Vacuum residue, coking

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. 

Feedstocks are light vacuum distillates and/or heavy ends from crude distillation or heavy vacuum distillates from other conversion processes visbreaking, coking, hydroconversion of atmospheric and vacuum residues, as well as deasphalted oils. 

Intermediate feedstock preparation processes such as direct hydroconversion of vacuum residues, solvent deasphalting, improved coking will also make their appearance. 

Stanislaus, A., Absi-Halabi, M., Khan, Z., Influence of Catalyst Pore Size on Asphaltenes Conversion and Coke-Like Sediments Formation During Catalytic Hydrocracking of Kuwait Vacuum Residues, In Catalysts in Petroleum Refining and Petrochemical Industries. Studies in Surface Science and Catalysis. 1996, Elsevier New York, USA. pp. 189-197. 

Fuel Coal Coal Pet coke/waste oil Vacuum residue Coal Lignite ... 

Carbon Residue. The carbon residue is one factor used to determine coke yield as a percentage of fresh feed, and is defined as the carbon residue remaining after evaporation and pyrolysis of the feedstock in a specified procedure. All other operating conditions being the same, as the carbon residue is increased, more coke will be produced. In recent years, as the quality of crudes has diminished, the carbon residue of vacuum residue feedstocks has increased from typical values of 10 to 20 weight % to 20 to 30 weight % and more. 

Because of the increased sulfur and impurity levels in crudes currently being processed, refiners in recent years have been considering residue desulfurization units upstream of the delayed coker. In addition to the reduction in sulfur content, residue desulfurization units also lower the metals and carbon residue contents. Due to the reduction in the carbon residue, the liquid product yield is increased and the coke yield reduced. In addition, the coke produced from a desulfurized residue may be suitable for use as anode grade coke. Table I shows the yields and product properties after coking Medium Arabian vacuum residue, with and without upstream residue desulfurization. 

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. 

J. Zhon and X. Zhang, Co-processing of waste polyethylene with vacuum residue in delayed coking process. Preprints, 43(1), 194-198 (1998). 

The type of unit described here can, if desired, be used to convert vacuum residues to lighter materials or to prepare feed stock for low sulfur coke production. These applications of the process have been discussed in several previous papers. A good commercial example of this flexibility is shown in Table II. These data show operations of the Lake Charles H-Oil unit when processing for conversion and for desulfurization. 

Vacuum residue Heavy oil Pet coke Anthracite Demolition wood Shell Texaco Texaco Lurgi Dry Ash Lurgi CFB... 

The nature of crude oils depends on their source. Initial separation into components is carried out by atmospheric and vacuum distillation. Heavy ends are particular boiling point cuts, which can include atmospheric gas oil (250-350°C), atmospheric residues (350°C+) vacuum gas oil (350-5S0°C) and vacuum residues (5S0°C+). The descriptions are based on boiling points and, within a particular distillation cut, various chemical species can be identified. These include saturated and unsaturated hydrocarbons, aromatic and polyaromatic hydrocarbons and inorganic atoms such as V, Ni, and S, which are associated with large organic molecules [5]. As a result of this complexity, the composition of the boiling cuts is often described in terms of their content of oils, resins and asphaltenes [6,7,8], the relative amounts of which vary depending on the cut and the source of the crude [6] Of these species, asphaltenes are particularly important in the present context since they are known to be associated with heavy coke formation [7,8]. 

Feed properties and operation conditions determine catalyst life in the residue hydrodesulfurization. In a high conversion operation of vacuum residue, catalyst deactivation due to coke is as important as the one due to metals. Though many researchers have worked on understanding and modelling deactivation of residue hydrodesulfurization catalysts, there has still been a controversy in a coke deactivation mechanism [2, 3]. Very few publications are available discussing an effect of a bed temperature profile on catalyst deactivation in large scale adiabatic commercial reactors. Most of the studies on deactivation of residue hydrodesulfiirization catalysts have been done with small-scale isothermal reactors [2,3,4,5]. The activity tests of the used catalysts were conducted to study the catalyst deactivation in the commercial reactors. This paper also describes an effect of a bed temperature profile on coke deactivation, which was tested in the commercial reactors. 

Effective solutions to the problems of the vacuum residue hydrodesulfurization unit equipped with the fixed bed reactors, such as a hot spot, pressure-drop buildup, and catalyst deactivation by coke fouling, were discussed. Improving liquid distribution can prevent hot spot occurrence. Dispersing inorganic solids throughout the reactors can control a pressure-drop increase in the first bed. For a high conversion operation, controlling the conversion in each bed can minimize the coke deactivation in the fourth bed. 

INFLUENCE OF CATALYST PORE SIZE ON ASPHALTENES CONVERSION AND COKE-LIKE SEDIMENTS FORMATION DURING CATALYTIC HYDROCRACKING OF KUWAIT VACUUM RESIDUES... 

To assess the effect of pretreatment on early coke formation, two sets of runs, R06/ R08 and R07/ R09, were conducted. In the first set, the runs were terminated immediately after pretreatment without introducing vacuum residue, while for the second set, the runs were terminated after vacuum residue was introduced for 6 hours only. The catalysts in runs R06 and R07 were presulfided as described in the Experimental Section, while those in runs R08 and R09 were simply soaked in recycled gas oil for 2 hours, simulating a practice adopted by the industry. A comparison of the carbon percentages of the spent catalysts (Table 4) of test runs ROl and R02 with those of test runs R07 and R09 shows that all four catalyst samples have nearly the same carbon content. This clearly demonstrates that almost all of the coke on the spent catalyst is deposited during the first few hours of the run. Furthermore, the surface area for both presulfided and unsulfided catalysts was significantly reduced during the early hours of the run. 

The observation that vacuum residue causes significant coke deposition on catalysts was further ascertained by conducting a special test run (Run R05) which is similar to Run R02 except straight run gas oil was used instead of vacuum residue as feedstock at 380 °C. The... 

Continued large increase in delayed coking capacity, particularly for processing of the lowest-quality vacuum residues. 

Delayed coke (Table 16.1) is produced during the delayed coking process—a batch process—from vacuum residua (Chapter 2) (Speight and Ozum, 2002). The carbonization (thermal decomposition) reactions involve dehydrogenation, rearrangement, and condensation. Two of the common feedstocks are vacuum residues and aromatic oils. 

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