Pass-through Distillation Pilot Plant

In 2014 two Canadian Companies collaborated to test Pass-through distillation experimentally.

One of the companies,Fielding Chemical Technologies Inc., is Canada’s foremost chemical recycler. Fielding wanted to learn through the pilot plant whether or not PTD could offer advanced treatment of hazardous industrial waste.

The other company, Drystill Holdings Inc., is a start-up technology company that has invented and patented equipment specifically for PTD, including the Stripper/Absorber Module (SAM) described in the fifth and sixth lectures of PTD 101. The pilot plant featured a mid-sized SAM capable of boiling 50 Kg of water per hour.

The pilot plant operated successfully for many months before being dismantled to make way for another project. For Drystill it furnished proof of concept for their proprietary technology. Fielding learned that the technology may be applied to the processing of industrial wastes, but has decided not to implement it for the time being.

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This is a picture of Drystill’s prototype SAM. It contains 100 heat pipes 25 mm in diameter and 550 mm in length.


Here is the same SAM installed on Fielding’s pilot plant framework.

Click here to see a video of SAM at work.

Click on the logos below to learn more about Fielding and Drystill.



About PTD

There are only so many unit operations known to chemical engineers, so it seems safe to say that every conceivable pair of them has been considered and, where appropriate, tested out in lab or plant. Some pairs exhibit such useful synergies that the combination becomes a unit operation in its own right, and it acquires a name of its own; witness evaporation + condensation = distillation.

I have searched the literature for references to current and previous attempts to combine evaporation and absorption. So far I have come up almost empty-handed. Yet this combination can provide low temperature distillations with half the energy used by current industrial practice. Has something been overlooked? As a community of professionals serving the world’s chemical engineering needs, we ought to talk.

To facilitate discussion, I have given combination of evaporation and gas absorption a name: “Pass-through distillation” and created this web site to serve as a clearing house for information on the topic. It is intended for the benefit of engineers and scientists in academia and industry world-wide.

If this field of enquiry ultimately generates wealth, some of it may rub off on Drystill and I will openly accept my share. In the meantime my activities relative to this web site will be directed toward shedding light on a topic that may benefit our planet.

Ian McGregor P. Eng.


Pass-through Distillation voted Project of the Year

POTYA sign


Keynote Address for PEO Gala

Good evening Ladies and Gentlemen. It is a great honour to address you tonight on behalf of Drystill, this year’s project of the year winner in the small company category. I would like to tell you our story. It is a story of how a very small Canadian company is making a big impact on world-scale problems. Removal of ethanol from watery mixtures, like fermentation broth, is an important industrial activity. Drystill has invented a device it claims can do a better job than conventional equipment. With funding assistance from NSERC, Drystill teamed up with Sheridan College to build and test a bench scale prototype under rigorous academic supervision. The testing validated Drystill’s claims. That is the project in a nutshell. Although it may not sound remarkable, I think you will be astonished by its significance to the world in general – and to Canada in particular. The removal of ethanol from watery mixtures is only one of thousands of tasks carried out every day by the process known as distillation. Although many people associate the term mainly with whiskey and other alcoholic beverages, distillation is in fact an industrial workhorse, touching almost everything we eat, wear and use. It is also a very old technology, used by the early Egyptians to prepare fragrances and medicines. Simplicity is its main virtue: You simply boil a liquid then cool the vapours to turn them back into a liquid.  Although mankind has become a lot smarter about the science behind distillation, the same ancient process is still in extensive use all over the world. Distillation, however, it has two serious drawbacks: Its enormous use of energy is costly both in financial and environmental terms, and its hot operating temperature is troublesome in some important applications. Drystill, in contrast, champions an alternative process called Pass-through Distillation. It enables economical low temperature distillation, reducing boiling temperatures down to 30 degrees Celsius while simultaneously reducing energy use by 50%. The Pass-through distillation process is in the public domain. It is not patentable because it is an obvious combination of well-known scientific principles. But it has never been implemented industrially for want of a suitable heat and mass transfer device with which to carry it out. Drystill has met that need with its invention of the SAM: an acronym for Stripper/Absorption Module. The SAM has patents granted or pending internationally. In industrial settings, distillation is not powered by renewable sources like wind or solar energy. It is powered by steam, generated by burning fossil fuels in a steam boiler. This means that where pass-through distillation displaces conventional distillation, the same results are achieved by burning half the fuel. The signing of the Paris Agreement last April marks the beginning of world-wide deliberate action against Global Warming. Each signatory country, including Canada, has pledged to cut back on the combustion of fuels to halt the progressive build-up of carbon dioxide in the atmosphere. No single measure will accomplish it. Some measures will be easy to bear – but others will be very painful. It will be painful to curtail our vacation travelling. It will be painful to have to cut back the thermostats of our domestic furnaces. But if manufacturers were to replace their conventional distillation equipment with Drystill equipment, the only pain would be its capital cost. That pain however would be offset by a fast payback due to reduced fuel costs. Government incentives could make that payback very rapid indeed. Once the investment has paid for itself, it will continue to save fuel costs for years to come. As countries around the world get serious about living up to their commitment to burn less fuel, this is the kind of measure that is going to be sought. It doesn’t affect anyone in a negative way. It is low hanging fruit in the quest for greenhouse gas reduction. But can Pass-through distillation be applied widely enough to make a significant impact on the global warming problem? Many people are surprised to learn just how much of our global energy budget is devoted to distillation. This pie chart, representing data from the US Energy Information Administration shows that 32% of all energy is used by industry. A third of that goes in to distillation! This is a whopping 9% of all energy used in the United States for any purpose. Pass-through distillation can cut that down significantly. It is applicable in many industries including food and beverage, pharmaceuticals, pulp and paper, specialty chemicals, wastewater treatment and others. It is reasonable therefore to say that Drystill’s technology is an attractive greenhouse gas control measure in the context of the current industrial environment. But industry itself is undergoing enormous changes. While coal, crude oil, and natural gas remain the dominant source of both energy and chemical feedstocks, there is a movement toward renewable sources such as agricultural crops and harvested biomass. Already in North America 10% of the fuel we burn in our cars is corn ethanol. Efforts have been underway for at least two decades to make ethanol economically from straw, wood chips, corn cobs and other waste biomass. When that effort finally succeeds, the world’s dependency on fossil fuels for transportation will be broken, and we will be well on our way to defeating the global warming menace permanently. Unfortunately there remain serious economic problems with the production of cellulosic ethanol. Many studies have shown that low temperature distillation enhances economics by keeping microorganisms alive and preventing destruction of expensive enzymes. Researchers have demonstrated this at lab scale but low temperature distillation through conventional means is not feasible at plant scale. A breakthrough technology is needed, and Drystill believes that pass-through distillation is the answer. The success of the cellulosic ethanol industry and victory over global warming may hinge on one question: Can pass-through distillation remove ethanol from a fermentation broth at a temperature of 30 degrees C? Here’s a news flash for you: Drystill and Sheridan College just tested it using a Drystill SAM. IT WORKED . For the good of our planet, this technology needs to be tested at demonstration plant scale as soon as possible. Since it is a Canadian invention, I am hopeful that a way will be found to do this testing in Canada, and that Canada will reap economic benefits by becoming the purveyor of pass-through distillation technology to biorefineries all over the world.

Akzo Nobel Announces Finalists in Imagine Chemistry Competition


Akzo Nobel logo


Developed in conjunction with KPMG, Imagine Chemistry was launched to help solve real-life chemistry-related challenges and uncover sustainable opportunities for AkzoNobel’s Specialty Chemicals businesses.

From January to March 2017, participants could submit solutions through a dedicated online challenge platform (The platform is now closed for submissions for 2017). Special challenge teams comprised of subject matter experts worked with participants through the platform to enrich and validate their solutions and determine if they are a good fit for AkzoNobel’s business.

An enthusiastic response resulted in more than 200 innovative ideas being submitted by chemistry start-ups, scientists, research groups and students around the world.

A jury made up of AkzoNobel business and R&D leaders and prominent international experts then selected the most promising ideas as finalists. This year’s finals will take place at AkzoNobel’s RD&I Center in Deventer, the Netherlands, from June 1-3, 2017.

Drystill is one of 20 finalists that will be participating at the event. Here is an excerpt from Akzo’s website, in which they describe Drystill’s submission:

“Pass-through distillation for wastewater

Steve Furlong, Drystill, Mississauga, Ontario, Canada

Challenge area: Wastewater-free chemical sites


Drystill is innovating thermal evaporation technology with its Stripping Absorption Module (SAM), a pioneering heat and mass transfer device.SAM can remove water from almost any stream or water source without heating it or putting it in contact with a drying material.


Conventional thermal evaporation of industrial waste has a high carbon footprint and costs. SAM offers a practical, low cost, low carbon solution that leads to wastewater-free chemical sites. This is achieved by dewatering effluent to produce clean, reusable water and high caloric-value/low-volume liquid residue.


Water vapor is transported to a compartment containing hygroscopic salt solution, which is then dewatered using conventional methods. SAM consumes no external heat or power: evaporation is caused by the exothermic absorption of the effluent vapors into a flow of concentrated brine (such as LiBr).

Pass-through Distillation, Drystill Chosen by Akzo Nobel

This year Akzo Nobel launched the first global innovation challenge for startups in chemistry. Through a competition called “Imagine Chemistry” Akzo is trying to accelerate the commercialization of important new ideas from around the world. On April 5, 2017 Program Manager Rinske van Heiningen announced that Drystill has been selected as one of ten finalists. She wrote:“We received over 200 submissions for more sustainable chemistry, which we have carefully reviewed with our team of researchers. We believe your solution ‘Drystill & Pass-through distillation: Improving the operating costs and carbon footprint of waste water concentration’ holds great potential and we are honoured to invite you to the finals, taking place in Deventer, The Netherlands from 1-3 June 2017. At the event, we will work together to further build on your idea during the workshops.

During the three-day event you will get personal feedback from our experts and AkzoNobel decision makers. The team will consist of senior level management, such as chief procurement officers, finance directors, operations managers, sales & marketing directors, R&D directors, etc. The composition of the team may vary depending on the phase and specifics of your startup. With this feedback, as well as the challenges/hurdles you perceive yourself, you will work in small teams over several rounds to further develop your concept.

Pass-through Distillation testing at Sheridan College

Sheridan College and Drystill Technologies are collaborating on a project to test pass-through distillation at lab scale, assisted by funding from NSERC.

The project formally commenced on May 30, 2016 and will operate until the end of the year. Many important industrial separations will be tested using room temperature pass-through distillation, including:

*separation of ethanol from fermentation broth

*separation of essential oils from aqueous slurry

*removal of water from corn ethanol plant syrup

*separation of butanol from fermentation broth

The SAM comprises 20 heat pipes 28 cm in length by12mm  in diameter, arranged vertically in a single column. It is capable of boiling 2.3 kg/hr of water. Drystill has built other SAMs in the past, but this one breaks new ground in that it includes an external stripping column between the evaporation and absorption chambers. Another new feature is mechanical distribution devices that permit even distribution of liquids over the heat pipes at extremely low flow rates.

The video below was taken on Aug 12, 2016 when the assembly of the apparatus was nearly complete and was being tested for leak tightness.


Drystill explains its version of Pass-through Distillation

Pass-through Distillation (PTD) is a powerful public domain concept, but to date its only commercial champion is the Canadian company Drystill.  Drystill’s contribution to the field is a proprietary piece of equipment called a SAM, which combines the first two process steps into a compact and efficient unit. When PTD is carried out using a SAM, the process is given a Drystill tradename,  TIEGA. This video is something of a Drystill infomercial,

Definition of Pass-through Distillation

Reduced to its essentials, distillation is a two-step process, evaporation and condensation. These two steps are “coupled” because they are carried out at the same pressure.



Pass-through distillation is a four step process, similar to simple distillation in that it begins by evaporating some feed liquid and ends by condensing it. These steps however are decoupled by an absorption step (step 2) and a desorption step (step 3) which involve a recirculating inventory of absorbent fluid. In step 2 this fluid absorbs the gases evaporated in the first step. In step 3 the absorbed material is boiled out of the absorbent fluid.


Decoupling permits the evaporator to operate at very low pressure (and a consequent low temperature) while the condenser operates at higher pressure (with consequent low cost cooling).

The four steps lend themselves to interesting heat economies. The absorption step runs at a higher temperature than the evaporator. This means that the heat released in the absorber may be used in the evaporator, permitting the first step to operate without an external energy source. The material absorbed by the absorption fluid must be boiled out in step 3 (desorption) through externally supplied heat. However if all the temperature sensitive material was left behind in step 1, step 3 may safely involve high temperatures, making it possible the use of low-energy multiple effect distillation (MED).

Please click here to play a 3 minute video that will acquaint you with the term “Pass-through distillation”. Then go to PTD 101 (a selection on the menu bar) to see companion videos that explain how PTD reduces energy costs and imparts other benefits, both direct and indirect.

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.



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.


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.


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.

Zero Water Consumption

In Industry the term “water consumption” comprises three main components: water that becomes part of the Product, water which becomes contaminated in the process and is sewered, and water that is evaporated in cooling towers. Distillation, per se, involves only the latter, and it does so in a very big way. Pass-through distillation can reduce or even eliminate this loss. To understand how, it is important to first understand why it exists in the first place. Cooling towers, and the water they “consume”, are part of a distillation plant’s energy flow.

Energy cannot be created or destroyed; it flows from a high temperature source to a low temperature sink. In most distillation plants it is provided to the evaporator as steam then removed from the condenser by a stream of cooling water. From the cooling water, the heat is wasted to the environment in an evaporative cooling tower where a portion of the water changes from liquid to vapour, carrying away waste heat at low temperature in the process. There are significant costs associated with the procurement and chemical treatment of the cooling water. The easiest way to mitigate these costs is to use less energy in the first place. If a conventional single effect distillation plant were retrofitted with a three-effect TIEGA process, it would produce at the same rate using half the energy input and consequently would use half the cooling water. Where there is evaporative cooling, water conservation automatically accompanies energy conservation.

A different approach to cooling is sometimes used: direct dry cooling. This operates in the same manner as a car’s radiator, and with very similar equipment. Many people know this type of equipment by the name “Fin fan”. This cooling method consumes no water at all. One of its drawbacks is higher capital cost than evaporative systems. But even when capital cost is not the most important consideration, direct dry cooling is often ruled out because it elevates operating temperatures some 20 Celsius degrees compared to evaporative cooling towers. Many distillations need to “run cool” because of the presence of temperature-sensitive materials. In some cases high temperatures cause heat exchangers to foul.  In other cases high temperatures cause delicate substances to thermally degrade, imparting objectionable odor or colour to products.

Pass-through distillation is ideal under these kinds of circumstances. The final condenser is completely decoupled from the process evaporator, so that their operating temperatures may be chosen independently. Suppose a conventional distillation operated with an evaporator temperature of 70C and a condensing temperature of 30C, furnished by evaporative cooling.  A retrofit for pass-through distillation could be configured to operate with those same temperatures using the same cooling system. The retrofitted plant would use half the energy of its predecessor, and would reduce the load on the cooling system to the same extent.

A second benefit of the PTD retrofit might be to reduce the process evaporation temperature from 70C to, say, 40C to eliminate fouling and improve product quality. That change would leave the triple-effect absorbent regenerator unaffected. Its first effect might operate at 190C while the water cooled condenser on the third effect would operate (as always) close to the temperature of the cooling water, 30C.

The third benefit would be to replace the evaporative cooling system with a fin-fan, and turn the plant into a “Zero Water Consumption” facility. This would raise the condensing temperature in the absorbent regeneration section to 50C, and that twenty degree increase would be felt all the way back to the first effect, raising its boiling temperature from 190C to 210C. The process evaporator however would be totally unaffected, and would continue to operate at 40C.


Zero water consumption is possible for any distillation process that can use direct dry cooling instead of evaporative cooling towers. Cooling towers, however, are more common partly because their capital cost is lower and partly because many processes cannot operate at the higher temperatures that direct dry cooling demands. A pass-through distillation plant overcomes these drawbacks. By virtue of using half the energy of a conventional plant, the capital cost premium of fin-fans is offset (a small fin-fan may even be cheaper than a large cooling tower).
A PDT plant can be configured to use “fin fan” cooling while reducing (rather than increasing) the temperatures seen by delicate process fluids.