This invention relates to a process for the manufacture of premium grade coke (needle coke) from petroleum-derived feedstocks. More particularly, this invention relates to a process for the production of needle coke from resid hydrotreated and deasphalted residual feedstocks and deasphalted fluid catalytic cracking unit (FCC) decanted oil (DCO) in a delayed coking process.
The delayed coking of conventional refinery residual components, generally having an atmospheric equivalent boiling point exceeding 1000.degree. F. generally produces a coke having a longitudinal coefficient of thermal expansion (CTE) of about 20.times.10.sup.-7 /.degree. C. or greater after graphitization. Graphitization generally refers to the process of exposing coke to high temperatures, generally ranging from about 2000.degree. F. to about 3500.degree. F., for the purpose of oxidizing or burning off impurities. Such impurities generally increase the CTE of the coke. The CTE of a graphitized coke is an important measure of its suitability for use in the manufacture of electrodes for electric arc steel furnaces. Electrode expansion attendant to high CTE coke can adversely change the arcing characteristics and performance of an arc furnace and can also be more susceptible to costly electrode breakage. For this reason, the steel industry standards for electric arc furnace electrodes generally require that the CTE be less than 8.times.10.sup.-7 /.degree. C., often less than 5.times.10-7/.degree. C, and for some services, less than 3.times.10.sup.-7 /.degree. C.
Some steel producers have begun to monitor the "dynamic puffing" characteristics of needle coke and either require that needle coke meet particular dynamic puffing specifications or debit or credit the price they are willing to pay for the needle coke based on dynamic puffing characteristics. Coke produced from conventional refinery residual feedstock components generally tends to have undesirable dynamic puffing characteristics. It has been found that puffing is generally correlated to the presence of elements such as sulfur in the coke which can form impurity pockets and become fracture sites. The presence of fracture sites present in coke having poor puffing characteristics, can and generally does reduce electrode life. For this reason, some steel manufactures are requiring that the needle coke they purchase be "non-puffing" or "slightly puffing", as defined by laboratory procedures which measure the difference between the minimum and maximum deflection points of a plug produced from needle coke that is heated across an extended temperature range. For example, some manufacturers require that such plugs produced from needle coke reflect dynamic puffing levels of less than 7 percent, often less than 4 percent, and, for some services, less than 2 percent, measured as a percentage of the length of the plug. Coke produced from conventional refinery residual feedstock components having such undesirable puffing characteristics generally has a sulfur concentration ranging from about 2.0 percent by weight to about 4.0 percent by weight whereas sulfur concentrations typical of coke having suitable needle coke puffing characteristics, are generally less than 1.6 percent by weight and typically less than 1.0 percent by weight.
Needle coke has a particularly high market value and can be worth from about 5 to about 10 times the value of its feedstock components, depending on the quality of the needle coke. Conventional refinery grade coke is generally used only as fuel and is valued accordingly. Moreover, refinery fuel grade coke can be high in sulfur content, further reducing its value to industry as a boiler fuel due to environmental regulations controlling emissions of sulfur-derived combustion products. Some better quality conventional refinery grade cokes can be used for anodes in aluminum smelting processes. Such cokes are generally referred to as anode grade delayed coke. The market value of anode grade coke is substantially lower than the market value of needle coke. Therefore, there is a great need in the refining industry for flexible and reliable processes for producing needle coke or for upgrading existing conventional coking processes to needle coking processes.
Conventional delayed coking processes utilized for producing needle coke are generally known in the art. In the usual application of the delayed coking process, refinery residual components are heated in a coking furnace and directed to a coking drum. During the coking process, the residual feedstock is thermally decomposed to a heavy tar or pitch which further decomposes into solid coke and vapor products. The vapor components formed during decomposition are generally recovered in a fractionating column to products such as coker wet gas, coker naphtha, coker distillates, and coker gas oil. The solid coke is left behind in the coke drum.
Delayed coking processes generally function in a semi-continuous manner such that while one coke drum or battery of coke drums fills with a mass of solid coke, a second coke drum or battery of coke drums is being purged of vapors, cooled, opened for removal of the solid coke, and prepared for refilling. When the first coke drum or battery of coke drums is filled, coke drum feed is redirected to the second coke drum or battery of coke drums which has been emptied of solid coke and prepared for coke drum feedstock. The solid coke is generally removed from the coke drums by means such as hydraulic or mechanical drilling.
It is also known that delayed coking feedstocks for the production of needle coke generally include refinery streams such as thermal tars, untreated straight run FCC decanted oil provided directly from an FCC operating facility, pyrolysis tar, minor amounts of high and low sulfur virgin residual components, other compositionally similar materials, and mixtures thereof. Moreover, other processing steps have been utilized upstream of delayed coking processes for the production of needle coke, to modify such feedstocks in a manner so as to produce needle coke under delayed coking conditions.
For example, U.S. Pat. No. 4,502,944 to Kegler et al. discloses a process for producing needle coke from a residual oil feedstock derived from a naphthenic crude oil. The residual feedstock is subjected to a demetallization step, followed by desulfurization, and delayed coking.
U.S. Pat. No. 4,178,229 to McConaghy et al. discloses a process for producing needle coke from a residual oil feedstock derived from a hydrogen donor diluent cracking operation. A gas oil fraction derived from the hydrogen donor diluent cracking operation is recycled back to the hydrogen donor diluent cracking operation as the hydrogen donor diluent.
U.S. Pat. No. 4,894,144 to Newman et al. discloses a process for the simultaneous manufacture of both premium needle coke and aluminum grade coke wherein a virgin heavy oil is hydrotreated, separated into a light and heavy fraction, and each component separately subjected to delayed coking conditions. The light fraction is coked under delayed coking conditions to premium coke and the heavy fraction is coked under delayed coking conditions to aluminum grade coke.
The above processes generally provide limited process control options for meeting the CTE and puffing specifications for needle coke described above or require that specific feedstock source constraints, such as a naphthenic crude source or a low sulfur vacuum residue derived from low sulfur crude, be satisfied in order to produce needle coke. These constraints and this limited flexibility can result in substantial process penalties and higher operating risk.
The processing and treatment of FCC decanted oil has also been the focus of U.S. patents.
For example, U.S. Pat. No. 4,832,823 to Goyal discloses a delayed coking process wherein untreated FCC decanted oil is conveyed directly to a delayed coker along with high and low sulfur vacuum resid. The combination of untreated FCC decanted oil and high and low sulfur resid results in reduced yields of low value fuel grade coke.
An article by Todo, Oyama, Mochida, Korai, Abe, and Sakanishi entitled "Cocarbonization Properties of Solvent Deasphalted Oil from a Petroleum Vacuum Residue in Production of Needle Coke" discloses a study of the production of needle coke using a deasphalted low sulfur vacuum residue derived from low sulfur crudes and untreated straight run FCC decanted oil.
U.S. Pat. No. 4,427,531 to Dickakian discloses a process for converting FCC decanted oil to a feedstock for carbon artifact manufacture, and in particular, carbon fiber production. The FCC decanted oil is vacuum stripped, solvent extracted, and heat soaked to provide a feedstock suitable for carbon fiber manufacture.
Processes utilizing untreated FCC decanted oil often incur difficulty meeting CTE specifications due to the presence of solids and catalyst fines in the decanted oil. Periodic shutdown of an FCC for cyclone repairs or replacements in order to minimize decanted oil solids content is costly as are auxiliary equipment for separating and removing solids from decanted oil.
Integrating a resid hydrotreating process with a solvent extraction process for increasing the yield of light hydrocarbon products is also the subject of U.S. patents.
U.S. Pat. No. 5,013,427 to Mosby et al. discloses a resid hydrotreating process wherein the vacuum reduced resid hydrotreater residual product is directed to a solvent extraction process for extraction and separation of the residual product into solvent extracted oil, solvent extracted resins, and asphaltenes. The solvent extracted resins are recycled back to the resid hydrotreating process for increasing the yields of lower boiling, higher valued liquid products. The solvent extracted oil is directed to a FCC process directly or by way of a FCC gas oil feed hydrotreating process.
U.S. patent application Ser. No. 07/616,218, filed on Jul. 18, 1989 and allowed on Jan. 15, 1992, now U.S. Pat. No. 5,124,077, discloses a process for removing catalyst solids and fines from FCC decanted oil by mixing the decanted oil with a residual fraction and processing the mixture in a solvent extraction process. The deasphalted oil from the solvent extraction process can be directed to a resid hydrotreating process or to an FCC for cracking.
Such processes have historically been utilized as an alternative to delayed coking processes and have generally not been sequenced or integrated with delayed coking processes in general, and particularly delayed coking processes for the production of needle coke.
It is therefore an object of the present invention to provide an integrated delayed needle coking process that provides a mechanism for consistently meeting needle coke CTE specifications.
It is another object of the present invention to provide an integrated delayed needle coking process that provides a mechanism for consistently meeting needle coke dynamic puffing specifications.
It is another object of the present invention to provide an integrated delayed needle coking process that provides a mechanism for meeting needle coke CTE and dynamic puffing specifications with minimal resid quality limitations and without substantially constraining the crude selection process.
It is another object of the present invention to provide an integrated delayed coking process that provides maximum flexibility for accommodating needle coke specifications with alternative process embodiments.
Other objects appear herein.