Gas separation: processes – Deflecting – Centrifugal force
Reexamination Certificate
2001-04-11
2003-03-18
Hopkins, Robert A. (Department: 1724)
Gas separation: processes
Deflecting
Centrifugal force
C055S345000, C055S419000, C055S457000
Reexamination Certificate
active
06533844
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to separation of two phases from each other, as well as an assembly suited for implementing said method. In particular, the invention concerns a method according to the preamble of claim 1 for separating solids and/or liquids from gas flows or, respectively, for separating solids from liquid flows. According to the present method, a gas-phase flow carrying, e.g., a catalyst or other solids or a liquid phase is passed to a separating means, wherein the other phase is then separated from said gas phase under the effect of a centrifugal force. To separate solids from a liquid-phase flow, the liquid flow is passed in a similar manner to a separating apparatus, wherein the solids are separated from the liquid under the effect of a centrifugal force.
The invention also relates to an apparatus according to the preamble of claim 15, suitable for separating solids and/or liquids from gas/liquid flows in fluidized-bed equipment.
Embodiments of highest commercial value used for separating two phases from each other are fluidized-bed reactors. Generally, fluidized-bed reactors are employed in the hydrocarbon conversion processes and energy generation. In these apparatuses a catalyst or similar solids-containing material capable of improving heat transfer or material fluidization is kept in a fluidized state by means of a gas-phase hydrocarbon or flue gas flow. Subsequently, the solids are separated from the gas flow by means of a cyclone.
The most generally used fluidized-bed reactor is a bubbling-bed reactor in which the linear flow velocity of the fluid medium is typically from 5 to 10 times the minimum fluidization velocity that can maintain the main portion of the solids in the fluidized bed of the reactor, whereby no significant amount of the solids can escape the reactor along with the hydrocarbon and flue gas flow. The term minimum bubbling velocity is defined as the linear gas flow velocity at which a portion of the gas flow begins to pass through the bed in the form of bubbles. This minimum bubbling velocity is dependent on the properties of the fluidizing gas and the solids involved.
When the gas flow velocity is increased above the minimum bubbling velocity, the top of the fluidized bed becomes less defined, in fact, being transformed into a gradient zone in which the solids content decreases upstream. At sufficiently high flow velocities, a fluidized flow is achieved in which practically all particulate solids are entrained in the gas flow that keeps up the fluidized state. Then, the solids separated by cyclones from the gas flow must be returned to the bottom part of the reaction space in order to maintain the mass balance unchanged.
As mentioned, the method and assembly according to the invention can be used, e.g., in processes employed for treating hydrocarbons. Examples of such processes include catalytic and thermal cracking, dehydrogenation, Fischer-Tropsch synthesis, manufacture of maleic acid anhydride and oxidizing dimerization of methane.
An application of the fluidized-bed reactor commonly used in energy generation is a boiler in which the fluidized material such as sand and/or solid fuel particles are fluidized with the combustion air flow and the flue gas released in the process. Also a liquid- or gas-phase fuel can be used. Circulating fluidized-bed (CFB) reactors of both the bubbling fluidized-bed and the entrained fluidization technique are conventionally used in the art. In these, the solids and unburnt particles are removed from the flue gas flow by means of cyclones. In this context, the term entrained fluidization refers to fluidization which takes place in both the turbulent fast fluidization range as well as the pneumatic transport range.
Hydrocarbon conversion processes are run using fixed-bed reactors and fluidized-bed reactors (fluidized catalytic reactors). In the present context, the term “fluidized catalytic process equipment” is used to refer to equipment used in processes having a fine-grained pulverized catalyst suspended, e.g., in a slowly upward rising gas flow, wherein the catalyst promotes the occurrence of desired reactions.
One of the most widely employed fluidized-catalyst reactor systems in the art is the FCC equipment, that is, fluidized-catalyst cracking equipment, comprising chiefly a riser pipe acting as a reactor operated in the fast-fluidization flow state and a regenerator operated in the dense-phase bubbling bed state.
In fluidized-bed reactors, the particulate matter of the suspended solids and the product gas are separated from each other in cyclones utilizing the effect of the centrifugal force. Typically, a number of cyclones must be connected in series along the gas flow in order to improve the overall collection efficiency, because single cyclones of normal construction exhibit inferior separation capability for particles smaller than 15 &mgr;m. Herein, a cyclone is rated effective if it can separate these small-diameter particles from the gas flow.
In addition to applications related to fluidized-bed reactors, cyclones are also used for, e.g., separating liquid droplets in steam systems, solids from flue gases of drying processes, phase separation on two-phase flows (demister equipment), separation of solids from gases (dust separators) and as hydrocyclones serving in the coarse separation of solids from waste waters.
Cyclone separators have either a coiled or spiralled structure in which the particulate matter suspension is directed as a tangential flow into the cylindrical section of the cyclone, whereby the catalyst particles are driven apart from the gas to a close distance of the cyclone inner wall when the flow typically circulates about 7-9 revolutions within the cylindrical section of the cyclone and the conical section forming a continuation thereof. Also axial cyclones are known in which the gas flowing through a pipe is forced into a circulating motion by means of vanes, whereby the solids under the centrifugal force are driven against the pipe wall and separated thereon from the gas flow.
The most common cyclone type is a single-port spiralled cyclone called the Zenz cyclone, in which the proportions of the different parts of the cyclone are standardized, thus permitting the dimensioning of the cyclone to be based on graphs and computational formulas. The collection efficiency of this cyclone can be enhanced by a large number of flow revolutions in the cyclone chamber, high flow rate at the inlet nozzle, higher density of solids, narrower inlet nozzle port and lower viscosity of the gas.
In the preseparation cyclone of a fluidized-catalyst cracking unit, tests have shown the gas residence time to be in the order of 1.0-2.0 s from the riser top to the cyclone outlet, after which the catalyst will further stay in the separation vessel at an elevated temperature for 5-40 s. During this time, valuable compounds will be lost as a consequence of thermal reactions. Resultingly, gasoline products will be converted by thermal cracking into combustible gases, particularly hydrocarbons of the C
2
type. Other byproducts of thermal reactions are dienes, such as butadienes, which in the alkylation unit cause a significant increase in the acid consumption. Pentadienes in turn are particularly reactive, whereby their detrimental effect is evidenced as a reduced oxidation resistance of FCC gasoline. Further problems hampering the use of conventional FCC units are related to their poor control of reaction time and the erosion of the catalytic particles/circulating solids and the reactor structures.
The problems are mostly related to such essential parts of the equipment as the separation units of gases from solids/catalysts, that is, cyclones, which in most cases are implemented as single-port units. Herein, the term single-port cyclone refers to a cyclone construction having only one inlet nozzle for feeding the gas flow into the cyclone. To achieve the desired through-flow capacity, a plurality of these units are generally connected in parallel and then two or three in series.
Eilos Isto
Hiltunen Jyrki
Niemi Vesa
Fortum Oil and Gas Oy
Hopkins Robert A.
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