Propylene oxide

MM: Welcome everyone to another Servomex podcast. I’m Maria Mokosch, Application Development Engineer here at Servomex, and I will be hosting this podcast. Today’s topic we are discussing is propylene oxide, or PO. This is part of our oxidation process series, so please have a look on our website. I am joined by two of our specialists. First, we have Karen Gargallo, our Applications Manager. Hi Karen.

KG: Hi Maria. Thank you for having me.

MM: Good to have you here. And we have with us also, Stephen Firth, our Global Business Development Manager. Hi Stephen.

SF: Good morning. Thank you for inviting me.

MM: Well, welcome Steven and Karen. So, we are going to talk about PO. Today, this colorless reactive liquid is used primarily as a building block in the manufacturing of derivative products, such as polyethers or polyether polyols, and propylene. The majority of PO produced is used for polyether polyol production. Polyethers are used in flexible or rugged foams; in 2020 propylene glycol had a market share of more than 20%.

Propylene glycol is mainly used in food processing pharmaceuticals and in antifreeze, anti-icing systems. So, propylene oxide, Karen, tell us a bit more about this component.

KG: So, propylene oxide, why is it important? As you’ve already said, Maria, it is highly versatile. It is the requirement to make polyurethane raw materials, which are then used in the manufacturing of thousands of everyday products. We are seeing that, especially in the Asia Pacific region, due to the requirement of customers with skincare, cosmetic products, you are seeing the growth in that part of the world, with regards to propylene oxide production.

In addition to what you’ve said with antifreeze, it is also used to make the rigid foams for the thermal insulation in the construction industry. And at the same time, when you look at upholstery, furniture, seat cushioning, and in your cars, those are all made from polyurethane, which of course propylene oxide is used for. An additional thing, just because of the structure of the component itself, it is also used as a building block in the production of other intermediate products.

MM: Stephen, tell us a bit more about the actual production processes.

SF: Propylene oxide is made via a number of routes. There seem to be as many different routes as there are companies out there making it. Originally the first viable commercial route was by chlorohydrin, and basically this was used up until the mid-Eighties, mid-Nineties, which then followed a hydroperoxide route to make, usually, probably an oxide and styrene monomer amongst many others.

The point being that the chlorohydrin route was quite nasty with carcinogenic materials, so it tended to be phased out. There are still a number of basic routes there, but they still all work on the principle of careful oxidation of various materials to produce propylene oxide. Propylene oxide itself is quite a reactive material.

So everything has to be done – with great caution is probably the best description – and great care as the process is effectively… You need enough oxygen in there to actually make it work, but not too much because if you put in too much, it tends to run away with you and tend towards exploding. Or there’s too little and it doesn’t actually work at all, you don’t get any efficiency in the process.

So it’s the classic Goldilocks reaction; you need to be right in the sweet spot with the oxygen within that process.

MM: So, basically, you would say that oxygen is one of the most important components to be controlled within this process.

SF: Yes, the propylene oxide production is very much about measuring oxygen. There are various points throughout the different processes where you actually do the measurements, but on any, propylene oxide plant, there are tens of oxygen analyzers, usually used in two-out-of-three voting systems, because the oxygen level has to be so carefully controlled at various points throughout the process.

So, when you go into any propane oxide plant, the oxygen analyzer is probably the most numerous of all of the analysis technologies that are used on that process. There are one or two others, and we’ll talk about those a little bit later, but predominantly the key measurement is the oxygen on this process.

KG: Yeah. Steven highlighted of course, the importance of oxygen measurements with the rack oxidation routes in making propylene oxide. We’ve also discussed this in the ethylene podcast, so I think just to highlight that in any type of oxidation, direct oxidation processes, we need to ensure the correct equipment and correct technology are utilized to make the… not just the process control, but also most importantly the safety application for the reaction.

MM: Looking at the processes, which challenges would you expect the customer to have to overcome and how can Servomex help them?

SF: So basically, as we’ve described, the production of propylene oxide is a gentle oxidation reaction. So oxygen becomes the key measurement, and that entails Paramagnetic technology, although there are Infrared and Zirconia measurements throughout the process, but essentially this is somewhere where you’ve got to be very careful. The customer is looking to control his process and do that safely, and then there are numerous quality measurements along the way.

From his point of view, it’s usually a hazardous area, so he’ll need suitable hazardous area equipment. As we’re dealing with safety measurements, there’s an increased requirement for a SIL – or a safety integrity level – approval to come along with the equipment, and if we just look at the application itself or the actual measurement, the response time is critical, from the point of view that they want to be able to control the reaction and clearly stop it before it gets into a dangerous state. So the speed of response is really important, and also, because the sweet spot on this reaction is relatively narrow, the accuracy of the equipment becomes quite important, so they know that they’re in the right window, um, when they’re doing the measurement. These are the key things that are important on this particular measurement.

KG: Based on, I think, the requirements that Steven has advised for the safety oxygen measurements in this process, the ideal technology is something that is not affected with the other components present in the sample, but at the same time provides the accuracy from low-level to high-level percent oxygen.

The Servomex OxyExact [2200] analyzer accuracy is 0.01% from across 0% up to 200% oxygen. This is also able of course, to measure enriched oxygen. That will give the customer the confidence that the analyzer that they’re using for this process will give them stable, accurate readings.

It also has SIL approval to then be able to use for the requirement for such a dangerous reaction to then have a one-out-of-three or two-out-of-three voting system for the safety measurements.

MM: Thank you, Karen. Steven, with a two-out-of-three voting system, how would you describe the typical system which a customer would require?

SF: What you find on propylene oxide plants – and actually it’s similar on ethylene oxide and terephthalic acid, PTA – is that two-out-of-three voting systems, so basically three analyzers on the same measurement, are totally independent of each other. So, there is no common point of failure. So the first that you see is effectively there are three sets of sampling, all independent. And the idea here is that… no common point of failure, any one of them can be taken offline and calibrated, and then the other two can still be used to monitor the process. And clearly that out-of-action process is kept to a minimum.

The actual sampling systems are relatively straightforward. You have to be a little bit careful of the materials of construction: propylene oxide is quite a reactive material, and so is the sample, wetted parts need to be of appropriate materials. But the actual sampling system itself is relatively straightforward.

Some of the measurements are done, basically, heated, so rather than use the standard 2200, which has a 60-degree cell, you make use of the 2222 version, which has a high-temperature cell, the Paramagnetic cell being heated up to 110 degrees centigrade. And at this point, you can keep everything in the gas phase. And this is quite important when you have various organic solvents, so you can keep everything in the gas phase, you’re not going to lose anything, you know, you’re making an accurate measurement of the oxygen in the actual process, and that’s quite important.

And then, also, it’s really important now that the sampling systems return the sample to the process. In years gone by, these things used to be vented to atmosphere regardless of the dangerousness or poisonousness of the materials. Now it’s very much the case, on processes of this nature, that the sample is returned to the process. And that means handling the sampling system with enough pressure to drive it back into an appropriate tapping point.

MM: Very good point, Steven. Apart from the oxygen measurement, Karen, which other measurements would be considered essential in the process? For example, for quality control, rather than only process control.

KG: Safety of course, as with any process, is the most important. We have to make sure that the process is safe, that there’s no runaway oxidation to then cause an explosion in the process, which, of course, is a big issue with propylene oxide production.

Process controls as you mentioned is also important. So as with any process, we have to ensure that to get a very good reaction, very efficient reaction, is to also measure the components that we are introducing to the reaction. So for the process then for the direct oxidation, which also called the hydro peroxidation route, we need to measure, of course, the oxidation needs oxygen from air.

It is essential to measure the air – that is, the oxygen – that is being introduced to the reactor, but also the same time is the other inlet components. So, for the SM PO routes, which is the styrene monomer propylene oxide process, ethylene is used as the other inlet to the reactor. So then, an infrared analyzer such as the Servomex SpectraExact [2500] is then required to measure the, the ethylene oxide introduced to the reactor.

In addition, of course, too, the most important application in this propylene oxide production is the safety for oxygen measurements. There are also process control and quality measurements that we have to consider, so I think it’s a good idea to divide the process into four to five smaller stages.

The first part is when you then make the ethyl benzene and for that you need the ethylene and the benzene to make the ethyl benzene. For the reaction to proceed efficiently, we need to measure the concentration of ethylene and benzene that are introduced to the reaction.

So, in this case, we measure ethylene with an infrared analyzer, such as the Servomex SpectraExact. With benzene, though, it is also important to measure the quality of this component by, in this case, instead of measuring the purity – like hundred percent benzene – but to measure the water concentration in the benzene, in that for the peroxidation reaction. Again, you’re seeing an infrared analyzer such as the Servomex SpectraExact.

The second part then – main stage – is now we have the ethylbenzene, that is now, then, the actual air oxidation, but in the importance of then having the oxygen measurement, not just for the safety of the reaction, but also to measure the air that is being introduced for oxidation.

And then the third main section is to dehydrate the MPC that is produced from the propylene of oxidation, and the last part is now that we have dehydrated the MPC to styrene, and then of course, styrene monomers then made into the propylene oxide.

So those are the main stages in the process. So we’ve spoken about… as with safety, we have to make sure of process control and quality measurements as well throughout the process, including the ethylene, the quality of the benzene, and at the end, the product as well, when you have the propane oxide, is to measure the quality of that by measuring the water content in propylene oxide.

MM: Thank you, Karen, for the description of the process, and where we can use measurements apart from the oxygen measurements. The SpectraExact 2500, which you mentioned, is one of Servomex’s flagship analyzers. Stephen, can you tell us a bit more about why this analyzer is idea for quality and process control for this application.

SF: The Servomex SpectraExact is an infrared analyzer, at its simplest. Perhaps its biggest advantage is that it has a sampling cell that is external to the electronics, so this means that we can put, basically, any type of gas – and some liquids – through the cell, be they flammable or toxic, and prevent any leakage from getting into the electronics. This means, effectively, we’ve got a separate sample from the electronics, meaning we can use it on a wide variety of applications by changing the optics, by changing the infrared filters, to measure a wide range of gases or liquids.

The benefit on a process such as the propylene oxide process, where there are many different gases to measure – you might want to measure ethylene or propylene or propane, and then you go on to maybe measure ppm moisture in liquid propylene oxide – is that you can use essentially the same analyzer for all of these measurements.

So, this means that you’ve got one analyzer on the process, you don’t have to have different analyzers to do different bits of the process. You can use the same analyzer with the same software, with similar sets of spares, and it gives you a certain commonality which helps with the maintenance and the ongoing training, that there’s only one basic analyzer to get used to.

As I’ve said, the great beauty is the sample cell is separate, which means we can operate at higher pressures, we can operate at different temperatures, and it gives a large amount of flexibility, which is really useful on these rather complex processes where at one point you’re doing one measurement, at a second point you’re doing a totally different measurement, but the advantage of using the same analyzer is great.

The 2500 analyzer, it comes in a hazardous area version, it’s got a SIL rating. it’s able to run auto-calibration, auto-validation and drive the solenoid. So, it’s a very flexible piece of equipment, which means it’s used quite a lot on the more toxic/corrosive applications because of its design. The sample cell is effectively a stainless steel bar with a hole down the middle, which means it’s very robust and can withstand the rigors of being on a process plant.

Besides that, the only… the other measurement to mention is that there are various process heaters on the process, which means that combustion gas is being produced and used to heat the actual process, and here we would use our 2700 combustion flue gas analyzer to measure the oxygen and carbon monoxide, mainly for efficiency, but there’s a safety measurement there as well. And by controlling that, and being able to control the temperature, then you, again, maximize the efficiency of the process.

MM: Thank you, Stephen, also for bringing up the point about combustion control. Another analyzer which we should mention at that point is the Laser 3 Plus. Stephen, do you want to elaborate a bit more about the advantages of the 2700, so FlugasExact 2700 versus – or maybe in favor of – lasers?

SF: Yeah, we’ve seen the Tunable Diode Laser start to be used on a number of measurements in different processes. The laser itself can either be used effectively in a process oxygen mode to measure a percentage oxygen in a process, or as a combustion oxygen mode of measuring combustion oxygen in a flue gas. What we’ve seen is that for the larger furnaces – ethylene crackers would be an example, that’s probably anything bigger than about a five-meter furnace – Tunable Diode Lasers tend to be preferred over the more traditional Zirconia technology. The main reason for this is the laser is an in-situ measurement – it goes across the process – and the speed of response is very fast, usually only a few seconds.

Because you’re going across the whole of the process, you get an average measurement and this can be for the oxygen, it can be for the CO, and occasionally for the methane measurement as well, because if these are fueled by natural gas, the methane measurement is used as a safety measurement. But it’s across the whole process, whereas Zirconias tend to be a spot measurement, and they’ll take a reading from about a meter to two meters around the tip of the probe. So, if you’ve got a big furnace with a Zirconia, you’re only taking a small part of the sample, but with a laser you’re covering the whole of the process, which is why on the bigger furnaces, the laser technology is starting to become the preferred technology.

MM: Thank you, Steven. Just to add to that, another point which our customers quite like is the low maintenance requirement of the Laser 3 Plus. It tends to require only a checking-up service about once a year. So that is one of the reasons why customers are looking more and more favorably to the laser.

KG: So the importance for making propylene oxide is to the production of urethanes and also other chemical intimidates. As we mention in our ethylene oxide podcast, ethylene oxide is the simplest epoxide, so an epoxide is a three-member ring, but propylene oxide is again another epoxide. If you look at the structure of the epoxide, it is what makes it highly reactive. That’s why it’s very good in using it to make more useful products such as urethane or polyurethane. But the disadvantage of having such a structure is that what makes it reactive is also what makes it unstable and extremely flammable.

And, if they’re not controlled with oxygen, when you have an oxidation reaction to make propylene oxide, it can then react violently and cause a fire and explosion. That is then why it’s important to make sure that we have a very careful control of the process, the oxidation, in the making of propylene oxide.

MM: So, Karen, do you want to give us an overview of the chlorohydrin process?

KG: Steven has mentioned chlorohydrin is the first method that was developed for the production of propylene oxide. If, again, you listen to our podcast or have read about ethylene oxide, chlorohydrin is also the process that was used for the production of ethylene oxide, and actually some plants for the production of ethylene using this process were then utilized to make propylene oxide.

Steven also said, of course, that it’s now being replaced by other processes, such as direct oxidation, because of economic reasons, but at the same time, the environmental concerns about some of the components that are utilized in the chlorohydrin process.

So, chlorohydrin is also called hydrochlorination, the reason being you used chlorine and water vapor to make propylene chlorohydrin, you combine propylene, chlorine, and water vapor to make propylene chlorohydrin. That’s why it’s called the chlorohydrin process. And then from that, once you have the chlorohydrin, you use components to then remove all the hydrochlorinate, the chlorohydrin, to make the propylene oxide. And the component that is used to dehydrochlorinate – that means remove the HCl – the component is caustic soda. It is still the process, but again, we are seeing the move towards more environmentally friendly and more efficient, more economic processes such as direct oxidation.

MM: Thank you Karen.

SF: Generally speaking, there are a large number of propylene oxide projects, mainly in Asia, as Karen’s already mentioned. And the growth seems to be between five and 6% per annum going on throughout the next decade. So, this is a process that will continue, that will be popular – if that’s the right word – going all the way through to, probably, the 2030s as we need more and more polyurethane effectively in various guises.

MM: That’s a very good outlook, Stephen. Thank you once again, everybody for listening to another Servomex podcast. And thank you, Karen and Stephen, and for your contribution. Don’t forget, we have lots of information available on our website. Please do visit Servomex.com to find out more about propylene oxide solutions and other oxidation processes. On the website, you will find videos and links to literature, so please have a look. Thank you once again and see you next time.