How Paper is Made

A schematic of the papermaking process is shown below. In this case, a chemical pulping process is depicted. However, if the digester, blow pit, and washer were replaced with a mechanical refiner, the drawing would also describe a mechanical pulp and paper mill. (Move your mouse across the drawings below to learn more about the processes shown.)

Note that once the wood has been converted to pulp, the pulp is beaten (more on this in a moment), refined in a Jordan refiner, and then sent to a large machine where a slurry of fiber is metered onto a moving wire. In this machine, called a Fourdrinier, a slurry that is 99% water by weight flows from the head box onto the wire, after which suction boxes pull water rapidly away. The result is a fiber mat that is then pressed, dried, coated and/or sized, re-pressed, and cut into desired sizes of paper.

A Fourdrinier unit is pictured here. A slurry of fiber is being metered onto the moving wire at the far right in this picture, with movement of the wire toward the left. The drier units can be seen at the far end of the production line. (Photo courtesy: American Forest and Paper Association)

The beating and refining processes are very important in the papermaking process. An examination of beating, in particular, provides several insights as to some of the technology involved.

This is a photo of softwood that has been chemically pulped. Note that the fibers are straight, smooth, and largely undamaged.

For the most part, however, smooth surfaces and rounded, undamaged fiber are not what is needed in making a quality sheet of paper. Fibers must be flattened to increase the contact area (and thus the bond potential) between them.
Flattened fibers can be readily seen in this highly magnified photo of the surface of paper. (Photo by John Crist and Ron Teclaw)
Moreover, by unraveling microfibrils from the cell walls, surface area (and thus hydrogen bonding potential) can be greatly increased as illustrated by this photo of mechanically produced fiber. (Photo: John Crist and Ron Teclaw)
The way that fibers are flattened, and subjected to a mechanical rubbing action that unravels microfibrils, is through the use of a beater. A simple beater (a Hollander beater) is shown here. A slurry of fiber goes around and around in the tub, each time passing between a fixed bedplate and the ribs of a rotating beater wheel. The opening between the bedplate and wheel is set to about the width of a single wood fiber, meaning that fibers are pounded and deformed with each pass. (Image adapted from TAPPI)

The longer fibers remain in the beater, the more beat-up they become, and the more the surface area of the fiber is increased.

Installation of Belt Conveyors in Place of Pneumatic Conveying System for Wood Chips Transfer

Background

The hard wood and soft wood logs (basic raw materials) are cut into small size by band saw. After wetting these logs with water spray to remove sand and dirt particles, they are fed into chippers. These small size logs are chipped to get chips of smaller size (1/2 " to 1").

These chips are then transported from the chipper house, located at the ground floor, to the top of the digester house (at a height of about 12 to 15 m), for cooking operations.

Conventionally, the chips were being transported pneumatically. The pneumatic conveying, though simple and easy to install, consumes more energy. Mechanical conveying is more energy efficient and consumes only 25 - 30% of the energy consumed by pneumatic conveying. Hence, many plants have converted their conveying systems, from pneumatic to mechanical and reduced their energy consumption. The existing layout of the chipper house and the digester house, governs the alternatives of mechanical conveying systems chosen. The installation of a belt conveyor can be taken up in those plants, where the horizontal distance between the digester and the chipper is sufficiently large. In case the horizontal distance is less and the inclination of conveying required is more, then a belt conveyor will not be suitable. In such cases, modified systems, such as, the cleated belt conveyors have been installed with good benefits.

This case study describes both the mechanical conveying alternatives, which have been implemented in two different paper plants in the country.

Previous status

Plant - A In an integrated paper plant, manufacturing paper & paperboards, the wood chips from the chipper house to the digester house, were transported using pneumatic conveying system. The horizontal distance between the chipper house and the digester house, was more and the inclination permitted the installation of a belt conveyor.

Plant – B In an integrated paper and paperboard plant, the wood chips from the chipper house to the silos and from the silos to the digester house, were transported using pneumatic conveying system.The horizontal distance between the chipper house and the digester was less. Also, the inclination of conveying was more, than what a conventional belt conveyor could accommodate.

Energy saving project

Plant – A The plant team replaced the pneumatic conveying system with mechanical belt conveying system.

Plant – B The plant team replaced the pneumatic conveying system with mechanical conveying system. As the inclination of conveying was more, the belt conveyor was not suitable. A modified belt conveyor called the 'cleated belt conveyor' was used for the conversion. This modified conveyor has 'cleats' on the surface of the belt and these cleats physically hold the chips from sliding down.

Concept of the project Pneumatic conveying is highly energy intensive and is at-least 3 - 4 times costlier than mechanical conveying. The latest trend among all the paper plants is to opt for mechanical belt conveying systems.

Implementation status, problems faced and time frame

Plant – A The pneumatic conveying system was replaced with mechanical conveying system. Belt conveyors were installed for the transportation of wood chips. In this particular plant, designing the layout of the belt conveying system was difficult, due to the lack of adequate space. The gradient however was favourable for installing a belt conveyor. The belt conveyor was installed after compromising certain minor factors, such as reduced passages and corridors. The project was implemented in 8 months time, once the layout was decided.

Plant – B In this particular plant, designing the layout of the belt conveying system was difficult, due to the lack of adequate space and steep gradient. Hence, cleated belt conveyors were installed for the transportation of wood chips. In the initial days of operation, problems were faced on account of the failure of pulley and belt. The supplier studied these problems and the design was improved, to avoid recurrence of these problems.

The project was implemented in two phases as below:

The cleated belt was installed, for chips transfer from the chipper house to silo, in the first phase. After successful commissioning and operation of the system, the second phase was taken up for implementation. During the second phase, the cleated belt was introduced for chip transfer, from silos to digester house. The implementation of each phase of the project took 6 months.

Benefits achieved

Plant – A There was a reduction in power consumption of 3868 units per day (equivalent to 193.4 kW per day), on installation of belt conveyors. Plant -B The summary of the benefits achieved is as follows:

Parameter	Units	Chipper to House of Silo	Silo to House Digester
Annual operating hours	hrs/yr	5600	4900

Average power consumption with pneumatic conveying	kW	77.00	63.25
Power consumption of cleated belt conveyor	kW	11.72	4.60
Power savings with the conversion	kW	65.28	58.65

Financial analysis

Plant - A (Belt conveyor) The annual energy saving achieved was Rs.19.59 lakhs. This required an investment of Rs.37.50 lakhs, and was paid back in 23 months.

Plant -B (Cleated belt conveyor) The annual energy saving achieved was Rs.38.30 lakhs. The investment made was Rs.75.00 lakhs, which was paid back in 24 months.

Benefits of installing belt conveyers

Lower power consumption -25% of pneumatic conveying power Belt conveyor for chips conveying Cleated belt conveyor for chips conveying.

Cost Benefit Analysis

Annual Savings - Rs. 38.30 lakhs with cleated belt conveyor

Investment - Rs. 75.00 lakhs

Simple payback - 25 months

Replacement of Dyno-drives with Variable Frequency Drives (VFD's) in Washer Drum Drives

Background

The contents of the digester, after cooking, are blown down to a blow tank. The blown pulp is then washed, to remove the dissolved lignin and chemicals.

Usually, washing is practised in counter current fashion, involving 3 or 4 stages of washing, using rotary drum washers. The washed pulp is then sent for bleaching and further processing. The rotary drum washers are operated under vacuum, utilising a barometric column. These drum washers are driven by a variable speed system, to achieve the desired speed variation, according to the throughput of the plant.

Previous status

In one of the old integrated paper plants, the washer drum drives were originally supplied with AC commutator motors. As these commutator motors had frequent maintenance problems, these were replaced with dyno-drives. The dyno-drives, though have lesser maintenance problems, are inefficient at lower speeds. As the washers were operating at 50 - 60% of the rated speed for majority of the time, the replacement of these drives with more efficient drives, such as, variable frequency drives (VFD) can result in substantial energy savings.

Energy saving project

The dyno-drives of the washers were replaced with variable frequency drives (VFD's).

Concept of the project

The dyno-drives are very inefficient at lower speeds. The dyno-drives also require special attention and maintenance, because of its semi-open construction. The variable frequency drives (VFD) are more efficient at lower/all speeds also and require lesser maintenance, in comparison to the dyno-drive.

Implementation status, problems faced and time frame

The dyno-drives in both the washer drums were replaced with 22.5 kW variable frequency drives (VFD's). A VFD can achieve the exact speed variation requirement energy efficiently depending on the process requirement. The problem faced during the implementation stage was the frequent tripping of the VFD's. The supplier studied this and suitable remedial action was taken, to solve the problem. The entire project was executed in 3 months time.

Benefits achieved

The replacement of dyno-drives with VFD's, resulted in a net reduction in power consumption. The net power saving achieved was 36,024 units/year (equivalent of 5.23 kW). The other major advantage is, the precise speed variation, which can be achieved.

Financial analysis

The annual energy saving achieved was Rs.1.08 lakhs. This required an investment of Rs.2.50 lakhs and had a simple payback period of 28 months.

Case 1 : Installation of High Capacity Chippers in the Chipper House Background

Chipper is the first major equipment in a paper plant. These chippers are used to produce wood chips, from the raw materials like hard wood, bamboo etc., for further processing in the digester house. Many of the old paper plants, in general, have chippers of lower capacity. The feed to the chippers were invariably, manual. This led to non-uniform feeding, which in turn affected the throughput through the system. This resulted in very high specific energy consumption in the chippers. The recent technological advancements have led to the development of high capacity chippers. These chippers are provided with mechanical feeding mechanisms, enabling consistent feed to the chipper and high throughput from the chippers. This results in lower specific energy consumption of the chippers. The installation of high capacity chippers is gaining increasing popularity, especially among the large integrated paper plants.

Previous Status

In a large integrated paper plant, the chipper house had a capacity to produce 25 TPH of chips. The chip preparation was performed by 5 numbers of 5 TPH (each) capacity disc chippers. This system had the following disadvantages:

Low capacity
Manual feeding leading to non-uniform feed distribution and throughput
High specific energy consumption (17-21kWh/ton)
Longer waiting time at the digester house, due to lower capacities and manual feeding
Very high dust generation and accumulation

The installation of high capacity drum chippers, can result in higher throughput and lower specific energy consumption.

Energy saving project

The high capacity drum chippers were installed in place of the older, low capacity disc chippers.

Concept of the project

These high capacity drum chippers have the following advantages over the low capaciy conventional chippers:

Higher capacity
Mechanical feeding leading to higher throughput
Lower specific energy consumption (about 7 kWh/ton)
High quality, uniform size chip preparation
Improved working conditions on account of low dust generation
Improved safety aspects, due to closed machinery moving parts

Hence, high capacity chippers can be installed in the chipper house to achieve maximum energy efficiency.

Implementation status, problems faced and time frame

Two numbers of high capacity drum chippers were installed in place of the earlier 5 numbers of disc chippers.
The implementation of this project was taken up in two phases. Initially, one drum chipper was installed and its performance was closely monitored. On achieving satisfactory operating results, the second drum chipper was installed. With this installation, all the old disc chippers in the chipper house were replaced.
There were no major problems faced during the implementation of this project. The implementation of this project was completed in 24 months.

Benefits achieved

The specific energy consumption reduced by one-third, with the installation of these high capacity drum chippers. A specific energy consumption of 7.0 kWh/ton was achieved with these drum chippers.
Besides, the energy savings, the installation of drum chippers also resulted in improved quality of chips produced, reduced dust generation and increase in throughput.

Financial analysis

The annual energy saving achieved was Rs.56.00 lakhs. This required an investment of Rs.240.00 lakhs, which had a simple payback period of 52 months.

Benefits of high capacity chippers

Lower specific power consumption - 7 kWh/ton
Mechanical feeding leading to higher throughput
Uniform chips size Cost benefit analysis
Annual Savings - Rs. 56.0 lakhs
Investment - Rs. 240.0 lakhs
Simple payback - 52 months