Black Liquor Evaporation Research Articles

ＫＰ操業技術アンケート調査結果 その３ 薬品回収工程（黒液濃縮，回収ボイラ，苛性化，石灰焼成）

In order to provide and share the technical information among Japanese pulp and paper industries. Pulping Technical Committee surveyed chemical recovery process operation in kraft mills of '99 fiscal year (Apr.'99 to Mar.'00), following the survey of kraft fiberline operations of '98. We received the answers from 33 mills, except one mill, out of 34 domestic kraft mills.This report summarizes result of the above investigation, concerning operational conditions and equipment of chemical recovery process which covers black liquor evaporator, recovery boiler, recausticizing and lime calcining.

Crystal Properties and Nucleation Kinetics from Aqueous Solutions of Na2CO3 and Na2SO4

Sodium carbonate and sodium sulfate are the major inorganic components in black liquor produced from alkali pulping, and these two compounds have been shown to be the major contributors to the formation of encrustations on the heat-transfer surface of black-liquor evaporators. Therefore, a determination of the variables affecting the nucleation and growth of these species is an important step in elucidating mechanisms by which such encrustations are formed. The metastable zone width for crystallization in the Na 2 CO 3 -Na 2 SO 4 -H 2 O system was determined as a function of the total solute concentration and the ratio of sodium carbonate to sodium sulfate. The polythermal method was used in the measurements, and supersaturation was generated by heating the system contents to increase the system temperature at a constant rate. Classical nucleation theory was combined with the metastable limit to correlate the effects of temperature and solute concentration on primary nucleation kinetics.

Separation methods for closed-loop manufacture of bleached kraft pulp — overview of an EU project

A summary of an EU project concentrating on separation processes for the closed-loop manufacture of bleached kraft pulp is presented. The project includes four separate tasks in which the need and function of specific “kidneys” are studied. In the first task, the reuse of condensates from black liquor evaporation in pulp bleaching has been studied, including a detailed characterization of the condensates. The discharge of accumulating non-process elements (NPEs) from acidic or neutral bleach filtrates was the focus of the second task of the project. Precipitation of NPEs occurs during a separate pre-evaporation of the filtrates to about 20% DS content. Further precipitation may be obtained by pH adjustment to a level where the precipitation and separation properties can be optimized to favour the discharge of some specific NPEs (e.g., silicon). The third task concentrates on the discharge of accumulating compounds from alkaline bleach filtrates by the use of ceramic nanofiltration membranes developed within the project. In the fourth task an overall system analysis of the closed cycle pulp mill is made.

Application of an In-Furnace NOx Removal (MACT) System for Recovery Boilers.

In recent years, the black liquor concentration in pulp recovery boilers has been markedly elevated to its high concentration of about 80% form its conventional level of about 70%, accompanied with improved performance of the black liquor evaporator in response to the requirement for an increased handling capacity of black liquor. This trend raised the combustion temperature in char bed, causing a problem of elevated NO x concentration level. In our present research using a field boiler, the authors verified that the NO x level can be decreased by designing improvements of the combustion process utilizing changes of the air distribution into the pulp recovery boiler. As opposed to the current recovery boiler where primary, secondary and tertiary air are used to complete combustion, our research was conducted on the application to the recovery boiler of the in-furnace NO x removal system where the NO x reduction process is designed to be enhanced in the char bed zone and in its upper zone by decreasing primary, secondary and tertiary air injected into such zones and by further injecting the fourth air into the upper zone of tertiary air to complete combustion. Results of the field boiler test showed that the NO x emission level can be decreased by about 30% compared with the conventional level by injecting the fourth air at a ratio of 8%, demonstrating that the in-furnace NO x removal system can also be effectively applied to the recovery boiler.

Heat transfer and heat transfer fouling in Kraft black liquor evaporators

A large number of experiments have been performed with New Zealand Forest Products Kraft black liquor to measure heat transfer coefficients and fouling rates during convective and subcooled flow boiling heat transfer as a function of surface temperature, bulk temperature, velocity, and solids concentration. Results from experiments with two chemical fouling inhibitors, with Teflon surface coating and in plate and frame heat exchangers, also are presented. The fouling deposits are analyzed with respect to appearance, composition, and process conditions for which they were obtained. With the assumption of chemical reaction-controlled fouling, a deposition model is developed and compared with the experimental data.

An energy efficient evaporation process for treating bleach plant effluents

Abstract Simulation results of an energy efficient evaporation process are reported for the treatment of bleach effluents in the pulp and paper industry. Due to the low concentration of the effluent stream, the evaporation process must have a high degree of energy efficiency in order to compete with other treatment alternatives, such as ultrafiltration, adsorption, ion exchange and biological treatment. For a pulp and paper mill with an annual capacity of 335,000 ton of bleached kraft pulp, the capacity of the spent black liquor evaporation plant is about 7.5 ton water/ton pulp if its concentration is increased from 16 to about 65 wt% dry substance. An evaporation plant for the bleach effluent was simulated for a flow rate corresponding to 7.2 m3/ton pulp and a concentration of 1.4 wt% dry substance. This stream is to be concentrated to 16 wt% resulting in an evaporation capacity of 245 ton water/h. The total evaporation capacity for both evaporation plants would be increased by 91%. Optimal energy conservation strategies were investigated where an absorption heat transformer unit (AHT) is integrated with both the existing black liquor evaporation plant and the proposed bleach effluent evaporation process. Different process configurations were simulated using a flow sheeting program, developed for simulation of general multiple-effect evaporation processes and absorption heat pump systems. Using real operating data from a major Swedish pulp and paper mill, simulation results are reported for the optimum location of the AHT unit. Consequently, the energy requirements for both evaporation plants would increase by only 6.6%. One of the main practical features of this process is that integrating the AHT unit would require minimum changes in the existing plant. An economic analysis resulted in a cost of 23 SEK/ton of pulp for the first alternative where the pre-concentrated 16 wt% effluent stream is concentrated further to 65 wt%, within the existing black liquor evaporation plant, and then recycled to the recovery boiler. The corresponding cost of 49 SEK/ton of pulp would result for the second alternative, where the 16 wt% effluent stream is sprayed on the waste from bark peeling and co-burned in the bark boiler.

クラフトパルプコンデンセート廃液のメタン発酵処理

Mixed foul condensate from cooking vessel and black liquor evaporator is treated, through methane digester with 2m3 effective volume.After 1year operation, efficient treatment was attained by the pilot plant with the porous carrier fixed bed digester.12-15kg-COD/m3/D for reactor load440-468Nl-Biogas/kg-COD removedca. 80% methane content in biogas86-88% COD removal rate andca. 95% total TRS removal rate are attained.Based on the pilot data, feasibility for full scale plant shows ¥59, 500, 000-¥160, 000, 000 anual net operation benefit for each case of 400ton/day BKP factory when replacing existing steam stripping facility.

Anaerobic Treatment of a Black Liquor Evaporator Condensate from a Kraft Mill in Three Types of Fixed-Film Reactors

A black liquor evaporator condensate from a kraft mill was anaerobically treated in a fixed bed reactor (= an anaerobic filter), an expanded bed and a fluidized bed. The condensate treated in the fixed bed reactor and the expanded bed had an average COD concentration of 1.4 kg/m3. Before treatment in the fluidized bed the condensate was diluted to 0.42 kg/m3. Efficient purification (80 % COD-reduction or better) was obtained at volumetric loads up to 2 kg COD/m3·d in the fixed bed reactor, 10 kg COD/m3·d in the expanded bed and 13 kg COD/m3·d in the fluidized bed. The specific methane production was 0.10-0.30 m3/kg COD added in the fixed bed reactor, 0.07-0.15 m3/kg COD added in the expanded bed and 0.07-0.20 m3/kg COD added in the fluidized bed.

直接か性化と回収ボイラの高効率化

Research Association of Pulp and Paper Technology was established on September, 1982 for the purpose of effective promotion of technical development of the pulp and paper industry in Japan.The objectives and the effects of direct causticization and improving recovery boiler efficiency, one of the research themes in our association, are as follows.1. Abbreviation of conventional causticizing process and consequently saving oil in lime kiln.2. Technical development of evaporator for ultra-higher concentration of black liquor.3. Reduction of pulp cost as the result of above-mentioned two objectives.Among several direct causticizing processes, we intend to make the study of ferric oxide process mainly as compared with borate process.Apart from thin-film evaporation of black liquor to higher solids and freeze drying evaporation etc., we will investigate the “disc evaporator” process.Our intention is to carry out R & D in accordance with the following schedule.1983 Bench-scale experiment (about 120 B.D.kg/hr dried solids) 1984 Construction of pilot plant1985 Operation of pilot plant (collecting and analysis of operational data for the design of commercial plant)

Adaptive Dead-Time Compensation

Some industrial processes are subject not only to varying gain and time-constants, but also to varying dead-time. For instance, in the pulp and paper industry the time delay between the percent solids and the feed liquor flow rate in a black liquor evaporator may change due to unexplained causes. For such a process, the now classical self-tuning regulator is inapplicable, since it requires the knowledge of the process time-delay. This report presents an adaptive dead-time compensator (ADTC) based on a recursive least-squares estimator and a pole placement algorithm. Simulations show that although it does not require the à-priori knowledge of the time-delay, this ADTC can perform near minimum variance. It is also shown that by the use of a variable forgetting factor, this ADTC can accommodate sudden and significant changes in the time-delay.

Simulation of the steady-state and dynamic behaviour of multiple effect evaporation plants. Part 1: steady-state simulation

A computer program for design and evaluation calculations of general multiple effect evaporation plants has been developed. The computer program, called INDUNS, is based on the principles of modular composition of flowsheets of evaporation plants. With the aid of a unit cell, flowsheets of arbitrary complexity are easily handled. Flowsheet data is entered via a special connection matrix. Input of data for known variables, such as physical properties, is made by a set of mnemonic symbols in order to facilitate the input procedure. These symbols are also used in the data output procedure, thus giving a very clear output matrix. All input data is carefully examined by INDUNS. When errors are detected, self-explanatory diagnostic messages are printed out. Thus, the risk of feeding INDUNS with wrong or incomplete data is small. INDUNS has been thoroughly tested in calculations of industrial plants, in the sugar as well as in the pulp and paper industry. Recently, several runs were carried out in the design of a number of Swedish black liquor evaporation plants. In these calculations, a special subfunction for the estimation of the overall heat transfer coefficients was used, developed at the Department of Chemical Engineering at Chalmers University of Technology. The results from these calculations show that INDUNS is easy to handle even for very complicated plants. They also show that relatively little effort and reasonably short computer running times are required to carry out a series of alternative design calculations using an initially given set of data.