Fluid Mixing

Liquid-liquid mixing

Liquid-liquid mixing is an important process in many industries, that is generally accomplished in mechanical agitation systems.

Liquid-liquid mixing in a mechanically stirred vessel is a very common industrial operation encountered in many food, pharmaceutical, petrochemical, and chemical industries.

The particular purpose of these industrial processes is to promote good contact between the two immiscible phases and extend the interfacial area in order to increase the mass transfer rate.

Liquid-liquid mixing performance in a stirred tank can be evaluated by different parameters, such as:

• Mixing time
• Agitation speed
• Circulation time
• Power consumption
• Drop size distribution
• Breakup and coalescence
• Interfacial area
• Phase inversion

Input parameters include:

• Flow pattern
• Impeller type
• Power number
• Number of impellers
• The dispersed phase volume fraction

Gas-gas mixing

Gas-gas mixing or gas blending is the mixing of gases for a purpose where the final gas mixture is specified. A gas mixture is defined in a molar gas fraction, by percentage, parts per thousand or parts per million.

In gas mixing, there are many variables and factors that play a role. Proper mixing and blending of gases saves money and increases repeatability of the end product and improves quality. The applications are scientific processes, industrial processes, food production, food storage, and breathing gases.

Various methods are available for gas blending. These may be distinguished as batch methods and continuous processes. Gas mixtures must be analyzed either in process or after blending for quality control.

Liquid-gas mixing

Liquid-gas mixing is an important and common unit operation in chemical and allied industries. Many of the most important chemical reactions in the industry involve mixing liquids and gases. Chlorination, hydrogenation, and organic oxidation are just a few examples. Liquids and gases are usually mixed to allow mass transfer to occur.

The main difficulty in most gas-liquid mixing is in how to maximize the contact between the two phases, and make sure that as many molecules as possible of the gas come into contact with molecules of the liquid.  This means making the gas bubbles as small as possible and ensuring that the bubbles are distributed evenly throughout the liquid. However, in practice, this is a very complicated matter.

Fluid Mixing Technology

Many industrial processes rely on effective and efficient agitation fluid mixing technology.

The application of fluid mixing technology covers the areas of mining, hydrometallurgy, biology, petroleum, food, pulp and paper, pharmaceutical and chemical process industry.

In these industries, in particular, we find typical chemical reaction engineering processes like fermentation, wastewater treatment, hydrogenation, polymerization, crystallization, flue gas desulfurization, and more.

Agitation

Generally, agitation refers to forcing a fluid by mechanical means to flow in a vessel.

Mixing

Mixing usually implies the taking of two or more separate phases, or two fluids, and causing them to be randomly distributed through one another.

There are a number of purposes for agitating fluids, for example:

• to optimize blending of two miscible liquids
• to dissolving solids in liquids
• dispersing a gas in a liquid as fine bubbles
• suspending fine solid particles in a liquid
• agitation of a liquid to increase heat transfer between the fluid and a coil or jacket in the vessel wall

Fluid mixing may also be important in reactors to ensure optimal operation conditions for some chemical systems requiring uniform temperature and species concentrations within the reactor volume.

Fluid mixing and blending devices therefore are important units both for reactive and non-reactive processes.

Fluid-Fluid Reactions and Reactors

Industrial fluid-fluid reactors may broadly be divided into gas-liquid and liquid-liquid reactors.

Gas-liquid reactors typically may be used for the manufacture of pure products (such as sulfuric acid, nitric acid, nitrates, phosphates, adipic acid, and other chemicals) where all the gas and liquid react.

Types of reactors

The types of reactors used for fluid-fluid, for example, gas-liquid and liquid-liquid reactions, may be divided into 2 main types:

1. Tower or column towers, and
2. Tank reactors.

Tower or Column Reactors

Tower or column reactors, without mechanical agitation, are used primarily for gas-liquid reactions. If used for a liquid-liquid reaction, the arrangement involves vertically stacked compartments, each of which is mechanically agitated. In either case, the flow is counter-current, with the less dense fluid entering at the bottom and the denser fluid at the top.

In the case of a gas-liquid reaction without mechanical agitation, both inter-phase contact and separation occur under the influence of gravity. In a liquid-liquid reaction, mechanical agitation greatly enhances the contact of the 2 phases. The emphasis here is only gas-liquid reactions.

Tank Reactors

Tank reactors generally employ mechanical agitation to bring about more intimate contact of the phases, with one phase being dispersed in the other as the continuous phase. The gas-phase may be introduced through a sparger located at the bottom of the tank; this is a circular ring of closed-end pipe provided with a number of holes along its length allowing multiply entry points for the gas.

In comparison with non-agitated reactors equipped only with spargers, mechanically agitated tank reactors have the advantage of providing a greater interfacial area for more efficient mass transfer.

Tank reactors are well suited for a reaction requiring a large liquid hold-up or a long liquid-phase residence time.

The operation may be continuous with respect to both phases, or it may be semi-continuous, i.e. batch with respect to the liquid. The simplest flow pattern for each phase is back-mix flow.

Tank reactors equipped with agitators (stirrers, impellers, turbines) are used widely for gas-liquid reactions, both in the traditional chemical process industries and in biotechnology.

They are also used for 3-phase gas-liquid-solid reactions, in which the solid phase may be catalyst particles; in this case, they are usually referred to as "slurry" reactors.