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PSE 476
Homework #3
1/26/07
1. Which of the following are not used to stabilize hydrogen peroxide during bleaching?
(1 point)
d) Elevated (pH>13) levels of NaOH
Under these conditions, the peroxide breaks down rapidly through the reaction:
H2O2 + HO2- H2O + HO + O22. Which of the following species produces the brightest unbleached groundwood pulp?
(1 point)
a) Norway Spruce
This is directly from the reading: Table 1 page 314. What I want you to get out of
this is that wood species makes a big difference to the brightness.
3. Are unbleached thermomechanical pulps typically brighter or darker than stone
groundwood pulps? (1 point)
b) Darker
This is from the reading, Table 3 page 317. The process of adding heat increases the
strength of the pulp but also reduces the brightness.
4. How much typically is brightness increased in mechanical pulps during peroxide
bleaching? (1 point)
b) 15-20%
This is directly from the reading: page 320. If you look at figure 5 on page 320 you
will notice that there is a maximum amount of attainable brightness increase.
Remember, though, you are balancing brightness increase with cost.
5. The pH of the peroxide bleaching solution is importance for brightness. An alkaline
pH of roughly 11.5 is optimum. Why is this? (1 point)
d) A large percentage of hydrogen peroxide is in the form of the reactive
perhydroxyl ion but the pH is not too high to cause darkening reactions.
This is found in the book and the lecture. H2O2 does not react with the
chromophores in mechanical pulp while HO2- does. The pka of hydrogen peroxide is
11.6 so you would like to be above this point to have the4 largest amount of HO2- in
solution. The problem is, however, at high pH the H2O2 breaks down. Therefore it
is safest to start bleaching at roughly 11.5.
6. True or false; peroxide bleaching is carried out at a pulp consistency of 15-20%
because of improved brightening performance over higher consistency. (1 point)
b) False
From the book, bleaching is better at higher consistencies but is typically run at this
range because of equipment issues.
7. Is dithionite pulping an alkaline or acidic process? (1 point)
b) Acidic
Typical pH for this bleaching is 4-6. Bleaching below pH 4 will cause deterioration
of the dithionite. (page 331 in the reading)
8. Typically, how long is the retention time in the dithionite bleaching process? (1 point)
a) 30-60 minutes
This is pretty typical time although almost all the bleaching occurs in the first 10
minutes. (page 332 in the reading)
PSE 476
Homework #3
1/26/07
9. True or false: Bleaching at high temperature with hydrosulfite (sodium dithionite)
increases bleaching rate but also increases the rate of brightness reversion. (1 point)
a) True
This is a true statement. Typical temperature range is 50-70C. (page 332)
10. True or false: The amount of brightness gain levels out at a certain use level of
hydrosulfite. Adding more will not increase brightness. (1 point)
a) True
Look at the figures in my notes and in the reading (Figure 17, page 331)
11. What is the most important variable for brightness increase when using sodium
dithionite? (1 point)
c) Chemical Charge
This is pretty typical for bleaching operations. This little piece of information can be
found in the reading on page 331.
12. Batch digesters are pressure reactors that are typically built to hold up to: (1 point)
c) 220 psi
This is directly from page A60 of the reading. Digesters are built to hold up to
roughly 15 bar which equates to about 220 psi. Because the cooking temperature
needs to be well above 100 C the reactors need to be pressure vessels.
13. A digester is packed with chips at 240 kg/m3. Are these chips most likely hardwood
or softwood? (1 point)
a) Hardwood
This is from page A62 of the reading. Hardwoods are typically more dense than
softwoods so more can be packed in the digester.
14. What temperature is the digester when steaming is stopped and it is sealed? (in
degrees Celsius) (1 point)
d) 90-110
This is from page A63 of the reading. Remember, this is for standard batch pulping;
not for displacement pulping.
15. Which of the following is not a characteristic of chip impregnation through liquor
diffusion? (1 point)
a) Occurs wherever water is present.
b) Diffusion is effective over long distances. This is the answer!!!!!!!
c) Diffusion is affected by wood density.
d) Diffusion is effective across the grain
Diffusion only occurs over short distances. All the other answers are true about
diffusion.
16. Which of the following is not a characteristic of chip impregnation through liquor
penetration? (1 point)
a) Penetration is affected by wood density. This is the answer!!!!!!!!!
b) Optimum penetration is in relatively dry wood.
c) Composition of liquor is relatively unimportant.
d) Penetration is effective over long distances in grain direction.
Since penetration has to do with liquid traveling through voids the density of the
wood is unimportant. The other three answers are all important aspects of
penetration.
PSE 476
Homework #3
1/26/07
17. Which of the following best represents the typical height of a Kamyr digester? (1
point)
b) 50-70 meters
This shows how tall these systems can be. This equates to 160-220 ft tall. From the
reading, the tallest continuous digester is 77 meters.
18. What is the range of capacities of continuous digesters? (1 point)
d) 200-2000 a.d. metric t/day
This is from the reading on page A515. This shows the large differences in size of
digesters.
19. Why is an excess of white liquor added to the chips in the high pressure feeder? (1
point)
b) To convey the chips.
This excess liquor is removed as the chips are charged to the digester.
20. How long do chips typically reside in the impregnation zone? (1 point)
c) 45 -60 minutes
This is from the reading on page A517. In my notes I gave a general value of 45
minutes. This time is important to get the liquor into the chips before the temperature
gets too high.
21. What is the temperature range in which chips undergo impregnation? (1 point)
b) 115-125 C
Once again this is from the reading (A517). At this temperature there is limited
lignin reactions occurring.
22. Depending on the mill, how long do chips reside in the column? (1 point)
c) 3-8 hours
This is from the reading on page A 515. The variation in retention time has to do
with many factors related to equipment and production requirements.
23. Why does the concentration of NaOH vary from the top to the bottom of a continuous
kraft digester? (1 point)
a) Consumption of NaOH during delignification.
b) Washing liquor dilutes the stream.
c) Different concentrations of liquor are charged into the digester in different parts
of the system.
d) All of the above. This is the answer. All these things are occurring.
24. The chips (fibers) are washed in the continuous kraft digester with: (1 point)
e) b & c Dilute black liquor is used first followed by water.