Atmospheric Distillation Unit

    The Atmospheric Distillation Unit also known as the Crude distillation unit (CDU) or the Topping Unitis at the front-end of the refinery. It receives high flow rates hence its size and operating cost are the largest in the refinery. Many crude distillation units are designed to handle a variety of crude oil types.

    The design of the unit is based on a light crude scenario and a heavy crude scenario. The unit should run satisfactorily at about 60% of the design feed rate. Seasonal temperature variation should be incorporated in the design because changes in the cut point of gasoline can vary by 20°C (36°F) between summer and winter.

    The capacity of the CDU ranges from 10,000 barrels per stream day (BPSD) or 1400 metric tons per day (tpd) to 400,000 BPSD (56,000 metric tpd). The economics of refining favors larger units. A good size CDU can process about 200,000 BPSD. The unit produces raw products which have to be processed in downstream unit to produce products of certain specifications. This involves the removal of undesirable components like sulfur, nitrogen and metal compounds, and limiting the aromatic contents.
    Typical products from the unit are:

    • Gases

    • Light straight run naphtha (also called light gasoline or light naphtha)

    • Heavy gasoline (also called military jet fuel)

    • Kerosene (also called light distillate or jet fuel)

    • Middle distillates called diesel or light gas oil (LGO)

    • Heavy distillates called atmospheric gas oil (AGO) or heavy gas oil (HGO)

    • Crude column bottoms called atmospheric residue or topped crude.


    The crude oil distillation unit (CDU) is the first processing unit in virtually all petroleum refineries. The CDU distills the incoming crude oil into various fractions of different boiling ranges, each of which are then processed further in the other refinery processing units. The CDU is often referred to as the atmospheric distillation unit because it operates at slightly above atmospheric pressure.

    Below is a schematic flow diagram of a typical crude oil distillation unit. The incoming crude oil is first injected with steam and preheated by exchanging heat with some of the hot, distilled fractions and other streams. It is then desalted to remove inorganic salts (primarily sodium chloride).

    Following the desalter, the crude oil is further heated by exchanging heat with some of the hot, distilled fractions and other streams. It is then heated in a fuel-fired furnace (fired heater) to a temperature of about 398 °C and routed into the bottom of the distillation unit.

    The cooling and condensing of the distillation tower overhead is provided partially by exchanging heat with the incoming crude oil and partially by either an air-cooled or water-cooled condenser. Additional heat is removed from the distillation column by a pumparound system.

    As shown in the flow diagram above, the overhead distillate fraction from the distillation column is naphtha. The fractions removed from the side of the distillation column at various points between the column top and bottom are called sidecuts. Each of the sidecuts (i.e., the kerosene, light gas oil and heavy gas oil) is cooled by exchanging heat with the incoming crude oil. All of the fractions (i.e., the overhead naphtha, the sidecuts and the bottom residue) are sent to intermediate storage tanks before being processed further.

    The Distillation Process
    The process flow diagram of a typical crude distillation unit is shown in the drawing above. Crude oil is pumped from storage tanks where it is freed from sediments and free water by gravity. It goes through a series of heat exchangers where it is heated with hot products coming out from the distillation column and by the exchange with heat from the pumparound liquid streams. The temperature of the crude feed can reach 120–150oC (248–302oF).

    The crude oil contains salt in the form of dissolved salt in the tiny droplet of water which forms a water-in oil emulsion. This water cannot be separated by gravity or through mechanical means. It is separated through electrostatic water separation. This process is called desalting. In the electrostatic desalter, the salty water droplets are caused to coalesce and migrate to the aqueous phase by gravity. It involves mixing the crude with dilution water (5–6 vol%) through a mixing valve.

    The crude is further heated in product heat exchangers. The preheating of the crude using the hot products cools down the products to the desired temperature for pumping to the storage tanks. This is essential for the economics of the unit in terms of energy conservation and utilization. Of course, preheating is not enough, as the crude has to be partially vaporized to the extent that all products, except for the atmospheric residue have to be in the vapor phase when the crude enters the atmospheric distillation column. Thus a furnace is required to boost the temperature to between 330 and 385oC (626 and 725oF) depending on the crude composition.

    The partially vaporized crude is transferred to the flash zone of the column located at a point lower down the column and above what is called the stripping section. The main column is typically 50 m (164 ft) high and is equipped with about 30–50 valve trays. The vapour goes up in tremendous amounts and at a high flow rate, necessitating a large diameter column above the flash zone.

    At the bottom of the stripping section, steam is injected into the column to strip the atmospheric residue of any light hydrocarbon and to lower the partial pressure of the hydrocarbon vapors in the flash zone. This has the effect of lowering the boiling point of the hydrocarbons and causing more hydrocarbons to boil and go up the column to be eventually condensed and withdrawn as side streams. As the hot vapors from the flash zone rise through the trays up the column, they come into contact with the colder reflux vapors comming down the column. In the overhead condenser, the vapors are condensed and part of the light naphtha is returned to the column as reflux. Further reflux is provided by several pump-around streams along the column.

    In the distillation tower, heat required for separation is provided by the enthalpy of the feed. For effective separation heat has to be removed from the tower, in this case, by the overhead condenser and several pump-around streams along the tower length. The pump-around stream is a liquid withdrawn at a point below a side stream tray that is cooled by the cold crude feed as part of the preheat exchangers train. It is then returned to the column a few trays above the draw tray. This pump-around cooling accomplishes a number of tasks.

    • First, the cold liquid condenses more of the rising vapors thus providing more reflux to compensate for the withdrawal of products from the column.

    • Second, heat is removed from the column at higher temperatures. This is in addition to the heat removal from the condenser which takes place at relatively lower temperatures, thus the thermal efficiency of the column is improved and the required furnace duty is reduced.

    • Third, pump-around streams reduce the vapor flow rate throughout the column. Therefore, the required column is smaller than what would otherwise be required if pump-around streams where not there.

    The drawback to using more pump-around streams is that they tend to reduce the fractionation because a more fractionated liquid is mixed after cooling with a less fractionated liquid a few trays above.

    The side draw products are usually stripped to control their initial boiling point. The strippers contain several trays and the stripping is done using steam at the bottom of the stripper or reboiler type side stream strippers. The end boiling point of the side stream is controlled by the flow rate of the side stream product.

    The overhead vapor is condensed at the top of the tower by heat exchange with the cool crude coming into the unit and by air and cooling water. The liquid product is called light straight run naphtha. Part of this product is returned to the column as an external reflux. Down the column,other products are withdrawn, such as heavy straight run naphtha, kerosene or jet fuel, LGO and HGO. All of these products are withdrawn above the feed tray. The atmospheric residue is withdrawn from the bottom of the column.

    Operation of Crude Distillation Units (CDU)
    The CDU can be looked at from the point of view of a process engineering as a multicomponent distillation column. Indeed, the commercial process simulation program models CDU as a case of multicomponent distillation with undefined pseudo-components instead of the normally encountered defined components. However, because we are dealing with a mixture of thousands of compounds and due to the limitation of any distillation column in terms of its capacity to fractionate these components, there are specific operational aspects which characterize the CDU operation. In addition, there are some practical aspects in meeting the required specifications and boiling range of the required transportation fuels. In this section, the factors which affect the design and operation of the unit are explored.

    The degree of fractionation in a crude unit is determined by the gap or overlap between two adjacent side stream products. Hence we can talk about the gap or overlap in the boiling point range between kerosene and LGO for example. In the ideal case there would be no overlap between these products and the end boiling point of kerosene would be the initial
    boiling point of the LGO. However, if we compare the ASTM distillation
    boiling points, and since ASTM distillation does not give perfect fractionation, the ASTM end point of kerosene is higher than the initial ASTM boiling point of LGO. This is called fractionation overlap.

    Since determining the initial and end point on the laboratory test is not always possible or accurate, the fractionation gap is defined as the difference between the ASTM 5% boiling point of the product and the 95% point of the lighter product. When this difference is positive, we have a gap indicating good fractionation. A negative difference is called an overlap indication that some of the light product is still in the heavier product and vice versa. By controlling the cut point of any two consecutive products we can affect the degree of fractionation.

    Cut Points
    The cut points in the CDU are controlled by the overhead vapor temperature which determines how much vapor goes to the condensers to produce light naphtha and by the flow rate of the various products straight from the column or the side stream strippers. The atmospheric residue level control inside the column determines its flow rate and thus its initial cut point.

    The amount of light naphtha is determined by the dew point of the naphtha at its partial pressure, which is close to the overhead temperature. Changing the draw-off rate of any product affects the cut points of the heavier product below it. For example, lowering the kerosene flow rate will lower its end point (make it lighter), but will also modify the initial cut points of the LGO and HGO and the initial cut point of the atmospheric residue. The residue flow rate, the internal reflux rate, the drawoff temperatures and the pump-arounds are also affected.

    Therefore, if the cut point of one stream is changed through a change in its withdrawal rate, the flow rate of the heavier product next to it should be changed in the reverse and by the same amount in order to make the changes in the desired stream only. For example, if the end point of kerosene is lowered by decreasing the kerosene flow rate by a certain amount, the flow rate of LGO has to be increased by the same amount. If this action is taken, only the cut point of kerosene is affected and the cut points of the other products remain unchanged.

    The side stream rate also affects the temperature at the withdrawal tray and lowers the internal reflux coming out of that tray. The internal reflux rate affects the degree of fractionation. It can be increased by increasing the heater outlet temperature, and by lowering the pump-around duty in the lower section of the column. When less heat is removed by the lower pump-around, more vapors will be available up the column and more internal reflux is produced as the vapors are condensed.

    Degree of Fractionation
    The fractionation quality between two consecutive streams is affected by several factors such as the vapor and liquid flow rates in the column zone between these two streams, the number of trays, and the heat extracted by the pump-around. Fractionation quality is formulated in terms of gap or overlap of the products. For perfect fractionation, zero gap and overlap are required. This means that the End Boiling Point (EBP) of the light cut would be the Initial Boiling Point (IBP) of the heavier cut and so on.

    In order to fractionate the crude oil into the various products, it has to be heated to a temperature between 330 and 385oC (626 and 725oF), depending on the crude composition. The partially vaporized crude is transferred to the flash zone of the column located at a point lower down the column. The furnace outlet temperature should be enough to vaporize all products withdrawn above the flash zone plus about 3–5 vol% of the bottom product.

    This overflash has the function of providing liquid wash to the vapors going up the column from the flash zone, and improving fractionation on the trays above the flash zone, thereby improving the quality of the HGO and reducing the overlap with the bottom products below the flash zone. This necessitates that there must be few trays in the region between the flash zone and the HGO drawoff. The overflash provides heat input to the column in excess to that needed to distill the overhead products. It also prevents coke deposition on the trays in the wash zone.

    The furnace outlet temperature is controlled to keep coking inside the furnace tubes and in the column flash zone to a minimum. However, the composition of the crude plays a part in determining the maximum temperature allowed. Paraffinic crude oils cracks more readily than an aromatic or asphalt-base crude. Therefore, the furnace outlet temperature for paraffinic crude oils is lower than that for other crude types.

    Column Pressure
    The pressure inside the CDU column is controlled by the back pressure of the overhead reflux drum at about 0.2–0.34 bar gauge (3–5 psig). The top tray pressure is 0.4–0.7 bar gauge (6–10 psig) higher than the reflux drum. The flash zone pressure is usually 0.34–0.54 bar (5–8 psi) higher than the top tray.

    Overhead Temperature
    The overhead temperature must be controlled to be 14–17oC (25–31oF) higher than the dew point temperature for the water at the column over- head pressure so that no liquid water is condensed in the column. This is to prevent corrosion due to the hydrogen chloride dissolved in liquid water (hydrochloric acid).

    Pre-flash Columns and Crude Column Capacity
    The crude flow rate to the CDU determines the capacity of the whole refinery. A crude column is typically designed for 80% loading, which means that the unit can be operated at 20% throughput more than the design value.

    The capacity of the column is limited by the vapor flow rate with a velocity between 2.5 and 3.5 ft/s (0.76 and 1.07 m/s). The vapor flow rate increases as the vapors rise from the flash zone to the overhead. To keep the vapor velocity within the limits mentioned above, the pump-arounds, which are installed at several points along the column, extract heat from the column. This results in condensing the rising vapors and reducing the vapor velocity.

    To expand crude capacity, the most used technique is to introduce a pre-flash column before the crude heater. The crude oil after preheating in the hot products and pump=around heat exchangers is flashed into a column where the lightest products are removed. The bottoms from the pre-flash column are introduced into the crude heater and then to the crude column.

    The amounts of the light ends in the crude are now less, and this reduces the vapor loading up the column. Although the unit throughput is increased, the furnace duty is not increased, since the crude rate going to the furnace is not affected due to the removal of the light ends. Pre-flash columns are also introduced in the original design of the CDU when the crude oil is light, and when it contains a lot of light ends in the naphtha range.