Why do we never get an answer
When we're knocking at the door?

("Question," by the Moody Blues, 1970)

 

ANSWERS TO THE DIY PTOA SEGMENT:

PTOA Readers and Students recently determined the service duty of shell and tube heat exchangers E103 through E106 in the below Benzene Plant PFD.

"Service duty" means the functional role of the HEx in the process, which process fluids flow into the exchanger, and which of the process fluids leave the HEx hotter or colder.

Guess what?

After reading just 35 PTOA segments, PTOA Readers and Students can truthfully claim that they have a little experience interpreting a PFD all by themselves!

E-103, The Tower Feed Heater

Q: Does the process stream feed to E-103 (Stream #18) enter the shell side or tube side?

A: The ISA PFD/PID for E-103 indicates that the Process Stream #18 enters E-103 shell side.

Q: Based on the name of this exchanger, do you think Process Stream #10 that exits the shell side of E-103 is going to be hotter or colder than Process Stream #18?

A:The label key states E-103 is a Tower Feed Heater. Process Stream #10 is the feed to the tower, T-101. Therefore, heat is transferred into Process Stream #10 and it exits E-103 shell side hotter than Process Stream #18 which flowed into the shell side inlet.

Q: The tube side process fluid is labelled "lps" (probably comes from an unidentified "low pressure separator").Do you think this process stream from "lps" leaves the E-103 tube bundle hotter or colder?

A: The E-103 tube side "lps" process stream is the hot process stream that exchanges its heat with the shell side tower feedstream. The "lps" will exit E-103 tube side outlet at a colder temperature than when it entered the tube side inlet.

 

E-104, The Benzene Condenser

Q: Does the process stream feed to E-104 (Process Stream #13) enter the shell side or tube side?

A:The ISA PFD/PID symbol for E-104 indicates that Process Stream #13 enters E-104 shell side.

Q: The name of the exchanger informs the PFD reader that E-104 condenses benzene vapors into benzene liquid. Do you think the Process Stream #13 that enters E-104 is going to leave E-104 hotter or colder? (Big Hint: Remember!...the process of changing state from a gas to a liquid evolves heat, which means the liquid process stream that has been condensed is cooler).

A:The benzene process stream (Process Stream #13) is condensed in this exchanger, which means benzene vapors are condensed into benzene liquid. PTOA Readers and Students already know that the Heat of Vaporization is required to vaporize a liquid into a gas and that Heat of Condensation is given off (evolves) when a gas is condensed into a liquid.

The Heat of Condensation is given off into the surroundings and the liquid that is condensed...in this case benzene Process Stream #13...will exit E-104 colder.

Q: The tube side process fluid is labelled "CW." What does "CW" mean?

A: The initials "CW" mean "Cooling Water." Cooling Water is a utility specifically made to provide cold water that removes heat in a HEx.

Q: Will the "CW" exit the tube side of E-104 hotter or colder?

A.The cooling water will exit E-104 hotter having indirectly absorbed the Heat of Condensation evolved from the benzene.

 

E-105, The Benzene Product Cooler

The process stream to E-105 is the same as Process Stream #14.  Process Stream #14 enters the shell side of E-105 and exits as Process Stream #15, now labelled  "Benzene Product."

Q: The tube side process fluid entering E-105 is labelled "CW." What does that mean?

A: PTOA Readers and Students already know that the initials CW mean "Cooling Water."

Q:Does "CW" exit E-105 hotter or colder?

A:The Cooling Water exits E-105 hotter having done its job cooling the Benzene product that flows on the shell side of E-105.

Q: The label for E-105 is Benzene Product Cooler.  Do you think the benzene product that leaves E-105 (Process Stream #15) is hotter or colder than the shell side Process Stream #14 that entered E-105?

A:Process Stream #15 (Benzene Product) exits E-105 cooler than Process Stream #14 that entered E-105 shell side.

 

E-106 The Benzene Reboiler

Q: Is the process stream that returns to the tower bottom hotter or colder than the process stream that entered the shell side of E-106?

A. The purpose of all reboilers is to heat up process stream vapors and return them to the of a bottom third of a column or tower.The process stream that returns to the bottom of T-101 is hotter than the process fluid that flowed into E-106.

Q: Does the tube side process fluid labelled "mps" exit E-106 hotter or colder than when it entered the tube bundle?

A: The tube side flow process fluid ("mps") exits Reboiler E-106 colder having transferred heat into the shell side process fluid that returns to T-101.

 

E7, The Last HEx in the CDU Feed Preheat HEx Train

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.

A: The "hottest-side fluid" that flows into E7 is labelled "Vacuum Residue."

"Residue" means "what is remaining when other stuff has been removed."

Vacuum Resid is made from crude oil...but all the lighter hydrocarbons are removed so the remaining stuff is made of big hydrocarbons that do not like to flow unless they are hot. This process stream is so heavy that it is used to make asphalt!

The graphic shows that Vacuum Resid enters E7 at 370 °C (698 °F), transfers heat into the CDU Tower Feed,  and exits E7 at 236 °C (457 °F).

After exiting E7, the Vacuum Resid is spit into two process streams. One process stream is further cooled and then goes to the Tank Farm for storage.

The remaining stream is kept hot and flows to the asphalt plant to make asphalt.

____________________________________________

Wow!

The focus study on introducing shell and tube heat exchangers (HExes) has taken up 7 PTOA segments! HExes must be very common process industry equipment!

PTOA Readers and Students are experts at recognizing that HExes swap thermal energy (aka heat) between hot streams and cold streams; one process stream becomes hotter and another becomes colder.

The next PTOA segments will focus on process equipment that is specifically designed to decrease process temperatures!

Take Home Messages: PTOA Readers and Students that self-evaluated the Benzene Plant PFD and the CDU Preheat Exchanger Train can rightfully claim that they have some experience interpreting PFDs.

©2015 PTOA Segment 00036
Process Industry Schematics

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