What is a Vapor Compression Still and How Does it Work?
“Water, water everywhere, nor any drop to drink.”
In his poem “The Rime of the Ancient Mariner”, Samuel Taylor Coleridge’s becalmed sailor laments the irony of being surrounded by plenty of water, but none of it being potable because it is the ocean. That is echoed by today’s world where even though 71% of the Earth’s surface is covered by water, only 2.5% of it is freshwater that can be used for drinking or agriculture. And even then, only 1% of the water is easily accessible.
Adding to the complications is the fact that populated areas can outgrow their local aquifers’ ability to refresh, leading to lower quality of available water. Agricultural run-off can also result in underground sources having higher amounts of salts and other elements, which must be removed before the water can be used in taps or fields. Furthermore, there are instances where it’s not a matter of treating wastewater, but of increasing the purity of the municipal water supply to meet the needs of biopharmaceutical manufacturers, medical uses and semi-conductor industries, to name a few.
Vapor Compression
Vapor compression (VC) distillation is conceptually similar in design to a heat pump or the more familiar mechanical refrigeration cycle. Major system components include the evaporator, compressor, heat exchangers, deaerator, and pumps. VC is inherently a thermally efficient distillation process, because it recycles a high percentage of the latent heat.
During the distillation process, feed water is piped into a boiler where it is heated to boiling point and the pure water vapor is then collected. The term “still” comes from distillation. While this is simple and relatively straightforward, bringing water to a boil is inefficient in terms of the amount of purified water collected compared to the amount of fuel used. The fuel must constantly be burned to maintain the temperature and any remaining heat is lost. In addition, the collected water then needs to be cooled so that it can be utilized, which also requires power.
The vapor compression increases the efficiency of the energy used by conserving the latent heat generated by boiling the water. This “leftover” heat would otherwise be lost to cooling or exhausted. It is used to pre-heat the incoming feed water. In some systems, the heat is further used to drive a compressor which pressurizes the steam and adds more heat to it as a result. All told, the same energy that is used to boil the water is reused in two to four other places within the system. In engineering terms, this recyclation of heat also reduces the load on the condenser and reboiler.
The Vapor Compression Distillation Process
A vapor compression still is a combined cycle system thanks to the reuse of the generated heat. The Aqua-Chem Vapor Compression Distillation process is as follows:
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Feed water is evaporated on one side of the tubes, and the generated steam passes through a disengagement space and separation system, to remove entrained water droplets, before the pure steam is drawn through the compressor.
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The energy imparted by the compressor results in compressed steam with an increased temperature of 7°F-10°F (4°C-5.5°C) which is equivalent to increased pressure of 3-5 psig (0.2-0.34 BARG).
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The higher energy steam is then discharged to the condensing side of the heat transfer surface. There the steam condenses giving up its latent heat, which is transferred through the tube wall to the feedwater. Additional water is evaporated, generating more vapor, as the process is repeated. The condensate produced is Purified Water and the portion of the feed water that is not evaporated is recycled.
Maintaining Quality
Like any system, the quality of what comes out of a vapor compression still is directly related to its “health” and hygiene. Because the feedwater is generally conventional tap or non-potable water the feed lines will accumulate salts or other contaminates over time and corrode the metal surfaces. Likewise, the waste lines will also have build-ups of materials within them. At the very least, this foreign material can affect the flow through of the respective lines and the heat exchange capacity. If something breaks loose from this accumulation, it can generate particles further down the system. At the same time, the evaporator can have baked on solids that precipitate out of the water as it is processed. This scaling reduces the efficiency of the operation, requiring additional energy to achieve the same levels of output.
This is where the expert technicians of Astro Pak can be called upon to perform chemical cleaning of the systems, equipment and components. Most customers passivate stills when newly installed and on a 2-5 year interval depending upon quality and volume of water needed to process with size of unit. Maintenance is required as efficiency is reduced with corrosion and deposits on the surfaces of the unit, especially on heat exchangers. The interiors of the heated elements are descaled using acid cleaning, removing the concentrated solids and exposing the metal surfaces so that it can operate as designed. The feed and waste lines, typically made from stainless steel, are flushed and checked for signs of corrosion (rust or rouge) and deposits that may have occurred. All affected areas are chemically derouged, cleaned and passivated to eliminate the rust and restore the steel’s corrosion resistance. In pharma applications it is usually 1-2 years; and in distilled water processes for lower quality applications, it is more like 3-5 year maintenance intervals.
The area of the boiler region where the liquid water is heated to boiling is where the corrosion and scale build-up is most prevalent.
The biggest problem in stills, is the presence of silica in the water (especially well water) which can cause silica or silicate build-up, which is very difficult to remove. Also, units that have gone 5 years or longer without maintenance service or derouging can be difficult due to large accumulations of scale and corrosion. The water may contain heavy minerals or special compounds that lead to insoluble materials and scale that also can be difficult to remove. Testing of the foulants is critical in most stills to determine the efficiency and optimum cleaning and derouging chemistry.
Regularly scheduled preventative maintenance and cleaning of the vapor compression still ensures optimal performance while largely preventing unplanned shutdowns, which can not only disrupt the supply of the distilled water for longer periods but can also result in even more costly repair and replacement of parts.
Sources:
https://www.nationalgeographic.com/environment/freshwater/freshwater-crisis/
