Oil and gas recovery issues in the oil unloading equipment

This post was last edited by Lao Jiang_SEKB on 2018-8-25 at 21:56. Closed-system oil unloading is now required by law in gas stations. During unloading, the gas phase of the oil tank is connected to the gas phase of the oil tanker. Filling oil tanks at oil depots is also done in a sealed manner.

Are you currently using top-loading or bottom-loading systems for loading and unloading tank trucks? If an oil and gas recovery system is to be installed, it is recommended to use a bottom-loading loading arm. The oil and gas recovery unit mainly consists of an oil and gas connection system and an oil and gas recovery system. 1) Oil and gas connection system: It connects and collects the oil and gas from various loading and unloading arms. Whether it is a top-mounted or bottom-mounted type, it is necessary to seal the gas phase space at the top of the tank truck during loading and unloading operations. Typically, a gas phase recovery pipe equipped with a rubber plug on the loading/unloading arm is used, along with a flame arrester (which must have flame arrestor certification). A main pipe is installed on-site, a pressure transmitter is fitted to this main pipe, and a buffer tank is used to store the collected oil and gas. When the pressure in the buffer tank reaches the specified level, the interlock activates the outlet valve of the buffer tank and starts the fan; the oil and gas are then pressurized and cooled by the fan before entering the liquid separation tank for separation, and finally being sent to the oil and gas recovery system. 2) There are many types of oil and gas recovery systems, such as condensation adsorption, CO, RTO, and so on. For details, see Figure 3. The main VOC recovery techniques include condensation, adsorption, absorption, and membrane separation. Based on the physical and chemical properties of VOCs, selecting different recovery methods or a combination of several methods to recover the organic compounds in VOCs can not only reduce environmental pollution but also yield certain economic benefits. 1.1 Condensation technology: The condensation method is the simplest approach for recovering VOCs. This process takes advantage of the different saturated vapor pressures of gaseous pollutants; by reducing the temperature or increasing the pressure, VOCs are condensed into liquid droplets and thus separated from the gas. Different condensation temperatures are used to achieve the gradual separation of the pollutants. The condensation method imposes relatively strict requirements on the boiling point and volatility of organic substances; generally, it requires a feed consisting of high-concentration organic substances with high boiling points and low volatility. The condensation efficiency mainly depends on the number of cooling stages of the condensation unit and the choice of condensing medium. The condensing medium is mainly cold water, frozen brine, and liquid nitrogen ； The condensation unit is composed of two or more single-stage refrigeration systems. The condensation temperature is generally achieved through steps such as pre-cooling, mechanical refrigeration, and liquid nitrogen refrigeration. The more refrigeration stages there are, the higher the recovery rate, but the greater the energy consumption as well. It has significant advantages in the treatment of high-concentration, single-component VOCs that are valuable for recovery, enabling both the purification of waste gases and their reuse. This process is currently widely used domestically and internationally for the recovery of high-concentration oil and gas. Edwards Engineering in the United States is a typical representative of the production technology for oil and gas recovery devices using the condensation method. 1.2 Adsorption technology: Adsorption technology makes use of solid adsorbents with a large specific surface area to capture VOCs in exhaust gases, thereby separating the harmful components from the gas. Once the adsorption reaches saturation, water vapor or hot air is used as a desorbing agent to remove the VOCs from the surface of the adsorbent and recover them. Adsorption is one of the mainstream technologies for treating VOCs in industry today, with key elements including adsorbents, adsorption equipment and processes, regeneration media, and post-treatment processes. The efficiency of adsorbents in capturing VOCs depends not only on the properties of the adsorbent itself but also on the type, concentration, and properties of the VOCs, as well as the temperature and pressure of the adsorption system. Generally, an adsorbent’s ability to absorb VOCs increases as the molecular weight of the gas increases; gases at lower pressures are more easily absorbed than those at higher pressures. Due to its broad-spectrum adsorption capacity for organic compounds, activated carbon is often used as a primary purification step in conjunction with other processes to treat large-volume, low-concentration VOCs exhaust gases with complex compositions. It serves to enrich and concentrate these organic exhaust gases, such as in the combined treatment technique of \"adsorption concentration + catalytic combustion\". The most common industrial adsorbents at present are granular activated carbon, honeycomb activated carbon, and activated carbon fibers. Furthermore, since molecular sieves have better safety performance during regeneration with hot air streams than activated carbon, a combination of hydrophobic molecular sieves and activated carbon beds is commonly used for the adsorption, concentration, and regeneration of low-concentration organic waste gases. The adsorption method is suitable for the recovery of VOCs at medium to low concentrations and at high throughput. It offers advantages such as high removal efficiency, thorough purification, low energy consumption, a mature processing technology, and ease of implementation, resulting in good environmental and economic benefits. The disadvantage is that the adsorbent has a low capacity, requires a large amount of adsorbent, and the equipment is bulky ； The adsorbent after adsorption not only requires regular regeneration and replacement, but there is also a risk of VOCs escaping during this process ； Due to the complexity of the entire process, the costs are relatively high. 1.3 Absorption technology: Absorption technology makes use of the principle of \"like dissolves like\" among organic substances; it involves bringing a low-volatility or non-volatile absorbent into direct contact with waste gases, thereby transferring VOCs into the absorbent solution and achieving the separation and purification of pollutants. Based on the mechanism, absorption processes can be divided into physical absorption and chemical absorption. The efficiency of absorption mainly depends on the properties of the absorbent and the structural characteristics of the absorption device. The absorbent should possess properties such as high solubility, no corrosiveness to equipment, low volatility, non-toxicity, chemical stability, low cost, and easy availability. It is usually a liquid substance, primarily a mixture of liquid petroleum products, surfactants, and water. Absorption devices mainly include spray towers, packed towers, various types of scrubbers, bubble towers, and sieve plate towers. 1.4 Membrane separation technology: This technology utilizes the difference in the dissolution and diffusion rates of various gas molecules through polymer membranes to achieve separation under certain pressures. The partial pressure difference between the gases on either side of the membrane is the driving force for membrane separation, which can be achieved by compressing the incoming gas or using a vacuum pump on the side where permeation occurs. Therefore, the membrane separation process is often integrated with condensation or compression processes. Membrane separation technology is currently in the stage of active development. Companies such as GKSS in Germany, MTR in the United States, and Nittō Denko in Japan have successfully achieved industrial-scale use of membrane technology for the recovery of VOCs from waste gases. However, its main applications in industrial settings are for treating substances such as gasoline vapor, ethane, and vinyl chloride, and the volume of gas treated is relatively small. The key to membrane separation lies in the selection of membrane materials; currently, silicone rubber membranes and hollow fiber membranes are commonly used. The common membrane separation processes for treating VOC waste gases include vapor permeation, gas membrane separation, and membrane contactors. Compared with traditional condensation, adsorption, and absorption methods, it offers advantages such as a simple process, high recovery rates, low energy consumption, and no secondary pollution, making it a separation method with great application prospects. This method is suitable for the separation and recovery of medium to high concentrations of VOCs.

New gas stations built these days all come equipped with vapor recovery systems