Patent Application
20210301211
Systems and methods are provided for controlling the morphology of coke formed in a delayed coker by addition of elemental sulfur to the input feed to the coker.
Delayed coking involves thermal decomposition of petroleum residua (resids) to produce gas, liquid streams of various boiling ranges, and coke. Delayed coking of resids from heavy and heavy sour (high sulfur) crude oils is carried out primarily as a means of disposing of these low value feedstocks by converting part of the resids to more valuable liquid and gaseous products. Although the resulting coke is generally thought of as a low value by-product, it may have some value, depending on its grade, as a fuel (fuel grade coke), electrodes for aluminum manufacture (anode grade coke), or in other uses.
In the delayed coking process, a feedstock is rapidly heated in a fired heater or tubular furnace. The heated feedstock is then passed to a coking drum that is maintained at conditions under which coking occurs, generally at temperatures above 400° C. under super-atmospheric pressures. The heated residuum feed in the coker drum also forms volatile components that are removed overhead and passed to a fractionator, leaving coke behind. When the coker drum is full of coke, the heated feed is switched to another drum and hydrocarbon vapors are purged from the coke drum with steam. The drum is then quenched with water to lower the temperature to less than 100° C., after which the water is drained. When the cooling and draining steps are complete, the drum is opened and the coke is removed after drilling and/or cutting using high velocity water jets.
For example, a hole is typically bored through the center of the coke bed using water jet nozzles located on a boring tool. Nozzles oriented horizontally on the head of a cutting tool cut the coke from the drum. The coke removal process adds considerably to the throughput time of the process. Thus, it would be desirable to produce a free-flowing coke in the coker drum that would not require the expense and time associated with conventional coke removal.
Even though the coker drum may appear to be completely cooled, areas of the drum often do not completely cool. This phenomenon, sometimes referred to as “hot drum”, may be the result of a combination of morphologies of coke being present in the drum, which may contain a combination of more than one type of solid coke product, i.e., needle coke, sponge coke and shot coke. Coke containing a combination of these morphologies is referred to as transition coke. Since unagglomerated shot coke may cool faster than other coke morphologies, such as large shot coke masses or sponge coke, it would be desirable to produce predominantly substantially free flowing shot coke in a delayed coker, in order to avoid or minimize hot drums.
Certain routes for controlling coke morphology include: 1) crude blending to produce compositions of vacuum residues suitable for producing the desired coke morphology, 2) operating the vacuum pipestill to produce deeper cuts, favoring shot coke formation during coking, 3) operating the coker drum at higher severity and lower pressure to favor shot coke formation, 4) blending the vacuum residue (i.e., the coker input feed) with heavy aromatic fuel oil (HAFO), 5) blending metal additives with the vacuum residue to produce shot coke during coking, and 6) air oxidation of delayed coker feed to produce shot coke during coking.
In various instances, the above methodologies can reduce or minimize the benefits of using coking in a refinery. For instance, crude blending is capable of producing a delayed coker feed that will form the desired morphology. However, due to constraints on the blended product properties, a refinery may have to forgo an opportunity to use an advantaged crude if it is unable to reliably achieve a desired coke morphology. Thus, using crude blending to control coke morphology can impose a limitation on refinery operation.
Altering the operation of the vacuum pipestill and/or coke drum also requires the careful consideration of the impact on other downstream units and possibly an increase in operational costs. Operating a vacuum pipestill to produce deeper cuts can result in an increased volume of vacuum gas oil (VGO), while also typically reducing the quality of the resulting VGO. This may result in a feed that is no longer within the specifications of a downstream processing unit, such as the fluid catalytic cracker (FCC). If the increased costs associated with these operational changes are not offset by an increase in value, then doing so makes little sense.
Operating a coker drum at higher severity and lower pressure may favor shot coke formation, but this strategy can also incur substantial additional costs. In particular, higher severity operation can lead to increased formation of coke and light ends at the expense of higher value liquid boiling range products, such as coker naphtha and/or coker distillate. Additionally, higher severity and lower pressure operation tend to reduce the net capacity of the coker, meaning that either lower volumes are processed in corresponding portions of the refinery or that additional coker equipment needs to be brought online to maintain a desired coking capacity.
While addition of HAFO to vacuum residue can be useful, addition of HAFO to the coker input feed displaces vacuum residue that could otherwise be coked. Thus, not only is a higher value feed (HAFO) introduced into the coker, but the resulting vacuum residue throughput is reduced in the delayed coker.
Addition of metals requires either access to a source of waste metals at the refinery and/or purchase of metal additives. Waste metals are not commonly available in a refinery setting. Purchase of metal additives can quickly become cost prohibitive for a commercial scale coker. More generally, due to the large volume of low value by-product (coke) that is generated, it is typically desirable to reduce or minimize the amount of additives or higher value feeds that are introduced into a coking process. For some additives, reducing or minimizing the amount of additives can provide benefits by reducing or minimizing the cost of the coking process. For other additives, reducing or minimizing the amount of additives can be beneficial in order to reduce or minimize the need for additional processing to handle the additives.
Some of the problems associated with air oxidation include: i) the concentration of dissolved oxygen is dependent on the applied pressure, ii) the concentration of oxygen in air is only˜21%, iii) diffusion of oxygen through a viscous liquid can result in significant mass transfer limitations and iv) oxygen radicals have relatively low selectivity for hydrogen-atom abstractions, so that competing side reactions can also occur.
U.S. Pat. No. 7,303,664 describe methods for controlling coke morphology based on addition of metal additives to the input feed to a delayed coker. The metal additives are added to the feed at a temperature lower than the coking temperature. The feed plus metal additives are maintained at the temperature lower than the coking temperature for a period of time prior to coking.
U.S. Pat. No. 7,306,713 describes another strategy for controlling coke morphology that involves addition of additives to the coker input feed. In U.S. Pat. No. 7,306,713 additives are introduced into a coker feed at a temperature ranging from 70° C. to 370° C., or 70° C. to 270° C. for a period of time to encourage dispersal. Elemental sulfur is described as an example of a non-metallic additive. The non-metal additives are added to the feed at a temperature lower than the coking temperature. The feed plus non-metal additives are maintained at the temperature lower than the coking temperature for a period of time to encourage dispersal prior to coking. In the Examples in U.S. Pat. No. 7,306,713, sulfur is added to a coker input feed at a temperature of 370° C. Optical micrographs of coke samples are provided that demonstrate an increase in shot coke formation as the residence time at 370° C. for the elemental sulfur is increased in a coker input feed. However, the optical micrographs at longer residence times still show significant variation in domain size for the particles and/or agglomerated particles in the resulting coke.
U.S. Pat. No. 7,374,665 describes methods for blending residual feedstocks to produce a coke during delayed coking that is easier to remove from the coker drum. The blending feedstocks are selected to maintain a minimum metals content in the blend while having a sufficiently low API gravity.
U.S. Patent Application Publication 2006/0006101 describes limiting the boiling range profile of material that is used in the coker feedstock, so that the feed to the delayed coker contains a limited amount of material that boils between roughly 480° C. and 560° C. A recycle stream of coker distillate is also added to the feedstock, so that the feedstock includes portions that are both below and above the boiling range of 480° C. to 560° C. The limitation on the boiling range profile of the feedstock is described as allowing for production of shot coke during the delayed coking.
A journal article by Siskin et al describes using air oxidation to treat a coker feedstock prior to delay coking in order to produce shot coke. See Siskin et al., “Chemical approach to control morphology of coke produced in delayed coking,” Energy & Fuels (2006) Vol. 20, No. 5, pages 2117-2124.
In various aspects, a method for controlling coke morphology during delayed coking is provided. The method includes providing a coker feedstock that forms transition coke under standard delayed coking conditions. The method further includes mixing the coker feedstock with 0.5 wt % to 5.0 wt % of elemental sulfur to form a mixed coker feedstock. The method further includes maintaining the mixed coker feedstock at a temperature of 160° C. to 260° C. for 0.3 hours to 7 days to form a treated coker feedstock. Additionally, the method includes exposing the treated coker feedstock to second delayed coking conditions to form a coker effluent and treated coke comprising shot coke. Under the standard delayed coking conditions, the provided coker feedstock can form transition coke comprising a lognormal domain size distribution having a value of a location parameter μ between 1.85 and 2.10.
In various additional aspects, a method for controlling coke morphology during delayed coking is provided. The method can include providing a coker feedstock that forms sponge coke under standard delayed coking conditions. The method further includes mixing the coker feedstock with 8.0 wt % or more of elemental sulfur to form a mixed coker feedstock. The method further includes maintaining the mixed coker feedstock at a temperature of 160° C. to 260° C. for 0.3 hours to 7 days to form a treated coker feedstock. Additionally, the method can include exposing the treated coker feedstock to delayed coking conditions to form a coker effluent and coke comprising shot coke. Under the standard delayed coking conditions, the provided coker feedstock can form sponge coke comprising a lognormal domain size distribution having a value of a shape parameter σ of 0.32 or more and a value of a location parameter μ of 2.10 or more.
In various aspects, a system for controlling coke morphology during delayed coking is provided. The system can include a delayed coker drum comprising a drum feed inlet and a coker effluent outlet. The system can further include a coker furnace comprising a furnace inlet and a furnace outlet, the furnace outlet being in fluid communication with the drum feed inlet. The system can further include a fractionator comprising a first fractionator inlet, a second fractionator inlet, a bottoms outlet, and a one or more product outlets, the first fractionator inlet being in fluid communication with the coker effluent outlet, the bottoms outlet being in fluid communication with the furnace inlet via a furnace conduit. The system can further include a mixed coker feedstock conduit in fluid communication with the second fractionator inlet, the mixed coker feedstock conduit being in fluid communication with a coker feedstock conduit and a sulfur input conduit. Additionally, at least a portion of the mixed coker feedstock conduit, the fractionator, and the furnace conduit can be maintained at a temperature of 160° C. to 260° C. Additionally, a residence time for feed in the at least a portion of the mixed coker feedstock conduit, the fractionator, and the furnace conduit can be 0.3 hours to 7 days.
All numerical values within the detailed description and the claims herein are modified by “about” or “approximately” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.
In various aspects, systems and methods are provided for controlling the morphology of coke produced during delayed coking. The morphology control is achieved in part by introducing elemental sulfur into the coker feedstock prior to coking. The elemental sulfur can be introduced into the feed under conditions so that the sulfur is well-dispersed within the feed for a sufficient period of time. This can allow for relatively even reaction of sulfur with components throughout the feed, resulting in a relatively small, uniform domain size distribution for the coke produced during delayed coking. This coke can correspond to shot coke. By producing coke with a small and relatively uniform domain size, the risk of uneven heating within the coke can be reduced or minimized. This can provide various benefits for a delayed coking process.
The amount of elemental sulfur that is added to produce a desirable coke during delayed coking can depend on the type of coke a feedstock produces without addition of elemental sulfur. In some aspects, systems and methods are provided for controlling the morphology of coke produced during delayed coking while reducing or minimizing the amount of elemental sulfur that is added to the coker feedstock prior to coking. For coker feedstocks that produce transition coke under standard delayed coking conditions, it has been discovered that a desirable shot coke phase can be achieved during delayed coking by using 0.5 wt % to 5.0 wt % of elemental sulfur as an additive prior to coking. In other aspects where a coker feedstock produces sponge coke under standard delayed coking conditions, systems and methods are provided for achieving a desirable shot coke phase by addition of 8.0 wt % to 10 wt % of elemental sulfur to a feedstock.
It has been discovered that dispersing elemental sulfur in a coker input feed at an unexpectedly low range of temperatures can provide substantially improved morphology for coke generated by delayed coking. In various aspects, maintaining a coker input feed including elemental sulfur at a temperature from roughly 160° C. to 260° C. can provide unexpected further improvements in coke morphology. In particular, maintaining a coker feedstock at 160° C. to 260° C. for a sufficient period of time to allow for reaction of sulfur with the feed can allow subsequent delayed coking to produce shot coke having an improved uniformity of particle size. The coke of substantially uniform particle size can be produced using coker feedstocks that would otherwise generate transition coke and/or sponge coke under standard delayed coking conditions.
The methods described herein present a solution to situations in which conventional routes may not be applicable. The methods are feed independent, permitting a refinery to run any economic blend of crudes desired. The methods have the option of not requiring a change in the operation of the vacuum pipestill or severity of the coker, and not requiring the addition of higher value feed components such as heavy atmospheric fuel oil, or costly metal additives. Additionally, since elemental sulfur is the active agent, for most refineries the purchase of an additive to control morphology is not required.
Without being bound by any particular theory, it is believed that the narrow range of temperatures provides a balance between two competing factors. First, it is believed that the temperature should be sufficiently high to allow for reaction between the elemental sulfur and various compounds within the coker feed. It is believed that, similar to oxygen, sulfur modifies coke morphology based on stoichiometric reaction with various compounds in a coker input feed. In order to achieve this reaction, the temperature should be sufficiently high so that the reaction of sulfur can occur at a meaningful rate. A treating temperature of 160° C. or more can provide a reaction rate of sulfur with the coker input feed at a sufficient rate to produce a desirable shot coke morphology. In order to achieve a sufficient level of reaction, a mixture of coker input feed and sulfur can be maintained at a temperature of 160° C. or more for a treating time of 0.5 hours to 7 days, or 0.5 hours to 5 days, or 0.5 hours to 1 day, or 2 hours to 7 days, or 12 hours to 7 days, or 12 hours to 5 days or 1 day to 7 days, or 1 day to 5 days, or 1 day to 3 days. The treated mixture can then be exposed to delayed coking conditions to produce a desired shot coke product having a small and relatively uniform domain size.
Second, if the sulfur is mixed with the coker feedstock at a temperature that is too high, it is believed that the sulfur may be less effective for modifying morphology due to loss of sulfur to the gas phase. The vapor pressure of sulfur at 175° C. is roughly 0.1 kPa, while the vapor pressure of sulfur is roughly 3.2 kPa at 271° C. Due to the long reaction time required for achieving the desired shot coke product, it is believed that the vapor pressure of sulfur at temperatures of 260° C. or more is sufficiently high so that substantial amounts of sulfur can transfer to the vapor phase. This can at least locally deplete the sulfur concentration in the mixture of coker input feed and sulfur. This local depletion can cause variation in domain size of the resulting shot coke that is formed during coking.
It is noted that relative to oxygen, elemental sulfur has a substantially higher solubility in benzene (0.57 M for sulfur in benzene at 25° C., 0.002 M for oxygen in benzene at 25° C.). This is believed to correlate with a substantially higher solubility for sulfur in coker input feeds. Thus, sulfur dissolved in a coker input feed can provide substantially more modification of the coker feedstock that an air oxidation process. Sulfur is also believed to have a higher selectivity for reacting with only certain weaker types of carbon-hydrogen bonds. A sulfur-hydrogen bond has an average bond strength of roughly 82 kcal/mol (roughly 340 kJ/mol) while an oxygen-hydrogen bond has an average bond strength of roughly 110 kcal/mol (roughly 460 kJ/mol). Without being bound by any particular theory, based on this difference in bond strength, it is believed that sulfur has a higher selectivity for reacting at the weakest carbon-hydrogen bond sites, such as alpha hydrogens that are adjacent to aromatic rings. By contrast, the higher reactivity of oxygen is believed to allow oxygen to react at a wider variety of locations.
In addition to producing a desired shot coke phase as the primary coke phase independent of the initial feed characteristics, for coker feedstocks that produce transition coke under standard delayed coking conditions, it has been unexpectedly discovered that a desired shot coke phase can be generated while reducing or minimizing the amount of sulfur that is added to a coker feedstock. In various aspects, a desired shot coke phase can be generated based on addition of 5.0 wt % or less of elemental sulfur to a coker input feed. This can allow the desired shot coke phase to be generated while reducing or minimizing the amount of excess gas phase sulfur that requires processing.
In this discussion, the naphtha boiling range is defined as roughly the boiling point of a C5 alkane (roughly 30° C.) to 177° C. The distillate boiling range is defined as 177° C. to 343° C. The gas oil boiling range is defined as 343° C. to 566° C. The vacuum residue boiling range corresponds to temperatures greater than 566° C.
In this discussion, domain sizes were measured within polarized light micrographs of coke by first segmenting the image to 4 separate levels of pixel intensities using a k-means clustering algorithm, effectively thresholding the image to 4 levels. Using this thresholded image, boundaries of every domain were identified by calculating the gradient of pixel intensity throughout the image. The locations of non-zero gradients correspond to the interface between neighboring coke domains. Finally, a two-dimensional convolution of the domains and their boundaries was used to effectively filter the domains by the size of an equivalent circle for diameters ranging from 2 to 50 μm. The resulting distributions were plotted in histograms. These histograms permitted calculation of a tail ratio (see Table 3 and 4), which is the ratio of the sum of the bin values above 10 μm to the sum of the bin values below 5 μm.
An alternative characterization approach is to convert the histogram to a probability density function by integrating the area under the histogram curve and normalizing the counts to this integrated value. The resulting plots can then be fit to a lognormal distribution to yield a shape parameter σ and the location parameter μ to describe the entire data and not just the tails of the distribution.
The shape parameter σ and the location parameter μ can be used to characterize whether a given region of coke corresponds to shot coke, transition coke, or sponge coke. Table 1 shows the definitions for shot coke, transition coke, and sponge coke based on σ and μ.
As shown in Table 1, in this discussion shot coke is defined as coke that has a value for shape parameter σ of 0.30 or less and a value for location parameter μ of 1.85 or less. Sponge coke is defined as coke that has a value for shape parameter σ of 0.32 or more and a value for location parameter μ of 2.10 or more. Transition coke, which corresponds to a mixture of sponge coke and shot coke, can have a variety of values for a depending on the nature of the transition coke. However, transition coke can be identified based on having a value for location parameter μ between 1.85 and 2.10.
In this discussion, the type of coke produced by delayed coking of a coker feedstock under “standard delayed coking conditions” is defined as the coke produced when a coker feedstock is processed in a delayed coker at a temperature of 475° C. and a pressure of 150 kPa-g.
A coker feedstock can correspond to a relatively high boiling fraction, such as a heavy oil feed. For example, the coker feedstock portion of the feed can have a T10 distillation point of 343° C. or more, or 371° C. or more. Examples of suitable heavy oils for inclusion in the coker feedstock include, but are not limited to, reduced petroleum crude; petroleum atmospheric distillation bottoms; petroleum vacuum distillation bottoms, or residuum; pitch; asphalt; bitumen; other heavy hydrocarbon residues; tar sand oil; shale oil; or even a coal slurry or coal liquefaction product such as coal liquefaction bottoms. Such feeds will typically have a Conradson Carbon Residue (ASTM D189-165) of at least 5 wt. %, generally from 5 to 50 wt. %. In some preferred aspects, the feed is a petroleum vacuum residuum.
Some examples of conventional petroleum chargestock suitable for processing in a delayed coker or fluidized bed coker can have a composition and properties within the ranges set forth below in Table 2.
In addition to petroleum chargestocks, renewable feedstocks derived from biomass having a suitable boiling range can also be used as part of the coker feed. Such renewable feedstocks include feedstocks with a T10 boiling point of 340° C. or more and a T90 boiling point of 600° C. or less. An example of a suitable renewable feedstock derived from biomass can be a pyrolysis oil feedstock derived at least in part from biomass.
In some aspects, the coker feedstock can correspond to a feedstock that, when processed under standard delayed coking conditions, produces a transition coke where the coke with a value for the location parameter μ between 1.85 and 2.10 In other aspects, the coker feedstock can correspond to a feedstock that, when processed under standard delayed coking conditions, produces a sponge coke with a value for the shape parameter σ of 0.32 or more and a value for the location parameter μ of 2.10 or more.
In various aspects, a coker feedstock that produces transition coke or sponge coke under standard delayed coking conditions can be processed under delayed coking conditions after treatment with sulfur as described herein. In such aspects, coking the treated coker feedstock under standard delayed coking conditions can produce coke corresponding to shot coke having a value for the shape parameter σ of 0.30 or less and a value for the location parameter μ of 1.85 or less.
In various aspects, elemental sulfur can be added to a coker feedstock to modify the morphology of coke that is formed by delayed coking. The sulfur can be added to the coker feedstock at a location where the sulfur can have a sufficient time to react with the feedstock to produce an improved coke morphology during coking. In some aspects, the sulfur can be added to the coker feedstock in a mixing tank. Additionally or alternately, the sulfur can be added to the coker feedstock in a conduit prior to entering a tank for holding the coker feedstock for a desired treating time. Further additionally or alternately, the sulfur can be added in a conduit sufficiently far upstream from the furnace for the coker drum(s) so that the desired treating time is achieved prior to passing the mixture of coker feedstock and sulfur into the coker furnace.
The amount of sulfur added to the coker feedstock can vary depending on the nature of the coker feedstock. Some coker feedstocks can correspond to feeds that produce transition coke under standard delayed coking conditions as defined herein. Such feedstocks can be treated with a reduced or minimized amount of sulfur corresponding to 0.5 wt % to 5.0 wt % sulfur, relative to a weight of the coker feedstock. This reduced or minimized amount can be sufficient so that the treated coker feedstock produces shot coke when exposed to standard delayed coking conditions. Other coker feedstocks can correspond to feeds that produce sponge coke under standard delayed coking conditions as defined herein. Such feeds require a greater amount of sulfur addition to convert the resulting coke to shot coke. Such feedstocks can be treated with 8.0 wt % or more of sulfur, such as 8.0 wt % to 12 wt % or possibly higher.
The sulfur can be mixed with the coker feedstock in any convenient manner. One option can be to mix the sulfur directly with at least a portion of the feedstock. Another option can be to mix the sulfur with a carrier solvent, and then mix the combined sulfur and carrier solvent with at least a portion of the coker feedstock. In some aspects, the mixing can be performed using one or more conventional mixers in a holding tank. Optionally, internal mixing structures can be included in conduits to assist with mixing the sulfur and the coker feedstock. Because the sulfur is soluble in relatively high concentrations in coker feedstock, dispersing the sulfur in the coker feedstock can be achieved by any type of convenient mixing that is sufficient to mix the feedstock itself.
After mixing the sulfur with the coker feedstock, the mixture of sulfur and coker feedstock can be maintained at a treating temperature for a treating time prior to passing the mixture of sulfur and coker feedstock into a furnace associated with a delayed coking reactor. The treating temperature can be a temperature of 160° C. to 260° C., or 170° C. to 260° C., or 180° C. to 260° C. The treating time can correspond to 0.5 hours to 7 days, or 2 hours to 5 days, or 12 hours to 4 days, or 1 day to 3 days. It is noted that maintaining the mixture of coker feedstock and sulfur at the treating temperature for the treating time can potentially be achieved while the mixture is passing through conduits toward the coker furnace, if the conduits are sufficiently long and/or the flow velocity is sufficiently low. It is noted that the coker furnace typically heats coker feedstock to a temperature near the desired coking temperature. Such coking temperatures are typically 400° C. or higher, and therefore well above the treating temperature of 160° C. to 260° C.
It is noted that in aspects where a separate tank, mixing vessel, or other holding volume is used for maintaining the coker feed and sulfur at the treating temperature, the holding volume can be operated in any convenient manner. For example, the holding volume can be operated in a batch mode, a continuous mode, or a semi-continuous mode. In batch mode, the holding volume (such as holding tank 110) can be filled to a desired batch level and then the entire batch can be held in the holding volume for a residence time corresponding to at least a portion of the desired treating time, such as up to all of the desired treating time. In continuous mode, coker feedstock 105 and/or elemental sulfur 102 can be added continuously to the holding volume. In such an aspect, an average residence time can be calculated for the coker feedstock based on the size of the holding volume relative to the input flow rate to the tank and/or the output flow rate from the tank. The average residence time can correspond to at least a portion of the treating time, such as up to all of the treating time. In aspects where the residence time in the holding volume is less than the treating time, the remaining treating time can correspond to time for transferring the mixture of coker feedstock and sulfur in one or more conduits that provide fluid communication between the holding volume and the coker furnace.
In configurations where a portion of the treating time corresponds to residence time within one or more conduits, such as the configuration shown in
Delayed coking is a process for the thermal conversion of heavy oils such as petroleum residua (also referred to as “resid”) to produce liquid and vapor hydrocarbon products and coke. Delayed coking of resids from heavy and/or sour (high sulfur) crude oils is carried out by converting part of the resids to more valuable hydrocarbon products. The resulting coke has value, depending on its grade, as a fuel (fuel grade coke), electrodes for aluminum manufacture (anode grade coke), etc.
Generally, a residue fraction, such as a petroleum residuum feed is pumped to a pre-heater where it is pre-heated, such as to a temperature from 480° C. to 520° C. The pre-heated feed is conducted to a coking zone, typically a vertically-oriented, insulated coker vessel, e.g., drum, through an inlet at the base of the drum. Pressure in the drum is usually relatively low, such as 15 psig (˜100 kPa-g) to 80 psig (˜550 kPa-g), or 15 psig (˜100 kPa-g) to 35 psig (˜240 kPa-g) to allow volatiles to be removed overhead. Typical operating temperatures of the drum will be between roughly 400° C. to 445° C., but can be as high as 475° C. The hot feed thermally cracks over a period of time (the “coking time”) in the coke drum, liberating volatiles composed primarily of hydrocarbon products that continuously rise through the coke bed, which consists of channels, pores and pathways, and are collected overhead. The volatile products are conducted to a coker fractionator for distillation and recovery of coker gases, gasoline boiling range material such as coker naphtha, light gas oil, and heavy gas oil. In an embodiment, a portion of the heavy coker gas oil present in the product stream introduced into the coker fractionator can be captured for recycle and combined with the fresh feed (coker feed component), thereby forming the coker heater or coker furnace charge. In addition to the volatile products, the process also results in the accumulation of coke in the drum. When the coke drum is full of coke, the heated feed is switched to another drum and hydrocarbon vapors are purged from the coke drum with steam. The drum is then quenched with water to lower the temperature down to 200° F. (˜95° C.) to 300° F. (˜150° C.), after which the water is drained. When the draining step is complete, the drum is opened and the coke is removed by drilling and/or cutting using high velocity water jets (“hydraulic decoking”).
Included below are results from the conversion of coker feedstocks corresponding to two different vacuum residues. Under conventional coking conditions, the first resid (Feed 1) produces sponge coke, while the second resid (Feed 2) produces transition coke. As shown in the examples below, addition of elemental sulfur as described herein results in formation of shot coke when coking both types of vacuum residue.
For the results presented below, samples of feedstock were coked according to the procedure in ASTM D4530. In some instances noted below, coking was held at 500° C. for 30 minutes according to the procedure employed by Siskin. Data was then generated using reflected polarized light microscopy images acquired on coke samples embedded in epoxy that were highly polished. Color was added by inserting a λ retardation plate between the cross polars. The acquired images were analyzed to produce histograms and to determine the parameters σ and μ for each sample.
In order to investigate the impact of sulfur addition on coke formation, samples with added sulfur were formed from both vacuum residues. For Feed 1 (produced sponge coke under standard delayed coking conditions), samples with added sulfur were formed by solution blending a toluene/sulfur solution with a toluene/vacuum residue solution to ensure homogeneous dispersion of sulfur. The resulting product had 8.8 wt % of S8 added to the vacuum residue. After sulfur addition, the mixture of toluene, sulfur, and vacuum residue was heated according to the following profile: 1) The temperature was increased from room temperature to 100° C. over the course of 10 minutes. 2) The temperature was further increased from 100° C. to 259° C. over the course of 20 minutes. 3) The temperature was maintained at 259° C. for 60 minutes to allow for reaction of sulfur with the vacuum resid. 4) The temperature was increased from 259° C. to a coking temperature of 500° C. over the course of 20 minutes. 5) The temperature was maintained at the coking temperature of 500° C. for 30 minutes. For comparison, a sample of Feed 1 without added sulfur were also coked using a similar heating profile.
For Feed 2 (produced transition coke under standard delayed coking conditions), samples with added sulfur were formed using solution blending as stated above by adding 5.0 wt % of S8 to the vacuum residue. After sulfur addition, the mixture of toluene, sulfur, and vacuum residue was heated according to the following profile: 1) The temperature was increased from room temperature to 100° C. over the course of 10 minutes. 2) The temperature was further increased from 100° C. to 306° C. over the course of 20 minutes. 3) The temperature was maintained at 306° C. for 60 minutes to allow for reaction of sulfur with the vacuum resid. 4) The temperature was increased from 306° C. to a coking temperature of 500° C. over the course of 20 minutes. 5) The temperature was maintained at the coking temperature of 500° C. for 30 minutes. For comparison, a sample of Feed 2 without added sulfur were also coked using a similar heating profile.
After forming coke, optical micrographs were taken from 20 different locations of each sample to provide a data set. The optical micrographs were then analyzed to produce histograms, to determine tail ratios, and to determine the values of the parameters a and p for each sample.
Based on the images obtained of coked formed from Feed 1 with and without treatment with sulfur, the domain sizes of the coke within the images were determined. This allowed a fit of a lognormal distribution to the domain size data.
The differences between the curves shown in
The differences between the coke formed with and without sulfur for Feed 2 are further illustrated in
The differences between the curves shown in
Additional feedstock samples and corresponding coke products were generated in order to further investigate the impact of sulfur addition on coke formation. This included investigating the impact of changing the reaction conditions and the impact of changing the amount of sulfur added to the coker feedstock.
To investigate the impact of reaction conditions, a batch of Feed 1 was prepared that included 9.0 wt % of added elemental sulfur. Samples of this mixture were then heat treated for roughly 1 hour at temperatures of 175° C., 200° C., and 225° C. Table 5 shows an elemental analysis of the resulting heat-treated mixture.
As shown in Table 5, at 225° C., a substantial portion of the sulfur added to the LLS resid was missing from the liquid heat-treated sample. This is believed to indicate loss of sulfur to the gas phase due to generation of H2S or evaporation resulting from the increasing vapor pressure of sulfur with increasing temperature. After heat treatment, samples were then coked according to ASTM D4530 to generate coke samples. Table 6 shows an elemental analysis of the resulting coke.
In Table 6, even though the sulfur content of the heat-treated mixture varied from roughly 4.0 wt % to roughly 9.3 wt %, the resulting coke had a relatively constant sulfur level. This is believed to indicate several things. The reaction of sulfur with the coke is stoichiometric. Thus, for a given type of coke, only a portion of the sulfur is incorporated into the coke structure. At some of the lower temperatures shown in Table 5, residual sulfur remains in the heat treated product, even if the sulfur has not been incorporated/reacted with the feed. At the higher temperature shown in
As shown in Table 7, the coke samples formed after sulfur treatment of Feed 1 corresponded to shot coke, independent of the temperature of sulfur treatment in the range of 160° C. to 260° C.
To investigate variations in the amount of sulfur added to a coker feedstock, varying levels of elemental sulfur were added to Feed 1. The added sulfur levels corresponded to addition of 0.5 wt %, 1.0 wt %, 2.0 wt %, 5.0 wt %, and 10 wt % of sulfur to the Feed 1. Table 8 shows estimated compositions of the resulting mixtures of Feed 1 and sulfur.
The samples shown in Table 8 that included added sulfur were then heat treated at 200° C. for roughly 1 hour. This resulted in the measured compositions shown in Table 9.
As shown in Table 9, the heat treatment reduced the sulfur to carbon ratio for the samples that originally included 5.0 wt % or less of added sulfur to similar levels. The samples shown in Table 9 were then coked according to ASTM D4530. The composition of the resulting coke is shown in Table 10.
As shown in Table 10, the sample derived from addition of 10 wt % sulfur to Feed 1 had a substantially different sulfur content in the final coke product. The remaining samples appeared to have somewhat similar sulfur contents in the final coke product. This indicates the ability to generate the desired shot coke phase using limited amounts of added sulfur.
Embodiment 1. A method for controlling coke morphology during delayed coking, comprising: providing a coker feedstock that forms transition coke under standard delayed coking conditions; mixing the coker feedstock with 0.5 wt % to 5.0 wt % of elemental sulfur to form a mixed coker feedstock; maintaining the mixed coker feedstock at a temperature of 160° C. to 260° C. for 0.3 hours to 7 days to form a treated coker feedstock; and exposing the treated coker feedstock to second delayed coking conditions to form a coker effluent and treated coke comprising shot coke.
Embodiment 2. The method of Embodiment 1, wherein the provided coker feedstock forms transition coke under standard delayed coking conditions comprising a lognormal domain size distribution having a value of a location parameter μ between 1.85 and 2.10.
Embodiment 3. A method for controlling coke morphology during delayed coking, comprising: providing a coker feedstock that forms sponge coke under standard delayed coking conditions; mixing the coker feedstock with 8.0 wt % or more of elemental sulfur to form a mixed coker feedstock; maintaining the mixed coker feedstock at a temperature of 160° C. to 260° C. for 0.3 hours to 7 days to form a treated coker feedstock; and exposing the treated coker feedstock to delayed coking conditions to form a coker effluent and coke comprising shot coke.
Embodiment 4. The method of Embodiment 3, wherein the provided coker feedstock forms sponge coke under standard delayed coking conditions comprising a lognormal domain size distribution having a value a value of a shape parameter σ greater than 0.32 and a value of a location parameter μ greater than 2.10.
Embodiment 5. The method of any of the above embodiments, wherein the treated coke further comprises a lognormal domain size distribution having a value of a shape parameter α of 0.30 or less and a value of a location parameter μ of 1.85 or less.
Embodiment 6. The method of any of the above embodiments, wherein the standard delayed coking conditions comprising a temperature of 475° C. and 150 kPa-g.
Embodiment 7. The method of any of the above embodiments, wherein the second delayed coking conditions comprise a temperature of 400° C. to 475° C. and a pressure of 100 kPa-g to 500 kPa-g.
Embodiment 8. The method of any of Embodiments 1-6, wherein the second delayed coking conditions comprise standard delayed coking conditions.
Embodiment 9. The method of any of the above embodiments, wherein the coker feedstock and the elemental sulfur are mixed in a holding volume, or wherein the mixed coker feedstock is maintained at the temperature of 160° C. to 260° C. for at least a portion of the 0.3 hours to 7 days in a holding volume, or a combination thereof.
Embodiment 10. The method of any of the above embodiments, wherein the mixed coker feedstock is maintained at the temperature of 160° C. to 260° C. for at least a portion of the 0.3 hours to 7 days in one or more conduits in fluid communication with a coker furnace.
Embodiment 11. The method of any of the above embodiments, wherein the mixed coker feedstock is maintained at the temperature of 160° C. to 260° C. for at least a portion of the 0.3 hours to 7 days by passing the mixed coker feedstock into a vacuum fractionator.
Embodiment 12. The method of Embodiment 11, wherein the coker effluent is passed into the vacuum fractionator.
Embodiment 13. The method of any of the above embodiments, further comprising separating the coker effluent to form at least an unconverted bottoms fraction, wherein at least a portion of the unconverted bottoms are exposed to the delayed coking conditions.
Embodiment 14. A system for controlling coke morphology during delayed coking, comprising: a delayed coker drum comprising a drum feed inlet and a coker effluent outlet; a coker furnace comprising a furnace inlet and a furnace outlet, the furnace outlet being in fluid communication with the drum feed inlet; a fractionator comprising a first fractionator inlet, a second fractionator inlet, a bottoms outlet, and a one or more product outlets, the first fractionator inlet being in fluid communication with the coker effluent outlet, the bottoms outlet being in fluid communication with the furnace inlet via a furnace conduit; and a mixed coker feedstock conduit in fluid communication with the second fractionator inlet, the mixed coker feedstock conduit being in fluid communication with a coker feedstock conduit and a sulfur input conduit, wherein at least a portion of the mixed coker feedstock conduit, the fractionator, and the furnace conduit is maintained at a temperature of 160° C. to 260° C., and wherein a residence time for feed in the at least a portion of the mixed coker feedstock conduit, the fractionator, and the furnace conduit is 0.3 hours to 7 days.
When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated. While the illustrative embodiments of the disclosure have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the disclosure. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty which reside in the present disclosure, including all features which would be treated as equivalents thereof by those skilled in the art to which the disclosure pertains.
The present disclosure has been described above with reference to numerous embodiments and specific examples. Many variations will suggest themselves to those skilled in this art in light of the above detailed description. All such obvious variations are within the full intended scope of the appended claims.
