Fluid mixing apparatus

Details Fluid mixing apparatus Fluid mixing apparatus

2001-08-29

2003-08-05

Soohoo, Tony G. (Department: 1723)

Agitating

Having specified feed means

Material introduced so as to cause rotary motion in mixing...

C366S340000

Reexamination Certificate

active

06601986

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to methods of static liquid mixing and more particularly to static mixing of liquid systems comprising a carrier fluid and one or more diluents. Such methods of mixing are most suitable for applications in semiconductor industry for dilution or concentration of etching, cleaning, or polishing solutions in semiconductor wafer fabrication.
2. Prior Art
Fluid mixing is employed in numerous applications with the goal to achieve uniformity of various physical and chemical properties such as density, temperature, viscosity, concentration, etc.
Fluid mixing could be accomplished by various methods. These methods may be broken down into three major categories: 1) mechanical agitation; 2) gas bubbling; and 3) static mixing.
Mechanical agitation involves usage of moving parts and therefore the reliability of devices that utilize it is inferior to that of devices utilizing static mixing methods.
Bubbling gases through liquids does not provide uniformity of mixed fluid parameters that could be achieved by other mixing methods.
The present invention relates to static mixing methods and devices. State of the art in static mixing is taught in Chemical Engineering courses, see for example chapter on static mixers in CHEMICAL ENGINEERING by J. M. Coulson and J. F. Richardson with J. R. Backhurst and J. H. Harker, Sixth Edition, Butterworth-Heinemann Publishing House, December 1999, volume 1, pp. 307-310. There, numerous static mixers are described comprising stationary helical blades contained within a pipe. Various combinations of lattices placed within a pipe are also described. Helical blades and lattices serve for cutting and twisting the flow to achieve better mixing. Multiple divisions and recombinations of fluid flow within a static mixer containing the above-mentioned elements (blades and/or lattices) secure homogenous mixing. Identified in that teaching are the most important characteristics of static mixing, namely a) mixing quality measured by the ratio of the standard deviation in fluid composition at a certain stage of mixing to the standard deviation at the mixer inlet; b) pressure drop factor measured by the ratio of pressure drop in a pipe without static mixing elements to the pressure drop in the same pipe but with static mixing elements, c) initial cost, and d) convenience of installation and easy maintenance.
A static mixing device, comprising a plurality of chambers, each chamber having an inlet and an outlet located in the opposite ends of a chamber displaced 180 degrees from each other, is described in U.S. Pat. No. 4,534,659 for “Passive fluid mixing system” issued to Theodore A. Dourdeville and Anthony Lymneos. This simple design could provide low initial cost, low maintenance cost, and low pressure drop, but good mixing quality is difficult to achieve utilizing this device.
In U.S. Pat. No. 4,753,535 for “Motionless mixer” issued to Tony King, a static mixer is disclosed comprising axially overlapping mixing elements that induce counter-rotational angular velocities relative to the axial velocity of moving liquids. This design may contribute to undesirable increase in pressure drop.
In U.S. Pat. No. 5,137,369 for “Static mixing device” issued to John Hodan, a static mixing device is described comprising a stacked arrangement of plates, the latter having channels that split flow of liquid and guide it in the direction generally normal to the primary direction of flow. The mixer is modular in a sense that it comprises a plurality of those plates to achieve the desired mixing effect. The disadvantage of the design lays in high pressure drop and elevated maintenance cost because the above channels should be periodically cleaned to secure consistent mixing quality throughout the operating life of the apparatus.
In U.S. Pat. No. 5,843,385 for “Plate-type chemical reactor” issued to Jeffrey Dugan, a reactor is described, in which static mixing is achieved by a plurality of serially joined chambers containing flow-splitting means. The device is easy to maintain. However, thoroughness of mixing in some applications could turn out to be inadequate.
In U.S. Pat. No. 5,863,129 for “Serial resin mixing devices” issued to Gary Smith, a disclosure is made to a family of inexpensive, easy to manufacture and easy to maintain static mixing devices, in which a multi-component liquid system flows through an elongated mixing chamber, the latter containing cylindrical mixing elements. This device is most suitable for such applications as mixing within spray guns or the like.
In U.S. Pat. No. 5,984,519 for “Fine particle producing devices” issued to Tadao Onodera et al., a device is disclosed where fluid flows through channels forming multiple high-speed streams, the streams colliding with each other creating pockets of turbulence. The device is claimed to be applicable only to conditions where high pressure could be applied to the fluid system.
In U.S. Pat. No. 6,000,418 for “Integrated dynamic fluid mixing apparatus and method” issued to Frederick Kern and William Syverson, a static mixer is described. The purpose of this mixer is to provide mixing means ensuring uniformity of cleaning and etching solutions used in semiconductor industry in the fabrication of integrated circuits on semiconductor wafers. The patented mixer employs multi-port venturi injectors. Such injectors provide easy to maintain and very accurate means of injecting required volumes of liquid chemicals into flow of carrier fluid. Using venturis generally entails higher power consumption if compared with mixers employing only static mixing elements.
A static mixing device comprising a plurality of axially extended helically twisted blades and impact bearing flat surfaces placed within a pipe is disclosed in U.S. Pat. No. 6,164,813 for “Static fluid mixing device with helically twisted elements” issued to Chiang-Ming Wang et al. The disadvantage of this device is in using the axial direction for static mixing, which does not allow proper usage of volume of the device, increasing pressure drop, as well as initial cost and maintenance cost.
It is to be understood that though known static mixers including those described above are very much suitable for their respective intended purposes, they do not achieve simultaneously consistent high quality mixing, low pressure drop, small initial capital investment, and easy maintainability, particularly in the applications related but not limited to thorough mixing of cleaning, etching, and polishing liquids used in semiconductor wafer manufacturing.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide excellent quality of mixing by reducing the ratio of standard deviation of liquid system component characteristic such as concentration of a certain component in multi-component fluid system at the outlet of the static mixer to that at the inlet to the values close to zero. The exact value of the ratio depends upon a specific application.
Another object of the present invention is to provide as low pressure drop as possible for fluid components flowing through a mixer while maintaining high quality of mixing.
Yet another object of the present invention is to reduce the initial capital investment in manufacturing of the static mixer by means of simplifying and standardizing means used for dividing, cutting, and swirling flow of fluids being mixed.
Yet another object of the present invention is to substantially reduce maintenance costs by making modular means used for dividing, cutting, and swirling flow of fluids being mixed and having each mixing module easily cleanable.
One more object of the present invention is to make static mixer easy to assemble and disassemble.
Still one more object of the present invention is to provide means of static mixing suitable for fluid systems comprising Newtonian liquids, such as deionized high purity water, and Non-Newtonian fluids, more particularly pseudoplastic fluids such as slurries and some polymer solutions. Those skil

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