Unit 6: Pulp and Paper Industries: Wood Chemistry, Kraft Cycle & Sheet Forming

Wood is an anisotropic, cellular natural composite engineered from three principal biopolymer fractions arranged within concentric plant cell walls (middle lamella, primary wall, and secondary wall layers $S_1, S_2, S_3$):

1. Cellulose ($40 - 45\text{ wt}\%$)

The structural skeleton of the fiber wall. A linear syndiotactic hom*opolymer of $\beta\text{-D-glucopyranose}$ residues linked exclusively via $\beta\text{-(1}\to\text{4)-glycosidic}$ bonds. Each anhydroglucose repeat unit ($\text{C}_6\text{H}_{10}\text{O}_5$, $M_0 = 162.14\text{ g/mol}$) is rotated $180^\circ$ relative to its neighbor, forming cellobiose repeat units:

• Native wood cellulose exhibits a high degree of polymerization ($\overline{DP}_n \approx 10,000$).
• Intramolecular hydrogen bonds ($\text{O(3)-H}\cdots\text{O(5)}$ and $\text{O(2)-H}\cdots\text{O(6)}$) stiffen the glucan chains into rigid, straight ribbons.
• Intermolecular hydrogen bonds pack parallel chains into crystalline elementary microfibrils ($3 - 5\text{ nm}$ width), conferring immense tensile strength ($> 1,000\text{ MPa}$).

2. Hemicellulose ($20 - 30\text{ wt}\%$)

Heterogeneous, branched, low-molecular-weight polysaccharides ($\overline{DP}_n \approx 100 - 200$) acting as a matrix compatibilizer between cellulose microfibrils and lignin:

• Softwoods (Gymnosperms, conifers): Predominantly Galactoglucomannans ($15 - 20\%$, linear backbone of $\beta\text{-(1}\to\text{4)}$-linked D-mannose and D-glucose, acetylated, with $\alpha\text{-(1}\to\text{6)}$-D-galactose side branches) and Arabinoglucuronoxylan ($5 - 10\%$).
• Hardwoods (Angiosperms, deciduous): Predominantly $O$-Acetyl-4-$O$-methylglucuronoxylan ($20 - 35\%$, linear $\beta\text{-(1}\to\text{4)}$-D-xylose backbone substituted with $4\text{-O-methyl-}\alpha\text{-D-glucuronic acid}$ residues) and Glucomannan ($2 - 5\%$).

3. Lignin ($20 - 30\text{ wt}\%$)

A three-dimensional, amorphous, highly crosslinked polyphenolic macromolecule that permeates the middle lamella and secondary cell walls, cementing fibers together and providing compressive strength and hydrophobicity. Lignin is biosynthesized via enzymatic dehydrogenative polymerization of three phenylpropane monolignol precursors:

1. $p$-Coumaryl alcohol $\longrightarrow$ $p$-Hydroxyphenyl ($\text{H}$) units.

2. Coniferyl alcohol $\longrightarrow$ Guaiacyl ($\text{G}$) units (dominant in softwoods, $> 95\%$).

3. Sinapyl alcohol $\longrightarrow$ Syringyl ($\text{S}$) units (hardwoods contain approximately equal mixtures of $\text{G}$ and $\text{S}$ units).

The monolignols are interconnected by diverse carbon-oxygen and carbon-carbon covalent linkages:

• $\beta\text{-O-4}$ (arylglycerol-$\beta$-aryl ether, constitutes $50 - 65\%$ of all linkages; most easily cleaved in chemical pulping).
• $\alpha\text{-O-4}$ ether linkages.
• $\beta\text{-5}$ (phenylcoumaran, $10 - 12\%$).
• $5\text{-5}^\prime$ (biphenyl, $5 - 10\%$).
• $\beta\text{-}\beta^\prime$ (resinol, $3 - 5\%$).

The primary objective of chemical pulping is to selectively dissolve and extract the inter-fiber lignin binder (delignification) while minimizing depolymerization of cellulose, thereby releasing intact, flexible, high-strength cellulosic fibers.

Comparison of Principal Pulping Technologies

| Parameter | Kraft (Sulfate) Process | Sulfite Process | Soda Process | |---|---|---|---| | Active Cooking Chemicals | $\text{NaOH} + \text{Na}_2\text{S}$ | $\text{H}_2\text{SO}_3 + \text{HSO}_3^-$ ($\text{Ca}^{2+}, \text{Mg}^{2+}, \text{Na}^+, \text{NH}_4^+$) | $\text{NaOH}$ alone | | Cooking pH Range | Strongly Alkaline ($\text{pH } 13 - 14$) | Strongly Acidic to Neutral ($\text{pH } 1.5 - 7$) | Strongly Alkaline ($\text{pH } 13 - 14$) | | Cooking Temperature | $165 - 175^\circ\text{C}$ | $130 - 150^\circ\text{C}$ | $160 - 175^\circ\text{C}$ | | Cooking Pressure | $7 - 9\text{ bar}$ | $5 - 7\text{ bar}$ | $7 - 9\text{ bar}$ | | Raw Material Versatility | Universal (all softwoods, hardwoods, bamboo, bagasse) | Restricted (resin-poor woods, spruce, fir, birch) | Non-wood annual plants (straw, bagasse) | | Pulp Strength | Outstanding (Highest tensile & burst) | Moderate (Lower tear strength) | Weak to Moderate | | Chemical Recovery | Closed-loop cyclic recovery ($> 97\%$) | Complex (Magnesium-base only) | Feasible |

The Kraft Cleavage Mechanism

The Kraft process relies on hydrosulfide ions ($\text{HS}^-$) acting as potent nucleophiles to accelerate the cleavage of ether linkages without excessive carbohydrate degradation:

1. Hydroxide ion ($\text{OH}^-$) deprotonates phenolic hydroxyl groups, generating a phenolate anion.
2. The phenolate induces neighboring-group expulsion of water or alcohol from the $\alpha$-carbon, forming a strained quinone methide intermediate.
3. Hydrosulfide ion ($\text{HS}^-$) nucleophilically attacks the quinone methide $\alpha$-position, creating a reactive benzylic thiol.
4. Intramolecular nucleophilic attack of the thiolate sulfur on the adjacent $\beta$-carbon cleaves the critical $\beta\text{-O-4}$ ether bond, fragmenting the lignin polymer into soluble phenolate fragments and liberating an episulfide intermediate:

``` Quinone Methide + HS⁻ ──> Benzylic Thiol ──> Intramolecular S attack cleaves β-O-4 bond! Fragments lignin into alkali-soluble phenolate ```

Without $\text{HS}^-$ (as in the pure soda process), quinone methides undergo rapid alkali-induced condensation with adjacent aromatic rings, forming refractory carbon-carbon bonds that arrest delignification.

In industrial Kraft pulping, cooking chemical concentrations are standardized by expressing all sodium salts in terms of equivalent sodium oxide ($\text{Na}_2\text{O}$, $M = 61.98\text{ g/mol}$) or sodium hydroxide ($\text{NaOH}$, $M = 40.00\text{ g/mol}$):

Canonical White Liquor Terminology

1. Total Titratable Alkali ($\text{TTA}$):

2. Active Alkali ($\text{AA}$):

Chemical species directly contributing to alkaline cooking:

3. Effective Alkali ($\text{EA}$):

Takes into account that sodium sulfide hydrolyzes to liberate only one equivalent of hydroxide:

4. Sulfidity ($\% S$):

The percentage of active alkali represented by sodium sulfide:

Optimal Kraft sulfidity ranges between $25\%$ and $35\%$.

5. Causticity ($\% C$):

Efficiency of sodium carbonate conversion in the recausticizing plant:

The H-Factor Kinetics Concept

Delignification in a Kraft digester depends non-linearly on cooking temperature and residence time. Kenneth Vroom (1957) synthesized these variables into a single dimensionless kinetic parameter, the $H\text{-Factor}$, by integrating relative reaction rates based on the Arrhenius equation with an activation energy $E_a = 134.0\text{ kJ/mol}$:

At $100^\circ\text{C}$ ($373.15\text{ K}$), $k_{\text{rel}} = 1.0$. At $170^\circ\text{C}$ ($443.15\text{ K}$), $k_{\text{rel}} \approx 870$. An entire cooking schedule (heating ramp + cooking plateau) yielding an $H\text{-factor}$ of $1200 - 1800$ ensures reproducible target Kappa numbers regardless of minor steam temperature variations.

The Kraft process owes its overwhelming global dominance to its closed-loop chemical recovery cycle, which achieves $> 97\%$ recovery of sodium and sulfur cooking chemicals while generating massive surplus electrical power and high-pressure steam from lignin combustion:

``` THE KRAFT CHEMICAL RECOVERY CYCLE Wood Chips + White Liquor (NaOH + Na2S) │ ▼ ┌──────────────┐ │ DIGESTER │ ──> Brownstock Pulp to Bleaching / Papermaking └──────┬───────┘ ▼ Weak Black Liquor (~15% solids) ┌──────────────┐ │ MULTI-EFFECT │ ──> Condensate to Washers │ EVAPORATORS │ └──────┬───────┘ ▼ Heavy Black Liquor (~70-80% solids) ┌────────────────────────────────────────────────────────┐ │ TOMLINSON RECOVERY BOILER │ <── Na2SO4 Make-up │ │ ──> High-Pressure Steam (80 bar) │ Upper: Steam Generating Tubes & Superheaters (Combustion)│ ──> Power Turbines │ Lower: Reducing Char Bed (950-1050°C) │ │ Na2SO4 + 2 C ──> Na2S + 2 CO2 (Smelt Reduction) │ └──────────────────────────┬─────────────────────────────┘ ▼ Molten Smelt (Na2S + Na2CO3 at ~850°C) ┌──────────────────────────┴─────────────────────────────┐ │ SMELT DISSOLVER (Water / Weak Wash) │ └──────────────────────────┬─────────────────────────────┘ ▼ Raw Green Liquor ┌──────────────────────────┴─────────────────────────────┐ │ CAUSTICIZING PLANT (Slaker + Causticizers + Lime Kiln) │ └──────────────────────────┬─────────────────────────────┘ ▼ Regenerated White Liquor (NaOH + Na2S) back to Digester! ```

1. Multiple-Effect Black Liquor Evaporation

Weak black liquor discharged from brownstock washing contains $14 - 17\text{ wt}\%$ dissolved solids (organics: lignin, hemicellulose breakdown acids; inorganics: $\text{Na}_2\text{CO}_3, \text{Na}_2\text{SO}_4, \text{Na}_2\text{S}$). It is concentrated to $70 - 80\text{ wt}\%$ dry solids in a 6- or 7-effect falling-film evaporator train, reaching high solids to enable stable combustion in the recovery furnace.

2. Tomlinson Recovery Boiler Thermochemistry

The concentrated heavy black liquor is sprayed through oscillating splash-plate nozzles into the furnace of a massive recovery boiler operating under two distinct chemical zones:

• Lower Reducing Zone (Char Bed at $950 - 1050^\circ\text{C}$):

Under substoichiometric primary air, carbon in the char bed acts as a powerful reducing agent, converting oxidized sodium sulfate back into sodium sulfide:

Sodium carbonate melts without reduction ($T_{\text{melt}} = 851^\circ\text{C}$). The resulting molten mixture of $\text{Na}_2\text{S}$ ($25 - 30\text{ mol}\%$) and $\text{Na}_2\text{CO}_3$ ($70 - 75\text{ mol}\%$) forms red-hot liquid smelt at $850^\circ\text{C}$ that drains continuously through water-cooled smelt spouts.

• Upper Oxidizing Zone ($1100 - 1200^\circ\text{C}$):

Secondary and tertiary air jets inject oxygen to burn volatile pyrolysis gases ($\text{CO}, \text{H}_2, \text{CH}_4$, and organic vapors), releasing intense thermal energy to superheater tubes, producing high-pressure steam ($60 - 100\text{ bar}$, $450 - 500^\circ\text{C}$) that drives turbo-generators.

Tomlinson Recovery Boiler Smelt & Black Liquor Combustion Energy Balance

Concentrated black liquor dry solids ($75 - 80\text{ wt}\%$ dry matter) combust with an adiabatic lower heating value $\text{LHV} \approx 13,500 - 14,500\text{ kJ/kg dry solids}$:

• Primary Air ($40 - 45\%$ total air): Injected directly at the hearth level at low velocity to maintain the reducing char bed temperature at $950 - 1050^\circ\text{C}$ ($\lambda_{\text{primary}} = 0.65 - 0.75$, substoichiometric).
• Secondary Air ($30 - 35\%$): Injected above the char bed to burn volatile pyrolysis gases and stabilize the bed geometry.
• Tertiary & Quaternary Air ($25 - 30\%$): Injected in the upper furnace at high velocity ($50 - 80\text{ m/s}$) to create intense turbulence, ensuring $100\%$ burnout of $\text{CO}$ and volatile sulfur species ($\text{H}_2\text{S}, \text{CH}_3\text{SH}$).

Superheated steam is generated at $450 - 490^\circ\text{C}$ and $60 - 90\text{ bar}$, generating $3.2 - 3.8\text{ tons of high-pressure steam per metric ton of black liquor dry solids}$.

The molten smelt flowing from the recovery boiler spouts is shattered with high-pressure steam jets and dissolved in weak wash water inside the agitated smelt dissolving tank:

The resulting solution is emerald green due to colloidal iron sulfide ($\text{FeS}$) complexes.

1. Dreg Separation & Slaker-Causticizer Chemistry

1. Green Liquor Clarification: Heavy insoluble particles ("dregs": unburned carbon, iron sulfide, silica) are removed in a rake clarifier or pressurized disc filter.

2. Lime Slaking & Causticizing: The clarified green liquor enters an agitated slaker vessel where reburned quicklime ($\text{CaO}$) is added:

• Exothermic Slaking Reaction:

• Causticizing Equilibrium Reaction:

Sodium sulfide ($\text{Na}_2\text{S}$) passes through completely unaffected, while $\text{Na}_2\text{CO}_3$ is converted into active $\text{NaOH}$, regenerating White Liquor.

Thermodynamic Equilibrium & Causticizing Efficiency

The causticizing reaction is reversible and limited by the solubility product ratio of $\text{Ca(OH)}_2$ and $\text{CaCO}_3$:

As white liquor total titratable alkali ($\text{TTA}$) increases, the high concentration of $[\text{OH}^-]$ drives the equilibrium backward, suppressing causticizing efficiency ($\text{CE}$):

In industrial practice, $\text{CE}$ is limited to $80 - 85\%$ to prevent unreacted slaked lime ("free lime") from blinding the lime mud filters.

2. Lime Mud Dewatering & Rotary Lime Reburning Kiln

The precipitated calcium carbonate mud ($\text{CaCO}_3$) is separated from white liquor in precoat vacuum drum filters, washed, and fed at $75 - 80\text{ wt}\%$ dry solids into a rotary lime reburning kiln ($70 - 110\text{ m}$ length) fired by natural gas, fuel oil, or gasified biomass:

The calcined quicklime is discharged and pneumatically conveyed back to the slaker, closing the inorganic lime cycle.

Unbleached Kraft pulp retains a dark brown appearance (ISO brightness $25 - 35\%$) due to residual modified lignin ($3 - 5\text{ wt}\%$ of dry pulp) containing chromophoric quinones, quinone methides, and conjugated stilbene structures. Bleaching achieves an ISO brightness $> 88 - 90\%$ through selective oxidative delignification and chromophore destruction.

Evolution from Elemental Chlorine to ECF & TCF

• Elemental Chlorine Bleaching ($C\text{-}E\text{-}H\text{-}D$): Historically used molecular chlorine ($\text{Cl}_2$) at acidic $\text{pH}$. Electrophilic aromatic substitution generated toxic, bioaccumulative polychlorinated dibenzo-$p$-dioxins ($2,3,7,8\text{-TCDD}$), dibenzofurans, and absorbable organic halides ($\text{AOX}$), leading to global environmental bans.
• Elemental Chlorine-Free (ECF): Replaces $\text{Cl}_2$ with chlorine dioxide ($\text{ClO}_2$). Operates as a selective one-electron oxidant, eliminating dioxin formation and cutting $\text{AOX}$ by $> 90\%$.
• Totally Chlorine-Free (TCF): Eliminates all chlorine-containing compounds, using oxygen ($\text{O}$), ozone ($\text{Z}$), and hydrogen peroxide ($\text{P}$).

Modern Bleaching Stage Chemistries

1. Oxygen Delignification ($\text{O}$-Stage, $90 - 105^\circ\text{C}$, $4 - 6\text{ bar O}_2$, $\text{pH } 11 - 12$):

Removes $40 - 50\%$ of residual lignin prior to the bleach plant. Oxygen reacts with phenolate anions to form hydroperoxide radicals, initiating oxidative aromatic ring cleavage into dicarboxylic acids (muconic acid derivatives).

2. Chlorine Dioxide Stage ($\text{D}$-Stage, $70 - 80^\circ\text{C}$, $\text{pH } 2.5 - 3.5$):

Chlorine dioxide is manufactured on-site (Erco R8 or SVP-Lurgi process) by reducing sodium chlorate ($\text{NaClO}_3$) with methanol or hydrogen peroxide in sulfuric acid:

3. Alkaline Extraction Stage ($\text{E}$-Stage, reinforced with $\text{O}_2$ and $\text{H}_2\text{O}_2$, $\text{EOP}$):

Extracts solubilized oxidized lignin fragments with aqueous $\text{NaOH}$ while peroxide cleaves stubborn carbonyl chromophores.

The papermaking machine transforms a dilute fiber suspension ($0.5 - 1.0\text{ wt}\%$ fibers in water) into a dry, smooth, continuous web of paper ($92 - 95\text{ wt}\%$ dry matter) at linear speeds exceeding $1,200 - 2,000\text{ m/min}$ ($70 - 120\text{ km/h}$).

``` THE FOURDRINIER PAPER MACHINE Stock Approach (0.8% solids) │ ▼ ┌──────────────┐ │ HEADBOX │ (Slice Jet Velocity Matching Wire Speed) └──────┬───────┘ ▼ ┌────────────────────────────────────────────────────────┐ │ FORMING SECTION (Fourdrinier Wire) │ ──> Gravity / Foil Dewatering │ (Continuous moving bronze or synthetic mesh) │ ──> Suction Vacuum Boxes └──────────────────────────┬─────────────────────────────┘ ▼ Wet Paper Web (~20% solids) ┌────────────────────────────────────────────────────────┐ │ PRESS SECTION (Shoe Press / Roll Presses) │ ──> Mechanical Expressing of Water │ (Felt-supported nip under 5-10 MPa pressure) │ (Web reaches ~45-50% solids) └──────────────────────────┬─────────────────────────────┘ ▼ Web (~45-50% solids) ┌────────────────────────────────────────────────────────┐ │ DRYER SECTION (40-60 Steam-Heated Drying Cylinders) │ ──> Condensing Steam (2-5 bar) │ (Enclosed hood, ventilation air removal of steam) │ Evaporates water to 93-95% solids! └──────────────────────────┬─────────────────────────────┘ ▼ ┌──────────────┐ │ CALENDER │ (Heated Steel Rolls: Compacting, Caliper Control & Gloss) └──────┬───────┘ ▼ ┌──────────────┐ │ REEL / WINDER│ ──> Jumbo Paper Rolls (Parent Reel) └──────────────┘ ```

The Three Progressive Dewatering Sections

1. Forming (Wire) Section ($0.8\% \to 20\%$ solids):

The pressurized headbox discharges an ultra-uniform turbulent jet of dilute stock through a precision slice opening onto a rapidly traveling endless loop of synthetic woven wire. Water drains gravimetrically through hydrofoils, vacuum suction boxes, and the couch roll, forming an interconnected, non-woven fiber mat. Dewatering cost is lowest here ($< 1\%$ total energy).

2. Press Section ($20\% \to 45 - 50\%$ solids):

The wet sheet is conveyed between continuous absorbent synthetic felts through rotating press nips (modern shoe presses exert pressures up to $5 - 10\text{ MPa}$ over an extended nip shoe width of $250\text{ mm}$). Water is mechanically expressed out of the sheet into the felt pores. Removing $1\text{ kg}$ of water mechanically consumes only $5 - 10\%$ of the energy required to evaporate it thermally.

3. Dryer Section ($45\% \to 94\%$ solids):

The remaining water ($1.0 - 1.2\text{ kg water / kg dry paper}$) cannot be expelled mechanically due to capillary retention within fiber lumen and cell wall micro-pores. The sheet snakes over $40 - 60$ cast-iron steam-heated drying cylinders ($1.5 - 1.8\text{ m}$ diameter) heated by condensing saturated steam ($2 - 5\text{ bar}$, $120 - 150^\circ\text{C}$). Steam consumption in the dryer section typically averages $1.3 - 1.8\text{ kg steam / kg water evaporated}$.

The contemporary pulp mill has transformed into an integrated forest biorefinery, producing advanced biomaterials and green biofuels alongside traditional paper grades:

1. Nanocellulose Engineering (CNF & CNC)

High-purity chemical wood pulps can be dismantled into nanoscale crystalline building blocks:

• Cellulose Nanofibrils (CNF): Produced by enzymatic or chemical pretreatment (TEMPO-mediated oxidation: selective conversion of $\text{C}_6$ primary hydroxyls to carboxylates, $-\text{CH}_2\text{OH} \to -\text{COO}^-$) followed by high-pressure homogenization ($1,000 - 1,500\text{ bar}$). Generates flexible, high-aspect-ratio nanofibrils ($5 - 20\text{ nm}$ diameter, several microns length) forming transparent, highly oxygen-impermeable barrier films.
• Cellulose Nanocrystals (CNC): Concentrated sulfuric acid hydrolysis ($64\text{ wt}\%\text{ H}_2\text{SO}_4$, $45^\circ\text{C}$) selectively digests amorphous cellulose domains, leaving pristine, defect-free crystalline nanorods ($d \approx 5\text{ nm}$, $L \approx 150 - 250\text{ nm}$). CNCs exhibit an axial Young's modulus exceeding $140 - 150\text{ GPa}$ (comparable to Kevlar and exceeding structural steel on a specific density basis).

2. Technical Lignin Valorization

Rather than burning all black liquor lignin for low-grade steam, modern processes (e.g., LignoBoost) precipitate high-purity Kraft lignin:

• Weak or semi-concentrated black liquor is acidified with $\text{CO}_2$ gas to $\text{pH } 9.5 - 10.0$, deprotonating phenolate groups and precipitating solid colloidal lignin.
• The recovered lignin is washed with dilute sulfuric acid and dewatered into dry brown powder ($> 98\%$ purity).
• High-Value Applications:
• Precursors for polyacrylonitrile-free bio-based carbon fibers.
• Phenol replacement in phenol-formaldehyde wood adhesives.
• Catalytic depolymerization into bio-vanillin and aromatic BTX chemical commodities.

3. Black Liquor Gasification (Chemrec Process)

Pressurized entrained-flow gasification ($950 - 1000^\circ\text{C}$, $30\text{ bar}$) converts concentrated black liquor with pure oxygen into high-quality synthesis gas ($\text{CO} + \text{H}_2$) and molten inorganic smelt. Gasification efficiency exceeds traditional Tomlinson boilers, enabling synthesis of renewable Bio-DME and synthetic green biomethanol.

University Honors Industrial Case Study: Smelt-Water Explosions in Recovery Boiler Dissolving Tanks

A catastrophic physical hazard in Kraft recovery boiler operation is the Smelt-Water Explosion:

• Explosion Physics: Molten smelt discharged at $850^\circ\text{C}$ consists of liquid ionic salts ($\text{Na}_2\text{S} + \text{Na}_2\text{CO}_3$). If a water boiler tube ruptures above the hearth, liquid water pours into the pool of molten smelt.
• Rapid Phase Transition (RPT): Because smelt temperature is far above the superheat limit of water ($300^\circ\text{C}$), water is trapped beneath the heavy liquid smelt. Violent nucleate boiling transitions into an explosive vapor explosion:

The resulting shock wave generates peak pressures exceeding $100\text{ bar}$, capable of ripping the heavy steel boiler shell apart.

• Mitigation: Automated Emergency Drain Systems (EDS), high-energy steam shattering jets at smelt spouts, and acoustic leak detectors on furnace boiler tubes.