New product R+D – mainly optical!

The latest Cryptospiridium scare in Northamptonshire produced a plea from GB Environmental that the use of UV disinfection systems should be reviewed in the UK, following the lead of the US (and various other EU countries) where potable water use of the technology is widely accepted. Many such case studies have been reported, for both drinking water and pharmaceutical pure water applications (Link). Another health topic comes from the University of Southampton report on the positive effects copper surfaces have on eliminating C.diff bacteria: whether this is ‘process industry’ related I’m not quite sure, but it interested me.

With the news this week that Physics is a dying subject in UK schools, with apparently no teaching capability even if the students had an interest in the topic, it is good to see so many reports on optical techniques and spectrometry being used to monitor materials and processes. A recent ABB report reviews the use of their FTIR spectroscopy equipment in satellites, semi-conductors, pharmaceuticals and gasoline blending (Link), and Wyatt Technology announce their annual meeting of light scattering users and researchers, to be held in October (Link). Malvern Instruments describe the use of their Insitec laser diffraction technology to monitor particle size on-line in a wet clay suspension, for process control on porcelain tile production (Link). To be fair I don’t yet know what microplate luminescence is, but Charm Sciences say it can be used to detect spoilage in samples from beverage products (Link): maybe they will also supply a technical article describing some of their other products, that are quoted to rapidly predict the hygiene status of surfaces and rinse waters in seconds. Another similar product is reported by SafeSol, offering a water disinfection system to control legionella and other ‘nasties’ in building water supplies, (Link).

One of the many things that fascinated me in a visit to see the activities at GCHQ (in the Doughnut at Cheltenham!) was a ‘Compound’ bow on display on their archery club stand (Link). The Long bow has been around for quite a while: but this compound bow was only designed and patented in 1967, so demonstrating that there are good mechanical inventions yet to be produced in all fields. By the way, judging by the number of Enigma style machines on display at GCHQ, they might still be working along these lines. ABB have been demonstrating that there are patentable ideas in us all with their “Race to Innovation”: 17 patents were filed in their latest 3-day session in China, brainstorming on the topic of “Better city, better life” (Link).

Optical spectroscopy techniques for gas analysis

Several Tunable Diode Laser gas analyser systems have been reported recently, in quite different applications! This is a review of the different systems, to illustrate the scope of the technology.

Recently we have seen several applications quoted for Tunable Diode Laser gas analyser systems, in quite different applications, and price levels! These are those known as at September 2007.

** The Yokogawa product launch in Europe of the ‘TruePeak’ tunable diode laser analyser, for gas analytical measurements, described the background to the technology: this unit uses a beam transmitted directly across a pipeline or flue.

Straightaway this gives a major advantage over GC and similar gas analysers, which have to use an extracted sample, which inevitably undergoes cleaning and conditioning: this can seriously affect the chemicals and relative concentrations that are present in the stream, before they reach the analyser.

The TruePeak is an in-situ optical transmission system, inserted into the process line, so it avoids the need for a sample, and also gives a reading in a few seconds, rather than the minutes or hours that sampling systems might require.

This unit is offered for monitoring O2, CO, CO2 and moisture on-line, in situ, using measurements at narrow absorption peaks specific to these gases, compared to reference measurements made at wavelengths away from absorption peaks, to give the zero reference.

This is where the tuning is required.

Once again the instrumentation industry benefits from telecommunications: without the mobile phone industry we would not have the components available that have made radar level measurement possible, or for that matter, wireless adaptations of pressure transmitters.

This time the tunable lasers that are used by this Yokogawa technology have been made available because of the need for tunable lasers for near infra-red fibre-optic transmission of telephone conversations.

It is pure serendipity that these devices can be tuned across these useful gas analysis absorption lines.

The near IR region is a good region for monitoring dual element gases, like H2O, CO, CO2, and the technique has been extended to H2S and Hydrocarbons as well.

Yokogawa admit that all the research and basic work has come from Dow Chemicals: the pioneers there have hundreds of these units installed, in Dow processes across the world.

The obvious application for these analysers is in combustion gas analysers, to ensure the boiler or furnace is operating at peak efficiency, with the correct fuel / air mixture.

But the Dow applications have gone further, their units are also used for monitoring water vapour in chlorine plants, to stop corrosion: and critically for the monitoring of acetylene at 1-2ppm in ethylene plants, which allows the plant efficiency to be much improved.

Further serendipity comes to answer the query that the more alert of you readers will be asking: how come a near IR analyser can measure O2 then, as it is not a dual element gas? Apparently this is one of nature’s quirks, that oxygen is paramagnetic, both outer electrons spin in the same direction, which is what makes oxygen so important maybe! But it produces a near IR absorption band, when it should not have one! Such analysers get immediate application in monitoring incinerators etc, preventing harmful emissions, because the unit can work at temperatures up to 1500C.

This TruePeak is not cheap, but it is not an inordinately expensive unit: and it must be one of the best of the ‘Boy’s Toys’ available to satisfy any Process Gas Analytical Research Chemist this Christmas!.

** Vaisala also introduced an oxygen monitoring system using a similar tunable diode laser detector earlier in the year: this is called the Vaisala Spectracap Oxygen Transmitter.

Again this can be flange-mounted directly into a process without sampling or sample conditioning equipment, but this unit uses a stainless steel probe in the flow stream, which tolerates chemicals and excessive amounts of moisture.

This Vaisala probe is quoted as suitable for process gas monitoring, in terms of monitoring oxygen in gas generation systems, inert gas blanket monitors, and monitoring the gas levels in composting and biological processes.

Their unit is not intrinsically safe, and I have the impression it would not be suitable for insertion into flue gas streams.

But a sampler system is available, to extract a small gas flow for analysis.

Currently Vaisala only offer this unit tuned for oxygen measurements.

** Offshore Europe in Aberdeen turned up yet another application of a Tunable Diode Laser (TDL) in a gas detection system from Allison Engineering.

Apparently their TDL system is used as a perimeter gas detection alarm, with the laser beam bounced around a whole area, like a chemical plant or a compound.

The typical gases detected this time are HF or H2S, in the atmosphere traversed by the light beam.

The concentration in the atmosphere is monitored to provide an alarm system to initiate a process shutdown of the plant should a major escape of these gases be seen, detected by the TDL, to protect the surrounding population, and environment.

Such systems as these are apparently in operation around critical areas on chemical plants in Coryton and Grangemouth.

** A Manchester based company, surprisingly called TDL Sensors, announced a process tunable diode laser spectrometer in 2006.

This product was quoted as designed primarily for the in-situ measurement of gases such as O2, CO, CO2, H2O, HCl, HF and NH3 from ambient temperatures up to 1500C, with both cross duct and single access point configurations available.

** Water vapour measurement in natural gas is particularly important as high moisture levels can encourage the formation of methane hydrate, a solid that forms in the pipeline and reduces flow or can even block the line completely.

The heart of the water vapour sensing system from IMA for this duty is a small tunable laser diode (about 2mm2) that produces a very narrow and specific wavelength of light tuned to a harmonic of the water vapour molecule in the near-infra-red band.

The light causes the molecule to vibrate and therefore absorb energy.

The laser is scanned through the specific wavelength of interest and, by comparing the light energy being absorbed at a water vapour wavelength to the light energy at surrounding wavelengths, a very precise measurement can be made.

What sets The IMA tunable diode laser (TDL) systems apart from other NIR analysers is the ability to get down to parts-per-million levels.

**Siemens has a Process Analyser that appears to use TDL techniques.

Up to three in-situ cross duct sensors, optionally available in an intrinsically safe version for operation in Ex zones, can be connected with their LDS analyser.

The list of gases able to be measured with their NIR diode lasers includes gases like O2, NH3, HCl, HF, H2O, CO, CO2, H2S, CH4, …and it is continuously growing with the state of the development in semiconductor laser technology.

Even traces of NH3, HCl or HF concentrations can be determined in wet process gases in front of any gas cleaning step.

See the LDS6 on https://pia.khe.siemens.com/index_process_analytics.products_solutions.products_systems.continuous_process_gas_analytics.in-situ-10231.htm.

Raman analyser at Huntsman Petrochemicals

 

Raman laser-based technology in Rosemount Analytical unit helps plant deliver 99.7% purity by reducing feedstock variability in paraxylene purification.

Huntsman Petrochemicals produces up to 360 Ktonnes per year of paraxylene at its Wilton site in the UK, a plant where control was recently transferred to a DeltaV digital automation system, a technology of Emerson Process Management. Further investment, in a Rosemount Analytical Raman on-line laser spectrometer from Emerson, has resulted in added process optimisation and performance improvement, and allowed site engineers more insight into their production process.

Paraxylene is a key starting material in the creation of polyester resin and fibre, used in the manufacture of clothing, films, drink bottles and food containers. The pure product is separated from the other two xylene isomers – orthoxylene and metaxylene – in a process that involves selective crystallisation from a chilled solution and centrifuging the resulting suspension. Optimum efficiency is maintained by controlling the composition of the incoming feed stream to the purification plant.

The previous method of monitoring the process stream composition used on-line melting point analysers, confirmed with frequent grab samples delivered for laboratory analysis. The delays inherent in providing meaningful data from either of these techniques led to a variability of 2 – 3% in feed composition. Early in 2003, a Rosemount Analytical Raman Analyser was installed, allowing full on-line composition monitoring of the feed to the purification plant. With composition information being updated every minute, process variability has been dramatically reduced by an order of magnitude to 0.25% with a consequent improvement in plant stability.

Data that improves plant performance

“The Raman analyser is definitely helping the plant performance,” says Tom Liddle, Plant Manager for the paraxylene plant at Wilton. “By reducing the variability of the process composition, we can run the plant at the optimum settings. We have improved plant efficiency, improved consistency, and we get the 99.7% quality required first time, all the time.”

Steve Gill, Process Engineer at Huntsman Petrochemicals, who pioneered this first use of a Raman spectrometer on-line at Wilton, says, “Considering that we are doing in-line dilution, I am very pleased with the performance. While the main benefit of the purification control scheme is to give consistent solids feed to our centrifuges, an additional benefit has been the ability to see the impact of upstream changes on variability. We’ve never been able to see that in real time before.”

Tom Liddle is also pleased to see the plant running smoother: “Without the on-line control provided by the Raman, variability in the process would occasionally lead to excessive solids loading in the centrifuges, resulting in vibration and potential bearing damage. Now we run at maximum output, and have reduced wear on the centrifuges.”

Raman Spectroscopy

Raman spectrometry uses single wavelength laser light to probe the sample stream. On the molecular level, a very small fraction of the light intensity is scattered. While most of this scattered light occurs at the same laser wavelength (Rayleigh scattering) an even smaller fraction of the incident light is shifted to longer wavelengths (Raman scattering).

The shift in wavelength from that of the laser source represents an exchange of energy with sample molecules. From the pattern of wavelength shifts and intensity of Raman scattering, both qualitative (molecular species) and quantitative (concentration) information can be determined. In practice a multivariate calibration model is developed for the application, allowing multi-component analysis to be performed.

Analysis of four streams

The Process Raman Analyser at Wilton will be used for simultaneous measurements at four separate process locations on the plant – feed and recycled material as described above, plus two final product streams to monitor paraxylene purity. “We are still learning what the analyser can do,” added Tom Liddle, “and have a little more to understand to get the on-line quality measurements on the product streams fully operational.”

The analyser is located in a control building. The laser light is transmitted through fibre optic cables onto the plant to the four measuring locations: optical probes provide the interface to the process streams. At each probe, light scattered by the sample is collected and transmitted back to the analyser through the return fibre. The derived analysis and concentration data are transmitted via Modbus communications to the Emerson DeltaV process control system, and the signals used to control plant feed dilution.

The Rosemount Analytical Raman support team, working on both sides of the Atlantic, worked with engineers from Huntsman Petrochemicals to provide project guidance from the early consultation and application engineering through to calibration and commissioning. The Rosemount Raman analyser is monitored remotely by Emerson engineers in Ohio, to allow on-line tuning and remote performance optimisation of the calibration model used.