Tissue: how to increase performance and quality

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Tissue: how to increase performance and quality

February 13, 2011 - 14:00

BRUSSELS, Feb. 14, 2011 (RISI) -Significant economic benefits can accrue from identifying and controlling process instability in tissue mills. The goal of this article is to introduce a practical methodology, illustrated by case studies, to help the tissue maker create economic value by identifying and controlling process instability. Ultimately, this could mean the tissue maker delivering more good quality cases of tissue per unit of installed capacity, but there may also be an intermediate focus on reducing raw material, energy or consumable costs.

BTG's approach has its basis in the appreciation of the economic factors involved in tissue making. Technical suppliers to the pulp and paper industry are too often concerned with the direct application of their product or service, and not nearly enough concerned with the economic impact of its application. On the other hand, some purchasing decisions are based only on cost and make no attempt to define the value a product may bring.

An economic model which considers cost in the context of market, quality and output may be helpful. It serves to identify the true value of a product or service and avoid purely cost-based decisions or an overemphasis on technical specification without considering their economic effect.

The market should be the first consideration, as it is from this that all else follows. Tissue covers a wide spectrum of commercial activities, from away-from-home to consumer tissue, branded or private label, premium and value grades and a whole host of subdivisions. Add to this the complexities of competition and the distribution channels for the tissue and it is clear there are many economic factors to understand before approaching the tissue manufacturing process.

Some generalities can be stated: premium branded producers will emphasize the very best quality above all else, as this underpins the brand, giving the consumer "reason to believe" in marketing parlance. On the other hand, private label producers are more than any other tissue producer part of the supermarket FMCG supply chain so need high production efficiencies and flexibility of operations to meet their customers' demands. And the cost context of the model is very much influenced by the market position of the producer as we will see.

Quality needs to be fit for purpose in market terms. The premium producer's brand will emphasize certain attributes: softness and hand feel can be paramount, but also strength and sometimes even the length of the roll. Private label similarly has exacting, but subtly different specifications, this time defined by the supermarket buyer. Furnish and consumable selection as well as product design and certain features of the manufacturing operation will contribute to quality.

Output is of course a critical economic factor as fixed asset utilization has a big bearing on unit cost and hence gross margin.

As discussed, and as a later example will show, increasingly the need is to start from converting output or cases of tissue per day or per line and work back from there. The productivity chain runs all the way back through to the tissue machine, with many issues manifesting themselves as converting problems, having their origin in the machine wet-end or even stock preparation area.

Cost or cost reduction is a fact of life for this industry. Even here costs need to be viewed through the market lens. Do we mean cost of quality? Or the cost of output? Furnish, consumables, labor, energy, efficiencies all have their role to play, but the key is to take a holistic view of the total cost of operations in the context of the market drivers.

The starting point for any project should define the desired outcome for the client; for example an improvement in productivity or quality or a reduction in raw material or consumable costs. In all cases the basis is that an economic benefit will derive for the client.

The initial consultation would review this and start, perhaps with some site-based work, to explore the critical success factors to achieve this objective. A strategy based on technical solutions is proposed, and an agreement to a project would define the scope, equipment, personnel and project schedule as well as the cost: benefit statement and expected payback. Typically a project would run for six months, but complex issues requiring multiple specialties may take longer to execute.

Reviewing some recent case studies will show how the process works in more detail.

Figure 1 - Existing tissue industry conditions

Case 1: Basis weight reduction

In the first case, the client needed to reduce the basis weight of its tissue by 0.3 g/m2whilst maintaining the caliper (bulk) of the sheet. Bulk was one of the important quality issues, and the actual sheet g/m2was driven by the need to obtain the bulk specification. The critical success factor was to maximize bulk: g/m2ratio, but the limiting factor was the need to make the sheet overweight to compensate for the variation in caliper.

Thus the strategy to be executed was to eliminate the bulk variation. There were two sources of bulk variation: periodic from the change of the crepe doctor every four hours and non-periodic from variation in thick stock consistency.

Installing a high-performance crepe doctor could take the blade change cycle from four hours to more than 50, thus consistent bulk was delivered throughout most of the blade life. This was thanks to the ceramic tip of the blade being resistant to impact wear, hence maintaining a constant crepe pocket angle. The blade geometry and material also enable the achievement of higher bulk for a given weight of tissue by altering the micro-crepe structure. Replacement of the static blade thick stock consistency transmitter with a modern optical total consistency sensor allowed proper consistency control and measurement at the machine chest, helping to eliminate the non-periodic g/m2variation.

The results were significant and achieved higher than the customer objective in terms of basis weight reduction. The savings in furnish cost on this grade would give an estimated economic benefit of Euro 360,000 ($480,000). Payback for the instrument is less than three months, and the overall Year 1 return on investment is better than 3:1.

Case 2: Improve converting yield

In the second case, the client's objective was to improve convertor efficiency. The critical success factor for this was to improve the quality of the sheet from the tissue machine; in particular, reducing the number of web breaks in the parent rolls, and reducing edge cracking which caused converting line web breaks.

The strategy was to improve on-machine crepe performance, as poor creping was the primary cause of web breaks. Analysis showed that many web breaks came about due to excessive crepe doctor wear giving a bypassing issue which would then trigger a blade change. The uniformity of the Yankee coating was thought to impair the operation of the high-performance blade, and after some investigation, fines management in the tissue machine wet-end was found to be a key factor. Use of a fixative chemical not only controlled anionic trash but also gave control of the level of fines and ash retention in the sheet; this in turn was seen to impact coating performance. Further trials established the optimum level of fines retention via monitoring WW consistency.

Wet end factors were shown to have a great influence on converting plant efficiency. During the trials, a reduction in web breaks, increased doctor blade life and improved machine speed all contributed to the final objective of improving converting line output. This was a more complex project, but still gave a payback in around six months and an estimated economic benefit of Euro 230,000/yr ($310,000).

Case 3: Reduce chemical costs

In this case, the client had no chemical strategy in place, which resulted in multiple companies supplying products to the tissue machine with no integration or control. It was clear that many chemicals were being overdosed and thus the critical success factor was reducing the dosage without negative impact on machine operation or product quality.

Charge measurement and control was the key to understanding and reducing chemical dosage. The priority was to optimize the function chemicals dosed to impart wet strength and dry strength to the tissue.

The tools used were zeta potential measurement and soluble charge measurement, using lab instruments. A new chemical addition strategy soon became apparent, employing a trash collector to neutralize chemical-consuming anionic charge and revised addition points at the area of maximum fiber receptivity. Following this, the same procedure was followed for the process chemicals. After the initial work by BTG specialists, the instruments were handed over and mill personnel were intensively trained in their use and interpretation of the results.

Such was the scope for improvement at this site that the chemical spend was reduced by more than 40% and the project payback time was less than two months. Unexpected benefits in terms of reduced cleaning, chemical usage and much improved machine efficiency were realized also. In this case chemical spend was reduced by Euro 800,000/yr ($1.1 million).

Recommended action

These short examples give an insight into how a holistic approach to problem solving in tissue mills with a strong focus on outcomes can quickly give a significant economic impact at the client facility.

Typically, operations management at a tissue manufacturing site have many day-to-day concerns and committing to a detailed analysis of critical success factors impacting economic performance takes some time. Therefore the strength of companies such as BTG is to augment the mill operations team for the period of a defined project and add its specialist skills to the process of improving performance without distracting from ongoing operations.