Tissue paper drying system

Abstract

A ssue paper drying system and a method for operang such a ssue drying system The ssue drying system comprises a yankee drying secon (11), a separator tank (21), a heat pump system (31; 31'; 31'') and a steam recirculaon arrangement (41; 41'; 41''; 41'''). Exhaust air from a yankee hood of the yankee drying secon and / or from another hot air dryer device such as a TAD secon (111) is used as heat source in the heat pump system. Liquid water transferred out from a yankee cylinder of the yankee drying secon is transformed into steam in a heat sink side of the heat pump system and the steam is recirculated back for use in the yankee cylinder.

Description

[0001] TISSUE PAPER DRYING SYSTEM

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates to a tissue paper drying system comprising a yankee drying section, a tissue paper system, a method for operating such a tissue paper drying system and to a computer program product.

[0004] BACKGROUND OF THE INVENTION

[0005] Within the field of tissue paper manufacture, a pulp is used to form a paper web that is transported through a series of treatment steps aiming to form and dewater the web to produce a tissue paper.

[0006] Some treatment steps include drying of the paper web in a yankee drying section in conventional or hybrid tissue machine and / or in a Through Air Drying (TAD) section in a TAD tissue machine.

[0007] In conventional and hybrid machine the paper web is transferred through the yankee drying section and heat is applied to the pa per web from a yankee cylinder of the yankee drying section and hot air is applied from a yankee hood of the yankee drying section to remove a portion of the water content in the paper web. The yankee cylinder is heated by steam.

[0008] In a TAD machine, the paper is transferred through one or more TAD sections where hot and dry air is blown through the paper web and the colder and more humid air is removed by means of a hollow and perforated roll and a hood, and then it is transferred to a yankee drying section similar to the one described above for a conventional or hybrid machine.

[0009] Both conventional and hybrid machines and TAD machines as described above comprise at least one steam heated device, in which steam is condensed to release energy to the paper web and ensure evaporation of the water in it. It is common practice in the paper industry to utilize fossil fuel boiler to generate steam for the drying process.

[0010] SUMMARY

[0011] It is an object of the invention to provide a more efficient, economic and environmentally friendly tissue paper drying system.

[0012] It is a further object of the invention to avoid or decrease the use of fossil fuels when preparing steam for a yankee drying device.

[0013] This is achieved in a tissue paper drying system, a tissue paper system, a method and a computer program product according to the independent claims.

[0014] According to one aspect of the invention a tissue paper drying system is provided comprising: a yankee drying section comprising a yankee cylinder and a yankee hood, wherein steam is used in the yankee cylinder for drying a tissue paper passing through the yankee drying section and hot air is used in the yankee hood for drying the tissue paper, wherein the yankee cylinder comprises a cylinder inlet for steam and a cylinder outlet for steam and liquid water and wherein the yankee hood comprises a hood inlet for air and a hood outlet for exhaust air; a separator tank comprising a separator inlet fluidly connected to the cylinder outlet for receiving steam and liquid water from the yankee cylinder, said separator tank being configured for separating steam from liquid water and for forwarding steam out from the separator tank via a separator steam outlet of the separator tank and for forwarding liquid water out from the separator tank via a separator liquid water outlet; a heat pump system having a heat source side and a heat sink side, said heat sink side comprising a heat sink inlet which is fluidly connected to the separator liquid water outlet and a heat sink outlet for transferring steam produced in the heat sink side from liquid water received from the separator tank out from the heat pump system, wherein the heat source side comprises a heat source inlet and a heat source outlet between which a fluid is to be transferred for being used as a heat source in the heat pump system, wherein said heat source inlet is fluidly connected to the hood outlet and / or to a TAD air outlet of a TAD section possibly provided in the tissue paper drying system, whereby exhaust air from the yankee hood and / or from the TAD section is used as heat source in the heat pump system; and a steam recirculation arrangement being configured for forwarding steam from the separator steam outlet and the heat sink outlet to the cylinder inlet of the yankee cylinder and further comprising at least one steam preparation device configured for preparing the steam for a suitable pressure to be used in the yankee cylinder.

[0015] According to another aspect of the invention a tissue paper system for the production of tissue paper comprising one or more tissue paper drying systems as described above is provided.

[0016] According to another aspect of the invention a method for operating a tissue paper drying system comprising a yankee drying section, possibly a TAD section, a separator tank, a heat pump system, and a steam recirculation arrangement as described above is provided, said method comprising the steps: forwarding exhaust air from the yankee hood of the yankee drying section and / or from the TAD section to the heat source inlet of the heat source side of the heat pump system; using said exhaust air in the heat pump system as heat source; transferring a mix of liquid water and steam from the yankee cylinder of the yankee drying section to the separator tank; separating the liquid water and the steam in the separator tank; transferring the liquid water from the separator tank to the heat sink inlet of the heat sink side of the heat pump system; transforming the liquid water received in the heat sink inlet into steam in the heat pump system; forwarding the steam produced in the heat pump system and the steam separated in the separator tank to the steam recirculation arrangement; preparing the steam in the at least one steam preparation device provided in the steam recirculation arrangement to a suitable pressure; and forwarding the prepared steam from the steam recirculation arrangement to the yankee cylinder.

[0017] According to another aspect of the invention a computer program product is provided comprising instructions which, when executed in a processor in a control system in a tissue paper drying system as described above, cause the control system to operate the tissue paper drying system according to the method as described above.

[0018] Hereby at least some of the energy needed for preparing the steam for the yankee cylinder is produced in a heat pump which utilizes energy from the air expelled from the yankee hood and / or TAD section. Hereby energy is better utilized within the system and the use of fossil fuels can be decreased. The integration of a heat pump in the tissue paper drying system can be beneficial in terms of reduction of fossil fuel consumption and reduction of CO2 emission related to paper making. It will further increase the flexibility of the system by differentiating the source for the production of the steam required for tissue drying and consequently increasing the flexibility of the tissue making process.

[0019] In one embodiment of the invention the heat source side of the heat pump system comprises an intermediate liquid loop and a heat exchanger, whereby the intermediate liquid loop is configured for circulating a liquid between the heat exchanger and an evaporator of the heat pump system, said heat exchanger comprising said heat source inlet and heat source outlet and being configured for transferring heat from the exhaust air received in the heat source inlet to the liquid in the intermediate liquid loop. Hereby heat is transferred from the exhaust air to the evaporator of the heat pump by means of the heat exchanger which is an air-to-liquid heat exchanger. The intermediate liquid loop is configured to transfer heat by means of a liquid media circulated in the loop, recovering heat from the heat exchanger by means of cooling down the exhaust air coming from the heat source inlet and evacuated at lower temperature at the heat source outlet, and delivering heat to the evaporator to evaporate the heat pump working fluid.

[0020] In one embodiment of the invention the heat source side further comprises a tank connected in the intermediate liquid loop for accumulating liquid utilized in the intermediate liquid loop. Hereby creating a thermal buffer for the process.

[0021] In one embodiment of the invention the tank is connected to renewable energy sources to enhance the performance of the heat pump system.

[0022] In one embodiment of the invention the steam recirculation arrangement comprises a thermocompressor comprising a first thermocompressor inlet which is fluidly connected to the separator steam outlet for receiving steam from the separator tank and a second thermocompressor inlet which is fluidly connected to the heat sink outlet and a thermocompressor outlet which is fluidly connected with the cylinder inlet for transferring steam from the thermocompressor to the yankee cylinder.

[0023] In one embodiment of the invention the steam recirculation arrangement comprises a mechanical vapor recompression, MVR, unit connected in a fluid line between the heat sink outlet and the cylinder inlet.

[0024] In one embodiment of the invention the steam recirculation arrangement comprises a mechanical vapor recompression, MVR, unit connected in a fluid line between the separator steam outlet and the cylinder inlet.

[0025] In one embodiment of the invention the tissue paper drying system further comprises a flash tank and a flash valve connected in a fluid line between the separator liquid water outlet and the heat sink inlet and a flash tank steam outlet of the flash tank is fluidly connected to the steam recirculation arrangement. In one embodiment of the invention the tissue paper drying system further comprises an air system which is fluidly connected with the hood inlet and the hood outlet and configured for preparing air to be forwarded to the hood inlet and to receive exhaust air from the hood outlet, said air system further comprising an air inlet and an exhaust air outlet, wherein said exhaust air outlet is fluidly connected to the heat source inlet.

[0026] In one embodiment of the invention the tissue paper drying system comprises more than one yankee drying sections and / or TAD sections connected to the same heat pump system.

[0027] In one embodiment of the invention the method step of using said exhaust air in the heat pump system as heat source comprises transferring heat from the exhaust air via a heat exchanger and an intermediate liquid loop to an evaporator of the heat pump system.

[0028] In one embodiment of the invention the method step of preparing the steam comprises adjusting a pressure of the steam in a thermocompressor and / or a mechanical vapor recompression, MVR, unit.

[0029] BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure la shows schematically a prior art tissue paper drying system in conventional or hybrid tissue machines.

[0031] Figure lb shows schematically a prior art tissue paper TAD section and yankee drying section in a TAD machine.

[0032] Figure lc shows schematically a prior art heat pump.

[0033] Figure 2 shows schematically a tissue paper drying system according to one embodiment of the invention.

[0034] Figures 3a-3b show schematically heat pump systems according to different embodiments of the invention. Figures 4a-4c show schematically three different embodiments of tissue paper drying systems according to the invention.

[0035] Figure 5 shows schematically how two yankee drying sections are connected to one heat pump system.

[0036] Figure 6 shows schematically a tissue paper drying system according to another embodiment of the invention.

[0037] Figure 7 is a flow chart of a method according to one embodiment of the invention.

[0038] DETAILED DESCRIPTION OF EMBODIMENTS

[0039] Figure la shows schematically a prior art tissue paper drying system 901 for conventional or hybrid tissue machine. The tissue paper drying system 901 may be provided in a tissue paper system and is used for drying the tissue paper to be produced in the tissue paper system. The tissue paper drying system 901 comprises a yankee drying section 11 comprising a yankee cylinder 13 and a yankee hood 15, wherein steam is used in the yankee cylinder 13 for drying a tissue paper passing through the yankee drying section 11 and hot air is used in the yankee hood 15 for drying the tissue paper as is well known in the art. An air system 51 is connected to the yankee hood 15 for providing hot air to the yankee hood and for receiving exhaust air from the yankee hood. The exhaust air is forwarded from the air system 51 via one or more heat recovery devices 61a, 61b. New air is heated up exploiting the hot exhaust air coming from the air system 51 and is provided into the air system, possibly via one of the heat recovery devices 61a. One of the heat recovery devices 61b may be used to heat up other fluids not directly used in tissue paper drying (heat up water or air for machine room ventilation, heat up water for stock preparation). Exhaust air at the outlet of heat recovery 61a is typically between 300°C and 100°C based on operative condition, and the humidity in the exhaust air can vary between 900 g_water / kg_dryair and 150 g_water / kg_dryair. A separator tank 21 is connected to the yankee cylinder 13 for receiving steam and liquid water (condensate) from the yankee cylinder. The separator tank 21 is configured for separating the steam from the liquid water. The steam (called blow-through steam) is then forwarded to a thermocompressor 43. The liquid water is forwarded from the separator tank 21 to a boiler 71 where it is transformed into steam and then transferred to the thermocompressor 43. This flow of steam is called motive steam and has a higher pressure than the blow-through steam. The two flows of steam (of different pressures) are combined in the thermocompressor 43 and forwarded back into the yankee cylinder 13 to be used for drying the tissue paper.

[0040] Typical operative conditions of a thermocompressor require a mass ratio between motive and blow-through steam of 1 to 0.8 to guarantee an efficient condensate removal from the yankee drying section, in this way the yankee drying section will not flood due to excessive condensation. The amount of steam sucked in by the thermocompressor is proportional to the motive steam flow and the motive to discharge pressure ratio. The latter is typically set to 2 to guarantee a pressure drop through the yankee drying section close to 1.6 bar. Thus if we have 8 barg in the yankee drying section the motive steam pressure is going to be 16 barg. Typical pressure level fo the yankee cylinder 13 is between 3 and 10 barg, typical pressure level of the separator tank is between 2 and 8 barg.

[0041] The boiler 71 commonly uses fossil fuels and the object of the present invention is to reduce the use of fossil fuels in tissue paper drying systems.

[0042] Figure lb shows schematically a prior art tissue paper drying system 902 for a TAD (Through Air Drying) machine. The tissue paper drying system 902 for a TAD machine comprises a TAD section 111 and a yankee drying section 11. The TAD section 111 comprises one or more TAD rolls 113 and one or more TAD hood 115, wherein hot air is blown through the paper web to dry the paper. A TAD air system 151 is connected to the TAD section 111 to provide the hot and dry air required for the drying process via a TAD air inlet Illa and remove the colder and wetter air generated in the drying via a TAD air outlet 111b. The air received in the TAD air system 151 from the TAD air outlet 111b is forwarded as exhaust air through a first and possibly a second heat recovery device 61a', 61b'. New air is heated up exploiting the hot exhaust air coming from the TAD air system 151 and is provided into the TAD air system 151, possibly via the first heat recovery device 61a'. One of the heat recovery devices 61b' may be devoted to heat up other fluids not directly used in tissue paper drying (heat up water or air for machine room ventilation, heat up water for stock preparation). Exhaust air at the outlet of the first heat recovery device 61a' is typically between 120°C and 100°C based on operative condition, and the humidity in the exhaust air can vary between 270 g_water / kg_dryair and 150 g_water / kg_dryair.

[0043] The paper web is transported through the TAD section 111 and then transferred to a Yankee drying section 11 to complete the drying process of the paper web. The yankee drying section 11 is similar to the yankee drying section 11 described in relation to Figure la and will not be further described here.

[0044] Figure lc shows schematically a prior art heat pump 131. A heat pump may be provided to recover heat from a heat source at low temperature and upgrade it to a higher temperature level by means of a working fluid 32a operated in a closed cycle. A heat pump 131 may comprise a heat exchanger denominated evaporator 32b in which heat is recovered from a heat source media through the evaporation of the working fluid 32a of the heat pump, a compressor 32e that compress to a higher pressure level the working fluid 32a vapor generated in the evaporator and increase its temperature, another heat exchanger denominated condenser 32c, in which heat is released from the working fluid 32a to a heat sink media through the condensation of the working fluid 32a, a lamination valve 32d which reduces the pressure of the working fluid 32a generating a two phase (liquid-vapor) flow. Either heat source and heat sink media can be a liquid or a gas based on availability and operative parameters of interest. A High temperature heat pump system can be defined as such whenever the heat sink outlet temperature overcome the 100°C.

[0045] Figure 2 shows schematically a tissue paper drying system 1 according to one embodiment of the invention. Figures 4a-4c show three other embodiments of a tissue paper drying system 1'; 1”; 1"' according to the invention. In all the embodiments as shown in Figures 2, 4a-4c the tissue paper system 1; 1'; 1”; 1"' comprises a heat pump system 31; 31'; 31”. Figure 2 shows schematically a general heat pump system 31 for use in the invention and in Figures 3a and 3b two more specific examples of heat pump systems 31'; 31” to be used in the invention are shown. Any one of the different heat pump systems 31; 31'; 31” can be used in all the examples illustrated in Figures 2, 4a- 4c. Figure 5 shows schematically how two yankee drying sections 11 are connected to one heat pump system 31”. Figure 6 shows how a TAD section 111 of a tissue paper drying system 101' of a TAD machine also can be connected to the heat pump system 31; 31'; 31'” according to the invention. Also in this embodiment any one of the different described heat pump systems 31; 31'; 31” can be used. The invention is first generally described with reference to all the drawings.

[0046] A tissue paper drying system 1; 1'; 1”; 1'"; 101; 101' according to the invention comprises a yankee drying section 11 comprising a yankee cylinder 13 and a yankee hood 15. As shown in Figure 5 a tissue paper drying system 101 may also comprise more than one yankee drying section 11 and as shown in Figure 6 a TAD section 111 may also be provided. The yankee hood 15 is mounted to the yankee cylinder 13 to cover at least a part of a yankee cylinder surface 14 whereby a tissue paper to be dried in the yankee drying section 11 will pass between the yankee cylinder 13 and the yankee hood 15 as is known in the art for yankee dryers. Steam is used in the yankee cylinder 13 for drying a tissue paper passing through the yankee drying section 11 and hot air is used in the yankee hood 15 for drying the tissue paper. The yankee cylinder 13 comprises a cylinder inlet 13a for steam and a cylinder outlet 13b for steam and liquid water. The liquid water removed out from the yankee cylinder 13 via the cylinder outlet 13b is condensed steam and may also be called condense. The yankee hood 15 comprises a hood inlet 15a for hot air to be used in the yankee hood for drying and a hood outlet 15b for exhaust air. As shown in Figure 2 there may be two hood inlets 15a and two hood outlets 15b, one for each side of the yankee hood 15. Another number of hood inlets 15a and hood outlets 15b is also possible and covered by the invention. The tissue paper drying system 1; 1'; 1”; 1'"; 101; 101' according to the invention comprises further a separator tank 21 comprising a separator inlet 21a fluidly connected to the cylinder outlet 13b for receiving steam and liquid water from the yankee cylinder 13. The separator tank 21 is configured for separating steam from liquid water and for forwarding steam out from the separator tank 21 via a separator steam outlet 21b of the separator tank 21 and for forwarding liquid water out from the separator tank 21 via a separator liquid water outlet 21c.

[0047] The tissue paper drying system 1; 1'; 1”; 1'"; 101; 101' according to the invention comprises further a heat pump system 31; 31'; 31''. This can be a heat pump system 31 as illustrated in Figure 2 or any of the heat pump systems 31': 31'' as illustrated in Figures 3a and 3b. Any one of the heat pump systems 31; 31'; 31''' can be used in all the shown embodiments in all drawings. The heat pump system 31; 31'; 31'' can suitably be a high temperature heat pump, HTHP. The heat pump system 31; 31'; 31'' has a heat source side 33 and a heat sink side 35. The heat sink side 35 comprises a heat sink inlet 35a which is fluidly connected to the separator liquid water outlet 21c. Hereby liquid water is received in the heat sink inlet 35a of the heat pump system 31; 31'; 31'' from the separator tank 21. In the heat sink side 35 the received liquid water is evaporated into steam by the heat produced in the heat pump system 31; 31'; 31''. The heat sink side 35 of the heat pump system 31; 31'; 31'' comprises further a heat sink outlet 35b for transferring the steam produced in the heat sink side 35 from the liquid water received from the separator tank 21, out from the heat pump system 31; 31'; 31''. The heat source side 33 of the heat pump system 31; 31'; 31'' comprises a heat source inlet 33a and a heat source outlet 33b between which a fluid is to be transferred for being used as a heat source in the heat pump system 31; 31'; 31''. The heat source inlet 33a is in some embodiments fluidly connected to the hood outlet 15b of the yankee drying section 11 whereby exhaust air from the hood outlet 15b is used as heat source in the heat pump system 31; 31'; 31''. In some embodiments the heat source inlet 33a can instead be connected to an air outlet 111b of a TAD section 111 provided in the tissue paper drying system 101' which is shown in Figure 6. In Figure 2 the most general form of a heat pump system 31 according to the invention is shown. This is illustrated as a heat pump according to prior art as shown in Figure lc, however the evaporator 32b' will have to be specifically designed for the purpose of this invention. The heat source inlet 33a will receive exhaust air, i.e. in gas form and the evaporator 32b' needs to be designed for handling heat exchange from gas form to liquid form. Other examples of heat pump systems 31'; 31'' usable in this invention are shown in Figures 3a and 3b.

[0048] The tissue paper drying system 1; 1'; 1"; 1'"; 101; 101' according to the invention comprises further a steam recirculation arrangement 41, 41'; 41''; 41''' which is configured for forwarding steam from the separator steam outlet 21b and the heat sink outlet 35b to the cylinder inlet 13a of the yankee cylinder 13. The steam recirculation arrangement 41, 41', 41'', 41''' comprises hereby fluid connections 42a between the separator steam outlet 21b and the cylinder inlet 13a and fluid connections 42b between the heat sink outlet 35b and the cylinder inlet 13a. The steam recirculation arrangement 41; 41'; 41''; 41''' comprises also at least one steam preparation device 43; 45, 45' configured for preparing the steam to be transferred into the cylinder inlet 13a for a suitable pressure to be used in the yankee cylinder 13. The steam recirculation arrangement 41, 41'; 41''; 41''' can be embodied differently based on the heat pump system capability in terms of maximum steam pressure which is illustrated by a few different examples in Figures 2 and 4a-4c. The steam recirculation arrangement 41; 41'; 41'' may in some embodiments (as shown in Figures 2, 4a and 4b) comprise a thermocompressor 43 comprising a first thermocompressor inlet 43a which is fluidly connected to the separator steam outlet 21b for receiving steam from the separator tank 21 and a second thermocompressor inlet 43b which is fluidly connected to the heat sink outlet 35b and a thermocompressor outlet 43c which is fluidly connected with the cylinder inlet 13a for transferring steam from the thermocompressor 43 to the yankee cylinder 13. Hereby steam of different pressures are received in the first and second thermocompressor inlets 43a, 43b and steam having a suitable pressure for the yankee cylinder 13 can be prepared by the thermocompressor and transferred out from the thermocompressor outlet 43c to the cylinder inlet 13a. The steam recirculation arrangement 41'; 41” may in some embodiments (as shown in Figures 4a and 4b) comprise a mechanical vapor recompression, MVR, unit 45 connected in the fluid connections 42b between the heat sink outlet 35b and the cylinder inlet 13a. The steam recirculation arrangement 41'” may in some embodiments (as shown in Figure 4c) comprise a mechanical vapor recompression, MVR, unit 45' connected in the fluid connections 42a between the separator steam outlet 21b and the cylinder inlet 13a. All the present embodiments can be either applied to a yankee drying section 11 in a conventional or hybrid tissue paper machine or to a TAD drying section in a TAD tissue paper machine.

[0049] According to some embodiments of the invention and as shown in the drawings 3a and 4-6, the heat source side 33 of the heat pump system 31' may comprise an intermediate liquid loop 37 and an additional heat exchanger 38. The intermediate liquid loop 37 is configured for circulating a liquid between the heat exchanger 38 and an evaporator 32b of the heat pump system 31'. The heat exchanger 38 comprises said heat source inlet 33a and heat source outlet 33b and is configured for transferring heat from the exhaust air received in the heat source inlet 33a from the hood outlet 15b (or from the air outlet 111b of a TAD section 111) to the liquid in the intermediate liquid loop 37. The heat exchanger 38 is hereby in this example an air-to-liquid heat exchanger. The intermediate liquid loop 37 fulfills the function to interconnect thermally the exhaust air stream (heat source) and the heat pump working fluid 32a in the evaporator 32b. Large scale heat pumps usually use liquid fluid as heat source media for the evaporator. This is justified by the low heat transfer coefficient of the air and the need of a compact solution due to the relevant amount of heat to be transferred. Moreover, for the specific application to tissue paper drying, due to the high temperature of the exhaust air and high volumetric flow, the adoption of a direct recovery from the exhaust air will result in a more compact and efficient system. Another relevant aspect of the liquid intermediate loop 37, is that it allows to easily and cost-efficiently transport the heat recovered from the exhaust air to the spot of installation of the heat pump system 31'. In tissue mills, the area of the air system 51 is typically crowded of equipment and it will be hard to fit more there. The heat pump system 31' with an intermediate liquid loop 37 can be positioned outside the tissue machine building with all the other ancillary services. By the use of an intermediate liquid loop 37 the exhaust air from the yankee hood 15 (or from a TAD section 111) can be efficiently used in a heat pump system 31' according to the invention. The heat pump system 31' will be efficient and compact, and the temperature in the intermediate liquid loop 37 can be easily controlled by changing the flowrate in the intermediate liquid loop 37 to guarantee the desired heat input to the heat pump evaporator 32b. The use of an intermediate liquid loop 37 in between air and working fluid 32a is hereby suitable and advantageous in a tissue paper drying system. This heat pump system 31' can be used instead of the heat pump system 31 as shown in the examples illustrated in Figure 2. The other parts of the heat pump system 31' are conventional parts of a heat pump as described in relation to Figure lc and will not be further described here.

[0050] Figure 3b shows schematically another example of heat pump system 31'' according to the invention, which can be used in all the examples of the invention. In this example the intermediate liquid loop 37 presented in previous embodiment (shown in Figure 3a) is integrated with a buffer tank 39, to which it is fluidly interconnected. The tank 39 comprises a first inlet 39a fluidly connected by the intermediate liquid loop 37 to the heat exchanger 38, a second inlet 39b fluidly connected by the intermediate liquid loop 37 to the evaporator 32b, a first outlet 39c fluidly connected by the intermediate liquid loop 37 to the evaporator 32b and a second outlet 39d fluidly connected by the intermediate liquid loop 37 to the heat exchanger 38. The tank 39 comprises liquid to be pumped in the intermediate liquid loop 37. Hereby more liquid is provided in the combination of the intermediate liquid loop 37 and the tank 39 compared to only an intermediate loop 37 whereby the system will have better capacity to make use of all energy during different circumstances such as fast temperature variations related to tissue process changes. The heat pump system 31'' with such a tank 39 will also have larger capacity and can be connected to more than one yankee drying sections 11, which is shown schematically in Figure 5. The tank 39 may be a simple tank but can also be connected to one or more extra energy sources. This is schematically shown in Figure 3b but is optional. An electric energy source 82, such as a resistance, can be connected to the tank 39 for heating the liquid. Another alternative is to connect a solar thermal panel device 83 by fluid connections and a pump to the tank 39 whereby the liquid can be heated by solar energy. Such extra energy sources 82, 83 are optionally connected to the tank 39 and may be useful to help in the start-up phase and / or to increase the performance of the system. Hereby the tank 39 gives increased flexibility to the system. This heat pump system 31” can be used instead of the heat pump system 31, 31' in all the examples illustrated in the Figures.

[0051] The tissue paper drying system 1; 1'; 1”; 1'”; 101; 101' comprises also suitably an air system 51 which is fluidly connected with the one or more hood inlets 15a and the one or more hood outlets 15b. The air system 51 is configured for preparing air to be forwarded to the hood inlet(s) 15a and to receive exhaust air from the hood outlet(s) 15b. The air system 51 further comprises an air inlet 51a and an exhaust air outlet 51b, wherein said exhaust air outlet 51b in some embodiments as shown in Figures 2-5, is fluidly connected to the heat source inlet 33a of the heat pump system 31; 31', 31” possibly via a first heat recovery device 61a.

[0052] Differences between the different examples as illustrated in Figures 2 and 4a-4c with regard to the different steam recirculation arrangements 41; 41'; 41”; 41'” will now be briefly discussed.

[0053] In the embodiment of a tissue paper drying device 1 as shown in Figure 2 the steam recirculation arrangement 41 comprises a thermocompressor 43 as already discussed above.

[0054] In the embodiment of a tissue paper drying device 1' as shown in Figure 4a the steam recirculation arrangement 41' comprises a thermocompressor 43 in the same way as in Figure 2 but also a mechanical vapor recompression, MVR, unit 45 fluidly connected between the heat sink outlet 35b and the second thermocompressor inlet 43b. Hereby the pressure of the steam delivered to the thermocompressor can be adjusted. If the pressure of the steam produced in the heat pump system 31; 31'; 31” is lower than the pressure required for the steam to be used as motive steam in the thermocompressor 43 such an MVR unit 45 may be necessary. Possibly, and as shown in Figure 4a, a part of the liquid water transferred from the separator tank 21 to the heat sink inlet 35a can be guided via an extra fluid connection 42c directly to the MVR unit 45 for being used to control the steam temperature at the outlet of the MVR unit 45.

[0055] In the embodiment of a tissue paper drying device 1” as shown in Figure 4b a thermocompressor 43 and an MVR unit 45 are provided in the steam recirculation arrangement 41” in the same way as for the embodiment shown in Figure 4a.

[0056] However, two additional components in the form of a flash tank 49 and a flash valve 50 are provided in between the separator tank 21 and the heat sink inlet 35a. A flash tank steam outlet 49a of the flash tank 49 is fluidly connected to the steam recirculation arrangement 41”. A flash tank liquid water outlet 49b is fluidly connected to the heat sink inlet 35a and will forward liquid water to the heat sink inlet 35a. By use of such a flash valve 50 the pressure of the condensate can be reduced to generate a two-phase flow of steam and condensate at lower pressure. This two-phase flow can be then separated in the flash tank in order to recirculate the low-pressure steam through the steam outlet 49a at the inlet of the MVR 45 and the low-pressure condensate at the heat sink inlet 35 a through the liquid water outlet 49b. This solution enables to decouple the pressure of the yankee cylinder separator tank and the pressure at which the steam is produced on the heat sink side of the heat pump 31; 31'; 31”. Moreover, this solution can guarantee higher efficiency of the heat pump because the pressure and temperature of the steam produced with the heat pump 31; 31'; 31” is lower.

[0057] Moreover, a lower pressure and temperature of the steam produced by the heat pump results in more market available solution for this application.

[0058] In the embodiment of a tissue paper drying device 1'" as shown in Figure 4c the steam recirculation arrangement 41'” does not comprise a thermocompressor but a mechanical vapor recompression, MVR, unit 45' which in this embodiment is positioned between the separator steam outlet 21b and the cylinder inlet 13a. Hereby, in some embodiments a thermocompressor may not be necessary because its purposes will be covered by the MVR unit 45'. The MVR unit will guarantee the recirculation of the blow-through steam, increasing its pressure from the separator tank pressure to the yankee cylinder pressure, while the steam generated by the heat pump system 31; 31'; 31” will be produced at the same pressure of the yankee cylinder.

[0059] In all the embodiments a boiler 71 is shown as optionally connected (shown by dotted lines). Hereby a boiler 71 may be connected as an extra resource if needed. In the embodiment shown in Figure 4c a thermocompressor 43 is also shown as optionally connected together with the boiler 71. This is because there is no thermocompressor in the ordinary system of Figure 4c.

[0060] According to one aspect of the invention more than one yankee drying section 11 can be connected to the same heat pump system 31; 31'; 31”. This is especially suitable for a heat pump system 31” having a tank 39 connected to the intermediate liquid loop 37 as shown in Figure 3b. In Figure 5 a tissue paper drying system 101 is schematically shown where two yankee drying sections 11 are connected to one and the same heat pump system 31”, via one heat exchanger 38 each. Both heat exchangers 38 are fluidly connected to the tank 39. More than two yankee drying sections 11 can also be connected to the same heat pump system 31”. The tank 39 will have additional fluid connections for each extra yankee drying section 11 that is connected in the system. Other hot air dryer devices, such as a TAD section 111 (as discussed in relation to Figure 6 below) can also be connected to the same heat pump system 31” as discussed in this embodiment. Steam produced in the heat sink side 35 of the heat pump system 31” is transferred to each of the yankee drying sections 11 via steam recirculation arrangements 41; 41'; 41”; 41'” connected to each of the yankee drying sections 11. The steam recirculation arrangements 41; 41'; 41”; 41'” may be any of the examples as discussed above in relation to Figures 2 and 4a-4c. In Figure 5 the steam recirculation arrangement 41 comprising a thermocompressor 43 is shown, however any of the other examples of steam recirculation arrangements 41'; 41”; 41'” can also be used. The possibility to add more yankee drying sections 11 to the system in Figure 5 is illustrated by the arrows A, B, C, D. At A liquid water from another yankee drying section 11 can be connected, at B steam can be forwarded to another connected yankee drying section 11 and at C and D another heat exchanger 38 can be connected for recover heat from exhaust air from another connected yankee drying section 11. This system 101 can be used in a flexible way. For example it can be used for utilizing heat from two yankee drying sections 11 to produce steam for one yankee drying section 11 or vice versa, i.e. to utilize heat from one yankee drying section 11 for producing steam for two yankee drying sections 11. The present embodiments can either be applied to a yankee drying section in conventional or hybrid tissue paper machine or to a TAD drying section in a TAD tissue paper machine.

[0061] Figure 6 shows schematically a tissue paper drying system 101' according to another embodiment of the invention. Many of the details are the same as in previous embodiments and will not be described again. For example the heat pump system 31; 31'; 31' can be any of the previous described heat pump systems 31; 31'; 31' and the steam recirculation arrangement 41; 41'; 41''; 41''' can be any of the previous described steam recirculation arrangements 41; 41'; 41''; 41'''. A difference is that in this embodiment the hood outlet 15b from the yankee hood 15 is not connected to the heat source inlet 33a of the heat pump system 31; 31'; 31'. In this embodiment another hot air dryer device, such as a TAD section 111 is connected in the system. More than one other TAD section 111 can also be connected in the system. The TAD section 111 comprises one or more TAD roll 113 and one or more TAD hood 115. The TAD hood 115 is mounted to the TAD roll 113 to cover at least a part of a TAD roll surface whereby a tissue paper to be dried in the TAD section 111 will pass between the TAD roll 113 and the TAD hood 115 as is known in the art for TAD system. Air is used in both the TAD roll 113 and the TAD hood 115 to dry the paper by means of pushing air through the paper web that is passing between TAD roll 113 and TAD hood 115. The TAD roll 113 and the TAD hood 115 are fluidly connected to a TAD air system 151 by means of a TAD section air inlet Illa and a TAD section air outlet 111b. Another number of TAD section air inlets Illa and TAD section air outlets 111b is also possible and covered by the invention. The TAD air system 151 is configured for preparing air to be forwarded to TAD section air inlet(s) Illa and to receive air from the TAD section air outlet(s) 111b. The TAD air system 151 further comprises an air inlet 151a and an exhaust air outlet 151b, wherein said exhaust air outlet 151b is fluidly connected to the heat source inlet 33a of the heat pump system 31; 31'; 31”, possibly via a first heat recovery device 61a' where heat can be recovered from the exhaust air and returned to the air system 151 as shown in Figure 6. A second heat recovery device 61b' can be connected after the heat pump system 31; 31'; 31”, i.e. connected to the heat source outlet 33b.

[0062] After the paper has passed through the TAD section 111 in this embodiment, it is transferred to a Yankee drying section 11 to complete the drying process.

[0063] The TAD section air outlet 111b is hereby in this embodiment fluidly connected with the heat source inlet 33a of the heat pump system 31; 31'; 31”. Hereby, in this embodiment, hot exhaust air from the TAD section 111 is used as heat source in the heat pump system 31; 31'; 31”. The rest of the tissue paper drying system 101' corresponds to previously described embodiments whereby the liquid water (condensate) retrieved from the yankee cylinder 13 is transferred to the heat sink inlet 35a of the heat pump system 31; 31'; 31” where it is transformed into steam and thereafter transferred for reuse in the yankee cylinder 13.

[0064] According to the invention a tissue paper system for the production of tissue paper is also provided, comprising one or more tissue paper drying systems 1; 1'; 1”; 1'"; 101; 101' according to any one of the embodiments described above. Other tissue paper drying devices than yankee drying sections and TAD sections as discussed above can possibly also be connected in the system, such as a through-air bonder (TAB), and / or floatation dryer, and / or impingement.

[0065] According to the invention a method for operating a tissue paper drying system comprising a yankee drying section 11 and possibly a TAD section 111, a separator tank 21, a heat pump system 31; 31'; 31”, and a steam recirculation arrangement 41; 41'; 41”; 41'” as described above according to any one of the different embodiments is also provided. A flow chart of one embodiment of the method is shown in Figure 7. The method steps are described in order below, however the steps are performed simultaneously:

[0066] SI: Forwarding exhaust air from a yankee hood 15 of the yankee drying section 11 and / or from a TAD section 111 to a heat source inlet 33a of a heat source side 33 of the heat pump system 31; 31'; 31”.

[0067] S2: Using said exhaust air in the heat pump system 31; 31'; 31” as heat source.

[0068] S3: Transferring a mix of liquid water (condensate) and steam from a yankee cylinder 13 of the yankee drying section 11 to the separator tank 21.

[0069] S4: Separating the liquid water and the steam in the separator tank 21.

[0070] S5: Transferring the liquid water from the separator tank 21 to a heat sink inlet 35a of a heat sink side 35 of the heat pump system 31; 31'; 31”.

[0071] S6: Transforming the liquid water received in the heat sink inlet 35a into steam in the heat pump system 31; 31'; 31”.

[0072] S7: Forwarding the steam produced in the heat pump system 31; 31'; 31” and the steam separated in the separator tank 21 to the steam recirculation arrangement 41; 41'; 41”; 41'”.

[0073] S8: Preparing the steam in at least one steam preparation device 43; 45; 45' provided in the steam recirculation arrangement 41; 41'; 41”; 41'” to a suitable pressure.

[0074] S9: Forwarding the prepared steam from the steam recirculation arrangement 41; 41'; 41”; 41'” to the yankee cylinder 13.

[0075] In some embodiments the step of using said exhaust air in the heat pump system 31; 31'; 31” as heat source comprises transferring heat from the exhaust air via a heat exchanger 38 and an intermediate liquid loop 37 to an evaporator 32b of the heat pump system 31; 31'; 31”. In some embodiments the step of preparing the steam comprises adjusting a pressure of the steam in a thermocompressor 43 and / or a mechanical vapor recompression, MVR, unit 45; 45'. The tissue paper drying system 1; 1'; 1”; 1"'; 101; 101' may also comprise a control system 203 (only shown in Figure 2 but may be provided in all embodiments). Said control system 203 is configured for controlling the tissue paper drying system 1; 1'; 1”; 1'"; 101; 101' to perform the method according to the invention as described above. The control system 203 is in communication contact, either by wire or wirelessly (as shown schematically by dotted lines), to for example pumps, valves and sensors provided in the tissue paper drying system for controlling the flows in the system. The invention further comprises a computer program product comprising instructions which, when executed in a processor in the control system 203 in the tissue paper drying system of the invention, cause the control system to perform the method as described above.

Claims

CLAIMS1. A tissue paper drying system (1; 1'; 1”; 1'"; 101; 101') comprising: a yankee drying section (11) comprising a yankee cylinder (13) and a yankee hood (15), wherein steam is used in the yankee cylinder (13) for drying a tissue paper passing through the yankee drying section and hot air is used in the yankee hood (15) for drying the tissue paper, wherein the yankee cylinder (13) comprises a cylinder inlet (13a) for steam and a cylinder outlet (13b) for steam and liquid water and wherein the yankee hood (15) comprises a hood inlet (15a) for air and a hood outlet (15b) for exhaust air; a separator tank (21) comprising a separator inlet (21a) fluidly connected to the cylinder outlet (13b) for receiving steam and liquid water from the yankee cylinder (13), said separator tank (21) being configured for separating steam from liquid water and for forwarding steam out from the separator tank (21) via a separator steam outlet (21b) of the separator tank (21) and for forwarding liquid water out from the separator tank (21) via a separator liquid water outlet (21c); a heat pump system (31; 31'; 31'') having a heat source side (33) and a heat sink side (35), said heat sink side (35) comprising a heat sink inlet (35a) which is fluidly connected to the separator liquid water outlet (21c) and a heat sink outlet (35b) for transferring steam produced in the heat sink side (35) from liquid water received from the separator tank (21) out from the heat pump system (31; 31'; 31''), wherein the heat source side (33) comprises a heat source inlet (33a) and a heat source outlet (33b) between which a fluid is to be transferred for being used as a heat source in the heat pump system (31; 31'; 31''), wherein said heat source inlet (33a) is fluidly connected to the hood outlet (15b) and / or to a TAD air outlet (111b) of a TAD section (111) possibly provided in the tissue paper drying system (101'), whereby exhaust air from the yankee hood (115) and / or from the TAD section (111) is used as heat source in the heat pump system (31; 31'; 31''); anda steam recirculation arrangement (41; 41'; 41”; 41'”) being configured for forwarding steam from the separator steam outlet (21b) and the heat sink outlet (35b) to the cylinder inlet (13a) of the yankee cylinder (13) and further comprising at least one steam preparation device (43; 45, 45') configured for preparing the steam for a suitable pressure to be used in the yankee cylinder (13).

2. Tissue paper drying system according to claim 1, wherein the heat source side (33) of the heat pump system (31'; 31”) comprises an intermediate liquid loop (37) and a heat exchanger (38), whereby the intermediate liquid loop (37) is configured for circulating a liquid between the heat exchanger (38) and an evaporator (32b) of the heat pump system (31'; 31”), said heat exchanger (38) comprising said heat source inlet (33a) and heat source outlet (33b) and being configured for transferring heat from the exhaust air received in the heat source inlet (33a) to the liquid in the intermediate liquid loop (37).

3. Tissue paper drying system according to claim 2, wherein the heat source side (33) further comprises a tank (39) connected in the intermediate liquid loop (37) for accumulating liquid utilized in the intermediate liquid loop (37).

4. Tissue paper drying system according to claim 3, wherein the tank (39) is connected to renewable energy sources (82, 83) to enhance the performance of the heat pump system (31”).

5. Tissue paper drying system according to any one of the preceding claims, wherein the steam recirculation arrangement (41: 41'; 41”) comprises a thermocompressor (43) comprising a first thermocompressor inlet (43a) which is fluidly connected to the separator steam outlet (21b) for receiving steam from the separator tank (21) and a second thermocompressor inlet (43b) whichis fluidly connected to the heat sink outlet (35b) and a thermocompressor outlet (43c) which is fluidly connected with the cylinder inlet (13a) for transferring steam from the thermocompressor (43) to the yankee cylinder (13).

6. Tissue paper drying system according to any one of the preceding claims, wherein the steam recirculation arrangement (41'; 41”) comprises a mechanical vapor recompression, MVR, unit (45) connected in a fluid line (42b) between the heat sink outlet (35b) and the cylinder inlet (13a).

7. Tissue paper drying system according to any one of the claims 1-3, wherein the steam recirculation arrangement (41'”) comprises a mechanical vapor recompression, MVR, unit (45') connected in a fluid line (42a) between the separator steam outlet (21b) and the cylinder inlet (13a).

8. Tissue paper drying system according to any one of the preceding claims, wherein the tissue paper drying system (1”) further comprises a flash tank (49) and a flash valve (50) connected in a fluid line between the separator liquid water outlet (21c) and the heat sink inlet (35a) and wherein a flash tank steam outlet (49a) of the flash tank (49) is fluidly connected to the steam recirculation arrangement (41”).

9. Tissue paper drying system according to any one of the preceding claims, further comprising an air system (51) which is fluidly connected with the hood inlet (15a) and the hood outlet (15b) and configured for preparing air to be forwarded to the hood inlet (15a) and to receive exhaust air from the hood outlet (15b), said air system (51) further comprising an air inlet (51a) and an exhaust air outlet (51b), wherein said exhaust air outlet (51b) is fluidly connected to the heat source inlet (33a).

10. Tissue paper drying system according to any one of the preceding claims, comprising more than one yankee drying sections (11) and / or TAD sections (111) connected to the same heat pump system (31”).

11. A tissue paper system for the production of tissue paper comprising one or more tissue paper drying systems according to any one of the preceding claims.

12. A method for operating a tissue paper drying system (1; 1'; 1”; 1'”; 101; 101') comprising a yankee drying section (11), possibly a TAD section (111), a separator tank (21), a heat pump system (31; 31'; 31”), and a steam recirculation arrangement (41; 41'; 41”; 41'”) according to any one of the claims 1-10, said method comprising the steps: forwarding exhaust air from the yankee hood (15) of the yankee drying section (11) and / or from the TAD section (111) to the heat source inlet (33a) of the heat source side (33) of the heat pump system (31; 31'; 31”); using said exhaust air in the heat pump system (31; 31'; 31”) as heat source; transferring a mix of liquid water and steam from the yankee cylinder (13) of the yankee drying section (11) to the separator tank (21); separating the liquid water and the steam in the separator tank (21); transferring the liquid water from the separator tank (21) to the heat sink inlet (35a) of the heat sink side (35) of the heat pump system (31; 31'; 31”); transforming the liquid water received in the heat sink inlet (35a) into steam in the heat pump system (31; 31'; 31”); forwarding the steam produced in the heat pump system (31; 31'; 31”) and the steam separated in the separator tank (21) to the steam recirculation arrangement (41; 41'; 41”; 41'”);preparing the steam in the at least one steam preparation device (43; 45;45') provided in the steam recirculation arrangement (41; 41'; 41''; 41''') to a suitable pressure; and forwarding the prepared steam from the steam recirculation arrangement (41; 41'; 41''; 41''') to the yankee cylinder (13).

13. Method according to claim 12, wherein the step of using said exhaust air in the heat pump system (31'; 31'') as heat source comprises transferring heat from the exhaust air via a heat exchanger (38) and an intermediate liquid loop (37) to an evaporator (32b) of the heat pump system (31'; 31'').

14. Method according to claim 11 or 12, wherein the step of preparing the steam comprises adjusting a pressure of the steam in a thermocompressor (43) and / or a mechanical vapor recompression, MVR, unit (45; 45').

15. A computer program product comprising instructions which, when executed in a processor in a control system (203) in a tissue paper drying system (1; 1'; 1"; 1'"; 101; 101') according to any one of the claims 1-10, cause the control system (203) to operate the tissue paper drying system according to the method according to any one of the claims 12-14.

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