At the first glance, if we compare the 2 kerosene and nitroglycerine we easily can observe that at as little as possible mishandling of nitroglycerine it can turn in a devastating results. Nitroglyceriny is highly explosive liquid and is difficult to keep under control. Yet Kerosene is 10 times more powerful is not immediatelly explosive becasue it can be fully controlled. As result finally humans can fly with the help of kerosene. But if Kerosene is out of control it can destroy much harder than nitroglycerin; So let’s see why and how this is happening.
For better or worse, though, all I could think about when I travel by plane is kerosene, and the remarkable trick those mid-19th century inventors used to transform crude oil: distillation.
In order to distil oil, Rhazes had used an apparatus called an alembic, which is what, in modern times, we call distillation vessels – the towers you see sticking up out of oil refineries. Crude oil is a mixture of differently shaped hydrocarbon molecules, some long like spaghetti, some smaller and more compact, others linked together in rings. The backbone of each molecule is made of carbon atoms, each one bonded to the next. Each carbon atom also has 2 hydrogen atoms attached to it, but there’s a lot of variety in their shape and size: the molecules vary in size from just 5 carbon atoms to hundreds. There are very few hydrocarbon molecules with fewer than 5 carbon atoms, though, because molecules that small tend to exist as gases: they’re called methane, ethane, propane and butane. The longer the molecule, the higher its boiling point, so the more likely it is to be a liquid at room temperature.
This is true of hydrocarbon molecules made up of up to 40 carbon atoms. If they get any bigger than that they can hardly flow at all and so become a tar. In distilling crude oil, the smaller molecules are extracted first. Hydrocarbon molecules with 5-8 carbon atoms form a clear transparent liquid which is extremely flammable: it has a flashpoint of -45°C, which means that even at sub-zero temperatures it will ignite easily. So easily, in fact, that putting this liquid into an oillamp is quite dangerous. Thus, in the early days of the oil industry, it was discarded as a waste product. Later, when we began to better understand the virtues of this liquid, it became more appreciated, especially for the way it mixed with air and ignited, producing enough hot gas to drive a piston. It was subsequently named petrol (or gasoline), and we began using it to fuel petrol engines.
Larger carbon molecules, with 9-21 carbon atoms, form a transparent clear liquid with a higher boiling point than petrol. It evaporates at a slow rate and so is less easy to ignite. But because each molecule is quite big, when it does react with oxygen it gives off a lot of energy, in the form of hot gas. It won’t ignite, however, unless it’s sprayed into the air, and it can be compressed to a high density before it bursts into flames. This is the principle Rudolf Diesel discovered in 1897 eventually giving his name to the liquid forming the basis of his tremendous invention: the most successful engine of the 20th century.
But in the early days of the oil industry, the mid-19th century, diesel engines hadn’t been invented yet, and there was a pressing need for a flammable substance for oil lamps. While searching for this oil, producers created a liquid that had carbon molecules with 6-16 carbon atoms. This liquid is somewhere between petrol and diesel. It has the virtues of diesel – it doesn’t evaporate so quickly as to form explosive mixtures – but it is a fluid with a very low viscosity, similar to that of water. As a result it wicks extremely weIl, aIlowing flame to be very bright. It was cheap and effective, and didn’t rely on olive trees or whales. It was kerosene, the perfect lamp oil.
But is it safe? I wonder myself – trying to relax everytime I travel by plane; While in my seat and start to read a book, before to take-off my attention snapps back to the flight attendants when they present the safety instructions at the beginning of the flight. They get to the bit in the safety briefing about the life jackets. They are aIl wearing one while pretending to blow a whistle. Then I wonder what it would feel like to survive a crash landing on the sea and be floating in water, perhaps at night, trying to blow the whistle, I also wonder what would happen to the kerosene in our fuel tanks in the event of such crash. Could it explode? I know one liquid that certainly could: nitroglycerine.
Like kerosene, nitroglycerine is a colourless, transparent, oily liquid. It was first synthesized by the Italian chemist Ascanio Sobrero in 1847. It didn’t kill him, which is a miracle, because it is a ridiculously dangerous and unstable chemical that can explode unexpectedly. Ascanio was so frightened by the potential uses of what he had discovered, he kept it quiet for a year and even then tried to deter others from making it. His student Alfred Nobel saw its potential though; he thought it could replace gunpowder. He eventually succeeded in creating it in a form that was relatively safe to handle. Alfred transformed the liquid into a solid that wouldn’t explode accidentally (although it did kill his brother Emil), and so created dynamite. This transformed the mining industry, making him a fortune. Prior to dynamite, mining companies had relied on manual labour to dig their tunnels, pits and caverns. He used his fortune – or, at least, apart of it – to create the most famous international award in the world, the Nobel Prize.
Like petrol, diesel and kerosene, nitroglycerine is made from carbon and hydrogen. But it comes with extras, too: oxygen (O) and nitrogen (N) atoms. The presence of these atoms, and their positions within the molecule, make nitroglycerine unstable. If the molecule comes under pressure from contact or vibration it can easily fall apart. When this happens the nitrogen atoms get together to form agas, and the oxygen atoms in the molecule react with the carbon to form carbon dioxide, another gas. They also react with the hydrogen to form steam, and whatever is left over forms still more oxygen gas. As the molecule decomposes, it creates a shock wave in the nitroglycerine, which causes the neighbouring molecules to fall apart too, creating more gas and sustaining the shock wave.
Ultimately, all of the nitroglycerine molecules decompose in a chain reaction that occurs at 30 times the speed of sound, transforming the liquid into a hot gas almost instanteously. This gas has a volume a 1000 times the volume of the liquid and so it expands rapidly, causing an enormous, hot explosion. Much of the devastation of the Second World War was caused by the widespread use of nitroglycerin based explosives.
The 100 ml limit on liquids carried on to aeroplanes is designed to prevent someone from bringing on board a large enough quantity of a liquid explosive such as nitroglycerine to destroy a plane. This amount of nitroglycerine will still explode, of course, but not with enough energy to bring the plane down. But, still, it is sobering to think that kerosene contains 10 times more energy per litre than nitroglycerine and there are tens of thousands of litres of it in the fuel tanks of an aeroplane.
Kerosene is not an explosive though – it will not spontansly explode. Unlike nitroglycerine, it doesn’t have any oxygen and nitrogen atoms in its molecular structure. It is a stable molecule that doesn’t readily decompose. You can bash it smash it or have a bath in it and it won’t explode. Unlike its less powerful cousin, nitroglycerine, if you want to harness the power of kerosene, you have to work for it – you need to make it react with oxygen. As the kerosene and the oxygen react, they will create carbon dioxide (CO2) and steam, but because the reaction is limited by its access to oxygen, the combustion can be controlled.
It is the huge power of kerosene, and our ability to burn it in a controlled manner, that makes it such an important liquid technologically. Global civilization currently burns approximately one billion litres of kerosene per day, mostly in jet engines and space rockets, but it is also still used for lighting and heating in many countries. In India, for instance, more than 300 million people use kerosene oil lamps to provide lighting in their homes.
Still, as much as we like to think we’ve got kerosene under control, there’s none the less a sinister side to it. The horrors of 11 September 2001, are a case in point. On that day I was at home, staring in disbelief at the television. In truth I can’t remember if I saw live footage of the second plane flying into one of the twin towers or whether what I saw was a news recap, but it stunned me. I stood looking dumbfounded at the telly trying to comprehend the scene. The two buildings were on fire, and there were reports of other planes being flown into targets elsewhere. It seemed like things couldn’t get any worse, and then they did: the 1st tower came down, collapsing in the type of slow motion that only giant objects can do. And then the 2nd tower came down. We were ready for it this time, but it was no less numbing.
It was the fuel from the aircraft that caused the towers to collapse. It wasn’t an explosion, because kerosene is stable. According to the FBI report, the kerosene reacted with oxygen from the winds blowing through the buildings’ damaged floors, raising the temperature on those floors to over 800°C. This did not melt the steel frame of the building – steel melts at temperatures exceeding 1500°C – but at 800°C, the strength of steel decreases to approximately half its original strenght and so it started to buckle. Once one floor buckled, the weight of the entire building above it collapsed on to the floor below causing it to buckle, and so on, like a house of cards. In total more than 2,700 people were killed in the collapse of the twin towers, including 343 New York firefighters. These terror attacks were a significant moment in the history of the world, not just because they initiated wars and all the horrors that go with them, but because the fall of those towers was such a powerful symbol of the fragility of democratic civilization. And the active ingredient of that moment of destruction was the planes’ kerosene.
So you can see why I would think they might mention it in the safety briefing. But like I said during the pre-flight saftey instruction on board, each time their presentation is ended, and they do not say a thing about the 150,000 litres of kerosene on board, nor comment on its dual nature: how, on the one hand, it’s a perfectly ordinary transparent oil, one so stable that you could throw a lighted match into the fuel tank and it wouldn’t ignite; and yet, on the other, mixed with the right amount of oxygen, it becomes an oil 10 times as powerful as the explosive nitroglycerine. Therefore most people traveling by plane have completelly no clue about all these and they just (want to) look relaxed. When in fact we are literally flying with a “bomb” that if well controlled can indeed bring us anywhere in the world faster than any other way of traveling. But if things are getting seriously out of control then flying is definitelly not anymore safe at all.
Although kerosene is not mentioned explicitly in the pre-flight safety briefing it occurs to me that it is nevertheless hidden in there somehow. If you think about it, the safety briefing is the one global ritual that we all share, whatever our ethnicity, nationaliry, sex or religion; we all take part in it before the kerosene is ignited and the plane takes off. The dangers that the briefing warns us of, such as landing on water, are so rare that even if you flew every day for a whole lifetime you would be unlikely ever to experience them. So that’s not really the point of it. Like all rituals, the language is coded and involves a special series of actions and the use of props. In religious rituals these props are often candles, incense burners and chalices; in the pre-flight safety ritual they are oxygen masks, life jackets and seatbelts. The message of the pre-flight ritual is this: you are about to do something that is extremely dangerous, but engineers have made it almost completely safe. The ‘almost’ is emphasized by all the elaborate actions involving the previously mentioned props. The ritual draws a Iine between your normal life, where you are in charge of your own safety, to your current one, where you are ceding control to a set of people and their engineering systems as they harness one of the most awesomely powerful liquids on the planet to shoot you through the atmosphere to a destination of your choosing. In other words, you need to trust them absolute. your life is in their hands; and so this ritual, performed before every flight, is really a trust ceremony.
In my plane trip to from Brussles to New Youk as the cabin crew began moving down the aisles, ostensibly checking that passengers’ seatbelts were correctly fitted and bags were stowed, I knew that the safety ritual was coming to a close – this was the final blessing. I nodded to the steward solemnly. The aircraft had arrived at the runway, and begun its takeoff procedure, and so the accumulated knowledge of more than a thousand years was being brought to, bear to turn liquid kerosene into flight.
If you have ever blown up a balloon and then let it go, allowing it to zoom and fart its way around a room, you will have a good grasp of how a jet engine works. As compressed air shoots out in one direction, the balloon is propelled in the opposite: this is Newton’s 3rd law of motion, which states that every action has an equal and opposite reaction. But storing enough compressed gas to power an aircraft would be pretty ineflicient: luckily, the British engineer Frank Whittle worked out how to solve this problem. He reckoned that since the sky is already full of gas, a plane shouldn’t need to carry it around; you just have to compress the gas that’s already in the sky as you fly along, and shoot it out the back.
All you need is a machine to compress the air. This compresor is what you see under the wing as you board a plane – it looks like a giant fan, and it is, but what you can’t see is that inside it are 10 or more fans, each one smaller than the last. Their job is to suck in the air and compress it. From there, the compressed air goes to the combustion chamber, in the middle of the engine, where it’s mixed with kerosene and ignited, producing a jet of hot gas that shoots out the back of the engine. The genius in the design is that, on its way out of the engine, some of the air’s energy is used to rotate a set of turbines – and it’s these turbines that rotate the compressors at the front of the engine. In other words, the engine harvests energy from the hot gas that it then uses to collect and compress more air as it flies through the sky.
The air shooting out the back of the engine allowed our plane, which weighed approximately 250 tons, to gain speed. It’s always hard to get a feel for just how fast you’re going when you’re looking out of the window of a speeding aircraft. The wings bob and wobble awkwardly at every bump of the runway, giving no hint of the engineering elegance that they’ll display once airborne. At 130 km/h, the intensity of the rattling and groaning cabin interior begins a worrying crescendo. If I had never flown in a plane, at this point I’d be very doubtful that we would ever get off the ground.
And yet the sheer embodied energy in the kerosene propelled us forward faster and faster; a fuel with more power than nitroglycerine was being harnessed at a rate of 4 litres per second. By now our aircraft was nearing the end of the 3,5 km-long runway, travelling at 260 km/h. This is arguably the most dangerous moment of the flight. There wasn’t much runway left, and if we didn’t get airborne quickly, we would run off the end, ploughing into the buildings there with thousands of litres of liquid kerosene in our fuel tanks. And yet majestically, like a goose taking off from a lake, we climbed into the sky, leaving behind all the buildings, cars and people on the ground in a matter of seconds. This is the moment I love most about flying – especially when it involves flying through the low clouds of Brussels into the bright sunshine above as we did that day. It feels like entering another realm of existence and I never tire of it.
A plane is, in a way, the modern magic lantern. Its genie is kerosene, which will grant your wish to go anywhere in the world, flying you there not on a magic carpet, but in something even better, a cabin that protects you from the extreme cold and wind, and is comfortable enough for you to relax, even sleep, in through your journey.
Of course, like all genies it has a dark side. We have fallen in love with the power of kerosene, but flying, and indeed the use of other products dependent on crude oil, are wreaking havoc with our global elimate: it is warming rapidly as a result of the carbon dioxide emissions from burning oils like kerosene. Globally we currently consume 16 billion litres of oil per day. Whether we will be elever enough to find a way of putting the genie back in the bottle is surely one of the most important questions of the 21st century. But above the clouds I wasn’t, if I’m honest, thinking about this. Instead I was marvelling at the cloudscapes and looking forward to having a drink from the trolley, which was now happily trundling down the aisle.
Wow! Great read! Very interesting and educational. I wish text books were written like this.
Thank you very much. I am glad you’ve enjoyed reading this. I also enjoy writing about materials science, this being one of the main reason why I created this blog. But one day who knows… probably I will publish my own text book about materials too, I will think about it. 🙂