Date of Graduation
Bachelor of Science in Chemical Engineering
Arkansas Razorback Distillers (A.R.D.) has developed a passive solar distillation system for treating acid rock drainage (ARD) from legacy (i.e., abandoned) mines. The solar still addresses the need to reduce both the metal sulfate contaminants as well as the acidity of acid rock drainage. During the design phase, A.R.D. addressed the need for the system to be low cost, simple, and effective for general use as well as a specified location. To demonstrate the applicability of the solar still, A.R.D. used the Freeport McMoRan Inc. Copper Queen legacy mine in Bisbee, Arizona, as a base case scenario. The mine was visited to gain insight about the problem and its solution.
Research was conducted to evaluate treatment technologies including, solar stills, bioreactors, solar ponds and reverse osmosis to determine the best way to clean contaminated water. The key factors in choosing the appropriate technology included long-term cost, durability, required maintenance, simplicity, and efficiency. A.R.D’s design is close to that of a traditional solar still with the exception that water vapor is not reclaimed. Evaporating pure water to the atmosphere is the most passive and cost-effective solution. Five gallons per minute of ARD water is being evaporated, and the vapor is not condensed because no beneficial or economical use was determined.
In the full-scale design, sunlight enters through a six-millimeter thick double pane polycarbonate roof, heating the water and vaporizing it. The water vapor/air mixture is forced from the still via a thermosiphon. The purpose of the thermosiphon is to maintain a low relative humidity within the still to increase the driving force for greater evaporation rates. In the bench-scale design, the thermosiphon effect is demonstrated by using exhaust fans. Rather than removing the salt brine continuously throughout the process, A.R.D. has decided to allow the salts to precipitate and collect at the bottom of the solar stills. The salts can be removed in a batch process every twenty years with little effect to the efficiency of the solar still. The removal of salts after a long period simplifies the operation of the still as well as reduces operating costs.
The solar still will be positioned near mining stockpiles where the acid rock drainage originates. Rather than building one large solar still, A.R.D.’s design uses multiple smaller solar stills in parallel to achieve the same results. Each solar still is 102 feet long, 22 feet wide and 10.5 feet high. The full-scale solar distillation unit was designed to handle the task mandated five gallons per minute of contaminated water. Each solar still is estimated to cost $40,000, which includes the cost of materials and construction. Twenty-seven stills are required to achieve five gallons per minute of evaporation and the total capital cost for twenty years for the system is $1,100,000. This corresponds to $18 per square foot, which is 28% cheaper than the average greenhouse cost of $25 per square foot.
The design parameters of the still were determined by testing a bench-scale apparatus and developing a mathematical model. At this point, A.R.D. has shown that the bench-scale can achieve close to the desired flow rate, and there are plans to improve the still to achieve ten mL/min. The maximum experimental flow rate obtained thus far is an average seven mL/min over twelve hours of sunlight. Experiments are planned to demonstrate the required ten mL/min.
This report provides a detailed explanation of the location, technology, process summary, economic analysis, experimental results, regulations, safety considerations, and scalability for a solar distillation system in Bisbee, Arizona.
Shreve, Stephanie; Galuska, Luke; Stoller, Anna; Raney, Alex; Caceres, Agustin; Chambers, Caitlyn; Graham, Chris; and Hazlett, Devin, "Passive Solar Distillation of Acid Rock Drainage" (2017). Chemical Engineering Undergraduate Honors Theses. 103.