CHE10710 Nitrogen

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Nitrogen/Inert Gas Systems

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already 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, Engineering Services, Saudi Aramco.

Chapter : Process File Reference: CHE10710

For additional information on this subject, contact R. A. Al-Husseini on 874-2792

Engineering Encyclopedia

Process Nitrogen/Inert Gas Systems

CONTENTS

PAGES

INFORMATION TYPES OF NITROGEN AND INERT GAS GENERATION ..............................................................1 Cryogenic 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.....................................................................................................10 PURGE REQUIREMENT CALCULATIONS ...................................................................................11 Pressure/Depressure Cycle....................................................................................................11 Purge Through ......................................................................................................................12 Tank or Vessel Blanketing....................................................................................................13 PURIFICATION-GAS QUALITIES FROM VARIOUS GENERATORS .........................................14 Purification Processes ...........................................................................................................14 DISTRIBUTION SYSTEM ................................................................................................................15 DESIGN CONSIDERATIONS...........................................................................................................16 PROCESS SELECTION ....................................................................................................................17 STORAGE..........................................................................................................................................18 SAFETY CONSIDERATIONS ..........................................................................................................19

Saudi Aramco DeskTop Standards

i



WORK AID WORK AID 1:ALLOWABLE CONCENTRATIONS OF OXYGEN................................................20 WORK AID 2: PURGE CALCULATIONS-PRESSURE/DEPRESSURE CYCLE ..........................21 WORK AID 3: PURGE CALCULATIONS - PURGE THROUGH CYCLE ....................................22 WORK AID 4: CONTAMINANT CONCENTRATION FROM VARIOUS INERT GAS GENERATORS .................................................................................................23 WORK AID 5: NITROGEN GENERATION RELATIVE COST VERSUS PURITY......................24 WORK AID 6: CRITICAL OXYGEN CONCENTRATIONS ..........................................................25 WORK 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


ii



LIST OF FIGURES Figure 1. Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8.

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


iii



TYPES OF NITROGEN AND INERT GAS GENERATION Four main processes are used for the generation of nitrogen or inert gas: cryogenic, combustion, pressure-swing adsorption, and more recently, polymeric membrane processes.

Cryogenic Nitrogen Generation In Saudi Aramco, there is a large cryogenic nitrogen generator at Qurayyah. Nitrogen produced there is shipped to the areas in Saudi Aramco. Cryogenic nitrogen generators are made to produce all-liquid nitrogen, all-nitrogen gas, or a mixture of liquid and gas. Typical sizes of cryogenic generators start at about 4,000 SCFH for all-liquid generators. Smaller all-liquid and very large all-gas sizes are available. Air products and Chemicals is a typical supplier of cryogenic nitrogen generators. Auxiliaries required for cryogenic nitrogen generators include vaporizers for the liquid nitrogen and compressors for the nitrogen gas. Storage can be in the liquid form in refrigerated storage vessels or as a gas in pressure storage. Schematics of a Linde cryogenic air liquefaction process and an air separator are shown on Figure 1.

Combustion Inert-Gas Generation There are two main types of combustion inert-gas generators, direct water cooled and indirect cooled. Indirect cooling can be either by air or water. Several fuels can be used. Natural gas is the most common. However, liquid fuels and even some wastes can be used. The combustion inert-gas generators can also be dual-fuel fired with liquid and gas. Typical sizes range from 800 to 150,000 SCFH.


1



With permission from D. Van Nostrand

Figure 1. Linde Dual-Pressure Liquefaction System


2



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


3



Auxiliaries required for combustion inert-gas generators include an air blower, a dryer similar to a compressed air 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


4



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

With permission from Permea, a Monsanto Company

Figure 3. Inert Gas Generator


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


5



Pressure-Swing Adsorption Nitrogen Generation The front end of a pressure-swing adsorption nitrogen generator is essentially the same as a combustion inertgas generator. This is followed by one or two dual-tower molecular sieve adsorbers. The adsorbers operate on a pressure/depressure cycle. They adsorb carbon dioxide and other contaminants Aftercoolers and oil and water filters are usually installed between the gas generator and the adsorbers. An ultrasorber or second adsorber can be added to increase the purity of the nitrogen. If very dry air is required, an additional dryer can also be added. Pressure storage is most common for this type of nitrogen generator. Typical sizes range from 500 to 50,000 SCFH. Figure 5 shows a pressure-swing adsorption nitrogen generator with compressor and two pairs of adsorber towers.

"3 " Molecular Sieve Adsorber "1 " Combustion Unit

"2 " Compressor System With permission from Permea, a Monsanto Company

Figure 5. Pressure-Swing Adsorption Nitrogen Generator


6



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 on the same frame. This unit produces 750 SCFH of nitrogen.

With permission from Permea

Figure 6. Adsorber Tower


7



MSC-0.75 producing 750 cubic feet per hour of nitrogen. Small capacity generators are shipped with combustion unit, compressor, and adsorption system on one steel frame. With permission from Permea

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's PrismR 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.


8



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% in hydrocarbon-rich atmospheres. Various chemical or process blanketing uses may have other limitations on such contaminants as carbon monoxide, carbon dioxide, hydrogen sulfide, and others.


10



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 to lower the oxygen concentration in a space. C1 – C o N = P2 P1 C2 – C o Co = % O2 in purge gas

[

]

C1

= % O2 initially in purged space

C2

= % O2 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 concentration must 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 – 0.5 = 7.8 1 N ] 5 – 0.5 [ 7.8 N = 4.11 N = 0.688 cycle (less than 1 full cycle )

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

[ ]

19 – 0.5 = 7.8 C 2 – 0.5 1

1

18.5 + C2 = 0.5 7.8 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 oxygen concentration 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 vessel to reduce the oxygen concentration. C –C C 2 = 1 V o + Co e

V = Ratio of purge gas volume to space volume Using the same example as before:

5 = 19 –V0.5 + 0. 5 e

e V = 18.5 = 4.11 4.5 V = 1.415 In this case 1.415 times the vessel volume of inert gas would lower the vessel concentration to 5%.


12



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.


13



PURIFICATION-GAS QUALITIES FROM VARIOUS GENERATORS (Also in Work 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

CO2

CO

H2

O2

Combustion-nonreducing

11.4-15

0-0.1

0-0.1

0.1-0.6

Combustion-reducing

11.4-15

0.1-0.6

0.1-0.6

0-0.1

Adsorption

0.002-0.1

0.1-3.0

0.1-3.0

0.12-0.001(1)

Polymeric membrane

<5

0.5-10

(1) 0.1% of CO and H2 is consistent with 0.12% O2 3.0% of CO and H2 is consistent with 0.001% O2

Purification Processes Several processes are available for further purifying the inert gas or nitrogen. • An MEA absorber will remove 99.5% of the CO2 and H2S in inert gas. •

A caustic absorber will remove all CO2 and SO2.

•

A catalytic purifier can reduce the concentration of O 2 to 2 ppm, NOx to 1 ppm, and CO to 20 ppm.


14



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 range from 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 ensure against incorrect connections.


15



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 oxygen content should be below 50% of the critical oxygen concentrations of materials frequently encountered are listed 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.


16



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 the required purity of the gas. The following table gives you relative capital costs of nitrogen generation systems versus the gas purity. Generation Process

CO2

Contaminant (ppm) H2Q

O2

Relative Capital Cost(1)

A

1,000

10

25

1.0

B

1,000

10

10

1.04

C

1,000

10

5

1.07

D

1,000

10

1

1.12

E

500

3

25

1.12

F

20

1

1

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 can supply about 1.8 million Btu per hour to a 125 psig steam generator.


17



STORAGE Cryogenic liquid generators require cold storage. At atmospheric pressure, storage must be at minus 320°F. 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 concentrations of hazardous material given in Work Aid 6. For a hydrogen-rich atmosphere, you should design for less than 2% O2. For a hydrocarbon atmosphere, you should design for less than 5% O 2. You should keep the inert gas concentration below 0.5% O2. 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 to detect excess nitrogen, therefore the lack of oxygen, by smell.


19



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, O2 should be limited to the following: • 2% in hydrogen-rich atmospheres •

5% in hydrocarbon-rich atmospheres


20



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 be required to reduce the oxygen percentage in a space.

C1 – C o N = [P 2 P 1 ] C2 – C o Co = % O2 in purge gas C1 = % O2 initially in purged space C2 = % O2 finally in purged space P1

= Low (min) pressure, atm

P2

= High (max) pressure, atm

N

= Number of pressure/depressure cycles


21



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.

C2 =

C 1 – Co + Co eV

V

=

Ratio of purge gas volume to space volume

Co

=

% O2 in purge gas

C1

=

% O2 initially in purged space

C2

=

% O2 finally in purged space


22



WORK AID 4: CONTAMINANT CONCENTRATION FROM VARIOUS INERT GAS GENERATORS The following table lists ranges of various contaminants that may be expected in the effluent gas from various types of inert gas generators. Inert Gas Generator Type

Contaminant Concentration CO2

CO

H2

O2

Combustion-nonreducing

11.4-15

0-0.1

0-0.1

0.1-0.6

Combustion-reducing

11.4-15

0.1-0.6

0.1-0.6

0-0.1

Adsorption

0.002-0.1

0.1-3.0

0.1-3.0

0.12-0.001

Polymeric membrane

<5


0.5-10

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WORK AID 5: NITROGEN GENERATION RELATIVE COST VERSUS PURITY

CO2

Contaminant Level (ppm) H2O O2

Relative Capital Cost(1)

A

1,000

10

25

1.0

B

1,000

10

10

1.04

C

1,000

10

5

1.07

D

1,000

10

1

1.12

E

500

3

25

1.12

F

20

1

1

1.40

(1)Based on 10,000 SCFH Cryogenic nitrogen cost is about five times the cost of combustion inert gas.


24



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

9.7

Pentane

12.1

Propane

11.4

Vinyl chloride

9

(1) Critical oxygen concentration (COC) is the minimum level of oxygen to sustain combustion. A mixture of flammable vapor and oxygen containing 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


25



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. Lower Explosive Limit (LEL) Vol. %

Upper Explosive Limit (UEL) Vol. %

Methane

5.0

15.0

Ethane

3.0

12.5

Propane

2.2

9.5

Butane

1.9

8.5

Hydrogen Sulfide

4.0

44.0

Hydrogen

4.0

75.0

Ammonia

15.0

28.0

Methyl Alcohol

7.4

36.0

Gasoline(1)

1.4

7.6

Naphtha(1)

0.8

5.0

Kerosene(1)

0.7

5.0

Gas

(1)Typical Values Reference conditions: 15°C at 101.325 kPA (60°F at 14.7 psia)


26



GLOSSARY blanketing combustible critical oxygen concentration (COC) explosive gas flash point ignition temperature lower explosive limit (LEL) purge upper explosive limit (UEL)


Maintaining a desired vapor concentration in a contained space such as in the vapor space of a process vessel or storage tank. A space could be inert-gas-blanketed, or it could be hydrocarbon-blanketed. Capable of being ignited with resultant burning or explosion. (Interchangeable with flammable.) The minimum level of oxygen to sustain combustion. Any combustible gas capable of being ignited and burned under certain conditions of quantity and/or confinement; normally used as a synonym for flammable gas. The minimum temperature at which a liquid gives off sufficient vapor to form an ignitable mixture with the air near the surface of the liquid or within the vessel used. 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. 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. Replacing vapor in a container or space with other vapor such as inert gas. 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.

27



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


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CHE10710 Nitrogen

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