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Summary Overview of Crude Units
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an Overview of Crude Units
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Overview of Crude UnitsCrude units are the first units that process petroleum in any refinery. There objective is to
separate the mixture into several fractions like naphtha, kerosene, diesel and gas oil. A
schematic diagram of an atmospheric crude fractionation unit is shown in Figure 1-1.
water
naphtha
PA1
steam
PA2 kerosene
steam
PA3 diesel
water DESALTER
Crude steam
HEN
gas oil
steam
sour water HEN FURNACE
Residue to Vac. Tower
Figure 1-1: Atmospheric Crude Unit.
Crude oil contains salts which can be harmful to downstream equipment and must be
removed. To remove the salts, water is mixed with the crude oil and typically heated to
temperatures between about 215 oF to about 280 oF and allowed to separate in the
desalter. The desalted crude enters another heat exchanger network. Both heat exchanger
networks make use of the vapors of the main column condenser, the pump-around circuit
streams (PA1, PA2 and PA3), and the products that need to be cooled. Then, the
preheated crude enters the furnace, where it is heated to about 340-372 oC (644-700 oF).
The partially vaporized crude is fed into the feed region (called flash zone) of the
atmospheric column, where the vapor and liquid separate. The vapor includes all the
components that comprise the products, while the liquid is the residue with a small
amount of components in the range of gas oil. These components are removed from the
residue by steam stripping at the bottom of the column. Products are withdrawn from the
side of the column and side strippers are used to help controlling the composition of light
components. In addition, to more effectively remove heat, liquid is extracted at various
points of the column and cooled down to be reinjected at a different position on the
column. Cooling water and sometimes air coolers are used in the heat exchangers PA1,
PA2 and PA3, but it is always more advantageous to have these streams release their heat
to the raw crude oil in the heat exchanger networks (HEN), usually called pre-heating
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,trains. Several different designs and configurations for the heat exchanger network in the
conventional crude oil distillation unit are possible. Figure 1-2 shows one particular
instance of a preheating train, not necessarily the best or most recommended one
(efficient ones are discussed later). In addition, in some oil distillation units, gas oil is not
produced and instead becomes part of the residue. Such units contain one less side-
stripper and one less pump-around than those shown in Figure 1-1 and Figure 1-2.
Further, in units in which gas oil is not produced, the diesel may be further separated into
heavy and light diesel.
Figure 1-2: Atmospheric Crude Unit-with preheating train.
The topped crude leaving the atmospheric tower still contains significant amount of
valuable oils. These oils cannot be distillated at atmospheric pressure because the
temperature required would be so high that severe thermal cracking takes place. Figure
1-3 depicts such a unit. Note first that this unit does not have a condenser and does not
feature side strippers either, simply because products do not have specifications on their
light end. However, side strippers can be used in specific cases, such as lube base oil
production.
2
, To Ejectors
VLGO
VHGO
Topped Crude
Vacuum Furnace
Vacuum Residue
Figure 1-3: Atmospheric Crude Unit-with Preheating train.
Figure 1-3 shows that light vacuum gas oil (LVGO) and heavy vacuum gas oil (HVGO),
are produced. Sometimes, depending on its properties, LVGO is blended with other
products like atmospheric diesel. Both are typically used as feed to fluid catalytic
cracking units. The vacuum distillation consists of the vacuum furnace, vacuum tower
and the vacuum producing system. The topped crude is heated up in the vacuum furnace
to about 400 °C. The temperature is controlled to be just below the temperature of
thermal decomposition. Although a single cut of vacuum gas oil (VGO) is allowed in
some cases, drawing LVGO and HVGO separately is more beneficial from the point of
view of energy savings, because the resultant HVGO draw temperature is 90-120 °C
higher than the corresponding draw temperature of a single VGO cut. Lighter
components are removed from the residue by steam stripping. In addition coke formation
is reduced by circulating partially cooled bottoms to quench the liquid to a lower
temperature. Because the heavy crude fraction contains metal complexes (asphaltenes
and porphyrines), which are catalyst poisons for downstream processes, sometimes a
recirculation of “wash oil” in the bottom part (not shown in the figure) is included to
prevent these compounds to reach the HVGO.
Vacuum distillation is widely used to produce catalytic cracking plant feed stocks of low
carbon content. It is also used to produce lube oil fractions. In practice, the uses of
vacuum distillates and residue depend on the type of crude oil feed, the type of refinery
and its downstream processing capacities. The major specifications for the most common
products are (Watkins, 1979):
• Catalytic cracking feedstock: Gas oils for catalytic cracking feedstock require a strict
separation between distillate and residue. The amount of carbon content in these oils
should be minimal while not sacrificing gas oil recovery. This is necessary to reduce
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, coke forming on the cracking catalyst. The metals content, particularly vanadium and
nickel, should be controlled because they are severe catalyst poisons.
• Hydrotreaters or Hydrocrackers Feedstock: These can tolerate a slightly higher metal
content because the hydrogenation catalysts contain these metals. However, the
amount of carbon and asphalt materials should be minimal to prevent coke formation.
• Distillate fuel oils: Distillate fuel oils to be used directly are specified by API gravity,
viscosity, metal content and flash point. Atmospheric boiling ranges can be used to
define the separation.
• Vacuum residue: The properties of vacuum residue can be defined in several ways.
When distillate production is to be maximized, the amount of gas oil allowed
remaining in the bottoms stream must be minimized. The residue is blended into
residual fuels. In this operation, one can normally set the volume percent of either the
whole crude or the topped crude that is to be yielded as vacuum residuum.
In the preheating train, the crude is under pressure to suppress vaporization. In the case of
a light crude, the pressure required to suppress vaporization is too high. The solution is to
separate some light components before heating the crude further in the preheat train
(Figure 1-4 ). The light components separated in the pre-flash drum are sent to the
column directly.
condenser
naphtha
PA1
steam
PA2 kerosene
Crude Oil
vapor
Desalter steam
PA3
diesel
steam
sour water gas oil
Preflash drum steam
Furnace AR to Vac. Tower
Figure 1-4 : Pre-flash design
In the pre-fractionation design (Figure 1-5), the light components are separated in a
pre-fractionation column. Thus, in the pre-flash design, components in the range of
naphtha are condensed in the condenser of the atmospheric tower, while in the pre-
fractionation design, these components are split into two fractions: light naphtha
condensed in the condenser of the pre-fractionation condenser and heavy naphtha
condensed in the condenser of the atmospheric tower.
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