If you don't respect yourself
Ain't nobody gonna give a good cahoot...
na na na na

("Respect Yourself," the Staple Singers, 1971)

 

YOU CAN DO IT...SO DO IT YOURSELF! 

PTOA Readers and Students were recently guided by Your Mentor while determining the service duty of shell and tube heat exchangers E101 and E102 in the Benzene Plant PFD.

Service duty means identifying the functional role of the exchanger in the process, knowing which process streams flow into the exchanger, and understanding which of the process streams exits hotter and colder.

PTOA Readers and Students are already capable of determining the service duty of E103, E104, E105, and E106 in the Benzene Plant PFD by themselves.

A little analytical reasoning never hurt anybody I know and Your Mentor had to learn to do it, too.

Knowing how each piece of processing equipment works and keeping the equipment online as long as possible are skills that Process Plant Owners expect Process Operators to possess.

The answers to the exchanger questions below will be revealed in a future PTOA segment.

 

E-103, Tower Feed Heater

• Does the process stream feed to E-103 (Process Stream #18) enter the shell side or tube side?
• Based on the name of this exchanger, do you think Process Stream #10 that exits E-103 is going to be hotter or colder?
• The tube side process fluid is labelled "lps" (probably comes from an unidentified "low pressure separator").Do you think the "lps" process stream will exit the E-103 tube bundle hotter or colder?

 

E-104, The Benzene Condenser

• Does Process Stream #13 enter E-104 on the shell side or tube side?
• The name of the exchanger informs the PFD reader that E-104 condenses benzene vapors into benzene liquid. Do you think Process Stream #13 that flows into the condenser 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 that goes into the surroundings... which means the liquid process stream becomes colder).
• The tube side process fluid is labelled "CW." What does "CW" mean?
• Will the "CW" exit the tube side of E-104 hotter or colder?

 

E-105, the Benzene Product Cooler

The process stream that flows into E-105 has the same composition as Process Stream #14. Process Stream #14 enters E-105 shell side and exits as Process Stream #15, now labelled  "Benzene."

Hey! Making benzene is the purpose of this plant! Benzene is the desired product made by upgrading toluene and hydrogen feedstocks!

• The tube side process fluid entering E-105 is labelled "CW." What does that mean?
• Does "CW" exit E-105 hotter or colder?
• The label for E-105 is Benzene Product Cooler.  Do you think the benzene product that exits E-105 (Process Stream #15) is hotter or colder than the shell side process stream (Stream #14) that entered E-105?

 

E-106, the Benzene Reboiler

PTOA Readers and Students were introduced to a steam reboiler in PTOA Segment 23 which featured industrial uses of steam.

The purpose of all reboilers is to heat up process stream vapors and return them to the  bottom third of a column or tower.

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

 

SERVICE DUTY GUIDELINES FOR HEAT EXCHANGERS

The following scenario happens to all Process Operators at some time or another.

It might be during the pass-on meeting discussing a shell and tube heat exchanger....

or

It might be when training a new recruit.

Whatever the situation, the Process Operator is trying to sound like the authority that s/he truly is...but just at the moment... the service duty of a process stream into the tubeside...(or is it shellside?)...seems to temporarily disappear from memory.

Never ever bluff and give out the wrong information. Just find the  the P&ID and review the actual situation.

To help in remembering shell and tube service duty, here are some shell and tube design guidelines:

 

Best on the Tube Side:

Corrosive process streams are usually on the tube side because it is easier to pull and replace a tube bundle than it is to replace the shell.

Replacing the shell means disassembling the entire exchanger and maybe even multiple exchangers because they are often stacked on top of one another.

Gooky process streams that can lay down deposits and foul things up also belong on the tube side because it is a lot easier to pull a bundle and blast it clean than to remove the pitting on the interior of a shell.

High pressure process streams flow through the tube side. That's because it takes a lot of energy to make all those moves through a tubesheet and tube bundle and...as PTOA Readers and Students will learn in future PTOA segments....you gotta have pressure to have flow.

 

Best on the Shell Side:

Low pressure process streams just don't have enough umph to get through the tube sheets. They have an easier time flowing through the shell.

Process streams with vapors (like the mixed Toluene+Hydrogen feedstream in the Benzene Plant PFD) need to flow on the shell side. Vapors expand and take up a lot of volume which means heat transfer would not be as effective on the tube side.

Process streams that are condensing...changing state from a gas/vapor to a liquid... flow into the shell side (for example E-104, the Benzene Condenser). To improve the separation of liquid and vapor in the shell, the shell may be designed much different than the familiar hot-dog shaped shell.

 

Take Home Messages:  PTOA Readers and Students are now experienced with regard to determining the service duty of shell and tube heat exchanger.

Tubes ide fluids are usually those that are corrosive, gooky, or have the higher pressure.

Shell side fluids are typically lower pressured, have vapors, or will have a liquid that is condensing into a vapor.

©2015 PTOA Segment 00032
Process Industry Temperature Changing Equipment

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