Question 5 of 6: Kraft Recovery Cycle — Evaporation, Recovery Boiler, Tall Oil, Digesters & Continuous-Digester Kappa Control

Question 5: Kraft Recovery Cycle — Evaporation, Recovery Boiler, Tall Oil, Digesters & Continuous-Digester Kappa Control (a)–(d) 10 marks each, (e) 20 marks

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Multiple-effect evaporation of Kraft black liquor

Weak black liquor leaves the brown-stock washers at roughly 12–18% dissolved solids (DS) — far too dilute to burn. A multiple-effect evaporator (MEE) train concentrates it to 45–55% DS (the feed to a direct- or non-direct-contact recovery boiler) using the vapour boiled off one effect as the heating medium for the next, so only the first effect needs live (prime) steam. Liquor and vapour normally flow counter-currently: the thinnest liquor enters the coolest, lowest-pressure effect and is progressively concentrated as it is pumped forward through hotter effects, while the vapour train runs the opposite direction, from the hottest (first) effect down to the coolest (last), whose vapour is finally condensed in a surface condenser under vacuum. Concentrating the liquor in the LAST (hottest) effects rather than the first avoids scaling/boiling-point-elevation problems at the point where the liquor is most viscous and most prone to organic fouling.

Steam economy is the ratio of water evaporated to live steam consumed, $\text{SE}=\dot m_{evap}/\dot m_{live\ steam}$, and scales roughly with the number of effects (each effect re-uses latent heat already paid for once): a well-designed 5–6-stage kraft black-liquor evaporator section typically achieves 4–5.5 kg water evaporated per kg live steam, the shortfall below the theoretical N-effect ceiling coming mainly from boiling-point elevation and flash losses.

(b) Kraft recovery boiler — steam generation and chemical recovery

The recovery boiler is simultaneously a power boiler and a chemical reactor. Concentrated (45–80% DS) black liquor is sprayed through liquor guns onto the lower furnace walls, forming a char bed on the floor; combustion is staged, with primary air admitted low (at/below the char bed) in a reducing, oxygen-starved atmosphere so the inorganic sodium/sulphur species are reduced rather than oxidized, while secondary/tertiary air admitted higher up completes combustion of the organic (largely lignin-derived) fraction, raising high-pressure steam in the boiler's water-wall tubes and superheater — typically fed to a back-pressure/extraction turbine-generator, making the recovery boiler the mill's principal source of both process steam and electrical power.

Chemically, the furnace converts the spent, oxidized sodium/sulphur compounds back into a re-usable, smeltable form. The dominant reduction reaction in the char bed is $$\text{Na}_2\text{SO}_4+2\text{C}\rightarrow\text{Na}_2\text{S}+2\text{CO}_2,$$ while sodium bound in organic salts is oxidized and released as inorganic sodium carbonate. The molten mixture of Na₂CO₃ and Na₂S (the smelt) drains continuously through smelt spouts into the smelt dissolving tank, where it dissolves in weak wash to form green liquor, which is then causticized with lime to regenerate white liquor for the digesters. A modern recovery unit's overall thermal efficiency (fuel heating value to useful steam) is typically 60–68%.

(c) Crude Tall Oil

Crude Tall Oil (CTO) is a mixture of fatty acids, resin (rosin) acids and unsaponifiable neutral compounds recovered as a Kraft by-product. During alkaline cooking, the wood's natural extractives (chiefly the fatty- and resin-acid esters in resin canals) are saponified by the free NaOH in the white liquor, forming sodium soaps, which are less dense than black liquor and, being surface-active, rise and skim off as a separate layer in the storage/evaporator-feed tanks — this skimmed material is crude tall oil soap. The soap is collected and acidulated (typically with sulphuric acid) to convert the sodium soaps back to free fatty/resin acids and split the emulsion, yielding CTO, which is then fractionally vacuum-distilled off-site into fatty-acid, rosin-acid and pitch/head fractions. Softwood species yield far more CTO than hardwoods — especially resin-rich pines — because resin content is concentrated in the resin canals unique to softwood anatomy; hardwoods have no resin canals and negligible extractives content. In a Kraft mill, CTO is sold as a raw material for downstream oleochemical, adhesive and ink manufacture; some fractions may also be used on-site as fuel.

(d) Batch digesters vs. the conventional Kamyr (continuous) digester

Batch digesters cook a discrete, weighed charge of chips and liquor as a single closed vessel taken through fill–heat-up–cook–blow steps, then emptied and refilled. Advantages: simple, robust, individually controllable (any one vessel's schedule can be adjusted or shut down without stopping the others), tolerant of chip-quality variability, lower capital cost per unit for smaller mills. Disadvantages: cyclic (not steady-state) heat and chemical demand surges the recovery cycle and requires blow-heat-recovery equipment; lower digester utilization (time lost to fill/heat/blow); typically higher specific steam consumption than continuous cooking, since each batch's sensible heat is largely lost at blow unless recovered.

The Kamyr (continuous) digester is a single tall pressure vessel through which chips and liquor move continuously at steady state. Advantages: smooth, steady steam/chemical demand on the recovery cycle; higher thermal efficiency (heat recovered internally via counter-current liquor extraction rather than lost at a blow); higher capacity per unit vessel volume; more consistent pulp quality at steady state. Disadvantages: much higher capital cost, more complex control, less tolerant of chip-quality upsets (a disturbance propagates down the whole column), and a full outage is required to correct a serious process upset. In general, a continuous digester is more advantageous for a large, steady-throughput Kraft line because of its lower specific steam consumption and higher utilization, while batch digesters remain more advantageous for smaller mills or those cooking varied furnishes/grades, where flexibility matters more than marginal steam economy.

(e) Raising a low Kappa Number out of a continuous digester

Kappa Number tracks residual lignin, so a Kappa reading below target means the pulp has been over-delignified relative to setpoint. The operator has several coupled variables available to bring Kappa back up (reduce delignification severity): (i) reduce cooking temperature in the cook zone (delignification rate is strongly Arrhenius-dependent on temperature, so even a small reduction slows lignin removal significantly); (ii) reduce residence time in the cook zone, by increasing chip throughput/production rate or adjusting the extraction-zone level that sets the cook-zone boundary; (iii) reduce effective alkali charge (lower liquor-to-wood ratio or lower EA% applied at the impregnation/circulation points), since less alkali available dissolves less lignin for the same time/temperature; (iv) reduce cooking (H-factor) severity more generally — H-factor combines time and temperature into the single target most digester control systems actually manipulate to hold Kappa on target.

Each of these adjustments carries a trade-off. Reducing temperature or H-factor to raise Kappa reduces the delignification rate, so — unless residence time is simultaneously increased — it also reduces achievable digester-house throughput/capacity at the higher target Kappa (less cooking work done per unit time). Reducing residence time (raising the extraction draw-off, i.e. speeding chips through the cook zone) has the opposite effect on throughput (MORE capacity) but risks less-uniform cooking. On the chemical recovery side, reducing EA charge directly reduces the alkali/black-liquor solids loading sent forward to the evaporators and recovery boiler (lower recovery-cycle throughput and, correspondingly, lower recovered steam generation per tonne of pulp), while reducing temperature/H-factor at a FIXED EA charge leaves recovery-cycle chemical loading essentially unchanged but slightly raises the black liquor's residual (unreacted) alkali content, which the causticizing plant must still process. All of these trims are made at the digester's liquor heaters/circulation-loop setpoints and at the extraction-zone level control, not by any single valve.