Abstract:
A method of cleaning a semipermeable membrane, the semipermeable membrane being configured for carrying a fiber web, includes the steps of providing a cleaning fluid and applying the cleaning fluid on the semipermeable membrane. Further, an air press configured for carrying the semipermeable membrane therethrough is provided, and the air press has pressurized air therein. The semipermeable membrane is conveyed through the air press and is subjected to the pressurized air within the air press. The pressurized air thereby flushes the cleaning fluid through the semipermeable membrane.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Prime Contract No. DE-FC36-01GO10622 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the cleaning of fabrics used in papermaking, and, more particularly, to cleaning fabrics with a low air permeability (i.e., semipermeable membranes). 
     2. Description of the Related Art 
     The need to clean fabrics of papermaking machines is well known. Since the flow of air and/or water through such fabrics is relied upon during the paper forming process, it is desirable that a maximum number of the fluid pathways available in such fabrics remain open. However, during transport of a paper web using such fabrics, various debris that is a by-product of the papermaking process can and does manage to get caught within the fluid pathways of such fabrics. This ongoing collection of debris in a fabric can reduce fluid flow rates therethrough to unacceptable levels and, if collecting near the surface of the fabric, can directly affect the surface quality of the paper being produced. Consequently, the need to effectively clean such fabrics exists. 
     A variety of methods and devices have already been developed for cleaning fabrics of papermaking machines. It has been disclosed to use different kinds of brushes, air jets and ultrasound spray bars. A rotatable needle jet has also been employed for cleaning fabrics. It has further been suggested to use water spray devices in combination with blowing air to clean a forming wire or screen. 
     However, these solutions have all been found suitable for cleaning forming wires, press belts and drying fabrics with a high air permeability. These solutions have not been found effective for cleaning low air permeability (i.e., semipermeable) fabrics. In fabrics with high air permeability, the fluid pathways tend to be both numerous and relatively large. As such, debris can be dislodged relatively easily from most such pathways, and there are enough fluid pathways available that it may not always be critical to achieve a high degree of cleanliness for the fabric to operate sufficiently. 
     However, in semipermeable fabrics, in order to obtain the desired low air permeability therethrough, the number of fluid pathways tend to be limited and/or relatively small, in comparison to high air permeability fabrics. Thus, removal of debris from pathways of semipermeable membranes tends to be much more difficult to achieve, and the margin for error in the number of pathways that can remain blocked and still maintain an acceptable permeability level is much smaller than it is for high air permeability fabrics. 
     What is needed in the art is an effective method and apparatus for cleaning semipermeable fabrics used in papermaking machines. Specifically, the method and apparatus needs to be vigorous enough to remove a high percentage of debris from a set of openings from which the removal thereof tends to be difficult. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for cleaning a semipermeable membrane in which a cleaning fluid is applied thereto and then flushed therethrough using an air press to thereby clean the semipermeable membrane. 
     The invention comprises, in one form thereof, an apparatus for cleaning a semipermeable membrane, the semipermeable membrane being configured for carrying a fiber web. The apparatus includes a source of a cleaning fluid and an applicator configured for applying the cleaning fluid to the semipermeable membrane. The apparatus also includes an air press configured for carrying the semipermeable membrane therethrough. The air press having pressurized air therein is thereby configured for flushing the cleaning fluid through the semipermeable membrane. 
     The invention comprises, in another form thereof, a method of cleaning a semipermeable membrane, the semipermeable membrane being configured for carrying a fiber web. The method includes the steps of providing a cleaning fluid and applying the cleaning fluid on the semipermeable membrane. Further, an air press configured for carrying the semipermeable membrane therethrough is provided, and the air press has pressurized air therein. The semipermeable membrane is conveyed through the air press and is subjected to the pressurized air within the air press. The pressurized air thereby flushes the cleaning fluid through the semipermeable membrane. 
     An advantage of the present invention is that it provides an effective way of cleaning a semipermeable membrane having a low air permeability. 
     Another advantage is that it provides an effective way of cleaning a semipermeable membrane without disturbing paper quality. 
     Yet another advantage is that the cleaning press of the present invention can be combined with an air press used for dewatering and/or can be used for impregnating/coating the paper web. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a side view of a first embodiment of a papermaking machine including an apparatus for cleaning a semipermeable membrane; 
     FIG. 2 is a side view of a second embodiment of a papermaking machine including an apparatus for cleaning a semipermeable membrane; 
     FIG. 3 is a side view of a third embodiment of a papermaking machine including an apparatus for cleaning a semipermeable membrane; 
     FIG. 4 is a side view of a fourth embodiment of a papermaking machine including an apparatus for cleaning a semipermeable membrane; and 
     FIG. 5 is a side view of a fifth embodiment of a papermaking machine including an apparatus for cleaning a semipermeable membrane in combination with an air press used for dewatering. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to FIG. 1, there is shown a papermaking machine  10  configured for cleaning a semipermeable membrane  12  used in a papermaking process. Papermaking machine  10  generally includes a cleaning fluid source  14 , a cleaning fluid applicator  16 , a press  18  and at least one transfer roll  19 . 
     The cleaning fluid provided by cleaning fluid source  14  is preferably an anionic detergent, a cationic detergent, a surfactant, a soap, a solvent and/or a solvent mixture. The cleaning fluid may include water admixed therewith. 
     Cleaning fluid applicator  16  is fluidly connected to cleaning fluid source  14  and is positioned adjacent to semipermeable membrane  12 . Cleaning fluid applicator  16 , shown schematically, is preferably a blade coater, a spray device or a transfer coater. Preferably, cleaning fluid applicator  16  is a spray device configured for applying the cleaning fluid under a high gas pressure, most preferably over a region of semipermeable membrane  12  which is greater than the space between adjacent holes therein (not shown). As such, cleaning fluid applicator  16  preferably produces a diverging spray, not a needle jet. 
     Press  18  is configured both for conveying and pressing semipermeable membrane  12 . The pressing action provided thereby flushes the cleaning fluid through and out of semipermeable membrane  12 . Press  18  includes at least one press roll  20 , one of which is illustrated in FIG.  1 . In this embodiment, press roll  20  has positioned thereagainst a doctor blade  22  for removing debris and used cleaning fluid after pressing and cleaning occurs. A trough  26  is positioned below doctor blade  22  for collecting the removed debris and used cleaning fluid. 
     A second embodiment of the invention, as shown in FIG. 2, discloses a papermaking machine  30  which is capable of both cleaning semipermeable membrane  32  and pressing paper web  34 . Papermaking machine  30 , in addition to semipermeable membrane  32 , includes a permeable layer  36 , a plurality of conveyor rolls  38 , air press  40  and at least one of cleaning fluid sprayers  42   a - 42   d . Papermaking machine  30  may be used solely for cleaning semipermeable membrane  32  or may be used for pressing, coating and/or impregnating paper web  34 , in addition to cleaning of semipermeable membrane  32  and permeable layer  36 . 
     Semipermeable membrane  32  and permeable membrane  36  are provided for carrying paper web  34 . Semipermeable membrane  32  has a low air permeability specially designed for displacement dewatering. Permeable membrane  36  has a high air permeability and may be a felt, a wire, a press belt, drying fabric or an anti-rewet layer. 
     Air press  40  includes a first main roll  44 , a second main roll  46 , a first cap roll  48  and a second cap roll  50 , which conjunctively define a pressurized air chamber  52 . The pressure of the air in pressurized air chamber  52  serves to flush the cleaning fluid through semipermeable membrane  32 . The pressure of the air therein is greater than atmospheric pressure (about 1 bar), advantageously more than about 2 bar and preferably greater than approximately 5 bar. First main roll  44  is vented (e.g., blind-drilled, grooved, etc.) so as to promote removal of water, used cleaning fluid and debris from semipermeable membrane  32 . 
     At least one of cleaning fluid sprayers  42   a - 42   d  is provided as part of papermaking machine  30 . Cleaning fluid sprayer  42   a  is directed toward semipermeable membrane  32  from a position just upstream of air press  40 , relative to web travel direction  35 . Cleaning fluid sprayer  42   b  extends into pressurized air chamber  52  and is configured to deliver cleaning fluid onto semipermeable membrane  32  from within pressurized air chamber  52 . Cleaning fluid sprayer  42   c  is located adjacent to second cap roll  50 . Cleaning fluid sprayer  42   c  and second cap roll  50  together are configured to act as a transfer coater for indirectly delivering cleaning fluid onto semipermeable membrane  32 . Similarly, cleaning fluid sprayer  42   d  and first cap roll  48  together also function as a transfer coater. 
     In yet another embodiment, papermaking machine  60  (FIG. 3) includes a semipermeable membrane  62  and a permeable membrane  64  for carrying a paper web  66 , an air press  68 , a cleaning fluid applicator  70  and conveyor rolls  72 . 
     Air press  68  includes a box arrangement  74  mounted adjacent a suction roll  76 . Box arrangement  74  and suction roll  76  coact to form an entrance nip  78  and an exit nip  80  therebetween, respectively through which semipermeable membrane  62 , permeable membrane  64  and paper web  66  are fed into and out of air press  68 . Box arrangement  74  and suction roll  76  together define an air pressure chamber  82 . Box arrangement  74  has an air inlet line  84  associated therewith for introducing air under pressure into air pressure chamber  82 . Conversely, suction roll  76  is provided with a vacuum line  86  for creating a negative pressure therein. 
     Cleaning fluid applicator  70  is positioned prior to entrance nip  78 , relative to a web travel direction  87 , and adjacent semipermeable membrane  62 . Cleaning fluid applicator  70  includes a sprayer  88  and an applicator roll  90 . Sprayer  88  delivers cleaning fluid  92  onto applicator roll  90  which, in turn, transfers cleaning fluid  92  to semipermeable membrane  62 . Alternatively or additionally to cleaning fluid applicator  70 , a cleaning fluid dispenser (not shown) could be provided within air pressure chamber  122 , in a manner similar to cleaning fluid sprayer  42   b  in FIG.  2 . 
     A further embodiment of the invention is shown in FIG.  4 . Specifically, papermaking machine  100  includes a semipermeable membrane  102  and a permeable membrane  104  for carrying a paper web  106 , an air press  108 , a cleaning fluid sprayer  110  and conveyor rolls  112 . 
     Air press  108  includes a box arrangement  114  mounted adjacent a press shoe  116 . Box arrangement  114  and press shoe  116  coact to form an entrance nip  118  and an exit nip  120  therebetween, respectively through which semipermeable membrane  102 , permeable membrane  104  and paper web  106  are fed into and out of air press  108 . Box arrangement  114  and press shoe  116  together define an air pressure chamber  122 . Box arrangement  114  has an air inlet line  124  associated therewith for introducing air under pressure into air pressure chamber  122 . Conversely, press shoe  116  is provided with a vacuum line  126  for creating a negative pressure therein. Additionally or alternatively to cleaning fluid sprayer  110 , a cleaning fluid dispenser (not shown) could be provided within air pressure chamber  122  in a manner similar to cleaning fluid sprayer  42   b  in FIG.  2 . 
     A yet another embodiment is set forth in FIG.  5 . Papermaking machine  130  includes a semipermeable membrane  132  and a permeable membrane  134  for carrying a paper web  136 , a two-stage air press  138  and conveyor rolls  140 . 
     Two-stage air press  138  is a cluster press that includes a first main roll  142 , a second main roll  144 , a third main roll  146  and four cap rolls  148 . Preferably, first main roll  142  and second main roll  144  are vented in order to promote removal of water, used cleaning fluid (initially applied in a manner shown in FIGS. 1-4) and/or debris. Stage one  150  of two-stage air press  138  is defined by first main roll  142 , second main roll  144  and a pair of cap rolls  148 . Stage one  150  has a first air chamber  152  associated therewith. In the embodiment illustrated, semipermeable membrane  132  is fed into first air chamber  152  adjacent first main roll  142  to maximize the time spent thereby in first air chamber  152 . Conversely, permeable membrane  134  and paper web  136  are fed in later, adjacent to second main roll  144 . 
     Stage two  154  is defined by second main roll  144 , third main roll  146  and a pair of cap rolls  148 . Stage two  154  has a second air chamber  156  associated therewith. In two-stage air press  138 , first air chamber  152  is for cleaning, and second air chamber  156  is for dewatering. 
     Stage one  150  has at least one of a first flushing direction  158  and a second flushing direction  160  associated therewith, and stage two  154  has an associated dewatering direction  162 . First flushing direction  158  and second flushing direction  160  are directed at first main roll  142  and second main roll  144 , respectively, within first air chamber  152 . Dewatering direction  162  extends toward second main roll  144  from inside second air chamber  156 . First flushing direction  158  is substantially the same as dewatering direction  162  (relative to the orientation of papermaking machine  130 ) but is substantially diametrical to second flushing direction  160 . Each direction  158 ,  160  and  162  signifies movement of fluid from a high pressure chamber side toward one of vented main rolls  142  and  144 . Stage one  150  may be chosen to be operated in first flushing direction  158  and/or second flushing direction  160  in order to achieve high cleanliness, especially if there are stickies in the pulp. 
     In cleaning semipermeable membrane  32 , a cleaning fluid is provided and is applied on semipermeable membrane  32 . Air press  40  is provided and has pressurized air therein. Semipermeable membrane  32  is conveyed into air press  40  and is subjected to the pressurized air therein. The pressurized air flushes the cleaning fluid through semipermeable membrane  32 , thereby cleaning semipermeable membrane  32 . 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.