Vacuum residue, coking products

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. 

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. 

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. 

Delayed coking is a thermal cracking process used in refineries to upgrade and convert crude oil residue known as vacuum tower bottom product (i.e. the bottoms fraction from a vacuum rectification tower) into liquid and gas product streams leaving behind a solid concentrated carbon material, coke. The vacuum towers referred to are generally used to further fractionate virgin atmospheric-... 

In contrast to delayed coking, fluid coking is a continuous process which uses the fluidized solids technique to convert vacuum residue to more valuable products, and coke formed during this kind of coking is a byproduct of the process... 

Coking is used for the conversion of crude oil vacuum residues and cracking residues into coke and a clean liquid product with a high H/C ratio. Typical products of coking are hydrocarbon gas, naphtha, gas oil, feed oil for downstream processing and coke. 

Table II shows the product yield for coking a vacuum residue at 3 temperatures and the percent of gas and liquid produced by the slow and fast reactions, respectively. Coking at 835°F gives proportionately more liquid in the total volatiles as a result of the slow reaction than at 915°F and 1035 °F. The data available sure insufficient to determine an overall rate constant for the process, equation 3.

Yields of Gas and Liquids Products and Their Rates of Formation for the Coking of Vacuum Residue at Different Temperatures... 

Figure 5. Product selectivity of the two stages in the coking of a vacuum residue...

Figure 6. Overall product yields for coking a vacuum residue at different temperatures. Key G, gas L, liquid and C, coke.

The Conradson coke residue in the non-distillable part of the samples (CCR/ND) 100 is in the limits from 23 to 33 % for vacuum residues, bitumens, and atmospheric residues. The statistics result in a mean x = 27.3 % and a coefficient of variation V = 9.6 % (relative). The products from conversion processes scatter from 33 to 58 %. The furfural extract (sample 24) stands out because it does not possess any coke residue. 

The Conradson coke residue in the simulated vacuum residue ((CCR/SVR) 100) for the vacuum residues and bitumens has a mean value x2l.6%( y= 7.01% relative). For the atmospheric residues the mean amounts to x = 13.8 % ( + y = 8.7 % relative). The products from conversion processes (samples 19, 20, and 22) have extremely high values demonstrating that they have been distilled exhaustively, whereas the distillate of the residue of a cat-cracker, sample 25, exhibits the extremely low value of 4.4 %. 

In a delayed coker, vacuum residue feed is heated to about 900 to 970°F (487 to 520°C) and sent to a large eoke drum. Cracking begins immediately, generating coke and cracked, vaporized products. Coke stays behind in the drum while the vapors rise to the top and flow to the product fi actionator. 

Coke can account for up to 30wt% of the product. For instance, the coke yield when a vacuum residue coming from a crude oil with 22°API and 10.79 wt% CCR is about 12.2 wt%, while for a vacuum residue from a crude oil with 12.7°API and 16.4 wt% CCR it is about 22.3 wt%. Coke can be shipped by rail, truck, or conveyor belt to a calciner, which converts green fresh coke from the drum into various grades of petroleum coke. Green coke can also be used for fuel. 

Table 5.7 presents the commercial results obtained with three feeds (vacuum residue from different crude oils, 538°C+) having CCR ranging from 22 to 31 wt% and 2.3 to 7.9°API. The typical behavior is observed, i.e., increasing the CCR of the feed causes increase in the production of coke, gas, and naphtha and decrease in the production of gasoil. 

Feedstocks for this very flexible process are usually vacuum distillates, deasphalted oils, residues (hydrotreated or not), as well as by-products from other processes such as extracts, paraffinic slack waxes, distillates from visbreaking and coking, residues from hydrocracking, converted in mixtures with the main feedstock. 

Thermal Cracking. In addition to the gases obtained by distillation of cmde petroleum, further highly volatile products result from the subsequent processing of naphtha and middle distillate to produce gasoline, as well as from hydrodesulfurization processes involving treatment of naphthas, distillates, and residual fuels (5,61), and from the coking or similar thermal treatment of vacuum gas oils and residual fuel oils (5). 

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