EP4686783A1

EP4686783A1 - Reduction of deposits in a paper machine

Info

Publication number: EP4686783A1
Application number: EP24192114.7A
Authority: EP
Inventors: Oliveira ARAUJO; Jukka ISOMETSA
Original assignee: Air Liquide Finland Oy; Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; Portuguesa Do Ar Liquido Lda Soc
Current assignee: Air Liquide Finland Oy; Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; Portuguesa Do Ar Liquido Lda Soc
Priority date: 2024-07-31
Filing date: 2024-07-31
Publication date: 2026-02-04

Abstract

The present application a method to prevent and/or eliminate the deposition of scale in the machine clothing and showers of a paper machine's press section and/or forming section. The method comprises adding CO₂ into the water that is used in the paper machine's press section and/or forming section. Advantageously, the method also allows to optimize the use of oxidizing antimicrobials.

Description

Technical field

This application relates to a method to prevent and/or eliminate the deposition of scale in the machine clothing and showers of a paper machine's press section and/or forming section.

Background art

Modern paper machine clothing is manufactured with a specific set of design and quality specifications for each paper machine's performance requirements. Such specifications, i.e., surface characteristics, open area, void volume, permeability, smoothness, etc. are engineered to achieve specific goals in the papermaking process. The need to implement an effective cleaning program has become increasingly crucial in recent years. This change is primarily due to increasing levels of recycled furnish, faster machine speeds and accompanying technology, elevated sheet quality requirements, and the desire for longer fabric life.
Forming fabrics must be kept free of contaminants in order to maintain surface characteristics, adequate open area, and to prevent sheet marking. Press felts must be cleaned, conditioned, and lubricated in order to maintain void volume, caliper and prevent wear; thus enabling the felt to take water and be de-watered uniformly throughout its operational life. Finally, dryer fabrics must be cleaned in order to maintain their permeability and prevent sheet streaking due to nonuniform drying profiles and sheet drop-offs in vacuum assisted transfers, uniruns and single tier dryer runs.
The objective of the felt/forming fabric conditioning system is to improve the overall fabric efficiency. The system can use either mechanical or chemical cleaning or a combination of both depending upon the conditions.
The major problems are:

All raw materials and additions used in paper making are potential fillers, especially with waste and secondary fibre stock;
Mills with closed systems add more contaminants causing problems in felt/fabric operation;
Frequent use of wet end starch, wet strength resins and various synthetic retention-aid polymers have increased the felt/forming fabric filling problems.

To combat these problems the paper makers need more than just showers and suction boxes. Mechanical cleaning and conditioning of the felts/forming fabric is attained through three methods:

High pressure showers (essential);
Low Pressure flooding showers (recommended);
Full width suction boxes (essential).

Low pressure flooding showers provide the flushing media for removing contaminants. Usually located on the inside of the felt/forming fabric, it is more effective when positioned in the nip formed by the felt/forming fabric and the press forming section rolls. A hydraulic wedge action thus created, forces the water through the fabric, flushing out the filling materials. Oscillating high pressure showers with needle nozzles are the most efficient means of cleaning available today. With proper positioning and technologically improved "No dwell time" synchronised oscillators, these showers provide Complete Coverage Cleaning.
Though these showers are more expensive than the conventional air and water cylinder and the crank arm oscillating types, their vastly superior performance, and the benefits to be gained from them, far offsets the initial higher expenditure. They are:

Maximum cleaning and increase in bulk;
Minimum water consumption;
Minimum felt/forming fabric damage;
Minimum shower maintenance;
Minimum energy requirements;

For optimum operation:

Place a high-pressure shower with needle nozzle 250-400 mm from the fabric;
Use continuous or intermittent showering depending on requirements;
Normally, position the shower on the paper side;
Most suitable nozzle diameter is 1.0 mm;
Water temperature should be equal to the temperature of the felt/forming fabric being cleaned;
Jet angle should be adjusted to suit individual application (start at 90 degrees, then experiment with or against the run by a few degrees).

In the forming and press section, showers are mostly used for cleaning and lubrication. For cleaning, high-pressure showers are used. Low pressure showers are mostly used for lubrication. Low pressure showers are directed to the rolls, doctor blades or toward suction box ceramics. This will reduce the friction and wear of the fabric, blade and/or rolls. If there is a clogged nozzle on a cross direction of the machine it means there is a dry part which will lead to uneven wear of fabric, blade or rolls.

High pressure showers

Oscillating needle showers, water pressure in forming section is usually 20 - 30 bars. In the press section high pressure shower pressure is usually 7 - 15 bars. Higher pressure can be used, up to 25 bars, for short periods of time.

Low pressure showers

Low pressure showers are stationary fan type of showers, with nozzle size of usually 3 mm. Water pressure used in low-pressure showers is between 2-3 bars.
For a standard paper machine with width 4-8 m, the water consumption is as follows in Table 1:

Shower Type Flow l/min / machine width m** Nozzle Pressure Water

HP shower Needle 20 - 35 0.8 - 1.0 mm 7 - 35 bar Make up

LP shower Fan 20 - 25 2.5 - 3.0 mm 2 - 3 bar Filtrate
In the forming section there is usually one high-pressure shower per forming fabric located on the sheet side. Each guiding roll has lubricated doctor blades. Suction rolls usually have one or two high-pressure showers. Last forming fabric before the press section has a knockout shower, which is used during web breaks. In one example, there can be three high-pressure showers, which means it uses 600 L/min of make up water. There can be eleven low-pressure showers (edge showers and knockout showers are excluded from this calculation), which can consume 2.200 L/min of filtered water.
In one example of the press section there is usually one high-pressure shower per press felt located on the sheet side. Sheet side guiding rolls have lubricated doctor blades. Suction rolls usually have one or two high-pressure showers. Press felts have one or two vacuum boxes which are mainly used for removing the excess water from the felt. Vacuum boxes have lubrication showers. In this example there can be 6 high-pressure showers, which means it can use 1.200 L/min of make up water. There can be seven low-pressure showers (edge showers are excluded from this calculation), which can consume 1.400 L/min of filtered water.
Document WO2008059006A1 discloses a process for reducing scaling by calcium carbonate precipitation on equipment in the pulp and paper industry. The process relates to an alkaline aqueous process fluid, which contains calcium ions and carbonate ions with a tendency for calcium carbonate scaling. Carbon dioxide is introduced into said fluid prior to its passage through the equipment. The invention also relates to the use of carbon dioxide for reducing scaling on equipment used in the pulp and paper industry. The alkaline aqueous fluid mentioned in the document is a mixture of water and pulp of various concentrations, and recycled water. However, the introduction of CO₂ in a fluid such as this is not enough to reduce scaling or to prevent calcium precipitation. CO₂ must be dissolved in a sufficient dosage in order to shift the calco-carbonic equilibrium to a non-scaling state, which does not occur with the technology described.
Additionally, this document mentions the inherent buffering capacity of CO₂, however no inherent buffering capacity exists in adding CO₂. The effect of dissolving CO₂ in said aqueous fluid is that the alkalinity of the fluid will not change, only the pH of the fluid will change. According to the US Geological Survey, the definition of alkalinity is: "The buffering capacity of a water body; a measure of the ability of the water body to neutralize acids and bases and thus maintain a fairly stable pH".
Thus, the mentioned technology does not solve the technical problem in an efficient manner.
In contrast, the presently disclosed method uses chemically treated warm fresh water with a neutral pH, with associated advantages to prevent and/or eliminate the deposition of scale in the machine clothing and showers of a paper machine's press section and/or forming section.

Summary

The present invention relates to a method to prevent and/or eliminate the deposition of scale in a machine clothing and showers of a paper machine's press section and/or forming section, comprising the following steps:

Providing fresh water with a pH between 7 and 9 and heated between 40 and 55°C;
adding CO₂ to said water in an amount between 5 and 1000 mg/L of water which is suitable to maintain the Langelier saturation index between -1.0 ± 0.2 ≤ LSI < 0 ± 0.2;
Feed the water to the high-pressure showers or to the low-pressure showers of the paper machine's press section and/or forming section.

In one embodiment, CO₂ is added between 80 and 200 mg/L of water.
In one embodiment, the purity of CO₂ is at least 97% v/v.
In one embodiment, CO₂ is added in gas, liquid or solid form.
In one embodiment, CO₂ is added cyclically into the water for a period of time between 0.5 and 2 hours per every 8-hour cycle.
In one embodiment, the pH of the water is maintained between 6 and 8.

General description

The present invention relates to a method to prevent and/or eliminate the deposition of scale in the machine clothing and showers of a paper machine's press section and/or forming section. Additionally, this method also allows to optimize the use of oxidizing antimicrobials, particularly the halogenated biocide treatment. The method is based on adding CO₂ into water that is used in the paper machine's press section and/or forming section.
The presently disclosed method aims to achieve the following goals:

1) Curative: use of CO₂ as a means to control the saturation index at the level of the machine clothing press section, forming section and shower/washing nozzles during water shower washing. The interstitial water inside the shower piping and machine clothing must have a low Langelier Saturation Index (LSI) so that the scaling is dissolved;
2) Preventative: use of CO₂ as a means to control the interstitial water inside the clothing and shower piping so that this water is slightly aggressive, i.e. -1.0 ± 0.2 ≤ LSI < 0 ± 0.2, and doesn't allow the scaling process to begin;
3) Provides an advantage to do the cleaning with the paper machine in operation, if scaling is already formed it's possible to dissolve it by using CO₂ so that the LSI can be ≤ -0.5;
4) The use of CO₂ avoids clogging the low and high pressure showers;
5) By controlling the saturation index, make up water use could be reduced;
6) The use of CO₂ to reduce water pH improves the effectiveness of oxidizing antimicrobials in process water systems thus improving the removal of slime and biofilms.

Carbon dioxide, as other weak and strong acids, can shift the calco-carbonic characteristics of a given water. The higher the dosage of CO₂ introduced, the lower the scaling properties of this water will be, to a state where the water is not scaling and not aggressive, i.e. the water attains an equilibrium. If CO₂ or other weak and strong acids continues to be added, the water becomes "aggressive" and thus will tend to dissolve calcium or magnesium carbonate particles attached to all solid surfaces or in suspension in the liquid.
Thus, for the technical problem the present invention aims to solve, CO₂ is dissolved into fresh water used in the shower nozzles of the paper machine, in order to maintain the LSI between - 1.0 and 0 (± 0.2). Even if the fresh water used has a LSI close to 0, the fact of heating the water before the shower nozzles will increase the LSI of said water and will increase scaling probability in the nozzles, thus the present invention can overcome this effect.

Detailed description of embodiments

Now, preferred embodiments of the present application will be described in detail. However, they are not intended to limit the scope of this application.
The present invention relates to a method to prevent and/or eliminate the deposition of scale in the machine clothing and showers of a paper machine's press section and/or forming section. Additionally, this method also allows to optimize use of oxidizing antimicrobials, particularly halogenated biocide treatments. The method is based on adding CO₂ into the water that is used in the paper machine's press section and/or forming section.
For the purpose of the present invention, the water used is understood as fresh water with a temperature between 40 and 55 °C and a pH between 7 and 8.5, to which CO₂ is added. This is also known as shower water.
Typically, but not limited to, the fresh water used in the presently disclosed method has a total hardness 10~45°F, total alkalinity 7~20 °F and conductivity between 2500-4500 pS/cm.
The method comprises the following steps:

Providing fresh water with a pH between 7 and 9 heated between 40 and 55°C;
Adding CO₂ to said water in an amount between 5 and 1000 mg/L of water, which is suitable to maintain the Langelier saturation index between -1.0 ± 0.2 ≤ LSI < 0 ± 0.2;
Feed the CO₂-treated water to the high-pressure showers and/or to the low-pressure showers of the paper machine's press section and/or forming section.

In one embodiment, CO₂ is added by injection into the shower water.
In one embodiment, CO₂ is added in an amount between 80 and 200 mg/L of water.
In one embodiment, the purity of CO₂ is at least 97% v/v.
In one embodiment, the CO₂ is added in gas, liquid or solid form. In a preferred embodiment, CO₂ is added in gas form.
It is the objective of the present method to prevent and/or eliminate scaling, maintaining the calco-carbonic equilibrium, i.e., the LSI, of the water within the disclosed range. This allows to create mildly aggressive conditions, i.e. -1.0 ± 0.2 ≤ LSI < 0 ± 0.2, in the interface of the water with the machine clothing and shower nozzles so as to reduce or even eliminate scaling formation that could entrain the attachment of pitch or stickies to the equipment.
Depending on the type of paper produced the objective of the presently disclosed method is not only to clean the paper machine clothing but also perform this method during paper production.
In one embodiment, the CO₂ is added cyclically into the water for a period of time between 0.5 and 2 hours per every 8-hour cycle.
In one embodiment, CO₂ can be added into the water at any point of the machine. In one embodiment, CO₂ is added after the high-pressure shower pumps of the paper machine's press section and/or forming section system.
The water used in the paper machine's press section or forming section can vary in origin. For high pressure showers, water is normally makeup water coming from the Water Treatment Plant of the mill pre-heated to the temperature of the machine circuits. For the other lower pressure showers, water is recycled white water with an additional step of filtration or flotation to remove suspended solids such as pulp fibers, fillers and other suspended solids like suspended CaCO₃.Either way, this water tends to be scaling in nature - since each time it goes to the nozzle of a shower it loses part of the free CO₂ it contains and thus increases pH and scaling ability. As water in the high-pressure shower circuit is preheated it also increases the scaling nature.
Every paper grade (tissue, magazine paper, office paper, cardboard) uses cleaning and lubrication showers and faces the same challenges. How much water is used depends on the width or paper grade. Rule of thumb is that a single layer (office paper, magazine paper, tissue) uses 700 L/min per cross direction meter. Nowadays most of the machines are 7 meter up to 11 meters wide giving the consumption of shower water between 4.900 - 7.700 L/min.
Multilayer machines (Cartonboard) have 2 to 4 wire sections, which increases water consumption by 80 L/min per CD meter each layer. Normal width of the cartonboard machines is from 4 to 8 meters. Shower water consumption in cartonboard machines varies from 3.500 L/min up to 6.900 L/min.
Also, as the recycled water tends to contain organic compounds like starch, there is a tendency to create biofilms in the circuit and particularly at the nozzles of the showers. Growth of the biofilm can lead to nozzle plugging or detachment to the felts and fabrics creating quality problems in the paper. Biofilm is normally controlled and/or eliminated by using a biocidal, such as oxidizing antimicrobials containing halogenated compounds.
The oxidizing antimicrobials commonly used in paper machine systems are the halogens, chlorine and bromine, in liquid and gaseous form; organic halogen donors; chlorine dioxide; and, to a limited extent, ozone. Oxidizing antimicrobials oxidize or accept electrons from other chemical compounds. Their mode of antimicrobial activity can be direct chemical degradation of cellular material or deactivation of critical enzyme systems within the bacterial cell. An important aspect of antimicrobial efficiency is the ability of the oxidizing agent to penetrate the cell wall and disrupt metabolic pathways. For this reason, oxidation potential alone does not always correlate directly with antimicrobial efficiency. Shower water pH affects oxidizing antimicrobial efficacy. pH determines the relative proportions of hypochlorous acid and hypochlorite ion or, in systems treated with bromine donors, hypobromous acid and hypobromite ion. The acid forms of the halogens are usually more effective antimicrobials than the dissociated forms. Under some conditions, hypochlorous acid is 80 times more effective in controlling bacteria than the hypochlorite ion. Hypochlorous acid predominates below a pH of 7.6. Hypobromous acid predominates below pH 8.7, making bromine donors more effective than chlorine donors in alkaline waters, especially where contact time is limited.
In this sense, the presently disclosed method allows to lower the shower water pH to between 6 and 8, which directly contributes to using less oxidizing antimicrobial.
In one embodiment, CO₂ consumption by applying the presently disclosed method would be from 1 to 2.5 ton of CO₂ / day depending on the grade and the machine width.
In one example, shower water was injected with CO₂ to achieve an LSI of -0.6. In this example the paper machine consumed 5.4 m³/min of shower water. To achieve LSI of -0.6 and to prevent scaling, 166 mg/L CO₂ were used. This led to a daily CO₂ consumption of 1.3 tn.
This description is of course not in any way restricted to the forms of implementation presented herein and any person with an average knowledge of the area can provide many possibilities for modification thereof without departing from the general idea as defined by the claims. The preferred forms of implementation described above can obviously be combined with each other. The following claims further define the preferred forms of implementation.

Claims

A method to prevent and/or eliminate the deposition of scale in a machine clothing and showers of a paper machine's press section and/or forming section, comprising the following steps:
- Providing fresh water with a pH between 7 and 9 and heated between 40 and 55°C;

- adding CO₂ to said water in an amount between 5 and 1000 mg/L of water which is suitable to maintain the Langelier saturation index between -1.0 ± 0.2 ≤ LSI < 0 ± 0.2;

- Feed the water to the high-pressure showers or to the low-pressure showers of the paper machine's press section and/or forming section.
Method according to the previous claim, wherein CO₂ is added between 80 and 200 mg/L of water.
Method according to any of the previous claims, wherein the purity of CO₂ is at least 97% v/v.
Method according to any of the previous claims, wherein CO₂ is added in gas, liquid or solid form.
Method according to any of the previous claims, CO₂ is added cyclically into the water for a period of time between 0.5 and 2 hours per every 8-hour cycle.
Method according to any of the previous claims, wherein the pH of the water is maintained between 6 and 8.