Come back home to the refinery...

("Born in the USA," by the Boss Bruce Springsteen, 1984)

 

THE LONGEST TRAINS THAT ARE ALWAYS RUNNING

The longest preheat exchanger trains in the processing world are found in the Crude Oil Distillation Units (CDUs) of a fuels refinery.

CDUs are the first processes in the long line that make hydrocarbon fuels and petrochemical products.

 

THE CRUDE DISTILLATION UNIT (CDU) TOWER

PTOA Readers and Students will learn much more about the CDUs and the CDU Tower in future PTOA segments covering Separation Processes. A schematic of a CDU Tower and the product slate from a fuels refinery is below.

The CDU Tower is the main feature of a CDU because its job is to upgrade the crude oil feedstock by separating it into   process streams that become the building blocks for making fuels like gasoline, jet fuel, and diesel fuel.

The CDU Tower also separates naphtha from crude. Naphtha is a process stream from which many petrochemicals are manufactured in a petrochemical plant.

 

Before the crude flows into the CDU Tower, the process temperature must be at least 735 °F (391°C). The fired heater that precedes the CDU Tower is purposely there to make certain that 735 °F is attained.

 

PREHEAT EXCHANGER TRAINS
HEAT LARGE VOLUMES OF PROCESS STREAMS

Do you have a 13 gallon trash can in your house?  A crude distillation unit that processes 100,000 barrels of crude daily is processing the equivalent volume of 323,000 trashcanfuls of crude every day!

323,000 trashcanfuls is a lot of crude oil that has to attain 735 °F (390 °C)! Depending upon the fired heater only for this duty would be way too expensive in fuel costs to stay competitive and remain in the processing business.

 

THE DESALTER
PREHEAT EXCHANGER TRAIN

There are two preheat exchanger trains in a Crude Distillation Unit. A Desalter Preheat Exchanger Train  and a CDU Tower Feed Preheat Exchanger Train are featured in the below schematic.

On the left side of the schematic, "Crude from tank" flows out of the green tank that would be located in the Tank Farm. The crude exits the storage tank with a process temperature of 45 °C (113 °F).

The crude flows through the Desalter Preheat Exchanger Train comprised of shell and tube exchangers E1, E2, and E3. The Desalter is the blue horizontal tank.

Note that the zig-zaggy lines that define the tube side of a shell and tube exchanger are missing from the exchanger symbols in this graphic.

In most cases (but not all!) the crude probably flows through the tubeside of these exchangers because crude oil is gunkier than the process streams that are used to indirectly heat the crude up.  The gunkier crude would be easier to clean from the tubes than the shell.

The tube side crude flow through the Desalter Preheat Exchanger Train is as follows in the order written:

• enters E1 at deg 45 °C (114 °F),exits E1 at 85 °C (185 °F) and flows into E2.
• exits E2 at 113 °C (235 °F) and flows into E3.
• exits E3 at 143 °C (289 °F) and flows into the Desalter.

The shell side flow through the Desalter Preheat Exchanger Train comes from several sources:

• E1: The light Top Pumparound process stream enters E1 at 152 °C (306 °F) shell side, exchanges heat into the crude, then exits E1 cooler and returns to the CDU Tower just above where it had been drawn off.
• E2: The LGO Product (Light Gas Oil) enters E2 shell side at  245 °C (473 °F), exchanges heat into the crude, then exits E2 cooler and flows toward the Tank Farm for storage.
• E3: The HGO Product (Heavy Gas Oil) enters E3 shell side at 339 °C (642 °F), exchanges heat with the crude, then exits E3 cooler and flows toward the Tank Farm for storage.

 

THE CDU TOWER PREHEAT EXCHANGER TRAIN

The de-salted crude exits the Desalter and is pumped into the CDU Tower Feed Preheat Exchanger Train comprised of E4, E5, E6, and E7.

The crude tower feed flow through the Tower Feed Preheat Exchanger Train is as follows in the order written:

• exits E4 tube side at 175 °C (347 °F) and enters E5.
• exits E5 tube side at 203 °C (397 °F) and enters E6.
• exits E6 tube side at 236 °C (457 °F) and enters E7.
• exits E7 and enters the  fired heater at 261 °C (501 °F). This flow might be on the shell side because the Vacuum Residue process stream is really, really gunky.

The hot process streams come from many sources as described below:

• E4: The LGO (Light Gas Oil)  Pumparound process stream enters E4 shell side at 249 °C (480 °F), exits much cooler, and is returned to the CDU Tower just above where it was drawn from the tower.
• E5: The HGO (Heavy Gas Oil) Pumparound enters E5 shell side at 339 °C (642 °F... logically the same temperature of the shell side heating stream for E3...because it is the same stuff!). This HGO exits E5 shell cooler, and is returned to the CDU Tower just above where it was drawn from the tower.
• E6: HVGO (Heavy Vacuum Gas Oil) Pumparound Plus Product process stream enters E6 at 281°C (538 °F). After exiting E6, part of the HVGO is sent to the Hydrocracker Reactor as feedstock (PTOA Readers and Students already know about Hydrocrackers).The remaining HVGO is further cooled, then some is sent to the Tank Farm for storage and the rest is returned to the CDU Tower just above where it was drawn off.

DIY!

PTOA Readers and Students can figure out what the hot stuff flowing to E7 is and what happens to it after it leaves this last exchanger in the CDU Tower Feed Exchanger Train.  Answers are coming up!

 

Take Home Messages: PTOA Readers and Students were introduced to an important separation process unit, the Crude Distillation Unit (CDU).

All CDUs have a Desalter Preheat Exchanger Train and a CDU Tower Feed Preheat Exchanger Train.

These preheat exchanger trains conserve the energy needed to raise the temperature that is required to separate fuels and petrochemical building blocks from crude oil. 

©2015 PTOA Segment 00034
Process Industry Schematics

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