Optimizing the wet end

How one newsprint mill has boosted output while reducing holes, breaks and fresh water consumption

November 2007
A PPI Special Report

A large North American producer of newsprint needed to increase production and paper machine efficiency.

Newsprint demand has been on a steady decline. Today's competitive business environment requires producers to become more efficient. Existing assets have to yield higher returns in order to remain cost competitive.

The newsprint manufacturer asked all current suppliers for ideas and input into how to improve operating efficiency. The producer was looking at all options - mechanical, chemical and operational - for improving the way it worked. Nalco had had a minor role at this mill in the past. Upon learning of the mills' goals, a team of the supplier's sales engineers and consultants reviewed the mill processes and determined that the mill needed to improve wet-end drainage. The machine was capable of running faster, but drainage was a limiting factor. From a chemical viewpoint, drainage was limited because the mill's retention/drainage aid was being added prior to the primary screen. The retention/ drainage aid had to be added prior to the screen because more sheet defects and breaks happened when it was added post screen.

The shearing of the flocculated fiber reduced its ability to enhance drainage, and hence, paper machine productivity was hindered.

Background

Previous attempts to improve the retention, drainage and formation (RDF) program at the mill had proven unsuccessful because of numerous runnability issues related to post screen addition of retention aids. Nalco's application strategy using PARETO allowed the mill to increase machine speed, increase retention and reduce holes associated with the RDF application with no appreciable increase in steam usage.

Program design

The first step with this customer was to determine the financial impact of increased production and increased machine efficiency. Certainly, the combination of higher machine speed and improved uptime would be a welcome improvement. With a clear understanding and agreement with the customer on the financial impact, based on improved productivity and efficiency, the next step was to complete a process simulation in the laboratory.

An investigation of the customer's process revealed the challenges it was facing. Water chemistry, retention aid choice, retention aid addition and retention aid application were factors hindering production.

These inputs led Nalco to recommend PARETO wet end optimization technology. The program is designed to provide a comprehensive approach to helping papermakers maximize the performance of chemical additives and to optimize their use. The analysis of the technical situation revealed that, for this North American newsprint mill, the retention/drainage aid needed to be added post screen to maximize its effect on drainage. Improved drainage would allow for greater machine speeds.

Previously, sheet holes and breaks had been a hindrance to adding the retention/drainage aid post screen. With the new program the mill was able to improve wet end drainage and increase machine speed.

Retention and drainage increased while dryer steam remained relatively unchanged - 0.1 kg/tonne of 1T60 (emulsion flocculant) enabled the retention of 8.8 tonnes/day of fines.

Another positive impact was a reduction in sheet defects and breaks - improvements directly attributable to the use of the Nalco solution.

Key performance indicators

• Speed
• Retention (wire pit set point, FPR, thick stock flow, couch solids)
• Sewer losses
• Drainage (steam, slice position, thin stock flow)
• Sheet defects and sheet breaks.

Program results

With the customer's cooperation, the wet end chemistry was "re-engineered" to maximize performance. Prior to PARETO, the machine's average speed ranged from 1,040–1,050 m/min. Afterward, the machine speed has been as high as 1,133 m/min.

The customer is using the drainage and retention provided to open the slice "significantly" and is making mechanical changes to deliver more speed. This has occurred without a significant increase in steam pressure and no loss of sheet brightness. The impact on sheet defects has been very positive. Web breaks caused by retention aid addition have been reduced. Fresh water consumption is also lower.

Maximizing machine performance, optimizing chemical performance and cost, reducing energy consumption and optimizing product quality requires an ongoing and constant commitment. A key to maintaining the value of the program is for the sales engineer to work closely with the customer to ensure documented gains are held and continuous improvement projects are part of everyday goals. Use of data collected both by the mill and the supplier helps to identify opportunities for improvement in the papermaking operation and chemical applications. Understanding key inputs helps the customer run its process to fully realize the paper machine's potential.

Fig. 1 – The average number of sheet holes/day before and after PARETO

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