Feb-2023

Optimising nitrogen utilisation in refinery operations

Technical aspects and insights on managing nitrogen by considering actual operational scenarios.

Rajib Talukder and Prabhas K Mandal
Aramco

Article Summary

Nitrogen gas, known for being chemically inert, non-flammable, colourless, odourless, and slightly lighter than air at atmospheric temperatures, has been extensively used in the chemical and process industry for many years. The oxygen content in empty vessels, equipment, and pipeline spaces is reduced by it to facilitate start-ups, or hydrocarbons are driven out by it on shutdowns. Due to its non-reactive nature and low solubility in liquids, it is commonly chosen as a blanketing gas, thereby virtually eliminating any risk of product contamination.

In refineries, most of the nitrogen is consumed in gaseous form. Liquid nitrogen is stored and then vapourised into gaseous nitrogen as needed. The base nitrogen demand load for a refinery during normal operation is met by gaseous nitrogen, a demand that is an order of magnitude smaller than the peak load observed during plant shutdowns. The peak load demand is met by liquid nitrogen.

This article does not address the methods by which the overall base or peak nitrogen demands of a refinery are determined. Instead, attention is given to several major contributors to the base or peak load where significant optimisation opportunities are believed to exist due to a high degree of conservatism associated with these contributors. In the following sections, major contributors, such as the nitrogen required for tank blanketing, surge drum blanketing, and starting up hydroprocessing units, are addressed.
Nitrogen blanketing is utilised in vessels/tanks containing liquids, such as a surge drum or tanks, for the following reasons:
• Safety: The use of nitrogen lessens the chance of oxygen penetration, thereby disrupting the formation of the fire triangle (fuel, heat, oxygen). This is especially relevant for vessels containing flammable liquid hydrocarbons.
• Protection against oxidation: Preventing oxygen from entering hinders the oxidation process, which could otherwise harm the quality of the liquid inside the vessel. This is particularly important for lean amine and wash water surge drums.
• Prevention of vapour loss: A nitrogen barrier restricts the amount of hydrocarbon vapour that can leave the vessel.

Estimation of nitrogen blanketing for surge drums
By maintaining an inert atmosphere over the stored liquid, nitrogen blanketing ensures quality, regulates pressure, and prevents incidents.
To correctly estimate the flow rate of the blanket gas, it is important to keep the pressure inside the vessel within safe and operational limits. Here is a basic guide on how to measure the blanket gas flow rate:
• Assess volume changes due to liquid outflow: The initial step involves identifying the maximum liquid volume that will exit the vessel over a certain period. This shift in volume results in a change in pressure inside the vessel, necessitating compensation via the blanket gas to maintain positive pressure. The Ideal Gas Law is used to convert the change in liquid volume into the needed gas volume.
• Consider pressure changes due to liquid contraction: The impact of major pressure changes in the vessel resulting from variations in incoming feed temperature is also accounted for, such as the volume contraction caused by the entry of cold feed when the supply of hot feed is interrupted.

When calculating the total need for blanketing gas for any surge drum, thermal inbreathing is generally not considered along with normal inbreathing resulting from liquid movement from the vessels, unlike in the case of tanks. This is primarily because the empty vapour space in a surge drum is much smaller than in tanks, and the likelihood of a coinciding loss of liquid inflow and contraction of the vessel’s vapour due to sudden ambient cooling from rain is extremely low.

However, it is essential to note that nitrogen blanketing of vessels is not designed to address certain circumstances:
• Using blanket gas as a safeguard for the vessel design for full vacuum: A total feed failure to the surge drum during the vessel’s emptying process is considered a severe situation. In such cases, according to API 521 standards, blanket gas cannot be used as a safety measure since control action credits (for instance, the blanket gas control valve) are not acknowledged as safety precautions. The vessel should be either constructed for full vacuum to ensure its safety or equipped with a safety instrumentation system to avert a vacuum under these emergency circumstances.
• Supporting NPSHa for the pump connected to the surge drum: While estimating NPSHa, the blanket gas’s solubility in the liquids being pumped is considered, and the vapour pressure is presumed to be higher than the actual vapour pressure, equal to the surge drum’s normal operating pressure. This is done to account for slight degassing in the liquid and to prevent pump cavitation.

Calculation of volume changes due to liquid outflow
The volume changes due to liquid outflow can be estimated using two different methods:
• Method 1: The flow of nitrogen blanketing is calculated to maintain the normal operating pressure in the surge drum when the outflow from the drum is continuous, but the inflow to the drum ceases. This estimation uses the API 2000 liquid outflow method. Licensor does not normally include thermal inbreathing.
• Method 2: The nitrogen blanketing flow is calculated to maintain the vessel at a slightly positive pressure (1.1 bara) when the outflow from the drum is continuous, but the inflow to the drum fails. This is calculated based on the vapour volume change due to the decrease from the normal liquid level to the very low liquid level at which the connected pump is stopped.

The operation of the surge drum is different from that of tanks. While tanks are either in receiving or dispatch mode, surge drums are always in both receiving and dispatch mode. They are frequently positioned between process units to help mitigate the impact of flow rate variations between interconnected process units. Unlike the typical control objective of maintaining a measurement at a set point, the goal of surge drum level control is to buffer the changes in controlled flow while keeping the liquid level in the vessel within limits. For surge drums, it is usually more important to allow levels to ‘float’ to minimise flow rate variations. Therefore, the level controller must permit this movement and try not to hold the level close to its set point. Instead, the controller should keep the surge vessel’s level between its upper and lower limits with the least possible change to its flow output.