Liquid purification or separation – Flow – fluid pressure or material level – responsive – Filter cleaning
Reexamination Certificate
1999-11-03
2003-10-28
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Flow, fluid pressure or material level, responsive
Filter cleaning
C210S110000, C210S143000, C210S192000, C210S203000, C210S253000, C210S277000, C210S284000
Reexamination Certificate
active
06638422
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for filtering particulates of various sizes from miscellaneous process liquids, and more particularly to an apparatus and method that utilizes a filtration bed formed from particles having a specific gravity lower than that of the liquid being filtered.
2. Description of the Prior Art
A preliminary patentability and novelty search regarding the invention described herein has revealed the existence of the following U.S. Pat. Nos.:
3,067,358
3,469,057
3,678,240
3,962,557
4,387,286
4,417,962
4,839,488
4,952,767
5,434,381
5,386,094
A careful review of the patents noted above has failed to reveal the concept, apparatus and method disclosed herein.
The need to remove particulates, whether contaminants or products, from process liquids is common to a wide range of processes. One such process is the necessity to filter metal particles from the solution used during electrical discharge machining. In the following description, the focus will be on the removal of particulate contaminants from such a solution, however, the same mechanisms can be applied to the filtration and harvesting of particulate materials which form the product(s) of a process. Although a variety of methods have been developed to remove particulates from such process liquids, the most popular method is media filtration. In media filtration, particulate contaminants are strained from the process liquid in one of two ways, either by pumping the contaminated liquid through a unitary permeable element, or by pumping the liquid through a filter bed which is itself composed of small particles.
In permeable unitary element filtration, the liquid is pumped through an element which has pores or channels that allow the liquid to pass through the element but prevent the passage of particulates larger than the pore/channel diameter. Permeable elements comprise a variety of materials, including fabric, paper, ceramic, metal and plastic. These elements filter the liquid primarily by capturing the contaminant particles on the surface of the element, thus building up a crust or layer of contaminants on the surface. As contaminants accumulate on the surface of the element, liquid flow through the permeable element is reduced because the crust or layer of contaminants acts as an obstruction and because an increasing number of the pores or channels become blocked. As the percentage of blocked pores or channels increases and the crust or layer of contaminants becomes thicker, the pressure required to maintain a specific rate of flow of liquid through the permeable element increases. Eventually, the pressure required exceeds the capability of the pump, or some other system component, and the contaminated element must be replaced with a new element in order to maintain the desired performance of the filtration system.
Alternatively, an attempt may be made to clean the filter element by backwashing it with clean liquid or air to remove the contaminant accumulated on the surface. However, even when the contaminant accumulation on the surface of such an element is removed by backwashing, there are usually contamination particles that remain lodged in the permeable element that backwashing is not totally successful in removing. Ultimately, the element must either be replaced with a new element or cleaned in a more rigorous fashion, i.e., by immersion in an acid or base solution to dissolve the contaminants. The more frequently such stringent cleaning or element replacement must be performed, the more costly this filtration process becomes.
In contrast, the second type of media filtration, namely, bed filtration, uses a filter bed composed of small particles such as sand or diatomaceous earth, and is one of the most common conventional methods of removing particulate contaminants from liquids. The sand filter uses sand particles that are about 0.35 mm in diameter and fairly uniform in size. Diatomaceous earth filters use a siliceous material formed from the skeletons of small (about 100 microns in diameter) marine algal cells called diatoms. Nominally, in a conventional bed filter, the process fluid is pumped, or allowed to flow via gravity, downward through a column, or bed, of material. This column may range from approximately one foot to several feet in thickness. As the particulate-laden liquid passes through the bed, the particulates are strained from the liquid and the cleaned liquid exits at the bottom of the bed.
The bed filter removes the particulate contaminants via one of two processes. First, the larger particulates, which are unable to pass through the spaces between the bed grains, are trapped at the top surface of the bed. This straining effect produces a layer, or crust (also called a cake), of large contaminant particles, which builds up on the surface of the bed, a mechanism called surface filtration. This cake can actually enhance the performance of the filter bed by helping to capture more contaminant particulates, which are retained in the crust itself because they cannot pass through the spaces between the contaminant particles which form the crust.
Second, smaller particulates which are carried into the bed by the fluid flow are intercepted by the bed's grains as they follow the convoluted flow pathways taken by fluid as it passes through the bed, a process called depth filtration. Although smaller particulates are captured in the bed material, the smallest particulates are not captured, as they continue to flow through the bed and exit with the semi-cleaned liquids at the bottom of the filter bed.
Ultimately, the particulates sequestered by the bed accumulate, making it more difficult for liquid to flow downward through the bed, and thus the flow rate declines. The pressure required to force liquid through the bed then increases, and presents an excellent indication of the growing need to cleanse the bed of the accumulated particulates. Cleansing is achieved by a process of backwashing or backflushing.
During backwashing, clean fluid is vigorously pumped upwards from the bottom of the particulate bed. This upflow of liquid causes the bed to expand slightly, freeing the captured particulates and washing them upwards and out of the bed. As the bed expands, the bed particles have less interference with each other and thus settle faster, matching the upflow rate of the liquid. This effect prevents the bed particles from being washed out of the bed along with the contaminant particulates. Typical backwash conditions are five to fifteen minutes duration with the bed volume expanded 15 to 30%.
Although sand and diatomaceous earth filters have been successfully applied to a wide variety of filtration problems, they have a number of limitations and drawbacks. For example, one of the most serious problems involves bed homogeneity. Non-homogeneous beds, for example, develop cracks that offer regions of less flow resistance in the bed. These cracks then typically enlarge and lead to the formation of channels in the bed, in turn causing poor distribution of the liquid flow through the bed, and thus very low particulate removal. Air may also be trapped in the bed, also leading to the formation of channels and poor distribution of the liquid.
In addition, the size and cleanliness of the bed particles are extremely important to the success of the filtration process; a bed composed of large particles allows significant numbers of small particulates to pass through the filter bed along with the filtered fluid. On the other hand, beds composed of smaller particles can become clogged with extremely small particulates, rapidly rendering the filter bed ineffective. Sand also adsorbs organic compounds on which microorganisms can feed. The highest nutrient concentration is on the surface of the sand granules, so that is where the microorganisms grow. This microbial growth clogs filters and shortens the time interval until cleaning is required
Finally, large volumes of clean liquid are required to backwash and clean conventional
Hruskoci Peter A.
Leavitt John J.
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