2 Oil and Gas Recovery Technology
2.1
Single Oil and Gas Recovery Technology Oil and gas recovery technology is primarily divided into four categories based on principle: condensation method, adsorption method, absorption method, and membrane separation method. Each of these technologies has its specific advantages and limitations. Numerous scholars both domestically and internationally have conducted in-depth research and analysis on these technologies, aiming to improve oil and gas recovery efficiency, reduce environmental pollution, and promote the sustainable use of energy.
2.1.1
Condensation Method
The condensation method is a widely used and highly efficient oil and gas recovery technology. It utilizes the characteristic that the vapor pressure of volatile components in oil and gas changes with temperature to achieve recovery. At room temperature, the primary volatile components in finished gasoline-C4 to C8 hydrocarbons-have relatively high vapor pressures, indicating they tend to evaporate from the liquid phase to the gas phase and volatilize into the atmosphere. Through the condensation method, when the temperature of the oil and gas drops below 0°C, the vapor pressure of hydrocarbons decreases with the decrease in temperature. The decrease in vapor pressure causes some hydrocarbons to exceed their saturated vapor pressure at that temperature, condensing from the gaseous state to the liquid state, thereby achieving oil and gas separation.
The condensation method is simple to operate, highly efficient, and does not cause secondary pollution, making it suitable for the recovery of high-concentration oil and gas [12]. Therefore, it is suitable for use in the front end of integrated processes with other oil and gas recovery technologies, but the equipment costs and operating expenses are relatively high. Condensation methods are primarily divided into mechanical condensation and liquid nitrogen condensation. The recovery efficiency, system energy consumption, and concentration of oil and gas after processing in condensation methods are influenced by various factors, with the primary influencing factors including condensation temperature, condensation pressure, initial concentration, and condensation process.
Numerous studies have shown that the condensation temperature of oil and gas is a key factor affecting recovery rates. To achieve higher recovery efficiency, lower condensation temperatures are often required, with some components needing to reach -110°C to condense [15]. Lower condensation temperatures imply higher energy consumption per unit of cooling capacity. Current research generally agrees that a three-stage condensation process effectively balances energy efficiency and recovery rate. Different scholars have determined optimal condensation temperature combinations through simulation and experimentation.
. Huang Weiqiu et al. found through Aspen software simulation that using a three-stage condensation process with temperatures of 2, −30, and −80 °C achieves an oil and gas recovery rate exceeding 95% with the lowest system energy consumption; if the condensation temperatures are adjusted to 2, −30, and −120 °C, the recovery rate can reach 99.62% without a significant increase in energy consumption. This parameter combination has been adopted by most scholars as the benchmark design. SHI et al. designed a three-stage
condensation process with condensation temperatures of 1, -40, and -110°C, achieving recovery rates of 99.73%, 99.79%, 99.82%, and 99.19% for four different gasoline vapor components, respectively. When the condensation temperature is within the range of 20 to -110°C, the total cooling load of the three-stage condensation process is reduced by 12.23%, 15.68%, 13.96%, and 15.65% compared to a single-stage process.
Zhao Zhiwei et al. found that setting the condensation temperature at 4, –50, and –110°C results in the lowest energy consumption and stable operation of the refrigeration system. Bi Jinbin et al. simulated and analyzed the three-stage condensation oil and gas recovery process using the PR model (a real gas state equation known as the PR equation), balancing oil and gas recovery efficiency with total system energy consumption. The optimal pre-cooling temperature and secondary and tertiary condensation temperatures were determined to be 5, -35, and -75 °C, respectively.
Moderately increasing the condensation pressure helps adjust the condensation temperature, achieve energy savings, and simultaneously enhance oil and gas recovery rates, a point that has also been widely recognized by the academic community. Lu Jieming et al. analyzed the effects of cooling temperature and pressure on condensation efficiency using a phase equilibrium equation model and proposed a multi-stage condensation recovery process. Research has found that the concentration of oil and gas emissions imposes stricter requirements on cooling temperature than recovery rates. At atmospheric pressure, cooling to below -100°C is required to meet standards, while pressurization to 0.5–0.7 MPa can increase the required cooling temperature by 20°C. Wang Dan et al. used Aspen Plus simulation to find that pressurization can effectively improve crude oil and gas recovery rates and reduce outlet concentrations. Addressing the high energy consumption of conventional condensation recovery processes, several scholars have conducted optimization studies on condensation processes. Ye Chao et al. used HYSYS software to establish a simplified simulation process, studying the effects of condensation temperature and pressure on condensation characteristics. The study found that pressurization has a more significant impact on high-temperature ranges than on low-temperature ranges. Through optimization of the oil and gas recovery process, residual heat recovery of exhaust gases was achieved, resulting in a 9.73% reduction in total energy consumption, an 8.11% decrease in cooling capacity, and an increase in the cooling coefficient from 1.04 to 1.08. Zhang Shanzhe [25] used single-factor experiments to determine the key process parameters affecting energy consumption and product quality. Through simulation and field verification using ASPEN HYSYS software, the comprehensive energy consumption was reduced from 1,329 kW to 1,253–1,255 kW, achieving energy savings of 5.57%–5.72%. LI et al. designed a novel VOCs deep condensation recovery (VOCs-DCR) system, with steady-state simulation showing a VOC recovery rate of 99.97%, energy consumption controlled at 35.67 kW, and VOC emission mass concentration of 45.17 mg/Nm³. SHRAM et al. designed a low-temperature steam recovery unit consisting of a double-chamber box with internal partitions, allowing alternating supply of steam-air mixtures to different parts of the unit, which can reduce oil and gas emissions by over 80%, reducing environmental impact while improving economic efficiency. GAO et al. developed a novel low-temperature VOC recovery system that integrates turbine expansion refrigeration technology and cold energy storage technology. For intermittent oil and gas emissions, steady-state and dynamic simulation analyses of the new system were conducted using HYSYS software. The results showed that the post-treatment non-methane total hydrocarbon emission mass concentration was 57.54 mg/Nm³, and the comprehensive recovery rate of oil and gas reached 99.99%, meeting current emission standards.
Mechanical condensation is limited by the refrigeration mechanism, resulting in higher refrigeration temperatures. In contrast, liquid nitrogen condensation can achieve refrigeration temperatures as low as -120°C or even -180°C, meeting stricter emission standards. Compared to mechanical condensation, liquid nitrogen condensation offers advantages such as rapid startup, lower deep-cold temperatures, higher recovery rates, and lower equipment maintenance costs. Xu Hao found during his research and analysis of the process and equipment of a three-stage liquid nitrogen condensation VOC recovery system that the system's cost-effectiveness ratio is not significantly related to the volume of gas processed but is closely related to the volume concentration and type of exhaust gas. When the volume concentration of exhaust gas increased from 3.8% to 19.0%, the economic benefit ratio rose from 0.38 to 0.59. Xing Chuan Sheng mentioned that using liquid nitrogen cooling can improve oil and gas recovery efficiency and reduce energy consumption costs, with cooling temperatures reaching -180°C to -160°C. Chen Song et al. compared the application of mechanical refrigeration and liquid nitrogen refrigeration in gasoline vapor recovery. Mechanical refrigeration cools the vapor from 30°C to -75°C, with lower energy consumption than liquid nitrogen refrigeration, but additional treatment facilities are required to meet emission standards. Liquid nitrogen refrigeration can directly cool the vapor temperature to -120°C, meeting emission standards. The combined condensation process of mechanical refrigeration and liquid nitrogen refrigeration improves recovery efficiency while increasing economic benefits by over 10% compared to liquid nitrogen refrigeration alone.
