Applications

Applications include oxygen delignification of pulp stock, cus-tody transfer (CT) of gas to steel mills, control of chemicalreactions- EO/PO, and production of oxygenated fuels (MTBE,and numerous others).

Nature of the Gas

In its most common form, oxygen (chemical formula O2) iscomposed of two oxygen atoms, and so is considered adiatomic (two-atom) gas. With a molecular weight of 32, oxy-gen is slightly heavier than air (mw = 28.8), and has a specificgravity compared to air of 32/28.8 = 1.11 SGU.

Oxygen ions are also possible (O-), as is a highly unstable formof oxygen called ozone (O3). These forms are not common forindustrial flow measurement. Molecular, diatomic oxygen (O2)will be referred to in the remainder of this paper.

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Measuring Oxygen Gas Using Coriolis Technology

Key Benefits

•Direct mass measurement achieves highaccuracy over a wide turndown.Independent of changes in pressure andtemperature. Uncertainties of volumetricmeasurement and conversion to standardconditions (inferred mass) eliminated. Third-party testing at 100:1 turndown and 60:1operating density range confirms accuracyand repeatability. Flow accuracy specifica-tion of ± 0.5% of actual rate ± sensor zerostability applies to all gases.

•Lower routine maintenance costs becauseCoriolis technology uses no moving parts.Calibration factor does not shift over time,eliminating need for routine proving. Masscalibration factor applies to any gas.Molecular weight changes do not affectmetering accuracy. Gas meters can beproved using liquid medium and traditionalproving devices.

•Wider usable flow range/turndown thanother gas measurement technologies. Thirdparty testing verifies 100:1 performancerange.

•Easy to install and maintain. No straight pipe-runs orfilters/strainers required. Insensitive to swirl effects. Sensorcan be installed in horizontal or vertical pipe runs. Largepotential for cost savings due to simple installation.

Overview

Oxygen is a valuable and widely metered industrial gas withnumerous important industrial uses. Atmospheric air containsapproximately 21% oxygen and 78% nitrogen, plus smallamounts of other gases. Pure oxygen can be separated fromair, generally via cryogenic or, more recently, using molecularsieve technology.

Oxygen is a valuable, reactive gas that is well known for itsability to support combustion. Therefore, reliable and accurateflow metering is important to assure safety and final productquality or yield. This application note describes some of themany ways Coriolis flowmeters from Micro Motion canimprove any process where oxygen gas is measured.

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Oxygen delignification of pulp stock

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Oxygen has no group classification, per the National ElectricCode (NEC). However, since oxygen supports combustion ofother substances, certain precautions are typically used in oxy-gen piping systems to increase safety. These precautionsinclude special cleaning of all piping components and velocityguidelines. Brochure G-4.4 from the USA Compressed GasAssociation is frequently used in the design, installation, andstartup of oxygen gas piping systems. CGA brochures are vol-untary guidelines reflecting current industrial practices in thechemical, steel, petroleum, and other industries, and as such,are designed to benefit users in the areas of safety and eco-nomic transmission and distribution of gaseous oxygen.

Selected excerpts from pamphlet G-4.4 will be referenced inthis paper, specifically as applied to Coriolis measurement ofoxygen gas. It is suggested that pamphlet G-4.4 be consultedin its entirety for additional guidelines beyond the scope of thispaper. Micro Motion wishes to express its appreciation to theCGA for the excerpts contained herein.

The Applications

O2 delignification of pulp stock

Exposing wood pulp to oxygen gas removes additional ligninsafter the Kraft digester—without attacking the wood fibersthemselves. Since lignins adversely affect pulp brightness,delignification effectively whitens the pulp stock. In manyprocesses, the pulp stock is bleached using chlorine after theO2 delignification stage. A typical process schematic is shownbelow:

The objective of the overall process is to use less chlorine.Bleaching lignins requires substantial amounts of chlorine, andremoving more of the lignins earlier in the process using oxy-gen is more effective and reduces the amount of chorinerequired. The oxygen reactor is essentially a finely tuned sec-ondary digester.

Accurate control of the oxygen feed gas using a Coriolis meterfrom Micro Motion enables good process control. Direct massmeasurement as well as elimination of routine maintenance

were cited by users as valuable attributes:

“It is a great benefit to have an in-line meter capable of measuringmass flow of O2 directly and accurately without the need to pres-sure and temperature compensate.”

“We forgot where the meter was because it has worked so reliablyin the 2+ years in service.”

Custody transfer of oxygen gas

Oxygen gas is commonly used in the production of steelthroughout the world. Metering the amount of gas consumedis a custody transfer (CT) application. Payment is based on thetotalized flowmeter reading. Pricing is usually on a mass orstandard volume (inferred mass) basis, making the Coriolismeter ideal for this application. Oxygen is usually generated“on-demand” by an air separation plant located on site. Flowrate can vary widely, depending on the demand from the oxy-gen blast furnace and the rate of steel production. High accu-racy over a wide range of flow rate, pressure, and temperatureis required for accurate CT. Coriolis meters have beenapproved by national governing bodies for this fiscal transfer.

Chemical reaction control

Critical chemical reactions can be controlled using Coriolismeters to measure the mass flow rate (standard or normal vol-ume) of oxygen feed gas. Good examples include ethyleneoxide (EO), propylene oxide (PO), and acetaldehyde.

Sizing and Selection

Sizing and selection of flowmeters for oxygen service is virtual-ly identical to sizing other Coriolis gas meters. The two primarycriteria are acceptable pressure drop and desired accuracyover the flow range of interest. Maximum recommendedvelocity per CGA Pamphlet G-4.4 can modify the flowmetersize selected, and cleaning is frequently considered by the enduser.

The use of the WindowsTM-based MMI WIN sizing programgreatly facilitates sizing and selection of the oxygen gas

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PFD: Oxygen delignification of pulp stock

Fiscal transfer of oxygen gas; air separation plant to steel mill

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Measuring Oxygen Gas Using Coriolis Technology

flowmeter. User-entered variables include flow rate, operatingpressure, and temperature. Several possible flowmeter mod-els are selected by the program. The final selection is based onallowable pressure drop, desired accuracy, and velocity.

Micro Motion’s ELITE® flowmeter is preferred for gas service(although numerous D-model flowmeters have been used inthe past), and are preferentially selected by the sizing pro-gram. With sizes ranging from the 1/4" (6 mm) model CMF025to the 3" (80 mm) model CMF300, oxygen flow rates from vir-tually zero to approximately 7000 scfm (600 lb/min) or 12 x106 NM3/h (16,000 kg/h) can be measured using a singleflowmeter (following the CGA velocity guidelines). For higherflow rates, two or more CMF300 sensors can be installed inparallel.

CGA velocity guidelines are shown in Figure 1. Per CGA, pipesize should be selected to adhere to the maximum recom-mended velocity as a function of gas pressure. This is based ontypical stainless steel piping. CGA guidelines also include infor-mation on system components such as flowmeters, wheredue to practical sizing concerns, the piping velocity guidelinesmay be exceeded.

The CGA suggests that upstream filters be considered to

remove particulate material. This prevents impingement andpossible sparking.

Impingement of particulates (weld slag, bits of corrosion, orrust particles) at high velocity is thought to create sparking,and is therefore a source of ignition energy. In the presence ofcombustible particulate material, this can initiate fires insidethe piping.

Oxygen service cleaning is frequently specified by the end-userand is intended to eliminate sources of combustible materialinside the piping. When internal organic, inorganic, and partic-ulate matter are removed, a system is generally consideredclean for oxygen service. The following excerpt from para-graph 3.5 Oxygen Service Cleaning of CGA Pamphlet G-4.4explains further:

3.5.1 General requirements

A system is considered clean for oxygen service when internalorganic, inorganic, and particulate matter are removed. Removalof contaminants such as greases, oils, thread lubricants, dirt,water, filings, scale, weld spatter, paints, or other foreign materialis considered critical. This can be accomplished by precleaning allparts and maintaining this condition during construction, by com-pletely cleaning the system after construction, or by a combina-tion of the two. CGA Pamphlet G-4.1 thoroughly describes thisprocedure.

Oxygen service cleaning of the sensor is available from MicroMotion for a nominal charge.

Performed by qualified outside vendors, the cleaned sensor isthen “bagged, tagged, and sealed.” Please contact the factoryor your local sales representative for more information on oxy-gen service cleaning.

References

User reference lists are available to help verify manufacturerclaims. Customer copies of third-party test data is also avail-

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PFD: Acetaldehyde production using oxygen process

Figure 1. CGA velocity guidelines

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Measuring Oxygen Gas Using Coriolis Technology

able. Please contact your local sales representative for moreinformation.

Summary

Oxygen is a commonly measured, valuable industrial gas andis used in a wide variety of processes. Reliable, accurate flowmeasurement is critical to insuring the safety of plant person-nel, product quality, and maximum yield. Existing volumetrictechnologies such as dP, turbine, and vortex are limited byturndown, accuracy, and reliability. Thermal mass meters haveoften been used, but are somewhat maintenance-intensiveand have the tendency to drift with ambient temperaturechanges.

More recently, Coriolis meters from Micro Motion have beenused in oxygen service with excellent results. With useableturndown of 100:1 and excellent accuracy over a 60:1 operat-ing density range, Coriolis meters from Micro Motion alsoeliminate straight-run requirements and routine maintenance.Direct mass measurement, synonymous with standard or nor-mal volumetric units, is fundamentally more accurate, andeliminates the need for flow correction factors based on pres-sure and temperature. Installation is simple and straightfor-ward, with no requirements for flow conditioning.

With an installed base of over 150,000 flow and density sen-sors worldwide, Micro Motion can provide the measurementexpertise and support you desire. Extensive third-party test-ing, and more than 7500 gas-phase applications around theworld, have increasingly shown the numerous benefits ofusing Micro Motion Coriolis technology to measure oxygengas.

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The contents of this publication are presented for informational purposes only and,

while every effort has been made to ensure their accuracy, they are not to be construed

as warranties or guarantees, expressed or implied, regarding the products or services

described herein or their use or applicability. We reserve the right to modify or improve

the designs or specifications of our products at any time without notice.

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