US20140000298A1

US20140000298A1 - Method and system for processing hot humid air resulting from an industrial process, prior to expelling it into the outside air, to recover water and remove the plume

Info

Publication number: US20140000298A1
Application number: US13/720,752
Authority: US
Inventors: Oscar Lopez
Original assignee: BRUNNSCHWEILER SA
Current assignee: Brunnschweiler Ltd
Priority date: 2011-12-29
Filing date: 2012-12-19
Publication date: 2014-01-02
Also published as: EP2609983A1

Abstract

Systems and methods are disclosed for processing hot, humid air resulting from an industrial process, such as paper production. These systems and methods allow a large amount of water to be recovered from the air and the plume emitted into the environment to be reduced, all with reasonable energy consumption. The exhaust stream of air is passed through two heat exchangers, which cool the exhaust air by heating respective streams of water. The heated streams of water are routed to and cooled by an absorption cooler before being returned as input to the heat exchangers. Some embodiments may also include a scrubber, which allows direct air-water contact, positioned between the two heat exchangers. The scrubber also aids removal of pollutants. Some embodiments may mix heated outside air with the exhaust stream of air prior to discharge in order to reduce the emitted plume.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Patent Application No. 11380107, filed Dec. 29, 2011. The contents of that application are incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to a method and system for processing hot, humid air resulting from an industrial process, prior to its expulsion into the outside air, in order to recover water and contaminants from the industrial process and remove the visible plume at the chimney outlet or other outlet of the industrial process.
2. Description of Related Art
Certain types of industrial facilities and processes, such as paper drying plants, generate large amounts of very hot humid air, i.e. very hot air loaded with a great mass of water vapor, as waste. This air, usually called “exhaust” air, is emitted directly into the atmosphere, producing some important negative effects.
On the one hand, the water contained in the air in the form of vapor is completely lost in the atmosphere, which means that industrial facilities waste huge amounts of water that could otherwise be reused. Water nowadays is an increasingly more valuable and costly asset and therefore wasting water in this way is becoming unacceptable even in the industrial sector.
On the other hand, the water vapor contained in the air can sometimes be corrosive or contain chemical pollutants, depending on the industrial facility. Releasing water vapor that is corrosive, polluted, or both into the atmosphere can have very serious environmental consequences, and such releases are often highly regulated.
Moreover, emitting large amounts of very hot humid air into the atmosphere causes the creation, at the chimney outlet of the industrial facility, of an effect known as the “plume.” The plume is a very high column of apparent white smoke that is created when the very hot humid air comes into contact with the much colder outside air, causing sudden condensation of the water vapor contained in the air. As a result of this condensation, the air contains a very large number of small water droplets in suspension, which agglomerate on tiny particles of dust and other materials present in the humid exhaust air from the process and in the environmental air itself. Once they agglomerate, the water droplets refract rays of sunlight in all directions and wavelengths to produce the white visual effect.
Plumes are undesirable for several reasons: on the one hand, they are unsightly and they are perceived by the population as a sign of pollution because they look like a column of smoke; on the other hand, if the temperature outside the chimney is very low, the drops of water that condense when leaving the chimney and come into contact with the outside air can freeze, causing the plume to behave like a snow cannon, which may cause safety problems (poor visibility, accessibility problems, etc.).
Some plume-removing systems are currently known, among which two can be highlighted. In a first type of system, condensation and later separation of most of the water contained in the humid exhaust air from the industrial process takes place by using condensers or heat exchangers, wherein a cold fluid (usually, although not exclusively, cold water) cools the humid exhaust air to below its dew point temperature, causing condensation of some of the water contained therein.
The lower the temperature of the water used in the condenser, the more efficient these systems are. Of course, the cheaper the water used, the greater the economy of the process, thus the tendency to use cold water. However, cold water between 10 and 18° C. does not in itself guarantee the complete removal of the plume effect, especially in cold climates. Thus, if water between 10 and 18° C. is used, the outlet temperature of the saturated humid air (and consequently its dew point) is slightly above 10-18° C., so that in the presence of negative outside temperatures, this air condenses and the water is deposited on small particles of environmental dust or transported by the process air itself, giving rise to the phenomenon of a smaller, but still present, plume. Moreover, using cold water at 10-18° C. usually involves the need for a very large exchange surface in the condenser given the slight temperature difference intended to be obtained between the water and the humid air to optimize plume removal. Of course, using such a large exchange surface in the condenser is typically impractical.
In other types of systems, the humid exhaust air is mixed with dry external air heated to a temperature usually somewhat higher than the temperature of the humid process air. The mixture or dilution achieved in this way has a humidity content midway between the outside air and the humid process air, thereby reducing its relative humidity and distancing it from saturation conditions. The air mixture so obtained is released directly into the atmosphere after passing through a mixing chamber. This system has the disadvantage of a high energy cost, caused in the process of heating the outside air. Furthermore, plume removal is never sufficiently complete in particularly harsh climates. In addition, the mixing or dilution between the re-heated outside air and the humid air from the process leads to load losses which in some cases are unacceptable for the operation of the equipment that enables extraction of the process air (including a fan whose flow would be reduced to the same extent that process efficiency is reduced).
To solve the issue of the energy cost associated with heating the dry outside air, an air-to-air heat exchanger is sometimes used (or, alternatively, an air-to-water exchanger with an intermediate cooling water circuit to the outside air exchanger), in which the heat of the humid air from the process air is used to heat the dry outside air.
Either of the above-described methods is usually used to remove the plume. However, for colder climates, mixed systems are known that combine both methods, first proceeding to condense the humid air with cold (or subcooled) water and later mixing with heated dry outside air.

SUMMARY OF THE INVENTION

Systems and methods according to embodiments of the invention provide methods for treating streams of hot, humid exhaust air from industrial processes, like paper manufacture, that may also contain one or more pollutants. Generally speaking, in systems and methods according to embodiments of the invention, the heat energy in the stream of exhaust air is used to drive the systems that remove heat, moisture, and pollutants from the exhaust air, and that heat often provides substantially all of the energy necessary to drive the systems and methods.
In a system according to one aspect of the invention, the exhaust air is routed through first and second heat exchangers. In each heat exchanger, a stream of cooler input water is heated by the stream of exhaust air as it passes through the heat exchanger. An absorption cooler is provided that accepts the streams of heated water from the two heat exchangers and cools them. The cooled streams of water are returned as input to their respective heat exchangers. In some embodiments, the hotter water from the first heat exchanger, which is first to receive the exhaust air, is used to provide energy for the generator of the absorption cooler, while the second stream of heated water from the first heat exchanger is used to provide energy for the evaporator of the absorption cooler. The absorption cooler itself is essentially isolated from the stream of exhaust air, in that it receives the heated streams of water from the heat exchangers, but does not interact with the stream of exhaust air itself. A water extractor removes remaining droplets of water in the cooled stream of exhaust air before the exhaust air is exhausted to the environment.
In a system according to another aspect of the invention, a scrubber may be provided in the path of the stream of effluent air between the first and second heat exchanger. While pollutants may condense in the heat exchangers, the scrubber provides for direct air-liquid contact and will generally filter particulate matter, volatile organic compounds, and pollutants like chlorides and sulfates, which are common byproducts of industrial processes like paper manufacture. Most advantageously, the scrubber operates at temperatures that also allow it to condense and remove significant amounts of moisture from the exhaust air.
Some systems according to aspects of the invention may also include structures and elements designed to reduce the visible plume at the outlet. In a system according to these aspects of the invention, an inlet for outside air and a heat exchanger may be provided to draw in outside air and heat that air. The heated outside air is mixed with the cooled exhaust stream of air in a mixer before being sent to the outlet. The stream of hot water that heats the outside air in the heat exchanger is heated in another heat exchanger that is in the path of the stream of exhaust air.
Some systems may include both a scrubber and a plume reduction system with heat exchangers. In some embodiments, an additional heat exchanger that heats the outside air may draw its heat from the condenser of the absorption cooler.
Other aspects, features, and advantages of the invention will be set forth in the description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with respect to the following drawing figures, in which like numerals denote like features throughout the figures, and in which:

FIG. 1 is a schematic block diagram of one embodiment of a system according to an embodiment of the invention;

FIG. 2 is a schematic block diagram of an absorption cooler used in embodiments of the invention;

FIG. 3 shows the evolution curve in a Mollier psychrometric diagram of the mass of water vapor contained in the humid air and the temperature of the humid air in the system of FIG. 1;

FIG. 4 is a schematic block diagram of a system according to another embodiment of the invention;

FIG. 5 shows the evolution curve in a Mollier psychrometric diagram of the mass of water vapor contained in the humid air and the temperature of the humid air in the system of FIG. 4;

FIG. 6 is a schematic block diagram of a system according to yet another embodiment of the invention;

FIG. 7 shows the evolution curve in a Mollier psychrometric diagram of the mass of water vapor contained in the humid air and the temperature of the humid air in the system of FIG. 6;

FIG. 8 is a schematic block diagram of a system according to a further embodiment of the invention;

FIG. 9 shows the evolution curve in a Mollier psychrometric diagram of the mass of water vapor contained in the humid air and the temperature of the humid air in the system of FIG. 8;

FIG. 10 is a schematic block diagram of a system according to another further embodiment of the invention; and

FIG. 11 shows the evolution curve in a Mollier psychrometric diagram of the mass of water vapor contained in the humid air and the temperature of the humid air in the system of FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic block diagram of a system, generally indicated at 10, for processing hot, humid air A_lfrom an industrial process, generally indicated at 12. The hot, humid air is at an elevated temperature denoted as T₁in FIG. 1. Some of the following description may assume that the industrial process 12 is a paper or tissue manufacturing process, although systems according to embodiments of the invention may be applied to other types of industrial processes. The system 10 recovers the water from the hot, humid air and lowers its temperature.
Because it reduces the amount of water vapor emitted, system 10 and other systems according to embodiments of the invention may also reduce the levels of any pollutants that are dissolved in, suspended in, or otherwise associated with the water vapor or the exhaust air itself. For example, paper and tissue production processes may result in exhaust air that contains chlorides and sulfates, which are often present in the cellulose that is being dried and in the solutions used during the process.
If the industrial process 12 is a tissue or paper manufacturing and drying process, the temperature T₁of the exhaust air A₁is often on the order of 400-500° C., with a humidity on the order of 0.4-0.6 kg of water per kilogram of dry air. As shown in FIG. 1, the temperature of the exhaust air A₁is reduced in a first heat exchanger 14, so that a cooler humid air A₂at a temperature T₂is obtained. T₂might, for example, be 120° C. In this first heat exchanger 14, heating of a cooler input water W₁also takes place in order to obtain a hotter output water W₂. For example, the temperature of the cooler input water W₁and the hotter output water W₂can be 100 and 110° C. respectively.
After it passes through the first heat exchanger 14, the air A₂is cooled down further in a second heat exchanger 16, so that an even cooler humid air A₃at a temperature T₃is obtained as a result of this second heat exchanger 16. T₃might, for example, be 40° C. The cooler humid air A₃contains suspended water. In return, this process heats a cooler input water W₃in order to obtain a hotter output water W₄.
After the second heat exchanger 16, at least some of the water suspended in the air A₃is extracted by means of a water extractor 18 (for example, a drift eliminator, cyclone separator, settling chamber, etc.). The extracted water, shown at 20, is taken from the extractor 18 and recovered. This enables some, most, or almost all of the water contained in the air A₃to be recovered, depending on the input temperature of the hot humid air result, air A₄with a lower suspended water content than the input air A₃is also obtained. The temperature T₄of air A₄is generally equal to the previous air temperature T₃. The air A₄is directed towards an outlet 22, for example a chimney, an ejector or any other device for extraction into the atmosphere, and only a very small plume is formed due to the very low humidity A₃, and enables this water to be reused for new purposes. The air A₄expelled from the outlet 22 may have a humidity of 0.05-0.25 kg water per kilogram of dry air at a temperature in the range of 40-70° C.
Meanwhile, the output water W₄from the second heat exchanger 16 is routed through an absorption cooler 24, which cools down the output water W₄from the second heat exchanger 16 and uses it as the input water W₃for the second heat exchanger 16. In the embodiment shown, the input heat of the absorption cooler 24 comes from the heat of the output water W₂from the first heat exchanger 14. After the temperature of this water has been reduced, it is used as input water W₁for the first heat exchanger 14. Notably, although the absorption cooler 24 receives the heated water streams W₂, W₄from the heat exchangers 14, 16, it is isolated from the exhaust stream of air itself.
An absorption cooling process is a process that cools by taking advantage of the fact that certain substances (called “coolants”) absorb heat on changing from a liquid to a gaseous state. To do this, the cooling process is based physically on the ability of a substance called “absorbent” (for example, lithium bromide) to absorb the coolant (for example, water) when it is in a vapor phase.
FIG. 2 is a schematic block diagram of the absorption cooler 24, in which an absorption cooling process takes place, illustrating its main parts. The main components of an absorption cooling machine 24 are: (i) an evaporator 242, in which the coolant passes from a liquid to a vapor state by absorbing heat from a hot input substance, which, as a result, is cooled; (ii) an absorber 244, in which the absorbent absorbs the coolant in vapor state, thereby obtaining a concentrated solution in which the absorber is the solute and the coolant is the solvent, and which releases a certain amount of heat with a low energy content; (iii) a generator or heater 246 which, by means of an external heat source, separates the coolant, again in a vapor state, from the absorbent solute, which is returned to the absorber 244 to restart the cycle; and (iv) a condenser 248, which receives the coolant in a vapor state from the generator or heater, and after converting it to a liquid state, delivers it to the evaporator, while also releasing a certain amount of heat.
In addition to those main components, an expansion valve 241 is interposed between the condenser 248 and the evaporator 242, and a pump 243 transports the absorber/coolant solution from the absorber 244 to the generator 246. The expansion valve 241 and the pump 243 create the two different levels of pressure used to create the desired cooling effects at adequate temperatures. A heat exchanger 245 is placed between the generator 246 and the absorber 244. The heat exchanger 245 preheats the absorber/coolant solution pumped from the absorber 244 by heat exchange with hot absorber chemical that is being returned from the generator 246 to the absorber 244.
In systems and processes according to embodiments of the invention, the input heat to the absorption cooler 24 is preferably used as a heat source specifically for the generator or heater 246 of the absorption cooler 24, while the output water W₄from the second heat exchanger 16 is used as a heat source for the evaporator 242 of the absorption cooler 24. This enables maximum use to be made of the excess heat from the industrial process, since the generator or heater 246 (the item that requires the most heat) receives the hottest input, whereas the evaporator 242 receives a heat input that is cooler but still hot enough to operate at the lower pressure level.
The method of the invention is therefore notably advantageous: the temperature of the hot humid air A₁generated by the industrial process 12 is reduced, with the lost heat being used to heat water W₁which, in turn, will be used to cool other water W₄that will enable the air A₂to be cooled even more. This means that the heat from the exhaust air itself helps cool the air, according to the invention. This enables the air to be cooled (in order to condense the water and be able to remove it) with optimum energy consumption. In this way, the water recovery and plume reduction system according to the invention entails a reasonable energy consumption that allows the system to be viably implemented.
Additionally, as was noted briefly above, as the water is cooled, many pollutants in the water can be separated from it. For example, chlorides and sulfates from paper and tissue drying processes may be removed from the exhaust stream, for example, as water vapor is condensed in the heat exchangers 14, 16. In other embodiments, other components may contribute to the removal of pollutants, as will be described in more detail below.
In an alternative or complementary manner, the input heat to the absorption cooling process 24 may come from other sources. For instance, the input heat may come from other processes, equipment, or points in the industrial process 12, such as vacuum systems (i.e., systems that remove water from paper by creating pressure differences that absorb water, which usually require the use of blower units to perform water extraction more effectively, with these blower units generating considerable residual heat); and co-generation systems (many industrial processes or facilities comprise co-generation systems normally composed of gas turbines or diesel engines that generate net electricity to be consumed in the industrial process itself, while also generating an important amount of residual heat).
FIG. 3 shows the evolution curve for the degree of air humidity (mass of water vapor per amount of dry air mass) and the air temperature of this first embodiment, starting from the initial hot humid air A₁and ending with the air A₄directed towards the outlet 22. As can be seen in the graph, the process begins with air A₁, provided with a high humidity W_inicand a high temperature T₁(for example, between 180 and 300° C.). The air is cooled in the first heat exchanger 14 and air A₂is obtained at a temperature T₂of, for example, 120° C., while humidity is preferably kept the same. Air cooling continues in the second heat exchanger 16, in such a way that the conditions of air A₃are reached along a process line that is secant to the condensation curve 26 at the dew point or minimum temperature of the wall on the air side of the cooler. Air A₃comprises dry air, water vapor in a concentration W_finaland suspended liquid water, all at a temperature T₃of, for example, 40° C. The liquid water is then extracted; the resulting air A₄maintains its temperature (i.e. it leaves at T₄=T₃) and water vapor content W_final, but contains hardly any suspended liquid water. Finally, this air A₄is directed towards the outlet 22.
FIG. 4 shows a block diagram of a system, generally indicated at 50, according to another embodiment of the invention. In system 50, those components that are similar to those of FIG. 1 are labeled with the same reference numerals and, unless otherwise specified, may be assumed to operate in substantially the same way. As shown, system 50 includes a first heat exchanger 14, a second heat exchanger 16, an absorption cooler 24, a water extractor 18 and an outlet 22, arranged in the same basic way as in system 10 of FIG. 1. Additionally, positioned between the two heat exchangers 14, 16 is a scrubber 52.
The scrubber 52 is essentially a washer which functions as an air-water exchanger by direct contact between air and water currents, producing an output air A₂″ at a lower temperature T₂″, as well as heated water. With this arrangement, the first and second heat exchangers 14, 16 serve to optimize the performance of the exchange that takes place in the scrubber 52, and essentially allow the scrubber 52 to take the exhaust stream of air at temperature and humidity conditions under which the scrubber 52 can be most effective.
In an exemplary embodiment, as was noted above, the drying equipment of the industrial process 12 may operate at 400-500° C. Given that, the air at A₁may have a temperature in the range of 200-250° C., with a humidity in the range of 0.4-0.6 kilograms of water per kilogram of dry air. Meanwhile, the water W₁entering the first heat exchanger 14 may be heated to approximately 100° C., and water W₂leaving the first heat exchanger 14 reaches a temperature of about 110° C. After the first heat exchanger 14, air A₂entering the scrubber 52 may be at a temperature T₂of about 150° C. and a humidity of about 0.5 kilograms of water per kilogram of dry air. The temperature of the water in the scrubber is in the range of about 20-50° C. The direct contact heat exchange offered by the scrubber 52 is effective and simple, because any water vapor that condenses as a result of the cooler temperatures within the scrubber 52 transfers its enthalpy and mass to the water in the scrubber 52, becoming a part of the water in the scrubber. Thus, some water recovery occurs within the scrubber 52, meaning that more water exits the scrubber 52 than enters it.
The scrubber 52 also has several other advantageous features and effects in system 50 and in methods of using system 50. For one, the heated water leaving the scrubber 52 can be drawn off and its heat reclaimed or used for other purposes, such as heating a building, or providing heat for another industrial process. Additionally, by its nature, the scrubber 52 naturally tends to eliminate solid particles and volatile organic components—which is why it is referred to as a scrubber or washer. In addition to basic solid particles and volatile organics, it is expected that pollutants like chlorides and sulfates will be separated in the scrubber 52, because the gases leaving the scrubber 52 are likely to be well below the dew point of these pollutants. The scrubber 52 itself is continuously self-washed to eliminate any buildup of particles that may occur. Typically, the heat exchangers 14, 16 and other components would also be provided with automatic washing systems to prevent substantial build-up of filtrates from occurring.
FIG. 5 shows the evolution curve for the mass of water vapor contained in the humid air and the temperature of the humid air in the second embodiment. If this graph is compared with the one in FIG. 3, it can be seen that the scrubber 52 or direct air-water exchanger has enabled the final humidity conditions to be reduced at the outlet, by having favored the removal of humidity from the input air. The hot humid air A₂at the inlet to the scrubber 52 first undergoes an adiabatic saturation process (almost isentropic, with an evolution curve close to a constant wet bulb temperature line, before reaching saturation, represented at point 28 in FIG. 5).
Once the air is saturated, heat continues to be extracted from it, by which the evolution curve follows the air saturation curve until reaching the output conditions (air A₂″); the conditions of output air A₂ ^″generally depend on the water flow to be heated in the scrubber 52, dimensions of the scrubber 52 and the existence or not of an inner fill to increase the time the water to be heated remains inside. As one example, air A₂″ leaving the scrubber 52 may be at a temperature of about 70° C. and a humidity of about 0.2-0.3 kilograms of water per kilogram of dry air.
Once the air A₂″, partially cooled and partially dehumidified, comes out of the scrubber 52, the air A₂″ is made to pass through a second heat exchanger 16, lowering the humidity and temperature yet again, similar to the arrangement of system 10 of FIG. 1. In the second heat exchanger 16, water W₃entering the heat exchanger 16 may be at a temperature of about 7° C., and water W₄leaving the heat exchanger 16 may be at a temperature of about 12° C. The humidity of the air A₃leaving the second heat exchanger 16 may be on the order of 0.05 kilograms of water per kilogram of dry air. Thus, much of the condensation of water vapor occurs in the second heat exchanger 16. As can be appreciated from FIG. 5, the temperature of the entering air A₂″ and the temperature of the exiting air A₃are not significantly different, primarily because much of the heat lost is latent heat. In other words, much of the energy is lost when the water changes phase. The exhaust temperature T₅of system 50 may be on the order of about 50° C. to about 70° C.
FIG. 6 is a block diagram of a system, generally indicated at 70, according to yet another embodiment the invention. System 70 includes optional additional mechanisms to reduce the visibility of the plume generated at the outlet 22 to a still greater extent. More specifically, compared with system 10 of FIG. 1, system 70 includes an inlet 72 for outside air, and a set of two heat exchangers 74, 76 that take heat from the exhaust stream of air to heat the outside air. Heated outside air E₁is added to and mixed with the air A₄resulting from extracting at least part of the suspended water from the air A₃, suitably mixing both airs E₁, A₄in a mixer 78, and expelling the mixture of both, represented as A₅, into the outside air E. As was noted briefly above, in system 70, the heated outside air E₁is heated, at least partly, by means of heat from the hot humid air A₁resulting from the industrial process 12, allowing the energy consumption for the outside air heating process and, therefore, for the plume reduction process, to be reduced.
FIG. 7, which shows the evolution curve for the mass of water vapor contained in the humid air and the temperature of the humid air in system 70, allows the positive effects achieved by these steps to be understood. When the suspended water has been recovered and the air A₄has been obtained, this air A₄is mixed with the heated outside air E₁, whose temperature is usually higher than that of the air A₄. The mixture A₅of both airs is a point that is farther away from the condensation curve. Then, on expelling this mixture A₅to the outside, the mixture comes into contact with the outside air E and the plume evolves along the evolution line 26 until the plume disappears, i.e. until the outside air point is reached E. The evolution of the plume takes place in a “safe area,” due to the fact that the evolution line 80 is not on the condensation curve 26 but to the left of it, i.e. in humidity/temperature values where there is no condensation risk; as a result, there is no condensation at the plume, making the plume (which is already very small because large amounts of water have been recovered) become invisible.
Optionally, the mixing of heated outside air E₁with the air A₄obtained after recovering condensed water is carried out as shown in FIG. 6. More specifically, outside air E is heated in a third heat exchanger 74 so that heated outside air E₁is obtained as an output. The third heat exchanger 74 that heats the outside air E is in fluid communication with a fourth heat exchanger 76 that heats and supplies the input hot water W₅to the third heat exchanger 74 to heat the outside air E.
The fourth heat exchanger 76 is in fluid communication with the first heat exchanger 14 and the second heat exchanger 16 and is situated between them such that it receives the exhaust stream of hot, humid air A₂after it has been discharged from the first heat exchanger 14. The third and forth heat exchangers 74, 76 are in a fluid circuit with one another, such that the fourth heat exchanger removes heat from the exhaust stream of air A₂exiting the first heat exchanger 14, creating the input hot water W₅for the third heat exchanger 74. Once the input hot water W₅to the third heat exchanger 74 has been used to heat the outside air E, the resulting cooled water W₆is used as input to the fourth heat exchanger 76 and is reheated by the exhaust stream of air A₂. The cooled air A₂′ at lower temperature T₂′ discharged from the fourth heat exchanger 76 is routed to the second heat exchanger 16 in FIG. 6.
Instead of the use of two heat exchangers 74, 76 to heat the outside air E, in another embodiment of the invention, the outside air E could be heated with an air-to-air exchanger, in which the hot humid air from the industrial process 12 transfers heat to the outside air E, i.e. the air from the industrial process 12 acts as a hot environment and the outside air E that is to be heated acts as a cold environment. This allows the outside air E to be heated with a single heat exchanger.
In the embodiment shown, the mixer 78 that mixes the heated outside air E₁with the cooled and dehumidified exhaust stream of air A₄is located before the outlet 22 that expels air into the environment; i.e. the mixer 78 is internal. Nonetheless, this aspect is not critical for the present invention, and the mixer 78 may, in practice, be located at different points if applicable.
FIG. 8 is a block diagram of a further embodiment of a system, generally indicated at 90, according to the invention, which simultaneously includes all the optional aspects described so far, i.e., it comprises a scrubber 52 for the exhaust stream of air, and a mixer 78 for heated outside air E₁. The outside air E is heated with third and fourth heat exchangers 74, 76 as described above, and may be heated to a temperature of, for example, about 30-35° C. The mixer 78 produces a turbulent mixture of air with humidity typically in the range of about 0.01-0.05 kilograms of water per kilogram of dry air and about 35-50° C., although the final exhaust temperature could be as low as 20-30° C. Combining all of these processes is an advantageous way to simultaneously combine maximum energy recovery at the facility, together with almost total recovery of the water carried in the process air flow and at the same time ensure optimal removal of the plume effect.
FIG. 9 shows the evolution curve for the mass of water vapor contained in the humid air and the temperature of the humid air in this fourth embodiment. This evolution curve provides maximum depletion of the energy and humidity content in the hot humid air circuit extracted from the process. Moreover, it may virtually eliminate the plume for the most adverse climate conditions, i.e. when the ambient air dew point is the lowest possible and the risks of creating condensation and, therefore, a plume, during the mixing that takes place between the extracted air and the ambient air increase, becoming even more critical.
In general, the energy consumption of system 90 and other systems 10, 50, 70 according to embodiments of the invention is reasonable, as already explained. The only items in the system which would normally have to consume energy are the absorption machine associated with the absorption cooler 24 and the circulating pumps used to ensure water transfer between the first heat exchanger 14 and the absorption cooler 24, between the absorption machine associated with the absorption cooler 24 and the second heat exchanger 16, and between the third and fourth heat exchangers 74, 76, if any.
In most embodiments of the invention, each process (heat exchange, absorption cooling, scrubbing, water extraction, etc.) will usually be performed in and using its own machine. Thus, the systems 10, 50, 70, 90 will comprise a number of machines adapted to work together. However, it is possible that a system according to an embodiment of the invention might include machines capable of performing more than one process. For example, an absorption cooler might be used that performs the functions of the absorption cooler 24 and the first heat exchanger 14, in such a way that it is directly supplied with hot air A1 from the industrial process 12.
FIG. 10 shows a block diagram of a fifth embodiment of a system 100 according to the invention. Generally speaking, system 100 has the features of system 70 of FIG. 6. In addition, in system 100, heat generated in the absorption cooler 24 (preferably in its condenser 248), is used to heat the outside air E. Thus, as can be seen in FIG. 10, outside air E is heated in an additional heat exchanger 102 in order to obtain heated outside air E₀, by means of a heat exchange with hot input water W₇that produces cooler output water W₈. This cooler output water W₈is then heated taking advantage of the heat provided by the condenser 248 of the absorption cooler 24 with the hotter input water W₇being obtained as a result. This characteristic of the invention makes it possible to achieve even greater energy efficiency in the process and helps to ensure removal of plume visibility, even in the harshest climates.
FIG. 11 shows this intermediate point of heated outside air E₀and how the temperature of the heated outside air E₁can be higher than in the embodiment shown in FIGS. 7 and 8 (although in practice it would depend on how both facilities are configured).
In this description, water has been described as the fluid used to exchange heat with air. Water is understood to mean any water-based fluid. This means that fluids such as pure water, water mixed with other agents (antifreeze, corrosion inhibitors, pH correctors, etc.), chemically treated and/or filtered water according to any applicable process, various types of industrial brine, etc. will be considered.
While the invention has been described with respect to certain embodiments, the embodiments are intended to be exemplary, rather than limiting. Modifications and changes may be made within the scope of the invention, which is defined by the appended claims.

Claims

What is claimed is:

1. A system for treating an exhaust stream of hot, humid air from an industrial process, comprising:

a first heat exchanger receiving the exhaust stream of air from the industrial process, the first heat exchanger cooling the exhaust stream of air to a first reduced temperature by use of a first stream of cooler water, the first stream of cooler water being heated by heat exchange with the exhaust stream of air in the first heat exchanger;

a second heat exchanger in fluid communication with the first heat exchanger to receive the exhaust stream of air at the first reduced temperature and cool the exhaust stream of air to a second reduced temperature by use of a second stream of cooler water, the second stream of cooler water being heated by heat exchange with the exhaust stream of air in the second heat exchanger;

an outlet in fluid communication with the second heat exchanger to receive and discharge the exhaust stream of air at a temperature equal to or lower than the second reduced temperature; and

an absorption cooler in association with the first heat exchanger and the second heat exchanger but isolated from the exhaust stream of air that (1) receives the heated second stream of cooler water from the second heat exchanger, and (2) receives the heated first stream of cooler water from the first heat exchanger and cools them by an absorptive process with a coolant, the absorption cooler being arranged with respect to the first and second heat exchangers such that the cooled first stream of water is returned as input to the first heat exchanger and the cooled second stream of water is returned as input to the second heat exchanger;

wherein the heated first and second streams of water provide input energy in the form of heat to the absorption cooler.

2. The system of claim 1, further comprising an outside air heating and mixing system including:

a third heat exchanger positioned between and in fluid communication with the first heat exchanger and the second heat exchanger that accepts the exhaust stream of air from the first heat exchanger and cools it by heat exchange with a third stream of water before passing the exhaust stream of air to the second heat exchanger;

an inlet arranged to accept a stream of outside air;

a fourth heat exchanger that accepts the stream of outside air and the heated third stream of water from the third heat exchanger and heats the stream of outside air; and

a mixer positioned upstream of the outlet that receives the heated stream of outside air and mixes it with the exhaust stream of air after the exhaust stream of air is discharged from the second heat exchanger and before the exhaust stream of air reaches the outlet;

wherein the third stream of water flows in a circuit between the third and fourth heat exchangers.

3. The system of claim 2, further comprising a fifth heat exchanger positioned between the inlet and the fourth heat exchanger, the fifth heat exchanger being in fluid communication with a condenser of the absorption cooler so as to receive a stream of water heated by the condenser and heat the outside air by direct contact heat exchange with the stream of water heated by the condenser.

4. The system of claim 2, further comprising a scrubber disposed between and in fluid communication with the first heat exchanger and the second heat exchanger to receive the exhaust stream of air, the scrubber providing direct contact between the exhaust stream of air and scrubber fluid.

5. The system of claim 4, wherein the scrubber removes one or more pollutants from the exhaust stream of air.

6. The system of claim 2, further comprising a water extractor disposed between and in fluid communication with the second heat exchanger and the mixer.

7. The system of claim 1, further comprising a scrubber disposed between and in fluid communication with the first heat exchanger and the second heat exchanger to receive the exhaust stream of air, the scrubber providing direct contact between the exhaust stream of air and scrubber fluid.

8. The system of claim 7, wherein the scrubber removes one or more pollutants from the exhaust stream of air.

9. The system of claim 7, further comprising a water extractor disposed between and in fluid communication with the second heat exchanger and the outlet.

10. The system of claim 9, wherein the system reduces the temperature of the exhaust stream of air from about 250-300° C. to about 40-70° C.

11. The system of claim 10, wherein the system reduces the water content of the exhaust stream of air from about 0.4-0.6 kilograms of water per kilogram of dry air to about 0.01-0.05 kilograms of water per kilogram of dry air.

12. The system of claim 1, wherein heat energy from the exhaust stream of air provides substantially all of the energy to operate the system.

13. The system of claim 12, wherein the heated first stream of water from the first heat exchanger provides an input heat for a generator of the absorption cooler and the heated second stream of water from the second heat exchanger provides an input heat for an evaporator of the absorption cooler.

14. The system of claim 1, wherein the industrial process comprises paper manufacture.

15. A method for processing an exhaust stream of hot, humid air from an industrial process prior to expelling it into outside air, comprising:

passing the exhaust stream of air through a first heat exchanger, thereby cooling the exhaust stream of air and heating a first stream of water;

receiving the exhaust stream of air at a second heat exchanger after said passing and cooling the exhaust stream using the second heat exchanger and a second stream of water, thereby heating the second stream of water;

extracting at least a portion of the water suspended in the exhaust stream using a water extractor;

exhausting the exhaust stream to outside air; and

cooling the first and second streams of water using an absorption cooler that is isolated from the exhaust stream of air such that the cooled first stream of water is returned as input to the first heat exchanger and the cooled second stream of water is returned as input to the second heat exchanger;

wherein the heated first stream of water and the heated second stream of water provide input heat to the absorption cooler.

16. The method of claim 15, further comprising, after passing the exhaust stream of air through the first heat exchanger, directing the exhaust stream of air through a scrubber in communication with the first heat exchanger and the second heat exchanger, the scrubber providing direct contact between the exhaust stream of air and scrubber fluid.

17. The method of claim 15, further comprising heating outside air and mixing the heated outside air with the exhaust stream of air prior to exhausting the exhaust stream to outside air.

18. The method of claim 17, wherein heating the outside air comprises passing the outside air through a third heat exchanger, the third heat exchanger being in fluid communication with a fourth heat exchanger in fluid communication with the first and second heat exchangers and in the path of the exhaust air, the fourth heat exchanger providing a third stream of heated water as input to the third heat exchanger.

19. The method of claim 17, wherein heating the outside air comprises providing a third heat exchanger that draws heat from a condenser of the absorption cooler.

20. The method of claim 17, further comprising, after passing the exhaust stream of air through the first heat exchanger, directing the exhaust stream of air through a scrubber in communication with the first heat exchanger and the second heat exchanger, the scrubber providing direct contact between the exhaust stream of air and scrubber fluid.