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.

ordinarydistillation

 

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.

PTDdiag

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).

PTD Covered in European Journal NPT

Tony Kiss, a researcher with Akzo Nobel, learned about PTD at a technology convention and asked to collaborate on an article about the technology.
Tony is certainly well qualified to publish important technical information. See www.tonykiss.com to learn more about his accomplishments. The article may be accessed in its entirety by clicking here . Here is an excerpt.

NPT_ProcessTechnol_2014_4_16-17_PTD_Page_1

PTD presented at Dutch distillation symposium

On June 17, 2014 a technical group known as NL-GUTS (Group of Users of Technologies for Separation) met in Veenendaal, the Netherlands, for a meeting dedicated to distillation and other thermal separation processes.Click here to learn more about NL_GUTS.
Ian McGregor and Steve Furlong attended to make a presentation on Pass-through Distillation. Both men were representing two Canadian Companies: Drystill Technologies which is a patent holder on PTD hardware known as SAM (Stripper/Absorber Module), and Fielding Chemical Technologies, a well established waste processing company that is collaborating with Drystill to test SAM technology in the processing of aqueous waste streams. Click here to hear the full content of the presentation, accompanied by the powerpoint slides.

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.

SUMMARY

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.

The Temperature Map

Almost every chemical plant has a boiler and an evaporative cooling tower. The boiler sources heat at a particular temperature. A portion of that heat is wrought into the the goods the plant produces, and the remainder (usually most of it) is dissipated into the environment by the cooling tower at a lower temperature. The source and sink temperatures, sometimes referred to as the “temperature rails”, are very important. They determine the plant’s capabilities.

The lower “rail”, the plant cooling water temperature, varies with the season and the weather. Plants must be designed to operate even in the hottest summer days, so the design value of the cooling water is often a near-worst-case value. We will use 30C.

While the plant’s “upper rail” is the steam temperature, evaporators may have a much lower temperature limit. We are going to consider one with a maximum allowable heating temperature of 85C. If we were to ask why the limit was set at that value, we would likely discover that it was a compromise. People responsible for product quality, equipment maintenance and production scheduling want low temperatures where delicate molecules don’t degrade and heaters don’t foul. But low temperatures are a costly luxury. The 85C limit is probably the point where product quality is only mildly affected and the fouling of the heater is just on the edge of acceptability.

single effect2

FIGURE 1    Single Effect Temperature Map

The diagram or “Temperature Map” above represents a single effect distillation heated by 85C steam and cooled by 30C water. The pink block represents the heater and the green block the condenser. The horizontal line at the 57C mark represents the phase change from liquid to vapour in the evaporator and from vapour to liquid  in the condenser (in this example boiling and condensing take place at the same temperature). The two heat exchangers here are well matched so they share the 55 Celcius degree temperature difference equally. The delta T of each is 27.5 C. This is a generous amount. For a given heat flow, the surface area required is inversely proportional to the delta T, so both heater and condenser are fairly small and inexpensive. But we want to see if we can save energy costs through the use of multiple effect distillation.

four effect2

Figure 2   MED Temperature Map

As we add effects, the delta T of each heat exchanger diminishes. If they are well matched, the five heat exchangers in the four effect unit will each have a delta T of 11 Celsius degrees. Each heat exchanger must therefore be larger and costlier than ones in a single effect plant of the same capacity, roughly by a factor of 27/11 or 2.5.

Fig 2 shows an idealized version of multiple effect distillation (MED). In real life there is always a gap between the temperature of the boiling liquid and the temperature of the condensing vapour. One cause of this is pressure drop through ducts and passages of real equipment. The other cause is boiling point elevation BPE), a difference in temperature between the boiling liquid and the vapour it produces.

Process and equipment designers work together to minimize the problem. To minimize pressure drop, huge vapour ducts are provided to convey gases from stage to stage. Boiling point elevation is a thermodynamic property of the fluid that cannot be mitigated. It dictates the kind of of process that may be considered. For example, if the fluid in the 4 effect process under discussion had a BPE of 10C, then each heat exchanger would have a delta T of only 1C. That plant would be an impractical monster, each heat exchanger having grown by a full order of magnitude. If the boiling point elevation were 11C, four effect distillation would be utterly impossible between the temperature rails specified.

PASS-THROUGH DISTILLATION TEMPERATURE MAP

Pass-through Distillation involves two independent pieces of equipment: a SAM and a multiple effect desorber. We’re going to consider first the desorber, shown below.

Desorber four effect

FIGURE 3        Four Effect Desorber Temperature Map

The first thing to notice is that since the desorber does not see any temperature sensitive process fluid, it is not bound by the 85C process temperature limit. I set an arbitrary metallurgical limit of 230C. (Above that temperature low cost alloys suffer excessive corrosion when exposed to the absorbent fluid in this study, but with different alloys or absorbent fluids the temperature limit could be greatly extended.)

The next thing to notice is that there are white spaces between the blocks representing heat exchangers. This is because the absorbent fluid, unlike the hypothetical process liquid of the previous example, has a strong boiling point elevation.

At the top of the map is the steam temperature of 230C. The heater has a 16C delta T. The boiling temperature is 214C. But the vapours do not condense at that temperature. There is roughly a 30C gap, and the condensation occurs in the first interstage heat exchanger at 185C. The pattern of a 16C delta T across a heat exchanger alternating with a 30C gap repeats until the bottom of the chart heat is discharged into the 30C cooling water.

Although there are other absorbent fluids available, Lithium Bromide and water is the one we will consider. It is widely used in the chiller industry. Its boiling point elevation is mainly a function of concentration,as shown on the Duhring Plot below. Every point on the chart represents a system state in which the lithium bromide solution is in equilibrium with the saturated water vapour surrounding it. A common way of expressing this is to say that the liquid is at its boiling point.  At any such point the addition of heat will cause some of the water in the solution to become part of the vapour (i.e. boil) while removal of heat will cause some of the vapour to be absorbed into the liquid.

ashrae duhring plot

FIGURE 4  Duhring Plot

The magnified portion of the chart shown below has been marked with a red horizontal line to represent system pressure of 200 Torr (that would read 22″HG on a vacuum gauge). Since the gas in the vapour space is pure water, we know from the saturated steam tables that its temperature is 66C. The chart tells us that if that this gas is in equilibrium with a 50% lithium bromide solution, the temperature of that solution will be 93C. It will also be in equilibrium with a 60% solution if that solution temperature is 115C. The boiling point elevation (BPE) values for these two cases are 27C and 49C respectively.

Duhring detail

FIGURE 5  Duhring Plot Detail

Referring back to the desorber temperature map (Fig. 3), we now know what the white blocks represent: the boiling point elevation (BPE) of the LiBr solution. We have seen that BPE is around 30C at 50% concentration and 50C at 60%. The process designer can choose the value of BPE by specifying the working concentration of the absorbent fluid. Now we will examine what will be affected elsewhere in the pass-through distillation process.

PASS-THROUGH DISTILLATION AND BPE

Fig. 6 below shows the schematic of a pass-through distillation plant comprising a SAM and a desorber. A feed liquid enters the top of the left hand chamber, splashes over warm tubes and exits the chamber at the bottom. The absolute pressure in the SAM and the feed temperature are matched such that the liquid is at its boiling point upon entering. All the heat added to the liquid by the warm tubes causes evaporation to occur. In this example the feed liquid is an aqueous slurry for which the boiling liquid and the vapour share the same temperature (i.e. the feed liquid has no BPE) and the vapour generated is saturated water vapour.

SAMptd2

FIGURE 6         Pass-through distillation Schematic

The blue arrows indicate the path of the vapours from the left hand (evaporation) chamber to the bottom of the right hand (absorption) chamber. As the vapours rise they are absorbed into the Lithium Bromide solution as it splashes downward over relatively cool tubes. The latent heat of evaporation carried by the vapours is imparted to the liquid but immediately removed by the cool tubes. As the solution descends its LiBr concentration drops, but at all points it is in equilibrium with the vapour, and its state can therefore be found on the Duhring plot.

There pressure drop in both chambers of the SAM is negligible, so the pressure and temperature of the vapour will be the same throughout. In the evaporation chamber the temperature of the liquid and vapour are equal. In the absorption chamber the temperature of the of the liquid solution is that of the vapour plus the boiling point elevation (BPE). Thus the BPE is the delta T which drives heat from the absorption chamber into the evaporation chamber. The tubes are heat pipes, which conduct the heat with very little thermal resistance.

Figure 7 below shows a temperature map for a SAM. This map is a little different from the ones we have seen so far. While the others were merely one dimensional bar charts, this one reflects the fact that as the LiBr solution falls through the absorber, its concentration and temperature drop.

SAMtempMAP1

FIGURE 7      SAM Temperature Map

Point 1 on the map corresponds to the brine entering the SAM while point 2 is the brine leaving it. The difference in the two temperatures depends upon the ratio of the mass flow rate of the brine to the mass flow rate of the water vapour it absorbs. At infinite brine flow the concentration would be unchanged and the equilibrium temperatures would likewise be unchanged. In practical systems point 2 will always be a few degrees lower than point 1. I arbitrarily chose to show a 5 degree drop.

Points 3 and 4 are the in and out temperature of the feed liquid respectively. In this example they are assumed to be equal.

The length of the vertical line between points 1 and 3 is the BPE of the absorption brine when it is in its regenerated state. In Fig.7 it is shown larger than the 30C of our example for the sake of legibility. Even so, the temperature map of the SAM touches neither the Process Limit of 85C nor the cooling water rail of 30C. That prompts the obvious question “How does the SAM temperature map relate to the plant temperature map?” The answer may surprise you: the two are independent.

Figure 8 below shows the same SAM with the same absorbent liquid operated at two different pressures. This time the maps are to scale for 30C BPE.

SAMtempMA21

FIGURE 8     Operating Pressure Dependency

It is the vacuum train that determines the position the SAM’s temperature map. The lower the absolute pressure of operation, the lower the temperature at which the feed liquid will boil. but whatever that temperature happens to be, the absorbent fluid will absorb the vapours at a temperature higher by the amount of the BPE.

At this point in the discussion, people with plant experience are likely to protest that 17 Torr is impractical for industrial distillations. That is true when industrial distillations involve condensers cooled by normal cooling water. Below 100 Torr the condensers may do an imperfect job, and process vapours will overwhelm the vacuum pump. In some plants where low temperature distillation is needed and the business can support the added operating cost, chilled water is fed to the condenser and lower operating pressures are obtained. Generally, any technique that minimizes the flow of uncondensed gas makes inexpensive vacuum equipment capable of very low plant operating pressures. Using a version of pass-through distillation, I ran a commercial pervaporation plant for several years at pressures normally below 20 Torr and sometimes below 10 Torr.

SUMMARY

A temperature map is a useful tool in visualizing how pass-through distillation differs from conventional distillation. PTD is a two step process. One of these is a conventional multiple effect distillation applied to an absorbent fluid. It is tied to the plant “temperature rails” in the same way as any other distillation. The other step is transfer of volatile components from the feed liquid to the absorbent fluid through evaporation and absorption. The temperatures involved in this step are independent of the plant steam or cooling water temperature over a wide range, making it possible to distillat very low temperatures, even below the temperature of the plant cooling water..

 

 

That Ugly Third Term

Chemists and Chemical Engineers, you may smile inwardly as you read this, but I am addressing my Mechanical Engineering colleagues who, like myself, learned in thermodynamics class that U, the internal energy of a system, is comprised of three  terms uand then forgot all  about one of them. It’s  easy to do when the  systems we work with –  heat engines, steam boilers, refrigeration etc. – deal only with the interplay of thermal energy (the TS term) and mechanical work (the PV term). That ugly looking third term, chemical energy, had little relevance to my world.

Some systems inhabit a realm too broad to fit that mindset, and and my study of Pass-through distillation has forced me to change my point of view. I have come to appreciate the power and importance of chemical energy, and that change can be compared to leaving Flatland and entering the real three dimensional world. Everything interesting in the universe, with exception of a few coarse industrial processes, hinges on that third term.

Pass-through distillation cannot be understood without accepting the counter-intuitive truth that a warm liquid can absorb a cool gas. This phenomenon, closely linked to boiling point elevation, is chemical in nature.

PTD vs.MVR

I have had the privilege of explaining Pass-through distillation to many clever and accomplished engineers during the past few years. One of them said suddenly “Aha! I see what this is! It is like mechanical vapour recompression (MVR) without the compressor”. Frankly, I didn’t understand his comment at the time, but now that I have grasped what he meant I find it very apt.

Mechanical Vapor Recompression (MVR)

The diagram below illustrates the MVR process: vapour from the evaporator vessel is brought to a higher pressure by means of a mechanical  compressor. At higher pressure, a fluid changes state at higher temperature. The higher pressure vapour condenses in the heating coils of the evaporator because its temperature is higher than the boiling liquid in the vessel.

mvr2

Notice that no external heat is being applied to the system. The latent heat demanded by the boiling liquid is fully supplied by the latent heat of the condensing vapour; the two quantities are nearly identical. Mechanical energy is being added to the system through the work of the compressor, but this amount is very small by comparison to the latent heat.The theoretical thermal efficiency of MVR can exceed that of a 100 effect evaporator. Even with the inefficiencies inherent in real equipment, efficiencies equivalent to 30 effect evaporators are reported.

Now let’s look at a SAM in operation. The green feed stream (assume water for our present purposes) enters the left-hand chamber at its boiling point falls over metal bars at a slightly higher temperature. Heat is imparted to the liquid causing evaporation. The vapour is ducted to the bottom of the right hand chamber. Induced by the pull of the vacuum train, the vapours rise counter-current to falling absorbent fluid, a brine solution, and become a part of the brine. This absorption involves a phase change; the vapour becomes liquid and its latent heat is released into the brine. But that heat is immediately conveyed by the bars to the evaporator chamber. ( Note: the “bars” are actually heat pipes which have very low thermal resistance)

 

SAM2d

Like MVR, the evaporation has taken place without the application of any externally applied heat. In both cases, the heat applied to the evaporator manifests itself as the mass flow rate of the vapour times its enthalpy of evaporation. In both cases the vapour is turned back into a liquid at a higher temperature that of the evaporator, and means have been provided to move that heat back into the evaporator.  So my friend’s comparison has a great deal of merit.

But our first diagram shows an MVR process in its entirety while the second diagram shows only a SAM, which is half of the pass-through distillation process. The other half is the brine desorber, which restores the diluted brine from the bottom of the SAM’s right hand chamber into a stream of pure water and the stream of concentrated brine which enters the same chamber at the top. The desorption process is a multiple effect distillation (MED). A three effect desorber is likely the practical choice, perhaps a four effect.

So MVR is simpler and more efficient than PTD. But it has some serious drawbacks that PTD overcomes.

First, it involves a large piece of mechanical equipment: a fan, blower, or compressor. Aside from their high capital cost, these pieces of equipment have reliability issues.

Second, MVR is not well suited for low pressure / low temperature operation. The compressor is already large when designed for operation at 1 atm. It must be twice as large for operation at 1/2 atm, because compressors are volumetric devices. By contrast, PTD can operate at 1 atm or at 1/30 atm without much change to equipment. Yes, it has a vacuum pump which must be upsized for lower absolute pressures, but since it only handles non-condensible gases, it is not a major piece of equipment at any pressure.

Third, practical compressors for MVR have modest compression ratios and consequently the delta T across the heating coil is small – on the order of 5C. This means the surface area must be large to compensate. No problem here – the large energy savings ensure rapid payback despite the high capital cost associated with large equipment. The real problem is that MVR can only be applied to fluids that boil and condense at roughly the same temperature. A few degrees of boiling point elevation can make the use of MVR infeasible. Pass-through distillation does not have this limitation. Concentrated Lithium Bromide brine can absorb water vapour that was generated 40 Celsius degrees cooler than itself. With that kind of delta T available, feed streams with high boiling point elevation are no problem, and smaller equipment can accomplish the same job.

So are the two processes, MVR and PTD, really similar? I would have to say “no”. Although both offer energy savings through the recycling of latent heat, they are quite distinct. Is one superior to the other? No, they each have attributes that make them the best choice for specific applications.

 

 

 

Notes on Part 4 (of PTD 101)

 

Part 4 contains the surprising truth that gives pass-through distillation its real energy saving potential. In a nutshell, gas absorption changes the state of the absorbed gas into a liquid; consequently the latent heat of evaporation must be rejected (as in a condenser). But this heat may be at a higher temperature than the contents of the evaporator. An appropriate heat exchange system then can supply the evaporator’s heat needs from the absorber. If this is done, we get the first boil-up “for free”.

We learned in part 3 that the second boil-up can use multiple effects to reduce energy consumption by 50%. So the combined energy use of the first and second boil-ups is 0 + 50% = 50% of conventional distillation.

I have found no instructive videos on Youtube to aid here, but I would refer you to any technical literature on Lithium Bromide Absorption Chillers to understand how a concentrated brine absortion fluid can absorb water vapours at extremely low absolute pressures even at warm temperatures.