Pressure Pressure Pressure Pressure
Temperature rise turn it up the heat feel the pressure

("Pressure," by Skindred, 2005)

 

USES OF PRESSURE IN THE PROCESS INDUSTRIES

Grab a cup of your favorite beverage!

This lengthy PTOA Segment is going to describe several good reasons to keep things under pressure in the process industries!

 

Catalytic Reactions Require Pressure

PTOA Readers and Students learned all about how chemical reactions convert reactants into desired products way back in PTOA Segment #26.

PTOA Readers and Students learned that the feedstocks that flow into industrial reactors are "reactants" and the totally different products that leave the reactor outlet are the products of chemical reactions that have taken place inside the reactor.

In PTOA Segment #28, PTOA Readers and Students learned how the beds of catalyst that are packed into reactors provide the surface area for the feedstocks to be converted into more valuable reactor products.

The exothermic reaction of hydrocracking was also featured in PTOA Segment #28.

PTOA Readers and Students learned that a Hydrocracker Reactor breaks up heavy hydrocarbons to make smaller, more useful valuable hydrocarbons that can be mixed into valuable jet fuel and diesel.

In a "fixed-bed hydrocracker" like the one shown to the left, high pressures are needed to encourage the feedstocks to convert into the desired products of jet and diesel.

 

 

The high pressure increases the probability of the reactants crash landing on the surface of the catalyst ... which of course increases the rate at  which the desired reactions occur.

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order just learned in PTOA Segment #139  that atmospheric pressure is the pressure everybody feels on their skin and is approximately 14.7 pounds per square inch.

Guess what?

The pressure that is required to encourage the desired reactions in a fixed bed hydrocracker to occur is 1500-2000 pounds per square inch!

Dang!

That's over 100 times the pressure that you feel on your skin!

If you had that much pressure on you it would surely be more difficult to float around ...  so it is easy to Imagine how high pressures encourage gaseous reactants to settle down on the catalyst surface so they can do their thing.

How that pressure is created in the first place will be covered in the upcoming focus on Rotating Equipment.

Are you a PTOA Reader or Student that is having a hard time relating to a pressure measured in "pounds per square inch?"

That must mean you don't live in the USA ... and you are in good company because the last time Your Mentor checked, the PTOA is being read in 93 countries.

So use the handy dandy tool below to convert psi to kiloPascals (kPa):

http://www.convertunits.com/from/PSI/to/kPa

 

 

Gotta Make Sure Flow is Going the Right Way

All the pipes shown in the above photo have pressurized fluids flowing through them.

Otherwise stated ...

99.9% of the pipes in any processing facility are operating with internal pressures that are somewhat above to substantially above atmospheric pressure.

The process streams in those pipes might be gas or might be liquid.

Guess what?

Solid granules can even be "fluidized" ... which means they can be made to flow as if they were a fluid.

The point is that none of the stuff flowing in any of those pipes can flow at the pressure PTOA Readers and Students feel on their skin ... atmospheric pressure.

That's because pressure has to be created to make a fluid flow through those pipes.

Whatever that process fluid is, it is going to flow from the area of highest pressure to the area of lowest pressure.

An "area of high pressure to an area of lower pressure" is called a Pressure Differential (aka Delta P and "dp" and ΔP).

If a fluid flows into an area of higher pressure ... the fluid reverses flow and starts flowing the other direction!

So one reason Process Operators must control pressure is to make certain the stuff in the pipes is flowing as is intended and not in the opposite direction!

 

Pressure Differential Is a BIG DEAL in FCC Units

Hey!

Time to get a little schematic-reading practice in while integrating what PTOA Readers and Students just learned about maintaining a crucial pressure differential for the purpose of making darn sure flow is going in the desired direction.

The catalysts shown in the nearby photo were first introduced in PTOA Segment #28.

Obviously they are solids... because they are definitely not liquids or gases!

There is a popular USA industrial process called a Fluid Catalytic Cracker (FCC) in which "fluidized" catalyst circulates continuously between a Reactor and a  (Catalyst) Regenerator.

USA refiners install FCC Units for the purpose of making high-octane gasoline blendstocks from heavy hydrocarbon feedstocks that are not very valuable.

The FCC feedstocks can be a lot heavier than the feedstocks used in the fixed-bed hydrocracker that was featured earlier in this PTOA Segment.

The photo directly above features a FCC Reactor, (Catalyst) Regenerator, and Main Fractionating Column.

A simple schematic of the process flow streams through the FCC is below. The Reactor is shown in red. The (Catalyst) Regenerator is shown in shown blue.

To understand the process flow ... find the Fresh Feed Arrow.

1. The Fresh Feed (heavy hydrocarbons) and hot, Regenerated Catalyst (fluidized with steam) are mixed at the bottom of the Reactor. The fluidized Regenerated Catalyst is in the line with the arrow slanting down to the right.
2. The Fresh Feed and Catalyst engage in cracking reactions all the while flowing up a riser in the Reactor. During their two second journey up the riser the heavy hydrocarbons are cracked up into smaller, more valuable hydrocarbons.
3. The desired hydrocarbon products flow out of the top of the Reactor, and the spent and coke covered catalyst falls downward into the annular area outside of the riser.
4. The spent and coked up catalyst at the bottom of the Reactor is fluidized and flows to the (Catalyst) Regenerator. The arrow slanting downward and to the left is spent catalyst flowing from the Reactor to the (Catalyst) Regenerator.
5. To burn the coke from the catalyst, air flows into the (Catalyst) Regenerator. Coke is burned from the reaction sites on the catalyst while the catalyst flows upward.
6. The hot combustion products exit the top of the (Catalyst) Regenerator and the newly regenerated catalyst is fluidized with steam and flows back to the Reactor through the line that is shown as a downward slanting arrow.

And now that PTOA Readers and Students 'get the gist' of the FCC feedstock, product, and catalyst process streams ... here's a You Tube from fccim.mov that will visually clarify some details:

Pretty nifty process this FCC, eh?

The ability to continuously regenerate the catalyst means that the feedstock to the Reactor can be really heavy oil because who cares if the catalyst gets coked up?

And the FCC operates at a much lower pressure ... would you believe 25 pounds per square inch?

Dang! That means there's no need to invest in the rotating equipment that is needed to create the high pressures that the fixed bed hydrocracker needs!

It's all good, right??

Well, let's take a moment to consider what would happen if the pressure differential between the Reactor and (Catalyst) Regenerator caused "a reversal" wherein the hydrocarbon feed made contact with the air that is used for catalyst regeneration.

Boom laka laka laka! Boom laka laka laka!

To greatly reduce the possibility of a dreaded "FCC reversal," the Differential Pressure between the Reactor and (Catalyst) Regenerator is crucially controlled and a safety interlock system based on pressure differential will shut down the flow through the standpipes that circulate the regenerated and spent catalyst in the event a flow reversal is detected.

Pressure Differential is important throughout many processing units but the FCC unit is the most crucial application of monitoring Delta P that Your Mentor could think of.

Hey ...

Let's Take A Few Paragraphs to

THINK GREEN!

Feedback Your Mentor has received indicates that some PTOA Readers and Students need to be informed that hydrocarbons come from any organic source ... not just crude oil.

FCC technology is currently being adapted for green energy biomass fuels which also contain hydrocarbons.

Most of the technology examples presented in the PTOA can be adapted for a GREENER WORLD.

It's too late for Your Mentor...

but PTOA Readers and Students will experience working with the revamped and exciting new technologies that will provide green energy for the world.

And everything that you are reading and learning in the PTOA will still apply to these revamped and future technologies!

So keep on reading and learning!

 

Optimizing Separating System Pressure

Reactors are nifty, safe places to change the chemical structure and properties of feedstocks.

But the stew of reactor products always requires separation into a slate of products ... like gasoline, jet fuel and diesel.

Each product is determined by the boiling point range of their smallest hydrocarbons and heaviest hydrocarbons.

The labelled Main (Fractionator) Column in the above photo and the labelled Fractionator in the nearby schematic perform the function of separating the combined reactor products into distinctly useful blendstocks.

Guess what?

The operating pressure of the Fractionator will determine how much gasoline versus jet fuel versus diesel versus heavier hydrocarbon is drawn from the Fractionator.

Operating Fractionators and any Separating Tower at the optimal pressure is a BIG DEAL!

Process Operators should adjust the pressure of the Fractionator or column to match market conditions so that the most valuable product will have the greatest production possible from the tower.

Otherwise ... moola is just flowing down the drain!

Guess what?

 

The pressure of the Fractionator Column is controlled by controlling how much off gas is allowed to escape from the Overhead Accumulator.

The Overhead Accumulator is the squatty, horizontal vessel shown at the top and to the right of the Fractionating Tower.

Light ends and gases that exit the top of the Fractionating Tower are condensed into liquids via a shell and tube heat exchanger and then flow into the Overhead Accumulator.

PTOA Readers and Students will soon learn about how pressure impacts boiling point and will learn all about distillation (aka fractionating) in a future PTOA Separating Systems Focus Study Area.

 

Pressure is Used to Maintain
a Desired Phase (aka Physical State)

Using the example of water, PTOA Readers and Students who are reading the PTOA in the intended sequential order learned way, way back in PTOA Segment #2 that phase changes are caused by temperature changes.

 

The industrial application of vaporizing water into steam was featured in PTOA Segment #24 entitled "What's in the Package?"

Even though the media in both of these PTOA Segments was water, every solid on the planet has a liquid and gas state.

You, too will vaporize if the temperature is hot enough ...

That's called cremation!

So that means ... when conditions are right ...

all the liquids flowing in the pipes could change into gases or vapors ...

And all the gases flowing in the pipes could condense into liquids.

 

Up to this point in the PTOA...

all of the information about water changing phase (aka physical state) has assumed that the ambient pressure surrounding the water while it changed phase was measured at sea level ...

and therefore approximately the same as the atmospheric pressure that you feel on your skin ...

the big assumption being that you live in a location near sea level!

However ...

When the surrounding ambient pressure is less than atmospheric pressure measured at sea level, a liquid will boil at a lower temperature than it would at sea level.

That means that when water is surrounded by a pressure that is lower than atmospheric pressure measured at sea level, it will boil at a temperature that is less than 212 °F (100 °C).

The below chart shows the variance in the boiling point temperature of water that corresponds to changes in atmospheric pressure.

Likewise, when the surrounding pressure is greater than atmospheric pressure at sea level, it will take a higher temperature to get the liquid to boil than it would at sea level.

Oh no!

Fred is confused again!

Don't stress, Fred!

The role that pressure plays with temperature that results in a phase change will be clarified soon.

For now just understand that unintended phase changes caused by pressure changes can be detrimental to process industry equipment.

For example:

PTOA Readers and Students will soon learn about Rotating Equipment, including pumps and compressors.

Compressors add energy in the form of pressure to gases.

Pumps add energy in the form of pressure to liquids.

But if a gas gets into a pump ...

or if a liquid gets into a compressor ...

these pieces of expensive rotating equipment ... or their drivers ...will be ruined or at a minimum, severely  damaged.

In summary ...

Maintaining the proper physical state of flowing fluids is another great reason to monitor pressure!

 

Take Home Messages: PTOA Readers and Students learned that an "area of high pressure to an area of lower pressure" is called a Pressure Differential (aka Delta P and "dp" and ΔP).

Fluids always flow from an area of high pressure to an area of low pressure.

Good reasons for Process Operators to monitor the PV Pressure include:

• High pressures must be maintained in some reactors to encourage feedstocks to convert into more valuable products.
• Process Operators must make absolutely certain that the  Pressure Differential favors forcing all process streams to flow in the desired direction.
• Process Operators must optimize the pressure of a fractionating tower to favor maximum production of the most valuable product.
• Process Operators must make certain a process stream stays in the desired phase (aka physical state) to protect equipment that is not designed to handle multiple phases.

 

©2016 PTOA Segment 0140
PTOA Process Variable Pressure Focus Study Area
PTOA Introduction to PV Pressure Focus Study

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