Is EDI Better Than Conventional ION Exchange? and How Do Cooling Towers Burn Down?
Compiled by Mike Henley
(Editor’s note: This article is based on recent discussions in the LinkedIn Ultrapure Water and Industrial Water Treatment groups. This column seeks to accurately reflect comments from each contributor. On occasion, there may be the need to edit contributor comments for clarity or length. An important purpose of each group is to provide a forum for practical examination of issues facing endusers of high-purity and industrial water.)
EDI versus IX
Mike: Since the 1980s, the producers of electrodeionization (EDI) systems have made great strides in developing this technology to the point that it is reliable and able to be sized for different treatment capacities. What are the reasons to use EDI instead of conventional ion exchange (IX)? And, when does conventional IX make more sense? Please share your thoughts, observations, and experiences.
V.K.: “In case highly pure water is required, it would be better to use an EDI system than ion exchange. Otherwise, it would be better to go for an IX system only. However, the advantage of using EDI is that it eliminates the need for the chemicals required for regeneration of IX resin, thereby saving that extra recurring cost.”
John: “EDI has improved dramatically in recent years with much greater selection and reliability. However, it is important to note that it produces a continuous waste stream. In areas where water conservation is a concern, service exchange DI may be a better choice, since it produces zero on-site wastewater. I have seen pharmaceutical customers replace EDI with service exchange DI to reap the benefits of operational simplicity and water savings.
On the flip side, service exchange DI poses a risk factor because resin is regenerated offsite by a service company. Regulatory agencies and guidance documents suggest that service-exchange resin should be obtained from companies that are either dedicated to clean applications or virgin resin should be used for each exchange. With increased focus on water conservation, service DI is being used in dirty applications, which should be not be reused in clean applications to prevent the potential for cross contamination.”
Johan: “Indeed, EDI can operate continuously, producing a constant water quality. Regeneration chemicals and disconnecting from the production line are not applicable. The reject stream (waste) is typically reused upstream RO. For pharmaceutical use, hot water sanitization is another practical feature.”
Matt: “EDI has its limits. One post states, "EDI is better for highly pure water." I have yet to see an EDI produce 18-megohm-cm water. We have always had to use IX or IX as a polisher to EDI. 16-megohm-cm is the best I have seen with EDI alone. EDI is less forgiving to feedwater fluctuations and normally requires RO as pretreatment. That said, EDI has definitely improved over the years.”
Michael: “Do not rule out the potential for EDI and/or IX fouling, which can lead to frequent cleaning and/or regeneration, even with best feedwater treatment practices. Know your source water intimately!”
Sergey: “EDI consumes less chemicals versus IX. But EDI strongly depends on RO permeate quality and is not capable to meet the demin water requirements in case of RO failure/operation problems. So, generally speaking, IX is more reliable, because it is able to compensate pretreatment defects.”
Roberto: “EDI sometimes is difficult to maintain because operational knowledge is necessary. But cost is lower because IX needs frequent replacement, and loses time/money.”
Industrial Water Treatment Discussion
Compiled by James McDonald, Chem-Aqua
Burning Cooling Towers
James: The thought of a burning cooling tower seems like an oxymoron. How can a water-rich cooling tower burn down? I ran across some pictures on the Internet, and it got me thinking. How can a cooling tower burn down? What are the possible causes? Does anyone have any stories to share?
Vinod: “It happens when it is not in operation. If structure is made of woods and fiber glass and maintenance is being performed, workers do it while using flaming torch or welding. In refineries hydrocarbon vapors may get trapped in the water line and burn. Here are some links discussing this matter:
The Ledger: Cooling Tower Catches Fire During Early Morning Repairs
F.E. Moran Special Hazard Systems: Cooling Towers: Learn the Root Cause of Pipe Decay
Oil Industry Safety Directorate: Case Study on Fire incident at Process Cooling Tower
Brett: “I don't see water. There is not too much to burn on a cooling tower. Perhaps light hydrogen?”
Mohamed: “Gas plants. This is possible when gas leakage happens, that's why most of cooling towers are provided with HC sensors at the fan area where gases will go.”
Christopher: “With our partners, we recently finished refurb works after a fire at Didcot Power Station, which made national news here in the UK. The fire was started by a motor fault. The tower was operational when the fire broke out, but you will notice (in the website link) that it is a plume-abated tower and that the fire only took hold in the dry section of the cooling towers. This is proof that even operational towers can catch fire!”
Maheboob: “Yes, this can happen for these reasons: 1. System construction—wood; and 2. Continuous process contamination entering in cooling towers (oil and gas). When the system is not in operation, a fire can happen as less water will be present. When the system is operational, chance of process contamination is possible. Monitoring can help avoid such situations through the use of gas sensors, temperature sensors, and turbidity readings.”
Tom: “Many basin liners are not fire retardant. Welding or cutting torches while working on cooling towers may cause fires.”
Ahmed: “In 1973, I saw a 30,000 cubic meter per hour cooling tower burn down because welders did not take elementary precautions. The fill was totally destroyed as well as the fans and fan housings.”
Zahid: “It can happen during stop/shut down if there are reckless maintenance activities such as unsafe welding.”
Shivanand: “It is fairly common. Dry wood and an ignition source is all that is needed. Flammable gases increase the probability of a fire. Continuous wetting of the wood, gas detectors, and proper hot work procedures will prevent this.”
Adnan: “I was working on a steel plant, during the shutdown, all the cooling tower fills were removed for cleaning purpose, just after a day when all the fills were installed back the fire broke… there were people inside the cooling tower working when one burning fill fell down, everyone was evacuated safely just in time...everything was gone. Only the cracked concrete structure was left.
More than 3,000 packings were burnt within less than 3 minutes. The shutdown extended for another 3 months. It was horrific. One unnoticed spark from the welding job on the top of the structure caused all of this damage.”
Andrew: “I've seen this on a number of occasions where the wooden structure around the fan area ignited the motor and bearings that while running were kept cool. However, on shutdown everything over heated. It is one big chimney, what an updraft!”
Emiel: “I saw a similar situation as explained by Ahmed, where a tower pack was ignited by welders during a maintenance.”
Alberto: “Normally, we prefer to have more pressure on the process side than on the cooling side. If we are cooling a flammable fluid and have a leak, we can have a risk condition and the ignition can come from the motor of the fans.”
Chris: “There are basin heaters in water cooling towers for freeze protection in the winter months. The standard practice is to isolate these in the summer months. I've seen a tower in Denver go down for cleaning and the heater caught the fill on fire, and the tower burnt to the ground.”
Mark: “I've seen the same thing as Chris, a basin heater was left on during the summer, and due to a massive construction project in Boston, dirt and debris caused the sump heater to short out or generate the heat needed to ignite the thin-film fill. The sump did not have water in it at the time. These sump heaters often get neglected and are left on in the summer. They can also corrode a lot faster than the rest of the sump.”
Alfonso: “What about the possibility of flammable process leaks entering the tower that are then ignited by spark emanating from the fan motor on the top of the tower?”
Steve: “I would have thought if the fire starts in a dry area like the top where fan motors are and gets hot enough, it would dry out area and the fire would spread. There would be a lot of factors like if the tower is running and materials of construction that could burn down any tower.”
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