Man it was brutal with plenty o' tissues
I guess you could say that I had issues.
But it's looking good I've dug my way out
I'm changing up what the story's about.

("New Alphabet," by EELS, 2013)

 

In the previous PTOA segment, PTOA Readers and Students focussed on the flows through the Reaction Furnace of a Steam Reformer.

An actual Steam Reformer is in the picture at the top of this page. The Steam Reformer has three parts as shown in the photo:

• The tall and boxy Reaction Furnace in the back left area of the photo.
• The convection section Duct is on the ground.
• The Chimney Stack is in the foreground.

PTOA Readers and Students already know that the desired Reaction Furnace product is Syngas.

Syngas is Hydrogen (H2 gas), Carbon Monoxide Gas (CO), and Carbon Dioxide gas (CO2).

 

WHAT HAPPENS NEXT AFTER REFORMING?

Sure.

Lots of fun ripping things apart and changing into newly bonded Syn Gas stuff ...but what happens next after all the heat and hullabaloo that takes place in the Reaction Furnace?

Waste heat recovery happens next!

The cost of producing Syngas would not make economic sense without transferring the heat out of the hot Syngas and hot flue gases and putting all that thermal energy to use.

PTOA Readers and Students are already familiar with the function and hardware components of the following heat recovery equipment:

• Preheaters.
• Steam Superheaters.
• WHBs.

Well...

Every Syngas-generating plant will have all-of-the-above heat recovery features!

You could say that heat transfer, heat recovery, and superheated steam production are major by-products of generating Syngas because otherwise it would be too expensive to build the plant in the first place!

 

FOCUS ON A SIMPLIFIED AMMONIA PLANT PFD

The below simplified Ammonia Plant PFD includes the three parts of a Steam Reformer:

• Reaction Furnace
• Duct
• Chimney Stack

Identify the Steam Reformer Components

PTOA Readers and Students need to identify the three functional sections of the Steam Reformer in the simplified Ammonia Plant PFD using the following hints:

• The Reaction Furnace of the Steam Reformer is the orange rectangle with the pink strip down the middle that is below the label "Steam reformer." The pink strip represents the Syngas production taking place inside the reactor-tubes.
• The orange rectangle (Reaction Furnace) is attached to the Duct that appears to be lying horizontally on the ground. The Duct effectively operates as the convection section between the Reaction Furnace and Chimney Stack.The Duct is hot red on the left side, cooler purple in the middle, and even cooler blue on the right side.The change in color from hot red to cool blue colorfully illustrates how the heat from the flue gases is exchanged as the combustion products flow through the Duct.
• The chimney stack is the tall blue A-frame shape.

Identify the Syngas Flow Path

PTOA Readers and Students need to trace out the Syngas flow path given the following hints:

• The hot Syngas produced in the Reaction Furnace flows out of the bottom of the Reaction Furnace and into the bottom of a Secondary Reformer that appears in the shape of an orange bullet.
• The hot gaseous product from the Secondary Reformer exits the right side of the Secondary Reformer and flows  into the tan (Process) Gas Cooler of the Waste Heat Boiler system.

Identify the Flue Gas Flow Path

There is not a line with an arrowhead to show the flow of the hot flue gases but take it from Your Mentor that they flow from the Reaction Furnace into the Duct.

The hot flue gases flow over outside of heat recovery equipment while on their way to the Chimney Stack.

From the hottest (left side) to the coldest (right side) part of the Duct, the following heat recovery equipment is identifiable in the PFD:

• A combined feed stream of  Feed (that has just been desulfurized) and MP Steam is preheated. This combined-feed line flows through the very hottest zone in the Duct and then enters the Reaction Furnace.
• Process Air needed for the Secondary Reactor is preheated in the left-most side of the purple zone of the Duct.
• Superheated Steam flowing from the vessel labelled "HP Steam Superheater" is made even hotter and drier by flowing through a superheater coil in purple zone of the Duct on the right side. After being superheated in the Duct, the High Pressure (HP) Superheated Steam flows off the PFD to the left. This Superheated Stream is suitably hot and dry to be used to drive a steam turbine!
• The Feed (which is Natural Gas aka Methane) is preheated in the blue zone of the Duct on the left side before it flows into the vessel that is labelled "Desulphurisation."
• The Combustion Air used in the Reaction Furnace is preheated in blue zone of the Duct on the right side.

Voila!

Using the Duct to preheat or superheat a total of five process streams stretches the money spent on creating the very hot temperatures needed for the endothermic reactions that make Syngas.

The recovery of waste heat simultaneously decreases the temperature of the flue gas that flows through the Duct en route to the Chimney Stack where it billows out into the atmosphere.

Hey! Reducing the temperature of the hot flue gases helps mitigate global warming!

That means saving money by recovering waste heat is also good for the environment!

 

The WHB System in an Ammonia Plant

PTOA Readers and Students need to locate the "Steam Drum" and "Gas Cooler" labelled in the upper right quadrant of the PFD.

The Steam Drum and Gas Cooler might look like the photo at the below right in real life.

Syngas Flow Path:

PTOA Readers and Students should follow the arrows and verify the following statements about the Syngas flow:

• The Secondary Reactor effluent product (Syngas) flows through the Gas Cooler. Although not labelled on the PFD, the Syngas will enter the  Gas Cooler at 1500-1600 °F and will exit at approximately 660 °F. PTOA Readers and Students already know that the heat from the Syngas is used to vaporize BFW into saturated steam.
• The 660 °F Syn Gas that exits the Gas Cooler  flows into the High Pressure (HP) Steam Superheater. The Syngas transfers heat into the Superheater coil and exits the Superheater cooler than when it entered the vessel.
• The Syngas enters the High Temperature (HT) Shift Reactor inlet.More steps are required to turn the Syngas into Ammonia but the focus on Syngas flow for this PTOA segment concludes at the HT Shift Reactor.
  

BFW Flow Path:

PTOA Readers and Students should follow the arrows to verify that the following statements about the BFW flow are correctamundo:

• The BFW flows from a different area of the processing complex and enters the Ammonia Plant PFD at the bold arrow labelled "BFW." Find the bold arrow labelled "BFW" on the right side of the PFD...approximately at the middle of the schematic.
• The BFW is preheated by flowing through the tube side of one HEx and then through a second HEx shell side.
• The BFW exits the second HEx and flows into the right side of the Steam Drum.
• BFW flows out of the left side of the Steam Drum and separates into two lines that enter the bottom of the Gas Cooler. The two lines represent several real-world lines that function like downcomers.
• The saturated steam generated in the Product Cooler rises into the Steam Drum via the two vertical "legs" that connect the Gas Cooler to the Steam Drum.

PTOA Readers and Students already know that any steam in contact with water is saturated steam. To produced superheated steam (aka useful hot and dry steam) the saturated steam must be removed from the water level in the Steam Drum and then heated.

• The saturated steam product flows out of the top of the Steam Drum and into the High Pressure (H.P.) Superheater.
• The superheated stream exits the H.P. Superheater and is further superheated in the mid part of the Duct.
• After this second stage of superheating the High Pressure Steam exits the PFD on the left.

Wow! Following all those process lines required a lot of concentration!

What was gained with the brain drain was an understanding of what Heat Recovery Equipment and Systems are and how they are incorporated into Syngas generating processes.

Well done!

 

TAKE HOME MESSAGES: Processes that use steam to reform light hydrocarbons require too much heat to be economical without heat recovery equipment and processes.

The Ammonia Plant PFD illustrated the three separate parts of a Steam Reformer: Reaction Furnace, Duct, and Chimney Stack.

The Ammonia Plant PFD also illustrated how the following heat recovery equipment is used to recover heat that would otherwise be injected into the atmosphere and wasted.

• Preheater Coils.
• Superheaters.
• Waste Heat Boilers

The Ammonia Plant PFD helped to illustrate why the PTOA classifies Heat Recovery Equipment and Systems as temperature-decreasing process industry equipment.

 

©2015 PTOA Segment 00045
Process Industry Schematics

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