EP2298988B1

EP2298988B1 - Drying and desiccating system for plants for producing web-like paper material

Info

Publication number: EP2298988B1
Application number: EP10176140A
Authority: EP
Inventors: Pietro Saccomano Rocchi Burlamacchi; Massimo Giannecchini
Original assignee: Novimpianti Drying Technology SRL
Current assignee: Novimpianti Drying Technology SRL
Priority date: 2009-09-16
Filing date: 2010-09-10
Publication date: 2012-02-08
Anticipated expiration: 2030-09-10
Also published as: PL2298988T3; ITMI20091582A1; ATE544904T1; ES2382118T3; EP2298988A1; IT1395745B1

Abstract

An evaporative drying and desiccating system (10) for a web-like paper material is described. The system (10) comprises at least one first drying device (12), of the pressurised rotating cylinder type fed by steam, on which the web-like paper material to be desiccated is dynamically wound. The cylinder (12) contains a pipe system, for the removal of the condensate, inside it. The system (10) thus comprises at least one second drying device (14), of the hood type, which at least partially covers the cylinder (12) and that comprises at least one first burner (16) capable of producing high temperature dry air. The hood (14) is both capable of blowing the high temperature dry air onto the web-like paper material wound on the cylinder (12), and sucking the moist air released from such a web-like paper material. Positioned along the pipes (26) for releasing the moist air from the hood (14), there are at least one waste heat boiler (34), fed by the moist air released from the web-like paper material and capable of generating high pressure steam for feeding a thermocompressor (36) and a make-up valve (70), which supply steam to the cylinder (12), and at least one auxiliary burner (58), capable of increasing the temperature of the moist air coming from the hood (14). The system (10) is finished off by an exhaust and recirculation fan (32) and by a recirculation and balancing pipe system (72) that, together with the release pipes (26) and through one or more adjustment proportioning valves (64; 38), make it possible to regulate and balance the air flows. It is thus superfluous to use a fuel boiler for feeding the cylinder (12), making it possible moreover, for the thermal conditions of the hood (14) and of the waste heat boiler (34) to be independently regulated, and the amount of energy required by the system (10) is reduced.

Description

The present invention refers to a drying and desiccating system for plants for producing web-like paper material, in particular but not exclusively, for plants for producing rolls of toilet paper.
As it is known, in the production process of paper in general, and in particular in the production of toilet paper, it is necessary to carry out an evaporative drying step of the product being processed, in order to extract the excess water content. The product that must be dried, normally made up of a paper pulp diluted with water, is prepared and sent to the drying and desiccating system after a preliminary pressing step. At the inlet of the drying and desiccating system, the sheet of paper being processed contains a low content of solid part, equal to about 35%-40% at a temperature of around 35°C. In other words, after the pressing step, the sheet of paper being processed can still contain up to 65% of water. The pressing step, therefore, is not capable of eliminating all the water from the paper fibres, which must therefore be removed through evaporation.
The finished product, in the specific case consisting of toilet paper, requires a content of solid part that is much greater than the values mentioned above. It is thus clear that in the evaporative drying step, it is necessary to extract most of the residual water content from the sheet of paper, so as to obtain a continuous sheet of paper that is sufficiently dry. After the evaporative drying and desiccating step, the sheet of paper is stored in reels so as to be subsequently processed (so-called "converting" step) and finally packed for delivery and for the final retail selling.
The evaporative drying and desiccating step of toilet paper is currently carried out through a system provided with two separate devices, which act simultaneously, however, upon the sheet of paper during such a drying and desiccating step.
A first drying device consists of a high yield hood, which provides for blowing hot air, at a temperature of up to about 650°C in hoods of the latest generation, on the sheet of paper being processed. The hot air is normally produced with a direct fire burner.
High yield hoods also have the need of discharging the water vapour extracted from the sheet of paper being processed into the atmosphere. The flow consisting of water vapour and of the air transporting it is called in jargon "mist". This "mist" has a high temperature, of about 400°C÷550°C in plants of the latest generation. Consequently, the dispersion in the atmosphere of the "mist" substantially cuts down the thermal efficiency of high yield hoods.
Simultaneously with the blowing, the sheet of paper being processed comes into contact with a cylinder heated with steam having a diameter that can vary from about 1.5 m to about 6 m. The cylinder consists of a container under pressure that contains process steam up to about 10 bar G and more, inside it. The steam is normally produced by conventional boilers, fire tubes, water tubes, or similar technologies, in turn equipped with (methane, LPG, gas oil, etc.) fuel burners that have a maximum combustion efficiency equal to about 90%.
In current drying and desiccating systems, all or part of the "mist" produced by the high yield hood can be used to preheat the process air and the comburent air with which the burners of the hood itself are fed. In some drying and desiccating systems, like for example that described in the patent application for an industrial invention N. IT2007LU000003A, to the same applicant, the "mist" produced by the high yield hood can also be used to produce low temperature and low pressure steam (about 3 bar G), which can be used for feeding a steam box that is suitable for correcting the humidity cross-section. In other solutions, like for example that described in the patent application for the industrial invention N. IT2009MI000364A, to the same applicant, the "mist" produced by the high yield hood can finally be used to produce high pressure steam (up to 20 bar G), which can be directly reused in the drying process thanks to the presence of a boiler that operates to aid the main one.
However, the energy of the "mist" is not capable of generating all the steam necessary for the drying process, but only 60%÷70% of such steam, therefore, it is unavoidably necessary for there to be an external steam generator so as to provide for the remaining amount. Moreover, such an external steam generator is necessary for the transient steps of the drying process, such as the start up step of the entire plant for producing the roll of paper (the so called "warm up" step) and/or the case in which the roll of paper itself is broken.
In the document US 4,615,122 , a method for feeding steam and hot air to the desiccating cylinder in a drying and desiccating system is described, in which a waste heat boiler is provided, potentially eliminating a remotely located external boiler plant. Such a solution has a configuration in series between the boiler and the hood, with the problem of not being able to carry out a separate regulation between the thermal conditions of the hood and those of the boiler, since the hood and the boiler have the air flows in series and are regulated by a single burner. It is thus possible to eliminate the external boiler plant, at the expense, however, of the flexibility of regulation of the thermal conditions of the entire drying and desiccating system and, consequently, at the expense of the production flexibility of the desiccated paper.
The general purpose of the present invention, therefore, is that of making a drying and desiccating system for plants for producing web-like paper material, in particular but not exclusively for plants for producing rolls of toilet paper, that is capable of overcoming the aforementioned problems of the systems made according to the prior art.
In particular, one purpose of the present invention is that of making a drying and desiccating system for plants for producing web-like paper material that has no external steam generator whatsoever, so as to be completely independent both during the normal drying step of the paper, and during the transient steps.
Another purpose of the invention is also that of making a drying and desiccating system for plants for producing web-like paper material, in which both the thermal conditions of the hood, and those of the boiler, can be regulated in a completely independent manner, making it possible to regulate the temperature, operation pressures and circulating air masses that are completely different for the two separate drying devices (hood and boiler).
A further purpose of the invention is finally that of making a drying and desiccating system for plants for producing web-like paper material, that is capable of reducing both the energy consumption of the overall plant, and the pollutant emissions that are dispersed into the atmosphere by the plant itself, since, as previously outlined, the independent regulation of the hood and boiler allows the thermal conditions to be configured in the most suitable way for the type of paper being processed.
These purposes according to the present invention are achieved by making a drying and desiccating system for plants for producing web-like paper material, in particular but not exclusively, for plants for producing rolls of toilet paper, as outlined in claim 1.
Further characteristics of the invention are highlighted in the dependent claims, which are an integrating part of the present description.
The characteristics and the advantages of a drying and desiccating system for plants for producing web-like paper material according to the present invention shall become clearer from the following description, given as an example and not for limiting purposes, with reference to the attached schematic drawings, in which the single figure is a schematic view of a preferred embodiment of such a system.
It should be made clear that, in the attached figure, identical reference numerals indicate elements that are the same or equivalent.
With reference to figure 1, an evaporative drying and desiccating system, which can be used inside a plant for producing web-like paper material is shown, made according to the present invention. The system is wholly indicated with reference numeral 10.
The drying and desiccating system 10 firstly comprises a first drying device 12, of the heated rotating cylinder type, also called "Yankee cylinder", on which the web-like paper material to be desiccated (not shown) is dynamically wound, made up of a sheet of paper having a high water content. The sheet of paper reaches the Yankee cylinder 12 from the previous processing station, typically a pressing station, which is well known in the state of the art and shall therefore not be described in detail in the present description. Depending on how the pressing step of the sheet of paper is carried out, however, the amount of water present in the sheet itself, can vary, up to more than 60% by weight.
The Yankee cylinder 12 typically has a diameter that can vary from about 1.5 m to about 6 m, whereas its width is equal to the width of the most common standard formats of reels used in plants for processing paper. The Yankee cylinder 12 is made in the form of a pressurised steam container, which contains a pipe system (not shown) inside it, for the removal of the condensate.
The drying and desiccating system 10, therefore, comprises a second drying device 14, of the hood type, which at least partially covers the Yankee cylinder 12 and that is capable of both blowing high temperature dry air on the sheet of paper wound on said Yankee cylinder 12, and sucking the moist air, also at high temperature,and in jargon called "mist", released from the sheet of paper itself during its desiccation step. The dry air blown by the hood 14 typically has a temperature that can reach about 700°C and more, whereas the moist air extracted from the sheet of paper can have a temperature of up to 450°C and more.
The high temperature dry air, blown by the hood 14, is produced by a burner 16 that uses a fuel (for example methane, LPG or gas oil) fed through a suitable feed pipe 18. Air, drawn from the atmosphere and conveyed, through a suitable inlet pipe 20 and by means of a fan 22, to the burner 16 itself, is used as a comburent. The flow of combusted gas and air in outlet from the burner 16 forms the dry air for drying and desiccating, which is sent in pressure to the hood 14 through one or more inlet pipes 24.
The moist air at high temperature, extracted from the hood 14 from the sheet of paper being processed, is on the other hand conveyed through one or more release pipes 26 and is at least partially sent, by means of a recirculation fan 28, to the burner 16 so that it can be heated and reused by the hood 14 itself for drying the sheet of paper. The remaining part of moist air at high temperature extracted from the hood 14 is finally sent, through the release pipes 26, towards an air-air heat exchanger 30 (not always present), where it gives up part of its heat to the outside air to be sent to the burner 16, and reintegration air, through the inlet pipe 20, and is therefore expelled from the system 10 by means of an exhaust and recirculation fan 32.
Along the pipes 26 for releasing the moist air from the hood 14, at least one waste heat boiler 34 is arranged, fed by such moist air or "mist" and that is able to generate high pressure steam (between about 10 bar G and about 20 bar G) so as to in turn feed a thermocompressor 36 and a reintegration or "make-up" valve 70, through a delivery pipe 40, which deliver the steam to the Yankee cylinder 12. The steam produced by the waste heat boiler 34 thus minimises, in the system 10, the use of a conventional fuel boiler for feeding the Yankee cylinder 12.
According to the invention, along the pipes 26 for releasing the moist air from the hood 14, upstream of the waste heat boiler 34, there is at least one auxiliary burner 58, which uses a fuel drawn through a first feed pipe 60 and that draws air, used as a comburent, directly from the atmosphere. The comburent air is conveyed to the auxiliary burner 58 through a second feed pipe 62, directly connected with the inlet pipe 20 which brings dry air drawn from the atmosphere to the burner 16 of the hood 14, and a fan 66 for the comburent air.
The auxiliary burner 58 makes it possible to obtain an increase of the temperature of the moist air, arriving from the hood 14 and from a recirculation and balancing pipe system 72 and to be sent to the waste heat boiler 34, from a value of about 400°C÷420°C up to about 600°C and more. Therefore, by introducing the auxiliary burner 58 before the tube bundle of the waste heat boiler 34, it is possible for the drying and desiccating system 10, according to the invention, to not have the conventional external boiler.
Moreover, by means of the pipes 72 and of one or more adjustment proportioning valves 38 there is a second recirculation and balancing circuit, in addition to that formed by the release pipes 26, also for the air flow of the waste heat boiler 34. The regulation of the flow of such recirculated air occurs through the adjustment proportioning valves 64 and 38 arranged on the release pipes 26 and on the recirculation and balancing pipes 72, respectively, with the purpose of increasing the mass of air circulating through the waste heat boiler 34 by an amount necessary so as to obtain the required amount of steam, without modifying the process conditions of the hood 14 and of the main burner 16. Consequently, the system 10 is provided with two separate groups for feeding the hood 14 in cascade on the waste heat boiler 34. By using two separate burners 16 and 58, two separate recirculation circuits 26 and 72 and a series of adjustment proportioning valves 64 and 38, it is possible, in real time, to obtain the balancing and the continuous and independent regulation of the air flows of the hood 14 and of the waste heat boiler 34 and, therefore, of the relative heat and energy conditions.
Along the pipes 26 for releasing the moist air from the hood 14, upstream of the auxiliary burner 58 and of the waste heat boiler 34, one or more gate valves can be foreseen, not illustrated in the figure, that make it possible to by-pass the waste heat boiler 34 and/or the auxiliary burner 58, for safety reasons.
The drying and desiccating system 10, therefore, comprises a first separator tank 42 that is able to collect the "flash steam" and the "blow-through steam", as well as the hot condensates, coming from the Yankee cylinder 12 through a pipe 44. From the first separator tank 42, the flash steam and the blow-through steam are reintroduced into the thermocompressor 36 through a pipe 46, whereas the hot condensates are sent, through a return pipe 48 and by means of a first circulation pump 50 for feeding the waste heat boiler 34, to a second accumulator and separator tank 52.
A second circulation pump 54 pushes the condensates contained in the second accumulator and separator tank 52, through a suitable pipe 56, in the tube bundle of the waste heat boiler 34, that is thus capable of generating a biphasic fluid. Such a biphasic fluid is sent to an upper cylinder body 68 of the waste heat boiler 34, that thus operates as a water tube boiler: the liquid part of the biphasic fluid is reintroduced into the accumulator and separator tank 52, operating as a lower cylinder body of the waste heat boiler 34, whereas the part of steam of the biphasic fluid itself, through the delivery pipe 40, feeds the thermocompressor 36 and the reintegration or "make-up" valve 70.
As well as the components described thus far and illustrated in the figure, the drying and desiccating system 10 can be equipped with all the devices for controlling the level, the temperature and the pressure (not shown) that are necessary for it to operate correctly, as well as a possible electronic programmable logic control system (PLC).
The waste heat boiler 34 is also equipped with all the foreseen safety equipment (safety valve, manual resetting thermostat, etc.) that allow it to be approved in compliance with the directive PED 97/23/EC relative to the safety of such devices.
It has thus been seen that the drying and desiccating system for plants for producing web-like paper material, according to the present invention, achieves the purposes previously highlighted. Thanks to the presence of the waste heat boiler, which uses the energy of the exhaust "mist" of the drying devices, and of the auxiliary burner, which in turn increases the energy of such a "mist", it is indeed possible to use the steam produced in the drying and desiccating production process, thus leading to a sensitive reduction of the incidence of specific energy per unit mass of finished product. Moreover, there is also a reduction of CO₂ emissions in the overall production process of the roll of paper, since the auxiliary burner, using "mists" at high temperature like the air in inlet to the combustion chamber, optimises the combustion process.
The scope of protection of the invention is defined by the attached claims.

Claims

Evaporative drying and desiccating system (10) for a web-like paper material, the system (10) comprising:
- at least one first drying device (12), of the pressurised rotating cylinder type fed by steam, on which the web-like paper material to be desiccated is dynamically wound, said cylinder (12) being internally provided with a pipe system for the removal of the condensate,

- at least one second drying device (14), of the hood type, which at least partially covers said cylinder (12) and that comprises at least one first burner (16) capable of producing high temperature dry air, said hood (14) being both capable of blowing said high temperature dry air onto the web-like paper material wound on said cylinder (12), and of sucking the moist air released by said web-like paper material, and

- at least one waste heat boiler (34), fed by moist air released by the web-like paper material and disposed along pipes (26) for releasing said moist air from said hood (14), said waste heat boiler (34) being capable of generating high pressure steam for feeding a thermocompressor (36) and a make-up valve (70) which deliver steam to said cylinder (12),
wherein along the pipes (26) for releasing said moist air from said hood (14), upstream of said waste heat boiler (34), there is additionally at least one auxiliary burner (58), capable of increasing the temperature of said moist air coming from said hood (14) and to be sent to said waste heat boiler (34).
System (10) according to claim 1, characterised in that said auxiliary burner (58) uses a fuel drawn through a first feed pipe (60) and draws the air, used as a comburent, directly from the atmosphere through a second feed pipe (62), directly connected with an inlet pipe (20) which brings dry air drawn from the atmosphere to said hood (14), and through a fan (66) for comburent air.
System (10) according to claim 1 or 2, characterised in that said auxiliary burner (58) is capable of increasing the temperature of said moist air coming from said hood (14) from a value of about 400°C-420°C to a value of about 600°C and more.
System (10) according to any one of claims 1 to 3, characterised in that said waste heat boiler (34) is capable of generating steam at a pressure comprised between about 10 bar G and about 20 bar G.
System (10) according to any one of the preceding claims, also comprising at least one exhaust and recirculation fan (32) and a recirculation and balancing pipe system (72) capable of forming a second recirculation and balancing circuit, in addition to a first one formed by the pipes (26) for releasing the moist air from said hood (14), for the air flows of said waste heat boiler (34) and of said auxiliary burner (58).
System (10) according to claim 5, characterised in that along the pipes (26) for releasing the moist air from said hood (14), upstream of said auxiliary burner (58) and of said waste heat boiler (34), and along said recirculation and balancing pipe system (72) one or more adjustment proportioning valves (64; 38) are foreseen capable of sending amounts of moist air released by the web-like paper material both into the atmosphere, and to said auxiliary burner (58), as well as being capable of reintroducing it into the circuits of said hood (14) even after having passed through said waste heat boiler (34), said one or more adjustment proportioning valves (64; 38) regulating the flows continuously and allowing the system to be balanced (10) in real time.
System (10) according to any one of the preceding claims, also comprising at least one first separator tank (42) capable of receiving the steam and the hot condensates coming from said cylinder (12), the steam coming from said cylinder (12) being reintroduced in said thermocompressor (36) through a pipe (46).
System (10) according to claim 7, also comprising at least one second accumulator and separator tank (52), said second accumulator and separator tank (52) receiving the hot condensates coming from said first separator tank (42) through a return pipe (48) and by means of at least one first circulation pump (50) for feeding said waste heat boiler (34).
System (10) according to claim 8, also comprising at least one second circulation pump (54) capable of sending to said waste heat boiler (34) the hot condensates contained in said second accumulator and separator tank (52), said waste heat boiler (34) thus being capable of generating a biphasic fluid.
System (10) according to claim 9, characterised in that said biphasic fluid is sent to an upper cylinder body (68) of said waste heat boiler (34), the liquid part of said biphasic fluid being reintroduced into said second accumulator and separator tank (52), operating as a lower cylinder body of said waste heat boiler (34), and the steam part of said fluid being sent to a delivery pipe (40) which connects said waste heat boiler (34) with said thermocompressor (36) and said make-up valve (70).