By Dr. David G. Palmer
Summary: Distillate quality is measurably improved by replacing the traditional volatile vent with a simple venturi device. The result is a lowering of TDS, raising of pH, and the ability to eliminate VOCs without having to use a carbon filter.
Distillation is an established technique for removing inorganic chemicals from raw water in order to improve its quality. Small, single effect distillers are currently gaining in popularity as a way of processing tap water to produce high quality drinking water. When water is heated, the steam produced can contain vapors of organic chemicals, present in the water, which have significant vapor pressures at the boiling temperature of water. The steam will also contain quantities of carbon dioxide that come from a shift in carbonate/bicarbonate equilibrium caused by the heating process. As the steam condenses and the distillate cools, these vapors and gases re-contaminate the product water.
Role of carbon dioxide
Carbon dioxide has an extreme affinity for water. Some dissolved carbon dioxide combines with water molecules to form carbonic acid.
CO2 + H2O —-> H2CO3 —–> H+ + HCO3-
This is why the water from small distillers is often quite acidic with pH levels of 5.0 or below. This low pH by itself may have a negative effect on taste.
The dissociation that occurs with carbonic acid is sufficient to increase the conductivity of the distillate. In fact the readings made on distillate using a total dissolved solids (TDS) meter are almost entirely due to dissolved carbon dioxide—not ‘dissolved solids’ at all!
This explains the high TDS readings obtained immediately after a batch distiller commences making distillate. This is when the rate of evolution of carbon dioxide is the greatest. It also explains why ‘double distillation’—re-distilling water—results in only a minor lowering of the bulk TDS. In fact, when the distillate starts to appear, the spot TDS can be higher than the bulk TDS of the water placed in the boiling tank! Carbon dioxide totally confuses interpretation of readings made with a TDS meter on distillate.
VOCs
Various techniques have been proposed for removing volatile organic compounds (VOCs) either before or after they have been reabsorbed into the distillate. One approach is to use a granular activated carbon filter to polish the distillate. A second technique is to use an activated carbon filter on the raw water supply so as to remove any organic compounds before the water is heated.
For many years small distillers have been equipped with a “volatile vent” at the beginning of the condenser. This small hole in the condenser tubing has a positive effect in reducing the re-absorption of certain VOCs but is unable to deal with others. Hence, small distillers must be matched with some form of carbon filtration to take care of VOCs. Advocates of volatile vents claim that VOCs present in the steam are “vented” through this small orifice. While this opening may act as a vent for gases evolved while the boiling tank is coming up to the boil, once steam is being produced in volume such gases are blended with the steam and will travel through into the condenser. For whatever reason, volatile vents are partially successful.
Inside the condenser
At the start of the condenser, the vapor entering consists almost entirely of water molecules. As the vapor moves down the condenser and the steam condenses, the concentration of carbon dioxide and organic vapors will increase. While the condenser is open to the atmosphere at its outlet, its shape dictates that the concentration of carbon dioxide and organic vapors will, with time, continue to build.
The amount of carbon dioxide reabsorbed into the distillate is governed by Henry’s Law, which states the concentration in the distillate will be proportional to the partial pressure (concentration) inside the condenser. Temperature also plays a role. If we halve the partial pressure, we will halve the concentration in the distillate. To minimize carbon dioxide re-absorption in the distillate, we need to minimize the concentration buildup inside the condensing coil. Because carbon dioxide is a gas under all the conditions of pressure and temperature encountered in distillation, the only mechanism whereby it can enter the distillate is governed by Henry’s Law.
The vapors of VOCs behave somewhat differently. They, too, are subject to Henry’s Law, but partial pressures and solubility constants are such that effectively no measurable quantities of such compounds can enter the distillate by this mechanism. How then can they appear mixed with distillate? Unlike carbon dioxide, VOCs are liquids under normal ambient conditions. Under certain conditions, their vapors can condense on the internal condenser surface and then mix with the distillate. It all depends upon actual and saturated vapor pressures.
To better understand this phenomenon, consider water—the most common volatile “inorganic” compound. While we are familiar with water present as a liquid, we also know that water molecules exist mixed in with the air as a vapor. This is what humidity is all about. Relative humidity is defined as the actual vapor pressure in the air at a given temperature divided by the saturated vapor pressure at that temperature. The saturated vapor pressure is a measure of the maximum water vapor the air can “hold” at that temperature. As the temperature is raised the saturated vapor pressure increases. As the temperature falls, the saturated vapor pressure also falls. The temperature at which water droplets suddenly appear on a surface whose temperature is being lowered is called the dew point. So a condition for water vapor to condense into water is that the actual vapor pressure in the air must be equal to, or exceed, the saturated vapor pressure.
Each VOC has its own set of vapor pressure characteristics. Vapors of any one VOC can only condense on the surface inside the condenser if its concentration is allowed to build up to where its vapor pressure becomes equal to, or exceeds, its saturated vapor pressure at that temperature.
So while the mechanisms for a gas to re-enter the distillate and a VOC to re-enter the distillate are quite different, both depend upon concentration. For the gas, the effect is proportional: halve the concentration in the condenser and you will halve the concentration in the distillate. For VOCs, the effect is more stunning: Keep concentrations below saturation, and you should get no VOCs appearing in the distillate.
By stopping the build up inside the condenser of carbon dioxide, and the vapors from VOCs, it should be possible to move the pH closer to normal and eliminate VOCs from the distillate.
Forced condenser ventilation
Keeping concentrations low inside the condenser can be readily achieved by replacing the traditional volatile vent with a simple venturi device as shown in Figure 1. The steam velocity passing into the condenser draws in a small volume of air which travels through the condenser sweeping with it carbon dioxide and organic vapors. These exit the coil and are not reabsorbed. The volume of air required is not large and the venturi creates only a minor increase in back pressure in the boiler.
The effectiveness of the venturi device for limiting build up can be readily monitored with a TDS meter. Figure 2 shows how TDS of the distillate varies following start up with an EcoWater distiller with (and without) a venturi.
Monitoring forced condenser ventilation effectiveness at removing VOCs requires sophisticated analytical equipment. Third party testing was carried out using two Pure Water Midi D distillers, one of which had been modified to include a venturi. Granular activated carbon (GAC) post filters were not used. A summary of the results is contained in Table 1.
Results
The first column of Table 1 contains the average measured VOC concentration in the feedwater, and the second the percent removal calculated from analyzing the distillate of the conventional unit with a volatile vent. Thirteen of the 50 VOCs show 100 percent removal, some of these being Carbon tetrachloride; 1,1-Dichloroethene; 1,1-Dichloropropene; and n-Propylbenzene. The 37 other VOCs were all detected at various levels in the distillate. Some of the poorer performers include Bromoform (46 percent removal); 1,2-Dibromomethane (54.7 percent removal); and Methylene chloride (69.6 percent removal).
Results with forced condenser ventilation showed 100 percent removal achieved in all but two cases:
Chloroform—In order to see what limits might exist, the concentration of Chloroform in the feedwater was increased by a factor of 200. The conventional unit achieved 74.9 percent removal; the unit with forced condenser ventilation achieved 98.7 percent removal.
Naphthalene—The conventional unit achieved 40.7 percent removal; the unit with forced condenser ventilation realized 87.9 percent removal. While falling short of 100 percent, forced condenser ventilation achieved over twice the removal percentage of the conventional unit.
The results demonstrate the remarkable effectiveness of forced condenser ventilation for eliminating a wide range of VOCs from the distillate.
Conclusion
Distillers that incorporate forced condenser ventilation technology will be capable of reliably producing potable water from feedwater containing all classes of contamination, including volatile organic compounds. Effective VOC removal will no longer be dependent upon the consumer having to remember to change a filter at some specified interval. The distillate produced by this technology will have lower TDS and pH closer to normal.
By not having to allow for a post-filter may prove a plus for future batch counter-top distiller designs, where space is at a premium and is another challenge altogether.
About the author
David G. Palmer, Ph.D., is president of Palmer Technologies Inc. (PTI) based in Lincoln, Neb., a specialist in distillation technology currently seeking patent protection for the approach explained in this article. PTI also manufactures treatment cartridges for eliminating scale problems associated with operating distillers on hard water. Palmer first came to the states in 1985 to work for Pure Water Inc., a manufacturer of small water distillers. He started PTI in 1992. Palmer holds a master’s degree in physics from the University of Otago (N.Z.) and a doctorate in physical chemistry from the University of London. He can be reached at (402) 474-7171 (phone/fax) or email: [email protected]