Technology news
Heat exchange of bulk materials, recovery and utilization of waste heat energy
2021-08-12
In recent years, as the preheating layer of the quenching and tempering tower, the plate heat exchanger has been widely used in the press plant to recover the waste heat. This technology was first introduced into China by Desmet Company and invented and patented by Solex Company of Canada.Your complete cooking oil project solution supplier and technology partner-- China Henan Huatai Cereals And Oils Machinery Co.,Ltd,The grain and oil machines produced by China Huatai have made full use of the theory of heat exchange of bulk materials and waste heat energy recovery and utilization, which achieved remarkable results.
In the "Process Heating" magazine that was just published in July 2021, Gerald Marinitsch of Solex Thermal Science introduced how indirect heat exchanger technology provides an environmentally friendly alternative to bulk solid materials in the form of waste heat recovery.
Moving bed heat exchangers can help reduce carbon and energy consumption in certain industrial processes by recovering and reusing waste heat
In many industrial processes, bulk solids require heat treatment to meet or provide final product specifications. For example, calcification is a high-temperature process that removes oxygen and volatiles to produce high-quality cement and other refractories.
Take the waste produced by metal production as an example. Waste heat is generated during the processing. The metal production process requires a lot of heat. When the waste is cooled from the molten material to the granular solid, a large amount of unused heat remains in the bulk solid. From 932°F to 1,472°F (500 to 800°C), energy is wasted or additional energy is needed to cool the product faster, and the product of the thermal pile can only be cooled slowly by itself. In most cases, water is sprayed on the thermal pile and energy is lost.
Wasting useful energy is no longer acceptable in the modern international community, as it runs counter to the increasingly prominent principles of environmental, social and corporate governance (ESG). ESG principles reflect the work of businesses, industries and communities to replace energy-intensive and often polluting processes with new methods to achieve efficient use of the primary energy consumed while minimizing negative impacts.
Bulk solid material heat exchangers, also known as moving bed heat exchangers (MBHE), have been used for decades. Waste heat recovery is being promoted as a technology to recover heat energy from granular solids and reuse it in a variety of ways. For example, heat generated by solid materials in bulk can be exchanged to other working fluids such as water, steam, thermal oil, air, or even supercritical carbon dioxide (sCO2). This heat exchange allows heat energy to be used in other processes in the plant. If the recovered waste heat cannot be used in the production or manufacturing process, the energy recovered at temperatures above 752°F (400°C) can be used in steam or gas turbines, organic Rankine cycles (ORC) and supercritical CARBON dioxide cycles for power generation. The low calorific value heat left in the power generation can be used to heat other areas of the plant or to operate absorption chillers that produce cold (cold) water. In some cases, low calorific value heat can provide heating systems for factories or areas.
However, it is necessary to understand some of the terms and technologies used in these processes before further understanding of some of the terms and technologies used in these processes.
The heat exchanger is a device that can exchange heat without mixing the moving fluid flow
Overview: Bulk solid materials and heat exchange
Most people understand terms such as "heat transfer" and "waste heat recovery". However, the understanding of "bulk solids" is relatively unfamiliar. By definition, bulk solids are mainly loose and dry particles collectively, these materials account for more than 80% of goods shipped around the world. Each of us is exposed to bulk solids every day, whether it is using salt, sugar, flour, pepper or pet food. From a broader perspective, bulk solids include everything from oilseeds, grains and minerals to chemicals, plastics, cement, metals and sand. The shape and size of individual materials may vary, and they can appear in the form of powders, granules, fragments, tablets, and crystals.
Before many bulk solids finally become final products (such as sugar), they usually pass through some processing steps that involve heating, cooling, or drying. This requires some form of heat transfer in the heat exchanger.
Traditional techniques for heat exchange of bulk solids include fluidized beds, drum drying, and fixed or moving bed heat exchangers. For fluidized bed and drum drying techniques, the heat transfer medium or working fluid is in direct contact with the solid. In the past heat exchangers were limited to "components that allow the transfer of heat from one liquid (liquid or gas) to another fluid", but indirect bulk solid heat exchangers, which allow the transfer of heat between a solid and a liquid (liquid or gas), are gaining popularity.
For indirect bulk solid heat exchangers, heat exchange is achieved by conduction to the surface of the plate or tube. In the moving bed heat exchanger, there is no contact between the solid and the working fluid, while the particulate solid passes through the heat exchanger by its own gravity. This feeding and discharging method can be compared with the continuous feeding and discharging silo. In principle, the plate heat exchanger can be heated or cooled with any heat transfer medium, including water, hot oil and steam. The indirect bulk solid heat exchanger does not include any moving parts other than the discharge device. The discharging device can ensure uniform mass flow and thus ensure uniform heating or cooling of solids.
Indirect bulk solid heat exchangers use heat conduction, where there is no contact between the solid and the working fluid as the granular solid flows through the heat exchanger by its own gravity.
Waste heat utilization of heating solid particles
Opportunities for waste heat utilization are not limited to processing in other sections of the plant. The heat recovered from the thermal product or air stream by a heat treatment step (such as a sintering machine) can greatly improve the thermal efficiency of the process by reusing the energy. It can also help to open the bottleneck of the equipment.
This energy recovery method is common in the oilseed processing industry. The oil at normal temperature is heated by the steam generated in the processing process or the waste heat generated by the steam from other processes. For this preheating step, the oilseeds are heated from the ambient temperature to the required temperature, usually 140 to 194 ° f (between 60 and 90 ° C), which reduces the overall energy consumption of preheating and provides a good return on investment for the installation of MBHE.
In addition, heat energy can be recovered from cooling solid bulk materials and used in another process step that requires heating solid bulk materials. In many process applications, temperature levels and heat loads do not match. By integrating the heat pump into the system, the required temperature level on either the hot or cold side of the system can be adjusted to meet the process conditions required at both ends. This operation depends not only on the exact temperature and the coefficient of performance (COP) of the heat pump system. They can provide an efficient double-acting energy boost: lowering the temperature of the cold side and raising the temperature of the hot side.
The picture shows a common energy recovery cycle design used to cool high-temperature bulk solids (such as ash, petroleum coke or graphite)
Heat exchange efficiency of solid materials
The heat balance around any heat exchanger is simple: the heat energy entering the bulk solid heat exchanger is the same as that leaving the heat exchanger. Therefore, if the heat exchanger is completely insulated, the heat transfer efficiency or thermal efficiency is the most ideal. Heat dissipation from the shell or outer wall is the only - path to reduce thermal efficiency, which is affected by the insulation around the heat exchanger.
The effectiveness of heat transfer is the energy that can be transferred or transferred from solids and working fluids to solids and working fluids. No difference from other heat exchangers, it is the heat transfer surface area that determines the amount of energy transferred (and the effectiveness of heat transfer). Indirect bulk material-solid heat exchangers are usually configured to transfer heat in countercurrent to maximize heat transfer efficiency.
The achievable temperature difference between the solid and the working fluid is an important consideration in evaluating efficiency. Assume that the operating temperature of the cryogenic heater or cooler is 302 to 392°F (150 to 200°C) or less. The temperature difference between the solid and the working fluid is usually between 18 and 27°F (10 and 15°C).
For applications where the product temperature is 752°F (400°C) and above, the difference in working fluid temperature is usually around 90 to 180°F (50 to 100°C), but may be higher. For example, if water is used to cool solid particles entering the heat exchanger at 1,832°F (1,000°C), the temperature difference with water can easily reach 1440°F (800°C).
In order to achieve the desired results of any bulk solid heat exchange process, careful consideration must be given when sizing the heat exchanger and the associated technical requirements as well as economic feasibility and return on investment must be taken into account.
In the oil press plant, three heat recovery circuits are used to recover the waste heat well and recycle it to preheat the oil
Example: feasibility of energy recovery
So, when is energy recovery? There is no universal answer to this question, but a simple example can illustrate the potential of major energy costs.
Considering approximately 100,000 kcal/hour (~120KW) of waste heat, this heat can be recovered from bulk solid cooling. Utilizing the waste heat of a production plant that runs continuously for about 320 days per year will generate about 780 g/h (920MWh) of usable energy
The cost of energy varies depending on the main source of energy. Taking into account that the average energy cost of 1 MCAL (0.042∈/kWh) per hour is approximately us $0.05, using available energy will save the plant us $46,000 (EUR 39,000) per year in direct costs. For larger factories, these figures can be easily calculated to get more cost savings. In addition to saving money, the use of waste heat has a very real environmental factor. By reducing input, especially when the input energy comes from fossil fuels, it also has a significant impact on reducing greenhouse gas emissions. If the main product being replaced comes from oil or natural gas, the reduction of greenhouse gas emissions is about 500 tons of carbon dioxide per year.
Moving bed heat exchanger or MBHE is a proven technology that has not yet reached its full potential in the utilization process. Such technology can effectively recover and utilize waste heat generated by cooling or heating of bulk solids.Your complete cooking oil project solution supplier and technology partner-- China Henan Huatai Cereals And Oils Machinery Co.,Ltd,The grain and oil machines produced by China Huatai have made full use of the theory of heat exchange of bulk materials and waste heat energy recovery and utilization, which achieved remarkable results.
