Storage tank nitrogen blanketing

Nitrogen blanketing is commonly used in industries and is widely applied in fields such as chemical, petrochemical, pharmaceutical, and food industries.

Nitrogen blanketing is a process where an inert gas, such as nitrogen, is filled into the headspace of storage containers and reactors. This alters the gas composition in the vapor space and is typically used to prevent oxidation, corrosion, polymerization, degradation, and formation of explosive mixtures due to the presence of oxygen.

When producing or storing unstable substances like olefins and diolefins, measures should be taken to prevent the formation of peroxides and self-polymerizing compounds. Substances such as butadiene, isoprene, and chloroprene can produce dangerously unstable polymeric peroxides in the presence of air, oxygen, or other catalysts. Styrene and propylene are also unstable compounds; over long storage periods in the presence of air or oxygen, they can self-polymerize and release heat, leading to overpressure and equipment rupture. In the production and storage of butadiene, measures are taken to prevent peroxide formation: it is strictly prohibited to have prolonged contact with air, nitrogen oxides, and oxygen-containing nitrogen, generally controlling the oxygen content in the butadiene vapor phase to less than 0.3%.

Nitrogen blanketing systems are often designed to operate at pressures above atmospheric pressure to prevent external air from entering the container. Since many processes and applications prefer the absence of oxygen and moisture from the air, nitrogen blanketing is adopted in various industries including petrochemicals, food and beverages, pharmaceuticals, and pure water manufacturing.

There are different types of tanks in plants, such as fixed-roof tanks, floating-roof tanks, and internal floating-roof tanks. Floating-roof tanks do not require nitrogen blanketing, fixed-roof tanks may require blanketing depending on the characteristics of the stored medium, and internal floating-roof tanks may sometimes consider nitrogen blanketing.

Figure-1 shows a simple method of nitrogen blanketing for fixed-roof tanks. In this design, the set points for the nitrogen blanketing valve and the breather valve (for inhalation and exhalation) must be chosen appropriately. The nitrogen blanketing valve’s set point is set between those of the breathing valve, and the settings of the breathing valve influence the nitrogen consumption.

For process equipment or storage tanks that are difficult to seal, a concentration control method can be used for nitrogen blanketing (Figure-2): an online oxygen analyzer is installed to continuously monitor the oxygen concentration in the gas phase of the equipment or container, and accordingly control the flow of nitrogen.

This method saves nitrogen and allows for a quick return on investment for the control system.

In chemical storage tank applications, the choice of nitrogen purity largely depends on the chemical properties, reactivity, and safety requirements of the stored substance. Here are some common uses of different purity levels of nitrogen gas in chemical storage tanks:

This level of purity is typically used for storing less sensitive or low-reactivity chemicals. These applications do not require complete elimination of oxygen but need a cost-effective solution to reduce oxygen concentration and thus lower reactivity:

This level of purity is suitable for storing chemicals that require higher safety and purity standards. These applications typically involve more reactive chemicals or higher safety requirements:

The highest purity nitrogen is typically used for extremely sensitive or high-value chemical storage, or in other special applications requiring high purity:

Nitrogen blanketing is an essential safety and quality control measure in various industries, particularly where chemical stability and safety are critical. The process involves using nitrogen to displace air in storage tanks or process equipment to prevent oxidation, degradation, and explosive reactions caused by the presence of oxygen and other reactive components of air.

Choosing the correct nitrogen purity depends on the specific requirements and sensitivity of the materials being handled. Lower purity nitrogen may suffice for general inerting of non-reactive substances, while higher purity levels are necessary for sensitive chemicals and applications requiring stringent control of contamination and reactions.

Ultimately, the implementation of nitrogen blanketing systems must be carefully designed and monitored to ensure they meet the safety, quality, and economic objectives of the facility, balancing effectiveness with operational costs.