The Use of Nitrogen Enriched Air (NEA) to Dry, Monitor and Control Fuel Moisture and Oxidizing Atmospheres in Lubrication and Bulk Fuel Storage Tanks
A study of the process of removing water (moisture) from an above or below ground bulk fuel storage tank by pumping (purging) Nitrogen Enriched Air (NEA) into and through the tanks primary empty space (ullage). It is important to monitor and or control corrosive low PH and moist atmospheres in tanks to protect fuel storage tank containment boundary integrity and quality of fuel or product being stored. Study results reveal that the absorption of moisture by a NEA mass-controlled flow through a tanks empty space decreases the moisture content in the tanks primary empty space as well as in the fuel or lubrication product stored during the procedure. This effect being the expected and desired process study outcome. Due to the relatively dry NEA entering the ullage and displacing the moist air, reduction of the partial pressure of water at the fuel/ullage interface is realized. This shift in equilibrium condition forces water to evaporate from the fuel’s entire surface. The amount of water migrating from the fuel directly into the ullage was during the study signiﬁcant and anticipated. The rate of decrease of the moisture content in the ullage is determined by the NEA mass ﬂow rate and this was found to be the dominant contributor to the tanks ullage and stored fuel surface drying time. When pumping NEA directly into the ullage a signiﬁcant decrease of moisture was realized even when the NEA is not pumped through the fuel. Realized as well, doubling the mass NEA ﬂow rate halves the drying time. The studied process for removing moisture from stored fuel and a bulk fuel storage tanks primary empty space is particularly attractive as a method to preserve quality of product stored and control tank boundary integrity degradation requiring very little bulk storage tank design modiﬁcation.
A major consideration in the design of bulk fuel and lubricating fluid storage tanks is the purity of the product stored. The ﬁrst line of defense is a region at the bottom of fuel or product storage tank where debris and water settle from the product being stored. Fuel storage systems also contain strainers and ﬁlters that monitor and or prevent large debris, sediment and water respectively, from entering the fueling delivery system and or consumers vehicle or equipment. Diesel, kerosene and lubricating fluids in bulk storage for example can absorb moisture which often enters the fuel tank ullage empty vapor space through the vent riser at the top of the tank.
Dissolved water (moisture) to a regulated degree is not considered to be a contaminant and is typically vaporized during combustion. During temperature and pressure ﬂuctuations however, moisture can separate from the solution forming a water-in-fuel emulsion: small water droplets suspended in the fuel. Separated water is a contaminant and can cause several problems.
Water that is drawn into a fuel and or lubrication system can interrupt the smooth and efﬁcient operation of the engines.
With temperature drops below product stability thresholds, free and entrapped water can gel or form ice, which can then attach onto the fuel lines and strainers introducing blockages.
Water also promotes the growth of bacteria, which can also block ﬁlters and create low PH and corrosive empty space atmospheres.
Due to the relatively greater density of water, these droplets gradually move down towards the tanks bottom forming a separate layer that may or may not be easily be removed from the system. The time of this settling process, as determined by the water and fuel density, viscosity and size of the water droplet and temporal condition can range from minutes to weeks. Such unpredictable and possible lengthy timescales are or can be unacceptable.
The main protocol for reducing the amount of moisture entering the fuel and or product storage tank is to reduce the tanks empty space, monitor and control the moisture in that space. Reducing the size of the ullage, and therefore, the amount of water vapor that can be dissolved into the fuel could be considered unreliable, as the fuel can also be contaminated with water during production and transport.
**The method being studied, and method suggested here involves pumping nitrogen enriched air (NEA) through a fuel or lubrication bulk storage tanks Empty (ullage) Space. The idea that, due to the absence of water in the NEA there will be absorption of any moisture in the empty primary storage space of the tank. ***NEA entering the ullage in a mass-controlled flow will exit the system through the tank vent riser carrying absorbed moisture (removing water) from the fuel tank.
The overall aims of the present study are to optimize a bulk fuel storage tanks ullage for a tank and fuel drying process that;
- Adequately monitors and removes moisture from the storage tanks primary empty space,
- Adequately monitors and controls excessive moisture dissolved in the product being stored.
- Adequately monitors and prevents in a tanks primary empty space, moisture and the formation of degradation low PH acidic atmospheres,
- Ultimately protecting and preserving quality of fuel or lubrication product being stored.
Objectives of Investigative Study and Testing:
Change in water (moisture) concentration in a fuel storage tanks ullage. The change in water concentration in the fuel storage tank away from equilibrium should be mitigated by the migration of water from the fuel at the fuel surface/ullage interface into the inert dry moisture purge gas flowing through and out of a tanks empty space. It is expected that the migration of moisture from the fuel at and over the fuel surface/ullage interface will reduce by evaporation the concentration of dissolved water (moisture) in the fuel itself.
Study and testing of theory will confirm that as reasonably expected, fuel drying times will correlate to a mass flow rate of dry inert gas (nitrogen) flowing through the empty space (ullage) of a fuel storage tank. As water concentration in fuel is reduced it is expected that a concentration difference between the ullage and fuel surface/ullage interface will be realized. Evaporation occurring on the entire fuel/ullage interface is the most important factor for the drying effect and reduction of water dissolved in stored fuel.
*Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase after reaching its boiling point. The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other based on how they collide. When a molecule near the surface absorbs enough energy to overcome the vapor pressure it will “escape” and enter the surrounding air as a gas.
**Henry’s law States that: At a constant temperature, the amount of a given gas dissolved in each type and volume of liquid is directly proportional to the partial pressures of that gas in equilibrium with that liquid.
***Fick’s law relates the diffusive flux to the concentration under the assumption of steady state. It postulates that the flux goes from regions of high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient (spatial derivative), or in simplistic terms the concept that a solute will move from a region of high concentration to a region of low concentration across a concentration gradient.
****Raoult’s law states that the vapor pressure of a solvent above a solution is equal to the vapor pressure of the pure solvent at the same temperature. This is called phase equilibrium, because the only thing changing is the phase that the substance is in. This leads to the conclusion that there is a vapor present above any liquid. The vapor pressure of a liquid is the pressure exerted by the vapor above the liquid.
HOW MUCH WATER IS OK?
The easiest answer would be none. All diesel contains some percentage of water. The most important thing is to keep the water below its saturation point so that it stays dissolved rather than entering equipment as free water. Equipment manufacturers specify that ZERO free water must reach the engine. Saturation points vary from roughly 50 ppm to 1800 ppm based on temperature and on the petroleum diesel/biodiesel ratio. As you can see on the chart following, biodiesel can hold significantly more water in saturation than its petroleum equivalent. Blending bio and petroleum diesel together, however, does not result in a mathematically proportional moisture content. The blend will hold less in solution that the sum of the parts, meaning that free water precipitation may occur when the two are mixed.
The best way to remove large volumes of settled water is to drain the tank. Ambient moisture and condensation can be prevented from entering the diesel fuel storage tank using good desiccant breathers in combination with and or a blanket of dry air (or nitrogen) fed into the tank’s headspace and out through the vent riser. The relative humidity of the diesel will tend toward the relative humidity *** (or “dryness”) of the air. Moisture in the diesel will, with time, be released back into the dry air until the diesel is just as dry as the air.
Mid-South Steel Products, Inc.