ChE 441: Paper Manufacture

Transcription

1 ChE 441: Paper Manufacture Lecture # 13 by Kazi Bayzid Kabir

2 Introduction Paper consists of a web of pulp fibers usually formed from an aqueous slurry on a wire or screen, and held together by hydrogen bonding Department of Chemical Engineering 2

3 Fourdrinier Paper Machine Wet End Operations Flowspreader: distributes the fibers uniformly across the machine from back to front Headbox: distributes the paper stock onto the moving forming wire Fourdrinier Wire: endless moving wire forms the fiber into a sheet and enables the furnish to drain by gravity Press Section: additional water is removed and the web is consolidated Department of Chemical Engineering 3

4 Fourdrinier Paper Machine Dry End Operations Dryer section: most of the remaining water is evaporated and fiber bonding develops as the paper contacts a series of steam-heated cylinders Calender section: sheet is pressed between metal rolls to reduce thickness and smooth the surface Reel: dried and calendered paper is wound onto a reel Department of Chemical Engineering 4

5 Fourdrinier Wet End Department of Chemical Engineering 5

6 Inlet to Headbox Flowspreader The inlet to the headbox must insure an even fiber slurry consistency and pressure across the width of the headbox to insure cross direction uniformity of paper A variety of techniques in the past has been used to insure this, including complex distribution piping or various types of manifolds Department of Chemical Engineering 6

7 Flowspreader The invention of the tapered inlet by J. Mardon in the 1950s greatly simplified this goal All modern headboxes use a tapered inlet pipe to accomplish this purpose Department of Chemical Engineering 7

8 Headbox Function of the headbox is to take the stock delivered by the approach system and transform the pipeline flow into a uniform rectangular flow equal in width to the paper machine and at uniform velocity in the machine direction Department of Chemical Engineering 8

9 Objectives of headbox system 1. Spread stock evenly across the width of the machine 2. Level out cross-currents and consistency variations 3. Level out machine direction velocity gradients 4. Create controlled turbulence to eliminate fiber floccing 5. Discharge an even flow from the slice opening and impinge on the wire at the correct location and angle Department of Chemical Engineering 9

10 Headbox Department of Chemical Engineering 10

11 Rectifier Roll Department of Chemical Engineering 11

12 Forming Fabric Forming fabric is a continuous loop or belt of finely woven screen made from wire or plastic Mesh size varies from 40 to 100 mesh Functions of a forming fabric are best described under the headings of drainage, sheet support and mechanical stability Department of Chemical Engineering 12

13 Breast Roll The breast roll is located under the headbox and serves to return the fabric to the forming area to receive the stock once again Department of Chemical Engineering 13

14 Forming Board The forming board consists of a large leading blade to reach in close to the slice followed by several smaller blades, usually with gaps between them Forming board supports to retard initial drainage Retarding early drainage facilitates uniform water removal and improves formation Department of Chemical Engineering 14

15 Deckle Board Deckle boards are used to prevent the stock from flowing off the two sides of the forming fabric when a thick layer of stock is delivered to the fabric by a large slice opening Department of Chemical Engineering 15

16 Table Rolls Table rolls are freely revolving rolls under the fourdrinier wire that support the weight of the wire and wet web Water is removed from the bottom of the web by a partial vacuum [45 kpa below ambient pressure at the meniscus that forms at the nip where the roll leaves the wire Department of Chemical Engineering 16

17 Table Rolls Department of Chemical Engineering 17

18 Lovacs/Wet Boxes A lovac develops suction using waterfilled drop legs to provide siphoning action These are sometimes called wet boxes The pressure gradient is proportional to the height of the water column Department of Chemical Engineering 18

19 Water Removal by Lovac Department of Chemical Engineering 19

20 Hivacs/Suction Boxes/Flat Boxes In the progression along the forming table the stock is subjected to increasing P to drain the sheet After the lovacs the stock encounters the suction boxes or hivacs Department of Chemical Engineering 20

21 Flatbox Installation Department of Chemical Engineering 21

22 Dandy Roll The dandy roll is a hollow, light, wire covered roll that rides on top of the fourdrinier wire just ahead of the suction boxes This roll has following purposes to improve the top surface for printing to improve formation by mechanical shear to increase the drainage capacity of the flat wire Some dandy rolls carry a pattern that is transferred to the sheet to provide watermarks or other special effects Department of Chemical Engineering 22

23 Couch Roll The couch roll is the guide or turning roll for the fourdrinier wire, where the paper web leaves the wire and the wire returns to the breast roll It has two purposes to transmit power to the fabric to increase the dryness of the sheet Department of Chemical Engineering 23

24 Couch Roll Department of Chemical Engineering 24

25 Pickup Felt The pickup felt is a traveling felt designed to pick the wet paper web off the wire and transfer it to press section web is supported at all times by either the wire or felt Department of Chemical Engineering 25

26 Twin Wire Formers Twin wire formers are machines that use a jet of stock imparted on two converging wires to accelerate water removal and maintain better web uniformity These are particularly useful for high speed machines, where the fourdrinier wet end would tend to give a two-sided sheet, since both sides are wire sides and the sheet is formed symmetrically on the two sides Department of Chemical Engineering 26

27 Several Types of Twin Wire Former Department of Chemical Engineering 27

28 Verti-former V twin wire former and headbox 3-Mar-08 Department of Chemical Engineering 28

29 White Water System White water system is the drainage from wet stock regardless of color Besides fiber white water system may contain a variety of other furnish derived materials The white water is richest from the area where the jet impinges on the wire and becomes progressively leaner Department of Chemical Engineering 29

30 White Water System Department of Chemical Engineering 30

31 White Water System Objectives Operating Guidelines Self-Study Department of Chemical Engineering 31

32 Fiber Recovery from White Water To meet the requirements of both ecology and economy the reusable fibers and fillers in paper machine white water overflow streams must be reclaimed into the machine furnish Drum type filters (sometimes) Flotation Disc filter Department of Chemical Engineering 32

33 Broke system Wet-end broke (Couch Pit) Dry-end broke Department of Chemical Engineering 33

34 Sheet Transfer from Wire to Press Section 3-Mar-08 Department of Chemical Engineering 34

35 Pressing Primary objectives are to remove water from the sheet and consolidate the web Other objectives nay be to provide sufficient smoothness, reduce bulk etc. Department of Chemical Engineering 35

36 Pressing Department of Chemical Engineering 36

37 Plain Press In a plain press, two smooth rolls are used and the capacity to remove water depends on the void volume of the press felt(s) to receive the water The first press is often a plain press and is often double-felted to receive more water Department of Chemical Engineering 37

38 Plain press nip showing limitation of water removal 3-Mar-08 Department of Chemical Engineering 38

39 Suction Press In a suction press, one of the press rolls is equipped with a suction box to remove water and hold it into the felt against centrifugal force until the sheet has separated from the felt Department of Chemical Engineering 39

40 Press Felt The press felt normally runs underneath the paper web going through a press section It is designed to absorb water squeezed out of the sheet and to support the sheet through the presses The felt is separated as quickly as possible after the press nip in order to avoid rewetting of the sheet from the felt Department of Chemical Engineering 40

41 General Aspects of Drying Removing water from the web is accomplished by adding heat to the web and circulating the air Heat is applied by the pressurized, steam filled circular steel or cast iron dryer drums Circulation of air over the web is very important Department of Chemical Engineering 41

42 General Aspects of Drying The rate of water removal depends on many factors the temperature and amount of steam entering the dryer adequate removal of the steam condensate and air from the interior of the drying can the amount of sheet-dryer surface contact, contact time, and contact pressure Cleanliness of the drum's exterior and interior surfaces Type of felt and condition of felt Circulation of hot, dry air Department of Chemical Engineering 42

43 Dryers Dryers are hollow, revolving, steam filled drums (about 2 m or 6 ft in diameter) designed to heat the web by direct contact and remove water by evaporation About kg of steam are used to remove 1 kg of water from the web Department of Chemical Engineering 43

44 Diagram of a Dryer Can Department of Chemical Engineering 44

45 Dryer Felts Dryer felts hold the paper web against the drums to improve heat transfer between the drum and web and absorb a portion of the water evaporated from the web Ideally, the felt is nonabsorbent, strong, flexible, and (as always) inexpensive Department of Chemical Engineering 45

46 Arrangement of Dryer Felts Department of Chemical Engineering 46

47 Pocket Ventilation The ventilation system is designed to keep hot, dry air over the free paper surface, especially in the pockets between dryer drums where water quickly evaporates Pocket ventilation systems blow hot, dry air into the pockets through air ducts and nozzles at high velocity Two general methods are used: Through the felt rolls Through an exterior duct Department of Chemical Engineering 47

48 Steam Control System The first step in controlling the heating of dryer drums is to remove the steam condensate from the drum The condensate tends to form pools or puddles at low machine speeds and rims at speeds above about 6 m/s (1200 ft/min) Larger amounts of condensate increase the speed required for rimming At intermediate speeds, the condensate will splash or cascade Department of Chemical Engineering 48

49 Steam Control System older, slow machines used stationary siphons that continuously collected condensate from the bottom of the dryer drum Modern, faster machines use rotating siphons that continuously removes rimming water Too much condensate, especially when rimming, decreases the heat transfer Department of Chemical Engineering 49

50 Dryer Hood The dryer hood is an enclosure around the dryer section and is used to improve drying efficiency by removing the moist air near the surface of the web If this air is not removed, it quickly becomes saturated with water, preventing further water removal from the web Department of Chemical Engineering 50

51 Yankee Dryer The Yankee dryer is a large dryer drum for drying tissue papers that are not strong enough to endure numerous felt transfers The Yankee dryer is normally the only dryer used to dry tissue An air cap is a hood mounted close to the surface of the Yankee dryer that forces heated air on the web, improving the drying efficiency The creping blade (doctor blade) is a thin metal blade that scrapes the dry tissue off the Yankee dryer Department of Chemical Engineering 51

52 Yankee Dryer Department of Chemical Engineering 52

53 Size Press The size press is located between dryer sections and consists of a pair of squeeze rolls mounted horizontally, vertically, or at a 45 angle It is used to apply surface size (usually a starch solution) to papers In the horizontal size press, the bottom roll rotates in a pan filled with size solution, and the same solution is delivered to the top surface of the web by a shower pipe The excess solution is squeezed off the sheet The base sheet is then redried Department of Chemical Engineering 53

54 Size Press Department of Chemical Engineering 54

55 Surface Sizing Surface size uses polymeric materials, commonly starch Surface sizing improves the water resistance of paper and bonds surface fibers to it to improve printing properties Except for wax, surface sizes are generally not hydrophobic like internal sizes They act by sealing the sheet, that is, filling the capillaries and pores with sizing formulation Department of Chemical Engineering 55

56 Calender The calender stack is a series of solid rolls, usually steel or cast iron, mounted horizontally and stacked vertically During machine calendering, the dry paper passes between the rolls under pressure, thereby improving the surface smoothness Department of Chemical Engineering 56

57 Calender Department of Chemical Engineering 57

58 Supercalender A supercalender is similar to the calender but uses alternate hard and soft, heated rolls It is usually used offmachine such as after paper coating Department of Chemical Engineering 58

59 Reel The reel is the last unit on the paper machine that collects the paper The paper is wound on a spool that rotates against a drum Department of Chemical Engineering 59