Dynamic modeling of a paper machine wet end

• Main Author: Yap, Eddy Fei Pang
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/8376

This thesis describes the development and properties of a dynamic model of the wet end of an operational paper machine. The need for such a model becomes paramount as paper makers strive to refine their processes to meet environmental constraints, and to overcome a combination of slumping markets and rising stumpage costs. Not only can a good model highlight deficiencies in the design, bottlenecks during operation, and regions of poor control, it also allows users to test their hypotheses and innovations without potentially causing major upsets and reducing throughput. A validated dynamic model of the paper making process is by far the best tool currently available to better understand and optimize such a process. The first concern in the project was to build a dynamic model of the paper machine wet end to describe the distribution of fines, fillers and fibers throughout the wet end system. IDEAS™, developed by Simons Technologies, was the simulation platform of choice due to its high fidelity, object oriented nature, versatility, and ease of use. In addition to the standard library objects, several unique objects had to be created to simulate the more difficult processes such as saveall operation, white water dilution network, and stock proportioning. Pump curves, pipe dimensions, valve characteristics, and tank sizes were incorporated into the model so as to create a proper representation of the process being modeled. The scope of the completed model covered the stock tanks to the end of the wire, including the white water system, broke system and stock reclaim system. The second step was to validate the dynamic model with mill data. Two sets of data were obtained from the mill for two different grades: low-ash and high-ash production grades. Analyzer results of the samples were used to estimate the components' separation ratios at every separator in the process. The model was then adjusted with the first set of data until it was considered satisfactory. Without changing the physical parameters in the model except the controller set points, the model was set to produce the second grade. For both data sets, over 70 % of the model results deviate by 5 % or less than the mill measurements. Once the model was satisfactorily validated, five plant scenarios were simulated and the responses analyzed. It was shown that fines and ash in broke enter the white water system very quickly via the white water chest as broke thickeners accepts. Increasing the ratio of broke to virgin feed will increase the relative fiber content of the main process streams. Changes in the wire pit consistency due to variations in the machine speed will cause the retention aid controller to react in the wrong direction. Stream compositions and consistencies are seriously affected by changes in the clay addition ratio. These results would be the first step to understanding, and eventually, optimizing the paper making process.