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In the distillation column, the heart of a petroleum refinery, there are many ongoing interesting processes. The challenge: there are many mixed gases. How do you separate them out? Well, if you can give a solution to this problem, the mix of crude oil can be separated into many useful components.
The tricky problem for engineers is the question, how do you approach the problem to begin with? And for simplicity, let's assume that there are only three gases. One solution is to cool down the gases and make sure that we will eventually reduce the temperature down to the boiling point of just one gas. Voilà, only that gas will condense and this liquid can easily be separated. If you want to separate out the next gas, you just reduce the temperature just below the boiling point of that gas and so on and so forth.
However, if you zoom into a distillation tower, you won't see any cooling coils. Without the coils, how did engineers condense gases?
To understand this clever engineering of the distillation column, first we have to assume that all of these trays are lined with condensed liquid with its lowest temperature liquid at the top and the highest temperature liquid at the bottom.
The crude oil is first fed into a furnace. Here, most of the liquid vaporizes. The fluid with mostly vapor and some liquid now enters the distillation column. Assume the liquid at the first tray is maintained at 370°C and the fluid which enters has a temperature of 400°C. As soon as this steam enters the column, the liquid portion falls down and only the vapor rises.
Now we have to closely observe the path taken by this vapor at 400°C. This kind of tray design with mushroom-shaped fittings is called a bubble cap design. The gas which enters via tube has to pass through the liquid in the tray. Remember the liquid is at a much lower temperature. Suppose one component in the gas has a boiling temperature of 375°C. This component will get condensed inside of the liquid pool as will all other gas components with a boiling point between 370 and 400°C. The gases which have a boiling temperature lower than 370°C escape this liquid portion and continue to rise.
Here the next liquid pool is waiting for them at a lower temperature, say 300°C. Once again, the process is repeated and all gases with boiling points ranging from 300 to 375°C get condensed in the liquid pool.
This is the way that engineers separate crude oil into different components. A practical distillation tower will have almost 15 trays. The heavier molecule, the higher the boiling point. This means that all of the heavy components in crude oil you find at the bottom of the tower and the lighter components you find at the top. For example, you will find gasoline at the top of the region and industrial fuel oil at the bottom region of the distillation tower.
At the very top of the distillation tower, a condenser is fitted. This is under active cooling. Some portion of the gas which is at the very top when passing through these coils will condense. The remaining gas which has a very low boiling point is collected as petroleum gas at the very top.
Unfortunately, the interesting physics of separating out the crude oil won't actually happen perfectly like described. An example of this can be found in the bottommost tray. All of the gases with boiling points higher than 370°C are supposed to get condensed here. However, some gas molecules with higher boiling points than 370°C will escape this liquid pool. They will eventually get caught at the top tray, but they're not supposed to be there.
The engineer's next tricky solution is to send this liquid back to the bottom tray via pipes. The tray, which is supposed to have higher boiling points, will vaporize the molecules which are supposed to be in the upper tray and trap only the needed molecules. This kind of separation happens continuously, returning all of the liquids back to the initial portion of the distillation column.
Now, let's focus on the residue which fell to the bottom of the distillation column. It contains some useful products. The fractions in the residue boil at very high temperatures, some at more than 1,000°C. However, at this temperature, the molecules will crack apart before they boil. Engineers still want to boil all the fractions of the residue but at lower temperatures.
The tricky problem for engineers is the question, how do you approach the problem to begin with? And for simplicity, let's assume that there are only three gases. One solution is to cool down the gases and make sure that we will eventually reduce the temperature down to the boiling point of just one gas. Voilà, only that gas will condense and this liquid can easily be separated. If you want to separate out the next gas, you just reduce the temperature just below the boiling point of that gas and so on and so forth.
However, if you zoom into a distillation tower, you won't see any cooling coils. Without the coils, how did engineers condense gases?
To understand this clever engineering of the distillation column, first we have to assume that all of these trays are lined with condensed liquid with its lowest temperature liquid at the top and the highest temperature liquid at the bottom.
The crude oil is first fed into a furnace. Here, most of the liquid vaporizes. The fluid with mostly vapor and some liquid now enters the distillation column. Assume the liquid at the first tray is maintained at 370°C and the fluid which enters has a temperature of 400°C. As soon as this steam enters the column, the liquid portion falls down and only the vapor rises.
Now we have to closely observe the path taken by this vapor at 400°C. This kind of tray design with mushroom-shaped fittings is called a bubble cap design. The gas which enters via tube has to pass through the liquid in the tray. Remember the liquid is at a much lower temperature. Suppose one component in the gas has a boiling temperature of 375°C. This component will get condensed inside of the liquid pool as will all other gas components with a boiling point between 370 and 400°C. The gases which have a boiling temperature lower than 370°C escape this liquid portion and continue to rise.
Here the next liquid pool is waiting for them at a lower temperature, say 300°C. Once again, the process is repeated and all gases with boiling points ranging from 300 to 375°C get condensed in the liquid pool.
This is the way that engineers separate crude oil into different components. A practical distillation tower will have almost 15 trays. The heavier molecule, the higher the boiling point. This means that all of the heavy components in crude oil you find at the bottom of the tower and the lighter components you find at the top. For example, you will find gasoline at the top of the region and industrial fuel oil at the bottom region of the distillation tower.
At the very top of the distillation tower, a condenser is fitted. This is under active cooling. Some portion of the gas which is at the very top when passing through these coils will condense. The remaining gas which has a very low boiling point is collected as petroleum gas at the very top.
Unfortunately, the interesting physics of separating out the crude oil won't actually happen perfectly like described. An example of this can be found in the bottommost tray. All of the gases with boiling points higher than 370°C are supposed to get condensed here. However, some gas molecules with higher boiling points than 370°C will escape this liquid pool. They will eventually get caught at the top tray, but they're not supposed to be there.
The engineer's next tricky solution is to send this liquid back to the bottom tray via pipes. The tray, which is supposed to have higher boiling points, will vaporize the molecules which are supposed to be in the upper tray and trap only the needed molecules. This kind of separation happens continuously, returning all of the liquids back to the initial portion of the distillation column.
Now, let's focus on the residue which fell to the bottom of the distillation column. It contains some useful products. The fractions in the residue boil at very high temperatures, some at more than 1,000°C. However, at this temperature, the molecules will crack apart before they boil. Engineers still want to boil all the fractions of the residue but at lower temperatures.
This is one easy example. Water generally boils at 100°C. However, if you reduce the pressure that surrounds the water, it will boil at a lower temperature, even as low as 50°C. The same trick is applied to the residue. Change the pressure of the distillation column and you'll be able to reduce the boiling point. This process is known as vacuum distillation. Here we still distill the residue into different fractions at a much lower temperature without breaking the molecules.
The lightest fraction of the vacuum distillation column goes into a catalytic converter. From the next fraction, wax and lubrication oils are collected. The heaviest fraction remains a liquid. Asphalt and industrial fuel are made from this fraction.
Now you might be wondering how they keep the vacuum continuous within this tower. This device, a steam ejector, does the crucial job. The steam ejector removes any non-condensible gases and air which is getting leaked into the tower. The device operates on the Venturi principle. When the high-pressure steam reaches the throttle region, its speed is increased drastically. According to Bernoulli's principle, high speed means low pressure. The low pressure region of the steam ejector draws air and gases, thus maintaining negative pressure within the tower.
You'll be surprised to know that pressure maintained inside the vacuum distillation is pretty low. Generally around 10 to 50 mm of mercury. Remember the standard atmospheric pressure is 760 mm of mercury.
Obviously, such low pressure demands special kinds of high-performance gaskets and ceiling systems. Meet the modern gasket born of the cutting-edge graphite technology, Grafoil. Grafoil are made from pure and expanded graphite. They conform to the microscopic imperfections of flanges very well and can withstand a high temperature. Common gaskets used for making joints airtight are the spiral wound gasket. Here, a V-shaped metal strip is spirally wound with a flexible material. It forms a resilient gasket which can handle high temperature and pressure well.
We've already seen how gasoline was produced from the main distillation column. Unfortunately, this gasoline has a very low octane number. If you were to use it in a car, there would be serious issues of knocking in the engine. However, if you purify the fuel oil in the heaviest of all of the fractions, we can produce a higher octane gasoline.
But even now we haven't unfolded the biggest mystery of refineries. In the beginning of the distillation process, we assumed that trays were filled with a liquid pool. How is this done? This is done by a process called preloading, also called pre-wetting. Preloading occurs by pumping a liquid from the top of the column. This pumping is done very slowly. The liquid flows down the column, cascading from one tray to the other until they are all filled. This also establishes the necessary liquid hold on each tray, creating the liquid seal required for proper vapor-liquid contact during normal operation. Now vaporized crude oil can be introduced into the distillation column.
This also means initially the liquid and all the trays are the same. However, this is a very temporary state. The moment vaporized crude oil is introduced into the system, everything starts to change. Within minutes, each tray will hold a liquid of a unique composition.
One barrel of crude oil is 159 L. However, it is interesting to note that the total volume of refined product is approximately 170 L. This volume increases due to processing gain. We broke down the crude oil with dense molecules into lighter molecule output. Of this 170l, 73l is gasoline, 43l is ultra low sulfur diesel and around 16l of kerosene and jet fuel. The remaining small players are represented in this picture.
It looks like magic. From this ugly, useless crude oil, we finally have produced highly valuable components. That's the magic of fractional distillation.
The lightest fraction of the vacuum distillation column goes into a catalytic converter. From the next fraction, wax and lubrication oils are collected. The heaviest fraction remains a liquid. Asphalt and industrial fuel are made from this fraction.
Now you might be wondering how they keep the vacuum continuous within this tower. This device, a steam ejector, does the crucial job. The steam ejector removes any non-condensible gases and air which is getting leaked into the tower. The device operates on the Venturi principle. When the high-pressure steam reaches the throttle region, its speed is increased drastically. According to Bernoulli's principle, high speed means low pressure. The low pressure region of the steam ejector draws air and gases, thus maintaining negative pressure within the tower.
You'll be surprised to know that pressure maintained inside the vacuum distillation is pretty low. Generally around 10 to 50 mm of mercury. Remember the standard atmospheric pressure is 760 mm of mercury.
Obviously, such low pressure demands special kinds of high-performance gaskets and ceiling systems. Meet the modern gasket born of the cutting-edge graphite technology, Grafoil. Grafoil are made from pure and expanded graphite. They conform to the microscopic imperfections of flanges very well and can withstand a high temperature. Common gaskets used for making joints airtight are the spiral wound gasket. Here, a V-shaped metal strip is spirally wound with a flexible material. It forms a resilient gasket which can handle high temperature and pressure well.
We've already seen how gasoline was produced from the main distillation column. Unfortunately, this gasoline has a very low octane number. If you were to use it in a car, there would be serious issues of knocking in the engine. However, if you purify the fuel oil in the heaviest of all of the fractions, we can produce a higher octane gasoline.
But even now we haven't unfolded the biggest mystery of refineries. In the beginning of the distillation process, we assumed that trays were filled with a liquid pool. How is this done? This is done by a process called preloading, also called pre-wetting. Preloading occurs by pumping a liquid from the top of the column. This pumping is done very slowly. The liquid flows down the column, cascading from one tray to the other until they are all filled. This also establishes the necessary liquid hold on each tray, creating the liquid seal required for proper vapor-liquid contact during normal operation. Now vaporized crude oil can be introduced into the distillation column.
This also means initially the liquid and all the trays are the same. However, this is a very temporary state. The moment vaporized crude oil is introduced into the system, everything starts to change. Within minutes, each tray will hold a liquid of a unique composition.
One barrel of crude oil is 159 L. However, it is interesting to note that the total volume of refined product is approximately 170 L. This volume increases due to processing gain. We broke down the crude oil with dense molecules into lighter molecule output. Of this 170l, 73l is gasoline, 43l is ultra low sulfur diesel and around 16l of kerosene and jet fuel. The remaining small players are represented in this picture.
It looks like magic. From this ugly, useless crude oil, we finally have produced highly valuable components. That's the magic of fractional distillation.