Abstract
Deposit control programs in pulp and paper mills are critical for machine runnability and paper quality. Although these programs are used in most systems, the management of their performance has historically been quite poor and complicated, entailing intensive analytical work and representing, in general, a reactive approach to the problem. This paper introduces the use of online and offline monitoring tools to enable a proactive approach to deposit control. Monitoring techniques are used for program dose optimization, trial evaluation and for studying the effect of process conditions on the deposit propensity of the media. Nalco’s innovation integrates online monitoring technologies with deposit control programs to monitor and control microbiological slime, pitch and stickies, and inorganic scale. With demonstrated success in the laboratory and at the mill sites, this approach enables paper makers to optimize chemical usage, ensure deposit control program performance, improve operating efficiency and ultimately reduce total cost of operation.
Background
Deposit problems are encountered at all pulp and paper mills, and are normally exacerbated by mill closure and the level of contamination entering the process via raw materials. Deposit formation from mill waters on equipment complicates pulp and paper making processes. The deposits cause plugging, increased utility costs, and inefficient usage of chemicals. Deposits in the approach system where the sheet is formed can result in lost production due to downtime, and downgraded products from increased dirt counts or sheet holes. Literature data show that growth of deposits in liquor lines and recovery system reduces flow rate; scale acts as a heat insulator and accounts for 10% loss in efficiency of heat transfer processes for every 1/16″ thickness of scale. The severity and location of the most troublesome deposits varies significantly from mill to mill. Mill audits should be conducted in order to develop a deposit control program to address the specific problem identified in each case.
All deposits that occur in the pulp and paper industry are normally grouped into three major categories: inorganic, organic and microbiological.
Inorganic scale is an old and persistent problem in many industries where technological processes involve large volumes of water. The most common inorganic scale at pulp and paper mills is calcium carbonate, calcium oxalate, barium sulfate, calcium sulfate, calcium hydroxide, burkeite, silica and silicates.
Organic deposits in the paper making process may originate either from wood (wood pitch) or from paper additives that are released during recycling (white pitch, stickies). They constitute a problem in mechanical pulping, recycling, brownstock washers, and a paper, tissue or board machine. The paper machine deposits are especially important because of the strong effect organic deposits have on runnability and paper quality.
The main sources of microorganisms in the paper making process are from mill water, furnish and additives. Uncontrolled growth of microorganisms can result in slime deposits on the machine. These deposits are initiated by the production of biofilm material that is used by these microorganisms to attach to machine surfaces. These biofilms in turn trap debris such as fiber, starch, fillers and other additives. As the deposits grow, they can form stringers. When the deposits become large enough they can detach and fall into the paper furnish or onto the sheet where they cause holes, spots, and breaks.
Usually, deposits are of mixed nature because scale that has accumulated from one type of component can form an excellent base for other scales or microorganisms to grow on. Organic admixtures in an inorganic scale are a common phenomenon; growing crystals can easily trap fiber, forming a base for new crystals to grow. Because the mechanisms of formation can vary, the approaches to control deposits are different. A comprehensive deposit control program is always multi component.
NalcoApproach to Deposit Monitoring&Control
Nalco currently offers a variety of chemicals that covers almost all types of typical deposits, inorganic, organic and microbiological. Nalco maintains a competitive edge with application know-how, and tailored to the customer’s needs chemical programs optimized via performance monitoring.
The uniqueness of the Nalco approach is in combining monitoring and control and its ability to demonstrate to the customer in a relatively fast on-site experiment the performance of the treatments and develop online monitoring tools to manage the program performance. By choosing a program that fits the customer’s needs, providing advanced warning of process problems and adjusting the treatment when necessary, the monitoring capabilities and process consulting give added value over a “chemical only” approach.
Nalco has developed monitoring and control of all types of deposits at all stages of paper manufacturing.
Deposit Monitoring Tools
In the absence of monitoring tools, evaluating the performance of a deposit control treatment is a time-consuming task and in general a reactive approach to the problem. Nalco has introduced proprietary deposit-monitoring instruments and technologies that are currently at different stages of development.
Application of the technology to inorganic scale as shown instruments are based on quartz microbalance technology and measure vibration frequency of the quartz crystal sensor. These two parameters are affected by the properties of the aqueous phase and by deposition on the crystal surface. The vibration frequency is proportional to the mass of a deposit on the metal surface open to the solution (slurry).
Here is a significant difference between the practiced online and benchtop applications. In the bench top test, the SRM is normally used as an electrochemically instrument: a pH shift is electrochemically induced to simulate pH-driven deposition of the scale that becomes less soluble at higher pH (calcium carbonate, calcium oxalate).In the online experiment, natural deposition is measured, the unit thus resembling a micro coupon. This technology is proven to be useful in bleach plants. A sample calcium carbonate mass accumulation graph.
Nalco monitoring instruments are also capable of measuring deposition of organic pitch and related materials using patent pending methodology. However, monitoring the deposition of stickies in a recycling process requires patent-pending modification to the sensor to promote absorption of the adhesive-based organic contaminants to the sensing surface. This proprietary method can be used online or in the lab to monitor the deposition behavior of stickies, as well as the response to treatment programs.
The DRM provides real-time continuous monitoring and is proven to be effective in paper machines. The on-line setup is a basis of the developmental monitoring technology that is a part of the Nalco Deposit Control technology for pitch and stickies control program. OxiPRO™ deposit control technology includes online instruments to measure surface deposition, microbial activity, and process conditions, such as oxidation-reduction potential (ORP) and pH, to ensure the maximum benefit from the deposit control program (Figure 8).
The Activity Monitor is a patent-pending instrument and method for monitoring microbial activity and ORP of the process stream. The monitor consists of a flow cell equipped with self cleaning probes. Data is collected continuously and can be accessed via the web or mobile phone. It can also send alarms via e-mail or text messages.
The Biomonitor is an online device for quantifying and recording the degree of soft-deposit accumulation on a transparent test disk that is partially immersed in a flowing process side stream. Since the test surface is not heated, deposits with associated mineral debris are the most common types of deposit adhering to the disk. The fouling recorded by the Biomonitor
indicates the fouling trend in the system. The Biomonitor allows for real time, on-line monitoring of fouling in the paper machine system correlating them with wet-end ORP and bioactivity. It includes wireless Internet communication, allowing instant access to program performance. The data gathered by the Bio-monitor improves control through dosage optimization and better timing of boil outs, before sheet breaks and paper defects occur.
A key factor for successful control of microbial growth is implementation of reliable and relevant monitoring. Online monitoring, coupled with tailored programs, application strategies, and on-site service with the capability of proactive program management provides a comprehensive strategy for maximum control of microbial activity and related deposits.
Use of online monitors allows the microbial control program to be adjusted in real time for optimal machine performance despite changes in raw material quality and process variability. The online monitoring tools detect upsets and send alarms allowing quick correcting action. The monitor also confirms the efficacy of the corrective action. The OxiPRO monitoring strategy can also be used for evaluation of a program, to confirm efficacy of the change, to allow for real time adjustments and to minimize transitional risks. The following case history demonstrates how online monitors can be used to minimize performance upsets during program changes.
The deposit control program the mill was using was not providing optimum machine performance. Improved monitoring of system fouling tendencies, allowed local engineers to predict/plan chemical boil-outs. The Oxi-Pro Biomonitor demonstrated improved system cleanliness allowing the mill to extend the boil-out period from 2 months to 98 days with no risk to machine efficiency.
Machine cleanliness also led to reduced downtime resulting from sheet breaks by approximately 38%, from an average of 49.25 to 30.5 hours per month. This is equivalent to a potential production increase of 7,320 tons per year, which is a 2.7% of production. This case study demonstrated that the OxiPRO monitoring strategy can be used to improve overall machine efficiency. The customer was able to save substantial profit losses by reducing downtime, extending the boil-out period, and reduce total cost of operation.
Conclusion
Machine deposits in pulp and paper production process can influence on-machine efficiency (OME). Integrating monitoring and control strategies to quickly identify the root cause of deposit-related problems allows for a proactive approach to deposit control. Comprehensive deposit control programs have been developed for inorganic scale, natural organic deposits, organic deposits from recycling and microbiological deposits. Monitoring technology and control packages have been developed for every stage in the papermaking process where deposits are a problem. This leads to improved control of deposit formation, fewer sheet breaks and defects, reduced downtime, and improved chemical efficiency. The overall result is increased OME, enhanced finished product quality and reduced total cost of operation.
