Retention & Drainage
Introduction
Retention and drainage aids are applied to the paper machine furnish to increase the efficiency of the papermaking operation and to improve the quality of the finished sheet. Retention and drainage programs can provide significant cost and operating advantages for virtually all paper manufacturers, and for producers of paperboard as well.
Retention is one of the most important processes on the wet end of the paper machine because of its wide-ranging effects on both production costs and sheet quality. Without an effective retention program, large quantities of fiber, fines and fillers can pass through the wire during sheet formation. Low retention results in inefficient use of expensive furnish components. Unretained filler and additives can create collection, disposal, and deposit problems.
A good retention program can provide more uniform distribution of fillers and size in the sheet, reduce sheet two-sidedness, and increase opacity factors that are important in a high quality sheet. Effective retention can also reduce machine deposits that can create wet end breaks or cause holes and spots in the sheet due to high levels of fillers and fines circulating in the wet end system.
FIGURE 1. Material balance around the wet end of a paper machine
illustrating retention
First-Pass Retention (%) = (0.70) - (0.18) x 100 = 74%
(0.70)
First-Pass Ash Retention (%) = 11 x 100 = 52%
21
First-Pass Ash Retention (%) = (0.70 x 0.21) - (0.18 x 0.39) x 100 = 52%
(0.70 x 0.21)
Drainage
Increases in machine speed and energy savings from steam reduction are two benefits that can be realized from better drainage. Formation improvements are also possible when headbox consistency can be diluted as the machine is capable of handling more water.
Stock drainage is the process of dewatering the fiber suspension to form a mat. In most fourdrinier applications drainage proceeds through filtration. Stock characteristics (e.g. freeness, temperature, additives) and consistency are among the variables that affect the rate of drainage on the wire. The fines fraction (less than 200 mesh) is primarily responsible for changes in drainage. Chemical drainage programs are often designed to keep the sheet open for drainage, and to prevent fines from depositing on the bottom of the forming sheet which can seal the sheet against the wire.
IMPROVING DRAINAGE WITH CHEMICAL DRAINAGE AIDS
Chemical drainage aids can influence both the rate of drainage and the amount of water removed. Chemicals that alter the hydrodynamic surface of the fibers will increase the amount of water that is removed on the wire. Products that influence floc size and sheet permeability can increase the drainage rate.
HOW SURFACE CHARGE AFFECTS DRAINAGE
Cationic polymers and alum affect drainage by altering the hydrodynamic surface of the fibers. Cationic polyelectrolytes can cause the collapse of internal and external void areas, or capillaries, and microfibrils on fibers. This decreases the amount of water associated with fiber and fines, which reduces the hydrodynamic specific surface area. This results in less drag as the water flows through the web. Coagulation or flocculation of the furnish components leads to a quick release of bound water and fast drainage from the forming web.
HOW FLOCCULATION AFFECTS DRAINAGE
Chemicals that improve the retention and distribution of fines and fillers often increase the drainage rate as well. Fines flocculated with polyelectrolytes are no longer free to plug interstitial spaces in the fiber mat, so water flows with less obstruction. Retention aids ensure fillers and colloidal materials are held firmly to the surface of fibers and fines. Drainage programs are designed to keep the sheet open for drainage, and to prevent fines that can seal the sheet against the wire from compressing onto the forming fabric.
Excessive flocculation should be avoided. Large floc size means that some exceptionally large pores are formed in the wet sheet. Large pores can bring about a disproportionately high drainage rate. An over-flocced stock will drain freely, but the water associated with the floc is difficult to remove in subsequent dewatering steps.
Some products designated as drainage aids will increase water removal over table rolls and foils, but result in a wetter sheet at the couch. Tying up fines through chemical flocculation enables water and air to flow more freely through the sheet. On exposure to vacuum, water is quickly pulled from the capillaries and displaced by air. The vacuum then works on air rather than water, reducing suction and decreasing compaction or squeezing of the sheet. If the stock is overdosed so that large fiber flocs are formed, this condition is aggravated. Water will be removed from the areas between the flocs and air can easily satisfy the vacuum. Water in the flocs is not removed. Therefore, the sheet is wetter at the couch than before the drainage aid was added.
DEMONSTRATION OF DRAINAGE AND DRYING EFFECTS
The following graphs clearly show the drainage and drying benefits of a retention and drainage polymer program. The mill produces corrugating medium using a 50% recycle (OCC) furnish on a fourdrinier papermachine. A Nalco cationic flocculant had been on the machine for a week-long evaluation. The four graphs depict data that was collected as the polymer was being removed from the machine. Polymer feed was reduced in 0.5 lb/ton (0.25 kg/t) increments in 1/2-hour intervals. The effects on reel moisture, reel speed, flat box vacuum, and dryer steam pressure as the polymer was removed from its addition to the wire pit are apparent.
DRAINAGE AND DRYING BENEFITS
Four ways of taking advantage of drainage benefits on the paper machine are:
Speed
Formation
Energy savings
Fiber substitution
SPEED
Increasing machine speed for production gains is one benefit of a drainage program. Greater dryness at the couch is a signal that machine speed may be increased in drying-limited situations. By increasing machine speed, papermakers are able to take advantage of the machine's increased ability to remove water. Increasing drainage to increase production can translate into a significant payback.
FORMATION
As sheet drainage is improved, the wet line on the machine moves back toward the headbox. This movement of the wet line indicates that the machine table is now able to handle more water. Increasing headbox dilution translates into more uniform distribution of filler and fiber throughout the sheet and better sheet formation.
ENERGY SAVINGS
Energy savings is another benefit from improved drainage. Increased dryness at the couch can reduce steam consumption. When there is less water to remove and it is easily released from the sheet, less steam is needed to accomplish evaporation in the dryers. Figure 4 shows one situation where steam pressure was reduced following the successful application of a microparticle program.
Increasing the rate of water removal to allow a reduction in headbox consistency can yield improvements in sheet formation, which can, in turn, affect energy costs. Improved formation and better starch retention can allow the sheet to be made with less refining, resulting in refiner energy savings. Conversely, if a mill needs to increase refining to improve formation, a drainage program can allow for additional thick stock refining without the loss of machine drying capacity.
FIBER SUBSTITUTION
Fiber substitution is another means of capitalizing on drainage improvement. Lucrative payouts can come about as higher freeness (and more expensive) fiber is replaced with lower freeness (and less costly) pulp. Drainage programs may be initiated as a way of maintaining drainage as groundwood or hardwood pulp is substituted for softwood kraft pulp, or old news is substituted for clippings. In some instances microparticle programs are absolutely critical in providing the extra drainage needed to maintain machine speed with these furnish substitutions.
SHEET QUALITY BENEFITS
A well-designed retention and drainage program can improve sheet quality in many ways. Formation improvement is a key quality benefit, particularly with high performance microparticle programs.
Increased drainage allows a reduction in headbox consistency. Increasing headbox dilution translates into more uniform distribution of filler and fiber throughout the sheet and better sheet formation. Improvement in optical properties, smoothness, porosity and printability are also available through better distribution and higher retention of filler and fines. The example in Figure 5 shows how porosity was reduced with the implementation of a high performance retention and drainage program, creating a sheet having improved coating hold-out and better print quality.
The substitution of shorter fiber pulps made possible by increased drainage can also produce formation improvement. A higher proportion of shorter fibers that are well distributed throughout the sheet provides a high formation. In the example in Figure 6, the amount of drainage afforded by a POSITEK program enabled a portion of the softwood fiber to be replaced with shorter hardwood fiber. Formation measurements taken with the MK formation gauge increased steadily as softwood was exchanged with shorter fiber.
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