Abstract
Two basic mechanisms of displacement washing of pulp fibres were investigated. Experiments were carried out in a laboratory washing cell that simulated a single stage of displacement washing. The step-change method has been employed in investigation of alkali lignin profile in outlet stream of washing effluents. The addition of urea to wash water resulted in a decrease of the liquid layer immobilized on the fibre surface. It had a positive effect on the washing efficiency expressed in terms of the void local efficiency.
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Abbreviations
- A:
-
cross-sectional area of pulp bed m2
- a V :
-
effective specific surface m−1
- B:
-
permeability coefficient m2
- C:
-
consistency of pulp kg m−3
- D:
-
axial dispersion coefficient m2 s−1
- E:
-
local efficiency
- Eɛ :
-
void local efficiency defined by eqn (6)
- h:
-
thickness of bed m
- K:
-
Kozeny constant in eqn (2)
- Lɛ :
-
mass of spent liquor in void volume of bed kg
- L 0 :
-
total mass of spent liquor in bed kg
- n:
-
number of measurements
- ΔP :
-
pressure drop Pa
- Pe:
-
Péclet number (= hu/Dɛ)
- RWɛ :
-
wash liquor ratio defined by eqn (8)
- t:
-
time s
- t m :
-
mean residence time defined by eqn (10) s
- u:
-
superficial wash liquid velocity m s−1
- V:
-
volume of wash liquid m3
- v:
-
effective specific volume of fibres m3 kg−1
- WYɛ :
-
void wash yield defined by eqn (7)
- w:
-
urea mass fraction in the wash liquid mass %
- δ:
-
mean relative quadratic deviation of void local bed efficiency, E ɛ , defined as
$$\delta = \sqrt {\frac{1}{n}\sum\limits_{i = 1}^{i = n} {\left( {\frac{{E_{\varepsilon ,\exp } - E_{\varepsilon ,calc} }}{{E_{\varepsilon ,\exp } }}} \right)_i^2 } } \times 100 \% $$ - ɛ:
-
average effective porosity of bed
- µ:
-
viscosity of wash liquid Pa s
- ρ:
-
solute (alkali lignin) concentration kg m−3
- τ:
-
space-time defined by eqn (9) s
- avg:
-
average concentration in bed
- calc:
-
calculated value
- e:
-
exit stream
- exp:
-
experimental value
- i:
-
inlet stream
- 0:
-
initial concentration in bed at t = 0
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