Gas separation – Multiple separators – each with discrete and longitudinally... – Centrifugal
Reexamination Certificate
2001-04-27
2004-01-06
Hopkins, Robert A. (Department: 1724)
Gas separation
Multiple separators, each with discrete and longitudinally...
Centrifugal
C055S396000, C055S457000
Reexamination Certificate
active
06673133
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a novel cyclone separator for removing fine solid particulates from a gas stream. The cyclone is especially applicable in a third stage separator apparatus, often used to purify the catalyst fines-laden flue gas stream exiting a refinery fluid catalytic cracking (FCC) catalyst regenerator.
BACKGROUND OF THE INVENTION
The emission of particulates in industrial gas streams must be carefully controlled in light of federal, state, and local regulations designed to curtail pollution. In the area of oil refinery operations, a major concern regarding particulate emissions lies in the flue gas exiting the catalyst regenerator section of fluid catalytic cracking (FCC) units. Current United States federal regulations limit particulate levels to 1 lb. of solids per 1000 lb. of coke burned in the catalyst regenerator, or the equivalent of a flue gas particulate concentration of approximately 80-110 mg/Nm
3
. Corresponding European regulations currently vary considerably, from 80-500 mg/Nm
3
; however, this value is expected to decline potentially to 50 mg/Nm
3
.
FCC technology, now more than 50 years old, has undergone continuous improvement and remains the predominant source of gasoline production in many refineries. This gasoline, as well as lighter products, is formed as the result of cracking heavier (i.e. higher molecular weight), less valuable hydrocarbon feed stocks such as gas oil. Although FCC is a large and complex process involving many factors, a general outline of the technology is presented here in the context of its relation to the present invention.
In its most general form, the FCC process comprises a reactor that is closely coupled with a catalyst regenerator, followed by downstream hydrocarbon product separation. A major distinguishing feature of the process is the continuous fluidization and circulation of large amounts of catalyst having an average particle diameter of about 50-100 microns, equivalent in size and appearance to very fine sand. For every ton of cracked product made, approximately 5 tons of catalyst are needed, hence the considerable circulation requirements. Coupled with this need for a large inventory and recycle of a small particle diameter catalyst is the ongoing challenge to prevent this catalyst from exiting the reactor/regenerator system into effluent streams.
Overall, the use of cyclone separators internal to both the reactor and regenerator has provided over 99% separation efficiency of solid catalyst. Typically, the regenerator includes first and second (or primary and secondary) stage separators for the purpose of preventing catalyst contamination of the regenerator flue gas, which is essentially the resulting combustion product of catalyst coke in air. While normal-sized catalyst particles are effectively removed in the internal regenerator cyclones, fines material (generally catalyst fragments smaller than about 50 microns resulting from attrition and erosion in the harsh, abrasive reactor/regenerator environment) is substantially more difficult to separate. As a result, the FCC flue gas will usually contain a particulate concentration in the range of about 200-1000 mg/Nm
3
. This solids level can present difficulties related to either the applicable legal emissions standards or the desire to recover power from the flue gas stream. In the latter case, the solids content in the FCC flue gas may be sufficient to damage turbine blades of an air blower to the regenerator if such a power recovery scheme is indeed selected.
A further reduction in FCC flue gas fines loading is therefore often warranted, and may be obtained from a third stage separator (TSS) device containing a manifold of cyclones. Electrostatic precipitators are known to be effective for this gas/solid separation but are far more costly than a TSS, which relies on the induction of centripetal acceleration to a particle-laden gas stream, forcing the higher-density solids to the outer edges of a spinning vortex. To be efficient, a cyclone separator for an FCC flue gas effluent will normally contain many, perhaps 100, small individual cylindrical cyclone bodies installed within a single vessel acting as a manifold. Tube sheets affixing the upper and lower ends of the cyclones act to distribute contaminated gas to the cyclone inlets and also to divide the region within the vessel into sections for collecting the separated gas and solid phases.
In the area of cyclone design, significant emphasis has been placed on so-called “reverse flow” types where incoming gas is added around a gas outlet tube extending from the inlet side of a cylindrical cyclone body. Particle-rich gas can be withdrawn from openings in the sidewall of the cyclone body, while clean gas essentially reverses flow from its initial path toward the end of the cyclone body opposite the gas inlet, back toward the gas outlet. The gas outlet is a tube normally concentric with, and located within the cyclone body. These types of cyclones are described in U.S. Pat. No. 5,514,271 and U.S. Pat. No. 5,372,707, where the inventive subject matter is focused on the shape and distribution of the sidewall openings in order to minimize turbulent eddy formation that can re-entrain solids into the clean gas outlet. In U.S. Pat. No. 5,643,537 and parent U.S. Pat. No. 5,538,696, devices are contemplated for use with this fundamental cyclone design to further extend, or improve the uniformity of, the vortex flow pattern and thereby increase separation efficiency.
Unfortunately, the requirement by itself for a gas stream to reverse direction and exit the cyclone body on the same side as the gas inlet imposes flow disturbances that are not easily overcome. Cyclones of the type described in U.S. Pat. No. 5,690,709, termed “uniflow”, eliminate the re-entrainment of solids associated with the reversal of gas direction. In this case, clean gas moves continually downward and exits the cyclone body below a lower tube sheet, which serves as the physical boundary between the separated particles and purified gas. This design, however, also promotes non-uniform flow patterns, which are here associated with the discharge of particles at essentially right angles to the particle-laden gas vortex, through the open bottom in the cylindrical cyclone body. Again, the basic operation of the cyclone in this case involves a change in direction of gas flow that should ideally be avoided. Furthermore, the open bottom design provides a relatively large surface area for exiting “dirty” gas to enter the bodies of adjacent cyclones in an overall arrangement of cyclones, such as in a TSS. This communication of gas among cyclones reduces separation efficiency.
Aside from general considerations about cyclone design, such as the induction of centripetal acceleration and the maintenance of a uniform flow pattern, further improvements in efficiency associated with any particular cyclone configuration must be verified through actual testing. Indeed, some proposed designs that were believed in principle to mitigate uneven flow patterns and localized eddy formation actually performed quite poorly in laboratory experiments. Even sophisticated computational fluid dynamics computer software has been found in some cases to be a poor predictor of TSS separation efficiency. Therefore, through extensive trial and error, coupled with the overall objective of refining the cyclone internal flow pattern, a significant improvement in fine particle separation from gas streams has been achieved.
SUMMARY OF THE INVENTION
The present invention is an improved cyclone for the separation of solid particulates from a gas stream. Many of these cyclones can be combined in a vessel for use as a third stage separator in the treatment of solid-contaminated gas streams, and in particular flue gas from a refinery fluid catalytic cracking unit or other solid-contaminated gas streams. The cyclone provides a high separation efficiency because a particulate-laden gas vortex is established and travels through the device with minimal flow pattern distu
Hedrick Brian W.
Sechrist Paul A.
Hopkins Robert A.
Paschall James C.
Tolomei John G.
UOP LLC
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