Patent Application
20190161684
This invention relates to a fluid coking process for converting a heavy hydrocarbonaceous feedstock to liquid products which uses high thrust feed nozzles for injecting feedstock into the circulating fluidized bed of heated coke particles.
Fluidized bed coking (fluid coking) is a petroleum refining process in which heavy petroleum feeds, typically the non-distillable residue (resid) from fractionation or heavy oils are converted to lighter, more useful products by thermal decomposition (coking) at elevated reaction temperatures, typically about 480 to 590° C., (about 900 to 1100° F.) and in most cases from 500 to 550° C. (about 930 to 1020° F.). Heavy oils that may be processed by the fluid coking process include heavy atmospheric resids, vacuum resids, aromatic extracts, asphalts, and bitumen from oil sands.
The process is carried out in a unit with a large reactor vessel containing hot coke particles that are maintained in the fluidized condition at the required reaction temperature with a fluidizing gas (e.g., steam) injected at the bottom of the vessel. The heavy oil feed is heated to a pumpable temperature, typically in the range of 350 to 400° C. (about 660 to 750° F.), mixed with atomizing steam, and fed through multiple feed nozzles arranged at several successive levels in the reactor. The steam is injected into a stripper section at the bottom of the reactor and passes upwards through the coke particles in the stripper as they descend from the main part of the reactor above. The feed liquid coats the coke particles in the fluidized bed, which make up the emulsion phase of the fluidized bed. As the thermal cracking reactions proceed, the liquid is transformed to vapour, which must migrate from the emulsion phase into the bubble phase in order to exit the system.
Liquid yields in fluid coking can be increased by reducing the reaction severity, or the time that molecules are exposed to process temperature. The typical approach taken to reduce reactor severity is to reduce reactor temperature. However, the downside of reducing temperature is increased stripper and sore thumb fouling, which can lead to reduced run lengths. Another approach to reduce reactor severity is to decrease the exposure time at high temperatures by providing short vapour phase residence times.
Long hydrocarbon vapour residence times are the most likely contributor to higher than expected “gas make”, defined as C4-components, in the fluid coking process. Vapour-liquid equilibrium suppression, coupled with less than adequate mass transfer between the emulsion and bubble phases, is the most probable mechanism responsible for high “coke make”, defined as the toluene insoluble solid by-product of the thermal cracking reaction. Both phenomena result in lower liquid yields, and preliminary estimates suggest that they can contribute to as much as 11 wt % liquid yield loss. Optimizing the rate of removal of vapour from the emulsion phase should reduce the overall hydrocarbon vapour residence time of the reactor, increase liquid yields, and reduce gas make. It is estimated that a 3-5 wt % liquid yield increase can be achieved through maximizing vapour recovery from the reactor dense bed.
Technologies that increase mass transfer between the emulsion and bubble phase and, thus, reduce the gas phase residence time and increase hydrocarbon vapour stripping, are required.
It has been discovered that the reactor section of a fluid coker is comprised of a dilute, upward-flowing stream of gas in the central (core) region of the reactor and a dense, downward-flowing, outer (annular) region of particles. This is due to the vaporized hydrocarbons rising primarily in the core. Thus, the core region has a high vapour and low solids concentration (solids lean) and the annular region has a low vapour and high solids concentration (solids dense).
The present invention is directed to the use of high thrust feed nozzles to transport unreacted hydrocarbon and coke present in the annular region of the fluidized bed to the high velocity core region of the fluidized bed to improve hydrocarbon stripping, reduce the gas phase residence time, and increase liquid yields. Thrust is a mechanical force that is generated through the act of accelerating a mass of fluid. In other words, it is the reaction force created by the ejection of fluid from a nozzle at high velocity. The fluid pressure is related to the momentum of the fluid and acts perpendicular to any imposed boundary, which in this case is the fluidized solids in the reactor. The amount of thrust generated depends on the mass flow rate and the exit velocity of the fluid. High thrust can be achieved by either slightly accelerating a large mass of fluid, or greatly accelerating a small mass of fluid.
Prior art nozzles that are presently used in fluid cokers have a limited ability to transfer solids from the annular region of the fluidized bed to the upward flowing core of the fluidized bed. Thus, a process is provided herein for converting a heavy hydrocarbonaceous feedstock to liquid products, comprising:
In one embodiment, the high thrust nozzles have a spray angle of about 3°-160° and a nozzle diameter between about 0.2 and 0.8″.
In the accompanying drawings:
The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments contemplated by the inventor. The detailed description includes specific details for the purpose of providing a comprehensive understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.
The present invention is directed to the use of high thrust feed nozzles in a fluidized coking operation to push unreacted hydrocarbon and coke to the high velocity core region of the fluidized bed to improve hydrocarbon stripping, reduce the gas phase residence time, and increase liquid yields.
Thrust is a mechanical force that is generated through the act of accelerating a mass of fluid. In other words, it is the reaction force created by the ejection of fluid from a nozzle at high velocity (John and Keith, 2006). The fluid pressure is related to the momentum of the fluid and acts perpendicular to any imposed boundary, which in this case is the fluidized solids in the reactor. The amount of thrust generated depends on the mass flow rate and the exit velocity of the fluid. High thrust can be achieved by either slightly accelerating a large mass of fluid, or greatly accelerating a small mass of fluid.
There are three main factors that affect thrust: friction effects, axial momentum loss and thrust loss due to the pressure difference between the nozzle exit plane and the background. When friction is considered, it is best to have nozzles with large exit angles. However, the axial momentum losses increase as the angle increases since a higher percentage of the exiting flow will be non-axial.
Cruz (Cruz, N., “Interactions between Supersonic Gas Jets and Gas-Solid Fluidized Beds”, MSc Thesis, The University of Western Ontario, 2009) found that the thrust force of gas in a convergent-divergent nozzle used to attrition particles in a fluidized bed had a strong relationship with the particle grinding efficiency. During the attrition process, particles are entrained into the gas jet and are accelerated to high velocity where they collide with the particles in the dense phase of the fluidized bed at the tip of the jet plume and cause particle breakage to occur. This concept can be used to enhance the movement of solids from the annular section of the reactor to the core region in the feed zone by using feed nozzles that produce a high thrust force.
The feed is injected through multiple high thrust nozzles located in feed rings 12a to 12f, which are positioned so that the feed with atomizing steam enters directly into the fluidized bed of hot coke particles in coking zone 11. Each feed ring consists of a set of high thrust nozzles (typically 10-20, not designated in
Steam is admitted as fluidizing gas in the stripping section 13 at the base of coker reactor 10, through spargers 14 directly under stripping sheds 15 as well as from lower inlets 16. The steam passes up into stripping zone 13 of the coking reactor in an amount sufficient to obtain a superficial fluidizing velocity in the coking zone, typically in the range of about 0.15 to 1.5 m/sec (about 0.5 to 5 ft/sec). The coking zone is typically maintained at temperatures in the range of 450 to 650.degree. C. (about 840 to 1200.degree. F.) and a pressure in the range of about 0 to 1000 kPag (about 0 to 145 psig), preferably about 30 to 300 kPag (about 5 to 45 psig), resulting in the characteristic conversion products which include a vapor fraction and coke which is deposited on the surface of the seed coke particles.
The vaporous products of the cracking reactions with entrained coke particles pass upwards out of the reaction zone 11, through a phase transition zone in the upper portion 17 of the vessel and finally, a dilute phase reaction zone at the inlets of cyclones 20 (only two shown, one indicated). The coke particles separated from the vaporous coking products in the cyclones are returned to the fluidized bed of coke particles through cyclone dipleg(s) 21 while the vapors pass out through the gas outlet(s) 22 of the cyclones into the scrubbing section of the reactor (not shown). After passing through scrubbing section which is fitted with scrubbing sheds in which the ascending vapors are directly contacted with a flow of fresh feed to condense higher boiling hydrocarbons in the reactor effluent (typically 525° C.+/975° F.+) and recycles these along with the fresh feed to the reactor. The vapors leaving the scrubber then pass to a product fractionator (not shown). In the product fractionator, the conversion products are fractionated into light streams such as naphtha, intermediate boiling streams such as light gas oils and heavy streams including product bottoms.
The coke particles that pass downwards from the dense bed 11 to stripper section 13 comprising sheds 15 are partially stripped of occluded hydrocarbons in the stripper by use of a stripping gas, usually steam, which enters via spargers 14. The stripped coke particles are passed via line 25 to a heater (not shown) which is operated a temperature from about 40 to 200° C., preferably about 65 to 175° C., and more preferably about 65 to 120° C. in excess of the actual operating temperature of the coking zone and recycled back to the fluid coking unit.
Current commercial fluid coking feed nozzles are designed to atomize the bitumen at the nozzle exit through shear from the high velocity and rapid decompression of the atomization steam upon exiting the nozzle. This decompression happens both axially and radially.
One such coker nozzle is described in detail in Canadian Patent No. 2,224,615, and is referred to herein as TEBM-2b with circular exit, or GEN2 nozzle. The GEN2 nozzle consists of a series of converging, diverging, and converging sections. The pressure drop across the exit of the GEN2 coker feed nozzle is on the order of 70 psi. The flow exiting the nozzle consists of bubbles dispersed in the liquid phase and the large decompression from the resultant pressure drop at the exit causes an explosive expansion of the bubbles, resulting in a phase inversion where the flow changes from liquid continuous in the nozzle to gas continuous in the jet, with liquid droplets and ligaments distributed in the gas stream.
In this example, the GEN2 nozzle and the 1.25GEN2, which is the same as the GEN2 nozzle except all of the dimensions are scaled up so that the throat area is 25% larger than the GEN2 nozzle, were tested in order to measured axial thrust force (lb) as a function of the nozzle pressure. In addition to the GEN2 nozzles, three commercially available fan spray nozzles, referred to herein as Nozzle B, Nozzle C and Nozzle D, and a curved throat fan nozzle used in the FCC process, described in detail in U.S. Pat. No. 6,199,768, referred to herein as CTF, were tested in this example. A GEN3 nozzle, which consists of the same internal geometry as the GEN2 nozzle but contains a diverging cloverleaf disperser at the tip of the nozzle, was also tested. A drawing of a GEN3 nozzle is shown in
Finally,
Experiments were conducted with the aforementioned feed nozzles having different equivalent throat diameters and exit angles by spraying water into open air over a range of liquid flow rates and nozzle pressures. Table 1 shows a summary of the nozzles that were tested and their specifications. The nozzles were mounted on a stand that allowed them to move freely in the axial direction. The reaction thrust force was measured using a 3000 lb thru-hole compression load cell, which was mounted on the nozzle conduit and was compressed between two plates while the nozzle was spraying.
In summary, the results in
In order to maximize the axial thrust force and reduce the expansion of the jet in the radial direction, a supersonic nozzle with a diverging/diffuser section was designed to accelerate the fluid axially in the nozzle exit, prior to injection into the fluidized bed (hereinafter referred to as the “Diffuser nozzle”). The Diffuser nozzle consists of the same internal geometry as the GEN2 nozzle but without the final constriction at the nozzle tip. The diffuser section now resulted in a much narrower jet plume. The phase inversion occurs within the nozzle, and the fluid acceleration through the nozzle will increase in the axial direction. In addition, a supersonic nozzle maximizes the velocity of the jet at a much larger cross sectional exit area compared to a subsonic nozzle.
Experiments were conducted using feed nozzles with different equivalent throat diameters and exit geometries by spraying air and water into open air over a range of liquid flow rates and nozzle pressures. Table 2 shows a summary of the nozzles that were tested and their specifications. The nozzles were mounted on a stand that allowed them to move freely in the axial direction. The reaction thrust force was measured using a 3000 lb thru-hole compression load cell, which was mounted on the nozzle conduit and was compressed between two plates while the nozzle was spraying.
A drawing of a GEN1 nozzle is shown in
Another nozzle geometry that would maximize the axial thrust force would be to add a diverging section to the GEN2 nozzle in order to accelerate the fluid to supersonic velocities before exiting the nozzle.
From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, the present invention is not intended to be limited to the embodiments shown herein, but is to be accorded the full scope consistent with the claims, wherein reference to an element in the singular, such as by use of the article “a” or “an” is not intended to mean “one and only one” unless specifically so stated, but rather “one or more”. All structural and functional equivalents to the elements of the various embodiments described throughout the disclosure that are known or later come to be known to those of ordinary skill in the art are intended to be encompassed by the elements of the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims.
| Number | Date | Country | |
|---|---|---|---|
| 62591678 | Nov 2017 | US |
