Page 185 - Design of Simple and Robust Process Plants

170 Chapter 5 Process Simplification and Intensification Techniques
nonwetted, microporous membranes it has been shown that the resistance due to
the membrane is low, or even insignificant. For wetted membranes, the resistance
of the membrane severely affects the absorption rate. Wetting is influenced by the
pore size, pressure difference across the membrane, and the interaction of the
absorption liquid with the membrane material. This phenomenon is described by
the Laplace equation:

DP = ±2 (c/r) cos H, (1)
2
where DP is pressure difference (N/m ), c is surface tension liquid (N/m), r is pore
radius (lm), and H is the contact angle.
The membrane will not be wetted if the contact angle is >90 and the pressure
difference is limited for a certain pore size. Membrane absorbers can be operated at
relative very low liquid flow rates which are not achievable by conventional absorbers
such as gas/liquid mass flow ratios of 1000. The low liquid loads has an impact on
the regeneration unit, which also can be smaller in size
The absorption liquid might contain salts of amino acids or promoted amino acid
solutions for CO 2 removal. Applications tested at the industrial scale include the
removal of CO 2 ,H 2 S and SO 2 . A ten-fold reduction in absorber size appears to be
realistic.

5.5.2
Increased Heat, Mass and Impulse Transport

More than just progress has been made to improve the design of heterogeneous
reactors such as stirred tank reactors (STRs), bubble columns, and trickle flow reac-
tors. Indeed, the challenge is to improve the rate-limiting step as heat or mass trans-
fer, and this forms the basis for reductions in equipment size. The improvements
made in this respect may be called ªspectacularº, and some examples (with indus-
trial applications) are mentioned briefly here. The improvements made will depend
on the ratio between transfer rate and reaction rate, starting from the historical situ-
ation.
Originally, the nitration of aromatics was a mass transfer limited operation
between two liquid phases ± the aromatics and the acid stream (HNO 3 in H 2 SO 4 )
(Hauptmann et al., 1995). The reaction is strongly exothermal, with the aromatic as
the dispersed phase. The reaction was executed in a series of isothermal CSTRs.
The HNO 3 is almost fully converted, while the H 2 SO 4 is recycled after removal of
the reaction water by flashing. To achieve high mass transfer rates in liquid/liquid
reaction systems, it is essential that the liquid phases are dispersed, and re-dis-
persed. This leads to the creation of a fresh interfacial surface that takes part in the
high heat and mass transfer rates. The design of a mixing system which accommo-
dates the dispersion and re-dispersion is crucial for performance.
Different technology suppliers have different solutions, but the final result is that
a series of CSTRs (three to five, with low performance mass transfer) were replaced
by a tubular reactor. The CSTRs design was executed under isothermal conditions,
as the ªhold-upº of these systems was considerable and could easily lead to a ªrun-