Paper 360
Papermaking is a complex, dynamic process with many possibilities of adjustments and influences in a production plant. Using high-level automation and production monitoring, it is possible to increase the efficiency of the process. Besides other parameters, also monitoring the charge environment—the particle charge and the zeta potential—in the wet end of the papermaking process improves the production efficiency. In such complex systems, this monitoring is extremely important.
Unclear charge conditions can lead to losses in quality of the base or the finished product, disturbances of the production process, and higher cost for additives. A permanent control is especially important if the facility uses different chemical additives meant to ensure a well-functioning production process and certain quality requirements of the base paper and finished product. Having information about the charge conditions available helps to optimize the dosage of additives throughout the complete wet end process and allows the mill technicians and engineers to get a good overview of the status.
Test devices that provide this necessary information include the CAS touch! Charge Analyzing System and the FPA touch! Fiber Potential Analyzer. These devices are used in the lab and are already well-known within the paper industry. However, as automation and digitalization play an increasing role in the paper and board industry (and most other industries), automatic measurements that provide reliable and accurate real-time process data become more and more important for meeting sustainable, ecological, and resource-efficient requirements.
Such online measurements offer several advantages at once. The results are available in real-time, which allows an immediate intervention if the process is not running well or quality issues occur. These real-time data cannot only be transferred to the process control system; they can also be transferred to any data cloud anywhere in the world, which enables networked handling of issues by experts who do not need to be onsite.
MEASURING ZETA POTENTIAL
An online device that has recently been introduced to the industry is the emtec FPO Fiber Potential Analyzer Online. The device measures the zeta potential of the fibers directly at the paper machine. The automatically-measured zeta potential is an important parameter for the level of interaction of the fibers and the chemicals added to the process.
The measuring principle of the online device is identical with the measuring principle of the laboratory instrument. Both determine the zeta potential of fibers in suspensions in the wet end area of paper and board machines, using the equation from Helmholtz and Smoluchowski.
The sample is sucked into the suction tube by the vacuum in the measuring cell. A fiber plug develops in front of the screen electrode and is solidified during the stabilization period. For technical reasons, an oscillating electrolyte flow is generated through the fiber plug by means of a periodically changing underpressure. The counterions on the fiber surface within the plug are sheared away and generate a streaming potential, which is gathered by the electrodes. The zeta potential is calculated by the Helmholtz and Smoluchowski equation from the streaming potential, the conductivity, the pressure difference, and a constant.
The complete test—sample taking, measurement, and cleaning—is done fully automatically by the FPO Fiber Potential Analyzer Online. The data are transferred to the process control system, where changes of the chemistry are immediately made visible by the measured zeta potential. This makes the device a very helpful tool for quality control and supports process optimization and quality improvement.
To optimize the use of chemicals in the process (therefore saving money), while the quality stays at the same level, the FPO Fiber Potential Analyzer Online helps determine the optimal quantity of chemical additives such as sizing agents, wet and dry strength agents, optical brighteners, and dyes. In addition, the knowledge of the zeta potential supports further applications: it is decisive for the maximum possible retention of fines, fillers, dyes, and resins; for optimal internal sizing; for the neutralization of negative effects of anionic trash, including pulp mill carryover; closure of the back water system, recycling of coated production, and so on; as well as for the maximal efficiency of single process steps, including dosing of anionic trash catcher (ATC) fixing agents and sludge dewatering.
CASE STUDY: TISSUE MILL
An example that clearly illustrates the benefit of using the device is a tissue paper mill that produces base tissue for kitchen towels and toilet paper on an alternating basis. Each type of final product must meet very different standards. Toilet paper must not contain any wet strength agent (WSA), because it needs to dissolve very fast once flushed; toilet paper containing WSA would lead to huge problems in wastewater systems. For the production of kitchen towel, WSA is indispensable to reach the necessary wet strength for its application, so WSA is added to the pulp in the wet end of the production.
Changing the production from one product to another product, for example from toilet paper to kitchen towel, requires changes to the mix of chemicals added to the process. The change of chemicals, or the adjustment of the amount of certain chemicals added to the tissue pulp, leads to changes in the charge environment and especially influences the surface charge of the fibers—the zeta potential. A lack of information about the surface charge of the fibers often leads to issues either with the process runnability or the quality of the final product. In this special case, a certain wet strength is necessary to guarantee an adequate quality of the kitchen roll.
The emtec FPO measures the zeta potential online and provides real-time data; these data help the mill reach the required quality of the respective type of tissue paper product before further tests, such as the ball burst test, are done in the lab. The surface charge of the fibers indicates very reliably whether the requested quality is likely to be reached. This does not mean that lab tests are no longer necessary; however, with the information provided by the device and the experience of mill personnel, the optimization of the process works out much faster, which can help save resources and time.
It is not necessary to get samples from the machine, which saves time. The data are automatically transferred to the process control system, where they can be used together with all other data to monitor the process and to ensure a good product quality.
The FPO, just as with other online tools that provide real-time data, helps the industry to further digitalize and automate the papermaking process. The device does exactly what a test device ready for Industry 4.0 needs to do: it measures fully automatically, with very low maintenance effort, reliable, accurate, process-relevant data and transfers this data automatically to the mill’s control system or a data cloud.
The instant availability of the test data at any place in the world allows networked discussions with people from different locations and fast solutions for potential issues— even by making automated changes to the process, if certain specifications get out of the optimal range.