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

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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

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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

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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.

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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 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

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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

With permission from Permea, a Monsanto Company

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

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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

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

With permission from Permea

Figure 6. Adsorber Tower

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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.

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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.

With permission from Permea, a Monsanto Company

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.

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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 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%.

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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 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