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Water Conservation Study, Redesign Aims at Limiting Impact While Boosting Production

By AL WATTERS

Case study shows how one multi-machine integrated mill was able to implement a water savings strategy with little operational risk on the machine production line

Environmental regulations, opportunities to save on chemical and steam costs, along with the general desire to "be a good and responsible neighbor," make it necessary for pulp and paper facilities to limit their environmental impact by reducing the amount of fresh water a mill uses and discharges. In most cases, environmental permits for increasing production capabilities actually require a reduction in effluent volume, concentrations, and, possibly, temperatures.

This article presents the process of implementing a portion of a millwide water conservation plan as part of a paper machine upgrade. The paper machine to be upgraded is one of several in a fully integrated bleached kraft mill. The project includes installation of major wet end machine track components and auxiliary equipment. The instantaneous production will increase by a minimum of 20% and a maximum of 38%, depending on the grade produced.

STUDY PHASE. The first step in the process of developing a plan for mill water conservation was through a millwide study of the water systems. The purpose of this initial study was to investigate where fresh water was used in services where current technology would allow the use of another type of process water through direct replacement or process modifications

Click here to see enlarged version FIGURE 1. At the case study mill, trends indicated that on an average daily basis, the central white water chest was overflowing approximately 9.5 million liters per day (lpd), or approximately 6,625 lpm.

Interactions between different "major" process areas were also studied to understand the dynamics of these processes. Studying both long-term averages and short-term dynamics would allow a more complete understanding of how water conservation in one area of the mill might impact other areas of the mill.

In this mill, the one major point of in-teraction between the paper machines and the bleach plant and pulp mill was the central white water collection tank. Excess white water from all the paper machines was sent to this tank. The bleach plant and pulp mill could pull water from this tank as required. Excess water from the tank overflowed to the sewer.

Long-term trends indicated that on an average daily basis, the tank was overflowing approximately 9.5 million liters per day (lpd), or approximately 6,625 lpm (Figure 1). While investigating the actual dynamics on a short-term basis, the tank was found to be in an overflow condition about 50% of the time and in fresh water makeup condition the other 50%. This meant that process swings in the paper mill and bleach plant could totally reverse the water balance. At times the paper mill produced more water than the bleach plant and pulp mill required, and at others the paper mill couldn't satisfy the demand of the bleach plant and pulp mill.

During overflow conditions, the overflow was, in fact, twice the average, or 13,250 lpm rather than 6,625 lpm. With these process conditions, one liter of fresh water saved on the paper machine only reduced mill water use by 0.5 liter. In other words, the only time fresh water reduction on the paper machine actually reduced overall mill consumption was when the tank was in the overflow condition.

To realize the full benefit of saving water on the paper machine, the bleach plant or pulp mill would have to complete a companion project that would reduce an equivalent amount of water. Again, this companion project would only save the mill water 50% of the time. From a capital standpoint, the mill would have to pay to reduce two liters of water in two separate locations to yield one liter of water reduction because of the process dynamics.

As a result of the study, projects identified as potentially having a cost-effective solution for the reduction of fresh water were estimated separately. The mill evaluated and prioritized each project based on a cost/liter of water savings. Specific projects were then chosen for appropriation grade estimates. Decisions were made on which projects to proceed with into detailed design based on the result of the detailed estimate project cost and cost/liter of water savings.

During the project study, the specific plan developed for this paper machine was removal of all fourdrinier and press section external showers from fresh water except high-pressure showers. The existing showers would be replaced with new showers designed for a high quality white water. The water conservation plan using the above criteria would reduce continuous water use by 1.5 million lpd, or 1,040 lpm.

DESIGN PHASE. Because the original water conservation study was done without the knowledge of the capital project and subsequent impact on fresh water requirements, the study concepts for water conservation were used only as a guide. The mill agreed that the original goal of reducing fresh water use by 1.5 million lpd was still valid, however. A plan to re-evaluate the water conservation concept was developed using the following criteria:

• Minimum impact to the papermaking process

• Maximum benefit to the application in reuse of a water source

• Minimum capital cost to reuse water

• Maximum flexibility of the designed systems for future expansion

• Maintain the original water conservation goal.

The paper machine's existing fresh water systems were investigated in detail to determine where fresh water was currently being used. A material balance was done to account for all the water used from that machine's fresh water header. This balance was then modified to account for services that would be deleted or modified because of the rebuild.

New shower or water requirements, including all process and cooling water services for the rebuild, were added back into the balance. To start with, it was assumed all new water requirements had to use fresh water (worst case). This would allow for a direct comparison of the true theoretical fresh water increase as a result of the new process equipment being installed.

The most significant changes in water requirements because of the machine upgrade included non-contact cooling water requirements (hydraulic unit cooling and lube oil cooling) and showers added for the top wire former. The new press section showers were about an even trade-off with the existing press section showers.

ANALYZING RISK. Using the mill's criteria for showers acceptable for substitution of white water for fresh water, showers were identified as to relative risk for use of white water. The showers that had the highest relative risk to the paper machine operation were the press section doctor and lube showers. (High-pressure needle showers were not an option according to the mill's criteria.) Showers run on white water in the press section can cause significant problems, even on these low-pressure showers. For example, if uhle box lube showers plug, this may create a dirty streak in the felt, ending up as a moisture streak in the sheet. Doctor lube showers could cause the doctor to wear the roll surface if a nozzle plugged, eliminating the doctor blade lubrication.

Because the results of the initial water balance indicated that the goal might be reached without total replacement of fresh water showers with white water, options for both white water and fresh water were obtained from the vendor. Typically, showers that run on white water require more flow than those that run on fresh water. Orifice sizes are larger for white water in an attempt to size the orifice large enough so that the fines and fillers won't plug the nozzles. Table 1 depicts the difference in shower flows for the same application as specified by the vendor for fresh water versus filtered white water (100 mesh screen, less than 100 ppm).

The difference in the shower requirements using filtered white water versus fresh water was required to understand the potential fresh water savings of each shower. This saving was used to try and justify the operational risk of using white water for press section applications. After discussions with the mill, the decision was made to try and keep the press section running on fresh water to maximize press cleanliness and efficiency and to minimize risks. This decision was based on the assumption that the total system design could still achieve the water savings goal.

MAXIMIZING HOT WATER REUSE. Based on the compilation of all the new water requirements, using the data for press section showers running on fresh water, the first area investigated for water savings was non-contact cooling water services. Reuse of this water has the minimum impact to the papermaking process and can have the largest benefit, since additional heat on certain showers provides better showering results and can reduce chemical precipitation from thermal shock.

The mill currently has a cooling tower system that provides a closed-loop source of cooling water for the paper machine chiller and other HVAC services. There is a minimum amount of process equipment cooling water on this tower. An option existed to expand this cooling tower system and upgrade the pumps and the headers to accommodate the additional load. The downside of this option was cost and loss of potentially beneficial heat in certain service areas. The decision was made to try and incorporate the available hot water back into services that could utilize the water prior to spending additional capital to provide a closed loop system for the new non-contact cooling water services.

Services that could use a source of heated water (and benefit from it) were high pressure needle showers, press uhle box showers, chemical cleaning (low pressure) fan showers, and doctor showers. The only hot water that was reclaimed to a hot water tank was the steam and condensate system condenser. This machine also received the hot water from another machine's condenser as well. Upon field investigation, the hot water tank was out of balance, causing approximately 570 lpm of hot water to overflow directly into the white water system.

	Click here to see enlarged version FIGURE 2. Field studies at the mill showed that the hot water tank was out of balance, causing approximately 570 lpm of hot water to overflow directly into the white water system. This information, along with the new non-contact cooling water requirements, was used to develop a new hot water balance.

This information, along with the new non-contact cooling water requirements, was used in developing a new hot water balance (Figure 2). The bulk of the non-contact cooling water requirement increase was because of the switch from an air loaded press section to a hydraulically loaded press section. Two new hydraulic units were required, along with several small lube oil units for some of the auxiliary equipment serving the new press section. The increase in cooling water demand, which in this case is fresh water demand, was approximately 510 lpm. This increase in water put the total normal available paper machine hot water at about 2,180 lpm. The concept was to put as many services on hot water as possible without designing a system that required more hot water than was normally available. Priority for hot water use was given to all high pressure needle showers, press section chemical cleaning showers, press section uhle box lube and doctor showers, and miscellaneous wet end misting showers.

The balance of excess hot water from this paper machine was sent to another paper machine that was short of hot water for use on its showers. The accounting of this water becomes a little tricky, since the water is measured as going to one machine but is effectively "passed through" the system and allowed for makeup to another. Data was obtained from the paper machine receiving this hot water to verify that it always required a minimum makeup greater than what would normally be available from this machine's hot water tank.

FILTERED WHITE WATER SYSTEM DESIGN. Based on the initial decisions made for hot water reuse and the resulting balance information, the white water filtration system design needed to satisfy the rest of the showers to allow for the fresh water savings goal to be reached. Showers to be put on filtered white water included fourdrinier wire return roll showers, top wire former doctor showers, top wire former lube showers, and the flooding nip shower (Figure 3). Chemicals that required dilution were also investigated. Most, as expected, were not able to utilize filtered white water because of the interaction between fines and filler with the chemicals, neutralizing their effectiveness. The retention aid, however, was able to use filtered white water of the quality available without adverse consequences.

FIGURE 3. Showers to be put on filtered white water during the redesign included fourdrinier wire return roll showers, top wire former doctor showers, top wire former lube showers, and the flooding nip shower.

The new showers were designed based on using white water with less than a 100 ppm concentration that had passed through a filtration system equivalent to a 100-mesh screen. The feed for this system was to be clear water from the save-all. The original design criteria supplied was questioned because of the high solids levels indicated for the save-all clear leg.

The mill investigated the data and found that the save-all performance was extremely poor when compared with past experiences. An audit was made on the save-all, and several maintenance issues were found that would cause the high solids levels. The mill committed to repairing the save-all. The save-all, when operating properly, would have significantly lower solids levels in the clear leg. Historical information was retrieved which indicated that the save-all clear leg had operated at solids levels near or below those required by the new showers. The past data was used as the new design criteria for the save-all clear leg.

There are two main types of process operations available to provide clarified or filtered white water of acceptable quality for reuse in paper machine showers. One method is through the use of a dissolved air flotation unit (DAF). DAFs have the capability to remove fibers, fillers, and fines from the water. Water quality leaving the unit can generally achieve solids levels of 50 ppm or less, with white water feed between 150 to 250 ppm of solids concentration.

The other method is through filtration or screening. Typically, solids loading in the feed water should be below 200 ppm for most effective use of these systems. Since these units only separate based on particle size, the solids concentrations from the units vary depending on the particle size distribution of the fluid.

The decision on which type of system to utilize depends on the specific showers to be used on white water and the type of grades produced on the paper machine. Typically, a high-ash content white water (from wet end ash or coating) would require a DAF system to remove the ash and prevent solids buildup in the system, yielding a cleaner operating system. Initial study estimate results indicated that the installation of a DAF unit for a single paper machine would be more expensive than a filtration system.

Both pressure filtration and gravity filtration equipment was investigated for this process. The method of filtration chosen for this application was a gravity type filter using 100-mesh media. The benefit of a gravity filter, as compared with a pressure filter, is that there is less chance of fiber or particles larger than the media openings being forced through the media. The disadvantages of this type of filter, as compared with a pressure type filter, are the space requirements and extra auxiliary equipment requirements, such as a tank and pump. Several motivating factors in this decision were:

• Bad experience the mill had with older pressure-type white water filters

• The capability to expand the system easily for addition of future showers

• The capability to selectively filter water to a higher quality with minimum additional capital if the save-all repairs did not clear up the clear leg solids issues.

Another area where the mill chose to accept some risk, at the benefit of reduced capital, was in the white water shower design. Quite often when one thinks of a shower on white water, a shower with a brush-either automatic or manual-is envisioned. The mill did not want to spend the extra money on these types of showers. The method for cleaning, if a nozzle were to plug, was removing a cap at the tending side of the shower and flushing it out.

The other design factor that can help minimize the risk in plugging shower nozzles is moving the shower farther away from the showered surface. This might allow the shower angle to increase, number of nozzles to decrease, and the shower orifice size to increase. This will allow a higher flow per nozzle and allow larger particles to pass through the nozzle orifice.

The system designed served showers with two different types of quality requirements (Figure 4). The flooding nip shower was placed on its own pumping system for two reasons: it had larger orifices, therefore allowing a higher level of solids, and it had two operating conditions (break and normal running conditions) which required two very different pressures. The other services' quality and pressure requirements were all basically the same: less than 100 ppm solids and a shower header pressure of 310 kilopascals (kPa). A dedicated pump serviced this system. All of the services on this system were constant pressure and volume services.

	Click here to see enlarged version FIGURE 4. The newly-designed white water distribution system served showers with two different types of quality requirements: a flooding nip shower that required variable pressure and other showers that required constant pressure and volume services.

Several additional considerations were used in the design of this system. The strainer chosen was slightly oversized for the existing condition to allow for the addition of more services utilizing filtered white water if the actual water use was not reduced as much as expected or more fresh water savings were desired. Also, if the save-all repairs did not eliminate the clear leg solids issues and an additional stage of filtration was required, all of the services requiring the higher level of filtration were supplied by a single pump. This system would be the only one requiring additional filtration. By breaking the system as it was done, only 50% of the normal machine filtered white water requirement would have to be improved with additional equipment.

The final white water filtration system design limited white water reuse to only less critical wet end showers and miscellaneous services, while still achieving the mill's goal for water conservation. Under normal operating conditions, a total of approximately 2,080 lpm of white water was reused for these services. The press section, with its relatively more critical shower applications, was allowed to remain completely on fresh water.

The showers in the press section, which could have been placed on filtered white water, could have theoretically reduced the fresh water demand by an additional 450 lpm. This would have increased the filtered white water system by 1,135 lpm (equivalent shower requirements if on white water). Also, from a mill balance standpoint, it may not have allowed all of the 450 lpm to be saved, further reducing the benefit of placing the press section on filtered white water.

AL WATTERS is staff engineer, BE&K Engineering, Birmingham, Ala. This article is based on a presentation at the 2000 TAPPI Papermaker's Conference and Exhibit, April 16-19, Vancouver, B.C.