Batch Digester
Quak Foo Lee
M.A.Sc., B.A.Sc. (Chem. Eng.)
Ph.D. Candidate
Department of Chemical and Biological Engineering



Batch digester
Figure 1: A line with washed stock screening after the washers. Some lines have full brown stock screening before the washers instead.



Several conical digesters are used, but their operation is staggered so as to provide both a fairly consecutive demand on inputs (example chips, cooking liquor, and steam) and a fairly constant pulp output. The digester cover-to-cover time for a batch is 2-4 hr, and conventionally involves the following procedure:

 The wood chips and cooking liquor are charged to the digester through the chip charging nozzle at the top, which may have a capping valve

 The chips are weighed during charging, by a weightometer (e.g. loadcell) under the chip conveyor.   Chip moisture is determined, to give the bone dry wood added to the digester. Off-line moisture measurement can take as little as 4 min. 

 The cooking liquor is a mixture of white liquor (mainly sodium hydroxide and sodium sulfide) and black liquor, and contains the desired total chemical charge. Example:

Bone dry wood

AA as Na2O (or NaOH)

Softwoods

14-19% 

Hardwoods

13-16% 

Here AA is the active alkali and equals NaOH + Na2S (both as Na2O or NaOH, in g/L). Some prefer to control by effective alkali (EA), which equals NaOH + Na2S (both as Na2O or NaOH, in g/L). Both AA and EA can be measured in-line. For example: 1054A Alkali Microprocessor Analyzer (Fisher-Rosemount), Cookstar (Honeywell).

Sulfidity, which equals (Na2S x 100) active alkali (both as Na2O or NaOH in g/L), falls into the range 22-45%. Chip penetration slows down below 17%. The unnecessarily large excess over this can result from high mill recovery of sulfur emissions and recycling certain chlorine dioxide generator effluent.

Liquor-to-wood ratio (hydromodulus) is the weight of total liquor to wood (using the bone dry wood basis).

Note: Na2O is generally used in North American, and NaOH in Europe. The conversion factor is:
 Na2O x 1.29 = NaOH.

 The digester is brought up to cooking temperature (approx. 170C) and pressure (700 kPa, 100 psig) by indirect heating via liquor heaters. Formerly, direct steam addition was used, but this added water that subsequently had to be evaporated.

 If cooking softwood, the digester is relieved to collect the turpentine.

 The digester is held at 170C for the required time. The area under the time/delignification rate curve gives a measure of the amount of delignification and is called the H factor. Each temperature has a different delignification rate relative to 100C = 1. [Vroom, K.E. "The H Factor: A means of expressing cooking times and temperatures as a single variable". Pulp Paper Mag. Can.,1957, 58 (C), pg. 228]. Example:

TABLE I

Temperature C 

H-factor

100

1

160

401

169

855

170

927

171

1005

H-Factor and the associated sequencing can be controlled by computer. Example, ABB AutoCook (ABB Automation), CyberCOOK (Orion CEM).

The degree of delignification of the pulp, and thus its bleachability (i.e. ease of bleaching), is measured by the kappa number. This is the mL of a 0.1 N strength potassium permanganate solution (KMnO4) consumed per gram of pulp. Kappa number can be measured in-line, using measurement principles such as lignin's absorption of UV light. For example:

ABB STFI OPTI-Kappa (ABB Automation), 

KNA-5100 (BTG), 

kajaaniKAPPAi

Kappa Analyzer (Metso Automation), 

NIR-Kappa (Procheck, Honeywell). 

In high yield applications, a small refiner/screen can break up knots and shives to give a more meaningful in-line value. Eg: REF-1100, which works before the KNA-5100 (BTG).

Similar delignification measures, that are used less nowadays, include the permanganate number (Kappa Number) and chlorine number.

 3 different types of hemicellulose degradation occur during the cook (eg: primary peeling). Some redeposition of lignin and xylan occurs on the fibers towards the end of the cook.

At the end of the cook, the black liquor consists of lignin combined with the sodium hydroxide (giving soluble sodium lignates) plus hemicellulose dissolved from the wood. It has a dark brown color and looks like thin brown treacle.

 The contents of the digester are forced through the blow valve by the pressure in the digester, and pass via the blowline into the blow tank. This mechanical action disintegrates the remaining chips.

 Heat is recovered, from the steam flashed during blowing, to provide hot process water for the mill. This hot water is produced and stored in an accumulator. Well-designed systems can give 99C water at the top of the accumulator.

 

Material of Construction

The materials of construction utilized are important, because of the corrosive environment. Batch digesters have traditionally been built of carbon steel, using a thickness that includes a corrosion allowance. Areas that subsequently erode are commonly weld overlaid with SS (eg: in situ by Uddcomb Method, Uddcomb Engineering). Cladding with SS (eg: 304L) is an option, but at greater initial cost. Nowadays, most NA carbon steel digesters are made from A516-Grade 70, because the corrosion rate increases with silica content. Worldwide, duplex stainless steels (either in clad or solid form. eg: 1803, 2205) are being used increasingly for new digesters because of their superior corrosion resistance. [Wensley, A. "Corrosion of batch and continuous digesters". Preprints of 9th International Symposium on Corrosion in the Pulp and Paper Industry, Ottawa, May 1998, pg. 27].

A 1999 CPPA Alkaline Pulping Committee survey questionnaire of Canadian mills' batch digesters found that:

 77% of the 100 digesters surveyed (at 11 mills) were 28-48 years old, and were mainly carbon steel.

 Inspection methods included: visual; liquid penetrate; ultrasonic thickness; wet fluorescent magnetic particle.

 The sulfidity employed ranged 22-34%. Measures against corrosion were strikingly more necessary at the upper end of this sulfidity range than at the bottom end.

 Eight mills had applied stainless steel weld overlay to at least some part of some of their digesters, and another mill was intending to do so. The use of 312 weld overlay seems to be becoming standard practice in many mills. The two other mills had applied thermal metal spray coatings.

[Clarke, S.J. and Nadezhdin, A. Canadian batch digester corrosion survey. Preprints of PAPTAC Annual Meeting, Montreal, Feb 2000, pg. A239].

A detailed evaluation (by Life Cycle Costing) of alternative approaches to protecting an old and corroded carbon steel batch digester has been provided by F-IPK. The 4 approaches evaluated were:

 Weld overlay. The gas metal arc welding process gives good results. The overlay material is commonly 312 duplex SS (Cr 29%, Ni 9%), while 309 SS (23% Cr) is also used. Eg: (Uddcomb Engineering).

 Thermal metal spray coating. New coating methods give low porosity (< 1%), good bonding strength, and dense stable oxide formation (continuous thin ceramic film). Eg: Combustion arc method with DenSys-CA 625 (contains Cr 20%, Ni 58%, Mo 8%, Fe 5%). But preparation of the carbon steel wall is important (eg: smooth clean surface). Eg: (Metalspray International).

 Anodic protection has been used for both batch digesters and continuous digesters. No special surface treatment is required. But the mechanical strength of the cathodes can be troublesome, and the potential and current must be customized to the cooking method.

 Purchase a new (and larger) digester.

Some basic assumptions were:

TABLE II

 

Weld Overlay

Thermal Metal Spray Coating

Anodic Protection

Lifetime (year)

> 20

5-7

about 20

at Corrosion rate (mm/yr)

0.2

0.08

0.2

They concluded that if the old digester was working well and there were no funds for a new digester, the old digester should be protected by either weld overlay or thermal metal spray coating (which had similar total costs). [Klarin, A. "Justifying better material with life cycle cost (LCC) methodology". PPI Paper Industry Maintenance Europe '98 Conference, Amsterdam, Nov 1998].

 

Batch Digester - Recent Process Improvements

Extended delignification is an approach of the 1980s that involves taking out additional lignin at the cooking stage. This allows the additional lignin removed to enter the chemical recovery cycle - rather than go to the bleach plant and then effluent treatment. Conventionally, extended delignification has been achieved either by more demanding cooking conditions (eg: extending the cooking time), or utilizing 1 of the optimizing systems outlined below. In a Kamyr digester, MCC etc are employed. These approaches cause a yield reduction. Recently, AQ/polysulfide pulping has appeared a possibly preferable approach - due to its aggravating neither the yield nor any chemical recovery bottleneck situation, and its low capital cost.

The table compares kappa numbers from conventional cooking and extended delignification.

TABLE III

 

Conventional

Extended

Optimized

Softwoods

28-35

15-25

8-15

Hardwoods

15-25

13-20

7-13

The digestion modifications described in this Topic and for Kamyr digesters follow 4 principles developed at STFI:

 The hydroxide ion concentration (OH-) should be lower during initial delignification than in conventional cooking, and as uniform as possible throughout the cook.

 The hydrosulfide ion concentration (HS-) should be as high as possible, both initially and at the beginning of the bulk delignification stage.

 The dissolved lignin and sodium concentration should be as low as possible, particularly at the residual delignification stage.

 The temperature should be kept "low", especially towards the beginning and end of bulk delignification.

These rules are additive. So a method that fully addresses 2 of the above 4 rules may be superior to one that partially addresses all 4. [Tikka, P.O. "Conditions to extend kraft cooking". Proceedings of TAPPI Pulping Conference, Boston, Nov 1992, pg. 701].

Proprietary systems exist that optimize certain aspects of conventional cooking. These aspects include:

 Round-bottomed, rather than conventional conical-bottomed, digester.

 Chip charging with steam to improve digester charge.

 Preheating (or impregnating) the chips with hot black liquor.

 Topping up cooking liquor strength during digestion, rather than adding the complete chemical charge initially.

 Doing part of pulp washing in the digester, by displacing cooking liquor with brown stock washer filtrate. It is sometimes called displacement cooking.

 Either pumping out the contents of the digester, or using cool black liquor to effect a cold blow (at about 90C) - since blowing at normal high digester temperatures (hot blow) reduces pulp strength.

Table III shows the kappa numbers achieved by such optimizing systems, although Headley (below) found "very few" mills cooking softwood below 20 (even for TCF pulps) - due to yield, strength, and recovery load considerations. Associated benefits are reduced steam consumption and knotter screen rejects, and the ability to tolerate lower quality wood without diminishing pulp strength. Example, Enerbatch, Rapid Displacement Heating, RDH, SuperBatch. A comparison of RDH and SuperBatch can be found in: [Headley, R.L. "Pulp cooking developments focus on fiber yield, lower chemical use". Pulp & Paper, Oct 1996, pg. 49].

TABLE IV: Superbatch cooking cycle breakdown at April in Riau Pulp mill, Kerinci,  where 30 hardwood species are cooked

Step

Time (min)

1. Chip fill

30

2. Warm liquor fill

30

3. Hot liquor fill

35

4. Cooking time (at max. temp.)

75

5. Wash liquor displacement

50

6. Pulp pump out

30

Total

250

For dissolving pulp, Visbatch (GL&V) provides a prehydrolysis stage before essentially Enerbatch kraft cooking. [Wizani, W. et al. "Kraft displacement cooking". World Pulp & Paper Technology 1995/96, pg. 65].

A 2-stage batch cooking sequence, that provides extended delignification with accompanying benefits, has recently been proposed by Paprican based on pulping fundamentals:

 1st-stage liquor is 2nd-stage spent liquor plus about half the normal conventional cooking chemical charge.

 2nd-stage liquor is the remaining chemical charge plus fresh water or brown stock washer filtrate.

Pilot 2-stage cooks showed a 25-30% reduction in kappa number using conventional cooking time and chemical consumption. At the same kappa number, pulp viscosity was 20-30% higher. Unbleached yield was greater below a kappa number of 20, but lower above it. Pulp strength at 16 kappa number was similar to that of conventionally-cooked pulp of 27 kappa number. See the EMCC and ITC Topic, for Kamyr digesters too old to be modified to these approaches, but able to benefit from the 2-stage batch approach. [Li, J. et al. "Improving extended delignification technology for kraft pulping. Part II. Co-current two-stage continuous cooking". Preprints of CPPA Technical Section Annual Meeting, Montreal, Jan 1998, pg. B9].

 

Turpentine

During the cooking of softwoods, the digester is relieved to collect turpentine in the turpentine recovery system. It is then either sold as crude turpentine or further processed to refined turpentine. Alternatively, it may be burned as fuel in the mill (e.g. lime kiln).

The composition of turpentine worldwide ranges 35-95% -pinene, 2-65% -pinene, 5-20% carene, and 2-3% limonene. [Grant, R.L. and Grant, A.C. Grant & Hackh's Chemical Dictionary. McGraw-Hill, New York, 1987].

Industrially, turpentine is used as a solvent (example paints), and also fractionated into pinene, terpene and other components - which are used in the manufacture of a variety of materials. Example ore flotation agents, perfumes, and detergents.

The sulfur content of crude turpentine has risen due to mill gas pollutant reduction measures. This has raised the average sulfur content of the turpentine received at International Paper's Jacksonville, FL, terpene and aromatics plant from 4,400 ppm S in 1989 to 7,000 ppm S in 1993. A value over 6,000 ppm S makes the foul smell (mainly TRS) difficult to process out of the turpentine, yet some mills' values go up to 40,000 ppm S. Several in-mill process measures for reducing the S content are given in: Foran, C.D. Impact of recent pulp-mill modifications on sulfur contents in crude sulfate turpentine. TAPPI Journal, June 1995, pg. 93.

 

Brown Stock Knotters and Screening

Brown Stock Knotters

Conventionally (see diagram), tree knots and other incompletely cooked wood in the pulp from the blow tank are removed by Jonsson-type vibrating screens. Eg: TR Screen (Andritz-Ahlstrom). These knotter screen rejects are either refined (example rejects refining) and then rescreened, or recooked.

Nowadays, these gravity knotters are replaced by pressure knotters similar to pressure screens. As closed systems, they are less messy and do not pull air into the stock to create foam. Eg: MODUScreen C (Andritz-Ahlstrom), Hi-Q (GL&V), DeltaKnotter (rotating screen basket), DeltaCombi (with fine screen) (Metso Paper/Fiber), Hy-Tec (Thermo Black Clawson).

A nonpressurized vertical screw type is also used. Eg: KW Washing Screen (Andritz-Ahlstrom).

Knots contain mainly compression wood.

Brown Stock Screening

Full brown stock screening (hot stock screening), involves 2-4 stages of screens, and may be used as an alternative to washed stock screening.

Screen room closure means that no discharges from this equipment go to the mill effluent treatment system.

 

Brown Stock Washing

Brown stock washing is used to remove most of the black liquor from the brown stock. The optimization of washer operation involves maximizing pulp throughput and black liquor solids content to the evaporators, while minimizing the chemicals and lignin passing downstream. Keeping chemicals and lignin losses low, respectively, reduces cooking chemical make-up and bleaching chemicals consumption.

The following 4 measures are commonly used to follow washer performance. The first 2 measures are inversely related, and so must be considered together:

 Washer loss, which takes 2 forms:

 Inorganic (soda loss). This is traditionally expressed as kg of salt cake (Na2SO4) per ad ton of pulp (from the last washer). It usually falls into the 6-18 kg range. The loss measured may be either that in the liquid hand-squeezed from the pulp, or it may also include that bound to the pulp. Diffusion washing gives lower values, because there is more time for chemicals to diffuse out of the pulp.

 Organic. Testing has become common, due to the need to minimize organic carryover downstream. Methods used include COD and Scandinavian Method SCAN-CM 45:91.

 Dilution factor (DF) is the net weight (in ton) of shower water added to the washers per ad ton of pulp. ie: after deducting the water leaving with the outgoing pulp. Typical traditional values are 2.5-2.8 ton/ad ton, while modern mills range 1-2 ton/ad ton.

 Displacement ratio (DR) is the ratio of the black liquor solids concentration reduction (between the vat and the outgoing pulp web) to the theoretical maximum reduction (ie: had the outgoing pulp web been at the concentration of the washer shower of that washer stage).

 Washer loading is a basic design parameter, and is measured as tons (either bd or ad) of pulp/day/m2 of washer area (or short ton/day/ft2). Because of their different fiber dimensions (eg: fiber length, fiber width) and freeness, loading is usually: softwoods > hardwoods > nonwoods. Eg: about 1.6 bd ton/day/m2 for Douglas fir (see also Mizell reference values below), but 0.6 for bagasse and reed, and 0.5 for wheat straw and rice straw. [Jeyasingam, J.T. "The need for improving the efficiency of brown stock washers used for agricultural residue pulp". Nonwood Plant Fiber Pulping, Progess Report No. 15, 1984, pg. 71. TAPPI Press]. Adding a small proportion of softwood pulp to hardwood pulp can improve washer loading (at the same washer loss and DF).

The Norden efficiency factor (after H.V. Norden) is a related parameter. It is the number of mixing stages required to give a specific displacement ratio at a given dilution factor. The resulting N Number, for each washing component in series, is additive and thus allows comparison between different washing systems. [Smook, G.A. Handbook of pulp & paper technology. 1990. Angus Wilde Publications, Vancouver]. It is defined in: [Antkowiak, J. et al. Discharge consistency impact on washer efficiency. Preprints of CPPA/TAPPI Pulp Washing '96 Conference, Vancouver, Oct 1996, pg. 13].

The 3 principles utilized to wash black liquor solids from the brown stock are:

 Displacement washing. Purer liquid displaces the liquid in the stock. Example, drum filter washing, in-digester washing.

 Successive dilution and thickening of the pulp. Example, single screw press.

 Diffusion washing of solids from inside the fiber. This is a slow process, and occurs in a diffuser. It is thus relevant to Kamyr continuous diffuser washing (which also utilizes displacement washing), rather than to the shorter retention time equipment described in this Topic.

Equipment used to wash black liquor from the brown stock is described here. Some are also used for pulp washing after an oxygen delignification stage and for bleach plant washing. For washing after a Kamyr digester, see Further Brown Stock Washing.

 Rotary vacuum drum filters (see diagram) are the traditional approach, utilizing 3 or 4 stages. The drum is covered by a mesh, through which the pulp drains from about 1% at the vat inlet to 12-20% consistency. A pressure difference is provided by the vacuum drawn as the filtrate passes down the barometric drop leg into the filtrate tank (seal tank). In theory, this leg should have a drop of at least 10 m (32 ft), but in practice as little as 6 m (20 ft) has reportedly proved adequate. Alternatively, the washer may be valveless. In this type there is no drop leg, due to the vacuum being created by the configuration inside the drum.

A valve cuts off this vacuum, in preparation for mat removal, about 30 past the highest point on the drum. This may be by take-off roll (ie: 1 or 2 driven rolls), air lift take-off, or underjet take-off (ie: air doctor, steam doctor, or water doctor. Air and water doctors are usually confined to bleach plant washers and deckers). For wood pulp, modern washer loadings should be about 6.8-7.3 bd ton of pulp/day/m2 (0.70-0.75 bd short ton/day/ft2) for brown stock washing, and 8.8-9.3 bd ton/day/m2 (0.90-0.95 bd short ton/day/ft2) for bleach plant washers. Short fibered nonwoods require lower loadings. [Mizell, W.K. "Improved pulp washing provides big savings for little expense". Pulp & Paper, April 1996, pg 133].

Hot shower water is introduced to the last stage, and the filtrates are passed countercurrent upstream. At each washer, the shower water is pulled through the pulp web, thus displacing the black liquor components in it. The final concentrated filtrate (weak black liquor), at 15-20% solids content, goes to the chemical recovery system.

 Rotary pressure drum filters resemble their vacuum equivalent above, except that 1 or more filters are totally enclosed by a hood. A positive pressure on top of the pulp mat then provides the pressure difference that assists drainage. Multistage washing can be effected either between the washers, or on a single drum (up to 4 stages) under its own hood. The hood facilitates HVLC gas collection. [Ricketts, J.D. "An overview of modern pulp washing systems". Preprints of CPPA/TAPPI Pulp Washing '96 Conference, Vancouver, Oct 1996, pg. 103]. Example Drum Displacer (DD Washer, Andritz-Ahlstrom). Stress corrosion cracking in such washers has been examined by finite element method analysis, using the NISA package (EMRC), in a joint project by Tristar Industries, Domtar and UBC. Design of the sensitive stress areas was modified using the ANSYS package (ANSYS). [Tayler, M. "Pressure washer drum replacement". Preprints of PAPTAC Annual Meeting, Montreal, Jan 1999, pg. A299].

More recently-developed alternatives are:

 Horizontal wire washer (belt washer). As on a PM forming table, the pulp flows from a headbox down a continuous traveling wire. Wash water is introduced at the far end of the wire. Displacement washing occurs as it is sucked through the pulp mat (by suction boxes aided by a blower and the totally enclosed hood). The resulting filtrate is pumped to the next section of the wire (upstream), and this is repeated to give multistage (eg: 5 or 6 stages) countercurrent washing. The discharge consistency, after a press section, is about 35%, and the dilution factor is low at about 1-1.5. Example Twin Wire Wash Press (Andritz-Ahlstrom), Chemi-Washer, Flex-S Washer (fewer washing stages, for incremental washing), Combi-Washer (provides additional washing/thickening, in return wire run, for a separate stock) (Thermo Black Clawson).

 Compaction Baffle Filter (GL&V). A pressurized totally enclosed rotary drum washer presses the pulp against a baffle to 18-22% consistency, and rotates it through a flooded wash pond to give displacement washing.

 Twin roll press. The material is pumped into the vat of 2 counter-rotating rolls. The mat is formed by drainage through holes and grooves in 1 roll (the other roll being plain), pressed by passing up through the nip, and then discharged by a doctor at 30-50% consistency. Eg: Vari-Nip Twin Roll Press (GL&V), Wash Press (Kvaerner Pulping), Displacement Press, TwinRoll (Metso Paper/Fiber).

 Single screw press, using a single screw and internal screens. Eg: Kvaerner Screw Press.  

[Ricketts, D. "Pulp washing system evolution offers better efficiency, lower capital cost". Pulp & Paper, Dec 1992, pg. 67]

[ Ricketts, J.D. "An overview of modern pulp washing systems". Preprints of CPPA/TAPPI Pulp Washing '96 Conference, Vancouver, Oct 1996, pg. 103].

Chemicals may be added to assist washing. Such as:

 Defoamers function by reducing the quantity of entrained air.

 Carbon dioxide functions by both slightly lowering the pH (thus reducing fiber swelling) and by liberating surfactants from the extractives. 3 mills are injecting CO2, by means of a dissolving sparger (ie: gas distributor) placed in the discharge zone of a pump near the end of the washing line. The mills are variously reducing carryover (soda loss and organic), increasing washer capacity, or increasing black liquor solids. [White, B. "Carbon dioxide on pulp during washing in the minimum impact mill". Preprints of CPPA/TAPPI Pulp Washing '96 Conference, Vancouver, Oct 1996, pg. 53] and [Chisholm, E. "Improving brown stock washing efficiency using CO2 to acidify post oxygen wash press filtrate". Preprints of CPPA Technical Section Annual Meeting, Montreal, Jan 1998, pg. B319].

 Others. Eg: Channel Block (a cationic polymer. Dorset), CustomSperse (Hercules).

Drum washer capacity has been spectacularly increased at Bowater's Catawba mill from 360 ton/day to 1,400 ton/day of bleachable softwood kraft pulp. This was achieved mainly by: Addition of a 4th washer to the three 1959 washers. An unconventional operating philosophy that runs a mat of only 5-15 mm (3/16-5/8 inch) thick at high individual drum speeds (eg: Drum 1 at 3 rev/min, ... , Drum 4 at 10 rev/min). Control through the DCS. Extending shower headers, increasing drop leg diameters, and replacing roll take-off with air. Good preventive maintenance. Their soda loss ranges 10.3-18.5 kg/ton, antifoam usage averages 0.9 kg/ton, and final black liquor solids has remained around 16%. [Dance, T.A. and Shipman, C.L. "Brownstock washing experience at Bowater's Catawba mill". Preprints of CPPA/TAPPI Pulp Washing '96 Conference, Vancouver, Oct 1996, pg. 77].