In the paper-making cycle of Kraft Paper Mills, wood chips undergo a "cooking" process under pressure in a liquid solution called white liquor to pulp the wood. White liquor is just one stage in a series of chemical reactions, including green and black liquor. Proper preparation of green liquor is key to making good white liquor. Smelt from the recovery boiler and water or weak wash combine to form green liquor which is processed in the slaker and through the causticizers where it produces NaOH (sodium hydroxide) which is a key component of white liquor. Lime mud (calcium carbonate, CaCO3) is also formed. The white liquor and steam is added to wood chips to create pulp for paper, and the waste from this process yields black liquor. Black liquor is sent to the recovery boiler, generating heat for steam and releasing smelt. When mixed with water or weak wash, smelt becomes green liquor, restarting the cycle.
Precise temperature control is crucial for efficiently converting green liquor into NaOH and calcium carbonate end products in the Kraft mill process. The green liquor feed temperature to the slaker must be optimized between 180˚F to 200˚F (82˚C to 93˚C) without causing a boil-over. Failing to reach the proper temperature can lead to poor sodium hydroxide conversion, reduced chemical recovery, and an increased need for makeup chemicals. Hence, re-evaluating green liquor heating efficiency is essential.
Importance of Perfect Temperature in Green Liquor Production:
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Insufficient conversion of green liquor to sodium hydroxide (white liquor) requires more makeup sodium hydroxide, affecting the causticizing process.
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Lower overall latent heat in the chemical cycle affects white liquor temperature and cooking efficiency of wood chips in the digester, leading to higher steam demand.
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Green liquor dropping to temperatures around 160-165°F (71°C-74°C) forces reliance on the exothermic reaction in the slaker for sodium hydroxide conversion.
- Cold green liquor necessitates more fresh makeup lime, leading to increased costs due to inefficient conversion and the high price of fresh lime and caustic.
Cold green liquor necessitates more fresh makeup lime, leading to increased costs due to inefficient conversion and the high price of fresh lime and caustic.
The most responsive and efficient method for heating the green liquor feed to the slaker is inline DSI (Direct Steam Injection) with a Hydro-Thermal Hydroheater.
Due to green liquor's corrosive nature, conventional surface-contact heat exchangers, such as shell and tube or plate and frame designs, are used. These heat exchangers are susceptible to pinholes or ruptured elements, and the condensate is often sewered to prevent contamination of the boiler feedwater. The sensible heat lost to the sewer in the condensate can be 15-30% and lends itself to a DSI heating method where 100% of the steam energy is transferred into the green liquor. In addition to corrosion, the green liquor scale can significantly reduce heat transfer surface efficiency, resulting in oversized units to account for the loss of efficiency between cleaning or replacement.
Internally modulated steam inline devices like the Hydro-Thermal Hydroheater/Jetcooker can take full steam pressure to the nozzle or orifice. Steam flow is controlled through a heater's stem plug and nozzle assembly. Changing the nozzle's steam discharge area directly varies the steam mass flow. Internal modulation eliminates the need for an external steam control valve.
A linear relationship exists between the nozzle's open area and steam's mass flow into the stream by not changing the steam pressure. To satisfy this relationship, a defined pressure differential must be maintained to achieve the choked steam flow through the nozzle. Choked flow steam velocities ensure turbulent mixing at the point of contact with the process stream (green liquor) for uniform and near-instantaneous heating at the nozzle and plug, minimizing the potential for fouling build-up in the heater.
In conclusion, maintaining proper green liquor feed temperatures requires a precise and reliable heating method. Using an inline, internally modulated Hydroheater is the best choice to achieve the desired degree of temperature control for the green liquor feed to the slaker.
