Through-Air Drying Apparatus with Dual Temperature Control
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Solution Overview
Problem
Existing through-air drying processes for paper webs face inefficiencies, particularly in drying non-uniform tissue webs and the limitations of air permeability and temperature, which result in variable moisture content and physical properties, and require increased capital investments for improved drying rates.
Innovation Solution
A tissue making machine with two noncompressive dewatering devices, where the temperature of the drying medium is separately controlled, allowing for elevated temperatures up to 700°F (371°C) in the first device and lower temperatures in the second, to optimize drying efficiency and maintain web properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the supply temperature is increased to improve drying rate, then the drying rate increases, but the web temperature may exceed the degradation temperature of wood pulp fibers (300°F) and cause scorching or burning
Solution Approach 1:
The drying process is divided into multiple zones with different temperature profiles. The first zone operates at higher temperatures (up to 500°F) for rapid moisture removal, while subsequent zones operate at lower temperatures to complete drying without scorching the web. This segmentation allows the system to achieve high drying rates while protecting the web from thermal damage.
Solution Approach 2:
The system dynamically adjusts drying parameters including temperature, air velocity, and moisture content thresholds. By monitoring web moisture content and adjusting supply temperature accordingly, the system maximizes drying rate when the web is wet and reduces temperature as the web approaches target moisture content, preventing scorching while maintaining productivity.
2Productivity
If the drying time is decreased to improve productivity, then the drying rate must be increased, but this requires increased capital investment in dryer dimensions
Solution Approach 1:
The system achieves faster drying times by optimizing operational parameters such as air supply temperature, air velocity, and moisture content control rather than increasing dryer size. This allows reduced residence time through enhanced mass and heat transfer efficiency, maintaining productivity improvements without capital-intensive dimensional expansions.
3Temperature
If high temperature steam is added to the drying medium to increase supply temperature, then the supply temperature can exceed web ignition temperature, but this introduces complexities to the manufacturing process and requires additional capital improvements
Solution Approach 1:
The system uses ambient air or process air that is heated and recirculated through the drying zones, eliminating the need for external steam generation equipment. The drying medium serves multiple functions including heat transfer and moisture removal, and the system automatically manages temperature and humidity control through integrated sensors and actuators, reducing manufacturing complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances drying rate and energy efficiency, reduces residence time, and improves physical properties like sheet bulk and surface texture without damaging the web, while minimizing thermal degradation and odor production.
Implementation Method 1
The air flow principally dries the embryonic web by evaporation
Implementation Method 2
Hot air flow passes through the web, then through the permeable fabric
Implementation Method 3
the heat transfer coefficient is influenced by the mass of air contacting the web during the drying process
Data Source
AI summary
Methods of improving the drying rate of a cellulosic web, such as a tissue web, by providing an apparatus having two noncompressive dewatering devices, such as two through-air driers, where the temperature of the drying medium supplied to each device is separately controlled. The temperature of the medium supplied to the first device may exceed 450° F., such as from about 450 to about 600° F. On the other hand the temperature of the medium supplied to the second device may be less than the temperature supplied to the first, such as from about 350 to 450° F. Drying the web in this manner not only improves drying efficiency, but also limits or prevents degradation of the web, such as the combustion of cellulosic fibers making up the web or monosaccharides associated therewith. As such, webs that are substantially free from furan and acetaldehyde may be produced by the present methods.