Stoves and furnaces – Solar heat collector – Energy concentrator with support for material heated
Reexamination Certificate
2002-09-04
2004-12-07
Cocks, Josiah (Department: 3749)
Stoves and furnaces
Solar heat collector
Energy concentrator with support for material heated
C126S634000, C250S435000, C422S258000
Reexamination Certificate
active
06827082
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to reaction chambers having ingress and egress openings, and a surface to be protected from contact with components of the reaction, in particular, for use in solar energy receivers for the protection of their transparent windows.
BACKGROUND OF THE INVENTION
Extensive work has been directed in recent years to the development of efficient ways to use concentrated solar radiation as the energy source driving endothermic chemical reactions, such as for example, the production of hydrogen and carbon black by pyrolysis of methane using solar energy for process heat.
One type of a solar reactor which may be used for such a purpose is a “surface receiver” wherein concentrated solar radiation is introduced through the receiver's aperture into its cavity, while the reactants flow through tubes staggered in different arrangements inside the cavity. In such a type of reactor, the radiation is absorbed at the surface of the tubes, and the heat required for carrying out the reaction is transferred through the tubes' walls to reactants flowing inside the tubes. However, such reactors are rather bulky and their working temperature is restricted by thermal limitations imposed by the tube material and the temperature gradient across the tube walls.
In an attempt to overcome these difficulties, another type of solar reactor has been developed, called a “volumetric receiver”. In such a receiver, the reactants are directly introduced into the reactor's chamber where they themselves are exposed to direct concentrated solar radiation that enters the chamber through a transparent window. The use of such reactors elimates the need for incorporating tubes, whereby the overall heat transfer efficiency of the process is increased. An example of such a solar volumetric receiver, designed for solar heating of compressed air, was described by J. Karni et al., Proc. ASME/JSME/JSES Int Solar Eng.Conf., 1: 551-556, 1995.
However, in many chemical reactions some of the reactants and/or products of the reaction are in the form of particles. This fact presents a problem when a volumetric receiver is considered for such a reaction, because particles will eventually be deposited on the surface of the transparent window of the receiver, reducing its transparency. Moreover, the radiation that will be absorbed by these particles will cause their immediate heating up, leading to the generation of hot spots at the window, and consequently to the disintegration of the window.
Many attempts have been made to overcome this problem, and the following are typical examples of such attempts described in the literature.
Litterst (Proc. 6th Inst. Symp. On Solar Thermal Concentrating Technologies, Almeria, 1992, pp. 359-369) experimented with a vertical fluidized bed reactor, having a transparent window mounted on a cylindrical wall of the reactor. Reactants are introduced in the reactor in a primary flow parallel to the window and an air curtain is provided parallel to the primary flow direction adjacent to the window's inner surface to protect it against contact with solid particles. This attempt failed as the thin air curtain adjacent the window's inner surface detached therefrom under the influence of the primary flow of reactants, a short distance from its entry port. An attempt to remove the window from the primary flow by mounting it on a T-type branch did not fair much better. Solid particles were transported in “pulselike eruptions” from the fluidized bed towards the window. An attempt to decelerate fast particles by injecting compressed air through radially positioned tubes near the window's inner surface showed that huge amounts of air, in the order of 50% of the primary reactant flowrate, were required to keep the window free of contact with solid particles.
A cylindrical volumetric solar reactor with a transparent window mounted adjacent a front end of the reactor's cavity and spaced away therefrom by an aperture plane, is described in the Paul Scherrer Institute, Final Report to Bundesamt fur Energie—Contract EF-REN (92) 033, p. 149. Here a suspension of ZnO powder in natural gas is injected into the reactor's cavity in a tangential primary flow adjacent a back end thereof. The products of reaction leave the cavity through a tangential outlet port located at the front end thereof. The window is kept clean of suspended particles by means of two auxiliary flows of gas, one injected tangentially at the window and one injected radially at the aperture plane. The design was optimized to minimze the auxiliary flows while keeping the window clear of particles, however, the total auxiliary gas flowrate was 83% of the primary gas flowrate. Such a high auxiliary gas flowrate can absorb the heat received by the reaction cavity and thereby interfere with the desired reaction.
A solar receiver described in WO 96/25633 comprises an axially symmetric annular chamber with an inner wall constituted by a fiusto-conical or cylindrical quartz window through which solar radiation is admitted into the chamber. A fluid mixture in the form of a particle suspension is injected into the chamber adjacent and tangentially to an end of its outer wall and the products of the reaction are withdrawn near an opposite end of the outer wall and tangentially thereto, whereby the suspension flows around the inner wall in a whirling manner. Due to the centrifugal force acting on the whirling particle suspension, contact between particles and the window is minimized. To cool the window, the inner surface of the window is swept with a particle-free pressurized fluid.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a novel solution for efficient protection of a surface in a reaction chamber.
In accordance with one aspect of the present invention, there is provided a method for protecting a surface at one end of a reaction chamber having a longitudinal axis transverse to said surface, the method comprising introducing a primary flow of reactants into the chamber in a manner whirling around said longitudinal axis, and withdrawing reaction products at an opposite end of the reaction chamber in a flow along the longitudinal axis, whereby said primary flow and said flow of reaction products approximate a free vortex flow, and introducing into the chamber a secondary protecting flow directed from a periphery of said surface towards said longitudinal axis, enabling thereby a pressure created by said vortex flow to keep said secondary flow adjacent said surface substantially over its entire area.
By virtue of the method of the present invention, a negative radial pressure gradient created by the vortex flow increases steeply towards said longitudinal axis and, therefore, towards the center of the surface to be protected, acting as an anchor to pull the secondary flow from the periphery to the center as a boundary layer without separation. This allows for the protection of the surface by the secondary flow with a significantly lower flowrate than that of the primary flow.
A further advantage of the present invention is that the path length of the whirling primary flow across the chamber is substantially extended when compared with the chamber's axial dimension, thereby contributing to achieving higher thermal and chemical conversion efficiencies, since the vortex flow provides strong mixing of the reactants. This mixing effect also helps in preventing strong local temperature gradients in the primary flow, which could lead to flow instability due to buoyancy.
The method of the present invention is particularly useful for reaction chambers wherein a reaction is carried out in which at least one component, a reactant, a product or a catalyst, is in a particulate form. The term “particulate form” as used in the present application denotes primarily a solid material being in the form of powder or particles, but may relate also to materials being in the form of liquid droplets.
The secondary protective flow may be an inert gas, but preferably is one of
Kogan Abraham
Kogan Meir
Browdy and Neimark , P.L.L.C.
Cocks Josiah
Kogan Abraham
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