CHE10710 Nitrogen

8/10/2019 CHE10710 Nitrogen

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Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcos

employees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Process For additional information on this subject, contactFile Reference: CHE10710 R. A. Al-Husseini on 874-2792

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Nitrogen/Inert Gas Systems


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Engineering Encyclopedia Process


Saudi Aramco DeskTop Standards i

CONTENTS PAGES

INFORMATION

TYPES OF NITROGEN AND INERT GAS GENERATION ..............................................................1Cryogenic Nitrogen Generation ..............................................................................................1

Combustion Inert-Gas Generation ..........................................................................................1

Pressure-Swing Adsorption Nitrogen Generation ...................................................................6

Polymeric Membrane Inert Gas Generation ............................................................................8 NITROGEN/INERT GAS REQUIREMENTS ............ .............. ............ .............. ............ .............. .....10

Allowable Concentrations.....................................................................................................10PURGE REQUIREMENT CALCULATIONS...................................................................................11

Pressure/Depressure Cycle ....................................................................................................11

Purge Through ......................................................................................................................12

Tank or Vessel Blanketing....................................................................................................13PURIFICATION-GAS QUALITIES FROM VARIOUS GENERATORS .........................................14

Purification Processes ...........................................................................................................14DISTRIBUTION SYSTEM................................................................................................................15DESIGN CONSIDERATIONS...........................................................................................................16PROCESS SELECTION ....................................................................................................................17STORAGE..........................................................................................................................................18SAFETY CONSIDERATIONS ..........................................................................................................19


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Saudi Aramco DeskTop Standards ii

WORK AID

WORK AID 1:ALLOWABLE CONCENTRATIONS OF OXYGEN ................................................20WORK AID 2: PURGE CALCULATIONS-PRESSURE/DEPRESSURE CYCLE..........................21WORK AID 3: PURGE CALCULATIONS - PURGE THROUGH CYCLE ....................................22WORK AID 4: CONTAMINANT CONCENTRATION FROM VARIOUS INERT GAS

GENERATORS .................................................................................................23WORK AID 5: NITROGEN GENERATION RELATIVE COST VERSUS PURITY......................24WORK AID 6: CRITICAL OXYGEN CONCENTRATIONS..........................................................25WORK AID 7: EXPLOSIVE LIMITS (SADP-J-503).......................................................................26

GLOSSARY

GLOSSARY.......................................................................................................................................27

REFERENCE

REFERENCES...................................................................................................................................28

Saudi Aramco Standards.......................................................................................................28

Saudi Aramco Design Practices ............................................................................................28

Exxon Basic Practices...........................................................................................................28


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Saudi Aramco DeskTop Standards iii

LIST OF FIGURES

Figure 1. Linde Dual-Pressure Liquefaction System .........................................................................................2

Figure 1. Linde Double-Column Air Separator (Cont'd) ...................................................................................3Figure 2. Combustion Inert-Gas System ...........................................................................................................4Figure 3. Inert Gas Generator ............................................................................................................................5Figure 4. Inert Gas Generator (Compressor and Dryer on Skid Mounting) ......................................................5Figure 5. Pressure-Swing Adsorption Nitrogen Generator ................................................................................6Figure 6. Adsorber Tower for Nitrogen Generator ............................................................................................7Figure 7. Adsorber Tower (Skid Mounted) .......................................................................................................8Figure 8. Membrane Inert Gas System..............................................................................................................9


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Saudi Aramco DeskTop Standards 2

With permission from D. Van Nostrand

Figure 1. Linde Dual-Pressure Liquefaction System


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Figure 1. Linde Double-Column Air Separator (Cont'd)


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Auxiliaries required for combustion inert-gas generators include an air blower, a dryer similar to a compressedair dryer, a compressor, and storage. Storage is often similar to a large compressed air receiver.A schematic of a combustion inert gas generator is shown on Figure 2.

With permission from Permea

Figure 2. Combustion Inert-Gas System


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Figure 3 shows a combustion inert gas generator alone and Figure 4 shows it with a compressor and dryer onthe same skid mounting.

With permission from Permea, a Monsanto Company

Figure 3. Inert Gas Generator


Figure 4. Inert Gas Generator (Compressor and Dryer on SkidMounting)


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Figure 6 shows a single pair of adsorber towers rated at 12,000 SCFH at 85 psig. Figure 7 is a skid-mounted pressure-swing adsorption nitrogen generator. The combustion unit, compressor, and adsorber towers are onthe same frame. This unit produces 750 SCFH of nitrogen.


Figure 6. Adsorber Tower


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MSC-0.75 producing 750 cubic feet per hour of nitrogen. Small capacitygenerators are shipped with combustion unit, compressor, andadsorption system on one steel frame.


Figure 7. Adsorber Tower (Skid Mounted)

Polymeric Membrane Inert Gas Generation

Polymeric membrane inert gas generators are a recent development. The heart of the generator is Monsanto'sPrism R separator. This separator selectively removes oxygen, water, and carbon dioxide from compressed air

by permeation through hollow fiber membranes.The equipment required includes an air compressor, the polymeric membrane separator, and storage.The separators operate at pressures between 100 and 1,450 psig. A typical operating pressure is 435 psig.


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Typical sizes range from 3,500 to 20,000 SCFH. Higher capacities are available using multiple units.Monsanto and Maritime Protection A/S are contacts for anyone interested in this equipment.A schematic of a membrane inert gas generator is shown on Figure 8.


Figure 8. Membrane Inert Gas System


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NITROGEN/INERT GAS REQUIREMENTS

Nitrogen and inert gas are used in the following service: Equipment purging

Tank or vessel blanketing

Feed to some processes

Solids conveying

Seal gas

Backup to compressed air systems

Others

Allowable Concentrations (Also in Work Aid 1)

A maximum of 0.5% of oxygen in inert gas is allowed to eliminate a possible explosion hazard.To prevent combustion, oxygen should be kept below 2% in hydrogen-rich atmospheres and below 5% inhydrocarbon-rich atmospheres.Various chemical or process blanketing uses may have other limitations on such contaminants as carbonmonoxide, carbon dioxide, hydrogen sulfide, and others.


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PURGE REQUIREMENT CALCULATIONS

One of the most frequent uses of nitrogen or inert gas is to purge equipment of explosive or hazardous vapors before maintenance. This can be done by using a pressure and depressure cycle or by continuous purging.

Pressure/Depressure Cycle (Also in Work Aid 2)You can use the following equation to determine the number of cycles of pressure and depressure required tolower the oxygen concentration in a space.

C1 CoC2 Co

= P2 P1[ ]N

Co = % O 2 in purge gas

C1 = % O 2 initially in purged space

C2 = % O 2 finally in purged space

P1 = Low (minimum) pressure in atm

P2 = High (maximum) pressure in atm

N = Number of pressure/depressure cycles

For example, assume a vessel at 1-atm pressure has an initial oxygen concentration of 19%. This concentrationmust be lowered to 5% to stay below the critical oxygen concentration of a hydrocarbon (see Work Aid 6).Inert gas with 0.5% oxygen is available for purging at 100 psig.

Co = 0.5%

C1 = 19%

C2 = 5%

P1 = 1 atm

P2 = 100/14.7 + 1 = 7.8 atm


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Thus:

19 0.55 0 . 5 = 7. 8 1[ ]

N

7.8N

= 4.11N = 0.688 cycle (less than 1 full cycle )

In this case, one cycle is adequate. To check:

19 0.5C2 0.5

= 7.81[ ]

1

C 2 =18.57.8

+ 0.5

C 2 = 2.87% O 2 after 1 cycle

One cycle would require 6.8 times the vessel volume of inert gas. This would lower the vessel oxygenconcentration to 3.87%.

Purge Through (Also in Work Aid 3)

The following equation can be used to calculate the quantity of inert gas or nitrogen required to purge a vesselto reduce the oxygen concentration.

C 2 =C1 Co

e V + Co

V = Ratio of purge gas volume to space volume

Using the same example as before:

5 = 190.5e V

+ 0. 5

e V = 18.54.5 = 4.11

V = 1.415

In this case 1.415 times the vessel volume of inert gas would lower the vessel concentration to 5%.


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Tank or Vessel Blanketing

The quantity of inert gas required for tank or vessel blanketing depends upon the maximum withdrawal of liquid or vapor from the vessel. The purge gas volume in must equal the liquid or vapor volume out.


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PURIFICATION-GAS QUALITIES FROM VARIOUS GENERATORS (Also inWork Aid 4)

The table below lists the typical concentrations of impurities that might be found in the gas produced by various

types of nitrogen and inert gas generators.

Contaminant %

Generator Type CO 2 CO H 2 O 2

Combustion-nonreducing

Combustion-reducing

Adsorption

Polymeric membrane

11.4-15

11.4-15

0.002-0.1


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DISTRIBUTION SYSTEM

The pressure of an inert gas or nitrogen system depends upon the use of the gas. The required pressure for tank blanketing is usually very low. For purging or for backup to a compressed air system, typical pressures rangefrom 60 psig to 100 psig. For solids conveying, the pressure frequently used is about 15 psig. However, higher and lower pressures are also used.Pressure drop in distribution piping is similar to pressure drop in a compressed air system. The average

pressure drop is normally kept below 0.2 psi per 100 ft of equivalent length.Check valves or non-return valves are usually installed at every unit battery limit and at each consumer to

prevent the backup of contaminants into the inert gas or nitrogen system.Breakaway connections are used for all consumers except for continuous or very frequent users.At utility stations, a connection different from air, steam, and water connections should be used to ensureagainst incorrect connections.


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DESIGN CONSIDERATIONS

In determining the system demand, you need to consider all users. You should also include: process feed, if any, tank blanketing, conveying, and purging. Be sure that you determine the maximum simultaneous loads for all consumers.The quality of inert gas or nitrogen used is very dependent upon its final use. For safety reasons, the oxygencontent should be below 50% of the critical oxygen concentrations of materials frequently encountered arelisted in Work Aid 6. Explosive limits for some commonly encountered gases are listed in Work Aid 7.For process feed or other uses where chemical contamination is a concern, the inert gas specification required

purity of the gas.


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PROCESS SELECTION (ALSO IN WORK AID 5)

The type of inert gas or nitrogen generator must be selected by balancing the cost of generation versus therequired purity of the gas.The following table gives you relative capital costs of nitrogen generation systems versus the gas purity.

Contaminant (ppm) RelativeGeneration Process CO 2 H 2Q O 2 Capital Cost (1)

A

B

C

D

E

F

1,000

1,000

1,000

1,000

500

20

10

10

10

10

3

1

25

10

5

1

25

1

1.0

1.04

1.07

1.12

1.12

1.40

(1) Based on 10,000 SCFH

Cryogenic nitrogen costs about five times as much as combustion inert gas.Waste heat recovery is possible with a combustion inert gas generator. A 20,000 SCFH inert gas plant cansupply about 1.8 million Btu per hour to a 125 psig steam generator.


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STORAGE

Cryogenic liquid generators require cold storage. At atmospheric pressure, storage must be at minus 320F.Vapor storage is similar to storage for compressed air.As a general rule, C.M. Kemp recommends 25 to 50 ft 3 of storage for each 1,000 SCFH of consumption.


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SAFETY CONSIDERATIONS

To avoid an explosive atmosphere, base your designs to stay below 50% of the critical oxygen concentrationsof hazardous material given in Work Aid 6. For a hydrogen-rich atmosphere, you should design for less than2% O

2.

For a hydrocarbon atmosphere, you should design for less than 5% O 2. You should keep the inert gasconcentration below 0.5% O 2.A list of gases, with their lower and upper explosives limits is contained in Work Aid 7.To ensure against asphyxiation or danger to breathing, you should keep the oxygen concentration above 19%O2 in areas where personnel can be present. Remember that nitrogen is odorless. It is usually impossible todetect excess nitrogen, therefore the lack of oxygen, by smell.


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WORK AID 1: ALLOWABLE CONCENTRATIONS OF OXYGEN

O2 should be limited to 0.5% or less in inert gas to eliminate the possibility of explosion.To prevent combustion, O

2 should be limited to the following:

2% in hydrogen-rich atmospheres

5% in hydrocarbon-rich atmospheres


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WORK AID 2: PURGE CALCULATIONS-PRESSURE/DEPRESSURE CYCLE

The following equation can be used to determine the number of pressuring and depressuring cycles that will berequired to reduce the oxygen percentage in a space.

C1 CoC2 Co

= P2 P1[ ]N




P1 = Low (min) pressure, atm

P2 = High (max) pressure, atm

N = Number of pressure/depressure cycles


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WORK AID 3: PURGE CALCULATIONS - PURGE THROUGH CYCLE

The following equation can be used to determine the amount of purge gas required in a once-through purge.

C 2 =

C1 Coe V

+ Co

V = Ratio of purge gas volume to space volume





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WORK AID 4: CONTAMINANT CONCENTRATION FROM VARIOUS INERTGAS GENERATORS

The following table lists ranges of various contaminants that may be expected in the effluent gas from varioustypes of inert gas generators.

Inert Gas Contaminant ConcentrationGenerator Type CO 2 CO H 2 O 2

Combustion-nonreducing

Combustion-reducing

Adsorption

Polymeric membrane

11.4-15

11.4-15

0.002-0.1


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WORK AID 5: NITROGEN GENERATION RELATIVE COST VERSUS PURITY

Contaminant Level (ppm) RelativeCO 2 H 2O O 2 Capital Cost (1)

A

B

C

D

E

F

1,000

1,000

1,000

1,000

500

20

10

10

10

10

3

1

25

10

5

1

25

1

1.0

1.04

1.07

1.12

1.12

1.40

(1) Based on 10,000 SCFH

Cryogenic nitrogen cost is about five times the cost of combustion inert gas.


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WORK AID 6: CRITICAL OXYGEN CONCENTRATIONS

LIST OF TYPICAL CRITICAL OXYGEN CONCENTRATIONS (%) (1)

Acetaldehyde 12

Acetone 11.6

Allyl chloride 12.6

Ammonia 15

Benzene 11.2

1, 3 Butadiene 10.4

Butane 12.1

1-Butene 11.4

Ethane 11

Ethanol 10.6

Ethylene 10

Gasoline (octane 100) 11.6

Heptane 11.6

Hexane 11.9

Isobutane 12

Methane 12.1

Methanol 9.7Pentane 12.1

Propane 11.4

Vinyl chloride 9

(1) Critical oxygen concentration (COC) is the minimum level of oxygen tosustain combustion. A mixture of flammable vapor and oxygencontaining less than the COC of oxygen will not sustain combustion.

Ref. "Combustion, Flames and Explosions of Gases," Bernard Lewis and Guenthe Von Elbe, Academic Press,Inc., NY, NY, 1961, Appendix


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WORK AID 7: EXPLOSIVE LIMITS (SADP-J-503)

For a given gas, the explosive limit is that volume percent of the gas present in air that will produce an

explosive mixture. Upper and lower explosive limits define the explosive range.

Gas

Lower ExplosiveLimit (LEL)

Vol. %

Upper ExplosiveLimit (UEL)

Vol. %

Methane

Ethane

Propane

Butane

Hydrogen Sulfide

Hydrogen

Ammonia

Methyl Alcohol

Gasoline (1)

Naphtha (1)

Kerosene (1)

5.0

3.0

2.2

1.9

4.0

4.0

15.0

7.4

1.4

0.8

0.7

15.0

12.5

9.5

8.5

44.0

75.0

28.0

36.0

7.6

5.0

5.0

(1) Typical ValuesReference conditions: 15C at 101.325 kPA (60F at 14.7 psia)


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GLOSSARY

blanketing Maintaining a desired vapor concentration in a contained space such as inthe vapor space of a process vessel or storage tank. A space could beinert-gas-blanketed, or it could be hydrocarbon-blanketed.

combustible Capable of being ignited with resultant burning or explosion.(Interchangeable with flammable.)

critical oxygen concentration(COC)

The minimum level of oxygen to sustain combustion.

explosive gas Any combustible gas capable of being ignited and burned under certainconditions of quantity and/or confinement; normally used as a synonymfor flammable gas.

flash point The minimum temperature at which a liquid gives off sufficient vapor toform an ignitable mixture with the air near the surface of the liquid or within the vessel used.

ignition temperature The minimum temperature required for a substance, whether solid, liquid

or gaseous, to initiate or cause self-sustained combustion independent of the heating or heated element.lower explosive limit (LEL) The minimum concentration of vapor or gas in air or oxygen below which

propagation of a flame does not occur on contact with a source of ignition.

purge Replacing vapor in a container or space with other vapor such as inert gas.upper explosive limit (UEL) The maximum concentration of vapor or gas in air or oxygen, above

which propagation of a flame does not occur on contact with a source of ignition.


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REFERENCES

Saudi Aramco Standards

SAES-J-503 Combustion Gas Monitoring Systems

Saudi Aramco Design Practices

SADP-J-503 Combustion Gas Monitoring Systems

Exxon Basic Practices

BP12-1-1 Inert Gas Generators

Documents

CHE10710 Nitrogen