solvent removed using the infrared lamp. The method, which in effect compares the paraffinic and aromatic nature of the sample, involves calculation of the following absorption intensity ratios ... 

Shell PER+, Pernis, Netherlands 127 MW Shell SGHP process Heavy residues Commissioned on NG in June 1997 and on syngas in November 1997... 

FIGURE 4.10 Typical distillation curve for (a) predominantly gasoline with trace heavy residual fuel oil and (b) predominantly gasoline with a middle distillate of diesel fuel. 

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