Page 209 - Design of Simple and Robust Process Plants

194 Chapter 5 Process Simplification and Intensification Techniques
1. Plug flow reactor provided with mixing elements and re-mixing elements in
order to achieve high mass transfer rates.
2. CSTR provided with high-efficiency mixing.

5.7.1.3 Heterogeneous reactions: G/S and L/S
Fixed bed and fluidized beds are the main reactor types selected for this category.
The fixed catalyst beds are mostly operated with top-to-bottom flow (to avoid fluidiz-
ing), and consist of large particle sizes. The opposite is true for fluidized beds: the
flow is upward, while the catalyst particles are small in order to accommodate the
fluidized state. The fluidized beds are selected for systems which need high-fre-
quency regeneration of the catalyst, and where the uniformity of temperature is a
stringent requirement, for example temperature-sensitive reactions. In some appli-
cations the fluidized bed is used in homogeneous reactions simply to control the
temperature; examples are the chlorination of hydrocarbons or the oxidation of gas-
eous fuels. The ranking of reactor types with increasing complexity is as follows:
1. Adiabatic packed bed; this is the cheapest type, and is mainly restricted by
energy transfer requirements.
2. Packed bed with cooling requirements; cooling may be provided in several
ways:
. Recycling of cooled, unconverted reactant or solvent (this makes it an adiabat-
ic bed).
. Cooling through intermediate injection of cold feed or solvent.
. Multi-stage beds with inter-stage cooling; these beds are often designed as
radial beds for high-capacity operations (e.g., NH 3 reactors and ethyl-benzene
de-hydrogenation). These beds may be built as a single embodiment.
3. Fluidized bed:
. With solids removal through the wall; this is applicable when the velocity se-
lected does not bring the fluid particles in the dilute phase.
. With solids removal through internal cyclones; this is applicable for high-
velocity systems to keep the solids in the fluidized bed.
. With solid removal through external cyclones, specifically if cooling is pro-
vided through recycling of the solid stream.
4. Multi-stage fluidized beds are used for: Catalytic reactions where a high con-
version is required, and which must be satisfied through plug flow and sup-
pression of by-passing; gas solid reactions or solid reactions (e.g., the calcina-
tion of lime). In this last application, different functions are combined in
order of processing of the solids: preheating and drying stages (to be seen as
a countercurrent heat exchanger with the flue gases); calcination stage fol-
lowed by cooling stages utilized for air preheating (see Figure 5.18). Any
imbalance in the heat recovery is often utilized for steam generation.
5. Multi tubular reactor applied to highly exothermal reactions such as oxida-
tion or hydrogenation reactions. The heat removal is performed respectively
with: