Pond Size Press - Paper making Overview

Pond Sizer - Paper making Overview

pond style size press is an older designed surface size coaters. Prior to Sym Sizers Starch was applied directly between two rolls creating a "pond" of starch. 

Paper passes through the liquid phase and is pressed between the size press rolls. prior to the Size press the Paper is dried to a Moisture content between 8 to 12 %. this is the ideal moisture range to accept liquid back into the paper. 

The moment the paper comes into contact with the Starch it starts to be absorbed into the paper. the advantage to pond size presses is that more starch can be applied to the paper sheet than sym sizers. this is due to the increased contact time with the starch. the pond level, or amount of starch within the two rolls typically can be adjusted to control the amount of starch absorbed into the sheet. 

With greater starch pick up from the pond the sheet will also contain more water after application. Leaving the Pond the sheet can contain almost as much as 40% Moisture.

The higher moisture content leaving the size press does have its disadvantages. the high moisture content will weaken the sheet. this section of the paper machine can be responsible;le for majority of the paper breaks incurred. 

PID Control loop parameters - Papermachine Automation

Control loop tuning

PID Control

PID control refers to the control system that alters the process outputs to bring the measured valued closer to the setpoint value.

The computer uses an error value (the difference between the setpoint and the measured value) to base the calculations on. PID is comprised of 3 parts, the proportionality value, the integral action and the derivate action. The majority of control systems tend to use PI control because of the limitations of the D action.

The Proportional Action

The proportional term produces an output that is proportional to the error value. The proportional response to the error value can be multiplied by the proportional gain constant (Value P on the control system).

A high proportional gain constant (Pk) relates to a larger change in the output to correct the error value. If the proportional gain constant value is too high it can make the control loop unstable. Likewise, if the value is too low the control action can be too small and results in a small output when responding to a higher input. This can lead to a less sensitive controller.

The integral action

The contribution to the PID control from the integral term is proportional to both the magnitude (highest value) of the error and the duration of the error. Plainly speaking the Integral value affects the time it takes for the proportional value to meet the set point by reducing the error value to 0. The controller works by adjusting the repeats per minute value. The bigger the integral action the quicker the proportional value meets the set point. By increasing the repeats per minute value the faster/ bigger the integral action is.

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The derivative action

The derivative action looks at the slope of the error value and tried to predict the measured value. This allows you to have higher P and me values while also keeping the system stable. The value of the derivative action describes how far in the future it should look so 20 would mean it looks 20 seconds into the future.

The issue with using the derivative action is if there is noise on the measured value (small spikes) this confuses the algorithm used and increases the effect of the Derivative value leading to an unstable control system. Most control systems do away with the Derivative action for this reason. The derivative function looks at the “steepness” of the curve, having noise on the MV leads to high steepness but no actual chance this is why the derivative action breaks down.

The PID control display page can be accessed through the face plates on the DCs system but cannot be altered. Fine tuning of the PID controls can be difficult. Fine tuning of the controls should dampen the oscillations created by the output so that the MV matches that of the SP quickly and stabilizes.

For more information of the devices that can be used with PID controllers within the papermaking industry take a look at this; Process Level Control - Paper machine Automation

Stone Ground wood Pulp - Mechanical Pulp Production

Mechanical Wood Pulping

All mechanical wood pulping makes no attempt to remove an impurity known as lignin from the fibers. A product made from mechanical wood pulp will not be durable and will degrade rapidly especially in the sunlight.

Stone ground wood Pulp

This is the most commonly used method mainly due to the simplicity of the machine. one large grindstone which breaks up the descending log into individual fibers and fiber bundles.

Mechanical pulp is produced from fiber defibrillations from a stone grindstone. the temperature around of the pulp is around 80 to 125 deg, locally at the stone where the fibers are being broken down the temperatures can reach up to 170 deg.

this high temperature is an advantage because the lignin in the logs binding the fibers together starts to become more malleable allowing for less damage to the fibers, more intact fibers are produced. 

Thermo-ground wood pulp

The thermo-ground wood mechanical pulp is an advance on the stone groundwood process where the logs are treated with hot water. the logs sit above the stone grinder in a hot water bath. the hot water is used to make the lignin softer. when the lignin becomes soft the break down of the logs into fibers becomes easier leading to less energy being needed and higher quality pulp.
Temperature around 80 to 95 can be generated locally at the grindstone. The water temperature is usually around 80 deg C. 

Pressurized groundwood

This is another advance on the original stone groundwood pulp. this is mechanical pulp production where the grinding takes place under compresses air pressure usually about 1 bar above atmospheric pressure. the water pressure is there for greater than 95 deg C (due to high-pressure atmosphere raises the boiling point of water). this allows for higher grinding temperatures without steam flashing. the high temperature promotes the softening of lignin. this improves fiber separation and reduces the specific energy consumption of the pulp.
Grindstone temp is about 124 – 130 deg.

Basic principles – Stone groundwood process

The fibers are torn from the logs and washed from the stone by means of spray showers. The supply of fibers and fiber fragments go through a screening process to remove the large particles these are known as shives. The stock is then thickened.

At grinding the logs are heated up from the frictions of the grind stone caused by pressing the logs against the stone the wooden structure is softened, the lignin becomes more malleable when heated. The bonding between the fibers will be less. The shearing forces between the stone surface and the wood mean the fibers are torn out.

This process uses very little or no chemicals but is very energy intensive (1200 – 2100kw/ dry ton)

Under the pulp stone, a basin collects the groundwood pulp

The pulp is screened after this process to remove shives (over sized particles) and then thickened

Varieties of Stone groundwood pulp

Fine groundwood pulp

·         Low moisture content in the logs

·         Gentle pressing of the log in the grinding zone (low speed of the chain drives) leads to a lower load consumption

·         Grit size – small dia grit size

·         Less sharpening of the grindstone with small dia grit on the abrasive layer

·         Less sharpening of the grindstone

·         Higher consistency in the basin

·         High specific energy consumption
As a result, the shopper riegler value is high, low dewatering ability

Coarse groundwood pulp

·         Fresh wood high moisture content

·         Intense pressing of the log in the grinding zone – increased the load on grindstone

·         Grit size :usage of grind stone with large diameter grit

·         Sharp stone surface

·         Low consistency in the basin

·         Low specific energy consumption

·         Result in good dewatering ability – high shopper riegler

Consistency Meters - Papermachine Automation

Consistency Meters

Rotor designed consistency meters

The most common consistency meter found, the rotor design CM (Consistency meter) sits just outside of the stock flow stream. A deflector rotor pulls stock into the recess where the measurement device rotates at a constant speed. The stock gets thicker the torque on the motor to maintain that speed increases. The consistency can then be measured against the motor torque. This is known as a strain gauge. These types of measurement devices require a certain flow to function correctly and can measure consistencies of 1% - 10%.

unlike fixed blade consistency meters the rotor design is not affected by the variations in stock flows because the device creates its own flow from the deflection rotor onto the measurment device.

Fixed blade Consistency meters

Fixed blade CM work in the same way as the rotor design except the paddle is placed within the fiber flow stream. The fixed paddles moves with the fiber flow. The consistency increases in the pip, this in turn increases the force against the blade. The force is measured by the meter and calculated to a consistency. This is another instance of a strain gauge. 

The limitation with this type of measurement device is the stationary aspect. As stock flows past the paddle, fiber and rejects can stick/ build up reducing the accuracy of the device.
Variable flow within the pipe will alter the consistency measurment. A higher flow will add a higher force onto the consistency meter resulting in a higher measured consistency.

The major advantage of this type of meaurment is cost - usually customers purchase a fixed blade/ dynamic blade consistency measurment will the intention of replaying it with a better model.

Microwave Consistency meters

Microwave transmitters work on the principle that sending microwaves through water the waves travel at a certain speed. When fiber is introduces the microwaves move faster through the stream. The consistency can therefore be measured depending on the speed of the microwaves being sent and received by the meter. 

Using the calculation;   Velocity = C / sqrt(e)   

Where; C = Speed of light in a vacuum

     E = Dielectric constant of liquid (water)

These devices are more accurate than the rotor/ blade design. The microwave Transmitter also has no moving parts for the pulp to affect/ build up on like the blade transmitter.

The CM is not affected by the flow rate, colour, and brightness, like traditional microwave ovens they are highly affected by metals. these consistency meters are used within very clean pulp systems like the aproach flow because the likleyhood of metals entering the stream are very low. The microwave transmitter needs to be the same size as the pipe being used. Due to the expensive nature of these devices microwave transmitters are typically used on smaller pipe work or substituted for cheaper models. 

For more Info on Instrumentation within paper-making check out my other blog posts!

Level transmitters;
https://www.papermakingbible.co.uk/2018/04/process-level-indicators-papermachine.html

PID controllers;
https://www.papermakingbible.co.uk/2018/02/pid-control-loop-parameters.htm

Consistency Measurement - Technical Papermaking

Consistency Measurement


Testing the consistency of a stock is the same as consistency of water solution. Consistency is the term given to the amount of solid matter within a body of liquid. The higher the consistency the more solid matter is present within the same volume of liquid. Consistency is measured in percentage; it’s a percentage/ ratio of solid matter to water in a certain volume.


To find the consistency we need to test the amount of dry content within a set volume of water. For example if we wanted the consistency of 100ml of water, and tested 25 gram of dry solids the consistency would be about 25%.

To capture the solid content within a solution a filter paper is used. Firstly the filter paper is weighed at 0% moisture (previously oven dried) and recorded. The stock sample is weighed (1g = 1ml) and recorded. The weighed stock sample is filtered through a vacuum and rinsed making sure all of the solids from the sample are on the sheet. The filtered solids and the filter paper go into the oven to remove all of the remaining moisture.

Once the sample leaves the oven it is weighed and recorded. This formula is used to calculate the consistency below.

Consistency=  (Mass of Solids)/(Volume of Liquid)

We have to calculate the mass of the solids captured on the filter paper, this is calculated below;

Mass of Solids=Total dry weight-Filter paper weight

Here are a list of results that I took from measuring the consistencies of a pulp preparation plant's stock flows through two fractionators and Long fiber screens.