EVAPORATION METHOD AND APPARATUS

Abstract: A new and improved method and apparatus for using the waste heat energy of a gas stream to heat a liquid so as to cause the evaporation of a volatile agent therefrom, the said method including means to prevent the liquid from flowing back with the source of the gas stream. In the illustrated embodiment the gas stream flows from a source (26) through a flue (28) to a channel (17) surrounding an inner chamber (14) of an evaporator (12). Sump means (20) receives and stores any liquid condensing from the gas stream or leaking from the inner chamber (14), so as to prevent the liquid flowing back to the source (26). A dam (22) is provided in the flue (18) adjacent the opening to the sump (20). Emergency discharge means (24) allow rapid draining of the sump (20). The apparatus includes a waste holding tank (10) from which liquid is supplied in a continuous or batch manner to the inner chamber (14) and a collection box (64) for concentrated waste from the chamber (14). Sensors (34,36,44) monitor the liquid level in the chamber (14) and a control system (50) controls the supply of liquid and gas to the evaporator (12).

Description: Background of the Invention This invention relates to heat recovery and more particularly to a method and apparatus for recovering said heat from waste gas streams. One current problem with many industrial processes is the safe and economical disposal of waste water and other waste fluid streams generated by the process. This problem is complicated by the fact that many of these streams are contaminated with one or more dissolved or suspended components which are present in amounts above those allowed by present day federal and/or state regulations. Where such contamination is present, the costs involved with either transporting large amounts of a relatively dilute solution to a treatment facility or of internally concentrating said solution so that the volume of material which must be handled is substantially reduced, are often a significant part of the total costs of the manufacturing process from which the waste fluid streams emanated. Further, many of these contaminants have a substantial economic value which, if they could be economically recovered, their recovery could be used to offset total costs. Accordingly, what is needed is a low cost process which utilizes a source of energy either to evaporate volatile agents such as industrial solvents for recovery, or to concentrate the waste fluid streams so that conventional precipitation, filtration and/or other methods of treatment and disposal can be more efficiently conducted. One such source of energy is the hot waste flue gas, exhaust air, and the like which are generated from industrial/commercial furnaces, ovens, incinerators, and similar devices and which often contain substantial quantities of heat energy. These gases are commonly discharged through convenient duct work (stacks) leading to the environment. While these "stack" gases may have temperatures as high as 800°C (1500°F) or more, this heat energy is frequently lost and abandoned due to the complexity, cost, and low efficiency of many present day methods for recovering and using their heat energy. Furthermore, many of these gas streams are themselves contaminated with large quantities of water vapor and carbon dioxide, as well as smaller amounts of carbon monoxide, unburned hydrocarbons, fly ash, acidic sulfur and nitrogen oxides, hydrogen chloride and other vaporous or solid entrained species, all of which act to render such streams obnoxious, corrosive or abrasive. As a result, direct use of such gas streams is generally avoided as a heat source even in potential applications where the above noted problems could be tolerated. In part, this avoidance can be particularly attributed to the large amount of water vapor frequently present in these gases. If the stack gases are piped directly from their point of origin to some application for recovery and use of their heat energy, the resultant cooling could easily cause at least some of this vapor to condense, resulting in a significant quantity of liquid water flowing back to the source of the heat energy. Such an event could seriously damage furnaces, ovens, or other equipment used to generate the heat. Indeed, in situations where molten metals are involved, contact with even a small amount of liquid water could result in a serious explosion. Further, if the gas is used to heat a quantity of liquid, any leakage from the vessel containing such liquid could lead to the same undesirable result. As a result, most conventional schemes to recover and utilize heat values from stack gases rely on the use of heat exchangers having a pressurized heat exchange fluid to transfer heat from the hot gases for use in another process. Since such a system often requires that the heat transfer fluid be circulated through a second heat exchanger located in contact with the process at the point where the heat energy is to be utilized it is inherently inefficient owing to the multiple heat transfers that are needed for the process to operate. Further, the aforementioned problems with free flowing condensate and/or system leakage are not really alleviated to any great degree and, in fact, may be aggravated since a second, pressurized heat transfer fluid is now involved. In view of these considerations, the routine utilization of waste stack gas as a source of heat energy for industrial use is often ignored or dismissed as impractical. Consequently, industries that deal in the refining, casting, and working of metals such as steel, aluminum, zinc, copper, and the like, frequently discharge substantial quantities of waste heat energy in stack gases with little regard being given to waste heat recovery schemes even for the evaporative purification of waste water which has been contaminated by their basic processes. Similarly, many large incinerators are operated for the purpose of reducing the volume of commercial and domestic wastes through combustion. Here, too, little regard is usually given to utilizing the heat generated by such operations for the evaporative purification of system cooling water or the drying waste sludges either for easier transport and/or resource recovery. Since the cost of treating waste water streams to meet EPA, RICRA and state requirements for the contaminants therein often have a significant impact on the competitiveness of many industrial processes, the advantages of either economically recovering the water or concentrating the waste for treatment are obvious. Further, in view of the number of such sources of waste heat, the magnitude of the annual thermal energy loss resulting by such discharge must be of truly staggering proportions. In addition, the aggregate heat content in such discharge, especially in urban areas, has the capacity to cause severe environmental perturbations. Clearly, advances in the techniques for stack gas heat recovery are required so that this energy resource can be viably applied for beneficial purposes. United States patent US-A-4 131 507 describes an evaporator which utilizes heat from waste gases to heat a liquid to be evaporated. The liquid is fed to an outer chamber surrounding an inner chamber through which the hot gases flow from a duct extending below the inner chamber. A disadvantage of such an evaporator is that liquid which may condense from the hot gases, or liquid which may leak from the outer container, can collect in the duct, with a risk that such liquid might flow back to the source of the hot gases. Object of the Invention It is an object of this invention to provide a method and apparatus for using the waste heat energy of a waste gas stream to evaporate a volatile agent from a waste liquid. Summary of the Invention This invention includes a method for using the waste heat energy of a waste gas stream to heat a waste liquid so as to cause the evaporation of a volatile agent therefrom, said method comprising: (a) inserting the waste gas stream into a container comprising a flue (18) for feeding the waste gas stream from a source (26) of heated gas, an inner chamber (14) to hold the waste liquid, an outer chamber (16) connected to said gas feeding flue (18) and forming a gas channel (17) substantially surrounding said inner chamber (14) through which the waste gas stream flows, and a sump (20) below said container and connected to said gas feeding flue (18) to receive and store any liquid condensing from the gas stream and any liquid leaking from said inner chamber (14), so as to prevent the liquid from flowing back to the source (26) of the gas stream; (b) feeding a sufficient quantity of the waste liquid into said inner chamber (14); (c) venting the evaporated volatile agent from said inner chamber (14); and (d) collecting any condensate from the gas stream and any liquid leaking from said inner chamber (14) in said sump (20). This invention also includes apparatus carrying out the process as defined above, comprising: (a) a flue (18) for feeding the waste gas stream from a source (26) of heated gas; (b) a container having an inner chamber (14) to hold the liquid solution for evaporation of said volatile agent and an outer chamber (16) connected to said gas feeding flue (18) forming a gas channel (17) substantially surrounding said inner chamber (14) through which the gas stream flows to supply heat to said inner chamber (14) and evaporate the volatile agent of said liquid solution; (c) a sump (20) below said container and connected to said gas feeding flue (18) to receive any condensate from the gas stream and any liquid leaking from said inner chamber (14), so as to prevent the condensate and leaking liquid from flowing back through the gas stream feeding flue (18) to the source (26) of the gas stream; and (d) a feed line for feeding the liquid solution into said inner chamber (14); and (e) a flue for combining and venting the evaporated volatile agent from said inner chamber (14) and said gas stream from said outer chamber (16) for discharge. The present invention thus provides a novel method and apparatus which simplifies the recovery of heat energy in stack gases and the like and utilises the energy for evaporation of a volatile agent from a waste liquid. For the purposes of this application, the term "stack gas" is meant to include any waste gas stream from any industrial, commercial, or domestic operation that contains potentially useful heat energy that is normally discharged to the environment. Detailed Description of the Invention A schematic flow chart of one embodiment of the method of the present invention is shown in the Figure. As shown therein, streams of process waste water collected from various industrial process operations such as washing, quenching, dissolution, etc., are collected in waste holding tank 10 for storage. Such waste water streams are typically contaminated with some amounts of dissolved inorganic materials, organic materials, such as cutting oils and solvents, acids, and various suspended materials. In the discussion which follows such waste water is treated either to recover a volatile agent therein or to increase the concentration of contaminants in the waste water for more economical treatment prior to disposal. The stored waste water is pumped into evaporator 12, which basically comprises an inner chamber 14 and an outer chamber 16 containing a channel 17 which substantially surrounds said inner chamber all of which fit into flue 18. Depending on the exact parameters of the process such an input may be either continuous or batch. Associated with chamber 16 is sump 20, which receives and holds any water and/or agglomerated material which condenses out of the gas stream as it passes through gas channel 17. Sump 20 is itself structured to prevent any of said condensate from finding its way back down flue 18 to the source of the gas. A dam 22 is provided to prevent liquid from flowing back along the flue 18 to the source. While materials of construction for evaporator 12, sump 20 and dam 22 are not critical, it should be recognized that many of the fluid waste streams, gases and condensates contain significant quantities of acidic materials which may be quite corrosive. Further, the fly ash and other solid contaminants in the stack gas can, at the temperatures and flow velocities involved, become quite abrasive and cause significant erosive damage to these components. Good design practice will take these matters into consideration. Where there is a more serious problem, such as the sudden influx of a large quantity of solution resulting from a rupture of inner chamber 14, sump 20 is positioned to intercept such influx and is also equipped with emergency discharge means 24 to assure its quick removal from flue 18. Such means can be as small as a counterweighted trap door or check valve which is adapted to open only when a preset weight of fluid has been accumulated in sump 20, with said means automatically closing when sump 20 is drained. Of course, more complex devices can be used should particular circumstances require them. Such devices are well known in the art. In practice, hot stack gases are picked up close to their source of generation by heat collector box 26 with a butterfly deflector flap 28. In the present invention, this is controlled so as to allow at least a portion of the gas to be directed into flue 18 for transit through gas channel 17. Further, deflector 28 can be counterbalanced such that, in the event of a system failure, it will automatically close off flue 18 and thus completely bypass evaporator 12 by discharging said gas stream directly into the ambient environment through chimney 30. This bypass mode is an important feature to prevent overheating of the evaporator and any contents therein when, for example, the liquid has been completely evaporated. Additionally, some industrial furnaces require periodic additions of corrosive fluxing agents. Such heat bypass would normally be used during fluxing to prevent corrosion of the components of evaporator 12 by vaporizing flux. In a preferred embodiment of the apparatus of this invention, evaporator 12 is arranged as a double chamber arrangement. The outer chamber 16 of the evaporator which forms gas channel 17 is essentially a cylinder set on a base plate (not shown). Firm mechanical attachment of the two chambers should be avoided to allow for normal thermal expansion and contraction as the apparatus is operated. Inner chamber 14 receives the liquid to be evaporated while the outer chamber 16 receives the hot stack gas at its base and guides it around the side of inner chamber 14 to permit heat to be transferred to inner chamber 14 and the liquid contained therein. In an evaporation application, the stack gases and vapors from inner chamber 14 combine at the top of evaporator 12 and exit together through a continuation of flue 18. If the vapors are to be condensed as a purified liquid (distillation application) a separate exit (not shown) is provided so that vapors can be directed to an air or water cooled condenser unit (not shown). It is of interest to note that with a water evaporation unit, the combined mixture of water vapor and gases exiting the evaporator could also be passed through a condenser unit before being exhausted so that both the water of evaporation and the water vapor in the stack gas could be condensed and recovered. Further, the heat energy released by such condensation could be itself utilized for some other function such as space heating. In either evaporation or distillation, it is preferred, but not necessary, that the level of liquid in the inner chamber can be controlled so that it does not either overflow due to excessive inflow rates or run dry due to over rapid evaporation. Once such a level is established, the rate of fluid input should be adjusted to a point where a boiling condition can be maintained with the available heat from stack gas alone without the need for an auxiliary heat source. In this manner, an optimum quantity of liquid is present in the inner tank to adsorb as much heat from the gases as possible or practical. This level is maintained by fluid level sensors 34 and 36 which are adjustably fitted into feed line 38 and monitor the fluid level through side arms 40 and 42. These sensors are supplemented by a sensor such as float valve 44 which acts as a fail-safe monitor in the event lower level monitor 36 fails. These monitors along with fluid pump 46 and motor control 48 for deflector flap 28 are all integrated in systems control box 50. By so doing it is possible to operate in a more or less optimum manner where the flow rate of fluid into chamber 14 is just equal to the rate at which the volatile component thereof is being removed by evaporation. Control box 50 can also be set up to operate various alarms (not shown) should a malfunction occur and to provide various status signals to a supervisory system such as a host computer (not shown). Techniques for exercising such control are well known. In the preferred embodiment, the liquid in inner chamber 14 is also agitated via a pump 60 which recirculates the liquid in a closed loop. The liquid can be still further agitated by air injection through a sparger 62 located at the bottom inner chamber 14. Such injection has been found to greatly enhance the evaporation process (30-40% improvement in evaporation rate). While pressurized air from any source could be employed, air blowers are preferred as a low cost air source. Further, as shown in the Figure, the air can be preheated by placing the delivery pipe within gas channel 17. By so doing even further quantities of waste heat can be recovered. The system of the present invention is completed when the concentrated residue of the aforementioned evaporation is drained from inner chamber 14 into waste collection box 64. Depending on the nature of the fluid being treated, this can be either continuous or, more usually, periodic depending either on when the quantity of stripped fluid reaches upper level control 34 or the concentration of contamination in the evaporated waste stream reaches some preset level.

Claims: A method for using the waste heat energy of a waste gas stream to heat a waste liquid so as to cause the evaporation of a volatile agent therefrom, said method comprising: (a) inserting the waste gas stream into a container comprising a flue (18) for feeding the waste gas stream from a source (26) of heated gas, an inner chamber (14) to hold the waste liquid, an outer chamber (16) connected to said gas feeding flue (18) and forming a gas channel (17) substantially surrounding said inner chamber (14) through which the waste gas stream flows, and a sump (20) below said container and connected to said gas feeding flue (18) to receive and store any liquid condensing from the gas stream and any liquid leaking from said inner chamber (14), so as to prevent the liquid from flowing back to the source (26) of the gas stream; (b) feeding a sufficient quantity of the waste liquid into said inner chamber (14); (c) venting the evaporated volatile agent from said inner chamber (14); and (d) collecting any condensate from the gas stream and any liquid leaking from said inner chamber (14) in said sump (20). The method of Claim 1 wherein step (c) comprises venting the volatile agent and combining it with said waste gas stream for discharge. The method of Claim 2, including the step of recovering said evaporated volatile agent from the combined discharge of volatile agent and gas stream. The method of Claim 1 wherein step (c) comprises venting the volatile agent and recovering it by condensation. The method of any preceding claim, including the step of preventing the collected liquid in the sump (20) from flowing back to the source (26) of the gas stream. The method of any preceding claim, including the step of draining liquid from said sump (20). The method of any preceding claim, including the step of controlling the quantity of the liquid which is fed into said inner chamber (14) between predetermined upper and lower levels. The method of Claim 7 including the step of controlling the amount of gas flowing through said gas channel (17), and coordinating said amount of gas with the liquid level in said inner chamber (14) to establish an optimum rate of evaporation. The method of any preceding claim, including the step of agitating said liquid in said inner chamber (14). The method of Claim 9 wherein said liquid agitation comprises sparging. The method of any preceding claim, further comprising the step of draining the residual liquid from said inner chamber (14). An apparatus for carrying out the process of Claim 1, comprising: (a) a flue (18) for feeding the waste gas stream from a source (26) of heated gas; (b) a container having an inner chamber (14) to hold the liquid solution for evaporation of said volatile agent and an outer chamber (16) connected to said gas feeding flue (18) forming a gas channel (17) substantially surrounding said inner chamber (14) through which the gas stream flows to supply heat to said inner chamber (14) and evaporate the volatile agent of said liquid solution; (c) a sump (20) below said container and connected to said gas feeding flue (18) to receive any condensate from the gas stream and any liquid leaking from said inner chamber (14), so as to prevent the condensate and leaking liquid from flowing back through the gas stream feeding flue (18) to the source (26) of the gas stream; and (d) a feed line for feeding the liquid solution into said inner chamber (14); and (e) a flue for combining and venting the evaporated volatile agent from said inner chamber (14) and said gas stream from said outer chamber (16) for discharge. The apparatus of Claim 12, wherein said sump (20) further comprises a dam (22) to prevent the liquid from flowing back to the source (26) of the gas stream. The apparatus of Claim 12 or Claim 13, wherein said sump (20) further contains a discharge door (24) to allow draining of said sump. The apparatus of Claim 14, wherein said discharge door (24) is a counterweighted trap door. The apparatus of any one of Claims 12 to 15, further comprising an upper and lower liquid level control system (34, 36) for controlling the quantity of the liquid solution which is fed into said inner chamber (14). The apparatus of any one of Claims 12 to 16, further comprising an agitator (62) associated with said inner chamber (14) for agitating said liquid. The apparatus of Claim 17 wherein said agitator (62) comprises a sparger. The apparatus of any one of Claims 12 to 18, further comprising a collection box (64) for receiving liquid from said inner chamber. The apparatus of any one of Claims 12 to 19, further including an exit for venting evaporated volatile agent directly from said inner chamber (14).

Inventors: KORENOWSKI, THEODORE F.
Applicants: TFK COMPANY, INC.
Application year: 1986
Main classification: B01D1
IPC codes: B01D3, B01D1, C02F1