Heat exchange – Plural casing-conduit units – line or common header connected
Reexamination Certificate
1999-02-23
2001-03-27
Atkinson, Christopher (Department: 3743)
Heat exchange
Plural casing-conduit units, line or common header connected
C165S108000, C210S609000
Reexamination Certificate
active
06206091
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to a method and apparatus for the treatment of waste solids, and more particularly to a method and apparatus for the treatment of waste solids that allows for the transfer of heat between hot waste solids and cold waste solids.
BACKGROUND OF THE INVENTION
Known methods of processing waste solids such as sewage sludge often involve the use of thermal energy or the production of thermal energy during one or more stages of a treatment process. For example, thermal conditioning may be used in such a manner as to improve the dewaterability of the sludge so that the solid and liquid phases are readily separated, as by sedimentation, filtration, and centrifugation. See, for example: U.S. Pat. No. 3,256,179 issued to Teletzke et al.; U.S. Pat. No. 3,697,417 issued to Teletzke et al.; and U.S. Pat. No. 3,824,186 issued to Erickson et al.
Other methods have been used to obtain heat by the biological decomposition of organic substances and to treat sludge by sterilization. See, for example: U.S. Pat. No. 4,493,770 issued to Moillet, and U.S. Pat. No. 3,959,125 issued to Teletzke.
For economic and environmental reasons, it is desirable to recover and reuse heat produced or used during the treatment of solid waste. Various methods and systems have been proposed to recover such heat energy. Traditional heat recovery systems often used an indirect two step heat transfer process wherein the first step involved transferring heat from treated hot solid waste to water in a first heat exchanger. In a second step, the heat in the water is then transferred to untreated cold waste solids in a second heat exchanger. Such systems are complicated, expensive, and inefficient.
Some heat recovery systems may employ a stainless steel tank placed inside of a larger stainless steel tank wherein each tank acts as a solid waste batch storage tank while heat is being transferred through the wall between the two sludge tanks. The heat is being transferred between the hot solid waste and cold solid waste without the use of an intermediate water media. Often, mechanical mixers are used to agitate the solid waste in each tank to facilitate the heat transfer. These heat recovery systems have very high fabrication costs, are very complex, have high ceiling requirements, and are difficult to maintain and clean. Additionally, the mechanical mixers have the further drawback of becoming clogged over time. As the mixers become clogged or as sludge scale coats the walls of the chambers, it becomes necessary to disassemble and clean the tanks to restore adequate heat transfer efficiency. Disassembly shuts down the whole system.
Spiral heat exchangers have also been used in sludge to sludge heat transfer applications, but present the problems of clogging and high maintenance requirements.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to overcome the shortcomings of the prior art by providing a method and apparatus for treating waste wherein heat can be effectively transferred from hot solid waste to cold solid waste without the use of an intermediate water media.
The invention provides a method and apparatus wherein a heat exchanger such as a tube-in-tube heat exchanger is used for transferring heat between one volume of sludge to a second volume of sludge. In one application, cold solids waste is moved or passed through one tube of a tube-in-tube heat exchanger, and hot solids waste is moved or passed through the annular space (i.e., the other tube) of the tube-in-tube heat exchanger, and heat is transferred from the hot solid waste to the cold solid waste. Although tube-in-tube heat exchangers are known for use in other, different applications, Applicant believes that they have never been used for a solid waste to solid waste heat transfer application as in the present invention. It is thought by Applicant that heat exchangers, particularly tube-in-tube heat exchangers, have never been used for a solid waste to solid waste heat transfer application because such an application was believed to be unworkable by those skilled in the art.
One aspect of the invention includes a method of treating waste. The method includes the acts of: (a) providing a cold waste solids source for maintaining cold waste solids, a hot waste solids source separate from the cold waste solids source for maintaining hot waste solids, and a heat exchanger (e.g., a tube-in-tube heat exchanger) remotely located from and connected to the cold waste solids source and the hot waste solids source; (b) passing a volume of cold waste solids from the cold waste solids source through the heat exchanger; (c) passing a volume of hot waste solids from the hot waste solids source through the heat exchanger to transfer heat from hot waste solids to cold waste solids; and (d) passing at least a portion of one of the volumes of cold and hot waste solids passed through the heat exchanger a second time through the heat exchanger to transfer heat from hot waste solids to cold waste solids.
Another aspect of the invention is to provide for a method for treating waste as discussed above that can be performed in a continuous manner.
It is another object of the invention to provide for an apparatus that may be used in a continuous treatment process.
It is another object of the invention to provide an apparatus for treating waste including a tube-in-tube heat exchanger that can be used for a direct solid waste to solid waste heat transfer application.
It is yet another object and feature of the invention to provide a method for treating waste using a heat exchanger wherein a volume of cold waste solids and a volume of hot waste solids are passed through separate conduits, channels or tubes of the heat exchanger, and wherein at least a portion of one of the volumes of waste solids (hot or cold) is passed through the heat exchanger more than once and, preferably, at least twice. In one embodiment of the invention, the cold and/or hot waste solids is passed through the heat exchanger and then returned to the cold and/or hot waste solids source. This may be referred to as recirculating the waste solids.
Several objects and advantages are realized by passing waste solids through the heat exchanger at least twice and/or to recirculate waste solids between the heat exchanger and the waste solids source (i.e., returning the waste solids to the waste solids source). One object realized is to be able to vary heat transfer capacities of the method and system using a single size heat exchanger. Another object is to minimize the heat transfer area and the size of heat exchanger required. This results in a smaller heat exchanger that will typically require a lesser number of return bends and, thus, require less maintenance. Further, the smaller heat exchanger will cost less than the typical heat exchanger.
One specific operational advantage of the method described above is that the waste solids may be recirculated for different periods of time in order to vary the temperature of waste solids. In one application that is explained in greater detail below, at least a portion of the waste solids is passed through the heat exchanger multiple times before being passed into a thermal conditioning tank, an anaerobic digester or other downstream process vessel or location. Such flexibility is not achievable with a one pass-through type system since the temperature of the waste solids is determined after the first pass through the heat exchanger. Further, the temperature of the waste solids passed to the anaerobic digester or other process vessel may have been varied to account for seasonal fluctuations and heating requirements. However, because the “multiple pass” or “recirculating” heat exchanger system is capable of obtaining varying heat transfer capacities, only one heat exchanger is necessary.
In a more specific example of an operational advantage attainable with the present invention, additional cooling and/or heating equipment is not required for the anaerobic digester. For a one-pass heat exchanger, the tempera
Atkinson Christopher
Michael & Best & Friedrich LLP
United States Filter Corporation
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