And the time will come when you see we're all one
And life flows on within you and without you.

("Within You and Without You," by G. Harrison of The Beatles, 1967)

THE TWO GAS LAWS THAT ARE ALWAYS & NEVER USED

PTOA Readers and Students have learned that the "Gas Laws" actually predict common-sense gas behavior ... so call them "laws" if you want to ... I guess.

Deconstructing the mystery of "The Gas Laws" has made it possible to focus on how these "common sense rules" apply in the process industries ... because gases are flowing through many of them thar pipes in a processing facility!

This PTOA Segment #154 features a couple "Gas Laws" that are essential to the design and operation of process industry equipment ...

...And yet the typical Process Operator and Control Board Operator are blissfully unaware that these "Gas Laws" are supporting their daily efforts to optimize upgrading a feedstock into a more valuable product.

Paradoxically then ...Process Operators and Control Board Operators use the below "Gas Laws" always ... like 24/7 ... and yet never consciously at all!

• Dalton's Gas Law of Partial Pressures.
• The Ideal Gas Law.

 

DALTON'S GAS LAW OF PARTIAL PRESSURES

How can the overall averaged Total Pressure of a gas be determined when the gas is really composed of several different types of gases?

A dude named Dalton figured out a common sense "Gas Law" that answers that question.

Dalton put pen to paper to document his observations and ...Voila! Physical Science text book fame forevermore!

 

In this case Dalton figured out that if you had a mixture of gases ... for example the red and blue gases shown in the below animated graphic created by former UCLA student Sam Freitas ...

... the contribution to the overall gas Total Pressure exerted on the walls of the container is partially from the red gas particles and partially from the blue gas particles.

Like ... Duh!

So who is stunned to learn that:

Total Gas Pressure =

Partial Pressure of Red Gas + Partial Pressure of Blue Gas

Like ... super DUH!!!

Admittedly it gets a wee bit harder when there's a dozen types of gases ...

but still kinda Duh!

 

ALL INDUSTRIAL SEPARATIONS EQUIPMENT IS DESIGNED WITH DALTON'S GAS LAW OF PARTIAL PRESSURES

Your Mentor knows that the brilliant PTOA Readers and Students are dying to know when they will use Dalton's Law of Partial Pressures.

The answer is both:

• Frequently and
• Never, Really!

PTOA's future Separations Processes Focus Study Area will explain in more detail how the lighter, smaller components of a liquid mixture are selectively separated from a complex mixture in a process called "distillation" and "fractionation."

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order were already introduced to "distillation" and "fractionation" in PTOA Segment 34.

There are two great reasons to put energy and know-how into Separations Processes:

• To separate the more valuable light ends from the less valuable feedstock; the more valuable light fraction can always be condensed back into a much more valuable liquid intermediate or final product.
• To strip highly volatile light ends out of desired heavier liquid product so that the heavier liquid product can be more safely stored, transported, and used.

The towers that are used for Separation Processes are designed using Dalton's Law of Partial Pressures; Your Mentor will demonstrate the use of Dalton's Law in a future PTOA Separations Processes Focus Study Area.

This PTOA Segment #154 is just alerting PTOA Readers and Student's that Dalton's Law is used to predict the contribution to the overall gas pressure that is made by each type of gas that has been vaporized and is hovering over the surface of a liquid mixture.

Who amongst the brilliant PTOA Readers and Students would be surprised to learn that this gaseous phase that is hovering above the surface of a liquid would have a higher concentration of light ends versus heavy ends?

Oh. Okay.

Fred is confused again.

Fred ...

 

Imagine the nearby animated graphic shows hexane and heptane gas particles that have been vaporized into the gas phase and are hovering over a well blended liquid mixture of hexane and heptane that is held at a temperature of 175 °F (70 °C).

Hexane and heptane look similar; from the outside these two hydrocarbons appear to be members of the same family of hydrocarbon chemical compounds ... and they are!

Hexane has 6 carbons and heptane has 7 carbons  ...and all the other bonds on both of them are with hydrogen; so heptane has one more Carbon atom and 3 more hydrogen atoms and is thus a bit heavier than hexane.

Chemically speaking ... if we could put them on a scale ...

Hexane has a Molecular Weight of 86.2 and Heptane is a bit more chubby with a Molecular Weight of 100.2.

A more significant difference between the two is boiling point:

Hexane will boil at 154.4 °F (68 °C), significantly lower than the 209.2 °F (98.4 °C) require to get Heptane to start vaporizing.

Both of the above traits mean that hexane is over twice as happy and inclined than heptane to free itself from the liquid bondage of the mixture at 175 °F (aka 79 °C = 353 °K = 635 °R).

Ergo,

Much more hexane vaporizes into the gaseous phase ...like the graphic shows.

And you know what that means, Fred?

Since more hexane has been removed from the mixture and vaporized into the gas phase ...

The contribution to the Total Gas Pressure above the liquid level is mostly from the partial pressure of the hexane (approximately 70%) and the contribution from heptane is quite a bit less (approximately 30%).

And one more important conclusion ...

More of the heptane remains and is concentrated in the liquid phase!

 

Duh!

 

 

Process Operations and Dalton's Law of Partial Pressures

In the real world of industrial processing, the stews of hydrocarbons that are being separated are more complex than the binary heptane/hexane example above, but the working theory behind the success of each physical separation by boiling point relies just the same on Dalton's Law of Gas Partial Pressures.

The Control Board Operator will receive feedback in the form of analytical results and make adjustments in the  PV Temperature, PV Flowrate, and the PV Pressure to maintain optimal operations of the Separations Process Tower ... all the while blissfully unaware that it is Mankind's understanding of Dalton's Law of Partial Pressures working in the background of any Separation Process.

 

 

THE IDEAL GAS LAW

The best PTOA Readers and Students are reading the PTOA Segments in the intended sequential order ...

And they noticed that Charles's Gas Law, Gay-Lussac's Gas Law and Boyle's Gas Law (featured in PTOA Segment #152, and #153, respectively) predicted how the Pressure, Temperature, and/or Volume of a specified amount of gas will change under certain conditions.

The Ideal Gas Law conveniently combines Boyle's Law and Charles's Law into one handy-dandy reference tool ...

and adds the specified amount of gas ... in mass units of moles (n) ... that would occupy the volume of The Ideal Gas at the Ideal gas's Temperature and Pressure.

The Gas Constant "R" is just another fudge factor constant whose purpose is to make the units work out so both sides of The Gas Law expression are equivalent. 

 

Demystifying The Ideal Gas Law

Don't let the adjective "Ideal" wig you out; it's not that complicated.

How would you define the "Ideal Child?"

Take a moment to develop the criteria that would be needed to define "Ideal" over a spectrum that has so much variance.

Likewise, the scientists of yore who studied gas behavior in the 1800s struggled to define the properties of a standard ... lo, "ideal" gas.

For example, the compared behavior of gases just prior to each of them condensing was not very uniform.

Still, it was helpful to define "an Ideal Gas" which would have predictable behaviors even if the effort just made it possible to classify gases that were "less than Ideal."

Basically, it helps to define some standard for "normal" before it is possible to recognize "the abnormal."

 

Limitations of the Ideal Gas Law in the Process Industry World

In the Real World we all live in, all gases are "Real Gases," not "Ideal Gases."

As the nearby graphs indicate, the Ideal Gas Law predicts the trend of most Real Gases albeit a definite deviance between the "Ideal" and "Real" gas behavior is evident as Pressure increases. The deviation of Real Gases from Ideal Gas Law behavior increases at extremely high Pressures and Temperatures.

Furthermore, The Ideal Gas Law assumes that there are no attractions between gas molecules.  Real Gases with high densities exhibit strong molecular attraction.

Nor does The Ideal Gas Law predict that most gases have a Temperature decrease when they are allowed to expand (aka experience an increase in Volume). The exception to this observation is the behavior of hydrogen gas.

This oversight is not trivial in the world of Steam Turbines as it does not predict that Steam ... a vapor ... could drop below its dew point upon being expanded.

And no Gas Law assumes that energy and/or work is expended when a gas is expanded (Volume is allowed to increase). In fact, gases DO expend energy (which can be used to perform work) while they are expanding. That thermodynamic principle makes the Gas Turbine possible. The the expansion of a gas can be accompanied by a decrease in its Temperature or Pressure.

The design and operation of Compressors and the behavior of flowing gases must likewise consider the expended energy and/or work in their design phase. After all, it is the very mechanical energy generated that is required to compress a gas!

Furthermore, Boyle's Law did not account for the fact that mechanically compressed gas molecules move faster. Their agitated state causes them to bang into each other which causes their Temperature to increase. So Boyle's Law did not predict the increase in Discharge Temperature observed at the Compressor Discharge. Truth is that the observed increase in Pressure and Temperature at the Compressor Discharge are evidence that energy and/or work has been expended to compress the gas.

In summary,

The Ideal Gas Law is better at predicting the behavior of low density gases in an environment of moderate Pressures and moderate Temperatures.

 

Process Operators and The Ideal Gas Law

The Ideal Gas Law is another gas law that Process Operators use everyday ... and also never at all!

No Process Operator or Control Board Operator is going to be consciously aware that s/he or they are using The Ideal Gas Law ... although the daily measuring and control of gas flowrates and the design of the facility are based upon The Ideal Gas Law theory.

Here's a few examples of the Real World applications of The Ideal Gas Law:

The Ideal Gas Law made it possible to define how much gas matter would be contained in a specific Volume of gas ... like a cubic foot or a cubic meter.

In the future PTOA PV Flowrate Focus Study Area, PTOA Readers and Students will become pros at referring to gas flow rates in terms of "Standard Cubic Feet Per Hour (SCFH)" or "Standard Cubic Feet Per Day (SCFD)" or in Cubic Meters (M³) per time unit.  The origin of this everyday Process Operator lingo is from The Ideal Gas Law.

Automatic Instrumentation Technicians will use a form of The Ideal Gas Law to make important adjustment in gas flow metering ... and yes indeedo these adjustments use Absolute Temperatures!

But ...ah me!

These PV Flowrate calculations are now in a "plug in" format ... or even automated ... so the reference to the origins of The Ideal Gas Law is hidden.

A Process Flow Diagram is an important process industry document that illustrates how the major process fluid streams flow through the major equipment and instrumentation while being converted into final products. PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order learned how to interpret a PFD way back in PTOA Segment #32.

A Mass Balance will be the cover sheet of a PFD.

The Mass Balance reveals how the compositions and properties of gas and liquid streams change as they flow through the major processing equipment and are step by step converted into more valuable products.

The Ideal Gas Law is used to predict the composition and properties of the lighter gaseous process streams listed on the PFD.

The most brilliant Outside Process Operators and Control Board Operators will be well aware of the "designed" process gas stream composition listed on the PFD Mass Balance so that they can compare the difference observed in the real world "actual" process gas streams.

Understanding the difference between "design" and "actual" gas stream composition and properties helps Control Board Operators understand the limits of the processing equipment.

 

TAKE HOME MESSAGES:  Although Dalton's Gas Law of Partial Pressures and The Ideal Gas Law were used to design the gas handling equipment of the processing plant, the Process Operator and Control Board Operator are not consciously aware of their daily impact on process operations.

Dalton's Gas Law of Partial Pressure is used to design gas-handling process equipment that physically separates light ends from heavy ends via differences in boiling point.

Control Board Operators unwittingly use Dalton's Law of Partial Pressures to make operating adjustments that optimize yield of the most valuable, physically-separated products.

Likewise, Control Board Operators unwittingly interface with The Ideal Gas Law when monitoring and making adjustments in gas Flowrates and consulting the Mass Balance of the PFD.

Automatic Instrumentation Techs will actually use a form of The Ideal Gas Law when calculating adjustments to flow meters; however the "plug in" format of the calculation disguises its origin from The Ideal Gas Law.

©2017 PTOA Segment 0154
PTOA Process Variable Pressure Focus Study Area
PTOA Introduction to PV Pressure Focus Study

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