CA1050317A

CA1050317A - Method and apparatus for treating fibrous material

Info

Publication number: CA1050317A
Application number: CA255,989A
Authority: CA
Inventors: Lars E. K. A. Tell; Per H. Malmstrom
Original assignee: Karlstads Mekaniska Werkstad AB
Current assignee: Metso Fiber Karlstad AB
Priority date: 1975-06-30
Filing date: 1976-06-29
Publication date: 1979-03-13
Also published as: NO762231L; JPS5853115B2; SE434859B; FI63976B; DE2627975A1; BR7604206A; SE7507449L; DE2660362B1; JPS5246101A; NO145923B; DE2627975B2; FI761883A; NO145923C; FI63976C; DE2627975C3; DE2660362C3

Abstract

ABSTRACT OF THE DISCLOSURE

Fibrous material is treated in apparatus com-prising a fluid permeable drum rotatably mounted within a housing shaped to form with the drum a peripheral treatment path converging in the direction of rotation of the drum from an inlet portion successively to a web forming zone, a washing zone, a treatment zone, a com-pression zone and an outlet. A fibrous suspension of cellulosic material is supplied to the inlet portion and forms a high fibrous concentration web on the rotary drum in the web forming zone, suspension liquid being dis-charged through the permeable drum; a washing liquid is applied to the web in the washing zone and displaces sus-pension liquid in the web; at the end of the washing zone, the web is compressed in the nip between the drum and a roll to discharge washing liquid from the web through the drum; after compression, the web is allowed to expand in the treatment zone where it becomes impregnated with a treatment liquid under pressure; at the end of the treatment zone, the web is compressed in the nip between the drum and another roll to express treatment liquid therefrom through the drum; after compression, the web is removed from the drum, allowed to expand in the presence of a gaseous medium, and shredded.

Description

~LIDS(~3~
BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for continuously treating fibrous pulp with a treatment fluid in the liquid or gaseous phase.

DESCRIPTION OF THE PRIOR ART

In the production of cellulose fibrous pulp, it is conventional practice to subject the pulp to a variety of treatments such as bleaching and washing, for example.
Apparatus has been proposed heretofore for treating fibrous pulp by exchanging one liquid contained in the pulp for another to be used in treating the pulp. For example, U.S. patent No . 3, 564, 631 describes apparatus in which liquid exchange takes place from both sides of a fibrous cake disF
posed between two feeder bands, the treatment being carried 15 out with a relatively low (estimated maximum 12%) pulp con-centration. The treatment liquid is fed into the ~ibrous cake and mixed with liquid contained therein, whereupon the liquid mixture is removed from the cake. The treatment causes a dilution of the liquid contained in the fibrous cake and the diluted liquid is partly removed. Web dewatering and rinsing apparatus has been devised in which the web is first rinsed, then compressed between an outer screen drum and an inner perforated drum, after which the web is again rinsed, as shown in U.SO patent No. 3,342,124. Alternatively, the web may be compressed at successive points on a drum by a plurality of angularly spaced apart rolls and sprayed with washing liquid at locations between adjacent rolls, as shown in German application No. 1,054,826.

- 2 -~5~3~7 Web treatment apparatus having multiple web washing zones in the form of diverging and converging duct portions for treating a pulp mat with like or different dilution liq-uids has also been proposed, as shown in U.S. patents Nos.

3,564,631 and 3,980,518 and Russian Author's Certificate No.
427,117. Such apparatus is relatively inefficient, however, and has a tendency towards plug formation.
U.S. patent No. 3,772,144 discloses a pressure filter or wash press including two rotatable filter drums enclosed in a positive pressure vat with a nip therebetween and doctor blades above the nip to remove solid material from the drum surfaces.
It has also been proposed to dewater a web to a dryness in the range from 30% to 70~ after a washing operation by passing it into the nip between a press roll and a rotat-able drum on which the web is supported, as shown in U.S.
patent No. 3,878,698.
A typical pulp bleaching treatment is described in U.S. patent No. 3,630,828 which gives comparative date for high concentration bleaching and low concentration bleaching.
Also, U.S. patent No. 1,957,937 discloses a paper pulp bleaching technique in which paper pulp in the dry state is introduced into a closed vessel, a vacuum is created in the vessel to withdraw air held in the interstices of the fibrous material, and then a suitable gaseous bleaching agent is introduced into the vessel. For continuous operation, a sheet or web of cellulose material in a substantially dry state is fed successively through a plurality of enclosed chambers in each of which air is evacuated and the web is then exposed to the action of a suitable bleaching agent in gaseous form. Before and after passage of the web through ~5~3~7 the treatment chambers, however, the web is fully expos~d to air, the known detrimental effects of which in a fiber sus-pension are discussed in the publication TAPPI 58 (1975): 2, p. 90-94 (May, O.W. and Buchman, S.J., "Practical Effects of Air in Paper-making").
Heretofore, it has usually been the practice in web treatment apparatus to remove the fibrous web from its carrier in the presence of air. In certain suction filters of the type disclosed in U.S. patent No. 2~689J653 and in Swedish patents NosO 194,958 and 194,959, however, the web is subject-ed to a vacuum in the web pick-off apparatus, with all of the problems that that entails.
SUMMARY OF THE INVENTION

It is an object of the invention to provide a new and improved method and apparatus for treating a fibrous pulp in which a treatment fluid in liquid or gaseous phase can be utilized to impregnate a high concentration (18 - 5~%) fibrous pulp in a rapid and highly effective manner.

Another object of the invention is to provide a new and improved method and apparatus of the above character which are capable of feeding a treatment fluid in liquid or gaseous phase rapidly into a high concentration fibrous pulp and distributing the fluid in the pulp in such fashion that it is brought into contact with the individual fibers.
~5 In general terms, the present invention provide~
in a method for continuouslv treating fibrous material comprising moving a liquid permeable screen element in an enclosed endless path including a web forming zone and a first treatment zone in that order, said first treatment zone being defined by an enclosure overlying and confining a Eirst area of the web, supplying a fibrous suspension of cellulosic material to the screen element and draining suspension liquid ~:)5~3~L~
therethrough to form a fibrous web on said screen element in said web forming zone, subjecting said first confined area of the web to a first treatment by introducing a first treatment liquid under pressure into said enclosure to displace liquid contained in the web through said screen element, moving the web after treatment in said ~irst treatment zone through the nip formed between the screen element and a rotatable compression roll having an impermeable surface, compressing the web in said nip to a degree sufficient to result in weh dry content in the ran~e of about 30% to 70% so as to produce a high concentration of fibrous material, moving the web as it leaves said nip directly through a second treatment zone defined hv a second enclosure overlying and confining a second area of the moving web, the high concentration of fibrous material in said nip forming a seal between the first and second zones substantially preventing the flow of treatment fluid from the latter to the former, permitting the compressed web to expand as it leaves said nip and enters said second enclosure, and subjecting the expanding portion of said web in said second enclosure to a second treatment hy introducing into said second enclosure a treatment fluid different from said treatment liquid, under pressure and in liauid or gaseous phase, whereby the treatment fluid penetrates into the web in a rapid, uniform and highlv effective manner and displaces fluid contained therein through said screen element.
Preferably, the treatment fluid is a fiher bleaching agent. According to another feature of the present invention, a method is provided as recited above, in which the web, directly after treatment in said second treatment zone, is passed through a press zone in which it is subjected ~S(~3~7 to compression in the nip between a second compression roll and said screen element so as to cause the treatment fluid impregnating the web to displace any treatment liquid remaining therein.
In another asDect, the present invention provides, in apparatus for continuously treating fibrous material comprising a liquid permeable drum rotatably mounted within a housing shaped to form with the drum an enclosed perinheral treatment duct, said duct having a first duct portion including means for supplying a fibrous suspension to an inlet end thereof to form a fibrous web on the surface of said drum by drainage of suspension liquid through said surface into said drum, and a second duct portion overlyi.ng and confining a portion of the web and including first means for introducing a treatment liquid under pressure thereinto to displace suspension liquid remaining in the web into the drum, a rotatable compression roll mounted downstream of said second duct portion for rotation about an axis parallel to said drum and having an impermeable surface forming a closure for an opening in said housing and cooperating with the surface of the drum to form a nip for applying pressure to the web to compress it to a dry content in the ranqe of about 30~ to 70~, means forming in said housing a third duct nortion overlving and confining a portion of said weh as it leaves said nip, and means for introducing into said third duct portion a treatment fluid other than said treatment liquid, under pressure, in liquid or gas phase, to displace treatment liquid in the web into the drum, the high concentration of fihrous material produced by the pressure of said nip on the web forming a seal substantially preventing the flow of treatment fluid from said third duct portion past said compression roll, and the radial distances between the drum surface and the housinq ~S1~3~
portions immediately preceding and following the compression roll exceeding the shortest distance between the drum surface and the compression roll.
In a still further aspect of the present invention, the present invention provides, in a method for continuously treating a fibrous weh of cellulosic material having liquid containing cavities between the -fihers in which the web is compressed in the nip between a movable, li~uid permeable screen element and a rotatable roll to press the li~uid out of said cavities, the improvement comprisin~ allowing the web, after compression in said nip and in the course of its removal from the screen eiement, to expand in an enclosure supplied with a pressurized gaseous medium other than air, whereby said fiber cavities are impregnated with the gaseous medium and absorption of air by the web is prevented.
In a yet another aspect, the present invention provides, an apparatus for continuouslv treating a fibrous web of cellulosic material having liquid containing cavities between the fibers comprising a liquid permeable screen element movable in an endless path, and a rotatable roll mounted to cooperate with said screen element to form a nip for compressing said web to press the liquid out of said cavities, the improvement comprising cover means substantially completely enclosing said web as it leaves said nip and expands with reformation of said cavities, and means for supplying a pressurized gaseous medium other than air to the interior of said cover means to refill said cavities and thereby ~revent the web from absorbing air.
DESCRIPTION OF PREFERRED
EMBODIMENTS
.
Additional features and advantages of the invention will be apparent from the following detailed description of several representative embodiments, taken in conjunction with ~' D

~35~3~L7 the accompanying drawings, in which:
Fig. l is a schematic view in side elevation, partly in section, of web treating apparatus constructed according to the invention;
Fig. 2 is an enlarged view of a portion of the ap-paratus shown in Fig. l illustrating the progress of the dis-placement of the washing liquid and of the treatment liquid at different locations in the treatment path.
Referring now to Fig. 1, fibrous pulp treatment apparatus embodying the invention comprises a rotatable drum 11 having a substantially cylindrical, li~uid permeable screen-like wall or shell 13. The drum 11 is mounted on a shaft 15 journalled in a machine stand (not shown) and is provided with conventional drive and speed control means 17.
Surrounding most of the drum 11 is a shell or casing 19 defining a peripheral treatment path 21 about the drum ll, the radial distance from the shell 13 to the casing 19 dimin-ishing in the direction of rotation of the drum ll from a maximum value at an inlet end of the treatment path 21 to a 2~ lesser value at the outlet and thereof. Controllable supply - 7a -~5~3~L7 devices 23 are provided in the casing 19 for introducing a substantially uniform flow of fibrous suspension to the inlet end of the treatment path 21 over the whole width of the shell 13.
The treatment path 21 includes an initial web forming zone 25, followed in the direction of rotation of the drum 11 by a washing zone 27, a compressing or compact-ing zone 29, a treatment zone 31 and a press zone 33. A
washing liquid chamber 35 located outside the casing 19 supplies washing liquid through perforations 30 in the lat-ter to the washing zone 27. Similarly, another chamber 37 also located outside the casing 19 contains a treatment fluid which is adapted to be supplied through perforations 38 in the casing 19 to the treatment zone 31.
The washing liquid chamber 35 is provided with liquid supply means 39 including at least one servo-governed control valve 41. Also, one or more supply conduits 43 pro-vided with servo-governed valves 45 are connected to supply treatment fluid to the chamber 37.
Mounted between the washing zone 27 and the treat-ment zone 31 is a rotatable press roll 47 having a liquid impermeable surface defining with the adjacent portion of the shell 13 of the drum 11 the compressing or compacting zone 29. A second press roll 49, also havi.ng a liquid im-permeable surface is mounted adjacent the shell 13 just be-yond the treatment zone 31. The rolls 47 and 49 are mounted onshafts 51 and 53, respectively, journaled in the machine stand (not shown) with the drum 11 and define with the shell 13 of the drum 11 press nips 55 and 57, respectively, which :: 30 have substantially constant gap widths during operation.

~C~5(~7 The press roll 49 is provided with conventional pressure gauge means 59 such as strain gauges, for example, for producing signals indicative of the linear pressure in the press nip 57.
From the press nip 57, the treatment path 21 leads to a web removal and feeding device 61 consisting of a cover 63 enclosing a combined shredding and feeding screw 65. The cover 63 is provided with a doctor blade 67 forming a closure against the surface 13 of the drum 11 and a sealing device 69 forming a closure against the press roll 49.
One or more steam supply conduits 71 provided with valves 73 are connected to supply steam to the space inside the cover 63. Also, the fiber feeding device 61 m~y be pro-vided with means such as a flange (not shown) to connect its output end to other treatment equipment. Within the drum 11 is disposed an outlet 85 for discharging liquid drained into the drum during the treatment process.
Control of the web forming, washing and treatment operations may be effected by a control system comprising a controller 75 connected by the conductors 77, 79, 81 and 83 to the pres~ure gauge means 59, the drum speed control de-vices 17, the control valve 41 for the washing liquid cham-ber 35 and the control valve 45 for the treatment liquid chamber 37, respectively. The specific type and construc-tion of the components of the control system and the system as a whole may be selected by one skilled in the art, as a matter of ordinary engineering skill, from a variety of elec-trical, hydraulic, pneumatic or mechanical components and systems that are well known per se.
__ g _ ~L~5~3~7 In operation, a suspension of fibrous material under pressure and at a constant rate of flow is fed through the supply devices 23 into the web forming zone 25 of the treatment path 21 while the drum 11 is rotated at a substan-tially constant speed. The pressure differential betweenthe treatment path 21 and the interior of the drum 11 causes the suspension liquid to flow through the shell 13 and a fibrous web 91 to be formed on the latter. As the web 91 is moved through the web forming zone 25 its thickness grad-ually increases until it completely fills the space betweenthe shell 13 and the casing 19, as shown in Fig. 1.
Since the treatment path 21 converges in the direc-tion of rotation of the drum 11, the fibrous web 91 is sub-jected to continued increasing compression as it is moved up to the compression zone 29 and suspension liquid continues to be drained from it through the shell 13.
As the fibrous web 91 is moved through the washing zone 27, washing liquid under pressure and at a constant rate of flow is supplied through the perforations 30 in the casing 19 and is forced into the web 91. The washiNg liquid 93 entering the web 91 displaces the suspension liquid 89 remaining therein to the level of a boundary layer defined by the dot-dash line A -A' in Fig. 2, the displaced liquid being drained through the shell 13. As the web 91 passes from the washing zone 27 to the compressing zone 29, only the web layer closest to the shell 13, i.e., below the line A - A', contains suspension liquid, the web portion there-above containing only washing liquid 93.

~35V3~L'7 As the web 91 continues along the treatment path 21 it is compressed in the nip 55 which forms the compress-ing zone 29, preferably to a concentration of 30 - 70~ and to a thickness less than the thickness of the web portion which contains only washing liquid 93 at the location where the web first enters the zone 29. The surface of the press roll 47 being liquid impermeable, all liquid expressed from the web 91 in passing through the nip 55 is drained through the shell 13 of the drum 11. Consequently, all of the sus-pension liquid 89 remaining in the fibrous web 91 is firstdrained through the shell 13 of the drum 11 and then part of the washing liquid 93 is also drained through the shell 13 and is mixed with the suspension liquid 89 in the interior of the drum 11.
Upon leaving the press nip 55 in the compression zone 29, the web 91 is allowed to expand freely in the treat-ment zone while treatment fluid 95 under pressure and at a substantially constant rate of flow is forced through the perforations 30 in the casing 19 into the web 91. Expansion of the web 91 creates cavities therein which are filled by the treatment fluid 95 so that penetration of the latter into the fibrous web occurs very rapidly and effectively.
In the downstream part of the treatment zone 31, taken in the direction of rotation of the drum 11, the treatment fluid 95 in impregnating the web 91 displaces any washing liquid therein. Such washing liquid will consist, to a large extent, of washing liquid that has rewet the web 91 after the latter has passed through the nip 55. The boundary layer between the treatment fluid impregnating the ~3~
web and the washing liquid therein is represented by the dot-dash line B - B' in Fig. 2. The displaced liquid drains through the shell 13.
The displacement of the liquid in the web reaches its culmination in the press nip 57, where the washing liq-uid 93 is pressed out and part of the treatment liquid im-pregnating the web penetrates the latter and drains into the interior of the drum ll. Because of the high compression of the web 91 in the press nip 55, the web itself serves as a seal between the washing zone 27 and the treatment zone 31 preventing the flow of treatment fluid from the treatment zone 31 into the washing zone 27.
The linear pressure in the press nip 57, the speed of rotation of the drum 11, the flow of the washing liquid 93 and the flow of the treatment liquid 95 are all coordinated in a control system so as to secure the desired displacements in the web of the washing liquid down to the line A - A' and of the treatment fluid down to the line B -B' (Fig. 2). Thus, with the gap width of the press nip 57 2~ and the supply of the fibrous suspension maintained substan-tially constant by means known per se (not shown), the nip ._ _ linear pressure responsive signals from the gauge means 59 are supplied to the controller 75. There they are compared with a set point value to produce a correction signal re-sponsive to any difference between the actual and set pointvalues. The correction signal is fed to the speed control devices 17 for the drum 11 to change the speed of the latter such that when the linear nip pressure decreases, the speed of rotation of the drum is reduced.

S~3~7 The controller 75 also produces, in dependence upon the set point value for the rotational speed of the drum, additional siynals for adjusting the servo-controlled valves 41 and 45 to correct the flow of washing liquid 93 to the washing chamber 35 and the flow of treatment fluid 95 to the chamber 37, respectively.
After the passage of the web 91 through the press nip 57, the web is removed from the shell 13 by the doctor blade 67 and is guided into the removal and feeding device 61. In the latter, the web is allowed to expand freely while a gaseous medium, preferably water vapor, is supplied to the device 61 under pressure through the supply conduit 71 In expanding, cavities are formed in the web which are filled with the gaseous medium, preventing the web from absorbing air.

3~7 The method and apparatus of the invention enable highly effective and eEficient washing of the web to be achieved. For example, when pressing a wet fiber web of ordinary pulp (e.g.~ softwood sulphate pulp) to a dryness of at least about 30 percentage by weight, not only is the bulk of the liquid existing between the fibers pressed out but part of the liquid from the fiber lumen (the cavity inside of the cell walls of a fiber) is also expressed. During the subsequent relaxation, when the pressure is relieved, the fiber lumen sucks in liquid from surronding free liquid existing between the fibers. The liquid being sucked into the fiber lumen will have a lower content of electrolytes (e.g., substances dissolved out of the wood during the cook-ing process which desirably are to be removed as far as pos-sible by the washing process) than the average electrolyte content in the surrounding liquid, since the cell walls act as molecular screens (filters) or semi-permeable diaphragms.
Thus, the electrolyte content of the remaining free liquid, i.e., liquid not contained in the fiber lumen, is increased.
A further pressing in a subsequent press nip, accordingly, presses out the liquid having the increased electrolyte con-tent. Such a washing process is of extremely great importance, since the pressing sequence makes high wash efficiency pos-sible.
If the expanding fiber web upon leaving the press nip is treated with wash liquid such as water vapor, for example, an increased effect is obtained and the subsequent pressing becomes still more effective.

-- l~L --~5~3~
Similarly, if the expandlng fiber web is treated with a treatment fluid containing electrolytically undis-sociated chemicals or other reagents, as in bleaching, for example, an extremely good infiltration of -these chemicals or reagents into the very vicinity of the fibers is obtained, whereby the reaction times are greatly reduced.
The invention thus provides a highly effective method and apparatus for the treatment of fibrous pulp. By forming the pulp into a web of high fiber concentration and impregnating the web with a reaction agent, both the reaction time and the quantity of agent required can be substantially xeduced.
Where in the following claims reference is made to a fluid useful for the bleaching of fibrous pulp, this includes in ad!dition to conventional bleaching agents such as chlorine, chlorine dioxide, hydrogen peroxide, oxygen and others, reaction agents and catalytic agents such as caustic soda, ammonium hydrate, magnesium carbonate, alkali metal borates and others, as well as combinations of such agents.
The specific embodiments described above are merely illustrative and are susceptible of modification in form and detail within the scope of the following claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a method for continuously treating fibrous material comprising moving a liquid permeable screen element in an enclosed endless path including a web forming zone and a first treatment zone in that order, said first treatment zone being defined by an enclosure overlying and confining a first area of the web, supply-ing a fibrous suspension of cellulosic material to the screen element and draining suspension liquid therethrough to form a fibrous web on said screen element in said web forming zone, subjecting said first confined area of the web to a first treatment by introducing a first treatment liquid under pressure into said enclosure to displace liquid contained in the web through said screen element, moving the web after treatment in said first treatment zone through the.
nip formed between the screen element and a rotatable com-pression roll having an impermeable surface, compressing the web in said nip to a degree sufficient to result in web dry content in the range of about 30% to 70% so as to produce a high concentration of fibrous material, moving the web as it leaves said nip directly through a second treatment zone defined by a second enclosure overlying and confining a second area of the moving web, the high concentration of fibrous material in said nip forming a seal between the first and second zones substantially preventing the flow of treatment fluid from the latter to the former, permitting the compressed web to expand as it leaves said nip and enters said second enclosure, and subjecting the expanding portion of said web in said second enclosure to a second treat-ment by introducing into said second enclosure a treatment fluid different from said treatment liquid, under pressure and in liquid or gaseous phase, whereby the treatment fluid penetrates into the web in a rapid, uniform and highly effective manner and displaces fluid contained therein through said screen element.

2. A method as defined in claim 1 in which the treatment fluid is a fiber bleaching agent.

3. A method as defined in claim 1 in which the web, directly after treatment in said second treatment zone, is passed through a press zone in which it is subjected to compression in the nip between a second compression roll and said screen element so as to cause the treatment fluid impregnating the web to displace any treatment liquid remain-ing therein.

4. A method as defined in claim 3 in which the compressed web, after passing through the press zone, is removed from the screen element and guided into a sealed enclosure in the presence of a gaseous fluid other than air and in the substantial absence of air.

5. In apparatus for continuously treating fibrous material comprising a liquid permeable drum rotatably mounted within a housing shaped to form with the drum an enclosed peripheral treatment duct, said duct having a first duct portion including means for supplying a fibrous suspension to an inlet end thereof to form a fibrous web on the surface of said drum by drainage of suspension liquid through said surface into said drum, and a second duct portion overlying and confining a portion of the web and including first means for introducing a treatment liquid under pressure thereinto to displace suspension liquid remaining in the web into the drum, a rotatable compression roll mounted downstream of said second duct portion for rotation about an axis parallel to said drum and having an impermeable surface forming a closure for an opening in said housing and cooperating with the sur-face of the drum to form a nip for applying pressure to the web to compress it to a dry content in the range of about 30% to 70%, means forming in said housing a third duct por-tion overlying and confining a portion of said web as it leaves said nip, and means for introducing into said third duct portion a treatment fluid other than said treatment liquid, under pressure, in liquid or gas phase, to displace treatment liquid in the web into the drum, the high concen-tration of fibrous material produced by the pressure of said nip on the web forming 2 seal substantially preventing the flow of treatment fluid from said third duct portion past said compression roll, and the radial distances between the drum surface and the housing portions immediately preceding and following the compression roll exceeding the shortest distance between the drum surface and the compression roll.

6. Apparatus as defined in claim 5 together with a second compression roll mounted immediately downstream of said third duct portion for rotation about an axis parallel to the axis of said drum and having an impermeable surface forming a closure for an opening in said duct portion and cooperating with said drum to form a nip for compressing the web to displace any treatment fluid remaining therein.

7. Apparatus as defined in claim 6 together with means cooperating with said housing and said second compres-sion roll to form a sealed enclosure disposed to receive the compressed web from the press roll, and means supplying a gaseous fluid other than air under pressure to said enclo-sure.

8. Apparatus as defined in claim 7 wherein the enclosure is provided with sealing means forming a closure against the press roll, a doctor blade forming a closure against the drum and conducting the web away from the drum, and the enclosure houses a combined shredding and discharg-ing screw, the enclosure and the screw forming a substan-tially gas tight device for removal and discharge of the fibrous web from the drum.

9. In a method for continuously treating a fibrous web of cellulosic material having liquid contain-ing cavities between the fibers in which the web is com-pressed in the nip between a movable, liquid permeable screen element and a rotatable roll to press the liquid out of said cavities, the improvement comprising allowing the web, after compression in said nip and in the course of its removal from the screen element, to expand in an enclosure supplied with-a pressurized gaseous medium other than air, whereby said fiber cavities are impregnated with the gaseous medium and absorption of air by the web is prevented.

10. A method as defined in claim 9 in which the impregnated web is shredded and transported out of the enclosure to a subsequent treatment station while absorp-tion of air by the shredded web is prevented.

11. A method as defined in claim 9 wherein the gaseous medium is water vapor.

12. In apparatus for continuously treating a fibrous web of cellulosic material having liquid containing cavities between the fibers comprising a liquid permeable screen element movable in an endless path, and a rotatable roll mounted to cooperate with said screen element to form a nip for compressing said web to press the liquid out of said cavities, the improvement comprising cover means sub-stantially completely enclosing said web as it leaves said nip and expands with reformation of said cavities, and means for supplying a pressurized gaseous medium other than air to the interior of said cover means to refill said cavities and thereby prevent the web from absorbing air.

13. Apparatus as defined in claim 12 in which the pressurized gaseous medium supplied by the supplying means is water vapor.

14. Apparatus as defined in claim 12 in which the cover means includes doctor blade means forming a closure against the screen element for removing the web therefrom and directing it into said cover means, and a sealing portion forming a closure with said roll.

15. Apparatus as defined in claim 14 together with fiber shredding and discharging means in said cover means and forming therewith substantially gas tight means for closed removal and discharge of the fibrous web from the screen element.