Gas separation: processes – Solid sorption
Reexamination Certificate
2003-04-14
2004-10-19
Lawrence, Frank M. (Department: 1724)
Gas separation: processes
Solid sorption
C095S273000, C095S285000, C055S524000
Reexamination Certificate
active
06805727
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to apparatus and methods for removing contaminating mists and particulates from gases in which such agents are dispersed, and more specifically relates to filtration devices and methods for removing oil mists and other organic vapor mists, as well as other solid and liquid particles from airstreams in which they are dispersed and/or with which they form an aerosol.
BACKGROUND OF INVENTION
Oil mist and organic vapor mists are very difficult to capture once airborne. By the term “mist” is meant a dispersion in gas (typically air) of liquid droplets sufficiently small to form a true suspension under ambient conditions, as well as dispersals of droplets above such size. Many industrial operations generate such oil mists, airborne hydrocarbon mists and volatile organic compound (VOC mists), resulting in harmful and costly pollution. Such pollution can affect indoor/outdoor air quality and create odor problems, generate HVAC/heat exchange fouling, cause gross contamination of roof-tops, and generate hazardous stormwater run-off and ground water contamination. Facilities experiencing these problems include metal working operations (machining and tooling), surface coating operations (solvent-based industrial painting), auto manufacturing operations, and oil refining and chemical production operations. Similarly compressors or the like associated with HVAC systems of buildings or even sea vessels often introduce oily mists into the HVAC system, and ultimately into the ambient environment.
In addition to liquid droplets of the type discussed above, solid particles dispersed in gases such as air, represent further common contaminants which contribute to pollution, and are often sought to be removed. Such solid particles are often present along with separate liquid droplets or particles. In other instances the liquid and solid particles can be present as combined entities, e.g., where the solid particles are enveloped or coated by the liquid.
A more general concern arises where one is concerned with filtration of complex waste streams where oil mist and suspended solid particles are present simultaneously, either as or with true aerosols and in the presence of suspended particulate matter which has been rendered static dissipative and/or neutral, which reduces the strength of the van der Waal's affinities. Conventional adsorbents and filter media rely heavily on such van der Waal's forces and other so called weak molecular interactions in order to perform as expected.
Conventional treatment methods of air streams contaminated with low concentrations of volatile organic compound (VOCs) mists, hydrocarbon and/or oil vapor mists, are expensive, complicated and difficult to operate. Yet many industries generate such air streams which must therefore be treated to meet the requirements of the Montreal Protocol and Clean Air Act Amendments of 1990. Conventional treatment technologies for removal of low mist concentrations from air streams include intense physical or chemical process such as multi-layer adsorption, catalytic conversion and photolytic degradation. Attempts have been made using bioscrubbers and/or an air phase bioreactors to remove hydrocarbons, but they are only able to achieve up to 75% removal because of flowrate/contact time issues, and performance varies significantly depending upon superficial gas velocity, gas inlet concentration, mass transfer coefficients and biokinetic constants.
In 1995, to comply with new EPA standards for organic air emission for hazardous waste treatment, storage and disposal facilities (TSDF's) and hazardous waste generators, Lawrence Livermore National Laboratory (LLNL) designed an organic removal and destruction treatment train to modify its existing wastewater treatment tank farm and achieve 95% TOC reduction. It consists of an air stripper, high-efficiency particulate air filter (HEPA), catalytic oxidizer, scrubber and mist eliminator.
The original high performance filters referred to as HEPA (High Efficiency Particulate Arrestors) were developed during the Manhattan project to prevent discharge of radioactive particles. Since then they have become the preeminent technology when high efficiency filtration is required in industry, medicine, military applications and more recently for household filtration devices. HEPA filters are composed of submicron glass fibers. A HEPA filter by definition will have a 99.97% particle removal efficiency for all particles of 0.3 micron diameter. HEPA filters exhibit higher efficiency for both smaller or larger particles. HEPA filters have high pressure drop performance characteristics and usually require a prefilter for optimum performance.
Similarly, work has been done with granulated activated carbon (GAC), but it has numerous well-known drawbacks such as clogging, re-release and absorption capacity. Utilizing zeolites in multi-layer design with GAC, while generally effective, is expensive and multi-layers cause large pressure drops (&Dgr; P) across filters based upon these combinations.
For purposes of this specification the term “dispersed” in reference to any of the several above types of particle/gas systems means that the mist, or solid particles etc. are dispersed in a gaseous phase such as air. The term “aerosol” can be defined as a system of solid or liquid or liquid-enveloped solid particles suspended in a gaseous medium, the particles having a negligible falling velocity.
Aerosols are further characterized herein by having liquid or solid (or combined liquid/solid) particles of less than 100 microns diameter. In a typical distribution, at least 40% by weight of the particles are less than 0.2 micron. At this size, assuming low surface charge which inclusion of organic and ionic compounds will promote, these droplets and/or particulates will stay buoyant in air indefinitely. The strength of the surface tension in a droplet is more than 100,000 times the mass of the droplet. Unless the droplet is opened, the pollutants entrapped inside remain inaccessible. It is been shown that when liquid aerosols are recirculated through a filter the droplet size distribution becomes much narrower and tends towards the smaller micron ranges. In other words if a liquid aerosol is not adsorbed onto a filter surface, smaller more robust droplets are formed with much higher surface tension to mass ratios.
Aerosols are formed spontaneously under ambient conditions in multiple ways. Particulate matter provides nucleation centers around which liquids and organic compounds self-assemble. Human activities like talking, breathing, and sneezing create liquid aerosols. This is the vector for transmission for all cases of tuberculosis. Movement provides particulate nucleation centers and household systems which move air create oily aerosol droplets. Other pollutants spontaneously coalesce and adhere to these initial aerosol droplets.
Air pollution in fact exists in many forms and each of these have different surface characteristics and affinities. A somewhat more complete picture of the complex interactions which occur in air is as follows:
Primary mechanisms of air pollution formation include:
1. Formation of volatile organic compounds through evaporation and incomplete combustion.
2. Biological generation of volatile organic compounds.
3. Formation of liquid aerosol droplets through shear and turbulence related processes, from cooking and household solvent based cleaners.
4. Anthropomorphic generation of particulate aerosol.
5. Biological formation of particulate aerosols through decomposition and spore formation.
6. Particulate aerosol formation from geological and weather based phenomena.
Secondary mechanisms of air pollution formation include:
1. Condensation—Liquid aerosol droplet formation due to condensation of gaseous phase components.
2. Adsorption—Adsorption of gaseous phase and liquid aerosol components onto particulate surfaces resulting in particulate aerosols with modified surfaces from adsorption of organic compounds and hydrous inorganic phase
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