CZ9703964A3 - Multiple zones limiting orifice drying of cellulosic fibrous structures, apparatus therefor and cellulosic fibrous structures produced thereby - Google Patents

Abstract

A limiting orifice through-air-drying apparatus for papermaking or other absorbent embryonic webs. The apparatus has a first zone (41) and a second zone (42). The first (41) zone is maintained at a differential pressure less than the breakthrough pressure, while the second zone (42) is maintained at a differential pressure greater than the breakthrough pressure. The residence time of the embryonic web to be dried with the apparatus is maintained at preferably less than 35 milliseconds on the first zone. Using the dual zone system described above, the overall energy required to run the apparatus can be reduced.

Description

Multi-zone restriction nozzles for drying cellulosic fibrous structures, apparatus and cellulosic fibrous structures produced by the apparatus.

Technical field

BACKGROUND OF THE INVENTION The present invention relates to absorbent papers which are continuously air dried and more particularly to cellulosic fibrous structures which are air dried continuously.

BACKGROUND OF THE INVENTION

Absorbent paper mills form a staple for everyday use. Absorbent pulp fibers contain cellulosic fibrous structures, absorbent foams, etc. Cellulosic fibrous structures have become chopped for daily use. Cellulosic fibrous structures are included in make-up wiping papers, toi summer papers and paper towels.

In the manufacture of cellulosic fibrous structures, the wet pulp of cellulosic fibers dispersed in a liquid suspension is deposited on a forming screen. The wet stock may be dried by any known drying agent, or a combination thereof, each drying agent affecting the properties of the resulting cellulosic fibrous structure. For example, the drying means and drying methods may affect the softness, thickness, tensile strength, and absorbency of the resulting cellulosic fibrous structure. The drying means and drying methods used to dry the cellulosic fibrous structure also affect the speed of its production without limiting it.

One example of a drying agent is felt webs. Felt drying belts have long been used to remove water from a pulp web by capillary flow through a liquid suspension into a permeable felt medium that is maintained in contact with the pulp. However, removal of water from the cellulosic fibrous web into the felt web and by means of a felt web results in a general compression and compacting of the pulp of the cellulosic fibrous web to be dried.

Felt belt drying may be assisted by vacuum or opposed press rolls. Press rollers maximize mechanical compression of the felt against the cellulosic fibrous structure. Examples of drying felt rolls are disclosed in U.S. Patent 4,329,201, issued May 11, 1982 to Bolton and U.S. 4,888,096, issued December 19, 1989 to Covan et al.

It is also known to dry the fibrous structure by removing water by means of vacuum, without using felt webs. Removal of water by means of the cellulosic fibrous structure by means of vacuum mechanically removes moisture from the cellulosic fibrous structure while the moisture is in liquid form. The vacuum further deflects the hidden areas of the pulp fiber structure into the deflection channels of the drying webs and strongly contributes to the distribution of different amounts of moisture into different areas of the pulp fiber structure. Similarly, it is well known to dry the cellulosic fibrous structure by means of a vacuum assisted capillary flow using a porous cylinder with a preferred pore size. Examples of such vacuum drying techniques are shown in commonly-assigned U.S. Pat

December 3, 1985 in the name of Chuang et al. and US 4,973,385, issued December 27, 1990 to Jean et al.

Considerable success has been achieved in another method of air-drying the pulp web.

continuous air drying is paper pulp,

In a typical to be dried, it is supported by an air-permeable perforated belt.

The hot air stream passes through the cellulosic fibrous structure and then through the permeable web or vice versa. The air stream essentially dries the pulp by evaporation. The regions of the perforation or deflected into the perforation in the air-permeable belt are preferably dried and the thickness of the resulting cellulosic fibrous structure increases. The areas at the joints of the air permeable belt are dried to a lesser extent.

made used

In the prior art it was air permeable, air drying. For example, the belt produce with a large open percent). Or, this air permeability can be achieved by applying the woven yarn with particles,

Drying belt in high ways

Re 28,459, US 4,172,910, 4,251, 928, in commonly 1985 on May 1, a paper pulp technique using a Yankyee dryer as shown in the patent name Sraith and US 4,942, the name Sundovist.

passport

The reduced plastic to increase its thermal conductivity of radiation or alternatively with the photosensitive plastic formed can be specially adapted to temperature, up to about 815 ° C (1500 ° F). Continuous air drying is reissued on July 1, 1975, issued October 30, 1979, issued February 24, said US patent name Trokhan, and in 1990, in the name of Todd, various attempts were made on the fiber structure to be dried.

belt or infrared dryer

Examples of profiated products

US 4,583,302, issued

675, issued 24, several belt improvements in a continuous air permeable process may be flat (at least forty can be manufactured with reduced air permeability with the joint sealant compositions may be impregnated with metal, and a reduction in its drying web through the continuous web may be impregnated).

to the airflow Examples of such disclosed in the US patent on the name of Cole and Knee name Rotar, US name Rotar et al., issued July 9, issued in a drying state still may be combined drying are April 22, 1986 on July 1990;

1981 na

No. 4,528,239, U.S. Patent 4,921,750,

In addition, they have been regulated to regulate, although this is still the case

In these experiments • 9 ·

9

The state of the art, even the state of the art, which is specifically designed for continuous air drying, does not concern the problem that may arise when a cellulosic fibrous structure with several regions is to be dried. For example, a first region of the cellulosic fibrous structure having a lower absolute humidity, density, and starting weight than the second region will usually have a relatively greater air flow than the second region. This relatively greater air flow occurs because the first region with lower absolute humidity, density, or starting weight has a relatively lower airflow resistance passing through this region.

This problem is exacerbated when the cellulosic fibrous structure with several regions to be dried is transferred to a Yankee dryer. In the Yankee dryer, the hidden hidden areas of the pulp fibrous structure are in direct contact with the periphery of the heated cylinder, and the hot air from the Yankee-dryer is fed to the surface of the pulp fibrous structure, which is on the opposite side than the heated roll.

However, usually to the most immediate contact with

The Yankee dryer occurs in high-density or high-starting areas that do not dry up as low-density or low-starting areas.

The preferred drying of the low density areas is by convective heat transfer in a Yankee dryer.

Therefore, the production rate of the cellulosic fibrous structure must also be reduced in order to compensate for the higher humidity in the higher density and mass area. In order to complete the drying of the higher density and weight areas and to prevent the burning or burning of already dried low density or low weight areas with air from the drying drum, the temperature in the Yankee dryer must be lowered and the dwell time must be extended over which the cellulosic fibrous structure is disposed in the drum, thereby reducing the production rate.

Another disadvantage of approaching the prior art is that, besides when mechanical compression is used, such as a felt web, it is all based on supporting the cellulosic fibrous structure to be dried. The air flow is directed to the cellulosic fibrous structure and is transferred through a support belt or alternatively passes through a drying web to the cellulosic fibrous structure. Differences in flow resistance when passing through a web or cellulosic fibrous structure increase differences in moisture distribution within the cellulosic fibrous structure. or create differences in moisture distribution that never existed before. However, no attempt has been made, according to the prior art, to adapt the air flow to differences in different areas of the cellulosic fibrous structure.

Some improvement in this problem is contained in commonly-assigned U.S. Patent 5,274,930, issued Jan. 4, 1994 to Ensign et al. and comprising a restrictor nozzle for drying the cellulosic fibrous structure in conjunction with continuous air drying, which is incorporated herein by reference. This patent discloses an apparatus using a microporous drying medium with a higher flow resistance than slits between the fibers of the cellulosic fibrous structure. Thus, the microporous drying medium forms a restrictor nozzle in a continuous air drying process, so that the same or, at best, a more uniform distribution of moisture in the drying process is achieved. This restrictor nozzle for an air drying device according to Esigne et al. discloses having one or more negative or positive pressure zones to promote airflow in both directions.

However, this patent also discloses that when the starting weight of the pulp increases, a longer residence time will be required after which the pulp remains in the microporous medium as would be logical. In particular, it was noted that tissue paper with plain

starting weight <12 pounds per 3 square feet) would require a residence time of at least 250 milliseconds in the microporous medium.

Applicants have unexpectedly found that this necessary residence time in the first zone can be reduced by providing a restriction nozzle in a continuous air-drying device that is divided into several zones. Furthermore, it has unexpectedly been found that the total energy consumption of the apparatus can be reduced by using suitable zones. In particular, less horsepower for the fan is required if the zones are appropriately sized and selected. With the new invention, a reduction in fan power consumption of up to 10 to 15 percent compared to the original device in said Ensign et al. At an annual operating cost of $ 200 to $ 250 per horsepower per year, potential savings can be significant.

Accordingly, it is also an object of the present invention to provide a restriction nozzle in a continuous air drying apparatus with microporous media that can be used in conjunction with continuous air drying to form a cellulosic fibrous structure. Another object of the present invention is to provide a limiting nozzle in a continuous air drying device that reduces the necessary residence time and requires less energy than previously disclosed in the art.

SUMMARY OF THE INVENTION

Said drawbacks are eliminated and said targets are achieved by a limiting nozzle in a continuous air-drying device in combination with an absorbent paper having a distributed moisture inside. The pulp may consist of a cellulosic fibrous structure. The stock may have a consistency of at least 18 percent. The apparatus comprises a restrictor nozzle for air flow through the pulp. The apparatus further comprises a plurality of different zones, arranged in order as at least one first zone and one second zone. The zones have different differential pressures relative to atmospheric pressure.

In one embodiment, the device has a water removal rate in the second zone of at least 5 pounds of water per pound of pulp per second. In a second embodiment, the device has a water removal rate in the second zone of at least 0.1 times greater than the water removal rate in the first zone in the second zone there is at least 5 pounds of water per pound of pulp per second. In a third embodiment, the device has a dwell time in the first zone of at least about 35 milliseconds.

BRIEF DESCRIPTION OF THE DRAWINGS ^ fl / k

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic side view of a microporous medium of the present invention incorporated into a permeate cylinder with internal sub-atmospheric pressure; FIG. 2 is a graphical representation of the relationship between consistency and residence time of the present invention; Fig. 3 is a graphical representation of energy consumption and water removal as a function of time for the present invention (CC), prior art microporous drying apparatus (BB), and prior art apparatus produced according to said US Patent 4,556,450, issued December 3, 1985 in the name of Chuang et al. CftA).

DETAILED DESCRIPTION OF THE INVENTION

The invention of FIG. 1 is comprised of a continuous air drying device 20 which is associated with a microporous medium 30. The device 20 and the medium 30 may be manufactured according to said U.S. Patent 5,274,930, the disclosure of which is incorporated herein by reference. The device 20 consists of a permeate roller 32 and a microporous medium 30 disposed around the permeate roller 32. A support member 28 such as a continuous air drying belt belts the permeate roller 32 from the supply roller 34 to the discharge roller 36 and surrounds the circular segment 40. This circular segment 40 it may be divided into several zones 41, 42 having different differential pressures from each other. Alternatively, the vacuum slots or dehumidification of the air drying can be particularly divided into several embodiments having two zones, the first pressures relative to the atmospheric device 20 consisting of separated from an endless belt. Paper machine.

The continuous nozzle device 20 according to the invention may be zones. The device 20 in the preferred zone 41 and the second zone 42. The embryonic web comes in successive contact with the first zone 41 and then with the second zone 42, and then with the other zones, if any. In the first zone 41, the pressure is kept lower than the pressure breakthrough of the device 20. In the second zone 42, the pressure is maintained higher than the pressure breakthrough of the device 20. The pressure breakthrough is determined according to Recommended Practice 901 of the Society of Automotive Engineers Aerospace

March 1, 1968 and called the Buble Point Test Method and adapted for use with a 50 mm immersion depth, which is incorporated herein by reference.

The first zone 41 and the second zone 42 may together encircle an arc of about 180 to 270 degrees, in particular 210 to 240 degrees. The first zone 41 may comprise up to 60 degrees of the total arc enclosed by the first zone 41 and the second zone 42, and in particular 20 to 30 degrees.

The support member 28 transports the absorbent paper to the apparatus 20 and through the first zone 41 and the second zone 42 at a rate providing a residence time in the first zone 41 of less than 35 milliseconds, particularly less than 25 milliseconds and most preferably less than 15 milliseconds. The residence time in the second zone 42 should be at least 125, and in particular at least 175 milliseconds.

The absorbent paper pulp, as formed or used herein, is any other papermaking paper that is wet stored and from which water must be removed to be functional in a dry state. Paper pulp is considered to be able to hold and retain water or remove water from the structure.

Cellulosic fibrous structures as used herein refer to structures such as paper having at least fifty percent cellulosic fibers and having a balanced ratio of synthetic fibers, organic fillers, inorganic fillers, foams, etc.

Suitable cellulosic fibrous structures for use in the present invention can be found in the commonly mentioned patent

US 5,245,025, issued September 14, 1993, in the name of Trokhan et al.

which is incorporated herein by reference.

When creating two different zones 41 and

42, where the first zone 41 is lower than 1.1 and than the pressure penetration from the device 20 through the restrictor drying nozzle and the second zone 42 has the stated dwell times, it has been found that the necessary power for the fan to create diffusion diffraction.

1.1 battery, can be significantly reduced.

Applicants have unexpectedly found that further drying and therefore increasing consistency does not substantially increase the aforementioned residence time in the first zone 41, as shown in FIG.

By appropriately selecting the residence time in the first zone 41 and further transporting the paper to the second zone 42, the efficiency of the drying process can be maximized and the power required for the fan can be reduced. In the invention described and claimed herein, the device 20 has a water removal rate in the second zone 42 of at least 5, and in particular at least 7 pounds of water per pound of pulp per second. A suitable point of transition between the first zone 41 and the second zone 42 is the point at which speed

4

the removal of water in the second zone 42 exceeds the rate of removal of the water in the first zone 41. The actual transition point is where the differential pressure in the device 20, relative to atmospheric pressure, runs from a pressure lower to a pressure higher than pressure. The system is optimized when the actual and appropriate transition point coincide. It has been found that the exact transition point will depend on the porosity and drainage capacity of the absorbent paper mill, the flow characteristics and the size of the orifices in the microporous medium, and perhaps other factors.

The second zone 42 may be further subdivided into one or more sub-zones, each having a separate fan, or may be provided without subdivision, and may have a single large fan, if desired. Alternatively, the individual first zone 41 or second zone 42 may each have their differential pressure exerted by two or more fans. The fans may be arranged one after the other or side by side. In general, it has been verified that the power requirement for two smaller fans or one larger, with the same overall power, is very similar in connection with the present invention. Since the first zone 41 operates at a lower pressure than the pressure breakthrough, it does not need a fan and can operate well with a vacuum pump. Thus, the first zone 41 in the device 20 of the claimed invention consumes only a minimum of energy. In this application, the power unit refers only to the power necessary to generate the doferential pressure in the device 20 and does not relate to the power necessary to convey the papermaking to the device 20U of the described and claimed invention is the ratio of drying speed in second zone 42 to drying speed in first zone 41. measured in pounds of water per pound of pulp per second, at least 0.10 times greater, and in particular at least 0.12 times greater. However, this ratio can be artificially increased by operating the inefficient first zone 41. For the purposes of the present invention, the first zone 41 has a speed

removing water of at least 40% of water per liter of paper

- 11 per second. For this rate of water removal in the first zone, minimum power is required, since the operation of the first zone 41 is based on capillary drainage that takes place at a pressure lower than the pressure breakthrough. pressure.

The aforementioned residence times are preferred for a stock having a pulp filter resistance (CPFR) of 5 to 20, in particular of 10 to 11. The filter resistance of the pulp is measured according to the procedure set forth in commonly-assigned U.S. Patent 5,228,954, issued July 20, 1993 to Vinson. et al., which is incorporated herein by reference.

Referring to Fig. 2, it has been found that the drying rate in the first zone 41 varies according to the filter resistance of the cellulose slurry (PFR). The drying rate in the second zone 42 is the same for all three curves A, B, and C. Curves A, B, and C in Fig. 2 show the order of increase in the filter resistance of the pulp slurry (PFR). In general, it has been found that the optimal residence time in the apparatus 20 is directly proportional to the filter resistance of the pulp slurry. The feed stock has a consistency of at least 18 percent and a possible consistency of at least 19 percent.

The apparatus 20 of the present invention has a higher water removal capacity for a given cellulosic filter resistance (CPFR) than can be achieved by prior art porous rollers which dry the web by capillary action and are maintained at a pressure lower than the pressure breakthrough as shown in U.S. Pat. No. 4,556,450, issued May

On December 3, 1985 in the name of Chuang et al., Incorporated herein by reference, comprising woven support members 28 and photosensitive plastic support members 28;

The rate of water removal is measured in pounds of water removed per pound of fibers divided by the time that the fibers are

exposed to this process.

Speed = Clibry of water removed / pound of fibers) / time in sec.

The water removal rate is determined by measuring the consistency of the pulp before and after the first zone 41 and the second zone 42 by gravimetric weighing and convective drying to achieve an absolutely dry base. The residence time can be easily calculated by knowing the length of the passage through the first zone 41 and the second zone 42 and the speed of the pulp mill.

Referring to Fig. 3, it should be noted that the rate of water removal in the second zone is noticeably higher with the apparatus of the present invention (CCC) than the rate of water removal from the cylinder of Chuang et al. CAA).

The device 20 of the present invention has a water removal rate of at least 5 pounds of water per pound of pulp per second, and in particular of at least 7 pounds of water per pound of pulp per second, in a second zone 42. pound of pulp per second, in particular at least 50 pounds of water per pound of pulp per second, in zone 41

The apparatus 20 of the present invention has a power consumption of less than 5 horsepower, and in particular less than 4 horsepower, per square foot of the paper stock exposed to the process, in the first zone 41. The device 20 of the present invention has a power consumption of less than 20 horsepower, and in particular less than 18 horsepower, and most preferably less than 16 horsepower per square foot of the area of pulp exposed to the process, in the second zone 42.

Claims (10)

Claims A papermaking device for continuous air drying by means of a limiting nozzle in combination with an absorbent paper having internally distributed moisture, said apparatus comprising a limiting nozzle for air flow through said papermaking, said apparatus further comprising a number of different zones arranged in the order of at least one first zone and one second zone, wherein the different zones have different differential pressures relative to atmospheric pressure, characterized in that the residence time of the stock in the first zone (41) is less than 35 milliseconds, especially less than 25 milliseconds, and in particular less than 15 milliseconds, and in particular that the zones consist of two zones, one first zone (41) and one second zone (42). 2. Continuous air drying apparatus in combination with absorbent paper having a moisture content and a consistency of at least 18 percent, said device comprising a restriction nozzle for air flow through said paper, said device further comprising at least two distinct zones, one first zone and one second zone having first and second zones separated from each other; differential pressures considering characterized s e t. water in the second zone (42) Yippee removal speed water in a rate of removal to atmospheric pressure, wherein the rate of removal is at least 0.10 times higher than the first zone (41), wherein the water in the second zone (42) is at least 5 pounds of water per pound of paper pulp per second; the second zone (42) is at least 0.12 times higher than the rate of water removal in the first zone (41), and in particular that the zones are comprised of two zones, one first zone (41) and one second zone (42). • · 3. A papermaking machine for continuous air drying through a confinement die, in combination with absorbent paper pulp. and having a moisture distribution therein, comprising a restrictor nozzle for the air flow through the pulp, the device further comprising a plurality of different zones arranged as at least one first zone and one second zone, the different zones having different differential pressures from each other. relative to atmospheric pressure, characterized in that the rate of water removal in the second zone (42) is at least 5 pounds of water per pound of pulp and in particular the rate of water removal in the second zone (42) is at least 7 pounds of water per pound of pulp second, and in particular that the zones consist of two zones, one first zone (41) and one second zone (42). Device according to one of Claims 1 to 3, characterized in that in the first zone (41) the differential pressure is lower than the pressure ingress of the device (20), and in particular that in the second zone (42) the differential pressure is a pressure higher than that of the device (20). Device according to claims 1 to 4, characterized in that in the first zone (41) the power consumption is less than 5 horsepower per square foot, and in particular in the first zone (41) the power consumption is less than 7 horsepower per square foot paper pulp. Device according to claims 1 to 5, characterized in that in the second zone (42) the power consumption is less than 20 horses per square foot of pulp, in particular in the second zone (42) the power consumption is less than 18 horses per square foot of pulp and, in particular, in the second zone (42) the power consumption is less than 18 horsepower per square foot of pulp. 7. A method of continuously drying a cellulosic fibrous structure through a confinement die, characterized in that an absorbent paper web is provided for drying having distributed moisture inside, providing means for generating air passage through the paper, providing a support member for supporting the paper, for continuously drying the air restrictor nozzle, on that side of the pulp opposite to the support member such that the stock is arranged between the support member and the device, the device being an air passage restriction nozzle, the device having several different air passage zones, these zones have differing differential pressures relative to atmospheric pressure, the pulp is placed on the support member, and air is passed through the paper and through the apparatus, and the pulp is conveyed to the apparatus, the residence time, for which the stock remains in the first zone is less than 32 milliseconds, particularly less than 25 milliseconds, and particularly less than 15 milliseconds. 8. A method of continuously drying a pulp web with a confinement die, comprising providing an absorbent paper web having a moisture content of at least 18 percent inside it, providing a means for creating air passage through the paper web, providing a support member for the web. supporting the stock, providing a continuous air drying device through the air restrictor nozzle, on that side of the stock opposite the support member such that the stock is arranged between the support member and the device, the air restrictor nozzle having several different zones different air passage zones having different differential pressures relative to atmospheric pressure from each other, the pulp is placed on the support member, and air is passed through the pulp and the apparatus to remove moisture the papermaking bone in the second zone at a rate of at least 5 pounds of water per pound of pulp per second. A cellulosic fibrous structure made on the apparatus of claims 1 to 6. A cellulosic fibrous structure produced by the process of claims 7 to 8. > «··. · ·· .- ··.