Scales and deposits in black liquor evaporation

Scaling in black liquor evaporators counts as one of the most notorious problems in the modern kraft pulp mills. The deposits commonly known as insoluble scales (mostly CaCO3) require a complicated removal procedure, usually acid or high-pressure washing. The soluble deposits are usually composed of double salts of Na-CO3-SO4 type or of sodium oxalate. Although easier to wash out, they may form in such large amounts that their sheer volume may effectively block the entire evaporation unit. In both cases, the scales may greatly reduce the heat transfer, increase the energy consumption and worsen the steam economy, leading eventually to expensive shutdowns. It is therefore important to understand how a deposit is formed and thus find a way to avoid it.

Scales and deposits in the fibre line

Precipitation of sparingly soluble salts, such as calcium oxalate, barium sulphate and calcium carbonate can severely reduce the availability of process equipment in the fibre line. The risk for the scale formation depends mostly on the total input of non-process elements, but also on the flow strategy and the degree of system closure. Recycling of process water reduces the water consumption but may also lead to an accumulation of non-process elements and thus increase the scaling risk. It is therefore very important to understand the technical conditions under which precipitation occurs.

RISE investigates scaling and precipitation problems

RISE supports pulp mills in investigating and mitigating problems related to scaling and precipitation through four complementary approaches:

Scale analysis
Analysing samples of existing scale deposits is often the first step in understanding a scaling problem. By identifying exactly what has precipitated, it is possible to form hypotheses about root causes and assess which mitigation measures are feasible. In addition to performing the analyses, RISE provides expert support in interpreting the results and advising on potential process measures at the mill.

Modelling
Process modifications typically alter the chemical composition of mill streams, potentially introducing or aggravating problems related to scaling and precipitation. At the same time, the chemistry is complex and difficult to predict. RISE has many years of experience studying such systems and has developed a chemical equilibrium model for the formation of sparingly soluble salts. This model can be applied already at the planning stage of a rebuild, using process simulation to assess precipitation risks and evaluate different mitigation strategies. We can also perform direct solubility simulations using OLI Studio software.

Solubility experiments
RISE can perform solubility experiments on specific process streams to assess precipitation risks under realistic conditions. A typical study includes precipitation of the most common sparingly soluble salts found in the fiber line, such as calcium carbonate, calcium oxalate, and barium sulfate, but other compounds can be investigated as needed. The effects of parameters such as temperature, pH, ionic strength, and COD levels can be evaluated. The results can then be compared with the RISE database and implemented in simulation models or used as supporting data in investigations.

Precipitation experiments
RISE has access to a unique, dedicated experimental setup for studying scaling and precipitation phenomena. The equipment enables studies both in laboratory environments and under full-scale industrial conditions.
Read more about this pilot facility.

Within paper mechanics, we aim at identifying the relevant properties of paper materials, developing test methods to quantify these properties, and predicting their effects in different applications. We combine our expertise in material mechanics and structural mechanics with knowledge on processes and products that are specific for paper materials and products, to investigate and propose solutions to problems in the paper and packaging industry. We work mainly with packaging materials, like paperboard and corrugated board, where the mechanical properties are crucial for the functionality.

Our research on paper mechanics is applied and is mostly performed in cooperation with industry partners, either bilateral or in consortia. We carry out larger research projects on paperboard and corrugated board within the RISE Bioeconomy research programme.

Advanced characterisation of paper materials

Our research laboratory in Kista gives us unique possibilities to develop new methods to characterise mechanical properties in materials like paper, paperboard and corrugated board, and study the structural mechanics of converting processes and of packaging. We have, for instance, developed methods for characterising properties in the thickness direction, fracture mechanics, and creep. By using climate chambers and climate rooms, we can perform testing in conditions different from the standard climate in our main laboratory.

Beyond standard measurements like tensile testing and SCT, we offer testing with methods that we have developed or improved. The following are some examples:

• Tensile and compression testing in the thickness direction of the paper or paperboard
• Measurements of transversal shear
• Determination of shear strength profiles through the thickness of paperboard
• Measurements of cohesive fracture properties
• Creep testing in tension and compression
• Hygroexpansion measurements using a method that does not add load to the test specimen

Local variations in material homogeneity and structure in the paper or board web are often associated with quality variations both in the product and in the conversion processes that follows. With different working methods, this connection can be clarified and then remedied.

Maps of variations produced with IR high-speed camera

The variation of the material homogeneity is reflected e.g., in the local heat radiation of the surface when the product comes out of the dryer. Our mobile IR camera system can be placed in a suitable measuring position to image the paper path in detail during many minutes of production. Variations over the surface can be detected without motion blur by high frame rate and short exposure times. Together with a description of the process, analysis of periodic variations and temporary patterns can lead to a better understanding of how the origin is linked to material flow and machine condition, respectively.

Maps of IR variations may also be synchronized with permanently installed measurement systems to model property variations for the areas that are not characterized by the measurement systems of the mill. In the long run, this improves the ability to find and understand the occurrence of temporary deviations online. This enables an earlier and more reliable performance classification of the various process sections at the mill. Another application may be faster start-ups and change of grades.

Detailed characterization of reels with automated testing and specialized analyses

As an alternative or complement, narrow product reels can be characterized in detail for a desired combination of product properties. By using automated paper testing equipped with automatic winding, hundreds of meters long test rolls can be characterized several times. In this way, you can get thousands of measurements for different properties measured in coincident positions and with step lengths down to the centimetre level.

In addition to standardized measurements, RISE can perform specialized analyses to identify problem factors in the product. Examples are local formation, anisotropy in the thickness direction or chemical analysis with a hyperspectral NIR camera.

Connection to extracted and structured process data

RISE platform for analysis of large amounts of data makes it possible to combine high resolution variability maps of the entire production with selected process data from the mill in a secure and controllable way. In this environment, different methods of advanced data analysis can be used on the common dataset for different purposes. These may range from simpler comparisons to the development of predictive models to use for causal analysis, anomaly detection or as a tool for operator support.

Development of tissue products in small-scale

The experimental paper machine, XPM, can produce paper in grammages between 15 and 300 gsm. As it is equipped with a Yankee cylinder it can also very easily be used for pilot scale production of tissue.

LRead more about tissue pilot

Even though board properties are measured in board mills, their relationship with printing is not fully established. Therefore, to develop knowledge and tools that enable one to identify the relevant material properties to print quality lie constantly in the focus of our research.

Expertise in printability for packaging materials

Through the years, we have developed unique expertise, knowledge and tools, on printability. This includes topography, absorption, compressibility and optics of the substrates and printing dynamics, applicable to conventional and digital printing techniques on board and corrugated materials.

The tools (methods and software) help our customers understanding of the complex processes that govern qualities of print. These understandings are also helpful for designing experimental investigations and for interpretation of the observed results. We have long term and well documented experiences in which paper and board surface properties are important for printing. By measuring of the board surface using image analysis tools, a prediction of the print quality can be achieved.

Simulation of printability

Since printing is a complex dynamic process in which multiple interactions are involved, it is important to understand the contribution from individual factors. For this, we have developed theoretical simulations based on physical models of printer/ink/substrate interactions. With these simulation tools, the effects of individual factors such as porosity, ink viscosity, nip pressure profile, compressibility of the board etc., can be studied.

Marine biomass and agricultural residues

Cellulose can be extracted from more than just forests. Marine biomass, such as algae and seaweed, and agricultural residues are promising sources for bio-based materials. By making use of these underutilised, renewable raw materials, we reduce pressure on forests while enabling sustainable, circular products. This opens new opportunities for high-performance packaging and material innovation – and RISE can support you in developing these solutions and bringing them closer to industrial application.