Optimization through storage, make down, and feed.

To win the Indy 500, it's not enough to drive a performance vehicle; you also have to know how to handle it. The same is true of performance chemicals. Over the years in wet end chemistry and paper machine operations, it has been this author's experience that the attention paid to the storage, make down and feed of performance chemicals is often too little and misdirected. This lack of focus is a major factor in chemical deposits, chemical inefficiencies, lowered machine efficiency, and poor product quality.

There are many generally accepted norms for the handling of performance chemicals. For our discussion, let's assume that the mill has a good fit of chemical function to capabilities, process needs, and product needs. In this scenario, the mill and the performance chemical supplier have agreed to chemical specifications, and the chemical has been delivered in accordance with those agreed-to quality specifications. This process of agreeing to chemical function, mill needs, process capabilities, and chemical specifications is continuous, which allows both parties to meet changing opportunities within the mill. The accepted performance chemical is in the mill's storage system, where our discussion begins.

STORAGE

The delivered product can be either solid, liquid, or emulsion. Chemicals are either dispersed or not. Some are sensitive to biological attack and some are not. Some are sensitive to heat or moisture, some are not. Once the chemical is accepted into the mill, it is the mill's responsibility to locate it in a staging area or tank that meets that particular chemical's specified storage requirements. Almost all performance chemicals deteriorate or lose efficiency over time (some faster than others), so keep inventories low and use them in a first in-first out manner.

Once the liquid or emulsion has been placed in its storage tank, agitation is important. Most liquid and emulsion chemicals have some degree of shear sensitivity, so slow agitation with large impeller blades is preferable to high speed and small impellers. The impeller should always be submerged and tank levels maintained to prevent splashing and chemical dewatering. Shear is a trigger for deposit formation, so large and slow impellers are less likely to have chemical deposits form on them.

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The tanks should be clean, with no chemical residue on the inside surfaces of the vessel. Tank boil outs should be performed regularly, followed by a good visual inspection to determine boil out effectiveness. In some cases where moisture is a concern, the tank should be padded with dry air or dry nitrogen. Liquids, emulsions, and/or their dispersants are potential foods for biological activity; therefore, treat stored chemicals with the recommended level of biocide to prevent bioactivity. Pump stored chemicals with low shear pumps to prevent chemical degradation and deposit formation.

Solids storage has a different set of needs. Because dry solids in bags or in a bulk tank can get dirty, mills must prevent contamination. Also, solids can easily become contaminated with water or moisture; enough moisture can even be picked up to alter the percent solids and thus the amount added to the process. Moisture can also change the solids particle size distribution and surface area. Keep them clean, dry, and in moderate temperatures.

MAKE DOWN

Liquid make down equipment and systems should be as small as possible to prevent long storage times, which can degrade the chemical and lead to inefficiency and chemical deposits. Longer storage times can also cause settling out, separation, evaporation, contamination, and biological attack.

Agitators should be low shear and submerged at all times. Low shear prevents chemical degradation and deposit formation, while being submerged prevents air entrapment. Dilution water quality is critical for successful make down. Water should be warm and clean. There should not be any color, solids or charge in the water that will interact with the chemicals. Warm water allows good solubility, whereas cold prevents good solubility and hot promotes chemical degradation. For make down, water volume should be at the supplier recommendation, and preferably more; excess dilution water on make down promotes better chemical solubility, efficiency, and distribution. If the mill or machine is going to be down for several hours, remove chemicals subject to time deterioration from service. Start up with fresh chemicals to maintain efficiency and prevent deposits.

Solids made down with water follow the same recommendations given above for liquids. There may be a longer agitation time for solids before they are completely in solution or dispersion. Eductors, often used in solids make down, are vital for wetting out and uniform distribution of solid particles. They must be well maintained and kept clean.

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Solids that remain as dispersions rather than going into solution tend to stratify, separate, or settle out, so agitation is required at all times. Since adding a solid directly tends to be non-uniform with poor distribution, solids should always be diluted out prior to machine addition. Solids often come with a supplier-added dispersant and/or biocide, so check for particle size distribution and bioactivity to verify these additives are working.

FEED

Now that the liquids, dispersions, and solids are made down, they must be added to the machine or process. Mistakes introduced between the made down chemicals and the machine can lead to inefficiencies and deposits. Tank agitators should be low shear and submerged; pumps should be low shear positive displacement; and dry air or nitrogen padding used where necessary.

Dilute the chemicals with clean water as much as possible prior to machine entry. This dilution should be at least 10 to 1--more, if possible. It should not be added to the run tanks but rather to the chemical line just prior to machine entry. This post dilution allows smaller run tanks and pumps, less dwell time, less chance of chemical degradation, less negative interaction of chemical and water, and easier wash up/clean outs at down times.

The pipe run from the tank to the process should be short and without dead spots to minimize chemical stagnation. Chemicals should never be added to the top of a chest because distribution is poor. If chest or tank addition is used, always add below the fiber surface where there is good agitation. If adding into the stock or water stream within a pipe, use a quill or distribution ring. A quill is preferred and should be designed by an engineer to match up with flow dynamics. Quills should be highly polished and of proper design to completely distribute the chemical without the possibility of fiber accumulations and break offs. Distribution rings are the next best method. These rings should be designed to match flow dynamics and never inject at the pipe/stream interface. This interface is a location of laminar flow, and poor mixing will take place. The points discussed above on temperature, time, dirt, bioactivity, etc. are also applicable for this feed section discussion.

SCREENING AND CLEANING

Performance chemicals are frequently screened and/or filtered as they are received, made down, and fed. Screening is a "band-aid" that proper handling can eliminate. However, since it is difficult to have a perfectly clean system, screening and cleaning will prevent chemical deposits and poorly dispersed chemicals from reaching the process.

Maintenance is also critical for storage, make down, and feed systems. Agitators that are not working or are not submerged, tanks that are not filled, open hatches, not boiling out, biological activity, evaporation, and contamination can all lead to deposits, chemical inefficiencies, increased breaks, off product quality, higher energy consumption, lower speeds, harder drying, and many more machine problems.

Mills frequently rely on suppliers to perform some maintenance/cleaning activities. In these arrangements, mill personnel should, at the very least, work with suppliers and ideally they should be directly responsible for system checks and maintenance.

SEQUENCE AND ADDITION POINTS

Now that the performance chemicals have been stored, made down, and fed properly to the machine, let's consider where they are added. Each chemical has a special function on the machine, whether for color/shade, biocide, strength, sizing, retention, drainage, defoamer/deaerator, pigment/filler, or charge. Chemicals should be added to a process stream that best allows the accomplishment of their function with as little interference from other factors as possible.

Each chemical should be added separately and with one shear point before the next additive. They should be added as dilute as possible and into as much turbulence as possible. Following these suggestions will help insure proper distribution and best efficiency for each additive. Below is a brief discussion of widely used chemical classes. The discussion is meant to share ideas that have worked, but does not imply that these suggestions will work in every case.

Color/shade control: Add 80% on the thick stock as far back in the system as possible. Add the other 20% from the stuff box forward for trim and grade change ease. Dyes can be mixed prior to machine addition, but only if recommended by the supplier. Some are compatible and some are not.

Strength: Experience here is mainly with starch, but dry strength and wet strength agents can be considered in a similar way. These additives are very substantive and are rapidly adsorbed on the fibers. There is little if any need to add them far back in the system to allow time for adsorption. Add them close to the headbox to limit the amount of shear from the pumps and cleaners. Another reason to add close to the head box is that they frequently assist retention and drainage.

Sizing: Synthetic alkaline sizes hydrolyze in water; this hydrolysis deposit increases as time and temperature increase. Thus, they should be added close to the headbox and prior to the high surface area of the pigments and fillers. This places them on the fibers where their function is best served. Some mills add size with the starch, which helps fix the size to the fibers. There are several different types of alum/rosin size and their locations depend on the type used. However, modern rosin/alum sizes are substantive enough to fibers that they should still be added up close to minimize the formation of alum/rosin insoluble deposits.

Retention: There are many retention packages, so I cannot suggest points of addition and sequence. This discussion focuses on retention aid shear sensitivity. If the retention aid is shear sensitive, it will be degraded with agitation, so add it close to the headbox to minimize exposure to machine pumps and screens/cleaners. If it is not shear sensitive, addition farther back in the thin loop is usually OK. Addition closer to the headbox promotes flocculation and harms formation.

Drainage: Drainage is affected by the retention package, but there are specific chemicals designed for drainage. Fines and small particles affect drainage the most, so the drainage aid is most effective when added directly to the wire pit white water or the save all cloudy water.

Defoamer/deaerator: Air--whether entrained, dissolved, or froth--is usually caused by the soluble organics and dispersants in the water phase. These chemicals are most effective when added to whitewater systems. Minimize the use of these products, which frequently show up in deposit analysis. Titrate these additives to their minimum use for entrained air control.

Pigments/fillers: Close after the centrifugal cleaners is the best location for these additives. The farther back they are, the more other additives coagulate them, making them less optically effective. Studies have shown that 10 to 20% of the added pigment/filler is lost on each pass of the centrifugal cleaners and other screens. Up close will allow good retention with the proper retention aid package selected, and provide the best optical properties with the lowest sewer loss. Add them after the sizing chemicals to keep size use at a minimum and on the fibers.

Charge control: These additives are often used to neutralize high charge on the machine so that other retention aids/additives can be used more efficiently. If used, these additives should be added to the stream that needs its charge reduced, not to the total furnish stream. Titrate to a minimum use for the lowest possible cost. Turbidity and/or charge tests can be used to determine the best treatment dose.

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The principles covered in this article have proven effective over the years to improve machine productivity and product quality.

RELATED ARTICLE

IN THIS ARTICLE, YOU WILL LEARN:

* Why chemical handling is critical to optimal performance.

* Specific guidelines for chemical storage, make down, feed and more.

* Recommended addition points for specific types of performance chemicals.

ADDITIONAL RESOURCES:

* "Synergistic effects from performance chemicals," by Kasy King, Solutions! April 2004. To access this article, enter product code 04APRS045 into search field on www.tappi.org.

* Managing Chemical Safety, by D.G. Nelson; available through TAPPI Press. Enter product code 08MCS into search field on www.tappi.org.

ABOUT THE AUTHOR

Kasy King is principal of Papermaking Process Consulting LLC, Appleton, Wisconsin, USA. Prior to founding his consultancy, King had a long career in the paper industry, including Scott Paper Co., SD Warren, Appleton Papers, and James River. He is a member of the Solutions! Editorial Board and the TAPPI Journal Editorial Board. King can be reached at [phone], or by email at [e-mail].