Reactor temperature cold shot

Assume that two adiabatic reactors are available with the same kinetics given in Section 7.3.3.1. Using an inlet temperature of 300 K, plot the PFR trajectory for the first reactor as well as the possible PFR trajectories for the second reactor if cold-shot cooling is employed. Use mixing fractions of A = 0.25, 0.5, and 0.75. Assume that the feed vector is given by Cj = [c j, t°] = [1,0], and that the exit concentration from the first reactor is Ca = 0.2 mol/L. 

Cold shot. Injection of cold fresh feed for exothermic reactions or preheated feed for endothermic reactions to intermediate points in the reactor can be used to control the temperature in the reactor. Again, the heat integration characteristics are similar to adiabatic operation. The feed is a cold stream if it needs to be increased in temperature or vaporized and the product a hot stream if it needs to be decreased in temperature or condensed. If heat is provided to the cold shot or hot shot streams, these are additional cold streams. 

Adiabatic with Cold-Shot Cooling. Some exothermic reactions are conducted in vessels with multiple beds of catalyst, which operate adiabatically (temperature increases through the bed). At the exit of each bed, a cold stream is mixed with the hot stream leaving the bed to bring the temperature back down to the desire inlet temperature for the downstream bed. This cold stream is typically some of the feedstream that has been bypassed around the reactor feed preheating system. 

If the system consists of a series of adiabatic reactors, there are two basic configurations. The first has heat exchangers or furnaces between each of the reactors to cool or heat the reactor effluent before it enters the next reactor. The second configuration uses cold shot cooling. Some of the cold reactor feed is bypassed around the upstream reac-tor(s) and mixed with the hot effluent from the reactor to lower the inlet temperature to the downstream catalyst bed. 

Openloop Response The openloop response of the seven-bed adiabatic reactor system with cold-shot cooling to a 20% increase in recycle flowrate FR is shown in Figure 6.17. The inlet temperature of the first reactor decreases because of the larger flow through the preheat system. However, the reactor inlet temperatures of the other reactors increase because the cold-shot flows are fixed. Then the first reactor inlet temperature starts to increase at 5 min because of the increase in the exit temperature of the seventh reactor. The pressure initially increases gradually because of the lower temperature in the first reactor. At about 12 min, the pressure drops rapidly because of the large increase in temperature in the first reactor. 

FIG. 19-3 Fixed-bed reactors with heat exchange, (a) Adiabatic downflow, (b) Adiabatic radial flow, low AP. (c) Built-in interbed exchanger, (d) Shell and tube, (e) Interbed cold-shot injection, (f) External interbed exchanger, (g) Autothermal shell, outside influent/effluent heat exchanger. (h) Multibed adiabatic reactors with interstage heaters, (t) Platinum catalyst, fixed-bed reformer for 5000 BPSD charge rates reactors 1 and 2 are 5.5 by 9.5 ft and reactor 3 is 6.5 by 12.0 ft temperatures 502 433, 502 => 471,502 => 496°C. To convert feet to meters, multiply by 0.3048 BPSD to m3/h, multiply by 0.00662. 

Figure 4.27 shows another method of controlling plug-flow systems. Instead of cooling the effluents from each adiabatic step in a heat exchanger, we introduce a cold shot of fresh feeds. The cold shot technique increases the concentration of reactant in all the segments. Mixing the cold feed with the reactor effluent lowers the inlet temperature to the next reactor. 

In discussing the general algorithm for a cold shot reactor with m simultaneous reactions we will return to the extent variables c and actual temperature T. Let Co and To denote the state of the cold unreacted feed and g, be the flow rate of reactants through the bed r of volume F,. The bypass that is mixed with the process stream between beds (r - - 1) and r is (g, — g, i) so that mass and heat balances give... 

---