separation processes are used to produce gaseous nitrogen and oxygen. They
are cost effective choices when demand is relatively small (tens of tons
per day) and when very high product purity is not required. A typical
purity for non-cryogenic oxygen systems is 93%. Non-cryogenic
nitrogen systems typically produce product at purity between 95% to 99.5%.
Non-cryogenic air separation plants are compact and operate at near-ambient temperature and pressure. Once installed, they can usually be brought on-line in less than half an hour.
Unlike cryogenic plants which use the difference between the boiling points of nitrogen, oxygen and argon to separate and purify those products, non-cryogenic air separation processes use physical property differences such as molecular size and mass, to produce nitrogen and oxygen at sufficient purity to meet the needs of many types of gaseous nitrogen and oxygen users.
The most common technologies are PSA (Pressure Swing Adsorption), which is used to produce nitrogen or oxygen (using different adsorbants); VPSA (Vacuum-Pressure Swing Adsorption), which is used in to produce oxygen; and Membrane Separation, used to produce nitrogen gas.
Nitrogen or Oxygen Generation Using Pressure Swing Adsorption (PSA)
PSA technology can be used to produce either nitrogen or oxygen
by passing compressed air, at several atmospheres, through a vessel containing adsorbent materials.
Adsorbants are chosen on the basis of their adsorption characteristics. A
desirable adsorbant will have much greater affinity for non-product
molecules than for the product gas (nitrogen or oxygen). This
characteristic results in most of the desired molecules passing through
the bed and into the product stream, while undesired components (product
captured by the adsorbant.
The PSA process is inherently a batch process, as the adsorbent bed requires periodic desorption. Consequently, PSA systems usually contain two adsorbant vessels to provide operational continuity. At any time, one of the vessels will be making product by adsorbing undesired components of air; while the other vessel is undergoing regeneration by de-pressurization to atmospheric pressure. When the adsorbing vessel approaches saturation, a set of valves quickly switches the services. A surge vessel (buffer vessel) downstream of the adsorbers ensures that delivery of product is continuous. While the two-bed system is most common, mono-bed and three-bed configurations are sometimes used. The mono-bed system provides capital savings while the three-bed system provides for greater continuity of production.
Nitrogen PSA vessels contain an activated carbon molecular sieve material which removes oxygen and other undesired components by adsorption. Nitrogen is typically delivered from the production unit at pressures of six to eight atmospheres and at a purity of 95 to 99.5%. If higher purity is required, both the equipment size and the ratio of air feed to product make go up. Alternatively, a "de-oxo" unit can be added that catalytically combines hydrogen with the oxygen in the nitrogen product leaving the PSA, producing water, which is removed by cooling and additional adsorption.
The waste stream from a nitrogen membrane or PSA plant is enriched in oxygen - often to about 40% oxygen. This stream is sometimes used for combustion enhancement or waste treatment equipment operation at the same site.
Oxygen PSA units typically use alumina to remove much of the water vapor from the air feed, together with zeolite molecular sieve to adsorb nitrogen, carbon dioxide, residual water vapor and other gases. Typical oxygen PSA delivery pressures exiting the unit are one to three atmospheres. Oxygen purity is typically between 90 to 95%; limited primarily by the argon content, which will be 4.5 to 5%.
Oxygen by Vacuum-Pressure Swing Adsorption (VPSA, VSA or PVSA)
Overall, VPSA systems are more costly to build, but more energy efficient than PSA systems for the same product flow, pressure and purity conditions.
VPSA units regenerate the sieve material under vacuum conditions because it results in more fully regenerated molecular sieve material, which is more selective than material subjected to the classic PSA regeneration process. As a result, a higher percentage of available oxygen is recovered, and less air has to be processed. Air compressor power is greatly reduced compared to an oxygen PSA unit due to lower air flow and lower compressor discharge pressure (typically less than half an atmosphere, gauge). There is an off-set to the air compression power savings however, due to power needed to operate the vacuum-generation machinery.
Oxygen VPSA units are usually more cost effective than oxygen PSA units when the desired production rate is greater than about 20 tons per day. They are often the most cost-effective oxygen production choice up to 60 tons per day or more, providing high purity oxygen is not required. Above about 60 tons per day, cryogenic plants are usually the oxygen production technology of choice, although in some cases, two oxygen VPSA plants allow for better matching of large step-changes in demand. VPSA specific power is about one-third less than that for oxygen PSA plants. It is usually similar to the specific power of a comparable capacity cryogenic oxygen plant.
Units using this type of non-cryogenic oxygen production process may be referred to as VPSA (Vacuum Pressure Swing Adsorption), VSA (Vacuum Swing Adsorption) or PVSA (Pressure-Vacuum Swing Adsorption) systems.
Nitrogen Membrane Systems
|Membrane nitrogen generators use tube bundles made of special polymers,
configured in a manner similar to a shell
and tube heat exchanger. The air separation principle is that
different gases have different permeation rates through the polymer film.
Oxygen (plus water vapor and carbon dioxide) are considered "fast gases"
that diffuse more rapidly through the tube walls than the "slow gases" argon and nitrogen.
This allows dry air to be converted to a product that is an inert mix of mostly nitrogen gas
and argon, and a low-pressure "permeate" or waste gas that is enriched in oxygen
(plus water vapor and carbon dioxide) and vented from the shell.
Nitrogen product emerges from membrane units at close to the compressed air feed pressure. In many applications this means that no supplemental product compression is required. Because there are no moving parts in the separation process, membrane units can be rapidly activated when needed and shut down when they are not.
Membrane separation units are typically made in standard-size modules, with nitrogen production ratings that depend upon the desired nitrogen purity. For a given standard module, nitrogen production rates increase with higher inlet air flow rates, but the purity of the product decreases. When required production capacity (at a specified purity level) exceeds the largest standard module size, a number of smaller units will be manifolded to allow them to operate in parallel.
Membrane units are most cost effective for relatively low demand applications. Because larger-capacity units are typically made up of multiple smaller capacity modules, membrane units have a close-to-constant cost per unit of separation/ production capacity over a wide range of production rates, in contrast to the declining cost for marginal capacity that is typical with PSA nitrogen generators and cryogenic air separation/ nitrogen generator systems.
There is a trend, however, toward using membranes at higher production rates. This has been made possible by manufacturers increasing the physical size and production capacity of their largest modules.
Universal Industrial Gases, Inc.
Universal Cryo Gas, LLC
2200 Northwood Ave. Suite 3
Easton, Pennsylvania 18045-2239 USA
Phone (610) 559-7967 Fax (610) 515-0945
All material contained herein Copyright 2003 / 2008 UIG.