The main process carried out in CDU is distillation, where a mixture is heated up and separated into several components or fractions by selective boiling and condensation. Relative volatility is the basis for the separation and there are no chemical reactions involved.
The crude oil feed is heated,
partially vapourised, and fed to a fractionating column. In the column, the
liquid particles flow down and the vapor rises up. As we are aware, those
components with lower boiling points vaporize first. This vapor cools down as
it rises from the bottom to the top, and contacts the liquid that is falling
down from the top. During this vapor-liquid contact, a part of the vapor which
is enriched in less volatile components may condense and fall down as liquid.
On the whole, the more volatile
light components vaporize, rise, and exit from the top of the column, and the
heavy liquid components in the feed get concentrated at the column bottom.
Temperature and pressure will be
higher at the column bottom and progressively decreases to the top. The vapor-liquid
equilibrium established by different components in the feed is the basis for
the separation. Trays or packings are used to enhance this process.
Main products from the CDU/ADU and taken for further processing are
- Straight run Naphtha
- Straight run kerosene
- Light atmospheric gas oil for diesel production or as jet aviation fuel
- Heavy atmospheric gas oil for diesel production
- Atmospheric Residue -Vacuum gas oil as feed to the VDU
Crude Preheat Train
Purpose: To raise the temperature
of the crude oil from the battery limit and make it suitable for desalting.
Process: Crude from the Battery
Limit (B.L) is pumped through a series of heat exchangers. The inlet temperature
of 35OC to 40Oc is raised to around 150OC. The
preheated crude is then sent to the Desalter.
The energy available in the
system, pump around, distilled products and overheads is recovered to raise the
initial temperature.
Desalter
Purpose: To remove the inorganic
salts that may be present in the crude oil as this may cause corrosion and
fouling problems downstream.
Process: The process is
separation through emulsification of the crude oil with wash water. Before the
feed enters the desalter, a demulsifying agent is injected into the preheated
stream in order to promote desalting through oil-water contact. The demulsifier
promotes desalter hydration, increases salt removal efficiency, and minimizes
oil content in the water effluent.
In the Desalter, the combined
crude-wash water feed enters through the bottom. The wash water is injected
into the crude oil stream before the preheating stage itself. A high-voltage
electric field is supplied which coalesces the inlet and separates it into two
phases – crude oil and water. The desalted crude is taken out from the top. The
water along with the brine/salt falls down under gravity and is removed as
effluent. The desalter pressure is to be maintained to prevent the
vapourization of crude.
Preflash Column
Purpose: The preflash column
cannot be found in all plants and is optional. The column is used to recover
light ends from crude oil to reduce the energy requirements and traffic in the
distillation column.
Process: The desalted crude is
preheated to about 200OC before being fed to the preflash column.
Stripping steam is injected from the bottom. The overhead vapor, which is
Naphtha, is partially condensed and sent to the reflux drum. The Naphtha is
then sent for further processing, preferably to the Naphtha Hydrotreater. A
reflux to the column is also given from the reflux drum. The rest of the crude
is taken out from the column bottom and sent again for heating.
Fired Heater
Purpose: Heats the crude to about
366OC and is partially vapourised until 33% wt to aid atmospheric
distillation.
Fired heater working is a complex
process, which will be dealt with later in a separate post.
Crude Distillation Column
Purpose: The distillation and
separation of crude components
Process: The partially vapourised
crude is fed to the column bottom at its flash zone.
In a distillation column, the bottom half is the stripping section and the top half is the rectifying section. The column is equipped with fractionation sections and pump-arounds with conventional trays or packing to obtain the different crude cuts. From top to the bottom, the section arrangements are
- Naphtha/Kerosene fractionation zone
- Kerosene pump-around zone
- Kerosene/Light Atmospheric Gas Oil (LAGO) fractionation zone
- LAGO pump-around zone
- LAGO/Heavy Atmospheric Gas Oil (HAGO) fractionation zone
- HAGO pump-around zone
- HAGO/ Feed Fractionation zone
- Bottom stripper zone
The overhead Naphtha vapors from
the column leave at about 120OC and is rooted to the reflux drum. A
part is sent back to the column as reflux and the rest is sent to the Naphtha
Hydrotreater.
The next cuts from top to bottom
are Kerosene, LAGO, and HAGO. These are drawn from the side trays and sent to
the respective strippers namely the Kerosene stripper, LAGO stripper, and the
HAGO stripper. Here, the light ends are stripped from the inlet and the
overhead vapors are sent back to the column as a pump-around, and the products
are taken out from the bottom of the strippers. Superheated LP Steam is used
for stripping LAGO and HAGO and the kerosene is just reboiled.
Another stream of stripping steam
is injected into the bottom of the main column which acts as the reboiler.
Atmospheric Residue collected at the bottom of the tower is sent to the Vacuum
Distillation unit for further processing.
Comments
Post a Comment