Paper and Pulp

Wet End Starch Cooking

Wet end starch – Starch added at the wet end to the paper slurry can help increase the sheet strength. The starch will also act as binder for various additives and fillers such as Alum, Calcium carbonate and TiO2. When properly cooked, starch swells and its viscosity increases. Wet end starch is produced to give it a cationic charge. This allows the cooked starch natural adhesion properties to create a bond with the paper fibers. By connecting the fibers and additives in small ‘clumps’, leaving relatively clean water in between (flocculation), faster drainage of water leading to increased wire and machine speeds.

Size press starch – Many paper processes utilise a surface treatment known as sizing. Typically this is accomplished by applying a thin layer of starch during the drying of the paper in a section of the paper machine known as the size press. The function of the surface size treatment is to fill the voids in the paper sheet so any printing inks will sit on top the paper instead of penetrating into the sheet. Sized paper produces a clearer image when printed, than does un-sized, and allows for printing speeds through the printing press.

Issues Associated with Starch Cooking

• Poor slurry make-down and batch cooking of starch experiences clumping of starch molecules. This can lead to uncooked starch granules being sent to the headbox, where they either plug the wire or drop into the white water system
• Inconsistent starch viscosity can lead to build up on machine components. This starch buildup may eventually fall back onto the sheet, causing quality problems. If uncooked starch is present at the size press, it may cook on the sheet in the dryer section causing picks and tears in the sheet
• Batch cooking of starch in tanks can lead to inconsistent viscosity and batch to batch quality issues

PSX Jet Cooker Key Benefits

• More consistent starch viscosities
• Inline starch cooking allows for better response to process changes such as solids (%) changes
• OptiShearTM design allows for higher solids concentrations of starch slurry to be cooked
• Adjustable condensing tube position allows for pressure drop adjustments to be made
• Jet Cooker design promotes better mixing of steam and starch slurry for better cookout
• Reduced starch consumption reduces costs. Better drainage reduces maintenance, lower white water contamination and improved product quality
• Reduces localised wear issues associated with traditional Jet Cookers

PSX Jet Cooker Solution

The PSX Jet Cooker is specifically designed for cooking starch slurries. The unique internal design of the Jet Cooker allows wet end starch slurry to be cooked at higher concentrations (6-10%) and higher temperatures of 220°F – 260°F (104°C – 127°C). Size press starch slurry can also be cooked at higher concentrations (up to 35% solids) and higher temperatures of 220°F – 320°F (104°C – 160°C). The PSX Jet Cooker design promotes a very thorough mixing of the steam and starch slurry, effectively cooking all of the starch instantly. The turbulent mixing affect breaks up clumps that would pass through other steam heating devices or batch processes. This helps to ensure that the starch entering the headbox is completely cooked and extremely consistent in viscosity and temperature.

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Green Liquor Heating

Kraft pulp mills produce pulp from wood chips. Within the pulp production, a chemical process produces green liquor. Green liquor is a combination of recovery boiler smelt and dilute white liquor. Dregs are removed and liquor is sent to recausticising tanks to produce white liquor. Flows vary based on the size of the pulp mill and can range from 350-1400 gpm.

Recausticising is a two stage process used to recover chemicals from in the pulp production process. First stage reaction occurs with lime in a highly agitated vessel known as a Slaker at high temperatures 180°F – 215°F (82°C – 102°C). The second stage reaction occurs in a series of agitated tanks known as Causticizers where the reaction is completed.

Process Heating Challenges

Green liquor must be heated prior to the lime addition and temperature plays a key role in the chemical reaction.

• Sparging also has a tendency to create ‘hot spots’ in the slaker or causticisers, resulting in uneven reactions within the vessel. This can cause incomplete regeneration, chemical carryover or excessive reaction times. Sparging can also exaggerate dust formation from the vessel during operation producing maintenance issues
• Heat exchangers are subject to damage as green liquor is corrosive at elevated temperatures. Stainless steel welds, in particular, are susceptible to stress corrosion which can lead to shorted heat exchanger life and cross contamination between the steam and liquor
• Eductor style steam injectors with limited turndown (2:1) are prone to steam cavitation and accelerated wear

A PSX heater can be installed in-line upstream of the slaker. The PSX heater assures high velocity steam injection for rapid and complete condensation of the steam via our internally modulated steam injection design. Our Radial Multi-port Jet Diffuser assures uniform heating of the liquor. This results in a more uniform causticising reaction, thus reducing lime costs, and allowing better control of the process. The end result will be higher quality white liquor for use in the digesters and reduced operating costs. The PSX heater can also be supplied in appropriate metallurgy to address the thermal cracking that can occur when heating green liquor.

Key ProSonix Heater Benefits

• Fewer process upsets and reduced steam vibration from more efficient internally modulated steam injection
• High liquid turndown up to 10:1 to match the mill’s operating conditions
• Precise temperature control of +/- 1°F (-17°C) produces a higher quality liquor with less carry over
• Lower production costs from more efficient slaker operation and lower lime feed costs
• Reduced maintenance costs and downtime with appropriate metallurgy and elimination of steam cavitation

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Whitewater Heating

White water is a fine particle slurry used throughout the paper making process. Its name derives from fine fiber particles in the water that give it a milky white appearance. White water drains from the paper stock into a pit below the paper machine and is used for a variety of uses in the paper making process.

White water systems must periodically be cleaned to remove bacterial build up that occurs over time. This process is known as boil-out, in which the white water is raised to near boiling temperatures of around 180°F – 190°F (82°C – 88°C) to kill any bacteria in the system. A caustic solution is sometimes added to shorten the process and increase the effectiveness. The water is held at temperature until all bacteria is killed.

White water systems are typically heated by heating tank contents with steam spargers in the white water chest. Steam Injection Heaters that use a steam pressure reducing valve (PRV) experience a pressure drop in the steam pressure which leads to low velocity steam flow. Low steam velocity leads to poor steam mixing. Externally controlled steam injection heaters may lead to:

• Ineffective condensation of the steam from the spargers, damaging the chest walls leading to expensive repairs
• Steam escaping from the tanks and venting to atmosphere
• Hot and cold zones to be present in the whitewater chest, leading to variable stock temperatures at the paper machine head box

PSX Heater Solution

A PSX Jet diffuser heater is installed in the white water system to quickly raise the white water temperature to near boiling and then throttle back to maintain the desired temperature.

ProSonix unique method of steam injection utilises an internal steam control to precisely deliver the appropriate mass flow of steam for the required heating. This is achieved via and integral Pneumatic Actuator, and a variable position stem plug in the steam jet diffuser. We do not throttle or regulate steam pressure. This design offers a precise method of steam control through a choked flow control delivery of the steam. Choked flow is the phenomenon of accelerating a vapor to maximum velocity by creating a pressure differential through an engineered nozzle. By establishing choked flow, the steam mass flow can be metered to precisely control the heating of the liquid.

Key Direct Steam Injection Benefits

• Energy savings resulting from faster tank time and reduced heat loss to atmosphere
• Lower maintenance due to the PSX heater’s self cleaning design
• Improved safety due to better steam injection heating methods (elimination of steam hammer)
• Reduced operating costs by reducing costly machine down time
• Faster tank heat up time (~ 6-8 times that of traditional sparging) equals less machine downtime

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