Crude Oil Distillation
The first essential task for the crude oil consisting of more than 108 compounds is to separate its major components based on boiling point differences.
Process flow-sheet
The conceptual process flow-sheet for the petroleum refinery (Figure above) consists of the following important sub-processes:
- Crude desalter
- Furnace
- Pre-flash column
- Crude distillation column supplemented with side columns. These columns produce the desired products
- Pump around heat exchanger units
- Heat exchanger network that facilitates energy recovery from hot product and reflux streams to heat the crude oil.
Desalting and Dewatering:
Crude oil is recovered from the reservoir mixed with a variety of substances: gases water and dirt (minerals). Desalting is a water – washing operation performed at the production field and at the refinery site for additional crude oil cleanup.
Crude desalter
- Crude oil consists of dissolved salts and they tend to cause operating problems such as fouling, plugging, catalyst deactivation and corrosion in various process equipments. Therefore, dissolved salts need to be removed using a separation process.
- The crude desalting unit is a separation process. Here, water along with other trace chemicals such as caustic and acid are allowed to enter a mixing unit along with the crude oil.
- The mixture of crude oil and water is subsequently passed through an electrostatic precipitator cum gravity settler. The electrostatic field enables the agglomeration of water droplets and aids faster gravity settling.
- An essential issue for the good performance of crude desalter is the temperature of the operation. Usually, high efficiency of salt removal is possible between 100 – 300 oF.
- Therefore, the crude oil is heated to about 250 oF before it enters the desalter unit.
- The clean desalted crude oil flows from the top of the gravity settler and the water along with other dissolved impurities is removed as a bottom product from the gravity settler unit.
- A high degree of salt removal is desired (95 – 99% removal of the dissolved salt in the crude oil). Usually, a two stage desalting process is deployed. When higher salt removal efficiencies are desired, three stage units are deployed.
Furnace:
The feed to a distillation tower is heated by flow through pipes arranged within a large furnace. The heating unit is known as a pipe still heater or pipe still furnace, and the heating unit and the fractional distillation tower make up the essential parts of a distillation unit or pipe still.
? Here, fuel oil and fuel gas (heavier products) obtained from the refining process itself are burnt to increase the crude oil temperature.
? Depending upon the quality of the crude, the desired temperature for the crude oil is about 600 - 700 oF.
? The pipe still furnace heats the feed to a temperature at which a portion of the feed will change into vapor.
? The vapor is held under pressure in the pipe in the furnace until it discharges as a foaming stream into the fractional distillation tower.
? Here the unvaporized or liquid portion of the feed descends to the bottom of the tower to be pumped away as a bottom nonvolatile product, whereas the vapors pass up the tower to be fractionated into gas oils, kerosene, and naphtha.
? Live steam is also used in the recent designs. The live steam is usually at about 50 psig.
The basic principles of using live steam are:
1- Upon condensation, oil and water are very easy to separate.
2- Steam can take significant amount of heat in term of enthalpy.
3- Steam enables enhancement in relative volatility.
- Live steam cannot be just fed at one section of the CDU. It needs to be fed at various sections to ensure both good heat distribution and reduce relative volatilities of the hydrocarbons at various sections of the main and secondary towers.
Therefore, live steam will enable good product quality as lighter hydrocarbons with higher relative volatilities in the bottom heavy product liquid streams will be easily stripped and carried along with the vapor.
? What primary disadvantage exists by using live steam in the CDU columns?
1- Live steam once it enters the column does not condense anywhere, as we don’t want any condensation to happen.
2- When live steam is used, vapor load increases significantly in the column.
3- This increases the diameter of the column at various sections. There will be of course a section that has maximum vapor load and this section will have the maximum diameter.
Pre-flash column
- The crude oil enters the pre-flash column after leaving the furnace
- The pre-flash tower separates the lighter fractions of the already heated crude oil.
- The heavier fractions of the crude oil leave from the bottom section of the pre-flash tower.
- Both lighter and heavier streams emanating from the pre-flash tower are fed to the main crude distillation column at various sections
- Pre-flash column enables better refluxes in the main column by distributing the streams effectively between various processing zones of the crude oil.
- Pre-flash column may or may not be included i.e., it is optional. In other words, the pre-flash column can be avoided and the heated crude oil from the furnace can be fed to the main column directly.
Distillation Process:
The distillation columns consisting of both main and secondary crude distillation columns.
? The first and the most fundamental step in the refining process (after the crude oil has been cleaned and any remnants of brine removed) is distillation, which is often referred to as the primary refining process.
? Distillation involves the separation of the different hydrocarbon compounds that occur naturally in a crude oil into a number of different fractions (a fraction is often referred to as a cut).
? In the atmospheric distillation process , heated crude oil is separated in a distillation column (distillation tower, fractionating tower, atmospheric pipe still) into streams that are then purified, transformed, adapted, and treated in a number of subsequent refining processes, into products for the refinery s market. The lighter, more volatile, products separate out higher up the column, whereas the heavier, less volatile, products settle out toward the bottom of the distillation column.
? The fractions produced in this manner are known as straight run fractions ranging from (atmospheric tower) gas, gasoline, and naphtha, to kerosene, gas oils, and light diesel, and to (vacuum tower) lubricating oil and residuum.
? The only condenser in the main column is a partial condenser to facilitate the production of both gas and naptha + water stream.
a-The main column consists of 45 trays and the secondary columns (side strippers) consist of 4 trays each. Three side strippers are used to strip the light ends from kerosene, LGO and HGO products.
b- The main column has two sections that are distinguished with respect to a flash zone. The flash zone is where the crude oil partially vaporized is fed to the main column. There are about 4 trays below the flash zone and 41 trays above the flash zone of the main column. The bottom most tray (residue stripping tray) is numbered as 1 and the top tower tray is numbered as 45. Trays 1 to 4 process the atmospheric residue portion of the crude in the section below the flash zone.
c-Trays 5 to 10 (6 trays above the flash zone) process the HGO product portion of the crude. From tray 10, HGO draw off product is taken out (as liquid) and enters the HGO side stripper unit. From tray 10 as well, the liquid stream is drawn and sent to tray 12 via a bottom pump around unit that enables cooling of the liquid stream. The steam + light ends from the HGO side stripper enter tray 11 of the main column.
d-Trays 13 to 22 (10 trays above the HGO processing zone) process the LGO product portion of the crude. From tray 22, LGO draw off product is taken (as liquid) and sent to the LGO side stripper unit. Also, from tray 22, another liquid stream is taken out and sent to tray 24 via a top pump around unit (TPA) that enables cooling of the liquid stream. The steam + light ends from the LGO side stripper enter tray 23 of the main column.
e-Trays 24 to 34 (10 trays above the LGO processing zone) process the kerosene product portion of the crude. From tray 34, the kero draw off stream is taken and sent to the kerosene side stripper unit. The steam + light ends of the kerosene side stripper enter tray 35.
f-Trays 34 to 45 (12 trays above the Kerosene processing zone) process the naphtha product portion of the crude.
g- It is interesting to note that steam enters main column at trays 1, 11, 23, 35 and therefore is present along with the vapor stream along with the hydrocarbons. Therefore, steam balances throughout the column are very important.
h- The cold naphtha stream obtained from the phase separator is sent back to the main column as reflux stream.
Kinds of Reflux :
Ways of removing heat are indicated in Fig below , the types of reflux:
1- Cold Reflux :is defined as reflux that is supplied at some temperature below the temperature at the top of the tower. Each pound of this reflux removes a quantity of heat equal to the its latent heat and the sensible heat required to raise it temperature from the storage tank temperature to the temperature at the top of the tower
2- Hot Reflux :Admitted to the tower at the same temperature . Reflux or over flow from plate to plate in the tower is essentially hot reflux because it is always substantially at its boiling point.
Hot reflux capable of removing only the latent heat because no difference in temperature is involved
3- Circulating Reflux :It is not vaporized. It is only able to remove the sensible heat thatis represented by its change in temperature as it circulate. This reflux is withdrawn from thetower as a liquid at a high temperature as a liquid and is returned to the tower after havingbeen cooled.
Heat Exchangers
A heat exchanger is a device that is used to transfer thermal energy (enthalpy) between two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid, at different temperatures and in thermal contact.
Shell and Tubes type:
The shell and tube heat exchanger is further divided into three categories as:
1. Fixed tube sheet
2. U tube
3. Floating head
Fixed tube sheet
The fixed tube sheet construction is its low cost because of its simple construction. In fact, the fixed tube sheet is the least expensive construction type, as long as no expansion joint is required.
U-Tubes type
- U-tube heat exchanger as one end is free, the bundle.
- Can expand or contract in response to stress differentials.
- In addition, the outsides of the tubes can be cleaned, as the tube bundle can be removed.
Floating head type
In this design, one tube sheet is fixed relative to the shell, and the other is free to ”float” within the shell. This permits free expansion of the tube bundle, as well as cleaning of both the insides and outsides of the tubes. Thus, floating-head SHTEs can be used for services where both the shell side and the tube side fluids are dirty-making.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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