Table of Contents
High Purity Ozone Generation
Mass Transfer Efficiency
Ozone generation is a part of several industrial processes. Failure of the ozone generating system may thus have costly consequences. However, it is reassuring to know that a little care in designing the ozone generation setup can prevent most of these errors. Primizone has brought out this article to boost understanding of several common mistakes which can be prevented or corrected at a low cost and enables reliable and cost-effective operation.
Among failure causes, the most common may arguably be the entry of process water from the point of injection of ozone into the gas pipe, to reach the ozone generator. This is potentially damaging and may lead to shut down of the ozone generation system for a significant duration, which will have a snowball effect on the subsequent process operations as well. This catastrophe is most easily prevented by installing a Backflow Protector.
Most backflow protectors have a sensor element to detect moisture or dampness, and a valve that prevents the entry of water. Many different varieties of backflow protectors are available, and being aware of the features that should be looked for will simplify the choice:
- The sensor is ozone-proof
- It can detect the presence of water rapidly
- The valve should have a default normally closed position
- The presence of water in the gas pipe should automatically prevent valve opening
- The protector should have indicators of the status of the system
- Valve status should be amenable to forcing as during maintenance operations, in order to allow functionality testing
- The backflow protector should be capable of being completely emptied
High Purity Ozone Generation
The ozone feed gas should be of very high quality to be used in an industrial process, which means it should be purified of particulates and hydrocarbons, water vapor and other impurities.
Water vapor is undesirable because it readily reacts with atmospheric nitrogen to produce nitric acid, a highly corrosive substance which can damage and even corrode the ozone generator. The oxygen piping may also carry impurities into the ozone system. These factors must all be kept in mind when choosing the source of oxygen supply, comparing the cost of generating oxygen on the production site vs. installing LOX systems.
To maintain purity, bottled oxygen or on-site generation of oxygen are two alternative oxygen sources for the system, provided an oxygen concentrator is fitted to reduce the dew point of the feed gas. Primozone is also interested in reducing the strain and cost of achieving an optimal oxygen supply, and one suggested solution is the installation of a filter kit to the piping just proximal to the ozone generator.
This type of filter not only prevents the entry of impurities from the gas line after the piping is welded, but those which may enter from the pre-PSA system compressor or the PSA system itself. The best filter kits will have:
- Oil and particle filters of 1 micron + 0.01 micron filter size
- The ability to change filter cartridges with ease and safety
- A pressure regulator installed distal to the filter kit
- A dual filter or a bypass to avoid system shutdown and downtime when the filter needs to be changed
Most ozone generators available currently are cooled by liquid, typically water. Process water may be used for this purpose in many applications and situations provided the water is at the right temperature. If the temperature is too high, however, it will impact the performance of the ozone generator, reducing the output or pushing up the power consumption as the system attempts to maintain production at prior levels. Lack of sufficient cooling will lead to automatic shutdown of the generator to protect itself against overheating. Thus the cost of using process water which is too warm must be balanced against the possibility of excessive power usage and process downtime. This is a point to be settled by individual manufacturers.
The most important point here is not where the water comes from, but the presence of a cooling skid with a closed loop which will make sure that only clean water runs through the generator.
Two primary types of cooling systems are in use:
- An active chiller which has a closed loop
- A heat exchanger with a closed loop and a circulation pump
The following features should be present when installing a cooling skid with heat exchanger:
- Ball valves which are extremely useful to isolate individual components, such as the skid pumps when they need to be replaced
- Appropriate position of the heat exchanger for easy cleaning and rinsing out of the piping
- A duty or standby pump to avoid downtime during maintenance
- The use of anticorrosion or antifreeze in the main loop
- An extra active chiller to allow maintenance without shutting down the operation – this may be better achieved by using two smaller chillers in place of a single larger one, or at least a chiller with a double compressor
Safety is an essential aspect in all operations involving ozone generation or use. This is facilitated by the use of appropriate components.
Primizone emphasizes the need for safety even before one begins to choose equipment for the setup. It is vital to choose a supplier who can walk the installer through the safety aspects beforehand, especially with regard to the safety studies. If the supplier is unwilling or unable to do so, possibly another choice should be made to reduce the risk of malfunction and safety issues, which involves both personal and process efficiency and cannot be glossed over to cut costs.
One primary aspect to consider in this regard involves sensors to detect ambient ozone and oxygen, which are universally installed but with little understanding of their role, the required number and the part they play once ambient ozone is detected.
The following outline may make it simpler to design a safe system:
- Is the ozone system in a locked room setup?
- How many people have the ability to enter the ozone room?
- What is the design of the ventilation?
Regarding the sensor itself, some important aspects include:
- The output – on/off or a 4-20 mA output signal
- Required frequency of sensor change
- Availability of a provision to recalibrate the sensor in a simple manner
Experience has shown the Primozone team that the floor-to-sensor distance, for instance, is critical in sensor efficiency, no matter how high the quality of the sensor used. If this is inappropriate, ambient ozone will not be detected early enough, potentially exposing the operators to it. If ambient oxygen levels are not monitored carefully, the risk of fire increases.
For these reasons, the sensor supplier must oversee sensor installation, and the sensors must be of the best quality, as this is an investment with immense long-term benefits. Safety is not an option in this area, but a lifesaving necessity.
Mass Transfer Efficiency
Once the ozone has been generated, it must be dissolved as much as possible, a process which determines the mass transfer efficiency or MTE. When this is higher, more dissolved ozone is present and less is wasted by remaining unused. It is impossible to reduce wastage to zero in the best of systems, and this ozone gas will emerge at the surface. This presents a second urgent need, to break it down safely.
Ozone destruction may be achieved by several techniques, such as:
The choice of technique must be made based on the specific application and volume of production. If much gas is collected over a large collecting tank or basin, the thermocatalytic or thermic system would do best. These differ in the initial cost of investment, which is lower for the thermic unit, and in the power each uses, which is higher with the thermic type of ozone destruction unit. When a good supplier is taken into confidence before buying a unit, the choice made is likely to be optimal.
Some important questions to face when choosing the destruction unit include:
- Is there an inbuilt temperature sensor?
- Is there an inbuilt pressure sensor?
- Is it possible to change out the catalyst?
- Can the used catalyst be recycled via the supplier?
- What is the catalyst lifetime?
- Can we change out the heating elements?
This information has been sourced, reviewed and adapted from materials provided by Primozone.
For more information on this source, please visit Primozone.