Methods to Enhance Vapor Recovery Efficiency

There are numerous methods for vapor recovery, including condensation, adsorption, and absorption, among others. However, using a recovery unit that employs only one method does not yield optimal results. This is because the volatilization of oils and gases is characterized by low airflow and high concentration, making a single vapor recovery technology insufficient for achieving high recovery efficiency.

The recycling unit utilizes a combination of condensation and adsorption processes, enhancing recycling efficiency and reducing costs.

The condensation recovery method utilizes refrigeration technology to extract heat from oil and gas, achieving a direct conversion of oil and gas components from the gaseous to the liquid phase. By taking advantage of the vapor pressure differences of hydrocarbon substances at different temperatures, cooling is applied to make some hydrocarbon vapors in the oil and gas reach a supersaturated state, where the supersaturated vapor condenses into a liquid, enabling the recovery of oil and gas. Generally, a multi-stage continuous cooling method is employed to lower the temperature of oil and gas, allowing it to condense into a liquid for recovery. The lowest temperature of the condensing unit is determined based on the composition of the volatile gases, the required recovery rate, and the concentration limits of organic compounds in the exhaust gas released into the atmosphere. The condensation process is typically achieved through steps such as pre-cooling and mechanical refrigeration. The pre-cooling stage aims to reduce the operational energy consumption of the recovery unit by lowering the temperature of the incoming gas from the ambient temperature to approximately 5°C, causing most of the water vapor in the gas to condense into water and remove moisture. After pre-cooling, the oil and gas enter the shallow cooling stage, where the gas temperature can be cooled to around -35°C, allowing 70-80% of the hydrocarbon components in the oil and gas to liquefy. Exiting the shallow cooling stage, the oil and gas proceed to the deep cooling stage, where the oil and gas can be cooled to around -70°C, enabling the recovery of over 95% of the oil and gas. The remaining gas at -70°C after condensation still contains a small amount of hydrocarbon components and does not meet the national standard emission limits. To achieve compliance with emission standards using only condensation methods, the temperature must be lowered to approximately -110°C, which only increases the recovery by 3-4% of the oil and gas but requires an increase of about 30% in energy consumption, making it cost-ineffective. Therefore, for the remaining gas after -70°C, the process of pressure swing adsorption is used, introducing it to an activated carbon adsorption unit for adsorption. After enrichment and concentration, the gas is then subjected to condensation treatment to achieve compliant tail gas emissions. When the adsorber becomes saturated, a vacuum pump is used to evacuate the adsorber, reducing the pressure inside and disrupting the adsorption equilibrium, releasing the adsorbed oil and gas from the adsorber. These are then sent through the vacuum pump to the very front of the condensation process for further condensation and recovery. In reality, there are two states of VOCs organic gas emissions: one is low flow rate and high concentration (flow rate below 2000 m³/h, concentration above 500 g/m³), and the other is high flow rate and low concentration (flow rate between 2000 and tens of thousands of m³/h, concentration into oil several g/m³ or lower). For the former emission state, the "condensation + adsorption combined process" is used, first condensing and recovering the liquid; for the latter emission state, the "adsorption + condensation combined process" is used, first using an adsorbent to concentrate the hydrocarbon components, allowing a large amount of air to be emitted, and then desorbing the concentrated hydrocarbon components to obtain enriched organic gas, which is then condensed and recovered. Since the process of oil storage and transportation is primarily the former emission state and treatment method, commonly referred to as oil gas recovery, we here mainly introduce the "condensation + adsorption combined process," where the adsorption process employs pressure swing adsorption desorption, hence also known as the "condensation + pressure swing adsorption process."