Page 189 - Design of Simple and Robust Process Plants

174 Chapter 5 Process Simplification and Intensification Techniques
Induced pulsing flow has also been applied for some time in extraction columns,
either by the introduction of pulsations with the feed pumps or with an external
pulsation device. Alternative designs applied here are the introduction of a rotating
disc or vertical displacement of discs which are assembled on an axis. These all have
the purpose of intensifying the mass transfer.
Currently, a supersonic, two-phase flow mixer is under development at Praxair
(Cheng, 1997). The concept is based on the creation of very small droplets in a gas
stream, through a supersonic, two-phase mixer The application area would be very
rapid gas-liquid reactions.

5.5.3
Benefits from Centrifugal Fields: ªHigeeº
Centrifugal fields have, for many years, been successfully exploited in phase separa-
tions. The application of cyclones in gas/solid, gas/liquid separations are well
known, their limitations being mainly due to particle size and scale-up (larger-scale
cyclones have a lower separation efficiency). Hydro-cyclones, which have found
application in liquid/solid separations, are very simple, robust devices and have a
wide field of application. Likewise, centrifugal devices are widely used for liquid/
solid and liquid/liquid separations. The application of multi-stage centrifuges for
liquid/liquid extraction is also an intense operation which is often applied where
there are only small differences in the density of the materials to be separated. This
technique is of particular interest in the pharmaceutical industry, where advantage
is taken of the small hold-up. Although during the past few decades several types of
centrifugal separation apparatus have been developed by the manufacturers, it was
only during the 1970s that process intensification initiatives were started, driven
mainly by ICI.
The idea was to reduce the size of equipment considerably in order to achieve
much lower capital costs (Ramshaw, 1987, 1995). It was concluded that the size of
equipment was often determined by phase separation, which in turn restricted the
speed at which transfer processes could be carried out. By applying a centrifugal
force, phase separation could be improved to provide higher processing speeds, and
this in turn should result in improved mass transfer. In terms of a gas/liquid col-
umn, the diameter should be decreased (higher velocity), but the height must also
be reduced by improved mass transfer and phase separation. The initial industrial
results for acceleration-induced gas/liquid separations were limited, and develop-
ments into the use of centrifugal forces over a wide range of operations continued
in research, especially as the potential benefits were realized and were not under
debate. Some details of these ongoing research efforts are summarized below.
Centrifugal forces have been applied for de-sorption, for example of de-aerators of
water streams. In the case of oil field flooding water, de-aeration vacuum towers
were installed. The first development of a so-called high-gravity (ªHigravº) machine
was at the university of Beijing, and this resulted in a range of plant-scale machines
that were tested successfully in the field (Zheng et al., 1997). The technique is also
applicable for de-aeration of boiler feed water.