Nitrogen displacement

An oil tank purged with nitrogen may contain a volatile medium; is it flammable and explosive? Can it be purged with nitrogen until a safe level is reached, and then purged again with nitrogen? This is because it is related to the residue amount in the pipe, its volume, and the amount of air intake and exhaust. It’s difficult to draw a conclusion regarding this dosage through calculation. Can I use water? The properties of the medium?

Assume that the volume of the oil tank is V (m3), and the gaseous phase remaining in the tank is mainly propane; now nitrogen is to be used for displacement, with a volumetric flow rate of Q (m3/min) during this process. During displacement, the gas in the oil tank flows out of the storage tank at the same flow rate. Assuming that the replacement process is smooth, that is, the gas composition is uniform throughout the tank, the gas composition expelled at any given moment will be the same as that inside the tank. The temperature of the gas used for displacement is set at T1 (K), and the pressure is at standard conditions of 0.1013 MPaG. The time required to displace 99% of the propane in the tank is denoted as t. Then: 1) The amount of substance in the gas phase filling the tank is n = V * (273/T1) * (1/22.4) kmol. 2) The molar flow rate of the fluid in and out is equal, namely m_in = m_out = Q * (273/T1) * (1/22.4) kmol/min. 3) The displacement time can be calculated using ln = -(m/n)*t; by solving this equation, the displacement time t can be determined. [Here, the principle of differentiation is applied, with a particular instant of displacement considered as a differential element.] This calculation is a typical example of unsteady-state mass balance. By using the formula above, you can assume the replacement time you plan for and the desired degree of replacement, and then determine the minimum required flow rate of nitrogen, Q. I hope this helps.

From this perspective, what is your replacement method: pressure increase/decrease, continuous replacement, or vacuum extraction? . .

It requires such complicated calculations. Replacement is carried out using pressure filling and release. For example, if the tank has a capacity of 30 cubic meters and is pressurized to a medium concentration of 2 kilograms, the nitrogen lost due to dilution will be three times that amount, resulting in 60 cubic meters. Repeat this process until the concentration required for safety is reached.