Ozone Generators Feed Gas Economics
In general, larger ozone generation systems benefit from the use of liquid oxygen (LOX). In general engineering
companies, especially in the drinking water field, automatically recommend LOX based systems. For larger drinking
water systems this is almost always the proper approach. In smaller drinking water plants and for some other ozone
water treatment applications this may not be the best option. This page discusses the economics of ozone generator
feed gas selection.
Oxygen based ozone generators are smaller and use less energy than an equivalent air fed system. They are also
simpler in the sense that gaseous oxygen is simply evaporated from LOX. For applications where ozone is mixed with
water using a side stream injection system, the size and pump energy also favors. This would be especially true where
the degassing takes place in the side stream versus the main contact vessel. As the size of the ozone generation system
increases the benefit increases as well.
The benefits of oxygen use are off-set by the cost of purchasing oxygen versus the freely available air. In addition, once
a system has been sized for oxygen there is no way to switch back to air feed except with a decrease in production
capacity. On the other hand and air feed system can almost double its capacity by switching to oxygen. This means that
the economics of a LOX fed ozone generators are dependent vagaries of the oxygen market. This is especially true if
oxygen supplier controls the LOX equipment.
Less tangible factors include the reliability of supply for LOX due to plant outages or problems with delivery (floods,
hurricanes, snow storms, etc.). Air, of course, is always available. For smaller users, even if there are small benefits to
the use of LOX they may be off set by potential supply issues and price uncertainty.
The following analysis evaluates the economics of air fed vs. LOX fed ozone generators for drinking water treatment
using bubble diffusers.
|No Increase in the price of LOX was considered in this analysis.
|Ozone Production Rate
||450 lbs/day (with 100% redundancy)
|Air Fed System
||10.1 kWh/lb ozone
|Oxygen Fed System
||4.5 kWh/lb ozone
|LOX System Rental
||$1,000/month (tank, evaporators, valves and controls)
In order to evaluate the two different systems, the lifetime costs in 2006 dollars was evaluated for each system at a
different LOX prices. The results are shown in the table below. The cost of the air fed system is constant since it is not
effected by the price of LOX. As LOX prices increase the lifetime cost of the LOX based ozone generator increases. The
break even point between the systems takes place around $90/ton of LOX. it is important to note that LOX price
increases were not considered, but it is likely that these prices will increase over time.
|Net present Value of System
|LOX Pricing ($/ton)
If instead of bubble diffusers a side stream injection system were used for ozone water mixing, the booster pump cost and
energy use would be higher for the air fed system since more gas would have to be injected into the water. The air fed
system would probably be changed to produce 3-4% ozone at a higher capital cost. This would shift the break even
point between the system to higher LOX pricing. Nonetheless, LOX prices can exceed $200/ton, so the air fed system
may still be competitive.
The point of the analysis is to demonstrate that the overall economics of the gas feed system should compared before
selecting the type gas supply. Other issues should also be considered such as reliability of supply, future pricing of LOX,
lifetime of the system, need for expansion, etc.