The viscosity of black liquor determines its handling properties, and is a critical parameter for the control of a recovery furnace. Viscosity is also an important factor in black liquor evaporation, as it affects the heat transfer rate. An increase in viscosity can lower the evaporation capacity. Softwood black liquors (e.g., pine) generally exhibit near-Newtonian behaviour (i.e., viscosity is independent of shear rate) but some hardwood black liquors (e.g., birch) can be thixotropic (i.e., viscosity is both time and shear dependent).
The viscosity of black liquor is affected by such factors as the wood species, the solids content of the liquor, the cook’s kappa number, the residual alkali and sulphidity of the liquor, as well as the temperature. As high solids firing can improve a boiler’s thermal efficiency, increase liquor throughput and reduce environmental emissions, many mills have increased their as-fired liquor solids concentration. However, black liquor viscosity increases significantly at solids concentrations over 70%, where NOX emissions also increase. If a mill is not equipped with a pressurized liquor handling system, in which high temperatures can be used to reduce viscosity, a viscosity reduction method may have to be employed in order to facilitate high solids firing. A viscosity of 200 cp and over is considered high, since 200 cp is a common practical limit for centrifugal pumps.
The simplest and most cost effective way of keeping viscosity in an optimal range (if the solids are not too high) is by alkali profiling. All you need is a suitable benchtop viscometer for black liquor, and a reliable way of measuring residual effective alkali (REA) and black liquor solids. To control black liquor viscosity in real time, you would need an on-line viscometer (or an on-line probe which provides an indirect measure of viscosity) and an on-line REA probe in a control loop.
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Could you tell me what is the method for measuring Black liquor viscosity, please. Thank you!
Black liquor viscosity can be measured directly using an on-line viscometer. Alternatively, an off-line, bench-top viscometer can be used. Viscosity can also be measured indirectly by monitoring the amperage of the black liquor recirculation pump. (see post for Jan 18, 2009).
Could you please let me know the range/value of Black Liquor viscosity generaly used in Pulp & Paper
The viscosity of black liquor depends on the wood species pulped, the solids content of the liquor, the nature of the organics, the amount of residual alkali, the temperature and the shear rate. The viscosity of softwood black liquor is generally higher than that of hardwood. A viscosity of 200 cp and over is considered high. The range could vary between 30 cp (hardwood) to 220 cp (softwood), at 70% solids and 120 C at a shear rate of 250/s.