Conversion Residues

The visbreaking process thermally cracks atmospheric or vacuum residues. Conversion is limited by specifications for marine or Industrial fuel-oil stability and by the formation of coke deposits in equipment such as heaters and exchangers. 

The coking process produces electrode quality coke from vacuum residues of good quality (low metal and sulfur contents) or coke for fuel in the case of heavy crude or vacuum residue conversion having high impurity levels. 

The feedstocks in question are primary distillation streams and some conversion products from catalytic cracking, coking, visbreaking, and residue conversion units. 

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. 

Phosphorylation is a common method of regulation. As described above, SH2 domains bind to phosphorylated tyrosine residues. Conversely, phosphorylation of serines and threonines proximal to SH3 and PDZ domains uncouples them from their target motifs. Therefore modulation of protein kinase activity in cells regulates interactions between adaptor proteins and their target proteins. 

There are commercial processes for the direct upgrading of residues under high severe hydroconversion conditions. Other alternatives consider the previous hydrotreatment of the residue, so that the hydrocracking stage does not need to be so harsh. Otherwise, residue conversion could also proceed via carbon rejection methods, these processes fall out the scope of the present book and will not be considered here. However, it is important to mention than VR coking is seen as a more economical alternative than HDP, especially for the more heavy crudes, for which concentrations of metals and nitrogen would require the toughest conditions. 

Reported residue conversion is significantly high for the five types of included reactors and largest for the slurry type of reactor. Besides, the slurry reactor together with the ebullated bed reactor can handle heaviest feedstocks and highest metal contents. Resid conversion requires higher temperatures, and pressure drop is essentially zero in these two reactors. However, product quality is better for the fixed and moving bed processes. 

Inorganic cations, although probably isolated by ion exchange, should not be soluble in the dichloromethane extract of the aqueous eluents and should probably remain therein. The experiment with lead(II) nitrate, which yielded <0.2 of the spiked Pb ion, supported this expectation. Therefore, heavy metal toxicity to bioassay systems should not be a problem for testing organic residues. Conversely, when inclusion of inorganics in a test residue is desirable, other recovery techniques should be considered. 

Rearrangements have also been reported for 1,4-dihydroarsenins in which the 4-keto group has been replaced by another double bond residue. Conversion to the oxime provides a route to 4-aminoarsenins (123), as illustrated by equation (25) (78TL1175). 

The difficulties attendant on the isolation of pure enzymes of known specificity is a major barrier to their routine use for the structural analysis of polysaccharides. As the specificities are separated and as the action patterns of carbohydrases become better defined, these enzymes may be expected to play an important and vital role in the investigation of the structure of synthetic polysaccharides containing ordered sequences of sugar residues. Conversely, it may be anticipated that synthetic carbohydrate polymers of known structure will aid in studies of the specificity requirements of purified enzymes. 

In the process (Figure 9-37), the residue feed is slurried with a small amount of finely powdered additive and mixed with hydrogen and recycle gas prior to preheating. The feed mixture is routed to the liquid phase reactors. The reactors are operated in an up-flow mode and arranged in series. In a once through operation conversion rates of >95% are achieved. Typically the reaction takes place at temperatures between 440 and 480°C and pressures between 150 and 250 bar. Substantial conversion of asphaltenes, desulfurization and denitrogenation takes place at high levels of residue conversion. Temperature is controlled by a recycle gas quench system. 

It has been proposed that, as an increase in the conversion of vacuum residue in the commercial fixed-bed reactors, a coke-controlled catalyst deactivation regime appears in the last bed, where coke blocks the active sites as well as decreases the diffusivity. The activity and diffusivity tests were conducted for aged and regenerated catalysts, which were used in the commercial reactors, to investigate mechanisms of the deactivation by coke and metal deposition. The effects of residue conversion, reactor position, and time on-stream on the deactivation were investigated, comparing the catalysts aged at different conditions. 

The Mizushima Oil Refinery of Japan Energy Corporation first implemented a high conversion operation of vacuum residue, versus a constant desulfurization operation, in the commercial residue hydrodesulfurization unit equipped with fixed-bed reactors, to produce more middle distillates as well as fuel oil with lower viscosity. The catalysts will be replaced when the sulfur content in the product oil reaches the allowable limit. Since we have believed that an increase in the residue conversion decreases the catalyst activity by coke deposition, we have been interested in controlling the coke deactivation to maximize the residue conversion during a scheduled operating period. 

Those deactivation models accounting for both coke and metal sulfides are rather simple. Coke and metals foul residue hydrodesulfurization catalysts simultaneously via different processes, and decrease both intrinsic reaction rate and effective diffusivity. They never uniformly distribute in the commercial reactors. We have examined the activity and diffusivity of the aged and regenerated catalysts which were used at the different conditions as well as during the different periods. This paper describes the effects of vacuum residue conversion, reactor position, and time on-stream on the catalyst deactivation. Two mechanisms of the catalyst deactivation, depending on residue conversion level and reactor position, are also proposed. 

Generally, an amount of coke on the catalyst increases from the entrance to the exit of the fixed bed reactors in residue hydroprocessing (1, 6, 7). Tamm et al. showed the highest remained catalyst activity at the outlet of the bench-scale fixed-bed reactor after a constant desulfurization operation (1), while Myers et al. found the highest catalyst deactivation rate in the last stage of three-stage pilot-scale expanded-bed reactors after a 60 - 70% vacuum residue conversion operation (7). These results from two typical reactor operations support that the catalyst deactivation in a lower... 

It has been believed that coke is produced by the precipitation of large molecular hydrocarbons such as asphaltenes when their solubility in oil is lowered (13, 14). An increase in the conversion of vacuum residue increases the aromaticy of the asphaltenes and decreases the aromaticy of the maltenes (15). Consequently, the solubility of the asphaltenes in the maltenes decreases. Absi-Halabi et al. propose that absorption of asphaltenes on the acidic sites of an alumina support is a major cause of the initial rapid coke deactivation, while a decrease in asphaltene solubility causes the following steady coke build-up (14). This explain that an amount of coke increases from the entrance to the exit of the reactors as asphaltene solubility decreases and that an increase in the residue conversion increases an amount of coke in the reactor exit. 

A study on the residue hydrodesulfurization catalysts used in the commercial reactors has suggested that there exists two deactivation mechanism such as metal-controlled deactivation and coke-controlled deactivation, depending on a residue conversion level. In the second and third bed, the deactivation is controlled by metal deposition. However, in the fourth bed, a coke-controlled deactivation appears at a high residue conversion. We also have proposed that there exist two stages in the metal-controlled deactivation. During the first stage, metal sulfides partially poison the active sites and... 

Figures 2 to 7 show PTOF s for the following reactions HDS, hydrodeasphalting (HDA), hydroderretallization (HDM), specifically hydrodev madization (HDV) and hydrodenickelization (HDNi), microcarbon residue conversion divided by total surface area in the reactor (HCR/A), and +525 C conversion divided by total surface area (+525/A) The HCR content of reaction product samples was used to calculate the fraction of +525°C material in the sample, using a correlation (ref, 5). Asphaltenes were measured by insolubility in n-pentane.

Figure 6 Microcarbon Residue Conversion per Unit of Catalyst Surface Area in the Reactor (wt%/m ) X 10 Versus Time on Stream (hours) symbols as given in Figure 2...

H-Oil units have been operated successfully at residue conversion levels above 60%. However, at the higher conversion levels there is increased fouling, sedimentation problems in the operating units. 

Tubular fouling by coke deposition on its inner wall has restricted the development of a high-conversion furnace for residual oil. Such a furnace is desirable from the viewpoint of economics and operability. The proposed tubular fouling model as shown in Figure 6 well represents the complex phenomena of tubular fouling of a residue conversion furnace by modelling a sedimentation of coke precursor and its reaction into coking material in and out the boundary film. 

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