Many processors rely on blanketing or padding to protect chemicals stored in tanks against contamination, degradation or chemical change as well as to prevent fire or explosions. Blanketing involves introducing a low-pressure (typically less than a few psig) flow of an inert gas above the liquid level of the chemical to fill the vapor space at the top of the tank. Nitrogen is the most commonly used gas because it's widely available and relatively inexpensive but other gases such as carbon dioxide or argon sometimes are employed. However, carbon dioxide is more reactive than nitrogen and argon is about ten times more expensive.
Typically, tanks with fixed roofs and unsealed tanks are blanketed while those with floating roofs aren't. The 95–99.9%-pure nitrogen blanket or pad creates a slight positive pressure in a closed tank, helping prevent the ingression of ambient air that could cause oxidative degradation or combustion.
A recent accident brings home the importance of tank blanketing. A North American paint manufacturer had used blanketing to protect select solvents but never instituted the technique as a standard practice. A tank not protected by a nitrogen blanket caught fire, resulting in significant damage, downtime and loss of revenue. Fortunately, no one was hurt. Needless to say, the facility soon made tank blanketing a standard practice. Not only does this help protect plant personnel and products, but it also eliminates the need for federal agencies such as the U.S. Chemical Safety and Hazard Review Board to investigate a facility.
TANK BLANKETING OPTIONS
A storage tank can be made inert several ways. One is by reducing the oxygen content in the vapor space to a value lower than the minimum concentration that supports combustion, i.e., the limiting oxygen concentration (LOC). Another is by keeping the fuel concentration in the headspace to a value lower than the minimum concentration that supports combustion, i.e., the lower explosive limit (LEL) or lower flammability limit. The third and final option is to increase the fuel concentration in the headspace to a value greater than the maximum concentration that supports combustion, i.e., the upper explosive limit (UEL) or upper flammability limit. A material's flammability envelope is bounded by its LEL, UEL and LOC. The values for specific chemicals can be found in publications such as material safety data sheets, the National Fire Protection Association's "NFPA 69: Standard on Explosion Prevention Systems" and chemistry handbooks.
How nitrogen is controlled usually depends upon the type of tank used. Methods include continuous purge, pressure control and concentration control. Continuous purge provides a constant flow of nitrogen and is probably the easiest method because a control device isn't required. However, nitrogen consumption is high. In pressure control blanketing, a valve allows the addition of nitrogen when the liquid level drops while a vent enables release of nitrogen when the liquid level rises. Concentration control blanketing relies on a feedback loop from an oxygen analyzer to the nitrogen generator to cycle the generator on or off. This method economizes the use of nitrogen because it shuts down the nitrogen supply until enough outside air infiltrates the tank headspace to raise the concentration of oxygen above the acceptable limit.
NITROGEN SUPPLY OPTIONS
Plants can obtain nitrogen supplies in several ways. These include receiving nitrogen as gas in high-pressure cylinders or as liquid in micro-bulk tanks (dewars) and bulk tanks. Nitrogen also can be generated on-site as a liquid by cryogenic means or as a gas on-demand by membranes or pressure swing adsorption (PSA). Following are the pluses, minuses and costs of each approach.
Single cylinders hold about 240 ft3 (6.8 m3) of gas at an average cost of $15 to $35 per 100 ft3 (2.8 m3) and are the most expensive option. Cylinders come in a wide variety of volumes and fill pressures. Fill pressures typically are about 2,200 psi (152 bar). Usable cylinder volume is about 205 std. ft3 (5,800 L). Note that not all the gas in a cylinder can be used. Also, high output pressures will result in even more wasted gas.
Cylinders can work well for low-flow applications such as welding that only need small amounts of gas, intermittently. However, cylinders can present safety issues, not only because of their bulk and weight. Should a cylinder be dropped, the canister can turn into a dangerous projectile.
Dewars are vacuum-insulated vessels that have an integral, internal evaporator to convert the stored liquid nitrogen to gas. Important performance parameters for dewars include flow rate and duty cycle. The internal evaporator limits the ability to maintain a relatively high flow rate. Also, because of the internal evaporator, you must allow time for the liquid to reach a sufficient temperature to ensure complete evaporation. It's not uncommon for the evaporator to frost up and lose its ability to deliver gas at the desired flow rate. A typical dewar of liquid nitrogen holds around 3,500 ft3 (99 m3). Dewars must allow some liquid to evaporate to atmosphere to relieve internal pressure. However, dewars evaporate at different rates (older ones faster than newer ones) and lose gas volume over time. Therefore, capacity depends upon how long the dewar stands unused. Typical losses are 10% or more. The average cost to the user is $1.50 to $2.50 per 100 ft3.