Measurably better
Moisture and oxygen analysis are critical to yield and quality in ethylene processing

World demand for ethylene has grown by almost five percent per year over the last 25 years—more than three times the growth rate of demand for crude oil. About half of all ethylene ends up as polyethylene. Accurate control of moisture and oxygen in the ethylene feed to the polyethylene process is essential to good product quality and profitability. An example from a world-scale polyethylene plant in China shows how to do it right.
Daryl Belock
Ethylene is colorless, flammable gas produced by steam cracking naphtha, ethane or other hydrocarbon gases. It is processed into many derivatives, but above all polyethylene.

During the polymerization process, accurate control of moisture and oxygen in the ethylene is important. High moisture content in the feed gas reduces catalyst activity and so decreases yield. Excess moisture can also reduce catalyst life.

High oxygen levels in the feed, meanwhile, can cause premature polymerization and undesirable branching or cross-linking of hydrocarbon products. Cross-linking causes problems during extrusion or molding, while branching leads to lower density and a softer product.

Oxygen can also poison the catalyst, decreasing reactor efficiency and lowering yield. And if large amounts of oxygen enter the process, whether from the atmosphere or from a plant source such as a compressed air leak, an explosion risk can result.

A real-world example

Shanghai SEECO Petrochemicals Complex has eight production units including a 900,000 t/y ethylene plant. The customer had 26 measurement points for trace moisture and 25 for oxygen; in some cases a single sample point was used for both measurement types.

For moisture analysis, aluminum oxide sensors were used to measure down to below 1 ppmv on high-purity ethylene streams, as well as 0–5 vol percent moisture for some “wet-end” applications such as vent gases containing nitrogen, hydrocarbons and hydrogen. Using a single technology for all moisture measurement applications reduces the plant’s spares inventory and simplifies maintenance.

For oxygen analysis in the ppm range, this manufacturer chose non-depleting electrochemical oxygen sensors. These work with bulk gas streams including nitrogen, hydrocarbons, and hydrogen. Compared to depleting galvanic fuel cells, non-depleting electrochemical sensors require less-frequent calibration and replacement. This is a great advantage in view of the large number of sample points.

For percent-range oxygen analysis, thermoparamagnetic oxygen transmitters were chosen. Thermoparamagnetic oxygen transmitters are cheaper than electrochemical sensors, are self-heated to well above ambient temperature, and are fairly resistant to wet gases that may foul electrochemical or paramagnetic sensors. This makes the technology a good fit for percent-range oxygen measurement in ethylene plants, which typically involves vent gases containing moisture and condensible hydrocarbons.

Conditioning is key

Proper sampling panels are a critical part of analyzer installation for a variety of reasons, discussed in turn below.

It must be possible to isolate the sensors for calibration and servicing. Sample panels are typically located at grade level, as close to the sample take-off point as possible while still allowing easy access. Probes extending into the middle one-third of the pipe are used to obtain representative samples. Needle or ball valves provide isolation, and check-valves are used on sample system outlets to prevent back-flow from flare or vent lines.

Pressure and temperature control are needed to get optimum performance from the various sensor types. Aluminum oxide moisture sensors, for instance, work best at the highest pressure available. A pressure sensor on each GE Sensing Moisture Image Series Probe (MISP) allows the analyzer to correct for the effects of pressure when calculating moisture readings.

All the sampling panels are exposed to varying outdoor temperatures. Daily temperature changes can cause moisture to adsorb to and desorb from the inside of the sample lines. This can lead to measurement swings between day and night, especially with sensors operating in the ppm range; trace heating of the sample lines can eliminate this problem.

Wide temperature swings also have a direct, if minor, effect on oxygen sensor readings, and can cause condensation inside instrument enclosures. Enclosure heaters can prevent this.

Some gas streams contain particulates such as dust from molecular sieves or catalysts, and even pipe scale. Stainless steel particulate filters rated at 7 µm ensure clean gas samples to all sensors. In some cases, coalescers are needed to knock liquid droplets out of the sample stream. The coalescers are fitted with bypasses to drain off the liquid they collect, ensuring low maintenance.

Proper control of sample flowrate is critical for the non-depleting electrochemical oxygen sensors, which are calibrated at a flowrate of 2 SCFH. If necessary, bypass loops are used to adjust the flowrate.

Certain sensors may be permanently damaged by exposure to excessive pressure or vacuum. Vacuum relief and over-pressure protection are provided by check valves located upstream and downstream of the sensor.

Most process sample systems use compression fittings for tubing joints. For applications that reach down into the parts-per-billion (ppb) range, however, customers often prefer VCR-type fittings, which use a metal-to-metal gasket design to ensure substantially lower leak rates. VCR fittings, which have been used in the semiconductor industry for many years, also tend to have low internal volumes and a high-quality electropolished surface finish.

Another way to improve performance is to join tubes by orbital welding to eliminate potential leak points. It is important to leave enough removable connections to allow equipment to be replaced if necessary. Both VCR fittings and welding are more expensive than compression fittings, so they are generally used only for connections upstream of critical sensors and for one or two connections downstream.

Mounting plates and enclosures are typically made from stainless steel for chemical and petrochemical plants. In less-aggressive environments, painted carbon steel is usually fine.

Isolation valves and branches on the inlet side of the panels are provided for those sensor types that can be field-calibrated. In some applications, three-way solenoid valves may be used instead to supply calibration gases. With the aid of appropriate electronics, the analyzer can then check that its performance is within specified limits, alerting the control room if this is not the case, or even calibrate itself automatically.

Performance in practice

SECCO has found good performance and reliability in the moisture and oxygen analysis systems provided by GE Sensing. Since it was installed in early 2006, the system has operated reliably, with only a few installation issues needing to be addressed by GE Sensing field services personnel. Overall, SECCO instrument engineers are pleased with the operation of the systems.

Oxygen sensors are periodically recalibrated by on-site technicians. For the non-depleting electrochemical oxygen sensors, the technicians use a one-point span gas at 80–100 percent of the sensor range. Thermoparamagnetic oxygen sensors are calibrated with both zero (100 percent nitrogen) and 100 percent span gases.

Aluminum oxide moisture sensors are swapped on a rotating schedule rather than being calibrated in the field. The old sensors are returned to the manufacturer for calibration against traceable standards. Newly-calibrated sensors carry their calibration data embedded in their smart electronics, so they require no configuration in the field.

In conclusion, analysis of moisture, oxygen, hydrogen and other gases is critical to optimizing production in ethylene and polyethylene plants. Solutions that provide low cost per measurement point, rugged technology that can handle installation in an industrial environment with a well-design sample handling system, and local service and support, help users to ensure that their plants perform reliably and economically.