June Spotlight: Ozone

Ozone Generators:
Is 'Apples to Apples' Performance Evaluation Possible
By Tim Teffeteller

Summary: There are many variables in choosing an ozone (O3) generator and oftentimes choosing what points to compare can be just as difficult as your final selection. The following is a primer on the topic.

Ozone as a technology offers a dilemma when trying to compare various manufacturing performance claims for demand sizing to a specific application. Several issues of concern when sourcing an ozone generator arise such as:
-- What technology to choose,
-- What ozone output and concentration is required,
-- How to measure ozone performance, and
-- What carrier gas requirements must be met.

Corona discharge or UV?
The two most prevalent technologies for ozone generation are corona discharge (CD) and ultraviolet light (UV). Both technologies have advantages over one another (see Table 1); however, CD is generally accepted as the preferred technology for potable water treatment. UV ozone generation offers initial economic benefits in non-potable water treatment, such as the spa industry.

Table 1. Ozone generation--UV vs. corona discharge (CD)

 Parameter

UV

CD
Maximum ozone production rate

1.94 gr/kWh
(using 185 nm bulb)

55 gr/kWh
(from dry air)
Concentration of ozone1 in output gas

1.8 gr/m:
~0.14% by weight
2

12-60 gr/m:
~0.1%-~4.8% by weight
2
Energy required to generate 1 kg of ozone

44 kWh

6-8 kWh
Ozone production

Variable decline

Constant
Capital costs

Low

Moderate
Ozone solubility

Very low

High
Operating costs (electrical energy)

High

Low

1. The concentration of ozone gas is determined at a standard room temperature of 20°C (68°F) and a standard pressure of 1 atmosphere (101 K\kPa).
2. 1.0% by weight is equal to 12.07 gr/m3.

True output and concentration
To make an informed purchasing decision about an ozone generator, an "apples-to-apples" comparison of ozone output and concentration by weight would seem to be in order.

First, however, a brief definition of terms would be appropriate.

Ozone output quantity: Units in mass or weight of ozone produced over time-generally rated in pounds per day (lbs/day), grams per hour (gr/hr) or milligrams per hour (mg/hr).

Ozone output concentration by weight: A measured amount of ozone contained in the carrier gas-percent by weight or volume and parts per million (ppm) by weight or volume, for example, milligrams per liter (mg/L) or grams per cubic meter (g/m3).

Carrier gas: For CD ozone generators, a minimum of 99 percent pure industrial grade bottled oxygen or, minus 62°C (-80°F) or lower dew point industrial grade compressed air (or equivalent bottled dry air) is required.

For UV ozone generators, air at a minimum of three relative humidity (RH) points-including 20 percent, 50 percent and 80 percent-is required. All RH conditions must be documented.

Although for the past two years the Water Quality Association's Ozone Task Force has been working to establish an ozone generator performance test procedure, manufacturers today aren't required to publish ozone output protocols or testing procedures that would factually validate marketing performance claims. A specific output quantity level is rarely reported with the corresponding output concentration, thus making ozone comparisons extremely confusing when researching generator outputs as the sole measurement parameter. Many published ozone performance ratings are based on the manufacturer's highest ozone output quantity level a given generator is capable of producing without reference to the output concentration percentage by weight at the specific performance rating.

The percentage of output concentration by weight measurement of an ozone generator is pertinent for two reasons:
1. Ozone solubility in water.
2. Direct ozone performance comparison.

As output concentrations by weight increase, the more soluble ozone becomes in water.

Figure 1. Solubility of ozone in water (5ºC)
generated by corona discharge and UV

Figure 1 illustrates the value of ozone solubility. As concentrations increase, more ozone is dissolved in water increasing the oxidative/disinfection/residual purpose for which it's utilized. If ozone generator output levels are non-consequential to ozone solubility, it could then be surmised that a 100-gr/hr ozone generator producing a 0.5 percent concentration of ozone by weight isn't as effective at producing soluble ozone when used in water as a 50-gr/hr ozone generator producing a 3.0 percent concentration of ozone by weight under similar conditions.

Low concentrations of ozone forced into water (typically found in UV ozone generation) will off-gas the vast majority of applied ozone into air. A common misconception finds that if an individual can smell the presence of ozonated water, ozone has reached a saturation point. Ozone contacted with water at poor concentration levels isn't able to reach a discernable degree of solubility. It's crucial to apply ozone in adequate concentrations in order to realize oxidation in water with the technology of choice.

Direct performance comparison
A direct-or apples-to-apples-ozone generator performance comparison is possible only when comparing similar output and concentration levels, tested under similar conditions.

Figure 2. Ozone output quantity
vs. concentration

As a sample chart, Figure 2 represents the concept of relating ozone output to concentration by weight. As the ozone output quantity increases, the output concentration by weight decreases. The same is true in reverse. As the ozone output concentration by weight increases, the output quantity decreases. The optimal crossover for both output and concentration should be noted for comparison purposes. This gives an apples-to-apples basis for comparing the maximum overall performance capabilities of an ozone generator and the ultimate value of the manufacturer's published claims. The ozone generator as illustrated above, would operate at an optimum level of ~1.75 gr/hr at ~1.25% output concentration by weight.

For marketing purposes, however, the ozone generator may be marketed as a 3-gr/hr ozone generator with no reference to the poor output concentration (percent by weight) at the "rated" output. This ozone generator when producing 3-gr/hr at 0.2 percent wouldn't dissolve ozone into water as effectively as it would at higher output concentrations with reduced output quantities under the same gas-to-liquid ratio and water temperature.

The missing parameter that determines the symbiotic relationship between the ozone output quantity and output concentration percentage by weight is the carrier gas flow. In CD ozone generation a finite volume of carrier gas is contained within an electrical field. The oxygen volume of the gas is then altered from its natural molecular state (O2), into an unnatural--or unstable--temporary ozone bond (O3). The volume percentage of conversion from oxygen into ozone and molecular strength of the ozone bond is dependent upon the oxygen atom time exposure to the electrical field under controlled conditions.

Carrier feed gas flow
For optimal carrier feed gas flow through the electrical field, the longer oxygen molecules are exposed to an electrical arc, the higher a percentage of ozone molecules are formed. An optimal flow rate of carrier gas is necessary to ensure against heat degradation. An electrical arc generates varied degrees of heat (dependent upon the CD technology). Heat destroys ozone. Once ozone is formed, its molecular bond will be broken if the gas flow through the electrical field stagnates or moves at a rate slow enough to allow thermal destruction.

If, on the other hand, the oxygen molecules contained in the carrier gas are not exposed to the electrical arc for an adequate time period in which to form ozone, minimal performance from the technology is recognized.

The optimum carrier gas flow rate would be dictated by the "cross over" point of output quantity and output concentration by weight ( ~4.5 SCFH as illustrated in Figure 2).

Oxygen concentration
Oxygen concentration contained in the carrier gas is also important. The higher percentage of oxygen contained in a volume of air exposed to an electrical arc, the higher the percentage of ozone that's created. The approximate composition of air at sea level is illustrated in Figure 3.

Figure 3. Air composition
at sea level

The majority of air consists of nitrogen. Thus, air is processed through a variety of available "pressure swing adsorption" technologies. These are processes using a molecular sieve material to adsorb nitrogen and moisture from air under pressure, using equipment known as oxygen concentrators or oxygen generation systems that incorporate varied technology derivatives. By this method, the ratio of oxygen to nitrogen is altered to provide a more favorable environment in which to create ozone (see Figure 4).

Figure 4. Air optimal
to ozone generation

This allows greatly enhanced output quantity and output concentrations from CD ozone generators. Keep in mind, conventional commercial air dryers do not purge nitrogen from carrier gas. Should a CD ozone generator comparison be made without similar carrier gas conditions, a true apples-to-apples performance evaluation is not possible.

Dew point level of carrier gas
The dew point is the temperature at which water vapor present in the air begins to condense (dew begins to form). Dew or condensed water that forms moisture create two major inhibitors in CD ozone generation as follows:

Humidity
Humidity is the lesser of the two evils. A greater expenditure of applied electrical energy is necessary in order to create optimal output quantities and output concentrations when moisture is present. It also creates conditions favorable for formation of nitric acids.

Nitric acid
Nitrogen present in air may convert to oxides, which then are able to dissolve in moisture. The moisture is then exposed to the electric arc and forms nitric acid. This by-product typically forms inside the electrical discharge chamber and begins to inhibit ozone production by corrosion properties to the electrical arc (CD glass or ceramic dielectric material) over time. The carrier gas should be dried to a -60ºC dew point to inhibit the formation of nitric acid.

Conclusion
One may ask, similar to author Gertrude Stein's query on roses, whether it's true: Ozone is ozone is ozone. Making an informed and educated decision requires a basic understanding of the product and/or service under consideration for purchase. The Latin phrase "caveat emptor"-let the buyer beware-should be held in high regard when the proverbial shopping cart includes a stroll down the ozone generator isle. The shopping list should include questions on:
-- Ozone technology requirement.
-- True ozone output quantity and output concentration performance.
-- Ozone solubility based on performance claims/data.
-- Direct ozone performance comparison.
-- Carrier gas requirements for rated ozone performance.
-- Maintenance, service life and warranty.

With the instant-purchasing-access now afforded by the Internet and other communication technologies, remember to shop smart. Know your specific ozone requirements. And most importantly, don't mix your fruits. Always make sure you're comparing apples to apples.

References
Ozone for Point-of-Use, Point-of-Entry and Small Water System Water Treatment Applications, Water Quality Association, Lisle, Ill., 1997.

Maintenance, Service Life and Product Warranty
Each ozone generator manufacturer's technology will require different levels of maintenance performed at varying intervals. What's important is that an understanding of each supplier's maintenance requirements be recognized and evaluated when comparing suppliers.

As an example, the heart of an ozone generator is the CD cell, where ozone is generated. Some technologies require the field technician to remove the CD cell, and return it to the factory for routine maintenance (i.e., cleaning). This leads to downtime, shipping charges, etc., which all increase the relative "cost" of the ozone equipment. Therefore, the added value of a field serviceable CD ozone generator should always be on the shopping checklist when comparing ozone generators. A few simple guidelines to remember should include:

-- Manufacturer's required frequency of maintenance in relation to the carrier gas (commercial air dryers utilized as air preparation generally require a higher frequency of scheduled maintenance).
--Manufacturer's expected service life when properly maintained.
-- Manufacturer's/distributor's availability of spare/replacement parts.
-- Manufacturer's/distributor's ability to provide technical support.
-- Documentation of operation and maintenance manual(s) with unit purchase.
-- Manufacturer's/distributor's product warranty coverage and duration.

About the author
Tim Teffeteller is the director of sales for Ozotech Inc., of Yreka, Calif. He has over 16 years of experience in sales and marketing, roughly half of that associated with ozone based equipment. He's an associate member of the Water Quality Association and a member of the International Ozone Association. Teffeteller can be reached at (530) 842-4189, (530) 842-3238 or email: sales@ozotech.com

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