Performance and Validation of Ozone Generation for Pharmaceutical Water Systems – Part 2
This article was originally published in the July – August 2017 issue of Pharmaceutical Engineering® magazine. Missed part one? Catch up now – Performance and Validation of Ozone Generation for Pharmaceutical Water Systems – Part 1. Get part three delivered straight to your inbox by subscribing to iSpeak.
Aqueous Ozone Process
The aqueous ozone process consists of four steps:
- Ozone generation
- Mass transfer
- Concentration and contact (residence) time
- Process control
As noted above, modern corona discharge ozone generators produce gas on-site, with control capability for increasing or decreasing output. They may include an air dryer or oxygen concentrator for feed gas preparation, as well as filtration, gas flow control, and gas concentration control.
In the mass transfer process, ozone is dissolved in water. Ozone is more soluble in water than oxygen, although its solubility is dependent on the temperature, pressure, and other factors.
When designing an efficient mass-transfer process, bubble size is a critical parameter. Mitani et.al. stated “[T]he smaller the bubble size, the greater the mass transfer rate of gas. The larger surface area to volume ratio of very small bubbles provides an overall larger area for ozone mass transfer to occur.”3 The interaction of the bubbles with the water promote higher mass transfer efficiency (MTE), resulting in higher oxidation and disinfection efficiency due to the greater ozone diffusion in the water.4 For best efficiency, bubbles should be 1 micrometer (μm) or smaller.
Larger bubbles reduce mass transfer and cause greater off-gassing of undissolved ozone, which translates into a loss of MTE. Bubbles coalesce, increase in size, and migrate to the upper surfaces of the water, accumulating in the head space of the tank or pipe. This off-gas must be collected, controlled, and decomposed to oxygen before it can be discharged.
A well-designed mass transfer system will create and manage the surface area for a reliable MTE ratio that is typically greater than 90%, which means that 90% of the ozone gas produced is transferred into solution. The gas-to-liquid ratio is paramount when calculating anticipated MTE of an ozone mass transfer system. An improperly sized and/or designed system can waste or fail to dissolve much of the ozone produced, causing inefficient and expensive operations.
This is the amount of ozone the generator produces, based on the dosage needed to establish proper oxidation and disinfection.
Calculations for sizing an ozone generator and mass transfer system are based on temperature of the water, contact time (see “Compensating for ozone half-life,” below), ozone concentration and half-life, volume of water to be treated, and flow rates. Considerations must include MTE, which is the ratio of ozone dissolved, total ozone demand (including the decay rate), and concentration of oxidizable material in the water that will consume dissolved ozone.5
Correct sizing is critical as MTE, decay rate, and ozone demand can change rapidly with organic loading and process changes including water temperature and pH variations.5 Pharmaceutical water quality and conditions tend to be less dynamic, but are susceptible to seasonal changes, enhancing the need for good sizing practices. These start with a proper mass calculation, acceptable process and safety margins, and sizing to account for dynamic operational ranges.
The concentration of ozone dissolved in the water (DO3) is expressed as a mass volume percent. DO3 is dependent on MTE and the mixing of dissolved ozone with the volume of water. To meet oxidation and disinfection goals, the ozone solution must be mixed promptly with the surrounding water in the tank to deliver a homogenous single-phase solution.
Due to ozone’s short half-life and the ongoing demand to oxidize organic material in the water, it is extremely important to replenish the ozone continuously to maintain a steady-state concentration. The “Solubility of ozone in fluids” sidebar below shows ozone solubility calculations using Henry’s Law. At 45°C, for example, ozone’s shorter half-life results in rapid decay and less contact time with the organics in the water. This reduces the level and efficiency of oxidation unless the decay rate is overcome by adding more ozone to maintain a steady-state concentration. Conversely, in colder water ozone has a longer half-life—although its reaction can be slightly slower—and less ozone is needed to maintain the concentration.
Ambient-temperature waters are ideal for ozone disinfection and maintaining concentration. Ozone is more soluble at ambient temperatures than at warm or hot temperatures, and more ozone gas produced by the generator is transferred into the water. This increases the efficiency of the mass-transfer process and consistency of the disinfection process.
Contact (or residence) time becomes a crucial parameter when trying to calculate concentration and time values following the MTE for any given water system. Concentration and time value is measured in milligrams of DO3 per liter of water multiplied by the reaction time in minutes. This is an accepted methodology for measuring and validating disinfection, and for defining and designing an ozone system. A known MTE from a well-designed and verified system with uniform mixing, therefore, will ensure the water is in contact with the ozone long enough and at a concentration high enough to deliver reliable disinfection results.
A well-designed ozone system must include the instruments necessary for control of the unstable and highly reactive gas. The technology required for good ozone process control includes instruments to measure DO3 (with feedback control), ambient ozone (for safety and OSHA compliance), ozone gas production flow and pressure, and hydraulic parameters that influence mass transfer.
By: Nissan Cohen and Brian L. Johnson
About the Authors
Nissan Cohen is a worldwide expert in total organic carbon, high purity, ultrapure, reclaim-and-recycle water systems, with profound expertise in instrumentation, automation, and organic contamination oxidation systems using ozone, UV, ion exchange, and catalysts. He has written over 35 published technical articles, and is a recipient of the Pharmaceutical Engineering® Article of the Year Award. An ISPE member since 1995, Cohen is a contributing author and chapter leader of ISPE Baseline® Guide Water and Steam Systems and Good Practice Guide (GPG) Ozone Sanitization of Pharmaceutical Water Systems, Co-Chair and coordinating author of the GPG Approaches to Commissioning and Qualification of Pharmaceutical Water and Steam Systems, a member of the Pharmaceutical Engineering® Committee, Chair of ISPE’s Water and Steam Forum, and Founder and Chair of ISPE’s Discussion Forums. He earned a BS in agriculture and genetics at the University of Wisconsin and Ruppin Institute, and an MS in agricultural water systems from Hebrew University.
Brian Johnson is the Director and CEO of Pacific Ozone, which he and his wife Karen Johnson acquired in 1997. Between 2005 and 2014 he served the International Ozone Association on the Board of Directors and the executive committee for the association’s Pan American Group. Johnson has also contributed to numerous articles and publications on commercial ozone applications. From 1994 to 1997 he served as a director of A Sport, Inc., a holding company that acquired Snowboards, Inc., as well as additional major sports product brands including Straightline. From 1986 to 1994, he was chief executive officer and shareholder of Snowboards, Inc., a sports products manufacturer that owned Avalanche Snowboards and Universal Bindings. Johnson has remained an active investor in various enterprises including power sports, technology and real estate development.
3. Mitani, Marie M., Arturo A. Keller, Orville C. Sandall, and Robert G. Rinker. “Mass Transfer of Ozone Using a Microporous Diffuser Reactor System”, Ozone: Science and Engineering 27 (February 2005): 45–51.
4. Roustan, M., R.Y. Want, and D. Wolbert. “Modeling Hydrodynamics and Mass Transfer Parameters in a Continuous Ozone Bubble Column”, Ozone: Science and Engineering 18, no. 2 (1996):99–115.
5. Wright, Philip Craig. “Mathematical Model for the Mass Transfer of Ozone into Water Systems.” University of Wollongong Thesis Collection, 1993. http://ro.uow.edu.au/cgi/viewcontent.cgi?article=3519&context=theses