Patent Application
20140000298
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.
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.
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.
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:
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 T1 of the exhaust air A1 is 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
After it passes through the first heat exchanger 14, the air A2 is cooled down further in a second heat exchanger 16, so that an even cooler humid air A3 at a temperature T3 is obtained as a result of this second heat exchanger 16. T3 might, for example, be 40° C. The cooler humid air A3 contains suspended water. In return, this process heats a cooler input water W3 in order to obtain a hotter output water W4.
After the second heat exchanger 16, at least some of the water suspended in the air A3 is 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 A3 to be recovered, depending on the input temperature of the hot humid air result, air A4 with a lower suspended water content than the input air A3 is also obtained. The temperature T4 of air A4 is generally equal to the previous air temperature T3. The air A4 is 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 A4 expelled 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 W4 from the second heat exchanger 16 is routed through an absorption cooler 24, which cools down the output water W4 from the second heat exchanger 16 and uses it as the input water W3 for 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 W2 from the first heat exchanger 14. After the temperature of this water has been reduced, it is used as input water W1 for the first heat exchanger 14. Notably, although the absorption cooler 24 receives the heated water streams W2, W4 from 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.
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 W4 from 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 A1 generated by the industrial process 12 is reduced, with the lost heat being used to heat water W1 which, in turn, will be used to cool other water W4 that will enable the air A2 to 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).
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 A1 may 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 W1 entering the first heat exchanger 14 may be heated to approximately 100° C., and water W2 leaving the first heat exchanger 14 reaches a temperature of about 110° C. After the first heat exchanger 14, air A2 entering the scrubber 52 may be at a temperature T2 of 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.
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
Optionally, the mixing of heated outside air E1 with the air A4 obtained after recovering condensed water is carried out as shown in
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 A2 after 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 A2 exiting the first heat exchanger 14, creating the input hot water W5 for the third heat exchanger 74. Once the input hot water W5 to the third heat exchanger 74 has been used to heat the outside air E, the resulting cooled water W6 is 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
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 E1 with the cooled and dehumidified exhaust stream of air A4 is 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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 11380107 | Dec 2011 | EP | regional |
