Method for separating solid particulates from a liquid

Details Method for separating solid particulates from a liquid Method for separating solid particulates from a liquid

1998-05-11

2002-05-14

Drodge, Joseph W. (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S500360, C210S798000, C210S636000, C210S321860

Reexamination Certificate

active

06387271

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention is directed to a method and apparatus for purifying aqueous liquids containing particulate matter and to a filter element, assembly, and method for effecting such purification. More specifically, the present invention relates to the treatment of aqueous liquids employed in power plants, more particularly nuclear power plants, to reduce the amount of insoluble iron present in such aqueous liquids.
Techniques, materials and devices for separating particulate matter from fluids have existed for centuries. Many such separations involve relatively low-level technology and simple materials. As science and technology have advanced, however, new materials and techniques have permitted separations to be achieved to meet the requirements for ever purer materials. Considering the developments in materials science in recent years, when viewed broadly, the separation of particulate matter from a fluid might seem to be a simple task. However, many such separations remain unresolved or, more typically, resolved only to the extent that the results obtained fall short of the purification sought.
Some of the factors which have resulted in less than complete separation include the large volumes of fluids being processed, the type of filtration media available for such separation processes, the nature and chemical composition of the particulate matter being removed, the fineness of the particulate matter and the nature of the fluids in which the particulate matter is found.
To illustrate some of the problems which result from such factors, one could consider any large scale industrial process in which large volumes of liquid are employed. The adverse effects of particulate matter present in the liquids being employed will vary from one process to another. Thus, while particulate matter may be tolerated in any amounts in certain processes, other processes require total elimination of particulate matter. Likewise, the particle size of such solid matter may be of little or no significance in some processes but critical in others. Intermediate these extremes, the specifications of some processes permit certain amounts of solid particulate matter as long as the particles fall above or below predetermined sizes.
Techniques and filtration materials have ranged at the low technology end of the separation spectrum from simple sieves or beds of readily available materials to the other end of the spectrum where new media have been developed to achieve separations of particulate matter from fluids in which the physical and chemical natures of the fluids, particulate matter and/or filtration media are carefully selected to achieve separation. The present invention relates to the latter type of separation.
One area in which removal of fine particulate contaminants is a major concern, is in the field of electric power generation systems. In such systems, which may be fossil fuel powered or nuclear powered, high purity feedwater is heated in a boiler to create either pressurized high temperature water or steam which is then expanded through a steam turbine. The shaft of the turbine is connected to an electric generator shaft which, when rotated, creates electrical energy. The steam which exits from the turbine is condensed in a heat exchanger and, typically, is subsequently purified and reheated. The condensed water is then directed back to the boiler as feedwater, completing a power cycle.
Frequently, the electric power generating plants purify the condensate to remove contamination, particularly ionic materials and particulate matter, which may either be present in the raw water supply, or may enter the feedwater, steam, or condensate from a variety of sources during the power cycle. Ionic materials may be removed by the use of demineralizers, to purify condensate through an ion exchange technique. Two types of demineralizers are used for condensate purification, namely, deep bed demineralizers and filter demineralizers.
Deep bed demineralizers use resin beads to remove dissolved ions in the condensate. Specifically, the condensate is passed through a bed of resin beads which are retained in a demineralizer vessel. The deep bed demineralizers typically have an effective pore rating in the condensate water of about 40 to 50 microns and are only marginally useful in removing particulates from the condensate.
Filter demineralizers employ powdered ion exchange resins and/or inert filter aids such as cellulosic fibers which are precoated onto fine porous elements and are sometimes referred to as “precoats”. Such elements typically include spirally welded metal elements, powdered metal elements, wedge wire elements and yarn or string wound elements. The condensate is passed through the precoated elements which remove dissolved contaminants and trap particulates. The precoats on the filter demineralizers typically have an effective pore rating of about 5 to 30 microns, and the underlying filter media have a pore rating of about 5 to 120 microns. The filter demineralizers have an overall effective pore rating of about 5 to 30 microns and are, therefore, somewhat more effective in removing particulates from condensate, as compared to deep bed demineralizers. Under certain operating conditions, however, solids levels are relatively high and lead to the need for extensive backwashing of the precoat resin, with the concomitant high cost of operation of the filter demineralizers.
Contaminants in feedwater, steam and condensate in fossil fuel-powered generating plants typically must be maintained at a level of total suspended solids of no greater than about 50 to 250 parts per billion (ppb), and most typically no more than about 50 ppb total suspended solids. In nuclear power plants tolerances for solid particulate contaminants are frequently much lower, typically about 0.025 to about 2.0 ppb. Certain types of contaminants, such as iron-containing contaminants are tolerated even less in nuclear power plants, particularly of the Boiling Water Reactor (BWR) type.
The nuclear power industry has recognized that as an important first step in reducing radiation fields and occupational exposure, it is necessary to reduce iron input to the BWR primary system. For feedwater iron, the recognized optimum concentration is not more than about 0.1 to 0.5 ppb. Several techniques have been employed to reduce feedwater iron levels for optimized water chemistry using condensate filtration.
Typically, iron found in the reactor vessel enters by way of the feedwater system and deposits on fuel cladding surfaces where soluble reactor water impurities, such as cobalt, are absorbed. Subsequently, the absorbed metal impurities become neutron activated and a portion thereof are later released to the reactor water as soluble or insoluble radioactive isotopes. Thereafter, they are transported by reactor water throughout the primary system and accumulate on piping and equipment surfaces, resulting in increased dry well and reactor building general area dose rates. Thus, by reducing the concentration of iron in the primary system, the amounts of deposited iron and non-radioactive cobalt (
59
Co) absorbed by the iron is reduced, resulting in a reduced source of activated
60
Co.
Although methods have been developed to limit radiation buildup on primary system surfaces, there has developed and still remains a strong need to limit feedwater iron concentrations. Thus, a technique has been employed recently in which zinc is injected into the feedwater to maintain a specified concentration of soluble zinc in the reactor water which thereby limits radiation buildup on primary piping components. In a modification of the technique which optimizes control of radiation buildup, a depleted zinc oxide method was developed to minimize the unwanted production of radioactive
65
Zn from neutron activation of
64
Zn. When the concentration of feedwater iron is high, however, the amount of depleted zinc oxide necessary to maintain the desired reactor water concentrations of zinc is increased due to the absorption of zinc

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