Fabrics for Improved Dewatering

In 2021, the world consumed over 620 EJ of energy. Approximately 2.6 EJ, or about 0.4% of that total energy demand, was just used to dry paper during its manufacture. In a climate of growing energy costs and increasingly limited energy supplies, finding ways of improving the efficiency of this process are desperately needed. Even in state-of-the art paper mills, the energy used to dry paper is well over twice as much as is theoretically needed. Clearly, there is a massive opportunity to make a major impact on global energy consumption. This thesis develops a technology that reduces the need for energy-intensive drying. By creating a fabric with enhanced dewatering capability, more water can be squeezed from the paper sheet mechanically. Thus, there is about 40-50% reduction in the amount of water that has to be evaporated in the dryer section, cutting the energy requirements of drying paper in half. Arriving at the mechanism ultimately capable of achieving this followed a meandering path that wound through the fields of flow in porous media, surface chemistry and wettability, partitioning of liquid droplets, and the physics of interfacial instability. The primary problem plaguing papermakers is that, after water is squeezed out of the paper sheet, the sheet resorbs water from the fabric that carried it through the press. Essentially, when pressure is released, the paper sucks water back out of the sink (the fabric) that was provided to remove it. No technology has yet been developed that can entirely avoid this rewetting tendency. Therefore, a fabric with one-way flow properties has been highly sought after in the industry for decades. Creating a fabric that lets water in, but doesn’t let it out, would drastically reduce the amount of water that has to be dried from the sheet later. The first attempt I made at achieving this goal was to use capillary forces to trap water in the fabric. By creating a wetting gradient in the fabric, it is theoretically possible to allow water into the structure at high pressures, but prevent it from leaving at low pressures (i.e. during decompression). To accomplish this, the physics of forced wetting in fibrous materials (e.g. non-woven fabrics, paper) was studied in detail in the second chapter. I found that simplistic, but widely used, models for these wetting barriers in fibrous media were inaccurate and therefore inappropriate for informing design choices. One outcome of this study was that I developed a method for accurately predicting the wetting resistance of hydrophobic fiber networks. This helped me discover that it was not practical to use capillary forces alone to control flow in the press fabric. However, the approach I developed is useful in other applications where barrier properties of papers and fabrics are essential. The second attempt to improve dewatering revolved around controlling the adhesion, or “stickiness” of the water to the press fabric. By making water more strongly adhere to the fabric, I reasoned, less water will go with the paper sheet when it is pulled away from the wet fabric. In this section, I indeed showed how altering adhesion can be used to control the transfer of water from two separating surfaces. I also illustrated the limits on using this approach, concluding that it is not a viable way of completely controlling flow in the press section. The third and, thankfully, successful attempt at creating a fabric with improved dewatering ability required a radical change of perspective on the problem of rewet. This involved inducing an interfacial instability in the liquid lying between the paper sheet and the press fabric. The details of encouraging this instability and its effect on dewatering are explored in depth in the fourth chapter. Essentially, rupturing the liquid bridging the fabric and paper destroys all paths for flow. If this process is carefully implemented, full dewatering of the paper under pressure occurs, without any water returning to the sheet upon decompression. With a successful and completely original method of controlling flow, I needed to determine the feasibility of implementing this technology in an industrial application. These preliminary studies occupy the focus of the fifth chapter. I find that the challenges associated with applying the insights of this thesis can largely be overcome with physically-informed design choices. However, there is a great deal of exciting work to be done in this area in the future. In summary, this thesis investigates many aspects of wetting and flow in fibrous materials: a prospect enticing enough to command the attention of any reader. Amidst its twists and turns, a few paths are uncovered that lead nowhere, many more paths that lead to exciting applications for adjacent problems and fields, and one final path ¬that leads to attaining this project’s ultimate aim: fabrics for improved dewatering.

Date

2022-12-06

Resource Type

Text

Resource Subtype

Dissertation

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