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Gas analysis in ethylene production
ES Magazine Issue 20 Industry Process & Emissions
Hydrocarbon Processing Magazine Issue 01
Ethylene Production Brochure
In ethylene production plants, it is essential to analyze process gases accurately and reliably to ensure safety and efficiency. Feed gas quality is critical to the overall process; it is also vital to monitor gas quality throughout the process to ensure a high product yield.
Failure to monitor the gas feed throughout the process can adversely affect the efficiency of the process – a less pure gas means a lower ethylene yield once the cracked gas is quenched and cleaned. There are also issues for safety and emissions if high levels of contaminants enter the wrong part of the process.
The SERVOTOUGH SpectraScan 2400 and SERVOTOUGH SpectraExact 2500 analyzers provide accurate gas quality monitoring at many points throughout the ethylene process. This allows optimization of the process reactions to ensure greater efficiency, delivering a higher yield and better-quality product. We also supply analytical solutions for safety, combustion control and emissions monitoring.
Our extensive experience in supplying analytical systems to the ethylene production industry means we understand your process. Our solutions are proven to deliver the results you need.
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With a wide range of gas analysis technologies available, we’re able to deliver the most precise, correct measurement for each point in the ethylene process.
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The latest podcast in our series looks at gas analysis solutions throughout the ethylene production process. Listen now to find out how Servomex provides optimum solutions.
Ethylene podcast transcript
MH: Welcome, once again, everybody to a Servomex podcast, today on the subject of ethylene. I’m joined once again by Maria Mokosch. Hi, Maria.
MM: Hi, Matt.
MH: So, just a reminder, Maria is our application development engineer looking after the EMEA region. Good to have you aboard again, Maria.
MM: Yep, thank you for inviting me back. I hope we’re going to have a really nice podcast and talk about the very serious subject of ethylene.
MH: You’re doing something right, because we keep getting you back, Maria. So you obviously know what you’re talking about!
Before we launch into talking about the process, and obviously, the analyzers, I just want to give a little background on the subject of ethylene. It’s a very, very important process for Servomex. There’s a lot of measurement points, there’s a lot of different analyzers used, it’s a long and relatively complicated process. But it’s also incredibly important in the world, it gives us a chemical that is used in so many different manufacturing processes for very, very important products.
Ethylene is the simplest alkene, I think, you can get which is an organic compound. An alkene is something with a carbon-carbon double bond. And basically, what happens, is ethylene is used as a monomer, so it’s used to build much larger chains of molecules, which is a process called polymerization.
One of the most common outputs we get from this polymerization process of ethylene is polyethylene. And polyethylene is used to make all kinds of plastic packaging, which is used in everyday products that you buy in the in the supermarket and grocery store. Plastic bottles, which are still widely used for the beverages that we drink. It’s also used to produce things like ethanol – which is industrial alcohol – ethylene oxide, which we’ve covered on a previous podcast and which is another incredibly important chemical, acetaldehyde, and vinyl chloride. You can also mix it with benzene and produce ethyl benzene, and this is used to produce things like styrenes, which are used, again, for more plastic manufacture, synthetic rubber manufacture, things like that.
Over 200 million tons was the estimate for 2020, produced globally. There are approximately 265 sites operating globally producing ethylene, and it’s very much a globally produced product, because it’s a commodity product. Many different countries will want to produce it themselves, to not be reliant on sourcing it from outside of their borders. And one interesting fact I found out, which I wasn’t necessarily aware of, is that the vast majority of the top manufacturers actually sit in Europe. It’s a very Europe-centric process: I think something like the top eight manufacturers were based in Europe, with the biggest supposedly placed in Germany, according to the data, and obviously that’s another really good reason why we’ve got you here today, Maria, that’s your homeland, Germany tops the league table.
It’s no secret these producers are some of the biggest names in chemicals, that that you would have all have heard of. People like LyondellBasell, Dow, Chevron, Exxon, Formosa, BASF, Shell, Total, you know, these are all huge names in petrochemicals. And they are the biggest producers of ethylene globally.
So, Maria, I mentioned it’s a very long complicated process, which it is, and you and I have got the process map in front of us right now, just to remind us, so we don’t get lost on our journey through ethylene. Could you try and break it down a little bit for us, and walk us through maybe what some of the key sections are, and then we’ll maybe go into a little bit more detail about the process.
MK: Sure. So first of all, as you’ve already mentioned, ethylene is one of the most important petrochemical intermediates, and indeed feedstock, of many end products: food containers, food packaging, you mentioned that, but also toys let’s not forget about that, carpets, insulation. So literally everywhere. There are five major licensors of ethylene plants such as KBR, Technip, Linde, Shaw, Stone and Webster, and Lummus. And, while the ethylene production differs slightly by licenses, the overall process is fairly similar. So I hope the licensors forgive me if there’s a slight difference in the actual process from what I’m about to explain.
MH: Maybe Maria, just before we carry on, as well, something I’ve realized from previous podcasts, is this term we’ve used in the past, which is licensor. So maybe it’s worth just explaining what we mean by that. Licensor is, you know… You can have lots of different companies that manufacture the chemicals, the names that I listed earlier. But there’s only a handful of companies that actually produce and manufacture the equipment that produces chemicals like ethylene, and also kind of own the manufacturing method to make that chemical. So using very specific chemicals or even catalyst materials, a lot of that can be quite proprietary as well.
So that’s what we mean by licensor, the people that actually make the equipment and own the methodology. And then the companies I listed are the ones that buy the equipment and use it to then make the product they then go on to sell.
MK: Indeed, yes. So, the overall process consists of four sections: cracking or pyrolysis, quenching, compression and cleaning up, and fractionation or separation of the products. Let’s start with the first step. The first step in the production of ethylene is to take the feedstock, which can be naphtha or natural gas, and crack it into ethylene and other various products in the furnace. The process is called pyrolysis; pyrolysis is the thermal cracking of hydrocarbons with steam, and is also called steam cracking.
The hydrocarbon stream is pre-heated and mixed with steam. The mixture is then heated further, up to about 500 degrees Celsius, and enters the cracker, where it is heated to 850 degrees Celsius, approximately. The tubes where the stream is in, are externally heated by gas-fired burners. Depending on the furnace design, they’re either on the bottom or on the side. The cracked gas then leaves the cracker at 850 degrees Celsius and is cooled rapidly to 230 degrees Celsius. This is done to prevent further cracking and loss of product.
The cooling produces steam, which is used for power generation – for example, driving compressors – and the gas is quenched with water, cooled, and routed to the primary fractionation section. Due to the high temperature in the furnace, the recovered heat can be used for pre-heating the reactants and for steam generation in the re-boilers.
So, let’s take a little stop here, and talk about the necessary monitoring steps. The feed quality needs to be monitored for high levels of CO2, because CO2 can freeze out at later process stages and damage equipment. The recommendation is to keep it below 100 ppm. Infrared analyzers, such as the SpectraExact 2500 can monitor the CO2 at the outlet of the feed treatment. And a tunable filter spectrometer, like the SpectraScan 2400 can monitor percent level CO2 and hydrocarbons in the feed itself.
In combination with the H2scan, the SpectraScan 2400 can provide a complete BTU analysis of feed and fuel. This is especially useful when hydrogen and methane rich tail gases are recycled to the burners.
Another important measurement for process control is the monitoring of oxygen and CO in the flue gas of the cracking furnace. This measurement can be done by combustion monitors such as a FluegasExact 2700 or in-situ TDLs such as the Laser 3 Plus, which provides an average path analysis across the furnace. If someone would like to know, measurement ranges are typically about 10% oxygen and 1000 ppm CO.
Further up the stack, legislation requires continuous monitoring of the emissions such as oxygen, CO, NOx, SOx, and, in addition, emission analyzers can be installed on either side of the SCR – which is selective catalytic reduction unit, for those not familiar with the abbreviation – to monitor the efficiency of the SCR in reducing NOx emissions. A cross-stack installed TDL, such as the Laser 3 Plus, monitors the ammonia levels which are allowed to slip through the catalysis during the NOx reduction.
So, coming back to the cracker itself, on the outlet of the cracker, a TLE – transfer line exchanger – rapidly cools the correct gas to preserve the gas composition, which should be monitored for conversion control to ensure the highest possible ethylene yield, and to prevent under- or over-cracking. This can be done, for example, with the SpectraScan in combination with the H2scan. This gives you a complete analysis of the correct stream, basically.
To prevent efficiency loss, decoking of the cracking furnace will be frequently necessary, on average about every 20 days, to get rid of the accumulated carbon coating. This is done by burning off the coke with air in a steam atmosphere. The progress of the decoking is monitored by measuring CO2 in the effluent. The CO2 content rises as the coke is burned off, and reduces then to zero when the furnace is clean again.
During the decoking cycle, a SpectraScan with an added CO2 channel, or SpectraExact with a range of about zero to 5%, can be utilized to ensure a short decoking cycle.
MH: So a short decoking cycle, presumably will be to… you want to be decoking for as little time as possible, because that affects your throughput, of course, so if you can decoke for half the time you can produce more products.
MH: So I just wanted to mention the cracking furnace, to go back to that in just maybe a little bit more detail, because I want to reference the podcasts that we’ve done in the past, Maria, on process heaters, and thermal oxidizers, because we spoke a little bit about cracking furnaces. Ultimately, you know, a cracking furnace system is a heater, it’s just serving a very specific purpose. And, obviously, the term cracking quite literally means taking a longer chain hydrocarbon like ethane, for example, and cracking it into smaller hydrocarbons like ethylene, and you get ethylene out of it. So that’s what that’s what we mean by cracking furnace.
There’s been an interesting trend over the last couple years of the use of more traditional Zirconia technology versus laser technology. It’s becoming quite common to utilize laser for the combustion control, because these are typically very large heaters, these can be 20 meters-plus on some of the bigger plants, maybe even longer. So having that cross-stack measurement, that average, is very beneficial compared to using traditional Zirconias, where you would need multiple measurement points – maybe four or five per side – to get an average.
There’s two other measurements, just to mention. One is the use of the methane measurement combined with CO on our Laser 3 Plus. Because these things are fired on natural gas, as you mentioned earlier, Maria, there’s a risk of natural gas build-up. If you have burner issues, flame-outs or burners not igniting during startup, you can flood the furnace with natural gas with fuel, which is a very dangerous condition. So having that extra methane measurement is there for safety.
The other measurement that’s quite common is above the tubes, where the ducting tends to narrow again, as you’re going effectively from your flue, where you’re taking your gases away to the next step. It’s actually quite common to install another, either a TDL or a Zirconia-type analyzer. And what you’re monitoring here, is for tube leaks. So, if any of your tubes – which are made of, as you know, they can be made of a ceramic-type material – because of the high temperatures involved in the furnace, they can break, they can age, they can crack.
If it cracks, what happens is you get feedstock inside the boiler. And what happens is that under high temperature that feedstock will actually ignite; it will combust on the tubes, which can cause serious coking issues. You can get increased coking rates, you also can get further tube damage. So a Zirconia or laser, placed post-tubes, will allow you to monitor if you’re seeing secondary combustion taking place of any feedstock, and it tells you that there may be some maintenance needed inside the furnace.
Okay, so I just wanted to mention those other two quite specialist measurements there that needed to be done. Back to you, Maria.
MM: Okay, the next step in the very long ethylene process is the primary fractionation and compression of the stream. So, going back to the cleaning of the correct gas before the fractionation step; the gas is cooled down in the quench tower to remove any heavy hydrocarbons. The gaseous stream is compressed in a multi-stage compressor, from which the liquid knockout is sent to the stripper for aromatics recovery. Usually, in the last gas compression stage, the gas is scrubbed with caustic soda to remove any remaining acid gas such as CO2 or hydrogen sulfide. The SpectraExact offers here a rapid measurement of trace CO2, with the minimum range of CO2 10ppm, to detect any CO2 breakthrough from the caustic wash.
After the caustic wash, the gas is dried with the molecular sieve, and sent to the product separation section of the plant. Fractionation and hydrogenation; at the product separation section, cleaned and dry cracked gas is fractionated into separate components. The section involves several distillation columns to separate the individual hydrocarbon products and other treatment steps which process the various streams such as cooling or hydrogenation. The dried gas enters the cryogenic cooler where it is cooled to minus 50 degrees Celsius and fed to the demethanizer.
The incondense, so the elements of the streams, are removed from the top of the demethanizer column and expanded. The top of the column should be monitored to ensure no valuable hydrocarbons are lost. This can be done by SpectraExact, or SpectraScan and H2scan, for ppm CO, percent levels C1 to C3 products, and hydrogen. Methane and hydrogen are separated by a cold box or chiller. It is recommended to monitor the purity of the product stream, or rather the remaining impurities such as CO and, as mentioned, C1-C2 and hydrogen, and the of a stream can be used in the plant fuel network. For example, as fuel for the furnace and the boiler.
The bottom of the demethanizer should be monitored for remaining methane, and before the stream enters the deethanizer, which is the next step where C2s are separated from the rest of the stream. So the bottom stream of the deethanizer then enters the depropanizer, and similar to that the bottom stream enters then the debutanizer, so it’s kind of a chain reaction.
Going back, the distillate of the deethanizer undergoes a hydrogenation, so basically what’s coming from the top of the column. So, the distillate of the deethanizer undergoes a hydrogenation reaction where for example, acetylene is converted into ethylene. The mixture of C2 species are separated in the C2 splitting column, and polymer-grade ethylene is obtained after heat recovery. So, essentially, after the C2 splitting column you only want the ethylene separated and everything else to be before either converted into ethylene or recycled.
At the fractionation columns for C1 to C3 – so we’re talking demethanizer, deethanizer, debutanizer – at each of the measurement points hydrocarbon should be monitored to minimize the loss from the top (so, the separated hydrocarbons at the bottom, and the remaining hydrocarbon stream) to make an optimization of the columns possible.
Then, before the ethylene product is compressed and stored, trace oxygen contamination should be measured, for example with the Delta F 300, for product quality but also for safety. Coming back to the distillate of the depropanizer, the distillate of the depropanizer is hydrogenated to convert methyl acetylene to propylene, which is another product and sent to the propylene purification, where polymer grade propylene is obtained.
The bottom streams of the C2 and C3 splitting columns contain ethane and propane, which are typically recycled as cracker feedstock, and acetylene is recovered in a separate section, or can be recovered in a separate section, using an extractive distillation and can be then subsequently used on site for the production for example. SpectraScan and H2scan are used to optimize the hydrogenation unit, which converts acetylene to ethylene and propane to propylene – so, by adding hydrogen, the triple bond is converted into double bonds, which is what we want. We want the product to be ethylene from the deethanizer after the C2 splitter, and propylene after the C3 splitter. It is important to monitor hydrogen here because adding too much hydrogen makes the process inefficient and risks over-compensation. So basically you are not only creating double bonds, you’re creating single bonds eventually, which is not what you want. So, you would create an ethane or propane. And the other reason to monitor hydrogen is because it can become a contaminant in the product. So, for quality of the product, basically.
MH: There’s an awful lot of chemistry in this process, that’s for sure. So you’re probably still with us if you’re a chemist, or you’ve got a chemistry background. If you’re like me, you’re trying to get your head around it a little bit as we go through it. But I think one of the important takeaways here is that, you know, Maria, you’ve been talking about these demethanizers, deethanizers, depropanizers , debutanizers, and then, of course, all the C1-C3 splitters. Basically, whether it’s the distillate coming out the top, or the dropout coming out the bottom, most of the chemicals that are coming out of these process steps are useful in some way. They are useful for either recycling like ethane, which you can recycle back into the… of course, to crack into ethylene, or they go off to be used to make other types of fuel, so gasolines and things like that.
MM: That’s right. That’s right. And one thing which I haven’t yet mentioned, the debutanizer, at the debutanizer the crude C4 stream from the steam cracking process can be upgraded to more valuable chemicals. So, butadiene can, for example, be recovered by extractive distillation or alternatively the mixed C4s can undergo selective hydrogenation of butadiene to produce butene. The produced butane can also be recycled as cracker feedstock. Then, other heavy hydrocarbons can be sent to the BTX, which is a mixture of benzene, toluene, and the three xylene isomers, for recovery. This can include a separate benzene extraction unit for benzene recovery, and the distillates from the debutanizer are mixed with other incondensable gases and used as fuel in the boiler. After the steps which are already mentioned, in the same way a depentanizer and deheptanizer, in theory, could be added. So, depending on the required product, we can really create an awful lot of products.
MH: Yeah, and that’s why the measurements are so important, I think, as sort of a summary on that. These aren’t just waste products. They’re waste products in terms of, it’s not the ethylene that you want, because that’s your goal here, is to produce ethylene. But everything that we’re producing is useful, as I said, in some ways. So monitoring their purity, and what’s coming out, is very, very key, which is why there are so many measurements on this. All of this stuff that’s produced is valuable to the plant operator. You either reuse or sell. It seems to me, Maria, there’s very, very little wastage in this process in reality.
MM: In reality, indeed, yeah, everything is either a product or can be recycled.
MH: I hadn’t really twigged that it was literally like, on the left-hand side, you’re putting in petrochemical products, and all that’s happening as you go through is, it’s basically just going, it’s cracking, and then it’s just kind of cooling everything out, so you get ethylene at the end, basically.
MM: You are literally separating, so you’re putting naphtha or natural gas in there, then you are separating into the chains. So you’re separating for C2s, then you’re separating C3s, then you’re separating C4s. And if you want to put it further, you can also separate C5, etc. So at any step, you can create a product. But, obviously, the most common ones are ethylene, and then the next one is propylene, both for polymerization purposes.
MH: Yep. One thing we haven’t done in a little while on the podcast is actually talk about the products in a little bit more detail. So, let’s talk about the products in some level of detail here, why they’re good products for these measurements. So we’ll start with the SpectraExact 2500, which is our process infrared-type measurement, suitable for hazardous areas, highly selective. There are different variations of the 2500 products, so NDIR technology, but sometimes utilized with gas filter correlation or gas cuvette – either a more straightforward NDIR, which uses filters to hone in on the wavelength of choice, or gas filter correlation technology, so an upscale version of our GFX bench technology, which uses gas-filled cuvettes, to give us a very highly speciated measurement for particular gases.
MM: Normally, what we’re trying to find is a line where you can measure without interference. So obviously most hydrocarbons are absorbing at, for example, wavelength 3.4. So, if you were to measure, that’s when you’re measuring, basically, total hydrocarbons, but you can’t really differentiate. If you were to differentiate between ethylene and other hydrocarbons, you would need to find a wavelength where they’re exactly not absorbing. Or alternatively, you could use the 2510 and essentially cut out the background. You’re using the highly selective spectrum.
MH: Yes, with the cuvettes. So, obviously, what that gives you in terms of analyzer behavior is a product that’s not just highly speciated and looking at a single gas, but also one that has very low drift, high accuracy. And it’s in a very, very rugged package, the 2500, which is suitable for hazardous area installations, which a lot of these types of petrochemical processes are quite often zoned in one way or another, because of the types of gases that we’re seeing here. So it’s a very solid analyzer for processes like the ethylene process.
The laser, which we mentioned is used on this ethylene process, is primarily used in the cracking side of things. Again, it’s another infrared-type measurement, ultimately, just using a different part of the spectrum. So again, we’re getting a highly speciated measurement, a laser tuned for oxygen or a laser tuned for CO with additional methane.
Our lasers are capable of being mounted what we call cross-stack, which means you have a transmitter on one side of the process and a receiver on the other and the laser beam is shone through the process from one end to the other, which means we can cover great distances – 20 meters, for example, which is the number I used earlier – and get a true average path measurement. And obviously, we have the ability to provide all kinds of specialist installation equipment to compensate for process wall movement: we can use diverge laser beams, for example, to cut that off. We’ve got years of experience with installing these on large process ducts.
We also have, in every laser now, what we call a reference cuvette. This is a small gas cuvette that’s filled with the gas of interest, so either oxygen or CO, and a very, very small part of the laser beam is split away from the main transmitter running through the cuvette to a separate detector. And what that basically ensures is that we retain lock of the line that we’re looking for, the laser wavelength of interest. And that’s because in a process where that line may disappear, for example, so CO could disappear, in theory, some lasers will start hunting around to find a line that isn’t there. Our reference cuvette means that we’re locked in on a wavelength, the laser is not going to go hunting, which means when that gas reappears, we’re already looking in the right place, for all intents and purposes. So that overcomes an issue with laser technology.
And then the 2700, which is a natural follow-on from the laser when we’re talking about combustion control – very tried and tested technology, zirconium oxide for oxygen and thick film catalysis, or thick film calorimetry, for combustibles in a close-coupled extractive system. And what we mean by that is we house the sensors in a temperature-controlled environment outside of the process conditions mounted directly onto the process duct where our sample is extracted at low flow, analyzed and then returned to the process.
This gives a number of advantages over much more traditional, in situ Zirconia technology, where you would find the Zirconia cell mounted in the probe tip, inside the process, running at very, very high temperatures in a harsh environment where sensor life would be significantly limited. So sometimes it wouldn’t be uncommon to see a 12 to 18-month lifespan on a Zirconia, whereas in our system, we’re looking at eight years plus, easily, because we have much more control over the sample conditioning.
This analyzer has been around since the late ‘90s, really. Obviously, it’s been developed over that time and has a number of new features now, but it’s a very tried and tested method, it’s well understood by the industry, and the product is very well reviewed by the industry, as well, over all those years. So, in summary, a really good product portfolio for combustion control, certainly in the in the cracker. And the Tfx is there, the thick film calorimetry sensor, paired with the oxygen just allows you to fine-tune that combustion process, so monitoring what’s known as CO breakthrough – when you’re at the point where you just run out of oxygen to fully oxidize your fuel, so instead of producing CO2, you produce CO – you can monitor that breakthrough, which normally occurs very, very quickly. You move from a state of very low CO, so baseline levels, a couple of 100 ppm, maybe to a couple of 1000 ppm in a very short space of time, certainly less than a minute, and you need to get that process back under control. If you didn’t have that sensor, you wouldn’t see the breakthrough, and you’d be controlling on oxygen alone, so it’s a very good package to have the two measurements together.
It’s worth mentioning the 4900 Multigas, which is a fairly traditional type of emissions analyzer, now at this point; it’s capable of housing four different sensors, different technologies, so typically Paramagnetic for oxygen, NDIR benches for CO2, and then much more speciated gas filter correlation, which we’ve mentioned in regards to the 2500, to monitor the lower level, the actual emission products that you’re trying to regulate. So low levels of SO2, for example, and NOx, which are typically in the sub-100 ppm range by regulation.
So you can have a number of different sensors in the bench, and the modern 4900 Multi gas, which is only a few years old, in reality. It’s a relatively new product in the Servomex portfolio, replacing a legacy-type system, which now has a very nice front end and intuitive touchscreen interface, and all of the digital communication options you could shake a stick at. And all of the older features, really, the old product had. It’s able to be fitted in in place of one of the old products if you’re upgrading. A very, very good product.
The SpectraScan 2400. TFS: tunable filter spectroscopy. So, this is a bit of a bespoke, infrared-type measurement, this one.
MM: It is. What it does, it’s basically using the unique wavelength while it scans the spectrums of each gas, and measures each gas individually. So it is measuring C1 to C5, mainly. Within these limits, it can measure no matter how many compositions, or how different the composition is, it can measure it.
MH: It uses rotate rotating filters. I think the benefit of the TFs technology, it’s quite often one of its sales pitches, I suppose you could say, is it’s often related to a process GC – so a traditional process GC, which can measure lots of different gases, but typically has a very long response time. You know, a GC has a sample time of several minutes. The SpectraScan is an online gas analyzer, it gives you a live measurement, it has a response time of seconds, like any other process analyzer really, or online instrumentation, and its capable, as you say, of splitting down those C1-C5, giving you the resolution of each gas separately.
MM: So, the most common instrumentation technology currently used for specification of hydrocarbons or hydrocarbon streams is gas chromatography, especially process GCs. This works by separating the hydrocarbon components through a long column and using a carrier gas. The different hydrocarbons exit the column at different times, and then they are detected. The SpectraScan uses a wavelength-scanning tunable filter spectrometer and measures, essentially, the spectrum of each gas. It is a rapid measurement, so it can measure, in less than a minute, anything between C1 and C5, so it provides a very fast response to a hydrocarbon stream measurement.
MH: Thank you everybody, for tuning in for our ethylene podcast. Thank you for joining us again, Maria.
MM: Thank you for the invitation.
MH: Always a pleasure to have you here, I’m sure we’ll have you back for another one. So please do visit servomex.com to find out more about our ethylene solutions. On the website you’ll be able to find the other podcasts, various bits of literature, application notes and process brochures, and all manner of things, so please do go and check out servomex.com. Thanks once again, and we’ll see you next time.
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