COOLING OF A CELLULOSE PULP WEB

Field of the invention

The present invention relates to a pulp dryer being operative for drying a web of pulp by means of hot air in accordance with the air borne web principle and comprising a cooling zone having a length along which the web is made to travel.

The present invention further relates to a method of cooling a web of pulp by forwarding said web through a cooling zone.

Background of the invention

Pulp used for the manufacture of paper and board is often dried in a convective type of dryer operating in accordance with the air borne web principle. An example of such a dryer is described in US 4,505,053. Hot air is blown onto a web of pulp by means of upper blow boxes and lower blow boxes, illustrated in Fig. 4 of US 4,505,053. The air blown by the blow boxes transfer heat to the web to dry it, and also keeps the web floating in a stabilized distance over the lower blow boxes. Hot air is supplied to the blow boxes by means of a circulation air system comprising fans and steam radiators heating the drying air. The air borne web transport in a stabilized and fixed position on a deck of blow boxes is also described in US 3,231 ,165. A complete pulp dryer is illustrated in WO 99/36615.

The dried pulp leaving the drying section of a pulp dryer has a rather high average temperature, often in the range of 55-100 ⁰ C, as measured in a bale of pulp collected after the drying section. Such a high temperature has been found to deteriorate the pulp quality, resulting in a reduction of the pulp brightness during storage of the pulp. To avoid problems of pulp brightness reduction attempts have been made to cool the dried pulp prior to packing it in a bale for transport. WO 02/50370 discloses a dryer which is divided into zones, and in which a last zone is supplied with cooling air in order to achieve a cooling of the pulp web.

Summary of the invention

An object of the present invention is to provide a method of cooling a pulp web, such method being more efficient than the prior art method. This object is achieved by means of a pulp dryer being operative for drying a web of pulp by means of hot air in accordance with the air borne web principle and comprising a cooling zone having a length along which the web is made to travel, said cooling zone being characterised in comprising a plurality of cooling blow boxes being distributed along the length of the cooling zone and being operative for blowing cooling air towards the web, and at least one liquid supply device being operative for supplying cooling liquid directly onto the web in said cooling zone.

An advantage of the cooling zone is that it provides for a very efficient cooling of the web combined with a low consumption of cooling liquid and cooling air, and a very limited moisture increase of the web. The efficient cooling of the web results in a cooler web leaving the pulp dryer, and reduces the problems of decreased colour quality that may be associated with hot webs of pulp.

According to a preferred embodiment said liquid supply device is located in a position along the length of the cooling zone at which position the web has travelled 25-80% of the length of the cooling zone, as seen from the beginning of the cooling zone. This position of the liquid supply device along the length of the cooling zone has been found to provide a particularly efficient cooling of the pulp web. According to one embodiment said liquid supply device comprises at least one spray nozzle. A spray nozzle is an efficient way of effectively distributing the cooling liquid over the web.

According to one embodiment said cooling zone comprises at least two liquid supply devices located in different positions along the length of the cooling zone, at least one of said liquid supply devices being located in a position along the length of the cooling zone at which position the web has travelled 25-80% of the length of the cooling zone. An advantage of this embodiment is that larger amounts of liquid can be applied onto the web

without causing wet areas inside the cooling zone. Preferably all of said different positions are located where the web has travelled 25-80% of the length of the cooling zone.

According to one embodiment said liquid supply device comprises an upper part being operative for cooling the upper face of the web, and a lower part being operative for cooling the lower face of the web. An advantage of supplying cooling liquid onto both the upper and lower faces of the web is that the cooling becomes more efficient.

According to one embodiment said cooling zone comprises a plurality of upper and lower cooling blow boxes being distributed along the length of the cooling zone and being operative for blowing cooling air towards the upper face and lower face, respectively, of the web. For the same reasons mentioned hereinbefore, cooling of the web becomes more efficient if the cooling air is blown onto both the upper and lower faces of the web, in particular for thick webs.

Another object of the present invention is to provide a method of cooling a web of pulp, which method is more efficient than the prior art methods.

This object is achieved by means of a method of cooling a web of pulp by forwarding said web through a cooling zone, the method being characterised in cooling said web in said cooling zone by forwarding said web past a plurality of cooling blow boxes being distributed along the length of the cooling zone and blowing cooling air having a temperature of 0-45 ⁰ C towards the web by means of said cooling blow boxes, and supplying cooling liquid directly onto one or both sides of the web in at least one position along the length of the cooling zone.

An advantage of this method is that cooling of the web is effective and quick. According to a preferred embodiment said method further comprises supplying cooling liquid directly onto the web in at least one position along the length of the cooling zone at which position the web has travelled 25-80% of

the length of the cooling zone, as seen from the beginning of the cooling zone.

According to one embodiment of the present invention cooling liquid is supplied directly onto the web in at least one position along the length of the cooling zone at which position the web has travelled 35-75% of the length of the cooling zone, as seen from the beginning of the cooling zone. It has been found that supplying the cooling liquid in this position is particularly effective as regards the cooling of the web.

According to one embodiment of the present invention the method further comprises measuring the temperature of the web downstream of said cooling zone, and controlling the amount of cooling liquid supplied to said cooling zone with regard to said temperature of the web downstream of said cooling zone. An advantage of this embodiment is that the amount of cooling liquid supplied is not higher than needed to obtain a suitable average temperature of the pulp after the cooling zone.

According to one embodiment of the present invention the control of the cooling liquid supplied accounts for the temperature of the web upstream of the cooling zone. An advantage of this embodiment is that changes in the drying process can be quickly accounted for. According to one embodiment of the present invention the method comprises measuring the temperature of the cooling air supplied to said cooling zone, and controlling the amount of cooling liquid supplied to said cooling zone with regard to said temperature of the cooling air supplied to said cooling zone. An advantage of this embodiment is that the control of the cooling zone may respond quickly to changes in the cooling air temperature, such that changes in the cooling air temperature will not affect the temperature of the web leaving the cooling zone in a negative way.

Further objects and features of the present invention will be apparent from the description and the claims.

Brief description of the drawings

The invention will now be described in more detail with reference to the appended drawings in which:

Fig. 1 is a schematic side view, and illustrates a pulp dryer.

Fig. 2 is an enlarged side view, and illustrates the area Il of Fig. 1.

Fig. 3 is a cross-section of a cooling zone, as seen along the line Ill-Ill of Fig. 2.

Fig. 4 is a schematic side view of the cooling zone, and illustrates various positions of spray nozzles.

Fig. 5 is a diagram, and illustrates the cooling effect by spraying cooling water at different positions.

Fig. 6 is a diagram, and illustrates the cooling effect of spraying cooling water at different positions. Fig. 7 illustrates, schematically, a control device.

Fig. 8 illustrates, schematically, two ways of controlling the spraying of cooling liquid.

Description of preferred embodiments Fig. 1 illustrates a cellulose pulp dryer 1 operating in accordance with the air borne web principle. A wet web 2 of cellulose pulp enters the pulp dryer 1 at an inlet 4. Arrows A indicate the direction of travel of the web 2 through the pulp dryer 1. The web 2 is forwarded through a plurality of upper blow boxes 6 and lower blow boxes 8. The blow boxes 6, 8 blow hot drying air, at a temperature of typically 110-200 ⁰ C, onto the wet web 2. The drying air blown by the lower blow boxes 8 keeps the web 2 in a floating condition, i.e., making the web 2 air borne during the passage between the blow boxes 6, 8. An example of a design of blow boxes can be found in US 4,719,708. As is illustrated in Fig. 1 the wet web 2 undergoes several drying passages through a drying section 9 of the pulp dryer 1. The pulp dryer 1 is provided with a cooling zone 10, which is located downstream of the drying section 9 and which will be described in more detail hereinafter. A separating wall 12 separates the cooling zone 10 from the drying section 9. The dried and cooled pulp finally leaves the pulp dryer 1 at an outlet 14 in the form of a dry and cool pulp web 16, said pulp web 16 typically having a temperature of less than 50 ⁰ C.

Fig. 2 is an enlarged view of the area Il of Fig. 1 and illustrates details of the cooling zone 10. The cooling zone 10 of the pulp dryer operates with a

specific combination of cooling air and cooling liquid in the form of water. The cooling zone 10 is provided with a plurality, i.e., at least two, of upper cooling blow boxes 18 and lower cooling blow boxes 20. The upper and lower cooling blow boxes 18, 20 have the same general design and operate in accordance with similar principles as the blow boxes 6, 8 described hereinbefore with reference to Fig. 1 , and, hence, the cooling air blown by the cooling blow boxes 18, 20 keeps the web 2 in an airborne condition also in the cooling zone 10. However, the air blown onto the web 2 by means of the cooling blow boxes 18, 20 is not a hot drying air, but a cooling air, usually ambient air having a temperature of typically 0-45 ⁰ C, more often 15-40 ⁰ C.

In addition to the cooling blow boxes 18, 20 the cooling zone 10 is also provided with a set of upper spray nozzles 22, and a set of lower spray nozzles 24. The upper spray nozzles 22, of which only one nozzle can be seen in Fig. 2, are mounted on an upper nozzle support 26 extending across the width of the web 2. The lower spray nozzles 24 are mounted on a lower nozzle support 28 extending across the width of the web 2.

Fig. 3 is a cross-section of a part of the cooling zone 10 as seen along the direction of the arrows III of Fig. 2. As can be seen the upper nozzle support 26 supports a number of upper spray nozzles 22. A pipe 30 is operative for supplying cooling water to the upper spray nozzles 22.

Furthermore, the lower nozzle support 28 supports a number of lower spray nozzles 24, a pipe 32 being operative for supplying cooling water to those nozzles 24. The spray nozzles 22, 24 are operative for spraying the cooling water onto the web 2 to cool it in a manner which will be described in further detail hereinafter. The upper spray nozzles 22 spray cooling water directly onto the upper face of the web 2, and the lower spray nozzles 24 spray cooling water directly onto the lower face of the web 2. The spray nozzles 22, 24 are evenly distributed along their respective nozzle support 26, 28 in order to provide a substantially even distribution of the sprayed cooling water across the width of the web 2.

One example of a spray nozzle that could be utilized for spraying cooling water onto the web is the nozzle TPU in size 1100050, which is a flat spray nozzle having a spray angle of 110°, and which is available from

Spraying Systems Co., Wheaton, Illinois, USA. Typically the nozzle would operate at a water pressure of 2-6 bar above ambient pressure. The median volume diameter would typically be in the range of 0.1 to 0.4 mm. The cooling water temperature would normally be in the range of 0-35 ⁰ C. The upper and lower nozzles 22, 24 described with reference to Figs. 2 and 3 form together a water cooling arrangement, and a cooling zone 10 could be equipped with one or several such water cooling arrangements located in different positions, as will be described hereinafter.

Fig. 4 illustrates various locations along the cooling zone 10 of one or several water cooling arrangements, of the type described hereinbefore with reference to Figs. 2 and 3, each such water cooling arrangement comprising upper and lower spray nozzles 22, 24. The cooling zone 10 comprises, as mentioned hereinbefore, a number of upper cooling blow boxes 18 and a number of lower cooling blow boxes 20. The cooling zone 10 may typically have a length L, from the first to the last of the cooling blow boxes 18, 20, of 20 - 140 m. The cooling blow boxes 18, 20 are substantially evenly distributed along the length L. The spray nozzles 22, 24 should, as will be illustrated hereinafter, be positioned in very specific positions along the length L of the cooling zone 10 to achieve optimum cooling of the web 2. An arrow A indicates the direction of travel of the web 2 through the cooling zone 10.

Fig. 4 illustrates four positions along the cooling zone 10 in which one or several water cooling arrangements comprising spray nozzles 22, 24 could be located. These four positions are named "0%", "40%", "60%" and "80%". The percentage number indicates the percentage of the length L of the cooling zone 10. Hence, "0%" refers to a position at the beginning B of the cooling zone 10, said beginning B being the position of the first cooling blow boxes 18, 20 of the cooling zone 10, as seen in the direction of travel of the web 2. Furthermore, "40%" refers to a position where the web 2 has passed through 40% of the length L of the cooling zone 10, etc. Hence, with a cooling zone 10 having a length L of 50 meters "40%" would refer to a position 20 meters from the beginning B of the cooling zone 10, "60%" would refer to a position 30 meters from the beginning B of the cooling zone 10, and "80%"

would refer to a position 40 meters from the beginning B of the cooling zone 10.

Fig. 5 is a diagram and illustrates test results obtained when locating the spray nozzles 22, 24 at the different positions along the length L of the cooling zone 10 in accordance with Fig. 4. It will be appreciated that some tests were performed with spraying water in only one position along the length L of the cooling zone 10, while in other tests water was sprayed in two different positions along the length L of the cooling zone 10. In those tests that the cooling water was sprayed in two different positions, each of said two different positions was equipped with upper and lower spray nozzles 22, 24 arranged in the manner illustrated in Figs. 2 and 3. Thus, in all of the tests water was sprayed onto both the upper and lower face of the web 2 at each of said positions. Table 1 summarizes the tests that were performed:

Table 1 : Spray positions and amounts of water sprayed at the various spray positions.

Fig. 5 illustrates the results of the tests in a diagram. On the Y-axis the surface temperature reduction, in ⁰ C, is illustrated in comparison to the case where cooling in the cooling zone 10 is performed only by means of the cooling blow boxes 18, 20. Hence, "0 ⁰ C" on the Y-axis would refer to a case where there is no effect of the cooling water at all, i.e., the only cooling effect comes from the cooling air. The temperature of the cooling air was about 28°C. Typically, the average pulp temperature, as measured in a bale of pulp collected after the cooling zone 10, was 38-42°C, when there was no cooling

effect of the cooling water. Thus, "0 ⁰ C" on the Y-axis of Fig. 5 would correspond to an average pulp temperature of 38-42°C. Furthermore, the cooling water had a temperature of about 15°C. The pulp web was a typical softwood pulp web and had a basis weight of about 830 to 860 g/m ² , as measured in accordance with TAPPI T410. The pulp web 2 had a total width of about 4.2 m, but the cooling tests were performed on a width of about 500 mm. The dryness of the pulp web 2 at the beginning B of the cooling zone 10 was about 89%, as measured in accordance with TAPPI T412. The length L of the cooling zone 10 was about 40 meters. The web 2 travelled through the cooling zone 10 at a speed of about 150 m/min. On the X-axis the total amount of water, in litres/h, sprayed onto the web 2 is indicated.

As can be seen from Fig. 5 tests no 1 -3 and 5 resulted in a cooling of the web 2 of about 8°C at a water spray flow of about 110 l/h. The test no 6, in which all of the cooling water was applied at the beginning B of the cooling zone 10, resulted in a cooling of the web 2 of only about 3-4°C at the same water spray flow. The test no 4 resulted in a cooling of the web 2 of 7°C at the same water spray flow. The moisture content of the web 2 as measured after the cooling zone 10 was substantially independent of the spraying positions. Hence, the position of the spray nozzles 22, 24 along the length L of the cooling zone 10 has a large impact on the cooling efficiency, but a limited effect on the moisture content of the pulp.

Fig. 6 illustrates the calculated cooling of a pulp web by means of combining blowing cooling air in a cooling zone, and spraying water at different positions along the length of the cooling zone. The calculations were made by means of a mathematical model which was based on test results similar to those illustrated in Fig. 5. The calculations have been made for a set-up similar to that illustrated in Fig. 4. The cooling water flow was fixed at about 115 I/hour, the cooling water had a temperature of about 15°C, and the temperature of the cooling air was about 28°C. The simulated web was a typical softwood pulp web and had a basis weight of about 850 g/m ² , as measured in accordance with TAPPI T410, and a width of about 500 mm. The dryness of the pulp web at the beginning B of the cooling zone 10 was about 89%, as measured in accordance with TAPPI T412. The cooling zone had a

length L of 40 meters. The web 2 travelled through the cooling zone 10 at a speed of about 150 m/min. In all cases the water was sprayed against the pulp web 2 in only one position, but both from above and from below the pulp web at that position. The X-axis of Fig. 6 refers to the position along the length L of the cooling zone 10 at which the water was sprayed. Hence, "0%" refers to a position at the beginning B of the cooling zone 10, as seen in the direction of travel of the web 2 and as illustrated in Fig. 4. Furthermore, "10%" refers to a position where the web 2 has passed through 10% of the length L of the cooling zone 10, etc. On the Y-axis of Fig. 6 the surface temperature reduction, in ⁰ C, is illustrated in comparison to the case where cooling in the cooling zone 10 is performed only by means of the cooling blow boxes 18, 20. Hence, "0 ⁰ C" on the Y-axis would refer to a case where there is no effect of the cooling water at all, i.e., the only cooling effect comes from the cooling air. As can be seen from Fig. 6, the cooling efficiency is best when the water is sprayed at a position located at 25%-80% of the length L of the cooling zone 10, as seen from the beginning B of the cooling zone 10 illustrated in Fig. 4. Thus, the spray nozzles 22, 24 of Fig. 4 should be located at a position which corresponds to 25-80% of the total length L of the cooling zone 10, as seen from the beginning B of the cooling zone 10. Hence, if the cooling zone 10 has a total length L of 50 meters, then the spray nozzles 22, 24 should be located at least 12.5 meters from the beginning B of the cooling zone 10, and no more than 40 meters from the beginning B of the cooling zone 10. Furthermore, referring again to Fig. 6, a more preferred range of the position of the spray nozzles 22, 24 is at 35% to 75% of the length L of the cooling zone 10, as seen from the beginning B of the cooling zone 10, and a still more preferred range of the position of the spray nozzles 22, 24 is at 45% to 70% of the length L of the cooling zone 10, as seen from the beginning B of the cooling zone 10. An absolute optimum position of the spray nozzles 22, 24 is at about 55% to 63% of the length L of the cooling zone 10, as seen from the beginning B of the cooling zone 10.

Fig. 7 illustrates, schematically, a control valve 36 which is operative for controlling the amount of cooling liquid that is supplied to the spray nozzles 22, 24, schematically indicated in Fig. 7, via a pipe 38. A control

device 40, such as a process computer, is operative for controlling the control valve 36. The control device 40 receives information from a first temperature sensor 42, which is operative for measuring the surface temperature of the pulp web 2 just upstream of the cooling zone 10, the direction of travel of the pulp web 2 being indicated by an arrow A, and a second temperature sensor 44, which is operative for measuring the surface temperature of the pulp web 2 just downstream of the cooling zone 10.

The control device 40 may control the amount of cooling liquid sprayed onto the web 2 in a feed-back manner, based on information from the second temperature sensor 44, and with the aim of keeping the temperature of the web 2 leaving the cooling zone 10 at a certain set-point, such as, for example, 38°C. The control device 40 may also, in a feed-forward manner, account for changes in the temperature of the web 2 upstream of the cooling zone 10, as measured by means of the first temperature sensor 42, when controlling the control valve 36. Furthermore, the control device 40 may also receive information from a central process computer 46 of the pulp dryer, such information including information about, for example, the present web travelling speed, the basis weight of the web 2, the drying air temperature, the steam pressure to the dryer, the drying capacity of the dryer, and other process parameters. The control device 40 may account for such information from the central process computer 46 in a feed-forward manner when controlling the control valve 36. The purpose of the feed-forward manner is to eliminate disturbances in the temperature of the pulp web 2 leaving the cooling zone 10, by using information indicating changes in the operating conditions, such as basis weight of the web, web travelling speed, and the drying conditions of the drying section 9 located upstream of the cooling zone 10. The feed-forward control may be implemented in several known manners including so-called pulse compensation.

Fig. 7 further illustrates, schematically, a fan 48 which is operative for supplying cooling air to the upper and lower cooling blow boxes 18, 20, illustrated hereinbefore with reference to Fig. 2. It will be appreciated that in practice there could be more than one fan 48 supplying the cooling air to the cooling blow boxes 18, 20. Returning to Fig. 7, an air temperature sensor 50

is operative for measuring the temperature of the cooling air supplied by the fan 48 to the cooling blow boxes 18, 20. The air temperature sensor 50 sends a signal to the control device 40. The control device 40 may thus account for the temperature of the cooling air, and in particular for changes in the temperature of the cooling air, when controlling the control valve 36.

Fig. 8 illustrates, schematically, two possible ways of including the feed-forward signal from the first temperature sensor 42 in the controlling of the control valve 36, illustrated in Fig. 7. Both ways of accounting for a feedforward signal in a control scheme are per se known from other technical areas. In a first alternative, indicated with unbroken lines in Fig. 8, a set-point and a feed-back from the output, i.e., the temperature of the web 2 after the cooling zone 10 as measured by the second temperature sensor 44, are inputs to a PID controller. According to a first way of controlling the control valve 36 a measured disturbance, i.e., the temperature before the cooling zone 10 as measured by the first temperature sensor 42, is taken via a filter q _tf and influences the output from the PID controller, such that the signal sent from the PID controller to the control valve 36, illustrated in Fig. 7, also takes said measured disturbance into account. According to a second way of controlling the control valve 36 a so called pulse-compensation is made, as illustrated by dashed lines in Fig. 8. The pulse-compensation comprises forwarding, potentially via the filter qtf, the measured disturbance, i.e., the temperature before the cooling zone 10 as measured by the first temperature sensor 42, to a model, such as a mathematical model. The output from the model influences the input data sent to the PID controller to account for the measured disturbance. It will be appreciated that a similar control scheme can be utilized for accounting, in a feed-forward manner, also for other parameters than the temperature of the web just before the cooling zone. For example, the control of the supply of cooling water could account for, in a feed-forward manner, the temperature of the cooling air as measured by the sensor 50 illustrated hereinbefore with reference to Fig. 7.

It will be appreciated that numerous variants of the above described embodiments are possible within the scope of the appended claims.

For instance, it would be possible to utilize a cooling zone having lower cooling blow boxes 20, but no upper cooling blow boxes. Such a cooling zone

would still operate in accordance with the air borne web principle, but would yield a slightly lower cooling effect than a cooling zone comprising both upper and lower cooling blow boxes 18, 20 in accordance with Fig. 4. The width of each cooling blow box 18, 20, as seen along the length L of the cooling zone 10, could be varied within broad limits. Typically, each blow box could have a width, as seen along the length L, of 100-500 mm.

Hereinbefore a cooling zone 10 has been illustrated in which the web makes a single passage, from the left to the right as seen in Fig. 4, through the cooling zone 10. It is also possible, however, to arrange a cooling zone 10 in which the web makes several passages, in a similar manner as has been described above with reference to Fig. 1 concerning the drying section 9 of the pulp dryer 1. Hence, the cooling zone 10 could either comprise one passage, as illustrated in Fig. 4, or several passages. In the latter case the total length of the cooling zone is the sum of all the passages in the cooling zone. Hereinbefore it has been described that the cooling water is supplied to the web in the cooling zone by means of spray nozzles 22, 24. It will be appreciated that other devices could also be utilized for supplying water to the web 2. Such devices include, e.g., wetted rolls. Applying the cooling water by means of spray nozzles 22, 24 is often preferred for practical reasons. Hereinbefore it has been described that one possible cooling liquid is water. While water is often the preferred cooling liquid, there are other possible cooling liquids. Examples of such other cooling liquids include alcohols, such as ethanol and glycol, and various ethers. A cooling liquid may also comprise a mixture, such as a mixture of water and glycol. Furthermore, the cooling liquid may also include chemical additives, normally constituting less than 10% of the cooling liquid, that could provide beneficial effects for the quality of the pulp.