Paper is one of the most commonly used commodities in existence. A huge range of products, from newspapers, books, magazines, to money, and packaging are made from paper. Whilst the electronic world, with activity increasing done online, the use of tablets and e-readers, has had an impact on its use, the demand for paper is still strong.
In this article we look at the role that steam plays in the production of paper. By concentrating on the actual paper production process we will describe the process from finished pulp onward. Pulp production is another subject in itself. It also utilizes steam, and to do it justice we will be covering it in a future article.
In basic terms, papermaking involves removing the water from the pulp, forming the pulp into sheets, drying it, coating it, drying it again if necessary, and then winding it into reels.
History
The ancient Egyptians grew a marsh grass called Cyperous Papyrus in the Nile river valley. The Egyptians cut thin strips from the plant’s stem and softened them in the muddy waters of the Nile. These strips were then layered in right angles to form a kind of mat. The mat was then pounded into a thin sheet and left in the sun to dry. The resulting sheets were ideal for writing on. Since they were also lightweight and portable they became the writing medium of choice of Egyptians, Greeks and Romans for record keeping, spiritual texts and works of art.
It is from papyrus that the word paper comes from. Although papyrus sheets were similar to paper in terms of function, being laminated sheets they were technically more like a mat and therefore not the same as the papers of today.
Experts believe that the very first paper was produced by a Chinese court official Ts’ai Lun, who is generally credited with ‘inventing’ paper in the form we know it. He mashed pieces of mulberry bark, cloth and hemp in water until they were reduced to pulp. He then drained away the water, pressing and drying the matted fibres. The result was paper.
The secret of papermaking remained in China for 650 years, until Arabs learned the art from Chinese prisoners of war. The process was brought to Europe in the 12th century, but the method of making paper from wood was lost along the way. Rags were used instead.
Modern papermaking began in the early 19th century in Europe with the development of Fourdrinier machine, which produces a continuous roll of paper rather than individual sheets. In 1844, Canadian inventor Charles Fenerty and German inventor F.G. Keller had invented the machine and associated process to make use of wood pulp in papermaking. This would end the near 2,000-year use of pulped rags and start a new era for the production of newsprint and eventually almost all paper was made out of pulped wood.
What paper is made of
Most paper is made using a plant material called cellulose, which is normally sourced from trees. Most wood material is formed of fibres, tiny cellulose strands stuck together with a natural adhesive material called lignin. It’s by separating and reorganizing those fibres that paper is made. To do so, it is necessary to break down the lignin and this is done using various different pulping methods.
Wood for paper comes from the following sources:
- Trees cut down
- Sawmill scraps and sawdust
- Recovered or recycled paper
Foresters divide trees into two broad categories, hardwood and softwood.
- Hardwood trees such as oaks and maples have wood with very short fibres. Paper made from these species is weaker than that made from softwoods, but its surface is smoother, and therefore better to write and print on.
- Softwood trees such as pine and spruce have wood with long fibres, and paper made from this type of wood is much stronger. This paper is ideal for making products like paper or paperboard, and boxes that require superior strength. But the finish is rougher, and that’s not as good for writing and printing.
A blend of hardwood and softwood fibre is ideal for making printing and writing paper, which needs to be both strong and smooth.
Trees used for paper generally come from managed forest called timberlands. It is common for two new trees to be planted for every harvested tree. Some managed timberlands are tree farms, where trees are planted and harvested over and over again. It is thought that this cycle can go on indefinitely guaranteeing a long-term supply.
As a rule of thumb; 2m³ of timber will produce 1 tonne of newsprint in a modern paper mill.
Recovered paper is a good source of fibre because it is readily available and easily recycled. Recycling also saves landfill space. A piece of recycled paper may contain new fibres as well as those which have been recycled once or several times. However recycling will never entirely replace using trees for papermaking. There are several reasons for this:
- There is not enough recovered paper to meet the world’s demand.
- Some paper is too contaminated to be reused.
- Fibres can only be recycled five to seven times before they become too short and weak to be used in papermaking. Eventually they will be washed out of the pulp during the recycling process.
Types and grades of paper
Many thousands of different types of paper are produced worldwide. Each type is produced to meet a specific use or purpose, for example glossy brochures require smooth, coated papers, while tissue papers must be soft and absorbent but must not dissolve when wet
The differences between paper types and grades are achieved by adjusting the following:
- The length of fibre used. Short fibres produce high quality smooth papers, while long fibres produce strong, rough papers.
- The thickness and weight of the paper.
- Whether or not a coating is used, and the type of coating used.
- Production processes (e.g. bulking or drying).
Paper making process
This section will cover the paper making processes itself, from finished pulp onwards. As described earlier in basic terms, papermaking means removing the water from the pulp, forming the pulp into sheets, drying it, coating it, and then winding it into reels.
The component processes include:
- Pulp preparation
- The paper machine itself, including reeling and cutting
Pulp preparation
At integrated pulp and paper mills, pulp is usually stored in high density towers before being pumped to stock preparation. Non-integrated mills use either dry pulp or wet lap (pressed) pulp, usually received in bales.
When the bales of wood pulp or waste paper arrive at the paper mill they are loaded onto a conveyor and passed into a circular tank containing water. This has a very powerful agitator at the bottom which breaks up the bales into small pieces. The pulp mass created begins to look like thick porridge. This machine is known as a hydrapulper. It operates automatically and when the disintegrating process is complete it discharges the pulp into large storage tanks. At this stage it is referred to as ‘stock’.
Before refining the fibres are stiff and inflexible and if this was made into paper it would result in a weak and bulky paper. The stock is therefore pumped through a refiner which has a series of revolving discs. This violent process cuts and opens up the fibres and makes the ends divide (known as fibrillation). The fibres become more pliable and this improves the fibre bonding. The properties of the resultant paper are directly related to the refining process.
After refining, the stock is screened and cleaned to remove small impurities which could ruin the finished paper. The stock passes to a blending tank where chemicals and dyes can be added to obtain the required characteristics of the finished paper. After passing through a second cleaning system the stock is now ready for the paper machine.
The paper machine
The paper machine itself has several sections. These sections either fall in the wet or the dry end.
Wet end:
- Wire section
- Dandy Roll
- Felts and press roll
Dry end
- Drying cylinders
- Size press
- Drying cylinders
- Calender
- Jumbo reel
The wire is a woven plastic mesh conveyor belt which can be up to 35 metres long and as wide as the paper machine. The stock by now is milky in appearance where it has been diluted with water. The stock flows from a flow box onto the wire which distributes the fibres evenly over the whole width of the paper machine. As the paper stock flows from the flow box onto the wire, the water drains away through the mesh leaving tiny fibres as a mat on top of the mesh. By the time the stock has travelled half way down the wire, a high percentage of water has drained away. From this point the removal of water has to be assisted by suction from underneath the wire. When the thin mat of fibres reaches the end of the wire, although it is still very moist and weak, it has become a sheet of paper.
A dandy roll is situated near the end of the wire section. It is covered with a woven wire and is in contact with the upper surface of the forming web. As the wet web of paper passes under the watermarking dandy roll, the design is impressed into the paper which results in a permanent watermark on the sheet.
The paper passes from the wire section to the press section which consists of a number of heavy rollers. The paper is conveyed through these rollers on thick felts of synthetic fibre. More moisture is squeezed out of the paper like a mangle, and drawn away by suction. At this stage of the process the paper is still very moist. As a general rule about 55% of the moisture is removed by the presses, while the dryer section reduces the moisture by about 45%.
Moving to the dry end of the machine, the paper passes through a large number of steam heated drying cylinders. Synthetic dryer fabrics carry the web of paper round the cylinders until the paper is completely dry. Part way down the bank of drying cylinders is the size press. It is here that a solution of water and starch can be added to the sheet in order to improve the surface for printing purposes. The paper then continues through the drying section.
The calender consists of a stack of polished iron rollers mounted one above the other. The function of the calender is to consolidate and polish or glaze the surface of the paper.
Still travelling up to 2000 m/minute, the paper now comes off the machine ready for reeling up into large reels, each of which may contain up to 20 tonnes of paper. These large reels are either cut into sheets or slit into smaller reels according to the customer’s requirements. The finished reels or sheets are then packed for despatch.
The role of steam in the paper machine
As mentioned earlier steam is used in the dry end of the paper machine. The dying cylinders which dry the paper are heated using steam. The drying cylinders are large metal cylinders heated by filling them with steam. The wet paper passes between these hot rollers, there can be up to 50 or 60 cylinders on a fast running paper machine.
Steam in the drying cylinder
While in contact with each dryer cylinder, the paper absorbs heat. As the paper travels across the open pocket between cylinders, this heat is dissipated through the evaporation of water. Heating and drying the wet sheet seals the fibres closer and closer together, turning them gradually from pulp into paper.
Dryer rolls are usually between 1.5 and 1.8 metres in diameter, and are supplied with steam between 1 and 11 barg, according to the type of paper or paperboard being produced.
Each group of dryers will use steam at a different temperature and therefore pressure.
Condensate removal from the cylinders
As the steam in the cylinder gives up its heat it condenses into water. This water (condensate) must be removed from the cylinder or proper temperatures will not be reached. The condensate situation in the cylinder itself can generally be described by four different terms:
- Puddling. On slow machines or where a considerable amount of condensate has accumulated it collects a as a “puddle” at the bottom of the cylinder.
- Climbing puddle. As the speed increases or as the amount of condensate is reduced the puddle starts to “climb”.
- Cascade. Once the puddle climbs beyond a certain level it starts to cascade.
- Rimming. As the speed increases more or as the quantity of condensate is reduced “rimming” starts to occur and the condensate forms a uniform thickness round the inside of the cylinder due to centrifugal action. Complete rimming is desirable in terms of uniform heat transfer.
This condensate situation stage in the cylinder can be affected by many factors including:
- Volume of condensate.
- The inside diameter of the dying cylinder.
- The condition (smoothness) of the inside of the drying cylinder.
- The acceleration of the drying cylinder (if coming up to speed).
- The speed of the cylinder.
- The differential pressure between the inside of the dryer and the condensate system.
Condensate, blowthrough steam, air and non-condensable gases are removed from the dryer using a device called a siphon. A siphon is simply a dip pipe which the condensate mixture is removed by. Siphons are stationary or rotary in type.
The ideal syphon should be designed and sized so as to keep the thickness and variation of condensate film to a minimum.
Condensate system outside the dryers
The two most commonly used systems for control and drainage across the drying section are the cascade method and the thermocompressor method.
To obtain the maximum heat from steam, ideally all the steam must be condensed. In practice, this never happens inside the dryer shell. Depending upon the dryer speed a percentage of the total steam entering the dryer shell is never condensed and leaves the dryer mixed with condensate as two-phase flow. The uncondensed steam in the condensate is called ‘blow-through steam’.
The cascade system, cascades the blow-through steam through succeeding dryer sections to a small group of dryers at the wet end. This last group discharges to a condenser and depending on the minimum pressures required, it might discharge to some other mill process. Several pressure control loops are used to throttle the steam each dryer section.
A jet compressor is a device that uses a fluid at high pressure to entrain a fluid at low pressure and discharge a mixture at some intermediate pressure. It can be used to recirculate either a compressible or non-compressible fluid. In their application in the paper machine the medium is steam. When both motive and suction mediums are steam the compressor is referred to as a “thermocompressor.”
A thermocompressor system is used to inject intermediate pressure steam into the dryer by using high pressure motive steam mixed with the low pressure blow-through steam. The pressure control of a section of dryers can be achieved by a pressure control instrument throttling the thermocompressor and the makeup steam. Thermocompressors when used on paper machine dryer sections, offer the advantages of machine flexibility and high production capabilities. Each separately controlled dryer section can be looked upon as a single paper machine that will operate within the design limits of the thermocompressor. A thermocompressor can be a useful tool if the correct information is provided for its design. An incorrectly designed unit can be very wasteful of high pressure motive steam, and also suction steam (blowthrough) if it cannot be recirculated.