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Every papermaker has the same dilemma of making a higher quality sheet on a more efficient machine without negatively affecting the environment or significantly changing the existing process! Although this portrays an extreme situation, it is a position more and more mills are finding themselves in on a daily basis.
Dramatically improving the filtration of the size press solution is one area that fits this profile. On an annual basis, one particular mill projected being able to reduce paper roll splices by 3,800. Fewer splices were made to cut out sheet defects, and the improved sheet quality helped reduce breaks (splices) at the winder, as well as providing numerous other benefits.
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Before (left) and after photos of the 0.002-in. slotted wedge wire element show the high removal efficiency of the Pulse Purge backwash system.
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MILL OVERVIEW. A Northeastern paper mill with a paper machine manufacturing roughly 250 tpd of printing and writing coated publication grades was experiencing problems with its sheet on the machine and at the off-machine coater. Many of the problems could be traced back to the pre-coat starch applied at the size press and the amount of debris present in the starch loop. The machine utilized a puddle-type size press to apply a "pre-coat" of starch.
By design, the excess starch from the puddle recirculated back to a machine run tank. This recirculating starch solution continually picked up fiber and felt hair from the sheet, uncooked starch from the cooker, and other miscellaneous debris, including large wads of paper during machine breaks. The rate at which this debris accumulated depended on grade, run-time, and various other factors.
The existing filter system running with 0.006-in. slotted wedge wire media was unable to maintain overall starch cleanliness over time. This resulted in debris continually accumulating and depositing on the sheet to be picked out on the paper machine and off-machine coater. The longer the run-time, the more severe the problems would become. To combat this, the day tank feeding the size press was routinely dumped to the sewer, and fresh, clean starch was introduced, and the problems would be minimized.
PROJECT OBJECTIVE. Since the root cause of the problems appeared to be debris buildup, the goal was to decrease the amount of debris present. Once this new level of cleanliness was established, it could be determined if any of the problematic issues were improved upon in a measurable quantity.
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The compact size of the Pulse Purge system (filter skid shown on left, hydraulic skid shown on right) lends itself to cramped areas that don't have room for large equipment.
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The existing filtration system utilized 0.006-in. (150-micron) slotted wedge wire. When finer media was used, excessive operator attention was needed to maintain the screen openness and adequate flows. Even at this slot size it was necessary to physically pull and clean the media at a minimum of once per shift. If this was not done, the screens would have a tendency to blind over and restrict flow to the size press.
Lab testing indicated that 0.002-in. (50-micron) filtration would remove a large percentage of the sludge and fiber present in the recirculating starch system. Therefore, an automated system that could filter at or below this level would achieve the desired conditions. An AES Engineered Systems Pulse Purge filtration system was chosen for a trial. This system provides automated operation, minimal operator attention, and 50-micron filtration. With the standard barrel pressure filter design, it could be placed directly into the system without modifying piping runs.
The trial equipment used at the mill was designed so that during filtration operation, fluid is fed via pressure through an appropriate number of barrels containing non-disposable filter elements. When either a timer or differential pressure setpoint is reached, the unit initiates a backwashing sequence. One barrel is taken offline at a time for backwash, leaving the remaining barrels to process the fluid without interrupting flow to the final end use. For backwashing, the barrel is drained while a water cylinder is filled with a fixed volume of backwashing fluid. This fluid is pressurized via a hydraulic skid, independent of the filtration process.
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FIGURE 1. During mill trials with the filtration system, splice ratios declined significantly but rebounded after the system was removed.
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Upon releasing this pressurized fluid into the now drained barrel, the energy equivalent of a 150-hp pump delivering more than 800 gpm for one second is utilized to remove the debris from the element. This low volume/high energy balance allows for finer filtration than previously allowable. For trial purposes, AES Engineered Systems provided an eight-barrel test skid.
MILL TRIALS. During an initial one-month trial, the filter system used 0.002-in. media. Either a differential pressure switch or timer could initiate automatic backwashing. Due to the much finer than industry standard level of filtration being trialed, a one-hour backwash initiation time was programmed to ensure adequate backwashing frequency.
Due to the closed loop configuration of the size press, as soon as the finer media was put online it began removing debris, and the starch was observed to be visually cleaner. After equilibrium was reached with the new level of filtration (0.002-in. instead of 0.006-in.), a step improvement in several key parameters was measured. Sludge in the run tank was reduced by 76% from pre-trial data, and, as a result, the pre-coat solids were increased from 9% to 12%.
One of the most significant improvements was the reduction in sheet holes and roll splices. Print testing of the sheet under the new conditions showed improvements in various visual ratings and physical testing. The need to dump the size press tank several times daily was eliminated. As a result, the mill starch sewer losses decreased by 260 gpd after the filter system backwash losses were taken into account.
During normal operation, the filter backwashed routinely once per hour as per the programmed setpoint. Flow to the size press was uninterrupted during backwashing and normal operation. During machine breaks or other upset conditions that introduced higher amounts of debris into the starch, the filter would initiate a backwash cycle due to differential pressure. This ability to backwash when needed resulted in the filter being able to maintain starch cleanliness without blinding the screens over.
Following an upset, the automatic controls allowed the filter to recover easily and quickly return to "normal" backwash intervals. During the month, the pre-coat run tank was not dumped to sewer nor were filter elements physically pulled for cleaning. Elements were removed for visual inspection and evaluation of operation. The visual inspections indicated a high removal efficiency of debris, along with an effective backwash efficiency that maintained consistent media regeneration.
Following this initial trial period, the historical filter system was put back online with 0.006-in. media. At this coarser filtration level starch quality diminished again, and the system required more operator intervention. To maintain a slightly comparable pre-coat starch quality as with the trial conditions, the run tank was dumped to sewer twice per shift, equating to roughly 1,000 gpd. Solids were maintained at 12% due to this frequent dumping to sewer and the continuous introduction of fresh starch. Figure 1 shows the splice ratio trend indicating the initial improvement of 23.3% to 15.0% when the unit went online, followed by its return to 23.9% when the unit was taken offline.
Following the placement of an order and prior to delivery of the Pulse Purge production unit, additional testing was done to determine if finer filtration would be possible. The 0.002-in. (50-micron) media was replaced with 0.001-in. (25-micron) slotted wedge wire, and the trial continued with the filtration system again providing uninterrupted service with an ease of operation.
Little to no operator attention was required, and the pre-coat starch quality was maintained through various machine grades. The splice ratio (Figure 1) once again showed a step improvement, dropping from 23.9% to 19.2%. As a result of this secondary trial, the production filter order was changed to include 0.001-in. media in lieu of 0.002-in. media.
RESULTS. By consistently filtering the pre-coat starch in the range of 25-50 microns, significant benefits were seen. When the trial unit was first put online, improvements were seen in overall runability and quality. After the unit was taken offline, this same step change occurred in the negative direction. The following data summarizes the tangible results of the two separate trials:
• Pre-coat size press solids increased from 9% to 12%
• Paper machine holes were reduced by 11%
• Off-machine coater holes were reduced by 11%
• Overall splice ratio was reduced by 23.7%
• Pre-coat run tank sludge was reduced by 68%
• Pre-coat sewer losses were reduced by 260 gpd
• Improvements were seen in mill print tests
• Labor savings were experienced due to de-creased filter maintenance.
To date, the filter has provided consistent starch quality at 25 microns. As with any papermaking process, the need to continually improve and optimize every system is an ongoing project. This data is strictly from the initial trial periods and does not reflect improvements seen following the installation of the production unit.
Equipment purchase justification details are confidential and were not disclosed by the mill. However, a detailed study was conducted following the trial, and equipment payback estimates led to the purchase of a five-barrel Pulse Purge system. Total purchase and installation costs can vary depending on the filter system options and other installation factors. However, the payback time on the entire filtration project was anticipated to be less than one year.
Areas that factored significantly into this payback calculation are listed below. The associated dollar values are typical industry standards that may vary and are not necessarily reflective of the exact savings associated with the mill providing the data. Note that the project was justified primarily on the sheet quality improvements and reduction of roll splices. Accurate dollar savings information is not currently available for either of these areas of improvement.
• Starch, water, and steam savings: $9,000/yr • Landfill avoidance savings: $11,250/yr • Labor savings for starch make-down, dump run tank, and filter maintenance: $12,000/yr • Sheet quality improvements for reduced holes/improved print quality: unknown savings • Elimination of 3,800 roll splices annually: unknown savings
The simplicity of installing a pressure filter in the existing piping means minor associated installation costs. No additional storage chests or pumping capability is required. This also lends itself to installation in cramped areas that do not have room for large equipment. The self-automated system means operator time can be spent where it needs to be, on the machine rather than the filter, especially during times of upset conditions and paper machine breaks.
Jonathan C. Pawlak is an applications engineer-filtration, cleaning & conditioning, AES Engineered Systems, Glens Falls, N.Y. He can be contacted at jonathan_pawlak@aestf.com.
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