Storage and Transportation Technology: [Weekly Topic] 10-12-8 to 12 – Concept of Gas Interconnection

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This post was last edited by flute6 on 2010-12-8 at 13:54. I am extremely grateful to everyone for their help; this situation has reminded me of an idea that has been in my mind for many years – namely, that the gases are interconnected. If we connected the gas phases of our existing large pyrolysis gasoline tanks using pipes, the gases entering and leaving the tanks could complement each other. This would not only reduce the amount of nitrogen that needs to be added through the nitrogen sealing system but also allow some of the gas phase to be supplied to other tanks before the breather valve opens, while the tanks are still being fed. Because when gas is exhaled, the nitrogen and light components in the tank are exhaled together; if the gases are connected, this also reduces the loss of volatilization of those light components to a considerable extent. The following is my idea from a few years ago regarding the gas connection of the internal floating roof tanks used for naphtha in the tank farm; it would be useful to know how to repair a tear in the tank roof ) Discussing the problems together is of great help to us. I. Concept for the modification to connect the naphtha vapor phase: The use of floating roof tanks and internal floating roof tanks is the main method for controlling hydrocarbon contamination from crude oil and light oils such as gasoline; compared to dome tanks, they can significantly reduce hydrocarbon evaporation losses. The gas-connection process is employed to link the gas phases of multiple storage tanks holding the same type of oil, so that the gas released when one tank is filling up is captured by another tank that is simultaneously discharging oil. Practice of using gas interconnection in crude oil tank farms has proven to reduce large breathing losses by 85%. During the emission of oil and gas, components such as the vent valve and breather valve on the tank top may carry away tiny oil droplets; or these droplets may form as a result of cooling by external cold air after the oil and gas are emitted, thereby causing localized contamination on the tank top. Our tank area currently has nine naphtha tanks with a total capacity of 65,000 cubic meters. Each tank is equipped with an independent nitrogen sealing system. Over the past two years, technical upgrades have been carried out to seal the vents on these tanks in order to reduce vapor losses. Compared to before, the amount of breathing losses has decreased significantly, but the problem has not been resolved completely. First, each tank for storing gas operates as an independent system; during the discharge process, as the gas space within the tank increases, the nitrogen sealing control system must continuously supply nitrogen into the tank in order to maintain a slight positive pressure. In other words, the volume of material discharged is equal to the volume of nitrogen gas added. Second, while the feed tank is in the feeding state, as the volume of the liquid increases and the gas space decreases, the gas portion inside the tank will be released into the atmosphere through the breather valve until feeding is complete. In other words, the volume of the inhaled air is equal to the volume of the exhaled air. It turns out that our naphtha tanks have been in use for over a decade now, and the sealing performance of their floating roofs is becoming increasingly poor. Even with internal floating roofs, some light hydrocarbons still manage to enter the upper layer of the floating roof through gaps, and are then released along with the nitrogen inside the tank. This phenomenon is most noticeable in summer; whenever feeding takes place, we can smell a strong odor of oil and gas. As the oil and gas entrained in the gas discharged by the vent valve on the tank top cool down upon contact with external cold air, fine oil droplets are formed, which can cause localized contamination on the tank top. At the same time, as oil and gas evaporate into the atmosphere, in addition to causing air pollution, it also increases the fire risk in the tank area. The evaporation of light components reduces the quality of the raw materials and increases losses. If we adopt a gas-phase interconnection approach, connecting the gas streams of these nine naphtha tanks together, we can use just one set of nitrogen sealing control system located at the bottom of the tanks, on the main gas interconnection bus. This system is used to control the nitrogen sealing on this gas bus. (This makes it easier to regularly inspect the nitrogen sealing control system; in the previous setup, such systems were located at the top of each tank, making regular maintenance difficult. If the nitrogen sealing system fails, it could result in the tanks collapsing.) In this way, the changes in gas volume resulting from our daily receipt of raw materials and the dispatch of naphtha complement each other, thereby overcoming and reducing the gas volume losses and safety risks associated with large-scale breathing effects. Since the gas volumes in each tank are connected, the risk of equipment damage caused by a failure in the nitrogen sealing control system in individual tanks is avoided, which in turn reduces environmental pollution. This reduces the amount of nitrogen used. Based on a current consumption of 65 tons of naphtha per hour, and assuming 330 days per year, this amounts to approximately 735,000 cubic meters. This does not yet include the loss of light hydrocarbons that are exhaled along with nitrogen. Someone may ask: Can the gas space in several tanks meet the gas replenishment requirement of the discharge tank? I say it’s more than sufficient; in each tank, the material is generally fed up to a safe height of 13.5–14 meters, which corresponds to 85% of the tank’s total capacity. There is still 3 meters above the floating disk, plus a 2-meter conical section, which constitutes the gas space. The combined gas spaces in all tanks are enough to meet the gas requirements of the discharge tank. Currently, the pressure set for our nitrogen seal control system is plus or minus 50 Pa. As per the design, we can set the nitrogen seal control pressure between 500–1000 Pa, which allows us to meet the production requirements while ensuring safety, further reducing energy consumption through less gas emission, and promoting environmental protection. I would appreciate it if everyone here could share their thoughts on my ideas; discussions are welcome. Thank you!