Down around the corner a half a mile from here...
You see them long trains runnin' and you watch them disappear.

("Long Train Running," by the Doobie Brothers, 1973)

In very large fuel processing plants, the shell and tube heat exchanger trains used for pre-heating crude oil for the distillation process can be quite long. A "half a mile long" as mentioned in the lyric snippet might be pushing it...but not by much.

 

SHELL AND TUBE EXCHANGERS SAVE $$$

The reason that the shell and tube heat exchanger is The Most Popular Piece of Processing Equipment is saving money. Without shell and tube heat exchangers, processing facilities would literally be burning money.

PTOA Readers and Students already know that a fired heater is relied upon to make absolutely certain that a required process temperature can be attained.

The natural gas and fuel gas used to generate the combustion products within a fired heater (and boilers, too) are expensive. Just like at your home, the cost of utilities to run the processing facility is a major expense and constant concern.

Guess what?

After spending the money in natural gas or fuel gas to attain the exact processing temperature required, the process stream usually needs to be cooled down directly after completing the processing step which required the the high process temperature in the first place!

PTOA Readers and Students have already observed the heat-up-and-then-cool-down processing scheme.

Who can forget the Benzene Plant PFD after staring at it for two PTOA segments in a row?

E101, the Feed Preheater, boosted the process temperature of the Toluene + Hydrogen combined feed.Then H-101, the fired heater, made certain the required reaction temperature was attained before the feedstock (aka reactants) flowed into the R-101 inlet.

Immediately after the reactions were completed, the reactor effluent was cooled down in E-102, the Reactor Effluent Cooler.

In the above case, the reactor effluent needed to be cooled down so that the upcoming processing steps to separate the desired benzene product from by-products would be more effective.

Another good reason to cool off hot final products, hot intermediate products, and hot by-products is to prepare them for storage. The hot vapors that would collect above the liquid surface of hot process streams would make the tanks in the Tank Farm go boom-laka-laka-laka, boom-laka-laka-laka!

In summary...the situation at a processing facility is:

• Some process streams must continually be heated up to convert feedstocks into desired products.
• Some process streams must continually be cooled down while on their way to further processing and/or polishing up.
• All process streams must be cooled down before being sent to the Tank Farm for storage.

The perfect solution is to exchange the heat between the process streams that are too hot with the process streams that need to be heated up. Exchanging the heat of hot process streams stretches the money spent on the fired heater fuel.

 

SHELL AND TUBE PREHEAT EXCHANGER TRAINS

With some forethought in design, the plan for exchanging heat between process streams can greatly reduce the expense for the natural gas and fuel gas needed for the fired heaters.

The plan to conserve energy...and save money... results in rows and rows of shell and tube heat exchangers which are known as "preheat exchanger trains."

A preheat exchanger train is designed into the processing plant scheme when a large volume of a process stream needs to be heated up significantly.

The heating process is broken up into steps; using the analogy of a real choo-choo train, each exchanger is a boxcar in the preheat exchanger train.

The process stream that needs to be heated up flows from one exchanger to the next exchanger in the train, gaining temperature all the while.

The process streams that are used for heating come from several different sources.

PTOA Readers and Students have the opportunity to  analyze a preheat exchanger train schematic in the next PTOA segment.

 

Take Home Messages:  PTOA Readers and Students already know that specific, targeted temperatures must be reached in a fired heater to successfully convert feedstocks into desired products. 

Directly after finishing the processing step that required a high temperature, the process stream usually needs to be cooled down to enhance separating desired products from by-products and/or to send products to storage in a tank farm.

Therefore, there are always process streams that need to be heated up and process streams that need to be cooled down in a processing facility.

Shell and tube heat exchangers conserve energy and save processing facilities money by exchanging heat between hot streams that need to be cooled down and cold streams that need to be heated up.

Significantly raising the temperature of a large volume of process fluid is accomplished stepwise in preheat exchanger trains.

 

©2015 PTOA Segment 00033
Process Industry Temperature Changing Equipment

 

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