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 THE CHALLENGE
The pulping process typically operates with excess heat, while the papermaking
process has to input heat in many places. Many of these facilities operate
separately and often waste the excess heat (from the pulping side) and/or are forced
to heat up various cold streams (on the papermaking side) to function efficiently.
For the facilities that operate both a pulping and papermaking process at the same
or adjacent sites, the two processes are “decoupled” due to incompatible chemistries
or fouling issues which challenge the efficient recovery of heat.
The stock passes through the pulping and washing steps, and is then dewatered and
sent to the papermaking process. The heat from the pulping process stays with the
water and returns to the pulp mill; the chemistry of the water is not compatible
with the paper making process, so operators strive to minimize carryover of any
pulping water to the paper machine. Heat builds up in the pulp mill and leaves
via hot humid vents and hot wastewater discharge.
The stock is diluted before it is sent to the paper machine forming section and
its viscosity effect governs paper mill speed. If the temperature of the diluted
stock is low, the viscosity of the dilution water will be relatively high and the
resultant drainage and paper mill speed will be slowed; on the other hand, if the
temperature of the stock is raised, the viscosity of the dilution water drops and
enhances the drainage in the paper mill forming section, thus allowing higher paper
mill speeds.
The Answer
For combined Pulp and Paper facilities, the Direct Contact Inc. (DCI) Heat
Recovery System overcomes the limitations of traditional heat recovery equipment
and captures the heat contained in the pulping process vent. Using a two-stage
version of the patent-pending design, the DCI Heat Recovery Vessel’s first chamber
captures particulate and condenses “high boiler” contaminates before sending the
gas stream to the heat recovery chamber (second stage). The custom-designed unit
then recovers both the latent heat and the sensible heat from the vents and
transfers it to the paper machine whitewater.
The Direct Benefits
The overall heat efficiency of the pulping and paper making processes both
increase dramatically while the waste streams emitted from the processes are
reduced as well. Power savings can quickly add to offset the initial purchase
and installation cost of the Direct Contact Inc. (DCI) Heat Recovery System and
start to contribute to the mill’s bottom line profitability. Additional benefits
can be realized when other hot water requirements are considered (e.g. Bleach Plant
Washers, preheating of Boiler Feedwater, ect.).
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