By Ronald G. Fink, RGF Environmental Group
Water evaporation was first used by the Phoenicians, Romans and Chinese to obtain salt from seawater. Large flats were filled with seawater and natural evaporation from the sun evaporated the water and left behind dry salt.
The first boiling water evaporators in the US are traced back to the Onondaga Indians from the Syracuse, New York area in 1654. Syracuse still has the nickname of 'Salt City'. The Onondagas used iron pots to boil local brine water down to a dry salt. Without realizing it, they also discovered server corrosion and evaporator meltdown, a problem that plagues traditional boiling water reactors to this day. The combination of water, salt and iron just does not mix well. Boiling water evaporators have not changed much over the last 350 years. Some industrial wastewater evaporators are using boiling water technology and are still plagued with corrosion problems.
Many applications utilizing evaporators assume and plan for a less than one-year life. Corrosion and meltdown are anticipated and accounted for.
Efficiency
Boiling water evaporation efficiency is based on some basic laws of physics:
Based on this very basic formula, it should cost about $.04 of fuel to evaporate one gallon of water under ideal conditions.
Traditional Wastewater Evaporation Methods
Natural Evaporation Ponds rely on the combination of solar heat and wind to naturally evaporate water. This method requires a great deal of area, is slow and is subject to the weather.
Concerns:
Forced Air Evaporators utilize blowers to force air in a counter current to a spray of water pumped to the top of a column and free falling downward. The evaporation rate is largely dependent on the water temperature and the dew point. This method is cost-effective and highly efficient when the waste stream to be evaporated is preheated by another process and the wastewater does not contain volatile compounds (VOCs) that would be readily transferred to the air, and thereby create air pollution. It is not recommended for water high in dissolved or suspended solids as they will deposit on the internals and block the water airflow. This design has no method of removing dried solids. Cost estimates depend on the temperature of the waste stream.
Concerns:
Boiler Blow-Off Evaporators can evaporate large volumes (2-4 gpm) at a low cost and are effective on pretreated waste streams. This is no method for collecting or removing suspended solids. Most of the dissolved solids are vaporized in the steam at 212°F and blown off into the atmosphere. VOCs will be vaporized and will create air pollution. Therefore, all VOCs and solids must be removed from this system. Efficiency is 70%-85%.
Concerns:
Basic Boiling Water Tank Evaporators basically heat the water to its boiling point 212°F and exhaust the steam via an exhaust pipe. This method has no way to remove dried solids other than baking the tank contents down to a cake, which insulates the heat and holds it into the steel, causing early tank failure. Efficiency is usually 65%-75% depending on design. With the addition of a condenser, you can distill the water for reuse.
Concerns:
Steam Tube with Water Exhaust Boiling Water Evaporator is basically the same concept as a Boiling Water Evaporator. However, the hot exhaust gases are bubbled through the wastewater to improve heat transfer efficiency. VOCs will be vaporized and air pollution will result. Dissolved and suspended solids are removed periodically by draining a slurry. Efficiency is 75%-85%.
Concerns:
Heat Exchanger Boiling Water Evaporators heat a coil filled with a high temperature oil, which is pumped to another coil inside a tank containing the waste. The advantage to this system is that the tank can be made of non-corrosive polypropylene with no direct flame contact. The vapour can be distilled. Efficiency is 70%-80%.
Concerns:
Steam Tube Boiling Water Evaporators utilize a hot tube as a heat source. The flame is directed inside a steel tube, thereby saving the tank from flame impingement and early failure. The tube will fail. However, it is easily replaceable and considerably less expensive than the tank. Efficiency is 70%-80%. The water is heated to 212F. The vapour can be distilled and reused.
Concerns:
A new technology could change the history of evaporators. It is a novel approach: no steel vessel, no pot of boiling water, no slurry concentrate to dispose of, no corrosion, and no meltdown.
New Technology
Thermo Oxidizer utilizes a ceramic chamber to flash evaporate atomized wastewater in a dry chamber. The atomized wastewater is heated in a chamber of hot gases to 800F - 1,400F resulting in a complete flash evaporation of the water, leaving behind all the contaminants as a dry ash. All volatiles in the wastewater are burned and actually add BTU value. The heat source can be oil, gas, diesel, or used oil. A secondary chamber thermally oxidizes the flu gases to eliminate any air pollution.
Considerations:
Definitions
Evaporation. The process in which a liquid dissipates or emits vapour, fumes or invisible minute particles.
Distillation. A process that consists of driving gas or vapour from liquids or solids by heating and condensing to liquid products once again.
BTUs (British Thermal Unit). Amount of energy required to raise a 1 lb. mass of water 1°F @ 1 atmosphere.
VOCs (Volatile Organic Compound). Organic compound, which readily dissipates into the air at room temperature, i.e., benzene, gasoline.
TDS (Total Dissolved Solids). The amount of ionic matter dissolved in a fluid and can be measured by electric current. Dissolved solids in water can be deceiving. For example: seawater contains 35,000 ppm of salt, yet it will appear crystal clear. 35,000 ppm is equivalent to 3.5% of contaminants.
Thermal Oxidation. High temperature breakdown of contaminants to carbon dioxide and water.
TSS (Total Suspended Solids). Substances suspended in a fluid large enough to be visible by the human eye and small enough to be kept in suspension by the movement of the fluid molecules.
Condenser. An apparatus in which gas or vapour is condensed to liquid form.
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