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DELTA F CORPORATION APPLICATION NOTE NO. 105 |
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DELTA F CORPORATION APPLICATION NOTE NO. 105 |
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By maintaining a low oxygen content in your olefin feedstock, you can produce a high quality product with less scrap. This saves producers, transporters, and users may thousands of dollars per year. Here’s why . . ..
Companies that produce, transport and use olefin feed stocks must be aware of the oxygen content in the gas in order to maintain high quality, superior process control, and higher yields of their polyolefin products.
Oxygen in olefins can cause the following polymeric deficiencies:
Oxygen can cause premature polymerization and undesired branching or cross linking. Cross linking causes processing problems during extrusion or molding, and branching of the polymer will lead to a lower density and a softer product. Oxygen can lead to long induction periods for the polymerization by consuming the initiator. It can also cause the polymerization to terminate prematurely, leading to a lower molecular weight, reduced viscosity, and reduced polymer toughness. Oxygen can poison catalyst sites which decreases process efficiency and causes lower yields. Oxygen contamination can also indicate a potentially dangerous air leak in reactors, pumps, valves, or interconnecting piping. A fast responding oxygen analyzer is essential to meet the necessary process control and safety requirements demanded for today's high quality products.
Conventional Approach to Monitoring Oxygen
Antiquated galvanic oxygen sensor technology uses two dissimilar metal electrodes, typically silver and lead, which are consumed in the act of measuring O2. The sensors have a relatively short life span of several months because:
The galvanic sensors operate on a battery principle wherein life expectancy is a function of usage. As these sensors age, they begin to read low due to a loss in sensitivity as electrode sites are depleted. For polyethylene and polypropylene production, this will mean poor quality or scrap product. False low readings lead operators to believe that the process normal when, in fact, higher oxygen levels are causing poor product quality. Analyzers with replaceable battery-type galvanic sensors must be recalibrated on a frequent basis because the silver cathode is poisoned by ppb level acid gases, typically halogens or sulfides, which are undetectable by most analytical equipment, but that are common in even the most pure olefin feed stocks.
These trace contaminants oxidize the sites of the sites of the silver cathode causing the analyzer to produce false low readings that can only be remedied by frequent recalibration and/or sensor replacement. Replaceable battery type cells have membranes that isolate the sample gas from the electrolyte. These make them less sensitive than open cathode cells to the gas constituents, but this includes oxygen! If oxygen levels of less than 10 ppm are required, battery type cells should be avoided. Oxygen transport into the sensor is controlled by permeation through a membrane, resulting in tremendous temperature and pressure dependence and limiting sensitivity to about 1 ppm O2. Although battery type cells have better life compared to open cathode cells, occasional increases in acid gas levels, still barely at detection limits, can cause cell life to drop 10 fold without warning! Hence, analyzer output can drop by up to 10% per day.
Open cathode type galvanic sensor (wet Hersch cell) analyzers are even more sensitive to acid
gases because the gas sample comes in direct contact with both cathode and electrolyte. Here,
even minute contaminant levels can cause drift due to the sensitivity of the O2 reactions. Without
warning, trace contaminants can degrade cell life from 3 months to 1 week. Further, the electrolyte level of open cathode cells is critical. Typically, dry sample gas will substantially reduce the level of the electrolyte over a period of days, affecting the oxygen readings. Thus, the sample must be humidified before it is measured. This causes a delay in the instrument's response time, is very temperature sensitive, and increases maintenance requirements.
The Delta F Difference
Delta F has solved the problems associated with both battery-type and open cathode galvanic sensors with its unique non-depleting coulometric sensor which never under-goes a net chemical change. It provides performance features unmatched in the industry, such as:
Unlike battery-type sensors, the Delta F sensor does not require periodic replacement and does not produce false low readings due to loss of measurement sensitivity. The sensors operate on a simple coulometric process whereby oxygen in the sample gas is reduced in an electrochemical sensor. Oxygen is reduced at the cathode to hydroxyl ions. Assisted by the potassium hydroxide electrolyte, the ions migrate to the anode, where they are oxidized back to oxygen which vents out of the sensor. Whereas conventional electrochemical sensors use a consumable lead anode as the driving mechanism for the reaction, an external voltage drives the reaction in the Delta F sensor, and, therefore, they undergo no chemical change.
As a result, much better measurement stability is achieved and periodic sensor replacement is eliminated. In contrast to galvanic sensors which require frequent (usually bi-weekly) calibrations, the Delta F sensor may only require semi-annual span checks and addition of water. The fast speed of response of the Delta F Sensor gives immediate knowledge of changes in the oxygen level whether they are caused by an ambient air leak or by poor quality feed stock. Delta F's STAB-EL™ option is available to protect the analyzer from the harmful effects of possible trace acid gas contaminants, such as H2S or halogens. STAB-EL™ counteracts the contamination effect on the electrolyte and the carbon electrodes are resistant to the poisoning effect of acid gases.
The Delta F sensor has oxygen sensitivity <3 ppb (Delta F Model FAH0050S) or <75 ppt (Delta F Model 560) in order to meet the stringent specifications tomorrow's processes will require.
Although Delta F's non-depleting electrode technology has remained fundamentally unchanged, over the years numerous improvements in sensitivity, reliability, and serviceability have been made. All Delta F Sensors Carry a Five Year Pro-rated Warranty.
The Delta F Platinum Series
The Platinum Series manufactured by Delta F Corporation ideally fits the needs of the olefin industry. In conjunction with Delta F's unique non-depleting sensor technology, the Platinum Series offers temperature compensation that allows the analyzer to function accurately in locations from the Sahara to Siberia. The analyzer's software corrects for the minor output changes caused by large temperature differences to within ppb's of accuracy. Easy menu setup, user scalable outputs, networking capability via serial communications, and hazardous area enclosures make the Platinum analyzers ideal for most olefin applications.
Join Amoco, Chevron, Soltex, Dow Chemical, Texaco, Exxon, Mobil, and Union Carbide in using the superior technology of Delta F oxygen analyzers for your oxygen measurement requirements in your olefin gas streams.
Recognized For Quality
Delta F's Quality Management System has been certified to ISO-9001 by Lloyd's Register Quality Assurance Ltd. This demonstrated compliance with an internationally accepted standard assures you of the highest quality in product design, manufacturing, and service.
| Benefits | Features |
| Accurate Oxygen Level Indication | Unsurpassed sensitivity (<0.2 ppb) gives high degree of credibility to mid-ppb readings. |
| Fast Start-Up/Rapid Response |
Sensor responds instantaneously to changes in oxygen concentration and is not susceptible to "oxygen shock." Sensor reads from air to 5 ppm in < 10 minutes. |
| Very Low Maintenance Requirements |
The non-depleting electrode sensor eliminates frequent recalibration and periodic replacement. documented cases have shown YEARS of operation with no span or zero adjustments needed. |
| Reliable Operation | 5 Year Sensor Warranty |
| No Scrubbing With Acid Gases |
The patented Stab-El™ Electrolyte Conditioning System option eliminates the need for scrubbing in olefin applications. |
| Certified Accurate Readings on Start-Up | NIST Traceable Factory Calibration guarantees initial accuracy with no user calibration required. |
| Best Oxygen Analyzer On-Line Availability | The Delta F sensor giver very stable long-term measurements, plus Stab-El™ eliminates the risk of getting "knocked" off line. |
Delta F Oxygen Analyzers can be ordered with a full scale range of 0-2 parts per billion (ppb)
to as high as 0-25 percent. For specific product recommendations, contact Delta F Corporation,
4 Constitution Way, Woburn, MA 01801-1087, Tel. (781)935-4600, FAX (781)938-0531, e-mail marketing@delta-f.com.