Bursting the bubbles in gas-liquid mixing

Most people don't have a great deal of time for froth. It's become a term of abuse: insubstantial, ephemeral, it takes up space. It's annoying — especially for engineers dealing with mixing systems. But industrial and academic researchers are finding ways of getting to grips with gas-liquid mixing, designing new techniques and devices that can mix the two phases quickly and efficiently.

There's no way of avoiding it — if you mix a gas and a liquid, you get bubbles. Such two-phase systems are notoriously difficult to work with. They don't flow like liquids; they don't conduct heat like liquids; they confuse instrumentation. However, for many processes, they are vital. Many of the most important chemical reactions in industry involve mixing liquids and gases. Chlorination, hydrogenation and organic oxidation are just a few examples.

The main problem to be addressed in most gas-liquid mixing is in how to maximise the contact between the two phases — in other words, to make sure that as many molecules as possible of the gas come into contact with molecules of the liquid. At its simplest, this means making the gas bubbles as small as possible, and ensuring that the bubbles are distributed evenly thoughout the liquid. In practice, however, this is a very complicated matter, and taxes even the most advanced computer simulation techniques.

Researchers at AEA Technology tackle the problem through a family of techniques known as power fluidics. This differs from other mixing techniques in that it uses the power of the fluid flow itself to mix the phases, rather than relying on mechanical mixing. Because of this, mixers using power fluidics have no moving parts. This makes them very attractive to the pharmaceuticals industry. Not only are they easy to maintain, with only the seals needing regular attention, they are also generally easier to clean than mechanical mixers. 'The internal geometry is completely open, with no static packings or vanes to foul, erode, corrode or cause cavitation, ' the researchers say.

According to AEA, the benefits don't stop there. Fluidic mixers are energy efficient, as they need no energy input from motors to power the mixing. They can deal with mixing times from less than ten milliseconds to over three hours, with high yields and reduced waste. And crucially for the pharmaceutical industry, they are suitable for continuous or batch operation.

As its name implies, the company's Vortex mixer, adapted from the V-tex off-gas treatment device, uses the vortex principle to power its mixing. Gas enters the flat, disc-shaped mixing chamber at a tangent and begins to spin towards the centre, while at the same time liquid is sprayed into the centre of the chambers. The droplets formed by the spray fly outward against the inward-bound gas, creating a counter-current effect which increases the turbulence inside the mixer. This makes mixing both very efficient and very fast, says AEA. The mixed liquid-gas stream leaves the mixer in the centre.The vortex mixer can be used for systems where the ratio of gas volume to liquid volumes is less than 1:3. According to AEA, the technology produces results comparable to a packed-column gas-liquid contactor, but occupies only a fifth of the space.

Impeller studies

Researchers at the BHr Group have taken a different tack, looking at the mixing characteristics of a range of driven-impeller mixers. Many parameters come into play here: the depth of liquid in the tank, for example, which can range from the diameter of the tank in a 'standard' mixer to three times that in a 'tall' tank. Different impeller types include turbines with pitched or concave vanes and hydrofoils, which can be used singly or 'stacked' on a single axle. In the latter case — found in tall tanks —the problem of segregation, where each turbine creates its own mixing environment and there is no mixing between these areas, needs to be tackled.

The media themselves also have an effect. For example, liquids whose viscosity decreases when subjected to shear forces behave differently to viscous Newtonian fluids when gases are introduced, and require different mixing strategies.

Intensity in-line

One research stream at BHr, the Hiline (High Intensity In-Line Mixing Research Consortium) group, is looking particularly at the use of in-line mixers, where mixing elements are incorporated within pipework rather than in a separate vessel or device. Hiline is investigating two main types of mixer for gas-liquid applications. Non-powered static mixers consist of shaped mixing elements inserted straight into pipework where mixing is needed. Some of these work by splitting the fluid up into individual streams and then recombining them into configurations with a different geometry. Others create turbulent vortices within the tube that interact with each other. These have no moving parts and are easy to maintain, although their complex geometries may cause problems for hygienic processes.

Ejectors, on the other hand, are multicomponent systems which work by transporting and compressing gas at the same time. These systems are made up from a nozzle, a gas chamber, a mixing tube and a diffuser. The nozzle injects a high-pressure jet of liquid into the mixing chamber, causing a sharp drop in pressure which draws gas into the mixing tube and entrains it into the liquid. As the gas and liquid pass through the tube, the two-phase flow changes structure, from a core of disintegrated liquid surrounded by a gas, to a dispersion of bubbles in a liquid. This is known as mixing shock, and as the name implies happens extremely rapidly. Large amounts of dissipated energy make the mixing intimate, and the mixing process carries on for some distance downstream of the mixing tube and the diffuser.

Hiline's current research centres around phenomena such as drop coalescence and the effect of the distance between mixers on their overall performance. The consortium, whose members include AstraZeneca, Procter & Gamble and DuPont as well as mixer manufacturers such as Silverson, Koch Engineering and Chemineer, is also working on methods for scaling-up equipment from pilot to industrial size.