Method for the devaporization of process exhaust air

Abstract

The invention relates to a method for the devaporization of process exhaust air generated by a plant that produces or processes a fibrous web, in particular a pulp web, paper web or cardboard web, wherein the installation has a dry end in which the fibrous web is at least partially dried by means of contact drying, wherein supply air is supplied to the dry end and exhaust air is discharged from the dry end, and wherein the supply air has a higher temperature and a lower relative humidity than the exhaust air, and a partial flow is branched off from the supply air and mixed with the exhaust air before being discharged into the surroundings, thus producing the process exhaust air.

Description

[0001] The invention relates to methods for de-vaporising process exhaust air from a plant for producing or processing a fibrous web according to the preamble of claim 1.

[0002] During the production of fibrous webs such as paper or pulp webs, a large amount of very humid exhaust air is generated, particularly during the drying process. If this air is released directly into the environment, visible fog can form under suitable climatic conditions.

[0003] This fog can fall as precipitation in the immediate vicinity of the turbine and lead to slippery roads, especially in winter. Low-hanging fog can also impair visibility in traffic.

[0004] Therefore, legal frameworks often require operators to ensure that no visible water vapor plumes are emitted from the plants.

[0005] In the past, reducing visible fog required additional energy expenditure to heat the exhaust air flow to a point beyond the condensation point, sometimes supported by pre-condensing some of the air humidity. Since this additional energy expenditure is not cost-effective, it was rarely implemented.

[0006] The state of the art for avoiding such a water vapor plume is, for example, the product ERCS - Type04 from Scheuch GmbH in Austria. (www.scheuch.com). Cold ambient air is preheated via a heat exchanger and then mixed with the moist exhaust air.

[0007] One disadvantage of the state of the art is that the technical effort required for vapor removal is not insignificant, as a separate heat exchanger and a blower system with suitable control must be provided.

[0008] Since a considerable amount of outside air must be additionally extracted, this vapor removal also increases the energy consumption of the system.

[0009] Finally, the cooling of the process air leads to condensation inside the exhaust air system, which can, among other things, lead to increased corrosion.

[0010] The object of the present invention is therefore to propose a simple and cost-effective method that effectively reduces or prevents the formation of fog.

[0011] It is a further object of the invention to propose a method which requires no or only minimal additional expenditure on energy and / or structural measures.

[0012] An improvement is described in the applicant's patent application DE102019114467. Therein, a method is described for dehumidifying process exhaust air from a plant for producing or processing a fibrous web, in particular a pulp, paper, or board web, wherein the plant comprises a drying section in which the fibrous web is at least partially dried by contact drying, wherein a first exhaust air is generated, and wherein the plant comprises a second source that generates a second exhaust air that is mixed with the first exhaust air, characterized in that the second exhaust air has a higher temperature and a lower relative humidity than the first exhaust air.

[0013] The object of the present invention is to provide an alternative to the method described in DE102019114467, which is cost-effective and very widely applicable, and in particular can also be retrofitted to existing systems.

[0014] The object is achieved by a method according to claim 1. Advantageous embodiments are described in the subclaims.

[0015] The method is a method for dehumidifying process exhaust air from a plant for producing or processing a fibrous web, in particular a pulp, paper, or board web. The plant comprises a drying section in which the fibrous web is at least partially dried by contact drying. Supply air is supplied to the drying section and exhaust air is discharged from the drying section. The supply air has a higher temperature and a lower relative humidity than the exhaust air. According to the invention, a partial flow is branched off from the supply air and mixed with the exhaust air before being discharged to the environment, thereby generating process exhaust air.

[0016] This process has the significant advantage that it can be retrofitted into an existing system without significant additional costs. In contrast to the state of the art in DE102019114467, it is irrelevant whether additional units are available in addition to the dryer section that generates the moist exhaust air. Vapor reduction can be achieved using the existing infrastructure for the dryer section alone. Only a line for diverting the partial flow is required.

[0017] As mentioned, this allows the retrofitting of a de-steaming system even in existing systems.

[0018] Preferably, it can be provided that so much of the partial flow of the supply air is mixed with the exhaust air that the dew point of the process exhaust air is below the ambient temperature of the system, in particular at least 5 ° below the ambient temperature of the system.

[0019] Alternatively or additionally, it can be provided that the relative humidity [%] of the process exhaust air is at least 5% points, preferably at least 10% points below the relative humidity of the exhaust air

[0020] It is advantageous if a supply air fan is provided in the supply air duct, and the partial supply air flow is diverted after the supply air fan. This eliminates the need for a separate fan to convey the partial flow, and the partial flow is conveyed by the existing supply air fan.

[0021] In an alternative design, the partial supply air flow can be diverted upstream of the supply air fan. In this case, an additional pusher fan is expediently provided in the duct for the partial supply air flow to convey the partial flow. This has the advantage that the amount of air discharged as a partial flow can be changed by changing the power of the pusher fan without having to change anything on the supply air fan. In some installations, for example, the supply air fan and the exhaust air fan are linked together as part of a dew point control of the dryer hood. In such cases, it is difficult to change the amount of partial flow without causing disruptions in the dew point control.

[0022] In order to be able to change the amount of partial flow conveniently and reliably using a thrust fan, it is advantageous if the thrust fan is designed to be frequency-controlled.

[0023] Furthermore, when using such a thrust fan, it is advantageous that the position of the branching of the partial flow can be freely selected and, in particular, can also be provided in front of the supply air fan.

[0024] Furthermore, it is advantageous for the supply air to be conditioned, particularly heated, and for the partial supply air flow to be diverted after or between the individual conditioning stages. This heating is usually necessary anyway for the use of the supply air in the dryer section, and the vapor removal effect of the partial supply air flow is thereby increased.

[0025] Furthermore, it may be advantageous if an exhaust air fan is provided in the exhaust air duct and the supply air is mixed with the exhaust air after the exhaust air fan.

[0026] Alternatively, it may also be advantageous to mix the supply air with the exhaust air upstream of the exhaust fan. For example, during renovations, local conditions may require the partial flow of supply air to be integrated upstream of the exhaust fan. The basic function of the process is retained.

[0027] The pressure generated by the supply air fan is usually higher than the pressure generated by the exhaust air fan. This ensures that the partial supply air flow is safely mixed into the exhaust air flow. This prevents unwanted backflow of the exhaust air into the supply air partial flow duct.

[0028] It may be advantageous to measure the temperature in the exhaust air and / or the supply air and / or the process exhaust air.

[0029] Alternatively or additionally, it may be advantageous to measure the humidity in the exhaust air and / or the supply air and / or the process exhaust air.

[0030] In a preferred embodiment, it can be provided that means, in particular adjustable flaps, are provided to change the amount of the partial flow of the supply air to the exhaust air.

[0031] Alternatively or additionally, these measures can also be implemented by installing a frequency-controlled thrust fan in the partial flow line, and using this control to adjust the amount of the partial flow. This adjustment via a frequency-controlled fan is usually more reliable and precise than via a controlled damper.

[0032] The diverted partial flow required for vapor removal typically comprises between 5% and 20% of the total supply air volume. Depending on weather conditions, however, it may be necessary to temporarily add no supply air at all (=0%) to the exhaust air. The diverted partial flow can be adjusted using adjustable dampers or other suitable means.

[0033] In very advantageous embodiments, it can be provided that the amount of the partial flow of the supply air to the exhaust air is controlled or regulated via the flow rate of the thrust fan, wherein in particular the flow rate of the thrust fan is increased or decreased depending on the flow rate of the exhaust air fan.

[0034] It may be advantageous to increase the output of the pusher fan when reducing the output of the exhaust air blower, or to reduce the output of the pusher fan when increasing the output of the exhaust air blower. A reduction in the amount of exhaust air conveyed per unit of time from the dryer section, under otherwise constant production conditions, results in an increase in the humidity of the exhaust air. The inventors surprisingly discovered that in this case, despite the lower amount of exhaust air, a larger amount of dry supply air should often be added to prevent steam formation.

[0035] Furthermore, this process allows for an increase in the discharge velocity of the process exhaust air at the roof outlet. In particularly advantageous designs, this process can be used to achieve a constant discharge velocity as described above. This is often particularly relevant with regard to emissions assessments.

[0036] In particularly advantageous embodiments, it can be provided that up to a certain threshold of the exhaust air fan (e.g., at maximum exhaust air fan power), the thrust fan is completely switched off or operated at a minimum speed. If the exhaust air fan power is reduced below this threshold, the thrust fan power is increased.

[0037] Suitable curves can be stored to determine the exact relationship between reducing the exhaust fan speed and increasing the thrust fan speed. These curves can be determined for each system.

[0038] For example, a 'bang-bang' control is possible, in which the thrust fan is run to maximum power as soon as the power of the exhaust air fan falls below a threshold.

[0039] Alternatively, a gradual increase is also possible when further thresholds are reached.

[0040] Alternatively, a functional relationship, for example a linear or quadratic relationship between the reduction of the exhaust air fan and the increase of the thrust fan, can be stored.

[0041] Alternatively, a functional relationship can be stored that is adapted to any necessary integration of the partial flow of supply air in front of the exhaust air fan.

[0042] Furthermore, it is expedient if the quantity of the partial flow of supply air that is mixed with the exhaust air is adjusted depending on the temperature and / or humidity of the exhaust air and / or process exhaust air and / or supply air and / or the environment.

[0043] In order to ensure good mixing of the exhaust air and the partial flow of the supply air, it can be provided that the partial flow of the supply air and the exhaust air are mixed by means of a static mixer.

[0044] Often, the process exhaust air is released into the environment at a height of more than 40m, especially 60m or more above the ground.

[0045] The invention is further explained below with reference to schematic figures. The invention is not limited to this embodiment. Figure 1 shows a scheme for a method according to one aspect of the invention. Figure 2 shows a scheme for a method according to another aspect of the invention. Figure 3shows a scheme for a method according to another aspect of the invention.

[0046] The dryer section 1 of a machine for producing a paper or pulp web is usually enclosed by a hood. The air in the hood must be able to absorb the water removed from the paper web. To prevent the hood from becoming saturated and no longer able to absorb the water, a portion of the hood air is continuously removed as exhaust air 2. Since this warm exhaust air 2 still contains some energy, the exhaust air 2 is cooled by one or more heat exchangers 3, 4. In the Figure 1 The design shown includes an air / air heat exchanger 3 for preheating ambient air 8 and an air / water heat exchanger 4 for preheating, for example, process water. An exhaust fan 5 is arranged behind it, which conveys the exhaust air toward, for example, a chimney, etc.

[0047] At the same time, fresh supply air 12 is continuously supplied to the hood or drying section 1. This supply air 12 should be as warm and dry as possible. For this purpose, supply air 8—usually ambient air 8—is drawn in by a supply air fan 11 and conditioned through one or more stages. Figure 1 For this purpose, a heat exchanger 3 is provided, which preheats the ambient air 8 using the exhaust air 2. In addition, a first heater 9 and a second heater 10 are provided, which further heat the air using condensate, steam, or other suitable heating media. The conditioning devices 3, 9, 10 are arranged in Figure 1all arranged upstream of the supply air fan 11. Between the supply air fan and the drying section 1, a partial flow of the supply air 13 is branched off from the supply air. This partial flow 13 is then mixed with the exhaust air 2. The resulting process exhaust air 7 is discharged to the environment, usually via a silencer 6 and a suitable chimney. An adjustable damper 14 or other suitable means can be provided to adjust or meter the amount of the partial flow 13.

[0048] The mixing of the partial flow of the supply air 13 with the exhaust air 2 takes place after cooling by the heat exchangers 3, 4, and after the exhaust air fan 5. Since the supply air fan 11 generally generates a greater pressure than the exhaust air fan 5, a flow of the exhaust air 2 'in the wrong direction', i.e. into the line of the partial flow 13, can be largely excluded.

[0049] The Figure 1This clearly demonstrates how easily the proposed dehumidification process can be integrated into an existing system. All that's required is the installation of the duct (shown in dashed lines) for the partial supply air flow 13, as well as, if necessary, an adjustable damper (14) for metering the partial flow 13. Furthermore, with the exception of the damper (14), this solution is virtually maintenance-free.

[0050] The volume of partial flow 13 is relatively small, both compared to the total supply air 12 and compared to the exhaust air 2, and usually comprises between 5% and 20%, often between 10% and 15%, of the total supply air and / or exhaust air volume. Depending on the weather conditions, however, at times no supply air (=0%) may be necessary to add to the exhaust air.

[0051] The following values ​​can be assumed for the different air flows: Table 1: Possible parameters of the air flows Air temperature Rel. humidity exhaust air 75-95 [°C], (from hood) e.g. 85 [°C] exhaust air 50-65 [°C] 90%- 98% (after heat exchangers) e.g. [58°C] e.g. 96% Supply air and 100-120 [°C] 1%-5% Partial flow of the supply air e.g. 110 [°C] e.g. 2% Process exhaust air 55-70 [°C] 70%-80% e.g. 6 [°C] e.g. 78%

[0052] In a typical application, exhaust fan 5 conveys approximately 156,000 m³ / h of exhaust air 2 at a temperature of 58°C and a relative humidity of 96%. If this exhaust air 2 were released directly into the environment, it would almost certainly result in the formation of vapor or fog.

[0053] If this exhaust air 2 is mixed with a partial flow 13 of the supply air of approximately 11,000 m³ / h at a temperature of 110°C and a relative humidity of 2%, the resulting process exhaust air 7 is approximately 167,000 m³ / h at a temperature of 61°C and a relative humidity of only 78%. The tendency to vapor formation has been massively reduced.

[0054] The execution in Figure 2 differs from the variant in Figure 1by branching off the partial flow 13 of the supply air 12 before the supply air fan 11. In order to convey the partial flow 13, an additional thrust fan 15 is provided in the duct. This thrust fan 15 can, in particular, be frequency-controlled. In this case, the amount of the partial flow 13 can be changed directly via the thrust fan 15, alternatively or in addition to a flap 14. In particular, the supply air fan 11 can be operated unchanged even if the partial flow 13 changes. In the design in Figure 2Partial flow 13 is discharged directly upstream of the supply air fan. Alternatively, this branch can be arranged further forward, for example, between heaters 9 and 10, or after heat exchanger 3. Partial flow 13 of the supply air may then have a slightly lower temperature than the main flow of supply air 12. However, this may be sufficient for the desired vapor removal, thus saving costs for heaters 9 and / or 10.

[0055] The Figure 3The diagram shown is particularly suitable for carrying out a process in which the amount of the partial flow of the supply air 13 to the exhaust air 2 is controlled or regulated via the flow rate of the pusher fan 15, wherein the flow rate of the pusher fan 15 is increased or decreased depending on the flow rate of the exhaust air blower 5. In such processes, the power of the pusher fan 15 can sometimes be reduced to such an extent - if necessary even switched off - that the partial flow of the supply air 13 is only conveyed at a very low pressure. If this pressure is below the pressure prevailing in the line of the exhaust air 2, this can lead to the moist exhaust air 2 being pushed back into the drying hood of the drying section 1 through the line of the partial flow of the supply air 13 or the supply air 12. To avoid this, as in Figure 3shown, it is generally advantageous if a flap 14a is provided to prevent such backflow. Since the flap 14a, in contrast to the flap shown in Figure 1 Since the design shown is not used to adjust the flow rate, it does not need to be designed as an adjustable flap 14. It is perfectly sufficient if the flap 14a can switch between the "open" and "closed" positions. Reference symbol

[0056] 1Dryer section 2Exhaust air 3Heat exchanger (air / air) 4Heat exchanger (air / water) 5Exhaust air fan 6Silencer 7Process exhaust air 8Supply air / ambient air 9First heater 10Second heater 11Supply air fan 12Supply air 13Partial flow of the supply air 14Adjustable damper 14aDamper 15Thrust fan

Claims

1. Method for demisting process exhaust air (7) of an installation for producing or processing a fibrous web, in particular a pulp, paper or board web, the installation comprising a drying section (1) in which the fibrous web is at least partially dried by means of contact drying, wherein a supply air (12) is supplied to the drying section (1) and an exhaust air (2) is discharged from the drying section (1), and wherein the supply air (12) has a higher temperature and a lower relative humidity than the exhaust air (2), characterized in that a partial flow (13) is branched off from the supply air (12) and is mixed with the exhaust air (2) to produce the process exhaust air (7) before being discharged to the environment2. Method according to claim 1, characterized in that so much of the partial flow of the supply air (13) is added to the exhaust air (2) that the dew point of the process exhaust air (7) is below the ambient temperature of the system, in particular at least 5 ° below the ambient temperature of the system.

3. Method according to one of the preceding claims, characterized in that a supply air blower (11) is provided in the supply air line (12), and the partial flow of supply air (13) is branched off downstream of the supply air blower (11).

4. Method according to one of the preceding claims, characterized in that a supply air fan (11) is provided in the supply air line (12), and the partial flow of supply air (13) is branched off upstream of the supply air fan (11), and an additional pusher fan (15) is provided in the line of the partial flow of supply air (13).

5. Method according to one of the preceding claims, characterized in that the supply air (12) is conditioned, in particular heated, and the partial flow of the supply air (13) is diverted after conditioning.

6. Method according to one of the preceding claims, characterized in that an exhaust air blower (5) is provided in the duct of the exhaust air (2), and the mixing of the supply air (13) with the exhaust air (2) takes place downstream of the exhaust air blower (5).

7. Method according to one of the preceding claims, characterized in that the temperature is measured in the exhaust air (2) and / or the supply air (12, 13) and / or in the process exhaust air (7).

8. Method according to one of the preceding claims, characterized in that the humidity is measured in the exhaust air (112) and / or the supply air (12, 13) and / or in the process exhaust air (7).

9. Method according to one of the preceding claims, characterized in that means are provided to change the quantity of the partial flow of the supply air (13) to the exhaust air (2), wherein the means can be designed in particular as adjustable flaps (14) or as frequency control of the pusher fan (15).

10. Method according to claim 9, characterized in that the quantity of the partial flow of the supply air (13), which is mixed with the exhaust air (2), is adjusted as a function of the temperature and / or humidity of the exhaust air and / or process exhaust air and / or supply air.

11. Method according to one of claims 9 or 10, characterized in that the quantity of the partial flow of the supply air (13) to the exhaust air (2) is controlled or regulated via the delivery rate of the pusher fan (15), the delivery rate of the pusher fan (15) being increased or decreased as a function of the delivery rate of the exhaust air fan (5).

12. Method according to claim 11, characterized in that when the delivery rate of the exhaust air fan (5) is reduced, the delivery rate of the pusher fan (15) is increased, or when the delivery rate of the exhaust air fan (5) is increased, the delivery rate of the pusher fan (15) is reduced.

13. Method according to one of the preceding claims, characterized in that the partial flow of the supply air (13) and the exhaust air (2) are mixed by means of a static mixer.

14. Method according to one of the preceding claims, characterized in that the process exhaust air (7) is discharged to the environment at a height of more than 40 m, in particular 60 m or more above the ground.

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