System for generating steam in a plant for the production of a web-like paper material

Abstract

Herein described is a plant for the production of a web-like paper material which comprises a system for generating steam at three or more pressure levels using the discharge coming from a Yankee hood. This system allows an energy recovery, given that the steam produced is generated through two heat exchangers, supplied by the Yankee hood discharge which would otherwise be disposed into the atmosphere. The first heat exchanger conveys the exit of the mixed phase in a first pressurised tank, where the steam is separated from water. From here the steam is introduced into the line for use in the paper drying process, while the water at balance temperature with the steam is transferred in a second pressurised tank, with lower pressure with respect to that of the first tank. Lastly, the residual water is introduced into a third pressurised tank, with lower pressure with respect to that of is the second tank, which collects the condensate from the Yankee cylinder together with the blow-through steam, so that this blow-through steam is recompressed to the operating pressure of the plant and it is re-introduced in the plant.

1 . Plant ( 10 ) for the production of a web-like paper material starting from a paper material slurry to be desiccated, the plant ( 10 ) comprising:
 a first drying device comprising at least one rotary Yankee cylinder ( 12 ), supplied with a pressurised steam, wherein said paper material slurry dynamically adheres on the lateral surface of said Yankee cylinder ( 12 );   a second drying device comprising at least one Yankee hood ( 14 ,  16 ) which at least partially surrounds said Yankee cylinder ( 12 ) and which is capable of both blowing dry air at high temperature on said paper material slurry wound on the lateral surface of said Yankee cylinder ( 12 ), and of sucking hot and moist air released from said paper material slurry;   at least one hydraulic discharge circuit ( 18 ), which is designed to discharge said moist air exiting from said at least one Yankee hood ( 14 ,  16 ) at a first predefined temperature value;   at least one hydraulic supply circuit ( 20 ), which is designed to supply said steam entering into said at least one Yankee cylinder ( 12 ) at a predefined operating pressure; and   a steam generation system in turn comprising:
 at least one first heat exchanger ( 22 ) of the air/water-steam type, which is positioned along said hydraulic discharge circuit ( 18 ) and which is designed to receive, as primary fluid, said moist air at said first predefined temperature value and to discharge said moist air at a second predefined temperature value, which is smaller than said first predefined temperature value, after heat exchange with a secondary fluid of said first heat exchanger ( 22 ); 
 a plurality of separator tanks ( 26 ,  28 ,  30 ), which are hydraulically connected to said first heat exchanger ( 22 ) and which are designed to separate the steam of the water supplied thereto, in water/steam mixed phase, and coming from said first heat exchanger ( 22 ); and 
 steam supply means ( 32 ,  34 ,  36 ,  38 ,  50 ), which are designed to supply the steam coming from at least one of said separator tanks ( 26 ,  28 ,  30 ) to said hydraulic supply circuit ( 20 ), 
   the plant ( 10 ) being characterized in that it comprises at least one second heat exchanger ( 24 ) of the air-water type, acting as a variable flow-rate economizer, which is arranged in series with respect to said first heat exchanger ( 22 ) and which is designed to receive, as primary fluid, a variable amount of said moist air at said second predefined temperature value and to discharge said moist air at a third predefined temperature value, which is lower than said second predefined temperature value, after heat exchange with a secondary fluid of said second heat exchanger ( 24 ), wherein said plurality of separator tanks ( 26 ,  28 ,  30 ) comprises:   at least one first pressurised tank ( 26 ), which is hydraulically connected to said first heat exchanger ( 22 ) through a first hydraulic connection circuit ( 40 ) and which is capable of delivering steam at a first predefined pressure value;   at least one second pressurised tank ( 28 ), which is hydraulically connected to said first pressurised tank ( 26 ) through a second hydraulic connection circuit ( 42 ) and which is capable of delivering steam at a second predefined pressure value, wherein said second predefined pressure value is smaller than said first predefined pressure value; and   at least one third pressurised tank ( 30 ), which is hydraulically connected to said second pressurised tank ( 28 ) through a third hydraulic connection circuit ( 44 ) and which is capable of delivering steam at a third predefined pressure value, wherein said third predefined pressure value is smaller than said second predefined pressure value.   
     
     
         2 . Plant ( 10 ) according to  claim 1 , characterized in that said second predefined pressure value is smaller by an amount comprised between about 4 bar G and about 8 bar G with respect to said first predefined pressure value. 
     
     
         3 . Plant ( 10 ) according to  claim 1 , characterized in that said second predefined pressure value is substantially equal to the operating pressure value of said Yankee cylinder ( 12 ). 
     
     
         4 . Plant ( 10 ) according to  claim 3 , characterized in that said operating pressure value of the Yankee cylinder ( 12 ) is comprised between about 4 bar G and about 10 bar G. 
     
     
         5 . Plant ( 10 ) according to  claim 1 , characterized in that said third predefined pressure value is smaller by about 1 bar G with respect to said second predefined pressure value. 
     
     
         6 . Plant ( 10 ) according to  claim 1 , characterized in that said third pressurised tank ( 30 ) is provided with a fourth hydraulic circuit ( 46 ) for connection with said Yankee cylinder ( 12 ), so that said third pressurised tank ( 30 ) is designed to collect the condensate coming from said Yankee cylinder ( 12 ). 
     
     
         7 . Plant ( 10 ) according to  claim 1 , characterized in that said third pressurised tank ( 30 ) is operatively connected to at least one thermocompressor ( 48 ), which is designed to suction the blow-through and flash steam from said third pressurised tank ( 30 ), to increase the pressure of said blow-through and flash steam and to deliver said blow-through and flash steam, for use, to said Yankee cylinder ( 12 ) through said hydraulic supply circuit ( 20 ). 
     
     
         8 . Plant ( 10 ) according to  claim 1 , characterized in that said steam supply means ( 32 ,  34 ,  36 ,  38 ,  50 ) comprise at least one first automatic motor-driven valve ( 32 ) for delivering the steam coming from said first pressurised tank ( 26 ), as well as one first hydraulic connection ( 34 ) for connecting said first automatic motor-driven valve ( 32 ) with said hydraulic supply circuit ( 20 ). 
     
     
         9 . Plant ( 10 ) according to  claim 1 , characterized in that said steam supply is means ( 32 ,  34 ,  36 ,  38 ,  50 ) further comprise at least one second automatic motor-driven valve ( 36 ) for delivering the steam coming from said second pressurised tank ( 28 ), as well as one second hydraulic connection ( 38 ) for connecting said second automatic motor-driven valve ( 36 ) with said hydraulic supply circuit ( 20 ). 
     
     
         10 . Plant ( 10 ) according to  claim 1 , characterized in that said steam supply means ( 32 ,  34 ,  36 ,  38 ,  50 ) comprise one or more circulation pumps ( 50 ) for supplying, to said hydraulic supply circuit ( 20 ), the steam coming from at least one of said separator tanks ( 26 ,  28 ,  30 ), said circulation pump ( 50 ) being provided with pressure control means. 
     
     
         11 . Plant ( 10 ) according to  claim 10 , characterized in that at least one of said circulation pumps ( 50 ) is installed at said third pressurised tank ( 30 ) and it is designed to supply water to said second heat exchanger ( 24 ).