type of mixing

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Liquid mixing

8.1 Introduction

Mixing is one of the most common operations in the chemical, biochemical, polymer

processing and allied industries. Almost all manufacturing processes entail some sort of

mixing, and the operation may constitute a considerable proportion of the total process-ing time. The term mixing is applied to the processes used to reduce the degree of non-

uniformity or gradient of a property such as concentration, viscosity, temperature, colour

and so on. Mixing can be achieved by moving material from one region to another. It

may be of interest simply as means of reaching a desired degree of homogeneity, but it

may also be used to promote heat and mass transfer, often where a system is undergoing

a chemical reaction.

At the outset, it is useful to consider some common examples of problems encoun-

tered in industrial mixing operations, since this will not only reveal the ubiquitous nature

of the process, but will also provide an appreciation of some of the associated diffi cul-

ties. One can classify mixing problems in many ways, such as the fl owability of the fi nal

product in the mixing of powders, but it is probably most satisfactory to base this clas-

sifi cation on the phases present: liquidliquid, liquidsolid, gasliquid, etc. This permits

a unifi ed approach to the mixing problems in a range of industries.

8.1.1 Single phase liquid mixing

In many instances, two or more miscible liquids must be mixed to give a product of adesired specifi cation, as for example, in the blending of petroleum fractions of different

viscosities. This is the simplest type of mixing as it does not involve either heat or mass

transfer, or indeed a chemical reaction. Even such simple operations can, however, pose

problems when the two liquids have vastly different viscosities, or if density differences

are suffi cient to lead to stratifi cation. Another example is the use of mechanical agitation

to enhance the rates of heat and mass transfer between a liquid and the wall of a vessel,

or a coil. Additional complications arise in the case of highly viscous Newtonian and

non-Newtonian materials.

8.1.2 Mixing of immiscible liquids

When two immiscible liquids are stirred together, one liquid becomes dispersed as drop-

lets in the second liquid which forms a continuous phase. Liquidliquid extraction, a

process using successive mixing and settling stages is one important example of this

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Liquid mixing 377

but care must be taken to ensure that the droplets are not so small that a diffuse layer is

formed instead of a well-defi ned interface; this can remain in a semi-stable state over

a long period of time and prevent the completion of effective separation. The produc-

tion of stable emulsions such as those encountered in food, brewing and pharmaceuticalapplications provides another important example of dispersion of two immiscible liquids.

In these systems, the droplets are very small and are often stabilized by surface-active

agents, so that the resulting emulsion is usually stable for considerable lengths of time.

8.1.3 Gasliquid dispersion and mixing

Numerous processing operations involving chemical reactions, such as aerobic fermenta-

tion, wastewater treatment, oxidation, hydrogenation or chlorination of hydrocarbons and

so on, require good contact between a gas and a liquid. The purpose of mixing here is to

produce a large interfacial area by dispersing bubbles of the gas into the liquid. Generally,

gasliquid mixtures or dispersions are unstable and separate rapidly if agitation is

stopped, provided that a foam is not formed. In some cases, a stable foam is needed (such

as fi re fi ghting foam); this can be formed by injecting gas into a liquid using intense agi-

tation, and stability can be increased by the addition of a surface-active agent.

8.1.4 Liquidsolid mixing

Mechanical agitation may be used to suspend particles in a liquid in order to promote

mass transfer or a chemical reaction. The liquids involved in such applications are usu-

ally of low viscosity, and the particles will settle out when agitation ceases. There is also

an occasional requirement to achieve a relatively homogeneous suspension in a mixing

vessel, particularly when this is being used to prepare materials for subsequent processes.

At the other extreme, in the formation of composite materials, especially fi lled

polymers, fi ne particles must be dispersed into a highly viscous Newtonian or non-

Newtonian liquid. The incorporation of carbon black powder into rubber is one suchoperation. Because of the large surface areas involved, surface phenomena play an

important role in these applications.

8.1.5 Gasliquidsolid mixing

In some applications such as catalytic hydrogenation of vegetable oils, slurry reactors,

three phase fl uidized beds, froth fl otation, fermentation and so on, the success and effi -

ciency of the process is directly infl uenced by the extent of mixing between the three

phases. Despite its great industrial importance, this topic has received only scant atten-

tion and the mechanisms and consequences of interactions between the phases are

almost unexplored.

8.1.6 Solidsolid mixing



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378 Non-Newtonian Flow and Applied Rheology: Engineering Applications

concrete, and of the ingredients in gun powder preparation, are longstanding examples

of the mixing of solids.

Other industrial sectors employing solids mixing include food, drugs, agriculture and

the glass industries, for example. All these applications involve only physical contacting,although in recent years, there has been a recognition of the industrial importance of solid

solid reactions, and solidsolid heat exchangers. Unlike liquid mixing, fundamental research

on solids mixing has been limited until recently. The phenomena involved are very differ-

ent from those when a liquid phase is present, so solidsolid mixing will not be discussed

further here. However, most of the literature on solidsolid mixing has been reviewed

elsewhere (Lindley, 1991; Harnby et al., 1992; van den Bergh, 1994; Xuereb et al., 2006).

8.1.7 Miscellaneous mixing applications

Mixing equipment may be designed not only to achieve a predetermined level of homog-

enity but also to improve the rates of heat and mass transfer, and the yield of chemi-

cal reactions. For example, if the rotational speed of an impeller in a mixing vessel is

selected so as to achieve a required rate of heat transfer, the agitation may then be more

than suffi cient for the mixing duty. Excessive or over-mixing should be avoided. For

example, in biological applications, excessively high impeller speeds or power input are

believed by many to give rise to shear rates which may damage micro-organisms. In asimilar way, where the desirable rheological properties of some polymer solutions may

be attributable to structured long-chain molecules, excessive impeller speeds or agita-

tion over prolonged periods, may damage the structure particularly of molecular aggre-

gates, thereby altering their properties. Similarly, the texture and structure of many

products (such as food, pharmaceutical, house-hold products, fermentation broths, etc.)

are extremely shear sensitive and obviously in such applications, the prediction and con-

trol of distribution of shear rates in the mixing vessel are much more important than the

estimation of power consumption (Benz, 2003, 2004). Yet in certain other applications,

the viscosity evolves with time thereby adding to the degree of diffi culty to achieve thedesired level of homogenization. For instance, lubricating greases are produced by car-

rying out in situ saponifi cation of glyceride fatty acids, followed by dehydration, heat-

ing it to the phase transition temperature of the soap, cooling it down to achieve soap

crystallization, milling and mixing various additives. At each stage, the contents of the

reactormixer vessel possess different rheological characteristics. Obviously, no sin-

gle impeller will be equally effi cient under all conditions. Similarly, lignin-based slurry

fuels offer a promising alternative to fossil fuels in kraft pulp processes. The prepara-

tion of the so-called lignogels entails effective mixing of lignin, fuel oil, water and sur-

factant, and indeed their non-Newtonian characteristics determine their atomization and

burning characteristics. In turn, the non-Newtonian characteristics are infl uenced by the

size of oil droplets produced during the mixing stage. As our fi nal example, similarly,

the quality of bread is strongly infl uenced by the gluten network present in the dough.

When wheat fl our and water are mixed, the protein in the dough forms gluten which is

a highly elastic cohesive mass About 20% of the wheat protein is soluble (albumen and



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type of mixing

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