Online Spectroscopic Monitoring in Recycled Stock Preparation Systems
Online spectroscopic sensors in recycled stock preparation provide continuous measurement of micro-stickies and fillers to optimize chemical dosing and yield.
Consistency Control
Stock Dilution
Continuous regulation of the mass ratio of dry suspended solids to water maintains hydraulic stability and basis weight uniformity across stock preparation systems. Automated consistency control manipulates dilution water valve positions in closed loops upstream of machine chests, refiners, and the paper machine headbox. Typical wet-end stock preparation balances furnish thickness between two and four percent solids by weight, whereas thick stock feeding the fan pump runs between three and five percent.
Shear-force, optical, and microwave sensors capture relative slurry viscosity or microwave phase shifts to establish raw signal inputs. The mechanism becomes ineffective below zero point one percent consistency, where sensor noise floor limits exceed measurable signals from dispersed fibres.
Dilution Loop
Upstream refiner performance depends upon steady mass flow through the plate gap. Feed valves inject clarified white water or fresh water into the suction inlet of stock transfer pumps, using the pump impeller to achieve instantaneous mechanical blending. A downstream sensor positioned between five and ten pipe diameters after the discharge port monitors the diluted slurry, signaling the field controller to adjust valve trims.
Delay between valve adjustment and downstream sensor registration represents the primary constraint on loop tuning. Excessive dead time leads to cyclic basis weight variation along the machine direction of the finished reel.
Operational Boundaries
Temperature shifts, furnish freeness fluctuations, and variations in filler content distort indirect shear measurements. Microwave transceivers eliminate mechanical drag biases caused by freeness variations, yet high-frequency measurements remain sensitive to free air bubbles entrained in virgin mechanical pulps. Proper degasser operation upstream of the measuring cell shields the calculation from gas-induced errors.
Correct stock thinning keeps refiner specific edge load uniform, suppressing fibre cutting while optimizing internal fibrillation.