Kraft Digesters Research Articles

Acid Cleaning of Corrosive Kraft Digesters and Evaporators

During scheduled shut downs of Kraft evaporator and digester, acid cleaning is required to remove scales that precipitate from chemical species present in liquors. Acid cleaning can affect the base metal of the Kraft equipment leading to significant localized corrosion, which can lead to unexpected failures. Burkeite was deposited on the carbon steel coupons and the x-ray diffractometer pattern of the scale was taken. 20% sulfamic acid solution with 0.5% inhibitor B was found to be effective in scale removal at either 25 o C or 50 o C.The rate of scale removal increased by either increasing the temperature or the rate of stirring (flow rate). Also changing the scale morphology was another way of enhancing its removal.

Interaction of metallic materials with simulated kraft digester white liquor – Towards the electrochemical detection of sulphide

The electrochemical behaviour of a range of metals (Pt,Pd,Ir,andFe) as electrode materials in simulated kraft digester white liquor (a concentrated sodium hydroxide solution with or without the addition of sodium sulphide at 170°C) has been compared by registering quasi-steady state current vs. potential curves, chronoamperometric responses at constant potential and impedance spectra. The results in the vicinity of the zero-current potential point to a two-step reaction with an adsorbed intermediate, possibly elemental sulphur, being rate limiting on the noble metals. At more positive potentials, a new process is detected on all the materials. According to relevant thermodynamic calculations, this process most probably corresponds to the oxidation of sulphide to polysulphide. Current densities measured on Pt and Fe in a wide potential range is linearly dependent on sulphide concentration. The associated chronoamperometric response is fast enough so that Pt can be regarded as a promising material for the electrochemical detection of sulphide in kraft digesters.

A hybrid capacitive pressure and temperature sensor fabricated by adhesive bonding technique for harsh environment of kraft pulp digesters

We have developed a compensated capacitive pressure and temperature sensor for kraft pulp digesters (pH 13.5, temperatures 25---175°C reaching a local maximum of 180°C and pressures up to 2 MPa). The gauge capacitive pressure sensor was fabricated by bonding silicon and Pyrex chips using a high temperature, low viscosity UV (ultraviolent) adhesive as the gap-controlling layer and heat curing adhesive as the bonding agent. A simple chip bonding technique, involving insertion of the adhesive into the gap between two chips was developed. A platinum thin-film wire was patterned on top of a silicon chip to form a resistance temperature detector (RTD) with a nominal resistance of 1,500 Ω. A silicon dioxide layer and a thin layer of Parylene were deposited to passivate the pressure sensor diaphragm and the sensors were embedded into epoxy for protection against the caustic environment in kraft digesters. The sensors were tested up to 2 MPa and 170°C in an environment chamber. The maximum thermal error of ±1% (absolute value of ±20 kPa) full scale output (FSO) and an average sensitivity of 0.554 fF/kPa were measured. Parylene-coated silicon chips were tested for a full kraft pulping cycle with no signs of corrosion.

Development of a combined piezoresistive pressure and temperature sensor using a chemical protective coating for Kraft pulp digester process monitoring

We have developed an integrated piezoresistive pressure and temperature sensor for multiphase chemical reactors, primarily Kraft pulp digesters (pH 13.5, temperatures up to 175 °C, reaching a local maximum of 180 °C and pressures up to 2 MPa). The absolute piezoresistive pressure sensor consisted of a large square silicon diaphragm (1000 × 1000 µm2) and high resistance piezoresistors (10 000 Ω). A 4500 Ω buried piezoresistive wire was patterned on the silicon chip to form a piezoresistive temperature sensor which was used for pressure sensor compensation and temperature measurement. A 4 µm thick Parylene HT® coating, a chemically resistant epoxy and a silicone conformal coating were deposited to passivate the pressure sensor against the caustic environment in Kraft digesters. The sensors were characterized up to 2 MPa and 180 °C in an environment chamber. A maximum thermal error of ±0.72% full-scale output (FSO), an average sensitivity of 0.116 mV (V kPa)−1 and a power consumption of 0.3 mW were measured in the pressure sensor. The sensors' resistances were measured before and after test in a Kraft pulping cycle and showed no change in their values. SEM pictures and topographical surfaces were also analyzed before and after pulp liquor exposure and showed no observable changes.

Development of a compensated capacitive pressure and temperature sensor using adhesive bonding and chemical-resistant coating for multiphase chemical reactors

We have developed an integrated capacitive pressure and temperature sensor for kraft pulp digesters (pH 13.5, temperatures from 25 to 175 °C, reaching a local maximum of 180 °C and pressures up to 2 MPa). The gauge capacitive pressure sensor was made by bonding silicon and Pyrex chips using a polymeric gap-controlling layer and a high temperature, low viscosity bonding adhesive. A simple chip bonding technique, involving insertion of the adhesive into the gap between two chips was developed. A platinum thin-film wire was patterned on top of a silicon chip to form a resistance temperature detector (RTD) with a nominal resistance of 1500 Ω. A silicon dioxide layer and a thin layer of Parylene were deposited to passivate the pressure sensor diaphragm and the sensors were embedded into epoxy for protection against the caustic environment in kraft digesters. The sensors were tested up to 2 MPa and 170 °C in an environment chamber. The maximum thermal error of ±1.74% (absolute value of ±36 kPa) full-scale output (FSO) and an average sensitivity of 0.257 fF/kPa were measured. Parylene-coated silicon chips were tested for a full kraft pulping cycle with no signs of corrosion.

Effect of sulphide on the corrosion behaviour of AISI 316L stainless steel and its constituent elements in simulated Kraft digester conditions

The electrochemical behaviour of AISI 316L steel and its constituent metals in simulated Kraft digester white liquor at 170 °C has been studied by registering slow scan rate voltammograms and impedance spectra at the corrosion potential. Interpretation of the results in terms of two approaches – the Mixed-Conduction Model for passive films and a two-step dissolution reaction – allowed for the estimation of corrosion currents and polarisation resistances as depending on the material and electrolyte medium. Tentative conclusions on the effect of sulphur-containing species in the white liquor on the corrosion mechanism of the studied materials are drawn.

Simulation of Pulping Process Using Neural Networks and Hybrid Model

A Neural Network Model is developed for a Kraft digester which is a fundamental stage in the pulp production. A detenninistic model is used to describe the main features of the process and will provide the data to verify the performance of the artificial neural network modeling. The paper shows that the artificial neural network is a good way to represent the process since it gives equivalent results when compared to deterministic model, and it is easier and cheaper to be developed.

Modeling of Continuous Kraft Digesters for Conversion to Optimum Modified Cooking

A computer model was developed to simulate modified kraft cooking digesters such as MCC, EMCC, ITC and Lo-Solids and to compare their performances with the conventional kraft digester. Results from the computer model showed that extension of cooking zone into the Hi-heat washing zone was a key to modified cooking. The content of xylan in modified cooked pulp was higher than that in the conventional pulp. As hardwood chips contain more xylan than softwood chips, it was predicted that the pulp yield increase in modified cooking would be more pronounced in hardwood kraft pulp than in softwood kraft pulp. An order of pulp yield and viscosity of hardwood kraft pulp from two-vessel digesters was as follows: Conventional<MCC<Lo-Solids<EMCC<ITC. A low packing degree of chips was obtained by simulation of Lo-Solid cooking, which explained the smooth downward movement of a chip column and a stable kappa number in the Lo-Solids cooking.

Crack development and corrosion in continuous kraft digesters: Strom, S. Proc. Conf. Engineering Solutions to Industrial Corrosion Problems, Sandefjord, Norway, 7–9 June 1993, 7 pp

Crack development and corrosion in continuous kraft digesters: Strom, S. Proc. Conf. Engineering Solutions to Industrial Corrosion Problems, Sandefjord, Norway, 7–9 June 1993, 7 pp

Stress Corrosion Cracking of ASTM A516 Steel in Hot Caustic Sulfide Solutions—Potential and Weld Effects

Abstract A combination of slow strain rate (tensile) testing (SSRT) and double cantilever beam (DCB) fracture mechanics testing techniques have been used to study the stress corrosion cracking (SCC) behavior of welded and unwelded pressure vessel quality steel plate (ASTM A516 Grade 70) in hot (92 C) caustic sulfide solutions (3.35 m NaOH + 0.42 m Na2S) of general relevance to the pulp and paper industry. Tests were conducted over a range of potentials (E) and stress intensities (KI). The most susceptible SCC situation occurred at potentials near the activepassive transition, where crack propagation rates were slightly lower in the fusion zone (FZ) of the weld than in the base material (BM). However, crack initiation on smooth specimens was far more frequent in the FZ. More noble potentials did not prevent SCC, but decreased the crack propagation rate and decreased crack initiation (i.e., crack density), particularly in the FZ. A fracture mechanics analysis of partial surface cracks and through thickness cracks was used to assess the cracking of kraft digesters in light of the results. It was concluded that anodic protection could play a beneficial role by decreasing the probability of crack coalescence and the resulting formation of a long critical (catastrophic) crack in the weld region. Instead, only small isolated cracks should form, which penetrate the vessel wall more slowly and lead to leakage before catastrophic failure.

Passivating Effect of Elemental Sulfur On Steel in Alkaline Pulping Liquors

Abstract Sulfur was added to alkaline pulping liquor and the time to passivation measured in two 3260 cu ft kraft digesters. The first digester was made of carbon steel partially coated with a stainless steel overlay while the second was of carbon steel only. With a sulfur addition of 0.32 lb per cu ft of liquor, complete passivation was obtained in both digesters. Without sulfur, passivation was delayed for 72 minutes in the carbon steel digester. The overall results showed that sulfur addition, either as such or combined with anodic protection, is another method of preventing corrosion in alkaline pulping digesters. 5.8.2, 6.2.3, 4.3.3

Review of Some Data on Corrosion of Stainless Steel and Higher Alloys in Pulp Mill Equipment

Abstract Allowable corrosion rates for equipment with low corrosion tolerance are discussed. Corrosion case histories in the various pulping processes are presented. Sulfite pulping solutions are most frequently-handled in 316 material. Stainless 316 is used because of good resistance to local attack under scale or deposits. In alkaline pulping, soda process digesters are usually made of carbon steel with heater tubing of either carbon steel or Type 316 alloy. Kraft digesters are commonly carbon steel, though many units are partially to wholly lined with alloy materials. Type 316 most frequently. Stainless 316 is used for heater piping, strong black liquor piping and green liquor piping. Carbon steel is most frequently used for weak black liquor. Corrosion in groundwood pulping varies greatly depending on wood being ground, water supply and treating chemicals added. Stainless 304 and 316 alloys are used frequently, both to resist corrosion and reduce contamination. Chlorine and hypochlorite bleaching usually pose severe corrosion problems. Hastelloy C alloy is most commonly used for high strength solutions. In more dilute solutions the molybdenum-bearing. stainless steels are satisfactory. Chlorine dioxide is even more corrosive to metals than hypochlorite, and some operators have replaced Hastelloy C with titanium. 8.5.3

Weyerhaeuser's Experience: Corrosion Resistance of Stainless Steel Overlays in Kraft Digesters

Abstract Success with hand weld overlays of Type 310 stainless to repair corrosion damage in bottom cones of Kraft digesters at Longview, Wash., induced Weyerhauser to use Type 310 stainless steel to repair complete digester shells of five units in 1956. Automatic welding guns were used with argon gas. At Everett, seven more digesters were overlaid in much the same manner. Type 316 was used for work at Everett because, by the time work was begun, some failures had been noted in the upper parts of the units at Longview. Analysis showed that the original weld material was being diluted by the base metal in the ratio of 1 to 2. In an effort to reduce the amount of dilution experienced, alterations were made in the method of deposit on the upper shell of the final unit at Everett. The gun was tilted 20 degrees to normal to effect greater impingement on preceding beads. Weight of deposit was increased, and rate of feed of argon ^ gas also was increased to effect better shielding. The top four feet of the shell was given a double overlay. Tilting the gun did not materially improve the quality of the deposit. Double overlay produced a surface with an analysis close to that of the overlay wire. Reports of inspections in several zones of the digesters are given. Significance of pits in overlaid material of this kind is emphasized. Tests are being conducted using various types of weld overlay to find one more resistant. Some design changes were made in the digester. Impingement plates were installed to absorb erosive force of liquor and to provide cathodic protection to the overlay. 3.7.3, 9.6.6, 8.5.3, 6.2.5.