How to treat the breathing emissions in oil depot tank areas? A tank area treatment solution combining membrane separation and condensation

The major emissions from the tank farm of an oil depot do not originate from the loading platform, but from the storage tanks themselves: during oil receiving operations, the gas phase inside the tank is pushed out, and the daily temperature difference causes the tank to breathe continuously. Additionally, there are leakage losses at the edge seals of floating roof tanks. Although these emissions are not large in volume, they persist daily and occur at multiple points across a wide area, making them a challenging aspect of tank farm emissions management. The recovery devices in the loading process cannot cover the breathing emissions from the storage tanks, and the tank farm requires its own collection and treatment system. This article explains the source composition of tank farm breathing emissions and a combined treatment solution using membrane separation and condensation, which can serve as a reference for the renovation of oil depot tank farms.

Where does the emission from the tank farm come from

Emissions from tank farms are categorized into three sources: large breathing occurs during oil receiving and dispensing operations, with gas phase emissions during oil receiving and air intake to dilute the gas phase inside the tank during oil dispensing; small breathing occurs continuously due to diurnal temperature differences and pressure changes, with higher emissions during seasons with large temperature differences on sunny days; sealed leaks from floating roof tanks and wear and tear of accessories constitute the third source, which are small but persistent. The first step in management is collection - the breathing vents of each tank are connected to the collection pipe network, with breathing valves and check valves preventing cross-tank air leakage and backflow. The collection main pipe enters the treatment unit. The tightness of the collection system determines half of the management effectiveness, and pipeline pressure testing and valve tightness are key points to be controlled during the renovation.

How to configure membrane separation and condensation

The characteristics of breathing gas in the tank area are low and fluctuating flow rates, and concentrations that vary with tank conditions. The combination scheme is designed based on these characteristics: the buffer tank first levels out the fluctuations, the condensation section collects the high-concentration periods, the uncondensed tail gas enters the membrane unit for enrichment and then returns to the compressor inlet for recycling treatment, and the retained side is discharged after meeting the standards. The membrane section follows the gas volume in the tank area, operating at low load during low flow rates at night to avoid unnecessary electricity consumption. The overall recovery rate can be high, and the emission concentration is stable within the limit. Compared to a pure condensation scheme, the combination configuration controls the refrigeration temperature level within an economic range, making both electricity consumption and investment more friendly.

How to arrange the renovation construction

The tank farm renovation is being carried out within an operational oil depot, where safety and operational continuity are two major constraints. Pipeline openings and connections are scheduled during windows when there are no receiving or dispatching operations, and hot work is approved and supervised in accordance with oil depot regulations. Each tank is connected to the collection pipe network one by one, with inspection conducted after each connection to ensure no impact on the receiving and dispatching of tanks in use. During the renovation period, temporary measures are taken for the breathing emissions from the tank farm, providing a basis for transition to compliance standards. Upon completion, the entire collection pipe network undergoes pressure testing, and the breathing valves and check valves are tested individually. After joint debugging with the processing units, they are put into operation.

There is another account that is easy to be overlooked in the renovation plan: after the expansion of the tank farm collection pipeline network, the specifications of the processing units need to be verified according to the long-term scale of the tank farm - if only the current tank capacity is considered for model selection, the processing capacity will reach its upper limit when new tanks are built in the later stage, and the downtime loss for secondary expansion will be much higher than the initial investment reserved. During the plan review, the tank farm planning map should be brought out for comparison, and this account can be calculated in ten minutes.

Reference for tank farm management configuration

Key points of operation management

1. Collect daily inspection data on pipeline network pressure and the status of breathing valves on each tank;
2. The liquid level of the buffer tank and the concentration of the incoming membrane are recorded on a shift basis;
3. Draw the trend of condensation temperature level and membrane front pressure difference on a weekly basis;
4. The membrane segment switching is linked with the gas volume in the tank area, and operates at low load at night;
5. The discharge outlets are inspected in batches, and the data is archived along with the tank farm ledger;
6. Special inspection of breather valves and grounding systems before the thunderstorm season.

Frequently Asked Questions

Should the internal floating roof tank also be connected to the collection system?

Consider tank type and local requirements: floating roof tanks still have edge seal losses and accessory emissions, and tanks with small gas phase spaces have relatively low collection value; prioritize based on the actual emission inventory of the tank area, and connect tanks with significant breathing first.

Can the media in different tanks of the tank farm be mixed and collected?

Media that are mutually incompatible or have significant differences in recycling value should be collected in separate groups. The collection pipeline network should be independent for each group, and the processing units should switch between groups to prevent mixed quality from affecting the value of the recycled oil products.

Do we need to shut down the warehouse for the renovation?

There is no need to shut down the entire warehouse. With the arrangement of tank-by-tank access and window construction, the receiving and dispatching operations are basically unaffected. For individual tanks, off-peak arrangements can be made on the day of access.

This article is compiled by Luoyang Wohong Petrochemical Equipment Co., Ltd., which provides design, manufacturing, and operation and maintenance services for condensation recovery, membrane separation recovery, and complete solvent recovery equipment to chemical and pharmaceutical enterprises. If you need process evaluation or scheme configuration, please feel free to contact us for discussion.