Paper Technology International 2024 - Journal - Page 112
PAPERTECHNOLOGYINTERNATIONAL
Process conditions are some of the worst working conditions
in the pulp and paper industry for all equipment. It must face
outstanding pH, temperature, pressure and with presence of shives,
stones, and sand. Nevertheless, solutions exist and are applied
successfully, notably with BTG’s rotating consistency transmitters.
In the bleach plant, bleaching chemicals are dosed typically
on mass 昀氀ow basis. Any variations in the incoming consistency
impact bleaching chemical dosage which then directly translates
to chemical concentration variation in the bleaching towers,
with negative impact on reaction kinetics and thereby bleaching
ef昀椀ciency. Let’s consider a 1000 Tpd sulfate hardwood pulp process
with a signi昀椀cant consumptions of chlorine dioxide (ClO2) and
sodium hydroxide (NaOH) as an example. Consistency variation
is reduced by installing rotating consistency transmitter instead
of static blade consistency transmitter. Based on the reduction
of standard deviation from 0.2 to 0.08, savings on chemical
consumption are estimated to approximately €325,000/year.
Consistency in Stock Preperation
OCC line
Stable process operations are fundamental to optimized
performance of paper or tissue machines. Therefore, stabilizing
consistency as early as possible in the stock preparation is
welcome. And the 昀椀rst step of consistency control starts at the outlet
of the main pulpers. Despite highly contaminated raw materials
when disintegrating waste papers, this is possible to get a workable
and sustainable consistency signal, with high availability. BTG’s
optical consistency transmitters with patented Peak method perfectly
ful昀椀ll this need. Stock preparation’s operators bene昀椀t from valuable
Figure 2: 1 ACT-2500 transmitter 2. Communication platform 3.
Controller/recorder (DCS) 4. Dilution water valve 5. Pulp chest
with suf昀椀cient mixing 6. Stock pump 7. BTG sampling valve
type MPS-1000
information to adjust dilution in anticipation of downstream stages.
Indeed, equipment for screening and epuration are designed to
work effectively at speci昀椀c consistency range and there are even
best performances at precise consistency target. Without good
consistency control, screening and epuration are less ef昀椀cient, with
negative consequences on overall machine operations.
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Re昀椀ning
Generally, and in an optimum way, re昀椀ning is controlled
by speci昀椀c energy applied (kWh/T). Any uncontrolled variation of
consistency directly impacts the energy applied on the 昀椀bers by the
re昀椀ning plates. A 1% change in 昀椀ber consistency inlet can create
10% change in 昀椀ber development, negative effects of sheet strength
development and all the machine runnability related with the change.
In re昀椀ning, maintaining the target consistency is critical for treating
the 昀椀bers to maximize 昀椀brillation and minimize 昀椀nes generation and
cutting.
Chemical dosage
Additives are either dosed as 昀椀xed 昀氀ow or as ratio
vs production in the stock preparation. As already described
beforehand in the paragraph dedicated to 昀椀berline, it’s important to
keep mass 昀氀ow constant while mastering the successive dilution
steps, so the correct amount of functional additives is introduced
at different process locations at various consistency levels.
Overconsumption or underconsumption of chemicals due to wrong
mass 昀氀ow information lead to chemical cost increases and system
overdosed, bad wet-end performances and machine runnability, and
out-of-speci昀椀cation production. Controlling consistency perfectly
enables accurate chemicals dosage with high ef昀椀ciency. This is fully
achievable with BTG’s oscillating blade consistency transmitter for
example.
Blending
Fiber recipe is determined to meet paper or tissue
speci昀椀cations and to keep cost at a certain level. For example, the
mixing chest can collect 昀椀bers from softwood line, hardwood line,
recovery system and broke line. For the reasons de昀椀ned before,
it’s primordial to apply recipe strictly. If one of the 昀椀ber mass 昀氀ow
deviates from its setpoint, this will impact the complete 昀椀ber mix. In
our example, there might be more softwood and less hardwood than
planned, and as a consequence it would lead to higher cost and
likely to lower quality of paper or tissue. A 0,5% error in blending will
impact furnish cost differential of €200,000.
Paper and Tissue Making
Basis weight
Generally, the basis weight of a paper or tissue machine
is managed via a control loop with the following inputs: basis
weight at the reel from the QCS (Quality Control System), 昀氀ow and
consistency of the thick stock. Any consistency variation after the
machine chest is fed forward to the basis weight controller. So,
when the consistency increases, the basis weight valve opening is
reduced or the feed-stock pump speed is decreased, to maintain a
constant amount of 昀椀ber 昀氀ow. In a continuous trend for delivering
lighter grammage with same mechanical properties, the closer the
actual basis weight is to the lower tolerance limit, the greater the
pro昀椀t. Because of its high accuracy on 昀椀ber consistency, BTG’s
rotating consistency transmitter is often a preferred choice for outlet
machine chest.
Retention
The retention at the paper or tissue machine is calculated
with consistency at the headbox and consistency in white water.
This calculation gives an indication about the percentage of raw
material kept in the sheet in the forming section. A high retention
means that most 昀椀bers, 昀椀llers, 昀椀nes and functional chemicals go
in the press section instead of turning around in the short loop.
Paper or tissue makers look for stability at an optimum level (not
necessarily the highest), and retention aid is often applied for this
purpose. Retention control is based on white water consistency only
because this value is more relevant than the retention calculation.
