You're hot then you're cold,
You're in then you're out.
You're up then you're down.

("Hot n Cold," by K.Perry, Dr. Luke, M. Martin, 2008)

 

IDENTIFYING EXOTHERMIC AND ENDOTHERMIC REACTORS

Exothermic Reactors

PTOA Readers and Students learned in PTOA Segment #28 that exothermic chemical reactions give off heat.

"Exo" is a Greek prefix that means "outside"; heat is released out of chemical bonds as they are broken.

The released heat is also an indication that the process stream is composed of different chemical structures that formed during the chemical reaction.

PTOA Readers and Students already know two tell-tale signs of highly exothermic reactions:

• The process fluid temperature of the reactor effluent product that flows out of the reactor outlet will be greater than the process fluid temperature that entered the reactor inlet.
• Quench gas lines will be present in the design to help control catalyst bed temperatures and prevent reactor temperature runaways.

 

Endothermic Reactors

Endothermic chemical reactions need heat to keep the chemical reaction going.

Endothermic chemical reactions rapidly decrease the process fluid temperature.

The Greek prefix "Endo" means "inside."

Endothermic chemical reactions constantly absorb heat and incorporate the thermal energy "inside" the new chemical bonds that are formed. 

For an endothermic chemical reaction to keep going, heat must be supplied.

Reactors that are built for endothermic chemical reactions are much shorter than reactors that are built for exothermic reactions.

Wait a minute!

A more accurate description would be that one tall endothermic reactor is broken up into three smaller reactors. This process architecture makes it possible to reheat the process stream in fired heaters.

Reheating the process stream in-between reactors returns the process stream temperature to the target required to continue the  desired reactions.

In summary, the following are tell-tale signs that an endothermic chemical reaction is being used to upgrade feedstocks into products:

The process stream will flow through a series of fired heaters and reactors.

The process temperature of the reactor effluent product will be much lower than the reactor inlet temperature. 

Reactors are labelled 1,2, and 3.

As the photo to the left shows, typically the three reactors become progressively taller: The first reactor is smaller than the second reactor and the last reactor is the tallest.

The reactors get progressively taller because the catalyst bed in each reactor is progressively longer.

Process temperature and process pressure are designed to make the most endothermic reactions happen first.

The first catalyst bed (in Reactor 1) must be shorter so that the process stream can be reheated before too much heat is lost and undesirable reactions start happening.

So the endothermic process architecture is accurately described by the lyric snippet listed at the beginning of this PTOA segment. The process stream...

• gets hot in a fired heater and then gets cold in a reactor...
• goes in and out of heaters and reactors in sequence...
• flows up to the top of a reactor and then down to the reactor bottom...then flows up to the top of a fired heater and then down to its bottom...yadda, yadda, yadda!

 

ENDOTHERMIC REACTION PROCESS PFD

The below simplified PFD has the tell-tale sign of an endothermic process.

This process reforms a feedstock (called "Naphtha" in the real world) into a gasoline blendstock called "Reformate."

PTOA Readers and Students should decode the PFD and notice:

• The Feed enters the PFD on the bottom left.
• Recycled Hydrogen Gas (H2 gas...not labelled in the PFD) combines with the oil Feedstock before flowing through the Heat Exchanger.

• PTOA Readers and Students already know how to decode whether or not the combined oil and gas Feedstock is being heated or cooled in the HEx and whether or not the combined Feedstock flows into the shellside or tubeside of the HEx because they've been reading the PTOA Segments in order!
• The now-hotter combined oil and gas Feedstock flows out of the Heat Exchanger and into Heater 1. The purpose of Heater 1 is to make certain that the combined oil and gas Feedstock attains the required reaction temperature.
• The Feedstock...now at the required reaction temperature...flows out of Heater 1 and into Reactor 1.

• Since chemical reactions take place in Reactor 1, the Reactor 1 effluent is chemically different than the combined oil and gas Feedstock that flowed into the Reactor 1 inlet.
• The reactions that take place in Reactor 1 are very endothermic; the process temperature of the Reactor 1 effluent product is much colder than the process temperature of the combined oil and gas Feedstock that entered Reactor 1 at the targeted reaction temperature. The Reactor 1 effluent product must be reheated so that the desired chemical reactions can continue.
• The Reactor 1 effluent product flows into Heater 2.Heater 2 reheats Reactor 1 effluent product to the desired reaction temperature. The reheated Reactor 1 effluent product flows out of Heater 2 and into the inlet of Reactor 2.

• Since chemical reactions take place in Reactor 2, the Reactor 2 effluent product is chemically different than the reheated Reactor 1 effluent product that flowed into the Reactor 2 inlet.
• Overall, the Reactor 2 reactions are endothermic.The process temperature of the Reactor 2 effluent product is less than the process temperature of the reheated Reactor 1 effluent product that entered Reactor 2. However competing reactions are occurring which make the temperature drop through Reactor 2 less severe. Still, the process temperature of the exiting Reactor 2 effluent product is too cold to continue the desired reactions.
• The Reactor 2 effluent product flows into Heater 3. Heater 3 reheats Reactor 2 effluent product to the desired reaction temperature. The reheated Reactor 2 effluent product flows out of Heater 3 and into the inlet of Reactor 3.

• Since chemical reactions take place in Reactor 3, the  Reactor 3 effluent product is chemically different than the reheated Reactor 2 effluent product that flowed out of Heater 3 and into Reactor 3.

Guess what!

In this naphtha reforming process, the  Reactor 3 effluent process temperature is a few degrees higher than the reheated Reactor 2 effluent product that entered the Reactor 3 inlet.

Yes indeedo!

The reactions that take place in Reactor 3 are slightly exothermic. The Reactor 3 effluent product does not need to be reheated.

The desired reactions have been completed.

Now the Reactor 3 effluent product must be cooled down to prepare for the separation processes that follow.

The Reactor 3 effluent product once again flows through the Heat Exchanger (shell or tubeside this time?).

Then the Reactor 3 effluent product flows through a HEx Condenser. The oil fraction of Reactor 3 effluent is condensed (the H2 will remain in the gas phase).

PTOA Readers and Students can already visualize how the Reactor 3 effluent product is cooled down by these two pieces of temperature-changing process equipment.

 

BRIEF INTRO TO GAS/LIQUID SEPARATION

The cooled, condensed Reactor 3 effluent product flows into a Separator. The Separator separates Hydrogen Gas from the desired liquid product, Reformate.

The desired Reformate product flows out of the bottom of the Separator and then exits the PFD.

Hydrogen gas (not labelled) flows out of the top of the Separator.

Most of the Hydrogen Gas (H2) is sent to a compressor to be compressed and recycled back into the process.

The recycling of Hydrogen gas means that the Hydrogen gas circulates in a loop. PTOA Readers and Students already know that heebie-jeebies must be removed from circulating systems.

Some of the Hydrogen Gas (H2) that is separated from the Reactor 3 effluent product flows to the right...the Off Gas line. Sending a little H2 to Off Gas prevents heebie-jeebie build up.

 

BRIEF INTRO TO COMPRESSORS & DEMISTERS

Compressors add pressure to gas streams.

PTOA Readers and Students should notice ISA symbol for a centrifugal compressor. The process gas stream (H2) enters on the side of the symbol that has a longer vertically drawn line and exits the symbol on the side that has a shorter vertical line.

The swaged-down structure of the ISA symbol for a centrifugal compressor makes it look like the gas is being squashed up into a smaller volume.

The ISA symbol for a centrifugal compressor is supposed to indicate that the process of adding pressure to a gas pushes the gas molecules together so that they are squashed together and more compact.

PTOA Readers and Students should also notice the criss-cross block in the Separator which is a demister. The demister knocks liquid particles that are entrained in the gas back into the Reformate. Liquids must never be sent to compressors.

 

TAKE HOME MESSAGES: Endothermic chemical reactions absorb heat and decrease the process temperature.

Endothermic reaction systems are identified by:

• Multiple fired heater and reactors in a series flow design which makes it possible to return the process temperature to the target needed to continue the desired chemical reactions.
• The outlet temperature of the reactor will be much less than the temperature at the reactor inlet.

PTOA Readers and Students were introduced to 2 phase gas/liquid separations and centrifugal compressors.

 

©2015 PTOA Segment 00037
Process Industry Stationary Equipment: Reactors

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