HOW BUBBLES CAN MIX LIQUIDS
from Craig Johnston

Contents of a liquid that separate are easy to deal with in small quantities. For example, a can of paint can be shaken at the hardware store And a bottle of salad dressing can be shaken before pouring on the salad. But larger containers of liquid call for industrial mixing solutions.

Twenty years ago a mechanical engineer from Seattle was challenged with coming up with an effective way of mixing brining salmon eggs. He knew mechanical mixer blades would crush the fragile eggs.

His solution was to feed pulses of compressed air in the bottom of the brining tank, creating bubble masses. As the bubble masses rose, they gently mixed the brining solution without damaging the salmon eggs.

Photo 1: Pulsair bubble

NO MOVING PARTS IN THE TANK

Many industrial mixing operations are 24 hour-a-day, seven day-a week operations. Mixer downtime can be expensive, not only for the cost of parts and repairs, but from process shutdown time due to emptying the tank and loss of product.

One of Pulsair's mixing application's primary attractions to industry is the fact it utilizes no moving parts within the tank. No moving parts means nothing to break, no need to empty the tank and no process downtime.

Photo 2: accumulator plate

In fact, the only parts inside the tank are flat disks, called accumulator plates, which form the bubble masses, and the plumbing to deliver the air pulses to the plates. Outside the tank is a compressor and the Pulsair equipment controlling the air pulses. "The Pulsair system is scalable," said Parks. "We can mix small containers, like 50-gallon drums, as well as some of the largest tanks in the world." Large indeed: Pulsair is used to mix two 2.9 million gallon, 90-foot diameter bunker fuel blending tanks at the Petronas facility in Kuala Lumpur. The company also mixes a 34-meter diameter, 300 ton red wine-fermenting tank at Lindemans Winery, Karadoc, Australia. (For winemaking and other oxygen-sensitive mixing applications, Pulsair can pulse nitrogen or other gasses instead of ambient air.)

HOW BUBBLES CAN MIX LIQUIDS

When Parks developed the Pulsair system, streams of bubbles had long been used in mixing liquids. But this continuous mass of bubbles process, known as sparging, uses a lot of energy and has a tendency to entrain oxygen in the liquid.

Parks found that if he sent a bubble mass up through the liquid, it pushed the liquid above it upward and dragged the liquid below along too. When the bubble mass broke the surface, the liquid it had lifted continued in motion across the top of the tank and down the sides. This momentum meant he could wait to send the next bubble mass until the liquid in motion slowed down, which saved on energy.

And while it seems counterintuitive, the Pulsair mixing system entrains little oxygen in the contents it mixes due to the relatively small surface area of the pulsed bubble mass. And as noted earlier, Pulsair can use nitrogen or other gasses to mix materials that are oxygen sensitive.

As Park's company continued to develop the process, Pulsair added sophisticated controllers to meter the amount of air delivered in each pulse. By controlling air pressure, duration of the pulse, and its frequency, technicians at a plant can vary the mixing properties to meet process requirements while minimizing energy costs.

Key to the formation of the bubble masses is Pulsair's accumulator plate, a disk of metal or other material ranging from 6" to 22" in diameter, which is fastened horizontally just above the bottom of the tank. When air is pulsed beneath the center of the accumulator plate, it wraps around the plate's edges and forms into a bubble mass over the top.

An additional benefit of the air pulse coming out of the accumulator plate is that it sends a shockwave across the floor of the tank, loosening solids that have settled on the bottom. Pulsair is the only mixing system that begins the mixing process at the very bottom of the tank, which allows it to begin mixing as the tank is beginning to be filled and conversely, can keep the product mixed as the tank is emptied.

The accumulator plate and in-the-tank plumbing can be constructed of steel, stainless steel, aluminum, fiberglass, PVC plastic or other materials, depending on the contents to be mixed in the tank. Some Pulsair applications can forgo the accumulator plate and inject the pulse directly in the bottom of the tank.

Abbildung 3:Schema Pulsair

MIXING LARGER TANKS

Pulsair's mixing properties vary with tank shape and depth, but a general rule of thumb is that the Pulsair system mixes most effectively within a 6' radius of the location of the accumulator plate. For large tanks, Pulsair employs arrays of accumulator plates.

For instance, in Lindemans' 34-meter diameter wine fermenting tanks, Pulsair employed 29 of its 22" diameter accumulator plates, using one plate in the center with three concentric circles of plates around it. Rather than pulsing all the plates at the same time, groups of plates are pulsed sequentially for maximum mixing effectiveness and energy savings.

Control of the pulsing of the mixing system can be done with either a Pulsair controller or incorporated Pulsair's software into a facility's plant automation system.

Pulsair's touchscreen controllers are designed to control a single tank or a whole tank-farm. Tanks can be individually controlled to start and stop mixing cycles, and to vary them according to the mixing process needs.

The touchscreen controllers also allow sa mixing facility to start small, controlling a single tank or several tanks, then scale-up to a larger number of tanks by reprogramming the touchscreen and adding modules.

In explosive environments, Pulsair can control its pulsing through pneumatic means, eliminating the possibility of an electrical spark.

One feature winemakers utilizing Pulsair mixing have found helpful is the Pulsair controller's ability to track and document the mixing settings and sequences for each batch of wine. This allows the winemaker to exactly duplicate the mixing cycle of a particularly fine batch of wine.

PORTABLE MIXING SOLUTIONS

The Pulsair mixing process is also ideal for portable mixing needs, and the company has developed a range of portable mixers for various applications.

For mixing 50-gallon drums and other small containers, Pulsair had the 5-55 Drum Stick. The Drum Stick has a small accumulator plate to allow it to be inserted through the bung fitting at the top of the drum.

For industry standard "tote" containers, the company developed the 10-55 Tote Stick, with an optional folding array of accumulator plates to mix along the edges of a tote container. Both the Drum Stick and Tote Stick can be used to mix other small containers.

Photo 6: Portable Hand-Held Mixer PTM-2000

For larger containers, such as tank cars and rail tank cars, Pulsair developed a range of Portable Hand-Held Mixers, the PTM-2000 series. The PTM-2000 series ranges from 5,000-gallon portable mixers to 30,000-gallon portable mixers. For tank contents that need to be heated in order to be mixed, heated air or steam can be pulsed into the contents to speed the warming process. For customers who have tanks that hold liquids for long periods of time, requiring only occasional mixing, such as wine fermentation tanks, Pulsair developed a Wine Cart, which allows the controller and injection valves to be moved and shared among a group of pre-plumbed tanks.

APPLICATIONS WORLDWIDE

Since its introduction over 20 years ago, Pulsair has become the preferred mixing process throughout the world. In addition to the bunker fuel and wine mixing applications mentioned earlier, Pulsair has been called upon to mix a wide variety of products.

In The Netherlands, Loder Croklaan uses Pulsair to mix palm oil. In Germany, Mûeller Industries employs Pulsair mixing for its pulp and paper operations. In the USA, the Greensboro, NC wastewater treatment plant utilizes Pulsair for three separate mixing processes.

Hot sauce makers, paint manufacturers, lubricant suppliers, chemical factories, and a host of other liquid mixing processes all use Pulsair which is in use today in over a thousand facilities around the globe.

CASE STUDY

Plant managers aren't the only ones who like the Pulsair mixing systems; the company's finance people love it as well. A case study from a Texas, USA lubricant mixing facility shows why.

The customer has an 84,000-gallon tank in which he blends bright stocks and neutral oils to be used as a base stock for other lubricants. He needs to have the mixed base stock ready for shipment at 9am each day.

Prior to the installation of Pulsair, the customer used a 30 horsepower side entry mixer to mix the tank, and a 100 horsepower pump that was used to recirculate the contents during mixing. In addition, a boiler was used to heat the oil to assist with the blending.

Pulsair replaced the mixer and pump with a 15 horsepower compressor, and turned the boiler down to half its original setting. Energy savings alone came to over 80-percent.

Using the mechanical mixer required a crew of two men starting work at midnight in order to blend the tank. With Pulsair the plant starts the crew at 5 in the morning and still meets the schedule, cutting manpower costs more than in half.

This resulted in real saving for the plant, over $300 per daily batch. Over a year's operation, that savings is nearly $80-thousand, more than enough to pay for the Pulsair system.

BUBBLE MASSES MIX EFFECTIVELY

Pulsair has come a long way since Dick Parks did his experiments, mixing the brining salmon eggs with air pulses. Today, customers large and small have adopted the most reliable and effective mixing technology using bubble masses to mix liquids around the world.