(36c) Characterization of Liquid-Pulp Fiber Suspension Mixing At Tee Mixers Using Electrical Resistance Tomography | AIChE

(36c) Characterization of Liquid-Pulp Fiber Suspension Mixing At Tee Mixers Using Electrical Resistance Tomography


Yenjaichon, W. - Presenter, University of British Columbia
Lim, J. C. - Presenter, The University of British Columbia

Characterization of
Liquid-Pulp Fiber Suspension Mixing at Tee Mixers using Electrical Resistance

Yenjaichon*, J.R. Grace, C.J. Lim and C.P.J.

of Chemical and Biological Engineering
of British Columbia
BC, Canada V6T1Z3

Mixing is an essential unit operation in
the pulp and paper industry. In bleach plants, pre-distribution of chemicals in
pulp suspensions before entering tower reactors is very important, and in-line
mixers have normally been used to ensure efficient contacting between chemical
and pulp in the tower to achieve the greatest lignin removal and optimal use of
the bleaching chemical. Chemical injection in pulp mixing operations is commonly
used as a pre-distributor to mix liquid chemicals into pulp suspensions ahead
of various mixers including static mixers, peg mixers and high-shear mixers.
The simplest type is the tee mixer. Understanding jet mixing behavior at a tee
junction provides basic concepts for further analysis of different types of
in-line mixers. However, few studies have been reported on the in-line mixing behavior
of liquid injection into pulp suspensions. This presentation will focus on a
study of mixing liquids into pulp fiber suspensions at tee mixers using a
non-invasive technique ?electrical resistance tomography (ERT)?.
The experiments were conducted in a
pilot-scale flow loop facility. Mixing quality of liquid injection into pulp
fiber suspension flow was evaluated after tee mixers for two pulp types
(softwood and hardwood kraft pulps) over a range of
suspension mass concentrations (Cm = 0 ? 3%), main stream or
pipe velocities (Up
0.5 ? 5 m/s) and side stream or jet velocities (Us = 1
? 12.5 m/s). The degree of mixing was determined from the measurement data in
cross-sectional planes along the pipeline using a modified mixing index based
on the coefficient of variation (CoV) of the
individual conductivity values in each image pixel. The results show
significant differences in jet mixing behavior of non-Newtonian pulp suspensions
and a Newtonian fluid (water). For Newtonian fluid in the turbulent flow regime,
mixing quality significantly improved with increasing jet-to-pipe velocity
ratio. In addition, at the same jet-to-pipe velocity ratio, the jet penetration
and mixing quality were almost independent of the main stream velocity for a
specific jet-to-pipe diameter ratio.

pulp fiber suspensions, on the other hand, the mixing quality strongly depends
on the main stream velocity since the flow regime varies with the main stream velocity
and the mixing quality of pulp suspensions strongly depends on the flow regime.
At low velocity, plug flow was approached and mixing was very poor due to the strong
fiber network, preventing tracer distribution downstream. At higher velocity,
the plugs disintegrated, and the mixing quality significantly improved
downstream in the pipe when the flow was in the turbulent regime. In addition,
the mixing quality depended slightly on the jet velocity unless the jet
momentum force was high enough to disrupt the fiber network and provide better
mixing. At higher mass concentration (Cm ≥ 2%),
mixing was very poor, even at a high main stream or jet velocity, since the
turbulent shear was not high enough to disrupt the plugs. Therefore, additional
shear was required to improve mixing.
Author to whom correspondence may be addressed.

address: wyenjaichon@chbe.ubc.ca