Gas hydrocarbon recovery process in atmospheric and vacuum distillation units and its practical application

Thread Content

During the operation of atmospheric and vacuum distillation units, the gaseous hydrocarbons produced at the top of the initial distillation tower and the atmospheric distillation tower represent valuable light hydrocarbon resources. These hydrocarbons include liquefied gas components as well as high-octane light hydrocarbons, and they are directly related to the energy consumption of the units, gas balance, and environmental emissions. Achieving efficient recovery of gaseous hydrocarbons is an important aspect for improving the quality and efficiency of operations in such facilities and optimizing their performance. Drawing on practical field applications and operational experience, this article provides a systematic overview of the current mainstream methods for recovering gaseous hydrocarbons in refineries, including their process characteristics, applicable conditions, and key operation points. It is focused on real-world production scenarios and practical operations, serving as a reference for frontline staff and technical management. I. Significance and Process Background of Gaseous Hydrocarbon Recovery: The gases generated in the initial distillate and regular distillate streams from atmospheric and vacuum distillation units are rich in light hydrocarbon components such as C3 and C4. In the past, many plants directly incorporated this portion of gas into the fuel gas system, which not only led to waste of high-quality resources but also increased flue gas emissions from the heaters, thereby affecting the overall efficiency of the plant. With the improvement in the level of integration between refining and chemical processing, as well as the need for more efficient management of operational costs, domestic refineries now generally carry out specialized recovery of normal and initial distillate gases. The light hydrocarbons obtained from this process are used as raw materials for the chemical industry or as liquefied gas products, thereby turning waste into valuable resources and reducing costs while improving efficiency. The choice of method for recovering gaseous hydrocarbons is not fixed; it must be determined based on factors such as the volume of gas, its composition, the plant’s overall capacity for recovering light hydrocarbons, the layout of the facilities, and the investment costs. Currently, there are two main approaches: the compressor-based pressure increase recovery process and the compressor-free pressure increase recovery process. Both methods have their respective applicable scenarios and are widely used in actual production. II. Gas hydrocarbon recovery process using compressors. Compression recovery is a traditional and most widely used method for recovering light hydrocarbons, and it is suitable for most atmospheric and vacuum distillation units with large processing capacities and high gas outputs. The core process involves feeding the overhead gas from the initial distillation tower and the overhead gas from the atmospheric pressure tower, either separately or together, into a gas compressor. By increasing the pressure, the partial pressure of light hydrocarbons is raised; thereafter, the gas enters a condensation and oil-gas separation system, where the C3 and C4 components are liquefied and recovered under certain pressures. The non-condensable gas is then sent to the fuel gas network. This process features high stability and strong adaptability, enabling good recovery rates regardless of the properties of the crude oil or the operating conditions; it is also the standard configuration for large-scale fuel-chemical refineries. In practical operation, it is essential to properly control the inlet pressure, outlet pressure, and cooling temperatures at each stage, in order to prevent liquid slugging caused by liquid present at the inlet, while also ensuring that the outlet pressure meets the requirements for the condensation of light hydrocarbons. This method achieves a high recovery rate and offers great operational flexibility, but the process is relatively complex, with high costs for equipment investment, maintenance, and electricity consumption. III. Compressor-free gaseous hydrocarbon recovery process. Compressor-free light hydrocarbon recovery is an energy-saving process that has seen rapid adoption in recent years; it is particularly suitable for small and medium-sized atmospheric and vacuum distillation units, retrofit projects, and situations where strict control over investment is required. The core technical principle behind this approach is to utilize pressure as a driving force. The key operation in this process is the operation of the initial distillation tower at increased pressure; by maintaining the pressure at the top of the tower between 0.35 and 0.4 MPa, the C3 and C4 light hydrocarbons in the vapor phase at the tower top dissolve directly into the light naphtha under this pressure, resulting in no separation between gas and liquid and complete carryover of the light hydrocarbons, with virtually no gas being discharged from the tower top. The naphtha containing light hydrocarbons then enters subsequent systems such as stabilizers and separation towers, where the C3 and C4 components are separated through distillation, thereby enabling the recovery of light hydrocarbons. From practical field operations, the compressor-free approach has clear advantages: it features a simple process, fewer devices, lower investment costs, no power consumption, and reduced operational and maintenance expenses. It also reduces the number of failure points in pump equipment, resulting in smoother operation. However, this method requires high precision in controlling the pressure of the initial distillation tower; it is necessary to maintain the pressure at the top of the tower within a specified range. Too low a pressure can result in incomplete dissolution of the light hydrocarbons, while too high a pressure can affect the efficiency of distillation as well as the load on the cooling system at the tower’s top. IV. Comparison of Application Scenarios and Practical Selection Between the Two Recovery Methods In plant design and actual operation, the choice of light hydrocarbon recovery method is determined primarily based on the following factors: The compressor-based process is more suitable when: the plant handles large volumes of material, with high yields of initial and normal distillate gases; the light hydrocarbon system across the entire plant needs to be recovered and processed centrally; there is a high demand for light hydrocarbon recovery rates, with an aim to maximize the recovery of C3 and C4 components; and the plant is equipped with a complete light hydrocarbon recovery system. The compressor-free process is more suitable when: the plant is of medium or small scale, with relatively low gas volumes; there are constraints related to space, investment, and energy consumption in renovation projects; it is not possible to add compressors on-site; and there is a desire to simplify the process and reduce operational costs. Both methods can meet the requirements for light hydrocarbon recovery; they merely differ in their approaches, but both can achieve satisfactory recovery results in actual production. V. Key operational control points for the light hydrocarbons recovery system: Regardless of the recovery method used, several core aspects must be addressed in field operations to ensure stable, efficient, and safe operation. First, maintain stable pressure at the top of the stabilizer. Pressure is the key parameter for the dissolution, condensation, and separation of light hydrocarbons; pressure fluctuations can directly affect the recovery rate. During operation, adjustments should be made in small increments, and large fluctuations must be avoided. Second, strictly control the tower top temperature. Excessively high temperatures can cause the volatilization and loss of light hydrocarbons, while excessively low temperatures increase the cooling load. The temperature should be adjusted appropriately based on pressure to maintain gas-liquid equilibrium. Third, enhance the liquid removal in the liquid separation tank. Prevent gas carrying liquid from entering pipelines or equipment, thereby avoiding abnormal conditions such as liquid slugging, freezing blockages, and tower overpressure. Fourth, closely monitor changes in the light hydrocarbon components. Adjust parameters such as pressure, temperature, and reflux in a timely manner based on test analysis to ensure stable recovery rates of C3 and C4. Fifth, in compressor-free processes, special attention must be paid to maintaining stable pressure in the initial distillation tower, ensuring it remains within the range of 0.35–0.4 MPa; this ensures that the light hydrocarbons are completely dissolved in the naphtha, thereby achieving zero gas emissions. https://mmbiz.qpic.cn/mmbiz_jpg/6eExS0Soicw1nB36cVJcsKf6Oby6ZAbkOWMFbf7WTqibYgyTntW2uOCkWrffwcI5Zhibzjia6SzQrdKVewzKS2ZAbKZUXoJWnl6Ms3VMGC29eWE/640?wx_fmt=jpeg&watermark=1#imgIndex=6 VI. Conclusion The recovery of gaseous hydrocarbons is an important aspect for improving the efficiency of atmospheric and vacuum distillation units, and it also serves as an indicator of the operational quality of such units. Compressor-based recovery and compressor-free pressure-raising recovery are the two main technical approaches in the industry today; the former is stable and reliable with strong adaptability, while the latter features a simple process that saves energy and costs. In actual production, the most suitable recovery method should be selected based on factors such as the scale of the facility, gas production volume, the overall setup of the plant, and investment and operating costs. Through precise operation to maintain key parameters such as pressure, temperature, and liquid level, maximum recovery of light hydrocarbons can be achieved, thereby providing strong support for reducing costs and improving efficiency while ensuring green and low-carbon operation of the facility.