PAPTAC Bleaching Committee

Section 6 - Technical Session

June 17, 2002

 

QUESTIONS ON D1 STAGE

 

1.         What is the effect of washing from the Eop stage?

 

Dryden – Affects Cl02 addition and tower pH.

Skookumchuck – Poor washing increases pH and therefore Cl02 usage in the D1 usage increases.

Celgar – Poor washing causes an increase in D1 Cl02 usage by 0.2 to0.2 kg. Cl02 per kg COD.  As washing fluctuates, acid use on D1 also fluctuates.

Western Pulp – Saves Cl02.  If washing poor, D1 bottom pH up and D1 bottom residual down.  Sometimes can’t add enough Cl02.

Terrace Bay – Affects pH control on D1.

St. Anne Nackawic – Poor washing – additional Cl02, but may be beneficial where buffer caustic used.  We do not use buffer caustic, end pH is 3.5.  Washers negative D.F., water does not displace filtrate.  Flow rate control, adjust setpoint based on manual brightness and vat residual, or manual Cl02 strength.  Used to have strategy involving on-line brightness and residual, after Cl02 added.  Unreliable.  Plans to use brightness Cl02 addition(FF), and 5 minute residual (FB).

Miramichi – Additional Cl02 usage.

Kamloops – Affects pH.  D1 filtrate recycled to Eop washer top shower.

KC Nova Scotia – No real data, but have seen some pH variation in D1 as carryover increases.

Tembec, Marathon – Using D2/D1 filtrate on showers.  Not measured.

Abitibi-Consolidated – We ensure a dilution factor of +1 and follow with a wash stage which was the mill’s previous hypochlorite stage.  A trial to save the power costs by shutting down and by-passing this wash stage resulted in a 10% increase in Cl02 usage.  $ 300,000. VS. power savings of $ 150,000.  We therefore do not by-pass this stage currently.

Irving – High chemical consumption, poor pH control, possible viscosity loss, pitch build-up and filtrate imbalance if sheet sloppy.

Cariboo – High chemical consumption pH can go above 5 which can cause off-grade.

Marathon -  Not measured.  Using D2 / D1 filtrate on showers.

 

 

2.         How do you control chemical addition on the D1 stage?

 

Dryden – Based on CEK and brightness.

Skookumchuck – Cl02 addition is manual, based on brightness in the Eop stage and brightness after the D1 sage.  Have brightness meters but do not run on auto.

Celgar – Kajaani (Metso) compensated brightness and control.

Western Pulp – Continuous residual analyzer at D1 bottom tower and at the D1 diffuser washer extraction filtrate.  Continuous pH analyzer at D1 bottom and D1 washer extraction filtrate.  Residual tests (manually) at same points every 4 hours.  D1 exit brightness test every 2 hours.

Terrace Bay – Ratio to Eop Kappa from analyzer.

St. Anne Nackawic – No answer.

Miramichi – Based on Kajaani residual output.  Residual setpoint adjusted by Operator to control final stage brightness at 82 – 84 (83( and Cl02 residual in the 0.l01 to 0.2 (0.1) range.  Caustic used to control pH in the 2.8 to 3.2 range.

Kamloops – Kajaani compensated brightness.

KC Nova Scotia – Dec. 2001 used Brite/Res comp Brite.  Had problems.  Switched to KF control only 6 months.  Good results.  Plan to add Brite/Res. to it.

Tembec, Marathon – KF control since Oct. 2001.  Based on Eop Kappa # by Kappa analyzer.

Abitibi-Consolidated – We control based on Kappa factor control with compensated brightness as back up.  KF Dosage – Flow (Dosage has an Operator input to offset addition based on brightness measurements (lab).  Note:  KF is compensated or biased using raw brightness as part of strategy.

Irving – lb./ton on % applied, feedback via manual tests.

Cariboo – Based on brightness and residual with production rate correction.

Marathon – Kappa factor control since October 2001 based on Eop kappa number by Kappa analyzer.

 

 

3.         Is Kappa Factor (using CEK) a valid method for D1 stage control?

 

Dryden – KF is an excellent method for D1 control.

Skookumchuck – No answer.

Celgar – No experience on this.

Western Pulp – Not used on routine basis.  Used for troubleshooting only.  Manual test.  Normally 2.3 to 2.6 (sequence DEoEopD).

Terrace Bay – A brightness sensor plus this would be better.

St. Anne Nackawic – Cl02 pH figures into pH/brightness relationship.  pH of stock from E1 washer and need for buffer caustic or acid also plays a role.  Too high pH gives inefficient use of Cl02.

Miramichi – Never attempted.

Kamloops – No!  We measure brightness instead in the Eop stage.

KC Nova Scotia – Yes.

Tembec, Marathon – Yes.  Works for MPI control 5.8 to 6.2 Kappa #.  Using .54 KF for 1 at this time.  D1 – 84 to 86 bright.  Unknown at MPI.

Abitibi-Consolidated – Yes, we’ve more than halved our brightness standard deviation and seen a 15% decrease in Cl02 consumption on the D1 stage.

Irving – Yes. Less effective if washing is poor or variable from previous E-stage.

Cariboo – Do not trust accuracy of tests.  If accurate enough, CEK would give a good indication of residual lignin, hence required Cl02 charge.

Marathon – Yes, works for MPI control to 5.8 to 6.2 Eop kappa number using 0.54 KF at the time for D1 stage 84 – 86  D1 brightness.

 

 

4.         What effect does Cl02 solution pH have on D1 stage efficiency?

 

Dryden – Usually none.  When generator is upset, Cl02 solution pH is very low, which affects D1 bleaching stage.

Skookumchuck – Do not monitor Cl02 solution pH.

Celgar – Have not investigated.

Western Pulp – We used to lower start up pH when making SBK to reduce shifts.  Typically D1 initial pH is 5.0 to 5.4 for end pH of 3.8 to 4.1.  Acid carryover to Cl02 generator level high, usage up.

Terrace Bay – Do not know.

St. Anne Nackawic -  Do not know.

Miramichi – pH normally 2.0 to 2.2 (2.1).  It does range from 1.8 to 2.5, but not long enough to see any impact.

Kamloops – Do not measure Cl02 pH.  Target pre-tower pH 3.5 to 4.5 and post tower to 2.0 to 2.5 by adjusting Cl02 and S02 antichlor dosages.

KC Nova Scotia – After testing, found optimum bleaching at 3.5 to 3.8 pH in vat.

Tembec, Marathon – Unknown at MPI.

Abitibi-Consolidated – I believe that if you control your chemical strength (Cl02), your pH should be constant.  As far as pH on the pulp solution, I think the science is quite explicit on the effect of pH on bleaching efficiency.

Irving – Loss of optimum brightness ceiling potential increase in shives. Can chase pulp pH if variable. Watch out for viscosity loss.

Cariboo – Control pH with caustic so Cl02 pH has no effect.  Low solution pH indices tower Cl02 generation efficiency.

Marathon – Unknown at MPI.

 

 

5.         What effect does Cl02 solution pH have on pulp properties?

 

Dryden – Usually none.

Skookumchuck – Do not monitor Cl02 solution pH.  Cl02 controls pH.  Have had problems when the pH was low in the tower and we were down for long periods.  Pulp viscosity fell.

Celgar – Have not investigated.

Western Pulp – Lower pH from carryover can result in low final brightness.

Terrace Bay – Have not measured.

St. Anne Nackawic – Cl02 pH low because modified Matheson process.  It contains H2S04 and HCl.

Miramichi – Never studied at our mill.

Kamloops – Do not measure Cl02 solution pH.

KC Nova Scotia – Not sure!

Tembec, Marathon – Unknown at MPI.

Abitibi-Consolidated – I believe that if you control your chemical strength (Cl02), your pH should be constant.  As far as pH on the pulp solution, I think the science is quite explicit on the effect of pH on bleaching efficiency.

Irving – None.  Pulp properties only affected if the pH of the pulp/Cl02 mixture is above 7 at any tube, e.g., at the point of addition.

Cariboo – Low pH helps to bleach out shives, while higher pH improves brightness.  Lower pH would increase AOX formation.

Marathon – Unknown at MPI.

 

 

6.         How do you handle production rate changes, in terms of different D1 stage bleaching conditions or setpoint?

 

Dryden – Usually cascade control from CEK to Kajaani to % on stock.  So on rate changes, addition rates will change the % on stock.

Skookumchuck – D1 stage is operated manually. Operator controls the rate and time lags through, then adjusts the D1 charge.

Celgar – No changes made – compensated brightness handles.

Western Pulp – Cl02 change is done through calculated % applied, table in DCS and feedback of lab tests for residual, brightness and continuous pH/residual measurement.

Terrace Bay – Gradual production rate changes.

St. Anne Nackawick  - Manually.

Miramichi – Conditions and setpoints are not changed.  At lower rates with higher retention times, Cl02 charge adjusted by lowering Kajaani setpoint.

Kamloops – Maintain same brightness target with minimum Cl02 dosage.

KC Nova Scotia – Kappa control uses bleach rate to call KF – feed forward.

Tembec, Marathon – KF control.  Do not generally adjust extensively on speed change.

Abitibi-Consolidated – We make production rate changes as an hourly rage change which cascades to the chemical additions, shower flows, dilutions automatically through Bailey NT DCS.  We offset some of the chemical additions by operator input to adjust for slight deviations in production rate.

Irving – As rate goes down, STMIX temperatures are reduced.

Cariboo – The production rate for D1 is calculated from the production rate to the bleach plant Do stage +/- any changes in bleach tower levels.

Marathon – Unknown at MPI.

 

 

7.         How do you handle species changes, in terms of different D1 stage bleaching conditions or setpoints?

 

Dryden – Automatically switch to the hardwood curve, when fiber length drops below 0.5 mm.

Skookumchuck – No answer.

Celgar – No changes made – compensated brightness handles.

Western Pulp – No difference.  Some Operators feel more Cl02 required for cypress.

Terrace Bay – Respond to D1 brightness (manual test).

St. Anne Nackawic – Different grades/species call for different setpoints.  Operator makes these changes based on changes in Bleaching Guidelines, which are broken down by grade.  Also, Operator experience.

Miramichi – No changes are made.  Kajaani setpoints changes to maintain final (83) brightness.

Kamloops – Maintain same brightness target with minimum Cl02 dosage.

KC Nova Scotia – No real change from softwood to hardwood except higher brightness target (for softwood)?

Tembec, Marathon – Unknown, don’t control species.

Abitibi-Consolidated – N/A – 90% pine, 10% balsam/spruce/tamarack blend.

Irving – Each grade has different setpoints.  Birch is the most aggressive, followed by softwood then maple.

Cariboo – Do not change species.

Marathon – Unknown.  We cook and bleach what we get.  Mixture Jackpine, spruce.

 

 

8.         How do you measure or quantify good mixing of medium consistency pulp?

 

Dryden  - Temperature profile around stock line after mixer outlet.

Skookumchuck – We don’t, but would like to know.

Celgar – Not done!

Western Pulp – Not done. Routine maintenance is important.

Terrace Bay – No answer!

St. Anne Nackawic – Don’t measure it.  Would think electrical load on mixer with regard to time and production rate as mixer and its elements age.

Miramichi – Not looked at.  Routine review against historical data can indicate washing or mixing problems.

Kamloops – No.

KC Nova Scotia – We don’t!

Tembec, Marathon – Not done.  Use MC pump for mixing.

Abitibi-Consolidated – Good question!  We currently suspect poor mixing in our oxygen mixer (Eop) and pretube.  Dropped oxygen from 15#/ton to 10#/ton with no effect on operation.  Looking for help.

Irving – Can only tell if poor mixing by highly variable residuals.  Cannot quantify between so-so and excellent mixing.

Cariboo – Compare the temperature around the stock line using an infrared gun.

Marathon – Not done!  Use MC pump for mixing.

 

 

9.         Do you have success stories regarding long loop feedback using tower end pH and residual?

 

Dryden – No.

Skookumchuck – We have a pH loop on T-20 with a Smith predictor – it works well.  Nothing on other stages.

Celgar – We do use feedback, but it’s not automated.  Operator looks at lab tests and adjusts pH or compensated brightness setpoints.

Western Pulp – Tried limited pH control, hard to control.  Now just monitor pH and residual.  Only able to add acid, sometimes need caustic.

Terrace Bay – We control these to control brightness or eliminate shives.

St. Anne Nackawic – No answer

KC Nova Scotia – No.

Tembec, Marathon – Only measure vat pH and residual.

Abitibi-Consolidated – No – looking for some though.

Irving – No.

Cariboo – Never tried it.

Marathon – Only measure vat pH and residual.

 

 

10.            Where is the best place to measure residual?

 

Dryden – Actually measure at about 40% of tower height.

Skookumchuck – Would like to know.

Celgar – Have D1 upflow tower with AD washer on top.  Best place should be before washer sampler, but don’t treat it.  Use D1 extraction filtrate instead.

Western Pulp – In D1 tower with diffuser, measure at tower bottom and extracted filtrate.

Terrace Bay – In the J-tube.

St. Anne Nackawic – Measure vat residuals.  Operators liked previous practice of tube residuals (20 to 30 minutes) but this was discontinued because of maintenance costs.

Miramichi – Actually measure them from washer vat.

Kamloops – Beginning and end of J-tube and in washer vat.

KC Nova Scotia – We measure in the vat because its easy.

Tembec, Marathon – Only measure vat pH and residual.

Abitibi-Consolidated – For Cl02 residual, we measure at the vat to determine remaining residual and keep this at a trace.  For control purpose, we measure approximately 5 – 10 seconds from the mixer.

Irving – We measure 5 seconds after Cl02 addition (D1) and 15 minutes (D1&D2- manual) after Cl02 addition.

Cariboo – It should be at the top of the upflow tube since 80% of reaction is complete.  Actually measure it shortly after Cl02 addition.

Marathon – Only measure vat pH and residual.

 

 

11.       Do you have any channeling issues?

 

Dryden – Not in D1 stage.

Skookumchuck – Most likely do, but they are not major problems.

Celgar – Some channeling is known to exist.

Western Pulp – Only if consistency drops below 8%.

Terrace Bay – At low production rates (800 ADT/day versus 1200).

St. Anne Nackawic – No answer!

Miramichi – No!

Kamloops – Don’t think so.

KC Nova Scotia – No!

Tembec, Marathon – J-tube upflow / downflow 25% level – 3.0 to 3.5 hours.

Abitibi-Consolidated – Not aware of any.

Irving  - Yes.

Cariboo – No!

Marathon – J-tube upflow / downflow 25% level used 3 to 3 ½ hours time.

 

 

12.       How high do you go on temperature?

 

Dryden – 70 to 72oC in pretube, 67oC in tower.

Skookumchuck – Constant 160oF (71oC).

Celgar – 75oC

Western Pulp - 77 – 79oC, but don’t directly control.

Terrace Bay – 160oF tower temperature.

St. Anne Nackawic – 81oC at STMIX.  Limited by blowback to E1 washer.  After Cl02 added, tower temperature is 75oC.

Miramichi – Inlet temperature is 186oF to control discharge temperature at 150oF.

Kamloops – 180oF

KC Nova Scotia – No answer.

Tembec, Marathon – 150oF cooler due to lots of retention time.

Abitibi-Consolidated – 75 to 85oC.

Irving – 185oF.

Cariboo – 75oC.

Marathon – 150oF cooler stage because lots of retention time available.

 

 

13.       Do you have any Cl02 decomposition in the tower?

 

Dryden – No!

Skookumchuck – Have not experienced any.

Celgar – No answer.

Western Pulp – Do not believe so.

Terrace Bay – Do not know.

St. Anne Nackawic  -  No answer.

Miramichi – None detected.

Kamloops – Don’t think so.

KC Nova Scotia – No answer.

Tembec, Marathon – No.

Abitibi-Consolidated -  No.

Irving – Yes, it is inferred by explosion hatches not reseating.

Cariboo – No.

Marathon – No.

 

 

14.       How long after Cl02 addition do you measure pH?

 

Dryden – Less than one minute.

Skookumchuck – In the J-tube, less than one minute after Cl02 addition, and in the tower after 2 to 2 ½ hours of retention.

Celgar – pH probe 20 seconds after Cl02 added.

Western Pulp – Tower starting and end pH continuously measured.  Operator checks every 4 hours.  Once/shift measure Cl02 solution pH.

Terrace Bay  - Tube 20 minutes.

St. Anne Nackawic – Vat – 2 to 2 ½ hours later.

Miramichi – Kajaani measures pH within seconds.  Manual tests done from tower top 3 hours later, every 2 hours and hourly washer vat tests.

Kamloops – About 1 minute.

KC Nova Scotia – No answer.

Tembec, Marathon – Vat pH.

Abitibi-Consolidated - No answer

Irving – Measure 5 seconds and 15 minutes after Cl02 addition and at end of tower.

Cariboo – Kajaani measures pH at location shortly after Cl02 addition.  Check vat pH manually, which is not good enough.

Marathon – vat pH.