Bulk tanks containing liquid nitrogen generally are between 3,000 gal (11,356 L) and 11,000 gal (41,639 L) in size. Bulk vessels comprise a vacuum-insulated liquid storage tank housed in an outer carbon steel shell. Bulk tanks, like the smaller dewars, must leak nitrogen to prevent pressure buildup. A rating plate on the vessel exterior indicates tank capacity. A typical installation includes the bulk vessel and a finned heat exchanger or evaporator, both of which are located outdoors, as well as pressure regulation to the application and remote monitoring of the liquid level. The evaporator, which is located next to the tank on the delivery line, relies on the outside temperature to heat up the liquid nitrogen. The evaporators often frost up on humid days and become inefficient.
Bulk tanks typically are rented. However, prior to a tank's installation, a plant will incur costs to pour a concrete slab to support it. The cost of nitrogen depends upon so-called "vaporization units" that relate to how much of the gas the user purchases annually. As of this writing, gas costs in the U.S. range from $0.35 to $0.90 per 100 ft3.
Delivering nitrogen as a liquid in micro-bulk or bulk tanks especially suits low-temperature applications such as flash freezing. By comparison, other freezing methods are slow.
Generating nitrogen on-site via a cryogenic plant usually is reserved for facilities such as large chemical complexes in remote areas. Air is liquefied by cooling to cryogenic temperatures, and split into argon, nitrogen and oxygen. The other gases and impurities are scrubbed out. An on-site cryogenic plant is best when a facility needs a high flow of nitrogen continuously and can utilize both the oxygen and nitrogen generated. An upside is that the facility doesn't pay for delivery and has almost a limitless supply of nitrogen. The production cost for nitrogen generally runs between $0.08 and $0.15 per 100 ft3. However, the approach requires a capital investment in a generator and possibly investment in a larger air compressor.
At $0.15 or less per 100 ft3, on-demand nitrogen generators often represent the most-cost-effective option for applications such as chemical blanketing of flammable liquids that need a steady flow of nitrogen. Note that on-demand generators provide gas at ambient temperature and so can't be used for freezing or refrigerant-type applications requiring cold liquid nitrogen. The method frees the plant from relying on outside suppliers and the attendant problems such as long-term purchase contracts and commitments, inflexible delivery schedules, price increases and long procurement processes. Moreover, adding an additional work shift doesn't require investment in more equipment. In contrast, adding shifts with a bulk delivered gas will double or triple the volume of gas that must be bought, increase the number of deliveries, etc.
An on-demand nitrogen generator also can help a plant meet sustainability objectives. Eliminating the need for another facility to generate nitrogen, store it and truck it reduces the entire carbon footprint required to supply nitrogen to the plant. Compared to third-party supplied bulk nitrogen, generation of 99.9%-pure nitrogen on-site with a PSA system uses 28% less energy. This lower demand for electricity translates into decreased creation of greenhouse gases.
ON-DEMAND NITROGEN GENERATORS
Nitrogen gas generators typically are free standing, housed in a cabinet or skid mounted, depending upon the application. A plant only needs to connect a regular compressed air line to the inlet of the generator (after ensuring that a sufficient supply of compressed air is available) and attach the outlet to a nitrogen line. Standard features often include high-efficiency coalescing pre-filters with automatic drains and a sterile-grade after-filter.
On-demand generators rely on either membrane or PSA technology. The choice largely depends upon the purity of nitrogen required for blanketing. Typically, applications such as fire prevention only need nitrogen of 95% to 98% purity and can use membrane generators. Applications such as the blanketing of oxygen-sensitive compounds, specialty chemicals and pharmaceuticals demand a higher purity stream and require use of PSA generators.
As an example of how membrane nitrogen generators work, a Parker Balston generator (Figure 1) separates the compressed air into component gases by passing the air through proprietary semipermeable membranes consisting of bundles of hollow fibers. Each fiber has a circular cross-section and a uniform bore through its center. Compressed air is introduced into the bore of the fibers at one end of the membrane module. Oxygen, water vapor and other gases permeate the membrane fiber wall and are discharged at low pressure through a permeate port; the nitrogen remains within the membrane and flows at operating pressure through the outlet port. The exiting nitrogen gas stream is 95%–99+%-pure and very dry, with a dewpoint of at least -58°F (-50°C). Membrane units need no electricity to generate nitrogen, so they can be used in Class 1 explosion-proof environments without any concerns.
In contrast, the Parker PSA nitrogen generator shown in Figure 2 uses high-efficiency pre-filtration to remove all contaminants down to 0.01 micron from the compressed air stream. The filters are followed by dual beds filled with carbon molecular sieves (CMS). In one bed at operating pressure, the CMS adsorb oxygen, carbon dioxide and water vapor. The other bed operating at low pressure releases the captured oxygen, carbon dioxide and water. Cycling the pressures in the CMS beds causes all contaminants to be captured and released while letting the nitrogen pass through. A final sterile-grade filter ensures removal of any microbial contamination. You can easily set purities with a flow control valve. For example, the DB-30 nitrogen system produces a flow of nitrogen as great as 1,530 std. ft3/h at 99.9% purity. The unit can achieve higher flow rates if gas of lower purity is acceptable. A built-in monitor measures the oxygen concentration of the nitrogen stream. The system requires a minimum feed pressure of 110 psi and can operate at pressures up to 140 psi. The resulting nitrogen has a dewpoint as low as -40°F (-40°C).