Currently, the condensation method still faces some technical challenges during its development, with energy consumption increasing by over 10% compared to liquid nitrogen cooling.
On one hand, the equipment costs and operational expenses of the condensation method are relatively high, especially when aiming for lower condensation temperatures to improve recovery rates, which significantly increases energy consumption and limits its large-scale application. Mechanical condensation methods have higher cooling temperatures, making it difficult to meet increasingly stringent emission standards. While liquid nitrogen condensation methods offer lower cooling temperatures, liquid nitrogen is expensive, and its storage and transportation pose certain safety risks. On the other hand, in practical applications, condensation equipment's adaptability to varying operating conditions for oil and gas needs improvement. For example, significant fluctuations in oil and gas composition and flow rates may affect the stability of recovery efficiency. In the future, condensation methods will focus on developing efficient and energy-saving condensation equipment and processes through research into new refrigeration cycles and high-efficiency heat exchanger technologies to reduce equipment costs and operational energy consumption. In the field of technological integration, further research should be conducted on how to more reasonably integrate condensation methods with other oil and gas recovery technologies to achieve synergistic effects and enhance overall recovery efficiency.
2.1.2
Adsorption Method
The adsorption method utilizes the adsorption capacity of adsorbents to separate different components in a mixture. In this process, one or more components are captured by the surface of the adsorbent, and separation is achieved through the adsorption of oil and gas molecules by the adsorbent. The adsorption method has the advantages of simple process and low energy consumption [33]. Additionally, this method can effectively recover organic solvents and recycle adsorbents, demonstrating significant advantages in environmental protection and resource conservation. Adsorption is effective for treating low-concentration oil and gas, making it more suitable for use in the downstream of oil and gas processing, where it can be integrated with other technologies to form a comprehensive treatment system.
In-depth research into the characteristics of adsorbents is crucial for significantly improving adsorption efficiency. Currently, commonly used adsorbents include activated carbon, silica gel, activated alumina, molecular sieves, carbon analysis sieves, and activated carbon fibers, each with its own unique characteristics. Additionally, research and development have been conducted on some emerging adsorbent materials to achieve more
efficient and environmentally friendly adsorption technologies. Huang Weiqiu et al. reviewed the adsorption performance and modification techniques of several commonly used oil and gas adsorbents, including activated carbon, activated carbon fiber, hydrophobic zeolite/mesoporous molecular sieves, and hydrophobic silica gel. They suggested that future research should focus on the development of new adsorbents, composite adsorbents, and adsorbent structure design. Xu Wei et al. systematically discussed the application of activated carbon adsorption technology in VOCs treatment, including pressure swing adsorption, high-temperature adsorption, temperature-pressure swing adsorption, and electric adsorption methods. Surface chemical modification of adsorbents can alter their adsorption capacity and selectivity toward VOCs. The match between the molecular size of the adsorbate and the pore size distribution of activated carbon is a key factor influencing adsorption efficiency. Process parameters during adsorption operations (temperature, gas concentration, and flow rate), environmental conditions (humidity, pressure), and the composition of the mixed gas all significantly influence the adsorption process. Xue Mengting et al. described the influence of different molecular sieve configurations on the adsorption of VOCs in exhaust gases. Cubic-configured molecular sieves exhibit the best adsorption performance for VOCs, followed by hexagonal-configured ones, while orthorhombic or monoclinic-configured ones show generally average adsorption performance. Microporous molecular sieves are suitable for small-molecule VOCs, while mesoporous molecular sieves are suitable for large-molecule VOCs. Feng Yongchao believes that the unique pore structure and adjustable silica-alumina ratio of MFI-type molecular sieves make them promising for VOC removal applications. However, current challenges include balancing hydrophobic VOC adsorption performance with catalytic activity, as well as low loading rates and easy deactivation due to the enrichment of active components on the carrier's outer surface. The aim is to address these issues by rationally designing the form of metal presence and modifying the elemental composition of the carrier.
Wang Yingxia et al. explored conditions for modifying silica gel to enhance gasoline vapor adsorption efficiency and reduce water absorption. They found that using tartaric acid as a modifier, with a heat treatment temperature of 550–650 °C and a treatment time of
3–10 h, and a heating rate of 3–10 °C/min, optimal results can be achieved. Additionally, the desorption rate of the modified silica gel increases with higher vacuum pressure, higher temperature, and increased desorption cycles.
Adsorption agent regeneration technology plays a crucial role in enhancing the sustainability, economic efficiency, and environmental friendliness of adsorption methods. Traditional adsorption agent regeneration technologies often suffer from significant drawbacks such as low efficiency, high costs, and environmental impacts. Researchers have conducted extensive studies on regeneration technologies. Currently, common activated carbon regeneration methods include thermal regeneration, oxidative regeneration, solvent extraction, biological regeneration, supercritical fluid regeneration, microwave irradiation regeneration, ultrasonic regeneration, and photocatalytic regeneration. Huang Weiqiu et al. found that for activated carbon adsorbing oil and gas,the influence of four factors-microwave power, irradiation time, activated carbon quantity, and vacuum pressure-on regeneration efficiency and loss decreases in that order. The optimal experimental conditions were determined to be microwave power of 300 W, irradiation time of 240 s, activated carbon quantity of 4 g, and vacuum pressure of 0.06 MPa. Zhang Yuzhou [41] reviewed the influence of three aspects-the inherent properties of activated carbon, different adsorbates, and varying control conditions-on microwave regeneration, emphasizing that in-depth research into these three aspects is necessary to advance the development of microwave regeneration. Han Yingjie et al. summarized the development characteristics of microwave-regenerated activated carbon technology, identifying factors influencing the microwave regeneration method, including microwave power, carrier gas atmosphere, regeneration time, adsorbent type, and adsorbate.
Sun Xianhang et al. explained the mechanism of supercritical CO₂ regeneration of activated carbon, summarizing the current status of VOCs desorption mechanism research from three aspects: pressure, temperature, and flow rate. They identified the microscopic mechanism of VOCs desorption under the action of supercritical CO₂ and the calculation model for the mass transfer coefficients of VOCs in activated carbon after desorption. Li Yichen found through comparative research that the microwave-vacuum coupled regeneration technology, involving intermittent microwave-vacuum isothermal desorption treatment with a 4.5-minute vacuum cycling process and a 0.5-minute nitrogen vacuum break process, is the optimal regeneration operation method for oil-bearing activated carbon. Hu Minggang established a first-order multivariate gray prediction model. This model was applied to predict the performance of adsorption-based oil and gas recovery units, demonstrating good predictive effectiveness. It provides important guidance for determining the optimal timing for replacing activated carbon and for modifying and optimizing oil and gas adsorption recovery units.
Numerous scholars have achieved certain results through their research. Chen Yaosi added a cooling device to the existing oil and gas adsorption recovery unit and, through experimental analysis, concluded that controlling the outlet temperature of the adsorbed oil within the range of -5 to 5°C
meets emission requirements. DWIVEDI et al. studied the breakthrough analysis of adsorption in tubular reactors and found that as the volume fraction of VOCs increases from 5,000×10⁻⁶ to
50,000×10⁻⁶, the breakthrough time significantly decreases. Activated carbon regeneration requires heating to a temperature range of 120–150°C and a regeneration time of 45–60 minutes to fully regenerate activated carbon pre-equilibrated with VOCs at a volume fraction of 8,000×10⁻⁶. Qiu Wenwu et al. studied the adsorption performance of dual-tower adsorption for oil and gas, using silica gel and activated carbon as adsorbents. The oil and gas first undergo buffer adsorption, followed by a pressure swing adsorption stage. Through experimental apparatus testing, it was found that in a pressure swing adsorption system containing 400 kg of adsorbent, liquid oil and gas could be adsorbed at a rate of 20–46 L/h, with hydrocarbon emission volume concentrations as low as 0.01%.
Currently, adsorption methods face two major challenges in oil and gas recovery: insufficient selective adsorption capacity of adsorbents and defects in traditional regeneration technologies. Adsorbents exhibit poor selective adsorption capacity for specific components of oil and gas, affecting overall recovery efficiency, and struggle to precisely recover target oil and gas molecules under complex oil and gas compositions; traditional adsorbent regeneration technologies are inefficient, costly, and prone to causing secondary pollution, limiting the sustainability and economic viability of adsorption methods.
Therefore, future adsorption research should focus on developing new adsorbents with high selectivity, high adsorption capacity, and environmental friendliness, precisely controlling material structure and surface properties to enhance specific adsorption capacity for oil and gas molecules.
Therefore, future research on adsorption methods should focus on developing new adsorbents with high selectivity, high adsorption capacity, and environmental friendliness; precisely controlling material structure and surface properties to enhance specific adsorption capacity for oil and gas molecules; conducting in-depth studies on regeneration mechanisms to develop efficient, low-cost, and pollution-free regeneration technologies; optimizing adsorbent structure to improve mass and heat transfer efficiency and adsorbent utilization rates; and enhancing adsorption system performance to adapt to different operational conditions.
