Process Tech & Oper Acad: Exothermic Rxns & Process Temps

We are never ever ever getting back together.
We are never ever ever getting back together.

("We Are Never Ever Getting Back Together," by Taylor Swift, 2012)

THE EXOTHERMIC HYDROCRACKING REACTION

The purpose of the hydrocracker reactor in the above photo is to break up big molecules made of hydrogen and carbon ("hydrocarbons") that are not very valuable and change them into smaller hydrocarbons that can be used as jet fuel and gasoline-blending components.

The reactor effluent is chemically changed and will never ever ever again have the same chemical structure or chemical characteristics of the feedstock.

A schematic of a big hydrocarbon being cracked up into small hydrocarbons is below. All of the atoms on the left hand side (the reactants) appear on the right hand side (the products); but the products are chemically very different than the feedstock.

An artist's illustration of a large, complex hydrocarbon on the left being cracked into smaller hydrocarbons on the right. Hydrogen gas atoms (H2) are the gray balls bonded together. The products will never ever ever be together like they were as feedstock.

The instant that the complex hydrocarbon molecules are cracked up, the smaller hydrocarbons that are formed need hydrogen to take the place of the carbon that they were "holding hands with" before those bonds were broken.

So there are two feedstocks that flow into the reactor inlet of the hydrocracker:

hydrogen gas (H2) and
the feedstock oil that is composed of complex hydrocarbons.

The cracking reactions that take place in the hydrocracker reactor are an example of very exothermic reactions.

The process stream temperature of the reactor effluent that flows out of the reactor outlet is much greater than the process stream temperature of the feedstocks that flow into the reactor inlet.

INTRODUCTION TO CATALYSTS

Feedstock, reaction temperature, and reaction pressure alone will not make a chemical reaction work in a reactor.

Reactors are filled with catalyst. Catalyst provide the surface area that is needed for the chemical reaction to happen.

: Catalyst can look like granulated sand, or pellets, or spheres, or be extruded like short pieces of spaghetti

Catalysts are made out of metals and acids stuck onto a substrate. This stuff attracts the feedstock components (aka: reactants) and promotes chemical reactions like bonding of reactants and/or ripping their chemical structure apart and rearranging.

This catalyst promotes bonding of the reactants into a product made of both reactants.

Molecules that are ripped up have a natural tendency to want to rearrange into something more complete and stable. So the catalyst chosen for a reactor is selected based on its ability to make the most of the desired products.

All of this ripping-and-bonding-and-rearranging action has to take place very quickly because the feedstock is continuously flowing into the reactor inlet, then over the catalyst bed where the reactions occur, and then out the reactor outlet.

A catalyst bed is the volume of catalyst loaded in the reactor. The vertical length of the catalyst bed is determined by how much catalyst is loaded.

The three factors that impact whether or not the chemical reaction has sufficient time to complete are:

The vertical length of the catalyst bed.
The flowrate of the feedstock/reactants.
The condition of the catalyst.

Process Operators and Control Board Operators must optimize the feedstock/reactant flowrate to make the most yield of the desired products. They must also make good judgments regarding daily operation of the reactor which will maximize the run length between turnarounds.

Artist's idea of catalyst surface and reactants.

Catalyst make it possible for a chemical reaction to occur but are not changed by the reaction. After the reaction is completed, the reaction products flow away from the surface of the catalyst and the catalyst is once again ready to interact with more reactants (feedstock).

THE TUBULAR STYLE FIXED-BED REACTOR

The feed pretreatment and hydrocracking reactors in a Hydrocracking plant.

Many reactors look like the tall silver tubes of the Hydrocracker Plant reactors in the picture to the right.

A schematic of the internals of the hydrocracker reactor is directly below.

Follow the process stream arrows at the top, bottom, and left side of the reactor.

In the upper left corner of the schematic, a feedstock labelled "Combined Feed" is shown flowing into the reactor inlet. The "Combined Feed" is made of H2 (hydrogen gas) and oil.

The schematic informs PTOA Readers and Students that the "Combined Feed" has just exited the fired heaters (not shown in the schematic).

PTOA Readers and Students that are reading the PTOA segments in the intended sequence already know why the fired heater was the piece of processing equipment that preceded the reactor.

PTOA Readers and Students also know that the H2 (hydrogen) needed for the hydrocracking reaction was made in the hydrogen plant; the hydrogen plant reactors used catalyst and the SMR and Water Shift reactions to convert steam (H20) and natural gas (CH4) into hydrogen.

Quench H2 can flow into the Combined Feed or areas below Catalyst Bed 1 and Catalyst Bed 2 to cool off the exothermic reaction in this Hydrocracker Reactor.

The reactor schematic shows another process stream that is labelled Quench H2.

The arrowhead on the Quench H2 process stream indicates that Quench H2 can flow into the reactor inlet with the feedstocks if needed.

The hydrogen for Quench H2 was also made in the hydrogen plant.

Quench H2 is hydrogen gas at a lower temperature because it did not flow through the fired heater with the oil.

Adding Quench H2 will cool the process stream temperature by diluting the process stream with a high volume of lower-temperature gas. The cool gas will absorb the heat from the process stream and keep the process stream from getting too hot.

Notice that Quench H2 can also flow into the "Contact and Distribution" areas that are situated below Catalyst Bed 1 and Catalyst Bed 2.

The need to use Quench H2 in between each catalyst bed is a big hint to PTOA Readers and Students that the reactions taking place are highly exothermic. The process stream can be cooled down in between catalyst beds when necessary just to maintain the desired reaction temperature and avoid a temperature runaway.

PTOA Readers and Students should also notice how much instrumentation is dedicated to temperature monitoring and control around an exothermic process.

TAKE HOME MESSAGES: A hydrocracker reactor is one example of a reactor in which exothermic reactions take place.

Exothermic reactions are identified by:

Reactor outlet temperature greater than reactor inlet temperatures.
Quench systems ready to quench a runaway temperature.
Temperature-control instrumentation all around the reactor.

Chemical reactions require catalyst. Reactors are filled with catalyst loaded into catalyst beds. The catalyst provides a surface area for the reaction to occur and is made specifically to promote the reactions that will chemically change the reactor feedstock into the desired products.

The time that the chemical reaction has to complete is determined by:

The vertical length of the catalyst bed.
The condition of the catalyst.
The flowrate(s) of the feedstock(s)...aka reactants.

The daily operation routines of Process Operators and Control Board Operators will impact the condition of the catalyst and therefore how long a run length will be between turnarounds.

Control Board Operators are also responsible for optimizing the conversion of reactor feedstock into desired products.

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