GENERON® IGS announces the use of Dehydration Membranes as a pre-treatment step to extend the operating range of Membrane Nitrogen Generators.
Houston, Texas April 29, 2005 – Generon® IGS, an affiliate of Innovative Gas Systems, has developed a new family
of Hollow Fibre Membranes designed to dry compressed air. These dryers have many industrial applications. Generon® IGS
has applied this unique technology to the production of Nitrogen by means of Membranes.
System Pretreatment is the Key for Membrane Produced Nitrogen
Membrane technology has largely emerged as a viable and versatile source of utility nitrogen over the years because
of the proper use of feed air pretreatment systems. The life and cost effectiveness of an on-site air separation
membrane system depends on the air pretreatment system that precedes it. This article outlines the importance of
existing pretreatment systems and introduces a new pretreatment option that uses membrane technology as a replacement
to refrigeration driers that promises to give the customer a more economical and reliable air separation system.
Introduction:
As we enter into our 20th year in offering non-cryogenic equipment for the on-site generation of industrial gases,
Generon® IGS has proven to be the industry leader in membrane produced nitrogen. With applications ranging from Oil
and Gas, to Marine, to Chemical Inerting, and to food preservation, 80-100 bscf of nitrogen gas is produced annually
using our products1.
Success in these applications has come from providing a portable, reliable and easy to operate technology at reasonable
costs. Membrane systems are inherently simple to understand and use. Our feed gas is all around us, air, and once it
is compressed and pretreated it is easily converted into a high purity nitrogen stream using current membrane technology.
While 95% nitrogen purity is adequate for most applications, today’s membranes can deliver 99.99% nitrogen purity with
little difficulty. The systems are modular so increasing capacity is as simple as adding modules.
The heart of the membrane system is of course, the membranes. The membranes work by selectively allowing O2 to
permeate faster than N2. Compressed air is fed to one side of the membrane and the O2 is selectively stripped from
the feed stream leaving an enriched N2 stream. By controlling the flow of high pressure feed passing over the membrane
surface, one can remove a small percentage or nearly all of the O2 from the stream. Controlling this flowrate allows
one to accurately control the product N2 purity. It is a simple one-valve operation. The pressure and temperature of
operation, the desired purity and the desired product flowrate determine membrane requirements. Fewer membranes are
required to produce lower purity N2 since the product flow can be increased with less O2 removal required. Higher
operating pressure and temperature can also reduce membrane requirements but they also lead to increased compressor
requirements. An optimum set of operating conditions is chosen for a particular application to minimize the cost of operation.
Membrane systems can also be compact and portable making them attractive for remote locations. Portable containers,
10’x 20’, have been made that can produce over 1500 scfm of 95% N2. All that is required is for the container to be
provided compressed air. Other nitrogen producing technologies, Pressure Swing Adsorption e.g., require nearly 10x the space.
Importance of Feed Air Pretreatment:
While membranes are generally simple to use and understand, there are some key requirements for reliable operation of
the membrane systems. The good news is that these requirements are well known. Liquids, water or oil, which are
typically found in air exiting a standard air compressor, can diminish the performance of the modules. This contamination
can lead to permanent performance loss so pretreatment of the compressed air going to the membranes is vital2. A typical
pretreatment system will use a moisture separator and/or refrigeration drier, coalescing filters, heaters and carbon
beds3 to protect the membrane modules. Long-term life for the modules is therefore dependent on the design and maintenance
of this pretreatment system. In well designed and maintained systems the module life can be as long as 10 years. In poorly
or under-designed systems, the module life can be measured in months.
Most compressed air is generated using oil flooded screw compressors, which super-saturate the air with both water and
compressor oil. The first part of the pretreatment system, the moisture separator along with the coalescing filters, is
designed to remove bulk liquids and particulate liquids from the air stream. For systems using a refrigeration drier,
the air is chilled to ~40F prior to removing the liquids so the air exits the drier at a water and hydrocarbon dewpoint
of 40F. Without this drier, the air is saturated exiting the coalescers and any further lowering in temperature will
lead to liquids forming in the feed air which present problems for the subsequent carbon bed and modules. This is why a
heater is required if a refrigeration drier is not used. Furthermore, the carbon bed works best if the air it is treating
has at least 7C of superheat (relative humidity less than 65%). The carbon bed is used to remove hydrocarbon vapors which are
typically ~.25 ppm after the coalescers but need to be lowered further to less than 10 ppb before going to the membrane modules2.
The carbon bed also treats the air for other fugitive vapors that the air compressor may draw in and prevents these contaminates
from contacting the membranes or altering the product gas.
Of these various pretreatment operations, the refrigeration drier is often the least desired, especially for remote locations.
It isn’t needed if one uses a properly sized heater that will superheat the air going to the carbon bed and membranes but there
is a catch. The membranes are most efficient and longer-lived when operating at lower temperatures (less than 55C). At sites
where the ambient temperatures may rise above 40C one often needs to heat the air to over 55C to give enough superheat to properly
protect the modules from contamination. Compression of the air raises the air temperature significantly and typical aftercoolers
only reduce this temperature to 7-10C above ambient. Since the compressed air at this point is saturated with water vapor, an
additional 7C of superheating is required to ensure that the carbon bed works well in removing oil vapors and prevent any
contamination of the modules or product gas. This situation often limits the operation of the membrane system.
New Pretreatment Technology:
We have just finished field trials using a different pretreatment technology that would avoid this ambient temperature limitation
without the need for refrigeration dryer. This new technology is based on a membrane based air dryer. These dryers are designed
to produce very dry air (-70C ADP) without altering the O2/N2 composition of the feed stream. When operating these membrane dryers
for this application, the dewpoint only needs to be suppressed ~10C so these dryers can be very efficient. A dryer can process
10x the flow of a comparably sized air separation module with only a 10 % purge loss.
Unlike the refrigeration drier or the heater, the membrane dryer requires no power to operate. The carbon bed and air separation
membranes operate best when they have 7C or higher superheated air, while the membrane dryer can operate effectively at near
saturated conditions. The membrane dryer will add ease and robustness to the pretreatment package. It can run at temperatures
up to 60C and would allow ambient temperatures up to 55C for the entire system without operational problems.
A comparison of this membrane technology relative to the refrigeration drier and regenerative desiccant driers shows why this
technology maybe the best fit for several applications. As the below table shows, one can have lower operating costs, space
and weight requirements as well as more dependable performance with a membrane drier system. The membrane technology also
minimizes liquid wastes, if that is a concern for the end user.
Dehydration Membrane Operation:
Saturated compressed air can be stripped of water vapor using the dehydration membrane module. Water vapor is selectively
removed because of it permeates faster than air through the membrane. A small fraction of the dry product stream is used as
a sweep stream to prevent water condensation on the permeate side of the fiber.
Typical System – 500 SCFM, 40F Pressure Dewpoint
| Drier Type | Size | Weight | Power | Air Loss | Moving Parts | Waste Water* |
| Refrigeration | 9 cf | 400 lb | 4-5 Kw | 0 | Yes | 30 gal/day |
| Membrane | 4 cf | 200 lb | 0-1 Kw | 10% | No | 0 gal/day |
| Desiccant | 70 cf | 1200 lb | 0-1 Kw | 12% | Yes | 0 gal/day |
* ~ 50 gallons/day of waste water may be generated by the prefilters/moisture separators positioned upstream to the
driers for all three drier types.
Upon first glance it appears that the 10% purge loss associated with the membrane dehydration technology will force
one to use a larger compressor to get the desired product flow. This is misleading however since this technology would
allow the air separation membranes to be run at near ambient temperatures instead of at temperatures15C higher. The
lower temperature of operation not only increases the lifetime of the air separation membranes but makes this separation
more efficient (less feed flow required for a given product flow). At higher purities (99% N2 and higher), less compressed
air is required with this type of system and at 95% N2 the compressed air requirement is only 3-4% higher; not the 10%
suggested by looking at the purge loss of the membrane dryer pretreatment alone.
Summary:
With membrane dryer technology, one can operate air separation membrane systems in very hot environments without the
need of refrigeration drier and still maintain system reliability and long term performance. The membrane drier is
compact, light-weight, has no moving parts and promises low operating costs relative to competing technologies.
This new technology will further expand the already large market for membrane based air separation systems. It can
also be used as a stand alone technology for the production of compressed dry air.
- Internal estimates of Generon® IGS based on module placements
- “Evolution of membranes in commercial air separation?, Journal of Membrane Science, Vol. 94, pp 225-248.
- Airliquide, Praxair, Generon® IGS websites
All of the IGS facilities are ISO 9001 Certified.