PTQ Q1 2024 Issue

equivalent displacement, especially when the assumed operating or ambient temperatures do not align with stand- ard or normal conditions. The calculated inbreathing con- siders the assumption of ambient airflow through the tank vent, where it is customary to consider the ambient air to be at normal or standard conditions. In cases where a medium other than air is utilised for vacuum relief, it might be necessary to convert the rate to an air-equivalent flow. Adjustments may be required for inbreathing if the inbreathing medium significantly differs from air. However, no adjustments are necessary for nitro - gen, as the molecular weight of nitrogen (28.02) and air (28.96) exhibits only a marginal difference (3.3% variation in molecular weight). In conclusion, facilities should assess the nitrogen inbreathing requirement for tanks using the appropri - ate method as described earlier. By doing so, they can potentially achieve significant reductions in overall nitro - gen requirement, leading to substantial savings in Capex and Opex while ensuring safety and tank integrity are not compromised. This prudent approach allows for efficient resource allocation while maintaining the highest standards of operational safety. Technical nuances and best practices in refinery units  Gaseous nitrogen for purging One of the most frequent uses of nitrogen is to purge equip- ment of explosive or hazardous vapours before lining up the vessel after maintenance or handing it over for maintenance. This is usually done using a pressure/de-pressure cycle. The cycles needed to ensure the oxygen concentration is below the Lower Explosive Limit (LEL) can be determined through the equation:

100% 98%

94%

100

87%

78%

66%

47%

33%

Vessel pressure (bara)

Figure 1 Reduction of nitrogen flow rate vs vessel pressure

a) Flow reduction: As equipment is pressurised, a notable decrease in the nitrogen flow rate is observed, particularly when vessel pressure exceeds half of the header pressures. This phenomenon can be seen in Figure 1 , which highlights the decline in flow rate under various pressures where the nitrogen header pressure is at 9 bara. b) Excessive consumption: To estimate the nitrogen vol- ume needed to achieve an oxygen content below 4 mol%, a comparative estimation is carried out for three different scenarios. In each scenario, the first cycle of nitrogen pres - surisation of the vessel, which is full of air, is considered. The vessel is pressurised from atmospheric pressure to different pressurisation scenarios. Here are three scenarios illustrating the differences:

Scenario 1: Pressurised to 9 bara Scenario 2: Pressurised to 5 bara Scenario 3: Pressurised to 3 bara

Upon reaching the targeted pressure, if the oxygen con- tent is below 4 mol%, the vessel is depressurised to its normal operating pressure of 3 bara, making it ready for hydrocarbon introduction. Otherwise, it is brought down to 1.5 bara and then pressurised to start the next cycle of pressurisation/depressurisation. This cycle continues until the oxygen content meets the desired threshold, after which the vessel is pressurised to 3 bara using nitrogen. The results are shown in Table 4 . In Table 4, it can be observed that for Scenario 1, the required nitrogen volume is the highest, being 1.5 times that of Scenario 2 and two times that of Scenario 3, even though only one cycle of pressurisation with nitrogen is required in Scenario 1. Key takeaway: Limiting pressurisation to half of the nitro- gen header pressure is both cost-effective and efficient, as supported by the provided scenarios.  Start-up nitrogen for hydroprocessing units’ high-pressure (HP) section The start-up of hydroprocessing units demands a sub- stantial amount of nitrogen, primarily for leak tests and inertisation. For a typical 1,000 m 3 volume hydroprocess- ing high-pressure loop, three cycles of pressurisations and depressurisations between 1.5 barg and 5 barg are essential. The goal is to reach O 2 <0.5 mol%, necessitating approximately 14,000 Nm 3 of nitrogen.

n = log[(Ci-Cn)/(Cf-Cn)]/log (Pf/Pi)

Eq. 3

Where: Cn = mol% oxygen in nitrogen

Ci = mol% oxygen initially in space to be purged Cf = mol% oxygen finally in space to be purged Pi = Initial pressure in bara Pf = Final pressure in bara n = Number of pressure/de-pressure cycles

For oxygen removal from equipment, a method involv- ing multiple pressurisation and depressurisation cycles is employed by refiners. The equipment is first pressurised to the maximum nitrogen header pressure. It is then depres- surised from several points until a slight positive pressure of approximately 0.5 barg is reached. This process of pres- surising and depressurising is repeated until an oxygen concentration below 4 mol% is achieved. For some cases, like reformer or hydrotreater, target oxygen is less than 0.5 vol%. Once this level has been reached, the equipment is pressurised to its normal operating pressure using nitro- gen, ensuring it is prepared for hydrocarbon introduction. The method of pressurising equipment to the maximum nitrogen header pressure is associated with the following drawbacks:

PTQ Q1 2024