Paper Technology International 2024 - Journal - Page 111
PAPERTECHNOLOGYINTERNATIONAL
TThe importance of consistency
Jeremy Devillers, Project Leader, BTG Instruments
INTRODUCTION:
In the shadow of the large pulp and paper process components and infrastructures, some transmitters operate 24/7, sometimes in
very tough process conditions, and deliver an essential information for process understanding and control: consistency – or the “thickness”
of the mixture. Let’s shine a light on it.
While energy and raw materials costs are prevailing as well as environmental considerations, pulp, paper, and tissue makers
recognize consistency’s importance to the process, and its ultimate effect on pro昀椀tability. An unstable process affects both quality and
productivity. Accurate consistency control is fundamental to achieve process stability because it sets the base for measurement and control
of so many other variables in 昀椀ber lines, stock preparations and wet-end of paper or tissue machines.
This importance is even reinforced with the development of new software-based solutions for control and optimization. Fortunately,
these innovations also bring advanced solutions for improving consistency control management.
The thickness of the mixture
All starts with water. As soon as 昀椀bers are detached
from each other in pulp mills or in pulpers, mixture with water is
necessary to process them in 昀椀ber lines and stock preparations. The
challenge for pulp and paper makers is to be able to know constantly
the proportion of 昀椀bers -and possibly other raw materials- in water
until it becomes a sheet of paper or tissue. Before expressing
the rate of 昀椀bers as dry content on the pulp dryer, paper or tissue
machine, the ‘thickness’ of the mixture is expressed as consistency.
Consistency is de昀椀ned as a percentage of solid mass in
a pulp slurry. The slurry can include 昀椀bers, 昀椀nes, 昀椀llers and water.
There are standards (ISO, TAPPI…) which de昀椀ne how to measure it
in the laboratory. If these methods are quite simple to execute with
suitable equipment, the laboratory measurements require human
resources, strictness and time. Whereas pulp and paper mills run
continuously by digesting, bleaching, blending, re昀椀ning, pumping
and diluting several tons of costly 昀椀bers per hours, it’s obvious that
more frequent, automatic and online consistency measurements are
required to keep control on the pulp and paper process.
Online consistency measurements are achieved through
calibrated consistency transmitters installed on pipes in most cases.
Frequency and quality of measurement depends on several factors
like technology used, installation, type of media, 昀氀ow conditions,
calibration, equipment, and services of transmitter’s manufacturer.
It is worth noting that all consistency transmitters must be calibrated
with laboratory measurements and that there is not a single
consistency transmitter technology that 昀椀ts all applications.
Control of consistency is then achieved by adjusting
dilution to the de昀椀ned consistency setpoint either manually or
more commonly automatically with a proper consistency control
loop. Its ef昀椀ciency depends on several factors as well: the online
consistency measurement as already explained above, but also the
sampling quality for calibration purpose, the mixing and dilution of
the pulp upstream, and the PID controller tuning. Typical number
of consistency loops can vary from 5-10 in a simple tissue machine
to 20-30 in a sophisticated 昀椀ne paper or board machine, and even
more than 50 in an integrated mill. Let’s review in detail what make
them so important.
Figure 1: BTG Optical consistency transmitter TCR-2511 installed
at the outlet pulper
Consistency in Fiberline
In 昀椀ber lines, medium consistency can be measured and
controlled already from the outlet of digester, setting already the
foundation of better stability in downstream stages such as brown
stock washing and oxygen deligni昀椀cation. Stabilizing the extracted
pulp 昀氀ow helps also to control continuous digester operations by
acting on the other cooking parameters (chip level, cooking liquors,
cooking time, H factor, Kappa number).
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