A Revolutionary Separation Technique
Drystill has invented and patented distillation technology. It is internationally a heat and mass exchange device called a SAM (an acronym for Stripper/Absorption Module). This device implements a revolutionary separation technique called “Pass-through Distillation” (PTD), a public domain concept that is being studied in universities around the world. PTD does not necessarily require a Drystill SAM in order to work. But in order to understand what a SAM does, it is necessary to first understand PTD. The best way to learn about it is to visit the website dedicated to disseminating information about it, PTdistil.com. It shows, through text and videos, how PTD brings about room temperature distillation using less than half the energy required by ordinary single effect distillation.
The website www.PTDistil.com is an educational website dedicated to the free dissemination of pass-through distillation, which is a public domain concept. The Drystill SAM, on the other hand, is a proprietary device.
It is an evaporator and gas absorber in a single low-pressure vessel with heat pipes passing through the bulkhead that separates them. The evaporator generates vapors that are ducted into the gas absorber and absorbed into a stream of absorbent fluid. Gas absorption generates heat and absorption uses heat. Roughly the same quantity of heat is involved by each process – the latent heat of evaporation.
Some absorbent fluids operate at temperatures much higher than liquid being boiled in the evaporator. As an example, if the evaporator feed is a fermentation broth (which is mostly water), it will boil at 30 degrees Celsius at 0.04 bar. At that pressure, a 55% Lithium Bromide brine will absorb water at a temperature of 60 degrees Celsius. Heat pipes conduct heat from the hot absorber to the relatively cooler evaporator. Under the right conditions, the heat demanded by the evaporator is exactly matched to that which the absorber must waste, and the SAM will operate with no externally applied heating or cooling.
In that process the absorbent fluid becomes diluted. In order to recycle the absorbent fluid in a continuous circuit, the diluted brine must be regenerated to its original concentrated state. This involves boiling out of it everything it absorbed during its transit through the SAM absorber chamber. Since the brine contains no temperature-sensitive substances, the high temperatures associated with multiple effect distillation (MED) are acceptable. The brine may then be regenerated using less than half the energy that was originally delivered by the heat pipes into the SAM evaporator.
Those are the basics. The details are even more fascinating. In fact, they constitute a body of knowledge of their own right, playfully called SAMology.
Basic SAMology
Pass-through Distillation involves an evaporator, a gas absorber, and an absorption fluid regenerator arranged as shown in Figure 1.
Figure 1
The heat required by the Evaporator is roughly equal in magnitude to that generated and thrown off by the Absorber. The temperatures within the absorber are higher than those in the evaporator. This offers the opportunity for thermal integration as shown in Figure 2.
Figure 2
Drystill has combined the evaporator and absorber into a single vessel with means of conveying the heat from one to the other, as shown in Figure 3.
Figure 3
The ducting of the vapors from the top of the Evaporator to the bottom of the Absorber may be done internally, as shown in Figure 4. The evaporator/absorber unit is called a SAM, and the process that combines it with a brine regenerator is called TIEGA (Thermally Integrated Evaporation and Gas Absorption).
Figure 4
Although other means could be used, Drystill normally uses heat pipes to convey thermal energy from the relatively hot Absorber chamber to the relatively cool Evaporator chamber. Figure 5 illustrates an internally ducted heat pipe SAM.
Figure 5
The SAM shown in Figure 5 is intended to convey the idea of how a SAM works, and to serve as a schematic representation on process flow diagrams. But a working SAM could be fashioned according to this basic geometry. If the heat pipes and the double bulkhead were made as a sub-assembly and loaded into a vacuum vessel with appropriate sealing means, it might look like this:
Figure 6
Figure 6 shows a dual bulkhead SAM with transverse heat pipes. The bundle is self-contained and may be pulled out of the shell (as shown here) for cleaning or inspection without the need to disconnect any piping.
Advanced SAMology
While simple in function, SAMs involve complex relationships between fluid flows and geometry. The heat pipes serve as coarse packing in both chambers, so there is the capability to separate not only solids from liquids but to concentrate the most volatile species in the vapor stream leaving the evaporator.
Tall SAMs
seek to take full advantage of this. They are constructed to give many vertically stacked rows of heat pipes. Drystill’s Pilot plant at Fielding Chemical was an example of a Tall SAM. A drawback of Tall SAMs is that they require a crane to pull the heat pipe bundle. This might be acceptable for service with clean fluids but bad if there is a need for routine cleaning.
For dirty service,
Long SAMs
are preferable. Fig 6 above is an example of a Long SAM. The bundle can be pulled, cleaned, and put back into service in under an hour. The long SAM shown in Figure 6 has transverse heat pipes. The capital cost could be saved if fewer, longer heat pipes were arranged axially. A bulkhead near the middle of the vessel would separate the evaporator and absorber compartments. If the vessel were 10′ long, both the evaporator and absorber chambers would be 5′ in length. This is a poor arrangement for horizontal heat pipes. Inside the heat pipe, liquid working fluid must move between the cold zone and the hot zone. The mechanism is the capillary action of the heat pipe’s internal wick. Capillary action is effective at short range but too slow to move working fluid long distances.
If a Long Sam with axial tubes were constructed with multiple bulkheads, evaporator and absorber chambers could be alternated and the travel distance for heat pipe working fluid would be kept short. A SAM made in this manner is called a
Multi-compartment SAM. A ten-foot-long SAM might have ten chambers, five evaporators, and five absorbers. A multi-compartment design affords extra design flexibility. One or more compartments can be used for heating or cooling other process fluids to influence the heat balance. For example, SAMs with supplementary heating could enable the use of a rectifier column in the vapor stream leading from evaporator to absorber. Applied to bioethanol, this measure would send a higher concentration of ethanol to the absorber leading to more economic operation in the absorption fluid regenerator.