Pulp & Paper

This is the important step of all cellulose processes, as the better the chips are cooked in the process, the less lignin in the pulp. This results in more efficiency in the final steps of the process, which will save money and customer operation time. Many measurements are used in the digester such as pressure, temperature, and conductivity, but the most problematic for operators are the flow measurements. 

Challenges

Black Liquor is an aggressive fluid used in the circulation and extraction lines of the digester and blow line. It’s a high concentration of pulp and high abrasive levels, result in extreme conditions that the operator must be aware of:

• High temperature 150 °C (300 °F)

• High pressure 20 Bar (290 psi)

• High alkaline consistency

• Adhesive fluid

Taking flow measurements often becomes very complicated due to:

• Damage to the lining from the changes in pressure and temperature conditions

• Damage due to the abrasive and adhesive fluids

• Leakage from electrodes, due to a deformation of the lining, resulting in a gap between the lining and electrode

Talking conductivity measurements often becomes very complicated due to:

• Plugging of the filters causes the conductivity to go up

Solution

Earth Rings

Minimizes lining abrasion from a slurry fluid

Welded Punch Plate

Prevents lining deforming at high temperature

PFA Lining, Electrode

Maintains sealing performance when the lining encounters abrasion from a slurry fluid

Epoxy Potting

Increases heat dissipation for high-temperature fluid measurement. Prevents the outflow of fluid when fluid leaks from the electrode.

The recovery plant is considered the most important unit or the heart of any pulp manufacturing; being directly related to the economic viability of the entire process. In this process, the strong black liquor is burned at the recovery Boiler (the heart of this process )and the reagents are fused and recovered. During the heating process, the chemicals generate heat and the chemical reagents fuse are then recovered. 
In the recovery boiler, heat is used to produce high-pressure steam, which is used to generate electricity in a turbine (co-generate plant). The turbine exhaust, low-pressure steam is then used for process heating. After the strong black liquor is burned, this becomes green liquor. This caustic soda is recovered and sent back to the chemical plant to return as white liquor to the process. The process cycle then starts again.

Challenges

Taking flow measurements often becomes very complicated due to:

• Instrument performance is at risk dues to the nature of the solidify of fluids causing build up on flow meters and conductivity measurements. 
• High temperatures and pressures of the system stretch the specifications of instruments 
• Particulate dust load in the rotary kilns can make O2 difficult

Solution

In a lime kiln, the control can be complicated as several elements need to be maintained to provide the proper and efficient drying. As the size of the kiln increases the more critical the items are. The control of the burner is paramount. It has to be hot enough to ensure that the lime mud is dry by the time it reaches the end of the kiln but not too hot that refractory damage could occur and poor product quality. The length of the flame in the cylinder directly affects these issues. 

There are two considerations in managing flame length, the amount for fuel supplied to the burner and the amount of air generated by the primary air and induced draft fans. These fans together control the total production capacity of the kiln, how far the flame is thrown into the kiln, and its shape. 

Timed Automation for blowback to prevent plugging and Oxygen, O₂ errors. 

Control for efficiency and energy cost savings

• Steam pressure control
• Combustion control
• Airflow
• Fuel flow
• Feedwater control
• Steam pressure control
• O₂ trim

Washing

After the screening process,  the washing process uses chemicals to help reduce the lignin and substances (black liquor) in the pulp. This stage’s objective is to remove the maximum amount of black liquor dissolved solids from the pulp while using as little water as possible. The remaining solids and chemicals in the pulp will interfere with later bleaching and other steps, increasing the cost of the final process.  Normally the process has 3 or 4 washing stages in series. The same washing process is located after the oxygen delignification process and after the bleaching stages, as well.

The lignin in the pulp will result in a bad quality of paper (in the case of the final product), because the lignin over time will oxidize in contact with the air, and this is what makes the paper looks “yellowish”.  Excellent paper quality will keep the bright and white aspect as a lower quantity of lignin remains in the final product.

Bleaching

The objective of the bleaching process is to remove only the chromophores (elements that provide color). This is possible because the structures responsible for color are also more susceptible to the oxidation and reduction process. In the bleaching process oxygen, hydrogen peroxide, ozone, and caustic Soda are used to reduce the lignin, but keep the integrity of cellulose fibers. The combination of these chemicals and the sequence of these steps will depend on each customer’s required level of brightness in the final product. Most facilities use the following process and wash with sodium hydroxide, then treated in sequence with alkaline peroxide and sodium dithionite.

Many measurements are used to monitor the process such as pressure, temperature, and flow. However, the process of bleaching highly depends on accurate pH/ORP measurements. 

Challenges

Taking pH/ORP measurements often becomes very complicated due to:

• Excessive Cl2O that weakens the paper intensity will also deteriorate a pH electrode.
• Installation locations are often proven to be difficult to get a pH sensor directly into the process.
• High pH 10-11, used to dissolve the reacted lignin, causes short reference life and thinning of the glass membrane resulting in unstable and inaccurate pH readings.
• The consistency of the process media causes plugging and coating of pH sensors resulting in sluggish and inaccurate readings.

Solution

The best fit pH solution is not a one size fits all. Yokogawa’s experts work side-by-side with each plant operator to determine which solution best aligns with their goals and existing installation constraints.

Heavy-Duty Glass
Mechanically strong sensor design for harsh and abrasive applications.

All-in-One Flat Surface Sensor 
Built-in silver ion trap to ensure the electrolyte remains completely free from silver ions and therefore eliminating the junction plugging problem while creating a double junction barrier at the same time.

Maintenance Manager >
The data-logging function provides a total process analysis by showing the record of the measured data before, during, and after the event occurred. The sensor wellness function predicts the next maintenance and sensor replacement for early detection of sensor failure, for pro-active maintenance. 

< Process Guard
Real-time online reference impedance dynamic check to detect early signs of coating

Immediately after the washing and bleaching section, the white pulp is further refined to specific specifications and consistencies in cleaning, storage, and blending chests before it is introduced to the headbox of the paper machine.  During each of these operations, additives such as resins, paper sizing materials, and colorants are blended into the pulp to meet customer specifications for fiber binding, wet tensile strength characteristics, and color in the finish paper sheet. As the paper stocks consistency has been significantly reduced through dilution of the pulp prior to the headbox, significant amounts of white water are removed as the pulp is applied to the Fourdrinier (wire bed immediately after the headbox) and as the sheet moves towards the drying section of the paper machine. 

Flow is commonly measured on the pulp stock into the headbox and on the white water lines; while level and pressure transmitters are used in the paper machine. The most critical measurement depends on accurate pH/ORP measurements. 

Challenges

Taking pH/ORP measurements often becomes very complicated due to:

• Coating of the pH sensor and clogging of the junction makes the tight control that is needed a very maintenance intense installation

Prior to the headbox, the tolerances allowed for pH can be relatively wide, in many cases +/- 1-2 pH units. However, at the headbox, the pH must be tightly controlled to typically within +/- 0.2-0.4 pH units.  pH control at the headbox is normally achieved with the addition of alum or soda ash.  Both are used to control the pH but they are also acting a flocculating agent to precipitate resins during the forming of the sheet of paper on the paper machines wire.  The control of pH at this point in the process is critical to the proper formation of the paper and also to prevent excess wear on the paper machine.  

Solutions

All-in-one pH Sensor 
Built-in silver ion trap to ensure the electrolyte remains completely free from silver ions and therefore eliminating the junction plugging problem while creating a double junction barrier at the same time. 

Differential pH 
The electrode is a 316SS rod coated with H+ ion and Na+ sensitive enamel allowing for pH measurements. There are no junctions to be clogged and no electrolyte to be depleted or replaced.

Digital SMART Sensors
There is no need to worry about unplanned outages and lost production time with the maintenance manager's function. The SENCOM platform utilizes the integrated "hot swap" and "plug and play" function to significantly decrease downtime, while the sensor wellness function predicts the next maintenance and sensor replacement. The SA11 SMART Adapter can integrate on top of the FU20 All-in-one pH sensor, giving you the ability to clean and calibrate offline. Giving you greater insight and enhanced capabilities to deliver more credible data throughout the entire product lifecycle.