CN107764037B

CN107764037B - Method and arrangement for recycling air in a drying process

Info

Publication number: CN107764037B
Application number: CN201710712734.6A
Authority: CN
Inventors: M·哈拉普洛
Original assignee: TM SYSTEM FINLAND Oy
Current assignee: TM SYSTEM FINLAND Oy
Priority date: 2016-08-18
Filing date: 2017-08-18
Publication date: 2020-10-30
Anticipated expiration: 2037-08-18
Also published as: CN107764037A; DE112017004101T5; FI128437B; WO2018033665A1; DE112017004101B4; FI20165617A; CN107764035A

Abstract

The present application relates to a method and an arrangement for recirculating air in a drying process, the application relates to a method for recirculating air in a drying process (1) within a plant, wherein, in the drying process (1), moisture is removed from a product into an air stream, producing an exhaust air stream (2 a). The exhaust air flow (2a) is directed to a cleaning unit (30) for cleaning the exhaust air flow (2a) and then to the device (3), wherein the moisture content of the exhaust air flow (2a) is reduced. Dividing the flow of exhaust air (2b-c) leaving the device (3) into at least: an air stream (2b) recirculated to the drying process (1) and an air stream (2c) reaching at least one chamber associated with the drying process (1). The application also relates to an arrangement for recycling air in a drying process (1).

Description

Method and arrangement for recycling air in a drying process

The present invention relates to a method and an arrangement for recycling air in a drying process.

Background

Drying is a high energy consuming operation due to the high latent heat of steam and the inefficiency of using hot air as the drying medium. In a typical industry, the drying process moisture is removed from the product into an air stream, producing a moisture laden exhaust air stream.

It is known to recover heat from moisture laden exhaust air and to use the recovered heat in plants that include drying processes. The cooled moisture laden exhaust air stream is then vented to the atmosphere, possibly producing an effluent. In some places, even a plume of clean water vapor is unacceptable. Visible water vapor plumes can be a concern for residential populations near the plant. Furthermore, plume-induced fogging and icing raises the risk of accidents and can be dangerous for crops and equipment.

If the moisture laden exhaust air is itself recirculated in the plant, this requires dehumidification and cooling of the exhaust air in combination with removal of unwanted particles, which can then be recycled. However, moisture removal during dehumidification is often inadequate and fresh dry air needs to be mixed with the dehumidified exhaust air to achieve acceptable moisture content levels for reuse. Furthermore, the costs of dehumidification and cooling installations make the recirculation of moisture laden exhaust air unprofitable.

Disclosure of Invention

It is an object of the present invention to address the above-mentioned drawbacks and to provide an efficient and cost-effective method, and an arrangement for implementing the method, for recirculating air in a drying process.

This object is achieved according to the method for recirculating air in a drying process of independent claim 1 and according to the arrangement for recirculating air in a drying process of independent claim 16. Preferred embodiments of the invention are disclosed in the dependent claims.

In a method for recycling air in a drying process of a plant, moisture is removed from a product into an air stream during drying, producing an exhaust air stream. The exhaust air stream is directed to a cleaning unit for cleaning the exhaust air stream and then to a device in which the moisture content of the exhaust air stream is reduced. Dividing the exhaust air stream exiting the device into at least: an air stream that is recirculated to the drying process and an air stream that reaches at least one chamber associated with the drying process.

The arrangement for recirculating air in the drying process of the plant comprises a stream of exhaust air from the drying process. The arrangement comprises a cleaning unit for cleaning the exhaust air flow and means for reducing the moisture content of the exhaust air flow. The cleaning unit is arranged in front of the device in terms of the flow direction of the exhaust air flow. The arrangement comprises means for dividing the exhaust air flow leaving the device into at least: an air stream that is recirculated to the drying process and an air stream that reaches at least one chamber associated with the drying process.

An advantage of the method and arrangement of the invention is that the amount of exhaust air flow to the atmosphere and the formation of visible water vapour plumes can be minimised.

Drawings

The invention is described in more detail in the following by means of preferred embodiments, with reference to the attached drawings, in which:

figure 1 shows an arrangement for recycling air in a drying process;

fig. 2 shows an arrangement for recycling air in a drying process with a heat recovery exchanger.

FIG. 3 shows an arrangement for recirculating air in a drying process comprising a cleaning unit;

FIG. 4 shows an arrangement for recirculating air in a drying process comprising a cleaning unit;

figure 5 shows an arrangement for recirculating air in a drying process comprising a clean room, a make-up air flow and a venturi scrubber.

Detailed Description

Figure 1 shows an arrangement for recycling air in a drying process. In the drying process 1, moisture is removed from the product into a hot air stream, producing a moisture laden exhaust air stream 2 a. Preferably, the drying process 1 is a continuous high energy consumption drying process, wherein the moisture laden exhaust air stream 2a may comprise for example 130-160 g of water vapour per kg of air. Examples of continuous high energy consumption drying processes are the drying of paper, pulp, wood or waste pulp.

The device 3 reduces the moisture content of the exhaust air stream 2 a. For example, device 3 cools exhaust air stream 2a to remove moisture, or absorbs moisture from exhaust air stream 2a, or adsorbs moisture from exhaust air stream 2a, or any combination of the three. The reduced moisture content of exhaust air stream 2a reduces visible water vapor in exhaust air stream 2a as it is discharged from device 3. As a result of the water vapor condensing and forming water droplets, the water vapor becomes visible. Clouds or fog are visible as atmospheric water vapor.

At least part of the exhaust air stream 2b-c leaving the device 3 is recycled to the drying process 1. The reduction of water vapor in the exhaust air streams 2b-c and the recirculation of the exhaust air streams 2b-c reduces the amount of exhaust air streams discharged to the atmosphere and the formation of visible water vapor plumes.

The plumes of moisture laden exhaust air stream 2a have a high moisture content and their discharge to the atmosphere under certain meteorological conditions without adequate moisture removal can result in the formation of visible plumes. It is seen that the formation of the water vapor plume depends on meteorological conditions such as wind speed, wind direction, atmospheric stability level, ambient temperature, mixing altitude, and relative humidity of the ambient air. By removing water vapour from the exhaust air stream 2a with the device 3, this reduces the generation of visible water vapour plumes and the length and height of the plumes.

The exhaust air stream 2b-c leaving the device 3 is divided into at least: air stream 2b that is recycled to the drying process and air stream 2c that reaches at least one chamber associated with the drying process. For example, the air stream 2b divided to reach the drying process represents 65-85% of the exit exhaust air stream 2b-c, and the air stream 2c divided to reach the at least one chamber associated with the drying process represents 35-15% of the exit exhaust air stream 2 b-c. The space surrounding the drying process 1 is not leak-tight and the amount of recirculating drying air 2b entering the space is typically smaller than the amount of moisture-laden exhaust air flow 2a which is drawn off (draw from) the space.

Typical examples of rooms associated with the drying process 1 are finishing rooms, service spaces, control and electrical rooms, machine rooms, finishing areas, special rooms (e.g. MCC and machine rooms). The at least one chamber associated with the drying process may comprise a production area of a plant in which the drying process is part of the production process. The air stream 2c that is separated from the dried and cooled exhaust air stream 2b-c leaving the device 3 can be used for heating, cooling or ventilating one or more chambers in connection with the drying process 1.

During winter time, in many geographical locations, the moisture content and temperature of the ambient air is low. For these weather conditions, the likelihood of visible water vapor plumes occurring becomes high. Furthermore, the inlet air from outside the chamber into the chamber associated with the drying process 1 needs to be heated and preferably humidified for maintaining the ideal working conditions of personnel and plant machinery. It is advantageous to also recirculate the exhaust air stream 2b to the chamber associated with the drying process 1, as this improves their inlet air quality and at the same time reduces the amount of exhaust air stream discharged to the atmosphere and reduces the formation of visible water vapour plumes. The method and arrangement for air recirculation in a drying process enables complete recirculation of exhaust air on a plant steady production stream, especially during the cold and drying seasons of the year.

In one embodiment of the method and arrangement for air recirculation in a drying process, a wet scrubber 28 is used to reduce the temperature and moisture content of the moisture laden exhaust air stream 2a from the drying process 1, i.e. the moisture laden exhaust air stream 2a is dried in the wet scrubber 28.

Scrubber 28 is advantageous for drying moisture laden exhaust air because it provides a large volume for heat and mass transfer. The wet scrubber 28 is cost effective because the heat and mass transfer surfaces required are rather low compared to conventional plate heat exchanger systems or shell-in-tube heat exchanger systems. In addition, scrubber 28 also removes unwanted material from moisture laden exhaust air stream 2 a. Examples of removable substances are ash, dust, odorous vapors and water-soluble chemicals.

In fig. 1, the moisture laden effluent air stream 2a from the drying process 1 is directed to a wet scrubber 28 for reducing the moisture content and temperature of the effluent air stream 2 a. Recycling part of the exhaust air stream 2b to the drying process 1 and introducing part of the exhaust air stream 2c into at least one chamber associated with the drying process 1.

The scrubber 28 comprises a scrubber tower having a circular bottom surface 4 and a cylindrical shell 5. Another bottom surface shape may be, for example, rectangular. Moisture laden effluent air stream 2a is directed horizontally to the bottom portion 6 of the wet scrubber tower via inlet opening 7. The directing of the flow may include pressurizing the moisture laden exhaust air flow with one or more blowers 8.

In the scrubber 28, the exhaust air stream 2a flows upward, and droplets 9 of the cooling water 10 are injected into the exhaust air stream from the plurality of nozzles 11. The cooling water droplets 9 flow downward in counter-current to the exhaust air flow 2 a. The moisture laden exhaust air flow 2a collides with the cooling water 10 creating a turbulent zone in which the air/water interface is constantly and rapidly renewed. By thorough mixing of the exhaust air flow 2a and the washing cooling water 10 in the scrubber 28, washing is effectively achieved.

Air is saturated when it reaches its water vapor capacity and contains all of the water vapor it can hold at a particular temperature. In saturated air, the water vapor content is equal to its capacity. At the dew point temperature, the air is saturated, and the dew point is the temperature at which the relative humidity of the air is 100%. In the scrubber 28, the dew point temperature of the cooling water 10 is lower than the dew point temperature of the exhaust air stream 2 a.

The dried and cooled exhaust air stream 2b-c is discharged from the scrubber 28 via one or more exhaust air outlets 12 located in the top portion 13 of the scrubber housing 5. Scrubber 28 may include an outlet 12 to atmosphere, as shown in fig. 4, which is used during, for example, start-up or pause of scrubber 28.

In another embodiment of the method and arrangement, the state of the exiting exhaust air streams 2b-c may be controlled. The arrangement comprises means for measuring the temperature and moisture content of the exhaust air stream 2b-c leaving the scrubber 28. The temperature and moisture content of the exit air stream 2b-c from scrubber 28 may be measured by, for example, wet and dry bulb thermometers, or other online humidity measurement devices may be used.

The arrangement comprises means 15 for comparing the measured temperature and moisture content values of the exiting exhaust air stream 2b-c with set values. The comparison may be performed using a comparison algorithm provided in the control device or the computer, for example.

In the method, the temperature and moisture content of the exit air stream 1b-c leaving the scrubber 28 are measured, the measured temperature and moisture content values are compared with set values for the exit air stream 2b-c, and the cooling water 10 flowing to the plurality of nozzles 11 and the pressure at the plurality of nozzles 11 are controlled based on the comparison results.

The set value of the exit air stream 2b-c from the scrubber 28 depends on the drying process 1 into which the exit air stream 2b-c is recirculated. However, the moisture content of the exiting exhaust air stream 2b-c needs to be sufficiently low to have free water vapor capacity. The maximum value of the set point range for the temperature of the exhaust air stream 2b-c exiting from the scrubber 28 preferably comprises 35 c, and the maximum value of the set point range for the moisture content of the exhaust air stream 2b-c exiting from the scrubber 28 preferably comprises 35 grams of water vapor per kilogram of air.

Furthermore, suitable set points for the temperature of the exhaust air stream 2b-c exiting the scrubber 28 may comprise 25-35 ℃, and suitable set points for the moisture content of the exhaust air stream 2b-c exiting the scrubber 28 may comprise 20-35 grams of water vapor per kilogram of air, preferably 25-30 grams of water vapor per kilogram of air. The arrangement further comprises means for controlling the cooling water 10 flowing to the plurality of nozzles 11 and the pressure at the plurality of nozzles 11 based on the comparison result. The control device 16 is capable of reducing or increasing the cooling water 10 flowing to the plurality of nozzles 11 and reducing or increasing the pressure at the plurality of nozzles 11. The control of the cooling water 10 flowing to the plurality of nozzles 11 and the pressure at the plurality of nozzles 11 may be performed with a pump 17 or a valve (or with, for example, an adjustable nozzle).

In fig. 1, the scrubber 28 comprises 3 consecutive modules 18a-c stacked on top of each other in the vertical direction y. The module 18 comprises an array of multiple spray nozzles 11 (for injecting droplets 9 of cooling water 10), mass transfer zones 19a-c (in which the exhaust air stream 2a collides with the droplets 9 of cooling water 10), droplet separators 20a-c (for separating droplets from the exhaust air stream 2), and water collection pans 21a-c (for collecting droplets separated from the droplet separators 20 a-c). Mass transfer zones 19a-c are created beneath each row of multiple nozzles 11. Droplet separators 20a-c and catchment trays 21a-c are arranged above the plurality of nozzles 11, as the exhaust air flow 2a moves upwards in the vertical direction y. The number of modules in a scrubber is preferably 2-5. In some cases, only one module 18a-c may be used if the required reduction in moisture content and temperature of the moisture laden exhaust air stream 2a is small.

In the method, droplets 9 of cooling water 10 are caused to be injected into mass transfer zones 19a-c created below a row of multiple nozzles 11, and droplets moving upward with the exhaust air stream 2a are separated in droplet separators 20a-c, and droplet separators 20a-c are drained to water collection pans 21 a-c. Droplet separators 20a-c are arranged above water collection pans 21a-c and may also include shapes for creating a falling film from the separated droplets.

The arrangement may also include means for controlling the size of the droplets 22 of cooling water 10 in each module 18 a-c. The droplets 9 are generally described as their perceived size, i.e., diameter. Control of the droplet size may be performed by means of the pump 17 and the tank 27 (or by means of e.g. an adjustable nozzle).

In the method, the size of the droplets 9 of the cooling water 10 may be controlled. The size of the droplets 9 of cooling water 10 is controlled by controlling the pressure at the row of multiple nozzles 11 and the size of the droplets 9 is controlled separately for each module 18 a-c. Preferably, the size of the droplets 9 is smallest at said plurality of nozzles 11 located at the bottom part 6 of the shell 5, increasing in a vertical y towards higher positioned nozzles, and largest at said plurality of nozzles 11 at the top part 13 of the shell 5.

In fig. 1, cooling water 10 is first fed to the plurality of nozzles 11 in the top portion 13 of the shell 5, i.e. to the module 18c in the vertically uppermost position. The subsequent modules 18a-b then use the cooling water 10 collected from the previous module 18c in a vertically descending order. This type of design minimizes the use of cooling water 10 in the scrubber 28. However, other designs are possible. For example, if the temperature of the cooling water 10 is high compared to the desired moisture content and temperature value of the exhaust air stream 2b-c leaving the scrubber 28, it may be desirable to provide the first two modules 18b-c located at the highest positions with cooling water 10 at its initial temperature.

To enhance the moisture removal of the exhaust air stream 2a, the scrubber 28 may comprise at least two droplet separators 20a-c, and the droplet separator 20c mounted vertically above in the y-direction is connected to a flowing cooling medium for providing a cooled separator surface. The method may then comprise the step of cooling the vertically upper droplet separator 20c with a flowing cooling medium for providing a cooled separator surface.

The arrangement comprises means 29 for recirculating at least part of the dried and cooled exhaust air stream 2b-c leaving the scrubber 28 to the drying process 1, wherein in the drying process 1 moisture is removed from the product into the dried and cooled exhaust air stream 2b, resulting in a moisture laden exhaust air stream 2a which is directed to the scrubber 28. The means 29 for recirculation may comprise, for example, a pipe and a blower.

The cooling water 10 may include raw water. In the scrubber 28, the cooling water used can then be used for the production of the product dried in the drying process 1. This arrangement reduces the energy consumption of the plant comprising the drying process, since the heat obtained from the reduction of the water vapour in the exhaust air stream is transferred to the cooling water 10.

In many geographical locations, the temperature of the raw water is low during winter. The low temperature of the cooling water 10 enhances the moisture removal of the exhausted air 2a in the scrubber 28. This is advantageous because at the same time the possibility of visible water vapour plume development becomes high if the exhaust air stream 2a is vented to atmosphere.

Preferably, the cooling water 10 is filtered to reduce the amount of possible solid particles therein, to avoid clogging of the plurality of nozzles 11. The cooling water 10 in the scrubber 28 may include water soluble substances and aqueous solutions, for example, liquid caustic soda. In addition, it may include chemicals for reducing the surface tension of water. The method for recirculating air may comprise the steps of: the cooling water 10 is filtered, and chemicals are added to the cooling water 10 for reducing the surface tension of the cooling water 10.

As the cooling water 10 flows downward in the scrubber 28, the temperature of the cooling water 10 increases. The temperature of the cooling water 10 leaving the scrubber bottom 6 is preferably 2-3 deg.c lower than the temperature of the moisture laden exhaust air stream 2a at the inlet 7 on the bottom 6 of the scrubber 28. The bottom 6 of the scrubber 28 comprises an outlet 23 for discharging cooling water 10 from the scrubber 28.

Figure 2 shows an arrangement for recycling air in the drying process 1 with a multi-stage heat recovery system 24. The arrangement of figure 1 is connected to a three-stage heat recovery system 24 comprising 3 heat recovery heat exchangers 25a-c arranged in series. The heat recovery heat exchangers 25a-c are equipped with washers 26a-c for batch cleaning.

In the first stage heat recovery heat exchanger 25a, the moisture laden exhaust air stream 2a transfers heat to the inlet air of the drying process, i.e. to the dried and cooled exhaust air stream 2b flowing out of the device 3. In fig. 2, the apparatus 3 comprises a scrubber 28. In the second and third heat

recovery heat exchangers

25b and 25c, the moisture-laden exhaust air stream 2a transfers heat to other flowing media. The moisture laden exhaust air stream 2a is directed from the multi-stage heat recovery system 24 to a device 3, such as a scrubber 28.

In fig. 2, the dried and cooled exhaust air stream 2b leaving the device 3, e.g. the scrubber 28, flows into the heat recovery exchanger 25a to be heated before it is recycled to the drying process 1.

The arrangement may comprise two or more devices 3 arranged in parallel for increasing the air recirculation capacity of the drying process 1. When several devices 3 are arranged in parallel, it is preferred that the set values of the exhaust air streams 2b-c exiting from each device 3 are substantially identical. The moisture laden exhaust air stream 2a from the drying process 1 is then directed to two or more devices 3 arranged in parallel.

In an arrangement comprising a scrubber, the dimensions of the scrubber 28 depend on the drying process 1 from which the exiting exhaust air stream 2a-c comes and is recirculated into, and on the available cooling water temperature 10 for drying and cooling of the moisture laden exhaust air stream 2 a. The method for determining the dimension comprises the following steps: determining a scrubber diameter based on the volumetric flow rate of the moisture laden exhaust air stream 2a and the velocity of the moisture laden exhaust air stream 2a through the scrubber 28; determining the number and height of modules 18a-c required for dehumidification and cooling based on the mole fraction of the exhaust air stream 2a at the inlet and outlet and based on the required residence time of the exhaust air stream 2a in the flow of droplets 9; the total volume of the spray sections is calculated based on the number of modules 18a-c, the total volumetric gas phase mass transfer coefficient, and the flow rate of the gas phase.

Fig. 3 shows an embodiment of an arrangement for recirculating air in a drying process, wherein the arrangement comprises a cleaning unit 30. In fig. 3, the moisture laden exhaust air stream 2a from the drying process 1 is directed to a cleaning unit 30 for cleaning the exhaust air stream 2a before it enters the device 3, which device 3 reduces the moisture content of the exhaust air stream 2 a. The cleaning unit 30 is arranged in front of the device 3 in terms of the flow direction of the exhaust air flow 2 a. The cleaning unit 30 cleans the exhaust air stream 2a by reducing the contaminant content of the exhaust air stream 2 a. The cleaning unit 30 cleans the exhaust air flow 2a by removing at least part of the undesired substances from the moisture-laden exhaust air flow 2 a. Examples of removable substances are ash, dust, odorous vapors and water-soluble chemicals.

The cleaning unit 30 includes a scrubber. In the scrubber 30, the exhaust air stream 2a flows upward, and droplets of washing water 31a are injected into the exhaust air stream from the plurality of nozzles 11. The discharge air stream 21a is washed by washing water 31 a. The wash water 31a is recycled, preferably with the addition of some fresh water 31 b. Cleaning chemicals 32, such as NaOH, may also be added. The water overflows 31c and the excess water overflows to water treatment.

The circulation of the washing water 31a is relatively closed, wherein the amount of fresh water 31b added represents 18-25% of the circulated washing water 31 a.

The cleaned exhaust air flow 2a is directed from the top of the cleaning unit 30 via the inlet opening 7 to the bottom part 6 of the device 3. As shown in fig. 3, the cleaning unit 30 and the device 3 are two separate apparatuses connected by a flow passage 33. The cleaning unit 30 is not contained within the housing 5 of the device 3.

The cleaning unit 30 reduces the pollutant content of the exhaust air flow 2a entering the device 3. Thus, the contaminant content of the cooling water 10 for the device 3 (e.g., scrubber 28) is kept at a low level. Thereby, the cooling water 10 with a low contaminant level can then be used more efficiently in the device 3, requiring less fresh water. For example, the washing water 31a consumed in the cleaning unit 30 accounts for 15 to 25% of the cooling water 10 of the apparatus 3.

Fig. 4 shows an embodiment of an arrangement for recirculating air in a drying process, wherein the arrangement comprises a cleaning unit 30. In fig. 4, the moisture laden exhaust air stream 2a from the drying process 1 is directed to a cleaning unit 30 for cleaning the exhaust air stream 2a before it enters the device 3, which device 3 reduces the moisture content of the exhaust air stream 2 a. The cleaning unit 30 is arranged within the housing 5 of the device 3. The cleaning unit 30 is arranged at the bottom part 6 of the housing 5 surrounding the device 3. The cleaning unit 30 is in front of the device 3, wherein the moisture content of the exhaust air stream 2a decreases in the flow direction of the exhaust air stream 2 a. The cleaning unit 30 cleans the exhaust air stream 2a by reducing the contaminant content of the exhaust air stream 2 a. The cleaning unit 30 cleans the exhaust air flow 2a by removing at least part of the undesired substances from the moisture-laden exhaust air flow 2 a. Examples of removable substances are ash, dust, odorous vapors and water-soluble chemicals.

The cleaned exhaust air flow 2a flows upward from the top of the cleaning unit 30. In fig. 4, the apparatus 3 comprises a scrubber 28 comprising 2 consecutive modules 18d-e stacked on top of each other in a vertical direction y. The cleaned exhaust air flow 2a flows upwards from the cleaning unit 30, i.e. from the washer 31 containing the washing water to the vertically lowest module 18c of the washer 28.

The vertically lowest module 18c of the scrubber 28 comprises an outlet 23 for discharging the cooling water 10 from the scrubber 28.

As shown in fig. 4, the cleaning unit 30 and the device 3 are two separate apparatuses housed in the same housing (the case 5 of the device 3). The scrubber 31 containing washing water that cleans the exhaust air stream 2a and the scrubber 28 that reduces the moisture content of the exhaust air stream 2a have separate washing water circulation, i.e. the washing water 31a is separate from the cooling water 10.

The cleaning unit 3 reduces the pollutant content of the exhaust air flow 2a entering the device 3. Thus, the contaminant content of the cooling water 10 for the device 3 (e.g., scrubber 28) is kept at a low level. Thereby, the cooling water 10 with a low contaminant level can then be reused more efficiently in the device 3, requiring less fresh water. For example, the washing water 31a consumed in the cleaning unit 30 accounts for 15 to 25% of the cooling water 10 of the apparatus 3.

The operation of the scrub column 28 is explained as described with respect to fig. 1.

The embodiment shown in fig. 4, in which the cleaning unit 30 is housed in the same housing of the device 3, provides space savings.

Fig. 5 shows an embodiment of an arrangement for recirculating air in a drying process comprising a clean room, a make-up air flow and a venturi scrubber. Figure 5 shows the arrangement of figure 3 provided with a supplementary air flow and a venturi scrubber between the cleaning unit 30 and the device 3. In fig. 5, a cleaning unit 30 for cleaning the exhaust air flow 2a is shown similar to the cleaning unit 30 shown in fig. 3, and the operation thereof is explained as described in relation to fig. 3.

The exhaust air flow 2a is cleaned in the cleaning unit 30 and the cleaned exhaust air flow 2a is directed to one or more heat exchangers 34. In the one or more heat exchangers 34, the exhaust air stream 2a is cooled with a make-up air stream. The heat exchanger 34 is an air-to-air heat exchanger of the type, for example, a cross-flow heat exchanger. The supplementary air stream preferably comprises outside air.

The cooled exhaust air stream 2a in the heat exchanger 34 is mixed with a make-up air stream in a mixing chamber 36. The mixture comprises 30-60%, preferably 45-55%, of the stream of exhaust air 2 a. The relative humidity of the air stream comprising cooled exhaust air stream 2a mixed with the make-up air stream is significantly below 100%. The purpose of the mixing is to reduce the relative humidity of the air stream comprising the exhaust air stream 2a, thereby effecting a phase change in the venturi scrubber 37.

An additional blower 8 may be used to balance the exhaust air stream 2a with the make-up air stream.

The exhaust air stream 2a mixed with the make-up air stream is directed to a venturi scrubber 37. In the venturi scrubber 37 the exhaust air stream 2a mixed with the make-up air stream is cooled. The venturi scrubber 37 includes a quench section that provides evaporative cooling. The venturi scrubber 37 further comprises a high pressure pump 38 to generate small droplets, i.e. mist, with high water pressure for the venturi scrubber 37. As shown in fig. 5, the air stream comprising the exhaust air stream 2a mixed with the make-up air stream flows downward in a vertical direction in the venturi scrubber 37.

As shown in fig. 5, the one or more heat exchangers 34, the mixing chamber 36 and the venturi scrubber 37 are mounted between the cleaning unit 30 and the device 3 in the flow direction of the exhaust air flow 2 a.

The use of a supplementary air flow and venturi scrubber 37 after cleaning of the exhaust air flow 2a functions as follows: the exhaust air stream 2a mixed with make-up air stream entering the device 3 is cooled and dried more than the moisture laden exhaust air stream 2a from the drying process 1.

In another embodiment of the arrangement for recirculating air in the drying process, the arrangement comprises an assembly as shown and described in fig. 5, with the difference that a heat exchanger 34 is used for cooling the exhaust air stream 2a with a supplementary air stream. After the exhaust air stream 2a is cleaned in the cleaning unit 30, the exhaust air stream 2a is directed to the mixing chamber 36. The make-up air stream is also directed to mixing chamber 36 where exhaust air stream 2a is mixed with the make-up air stream.

The use of a supplementary air flow increases the volume of air flow from the device 3. Then, for example, the air stream 2b divided to the drying process represents 32-18% of the exiting exhaust air stream 2b-c, and the air stream 2c divided to the at least one chamber associated with the drying process represents 68-82% of the exiting exhaust air stream 2 b-c.

In the fig. 3 and 5, the device 3 for reducing the moisture content of the exhaust air flow 2a is similar to the device 3 shown in fig. 1 and its operation is explained as described in relation to fig. 1.

The embodiments of the arrangements shown in figures 3, 4 and 5 for recycling air during drying are also applicable to the arrangement shown in figure 2 comprising a multi-stage heat recovery system 24. The moisture laden exhaust air stream 2a is directed from the multi-stage heat recovery system 24 of fig. 2 to a cleaning unit 30. Furthermore, in fig. 3, 4 and 5, the shown exhaust air stream 2b leaving the device 3 then flows into a heat recovery exchanger 25a as shown in fig. 2 for heating, before it is recycled to the drying process 1.

With the arrangement and method for recirculation, moisture laden exhaust air can be recirculated within a plant containing the drying process without venting it to the atmosphere. Most of the recirculated exhaust air may be recycled in the drying process. This increases system efficiency and saves energy. Atmospheric emissions and the formation of water vapor plumes can be avoided.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

List of parts: 1 drying process, 2a-c exhaust air stream, 3 device, 4 bottom surface, 5 shell, 6 bottom part, 7 inlet opening, 8 blower, 9 droplets, 10 cooling water, 11 multiple nozzles, 12 outlet, 13 top part, 14 device for measuring temperature and moisture content of air stream, 15 contrast device, 16 control device, 17 pump, 18a-c module, 19a-c mass transfer area, 20a-c droplet separator, 21a-c water collection tray, 22 droplet size control device, 23 discharge outlet, 24 heat recovery system, 25a-c heat recovery exchanger, 26a-c washer, 27 tank, 28 washer, 29 recirculation device, 30 cleaning unit, 31a-c cleaning water, 32 cleaning chemicals, 33 flow channel, 34 heat exchanger, 36 mixing chamber, 37 venturi washer, a 38 venturi tube scrubber pump,

y is the vertical direction.

Claims

1. A method for recirculating air in a drying process within a plant, wherein moisture is removed from a product in the drying process into an air stream, producing an exhaust air stream, characterized in that the exhaust air stream is directed to a cleaning unit for cleaning the exhaust air stream, and then to a second scrubber, wherein the moisture content of the exhaust air stream is reduced, and the exhaust air stream leaving the second scrubber is divided into at least: an air stream that is recirculated to the drying process and an air stream that reaches at least one chamber associated with the drying process,

cleaning the exhaust air stream comprises washing the exhaust air stream with wash water in a first scrubber,

after cleaning, the exhaust air stream is mixed with a make-up air stream so that the mixture contains 30-60% of the exhaust air stream, and then cooled in a venturi scrubber.

2. Method for recycling air in a drying process within a plant according to claim 1, characterized in that said cleaning unit is arranged in the shell of said second scrubber.

3. Method for recycling air in a drying process in a plant according to claim 1, characterized in that after cleaning the exhaust air stream is cooled with a supplementary air stream in one or more heat exchangers.

4. The method for recycling air in a drying process within a plant of claim 3, wherein an exhaust air stream is mixed with the make-up air stream after the exhaust air stream is cooled.

5. Method for recycling air in a drying process within a plant according to any one of claims 1 to 4, characterized in that the air flow divided into the air flow recycled to the drying process represents 65-85% of the exhaust air flow leaving the second scrubber and the air flow divided into the air flow reaching at least one compartment associated with the drying process represents 35-15% of the exhaust air flow leaving the second scrubber.

6. The method for recycling air in a drying process within a plant of claim 1, further comprising the step of heating, cooling or ventilating at least one chamber associated with the drying process with an air flow to the at least one chamber associated with the drying process.

7. The method for recycling air in a drying process within a plant of claim 1 further comprising the step of heating the exhaust air stream exiting the second scrubber in at least one heat recovery heat exchanger prior to recycling the exhaust air stream exiting the second scrubber to the drying process.

8. The method for recycling air in a drying process within a plant of claim 7, further comprising the step of directing the exhaust air stream from the drying process to the at least one heat recovery heat exchanger prior to directing the exhaust air stream to the second scrubber.

9. Method for recycling air in a drying process in a plant according to claim 1, characterized in that in the method the exhaust air flow is directed to two or more second scrubbers arranged in parallel.

10. The method as claimed in claim 1, for recycling air in a drying process in a plant, wherein the drying process comprises a continuous high energy drying process, wherein the resulting exhaust air stream contains 130-160 g of water vapour per kg of air.

11. The method for recycling air in a drying process within a plant of claim 1, wherein directing the exhaust air stream comprises pressurizing the exhaust air stream with at least one blower.

12. An arrangement for recirculating air in a drying process within a plant, the arrangement comprising an exhaust air flow from the drying process, characterized in that the arrangement comprises: a cleaning unit for cleaning a stream of exhaust air and a second scrubber for reducing the moisture content of the stream of exhaust air, and the cleaning unit comprises a first scrubber containing wash water and is arranged before the second scrubber in the flow direction of the stream of exhaust air, and the arrangement comprises: means for dividing the exhaust air stream leaving the second scrubber into at least an air stream that is recirculated to the drying process and an air stream that reaches at least one chamber associated with the drying process,

the arrangement comprises: a mixing chamber located after the cleaning unit, in the direction of flow of the exhaust air stream, for mixing the exhaust air stream with a supplementary air stream such that the mixture comprises 30-60% of the exhaust air stream,

the arrangement comprises: a venturi scrubber located after the mixing chamber, in the flow direction of the exhaust air stream.

13. An arrangement for recirculation of air in a drying process within a plant according to claim 12, characterised in that the arrangement comprises: one or more heat exchangers located after the cleaning unit in terms of the flow direction of the exhaust air flow.

14. An arrangement for recirculation of air in a drying process within a plant according to claim 12, characterized in that the cleaning unit is arranged in the housing of the second scrubber.