OPTIMIZED DEAERATION LEADS TO SUBSTANTIAL PROCESS AND QUALITY IMPROVEMENTS IN PAPER MANUFACTURING

ABSTRACT

High air or gas contents in pulp suspensions constitute a serious problem area in papermaking. They are responsible for higher pump energy demand, deposits, pinholes, foaming and several other problems. An underestimated source of these problems is dissolved gases.

High contents of dissolved gases mainly originate from a decomposition of calcium carbonate where carbon dioxide is formed. In comparison to air and its components, carbon dioxide is far more easily dissolved in water. The immense pressure drop at the wire and at the foils leads to dissolved gases being released.

The measurement results achieved in various mills show that determining the levels of entrained and dissolved gases online enables the optimization of prevention measures as well as mechanical and chemical deaeration. The results also show that online control of deaerator chemicals based on the measurement values leads to substantial cost reductions, process improvements and quality enhancements in paper manufacturing.

PROBLEMS CAUSED BY HIGH AIR/GAS CONTENTS DURING PAPER MANUFACTURING 

High air / gas contents are a serious challenge on papermakers in the paper manufacturing process [1].

Typical production problems arising from high air / gas contents are the need for higher pump capacity, deposits in the wire, press and dryer sections, increased foaming, accumulation of hydrophobic substances (localized dirt islands), web breaks originating from floated resinous and sticky substances, and poor drainage on the wire.

Paper quality losses that are attributable to high air / gas contents during paper manufacturing are generally speaking poor formation, porosity and printability, specks, pinholes / holes, and strength losses.

In a mill environment many factors have brought about a rise in gas contents: closed water loops, ever-higher machine speeds, hydrophobic material accumulating in the system and the use of calcium carbonate fillers. They all contribute to higher air and gas contents.

Paper manufacturing operations are upset not only by entrained air that are present in the form of bubbles, but to a major extent also by dissolved gases that are released whenever a pressure drop or temperature increase occurs in the process. The growing use of calcium carbonate as a filler and coating pigment but also high water hardness is responsible for CO₂ to be released, especially if aluminum sulfate, PAC, wet strength agents or other acidic chemicals are introduced to the process. Dissolved CO₂ constitutes a major problem in paper manufacturing. This is the reason why we talk about gas contents and not air contents.

As reflected in the equations below, calcium carbonate starts to decompose at pH´s around 8. Coming closer to pH 7, the equilibrium moves towards hydrogen carbonate. For pH's around 5, the equilibrium lies totally on the side of CO₂ and H₂O, which means that nearly all calcium carbonate is transformed into CO₂ present in the form of dissolved gas.

What aggravates the situation is that the solubility of CO₂ in water is 50 – 100 times higher compared to that of air. At the paper machine wire, the pressure drops from approximately 200 –300 kPa in the headbox to approximately 50 kPa at the suction pipes (Fig. 1). The solubility, which has a linear correlation to pressure and is zero at zero pressure, decreases and the dissolved carbon dioxide forms small bubbles, which increase rapidly. Selective gas measurements in pulp suspensions have shown that in most cases the measured dissolved gases consist of more than 90% out of CO₂ [2].

Figure 1. Danger potential of dissolved gases in paper production

Dissolved gases that are released owing to the sharp pressure drop on the paper machine wire tend to produce pinholes (Fig. 2) and holes in the paper product with critical effects in subsequent coating and printing operations. In the defective areas of coating base, color strike-through occurs and the color builds up on the backing roll – a phenomenon that necessitates frequent downtimes. In newsprint and SC paper production, pinholes lead to missing dots and color build-up in the printing system.

Figure 2. Pinhole [3]

Low-grammage papers are particularly sensitive to high air and gas levels in a system. Owing to their high affinity to hydrophobic substances, air bubbles cause undesirable flotation effects with resulting deposits of white pitch or other types of hydrophobic material in the white water circuit. Additionally, particle agglomerates of this kind become noticeable as specks in the paper (Fig. 3).

Figure 3. Speck consisting of flotated hydrophobic particles [3]

They can also build up in the white water circuit and are found as deposits in pumps and in the wire, press and dryer sections especially in processes with high contents of dissolved gases in the stock (Fig. 4).

Figure 4. Resinous deposit found in a pump [3]

If specks as shown in Figure 3 appear on coating base, the color binder is prevented from penetrating into the paper so that the coating becomes nonuniform with matt or glossy spots. Apart from this, because of an unsatisfactory binder penetration, the coating adheres less firmly to the basepaper and coating ruptures or broken-out particles may occur (Fig. 5).

Figure 5. Coating rupture [3]

MEASUREMENT OF FREE AND DISSOLVED GASES IN PAPER STOCK SUSPENSIONS

A few instrument manufacturers supply high-precision online paper machine equipment for measuring gas contents in paper stock suspensions and in coating colors. These devices are based upon various measuring principles such as ultrasonics or the ideal gas law.

The authors used an online gas analyzer capable of measuring both entrained gas and dissolved gas that may be released in paper stock suspensions at consistencies up to 3 % (Fig. 6). Providing a measuring range of 0.00 – 8.00 % by volume and a measuring accuracy of 0.02 % by volume, the system tests stock samples by compressing and expanding the gases in a measuring cycle.

Figure 6. Online gas analyzer with multisampling

The entrained gas (gas bubbles) is measured by compressing a sample in the measuring cell. The volumetric share of the entrained gas is calculated by Boyle's law (pV = const.). On the other hand, the dissolved gas that may be released is measured by expanding the sample. This simulates the processes on the paper machine wire where the stock suspension is subject to a sharp drop in pressure. As the pressure in the measuring cell is reduced, the solubility of gases in the stock suspension decreases and are released. Thus the volume of the suspension increases. In this physical state, the Boyle's law can calculate the released proportion of the dissolved gases. System operation is fully automatic. An integrated rinsing cycle that is optionally adjustable assists the high accuracy and reproducibility of measurements.

Figure 7. Installation of an online gas analyzer in a paper machine circuit

The gas analyzing system may be installed at optional locations in the approach flow of a paper machine. It is of special interest to the papermaker to know the gas content at the headbox, because the gases existing at that point directly influence the sheet forming process. In the example shown (Fig. 7), the gas analyzing system is connected to two sampling points (multisampling), i.e. ahead of the mechanical deaerator and at the headbox. This enables the degassing effect of the mechanical deaerator to be assessed and optimized. Dosages of deaerator chemicals are controlled in a closed loop via the output signal of the device.

DEGASSING METHODS FOR PULP SUSPENSIONS

As already outlined, high gas contents – alone or in combination with other factors - may be responsible for a large variety of problems in papermaking. If avoidance measures prove inadequate to achieve the desired degassing effect , papermakers may apply mechanical deaerators and deaerator chemicals in order to further reduce the levels of entrained and dissolved gases [4].

Deaerator chemicals are broken down according to deaerators and defoamers. The latter are primarily intended to destroy surface foam, whereas chemical deaerators help to avoid and eliminate dispersed air bubbles. The chemical characteristics of the additives differ depending on their intended purpose – i.e. destruction of macro-foam or removal of finely dispersed air.

Pure defoamers are of a hydrophobic nature and are insoluble in water. They penetrate into the monomolecular layer of the foam at the phase interface where they destroy the foam lamellae. Deaerators by comparison are hydrophilic or dispersible to some extent. They are expected to become effective inside a stock suspension. Their non-polar chains bind the hydrophobic gas bubbles and are then transported to the phase interface in a targeted flotation process. This helps to destroy the foam as it emerges.

Deaerator chemicals generally consist of non-polar hydrocarbon chains modified by polar groups [5]. The type and extent of modification decisively influences the performance of the products. This concept enables chemical suppliers to design substances with a large variety of properties. Table 1 lists the dominant families of compounds that are currently used for the degassing and defoaming of paper stock suspensions.

Table 1. Dominant classes of substances used as deaerators or defoamers for paper stock suspensions

Defoamers	Deaerators
Silicone oils	Aliphatic alcohols
Paraffins	Fatty acid esters
Fatty alcohol alkoxylates	Fatty acid ethoxylates
Polyglycols	Fatty acid polyethers
	Fatty alcohols

Just like the foaming effect of surfactants, the performance of defoamers and deaerator chemicals is temperature -dependent. Most defoamers and deaerators perform optimally within a certain temperature range, but they lose effectiveness outside this range [6]. Against the background of increasingly closed mill water loops with ever-rising temperatures, the effectiveness of deaerator / defoamer chemicals at elevated temperatures (45 – 55°C) deserves priority attention.

Figure 8. Performance of different deaerator chemicals

Deaerator chemicals have to be expertly dosed to successfully reduce gas contents in stock suspensions [7]. When overdosed, they give rise to undesirable phenomena such as sizing losses. In the extreme, they even produce the opposite effect of increased take-up of entrained air. Deaerators belonging to different families of compounds may perform altogether differently in various manufacturing processes (Fig. 8). The degassing effect is primarily determined by influences of stock suspension components, anionic trash levels in circuit waters and production temperatures. Conducting targeted trials with online gas content measurements is the only way for papermakers to identify the best-performing deaerator chemical for a specific application and to determine its optimal dosage amount.

Basically, there are two locations where dispersed gases may be released from the paper machine circuit: the silo and the mechanical deaerator.

Under normal pressure conditions (ambient pressure plus height of the water column) in the silo and under suitable hydrodynamic flow conditions, trapped air may rise to the surface. However, this degassing effect varies in efficiency depending on the plant design. Mechanical deaerators by comparison are high-performance systems which employ either vacuum pressure or centrifugal forces.

A vacuum degassing system and a centrifugal degassing system of the types applied in today's paper mills are illustrated in examples, below.

Mechanical vacuum deaerators are installed in the approach flow to a paper machine for the purpose of degassing the stock suspension. If a mechanical vacuum deaerator has a cleaner system installed upstream of the deaerator, the cleaner accept is conveyed to a vacuum vessel mounted at the top (Fig. 9). Mechanical deaerators without a cleaner system only comprise a vacuum vessel [8]. Both types are designed to remove both free and dissolved gases from paper stock suspensions [9]. There are various designs available. Most of them feed the stock suspension via spray nozzles into the tank of the mechanical deaerator where it impinges on the tank wall. In this manner, gas bubbles are separated from the fiber [10]. Under vacuum pressure, the dissolved gas is transformed into free gas for removal from the suspension, together with the residual free gas, via the vacuum system of the deaerator. The water vapor that is formed in the process is condensed for recirculation. The degassing effect is crucially dependent on the tank pressure and the dwell time of the stock.

Figure 9. Example of a mechanical deaerator with cleaner connected ahead

Alternatively, modified centrifugal pumps are commercially available for mechanical deaeration, which separate entrained gas from the white water [11]. In a rotating chamber, the gas bubbles are removed from the stock suspension under the action of centrifugal forces and are eliminated from the system (Fig. 10).

Figure 10. Centrifugal deaeration of white water [11]

MILL EXPERIENCE

In a Swedish paper mill, trials were conducted to investigate the impact of free and dissolved gases on the white pitch deposit tendency (Fig. 11). The machine produces medium weight coated (MWC) paper. Deposits consisting of coating binder and pigments in the first dryer section constituted a major problem in the manufacturing process limiting machine runnability. The machine had to be stopped for cleaning in very short periods because the deposits lead to an immense increase of web breaks in the first dryer section.

Figure 11. Decreasing white pitch deposits optimizing the deaeration of a MWC pulp suspension

Figure 11 shows some results of the trials. In the first two cases, different gas contents were adjusted by varying the dosage amount of deaerator chemicals. The white pitch tendency almost doubles with a slight increase of free and dissolved gases in the stock suspensions. It is also very dependent on the coated broke ratio used as raw material input. Optimizing the dosing amount of chemical deaerator relieved the problems with white pitch deposits. Gas content was decreased to levels of 0.1 % by volume free gas, and 2 – 3 % by volume dissolved gas, by adding the optimum amount of deaerator chemicals in each production situation. Web breaks were continually decreased with these measures.

In a German mill producing LWC coating base stock, degassing was optimized. Targeted measures gave a 94 % reduction in gas contents at the headbox. At the same time, cost savings of approximately 30 % were achieved for deaerator chemicals. As a result of lower gas levels, the pinhole problem was completely eliminated.

The optimization measures were taken for a 40 g/m² LWC coating base that was produced from a furnish of approximately 50 % chemical pulp, 40 % mechanical pulp, 10 % DIP and coating broke containing primarily calcium carbonate pigments. 1.5 % aluminum sulfate was added upstream of the mechanical vacuum deaerator which was run at a constant pump speed. pH's in the approach flow were at 6.9 – 7.0, prior to optimization. To achieve the desired results, combined use was made of two deaerator chemicals, one of which had a stronger degassing effect and the other a stronger defoaming effect. For online measurements during optimization, an online gas analyzer with multisampling was connected ahead of the mechanical deaerator and at the headbox (Fig. 7).

In the course of the trials, the two dosages of deaerator chemicals were varied in steps. Optimum results were obtained by the sole addition of the product with a primarily deaerating effect. When applied in optimal dosage amounts, the chemical gave a 94 % reduction in overall gas levels at the headbox. Simultaneously, total chemical costs for deaeration were cut by 30 % (Fig. 12). The diminished gas levels met the original objective of optimization: to eliminate pinholes in the paper. The drastic reduction in dissolved gas contents by optimized chemical use demonstrates that a controlled application of deaerator chemicals improves the performance of mechanical vacuum deaerators.

Figure 12. Gas contents before and after optimization

These results strikingly demonstrate the benefits of using an online gas analyzing system for optimizing stock deaeration which are reflected in cost savings and enhanced process stability.

When grammages were increased from 40 g/m² to 42 g/m² during the test runs, the amounts of dissolved gas released at the headbox underwent a significant rise, and the pinhole problem became acute again (Fig. 13).

Figure 13. Grammage increase at constant deaerator dosages

Since optimal dosage amounts obviously differ widely for various paper grades and grammages, constant additions of deaerator chemicals fail to continually meet the goal. Instead, controlled deaerator dosages based on online measurements have proven to be the right approach to improve paper machine processes in the long term, thus enabling paper manufacturers to reap the associated cost-cutting benefits.

Figure 14 illustrates the costs, verses the deaerator chemical dosages for a LWC production.

Figure 14. Example for ROI of an online gas analyzer in LWC production

For a production of 100,000 ton per year (tpy) of LWC paper, a reduction in deaerator dosages from 0.15 % to 0.05 % - e.g. by adding chemicals strictly to demand instead of in constant amounts – helps to diminish the operating costs of deaeration by up to per year. Papermakers achieve a sustainable cost reduction by closed-loop control of deaerator chemicals - a system that provides for very short ROI periods of online gas analyzers. Figure 15 shows how the ROI period for a gas analyzer rapidly goes down with increased production rates and/or higher dearator dosages.

Figure 15. Example for ROI of a online gas analyzer in newsprint production

CONCLUSIONS

High air and gas contents constitute a major problem area for papermakers in paper manufacturing. Elevated gas levels in stock suspensions are a major source of problems in papermaking, such as poor drainage and formation, pinholes, web breaks and deposits. Technologists trying to correlate these problems to gases or air present in paper stocks can conduct online measurements of entrained and dissolved gases in the process. Especially the impact of dissolved gases on production and quality problems is underestimated in most cases.

Mill practice proves that if degassing of stock suspensions is optimized, these problems are successfully combatted or altogether avoided. Adding deaerator chemicals in automatic closed-loop control with the signals of a highly accurate and reliable online gas analyzer will achieve increased machine runnability, improved paper quality and significant production savings.

LITERATURE

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2. R. Rauch, Verringerung von Störungen durch hohe Kohlendioxidgehalte bei der Herstellung von Papieren unter Verwendung von Calciumcarbonat als Füllstoff, PTS Research Report No. 17/97 (1997)

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Wochenblatt für Papierfabrikation, pp. 1137-1141 (1998) No.22

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11. P.O.Meinander, Zentrifugale Entlüftung - eine neue Möglichkeit zur Prozessverbesserung, Wochenblatt für Papierfabrikation 127, No.11/12 (1999)

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