Something new under the sun?

There is much to be said regarding pas-through distillation as a separation tool. This post is going to deal with only one aspect of the topic. The internals of the SAM may constitute a brand new type of fractional distillation apparatus, eliminating the reboiler in favour of heated “packing”.

Let’s first of all review what we know about two simple forms of distillation which will serve as points of reference: the flash tank and the stripping column.

flash

 

FIGURE 1 Flash Distillation

As shown in Figure 1 above, a flash distillation unit heats the feed outside the vessel to a temperature above its boiling point at the pressure in the tank. When the liquid passes through the pressure reducing valve, some of the liquid vapourizes, and the liquid quickly drops to its boiling point. A stream of liquid L is removed from the bottom and a stream of vapour V from the top. These two streams are in equilibrium with each other both thermally and chemically.

stripper

FIGURE 2   Stripper Column

Figure 2 shows a stripping column with its reboiler. The column is contains trays or packing upon which descending liquid and ascending gases can exchange mass and energy. Like the flash operation, it too generates a liquid stream at the bottom and a vapour stream at the top, but this time the vapour stream will  be in thermal and chemical equilibrium not with the bottoms product but rather with incoming feed at the top.

Now consider Figure 3, a SAM. The acronym stands for Stripper/Absorber Module. We are considering the left-hand compartment which I often call the evaporator section but which is, arguably, a stripper column. The heat pipes serve as a coarse packing, causing mass exchange between ascending vapours and descending liquids. The feed liquid is the last thing the vapours (shown in blue) see before leaving the compartment. We should expect then that the vapour stream should be in equilibrium with the feed, or at least nearly so. Most importantly, the vapour stream will be richer in the most volatile components than the bottoms. So is this a stripper? I say yes, but one you will never encounter in a chemical engineering textbook.

SAM2d

FIGURE 3    Stripper/Absorber Module

What makes it distinctive is that at every “tray” heat is added. This leads to strange flow behaviour. At the top tray both the descending liquid stream L and the rising vapour stream V are at their maximum. A portion of the feed is evaporated on this top tier of heat pipes, but the vapour flow rate is the cumulative total evaporation on this “tray” and all the trays beneath it. The flow of liquid descending from the top row of heat pipes to the second is reduced from the feed flow rate by the amount that evaporated. Similarly the amount falling from all subsequent tiers will be reduced until a minimum flow rate is reached at the bottom. At that point no further evaporation takes place and the flow of vapour is zero.

McCabe-Thiele analysis will not work on this “stripper”. Its underlying assumption of constant molal overflow does not apply. Other tools will have to be used to describe its behaviour. I am hopeful that someone reading this post may find the task interesting enough to undertake.

In the meantime we must be content with what we know from first principles:  1)  All the heat brought into the chamber by the heat pipes will show up in the enthalpy of the vapour leaving the chamber, assuming that the process liquid enters and leaves the chamber at its boiling point, and 2) The vapour leaving the chamber will tend to find equilibrium with the feed.

Spread the Word

New Energy saving technologies in the realm of evaporators and distillation equipment should interest many people around the world. After all, evaporation and distillation are number one and two among unit processes in terms of energy consumption. If there exists a global will to address the threats of greenhouse gas and fossil fuel dependency, no stone should be left unturned in the quest for reductions in our global consumption of energy; but extra attention should be given to the Big Users: evaporation and distillation.

Should a technology arise from any quarter claiming significant reductions in Big User energy consumption, those claims should be tested and either debunked or validated. The question is “whose has the responsibility to do so?”. I have arrived at the sad conclusion that there is no such individual or group on this planet. An energy catastrophe could occur while technologies that might prevent it remain in obscurity.

But there are grounds for optimism as well. This world is populated with many intelligent people (like yourself) who care about global problems. As individuals they may little say in the affairs of technology but through their on-line associations they can have great influence in lifting an idea out of the shadows and into the light where its true merits can be determined.

I experienced the power of the internet first-hand quite accidentally. A singing group I belong to gave an impromptu performance in a coffee shop and was caught in the act by a patron with a cell-phone video recorder. He posted the clip on YouTube for the enjoyment of his friends, but the video “went viral” and was soon viewed by over a million people around the world (check out “Old Men Singing at Tim Hortons” on YouTube). No single person determined the extent of its reach. It was the “many people” who thought it worthy of the attention of friends and associates.

You know someone who might benefit from knowing about PTD. Why not send him the link to this site?

What’s “new”?

Distillation is a one of the oldest and most widely practised industrial practices known to man. Could there really be something new in such a well-worn field of study?

“Newness” has a particular meaning to patent attorneys, and in that sense pass-through distillation is not new. But there is also a subjective sense of the word, as in “That’s new to me!”  For most of us, this category includes many great inventions that failed the test of commercialization  and vanished into obscurity. In this sense of the word, pass-through distillation may be new to you.

Once you have learned what it is and how it works, you will want to know if it is relevant to you. You might ask: Can it be applied to my process? What are  its operating costs and capital costs as compared to conventional technology? If it enables me to reduce energy consumption, will I get carbon credits? Are there any case studies in my industry? Who is currently active in this field?

I am hoping to make this kind of multi-disciplinary information available on this site over time. Your questions and comments will be helpful.