An Assessment of Cellulose and Paper Production

Paper is an essential material in meeting the communication needs of societies and has retained its importance since its discovery. The demand for paper products has increased from the time of its initial use, and this growth is expected to continue in the future. Today, global paper consumption has exceeded 400 million tonnes per year. Paper's contribution to the technological and cultural development of countries is significant. The cellulose and paper industry is classified as a heavy-scale production industry requiring intensive capital and energy. The paper industry uses woody and herbaceous lignocellulosic materials and waste paper as raw material sources, converting them into intermediate products of cellulose and paper pulp through mechanical, chemical, or a combination of these methods. These intermediate products are ultimately converted into paper. From this perspective, paper production ranks among the processes that derive the highest proportion of benefit from plant material and convert it into high value-added products. Today, the paper industry using the most advanced technological approaches has very high investment and operational costs. If one considers that approximately five billion particles per second (fibres, fillers and additives, etc.) are formed at the headbox at speeds of 60-80 km/hour, with bonding occurring between these particles, and approximately two kilometres of paper being produced per minute from the paper machine, one can get an idea of how precise and technological this production technology is. Thousands of different types of paper products can be produced for various applications. Paper and board products can be classified in many different ways depending on the properties of the raw materials used, the technological approach selected, and the final product characteristics. The following classification system is provided in its most general and simplified form. These main paper classes are further subdivided into many subgroups: By basis weight (gramme weight): Low, medium or high basis weight papers. By colour properties: Coloured, white, bleached or unbleached (brown) papers. By end use: Industrial, cultural and food papers. By raw material characteristics used in production: Papers produced from woody, herbaceous or secondary fibres. By surface treatment applied: Coated or uncoated, supercalendered papers.

2. Raw Material Sources Used in Paper Production

Cellulose, which is the main skeletal element of paper structure and is present in quantities of 70-100% depending on type, is one of the most important natural polymers produced by living plant organisms on Earth. The structure of cellulose is formed by D-glucose sugars forming β-1-4 glycosidic bonds. From this perspective, cellulose is a homopolymer. Cellulose shows a very wide distribution on Earth, from primitive plants (algae, mosses, ferns, etc.) to highly organized plants (woods) and certain bacteria. Although widely found in the plant kingdom, it is found in the highest concentration and purity in the cotton plant. In other lignocellulosic plants (wood, agricultural crops and grasses), it can be found in quantities of 30-70% along with chemical compounds such as lignin, hemicellulose and extractives. In morphologically less developed plants such as mosses and algae, it is found in much lower proportions (10-40%). Obtaining commercially significant amounts in pure form is preferred from plants where it is widely distributed, in order to prevent degradation of its chemical properties. Important plant sources used globally for the paper industry and their characteristics are presented in Table 1. Table 1. Plant Sources Used in Paper (Cellulose) Production and Their Characteristics Paper pulp (cellulose) production from woody and herbaceous plant sources, depending on the technological approaches used, can be examined under two main groups: chemical and mechanical paper pulp production. These groups are further subdivided into different subgroups. Table 2 shows paper pulp production methods from plant material in the most general form. Table 2. Processes Used in Cellulose and Paper Pulp Production from Biomass

3. Chemical Paper Pulp Production (Cellulose Production)

In addition to the degree of purity, one of the basic conditions for producing high-quality paper and board products with high physical and mechanical properties is the complete and effective removal of lignin, which is the most commonly found component after cellulose in lignocellulosic plant cells. This is only possible through the application of fully chemical processes and subsequent multi-stage bleaching operations. Two fully chemical cellulose production methods are most commonly used worldwide.

These are:

• Acidic sulfite fully chemical paper pulp (cellulose) production method. • Alkaline sulfate (Kraft) fully chemical paper pulp (cellulose) production method. Although the sulfite chemical paper pulp production method was widely used initially, from the 1950s onwards, when it became clear that the sulfate method could be used with all types of woody and herbaceous plants and offered many advantages over the sulfite method, more than approximately 80% of global total chemical paper pulp and cellulose production is still performed using the sulfate method today. Regardless of which chemical approach or compound is used in delignification processes, the breakdown of lignin's polymeric structure generally depends on the cleavage of all types of aryl ether bonds (aliphatic C-O-C aromatic). This is because the majority of bonds (50-70%) forming the polymeric structure of lignin found in both hardwood and softwood are alpha and beta aryl ether bond types. Additionally, the solubility of lignin can be facilitated through oxidation or derivatization (sulfonation) of functional groups such as carbonyl, carboxyl and hydroxyl in its structure, thereby allowing it to move away from the cell wall.

During these processes, while ensuring that undesired lignin moves away from the cell wall as much as possible (delignification) in paper production, chemical reaction conditions suitable for cellulose production must be used that will cause the least damage to cellulose.

The reactivity of lignin from different sources also varies. In particular, delignification of softwood is more difficult compared to hardwood and herbaceous plants. For this reason, in some cases, harsher reaction conditions (>170°C temperature and high pressure) may be required to obtain cellulose with the desired level and high degree of purity. However, this is disadvantageous in terms of the economics and efficiency of the process because fiber strength reduction in cellulose fibres may occur along with yield loss due to degradation of cellulose and polysaccharides. Generally, the establishment of a large-scale chemical paper pulp facility with a capacity of 1,000 tonnes per day (e.g. Kraft) costs over approximately USD 800 million. Additionally, the process requirements during operation of this facility and the approximate costs per tonne of wood pulp are shown below in general terms in US dollar pricing to provide an idea: Raw material: $90 Electrical energy: $0.0 (Assuming power generation from own waste sources) Chemicals: $70 Labour: $40 Depreciation and maintenance: $135 Total cost: $335/tonne Paper pulp costs and selling prices vary seasonally, regionally and over years. Generally, when the selling price of fully bleached Kraft paper pulp is taken at $600/tonne, approximately $265 profit can be made per tonne of paper pulp according to the above calculation. Table 3 outlines some advantages and disadvantages of chemical cellulose production methods. Table 3. Characteristics of Chemical Paper Pulp Production

4. Mechanical Paper Pulp Production (Cellulose Production)

Unlike chemical paper pulp production, these methods are utilized in the production of inexpensive paper and board products where the degree of purity is not important and physical and mechanical properties are low. This is because in this approach, the aim is not to remove lignin from the cell wall, but to make plant fibres individual through mechanical effects (crushing, grinding, shearing, etc.). Two main mechanical paper pulp production methods are most commonly used worldwide.

These are:

• Stone mechanical paper pulp production (SGW). • Refiner mechanical paper pulp production (RMP). These methods contain many subgroups and newly developed technological paper pulp production forms. Stone mechanical paper pulp production is the simplest and first mechanical method developed at the beginning of the 1900s. In this method, wood blocks are fibrillated by being pressed against large discs rotating in a water pool containing surface-abrasive stones. Subsequently, although new approaches were developed to make the system more effective and improve the quality of the obtained fibres, it is a mechanical paper pulp production method that is gradually declining and losing importance today. In the 1950s, refiner mechanical paper pulp production was developed, based on the principle of paper pulp production by subjecting plant raw materials to mechanical effects between two rotating discs. Subsequently, many modifications were made to this method, and fibre quality properties were improved through certain approaches during production. Today, more than 70% of global total mechanical paper pulp and cellulose production is still performed using refiner methods. Yield in mechanical pulp production is quite high, approximately between 85-95%. However, low-strength papers are produced because the obtained fibres contain high levels of lignin along with cellulose and can easily change colour. For this reason, they are used mixed in different proportions with chemical pulps in the production of printing/writing papers. Generally, the establishment of a medium-scale mechanical paper pulp facility with a capacity of 500 tonnes per day (e.g. TMP) costs over approximately USD 200 million. Additionally, the process requirements during operation of this facility and the approximate costs per tonne of wood pulp are outlined below in general terms to provide an idea: Wood raw material: $40 Electrical energy: $100 Chemicals: $50 Labour: $40 Depreciation and maintenance: $100 Total cost: $320 Paper pulp costs and selling prices vary seasonally, regionally and over years. Generally, when the selling price of fully bleached Kraft paper pulp is taken at $450/tonne, approximately $130 profit can be made per tonne of paper pulp. Table 4 outlines some advantages and disadvantages of paper pulp production using mechanical methods. Table 4. Characteristics of Mechanical Paper Pulp Production

5. Conclusion and Recommendations

Paper, an important intermediate material in the development of societies and meeting communication needs, has retained its importance from when it was first used thousands of years ago until today. In the future, demand for paper products is expected to continue increasing. Turkey's paper industry has made significant advances in recent years with the privatization of SEKA and the dominance of the private sector in the market. However, today, Turkey's paper industry is dependent on imports in terms of raw material cellulose. To reduce this external dependency, priority should be given to waste paper recycling technology, which could provide a way forward. This is because waste paper recycling rates in our country are considerably lower compared to developed countries. Prof. Dr. H. Turgut Şahin - Isparta University of Applied Sciences / Faculty of Forestry - Forest Industry / Engineering Department

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