As its name indicates, a Reversed – Flow
Reactor (RFR) operates with periodical reversals of gas flow through beds
of catalyst and, when necessary, ceramic packing. Figures in the sidebar
illustrate the simplest configuration of the RFR. More complex schemes are
often used for practical purposes.
Cold incoming gas flow enters the reactor
through one of the valves V1 or V2. During the first half-cycle (Phase 1),
the valves V1 and V4 are opened while the valves V2 and V3 are closed. This
sends the gas flow in the direction shown by arrows. While passing through
solid material, the gas accepts heat from the hotter solid material. Thus,
gas temperature rises while the solid material is cooled. Dramatic
difference in heat capacity of solid and fluid phases allows for preheating
large volumes of gas using a relatively small bed.
Once the gas flow is sufficiently heated,
catalytic reaction begins and, in the case of exothermic reaction, heat is
released. After leaving the reaction zone, hot gas flow passes through
outlet portion of the bed. Heat is transferred from the gas to colder
catalyst. Cooled gas flow then leaves the reactor through the valve V4.
After the inlet portion has been
sufficiently cooled, the flow is reversed and Phase 2 begins. The gas flow
now enters catalyst bed from the side preheated during the previous phase,
through the valve V2. After reaction zone, it gives up heat energy to
previously cooled portion of the bed and leaves the reactor through the
valve V3. The valves V1 and V4 are closed.
Periodical reversals of the gas flow help
maintain the catalyst temperature at design conditions as long as enough
heat is released by the reaction. The resultant temperature profile (shown
on the left) has a characteristic shape: the temperature is at maximum in
the central portion of the bed, decreasing towards the inlet and outlet.
The ‘heat wave’ moves periodically through the bed in the direction of gas
flow, oscillating between two symmetrical end positions.
In-depth discussion of theoretical
fundamentals, laboratory and pilot tests, and design and operation of
commercial plants using reversed-flow reactor technology can be found in
numerous publications by Matros