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 timetemperature 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]
Temperature ( C )
180
170
160
150
140
130
120
110
100
90
80
70
60
50
0
1
2
3
Time (Hr)
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)
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
2.25
2.5
2.75
3
Deg C Rel Rxn Rate Avg rate Delta T (h)
80
0
0
0.25
95
1
2
0.25
110
3
9
0.25
125
15
40
0.25
140
66
162
0.25
155
258
590
0.25
170
923
923
0.25
170
923
923
0.25
170
923
923
0.25
170
923
923
0.25
170
923
923
0.25
170
923
923
0.25
170
923
461
0.25
H
0.080447
0.456381
2.274984
10.10378
40.46054
147.6096
230.7293
230.7293
230.7293
230.7293
230.7293
230.7293
H-Factor
662.4443
893.1736
1123.903
1354.632
1585.362
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. Recovery
a. 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, Na2S
c. Explain how Smelt is converted into white liquor.
Answer: Through the following recaust reaction … converts Na2CO3 to NaOH
3. Drying
a. 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 below
Machine speed
2000 m/min
Machine width
10 m
Reel 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  k 2 
R
w
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/m 2 and an
after pressing consistency of 40%. It has been determined that the degree of rewetting is
R  15 g / m 2 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:
RS 
CH  C W
CH
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.