The Science of Defoaming

389

7

Mechanical Methods

for Defoaming

7.1 INTRODUCTION

Use of antifoams in controlling unwanted foam is in some cases considered to involve

serious disadvantages. An obvious example concerns aerated fermentation processes

where gas is sparged into reaction vessels by various means. Foaming should be mini-

mized in such processes to maximize the working volumes of containing vessels and

minimize any escape of foam leading to loss of reactants or products accumulated

therein. Minimal foam requires low rates of aeration and agitation or, preferably, the use

of some means of foam control [1–4]. Although foam in this context can be eliminated

by antifoams, their presence can be undesirable [3–7]. Antifoams are, for example,

known to reduce mass transfer rates and inhibit fermentation reaction rates (presum-

ably as a result of contamination of gas–liquid surfaces). In some cases, they may

also exhibit toxicity problems, although use of food-grade or pharmaceutical-grade

polydimethylsiloxane (PDMS) antifoams should minimize that factor. Antifoams may

also contaminate products so that effective separation and purication is adversely

affected. In consequence, foam control by use of mechanical devices is often used to

avoid these problems. Perhaps an extreme illustration of the need for such approaches

is revealed by a patent application concerned with defoaming, using a centrifuge, of

blood circulating externally from an articial lung [8] (see also Chapter 11). However,

in severe cases of overfoaming, the limitations of mechanical defoaming devices mean

that they can often only be used successfully in combination with antifoams.

Of the various types of mechanical defoaming devices, the most commonly

used employ a rotary action. Such devices include rotating discs, various impel-

ler arrangements, centrifugal baskets, spinning cones, and cyclones. However, other

types of devices have also been investigated. These latter include ultrasonic devices

(see, e.g., reference [7] and references cited therein), application of vacuum through a

suitable membrane [1,9–11], use of packing materials of appropriate wettability (see

Section 7.4), and even application of liquid sprays [1,10,12]. Here we review the use

of most of these mechanical defoaming devices and include speculation concerning

the mechanisms by which they function.

7.2 DEFOAMING USING ROTARY DEVICES

7.2.1 deSiGnS of roTary deViceS deScribed in ScienTific liTeraTure

Rotary devices are generally used to apply centrifugal and shear forces to foam to

produce lm rupture. As we discuss in Section 7.2.2, understanding of precisely how