RUSSIAN GAS PLANT GETS ADVANCED UV MONITORING

At Russia's Astrakhan gas plant, installation and commissioning have been taking place this year on 40 real-time monitoring systems that employ UV scanning.

The systems provide a higher level of control for the plant's Claus sulfur-recovery process, says developer NovaChem B.V., Arnhem, The Netherlands.

In the Phase 2 expansion project at the southeast Astrakhangazprom natural gas field near the Caspian Sea, the advanced UV scanning systems will simultaneously monitor five key gas components on each of eight separate process streams.

The goal is continuously to optimize the sulfur-recovery process, balance the stoichiometry, and reduce air demand to a minimum.

The Astrakhan plant, built in the mid-1980s for about 80 billion rubles (about $400500 million), was designed to process 6.9 billion cu m/year (661.5 MMcfd) of separated gas.

Design also called for production of 13.2 million cu m/day (462 MMscfd) of commercial gas, 3.2 million metric tons/year (tpy) of stabilized condensates, and 360,000 tpy of LPG.

NovaChem says it currently produces 620,000 tpy of natural gasoline, 400,000 tpy of diesel oil, 480,000 tpy of fuel oil, 172,000 tpy of butane-propane, 29,000 tpy of raw butane, and 3.4 billion cu m (2.7 billion cu yd) of fuel gas.

Desulfurization extracts 2.25 million tpy of sulfur from a produced gas that contains, among other sulfur compounds, levels of H2S that average 100 mg/l. (Fig. 1).

UV IN NEW ON

The new monitoring system combines a diode-array UV spectrometer, a fiber-optic optical path, and a 150 C./302 F. sampling chamber able to be located right at the processing tine.

Together these elements continuously monitor concentrations of SO2, H2S, sulfur vapor, carbon disulfide (CS2), and carbonyl (COS) to accuracy levels of parts per million.

Relative SO2 and H2S levels indicate stoichiometric balance. NovaChem says that levels of sulfur vapor, whose presence can throw off readings in UV spectrophotometry because of its significant absorbance in the UV range, are monitored so that analyzer software can correct the signal automatically.

CS2 and COS concentrations are monitored to protect the costly catalyst against poisoning.

By contrast, the dozens of older monitoring systems in place at Astrakhan Phase 1, which was built in the mid-1980s, are limited to single wave length photometers measuring SO2 and H2S levels only.

That is the typical technology found on most existing sulfur-recovery units today, says NovaChem, and was state-of-the-art when Astrakhan was built.

The limitations inherent in single-wavelength photometry make the air balance always prone to wide swings. Possible catalyst poisoning goes undetected until after the fact.

Lack of fiber optics for transmission and collection required bringing sample and instrument together. Long sampling lines with heat tracing were necessary to maintain temperatures at greater than the sulfur dew point (150 C./302 F.). Cooling, condensation, and even solidification of sulfur in the sampling lines caused frequent blockages.

Thus the benefits were limited. Efficiencies, yields, and catalyst costs suffered accordingly.

REGULATIONS

As sulfur emissions became a worldwide environmental issue in the past 30 years, the Claus sulfur-recovery process emerged as the processing industry's method of choice for achieving compliance.

The two-stage catalytic process and analyzers are shown in (Fig. 2).

Recovered sulfur is sold to offset emission control costs. The only other reaction product in water.

The gas stream is reactive, corrosive, and saturated with sulfur mist because the stream temperature is very close to the sulfur melting, point (150 C./302 F.).

Concentrations of sulfur compounds in the feedstock may vary by 10 or 20 to 1 from second to second, making it difficult to keep the sulfur-recover,y process under control, especially levels of oxidizing air for the first stage.

The usual answer, says NovaChem, has been to inject large amounts of excess air, which greatly raises energy costs for fan power and process heat.

And the same swings in H2S and SO2 levels that complicate air balancing also raise the risk of catalyst poisoning, which also hurts process economics.

FIVE AT ONCE

The new NovaChem monitoring systems at Astrakhan Phase 2 are aimed at overcoming these problems, reducing balancing air requirements to 0 most of the time, and better protecting against catalyst poisoning.

Simultaneously, they monitor the five key components, as shown in Table 1.

As with any UV monitoring method, absorbances over the full UV range for the components at various concentrations were established as standards. They are given in Fig. 3.

Note that Fig. 3d covers both catalyst poisons, CS2 and COS.

On the actual analyzer display, spectra for all five are shown simultaneously. Analyzing the full spectral range enables the operator to differentiate among all components, a capability not possible with single-frequency photometer monitoring.

On each of the eight streams at Astrakhan, six different NovaChem analyzers monitor conditions at a different point in the process, all they way from sales gas to tail gas.

Locations include the following lines: sales gas outlet, Sulfrine unit, feed-forward analysis at the Claus plant inlet, sulfur pit, tail gas, and stack gas.

The main difference among the analyzer packages is design of the sampling module sampling

The sampling modules for Sulfrine and tail gas contain the flow cell, a demister, portions of the fiber optic cable, steam ejector, and heaters to keep chamber temperature at 150 C. 2 C. (302 F. - 4 F.).

The flowcell's pathlength varies between 5 and 25 cm (1.9 and 9.8 in.) depending on location of the sampling point in the process and concentrations in the sample.

A steam back flush and sample-line preheating system are also incorporated to keep sampling lines clear and, when necessary, quickly to unclog them.

Because fiber optic cables constitute the optical path between flow cell and instrument, the entire chamber can be heated to 150 C./302 F. and located close to the process stream. There are no mechanical, electrical, or electronic components in the sampling chamber that could be harmed by 150 C. temperatures or the corrosive, reactive plant environment.

Heating the entire chamber to 150 C. also keeps both sides of the flow cell quartz lenses at the same temperature, retarding sulfur vapor condensation on them.

In all cases the sampling module is located no further than 0.5 m (20 in.) from the process stream, and the sample path never exceeds 1 m (3.3 ft). The analyzer, therefore, "sees" a sample very close in both time and composition to conditions in the process stream.

This capability, and instantaneous readings, account for the system's

Response, in fact, is fast enough that the output signal could be used for true closed-loop control.

In Astrakhan Phase 1 and in most other sulfur-recovery operations with UV photometers, by contrast, sampling circuits measure 10 m (39 ft) or longer, drastically slowing response time.

In the new Phase 2 analyzers, the sample in gaseous form enters the enclosure at 300-500 l./hr (10.39-17.66 cu ft/hr) through a probe in the process stream, goes through the demister, and then through the flow cell for measurement.

Temperature throughout is maintained to prevent sulfur solidification in the lines. Thereafter the sampled gas returns to the processing line through a steam ejector.

Limiting flow rates to 300-500 l./hr also minimizes clogging problems, which are common at higher flow rates.

SOURCE, DETECTOR, PC

NovaChem's OPPA-203 process diode-aray analyzer lies at the heart of the monitoring system at Astrakhan (Fig. 4).

It is an on-line UV/VIS spectrometer operating in wave-length ranges of 200-800 nm or 400-1,100 nm. It sweeps the entire UV spectrum at up to 5 times/sec with a resolution of 1 nm, detecting impurities to the ppm level.

The modular-design analyzer contains a pulsed xenon flash lamp as light source, diode array detector, and built-in PC-AT microprocessor and 144-mb disk drive, display, and membrane-pad controls.

Control outputs are industry standard 4-20 ma, RS485, and TTL interfaces. Standard enclosure meets IP 55 (NEMA 4) requirements.

Linking the sampler and analyzer are 4-m fiber optic cables. NovaChem says they provide a clear, interference-free optical path and permit remote sampling. Outputs from the analyzer feed to the main control room at Astrakhan.