Chemical Recovery in Pulp and Papermaking by Pira International Ltd

Chemical Engineering

By Pira International Ltd

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8). 8 Summary of evaporator types Film evaporators  Rising-film type  Long-tube-vertical type  Falling-film type  Long-tube-vertical type  Plate-type  Horizontal-tube – spray-film  Combination type  Rising-/falling-film  PFR* type (P = preheat, F = falling film, R = rising film)  Short-tube-vertical type Natural circulation evaporators Special evaporators using forced circulation  LTV with recirculation  Forced-circulation type  Integrally mounted vertical-tube section in single pass  Externally mounted tube section in single pass or two passes:  Horizontal tube  Inclined tube  Vertical tube  Evaporator/crystalliser Note: *PFR is a registered trademark of the Unitech Division, Ecodyne Corp.

A proprietary spiral rib-type design was developed and tested. The design has allowed for an increase of 50–75% in the heat transfer coefficient, and a reduction of 50% in power when compared with the conventional forced-circulation design (Rieke 2002). In the US, Weyerhaeuser fires up to 80% solids liquor in the recovery boilers at several of its mills. As such, reliable and economical evaporation technology is required to meet the concentration and throughput demands for black liquor consistently.

However, the supersaturation developed in the tube of a forced-circulation unit is lower than that in a falling-film system because boiling within the forced-circulation tube is suppressed. In addition, the temperature rise through the heater is carefully selected to minimise any adverse temperature effects (sodium carbonate and burkeite salts exhibit an inverse solubility). The high tube velocity typically required for the forced-circulation design also counters the effects of high viscosity via shear thinning and increased turbulence.

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