Page 210 - Design of Simple and Robust Process Plants

5.7 Simplification and Ranking per Unit Operation 195
. boiling liquid, such as water in a thermo-syphon design. In some applica-
tions this is done with different boiling zones, which adds to the complexity;
and/or
. circulating liquid, such as molten salt for high-temperature applications (e.g.,
dehydrogenation of ethyl benzene).

5.7.1.4 Heterogeneous; G/L
For heterogeneous G/L reactor systems, several very different forms are possible.
Especially important in the design of the system is the volumetric ratio of gas versus
liquid. Fundamentally, there are three different modes of gas liquid contact to be
recognized:
1. Gas bubbles dispersed in liquid, as in bubble columns.
2. Liquid dispersed in gas, as in spray columns.
3. Gas and liquid in film contact, as in packed columns.

A selection criterion, b, is the ratio of the liquid volume to the volume of the diffu-
sion layer within the liquid phase (Krishna and Sie, 1994):
" l
b ˆ
l a
where e l is the fractional hold-up of the liquid phase, a is the interfacial area per
±1
unit volume, m ,
d l = thickness of diffusion layer of liquid phase d l = D l m
k l
2
where D l is liquid diffusion coefficient m sec ±1 and k l is liquid phase mass transfer
±1
coefficient m sec .
4
3
For gas bubbles dispersed in the liquid phase, b =10 ±10 , while for gas in liquid
film contact and the liquid dispersed in gas, b = 10±40. The overall aim is to choose
the value of b such that the reactor volume is effectively utilized.
For slow, liquid-phase reactions the liquid phase volume is increased at the
expense of the interfacial area. A high value of b is achieved in bubble columns and
tray columns operating in the froth regime, occasionally with higher weir heights to
increase the liquid hold-up on the trays. An example of the bubble regime applica-
tion is wet air oxidation, it might also be used in biological aeration basins and fer-
mentation reactors. The HNO 3 and H 2 SO 4 absorption towers are other typical appli-
cations in this field where the hold-up is increased by application of excessive high
weirs heights; it should be noted that in these cases the heat removal is extensive
and is carried out on the trays with cooling spirals.
For rapid reactions, the reaction is mainly limited to the diffusion layer, and low b
values are preferred (spray columns and packed columns). The aim is to increase
the interfacial area, but there is no need to overemphasize turbulence in the liquid
phase. The absorption of CO 2 in caustic solution is carried out in packed columns.
Within the consideration of the above discussion, the following ranking is select-
ed respectively for slow reactions which take place in the liquid phase (liquid phase-
controlled), and for rapid reactions which take place at the interface (interfacial-con-
trolled).