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Distillation Makes Water Purification Easy.

Pure laboratory water is one of the most basic needs. To get it, many facilities use distillation. This removes a broader range of impurities in a single step than any other water-purification method. It removes most inorganic solids, all organics with a boiling point [is greater than] (100 [degrees] C), bacteria, and pyrogens. It does not remove gases and organics that undergo the same phase changes as water, but these are removed before and after distillation using other technologies.

Distillation differs from other water-purification methods, because it removes water from the impurities. Other methods remove the impurities from the water. In distillation, the phase change from liquid to vapor separates water from its impurities, which remain in the boiler. Stills can produce water with a purity of 2,000 - 300,000 ohms.

Still parts include a boiler, electric immersion heaters, a pyrogen-reducing baffle, a condenser, and a constant level device. Optional accessories--an electric inlet valve, low-water cutoff, an electric drain valve, and fully automatic control--let the still work with pretreated feedwater and a storage tank.

Water to be distilled follows a complex path. Incoming water serves as cooling water for the condenser. This preheats the water before it enters the boiler. It then flows into the constant level device and then into the boiler. The constant level device maintains the proper boiler water level. Heating produces pure vapor that moves from the boiler through a pyrogen-reducing baffle into the condenser tubing. The baffle removes contaminant-laden water droplets from the vapor. The pure vapor condenses in the condenser tubes, giving distilled water. The distilled water exits the condenser and is stored in a tank.

Every still must separate gaseous water without carrying over solid particles in suspended liquid water droplets. This problem, known as entrainment, occurs with rapid boiling. To prevent entrainment, the still evaporator should be the right diameter and design to give optimum conditions at the interface of the heated liquid. In addition, the baffling systems must be designed so that the velocity is sufficient to insure removal of any entrained particle. The purified steam must be protected from all contamination, and the condenser design must ensure optimum removal of volatile impurities. Otherwise, these will reduce distillate water purity.

Turning 1 kg of liquid water into steam requires 2200 kJ of energy. Consequently, the still heat source is always an important consideration. Stills with capacities [is greater than]38 L/hr are generally steam heated. For large stills, conserving the latent heat of the steam and reusing it one or more times to boil water in additional evaporators saves money. Laboratories operate these stills, called multiple-effect stills, using pressure and vacuum to ensure that the temperature required to boil water drops from one evaporator to the next. Operating costs for each evaporator decrease with the total number of evaporators. Large saline-water stills operate in this manner.

Many different water stills are available, from single-effect laboratory models with a capacity of about 2 L/hr to vapor-compression and multiple-effect stills producing more than 3,800 L/hr.

When dependable purity is more important than low-cost operation, laboratories employ multiple stills. Each still feeds the next one. Thus, the first still feeds the second, and the second feeds the third. Multiple distillation equipment is used mainly in hospitals and minimizes the risk of water contamination caused by a temporary condition, such as a boil-over.

Multiple stills produce a better grade of water, distilled-water purity varies with the feed-purity. Consequently, laboratories almost use double distillation when extremely high purities are needed. Operating costs for multiple stills are directly proportional to the number of stills.

There are other distillation methods for producing various grades of distilled water. Some of them are very complex but offer the advantage of energy conservation. These include vapor-compression stills, centrifugal-rotating heat-exchanger stills, and falling-film evaporator stills. They may be combined using the multiple-effect principle and vacuum operation.

Stills that produce potable water usually do not contain tin-coated parts. Because a tin coating on metallic components minimizes water contamination from the system, these stills may contain metallic impurities from the equipment parts. Distillation and storage systems for the highest-purity water should be fabricated from tin-coated components.
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Author:Karet, Gail
Publication:R & D
Article Type:Brief Article
Geographic Code:1USA
Date:Dec 1, 1998
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