The attached
questionnaires were sent out to Pulp Bleaching committee members. The responses to the survey are
attached. The responses are coded to
protect the confidentiality of the mills.
Eighteen (19) mills
reported on the operation of twenty (22) generators. Of those, there were ten (10) R8, three (3) R10, one (1) SVP, one
(1) SVP-SCW, three (5) Mathieson and two (2) Solvay generators.
Performance factors
Changes in rates of
production, shutdowns, standby operation, boil outs, puffs and white outs
reduce the plant yield.
Operating rate as a
percent of designed rate varied from 0.5 to 1.8 times the design rate.
The frequency of rate
changes range from zero (0) per week to ten (10) per week.
Standby operation on
sub-atmospheric units is not an infrequent occurrence, two-three (2-3) per week
being not uncommon.
In the sub atmospheric
units, frequency of boil outs range from a low of twice (2) per year to
twenty-six (26) times per year.
Maintenance shut downs
range from annual, every six (6) months to once a month. Reasons for shut down were preventative or
corrective maintenance on all major items of equipment and instruments.
Puffing frequency
Several mills reported
no puffs in the prior year. Others
reported up to three (3) puffs per week.
The predominant location of the puff was the generator and contaminants
in the chemical feeds and sodium chlorate dissolving water as the source of the
puffing.
Raw material specifications
Knowledge of the
specification for the raw material was sparse.
It could be that specification is included in the purchase contract and
was not readily available to the respondent.
Turbidity of sodium chlorate, colour and clarity of sulphuric acid and
colour of methanol are quick qualitative assessment tools.
Sodium chlorate
Four (4) mills took
delivery of sodium chlorate solution.
The mill’s hot water tank was the predominant source of sodium chlorate
dissolving water. This can become
contaminated with extraneous mill process fluids, black liquor or other
filtrates.
A few mills (4) filtered
the water prior to its use to dissolve the sodium chlorate crystal. Slightly more (5) filtered the sodium
chlorate during the transfer from the unloading tank to the generator feed
tank.
The filter elements used
were 10 micron. All mills filtered the
sodium chlorate feed solution prior to its entry into the generator. 11% used 5 micron , 33% used 10 micron, 44%
used 20 micron. Two (2) mills still
used porastone filter elements at 55 micron.
The change out frequency
for sodium chlorate solution filter elements ranged from three (3) days to four
(4) weeks.
The change out frequency
for sodium chlorate feed filter elements ranged from two (2) days to eight (8)
weeks. Four (4) mills change on high
pressure, low flow.
Sulphuric acid
Only four (4) mills
reported filtering the sulphuric acid during the unloading. Seventeen (17)
mills filtered the sulphuric acid prior to its injection into the generator.
The change out frequency
for sulphuric acid unloading filter elements was weekly. 38% used 10 micron filters, five (5) mills
used porostone filters.
The change out frequency
for sulphuric acid feed filter elements ranged from two (2) to twenty-six (26)
weeks and five (5) mills changed on low flow.
Methanol
Fifteen (15) of sixteen
(16) methanol based units filtered the methanol feed. 54% 10 micron, 13% 20 micron, 33% 40 micron. One (1) mill changed and therefore inspected
the methanol quality every 12 months and another every 6-7 months. 45% of mills that diluted the methanol with
water prior to entry into the generator room filtered that water. The filter elements were 41% 10 micron, 20%
20 micron, 20% 40 micron. One (1) mill
used a stainless steel mesh screen.
Sub-atmospheric systems
use hot water to flush slurry lines and wash the saltcake by-product. Hot water tank was the major source of
water. Only five (5) mills filtered the
water prior to its entry into the generator or filter process streams.
Sub-atmospheric
generator operators reported problems with high pressure kick-outs and mills’
frequencies of six (6) and fourteen (14) per month were reported.
White outs are a
potential problem in generators that use methanol and in R8 and R10
generators. Several mills reported
frequencies of three (3) and eight (8) per month.
Chemical usages
Seven (7) mills reported
chemical usages that were superior to typical performance guarantees for the
respective processes.
The lowest sodium
chlorate usage for sub-atmospheric generator was 1.62 kg NaClO3/kg
ClO2 (97% yield) and the atmospheric Mathieson was 1.65 kg NaClO3
(95% yield) and the Solvay 1.75 kg NaClO3/kg ClO2.(90%
yield).
The lowest methanol
usage for sub-atmospheric generators was 0.15 kg CH3OH/kg ClO2.
The sulphur dioxide
usage for Mathieson was 0.57 kg/kg ClO2.
There is significant
room for improvement. At 15.0 ton/day,
a 1% improvement in yield could result in annual savings of $55,000/year,
$110,000/year for a 30 ton/day plant and $165,000 for a 45 ton/day plant.
Emergency quench water
Sub-atmospheric
generators use water to quench the reaction after a decomposition or puff or
after an interlock has been activated.
The mill process water was the source of this emergency water. Three (3) mills reported an emergency quench
water tank as its back-up and three (3) mills had the mill’s fire water main as
an emergency water source back-up. Note.
Safety incident report on consequences of lack of back up.
Generator chemical targets and control
The range of targets for
each process were significant. The
deviations from the set targets were reasonable.
Defoamer
Sixteen (16) mills used
tributyl phosphate addition. Nine (9)
mills use small batch additions 25-75 mls once/twice/thrice/shift, nine (9)
added it continuously through the methanol feed at concentrations ranging from
19 to 50 ppm. Two (2) mills added as
needed.
Saltcake filter operation
Only five (5) mills
monitored the % NaClO3 lost in the cake.
Chlorine dioxide absorption tower
The water source for the
towers in twelve (12) mills came from a chiller. Four (4) took the water from the river and three (3) took its
water from wells. Only nine (9) mills had
a back-up water supply.
Chlorine dioxide solution concentration
Four (4) mills were
targeting a concentration ³12 gpl; seven (7) were 10 gpl and the remainder
were at 8-9 gpl.
Chlorine dioxide solution storage
The capacity range from
3 hours for atmospheric units to 19 hours for one (1) sub-atmospheric unit.
Tail gas scrubber
It was vented to bleach
plant scrubber in five (5) mills.
White liquor (2),
caustic soda (2) and water were the scrubbing liquors of choice.