Liquor Evaporation Research Articles

EVATool: A novel software for aiding in operational decision-making of evaporation plants in kraft pulp mills

The energy recovery capability of a kraft pulping process is crucial for its feasibility, and the evaporation process of black liquor is very important for a successful energy recovery since it is the first stage of the recovery system. Despite the apparent simplicity of the process, black liquor evaporation involves multiple evaporators and complex unit operations. Therefore, analyzing and simulating different operational scenarios is essential before proposing changes. A new software tool was developed to address the limited offer of specific software for analyzing black liquor evaporation plants. It focuses on fast analysis of these plants by estimating the heat transfer coefficient, steam consumption, steam economy, black liquor mass fraction between effects, and others. The tool was developed in Python and provides an interface resembling the supervisory system used by operators and offers equipment widgets for creating plant diagrams. EVATool also presents a dashboard for easy visualization of the main results. Software validation using examples of evaporation plants has confirmed its capability to simulate plants in different modes. In an industrial setting, the dashboard enables the comparison of simulated values with project indicators unavailable through operational instrumentation. While improvements are possible, this software shows great potential for industrial implementation, contributing to operational decision-making in daily operations of the evaporation process in pulp mills.

Black liquor evaporators upgrade - How many effects?

Black liquor evaporation is generally the most energy intensive unit operation in a pulp and paper manufacturing facility. The black liquor evaporators can represent a third or more of the total mill steam usage, followed by the paper machine and digester. When considering an evaporator rebuild or a new system, the key design question is how many effects to include in the system. The number of effects is the main design feature that determines the economy of the system and the steam usage for a given evaporation capacity. A higher number of effects increases steam economy and reduces energy cost to a point, but additional effects also have higher initial capital cost and increased power costs. This research paper uses life-cycle cost analysis (LCCA) as a method to determine the optimum number of evaporator effects for a new evaporator system. The same basic principles and method can also apply to existing evaporator rebuild projects.

Effect of resin and fatty acids on sodium salt fouling

The evaporation of black liquor is an important, energy intensive unit operation in the kraft chemical recovery process. Between 50 and 85% dry solids, black liquor evaporators are susceptible to heavy sodium salt fouling leading to reduced heat transfer efficiency and in extreme cases, plugging. Recent mill experiences indicate that the addition of small amounts of resin and fatty acids containing streams (tall oil soap, tall oil, tall oil brine) to the concentrators can reduce sodium salt fouling. The underlying mechanism for this is unknown, which limits broader application and control. In this work we use model solutions of sodium salts in a bench-top scaling setup to investigate the impact of resin and fatty acids on sodium salt fouling. The results show that low concentrations of abietic acid and oleic acid can inhibit sodium salt fouling during nucleation and in the presence of a stable crystal population.

Black liquor evaporator upgrades— life cycle cost analysis

Black liquor evaporation is generally the most energy intensive unit operation in a pulp and paper manufacturing facility. The black liquor evaporators can represent a third or more of the total mill steam usage, followed by the paper machine and digester. Evaporator steam economy is defined as the unit mass of steam required to evaporate a unit mass of water from black liquor (i.e., lb/lb or kg/kg.) The economy is determined by the number of effects in an evaporator train and the system configuration. Older systems use four to six effects, most of which are the long tube vertical rising film type. Newer systems may be designed with seven or even eight effects using falling film and forced circulation crystallization technology for high product solids. The median age of all North American evaporator systems is 44 years. Roughly 25% of the current North American operating systems are 54 years or older. Older systems require more periodic maintenance and have a higher risk of unplanned downtime. Also, older systems have chronic issues with persistent liquor and vapor leaks, shell wall thinning, corrosion, and plugged tubes. Often these issues worsen to the point of requiring rebuild or replacement. When considering the age, technology, and lower efficiency of older systems, a major rebuild or new system may be warranted. The intent of this paper is to review the current state of black liquor evaporator systems in North America and present a basic method for determining whether a major rebuild or new installation is warranted using total life cycle cost analysis (LCCA).

Определение математической зависимости плотности черного щелока от содержания сухих веществ (хвойная ЦВВ)

A modern evaporator station is an energy-saving production of evaporated liquor with high dry matter content for ensuring the minimum SO2 emissions from the soda recovery boiler. Such stations are equipped with Falling Film devices and consist of 5–7 stages. The necessity to improve and create new black liquor concentrators is driven by the increased attention to environmental issues and the wish to produce liquor with a higher calorific value. The physical properties of black liquor depend on its composition and have a significant impact on the selection and design of evaporators. The main properties considered in the evaporation and combustion processes are density, viscosity, surface tension, heating value and boiling temperature. One of the most important characteristics of the liquor evaporation is its viscosity, since it determines the ability of the liquor to pump and affects the design features. Another equally important characteristic is density. With its help, the dry matter content and evaporation process in the main stages of the liquor regeneration are regulated. The study was carried out to determine the mathematical dependence of the black liquor density on the dry matter content for high yield softwood pulp. The study objectives were the following: analyze the effect of the dry matter temperature and concentration on the density of black liquor obtained from cooking high yield softwood pulp according to the technological flow of production; develop a mathematical model of dependence of the black liquor density of high yield sulphate softwood pulp on the concentration and temperature; conduct the TAPPI comparative testing of the results of mathematical and correlation dependences. A mathematical dependence of the black liquor density on the temperature and dry matter content required for immediate technological calculations of chemical regeneration departments in sulphate production of semi-finished products for cardboard was obtained. The following equation of mathematical dependence based on the conducted research, regression analysis, and mathematical processing of the results was obtained: ρ = 0.974 + 0.0071x – 0.0002t – 0.000007xt – 0.00000045t2 – 0.0000045x2 (where ρ – density, g/cm3; x – dry matter content, %; t – temperature, °C). It allows calculating with the highest accuracy the density of black liquor obtained from cooking high yield softwood sulphate pulp. Comparative testing of the results of the developed mathematical dependence and correlation dependence (published by TAPPI) of black liquor density on the dry matter content was carried out. A high level of comparability of the proposed mathematical equations was found.

Sodium salt scaling in black liquor evaporators and the effects of the addition of tall oil brine

AbstractSodium salt scaling, i. e. the formation of doubles salts comprised of sodium, carbonate and sulphate on the heat transfer surfaces, is a common problem that occurs during black liquor evaporation. In this study, experimental results are presented that provide new insights into the formation and composition of such scales and how they are influenced by the addition of tall oil brine. It was found that increased content of sodium carbonate and sodium sulphate in the black liquor increased scaling, while the ratio between carbonate and sulphate had a lesser influence than reported in other studies. Black liquor created loose clay-like scales comprised of aggregated crystals and black liquor, whereas salt solutions created hard mineral-like scales. The scales formed by both the black liquor and the salt solution showed a tendency to fall off during formation after primary nucleation. It was also found that both tall oil soap and alkalized tall oil brine could inhibit the formation of scales. The inhibition effect is stronger if adding the soap or brine just before scaling starts, but also depends on the amount added, the sodium carbonate and sodium sulphate content in the liquor as well as other factors.

Evaluation of sodium salt scaling in black liquor evaporators using existing process data

Abstract Scaling caused by sodium carbonate and sodium sulphate salts is a common problem during black liquor evaporation and mills currently have no proper tools to monitor or follow it up. The method proposed enables automatic evaluation of scaling rates and behaviours, together with washing performance, by using measurements that are already available at most mills, such as the boiling point elevation and the overall heat transfer coefficient. The method identified the scaling correctly in most cases, normally >90 %, although fast and sudden scaling proved to be challenging; its performance was also dependent on the quality of the data used. Historical data from three mills was evaluated and it was found that the scaling rate generally increased at higher concentrations of black liquor: one of the mills had almost no scaling below 70 % dry solids content. Another mill, however, deviated from this trend and had significant problems with fast and sudden scaling in one effect at around 60 % dry solids content. The correlation of process parameters revealed that this scaling was triggered, to a large extent, by the absence of tall oil brine, which is a side stream from the production of tall oil that is normally fed to the evaporators.

Estimation of energy efficiency of kraft pulp production with the exergy increment method

This article contains results of exergic analysis of kraft pulping flow chart. The results of exergic balances of main kraft pulping processes such as alkali recovery at recovery boilers, black liquor evaporation, chips cooking, lime decarbonation are considered in details in the article. The analysis of the process flow chart makes it possible to determine the bottlenecks in the use of heat energy and to substantiate principal lines for increasing energy efficiency of the processes under study. A main share of the exergy expended in the existing pulping process is due to alkali recovery in the recovery boiler and comprises 70% of the total exergy available in the system. A procedure of hydrothermal production of chemicals in the process of kraft pulping is studied. A schematic diagram and analysis of heat technique of the kraft pulping process which in fact consists of organic component removal from black liquor through its autoclave carbonation with flue gases releasing from lime kiln at the temperature 80-90 oC are given in the article. The removal of organic components under these conditions can reach 70 %. In the studied version the exergic efficiency ηe = 80 % is considerably higher than that of the flow chart existing for chemicals recovery which is equal to ηe = 48 %. This is the evidence of high energy efficiency of the method developed.

Solids Content of Black Liquor Measured by Online Time-Domain NMR

Black liquor, a valuable by-product of the pulp production process, is used for the recovery of chemicals and serves as an energy source for the pulp mill. Before entering the recovery unit, black liquor runs through several stages of evaporation, wherein the solids content (SC) can be used to control the evaporation effectiveness. In the current study, the time-domain nuclear magnetic resonance (TD-NMR) technique was applied to determine the SC of black liquor. The TD-NMR system was modified for flowing samples, so that the black liquor could be pumped through the system, followed by the measurement of the spin-spin relaxation rate, R2. A temperature correction was also applied to reduce deviations in the R2 caused by the sample temperature. The SC was calculated based on a linear model between the R2 and the SC values determined gravimetrically, where good agreement was shown. The online TD-NMR system was tested at a pulp mill for the SC estimation of weak black liquor over seven days without any fouling, which demonstrated the feasibility of the method in a harsh industrial environment. Therefore, the potential of the TD-NMR technology as a technique for controlling the black liquor evaporation process was demonstrated.

Biological treatment and ultrafiltration of woodchip pre-hydrolysis liquor from dissolving pulp mills

Abstract Dissolving pulps could be considered as the future biorefineries, which normally generate liquor during the wood chip pre-hydrolysis (PHL). PHL has high chemical oxygen demand (COD) and biochemical oxygen demand (BOD). Mills do not have efficient means for disposal of PHL, being in general burned in the recovery boiler with a high cost due to its low consistency. The objectives of this work were to evaluate i) the biotreatability of PHL and the effects it would have on a kraft mill effluent biological treatment plant and ii) the use of ultrafiltration (UF) membrane to treat the PHL prior to biological treatment. PHL was generated into lab with a hydrothermal process and was mixed with kraft pulp mill effluent in different proportions and added to sequencing batch reactors (SBRs). The SBRs achieved high rates of COD removal (>75 %). However, treated effluent COD increased with the increase in PHL dose. This treatment using UF membranes reduced the COD load. Biotreatability of the UF permeate was higher than that of PHL. UF retentate, with 28 % of the volume and a much higher solids content than the initial PHL, would have a significantly lower evaporation energy demand if sent to the black liquor evaporators.

Evaluation of effect of evaporation on supercritical water gasification of black liquor by energy and exergy analysis

Supercritical water gasification (SCWG) is a promising innovative black liquor handling method. The low concentration of black liquor is a burden for the system scale and investment cost. In this study, we introduced black liquor evaporator to increase the concentration using self-generated steam and power, and studied its influence on the energy and exergy efficiency. The results showed that increasing black liquor concentration from 10 wt% to 20 wt% reduced the energy efficiency but increased the exergy efficiency of the system without evaporator. With the evaporator, an optimal target concentration existed for the exergy efficiency owing to the influence of the target concentration on the balance between the energy consumption and saving brought by the evaporator. With black liquor condensed from 15 wt% to 21.82 wt%, maximum exergy efficiency of the system (41.95%) was obtained. For lower-concentration black liquor, more water needs to be evaporated to get the optimal concentration and evaporator was more desired because it brought greater improvement on the exergy efficiency. The increasing effect number of multi-effect evaporator shifted the optimal concentration to higher values, which increased from 19.93 wt% to 23.13 wt% when the effect number increased from 4 to 7. The exergy efficiency of the system was also improved, and the improvement was more significant at higher target concentration.

Enhanced process integration of black liquor evaporation, gasification, and combined cycle

Energy recovery from black liquor (BL) can be performed through gasification at temperatures above the melting point of inorganic chemicals. Complementarily to BL gasification experimental research, this study is conducted to simulate the thermodynamic modeling of an integrated system for BL evaporation, gasification, and combined cycle for power generation. For BL evaporation, a novel system is proposed based on the concept of exergy recovery to minimize exergy loss, and thus, lower the required energy input for evaporation. From the process design and calculations, higher target solid content leads to lower total required energy for BL evaporation. The lowest required total energy for evaporation can be achieved at a target solid content of 75wt% wb. Furthermore, an integrated power generation system adopting gasification and combined cycle is modeled, and an application of different BL evaporation technologies is also evaluated in terms of net energy efficiency. The integrated system with exergy recovery-based evaporation can achieve a net energy efficiency of 34.5%, which is significantly higher than those of multi-effect evaporators (24.5%) and conventional boiler-based evaporation (14.7%).

The Effect of Bulk Crystals on Sodium Salt Scaling in Black Liquor Evaporators

Black liquor evaporation is an essential unit operation in the chemical recovery cycle of a pulp mill. The high concentrations of sodium carbonate and sodium sulphate in black liquor cause crystallization fouling, or scaling, in the concentrator effect, which affects the operation negatively. The focus of this work was to evaluate the effect the content and size of the bulk crystals had on the scaling rate of the sodium salts. This was done experimentally by evaporating both pure sodium carbonate solution (to crystallize anhydrous sodium carbonate) and black liquor. It was found that bulk crystals were required to suppress rapid scaling during primary nucleation (when passing the metastable limit), which was tenfold higher than scaling during continuous crystallization under steady-state operation. The amount of bulk crystals did not, however, affect the scaling rate during continuous crystallization. It was also found that the scaling rate increased when the largest crystals were removed, in this case by a hydrocyclone. The results were similar for both sodium carbonate solutions and black liquor, although the latter had a finer crystal size and scale structure.

Separation and recirculation of bulk crystals to potentially mitigate sodium salt scaling in black liquor evaporators

Black liquor contains high concentrations of sodium carbonate and sodium sulfate. In black liquor concentrators, these salts begin to crystallize and have the potential to rapidly form scales on the surfaces of heat exchangers. The highest potential for scaling is when a supersaturated liquor begins to nucleate, which occurs when no crystals are available in the bulk. A mitigation strategy used in some mills is to recirculate thick liquor, which increases the bulk crystal concentration. However, a more efficient solution would be to separate bulk crystals and add them selectively to mitigate scaling directly. This study investigated the separation of bulk crystal from black liquor using a hydrocyclone. Separation was shown to be possible and was proven for a dry solids content of up to 65%, which corresponded to a viscosity of 20 mPa·s at 120°C. The two most important parameters relating to separation were viscosity and flow velocity. The cyclone Reynolds number encompasses both of these parameters and can be used to estimate the degree of separation. An implementation strategy for bulk crystal recirculation has also been developed and discussed. The recirculation of bulk crystals is most beneficial when operating close to critical solids and when reseeding crystals after cleaning.

Ruthenium Volatilisation in Nuclear Waste Systems - Studying the Baseline Thermodynamics of Ru(III)

Spent Fuel Management at Sellafield includes the reprocessing of spent nuclear fuel from stations across the UK and also the reprocessing of spent fuel from overseas customers. At Sellafield, methods have been developed for the processing of high level wastes, including highly active liquors (HAL), which is a result of reprocessing irradiated nuclear fuel. This Highly Active (HA) raffinate / waste stream is concentrated in evaporators and storage tanks in the Highly Active Liquor Evaporation & Storage (HALES) facility before feeding to the Waste Vitrification Plant (WVP). Here, the resultant HAL feed is calcined and combined with glass before pouring into containers to produce an immobilised HA wasteform. Ruthenium is a fission product possessed of two relatively long lived stable isotopes: Ru-103 (t1/2 = 39.8 days) and Ru-106 (t1/2= 1 year). Both isotopes form part of the inventory of HA waste raffinate during reprocessing of spent fuel. Volatilisation of fission products in nuclear waste generally occurs at high temperature – apart from ruthenium where volatilisation occurs at the lower temperature stages of the vitrification process. Given its volatile nature and high specific radioactivity, ruthenium presents a strong challenge to the nuclear industry in effectively managing its abatement. Part of the challenge is to fully understand the highly complex solution chemistry under conditions relevant to HA waste streams and associated abatement systems. Experimental work within the National Nuclear Laboratory (NNL), UK has demonstrated that the presence of oxidising metal ions in HA waste (e.g. Ce(IV)) can enhance the volatility of ruthenium through a chemical conversion of Ru(III) species to what is assumed to be RuO4. A better understanding of these species, their electrochemical processes and reaction kinetics is required to underpin the empirical evidence gathered to date, in particular to develop gravimetric, electrochemical and spectroscopic analytical methods that will improve the understanding of ruthenium speciation in high nitric acid environments, establish the kinetics of inter-conversion between ruthenium species and establish the mechanism by which metal ions such as Ce(IV) may oxidise ruthenium. We have studied the baseline electrochemical behaviour of ruthenium and present here for the first time the intrinsic thermodynamics of uncomplexed Ru(III) using electrochemical methods to determine as bought RuCl3 to be a mixture of Ru(III) and Ru(IV). We have achieved this via cyclic voltammetry of RuCl3 in HClO4 which yielded two peaks, namely at 0.55 and 0.95 V. A similar peak has been seen by Maya [1] at 0.9 V, reported to be tetramer oxidation. Analysis of the solution using UV-vis shows as bought RuCl3 to be a mixture of Ru(III) and Ru(IV). Subsequently a successful method to electroreduce the mixture to a pure solution of Ru(III) was developed. Cyclic voltammetry of the electroreduced Ru(III) shows an absence of the previously seen peak at 0.55 V, suggesting this peak was associated to the oxidation of Ru(IV), whilst the previously observed peak at 0.95 V had been shifted to ~ 0.8 V suggesting that this peak was not in fact an observation of tetramer oxidation, as proposed by Maya. The latter peak was further interrogated using a scan rate dependence study, which shows the peak potential increasing as scan rate increases showing we have an irreversible, kinetically slow process occurring. A linear relationship between the square root of the scan rate against peak current indicates the oxidation is occurring in solution. The n number of this transition was experimentally determined to be 0.33 and we have confirmed a diffusion coefficient of 4x10-10 m2 s-1 for RuCl3. This shows we have a trimer in solution which is oxidised from: 3 Ru(III) to Ru(III)-Ru(IV)-Ru(III). Future work will focus on the ruthenium nitrosyl species found in spent fuel to understand what effect NO complexation has on the system with the aim of comparing the uncomplexed ruthenium system with the complexed ruthenium system. [1] Maya, L., J. Inorg. Nucl. Chem., 41, 1978, 67

Cleaning of Water-Soluble Scales in Black Liquor Evaporators – A Modeling Approach

In the pulp and paper industry, the evaporation of black liquor is an important step to recover heat and chemicals. Due to a substantial amount of sodium carbonate and sodium sulfate in the black liquor, evaporators typically have to address the scaling of these water-soluble salts at concentrations above approximately 50% dry solids content. To maintain their operation, the evaporators are cleaned regularly using condensate or weak black liquor. In this paper, a model for evaporator cleaning was developed based on a dissolution model established via experimental measurements. Two new correlations for black liquor were also developed from experimental measurements: the solubility limit of sodium scales in black liquor and the density of black liquor containing additional dissolved salts. The simulations indicate high dissolution rates, meaning that the limiting step for the cleaning is the feed of free water with the wash liquid. The simulation results were insensitive to the distribution of scales, the mass transfer coefficient, and the temperature. However, the results were sensitive to the solubility limit and natural salt content for wash liquors above 30% dry solids content.

Formation of Soluble Calcium in Alkaline Pulping of Acacia

Calcium is the primary inorganic element in wood. During chemical pulping, in which wood is delignified to free cellulose fibers for pulp, a significant portion of the calcium in wood is released. The released calcium is mostly soluble at first, going through a metastable limit before reacting with carbonate to form calcium carbonate. The precipitation of calcium forms scale in the pulping digesters and in downstream processes such as bleaching and black liquor evaporation. This work provides new data for the tropical hardwood Acacia and gives equations for the release of calcium as a function of H factor in Kraft and soda pulping. Higher levels of soluble calcium were seen in the pulping of Acacia than has been seen in the batch pulping of pine in earlier studies, and this soluble calcium forms a more stable intermediate than is formed during the pulping of pine. The implication is that scaling in mills pulping tropical hardwoods such as Acacia can be more severe and more of this soluble calcium may leave the digester to result in fouling of black liquor evaporators.

Study of Sodium Salts Solubility in Eucalyptus Black Liquor to Understand and Prevent Scale Formation in Evaporators

Black liquor is a by-product of wood digestion process in pulp and paper mills and it is composed by organic and inorganic products. After this digestion process, black liquor has a solid content of 15 wt/% and to be used as fuel in the recovery boiler it is necessary to be concentrated above 75 wt/% of solids. This concentration process is performed in an evaporation plant using multiple effect evaporators. Some black liquor physical properties, such as density, solids content, viscosity and sodium salts content, are strongly dependent on the kind of wood processed (hardwood or softwood) and on operating conditions during the digestion process. Knowledge and comprehension of the relationship between these physical properties of black liquor are essential for studies aiming at a greater energetic performance of the black liquor evaporation unit. When black liquor reaches higher solids content (above 50 wt/%), scaling formation is observed on the heat transfer surfaces of evaporators and concentrators, due to the precipitation of sodium salts, reducing the overall efficiency of this equipment. The aim of this work is to investigate and comprehend the mechanism of formation and precipitation of sodium salts in eucalyptus black liquor evaporators, through the study of the relationship between some physical properties (density, viscosity, total solids content) and the solubility of sodium salts, evaluating the influence of these variables. In this study industrial samples of black liquor taken from different process streams of the evaporation plant were used. A greater comprehension of the mechanism which is responsible for the reduction of the solubility of these salts is very important for the development of alternative methods to avoid or reduce scaling formation. Experimental data were used to develop a model based on chemical equilibrium of salts dissociation reactions, considering also temperature and pH of the system, which is capable of estimating sodium sulphate solubility limit and comprehend the process that causes the formation of the first scaling and the behaviour of the salts in the system. This model may be used to establish some operating parameters, trying to avoid or control scaling formation in industrial evaporators.

Crystallization fouling of sodium salts in black liquor falling film evaporators – development of experimental method and first results

The black liquor evaporation plant is the largest steam consumer at the pulp mill. At the same time, its performance is heavily influenced by fouling, where crystallization of sodium salts (double salts of Na2SO4 and Na2CO3) is a major contributor. In this work, an experimental method was developed for a pilot evaporator that greatly improved the possibility to measure crystalizing fouling. The main improvements were that the heat transfer surface could be cleaned online to reset the fouling, and that the scales’ chemical composition could be determined from the wash liquid, i.e. no need to open the evaporator and take scale samples. The method was evaluated at different operational modes and a wide range of operational conditions where crystallization of sodium salts can occur, i.e. 50-85% dry solids content. The method enabled fast cleaning of the heat transfer surface up to about 72% dry solids content. In addition, the measured rate of fouling showed no increase at higher dry solids contents. The observed fouling was mainly formed due to primary nucleation at operation close to metastable limits, where the first metastable limit showed the highest fouling rate. It was also found that the fouling rate measurement results could not be repeated, despite operation under the apparent same conditions with the same liquor. This means that the fouling mechanism is more complex and the rate cannot be predicted using only liquor composition and operational conditions.

Fate of biosludge nitrogen in black liquor evaporation and combustion

At many mills, biosludge, which has a high nitrogen content, is added to black liquor and burned in kraft recovery boilers. The aim of this work was to determine the fate of biosludge nitrogen in the high solids black liquor concentrators and in the recovery boiler. Specifically, does biosludge addition result in higher nitric oxide (NO) and cyanate formation during black liquor combustion? To obtain this information, samples were collected from the chemical recovery cycle of a Finnish kraft pulp mill along with relevant process data. Laboratory combustion experiments clearly showed an increase in NO formation for the mill black liquor with biosludge, but no clear increase in nitrogen oxide emissions was detected in the recovery boiler after biosludge addition. Analysis of the green liquor samples from the dissolving tank showed a significant increase in nitrogen exiting the recovery boiler as cyanate. This finding was supported by laboratory tests studying cyanate formation. The increased cyanate results in increased ammonia formation in the recausticizing cycle, which can lead to higher NO emissions, as seen in the noncondensible gas incinerator at the mill.