Reactor network (RN)

Due to the wash out effect and the complex reactor operation different reactors configurations were tested in order to obtain the auto-thermal operation. Intense investigations were made in this direction and for the first time Vanden Bussche and Froment [119] introduced the concept of “star reactor” which can operate in a transient mode giving practically constant exit concentrations and higher conversion than the RFR.

Another way to obtain a sustained dynamic behavior [287] is to implement a reactors network (RN) which consists of a closed sequence of catalytic fixed bed reactors.

This configuration which has been also called “ring reactor” was also previously addressed by Barresi’s group [116, 117, 118] who proposed a RN made of two or three reactors connected in a closed sequence as an alternative configuration to RFR. The RN is operated by means of a set of valves which enable to change the feed position, thus the sequence of reactors is modified, simulating the behaviour of a moving bed and achieving a sustained dynamic behaviour. Contrary to the RFR, the flow direction is maintained in this way ensuring uniform catalyst exploitation and avoiding the wash out effect.

The working principle and the practical implementation of a network made of three reactors are represented in figure 4.3.2.1. The system is feed through the lower reactor (reactor number 1) then the flow passes through the middle reactor (reactor number 2) and exits through the upper reactor (reactor number 3). In this case the order of

reactors is 1-2-3. After a time period (switching time tc) the feed position is shifted to the second reactor by a set of valves. Now the first reactor of the sequence becomes the second one thus changing the order in the close sequence to 2-3-1. A further change of the feed position leads to the sequence of reactors 3-1-2. By this way it is possible to create a closed cycle which prevents the heat front from leaving the system; the flow direction is maintained in the same way, contrarily to the RFR this ensures uniform catalyst exploitation because temperature and concentration profiles migrate throughout the entire length of the system. The strategy of changing the feeding position is flexible.

Figure 4.3.2.1 Practical implementation of a network of three catalytic fixed bed reactors with periodically variation of the feeding position [119].

Instead of switching from one reactor to the next one in the close sequence, it is possible to switch the feeding position from the first one directly to the last one and to have the products leaving the system from the second. In this case the order in the close sequence is 1-3-2 and the flow direction is also maintained on a single sense. In this way it is possible to exploit the catalyst length of one reactor more efficiently in one cycle,

until the disturbances caused by the switching appear. This configuration has not been investigated in the literature yet and it will be stressed in the present thesis.

The simulated moving bed reactor received little attention up to several years ago.

Haynes and Caram [133] presented some theoretical results concerning the operation of a two reactors network compared with reverse flow operation, showing the applicability to mildly exothermic processes both for generic reversible and irreversible reactions. Auto-thermal behavior with a nearly uniform catalyst utilization are the main advantages of the network; it however presents a small range of switching times which allow to reach and maintain a pseudo-steady state of operation. The performance and behavior of a network of three beds applied to non-stationary catalytic destruction of volatile organic compounds (VOC) have been investigated by means of numerical simulations by Brinkmann et al. [134]. Each reactor presented a large inert section for heat exchange followed by the catalytic active part. The effect of transport parameters on conversion and the maximum bed temperature have been studied as well as the influence of the design variables. The results suggested that good conversion and auto-thermal behavior can be obtained in certain conditions even at low VOC concentration but safe operation is related to a narrow range of switching times. This aspect has been investigated in detail and a more robust control policy than the open loop strategy based on fixed switching time has been proposed [288, 289; 290]. Brinkmann et al. [116] and Barresi et al. [288]

suggest that a network of catalytic fixed bed reactors can be a suitable alternative to reverse flow operation because it can reduce the emissions of unburned gas related to the phenomenon of washout, i.e., in that case, the drop in combustion efficiency upon each flow reversal, due to the removal of unconverted gas immediately after the change of the feeding position [291]. Velardi and Barresi [117] investigated the application of the reactors network to methanol synthesis showing that this device allows for higher conversion than the RFR and it is not significantly affected by the wash-out at the beginning of the cycle. Furthermore, a proper choice of the switching time allows for slight variation of the outlet gas temperature along the cycle, differently from the RFR, thus reducing the potential disturbances of the equipment downward. Fissore et. al. [292], Fissore and Barresi [293] summarized the main characteristics and performances of the

RN with respect to both the combustion of lean VOC mixtures and to the exothermic equilibrium-limited reactions.

All previous studies involved high exothermic reactions in case of isothermal conditions; the authors did not study low adiabatic temperature rise reactions and the influence of thermal balance on overall reactor performances. This type of reactions will be the case study of this thesis; i.e. the selective catalytic reduction of NOx with ammonia. For this reaction the adiabatic temperature rise is about of 10-20 K but the temperature rise in a forced unsteady state reactor will be a multiple of this value, thus allowing auto-thermal operation when low temperature gas is feed to the reactor. In these conditions, as it was also stressed in the conclusion section of the work of Yeong and Luss [115], the choice of the switching time will be affected by the dynamics of the heat wave.

In order to emphasize the performance of the RFR and RN and the possibility of using the RN, as a successful alternative to the RFR, a comparative analysis will be presented in the following section.