Page 449 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: GPB/GAM P2: GQT Final Pages
Encyclopedia of Physical Science and Technology EN013A-619 July 26, 2001 19:32

254 Pulp and Paper

−
−
In Kraft pulping the active reagents are HS and HO . 3. Other Options
The Na 2 S exists in equilibrium with H 2 O and serves not
There are numerous alternatives that have been proposed
only as a source of HS , but also as an additional source
−
and that are being implemented. These include the addi-
of NaOH according to the following:
tion of anthraquinone to soda and Kraft processes; the use
H 2 O + Na 2 S NaHS + NaOH. (1) of ferric oxide in the soda pulping process (DARS pro-

cess) and the substitution of sodium metaborate (NaBO 2 )
The actual mechanisms of Kraft delignification are highly for NaOH in Kraft pulping (borate based autocausticiz-
complex, revolving around the ionization of acid phenolic ing). These options largely are designed to achieve pro-
−
units in lignin by OH and nucleophilic displacement of cess advantages. Anthraquinone (AQ) addition improves
lignin units with HS . The chemistry of delignification is pulping yield by 1–3%. Its utility, however, is limited to
−
reviewed in detail elsewhere. It is sufficient to note here the alkaline systems and its economics are dependent upon the
conditions specified above and the pulp yields; typically trade-off between raw material and chemical costs. DARS
45–55% of the dry weight of wood furnish is produced as and borate-based Kraft pulping are designed to simplify
Kraft pulp. chemical recovery. The DARS process is applicable only
to sulfur-free systems.
Other options include oxygen pulping as well as oxy-
2. Sulfite and Soda Pulping
gen bleaching, discussed later in the chapter. A consid-
It is useful to compare Kraft pulping to sulfite pulp- erable amount of research has been expended on totally
ing as a means for understanding differences among new approaches to pulping wood and agro-based mate-
these systems. Such a comparison is shown in Table III. rials and include sulfur free organosolv (organic solvent)
Conditions and results for soda pulping are also shown pulping and biopulping. Organosolv pulping typically em-
in Table III. Kraft pulping is presented to facilitate ploys aqueous organic solvents such as ethanol, methanol
comparison. or acetic acid as the pulping liquor. Pollution problems
From Table III, the similarities and differences among are considerably reduced with these methods because the
processes become apparent. Certainly the domination of solvents have to be completely recovered for economic
sodium as a base, and sulfur as an active reagent, become reasons; and consequently, this also results in recovery
obvious. The narrow range of cooking temperatures and and usage of all the formerly discarded wood compo-
yields also becomes apparent. What is not shown is the nents. Another advantage is the potential for developing
strength advantage of Kraft pulp. Also not shown are such small, competitive pulp mills with lower capital invest-
process considerations as chemical and energy recovery. ment. Two organosolv pulp mills, one each based on 50%

TABLE III Pulping Conditions and Results for Sulfite and Soda Pulping a

pH “Base” Active Max temp Time at max Yield
◦
Pulping method range alternatives reagents ( C) temp (min) (percent)
+
Acid bisulfite 1–2 Ca 2+ ,Mg 2+ ,Na ,NH + HSO 3 −H + 125–145 180–420 45–55
4
Bisulfite 3–5 Mg 2+ ,Na ,NH + HSO ,H + 150–170 60–180 50–65
−
+
4 3
Two-stage sulfite (Stora type) 50–60
−
Stage 1 6–8 Na + HSO ,SO 2− 135–145 120–360
3 3
Stage 2 1–2 Na + HSO 3 ,H + 125–140 120–240
Three-stage sulfite (Silvola type) 34–45
−
Stage 1 6–8 Na + HSO ,SO 2− 120–140 120–180
3
3
Stage 2 1–2 Na + HSO 3 ,H + 135–145 180–300
Stage 3 6–10 Na + HO − 160–180 120–180
NSSC 5–7 Na ,NH + 4 HSO ,SO 2− 160–180 15–180 75–90 b
+
−
3
Alkaline sulfite 9–13 Na + SO 2− ,HO − 160–180 180–300 45–60
3
Soda 13–14 Na + HO − 155–175 120–300 50–70 b
(Kraft) 13–14 Na + HO ,HS − 155–175 60–18 45–55
−
a From Sjostrom (1981). “Wood Chemistry: Fundamentals and Applications,” Academic Press, New York.
b Hardwood.