MACHINE FOR DRYING TISSUE PAPER PROVIDED WITH A COGENERATION SYSTEM

The present invention refers to a machine for drying tissue paper provided with a cogeneration system.

BACKGROUND OF THE INVENTION

In the paper industry machines for drying tissue paper are known, that are provided with a hood that blows hot air with a great velocity against the tissue paper, and a rotatable drying cylinder called Yankee partially covered by said hood. The tissue paper is dried thanks to the combination of the drying cylinder that transmits heat by contact and the hood that dries by heat and mass transfer.

Generally, the machines for drying tissue comprise hood that operate with air at temperatures of 300, 400 and 500 ⁰ C. Recently, hoods that operate at 650 and 700 ⁰ C were made, but they have problems regarding the material expansion, so that the manufacturers of said hoods use materials and manufacturing systems developed with finite element models. The speeds of the blown air can reach values from 90 to 160 m/s.

The hood must be placed very near from the drying cylinder and the paper, leaving a distance of about one inch. However, the hood must have a separation mechanism when there is no paper, to prevent the air to be directed against the opposed cylinder because it would be dangerous, and also the separation mechanism permits the access to an operator for service.

The hood is divided into two parts, called respectively wet part and dry part, so that the tissue paper firstly passes by the wet part and then by the dry part of the hood. The division of the hood is carried out

according to the cylinder axis, because the wet and dry parts are symmetrical. At the wet part of the hood the yield is higher and more water is extracted, so it seems that a symmetrical hood is not suitable. However, in the market there are symmetrical hoods for spare parts reasons, decreasing the costs.

Each part of the hood has its own air circuit, so that each circuit comprises a motor fan that blows air, which is heated by heating means before it enters inside the corresponding part of the hood. Said heating means of blown air can be of several kinds, such as vapour radiators, thermal oil radiators, or more usually gas burners in air vain. In the case of gas burners, each receives a mixture of combustible gas, e.g. natural gas, and air to feed the flame of the burner.

The hood extracts water evaporated from the paper. This vapour is concentrated in the circulation air so a quantity of air must be drained to outside. To compensate it, fresh air must enter from outside. The hood includes respectively a feeding air input for the wet part and a feeding air input for the dry part . The part of the hood on the Yankee cylinder comprises, at its external surface, a plurality of openings that are communicated with the internal part of the hood. Some openings permit the air to pass and other ones recover the wet air.

In operation, the wet tissue band enters the drying machine adhering it on the surface of the Yankee cylinder to be dried, and the hot air enters inside the hood to be applied against the cylinder to enter in contact with the wet tissue band. Therefore, when the cylinder rotates, the tissue band is dried, passing firstly under the wet part of the hood and then under the dry part of it .

From both parts of the hood, the wet one and the dry one, is extracted a portion of air, the exhausted one, to prevent air in the circuit with a humidity of 100%.

Until now, to do this there are two practices: one consists on extracting the air purge from both wet and dry hoods and send it to a heat recuperator, and the other one consists on circulate the purge from the dry hood, where the air has less content of humidity, to the wet hood, and from it to outside. This is carried out this way because the two parts of the hood are symmetrical.

Up to date it is feasible in a tissue paper plant to use a cogeneration central, because the greater thermal consumption is carried out in the hood. As stated previously, this consumption is carried out normally with heated air by the combustion of natural gas in air vain. Therefore, temperatures of 500 ⁰ C are achieved quickly, even though hoods with temperatures up to 700 ⁰ C are also reached.

Because the production of paper and the evaporation depend on the humidity difference between the limit layer of the air in contact with the paper and the blown air, and also on the quantity of heat transported by the air, it is very difficult to find a solution for the cogeneration. The hoods have been designed up till now by the recirculation of wet air with the extraction of the purge to maintain a maximum humidity.

Currently, the development of the hood to be adapted to a cogeneration central is carried out passing all the cogeneration combustion gases by the two parts of the hood, the wet one and the dry one. Optionally, a burner in vain of these gases increases the temperature. At the exit of the hood, without recirculation, the gases pass by the aquotubular boiler to produce the vapour necessary for the Yankee cylinder. In a circuit like this, the humidity is very low.

SUMMARY OF THE INVENTION

The object of the machine for drying tissue paper provided with a cogeneration system of the present invention is to solve the drawbacks of the drying machines of the state of the art, providing a greater efficiency and permitting to be adapted to a cogeneration central.

The machine for drying tissue paper provided with a cogeneration system, object of the present invention, comprises a drying hood divided into two parts, a wet part and a dry part, that blows hot air with a great velocity to a rotatable Yankee cylinder on which the tissue paper is placed to dry it, and each part of the hood comprising a motor fan and heating means that heat the air from the fan before it enters the corresponding part of the hood, and the hood including an exit for extracting a purge for exhaust air, and it is characterised in that the hood is geometrically asymmetrical, the wet part presenting a greater drying surface and operating at a greater temperature with respect to the dry part, in that the air purge is recirculated from the wet part to the dry part before it exits the hood, and in that the dry part of the hood receives the gases from the cogeneration system.

Therefore, it is obtained a machine for drying tissue paper with a greater efficiency, obtaining also the following advantages. The asymmetric hood permits to obtain a wet part with a greater drying surface and operating at a greater temperature, to obtain a greater yield in the drying process .

The cogeneration is carried out at the dry hood, because it has a lower yield and it is where the most savings can be obtained. The wet hood is for increasing the production, because it has a better yield.

As the air purge is conducted from the wet part to the dry part, contrary to the teachings to the state of the art, it is possible to increase the flow rate of said

dry part and, therefore, the blowing velocity, increasing the yield. Furthermore, it is possible to keep a necessary good humidity level, because said dry part operates with gases from the cogeneration, with a relatively low humidity.

According to a preferred embodiment of the invention, the wet part, with an embrace angle with the cylinder of 145°, operates at 65O ⁰ C and at an air velocity of 160 m/s, and the dry part, with an embrace angle of 105°, operates at 405 ⁰ C and at an air velocity of 145 m/s.

Preferably, the heating means for the air that enters into the hood are gas burners in air vain. Both circuits, the wet part and the dry part, of the hood have a closed operation, so that the air at the exit of the fan passes by the gas burner in vain, enters the hood, and it exits to enter again inside the fan.

To start the machine it is necessary to star the air circuit with the burner to finally permit the entrance of the cogeneration gases. Advantageously, the machine comprises a recovery boiler connected to the air purge exit of the dry part of the hood.

Said recovery boiler permits to obtain vapour at a temperature and pressure suitable for the machine. Preferably, the recovery boiler is of pyrotubular kind. However, another kind of boiler can be used such as, e.g. aquatubular boilers.

Advantageously, the recovery boiler comprises a gas recovery zone and another zone with a housing with a gas burner.

Preferably, both zone of the recovery boiler are placed into the same shell. Therefore, the quality of the vapour does not change if it is carried out with recovery or not . Advantageously, at the exit of the recovery

boiler two economisers are provided, one for each zone of said boiler.

Advantageously, at the exit of the recovery boiler an air-to-air recuperator is provided, that heats by heat exchange with the gases form the recovery boiler, a fresh airflow that is blown to said burners of the wet and dry parts of the hood, and to the fresh air entrance of the wet part of the hood.

Also advantageously, at the exit of the air-to- air recuperator a low-pressure, preferably 0.5 bar, vapour boiler is provided, to heat the process water.

Advantageously, a thermo-compressor is provided, that mixes the vapour produced by the recovery boiler and the flash or vapour evaporated of condensates from the Yankee cylinder and extracts vapour at the feeding pressure .

Advantageously, a first expansion tank is provided through which the condensates of the Yankee cylinder pass to produce vapour flash to be recovered in the thermo-compressor, and a second expansion tank, at a lower pressure, through which a second flash at a lower pressure, preferably 2 bar, is produced, to feed the vapour box.

Advantageously, the vapour produced at the exit of the second expansion tank serves to feed a vapour box that heats the tissue paper before it enters into the Yankee cylinder, and also to heat the machine water.

This solution, even though is usual for a conventional paper machine, is new for tissue paper machines, where vapour boxes are usually not used.

BRIEF DESCRIPTION OF THE DRAWINGS

To facilitate the description of what has be

disclosed previously, some drawings are attached in which, diagrammatically and only as a non-limitative example, a practical case of embodiment of the machine for drying tissue paper provided with a cogeneration system of the invention is shown, in which fig. 1 is a diagrammatical view that shows the machine for drying tissue paper of the invention .

DESCRIPTION OF A PREFERRED EMBODIMENT

As it can be seen from fig. 1, the machine 1 for drying tissue paper 2 of the invention comprises a drying hood 3 divided into two parts, a wet part 3a and a dry part 3b, that blows hot air with a great velocity into a Yankee rotatable cylinder 4 on which the tissue paper 2 is placed, and each part 3a, 3b of the hood 3 including a motor fan 5a, 5b and a gas in vain burner 6a, 6b that heats the air from the fan 5a, 5b before it enters into the corresponding part of the hood 3a, 3b. Both circuits, wet part 3a and dry part 3b, of the hood 3 have a closed operation, so that the air at the exit of the fan 5a, 5b passes through the gas in vain burner 6a, 6b, it enters inside the hood 3, and it exits to return to the fan 5a, 5b.

The hood 3 is geometrically asymmetric, so that the wet part 3a presents a greater drying surface and it works at a higher temperature with respect to the dry part 3b. Hence, it is obtained a higher yield in the drying process .

At the hood 3 it is provided an exit for extracting a purge of exhaust air 7, in this case the purge 7 is recirculated from the wet part 3a to the dry part 3b before it exits said hood 3.

Furthermore, the dry part 3b of the hood 3 receives the gases 8 from a cogeneration system. The cogeneration is carried out in the dry hood, because it is

the one that has a lower yield and it is where more savings can be obtained. The wet hood is specialised in increasing the production, because it has a better yield.

As the purge is conducted from the wet part to the dry part, opposed as it is done in the state of the art, it is possible to increase the flow rate of said dry part and, therefore, the blowing velocity, increasing the yield. Furthermore, it is possible to keep a good humidity level because the dry part operates with gases from the cogeneration.

Hence, it is obtained a machine for drying tissue paper with a greater efficiency.

The wet part 3a of the hood 3, with an embracement angle with the cylinder of 145°, operates at 65O ⁰ C and with an air velocity of 160 m/s, and the dry part

3b, with an embracement angle with the cylinder of 105°, operates at 405 ⁰ C and with a velocity of 145 m/s.

On the other hand, the machine comprises a recovery boiler 9, of pyrotubular kind, connected to the air purge exit 7 of the dry part 3b of the hood 3.

Therefore, if the cogeneration gas enters at 405 ⁰ C, at the exit the dry part of the hood the gases are at 31O ⁰ C yet, with a humidity of 0.0851 Kg H ₂ O/Kg dry.

Said recovery boiler 9 is a mixed boiler, with a gas recovery zone and another zone with a housing with a gas burner, both zones being placed inside the same shell, so that the quality of the vapour does not change if it is carried out with recovery or not. At the exit of the recovery boiler 9 there are two economizers 10, one for each zone of the boiler 9. The recovery boiler 9 provides vapour at 17 bars and at a temperature of 222 ⁰ C.

Furthermore, at the exit of the recovery boiler

9 there is an air-to-air recuperator 11 that heats, by heat exchange with the gases from the recovery boiler 9, a fresh airflow 12 that is blown to the burners 6a, 6b of the hood

3 and to the entrance of the wet part 3a of the hood 3.

At the exit of the air-to-air recuperator 11, downstream in the circuit, a vapour boiler 13 at 0.5 bars is provided to heat the process water and the manufacturing section.

A thermo-compressor is provided, that mixed the vapour at 17 bars produced by the recovery boiler 9 and the condensate flow from the Yankee cylinder 4, and it extracts vapour at 8 bars, that is the feeding pressure. It is also provided a first expansion tank, through which the condensates of the Yankee cylinder pass to promote the flow to be recovered, and a second expansion tank through which said flow to be recovered passes to produce vapour at 3 bar. The vapour produced at the exit of the second expansion tank serves to feed a vapour box that heats the tissue paper before it enters inside the Yankee cylinder, and to heat the machine water. This solution, even tough is usual for a conventional paper machine, is new in the case of tissue paper machines, where vapour boxes are usually not used.

The exit gases from the boiler are finally at 155 ⁰ C.

Finally, the water of the recovery boiler, whose entrance is at 47 ⁰ C, will be heated by a plate exchanger with the water from the cogeneration central, it will pass by a degasifier and it will enter inside the boiler at 105 ⁰ C. The gases at the degasifier exit will be at 148 ⁰ C.

To calculate the energy used in the high yield hood 3, the model with no post-combustion has been fixed, operating the hood 3 only with the recovery gases. Once fixed the reached production, another model is carried out, this time with cogeneration central, keeping the production of the paper machine 1, and therefore the evaporation inside the hood 3.

The boiler 9 in these conditions produces less vapour than that the machine 1 needs to operate. The production calculation serves to determine the yield of the assembly. Only in the case of heating of water, it is supposed that it will be for all the produced vapour.

The production will be of 4,800 kg/h of vapour at 17 bars and 3,510 kg/h vapour at 2 bars, with a recovery in the boiler of 5,503 kw. The used hot water will be used to heat the loss of condensates in the operation of live vapour (vapour box) 4,000 kg/h of fresh water of the boiler from 15 ⁰ C to 8O ⁰ C, temperature at which it will enter inside the degasfier. Furthermore, 16 m3/h of fresh feeding water of the paper machine will be heated to 7O ⁰ C. In this step of heating water 1,345 kw will be recovered. During the transit of the gases by the high yield hood 3 the heat will be recovered, passing from 405 ⁰ C to 31O ⁰ C, using 3,614 kw. The recovered assembly will be of 3,614 + 5,503 + 1,345, that is a total of 10,462 kw. If the gases at the exit of the cogeneration central had a power of 10,563 kw, the recovery yield is, therefore, of 99,9%.

The cogeneration central is based in an 18V34SG motor .

The management of this complex cogeneration central-forced air hood must permit to operate if the cogeneration central is stopped. This is a cogeneration principle that never can be omitted. Therefore, it is necessary that the hood could operate with recirculation of air and that the vapour boiler could work without the addition of gases from the hood.