Views: 1 Author: Site Editor Publish Time: 2023-07-06 Origin: Site
New Studying of Drying Technology and evaluation of the Suitable Drying Equipment
Abstract: The role of the drying curve was analyzed, and the specificity of the drying curve was emphasized.
The importance of reasonable evaluation of drying equipment was elaborated, and efficient, energy-saving, and environmentally friendly drying technologies, processes, and equipment were developed among various drying equipment.
Drying Technology involves many disciplines and is a highly practical technology. Drying engineering can be said to be a combination of art and pratical experience. It is of practical significance to correctly understand and evaluate drying technology and equipment. Based on the actual application of drying engineering, we summarize some opinions and understandings, including the understanding and application of drying curves, analysis of drying efficiency, and environmental issues and certain limitations of drying technology.
1. About the drying curve
The classic drying curve is the relationship curve between the moisture content of materials and time obtained through drying with a small amount of materials, high air volume, and static state, which has great limitations in guiding practical engineering design. Before the breakthrough of drying theory (including foundation, model, and simulation), practical and reliable engineering design and system selection must also rely on drying curves. But the drying curve is obtained by the material under specific equipment and process conditions, and the main parameter settings are selected according to experimental design methods. Based on the test results, a reasonable drying curve is optimized and designed as the basis for actual engineering design. This aspect of work is done better on vacuum freeze-drying equipment, which is commonly referred to as the freeze-drying curve.
Because the purpose or definition of drying is to remove moisture (solvent) from the material, the final moisture content of the product is an important performance indicator, and the removal of moisture is a process of mass transfer. Therefore, the drying time is mainly determined by mass transfer, and the purpose of heating is also to accelerate mass transfer. Drying the same material using different equipment and processes requires a significant difference in drying time, which can also be intuitively reflected in the different drying curves. The drying curve may even be obtained by combining two or more equipment and processes for drying.
2 Performance evaluation
There are many kinds of drying equipment. How to evaluate their performance is a comprehensive problem, which should be considered from three aspects: thermal efficiency analysis based on the First law of thermodynamics, drying efficiency analysis based on the Second law of thermodynamics, and drying environmental load analysis. This is a requirement for the development of efficient, energy-saving, high-quality, and low-pollution advanced drying technologies.
Thermal efficiency and drying efficiency analyze a drying system from both the quantity and quality of energy consumption. The current evaluation of drying equipment (systems) mainly considers the amount of energy utilization, mainly including unit water loss energy consumption (3350-6280kJ/kgH2O) and thermal efficiency (30-50%), which is the part of energy that can theoretically be converted into useful work under environmental conditions. The analysis method attempts to form a practical and feasible thermodynamic and economic calculation method in the design of drying systems, with the goal of optimizing the entire system design.
For hot air drying systems, the energy-saving direction should first consider utilizing exhaust heat, and then try to reduce heat transfer temperature difference. We should vigorously promote the use of low-grade heat sources, utilize solar energy and industrial waste heat, adopt heat pump dehumidification hot air circulation technology, develop combined drying equipment, and use combined drying processes. Exhaust heat recovery can save 10% to 15% energy.
The steam conduction heating system generally adopts the method of throttling and reducing pressure to adjust the steam supply pressure to meet the equipment requirements, causing energy depreciation and making it difficult for condensate to be discharged smoothly, thereby reducing the strength of the heat exchange equipment. The recovery and heating of condensed steam from heat pumps can replace traditional steam systems to save energy and reduce consumption, which can save 15% energy for traditional drying.
Hot blast furnaces are widely used as drying heat sources in China. Considering the First law of thermodynamics, the thermal efficiency is very high, reaching 65%~75%, but the drying efficiency is much lower (especially high temperature drying), which is a great waste in terms of energy quality. Developing medium and low temperature drying equipment as much as possible, although increasing equipment investment in the initial stage, is beneficial for reducing drying costs, rational energy utilization, and sustainable economic development.
Enterprises with multiple heat consuming equipment such as evaporation and concentration, drying, etc. can also apply network optimization and narrow point (narrow point) technology of process integration methods for optimization design.
In short, waste heat recovery, medium and low temperature drying, and combined drying all bring about an increase in equipment, leading to an increase in initial investment. Therefore, it must be considered in conjunction with drying costs, which is a comprehensive issue faced by modern drying technology. From the current technological level, the increased equipment investment can be recovered within 3 months to a year, and the effect is very obvious for medium and large drying equipment systems
3. Environmental load and limitations of drying technology
Environmental issues are related to the sustainable development of the economy, and many projects consider environmental assessment as an important project. The noise of drying equipment systems, dust from materials, sulfur dioxide content in coal combustion, and exhaust emissions are all environmental issues faced by drying technology. The environmental load analysis of the drying device shows that using clean energy, fully recycling and utilizing exhaust waste heat, and reducing the exhaust temperature of the exhaust while ensuring the drying rate can significantly reduce the environmental load of the drying process.
Solving environmental problems has posed new challenges for the development of drying technology. Pollutants in many industries need drying equipment for treatment, such as waste water and residue treatment in fermentation and brewing industry, sludge drying after sewage treatment, spray desulfurization technology for coal combustion, etc.
Drying is a Unit operation that consumes a lot of energy, and it has great limitations. With the development of packaging and storage technology and the reduction of transportation costs, materials that previously required drying treatment do not need to be dried, such as milk, vegetables, etc. Mechanical dehydration and evaporation processes have great advantages in handling high moisture content materials, which requires drying technicians to choose appropriate drying methods and evaporation concentration processes to handle materials in a targeted manner.
Based on the above understanding and evaluation of drying, the following focuses on three issues:
(1) Drying test
The drying process is quite complex, and the existing method to verify the applicability of drying equipment is to conduct material tests on the selected drying equipment, usually using the orthogonal test method. This requires designers to design experiments and analyze results from the perspective of drying technology to verify the degree of influence of various parameters on drying.
After long-term practice, humans have accumulated a large amount of application results in drying engineering, and can also infer from engineering data to simplify experiments. Based on the old parameters and some new parameters, other new technical conclusions can be derived.
⑵ Combination of Different Drying Equipment
Engineering and technical personnel are increasingly realizing the significance of combined drying, which is mainly reflected in the increase of production capacity and the improvement of thermal efficiency. Typical applications mainly include the combination of spray and vibrating fluidized bed drying of milk powder, which can save 20%~30% energy. The combination of box drying and flash drying can double the production of Yonggu purple pigment. Internally heated fluidized bed drying of refined salt increases thermal efficiency by 50%.
(3)Application of regenerative combustion technology
The regenerative combustion technology and high-temperature air combustion technology have the characteristics of high thermal efficiency, reduced pollution emissions, and small equipment size; In the past decade, this technology has mainly been implemented and promoted in developed countries. China has also taken action. This technology has pioneered revolutionary burner and furnace designs, with significant energy-saving and clean combustion effects in the field of steel heating. As an important component of drying equipment, hot air furnaces can learn from and apply this new combustion technology. This technology is also known as environmentally coordinated combustion technology.
① The study of scientific experimental methods is very important for the design and selection of drying equipment.
② A comprehensive evaluation of drying equipment has a promoting effect on the development of modern drying technology.
③ In practical work, drying technicians should pay attention to environmental load issues.