TAD system for paper production.

Abstract

TAD system for paper production comprising a drying station (DS) with multiple perforated cylinders or TAD cylinders (1 A, IB, 1C) through which air passes from a hood (2) on which an air recirculation line (9, 11) is arranged. A primary heating unit (15) is arranged on the air recirculation line and, upstream of the primary heating unit (15), a by-pass duct (14) is provided on the recirculation line (9, 11) such that a part (F13) of the recirculated air (F9) passes through the primary heating unit (15) before being introduced into the hood (2) and another part (F14) is introduced into the hood (2) via the by-pass duct (14) without passing through the primary heating unit (15).

Description

[0001] TITLE

[0002] TAD system for paper production.

[0003] DESCRIPTION

[0004] The present invention relates to a TAD system for paper production.

[0005] In particular, a TAD system according to the present invention can be used for the production of tissue paper.

[0006] It is known that tissue paper is particularly intended for the production of articles for hygienic or sanitary use, such as toilet paper rolls, kitchen paper rolls, napkins, handkerchiefs, etc. One of the factors that most characterize tissue paper is its bulkiness or “bulk”. Other things being equal, such as the chemi cal -physical structure of the pulp from which the paper is obtained, the bulk can be significantly influenced by the dehydration process of the wet fibrous veil obtained from the pulp.

[0007] In general, in the so-called TAD ( Through -Air Drying ) systems, the wet sheet, transported by a structured belt, passes over a perforated cylinder or “TAD cylinder” through which hot air passes, coming from a hood, which produces a drying phase without significant compression of the sheet. In the latter, therefore, the structured profile imparted by the support belt is maintained. Fig. 1 of the attached drawings is an explanatory diagram in which the reference “T” denotes a TAD station as a whole, the reference “WW” indicates the wet sheet, the reference “F” indicates the structured belt that supports the wet sheet entering the TAD station, the reference “C” indicates a TAD cylinder or drum rotating in the direction “RC”, the reference “H” indicates the hood that supplies the hot air “A”. Depending on the chosen configuration, the direction of the hot air can be entering or exiting with respect to the cylinder “C”. In Fig.l the hot air “A” passes through the cylinder “C” from the inside. The reference “DW” indicates the ply exiting the TAD station. The ply exiting the TAD station is typically passed over a Yankee dryer.

[0008] In practice, in TAD processes the sheet is dehydrated and dried by non- compressive means, thus avoiding the compaction that occurs during wet pressing.

[0009] The improvement of the properties of the finished product in terms of volume and absorbency, compared to processes that involve a prevalence of compressive actions for the extraction of water from the wet sheet, makes the production of TAD paper particularly suitable for hygienic and sanitary use, as well as for uses in which a high absorption capacity is generally required.

[0010] Despite the advantages in terms of product quality, the TAD process suffers from the disadvantage of high energy consumption compared to wet pressing. The high energy consumption is mainly due to the fact that non-compressive drying involves drying by hot air pushed through the sheet, rather than by a mechanical action designed to push the water out of the sheet as is the case in compressive drying processes. This makes TAD drying operations more energy-consuming than wet pressing.

[0011] The cylindrical surface of a TAD cylinder usually has a honeycomb structure, that is, an open-cell cellular structure with very high permeability, to allow a more uniform and effective drying of the wet sheet while maintaining an acceptable support capacity or structured belt, avoiding the achievement of high pressures in the contact areas with the paper or structured tape.

[0012] The hood is supplied with relatively warm and dry air. The flow of air through the wet ply and the structured belt is induced by the pressure difference between the internal surface and the external surface of the TAD cylinder (or TAD cylinders in configurations that include a plurality of TAD cylinders).

[0013] The colder and more humid air that is formed as a result of this heat exchange is partly expelled from the system by an exhaust fan, while another part of it is recirculated through the so-called “air system” made up of ducts and devices for conveying and controlling the temperature and flow rate of the air supplied to the hood.

[0014] A predetermined amount of fresh air taken from outside is introduced into the air system, and both the fresh air and the recirculated air are then heated, typically by gas burners. This produces the warm and relatively dry air that is introduced into the hood. The air temperature in the hood is usually between 150°C and 220°C.

[0015] Since the wet sheet feed rate through the TAD station can be in the order of 1500 m / min and the wet sheet is in contact with a TAD cylinder for a time that is generally less than 1 sec, a TAD system must be designed to provide very high drying rates. However, at the same time it is necessary to reduce as much as possible the production costs related to the energy used to heat the air introduced into the hood. The main purpose of the present invention is precisely to provide a TAD system for the production of tissue paper that allows for the optimization of energy consumption without compromising the correct execution of the drying process and / or allowing for an increase in the drying capacity of the sheet and therefore of production, as well as greater operating flexibility of the system.

[0016] This result has been achieved, in accordance with the present invention, by adopting the idea of making a system having the characteristics indicated in claim 1. Other characteristics of the present invention are the subject of the dependent claims.

[0017] Thanks to the present invention, it is possible to control the temperatures of the air flows conveyed to the hood more effectively and with greater operational flexibility, offering, at the same time, the possibility of using alternative heating systems that can be conveniently selected based on the programmed production and / or the availability of energy sources that are time by time more convenient from a cost point of view, all this without introducing significant system complications.

[0018] These and further advantages and characteristics of the present invention will be better understood by any person skilled in the art thanks to the following description and the attached drawings, provided by way of example but not to be considered in a limiting sense, in which:

[0019] - Fig.l is an explanatory diagram relating to the operation of a TAD section known per se;

[0020] - Figs.2A-2B-2C are schematic representations of a TAD system according to three possible embodiments of the present invention;

[0021] - Figs.3A-3C are schematic representations of a TAD system in accordance with further possible embodiments of the present invention.

[0022] The present invention relates in particular to systems with multiple TAD cylinders arranged in series. The invention can be applied to a plurality of TAD cylinders in order to allow differentiated and independent control of the temperature of the TAD cylinders. This solution allows to set the most convenient temperature for the degree of humidity of the sheet that passes over each of the TAD cylinders.

[0023] The following description concerns possible ways of implementing the present invention. Reduced to its essential structure and with reference to the diagrams of Figs.2A-3C, a TAD system in accordance with the present invention comprises a drying station (DS) in which a plurality of perforated cylinders or “TAD cylinders” (1A, IB, 1C) are arranged through which hot air passes, coming from a hood (2), which produces a drying phase without significant compression of a continuous wet sheet of paper (3). The latter passes over the TAD cylinders (1A, IB, 1C) and is produced in a papermaking machine (schematically represented in the drawings by the reference PM) upstream of the TAD system. In the context of the present invention, the specific methods of production of the wet sheet (3) are not relevant, so that such production can be carried out with any method known to those skilled in the art. The TAD cylinders (1 A, IB, 1C) each rotate around its own longitudinal axis, promoting the advancement of the sheet (3) through the drying station (DS). Typically, the wet sheet (3) is supported by a structured belt (4) that is permeable to air as it passes through the station (DS).

[0024] The system also includes means for introducing hot air into the hood (2) and an air recirculation unit consisting of ducts in which part of the exhaust air exiting the TAD cylinders (1 A, IB, 1C) is recirculated through the hood (2).

[0025] In the attached example diagrams, the recirculation unit comprises a duct (9) for recirculating the exhaust air and a flow divider (12) on the outlet of said recirculation duct (9). An external exhaust duct (7) is also provided with a respective exhaust fan (8) to discharge to the outside the air that is not recirculated. Through the recirculation duct (9) the air extracted from the hood (2) is recirculated, i.e. it is returned to the hood (2). On the recirculation duct (9) is inserted a respective recirculation fan (10) on whose outlet (11) the flow is provided the divider (12). A primary duct (13) and a by-pass duct (14) are connected to the two outlets of the latter, into which the flows (Fl 3, Fl 4) of the air exiting the divider (12) located on the outlet of the recirculation fan (10) are respectively conveyed. The first flow (Fl 3) is conveyed, via the primary duct (13), to a heater (15). For example, the heater (15) can be a gas burner but can also be of any other type. The second flow (14) can be conveyed, via the by-pass duct (14), to a supplementary heating unit (16) which can be, for example, a steam, gas or electric heating unit. In the diagrams of Figs. 2A-2C the supplementary heating unit (16) is not foreseen, while it is foreseen in the diagrams of Figs. 3 A-3C. A corresponding fan (14V) can be inserted on the bypass duct (14). The outlet of the heater (15) is connected to the air inlet of the hood (2) by means of three ducts (18A, 18B, 18C) each of which supplies air to a respective TAD cylinder (1A, IB, 1C). In this way, with reference to the diagrams of Figs.3A-3C, a TAD cylinder is supplied with air, differently treated by the heater (15) and by the supplementary heating unit (16), resulting from the mixing of the flows (F15, F16) exiting the heater (15) and the supplementary heating unit (16), while the other TAD cylinders are supplied with the air coming from the heater (15). Or, with reference to the diagrams in Figs.2A-2C, on one (18A) of the ducts (18A, 18B, 18C) a flow of air coming from the heater (15) and from the by-pass duct (14) is mixed while the other two ducts ( 8B, 18C) convey air coming from the heater (15) to the respective TAD cylinders (IB, 1C). The references “F18A”, “F18B” and “F18C” in the attached drawings represent the flows of the recirculated air supplied to the cylinders (1A), (IB) and (1C) respectively.

[0026] Therefore, the temperature of the air supplied to the individual TAD cylinders is differentiated.

[0027] In practice, in the configuration described above, the flow (F9) of the recirculated air is divided into two parts, i.e. a primary flow (F13) and a bypass flow (F14) downstream of the recirculation fan (10). One part (F13) of the recirculation flow (F9) passes through the heater (15) which can also be switched off or configured to operate with reduced heating capacity if necessary, while the other part (F14) is heated by the supplementary heating unit (16) or is fed directly into one of the ducts (18A, 18B, 18C) supplying the TAD cylinders (1A, IB, 1C).

[0028] For example, said first part (F13) and said second part (F14) constitute respectively 70% and 30% of the recirculation flow (F9). For example, the recirculation air flow (F9) is at a temperature between 100°C and 110°C, at a pressure between 15 mbar and 20 mbar. Therefore, by bringing the air of the by-pass flow (F14) to a temperature between 400°C and 500°C by means of the supplementary heating unit and leaving the temperature of the primary flow (Fl 3) unchanged, the air supplied to the hood (2) will be at a temperature between 190°C and 200°C. Thanks to the splitting of the recirculation flow (F9) it is possible to use the supplementary heating unit (16) to supply air to the hood (2) using more convenient energy sources and , in particular if the heater (15) is a gas burner, avoiding the use of gas. For example, if the supplementary heating unit (16) is a steam heat exchanger, the steam can be taken from other points of the paper machine from which it should be discharged outside. Or, for example, if the supplementary heating unit (16) is an electric heat exchanger, the electric energy can be produced from renewable sources using photovoltaic panels and / or wind generators. Or, depending on the current cost of gas, the supplementary heating unit can be deactivated to use the gas burner exclusively.

[0029] Fig. 2A shows a configuration with multiple TAD cylinders (1A, IB, 1C) arranged in series along the direction (MD) followed by the wet sheet (3), in which the proposed innovation affects the first of them; this allows the first drum (1 A) to be supplied with air at a different temperature than the others (IB, 1C). The invention can however be applied to any other cylinder in the series or to multiple cylinders, obtaining the widest operating flexibility.

[0030] With reference to the exemplary diagram in Fig.3A, the drying station (DS) comprises a plurality of perforated cylinders or TAD cylinders (1A, IB, 1C) and the hood (2) serves all the TAD cylinders (1A, IB, 1C) which are appropriately arranged so as to define a longer path for the wet sheet (3) and receive the hot air introduced into the hood itself. In the example shown in Fig.3 A the TAD cylinders (1 A, IB, 1C) are three in number but it is understood that they may be in a different number from that shown in the figure. The TAD cylinders (1A, IB, 1C) rotate around their own longitudinal axes facilitating the advancement (MD) of the wet sheet (3) through the station (DS). A part of the air flow coming out of the drying station (DS) is discharged to the outside through an exhaust duct (7) on which a corresponding fan (8) is inserted, while another part is recirculated through the recirculation duct (9) on which the recirculation fan (10) is inserted. The duct (11) of the air outlet from the recirculation fan (10) is crossed by the recirculation air flow (F9) and the flow divider (12) is inserted on it. The primary duct (13) and the by-pass duct (14) are inserted on the flow divider (12) into which the flows (F13, F14) of the air coming out of the divider (12) are respectively conveyed. The primary flow (F 13), i.e. the flow that passes through the primary duct (13), feeds the burner (15), which can be of the gas type, which in this example constitutes the primary heating system for the recirculation air. In this example, the air coming out of the gas burner (15) is conveyed to the hood (2) along three parallel lines (18A, 18B, 18C) each of which supplies air to a respective TAD cylinder (1 A, IB, 1C). The by-pass flow (F14) is conveyed, via the by-pass duct (14), to one (18A) of the three lines (18A, 18B, 18C) serving the gas burner (15) to differentiate the temperature of the first TAD cylinder (the TAD cylinder 18A furthest upstream of the trio with respect to the MD direction followed by the wet sheet 3). The auxiliary heater (16) is arranged on the by-pass line (14). Combustion air is supplied to the burner (15) via a corresponding supply duct (15 A). For example, the recirculation flow (F9) is at a temperature between 100°C and 110°C, at a pressure between 15 mbar and 20 mbar. The temperature of the flow (Fl 5) immediately downstream of the gas burner (15) is between 190°C and 210°C, at a pressure between 1 mbar and 3 mbar. With a subdivision of the recirculation flow (F9) similar to that of the previous example (F 13 equal to 70% of Fl 1; F14 equal to 30% of Fl 1), due to the injection of the by-pass flow (Fl 4) in the first line (18 A), downstream of the respective injection point (K), there is a temperature of approximately 180°C.

[0031] In Fig.2B and Fig.3B the by-pass flow (F 14) is instead conveyed onto the duct (18B), while in Fig. 2C and Fig.3C the by-pass flow (F14) is conveyed onto the duct (18C).

[0032] It is understood that the bypass flow (F 14), regardless of whether or not it passes through a supplementary heating unit, may also be conveyed to more than one of the ducts (18 A, 18B, 18C).

[0033] With reference to all the examples described above, control valves can be inserted on the various ducts indicated to allow the air flow rates to be adjusted according to programmed or desired values.

[0034] More generally, the flow rate of the part (Fl 3) of the recirculated air (F9) that passes through the primary heating unit (15) is between 50% and 100% of the flow rate of the recirculated air (F9).

[0035] From the above description it is evident that a TAD system in accordance with the present invention comprises a drying station (DS) wherein a plurality of perforated cylinders or TAD cylinders (1; 1A, IB, 1C) are arranged through which passes air coming from a hood (2), wherein the hood (2) is supplied with air heated to a pre-set temperature by means of a hood air supply circuit comprising a recirculation line (9, 11) by means of which a part of the air supplied to the hood (2) is collected at the outlet of the hood itself and recirculated in the hood air supply circuit, wherein on the recirculation line (9, 11) is arranged a primary heating unit (15) configured for bringing the temperature of the recirculated air (F9) to a pre-set value, wherein, upstream of the primary heating unit (15), on the recirculation line (9, 11) a by-pass duct is inserted (14) such that a part (F13) of the recirculated air (F9) passes through the primary heating unit (15) before being fed into the hood (2) and another part (F14) is fed into the hood (2) via the by-pass duct (14) without passing through the primary heating unit (15), and wherein the flow (Fl 5) of the air exiting the primary heating unit (15) is conveyed to the hood (2) along a plurality of conveying lines (18 A, 18B, 18C) each of which supplies air to a respective perforated cylinder or TAD cylinder (1A, IB, 1C), and the flow of air (F14) passing through the by-pass duct (14) is conveyed, via the by-pass duct (14) to one or more of the perforated cylinders or TAD cylinders (1 A, IB, 1C) through one or more of the conveying lines (18 A, 18B, 18C).

[0036] From the preceding description it also appears evident that a TAD system in accordance with the present invention may comprise one or more of the following additional features, even combined with each other: a secondary heating unit (16) is inserted on the bypass duct;

[0037] - the primary heating unit (15) comprises a gas burner;

[0038] - the secondary heating unit (16) is a steam heating unit;

[0039] - the secondary heating unit (16) is an electric heating unit;

[0040] - the flow rate of the part (Fl 3) of the recirculated air (F9) passing through the primary heating unit (15) is between 50% and 100% of the flow rate of the recirculated air (F9);

[0041] - the air temperature immediately downstream of the primary heating unit

[0042] (15) is between 150°C and 210°C;

[0043] - the air temperature immediately downstream of the secondary heating unit

[0044] (16) is between 100°C and 500°C.

[0045] In practice, the execution details may however vary in an equivalent manner with regard to the individual elements described and illustrated, without departing from the idea of the solution adopted and therefore remaining within the limits of the protection offered by this patent in accordance with the claims that follow.

Claims

CLAIMS1) TAD system for the production of paper comprising a drying station (DS) wherein a plurality of perforated cylinders or TAD cylinders (1; 1A, IB, 1C) are arranged through which passes air coming from a hood (2), wherein the hood (2) is supplied with air heated to a pre-set temperature by means of a hood air supply circuit comprising a recirculation line (9, 11) by means of which a part of the air supplied to the hood (2) is collected at the outlet of the hood itself and recirculated in the hood air supply circuit, wherein on the recirculation line (9, 11) is arranged a primary heating unit (15) configured for bringing the temperature of the recirculated air (F9) to a pre-set value, wherein, upstream of the primary heating unit (15), on the recirculation line (9, 11) a by-pass duct is inserted (14) such that a part (Fl 3) of the recirculated air (F9) passes through the primary heating unit(15) before being fed into the hood (2) and another part (F14) is fed into the hood (2) via the by-pass duct (14) without passing through the primary heating unit (15), and wherein the flow (Fl 5) of the air exiting the primary heating unit (15) is conveyed to the hood (2) along a plurality of conveying lines (18 A, 18B, 18C) each of which supplies air to a respective perforated cylinder or TAD cylinder (1 A, IB, 1C), and the flow of air (F14) passing through the by-pass duct (14) is conveyed, via the by-pass duct (14) to one or more of the perforated cylinders or TAD cylinders (1A, IB, 1C) through one or more of the conveying lines (18A, 18B, 18C).2) TAD system according to claim 1 characterised in that the air temperature immediately downstream of the primary heating unit (15) is between 150°C and 210°C.3) TAD system according to claim 1 characterised in that the flow rate of the part (F13) of the recirculated air (F9) passing through the primary heating unit (15) is between 50% and 100% of the flow rate of the recirculated air (F9).4) TAD system according to claim 1 characterised in that a secondary heating unit(16) is inserted on the by-pass duct (14).5) TAD system according to claim 1 characterised in that the primary heating unit(15) comprises a gas burner.6) TAD system according to claim 2 characterised in that the secondary heating unit(16) is a steam or gas heating unit.7) TAD system according to claim 4 characterised in that the secondary heating unit (16) is an electric heating unit.8) TAD system according to claim 4 characterised in that the air temperature immediately downstream of the secondary heating unit (16) is between 100°C and 500°C.

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