As the demand for hydrocarbon-based fuels has increased, the need for improved processes for desulfurizing hydrocarbon feedstocks of heavier molecular weight has increased as well as the need for increasing the conversion of the heavy portions of these feedstocks into more valuable, lighter fuel products. These heavier, “challenged” feedstocks include, but are not limited to, low API gravity, high sulfur, high viscosity crudes from such areas of the world as Canada, the Middle East, Mexico, Venezuela, and Russia, as well as less conventional refinery and petrochemical feedstocks derived from such sources as tar sands bitumen, coal, and oil shale. These heavier crudes and derived crude feedstocks contain a significant amount of heavy, high molecular weight hydrocarbons. A considerable amount of the hydrocarbon of these heavy oil streams are often in the form of large multi-ring hydrocarbon molecules and/or a conglomerated association of large molecules containing a large portion of the sulfur, nitrogen and metals in the hydrocarbon stream. A significant portion of the sulfur contained in these heavy oils is in the form of heteroatoms in polycyclic aromatic molecules, comprised of sulfur compounds such as dibenzothiophenes, from which the sulfur is difficult to remove.
The high molecular weight, large multi-ring aromatic hydrocarbon molecules or associated heteroatom-containing (e.g., S, N, O) multi-ring hydrocarbon molecules in the higher molecular weight oils are generally found in a solubility class of molecules termed as asphaltenes. A significant portion of the sulfur is contained within the structure of these asphaltenes or lower molecular weight polar molecules termed as “polars” or “resins”. Due to the large aromatic structures of the asphaltenes, the contained sulfur can be refractory in nature and is not very susceptible to removal by conventional alkali salt solution complexes such as potassium hydroxide or sodium hydroxide solution treatments under conventional operating conditions. Other intermediate refinery crude fractions, such as atmospheric resids, vacuum resids, and other similar intermediate feedstreams containing boiling point materials above about 850° F. (454° C.) contain similar sulfur polycyclic heteroatom complexes and are also difficult to desulfurize by conventional methods. These heavy crudes, derived refinery feedstocks, and heavy residual intermediate hydrocarbon streams can contain significant amounts of sulfur. Sulfur contents of in excess of 3 to 5 wt % are not uncommon for these streams and can often be concentrated to higher contents in the refinery heavy residual streams.
These high sulfur content hydrocarbon streams can be excessively corrosive to equipment in refinery and petrochemical production and/or exceed environmental limitations for use in processes such petroleum refining processes. If a significant amount of the sulfur is not removed from these feedstocks prior to refining, significant costs in capital equipment may be required to process these corrosive crudes and the sulfur is generally still required to be removed by subsequent processes in order to meet intermediate and final product sulfur specifications. Additionally, most conventional catalytic refining and petrochemical processes cannot be used on these heavy feedstocks and intermediates due to their use of fixed bed catalyst systems and the tendency of these heavy hydrocarbons to produce excessive coking and deactivation of the catalyst systems when in contact with such feedstreams. Also, due to the excessive hydrocarbon unsaturation and cracking of carbon-to-carbon bonds experienced in these processes, significant amounts of hydrogen are required to treat asphaltene containing feeds. The high consumption of hydrogen, which is a very costly treating agent, in these processes results in significant costs associated with the conventional catalytic hydrotreating of heavy oils for sulfur removal.
Due to their high sulfur content, high viscosities, and low API gravities, these heavy hydrocarbon feedstreams cannot be readily transported over existing pipeline systems and are often severely discounted for use as a feedstock for producing higher value products. Another alternative utilized is to make these heavy oils suitable for pipeline transportation or petrochemical feed only after significant dilution of the heavy oil with expensive, lower sulfur hydrocarbon diluents.
As a result, many process methods have been utilized in the art to desulfurize the very “heavy” or “high molecular weight” hydrocarbon containing streams. Due to the problems discussed with the use of fixed bed catalyst systems for use in desulfurizing these heavy hydrocarbon streams, alkali metal hydroxides have been used in aqueous solutions and contacted with the heavy hydrocarbon stream under specific conditions resulting in the formation of a desulfurized hydrocarbon product wherein a portion of the sulfur has been removed from the heavy hydrocarbon stream and a spent alkali metal hydroxide. Herein, the spent metal hydroxide is most predominantly in the forms of an alkali metal sulfide and/or alkali metal hydrosulfide.
A significant drawback to the alkali metal hydroxide desulfurization process of the art is that the alkali metal hydroxides that are spent in the processes (i.e., either in the form of a sulfide or hydrosulfide) are not easily converted back into their active hydroxide forms. In most conventional processes for alkali metal regeneration, the alkali metals sulfides are typically steam stripped, removing the sulfur from the alkali metal compounds by forming hydrogen sulfide. Not only is this stripping process typically inefficient and costly, but it also results in the formation of hydrogen sulfide (H2S) which in turn must also under go an additional costly process for removal of the elemental sulfur, such as a typical Claus process. Besides requiring additional equipment and expenses, the use of steam stripping and subsequent Claus processing results in much of the hydrogen fraction of any hydrogen supplied to the hydrocarbon desulfurization process being lost as water in the H2S disposal process.
Therefore, there exists in the industry a need for an improved process for removing sulfur from bitumens, heavy crudes, derived crudes and refinery residual streams utilizing a metal hydroxide and a regenerating the spent metal hydroxide compounds for reuse in the desulfurization process.