Increasing the efficiency of dust extraction at the dry end of a dissolving pulp sheet machine.
Optimization and re-design of the suction pipe. Suggestions for ideal positioning of the suction pipe to minimize dust production based on a thoroughly flow simulation and particle flow analysis.
An increased dust extraction rate of 50 percent, mainly on small particles.
Modern paper machines are based on the principles of the Fourdrinier Machine, which uses a moving woven mesh of cellulose fibers to create a continuous paper path by filtering the fibers held in a paper stock and producing a continuously moving wet fiber mat. This is dried in the machine to produce a strong paper web. Pulp sheets are cut after the drying section. This is done by a rotating knife. The cutting process creates a lot of fine particles producing a large quantity of dust (approx. 200 kg within 24 hours). This dust causes problems as it has a detrimental effect on the operation of the machine and impairs occupational safety and needs to be extracted. Andritz wanted us to optimize the dust extraction.
Figure 1: Sample of flakes with loose fibers
Figure 2: Dust Extraction
In a first step we analyzed the particles sizes and distribution. Therefore, we took real samples from the production site, provided by Andritz. We then used a sifting process to characterize the different particle classes. The sifting procedure was used to separate the sample into classes of different sizes, shapes and to estimate the quantity distribution as all these features influence the resistance and have a massive impact on the flow properties. The structure of pulp fibers leads to a variety of different dust particles. The particle distribution is later used in the flow analysis. The next step was the flow simulation of the airflow in the suction area. The complex geometry of the 8m wide cutting section was simplified and a numerical mesh was created to solve the flow field via a finite volume approach.
To calculate the suction rate a particle simulation was done. We used one way coupling and the air flow field was frozen. The particle classes defined by the analysis before were injected in the flow field. The flow path of the particles is calculated by solving their equations of motion. According to the particle size and form each class of them has a different flow resistance which defines the trajectories in the frozen flow field. Flow conditions and the particle trajectory can change depending on the velocity, mass and defined resistance of the particle. We solved the problem by repositioning the suction pipe in the suction area and geometry changes of the pipe to get an improved suction behavior. Various detail simulations, 2D and 3D, were used to study the relevant parameters and to solve the problem. For small particle classes an increase of 50% suction rate was achieved. Furthermore, it became clear that large particles needed a completely different form of extraction method. Different possibilities are still under investigation.
Based on the particle evaluation and the simulations carried out, we analyzed the air flow and the suction rate in the system, that is particle distribution and number of small-, medium- and large- sized particles to evaluate the design of the suction pipe and to optimize it. Based on the results we were able to develop proposals for the suction pipe – its re-design and the ideal positioning as well as its inlet pressure to maximize the suction effect. In our simulation we demonstrated that an increased dust extraction rate of 50 percent, mainly on small particles could be realized by implementing our suction pipe improvements.
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