Petroleum Refining Processes for Crude Distillation

Distillation is the first step in the processing of crude oil and it takes place in a tall steel tower called a fractionation column. The inside of the column is divided at intervals by horizontal trays. The column is kept very hot at the bottom (the column is insulated) but as different hydrocarbons boil at different temperatures, the temperature gradually reduces towards the top, so that each tray is a little cooler than the one below.

The crude needs to be heated up before entering the fractionation column and this is done at first in a series of heat exchangers where heat is taken from other process streams which require cooling before being sent to rundown. Heat is also exchanged against condensing streams from the main column. Typically, the crude will be heated up in this way upto a temperature of 200 - 280 ⁰C, before entering a furnace.

As the raw crude oil arriving contains quite a bit of water and salt, it is normally sent for salt removing first, in a piece of equipment called a desalter. Upstream the desalter, the crude is mixed with a water stream, typically about 4 - 6% on feed. Intense mixing takes place over a mixing valve and (optionally) as static mixer. The desalter, a large liquid full vessel, uses an electric field to separate the crude from the water droplets. It operates best at 120 - 150 ⁰C, hence it is conveniently placed somewhere in the middle of the preheat train.

            Part of the salts contained in the crude oil, particularly magnesium chloride, are hydrolysable at temperatures above 120 ⁰C. Upon hydrolysis, the chlorides get converted into hydrochloric acid, which will find its way to the distillation column's overhead where it will corrode the overhead condensers. A good performing desalter can remove about 90% of the salt in raw crude.

Downstream the desalter, crude is further heated up with heat exchangers, and starts vaporising, which will increase the system pressure drop. At about 170 -200 ⁰C, the crude will enter a 'pre-flashvessel', operating at about 2 - 5 barg, where the vapours are separated from the remaining liquid. Vapours are directly sent to the fractionation column, and by doing so, the hydraulic load on the remainder of the crude preheats train and furnace is reduced (smaller piping and pumps).

Just upstream the preflash vessel, a small caustic stream is mixed with the crude, in order to neutralise any hydrochloric acid formed by hydrolysis. The sodium chloride formed will leave the fractionation column via the bottom residue stream. The dosing rate of caustic is adjusted based on chloride measurements in the overhead vessel (typically 10 - 20 ppm).

At about 200 - 280 ⁰C the crude enters the furnace where it is heated up further to about 330 -370 ⁰C. The furnace outlet stream is sent directly to the fractionation column. Here, it is separated into a number of fractions, each having a particular boiling range.

At 350 ⁰C, and about 1 barg, most of the fractions in the crude oil vapourise and rise up the column through perforations in the trays, losing heat as they rise. When each fraction reaches the tray where the temperature is just below its own boiling point, it condenses and changes back into liquid phase. A continuous liquid phase is flowing by gravity through 'downcomers' from tray to tray downwards. In this way, the different fractions are gradually separated from each other on the trays of the fractionation column. The heaviest fractions condense on the lower trays and the lighter fractions condense on the trays higher up in the column. At different elevations in the column, with special trays called draw-off trays, fractions can be drawn out on gravity through pipes, for further processing in the refinery.

At top of the column, vapours leave through a pipe and are routed to an overhead condenser, typically cooled by air fin-fans. At the outlet of the overhead condensers, at temperature about 40 ⁰C, a mixture of gas, and liquid naphtha exists, which is falling into an overhead accumulator. Gases are routed to a compressor for further recovery of LPG (C3/C4), while the liquids (gasoline) are pumped to a hydrotreater unit for sulfur removal.

A fractionation column needs a flow of condensing liquid downwards in order to provide a driving force for separation between light and heavy fractions. At the top of the column this liquid flow is provided by pumping a stream back from the overhead accumulator into the column. Unfortunately, a lot of the heat provided by the furnace to vaporise hydrocarbons is lost against ambient air in the overhead fin-fan coolers. A clever way of preventing this heat lost of condensing hydrocarbons is done via the circulating refluxes of the column. In a circulating reflux, a hot side draw-off from the column is pumped through a series of heat exchangers(against crude for instance), where the stream is cooled down. The cool stream is sent back into the column at a higher elevation, where it is been brought in contact with hotter rising vapours. This provides an internal condensing mechanism inside the column, in a similar way as the top reflux does which is sent back from the overhead accumulator. The main objective of a circulating reflux therefore is to recover heat from condensing vapours. A fractionating column will have several (typically three) of such refluxes, each providing sufficient liquid flow down the corresponding section of the column. An additional advantage of having circulating refluxes is that it will reduce the vapour load when going upwards in the column. This provided the opportunity to have a smaller column diameter for top sections of the tower. Such a reduction in diameter is called a 'swage'.

The lightest side draw-off from the fractionating column is a fraction called kerosene, boiling in the range 160 - 280 ⁰C, which falls down through a pipe into a smaller column called 'side-stripper'. The purpose of the side stripper is to remove very light hydrocarbons by using steam injection or an external heater called 'reboiler'. The stripping steam rate, or reboiled duty is controlled such as to meet the flashpoint specification of the product. Similarly to the atmospheric column, the side stripper has fractionating trays for providing contact between vapour and liquid. The vapours produced from the top of the side stripper are routed back via pipe into the fractionating column.

           

The second and third (optional) side draw-offs from the main fractionating column are gas oil fractions, boiling in the range 200 - 400 ⁰C, which are ultimately used for blending the final diesel product. Similar as with the kerosene product, the gas oil fractions (light and heavy gas oil) are first sent to a side stripper before being routed to further treating units.

At the bottom of the fractionation column a heavy, brown/black coloured fraction called residue is drawn off. In order to strip all light hydrocarbons from this fraction properly, the bottom section of the column is equipped with a set of stripping trays, which are operated by injecting some stripping steam (1 - 3% on bottom product) into the bottom of the column. 

Importances of Shell & Tube Exchangers

Majorly used for high pressure applications within plants and everything as such, shell & tube are the types of heat exchangers that are being used for many industrial applications in the present times. These are not only highly efficient, but they are also effective in terms of heat transfer at all times. Individuals who wish to know everything there is about these heat exchangers should be aware of the fact that they are available in different lengths, sizes as well as diameters. Therefore, checking those out is definitely recommended before purchasing one for future use.

Description & History of Shell & Tube

Mostly, the shell & tube heat exchangers are made of carbon steel and copper tubes in general. These allow swift and very effective transferal of heat and have been used for this particular task since a long period of time now. Due to their effectiveness in the process, these come in handy for high pressure applications as well, which is what makes them a very unique commodity that can be found on high end plants of different industries these days. Moreover, these are also used for liquid of steam and steam to liquid applications, whenever needed. Therefore, it can be easily said that they have many uses, all of which are unique in their own way.

Shell & tube heat exchangers have been around since a very long period of time. While many people may be thinking that is has been over a couple of years, these heat exchangers have been used by men for more than one-fifty years, which shows how accurate, efficient and effective they are and always have been in the process of the transferal of heat and much more. Manufacturers of various commodities may find themselves investing in these heat exchangers due to the fact that they are immensely durable and come in handy in the long run.

Industrial Uses

On the other hand, the major uses of shell & tube include processing heat removal, liquid and steam, liquid and gas cooling, steam condensing and much more. Moreover, it is also widely used for lube oil cooling on a large scale. All of these industrial applications require these heat exchangers in order for the process to go smoothly and rather quickly. Turbine as well as engine cooling is another very prominent application of these heat exchangers that many manufacturers have been taking advantage of since a very long time. While there are many other prominent applications of these heat exchangers, these are the most common ones that are being used in the present times.

Less Expensive

Another very important thing for people to be aware of is the fact that these are much cheaper in comparison with all the other types of heat exchangers out there. This allows manufacturers to save a lot of money in the process of purchasing them, only to use them for significant and high end industrial uses in both the short and long run. As a part of cooling down different types of commodities, these are by far the best and the most efficient, specifically on the price they are available at.

Conclusion

These are also much easier to use in comparison with other coolers and heat exchangers in general. Therefore, due to all that they have to offer in in the near future, these are highly recommended for all manufactures who wish to have the perfect heat exchangers for different industrial uses. From oil cooling, to gas cooling and much more, these exchangers actually tend to carry out all of this for individuals who are interested in fulfilling these tasks on a day to day basis. The best part is that their capacity can always be increased. 

Shell & Tube Heat Exchangers Features

Shell & tube are the types of heat exchangers that are used in many high end industrial applications these days. While these are used for many different reasons, one of the most prominent ones is that they allow the transfer of heat and ensure seamless use in the long run. There are other kinds of heat exchangers as well, but these are special in their own way, which is why all those who are willing to purchase these should be aware of their features beforehand. A very important thing to do is to conduct a good amount of research on these heat exchangers.

Heat Efficiency/Rate

The heat efficiency of the shell & tube heat exchangers is said to be rather low in comparison with all the other types of heat exchangers. This is both beneficial and not so much, as well. However, individuals who are looking for heat exchangers that have low heat efficiency cannot find something better than these ones as they are truly very efficient and effective, too. Their heat efficiency can be increased but not by a long shot, so these actually make the perfect heat exchangers where less heat efficiency is required for a specific task.

Assembly

On the other hand, the shell & tube heat exchangers are rather complex in comparison with other kinds of heat exchangers. This is due to the fact that their assembly is not as easy. With a lot of connections, bolts as well as tubes to assemble, it may take up some time. However, the best part is that the disassembling is not nearly as difficult as one may expect it to be. Therefore, making these heat exchangers the best for anyone who wishes to use them for both commercial as well as industrial uses in the near future.

Cost & Installation

Another feature of the shell & tube heat exchangers that individuals should be aware of is the fact that they are of high cost usually. However, with the initial cost being high, it is completely worth it in the long run since these last for a long period of time because of the fact that they have high end durability. Since they are not deteriorated easily, it means that the initial investment being high does not matter as much – since it is a good investment in the long run. Using these heat exchangers may as well cost a lot in the first place but it definitely provides the best possible results in all applications.

Weight & Size

The size and weight of shell & tube heat exchangers is normally high, which is another feature of this equipment that individuals should be aware of before investing in it for good. Due to the fact that it weighs more, it is used for higher end industrial applications. With their heat transferal properties, these heat exchangers come in handy for all kinds of processes that require immediate or long-term exchange of heat. However, as their weight is more than the other sorts of heat exchangers, their installation costs will be higher as well but those are, as always, eventually covered in the long run.

Efficient Chemical Cleaning

The best part is the fact that these heat exchangers can be cleaned as well, most importantly, through chemical cleaning. The process has to be done carefully, but it eventually brings out positive results in a short period of time. These heat exchangers are being sued by many high end industries as well as in other sectors; therefore, those who wish to purchase these should most definitely c heck out their features in the first see in order to see what they are paying for, and all the benefits they offer.   

About Heatec Holding.

Our subsidiary company, Heatec Jietong, has been providing heat transfer solutions to the marine industry since 1990s. Our customers are worldwide, including the major shipyards, ship owners, ship managers and ship charterers etc. With this global clientele, our product and services extend across the globe, enabling our customers to operate their vessels efficiently.

Investing in Air Cooled Heat Exchangers?

For many for are not aware, the main reason for using an air cooled heat exchanger is the fact that through it, clean water can be cooled down almost immediately. A lot of places in different parts of the world lack clean as well as cold water; therefore, this device truly comes in handy then. On the other hand, clean and cold can either also be expensive or just corrosive due to natural reasons. In such circumstances, the usage of this heat exchanger enables high end water cooling at a swift rate, which is why it has become a popular device amongst all these days.

High End Reliability

A prominent benefit of using an air cooled heat exchanger is the fact that it stands undisputed as one of the most reliable sources for cooling water. Amongst all the other ways through which human can fulfill this purpose, using these heat exchangers is very much utilized these days as it is not only the most reliable but it is also the most convenient. While a lot of individuals may not yet be completely aware of these air coolers, they are slowly becoming everything that people would need when there is no cold and clean water around.

Free Air

The air used by these heat exchangers is free in general. This means that individuals do not have to spend anything in this matter, saving a good amount of money in the long run. This kind of air cooled heat exchanger has air that can be used for free whenever required and without having to struggle too much. While using it, the heat exchanger is likely to make no sound at all and it is considered to be one of its prime benefits. The sound it makes is barely sound in comparison with other such machines.

Wide Range of Unique Types

All sorts of an air cooled heat exchanger arrive with different features. Individuals should always choose the kind that they prefer the most in order to see the way it will benefit them in the near future. These heat exchangers definitely vary from model to model, and while some have different temperatures, the others have various sorts of design pressure. As these machines are used for the purpose of the removal of heat that is not required, these are used in areas where the temperature is a lot more than 20 degrees. Therefore, this piece of information should definitely be kept in mind before purchasing them for good.

Top Notch Performance

One of the most well-known benefits of this heat exchanger is the fact that it offers spectacular performance, which is precisely why people prefer it over all the other methods of heat removal from various commodities and natural sources. The high end electric motors found in these heat exchangers shows how swift they work and how very durable they can be in the long run. According to many user reviews, these devices can be really high end to configure and use sometimes, but with the right amount of knowledge and some experience, it really gets easy in a short period of time.

Better than all Competitors

In comparison with water-cooled exchanger, these have a lot more ability for heat removal – and they have also more effective cooling properties which make them a suitable choice in comparison with all the other machines and devices that can carry out the same job that they can do. Before purchasing an air cooled heat exchanger, one must always check out its performance and top features. Currently, a lot of these devices have advanced technology features – which truly do increases the levels of the overall performance by a long shot; hence, making these machines a wise investment. 

Heat Exchangers and their Pros and Cons

A heat exchanger can simply be called one of the most important components of any machine. It protects appliances and equipment from overheating and keeps it cool. Since heat exchangers are used in a number of different machines, there are a number of different types of heat exchangers available in the markets today. It is important to be able to know the benefits and drawbacks of all these types so when one has to select one for a certain kind of equipment, they will know which one will work best. The application of the exchanger, the power of operations, the pressure of the fluids, the temperature driving force and many such things should be considered.

First up, there is the shell and tube heat exchanger which is easily the most commonly used exchanger out there. Since it is quite common, it is also widely understood and people generally have an idea about how this works. It is also highly versatile and can be used in a number of different equipment because of its flexibility. Most of these are small equipment, however. Also, the design pressures it allows and the temperatures it can withstand are the more diverse than any other type of exchanger. Since it is made of rugged material, it can withstand aggressive wear and tear. On the downside, it is not as thermally effective as other exchangers and it can fall prey to flow induced vibration and fail to work. It is also not good for temperature cross conditions since a number of different units will have to be used in the heat exchanger. There are also a number of stagnant areas on the shell side that can cause corrosion and it can also fall prey to flow mal-distribution.

Moving on, another common type of exchanger is called the compact heat exchanger and as one can understand by its name, it is quite small and can be fitted in smaller equipment. First and foremost, it is very cheap to buy and the initial costs of plate type purchase are very low. It also allows for a number of different configurations like spiral, gasketed, semi-welded and fully welded. The heat transfer coefficients are very high since the wall sheer stress is very high and thus, it can withstand nearly three times the amount of temperature that shell and tube exchangers can withstand. Fouling characteristics are comparatively low since the turbulence within the exchanger is quite high. This exchanger also allows cross temperatures to be achieved and is also very easy to install, setup and start. On the downside, though, since it is so small it does not allow a high range of temperatures and pressures to work through it. Also, since the path of flow is very narrow, there is a lot of clogging and plugging. The gasket units in the heat exchanger require special opening and closing up measures.

Lastly, there are the air cooled heat exchangers. They are great for areas where cooling water is scarce and one has to spend a lot of money to treat water and let it cool down. It is very effective in cooling down fluids of very high temperatures and it can bring down water bodies of eighty degrees Celsius down to nothing. The maintenance costs and operating expenditures are very low. Typically, they amount to thirty or thirty-five percent of what it would ordinarily cost to cool water. On the downside, however, the heat exchanger is very expensive since it is considered to be an important car component. Also, it is a lot bigger than a compact exchanger and will need a larger footing on which it will be installed. Lastly, the process outlet temperature is also quite high. 

More heat exchangers information at www.heatecholdings.com 

What one needs to know about Heat Exchangers

A heat exchanger, as the name implies, is basically a piece of equipment that serves the purpose of transferring heat from one medium to another. This media might be separated by a wall that is solid in nature so as to prevent mixing or they can also be in direct contact. These are used widely for space heating, air conditioning, refrigeration, in power, chemical and petrochemical plants along with in petroleum refineries, sewage treatment and in natural gas processing. A fluid actually does the hard work of carrying heat. The secret of their efficiency lies in two things. Although a little scientific, they are still easy to understand. The wall that separates the heat exchanger tubes needs to have a wide surface area. At the same time, it is equally important to reduce the flow of these fluids while passing through these tubes. Fins can be added in one or both directions to increase the exchanger’s performance manifolds. These fins also increase the surface area and also create possibility of turbulence during fluid flow.  

One can find the classic example of a heat exchanger in the internal combustion engine where a circulating fluid that is known as an engine coolant flows through the radiator coils and the air flows past these coils and cools the coolant while heating the incoming air. There are many kinds of heat exchangers like the shell and tube ones. These consist of a series of tubes. One of the sets of these tubes contains the fluid that is either to be heated or cooled. It is the second fluid that runs over these tubes which are either being heated or cooled in order for it to provide the heat or absorb the heat that is required.

In this kind of a heat exchanger, there is also a set of tubes that is known as a tube bundle which can be made from many different kinds of tubes. For example they can be plain or longitudinally finned. These tube and shell heat exchangers are used typically for high pressure applications that usually have pressures more than the thirsty bar along with temperatures that go above than two hundred and sixty degrees Celsius. The reason behind this is that tube and shell heat exchangers because of their shape are robust.

When designing these tubes in the shell, there are several thermal design features that have to be considered. There can be many different kinds of variations on the tube and shell design. It is typical though that the ends of each of the tubes are connected to plenums that are also known as water boxes some times, through the holes that are there in the tube sheets. These tubes can either be in the shape of a U and called U tubes and they can also be straight.

These tubes have a small diameter that makes the heat exchanger compact as well as economical. Wider diameter makes the tubes far less economical. Though there are more chances for the heat exchanger to foul up faster and the small size also makes the cleaning of such a fouling much more difficult. So to overcome these problems, tubes of larger diameters can be used as well. So it can be said that in order to determine the diameter of the tube, the cost and fouling nature of the fluids as well as the available space and cost are all taken into consideration. Baffles are also used in the tube and shell heat exchangers so as to direct the fluid across the tube bundle. They prevent the tubes from vibrating too.