Paper machine
Most modern papermaking machines are based on the principles of the Fourdrinier Machine. It has been used in some variation since its conception. The Fourdrinier uses a conveyor belt in the wet end to create a contineous paper web transforming a source of wood pulp into a final paper product.
The main other paper machine type is the cylinder mould machine[1] using a rotating cylinder partially immerge in a tank of fiber slurry in the wet end to form a paper web, giving a more random distribution of the cellulose fibres. This type of paper machine give lower production speeds and today few are used worldwide producing mostly high quality security paper, bank notes needing very detailed watermark and paper board for higher strength.
The main variation on the Fourdinier machine is called the Twin Wire Machine [2] or Gap former using two screens in the wet end section increasing the dewatering speed of the fiber slurry . Other variation in the wet end section are called hybrid formers (top former), mostly rebuilds of classic Fourdrinier former using also two screens technology.
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Contents
History
In 1799, Louis-Nicolas Robert of Essonnes, France, was granted a patent for a continuous paper making machine.[3]. At the time Robert was working for Saint-Léger Didot, with whom he quarrelled over the ownership of the invention. Didot thought that England was a better place to develop the machine. But during the troubled times of the Fench revolution, he could not go there himself, so he sent his brother in law, John Gamble, an Englishman living in Paris. Through a chain of acquaintances, Gamble was introduced to the brothers Sealy and Henry Fourdrinier, stationers of London, who agreed to finance the project. Gamble was granted British patent 2487 on 20 October 1801.
With the help particularly of Bryan Donkin, a skilled and ingenious mechanic, an improved version of the Robert original was installed at Frogmore, Hertfordshire, in 1803, followed by another in 1804. A third machine was installed at the Fourdriniers' own mill at Two Waters. The Fourdriniers also bought a mill at St Neots intending to install two machines there and the process and machines continued to develop.
In the USA the first recorded paper machine was Gilpin's at Brandywine Creek, Delaware in 1817. This machine was also developed in England, but it was a cylinder mould machine. The Fourdrinier machine was introduced into the USA in 1827.[4]
Pulp preparations
Harvested tree trunks are cut into logs of four to eight foot lengths, then sent to a very large horizontal debarking drum, which rotates and strips the logs bare; or a vertical ring debarker which removes bark mechanically in a single pass. In some cases, whole tree length logs can be debarked. The freshly debarked logs are then fed into a chipper, which reduces the logs to handheld-sized chips. The chips are then passed along to a digester where they are cooked for a number of hours, a process that softens the wood to a large degree. The digester can be one of two types: sulfite or sulfate. In a sulfite digester, the principal chemical constituent is calcium acid sulfite and the method is referred to as the acid process. The sulfate, or Kraft, process is the younger of the two, and uses an alkaline system that reduces cooking time.
After the cooking is complete and the lignin content has been removed, the softened chips are fed at high pressure into refiners where the chips are forced between rotating steel plates. The refiner plates shatter the chips into a soup of brown fibres. Chlorine is used to bleach brown fibres to a brighter white colour, and calcium hypochlorite (sulfite process) or chlorine dioxide (sulfate process) are also used for whitening. Caustic soda (sodium hydroxide NaOH) (lye) is used to wash the pulp of any impurities, and the steps are repeated in order to obtain the desired brightness.
Sections of the paper machine
There are four main sections to a paper machine.
Wet end
The first section is typically known as the wet end. Pulp may be delivered to the paper machine in a slurry form (a mixture of fiber and water) directly from the pulping process. Alternatively, pulp may be supplied in dried sheets which are then broken down in water to produce a similar slurry, before being fed to the refiners in the wet end where the fibers are subjected to high pressure pulses between bars on rotating refiner discs. This action causes the fibrils of the fibers to partially detach and bloom outward. After refining the pulp is mixed with some of the following: sizing, fillers, colors, retention aid and waste paper called broke to a stock, and passed on. Washing is done in pressurized screens and hydocyclones and also deaeration is done.
The stock then enters the headbox, a unit that disperses the stock and loads it onto a moving wire mesh conveyor with a jet from an opening called the slice. The streaming in the jet makes some fibres align. This alignment can partly be taken away by adjusting the speed difference between the jet and the wire. The wire revolves around the Fourdrinier table, from breast roll under the headbox over the couch to the forward drive roll, foils under the wire are creating low pressure pulses that will vibrate and partly deflocculate the fibres while water is removed. Later on Suction boxes below the wire gently remove water from the pulp with a slight vacuum and near the end of the wire section the couch will remove water with higher vacuum.
Press section
The second section of the papermachine is the press section, which removes the most water via a system of nips formed by rolls pressing against each other aided by press felts. This is the most efficient method of dewatering the sheet as only mechanical pressing is required. Press felts historically were made from cotton. However, today they are nearly 100% synthetic. They are made up of a polyamid woven fabric with thick batt applied in a specific design to maximise water absorption.
Presses can be single or double felted. A single felted press has a felt on one side and a smooth roll on the other. A double felted press has both sides of the sheet in contact with a press felt. Single felted nips are useful when mated against a smooth top roll, which adds a two-sidedness—making the top side appear smoother than the bottom. Double felted nips increase roughness, as generally, press felts.
Conventional roll presses are configured with one of the press rolls is in a fixed position, with a mating roll being loaded against this fixed roll. The felts run through the nips of the press rolls and continues around a felt run, normally consisting of several felt rolls. During the dwell time in the nip, the moisture from the sheet is transferred to the press felt. When the press felt exits the nip and continues around, a vacuum box known as an Uhle Box applies vacuum (normally -60 kPa) to the press felt to remove the moisture so that when the felt returns to the nip on the next cycle, it does not add moisture to the sheet.
Pickup roll presses are vacuum assisted rolls loaded against plain press rolls (usually a roll in a centre position). While out of favour, these are generally found in machines built in the 1970s–1980s. Pickup roll presses normally have a vacuum box that has two vacuum zones (low vacuum and high vacuum). These rolls have a large number of drilled holes in the cover to allow the vacuum to pass from the stationary vacuum box through the rotating roll covering. The low vacuum zone picks up the sheet and transfers, while the high vacuum zone attempts to remove moisture. Unfortunately, centrifugal force usually flings out vacuumed water—making this less effective for dewatering. Pickup presses also have standard felt runs with Uhle boxes. However, pickup press design is quite different, as air movement is important for the pickup and dewatering facets of its role.
Crown Controlled Rolls (also known as CC Rolls) are usually the mating roll in a press arrangement. They have hydraulic cylinders in the press rolls that ensure that the roll does not bow. The cylinders connect to a shoe or multiple shoes to keep the crown on the roll flat, to counteract the natural "bend" in the roll shape due to applying load to the edges.
Extended Nip Presses (or ENP) are a relatively modern alternative to conventional roll presses. The top roll is usually a standard roll, while the bottom roll is actually a large CC roll with an extended shoe curved to the shape of the top roll, surrounded by a rotating rubber belt rather than a standard roll cover. The goal of the ENP is to extend the dwell time of the sheet between the two rolls thereby maximising the dewatering. Compared to a standard roll press that achieves up to 35% solids after pressing, an ENP brings this up to 45% and higher—delivering significant steam savings or speed increases.
Dryer section
The dryer section of the paper machine, as its name suggests, dries the pulp by way of a series of steam-heated rollers that stretch the web somewhat, removing the moisture. Additional sizing agents, including resins, glue, or starch, can be added to the web to alter its characteristics. Sizing improves the paper's water resistance, decreases its ability to fuzz, reduces abrasiveness, and improves its printing properties and surface bond strength. Some paper machines also make use of a 'coater' to apply a coating of fillers such as calcium carbonate or china clay.
Calender section
A calender consists of a number of rolls, where pressure and heat is applied to the passing paper. Calenders are used to make the paper surface extra smooth and glossy. It also gives it a more uniform thickness. The pressure applied to the web by the rollers determines the finish of the paper.
After calendering, the web has a moisture content of about 6% (depending on the furnish). It is wound onto a roll called a tambour, and stored for final cutting and shipping. The roll hardness should be checked, obtained and adjusted accordingly to insure that the roll hardness is within the acceptable range for the product.
References
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- Fourdrinier machine description from Paper Manufacturing in the United States, 1916
- Biography of Henry Fourdrinier from Dictionary of National Biography, 1889
- British Association of Paper Historians
- Video: Frogmore Mill in Apsley; Victorian era Fourdrinier machinede:Papiermaschine
fr:Machine à papier nl:Papiermachine ru:Бумагоделательная машина fi:Fourdrinier
sv:Pappersmaskin- ↑ Paper Machine Clothing: Key to the Paper Making Process Sabit Adanur, Asten,CRC Press, 1997, p 120-136, ISBN 9781566765442
- ↑ Technology choice in a global industry : the case of the twin-wire in Canada, Ofori-Amoah, Benjamin, 1989 Thesis (Ph.D.) - Simon Fraser University, 1990, http://ir.lib.sfu.ca/handle/1892/6373
- ↑ Larousse Encyclopaedia - les frères Robert, Mécaniciens français.
- ↑ Hills, Richard, "Papermaking in Britain 1488–1988", Athlone Press, 1988.
