METHOD AND APPARATUS FOR REDUCING THE WATER AND ENERGY CONSUMPTION OF A PAPER MACHINE WITH THE HELP OF A VACUUM SYSTEM AND OPTIMIZATION OF SOLIDS CONTENT AS WELL AS USE OF THE SAME

Abstract

A method for reducing the water and energy consumption of a paper machine with the help of a vacuum system and optimization of web solids content. On the paper machine wire section, water is removed with the help of a hybrid vacuum system serving first the vacuum locations needing a lower vacuum level and then those requiring a higher vacuum level in such a fashion that the hybrid vacuum system removes water on different sections of the paper machine at the vacuum levels rendering the Individually maximized energy efficiency. Additionally the solids content on the wire section is optimized with fee help of an unfelted and smooth press roll adapted above the wire suction roll.

Description

The invention relates in accordance with the preamble of claim 1 to a method for reducing the water and energy consumption of a paper machine with the help of a vacuum system and optimization of solids content. Furthermore, the invention relates to the use of an apparatus in accordance with the preamble of claim 5 and a hybrid vacuum system in accordance with claim 10.

As is generally known, a papermaking process is highly energy-intensive. The greatest energy consumers can be basically listed as the heating of raw materials, pump units at the wet end and the dryer section itself.

In the heating of raw materials for papermaking, fiber and fillers are taken to the paper mill from outdoors storage. Hence, they must be heated to the process temperature of 40-50° C. The stock is mixed with water, whose amount typically by weight is 10 to 20 times greater than the amount of wood fiber and filler materials in the resulting production furnish (furnish consistency in the headbox is generally 0.3-1 %, max. 2 %, which translates into 1 part of fiber and similar solids in 99-300 parts of water). This water volume is recirculated in the process even if the process does not have a closed circulation system. Water consumption per produced ton of paper is about 10-20 m³/t. The necessary amount of water is generally taken from a lake/river at a temperature of 0-25° C., depending on the season of the year. Today, the process temperature is elevated up to 40-50° C. in order to improve dewatering drainage. Hence, the effluent water passed from the mill to wastewater treatment contains a great amount of low-value thermal energy whose recovery concurrently is unprofitable or complicated.

Additionally, papermaking needs a lot of air that also must be heated. Many mills use large volumes of hot air which is exhausted from the process without being utilized by heat recovery.

The above-mentioned great amount of circulation water requires massive pumping power that is another major energy consumer. Eventually, this energy is converted into heat thus providing a major portion of the thermal energy required by the process. Drying the paper web on the wire and press sections is partially implemented with the help of vacuum. This portion of the process is known as the process vacuum system. Also the vacuum system is a prominent energy user that typically consumes 20% of the mill's overall power.

After passing through the wire and press sections, the solids content of the paper web is generally in the range of 40-50%. The final moisture content of the paper web is 6-8%. Drying the web after the press takes place by evaporation. Evaporation of water is a highly intensive consumer of energy that is obtained from steam. Hence, the solids content of the paper web is advantageously maximized upstream of the dryer section in order to minimize steam consumption.

As said, an important portion of a paper machine is its vacuum system. The vacuum systems of a paper machine can be divided into two basic arrangements: a water ring pump system and a turboblower system.

In FIG. 1 is illustrated a typical paper machine equipped with water ring pumps. The drawing also shows conventional dewatering equipment of a paper machine that will be discussed later in the text. The system generally comprises 6-15 pcs. pumps that serve the locations of the paper machine requiring vacuum at different sub-atmospheric pressure levels. In a water ring pump, the rotating water acts as a piston that compresses the entering gas. The compression takes place isothermally, whereby the thermal energy released from compression is mainly absorbed by the seal water passed into the pump. A great amount of seal water is required, 100-400 l/min per pump, but 80-90% thereof can be recirculated if elevation of water temperature can be prevented through cooling the liquid circulation. However, thereby will be lost the thermal energy recoverable from the compression cycle and also from a portion of the heat released in the process. As the spent seal water contains fiber and impurities, it is generally passed out from the mill's water circulation. The overall efficiency of a water ring pump is dependent on the vacuum level and pump rotation speed. Efficiency at a high vacuum is better than at low vacuum.

Alternatively, the vacuum system in paper machines can be implemented with centrifugal blowers of the type generally known as turboblower. The vacuum locations in a paper machine are generally operated at a high vacuum in excess of 60 kPa. Hereby one or two multistage blowers are required to reach the highest vacuum levels. The number of stages, or impellers, arranged in series is typically 4 pcs. Generally, at each impeller is arranged an intermediate port for connection to lower vacuum locations. The capacity of this blower type cannot be varied by changing the speed of rotation. The only possible way of adjustment is by throttling the air flow. In terms of energy efficiency, this kind of capacity control is uneconomical. Additionally, the system also has at least one single-stage blower for medium vacuum locations. As the exhaust air from the blower system is hot, its thermal energy can be recovered by heat exchangers.

In FIG. 2 is illustrated a typical turboblower system. The pumping efficiency of a turboblower system is slightly better than that of a water ring pump. On the other hand, when the system has a smaller number of blowers, the flows to the vacuum locations must be controlled with throttle valves, whereby the overall efficiency of the system is impaired. The greatest benefit is appreciated therein that the blower has no rotating water ring, i.e., it does not need seal water at all. In contrast, a significant problem arises from cost of the system due to the multistage blowers, whose higher price of acquisition and installation together with their auxiliary equipment increase the investment costs.

In addition to the above systems, the dewatering equipment of a paper machine is a vital element. On the wire section, water is initially removed from the web by gravitation and with foil effects and centrifugally. Thereupon more differential pressure must be applied across the web, since a major portion of water is transferred from the paper web to the dewatering equipment by compression. Typically, downstream of the felt water level are arranged flat suction boxes having a vacuum therein. At the end of the wire section is generally located a suction roll. In FIG. 3 is shown the conventional construction and operating principle of flat suction boxes and wire suction roll.

The vacuum system produces the vacuum in the flat suction boxes and the wire suction roll. Air flows into the suction boxes through the web, whereby a pressure differential is established. This function is highly energy-intensive. The pressure differential causes friction between the suction box and wire thus increasing the energy consumption of the wire drive motors. Water is removed from the paper web partially along with the air flow, but also due to caliper compression of the web, particularly for thicker paper grades having a basis weight greater than 80 g/m².

The wire suction roll has holes drilled thereto for suction of air through the web. The passing-through air does not retard the web travel but requires more capacity from the vacuum pumps due to the elevated vacuum level and additional air sucked through the drilled holes. Water is collected from the web through the wire into the holes of the roll wherefrom it is ejected centrifugally to a water collection pan adapted about the roll. In practice has been found that no water will pass through the roll holes if the web speed exceeds 200 m/min. Another practical experience relates to the crucial roll of the pan as all water ejected from the roll must be collected into the pan, wherefrom it is returned to the water circulation of the machine. Since the water tends to adhere to the roll surface and holes due to surface forces, it must be separated with the help of different doctoring arrangements.

It must be noted that while no fresh water is needed for the suction box, the wire suction roll consumes water by about 100-200 l/min as lubrication for the seal strips of the vacuum chamber of the roll.

Typical solids content after the wire is 15-20%. Solids content is preferably maximized in order to minimize energy consumption at the later discussed press and dryer sections, simultaneously achieving improved machine runnability. A general rule of thumb in the art is that in paper webs a change of 1% downstream of the wire section results in 0.25% increase in solids downstream of the press section.

In the appended FIG. 4 are shown the results of a study performed by Ph.D. Räisänen on the effect of vacuum level and dwell time on the solids content downstream of the wire section. The diagram is reprinted from the annual report 1994 of research program Sustainable Paper (Kestävä Paperi). As has also found in practice, the diagram clearly indicates the optimum running condition, namely, that the vacuum level must increase toward the trailing end of the wire travel. Another important aspect relates to the maximum solids content attainable at a given pressure differential. As a longer dwell time cannot offer a higher solids content, a higher vacuum level must be applied. Since a vacuum level of about 70 kPa is a practical maximum, compression must be applied to the web downstream of the wire section in order to reach a higher solids content. Down-stream of the wire section on the press section, this function is implemented with the help of presses that remove water from the web to felts and/or suction roll holes and/or grooves of a grooved roll. In some paper machines, a press and endless felt are adapted above a suction roll. This arrangement typically achieves a solids content of about 24% downstream of the wire section.

As elucidated above, running a paper machine involves an extremely great number of factors affecting water and energy consumption. In prior-art arrangements the goal has been to solve one problem at a time. Now the present invention attemps to find a solution by way of examining all the different factors separately and then combining their effect in the overall performance. This approach proved that in the art there still are substantial possibilities of improvement in the various properties of a paper machine. The essential feature of the invention is particularly a reduced consumption of water and energy by virtue of an improved vacuum system and optimization of web solids content. Resultingly, the invention offers reduced energy consumption in a paper machine through increased solids content downstream of the wire and press section and, further, by reducing raw water consumption in the mill. This goal is attained by utilizing conventional equipment in an entirely novel and innovative fashion and enhancing the operating practices of the paper machine. In addition to the economical benefit resulting therefrom, the invention facilitates reduced investment costs with regard to the present situation.

The essential features of the invention are crucial factors in the arrangement defined in the claims. The invention provides plural significant benefits while simultaneously avoiding the problems hampering the prior art.

The arrangement according to the invention implemented using a so-called hybrid vacuum system, whereby a significant improvement is achieved in the energy consumption of the paper machine in its front end, on its wire section through reduced water and electricity consumption and increased web solids content. A significant feature is that the invention aims to provide a comprehensive improvement of energy consumption rather than simply attempting enhanced energy use in a single component such as a pump.

More specifically, the invention is characterized by what is disclosed in the appended claims. The invention is next described in more detail by making reference to the annexed drawings wherein

FIGS. 1-3 show some embodiments of prior-art constructions;

FIG. 4 shows change of solids content as a function of vacuum level and dwell time;

FIG. 6 shows the results of a solids content measurement on a double-layer wire section; and

FIGS. 5 and 7 show some embodiments of an arrangement according to the invention.

The above-discussed conventional vacuum system implemented with the help of turbo flowers does not consume water at all. However, the system is hampered by the costly multistage blowers that are required to achieve the high vacuum levels needed in a paper machine, as well as by the energy-wasting throttle control. In the embodiment according to the invention, a so-called hybrid vacuum system 1 is configured, said vacuum system being implemented primarily with cost-effective single-stage or two-stage blowers 2. The blower impellers are mounted on the motor shaft, which also makes facilitates the capacity control of a possible two-stage blower by varying the rotation speed. An essential detail herein is that said blowers are used for serving only those vacuum locations of the paper machine that can utilize their salient energy efficiency, typically at a vacuum level of 0-60 kPa, namely, all other vacuum locations except wire suction roll 3 and press suction roll 13 requiring a higher vacuum. To serve the suction rolls that require an elevated vacuum level, a water ring pump 4 is used that for generation of a high vacuum is a very good and energy-efficient device. No water separation is provided between the water ring pump 4 and paper machine 5, since only clean lubrication water 6 of the seal strips is pumped therein. The seal strip lubrication water travels to the water ring pump 4, wherein it is used as seal water 7 of the pump. This water volume, however, is not sufficient to the function of pump 4 as its flow also varies according to the running conditions of the paper machine. Hence, a certain amount of supplementary water 8 must be introduced. Since the function of pump 4 presumes a correct amount of seal water, the flow of water must be controlled in the following fashion: downstream of the pump is installed a water separator 9 whose water volume is measured. This measurement is used to control the amount of supplementary water 8 to be delivered to the pump as shown in appended FIG. 5. An alternative method of controlling the admission of supplementary water to the pump is to measure the temperature of the pump 4 that begins to arise if the amount of seal water is insufficient.

As no process water is discharged from the roll 3, the water coming out from water separator 9 is clean. The water temperature has risen in the pump and can thus be passed to the process or used as cleaning shower water that must be warm. Accordingly, the water consumption in the factory is decreased and its thermal energy can be recovered. Other vacuum locations of the paper machine can be served by speed-controlled turboblowers 10 having single-stage or maximally two-stage construction. Speed control is the most energy-efficient control method, which lowers the mill's operating costs. The outcome is an especially energy-efficient hybrid system that provides significant savings in water consumption. By virtue of the hybrid vacuum system, electricity consumption is typically reduced by about 30%.

The cost of the turboblower 10 is dictated by the number of impellers (i.e., blower stages or steps), as well as by the foundation and auxiliary equipment costs thereof. As to investment costs, the hybrid vacuum system is clearly more profitable than prior art systems. Investment costs are about 20-40% lower that in the system illustrated in FIG. 2.

Energy efficiency is also enhanced by way of passing the hot discharge gas exiting from the turboblowers 10 via heat exchangers 11 to the ambient. In appended FIG. 5 is illustrated the flow diagram of the hybrid vacuum system 1. Recovered thermal energy is 50-60% of electricity input. The recovered heat is used for preheating the intake air or fresh water inflow of the mill.

In appended FIG. 6 is shown the maximization results of solids content on the wire section. The measurement has been carried out in a practical test performed on a paper machine equipped with another energy utilization enhancement scheme according to the present invention. The solids content measurement plotted in FIG. 6 was performed on a two-layer wire section having 16 pcs. of dewatering elements (on the lower wire). In the diagram is depicted the measurement of solids content and water removal on the wire section 12. At the end of wire section 12, downstream of wire suction roll 3, a solids content of 30% is attained. An essential prerequisite to this end is that the solids content shall be about 10% upstream of the wire suction roll 3.

This process behavior can be traced to the configuration in which above the wire suction roll 3 is adapted a wire press 18 running unfelted. Rewetting is a common phenomenon known to hamper the operation of a felted press. When the press nip opens, water reflows from the felt back to the web. This rewetting is the stronger the wetter the web. Consequently, a felted wire press cannot reach as high a solids content as a press having a smooth unfelted roll 18. In the configuration illustrated in FIG. 5, the roll is further mounted at the end portion of the vacuum chamber 14 of the wire suction roll. With the help of a water ring pump 4, the roll interior space is kept at a maximized vacuum level of about 70-75 kPa. The wire press covers a portion of the wire suction roll 3 thus reducing water flow through the web to the suction roll. However, these arrangements alone are not sufficient for attaining a desired solids content. This requires more effective water removal. In practice, when water is transferred from the web or wire to the suction roll holes 15, the entrained air lands the water as a thin film onto the hole rims. Then, water cannot properly be expelled centrifugally from the roll, but rather forms a “mist” about the roll. Instead, when a sufficient amount of water is present, the wire press forces the water into the roll holes 15 as a plug 16 that readily leaves the roll centrifugally.

In FIG. 7 is illustrated the principle of water removal as well as the water film and water plug 16. The most advantageous precondition for forming a water plug is that the roll hole 15 is straight-walled without a beveled rim. The minimum amount of water can be estimated when the open surface area of the roll 3 is known. In a practical arrangement, the discharge pipe of the water collection pan of the wire suction roll is provided with a flow meter. By practical tests, a set value is determined for the flow. With the help of this set value of water flow, the vacuum level of flat suction box 17 preceding the roll can be controlled in order to pass a desired amount of water to be removed at the wire suction roll 3. Then, there is no need to elevate the vacuum level of suction boxes unnecessarily, whereby savings are attained in the energy consumption of the vacuum pump and the wire drive motors. This kind of operation is entirely different from that today used in the art wherein the goal is to maximize the solids content increase at each water removal element. The control scheme of the wire suction roll 3 and the flat suction boxes is illustrated in FIG. 5.

As an additional verification of the concept, the table below proves how the arrangement according to the invention achieves higher solids content down-stream of the press and thus reduces steam consumption on the dryer section.

As listed below, typical distribution of water removal percentages on the press section in a three-nip press divides as follows: first press 20%, second press 50% and third press 30%. In the table is also estimated the increase of solids content for two cases wherein downstream of the wire the solids content is 20% and 30%, respectively (with the assumption that each 1% increase downstream of the wire causes a 0.25% increase downstream of the press).

Results obtained from a paper machine illustrated in FIG. 5 are listed below.

	Prior art paper	Arrangement acc.
	machine	to invention.
Wire solids content (%)	20	30
Press solids content (%)	46	48.5
Press water removal (l/min)	1000	450
at press #1	200	0
at press #2	500	270
at press #3	300	180

As is evident from the table, the arrangement according to the invention achieves a higher solids content downstream of the press and resultingly provides a substantial reduction of steam consumption on the dryer section. A salient feature is that as the solids content increases by 10% downstream of the wire, it is possible to obtain a higher solids content downstream of the press even if the number of felts is reduced from three to two.

The present arrangement offers substantial benefits by optimizing the different individual elements in a novel way into an overall solution disclosed in the invention. The essential feature of the invention is that, on the paper machine wire section 12, water is removed with the help of a hybrid vacuum system serving first the vacuum locations needing a lower vacuum level and then those requiring a higher vacuum level. Additionally, the solids content on the wire section 12 is optimized with the help of an unfelted press roll 13 adapted above the wire suction roll 3 and finally at the other vacuum locations of the paper machine in such a fashion that water removal is carried out at different sections of the paper machine with the help of different vacuum systems running them at their optimal energy efficiency levels.

The hybrid vacuum system is more particularly implemented so that single-stage or two-stage blowers 2 are run at their optimal energy efficiency speed to serve vacuum locations needing a lower vacuum level, typically at a vacuum of 0-60 kPa. Water removal at locations requiring a higher vacuum is carried out with the help of a water ring pump 4 at a vacuum level of about 70-75 kPa, while other vacuum locations are served by speed-controlled turboblowers 10. Simultaneously, the solids content on the wire section 12 is optimized by mounting above the wire suction roll an unfelted press roll 13 located at the end portion of the vacuum chamber of the wire vacuum roll. A further essential feature is that wire press 13 forces water into holes 15 of wire suction roll 3 so as to form water plugs 16 therein thus further increasing the solids content. The overall result is a combined use according to the invention of a hybrid vacuum system and optimization of solids content on the wire section in order to reduce water and energy consumption on a paper machine.

To a person skilled in the art it is obvious that the invention is not limited by the above-described exemplary embodiments, but rather may be varied within the inventive spirit and scope of the appended claims.

Claims

1. A method for reducing the water and energy consumption of a paper machine with the help of a vacuum system and optimization of solids content, comprising the following step: removing water on the paper machine wire section with the help of a hybrid vacuum system serving first the vacuum locations needing a lower vacuum level and then those requiring a higher vacuum level in such a fashion that the hybrid vacuum system removes water on different sections of the paper machine at the vacuum levels rendering the individually maximized energy efficiency and the solids content on the wire section is optimized with the help of an unfelted and smooth press roll adapted above the wire suction roll.

2. The method of claim 1, wherein in the method water is removed on the wire section of the paper machine with the help of a hybrid vacuum system so that single-stage or two-stage blowers are used to at their optimal energy efficiency to serve vacuum locations needing a lower vacuum level and water removal at locations requiring a. higher vacuum Is carried out with the help of a water ring pump, while other vacuum locations are served by speed-controlled turboblowers and, simultaneously, the solids content on the wire section is optimized by mounting above the wire suction roll an unfelted press roll located at the end portion of the vacuum chamber of the wire vacuum roll.

3. The method of claim 1, wherein in the method single-stage or two-stage blowers are most advantageously used to serve vacuum locations needing a lower vacuum level, typically at a vacuum of 0-60 kPa, while the higher vacuum level is elevated to a level maximized with the help of a water ring pump to about 70-75 kPa and, additionally, the wire press forces water into the holes of wire suction roll so as to form water plugs therein thus further enhancing the solids content.

4. The method of claim 1, wherein in the method the seal strip lubrication water is used as the seal water of the water ring pump, while the required supplementary water is metered either at a water separator located downstream of the water ring pump or by monitoring the pump temperature, whereby the water leaving the water separator is returned to the process or used as the cleaning shower water, and the hot discharge gas exiting from the turboblowers is passed via heat exchangers to the ambient.

5. An apparatus for reducing the water and energy consumption of a paper machine with the help of a vacuum system and optimization of solids content on the wire section, wherein the apparatus comprises a hybrid vacuum system operating in combination with an unfelted, smooth press roll located above a wire suction roll.

6. The apparatus of claim 5, wherein said hybrid vacuum system comprises single-stage or two-stage blowers offering their optimal energy efficiency while serving vacuum locations needing a lower vacuum level, a water ring pump serving locations requiring a higher vacuum as well as speed-controlled turboblowers and, further, mounted above the wire suction roll of the wire, an unfelted press toll located at the end portion of the vacuum chamber of said wire vacuum roll.

7. The apparatus of claim 5, wherein the impellers of blowers are mounted on the pump motor shaft so to allow the control of their rotation speed.

8. The apparatus of claim 5, wherein, when there is no water separation provided between the water ring pump and the paper machine, the seal strip lubrication water is used as the seal water of the water ring pump, while the required supplementary water is metered either at a water separator located downstream of the water ring pump or by monitoring the pump temperature, whereby the water leaving the water separator is returned to the process or used as the cleaning shower water, and the hot discharge gas exiting from the turboblowers is passed via heat exchangers to the ambient.

9. The apparatus of claim 4, wherein said speed-controlled turboblowers have one or two stages and the hot discharge gas exiting from the turboblowers is passed via heat exchangers to the ambient.

10. Use of a hybrid vacuum system in an optimized fashion to control solids content on a paper machine wire section.

11. The use of claim 10, wherein on the wire section of the paper machine is employed a hybrid vacuum system with the help of which water removal is firstly carried out at vacuum locations needing a lower vacuum level and next at locations requiring a higher vacuum using the vacuum system units at their optimal energy efficiency, while simultaneously, the solids content on the wire section is optimized with the help of an unfelted press roll mounted above the wire suction roll.

12. The use of claim 10, wherein on the wire section of the paper machine is employed a hybrid vacuum system, whose single-stage or two-stage blowers are used to at their optimal energy efficiency to serve vacuum locations needing a lower vacuum level and water removal at locations requiring a higher vacuum is carried out with the help of a water ring pump, while other vacuum locations are served by speed-controlled turboblowers and, simultaneously, the solids content on the wire section is optimized by mounting above the wire suction roll an unfelted press roll located at the end portion of the vacuum chamber of the wire vacuum roll.

13. The method of claim 2, wherein in the method single-stage or two-stage blowers are most advantageously used to serve vacuum locations needing a lower vacuum level, typically at a vacuum of 0-60 kPa, while the higher vacuum level is elevated to a level maximized with the help of a water ring pomp to about 70-75 kPa and, additionally, the wire press forces water into the holes of wire suction roll so as to form water plugs therein thus further enhancing the solids content.

14. The method of claim 3, wherein in the method the seal strip lubrication water is used as the seal water of the water ring pump, while the required supplementary water is metered either at a water separator located downstream of the water ring pump or by monitoring the pump temperature, whereby the water leaving the water separator is returned to the process or used as the cleaning shower water, and the hot discharge gas exiting from the turboblowers is passed via heat exchangers to the ambient.

15. The apparatus of claim 6, wherein the impellers of blowers are mounted on the pomp motor shaft so to allow the control of their rotation speed.

16. The apparatus of claim 6, wherein, when there is no water separation provided between the water ring pump and the paper machine, the seal strip lubrication water is used as the seal water of the water ring pump, while the required supplementary water is metered either at a water separator located downstream of the water ring pump or by monitoring the pump temperature, whereby the water leaving the water separator is returned to the process or used as the cleaning shower water, and the hot discharge gas exiting from the turboblowers is passed via heat exchangers to the ambient.

17. The apparatus of claim 5, wherein said speed-controlled turboblowers have one or two stages and the hot discharge gas exiting from the turboblowers is passed via heat exchangers to the ambient.

18. The use of claim 11, wherein on the wire section of the paper machine is employed a hybrid vacuum system, whose single-stage or two-stage blowers are used to at their optimal energy efficiency to serve vacuum locations needing a lower vacuum level and water removal at locations requiring a higher vacuum is carried out with the help of a water ring pump, while other vacuum locations are served by speed-controlled turboblowers and, simultaneously, the solids content on the wire section is optimized by mounting above the wire suction rod an unfelted press roll located at the end portion of the vacuum chamber of the wire vacuum roll.