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Solving the Colebrook
Equation for Friction Factors - Part 2
Vaporization of an industrial fluid has always been a topic that many process engineers, especially those who have not faced the challenge before, find daunting. When it comes to vaporizing a fluid that could be considered "less than perfectly clean", there's always the fear of fouling the heat exchanger being used....and with good reason!
Now, vaporization is a very broad topic. There's vaporization (typically called reboiling in this context) at the bottom of all distillation columns. There's also vaporization when the goal is to concentrate a solute dissolved in water (this is more correctly referred to as evaporation). You can learn all about different types of reboilers at The Distillation Group hosted by Mr. Andrew Sloley. For a closer look at the heat exchangers used for vaporization specifically, check out the Wolverine Tube Heat Transfer Databook (Adobe Acrobat required).
Also realize that there is more than one way to vaporize a fluid, as is typically the case in the engineering profession. For a fluid that is considered to have "average fouling tendencies", using a thermosiphon setup is quite common. Below is a typical setup that utilizes a vertical shell and tube heat exchanger.
The "golden rule" in employing this type of configuration is that the maximum vapor fraction should be no more than 30%. Use a lower vapor fraction for more fouling fluids. Why a 30% vapor fraction? Simple...experience has shown that a 30% vapor fraction helps keep a 6-8 foot long tube wet along it's entire lenth (or nearly the entire length). This is key, for when the tube begins to dry out, fouling occurs. Note the use of a separate vessel to allow the two-phase mixture leaving the heat
exchanger to separate. The mounting of the components is such that the liquid level in the vessel is aligned with the top of the tubes (the vaporization is usually done in the tubes to aid in the cleaning of the exchanger). This is what allows the thermosiphon motion of the setup. Note the absence of a pump. When the steam is turned on, the tubes are flooded with liquid. As the liquid begins to vaporize, a natural temperature gradient is created and the fluid is "pulled" in the exchanger. This is a very common method of vaporizing industrial fluids that has been proven effect time and time again.
Now, if the fluid that you need to vaporize is considered "extremely fouling", then another method may work best. This method is commonly referred
For our readers outside the U.S., you may not be as "in tune" with the increased popularity of ethanol as those in the states. In short, ethanol plants are being built at an amazing rate in the U.S. Why are companies in the U.S. making ethanol? There's been a huge demand created by the gradual phase out of MTBE as a fuel additive. I won't even think about going into the political topics surrounding ethanol, nor will I comment on why MTBE is being phased out. Every member of our audience is bound to have very strong opinions on both topics and frankly they've both been discussed sufficiently.
So, why am I bringing up the topic here in our newsletter? Well, for all of the reasons to use ethanol or to not use ethanol in gasoline, there was one fact that the industry simply couldn't avoid in the past. It took more energy to make a gallon of ethanol from biomasses than the ethanol had energy value. On an industrial scale, that was enough to make any company dedicated to large scale production very nervous. However, with the advancement of ethanol technologies, that may be changing. Hosien Shapouri, James Duffield, and Michael Graboski have published a paper titled "Estimating the New Energy Balance of Corn Ethanol" that compares the current production costs of those of the past. It's an interesting article and, if nothing else, the results area a testament to the power of ingenuity.
Recently in our online forum, the following inquiry was posted (edited for publication):
"In our compressed air system, it has been assumed that if the intake air temerature is reduced by 4°C, the power consumption of the compressor is reduces by 1%. I am unsure how the air temperature affects the power consumption?"
One of our senior forum advisors, Art Montemayor, offers the following response:
"You have good cause for doubting that compressor power
requirements are lessened by a drop in suction air temperature. If anything, the power to
a compressor will increase should the suction temperature be lowered. This is
assuming what you failed to state: the compressor is a reciprocating type with fixed
capacity and at constant speed. This is logically so and easily explained by the fact that
the air density increases with a lower temperature, thereby allowing the machine to
suck in and deliver more air - and thereby necessitating more horsepower.
books and thermodynamics books) deal with theoretical situations
- not the real life. For example, the books don't state it outright, but they are assuming
that you can vary the capacity of a given compressor at will -- and differentially at that
too! This is not the usual case in practice. That's why, if you state the specific case of
sucking in and delivering a given quantity of air (identified at a specific condition -
such as SCFM, 70 °F and 14.696 psia), then it is true that you can "save"
horsepower by lowering the suction temperature - but based on volumetric capacity - not
the empirical mass capacity that a compressor realizes. The fact of the matter is that
horsepower represents work done at a certain rate on a specific mass of gas
- not volume. Therefore, if you suck in more mass (due to a density increase and a
constant, positive volume displacement), you will require more horsepower. There is no
alternative to this fact."
I have to admit something...I've always hoped that I'd live long enough to see what shape the world takes after the appreciable oil reserves are depleted. Now, before I go on, let me say....I am in no way anti-oil! Look at what oil has done for the entire world, especially the developed countries around the world. So much of every aspect of our lives is impacted by the precious energy locked up in oil. The reason I'd like to be around for this transition is because it will force engineers and consumers to go through an adaption the likes of which don't come along very often!
Aren't you, as an engineer, eager to see what changes take place? What technology will rule the automotive industry? Will corporations own millions of acres of farmland dedicated strictly to producing crops that are destined to become plastic (not too far from the way the paper industry is today)? What materials will step up and take the place of the thousands of materials derived from oil? It will indeed be an interesting ride!
You can just about drive yourself crazy trying to guess when this might happen considering that you have to take into account variables such as global population growth, rate of consumption increase, outputs, new technologies, etc. While I'm by no means old, it's amazing to see the progress that's being made right now and I'm sure that there's more to come. I believe that the short term goal should be to minimize the use of oil for transportion, thereby giving the entire chemical industry more time to complete it's evolution. There's much research and development needed before we're ready to face the challenges of tomorrow's chemical processing industry. One thing is for sure, chemical engineers will become even more important in the future and we have to be ready to evolve along with our industry.
We hope that everyone is enjoying the redesign of our site and our new logo. Every so often, it becomes necessary to streamline the site and it's enormous amount of content. In addition to ChE Links (our flagship link directory managed by Dr. Bernhard Spang), we've also introduced a reciprocal link directory as our way of saying thanks to all of the websites out there that link to Cheresources.com. Our reciprocal link directory give us a chance to present even more great websites to our visitors. If you have a site and you'd like to link to Cheresources.com, let us know and we'll include your link in our directory.
Here are some great links that you can find in these directories.
From ChE Links:
John French's Art of Distillation
Electronic text of a book about distillation from 1651.
From our reciprocal link directory:
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1. How can I determine the length of a pulley belt without stopping our machine?
L = Pitch length of belt
2. What is a good estimate for a roughness factor for a pipeline that is severly rusted?
As pipes age, their roughness can increase substantially which makes fluid dynamic calculations quite a challenge. Before are some good estimates to use for aged piping:
Reference: Online Pressure Drop Calculator (Check
this site out, they've got some great tools!)
3. What are some of the best sites on the internet for learning more about all aspects of sulfuric acid?
Since more sulfuric acid is produced on the planet than
any other chemical, this is a good question. The following sites offer a wealth of
information on the topic:
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