Patent Application
20050201905
1. Field of the Invention
This invention relates to a method and apparatus for improving fluid catalytic cracking (FCC) feed nozzle performance and reliability. More particularly, the invention is directed to a nozzle assembly located near the bottom of the riser reactor for achieving a desirable feed/catalyst contacting condition similar to the modern side entry feed nozzles located at an elevated riser location, but without the need to change the riser, thus substantially reducing the capital cost required to benefit from achieving such a desirable feed/catalyst contact condition.
2. Description of Related Art
As is well known, a typical fluid catalytic cracking (FCC) unit consists of a reactor and a regenerator. Long chain hydrocarbon, such as vacuum gas oil, is injected into the riser reactor through feed nozzles to contact with hot, regenerated catalyst to crack into small, more valuable products, such as gasoline. The catalyst is temporarily deactivated in the riser reactor during the reaction due to the deposition of a coke layer on the surface. The product gas is separated out at the top of the reactor and sent to the product recovery system. Spent catalyst is then transported to the stripper where steam is introduced to recover gas products trapped in the spent catalyst. The stripped spent catalyst is then transported to the regenerator where air is introduced to burn off the coke layer on the surface to reactivate the catalyst. The regenerated catalyst is then transferred back to the riser reactor, which completes the catalyst circulation cycle.
An FCC feed nozzle is critical to unit performance. It determines the uniformity of initial contact of feed with the regenerated catalyst, which has profound effects on dry gas yield and coke selectivity. Most modern FCC feed nozzles are side-entry nozzles that are installed on the periphery of the vertical riser section at an elevated location. This is the preferred option for a grass-root FCC. However, some older FCC units still have feed injection systems located at the bottom of the riser and there could be limitations, such as high capital cost and the pressure-balance of the unit, to convert these units to vertical risers with modern side entry nozzles. The riser, at the location of the bottom injection feed nozzles, may be oriented either horizontally, vertically or inclined.
Applicant's U.S. Pat. No. 5,979,799 discloses a side entry nozzle having a spray pattern that is angled upwardly at an angle of up to 30° from the axis of the nozzle.
Applicant's U.S. Pat. No. 6,387,247B1 discloses a nozzle tip design for a single, centrally located bottom entry nozzle having an upward discharge of from about 10° to about 80° from the axis of the nozzle.
U.S. Pat. No. 4,778,658 to Nielsen discloses a single, centrally located bottom-feed nozzle having angled injection quills for injecting hydrocarbon into an enclosure mixing chamber inside the feed nozzle for better mixing of hydrocarbons and steam, and better atomization prior to injecting the hydrocarbon/steam mixture into the riser for contacting with the catalyst.
U.S. Pat. No. 4,808,383 to Buyan discloses a bottom-feed injection system consisting of multiple feed nozzles with their axis and discharge ends parallel to the axis of the riser.
A feed nozzle assembly located near the bottom of an FCC riser reactor includes a plurality of feed nozzles, each feed nozzle including a discharge end which is angled toward the center of the riser. The angled discharged ends provide a combined discharge of hydrocarbon feed and atomization gas into the central portion of the riser thereby providing a desirable feed/catalyst contact condition while preventing impingement of the feed upon the riser walls
One objective of the instant invention is to improve catalyst/feed contacting for FCC units with a feed injection system located near the bottom of the riser reactor. Another objective of the instant invention is to improve feed distribution in the riser. Yet another objective is to reduce the rate of fouling (coking) of riser walls. Yet another objective of the instant invention is to improve catalyst circulation of the unit.
Applicants' invention is primarily directed to improving feed/catalyst contact. Thus, in Applicants' invention, the catalyst makes contact with the hydrocarbon/steam spray immediately after its exit from the discharge end of the individual nozzles. One essential element is thus that the discharge end of the nozzle is surrounded on all sides by catalyst. The instant invention with multiple nozzles providing multiple contacting areas with the catalyst is an improvement over the prior arts of a single, centrally located bottom entry nozzle, such as Applicant's U.S. Pat. No. 6,387,247B1 and U.S. Pat. No. 4,778,658 to Nielsen.
The instant invention is also a new and improved design of the prior art of U.S. Pat. No. 4,808,383 to Buyan noted above. The prior art of U.S. Pat. No. 4,808,383 discloses a bottom-feed injection system consisting of multiple nozzles, but with their axis and discharge ends parallel to the axis of the riser. Due to the fact that the discharges from these nozzles are parallel to the riser axis, it cannot achieve desirable feed distribution in the riser. If the discharge of each nozzle in U.S. Pat. No. 4,808,383 were to expand quickly to cover a larger area, it will impinge on the riser wall, leading to mechanical damages. One essential element of the instant invention is that the discharge ends of individual nozzles are canted toward the center of the riser. This allows the individual discharges of the instant invention to expand quickly and to provide desirable feed distribution in the riser, but without the drawback of impinging on the riser wall.
The instant invention provides an alternative to achieve a desirable feed/catalyst contacting condition near the bottom of the riser similar to the modern side entry feed nozzles located at an elevated riser location (see Applicant's U.S. Pat. No. 5,979,799 noted above), but without the need to change the riser, thus substantially reducing the capital cost.
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In many riser inlet configurations there is a zone below the discharge end 8 of the feed nozzles 4 where there is a risk that catalyst might settle and lose its fluidity. This would have a negative impact on the ability of the operator to circulate catalyst through the unit at any desired circulation rate. In extreme cases this might even lead to complete stagnation of the catalyst circulation. Clearly, this is a risk that is to be avoided. Designers of FCC units have found various ways and means to overcome these problems. A solution to this problem is to install a device to distribute fluidizing gas at a low elevation in the riser. Usually, steam is used for this purpose, but other gases could be used as well. These so-called pre-fluidizing devices, shown at 2 in
Although application of the invention disclosed herein might be less vulnerable to the risk of reduced circulation than side-entry nozzles at a higher elevation, these risks may still exist. A designer will minimize these risks by the use of a pre-fluidizing device, as described above. He/she will consider carefully how to make the layout of the bottom of the riser so as to accommodate all the different components—the different feed nozzles, optionally a separate emergency steam nozzle and optionally a separate pre-fluidizing distribution device.
Although the nozzles are called bottom-entry nozzles and although the feed nozzles are usually straight tubes that perforate the riser bottom, this need not necessarily be the case. They could equally well be tubes that perforate the riser walls and then make a 90-degree turn, from perpendicular to the riser axis to parallel to the riser axis.
