Assignment 3 – kraft pulping and papermaking

1. Kraft Pulping a. What is the chemical composition (and percent composition) of a typical softwood? Use

it to estimate the theoretical yield from the kraft process. What is the composition of the pulp following the cook? (Give compositions in units of percent by mass)

Answer: Softwood: 45% Cellulose, 25% Hemicellulose, 30% Lignin and 1% extractives. Almost all cellulose is retained along with some (8%) hemicellulose and very little lignin (3%). Therefore total yield would be approximately 56%. (Note these are all fairly approximate).

b. What is the kappa number and what does it measure? Answer: Kappa number indicates the amount of lignin in the pulp. %Lignin is 0.15 x Kappa #.

c. Define the H-factor. Assume you want to make pulp at 50 kappa using white liquor containing 8.8% active Alkali. When would you stop the cook if it had the time-temperature profile given below (linear from 80C to a plateau of 170C at 1.5 hours)?

Wikipedia: H-factor is a kinetic model for the rate of delignification in kraft pulping. It is a single variable model combiningtemperature (T) and time (t) and assuming that the deligification is one single reaction.[1]

0 0.5 1 1.5 2 2.5 35060708090

100110120130140150160170180

Time (Hr)

Tem

pera

ture

( C

)

Figure: Time-temperature profile of kraft cook.

Answer: From the plot of Kappa and H-Factor we see that a 50 Kappa require an H-Factor of 800.

time (h) Deg C Rel Rxn Rate Avg rate Delta T (h) H H-Factor 0 80 0 0 0.25 0.080447

0.25 95 1 2 0.25 0.4563810.5 110 3 9 0.25 2.274984

0.75 125 15 40 0.25 10.103781 140 66 162 0.25 40.46054

1.25 155 258 590 0.25 147.60961.5 170 923 923 0.25 230.7293

1.75 170 923 923 0.25 230.7293

0 0.5 1 1.5 2 2.5 3 3.50

20

40

60

80

100

120

140

160

180

Time (Hr)

Tem

pera

ture

( C

)

From the embedded Excel spread sheet (also on website), we see that you need almost 2.25 hours of cook time say 2.2hours.

2. Recoverya. Define White Liquor, Green Liquor, Black Liquor and list the main chemical components

(from Wikipedia)

White liquor is a strongly alkaline solution mainly of sodium hydroxide and sodium sulfide. It is used in the first stage of the Kraft process in which lignin and hemicellulose are separated from cellulose fiber for the production ofpulp.[1]

NaOH and Na2S.

Green liquor is the dissolved smelt of sodium carbonate, sodium sulfide and other compounds from the recovery boiler in the kraft process.

The weak black liquor is further evaporated to 65% or even 80% solids ("heavy black liquor"[5]) and burned in the recovery boiler to recover the inorganic chemicals for reuse in the pulping process.  The black liquor is an aqueous solution of lignin residues, hemicellulose, and the inorganic chemicals used in the process. The black liquor comprises 15% solids by weight of which 10% are inorganic and 5% are organic. Normally the organics in black liquor are 40-45% soaps, 35-45% lignin and 10-15% other organics.

b. What are the main chemical in the smelt leaving the recovery furnace

Na2CO3, Na2Sc. Explain how Smelt is converted into white liquor.

Answer: Through the following recaust reaction … converts Na2CO3 to NaOH

3. Dryinga. The “constant” drying rate of newsprint on a hot plate is 11 kg water/m2h. If the

consistency of the paper at critical moisture content is 80%, determine the time required to dry 1 m2 from its initial consistency of 40% to the critical point (basis weight 40 g/m2)

b. Calculate the cost/hour for steam to dry paper on the machine described belowMachine speed 2000 m/minMachine width 10 mReel moisture content 8%Cons. After press 40%Basis Weight 40 g/m2

Steam Consumption 1.5 kg steam/kg water evap.Steam cost $5.00/100 kg

c. If the consistency after pressing could be raised to 42%, what would the cost savings in steam consumption per year, based upon 350 days of continuous operation?

4. Pressing The degree of re-wetting after the press nip can be determined from the linear portion of Sweet-Wahlström equation, that is,

k1=k2+Rw

where k1 is ratio of the mass of water to the mass of fibres after the nip, k 2 is a constant, R is the degree of re-wet in units of g water per unit area of paper, and w is the basis weight of the paper. A press nip operates with newsprint with a basis weight of 49 g/m2 and an after pressing consistency of 40%. It has been determined that the degree of rewetting is

R=15 g /m2 for the press felt used in this application. Using the Sweet-Wahlström equation, estimate the change in basis weight required to achieve an after press consistency of 40.5%.

5. Headbox Manifold:a. Assuming no friction or minor losses, derive the equation for the shape of the

manifold on a paper machine for a constant stock flow velocity. b. A technician has measured the stock flow velocity near the inlet to be 8.2 ft/s and

5.8 ft/s at the outlet (120 inches away from the inlet). The pressure at the inlet is 9.7 feet of water. Assume that there is no friction losses in the manifold. Determine the static pressure at the outlet?

6. a) First-pass retention, RS , can be defined by the expression:

where CH is the consistency of the paper machine headbox and CW is the consistency of the white water in the tray section of the paper machine. Derive the above equation from a simple material balance. (Assume all of the white water to have the same consistency, CW.)

b) The “total retention” of a paper machine is defined as the mass of material on the wire leaving the couch as a fraction of the mass delivered to the paper machine. This retention is always higher than the first-pass retention. Explain why.