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I. Introduction I. Introduction Against the backdrop of global energy shortages and an increasing awareness of environmental protection, refineries, as industries that consume large amounts of energy, are under close scrutiny regarding their energy efficiency. Low-temperature waste heat, as an important component of the energy system in refineries, holds great potential for development. The rational utilization of low-temperature waste heat from refineries not only helps to reduce a company’s energy consumption and production costs, but also decreases thermal pollution in the environment, contributing to the achievement of sustainable development goals. This report will explore in depth the sources of low-temperature waste heat in refineries, the methods for utilizing it, the principles behind its utilization, and its future development directions. II. Sources of low-temperature waste heat in refineries The low-temperature waste heat in refineries mainly comes from units such as atmospheric and vacuum distillation, catalytic cracking, delayed coking, hydrogenation units, as well as condensate water stations. In these devices, many components that require cooling generate low-temperature waste heat ranging from 85 to 150°C. The initial distillate gas from the atmospheric and vacuum distillation units, as well as the first and second atmospheric streams and cold paraffin oil, release large amounts of low-temperature waste heat during the cooling process. The overhead gas and oil, overhead circulation, mid-stage reflux, diesel, etc., from the catalytic cracking unit are also important sources of low-temperature waste heat. The overhead gases and streams from the fractionation towers of delayed coking units, as well as diesel, wax oil, stabilized gasoline, etc.; and the overhead gases from the stripping towers, hot light ends gas, fractionation tower overhead gas, and various products from hydrogenation units (including hydroprocessing of gasoline and diesel, hydrocracking, hydrotreating of residue, hydroreforming, etc.), also generate large amounts of low-temperature waste heat during cooling. Furthermore, the saturated condensate generated in the condensate station after the process steam releases heat also carries a large amount of heat, with the heat content in it accounting for a certain proportion of the total heat from the steam. Some units with different processes, such as continuous reforming, hydrogen production, and aromatic extraction, as well as flue gases from heating furnaces and residual circulating water, also generate low-temperature waste heat. According to statistics, petrochemical plants with a production capacity of tens of millions of tons can generate low-temperature heat amounting to 970 GJ/h. If such substantial amounts of low-temperature waste heat can be made full use of, it will bring significant benefits to the enterprises. III. Ways of Utilizing Low-Temperature Waste Heat (1) Utilization of waste heat from steam condensate: Steam condensate has a high temperature and contains a large amount of heat; currently, most oil refineries use this waste heat to heat circulating water and deionized water. This method not only replaces part of the heat obtained from fuel combustion but also reduces water consumption, which is of great significance for refineries in terms of energy savings and cost reduction. However, there is still room for improvement in the utilization of the waste heat from steam condensate, and further exploration of more efficient utilization methods is needed to enhance its energy efficiency. (II) Combined utilization of dry gas and steam waste heat: In some refineries, excess steam and high-temperature flue gases generated by the plants were previously released directly into the atmosphere, and excess dry gas was also burned in flares, resulting in energy waste and environmental pollution. Today, some refineries are beginning to pay attention to the recovery and utilization of this waste heat. The waste heat from flue gas, dry gas, and steam is used for heating various furnaces in the refining plants, while the remaining heat is fed into steam boilers as fuel. The steam generated is utilized in a balanced manner, first for heating and power generation, and then for producing electricity. Priority should be given to using steam engines to directly drive mechanical devices such as compressors and pumps; the heat remaining from the steam can then be used to rotate turbines, which in turn drive generators to produce electricity, thereby minimizing the waste of excess heat and energy. (III) Heating using low-temperature waste heat: The annual growth rate of heating infrastructure in China’s urban areas is 10%, resulting in a high demand for natural gas and fossil fuels. This not only leads to significant pollutant emissions and haze formation but also causes a shortage of heating sources in the northern regions. During the operation of refineries, there is a large amount of low-grade waste heat that is not fully utilized; this heat can be used as a substitute for natural gas or fossil fuels in order to meet the heating needs during winter, as well as to heat domestic water, and to supply heat to buildings such as offices, control rooms, and laboratories. This not only solves the problem of utilizing waste heat in refineries but also reduces the consumption of fossil fuels and environmental pollution, achieving a win-win situation in terms of economic and environmental benefits. (IV) Development history of low-temperature waste heat power generation: Abroad, research on low-temperature waste heat power generation technology began in the late 1960s, and the equipment systems and thermal systems were put into use in the mid-1970s. After more than 40 years of research and development, Japan has made significant progress in low-temperature waste heat power generation technology, thus forming a unique industrial chain. As of 2009, the total installed capacity of organic rankine cycle waste heat power generation systems in Japan had reached 160×10^4 kW. The domestic waste heat power generation industry started late, but after more than 10 years of development, research, and practical operation, it has seen a certain degree of development and application across various industries. Organic Rankine cycle power generation technology: The conventional Rankine cycle uses water as the working fluid. Since water has a high boiling point at normal pressure (100°C), it is difficult to vaporize water using industrial low-temperature waste heat; therefore, it is challenging to implement Rankine cycle power generation technology that utilizes low-temperature waste heat as a heat source. The organic Rankine cycle (ORC) uses organic substances with boiling points much lower than that of water as working fluids, such as Freon, propane, or chloroethane. Among these, pentafluoropropane (R245fa) has a boiling point of 15°C at atmospheric pressure, and it is currently the preferred working fluid for ORC systems due to its high efficiency. The basic principles of the organic Rankine cycle are the same as those of the conventional steam Rankine cycle, but organic working fluids can vaporize at lower temperatures; thus, industrial waste heat at low temperatures can be used as a heat source, and lower temperature requirements are imposed on the condensation system. This technology can convert the waste heat from low-quality flue gas in refineries into electrical energy, enabling deep recovery of this waste heat; it is an efficient energy-saving technique. Low-temperature waste heat heating – power generation combined use: Some refineries have adopted a low-temperature waste heat heating – power generation combined use approach. During the heating season, hot water is first used to generate electricity before being utilized for heating; the power generation by the hot water generators is adjusted according to temperature changes, with the generators employing ORC technology ； During the non-heating season, in accordance with the principle of \"using high-temperature heat for high-energy applications and low-temperature heat for low-energy applications,\" the hot water at higher temperatures is used for power generation. This combined utilization scheme fully exploits the value of low-temperature waste heat, thereby improving energy efficiency. (V) Low-temperature waste heat refrigeration: During the non-heating season, refineries can employ low-temperature waste heat refrigeration technology to utilize the remaining heat after power generation from low-temperature waste heat to produce chilled water, which is then used as cooling water for the heat exchangers in refining units. Low-temperature waste heat refrigeration technology is based on the principle of refrigeration through the vaporization of a liquid refrigerant under low temperature and pressure. The system consists of two circulation loops: one for the refrigerant and one for the absorbent, and it includes four components: a condenser, an evaporator, an absorber, and a generator. Common absorbent-refrigerant pairing fluids include lithium bromide aqueous solutions (lithium bromide – water) and ammonia aqueous solutions (water – ammonia). In this way, the effective utilization of low-temperature waste heat during the non-heating season is achieved, reducing energy waste. (VI) Desalination of seawater using low-temperature waste heat. Desalination is an effective way to address the shortage of fresh water resources, and it is particularly important in islands and coastal cities. However, traditional seawater desalination methods are energy-intensive, resulting in high water production costs. Low-temperature multi-effect evaporation seawater desalination technology has become the mainstream technology for second-generation seawater desalination plants, with steam costs accounting for the largest share of their operating expenses. If the low-temperature waste heat from refinery units can be used to generate steam for seawater desalination plants, the cost of seawater desalination can be significantly reduced. An offshore refinery increased the temperature of some of the fresh water produced by desalination units by passing it sequentially through a gasoline cooler, a diesel cooler, a paraffin oil cooler, and an asphalt cooler. This heated water was then fed into a flasher to generate initial steam, which served as the steam power for the evaporators in the low-temperature multi-effect desalination system. This approach not only reduced the costs associated with desalination but also prevented thermal pollution and energy waste. It also minimized scaling issues and extended the service life of the coolers, thereby helping to address the plant’s water shortage problems to some extent. IV. Principles for the utilization of low-temperature waste heat (1) Source control: During the production process, it is necessary to reduce low-temperature waste heat at its source first, by optimizing processes to improve energy efficiency and thereby decrease the amount of such waste heat generated. For example, optimizing the process flow to reduce unnecessary heat loss, and adjusting the operating parameters of the equipment appropriately, so as to make more efficient use of energy during the production process. (II) Energy efficiency first: The energy efficiency of utilizing low-temperature waste heat at the same level in refineries is higher than that of upgrading such utilization methods. Therefore, when selecting a utilization scheme, priority should be given to those that allow for long-term use at the same level, such as air preheating for heaters, preheating of demineralized water, and maintaining the temperature of storage tanks and reboilers. These peer utilization methods can apply waste heat more directly to the production process, reducing losses in energy conversion. Secondly, consider the utilization plans at the same level required during certain periods of the year, such as domestic hot water and heating. Finally, consider upgrading utilization options such as low-temperature waste heat refrigeration and organic Rankine cycle power generation. Although upgrading utilization can convert low-temperature waste heat into higher-quality energy or enable additional functions, its relatively low energy efficiency means it should be given lower priority when planning utilization schemes. (III) Adapt to local conditions: Refineries in different regions should select appropriate schemes for utilizing low-temperature waste heat based on their specific regional characteristics. In the southern regions, heating is not required in winter; the amount of heat needed for heating oil storage and transportation as well as for pipeline insulation is low, resulting in plenty of excess low-temperature waste heat. At the same time, the southern regions have abundant water resources but high electricity prices; therefore, for refineries in these areas, power generation schemes that make use of low-temperature waste heat offer good economic benefits. For oil refineries located on islands and along coasts, using low-temperature waste heat for seawater desalination not only helps to address their own water shortages but also makes full use of the value of that low-temperature waste heat, making it an economically viable option. V. Conclusions and Outlook: Refineries possess significant amounts of low-temperature waste heat, and making full use of this heat is of great importance for refineries to reduce energy consumption and tap into energy-saving potential. Currently, there are various ways to utilize low-temperature waste heat, including the use of waste heat from steam condensate, combined utilization of dry gas and steam waste heat, heating using low-temperature waste heat, power generation, refrigeration, and seawater desalination. During utilization, it is necessary to follow the principles of reducing low-temperature waste heat at the source, giving priority to energy-efficient utilization methods, and adapting approaches to local conditions. In the future, as technology continues to advance, greater breakthroughs are expected in the utilization of low-temperature waste heat in refineries. On the one hand, it is necessary to develop more efficient waste heat recovery equipment and technologies to improve the efficiency of recovering and utilizing low-temperature waste heat, as well as to further optimize technologies such as organic Rankine cycle power generation and low-temperature waste heat refrigeration, in order to reduce equipment costs and operational energy consumption. On the other hand, it is necessary to strengthen research on the comprehensive utilization of low-temperature waste heat, explore more innovative utilization methods, and integrate the use of such waste heat with other industries in order to achieve hierarchical and circular use of energy, thereby injecting new momentum into the sustainable development of oil refineries. Chemical Engineering Discussions