制浆造纸专业单词

Chapter 11 Paper Manufacture – Dry End Operations
11.1 PAPER DRYING
After pressing, the sheet is conveyed throught the dryer section where the residual water is removed by evaporation. On conventional paper machine, the thermal energy for drying is transferred to the paper by wrapping a series of large-diameter, rotating, staem-filled cylinders. A representtative system-cylinder drying system for lightweight paper is shown in Figure 11-1.
The massive dryer section is the most expensive part of a paper machine in terms of capital cost. It is also the most costly to operate because of the high energy consumption. Therefore, efforts to increase evaporation rate (to reduce the number of dryers) and conserve energy ( to reduce steam usage ) are uauslly well justified. Unfortunately, the drying operation does not appear to receive as much attention as other parts of the process, and many opportunities to improve efficiency are lost.
Criteria of Performance
Two indices are important in assessing the performance of a dryer section: evaporation rate and steam economy. Evaporation rate is measured as pounds of water evaporated per hour per square foot of dryer surface contacted ( or equivalent metric units). A high evaporation rte is desirable with respect to equipment requirements, but drying must alwys be carried out within the constraints of the relatively low drying rate is required on some grades to ensure product quality. Generally, uniform evaporation across the machine is desired. However, if it is necessary to compensate for moisture streaks or other profile problems, techniques are available to raise or lower the evaporation rate at selected positions across the machine.
The evaporation drying rate is greatly influenced by the steam pressure used inside the drying cylinders. When assessing the operation of different machines making the same type of paper, comparisons should be made on the basis of “lines of equivalent performance” as illustrated in Figure 11-2. Actual paper machine evaporation rate data for different products are provided in a seroes of TAPPI data sheet (e.g., Figure 11-3).
Steam economy is measured as thousands of BTU’s per pound of water evaporation ( or kJ per kg) or as mass of steam per unit mass of water evaporated. Obviously, a low steam usage is desirable for the most economical operation. A value of 1.3 kg steam per kg water evaporated is typical for a modern well-designed, well-maintained system, but many machines have significantly higher usage. The steam/condensate and air handling systems have the greatest impact on energy consumption.
Zones of Evaporation
The drying rate varies along the machine. The first two or three cylinders into the drying section serve principally to raise the temperature of the sheet ( the “warm-up zone”). Evaporation then quickly reaches a peak rate which is maintained as long as water is present on the fiber surface or with the large capillaries (“constant rate zone”). At the point where the remaining free moisture is concentrated in the smaller capillaries, the rate begins to decrease (“falling rate zone”). Finally, at about 9% moisture, the residual water whthin the sheet is more tightly held by physicochemical forces, and the evaporation rate is further reduced (“bound water zone”). The various zone are illustrated in Figure 11-4.
All factors being equal, the more evaporation which takes place in the constant rate zone, the higher will be the average evaporation rate. By the same token, the average rate will be lower if a significant amount of time is spent in the bound water zone. Unfortunately, some machines were forced to “over-dry” to compensate for poor drying uniformity. A natural levelling efect in the profile occurs at lower moisture contents because the physicochemical nonds become progressively more difficult to break.
Description of Drying Process
The wet web from the press section containing 55-60% moisture (40-45% dryness) is passed over a series of rotating steam-headed cylinders (usually 60 to 72 inches in diameter) where water is evaporated and carried away by ventilation air. The we web id held tightly against the cylinders by a synthetic, permeable fabric called a dryer
felt. The fabric also serves to support and guide the sheet through the dryer section (In some cases, it is also aids in controlling cross-direction shrinkage and keeps the sheet flat, i.e., prevents sheet cocking).
Most paper machine have three to five independently-felted dryer sections, each with independent speed control to maintain sheet tension between sections and adjust for any sheet shrinkage that occurs. All top and bottom felt runs are equiooed with tensioning nd poistioning rolls. Usually, three to five sections are also grouped for independent steam pressure control; three may be the same or different from the felt groupings. A typical configuration for conventional two-tier drying is illustrated in Figure 11-5.
Attention should now be directed to a “dryer pocker” (Figure 11-6). Paper drying can best be visualized as a a repetitive two-phase process. In phase 1, the sheet picks up sensible heat while in contact with the steam cylinder. In phase 2, the sheet flashes off steam in the open draw between the top and bottom cylinders, thus causing the sheet to spontaneously cool and become ready to pick up sensible heat again.
A typical temperature profile between the steam inside the dryer cylinder and the paper wrapping the cylinder is illustrated in Figure 11-7. The major thermal resistances are usually provided by the condensate layer inside the cylinder and the air layer between sheet and cylinders. The din film can also be significant for certain machines and/or paper products, and doctors may be required to keep the surface clean.
The air layer is minimized by utilizing adequate felt tension to keep the paper web firmly against the dryer cylinder surface. It has been found that increasing felt tension beyond a certain point does not further reduce the air layer. The tension needed is directly proportional to machine speed and varies with cylinder diameter to the 1.5 power. Figure 11-8 provides machine as a function of machine speed and cylinder diameter.
The condensate layer is probably the most significant resisitance to heat transfer on high-speed machinees; this aspect will be covered later in this section. If non-condensibles are allowed to accumulate within the steam cylinders, they can adversely affect heat transfer and can also cause nonuniform drying.
The major resistance to steam flashing off in the pocket is the buildup of humidity, causing a lower differential of partial pressures. The problem of achieving adequate pocket ventilation will also be covered later in this section. Steam and Condensate System
The heat energy gor paper drying comes from steam as it condenses inside the dryer cylinders. This type of heat is referred to as “latent heat”. The temperature at which steam condenses and the relative amount of latent heat depend on the steam pressure, as illustrated by the steam table data in Table 11-1. Steam always condenses at the “saturation temperature”as defined by the pressure in the system; this is important with respect to controlling drying uniformity across the machine. Steam is usually transported at a temperature considerably above the saturation level (i.e., it is “superheated” ) to prevent condensation with pipe lines.
As steam pressure increases, the condensing (saturation ) temoerature increases and the latent heat decreases. Therefore, the heat transfer rate increases with pressure, but more steam must be condensed gor a given amount of heat transferred.
The condensate that forms in the dryer cylinders is removed by a specially-designed syphon assembly. On slow machines, the condensate collects in a puddle at the bottom of the cylinder; a stationary syphon angled into the puddle is often used on these machines. With increasing speed, the puddle begins to cascade; and finally a true rimming condition is reached where the condensate covers the entire inside surface due to centrifugal force (Figure 11-9).
For high-speed machines (and some slower machine), rotating wphons are used where the syphon assembly is fixed to the dryer shell to minimize the syphon-to-shell clearance (Figure 11-10).
On slow machines, puddling condensate within the cylinder has a positive effect on heat transfer rate. With the onset of rimming conditions, there is still considerable turbulence in the condensate layer which helps to maintain heat transfer.
But with increasing machine speed, the condensate layer becomes more immobilized (due to greater centrifugal force) and it becomes more important to reduce syphon clearaness to minimize the thickness of the condensate
film, Hower, at the high speed of modern paper machines, the condensate layer has become so immobilized that even a very thin layer is a significant impediment to heat flow.
Fortunately, the condensate layer still has some peripheral motion at high machine speed. This motion can be converted into a wave action by means of “dryer bars” attached axially to the interior surface of the dryer cylinder (Figure 11-11). This wave action (called “sloshing”) greatly decreases the thermal resistance of the rimming condensate, and thus allow an increased rate of heat transfer. Dryer bar segments can be installed in certain dryer cylinders to selectively improve evaporation rate at cross-machine locations for moisture profile correction.
On most existing paper machines, the steam feed and the condensate removal both take place through the same side, usually the front journal. On wider machine, this arrangement may favor accumulation of non-condensibles. In more recent designs, steam is intruduced at the backside of the cylinder and condensate is removed at the frontside (tending side) as shown in Figure 11-12. The current trend for wide paper machines is to use two syphones, one at each end of the dryer cylinder to ensure more even condensate removal.
The pressure differential required to “pump” pure condensate from the dryer shell to the centerline jorrnal against centrifugal force for a high-speed machine is a very large (e.g., 20 psi at 3500 ft /min). In practice, the ifferential is considerably reduced by allowing steam to entrain the condensate and thereby reduce the effective density (as illustrated in Figure 11-10). This so called “blow-through steam”also serves to evacuate the non-condensible gases from the dryer cylinder.
To ensure that condensate is removed continuously, the differential pressure (as measured between the steam and condensate lines) is usually controlled at a level somewhat above the minimum required. The main effect of higher differential pressure is to increase the blow-through steam, which typically amounts to 15-20% mass fraction of the condensation rate. Althrough a substantial amount of blow-through steam can be tolerated, excessive rates will erode the piping components, and reduce the flexibility of the blow-through steam handling system.
Systems
There are two common systems for handling blow-through steam. In the cascade system, the steam is separated from the condensate and then re-used in the lower-pressure dryer secions, as illustrated in Figure 11-13. The main disadvantage of this system is the interdependency of sections: only the highest pressure section can be varied independently. The second approach, which is now the most popular, is the “thermo-compressor”system, as illustrated in Figure 11-14. Here, the lower-pressure blow-through steam is “boosted” in pressure by mixing with high-pressure motive steam and then re-used, usually in the same section. This system provides totally independent pressyre control for each section and freedom in partitioning of cylinders, but at the cost of losing some electrical generation from the high-pressure motive steam. The thermo-compressor nozzle is illustrated in Figure 11-15.
Pocket Ventilation
Prior to 1960, dryer felts were woven from wool and were virtually non-permeable. When dryer were clothed with these “conventional” felts, the pockets were essentially sealed except at the ends, and ventilation to remove humid air was difficult. Typically, high-velocity air was injected into one end of the pocket, which induced a flow of air across the pocket to flash out the humidity. This system was rather ineffective, especially on wider machines.
In the early 1960’s, clothing manufacturers began to introduce synthetic fabrics of more permeable construction. These “open” fabrics were found to provide ventilation by spontaneously carrying air into and out the pocket, as illustrated in Figure 11-16. In vestigators found that the amount of air displacement was primarily a function of felt permeability and machine speed, as shown in Figure 11-17.
The permeable fabric was further exploited by supplying hot dry air expressly into the pocket. Two general methods are used: through the felt roll as illustrated in Figure 11-18, and through an exterior duct as illustrated in Figure 11-19. There are many variations to these basic designs. By providing variable supply air across the
machine, these systems can also be utilized for moisture profile correction.
The improved pocket ventilation achieved with modern clothing and air supply systems has been respomsible for greatly increased evaporation rates. The impact on the dryer pocket humidity profile is quite dramatic, as shown in Figure 11-20.
Sheet Flutter
After synthetic fabrics were introduced, the permeabilities were soon raised to take advantage of the improved ventilation. However, the strongeraie currents induced by these fabrics compounded an existing problem with sheet flutter on faster lightweight machines. (Sheet flutter refers to the general billowing and flapping of the paper sheet in the open draws of the dryer section. Flutter at the sheet edges leads to creases and bresks.) The sheet flutter problem promoted the clothing manufacturers to offer dryer fabric constructions covering the complete range of permeabilities, including some with reduced permeability at the edges where flutter problems are most severe. In the meantime, the ever-present trend toward increased machine speed was also exacerbating the flutter problem.
A number of design approaches were taken to control the sheet flutter problem, including changes in the pocket geometry and the use of air doctors to divert the more damaging air currents. However, the most widely utilized stratagem on existing machines was to switch to a single-felt configuration for the first dryer section where the sheet is most prone toward wrinkling. In the so-called serpentine felt run (Figure 11-21), the bottom felt is eliminated and the top felt follows the sheet and wrap both the top and bottom dryers. Thus, the felt supports the sheet in the draws between top nad bottom dryer cylinders, but lso runs between the sheet and the bottom dryer surfaces. The serpentine felt run is effecitive in controlling sheet flutter up to a point, but instabilities are still evident at higher speed. Unfortunately, with this arrangement, the bottom tier of dryer cylinders becomes redundant.
On a typical modern-design, high-speed machine producing lightweight sheet, the bottom tier of dryers has now been eliminated in the wet-end sections; and the sheet wraps a suction roll between dryer cylinders for greater stability (refer back to Figure 17-1). Blow boxes may also be used to control the boundry air layer. For the next generation of high-speed machines, some designers are proposing a complete single-tier arrangement with a number of “reverse sections” to provide drying from both side of the sheet.
Hood Ventilation
Air is an important part of the paper drying process. Depending on what type of hood arrangement is used, from 7 to 20 pounds of air utilized for each pound of water evaporated. To prevent drips, buildups and corrosion within the hood, the volume and temperature of the exhaust air must be sufficient to avoid loalized condensation. The supply air should be strategically introduced into the drying process for best utilization.
The early dryer hoods consisted of little more than a false ceiling with exhaust fans. All the process air was sucked from the machine room and was not effectively utilized. Partially-enclosed hoods were an improvement, but totally-enclosed hoods provide much better control of supply and exhaust air flows and ensure a more comfortable working environment (refer to Figure 11-22). The modern generation of hoods (the so-called “high-dew point hoods”) are well sealed and insulated. Diffusion air is totally eliminated, and the amount pf fresh makeup air is sharply reduced by operating at high temperature with partial recycle.
Steam Economy and Heat Recovery
Although some steam may be wasted by a poorly operated steam/condensate system, most of the heat energy expended for paper drying ends up with the exhaust air. Steam economy is, therefore, strongly affected by the amount of air used and extent to which heat can be recovered into the supply air. All modern hoods are equipped with a heat recovery system similar to that oictured in Figure 11-23. The primary element is the air-to-air heat exchanger for transferring heat from the hot, humid exhaust air into the fresh, ambient supply air.
Steam economy can be optimized in the case of a high-dew point hood because less air is used and a higher level
of heat recovery is possible from condensing vapors. Unfortunately, the amount that can be recovered in the supply air is typically limited to 10-15% (see Figure 11-24). Heating of both machine room supply air and process water is often incorporated into the heat recovery system; although useful energy is recovered, this “low-level heat” is normally not credited against steam economy.
Alternate Methods of Drying
Other methods of paper drying (besides steam cylinder drying) are used for certain applications. Air-impingement drying and infrared drying are commonly used for coating where contact with a steam cylinder would cause adhesion problems. Airborne drying, a technique commonly used for pulp drying, is also utilized in the production of extensible papers; since little tension is applied in the machine direction, the paper is not pre-stretched as it is during conventional drying (refer to Section 20.3). Air-through drying has found application for the drying of lightweight porous papers, mainly tissues (refer to Section 20.7).
Microwave or dielectric drying would appear to have application at the dry end of the machine. Free water (as opposed to bound water) preferentially absorbs the microwave energy to provide self-compensating profile correction, as illustrated in Figure 11-25. However, this technique has not caught on, perhaps because the economics are not favorable.
Press Drying and Impulse Drying
Press drying refers to any process that utilizes the combined application of pressing and drying under restraint. Arange of technologies are under active development, but none is known to be used commercially as yet. This approach holds the promise of more economical water removal along with improved product quality.
Impulse drying is a form of press drying, but the conditions of temperature and pressure are more intense. The basic idea is to provide an additional driving froce within the press nip: this force is created by generating a high-pressure steam front at one surface of the sheet (in contract with a very hot press surface) that literally pushes out water toward the cold press surface. Drying rate two or three orders of magnitude greater than those for conventional drying are claimed; but again, the technique has not yet been applied commercially.
11.2 CALENDERING
Calendering is a general term meaning pressing with a roll. Most, but not all, paper grades are calendered with the principal objective being to obtain a smooth surface for printing. Some sheet compaction always occurs during calendering: in some cases (as for example, with foodboards and tissues) this may be seen as a diaadvantage. But thickness reduction is an objective with newsprint-type sheets where it is important to obtain a specified length of paper in a standard-diameter reel. Generally, calendering is performed on dry paper, but some calendering treatments may be carried out on partially-dried paper. ( it has been suggested that a smoothing press or fourdrinier lump breaker roll are types of calerdering equipment, since their primry role is to smooth the sheet). Another common objective of calendering is to improve the cross-direction (CD) uniformity of certain properties, particularly of thickness, which is important for reel-building and converting. It must be noted that where calendering pressure is varied to compensate for non-uniformity in one CD property (e.g., caliper), then the CD profiles of other properties (e.g., density, smoothness) are made lessuniform. Generally, the induced non-uniformities are small, and caliper control remains an important aspect of calendering. (Cross-direction profile control methods will be discussed in Section 11.3).
Calendering changes the surface and interior properties og the sheet by passing the web through one or more two-roll nips where the roll may or may not be of equal hardness. The pressure are extreme and the time that any section of the web actually spends in the nip is infinitesimally small. The basic objective is to press the pper against the smooth surface with sufficient force to deform the paper plastically and replicate the calender roll surface onto the paper. The replication of greater pressure and/or shear forces to make them more pliable. At one time, calendering was likened to flat-ironing a cotton shirt, but, in fact, virtually no burnishing action occurs with roll-nip calendering (The only type of calender still in common use that works by burnishing is the brush calender,
in which the suface of ppaerboard is contract by a rapidly ritating brush.)
On-Machine vs. Off-Machine Calendering
For reason of economy and efficacy, most calendering operations are carried out on-machine. Calendering that is concerned with reel-buikding must,of necessity, be part of the ppaer machine process. Off-machine calendering is a relatively costly process, and is only resorted to when adequate paper suface finishing cannot be obtained on-machine. The calssic off-machine operation using alternating iron and compressed fiber rolls is known as supercalendering (refer to Section 18.3 for dicussion).
Up to the mid-1970’s, on-machine calendering was limited to the traditional operation where paper was passed through one or more nips formed by a set of iron rolls. The difference in printing qualities between conventional machine-calendered and supercalendered ppaers are very substantial. However, during the late 1970’s and 1980’s, thanks to new elastomeric roll coverings and innovative equipment designs, the disparity between on-machine and off-machine capability has been narrowed. The new on-machine technology has enabled paper producers to provide a range of machine-calendered finishes to serve more market niches. Some mill references have even suggested that on-machine soft-nip calenders can replace supercalenders for centain less critical grades.
Types of Machine Calenders
In traditional hard-nip machine calenders, the web is calendered to a uniform thickness, as illustrated in Figure 11-26.Small-scale fiber concentrations (floes) are forced to occupy the same thickness as lightweight spots because the high points. Hence, basis weight variations also become density vriations; these in mm become vriations in surface properties, which for example, are visible as print mottle.
In supercalenders, the nips are formed by mating a hard roll with a soft roll. The covers of the soft rolls are made of compressed paper, rubber or cast polymers, which have approximately the same hardness as paper under compression in the nip. The soft cover serves to distribute the pressure in the nip more evenly over the hills and valleys in the paper web and produces a product with more constant density rther than constant caliper as illustrated in Figure 11-26. Consequently, the surface properties will be more uniform, the basis weight variations will show up as thickness variations.
Although many different arrangements and configurations of hard-nip machine calenders are available the equipment depicted in Figure 11-27 is reasonably typical. The up-to-date features of this “stack” are heated rolls and the variable0crown rolls. Calendering at high temperature is desirable because the paper becomes more pliable and can be calendered at low pressure. Therefore, the first two wrapped rolls of the calender stack should be heated (Figure 11-28). The king variable-crown, but it is also desirable to have one itermediate roll with a variable-crown to facilitate changing nip loads. The queen roll (second from bottom) is usually the driven roll.
On-machine soft-nip calendering is an attempt to get some of the benefits of supercalendering without paying the price. This technique was widely adopted in the 1980’s as means of improving paper suface qualities. A number of configurations are used, but the arrangement shown in Figure 11-29 is probably the most common. Since the side contracting the metal roll receives a much better finish than the side contracting the resilient roll, it is necessry to have two nips for equal finish. The resilient roll is uaually water-cooled to remove heat generated in th ecover. The hard roll is equipped with variable crown. An important design feature o fthese sofr-nip calenders is that allrolls are individually driven; this is to enable the nips to be opened and closed at machine speed to faciliate threading of the web without damaging the roll covers.
Calendering Variables
Clearly, the objectives in calendering are to control the chickness and surfeac properties of the paper, without adversely affecting strength properties. The major variable which bear on the capability of the calender to achieve thes objectives of paper properties and operating parameters. Obviously, the properties of the paper ebtering the calender influence the calendering operation and the properties of the calendered paper. The initial values will have an influence on the final values, as will the characteristica of the furnish.
The moisture content of paper has a profound effect on its compressibility. Consequently, various methods to
utilize moisture for greater cakendering effect have been utilized. On some machines, the web leaving the dryer section is passed over a cooled “sweat roll”where moisture is condensed and transferred to the surface of the paper. Steam showers or mist showers are also used to increased the surface moisture content. For certain paperboard grades, moisture is sometimes added to the sheet surface by means of water boses prior to the first calender nip. In some instances, one or two calender.
Nips may be installed between th elast two dryer sections to provide calendering action at higher sheet moisture content, in which case the equipment is called a “breaker stack”.
The reductions in sheet roughness and caliper are inter-related, and are both functions of nip pressure, retention time and number of nips. Generally, there is limited scope for independent control of either calper or smoothness when using concentional machine calenders; however, temperature is known to have a somewhat disproportionate effect on roughness as illustrated in Figure 11-30. Of course, the smoothing effect also depends on the finish of the roll; most calender rolls are “superfinsihed” following grinding by utilizing belt polishers.
Paper quality variability is measured and controlled in two dimensions: the machine direction (MD) and cross direction (CD). During the production process, the former varies with time and the latter mainly with respect to cross-machine position. The ultimate goal in papermaking is, of course, to produce a uniform product, and this requires that all variations be reduced to negligible levels.’
Virtually all MD variations can be tracted back to high-frequency pulsation in the headbox approach system. Significant progress has been made in recent years in identifying the sources of these disturbances, and by means of process or equipment modifications either eliminating the cause or dampening the signal. For example, by properly positioning the rotating foils on some approach system pressure screens, not only can reinforcement of potentially troublesome pressure pulses be avoided, but in some cases, nullification of one pulse by another can be achieved.
The present discussion will be concerned only with CD profile control. However, it must be recognized that when an on-machine CD sensor is scanning the sheet, it traces a diagonal path, measuring a “now” profile that includes both MD and CD variations, as illustrated in Figure 11-31. A typical series of scan profiles is shown in Figure 11-32. Until the superimposed MD variability is eliminated from the measured profiles, a “pure” CD profile can not be idenfied and successfully controlled. The conventional procedure for doing this, called exponential multiple-scan trending, weights the current measurement at each CD position to the long-term historical value rather than to the most recently measured value. Using this method, approximately 10 scans are needed to register 90% of a step-wise profile change. Various techniques designed to provide more rapid CD measurement resolution are under development.
Sensors
Many different types of sensors are used to measure paper properties on-line. The sensor usually does not measure the deisred property directly, but rather a related characteristic that is well matched in sensitivity. The sensing methods include nuclear, infrared, nicrowave, visible light, magnetic reluctance and ultrasonics. In a few cases, two or more sensors are combined for a partticular measurement.
Typically, a cluster of sensors measuring different properties are mounted on a traversing platform which scans the sheet. Each sensor will generally consisit of a radiation source (e.g., nuclear isotope, light bulb, etc.), radiation detector, analog and digital electroics, meaasurement algorithm, software routines and power supplies. The packing and design of each sensor must provide for accurate measurement in environments which include high variable temperature and humidity, dirt and dust.
The most common on-line measurements are for grammage, moisture content, and sheet caliper. Grammage is typically measured by beta-particle attenuation, where the paper between the source and the detector attenuates the radiation according to its mass per unit area. The detector is usually an ionization chamber, and a suitable beta-particle emitting isotope is selected based on the ppaer or ppaerboard grammage. The moisture content of lightweght low-moisture ppaer is generally measured by infrared radiation absorption. For higher weght ppaers。