1. Field of the Invention
This invention relates to a method and an apparatus for separating fine particles from a carrying gas by accelerating the carrying gas to a supersonic speed and impacting it against a virtual impactor so as to form a shock wave at the surface of said virtual impactor and collecting the fine particles that pass through the surface of the virtual impactor in the interior chamber of the virtual
2. Description of the Prior Art
The separation and collection of fine particles from gases is of intense interest in material sciences where fine particles may have unique and valuable properties. Often such particles are produced along with byproduct vapors in high temperature environments. It is frequently of importance to preserve the purity of the particles by separating them from the byproduct vapors which may condense on them if the particles are simply filtered.
The separation and collection of fine particles from gases is also important in preventing such particles from entering the atmosphere as an unintended consequence of manufacturing or power generation processes, as in the manufacture of cements and the fly ash produced from coal-fired electrical generators. Large investments are made in filtration systems and/or high voltage electrical devices to separate and collect the fine particles that can cause pollution.
The separation and collection of fine particles from gases is also important in research on atmospheric aerosols and particulates and in preparing powders comprising particles of uniform size for such applications as advanced materials processing.
Prior art devices known as virtual impactors are often used to classify fine particles suspended in carrying gas according to their sizes. In these devices, a subsonic gas stream containing particles is caused to impinge upon a surface containing an aperture. The flow of the impinging gas/particle stream into the aperture is controlled in such a way that only a small fraction of the impinging flow passes into the aperture. The majority of the gas in the stream and the small particles which follow the gas flow are forced to turn away from the aperture. Larger particles, with greater momentum, cannot negotiate the turn and follow the smaller (minority) flow into the aperture. The minority flow and the larger particles are carried through the aperture to a collection device for the particles, such as a filter. The majority flow and the smaller particles are passed into a separate collection device. Very accurate and balanced control of the majority and minority flows is used to determine the size cutoff between particles passing through the aperture with the minority flow and those which follow the majority flow. Successive stages of virtual impactors may be used to further classify the smaller particles in the majority flow. The smaller the particles, the greater their tendency to follow the gas flow, and thus the more difficult they are to classify, requiring ever more accurate flow control and geometric tolerances.
Another kind of prior art virtual impactor, called a counterflow virtual impactor, is used to attain closer control of the size cutoff between particles collected from the minority flow and those retained in the majority flow. In the counterflow virtual impactor, a particle laden gas flow is caused to impinge upon a surface containing an aperture as described. In this case, the apertured surface is formed by a solid plate joining together two concentric tubes. The outer tube has a solid wall and the inner tube has a porous wall for a short distance near its end joining the solid plate. The inner tube forms the aperture and the solid plate joining it to the outer tube forms the solid surface of the virtual impactor. Gas is supplied to the annular space between the tubes and passes through the porous part of the inner tube into the space behind the aperture. A suction is applied to the end of the inner tube away from the aperture. Part of the gas added through the porous wall is drawn into the suction and part flows toward the aperture. Because of this difference in flow direction, there is a plane in the porous tube at which the added gas flow has zero velocity along the axis of the porous tube. This plane lies within the porous tube behind the aperture and solid plate which form the surface of the virtual impactor. Particles impacting the apertured surface either penetrate the aperture or are turned aside depending on their size and velocity. The counterflow of gas from the porous tube provides an additional selection method by forcing those particles which are collected to travel some distance within the porous tube in order to pass the plane of zero gas velocity. Those which do pass this plane are entrained in the gas flow drawn by the suction and may be collected by a device such as a filter. Those particles which do not reach the zero velocity place are expelled to rejoin the majority flow turned aside at the virtual impactor surface. The gas flow through the porous tube may be varied in order to move the position of the plane of zero velocity, thereby selecting the penetration distance of the particles which are collected. The penetration distance depends upon the particle size, and thus selecting the penetration distance is effective in selecting the size cutoff of the particles collected--for a given penetration distance, particles larger than a certain size will be collected while smaller particles will be rejected.
There is some limited discussion in the prior art patent literature with regard to supersonic flows in the context of virtual impactors, however, the purpose and function of those flows is entirely different from the subject matter of the present invention. For example, U.S. Pat. No. 4,806,150 entitled DEVICE AND TECHNIQUE FOR IN-PROCESS SAMPLING AND ANALYSIS OF MOLTEN METALS AND OTHER LIQUIDS PRESENTING HARSH SAMPLING CONDITIONS by Joseph L. Alvarez and Lloyd D. Watson discloses the use of supersonic flows in a device that includes a virtual impactor. The purpose of the supersonic flow is to break up injected liquid and to provide more effective cooling of the particles in the flow than is found in subsonic atomizers. There is no teaching or suggestion of impacting the supersonic flow against a virtual impactor so as to provide for particle separation or sizing.
U.S. Pat. No. 5,021,221 discusses a solid plate impact separator utilizing the impingement of a particle laden supersonic stream onto a solid plate for the separation of fine liquid particles from gases That device is incapable of separating solid particulates from gases because it requires the material being collected to "stick" when it strikes the surface, so that it, for example, forms a thin liquid film on the surface of the solid impactor plate to aid in sticking of newly arrived liquid particles. In addition, the disclosure in the cited patent does not teach or suggest collection or separation of solid particles. The present invention will separate either solid or liquid particles from gases, but the separation and collection of solid particles is the preferred use.
Similarly, Russian Patent SU 693 162 discusses an impactor that uses supersonic flows to collect small particles on a side wall, but otherwise appears to be irrelevant to the subject matter of the present invention.
The following patent references appear to be of lesser relevance: U.S. Pat. Nos. 2,793,282; 3,077,307; 3,430,289; 3,602,595; 3,659,944; 4,301,002; 4,670,135 and 4,767,524.
A useful discussion of the physical principles relevant to the present invention is set forth in the following reference texts: THE DYNAMICS AND THERMODYNAMICS OF COMPRESSIBLE FLUID FLOW, VOL. I AND II, by A. H. Shapiro (Ronald Press, N.Y. 1954) and HYPERSONIC FLOW THEORY by W. D. Hayes and R. F. Probstein (Academic Press, N.Y. 1966).
It was in the context of the foregoing prior art that the present invention arose.