Page 188 - Design of Simple and Robust Process Plants

5.5 Intensification of Process Functions 173
three-phase systems, where the solid phase is the catalyst. However, it should also
be realized that during hydrogenation heat must be removed, as this can also-dom-
inate the design, specifically during total hydrogenation. The options for heat
removal are:
. Adiabatic reactor, where the temperature increase of the reaction medium is
controlled with recycle of (preferably) the liquid phase due to is higher specif-
ic heat and density. This is commonly applied in packed bed reactors.
. Boiling reactor applied with slurry reactors for CSTR and bubble column
designs.
. Tubular reactor.

The above ranking order should be seen in perspective of the discussion on the rank-
ing order of reactor design in Section 5.7.1.
The intensification from a mass transfer perspective has been studied by Marwan
et al. (1997) in co-current, down-flow contact reactors (gas dispersed in liquid) for
different reaction systems. This contact system can be applied for slurry as well as
packed bed reactors, although a packed bed reactor will often be favorable to avoid
the need and cost of catalyst recovery. The principle is that the gas is co-currently
with the liquid dispersed by high-velocity jet in down flow, while the liquid is the
continuous phase. The high degree of shear and turbulence created results in good
gas-liquid contact and high interfacial area, resulting in high mass transfer rates.
The dispersion in the column extends down the column, where coalescence of bub-
bles occurs; these rise back up the column and re-disperse higher up. This concepts
leads to much smaller reactor compared with CSTRs, and also higher selectivity,
depending on the reaction mechanism.
Another solution was reported by Turunen (1997). A packed bed reactor was
developed with the catalyst fixed in pockets inside a structure made of metal gauze.
Gas liquid is moving co-currently in the open channels, and gas is dissolved in the
liquid. The liquid penetrates through the gauze into the catalyst pockets, where reac-
tion takes place. The disadvantages of catalyst recovery in a slurry reactor may be
overcome by this solution.
The application of co-current, down-flow gas liquid reactor through packed beds
(trickle beds) with self-generating pulsing flow is a technique to enhance mass
transfer (Tsochatzidis and Karabelas, 1995). The advantages are the intensive inter-
action between the phases, resulting in high heat and mass transfer rates through
renewal of interfacial areas, reduced back mixing, and elimination of hot spots.
However, this has as a disadvantage that relative high flow rates are required to
achieve pulsing flow with short contact times. The application is therefore limited to
fast chemical reactions.
Induced pulsing flow was also studied by Tsochatzidis et al. (1997). These studies
emphasized the introduction of induced pulses with the liquid stream at a relative
high frequency. The results indicated an enhancement of mass transfer in an oper-
able area where self-generating pulsing flow was not applicable. The technique
appears promising, but needs further investigation.