Folded-path gas analysers: making the analyser fit the process
By Process Online Staff
Thursday, 14 September, 2017
In chemical plants, petrochemical plants and refineries, tunable diode lasers (TDLs) are becoming an increasingly common sight. Their high reliability and low maintenance has made them the gas analyser technology of choice for many companies. However, installation locations and conditions encountered in some processes have limited their application range.
The rapid rise of tunable diode laser (TDL) analysers over recent years has led to them becoming established as a core measurement technology within the portfolio of gas analysis techniques available today.
The ability of TDLs to interface directly to the process, which eliminates the need for costly and high-maintenance sample handling systems, gives them an inherent rapid speed of response. This makes them ideal for real-time dynamic measurement of process conditions. Conversely, one aspect of TDLs has not advanced at the same pace as the measurement technology itself: the process interface available for cross-stack, in situ units.
Installing an optical, cross-stack TDL directly in a process pipe or vessel creates some installation and operational challenges and limitations which need to be considered carefully before considering their deployment. Nowadays, new innovative process adaptions have been developed that allow the operation of TDL analysers in locations and applications previously considered impractical, if not impossible.
Installation point selection and unit size
The first consideration when planning a cross-stack/pipe TDL is the installation point. The decision has to be based on executing the measurement at the point in the process where the most pertinent analysis data can be collected. However, this can create the first challenge if there are space constraints or if the diameter of the process pipe is small.
Optical restrictions mean that an elongated housing is needed to provide the required focal length between the laser and the receiver. These long optical housings not only create installation and measurement limitations on small pipes but, due to their large internal volumes, also exacerbate the stability of alignment, and provision and consumption of purge gas required to keep the analyser’s optics clean.
Even if space limitations are not a concern, alignment of the transmitter and receiver units across the pipe is always a consideration. A cross-stack TDL requires careful alignment, which means mating flanges have to be welded onto the pipe prior to installation, adding cost and complexity to the procedure.
To aid alignment of the transmitter and receiver units, various alignment mechanisms are available, ranging from simple, large, compression O-rings to more complex, flexible, metal sealing designs. Care has to be taken to ensure that satisfactory alignment has been achieved, and that the integrity of the process line has not been compromised through introducing leak paths during the alignment process, which could allow the escape of process gases into the atmosphere.
An issue that is particularly relevant when considering installing a cross-stack TDL relates to its size. Firstly, the optical alignment can degrade over time as the alignment mechanism sags due to the weight of the transmitter and receiver body together with the long optical housings (Figure 1). Additionally, if the vessel wall is thin it is essential to add braces to provide additional stability.
Even when these considerations have been thoroughly accounted for, alignment can still be compromised if the process temperature varies greatly, for example, during start-up or when using the analyser on a batch process where there is a significant temperature ramp profile. This is because the walls of the process vessel will flex as they heat up, and although this may not be visibly noticeable it can lead to the laser becoming misaligned with the receiver once the process is running, necessitating expensive and time-consuming realignment.
In an attempt to mitigate this effect, the laser beam profile can be altered to create a divergent optical spread. While this can help to ensure some signal always reaches the detector, the received signal intensity is inevitably reduced.
The next major consideration is the provision of purge gas to protect the analyser’s optical windows from particulates or condensation entrained in the process gas stream.
The majority of cross-stack TDLs require an optical purge, and due to the diameter of most TDL optical housings there is a large purge volume to fill and a sizable optical surface area to be kept clean. This leads to significant purge flow requirements, sometimes as much as 50 litres/min for each side. Such a high level of consumption obviously brings cost implications, and additionally introduces substantial quantities of diluent gas into the process stream, which may create process quality or other problems downstream.
For many processes it would be ideal if it were possible to easily employ a TDL analyser directly across a small bore process pipe. This is a challenge for the majority of in situ TDLs due to their size, weight, sensitivity and optical design limitations. Even the best instruments available are usually limited to a minimum pipe diameter of 12″ (DIN 300). This restriction often means that a suitable expansion pipe section has to be installed, which brings its own concerns (pressure reduction and reduced velocity), or a slipstream or bypass installation must be fitted.
Although the bypass approach can enable a cross-stack TDL to be installed on a small pipe, it is not an ideal solution. Firstly, in such an arrangement the optical purge on one side is working in the direction of process flow, while the other is working against the flow. This can lead to instability of the optical path length, which in turn will result in measurement error. Secondly, the purge gas will have a pronounced dilution effect on the process gas as it passes through the bypass arm, and there are other considerations such as process flow control effects, especially as the valves can introduce undesirable pressure drops in the line.
The final consideration is more subtle but nonetheless is an important phenomenon that is relevant to TDL technology. When a laser beam passes through gases at various temperatures and therefore different densities and indices of refraction, the beam will be diffracted at the interface where the density changes. When using a cross-stack TDL to analyse a hot gas stream, this is exactly what occurs, since typically there will be a non-homogeneous temperature distribution and in addition, a high flow rate of cold purge gas at each end of the optical path. This diffraction of the laser light is known as “beam steering” and contributes to signal noise. It affects the measurement stability of the analyser as well as potentially adding complications to the alignment process.
As can be seen in Figure 2, beam steering leads not only to loss of received energy at the detector, but importantly, laser energy will arrive at the receiver from various optical paths and some rays may even miss the detector entirely. The cumulative effect is random fluctuations of the laser beam at the receiver and therefore noise on the signal. In addition, the multiple signal paths further contribute to increased signal noise and measurement instability. In general, beam steering increases with temperature and pressure in the process and also with the length of the optical path through the hot gas. Even if there is no cold purge gas, beam steering can still cause a significant decrease in measurement quality.
Make the analyser fit the process
Where can choices be made to overcome the limitations outlined above? As a general rule a reduction in the weight of the TDL can go a long way to eliminating concerns regarding alignment stability and the use of TDLs in tight spaces. Some lighter cross-stack analysers are available, but they still require alignment, which can become more difficult if the optics and beam diameter have also been reduced. There also still needs to be opposing flanges fitted across the pipe and significant volumes of purge gas.
There is an alternative to cross-stack devices. For the majority of process applications, a folded-path TDL (where the laser beam from the sensor head is reflected back to a receiver also in the sensor head) offers a host of advantages:
- Transmitter and receiver in a single unit and no need for an expensive interconnecting cable.
- Usually single flange installation.
- No tricky alignment across the pipe or vessel.
- Reduction in purge gas requirement.
- Small size, so easy to install in tight spaces.
- Greater accuracy (as the laser beam passes through the sample twice).
- Lightweight design removes stress on flanges and seals.
Folded-path TDLs (Figure 3) address many of the problems of cross-stack analysers. The advantages they offer have significantly opened up measurement opportunities in situations that have previously proved difficult or impossible to overcome. Such TDLs may still require purge gas to protect the optics, but due to the smaller diameter of the in situ probe and low internal volumes the purge requirement is significantly lower, sometimes as much as an order of magnitude lower. For gas streams containing entrained particles or where there is potential for condensation, purge will remain a necessity, at least for the foreseeable future, but not all processes fall into this category.
Headspace monitoring using a TDL
When considering a TDL for headspace monitoring, the matter of purging is critical. This is because the purge gas will dominate the optical path of the TDL (Figure 4) as the sample is largely static and there will typically be insignificant circulation or velocity in the headspace to displace the purge gas. This would be a hazardous situation, since the analyser will always show an erroneously low target gas value. Since many headspace and inertisation applications are free of particulates, not using purge gas can be viable, but for cross-stack analysers two ports are still required. This eliminates simple attachment to the tank and means elaborate bypass or extractive systems are necessary.
A better solution is a folded-path TDL that does not require purging and that can interface through a single port into the headspace itself. The separation between the optical windows of the device fully defines the optical path, and as the laser beam passes twice through the gas, good sensitivity can be achieved with a compact probe.
Hot and dusty applications
Another application that poses issues for deploying TDLs is hot and dusty processes. If purge gas entering the gas matrix is not a problem then it can be used to protect the optics on either a cross-stack or purged folded-path TDL. But the combination of cold purge gas and hot process gas raises the problem of beam steering. The ideal solution would be to remove the purge requirement if possible (with the added benefit of simplified installation and reduced running costs). However, removing the purge means another form of protection for the optical windows is necessary. The answer here is use of a non-purged, folded-path TDL with the addition of a sintered metal filter and baffle.
TDL on pipes down to DIN 50
In the past, a typical ‘no-go’ location for TDLs has been in situations where there is a need to interface directly into a small diameter process pipe. As has been discussed, cross-stack analysers require complex and expensive bypass configurations, and even folded-path analysers are typically limited to a minimum diameter line of 4″ (DIN 100). An innovative inline process adaption: the wafer cell (Figure 5), provides the solution.
This adaptor allows a TDL to be installed on pipes down to 2″ (DIN 50) and offers no obstruction to the process flow, which brings the additional benefits of compatibility in high velocity operation (>25 m/s) and immunity to vibration.
Process adaption for extractive systems
There are occasions when there might be a fully serviceable sample handling system — perhaps one previously used with a paramagnetic or NDIR analyser — which a site would like to continue to use with a TDL installation.
Previously, adding a TDL analyser in these circumstances has usually meant either installing a specialised, fixed, extractive-style TDL or adding a sample cell to which the transmitter and receiver housings of a cross-stack analyser can be attached. This is a viable solution, but will often mean a large panel is required to hold the sample cell and analyser hardware and can be a significant problem if wall space is at a premium.
An elegant approach in these circumstances is an analysis device that can be easily adapted between an extractive or in situ interface, whether it is a purged or non-purged probe, or an inline wafer cell. Such an extractive process adaptor (Figure 6) attaches directly to the TDL analyser to create a stable, fixed and prealigned optical cavity. The analyser can be operated continuously as an extractive analyser or modified later simply by changing the process adaptor to offer the most appropriate interface for the final in situ location.
The benefits of TDL measurement are not always achieved in practice due to the limitations of the process interface of typical cross-stack, in situ TDLs. These limitations create challenges and often lead to compromises regarding measurement stability and integrity in many potential TDL applications.
By recognising these limitations and re-imagining a better solution, in the form of folded-path TDLs, it is now possible for a compact, lightweight TDL analyser to be utilised across a vast range of installation locations with confidence and without compromise.
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