5
1 Dal legno alla lignina
1.1 Legno
Il legno è un materiale biologico eterogeneo, igroscopico e anisotropo costituito da fibre
di cellulosa (40-50%) ed emicellulosa (15-25%) impregnate da lignina (15-36%).
Il legno è la più importante risorsa naturale rinnovabile non solo in termini di quantità,
ma anche grazie alla sua composizione e alle sue versatili proprietà e ai suoi usi come
combustibile, materia prima per la produzione della carta, elemento strutturale,
rivestimento e per le altre molteplici applicazioni.
Cellulosa
La cellulosa è una molecola lineare con una forte tendenza ad interagire con se stessa
tramite legami ad idrogeno cosicché microfibre possono formarsi creando sia regioni
amorfe e sia regioni cristalline altamente ordinate. Il grado di polimerizzazione della
cellulosa è compreso tra 8000 e 10000.
Emicellulosa
A differenza della cellulosa, l’emicellulosa è costituita da un gruppo di polisaccaridi.
Tuttavia, esistono notevoli differenze strutturali tra emicellulose a secondo del tipo di
legno utilizzato.
Il tipico grado di polimerizzazione è tra 20 e 200, molto più basso rispetto a quello della
cellulosa.
Lignina
La lignina fu definita per la prima volta nel 1971 da Sarkanen and Ludwig come un
prodotto polimerico naturale ottenuto da una polimerizzazione deidrogenativa di tre
precursori primari: alcool cumarilico, coniferilico e sinapilico.
1.2 Il processo di delignificazione
I processi più comuni usati per estrarre chimicamente la cellulosa dal legno sono: quello
alla soda, noto anche come processo Kraft e il processo al solfito. Tuttavia, il processo
kraft, oggigiorno è la tecnologia dominante per la superiore resistenza della pasta di
legno che si ottiene.
La delignificazione è divisa in tre fasi, chiamate: iniziale, del bulk e residua a causa
della differente selettività durante il processo di trattamento del legno. Già nelle prime
due fasi il grado di delignificazione è del 90% e la maggior parte della lignina è
rilasciata nel black liquor.
La kraft lignina è preferibilmente ottenuta per precipitazione dal black liquor per mezzo
di acidi minerali o anidride carbonica, la cui composizione è molto complessa da
determinare dovuta al miscuglio di legno degradato e alla grande fluttuazione della
composizione originata dal tipo di processo e dal tipo di legno utilizzato.
6
1.3 Lignina
La macromolecola della lignina è principalmente un polimero aromatico senza
un’evidente unità ripetitiva e la polimerizzazione dei tre precursori produce una
struttura tridimensionale.
Nonostante siano stati proposti diversi schemi molecolari, una struttura della lignina ben
definita non può essere determinata poiché si hanno delle reazioni di polimerizzazioni
casuali che generano un polimero amorfo senza una regolare struttura. Tuttavia, alcuni
modelli strutturali statistici sono stati sviluppati basati sulla frequenza dei più comuni
legami presenti tra i gruppi aromatici.
Tra tutti i legami interunità presenti nella lignina il legame etereo Ε-O-4’ è il più
frequente sia in strutture fenoliche che in strutture non fenoliche.
Fig. 1-1: struttura della lignina
La lignina kraft contiene più gruppi fenolici e carbossilici rispetto alla lignina nativa,
dovuto ai cambiamenti durante il trattamento di delignificazione del legno. Questo
determina un peso molecolare inferiore della lignina degradata che corrisponde
approssimativamente a 3000-5000 g/mol, mentre per la lignina nativa è stato proposto
un peso molecolare di circa 20000 g/mol.
7
2 Prodotti e sottoprodotti del cracking catalitico della lignina
2.1 Fenoli
Oggi, il 99% della produzione mondiale dei fenoli è ottenuto da processi sintetici e il
processo più comune è basato sull’ossidazione del cumene, prodotto dalla reazione tra
benzene e propene, entrambi derivanti dalla distillazione del petrolio.
Nel 2006 è stata stimata una produzione di fenoli di circa 8.2 milioni di metri cubi con
un valore approssimativamente di 7 miliardi di dollari. Tali composti organici sono
utilizzati principalmente per il 41% per la produzione di bisfenolo A, per il 28% per la
produzione di resine fenol-formaldeide e il resto per la produzione di caprolattame,
alchilfenoli, anilina e acido adipico.La crescita del mercato del fenolo è stata dal 2001 di
circa 5% annuo dovuto all’alta domanda di BFA usato per la produzione di
policarbonati e per la produzione di resine epossidiche. Inoltre, nel periodo 2007-2011 è
stata stimata una continua crescita del 4% annuo.
Come già accennato, il petrolio rimane la principale fonte per la produzione di fenoli
che con il suo continuo aumento di costo influenza pesantemente il prezzo dei fenoli sul
mercato. Il trend del costo dei fenoli durante gli ultimi anni è presentato nella tabella
seguente ed è possibile notare come il prezzo dei fenoli sia estremamente dipendente dal
prezzo del petrolio.
Anno Fenoli US $ / t US $ / m3 US $ / barile
1999 551-794 107 17
2001 661-882 151 24
2003 882-992 164 26
2005 1102-1543 377+ 60+
2008 1500-1620 628+ 100+
Tabella 2-1: Trend dei prezzi dei fenoli
2.2 Altri prodotti
Altri prodotti del cracking catalitico della lignina sono presentati di seguito con le loro
applicazioni e il loro costo sul mercato.
Prodotti Applicazione Prezzo
Cresoli Fonti per erbicida selettivi
Produzione di farmaci
Solventi
1600$/ton
(USA)
Etil- e propilfenoli
Materie prime per resine
Erbicida,insetticida
Inibitore di polimerizzazione
1500 $/ton
(USA)
Xilenoli Farmaci, pesticidi,
Resine, fonti per la produzione
sintetica di antiossidanti
1600$/ton
(Europa)
Acidi cresilici
Materie prime per resine
fenoliche,solventi
Agenti sgrassanti e flottanti
1600$/ton
(USA ed
Europa)
Tabella 2-2: prodotti del cracking catalitico della lignina
5
Introduction
The extraction of lignin, a by-product from the wood in the production of chemical
pulp, from black liquor is a cost effective way either to increase the production of a
normal pulp mill or to produce valuable chemicals.
Nowadays the black liquor is disposed in recovery boilers, which in many Kraft pulp
mills represent the limiting step in the production chain. Building the recovery boilers is
very complex and costly and those built in Sweden in the 80s, 70s or even in the 60s
fitted the production of that time without considering the possible future production
increases. In such cases, the recovery boiler is often one of the major bottlenecks of the
production chain. Therefore withdrawing the lignin, the recovery boilers would work
with a lower load and there would be the possibility to increase the production of pulp
and paper at many mill sites.
Moreover, the lignin could be used directly in many applications or used as feedstock
for value added products such phenol, benzene and composite materials.
With the increasing of the oil and energy prices the opportunities of lignin as a foreseen
raw material arises. Instead of burning all the black liquor, at least some of the lignin
can be used as raw material as a substitute for oil derived feedstock in several different
applications. Today it seems possible put into practice a profitable process to convert
the lignin into higher value products, especially due to the introduction of the
advantageous green-energy certificates on the energy market, with the purpose of
limiting the oil utilization.
One groundbreaking process includes a catalytic hydrocracking route to convert lignin,
extracted from the black liquor via the LignoBoost-process, into more valuable
chemicals such monophenols.
7
1 From wood to lignin
1.1 Wood structure
Wood is a heterogeneous, hygroscopic, cellular and anisotropic biological materials
consisting of different types of cells. It is composed of fibres of cellulose (40-50%) and
hemicellulose(15-25%) impregnated with lignin (15-36%).
Wood is the most important renewable natural resource not only in terms of quantity,
but because of its composition and versatile properties, its uses as fuel, raw material for
pulp, structural timber, sawn wood, panels, furniture, feedstock for chemicals and other
many purposes.[1]
Cellulose
The cellulose is a linear molecule with a strong tendency to interact together by
hydrogen bonding so that microfibrils can be formed creating highly ordered
crystalline- and less ordered amorphous regions. It consists of Ε-D-glucopyranose units
linked together by (1-4)-glycosidic bonds. Around 8000-1000 is the degree of
polymerisation of wood cellulose. [1]
Hemicellulose
Unlike cellulose, hemicelluloses are a group of branched polysaccharides. Structural
and quantitative differences exist between hemicelluloses in softwoods and hardwoods.
The typical degree of polymerization is between 20 and 200, much lower than that of
cellulose.[1]
Lignin
Lignin is a polymeric natural product obtained from a dehydrogenative polymerization
of the three primary precursors: coniferyl-, sinapyl- and coumaryl- alcohols. [2]
1.2 Processing of wood to pulp
It lasts one or two days to convert the wood into pulp. By grinding and refining, the
wood is disintegrated into fibres. However, other alternatives process can be used such
as chemical processing wood where the lignin is degraded and dissolved in solvents
releasing free fibres. The most prevalent pulping processes are: the sulphate or know
commonly as kraft pulping and the sulfite pulping. However, Kraft pulping is nowadays
the dominating technology used to produce unbleached pulp.
It is preferable to the others pulping process for the superior strength imparted to the
produced pulp.
In an alkaline solution with hydroxide- and hydrosulphide ions as active de-lignifying
agents, the wood chips are pulped at 150-170 C.
Delignification process is divided in 3 phases, namely: initial, bulk and residual
delignification because of different selectivity during the kraft pulping. [3] [4]
Nevertheless, only in the first two stages the lignin reactions occur and at termination,
the degree of delignification is around 90%, but may depend on the pulping conditions.
8
0
5
10
15
20
25
30
35
40
45
0 20 40 60 80 100
Delignification of wood [%]
D
i
s
s
o
l
v
e
d
c
a
r
b
o
h
y
d
r
a
t
e
s
[
%
o
n
w
o
o
d
]
Final phase
Bulk phase
Initial phase
Fig. 1-1:The selectivity of delignification in the three phases of Kraft pulping
(Gallerstedt and Lindfors 1984)
Many investigations have been carried out to explain the retarded delignification
observed during the final phase and the main possible reasons can be:
I. the presence of alkaline stable native lignin structures;
II. condensation reactions occur in lignin;
III. the presence of alkaline stable covalent linkages between lignin and
carbohydrates.[4]
1.3 Black liquor composition
Kraft lignin is preferably obtained by precipitation from “black liquor” with mineral
acids or carbon dioxide, whose composition is very complex to determine due to the
mixture of wood degradation with strongly fluctuating composition depending on the
process and the raw materials. Whereas, crude lignosulfonates are partly used directly as
“spentsulfite liquor”.
In the following tables are shown the typical composition of pulping liquor from
hardwoods and softwoods obtained by kraft and sulfite processes.
Component Percentage of total solids
Softwood Hardwood
Lignosulfonate 55 42
Hexose sugars 14 5
Pentose sugars 6 20
Acetic and formic acid 4 9
Resin and extractives 2 1
Ash 10 10
Others 9 13
Table 1-1: Example for the composition of spent sulfite liquors from softwood and hardwood
(B. Saake, R. Lehen, 2007)
9
Component Percentage of total solids
Softwood Hardwood
Kraft lignin 45 38
Xyloisosaccharinic acids 1 5
Glucoisosaccharinic acid 14 4
Hydroxy acid 7 15
Formic acid 6 6
Acetic acid 4 14
Resin and fatty acids 7 6
Turpentine 1
Others 14 12
Table 1-2: Example for the composition of kraft black liquors from softwoods and hardwoods
(B. Saake, R. Lehen, 2007)
A representative elementary composition of pine black liquor is presented in the
Table 1-3.
Element Typical amounts
(%)
Carbon (C) 35,0
Oxygen (O) 33,9
Sodium (Na) 19,0
Sulphur (S) 5,5
Hydrogen (H) 3,6
Potassium (K) 2,2
Chlorine (Cl) 0,5
Nitrogen (N) 0,1
Inert material 0,2
Table 1-3: Elementary composition of pine black liquor solids (E. Vakkilainen, 1999)
The higher sulfur content present in the lignosulfonates (4-8%) compared to that in the
kraft lignin (1-1.5%) permits a higher solubility in water over the entire pH range, but
with drawback to be insoluble in many organic solvents. On the other hand kraft lignin
is soluble in organic solvents, but not soluble in water with a pH lower than 10.5 [7].
1.4 Lignoboots process
Lignoboost, a new promising technique to extract lignin from black liquor, has been
developed by the Swedish STFI-Packforsk AB together with Chalmers University of
Technology and industrial partners.
Around 30-40% dry solid is removed from the black liquor. Injecting carbon dioxide,
which decreases the pH to about 9-10, the lignin precipitates and it is separated from
black liquor over a press filter. In order to purify the lignin, the solid cake formed is
then washed with acidified water in one or several steps [8].
10
Fig. 1-2:Possible extraction sites in a Kraft pulp mill (STFI-Packforsk, 2005)
The remaining liquid phase, which consists of mainly cooking chemicals and soluble
organics, is returned to the recovery cycle.
An affective wash step is crucial after filtration because sodium has to be washed out
from the lignin and returned to the mill to avoid an excessive demand of make-up
chemicals. Moreover, the sodium content has to be low to prevent corrosion and other
problems associated with low melting ash if the lignin is used as biofuel.
The washing liquor is enriched of sodium by the acidifying agent sulphuric acid, which
decreases the sodium content in the lignin, so that not a large net sodium removal is
necessary, especially when the washing liquor is also recycled.
Addition of sulphur is somewhat problematic because it is partly covalently bonded to
the remaining lignin and tedious to remove under the acidic washes. Moreover a surplus
of sulphur increases the costs for sodium make-up [9].
A separation method using ultrafiltration has showed good results in terms of separation
of lignin from the cooking circulation with the advantage to produce fraction of certain
molecular weight. However, higher capital and operating costs compared to acid
precipitation are required for this separation method.
Utilization of Kraft lignin separated from black liquor, as feedstock for phenol
production is interesting due to the low price compared to an oil-derived feedstock.
Furthermore, there are many other fields where the lignin held from the lignoboost
process can be used. For instance, in fuel application the lignin, preferably pelletted or
suspended in oil, can be co-fired with other fuels or it can be used even to produce
methanol, DME or bio-diesel by pyrolysis or gasification.
The demonstration plant in Bäckhammar, Sweden produces 4,000 tonnes of lignin per
year, which it would be enough to provide electricity, and heating to 1,200 families
houses and replace 2500 tonnes of fuel oil [10].
11
2 Lignin
Lignin (derived from the Latin word ”lignum” meaning wood) is a natural polymer that
with cellulose and hemicelluloses is one of the three major constituents of vascular
plants. The content of lignin ranges from 15%-36% and after cellulose is the second
most abundant natural polymer.
Lignin for its proprieties performs several important functions that are extremely
essential to the plant. Due to the lipophilic character, lignin decreases the permeation of
water across the cell walls, which consist of cellulose fibres and amorphous
hemicelluloses, thus enabling the transport of aqueous solutions of nutrients and
metabolites in the conducting tissues. Lignin also imparts strength to the cell walls by
acting as a glue binding the fibres together into a composite of good mechanical
strength.
Finally, lignin impedes the penetration of destructive enzymes into the cell walls
resisting the attack of microorganisms.
The lignin was first defined by Sarkanen and Ludwig 1971 as “polymeric natural
product arising from an enzyme-initiated dehydrogenative polymerisation of the three
primary precursors: coniferyl-, sinapyl-, and coumaryl alcohol”[11]
Fig. 2-1:The three primary precursors of lignin (B. Saake, r. Lehen 2007) [5]
The macromolecule of lignin is basically a crosslink aromatic polymer with no evident
single repeating unit and the polymerisation produces a three-dimensionally crosslinked
network, which is formed both in the cell walls and between the cells [12].
The composition of lignin varies with the plant source. In coniferous trees, which are
classified as soft wood, are constituted mainly by the polymerisation of coniferyl
alcohol, whereas hardwood lignin is a result of guaiacyl and sinapyl structures and grass
lignin of coumaryl structure in addition to the former two types [13].
12
2.1 Lignin structure
The designations of carbon atoms in lignin chemistry does not follow the IUPAC
nomenclature therefore the OH group in the aromatic ring occupy the position 4. The
propane side chain is labelled with Greek letters, starting from the benzylic carbon,
termed ∆ carbon followed by the second carbon atom labelled by the letter Ε.
Fig. 2-2:Nomenclature of coniferyl alcohol (B. Saake, R. Lehen 2007) [5]
The irregular structure, due to the random combination of phenoxy radicals explains the
optically inactivity, even though both carbon ∆ and carbon Ε in the side chains of the
phenylpropane structural units are asymmetric [14].
Although several schemes have been proposed, no a definite lignin structure can be
determined due to the random coupling reaction, which leads to a three-dimensional,
amorphous polymer without a regular structure or repeating unit.
However, some statistical structural models have been developed based on the
occurrence in the lignin structures showing the most common interunit linkages present
between the aromatics groups [15].
Linkage type % of linkage in Softwood lignin
Arylglycerol Ε aryl ether unit ( Ε-O-4) 36
a
Phenylcoumaran unit ( Ε-5) 12
a
5-5 units 11
a
Dibenzodioxin (5-5-O-4) 5
a
Diaryl ether units (4-O-5) 4
b
Ε- Ε units 3
a
Ε-1 units 2
a
Table 2-1: Proportion of the main lignin inter-unit linkages in softwood. (Lawoko 2005)
a
Determined using spruce MWL (Zhang and Gellerstedt 2000)
b
According to Brunow et al. 1999
Ricevi informazioni sui nostri servizi, sulle offerte e non perdere news e consigli su università e lavoro.
