Towards a More-Cost-Efficient Paper and Board Making using
Microfibrillated Cellulose
Eva Ålander, Ida Östlund, Karin Lindgren, Malin Johansson, Magnus Gimåker RISE Bioeconomy, Box 5604, SE-11486 Stockholm, Sweden
Abstract
The potential uses of microfibrillated cellulose (MFC) in the paper and board industry have been known for decades, but not come into operation, largely due to the high-energy consumption associated with its production. This problem has now been alleviated, and today there are many companies in the process of commercialising and planning for its up scaling of various MFC applications.
Techno-economic analysis based on a pilot scale trial at the FEX paper machine reveals in this work a positive impact on the variable costs using MFC as a wet end additive in fine paper production. It is shown advantageous to increase the filler content with addition of MFC in two different scenarios: without and with wet end starch addition. Alternatively, the total grammage of the product could be decreased without changing the filler content and still maintaining the strength of the product.
To facilitate evaluation of MFC at mill scale, a mobile demonstration plant for production of MFC is now available through RISE Bioeconomy. This gives an opportunity to easy and cost-efficient verify the use of MFC in various processes and products at the mill.
Introduction
MFC was first produced in the beginning of the 80’s by Turbak and co-workers [1, 2]. The process for producing MFC consisted of pressing a pulp through a high-pressure
homogeniser resulting in a highly viscous gel that was referred to as “MFC”. Since then, many application areas have been identified, such as cosmetics, pharmaceuticals, paints, oil field services, paper additives, barrier coatings, nanocomposites, hygiene and absorbent products [1, 3].
A major obstacle for the commercial use of MFC has been the high-energy consumption (excess of 25000 kWh/t MFC) in its manufacturing [4]. In the past decade, this problem has however been alleviated by development of different pre-treatment procedures of the fibres before the homogenisation. The first generation of nanocellulosics requiring a low energy consumption and causing less clogging in the homogeniser was reported in 2007 [5]. The energy consumption was estimated to 15000 kWh/t and it was later shown that a reduction down to 1800 kWh/t was possible [6]. The low energy consumption was due to enzymatic and mechanical pre-treatment steps before the homogenisation. A number of other pre-treatment methods do exist, e.g. carboxymethylation [7] and TEMPO-mediated oxidation [8-10]. MFC with enzymatic and mechanical pre-treatment step before
homogenisation is produced without using solvents or hazardous chemicals which makes it relatively environment friendly.
Considering the extensive research on MFC, there are comparatively few publications on the use of MFC or nanocellulose as a wet-end additive in paper making [11-18]. These publications were all based on lab studies but there are also a few studies carried out in
In this study the economic potential in terms of variable cost needed for production of fine paper has been evaluated for a case where MFC was added in order to increase the filler content at maintained grammage and tensile index. A solution for demonstrating the impact from MFC in mill trials is also presented.
Manufacturing of MFC
The MFC production based on a mechanical enzymatic pre-treatment procedure prior to high-pressure homogenisation is carried out as illustrated in Figure 1. The MFC quality is improved with increased homogenisation pressure and number of passes through the homogeniser, as well as decreased homogenisation concentration. In fact, the MFC quality, for example measured as gel strength, is proportional to the energy consumption of the homogenisation step, Figure 1. A more detailed description of the MFC production for the herein described case study is given by Ankerfors et al [19]. The MFC produced and used was homogenised with an energy consumption of 1800 kWh/t (2.3% consistency and 1500 bar in pressure), represented by the arrow in Figure 1. This is also representing the required level of minimum energy input to obtain a gel-like product, below this point the products act more like fibre suspensions.
Figure 1. Procedure for MFC production (left), and MFC quality development with increased energy input at homogenisation (right).
Increased filler content using MFC as strength additive
When using MFC as a wet-end additive, the dry strength properties increase. This strength increase can be used by a paper or board producer to reduce the raw material cost by decreasing the grammage, i.e. use less pulp, at maintained strength or to increase the filler content at constant grammage, i.e. use less pulp and more filler, at maintained strength. Another alternative is to use a weaker (and cheaper) pulp in combination with MFC at constant grammage and maintained strength.
Since energy, and thereby money, is used when producing MFC, it is of interest to look at the variable cost when using MFC as a strength additive in paper and board making. Data used for this evaluation were taken from a pilot scale trial at the FEX paper machine [19]. In Figure 2 it can be seen, that it is possible to increase the filler content by 7%-units when adding 3.2% MFC and still maintain the same tensile index. If combining the same amount of MFC with 2% C-starch, it is possible to increase the filler content even further (12%-units in total). These increases in filler content will compensate for the decrease in dry solids content after press caused by the addition of MFC, see Figure 2.
The cost evaluation is made for a non-integrated mill where MFC is produced at the mill. Any transports have for instance not been included in this evaluation. A basic process layout is shown in Figure 3 together with energy inputs and approximate consistencies/dry solids content.
Figure 2. Results from pilot scale trial with MFC as a wet end additive showing the tensile index (left), and the dry solids content after press (right). Both diagrams are from
Ankerfors et al. [19]. MFC contents are based on fibres which means that, at 35% filler content, the amount of MFC in the sheet is 1.6% or 3.2%.
Figure 3. Process layout for the pilot scale trial together with energy inputs used. Dry solids contents at different positions are shown. HW = hardwood pulp, SW = softwood pulp and LP steam = low pressure steam.
Two different scenarios have been studied; without and with wet end starch. The input data and prices used in the cost evaluation of these two scenarios are summarised in Table 1 and Table 2. Please note that the evaluation of costs for production does not include investments, labour costs or transportation costs.
The following main assumptions have been made for the cost evaluation:
- The cost for producing MFC has been calculated from the pulp and enzyme price, and energy needed for refining and homogenisation.
- The pulp used for producing MFC was a never-dried bleached softwood sulphite pulp, but the price has been assumed to be equal to the price for bleached kraft pulp.
- The drying energy has been assumed to be equal to the energy needed to evaporate water from the pressed web, based on the difference in the dry solids content from the actual dry solids content after press to 93%.
Table 1. Data from Ankerfors et al. [19] that was used in the present cost evaluation.
Scenario 1, effect from MFC (no C-starch)
Scenario 2, effect from MFC together with C- starch Reference MFC Reference MFC + C-starch
Hardwood pulp, % 58.4 50.2 62.4 50.2 Softwood pulp, % 14.6 12.6 15.6 12.6 Filler: PCC, % 27 34 22 34 MFC, % 0 3.2 0 3.2 Total sheet, % 100 100 100 100 Chemicals: C-starch, % 0 0 0 2 C-PAM, % 0.03 0.03 0.03 0.03 Silica, % 0.06 0.06 0.06 0.06 Tensile index GM, Nm/g 23.8 23.8 27.4 27.4 Dry content after press, % 51.8 52.0 51.2 50.9 Table 2. Prices used in the present cost evaluation.
Resource Cost
Bleached kraft pulp 670 €/ton (market price)
MFC 794 €/ton*
PCC slurry 160 €/ton** Cationic starch 520 €/ton** Retention aid (C-PAM) 2 500 €/ton** Colloidal silica 2 000 €/ton**
Electricity - Sweden 0.05€/kWh (common market price) Low pressure steam 3.6 €/GJ*
Investments, labor, transports Not included *Estimated by RISE Bioeconomy, **Alibaba.com Nov 2016
The variable costs for the two scenarios are shown in Figure 4. In both cases, a large reduction of variable costs was seen when adding MFC and increasing the filler content. For the 1st scenario with an addition of 3.2% of MFC and 7% increased filler content the variable cost is reduced by 5.5%, while for the 2nd scenario combining the same amount of MFC with 2% C-starch and 12% increased filler content the variable cost is reduced as much as 9.4%. In general, pulp costs decrease and costs for filler and MFC increase. Due the similar dry solids content after press, the costs for steam and electricity are almost unchanged when adding MFC and simultaneously increasing the filler content. To note, the energy consumption of 2000 kWh/t for MFC production is included in the MFC price.
Figure 4. Estimated variable costs for the two scenarios where MFC was used to be able to increase the filler content at maintained tensile strength index.
In summary, the use of MFC as a strength additive has been proven to have a positive impact on the variable costs for a fine paper case when increasing the filler content.
Mobile demonstration plant for on-site trials
To conduct full-scale trials is an important and crucial step towards industrial
implementation. Typically, MFC contains only 2% solid material and as much as 98% water. Large quantities of material at low consistency are expensive to transport, and the volume reduction of MFC is associated with difficulties. Already at consistencies above 5% the MFC becomes highly viscous and hence difficult to pump and handle.
Furthermore, it is well known that drying causes irreversibly hornification and loss of the unique properties of MFC [22]. The need for large-scale on-site production then means high investment costs, and makes a significant barrier to innovation that is difficult to overcome for most companies. To facilitate evaluation of MFC in different processes and products at mill scale in a cost-effective way, a mobile demonstration plant has been established. With a capacity to produce 100 kg MFC/hour, the mobile plant makes it possible to evaluate the use of MFC at the mill. This offers a fast track to more resource efficient and competitive products at none or low investment costs, without need for transportation of materials. The established demonstration plant consists of a three-stage process: enzyme treatment, refining and homogenisation. At a trial, the mill simply supplies the on-site plant with ready-to-use pulp, electricity and water, and the demonstration plant delivers MFC back to the mill.
Acknowledgement
The authors wish to thank the financing companies within the pre-competitive research of InnRP2015-2017 Source Efficient Paper and Board Making at RISE Bioeconomy
(Andritz, BillerudKorsnäs, Fibria, Hansol Paper, Holmen, ITC, Metsä Board, Metsä Fibre, Saica, Smurfit Kappa, Solenis, Stora Enso, Tetra Pak and UPM). Furthermore, the funding by VINNOVA of the project “Mobile demonstration factory for nanocellulose” is fully acknowledged. BillerudKorsnäs, the partner in this project, is gratefully acknowledged for their invaluable contributions.
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