PSA Nitrogen Generation

IntroductionPressure Swing Adsorption (PSA) is a nitrogen gas generation method with a specially designed adsorbent. This adsorbent is

called a Carbon Molecular Sieve (CMS) having micro pores in its surface to adsorb O2, CO2 and H2O molecules when under a

certain pressure. After the adsorption process, the adsorbent is regenerated by depressurizing the vessel containing the

adsorbent. PSA can produce the nitrogen gas continuously by repeating above adsorption and regeneration.

The use of the PSA process has seen immense growth during the last few decades, mainly due to its simplicity and low operating

costs. Major applications outside of nitrogen have been the recovery of high purity hydrogen, methane and carbon dioxide and

oxygen.

The number and size of the CMS will dictate the purity of the nitrogen produced and the flow rates possible. PSA has been

successfully used to produce nitrogen from a rate of 5,000 CFH to 60,000 CFH with purities ranging from 95% - 99.9995%.

　

When to Use PSAThere different options of nitrogen production, such as permeation membrane systems and cryogenic distillation. This means it is

important to first establish if PSA is the most efficient and economic method for your application. In order to do this you will need

to determine the day to day use of nitrogen that is anticipated and the purity of the nitrogen that is required. Figure 1 shows the

areas where PSA is likely to be a good option (Adsorption onsite generation).

Figure 1 – Map of areas covered by different methods1

When to Use PSAPSA can produce nitrogen in a range of purities as shown in figure 1. However, figure 1 does not take into account the financial

aspect (i.e. the cost of production). Figure 2 can give an idea of the general trend of how costs change with increasing levels of

purity and increasing flow rates. It is not always necessary to produce highly pure nitrogen. For example blanketing vegetable oils

can be done with purities of around 99.5%.

Figure 2 – Effect of Purity on cost1

The purity of nitrogen required to blanket a

flammable material is determined by the material’s

Limiting Oxygen Concentration (LOC) or Lower

Flammability Limit (LFL). These values can be

obtained from the National Fire Protection

Association’s (NFPA) NFPA 69: Standard on

Explosion Prevention Systems. In this guideline it

can be seen that sometimes the purity required can

be below 95%.

During the course of a day the demand for nitrogen

can vary. This is bad news for PSA as PSA works

most efficiently when working at its full design

capacity.

When to Use PSAIn order to gain the maximum economic benefits from an on site PSA system it is incredibly important to ensure that it is working

at maximum capacity for as long as possible. To allow this to be done it is vital to establish the flow pattern on a daily basis that is

required. As well as knowing figures like the number of hours of operation during a day, it is just as important to know the

fluctuations in the load during the day. If fluctuations are present in the load, it will be the nature of these fluctuations that will

determine the size of the PSA system, and the buffer tank if required.

PSA systems are outstanding when it comes to supplying a steady flow pattern. This kind of flow pattern allows the unit to be

sized very easily and constantly work at full capacity. When the loading pattern contains instantaneous peaks and troughs a PSA

system will struggle. Sizing for the lower demand will mean that a buffer tank will be required and sizing for peak demand will

mean the PSA system is working at part capacity or idle, bringing high operating costs. An erratic flow pattern can usually be

handled by a combination of a PSA system and liquid nitrogen supplements. PSA systems are usually aimed at having a utilisation

of 90% or higher.

Figure 3 –

Possible nitrogen

flow patterns. 1

How Does PSA WorkPSA systems are actually very easy to understand in concept. The basic description would be that air is taken from the

atmosphere, filtered to remove solids (dust particles etc.), compressed to achieve the required pressure by the PSA system, put

through the absorbent at high pressure and then either collected in a tank or distributed around the network. Figure 4 shows the

general system schematic for a PSA system.

Figure 4 - Basic Schematic for PSA system2

How Does PSA WorkLooking in more detail at the PSA system (the nitrogen generator module in figure 4) the process used by PSA can be condensed

into 6 steps. The PSA system in general is constructed of two vessels containing an absorbent. The absorbent is there to remove

Oxygen, Carbon Dioxide and other gases found in air, apart from Nitrogen, which will pass straight through the absorbent

unaffected. Only one of the vessels is in operation at any given time, while the other is regenerating.

For the purpose of explanation and clarity, from figure 5

the vessel on the left will be referred to as vessel 1 and the

vessel on the left will be referred to as vessel 2. The first

step of the process will involve pressurising vessel 1 and

depressurising vessel 2. This will allow vessel 1 to be

prepared for adsorption and vessel 2 to be ready for

regeneration. The pressure required for pressurisation can

vary from system to system, and the 8 bar indicated in

figure 5 is not compulsory.

In figure 5 the green particles are nitrogen while the red

are other gases present in air. In between the two vessels

there is a release to the atmosphere. Figure 5 – Repressurisation/Depressurisation3

How Does PSA WorkThe next step of the process is shown in figure 6. This step is

called adsorption and regeneration. Vessel 1 is now pressurised

and compressed air is being passed through it and the

absorbent is active. So at the exit point of vessel 1 pure

nitrogen is emitted. Some of the nitrogen emitted is sent to

vessel 2. Vessel 2 is depressurised so passing a nitrogen

through it will remove any absorbed gases from the absorbent

and then this is released to the atmosphere.

Figure 6 -

Adsorption and

Regeneration3

Step 3 is known as Equalisation. As the name suggests all

that is happening during this time is that the pressure in

vessels 1 and 2 are being equalised. During this step no

nitrogen is produced and no gases from regeneration are

released to the atmosphere as regeneration and absorption

will have ceased. After this step the whole cycle is

repeated but with vessel 2 doing the adsorption and vessel

1 doing the regeneration.

Figure 7 -

Equalisation3

Safety ConsiderationsDuring operation Nitrogen generators such as PSA system can produce oxygen rich and nitrogen rich atmospheres. This is

particularly dangerous when the system is housed within a building. It is important that the area the system is housed in is well

ventilated and provided with adequate fire protection. This is because if an oxygen rich atmosphere is produced there is a

significantly increased risk of fire. It is recommended that the oxygen concentration within the building should not exceed 23.5%.

As there is also a possibility of nitrogen rich atmospheres being produced, hence a reduction in the oxygen level, the oxygen level

should be monitored to ensure it stays above 19.5% as lower than this constitutes hazardous working conditions.

When installing the PSA it is vital to install proper ventilation, always ensure the nitrogen exhaust and the waste gases are piped

out of the building. Sensors for the oxygen level should also be linked in with an alarm system, and signs should be posted to warn

personnel of a possible oxygen deficient area.

An important note would be to consider having an oxygen level alarm in rooms where tools

are powered by gases other than oxygen, as leakages from pipes and tools can also result in

an oxygen deficient area.

If it is not possible to maintain an oxygen level that is adequate then the correct breathing

apparatus should be provided. As previously mentioned that it is possible to have an oxygen

enriched area, in which fire hazards are increased, it is recommended that personnel working

in the building where the PSA is housed should wear flame retardant clothing such as NOMEX,

and that the building and directly surrounding area be kept free of hydrocarbons and other

combustible materials.

Installation 1The first part of installation is site selection. One of the primary factors in site selection is the quality of air. Many PSA systems are

located in or near industrial areas, resulting in the air quality be fairly low. It is likely the air will be contaminated with

hydrocarbons, acid gases and particulate matter. This will have a negative affect on the operation of the PSA system and in the

case of hydrocarbons and other flammable particles could have a serious impact on the safety of the operation of the PSA system.

In industrial areas some level of contamination is to be expected, and the manufacturer should be consulted to determine

acceptable levels of contamination.

When selecting a site, survey of the surrounding area should be performed. The survey should include but not be limited to,

looking at how future development might affect the air quality and investigations into potential fire hazards. It is important to

ensure that around the PSV there will be enough space for other operations and maintenance to continue.

As noted in the ‘Safety Considerations’ section the ventilation of the building that

houses the PSA is of paramount importance. It is recommended that a minimum of 6

air changes an hour are performed. The release of the oxygen rich waste gas should

be funnelled away from any possible sources of ignition (i.e. road traffic).

Materials used for the construction of the PSA and interconnecting pipework are

usually carbon steel or copper. When selecting materials the presence of high velocity,

oxygen rich gases should be considered. Any non – metallic material will react with

oxygen, so when routing exhaust gases, contact the manufacturer for suitable materials

of seals and gaskets are required.

Installation 2Before the installation is completed and the PSA turned on, pipework needs to be cleaned,

especially in the waste gas routing. The removal of rust, dirt, weld slag and oils is vital for

minimising the fire risk and maximising the life and performance of the PSA. Under EN60079

Nitrogen generators are not thought to be a hazard to electrical equipment, so it is acceptable

to use general purpose wiring depending on if the location in in or out doors. All equipment

must be grounded.

The fire protection that would need to be installed for the PSA system is very simple. A

large, readily available supply of water is usually sufficient if in the form of a number of fire hydrants or hoses. The fire protection

system should allow the fire to be approached from all angles. It common practice to have an emergency shut down system on a

PSA, and on larger installation there may be more than one place to trigger the system.

Something that is sometimes overlooked is the noise produced by compressors and high gas velocities, this should be considered if

placing near a residential area. Also the venting of the building should be directed away from personnel and exhaust from pressure

relief valve should directed away from personnel. Waste gases from nitrogen generators are typically high in water content, so

drainage and freezing protection should be considered.

When reviewing hazards with in PSA supplier ‘dusting’ (deterioration of the adsorbent) should be taken into account. The particle

produced are not normally hazardous but are frequently a low level irritant and the particle can cause measuring instruments to

provide inaccurate readings.

Installation 3

PSA systems will have air compression systems that will

generate water condensation via processes such as drying. The

condensate stream do sometimes contain small quantities of

oil, glycol and molecular sieve dust. Provisions should be made

for the disposal of such solids or fluids, that comply with all

national and local environmental practices.

Finally before installation is started, a hazard review should be

performed to reduce any possible hazards. It is preferable that

more than one person performs this.

Storage – Cryogenic LiquidEach site will have their own issues with the storage of nitrogen. This is due to the fact that sites across the world all have

different environments to cater for. Therefore every storage system needs to be designed with the on site environment

considered. The characteristics of nitrogen that can it to be a hazard when stored in large quantities are that it is colourless,

odourless, tasteless and more importantly does not support life.

Nitrogen can be stored on site as a cryogenic liquid in a tank. Usually this is only an option when the amount of nitrogen to be

stored is quite high. EIGA DOC 127/13 will provide good guidance for the storage of cryogenic nitrogen for up to 125,000 litres. for

To reduce the dangers on handling stored cryogenic liquids (in this case nitrogen) it is important to undertake a risk assessment in

accordance with the Confined Spaces Regulations (SI 1997 No. 1713). Refer to BCGA Code of Practice 36 for information on the

design of cryogenic gas storage on site.

When considering small quantities of nitrogen (50 litres or less) the use of dewars is appropriate. BCGA COP 30 is able to give

good advise on storing cryogenic nitrogen on a small scale in dewars. Before choosing the option of storing small quantities

cryogenically, it is important that cost implications are considered.

Storage - GaseousAs mentioned in the Cryogenic storage section nitrogen is only

a treat to life because it is a gas that does not support life. This

means when storing nitrogen on site in cylinders, as gas form,

the treats are that it may be compressed in a cylinder,

therefore likely to explode when heated and it is an asphyxiant

stored in a high concentration.

This means it is important to keep the nitrogen cylinders in a

cool, well ventilated area. The cylinders should not be allowed

to reach temperatures above 50oC. The cylinders should be

stored in a vertical orientation and should be secured to

prevent them from falling over. Frequent inspections of

cylinders should be performed to check for leaks and the area

they are stored in should preferably be away from sources of

heat and ignition. For further advice see BOC Doc 8347.

Piping and Distribution 1Care should be always be taken when designing piping and

distribution for nitrogen. Consideration of local temperatures

and pressures involved should be taken into account. When

pumping large quantities of nitrogen around a large network, it

may be suitable to have pressure reducing stations outside of

workshops or any other site where the nitrogen is to be used,

This is to reduce the pressure of the nitrogen to the correct

pressure for use. The pressure reducing station should have a

Pressure Reducing Device (PRD) on the low pressure side of the

pressure reducing station as a back up if the station should fail.

The components of the pipework system itself should have

isolation valves where necessary to allow for testing and

maintenance.

Piping and Distribution 2When piping pressurised nitrogen above ground it is important

to keep piping away from sources of heat, and to design the

distribution network with expansion and contraction due to

heat in mind. Other things to bare in mind are sources of

damage and vibration.

Underground pipework should also have consideration of how

temperature changes will expand and contract the pipework.

All underground pipework must not have threaded or flanged

connections in order to prevent gas from leaking and cathodic

protection should be used to prevent corrosion. When

underground pipes are to go under load bearing roads or

paths, it is best to encase them in pipe sleeves that are then

vented to atmosphere.

Refer to EIGA Doc 149/10 section 8 for more guidance.

Operation of the PSAFor the operation of the PSA to be done in a safe manner it is

generally a case of getting the correct practices and procedures

in place. The creation of operation checklists and log sheets

should be created for the start-up and operation of a generator

and all of its components. The log sheets are there to monitor

temperatures, pressures, vibrations, power and capacity.

Keeping a log sheet will help to diagnose any problems early

and adjust any characteristics to optimise performance.

The actual operation of the PSA should be performed by a

properly qualified persons, who will have developed a list of

procedures to cover different failures of the PSA. For example

what should be done in case of a fire in the compressor. For

further guidance see EIGA Doc 149/10 section 9.

MaintenancePerforming maintenance on these system can be very dangerous. The tanks containing the adsorbent are usually stored in

cabinets. This means the creation of a nitrogen or oxygen enriched atmosphere is likely in these areas. Also it means that work

may have to carried out in confined areas.

If repairs are necessary and require piping or vessel to be opened or have hot repairs done, the pipe work or vessels concerned

should be purged with clean air until an oxygen concentration of 19.5% - 23.5% is achieved and can be maintained. After new

equipment has been installed, before restarting the PSA a good cleaning process to remove any contaminants should be

performed. For further guidance see EIGA Doc 149/10 section 10.

References1) http://www.airproducts.com/~/media/Files/PDF/products/producing-nitrogen-via-psa-CEP-Article_20120638.pdf [online] Accessed 12/09/2013

2) http://www.gastec.com.my/Nitrogen/N2%20PSA/GasTec%20PSA%20N2%20Gen%20Systems%20Presentation.pdf [online] Accessed 13/09/2013

3) http://www.atlascopco.com/nitrogenus/products/nitrogen_generators/psa_nitrogen/ [online] Accessed 13/09/2013

Useful Documents1)BCGA CODE OF PRACTICE CP362)BCGA CODE OF PRACTICE CP303)BCGA CODE OF PRACTICE CP254)BULK LIQUID OXYGEN, NITROGEN AND ARGON STORAGE SYSTEMS AT PRODUCTION SITES IGC Doc 127/13/E 5)BOC gases Datasheet (Nitrogen – oxygen free)6)SAFE INSTALLATION AND OPERATION OF PSA AND MEMBRANE OXYGEN AND NITROGEN GENERATORS IGC Document 149/10/E7)BULK LIQUID OXYGEN, NITROGEN AND ARGON STORAGE SYSTEMS AT PRODUCTION SITES AIGA 031/06