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This Simple Process Can Turn Water Into an Environmentally Friendly Disinfectant

Hydrogen peroxide, water's short-tempered sibling, has helped humans to fight infection since we discovered its antiseptic properties nearly two centuries ago. Unfortunately, it's pretty unstable, making it tricky to transport where needed.

Now, researchers have found a rather simple way to generate the chemical out of water, air, and electricity, and it could be just what remote communities need.

The process, developed by chemical engineers at MIT, could one day be used to create portable devices that use non-toxic catalysts to electrochemically transform a water supply into an environmentally friendly germ killer.

The chemical formula of hydrogen peroxide is simple - it's just a water molecule with some additional oxygen, or H₂O₂. Making it isn't exactly a complicated process, either. When it was first isolated back in 1818, chemist Louis Jacques Thénard simply reacted barium peroxide with nitric acid.

These days, hydrogen peroxide is valued for its bleaching and sanitation properties, proving effective at killing microbes at relatively low concentrations while degrading relatively quickly and safely in the environment.

To meet demand, most industrial-sized methods today rely on what's known as the anthraquinone process, which reduces oxygen molecules using hydrogen taken from methane with a little help from a catalyst.

It makes plenty of hydrogen peroxide, but is also an energy-intensive process that relies on fossil fuel resources. Importantly, it doesn't scale down very easily.

If you're a wood pulping business or medical unit stationed in the middle of nowhere, that leaves just two ways to get your hands on the material – risk importing it in sufficient amounts, or use another method to produce your own.

"There's a growing community interested in portable hydrogen peroxide, because of the appreciation that it would really meet a lot of needs, both on the industrial side as well as in terms of human health and sanitation," says chemical engineer Yogesh Surendranath.

Hydrogen peroxide's loosely attached second oxygen is keen to marry up with other molecules without much introduction. For the organic chemicals in living material, this is a destructive process that makes the peroxide perfect for killing pathogens.

But it also means it's quick to react with other materials in an energy-intense fashion. Transporting concentrations over 35 percent over a long distance is a recipe for disaster.

As for making your own, most existing processes are slow and inefficient, producing large amounts of water for just a trickle of hydrogen peroxide. To get around this, processes can make use of toxic additives like lead or mercury.

To develop a better, more environmentally friendly method, Surendranath and his team went back to the anthraquinone process to see what could be improved.

Instead of taking hydrogen from methane, water can be broken down into hydrogen and oxygen through electrolysis, using nothing more complicated than an electrical current being passed between a pair of electrodes.

The hydrogen can then be passed to the anthraquinone catalyst as usual, which is transported through a system where it's mixed with a solvent and then introduced to an oxygenated solution to form hydrogen peroxide.

Theoretically, other materials could serve the same mediator role as anthraquinone. The process of electrolysis coupled with the step-by-step transfer of the hydrogen-carrying catalyst is the key to making the process efficient without the need for any potentially hazardous elements.

"It's kind of an amazing process because you take abundant things, water, air and electricity, that you can source locally, and you use it to make this important chemical that you can use to actually clean up the environment and for sanitation and water quality," says Surendranath.

Now, we can't yet expect to see anything on the market using this technology, at least in the near future - but it's an exciting concept nonetheless.

The team managed to show it works through a proof-of-concept mock up in the lab that used a rather modest amount of energy, and they note there's still plenty of room for improvement.

"One of the ways to do that is to just increase the concentration of the mediator, and fortunately, our mediator has already been used in flow batteries at really high concentrations, so we think there's a route toward being able to increase those concentrations," says Surendranath.

Original article at 

https://www.sciencealert.com/this-simple-process-could-turn-water-into-an-environmentally-friendly-disinfectant-anywhere

Power plant water issues: Effectively cleaning cooling tower fill

While once-through cooling was a common feature at many power plants in the last century, environmental regulations regarding intake and discharge issues have basically forced a transition to cooling towers, or in some cases air-cooled condensers, for new projects.  Essential to steady heat transfer in cooling towers, and also their physical stability, is proper chemistry control.  But even with diligent chemical feed and monitoring, cooling towers, and especially the tower fill, can accumulate scale and microbiological deposits that inhibit heat exchange, and, in worst case scenarios, may induce partial collapse of the tower.  This article examines methods to clean tower fill before fouling causes irreversible damage.

Background

Cooling tower performance is highly dependent on the efficiency of contact between the hot return water from heat exchangers and the cool air being pulled or blown through the tower.  Heat transfer is enhanced by the use of cooling tower fill, which, over the decades, has evolved into sophisticated designs to maximize air-water contact.  An illustration of high efficiency PVC film fill is shown below.

 

 

Figure 1. High efficiency cross-fluted fill (Photo courtesy of Brentwood Industries)

The transition from early splash fill designs to modern high efficiency types reduced cooling tower capital and operating costs.  However, the generally tortuous path that provides good contact between air and water also makes these fills highly prone to fouling.  Advanced fouling results in a ~10x weight gain, leading to fill collapse into the sump and expensive fill replacement. 

Proper Chemical Treatment

While this article focuses on methods to clean tower fill that has begun to accumulate deposits, it is paramount to understand that proper chemical treatment during normal operation is essential to prevent severe or sudden scaling and fouling problems.  With regard to scale (and corrosion) control in cooling systems, the four-decade long methodology of phosphate/phosphonate treatment is giving way to polymer-based, non-phosphorus chemistry for two primary reasons.  One is that phosphorus discharges to the environment are being increasingly regulated and restricted due to problems with toxic algae blooms that have afflicted numerous bodies of water.  Secondly, the new polymer programs are proving to be more effective for scale and corrosion control in cooling systems.  (Post, R., Kalakodimi, R., and B. Buecker, “An Evolution in Cooling Water Treatment”; PowerPlant Chemistry Journal).

The most serious issues in cooling systems are usually related to microbiological fouling.  Thus, virtually all systems have as primary treatment some form of oxidizing biocide, most commonly bleach but also possibly gaseous chlorine, bleach/sodium bromide, chlorine dioxide, monochloramine, and monobromamine.  A problem at many facilities, and this is particularly true in the power industry, is that the regulations developed for and by the United States Environmental Protection Agency (USEPA) allow no more than 0.2 ppm free available chlorine average residual for 2 hours per day as “Best Available Technology.”  For plants so constrained, treatment is only allowed for less than 9 percent of any day, thus giving microbes a chance to settle and begin forming protective slime layers.

 

Figure 2

 

Options for dealing with fouled fill

Many facilities have suffered from fouled cellular plastic fill.  Replacement of the fill in kind is a potential solution, but potentially sets up repeat situations.  Another option is a switch to low-fouling fill designs that generally feature a more vertical flow pattern, less surface texturing, and sometimes wider spacing between the plates, all at the expense of some cooling efficiency.  Since fill replacement can be expensive in terms of both materials and outage time, others have chosen to clean the fill chemically, or sometimes, mechanically.  The choice of replacement vs. cleaning, as well as the cleaning methodology, requires careful consideration.  The decision depends on the extent of the fouling, the physical and chemical nature of the foulant, the type of fill, and environmental considerations in dealing with cooling tower blowdown.  For example, in heavily fouled film packs, some passages may be completely blocked, preventing the cleaning solution from flowing through, and perhaps acting as a filter for solids removed in other parts of the pack.  The total mass of deposits, if released at once into the recirculating water flow will result in very high suspended solids, and blowdown may have to be diverted or treated prior to discharge.  The type of foulant also varies considerably depending on the nature of the circulating water and the treatment chemistry employed.  Over time, the fouling matrix behaves as a filter media, trapping additional suspended solids in the crevices of the fill pack and impeding air and water flow.  At this point, the efficiency criteria that constituted the driving force for selecting the fill has become null. 

Figure 3.  Fouled film fill that is no longer effective.

The loss in cooling tower capability as a function of weight gain for a fill of offset flute design is trended in Figure 4.

Figure 4.  Tower capability loss vs. fill weight gain for a standard offset flute cellular plastic fill pack. (Monjoie, Michel, Noble, Russell, and Mirsky, Gary R., Research of Fouling Film Fill.  Cooling Technology Institute, TP93-06, New Orleans, LA, 1993.)

Over time, the high efficiency fill becomes increasingly less efficient, may gain as much as 10x its initial weight, begins to extrude around the supporting beams, and ultimately collapses into the sump.  At the point where performance loss becomes obvious to operators or the fill begins to deform, it is too late to consider cleaning as an option; fill replacement is required.  However, if the fouling is detected in its early and moderate stages, several cleaning options are available, depending on the nature of the foulant.

Cleaning Options For Cellular Plastic Fill

The most appropriate method for cleaning tower fill depends on several factors, including safety concerns, system metallurgy, in-service vs. out-of-service cleaning, potential impact on plant operations, disposal options for the cleaning solution, impact on the environment, and the chemical and physical nature of the foulant.

Mineral Scales

Hard mineral deposits most commonly consist of silica/silicates or calcium carbonate (calcite).  Silica solubility is lowest at low temperature, and deposits often occur near the bottom of the counterflow fill pack where the temperature is lowest, the water is most concentrated, and uneven water/air distribution can lead to dry spots or locally concentrated areas.  Calcite deposits often occur throughout the fill pack, but are generally heaviest toward the bottom.  Higher temperature near the top of the fill pack has the lowest calcite solubility and promotes faster deposition kinetics.  However, as the water passes through the fill, the minerals are concentrated slightly by evaporation, and the pH will rise slightly as excess CO₂ is stripped.

One technique that can be used effectively on either type of hard scale in its early stages is to apply certain types of surfactants that penetrate the hard deposit and induce it to spall from the slightly flexible plastic substrate. The surfactant is typically applied in addition to the normal scale inhibitor program for an extended period of 60-180 days. This program is never 100% effective, but will often result in removal of 70-80% of the fouling minerals.  Prior to implementing the cleaning process, it is imperative to identify and correct the scaling condition.

For large cooling systems, where the predominant scale deposit is calcite, the fill can be cleaned by reducing the operating pH and/or cycles of concentration until the water is undersaturated with respect to calcite at the fill conditions.  Calcite often serves as the binder for the deposit matrix, so dissolving the calcium carbonate in the deposit matrix can be disproportionately effective.  In principle, any degree of undersaturation will be effective over time.  Sulfuric acid is an obvious choice for many plants that already use it for pH control, but very careful planning involving plant personnel, the chemical supplier, and any outside contractors is required before using such a hazardous chemical. Other plants may prefer to use safer, less corrosive acids such as organic acids or inhibited sulfamic acid. [4]  Some organic acids are more effective than mineral acids at intermediate pH, and are synergistic with sulfuric acid.  At pH 5, application of the appropriate organic acid will accelerate the rate of calcite dissolution by 10-20x as compared to sulfuric acid alone.

For predominantly light calcium carbonate scaling, off-line foam acid cleaning has been used very successfully, at least on smaller towers. Strong acid foam is applied by skilled specialists from the top of the fill pack. The nature of the foam allows the acid to contact the scale as it slowly passes downward through the fill.  The relatively low volume of spent and mostly neutralized foam cleaning solution is either collected in the sump and disposed of, or is allowed to mix with other circulating water from neighboring tower cells that may be in service, depending on plant safety and environmental requirements.

Mineral scales can also be mechanically cleaned with some success in situ or ex situ.  Due to its brittle nature relative to the flexible PVC, the scale can be dislodged with some success by mechanically cleaning the fill pack in-situ from below.

Microbiological/Organic Deposit Matrices

Deposits where microbiological growth or organics serve as the binder for the deposit matrix are characterized by a soft, sometimes putty-like consistency.  Unlike mineral scales, deposits of microbiological origin tend to accumulate primarily in the middle of the fill pack.  Water velocities directly under the spray nozzles are generally high enough to discourage microbiological adhesion.  For this reason, microbiologically initiated fouling sometimes goes undetected because it is not visible on inspections from the top looking down beneath the spray headers.  As the water velocity slows down several inches into the fill, microorganisms begin to colonize the surface, acting as a filter for suspended solids passing through the fill.  Fouling tends to be more intense in the middle of the fill than at the bottom because suspended solids are filtered out prior to reaching the bottom layer, and because the last few inches of fill do not physically support a thick, soft deposit mass.  The inability to clearly view microbiological fouling from either top or bottom, combined with the difficulty of inspecting the middle layers of fill, often allows this type of fouling to progress undetected until it has reached an advanced stage.  Plant personnel have attempted to monitor fill fouling during tower operation using sections of fill suspended from load cells, or by cutting an access window into the end of the tower casing to allow a middle section to be removed periodically for inspection using a man lift, or by suspending a section of fill beneath the main fill pack to allow it to be easily inspected and weighed.  All of these methods can work, but none have proven to be totally satisfactory.

Several effective methods exist to remove biological-silt matrix deposits from cooling tower fill.  Hyperhalogenation is a widely attempted method, but its effectiveness is usually disappointing.  Potential corrosion of system components and the need to dechlorinate prior to discharge are important considerations.

Microbiological matrices often have high water content and will shrink and detach from surfaces when thoroughly dried.  However, effectively drying out cooling tower fill can prove problematic even with the help of fans, even if the tower is located in a low humidity climate.  Chlorine dioxide has also been used as a cleaner for cooling tower biofilms with some success.  However, the most widely practiced and effective cleaning method for deposits with microbiological or organic binders is hydrogen peroxide (H₂O₂) due to its oxidizing strength and the physical action of the oxygen micro-bubbles produced as the chemical reacts with organic deposits.  The positive environmental profile of hydrogen peroxide involving rapid breakdown to water and oxygen, and its ease of application are additional factors favoring peroxide as a tower fill cleaner.  Typical dosages are in the range of 500-3,000 ppm active H₂O₂.  As with most cleaning operations, the addition of low levels of surfactants will help loosen deposits.  Polymeric dispersants are generally added to assist in keeping the removed solids in suspension until they can be blown down.

Much of the biomass consists of extracellular and intracellular water and organics that will dissolve with peroxide cleaning.  A substantial portion of the deposit typically contains much mud and silt that will be released into the water.  Figures 5 and 6 illustrate the appearance of a slime-clay matrix on moderately fouled high efficiency cooling tower fill before and after cleaning. 

In cases where the deposit contains a high percentage of inorganics, the circulating water can be expected to become highly turbid.  The potential for high suspended solids in the cooling tower blowdown should be anticipated when cleaning a severely fouled system and taken into account in the job planning scope.

Summary

• Every effort should be made to prevent deposition from occurring in the first place.  A fill type should be specified that is compatible with reasonable expectations for the system, considering influent water quality, microbiological control, presence or absence of pre-treatment equipment, and the possibility for external foulants that might enter the tower through airborne contamination or process fluid leaks. 
• The microbiological and deposit control program should be diligently monitored to ensure that it is within expectations and delivering the required results. 
• The performance of any pretreatment and sidestream solids removal equipment should be reviewed to ensure such equipment is delivering and maintaining suspended solids within specifications.
• Plant personnel should be proactive with inspection and monitoring.  There are more options, and less expensive ones, if the fouling is detected at an early stage.
• Periodic, light, preventative maintenance tower fill cleanings should be considered.  Most high efficiency fills tend to gain weight slowly over time. Annual preventative maintenance cleanings can stabilize or reverse that trend.
• If cleaning is indicated, safety and environmental considerations must be adequately addressed.  Cleanings require careful planning and coordination between plant personnel and the chemical supplier/consultant.  Personnel safety is critical.  Also, cleanings can release many suspended solids that if not carefully controlled can foul equipment or present disposal problems without proactive planning.
• If fouling occurs, all remediation options should be considered, but, generally, the least costly and least aggressive methods applicable to the nature and quantity of the deposit should be the starting point.  Identifying and correcting the fouling conditions at an early stage is least expensive, and preferable to aggressive cleaning or ultimate fill replacement if the fouling conditions are allowed to persist.
• All systems are different, and careful consultation with the cleaning vendor and water treatment experts should be a priority.

 You can find the original article @ https://www.power-eng.com/2019/08/21/power-plant-water-issues-effectively-cleaning-cooling-tower-fill/#gref

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Is your water treatment controller misbehaving?

Recently we had a customer that complained about weird behavior in several of the controllers he purchased. He is a first time user of Lakewood Controllers and the behavior was attributed to defective controllers. After several service visits, nothing wrong was found with the controllers so we decided to spend more time training the customer as we thought the customer's expectations of the operation of the controllers, were somehow different than the way the controller operates.

After several visits and email exchanges, we were able to figure out what the problem was. It was electric noise in the power grid. We don't think about it but, these controllers were installed in  New York City, where the grid is old and overloaded. 

The quick solution was the installation of an AC Power conditioner at one of the controller's installations. The conditioner installed was the Furman PST-2( https://www.furmanpower.com/product/15a-8-outlet-surge-suppressor-strip-PST-2+6 ) Once the conditioner was installed, the unit behaved correctly and reliably, while the rest of the controllers were failing.

This story is basically an introduction to an article about about noise in the process control applications (below) We hope our hard learned lesson, and this article, can help you crack hard to solve process control issues.

Noise and disturbances in process control

By Vance J. VanDoren, Ph.D., P.E., consulting editor March 15, 2001

Were it not for noise and disturbances, a feedback controller could fairly easily maintain output of the controlled process (process variable) close to its desired value (setpoint). However, forces other than the controller’s efforts can often change the process variable. An example is sun shining in on a room cooled by an automatic air conditioner. In spite of the thermostat’s efforts to lower the room temperature to a 72°F setpoint, the room may actually get hotter. These uncontrollable influences are known as disturbances.

A crosswind can disturb a truck’s lateral position, requiring the driver to compensate with a control effort. Visual ‘noise’ such as snow can obscure the driver’s measurement of his actual position, causing him to make unnecessary course corrections.

Noise, on the other hand, makes the process variable appear to deviate from the setpoint whether any real disturbance is at work or not. Noise is generally a result of the technology used to sense or measure the process variable. With electrical signals, measurement noise is often due to interference from other electrical sources. Noise can also be caused by wear and tear on the sensor or some physical obstruction that causes the sensor to send an inaccurate reading to the controller.

Errors between the setpoint and the process variable also occur when the setpoint changes. However, setpoint changes are relatively easy for a controller to implement if it’s programmed with enough data about the dynamic behavior of the process. It is the random nature of disturbances and the fictitious effects of noise that make feedback controllers work so hard.

An everyday example

Consider a large truck being driven down the highway through a crosswind. The driver’s brain is the controller and his eyes are the sensors that measure the truck’s position. Based on what he sees, the driver uses his steering wheel to maintain the truck’s lateral position between the white lines.

If the wind is light and visibility is good, the driver need only concern himself with updates to the truck’s desired position necessitated by the curves in the road. Such setpoint changes are relatively easy for him to make, assuming he knows how the truck will react when he turns the steering wheel.

However, a strong wind can blow the truck off the course the driver has chosen to follow. To compensate for such disturbances, the driver must adjust his steering to correct his position errors. Worse still, if the wind’s speed or direction changes at random, the driver’s corrective efforts will have to be more frequent and more dramatic.

Now suppose the wind is accompanied by snow. The driver may not be able to see the white lines very well, so he may end up changing his course to compensate for a nonexistent position error. Snow constitutes a visual ‘noise’ that corrupts the position measurements taken by the driver’s eyes.

PID control

In a more typical process control situation, a PID controller is responsible for applying a corrective effort in proportion to the error between the process variable and the setpoint plus the integral and derivative of that difference. It is the derivative action that is most affected by noise and disturbances.

On the plus side, a PID controller tuned to provide aggressive derivative action can react quickly to an error and start the control effort moving in the right direction immediately after a disturbance begins. This can shorten the time required to compensate for a disturbance compared to a controller that uses only proportional and integral action.

However, the derivative action will amplify any noise embedded in the measurement, since the derivative of a fluctuating signal also fluctuates. This can lead to unnecessary and potentially counter-productive control efforts. Most PID controllers are equipped with noise filters to suppress extraneous fluctuations that the derivative action would otherwise generate.

 

You can find the original article here:  https://www.controleng.com/articles/noise-and-disturbances-in-process-control/

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