BACKGROUND
Pulping processes typically operate with excess heat, while papermaking processes require heat in many places. Many facilities operate these processes separately and often waste excess heat from the pulping side and are forced to heat up various streams on the papermaking side. For facilities that operate both pulping and papermaking processes, the processes are not coupled, due to incompatible chemistry or fouling issues, which challenge the efficient energy recovery.

Stock passes through pulping and washing steps, and is then de-watered and sent to papermaking. The heat from pulping stays in the water and returns to the pulp mill, since the chemistry of the water is not compatible with papermaking. Operators strive to minimize carryover of pulping water to the paper machines. Therefore heat build-up from the pulp mill is discharged through vents and to drains.

Stock is then diluted before being sent to paper machine forming sections and its viscosity affects the paper mill speed. If the temperature of the diluted stock is low, the viscosity of the mixture is 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 mixture drops and enhances the drainage from the paper mill forming section, thus allowing higher paper mill speeds.

SOLUTION
For combined Pulp and Paper facilities, the Direct Contact LLC Heat Recovery System overcomes the limitations of traditional heat recovery equipment and captures the energy contained in the pulping process vents. Using a two-staged version of the patented design, the DCLLC Heat Recovery Vessel’s first chamber captures particulates and condenses high-boiling contaminates before sending the gas stream to the heat recovery chamber (second stage.) The custom-designed unit then recovers both the latent and sensible heats from the vents and transfers it to the paper-machine whitewater.

DIRECT BENEFITS
The overall heat efficiency of the pulp and paper making processes increase dramatically, while waste streams from the processes are reduced. Energy savings quickly add up to offset purchase and installation costs of the DCLLC Heat Recovery System and contribute to the mill’s long-term profitability. Additional benefits are realized when other hot water uses are considered (e.g., Bleach Plant Washers, pre-heating boiler feed-water, etc.)