PSE 476: Homework #3 - Hydrogen Peroxide Bleaching and Pulp Production, Assignments of Materials science

Download PSE 476: Homework #3 - Hydrogen Peroxide Bleaching and Pulp Production and more Assignments Materials science in PDF only on Docsity! 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 + O2- 2. 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.