Water-saving and emission-reduction technologies for refineries

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1 Problems faced by oil refining enterprises in water conservation and emission reduction (1) Lack of sufficient subjective emphasis. Enterprise managers and employees lack sufficient awareness of the importance, urgency, and long-term nature of water conservation and emission reduction efforts. (2) The market mechanism is imperfect. A water pricing mechanism that conforms to the laws of a market economy has not yet been established, with industrial water prices, water resource fees, and pollution discharge fees being low. (3) **No timely and accurate water price warnings.** An industrial water conservation statistics system has not yet been established, which poses difficulties in strengthening water conservation management and setting up a water price early-warning system. (4) The enterprise’s water conservation and emission reduction management systems are inadequate. At present, many enterprises have weak water conservation management practices, and waste is widespread. 2 Main water-saving and emission-reduction technologies for oil refining enterprises: In terms of water utilization, oil refining enterprises should adhere to the principle of \"using more water where it is available in large quantities, using less water where it is scarce, and reusing wastewater.\" ； It is necessary to continue carrying out water balance testing and strengthen the metering management of water use and conservation ； Ensure online monitoring of water supply, drainage, and water treatment to eliminate leaks and losses” ； Accelerate the upgrading and application of water-saving and emission-reduction technologies, and enforce strict assessments to ensure compliance with water-saving and emission-reduction standards. 2.1 Converting direct-flow water to circulating water: Many process steps in oil refining plants still use direct-flow cooling water, with the most typical application being in the cooling of pump systems. The cooling water for mechanical pumps is mainly used to cool the bearing housings of the pumps. Currently, the supply temperature for industrial circulating water is generally between 25 and 30°C. Although this is higher than the temperature of fresh water, it is fully suitable for use as cooling water for machine pump bearing housings. It is feasible to replace the direct flow water used for cooling machine pumps with circulating water, and this approach is both economical and safe. 2.2 Seawater and brackish water desalination technologies: Seawater or brackish water can be desalinated using multi-effect evaporation technology (which can take advantage of the waste heat generated in oil refining) or membrane separation technology. It offers significant economic benefits and important social benefits in enterprises facing water shortages but abundant seawater or brackish water, as well as a surplus of low-temperature heat. Currently, seawater and brackish water desalination technologies are energy-saving and water-conserving technologies that receive significant support in China. 2.3 Reuse of sulfur-containing water after steam stripping: Sulfur-containing wastewater in oil refining plants generally comes from units such as distillation, catalytic cracking, hydrocracking, hydrorefining, and delayed coking. These wastewater streams generally contain hydrogen sulfide, ammonia, carbon dioxide, as well as small amounts of oil and organic substances. Sulfur-containing wastewater generally needs to be treated by stripping before being discharged outside the wastewater treatment plant. To reduce wastewater discharge and achieve water conservation, the reuse of sulfur-containing wastewater after stripping has received widespread attention. The more appropriate reuse methods are: separation of clean and polluted water, and stripping for reuse. 2.4 Increasing the concentration ratio of circulating water: With economic development, industrial water consumption is on the rise, and circulating cooling water accounts for about 70% of the total industrial water usage; therefore, there is great potential to improve the efficiency of circulating cooling water. Operating at a higher concentration factor is currently recognized as an effective method for saving water. Increasing the concentration factor from 1.5 to 2 saves 50% water; increasing it from 2 to 3 saves 30%; increasing it from 3 to 4 saves 15%; and increasing it from 4 to 5 saves 6%. Raising the concentration factor above 5 no longer yields any water-saving benefits, and instead it tends to increase corrosion and scaling. Therefore, it is generally appropriate to increase the concentration factor to 5. However, as the concentration ratio increases, the scaling and corrosion factors in the circulating water system also increase exponentially; therefore, the development and application of water treatment technologies for high concentration ratios are key to water conservation and emission reduction technologies. Ways to increase the concentration ratio of circulating water can be considered from the following aspects. 2 4.1 Appropriate water treatment agents: Based on the enterprise’s requirements for make-up water and the type of water quality, a formulation of corrosion and scale inhibition agents is determined; after thorough testing and evaluation, it is put into use ； In recent years, as the concentration ratio of circulating water has increased, bacterial resistance has significantly reduced the efficacy of non-oxidizing bactericides. The sludge problems caused by microbial growth in the systems have become increasingly severe, making it essential to carry out efforts to select the most effective bactericides. 2.4.2 Advanced side-stream treatment: Optimize system operation based on water quality and actual system conditions, select cost-effective chemicals for circulating water treatment as well as efficient water-saving facilities, in order to increase the concentration ratio of circulating water. Increasing the amount of water treatment agents and adding appropriate high-performance dispersants and scale inhibitors can improve the scale inhibition effect, but this is only a temporary and passive treatment method suitable for systems with lower concentration ratios. For cooling water systems operating at high concentration ratios, appropriate bypass treatment processes should be selected. By using side-stream treatment processes (including filtration, membrane separation, chemical precipitation for softening, ion exchange, etc.) to purify and soften the wastewater from the circulating cooling water system before reusing it as make-up water, the amount of waste discharged can be significantly reduced and the stability of water quality can be improved. In recent years, the role of new types of ion-exchange fiber filter media in the side-stream treatment of circulating cooling water has gradually attracted attention. In addition to their filtering function, they can also undergo ion exchange with calcium and magnesium ions in water, thereby serving to soften the water quality. For medium and small-scale circulating cooling water systems, a chemical precipitation-fiber filtration controlled-release dosing process that can remove suspended solids, reduce turbidity, and simultaneously enable automatic dosing can be employed ； For large-scale circulating cooling water systems, a chemical precipitation-fiber filtration-weak acid resin exchange process that can simultaneously remove suspended solids and reduce hardness can be employed to fundamentally improve water quality and achieve water conservation and emission reduction. 2.4.3 Innovations in methods for dealing with material leaks In the case of leaks in traditional water coolers, the following methods and steps are generally used to address such issues: ① Identify and eliminate the source of the oil leak ； ②Stop adding corrosion and scale inhibitors as well as biocides; replace the water and remove oil on a large scale until the water meets the required standards ； ③Add degreasing cleaner and sludge remover for cleaning ； ④Clean until the end, then change the water in large quantities to ensure it meets the required standards ； ⑤Operating normally; the corrosion and scale inhibitors as well as the biocides have begun to be added as scheduled. This method involves a large amount of water replacement; stopping the addition of corrosion and scale inhibitors as well as biocides has a significant impact on the corrosion inhibition, scale prevention, and bactericidal/algicidal effects of the circulating water, resulting in a so-called period of water quality instability. During the treatment period, the corrosivity of the circulating water increases, leading to more severe corrosion of the water coolers and more frequent material leaks; this creates a vicious cycle that poses a serious threat to increasing the concentration ratio of the circulating water as well as to the proper operation of the water coolers. The method generally recognized as reasonable at present is: ① Identify and remove the source of oil leakage ； ②Add degreasing cleaner and sludge remover for cleaning ； ③Clean until the end, then slowly replace the water; during this water replacement process, add corrosion and scale inhibitors as well as biocides normally. This method for dealing with material leakage has no significant impact on the quality of circulating water, and the concentration ratio continues to increase steadily. 2.4.4 Strengthening systematic scientific management: While carrying out research and monitoring activities, it is essential to establish and improve management systems for circulating cooling water. Having effective scientific management methods is crucial for increasing the concentration ratio of circulating water, improving its quality, and achieving water conservation and emission reduction. 2.5 Condensate Recovery Condensate recovery is generally divided into open and closed types. An open-loop recovery system recycles condensate back into the boiler’s feed water tank. During the process of recovering and utilizing condensate, one end of the recovery pipeline is open to the atmosphere; typically, the condensate collection tank is open to the atmosphere. When the pressure of the condensate is low and it cannot reach the reuse location by its own pressure, a pump can be used to pump the condensate. The advantage of this system is its simple equipment, easy operation, and low initial investment ； However, the economic benefits obtained by the system are poor, and since the condensate comes into direct contact with the atmosphere, the dissolved oxygen concentration in it increases, leading to equipment corrosion. This type of system is suitable for small steam supply systems with low condensate production and low amounts of secondary steam. When using this system, the amount of vapor generation should be minimized in order to reduce thermal pollution as well as losses of working fluid and energy. A closed recovery system maintains a constant positive pressure in the condensate collection tank as well as all pipelines, making the system sealed. Most of the energy contained in the condensate in the system is directly recovered and returned to the boiler through specialized recovery equipment; the only energy loss occurs due to the cooling of the piping network. Thanks to the closed system, the water quality is maintained, which reduces the costs associated with treating the water before it is fed into the boiler. Its advantages are good economic benefits from condensate recovery and a long service life for the equipment, but the initial investment for the system is high and operation is inconvenient. Condensate recovery water is a high-quality reusable water; after oil and iron removal, it can be used as boiler feed water. The temperature of the condensate water in closed-loop recovery is relatively high (120–150°C), and it can be used as a heat source to heat the raw water in the water treatment unit. The methods for removing oil and iron in the condensate recovery process of refineries include cyclone separation and gravity sedimentation for oil removal, composite double-layer membrane systems for oil and iron removal, resin extraction and membrane filtration for oil removal and iron removal, adsorption for oil and iron removal, as well as interception-based methods for oil removal. Currently, interception-based methods show good performance in industrial applications. 2.6 Wastewater reuse technology: The main pollutants to be removed in wastewater reuse include suspended particles, organic matter, nitrogen and phosphorus compounds, inorganic anions and cations, salts, as well as viruses and bacteria. The commonly used unit treatment methods consist of a combination of three physical-chemical and biochemical approaches: namely, the coagulation (air flotation) and sedimentation unit technology, the biological filter (oxidation) method, and the filtration and disinfection unit technology. Other methods and techniques are applied depending on the quality of the wastewater to be treated and the standards for wastewater reuse. Table 1 lists the current mainstream technologies for wastewater reuse. Table 1 Common Treatment Technologies for Wastewater Reuse in China: Treatment Methods and Targets Removed
Unit Treatment Technologies
Physicochemical Methods: Suspended solids – Coagulation (air flotation), sedimentation, rapid filtration, MF (microfiltration), UF (ultrafiltration); Organic matter – Coagulation sedimentation, catalytic oxidation, adsorption; Inorganic substances – Distillation, freezing, ion exchange, electrodialysis, RO (reverse osmosis)
Phosphorus – Alumina adsorption, lime coagulation, or iron salt coagulation, ion exchange
Ammonia nitrogen – Stripping, ammonia desorption, zeolite adsorption, ion exchange, breakpoint chlorination
Deodorization – Biological deodorization, ozone oxidation, activated carbon adsorption
E. coli – Chlorine, ozone oxidation, UV (ultraviolet disinfection), UF (ultrafiltration)
Biological Methods: Organic matter – Biological aerated filters, biological contact oxidation, extended aeration; Nitrogen, phosphorus – Biological A/O, A2/O, SBR
Emerging membrane separation technologies have become a focus of development for advanced wastewater treatment due to their ability to produce high-quality effluent, require less space, and offer high efficiency. There are currently several industrial-scale applications of such technologies, including ultrafiltration (UF), reverse osmosis (RO), and membrane bioreactors (MBR). 2.7 Other water-saving and emission-reduction technical approaches 2.7.1 Water pinch point technology: A model of the entire plant’s water system is developed; water pinch point technology and mathematical programming methods are applied to conduct an integrated analysis of this system, identify bottlenecks, and develop and implement optimization plans. The water pinch point technique consists of three aspects: the first is to determine a minimum value for pre-determined fresh water consumption and wastewater generation by analyzing water usage operations ； Second, through water reuse and recycling technologies, a water network is designed to achieve the target values for fresh water consumption and wastewater generation ； Third, the existing water supply network is modified through effective process changes to maximize water reuse and minimize wastewater generation. 2.7.2 Rainwater Collection and Utilization During the rainy seasons, considering the collection and utilization of rainwater has also become an area that oil refining companies are exploring. The key assessment criterion for rainwater collection and utilization plans is whether the costs of storage and reuse are reasonable. 3 Achieving a win-win situation of water conservation and efficiency improvement in oil refining enterprises. There are various technologies available for water conservation and emission reduction in oil refining enterprises; to truly achieve a win-win outcome in terms of water conservation and efficiency improvement, it is necessary to pay attention to the following aspects: First, for new, renovated, and expanded industrial projects, the “three simultaneities and four requirements” principle must be strictly followed, meaning that water conservation facilities must be designed, constructed, and put into operation simultaneously with the main project itself. Secondly, oil refining companies should actively pursue clean production to reduce wastewater generation, and truly control the creation of sewage and waste water from the very beginning of the production process. Third, oil refining companies need to plan scientifically, adopting a combination of decentralized and centralized treatment methods for industrial wastewater, to ensure that the wastewater is discharged in compliance with standards. This will help to gradually improve the conditions related to water extraction and use, thereby preventing water quality-related shortages. Fourth, oil refining enterprises should adopt various effective methods to carry out extensive, in-depth, and sustained publicity and education efforts, so as to help people develop a correct understanding of water, and to foster a positive atmosphere centered on water conservation, rational use of water, prevention of water pollution, and protection of water resources. This post was last edited by uway on 2008-5-13 15:47]