Hot-Reaction Case

We are considering an irreversible reaction in this book. For reversible reactions, Luyben2 demonstrated the opposite effect of recycle flowrate on production rate, specifically, increasing recycle flowrate increases production rate. With reversible reactions, the impact of temperature is less important than the impact of concentrations because of the reaction equilibrium constraints. 

Thus the absence of the furnace to provide heat input when the bypass valve is completely shut produces a process that is not as robust and not as easy to operate. Another problem with just using heat exchanger bypassing is the impact of changes in heat transfer coefficients over time due to fouling. The stability of the process will degrade as the FEHE fouls. 

The linear analysis in Section 7.3 predicted that the FS2 flowsheet should be able to handle systems with reactor gains up to 6. To test this prediction, kinetic parameters are modified to give reactor gains of this magnitude in a new design. Both the activation energy and the heat of reaction are increased by a factor of 1.9. The specific reaction rate at 

Luyben, Design and control of gas-phase reactor /recycle processes with reversible exothermic reactions, Ind. Eng. Chem. Research 39, 1529 (2000). 

TABLE 7.4 Optimum Design Parameters for Hot-Reaction Case 

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If a much higher specific reaction rate is used, the reactant concentration is lower and the tendency for a runaway is reduced. This is illustrated in Figure 4.13, where the preexponential factor k0 is increased by a factor of 10. In this hot reaction case, the reactant concentration CA builds to only 1 kmol/m3 and no runaway occurs. Note that the fresh feed is cut off when the reactor is full (VR = 12.57 m3). The batch is terminated when the desired conversion (99.99%) is attained at 460 min. 

Figures 7.13 and 7.14 give results using the FS2 flowsheet with the furnace for this hot-reaction case. Figure 7.13 shows that a 10% decrease in recycle flowrate can be handled, but a 20% decrease produces a reactor mnaway. This occurs despite the fact that the reactor inlet temperature increases only slightly ( 0.5 K) during the transient. Figure 7.14 gives results for changes in the setpoint of the reactor inlet temperature controller. Rather surprisingly, inlet temperature can be increased by 2 K without a runaway. This is unexpected since the isolated reactor (Fig. 7.12) showed a runaway with a +2 K change in Tm. The difference may be due to the effect of pressure. In the isolated reactor simulation, pressure is held constant at 50 bar. In the simulation of the whole process, pressure drops as reactor temperature increases due to the increased consumption of reactants. Since the reaction rate depends on the square of the total pressure (P2), the decrease in pressure lowers the reaction rates. However, a 3 K increase cannot be handled.

Vote 3. If more HMPT is used the yield of the cumulene will be lower the compound can remain longer in the hot reaction mixture, where it can polymerize. Mote 4. It is essential to follow the instructions given. The reaction sometimes starts very soon if in such a case the desired pressure (10-20 mmHg) has not yet been achieved, part of the cumulene may polymerize in the hot reaction mixture. 

However, in this case, the hot reaction mixture is filtered by suction before cooling. 

The first effective synthesis of CgFgAsFfi employed 02AsF as the oxidizer of C F dissolved in liquid WPg. This solvent not only provided the desirable diluent effect for this hot reaction but its relatively high heat capacity also aided in preservation of a lower temperature. These are essential requirements for high-yield syntheses of C5F(AsF and its monocyclic relatives, since all are thermally unstable at ordinary temperatures. In more recent work sulfuryl chloride fluoride has been used as the diluent and moderator and the low working temperatures have resulted in greatly improved yields. Nevertheless, even with SO2CIF, pyrolysis products from the salts are always observed and a quantitative yield has never been obtained for any of the monocyclic cation salts. For these and other reasons the salt composition in each Case has been determined from the stoichiometry of the salt pyrolysis products and other reaction stoichiometries. 

The effect of scavengers. The presence of a compound with a high rate constant for reaction with thermal atoms (scavengers) in minute concentration will decrease substantially the yields of thermal chemical reactions and only very little the yields of the hot reactions. In the case of product produced both by thermal and hot reactions, the addition of minute concentration of scavengers (1-2 % mole fraction) is necessary for elimination of the thermal reaction and to enable to measure only the yield of the hot reaction. 

From the genenil form of the correlation diagram in Figure 6.18 it is evident that since barriers cun be overcome with the aid of thermal energy the initial excitation need not be into that stale that is represented by the characteristic configuration. In this case low temperature can prevent the reaction from taking place. Excitation of higher vibrational levels of one-and-the-same absorption band can enable the nuclei to move over the barrier in a hot reaction. 

Addition of HF to acetylene homologs, giving difluoroalkanes, has been described both for small218 and for large batches (5-10 moles).220 In the latter case it is necessary to avoid formation of hot reaction zones (resinification) and decrease in temperature which can lead to dangerous accumulation of unchanged reagents. Propyne and 1-butyne are led as gas into liquid ice-cold anhydrous HF (2.5 moles per mole of alkyne) pentyne and hexyne are dropped into HF at — 50° (copper vessels). 

The hot-atom chemistry of phosphorus in sulphur and phosphorus compounds, reactor-irradiated in benzene solutions, indicates that a wide range of products is produced. - In one case, greater oxidation of the recoil P is observed in dilute solutions, whereas in the other case thermal reactions of P with carbon atoms dominate for low concentrations but hot reactions seem more important in the higher concentration range. A detailed reaction sequence is given for the phenyl-phosphorus compounds. ... 

Several investigations " of neutron-irradiated aqueous solutions of XeOa have given results which indicate a high yield of xenon, isolated because of its non-reactivity, and because of the instability of O species formed by hot reactions. In the case of isomeric transitions, Xe, Xe, and Xe,... 

Adiabatic operation. If adiabatic operation leads to an acceptable temperature rise for exothermic reactors or an acceptable fall for endothermic reactors, then this is the option normally chosen. If this is the case, then the feed stream to the reactor requires heating and the efiluent stream requires cooling. The heat integration characteristics are thus a cold stream (the reactor feed) and a hot stream (the reactor efiluent). The heat of reaction appears as elevated temperature of the efiluent stream in the case of exothermic reaction or reduced temperature in the case of endothermic reaction. 

If the reactor can be matched with other process streams (which is unlikely), then the reactor profile should be included in the heat integration problem. This would be a hot stream in the case of an exothermic reaction or a cold stream in the case of an endothermic reaction. 

If indirect heat transfer is used with a large temperature difference to promote high rates of cooling, then the cooling fluid (e.g., boiling water) is fixed by process requirements. In this case, the heat of reaction is not available at the temperature of the reactor effluent. Rather, the heat of reaction becomes available at the temperature of the quench fluid. Thus the feed stream to the reactor is a cold stream, the quench fluid is a hot stream, and the reactor effluent after the quench is also a hot stream. 

Preparation of 2 4-dinitrophenyl-sulphides. Dissolve about 0-5 g. (or 0 005 mol) of the mercaptan in 10-15 ml, of rectified spirit (or in the minimum volume necessary for solution warming is permissible) and add 2 ml. of 10 per cent, sodium hydroxide solution. Mix the resulting sodium mercaptide solution with a solution of 1 g. of 2 4-dinitrochlorobenzene in 5 ml. of rectified spirit. Reaction may occur immediately with precipitation of the thioether. In any case reflux the mixture for 10 minutes on a water bath in order to ensure the completeness of the reaction. Filter the hot solution rapidly allow the solution to cool when the sulphide will crystaUise out. RecrystaUise from alcohol. 

Carbanions stabilized by phosphorus and acyl substituents have also been frequently used in sophisticated cyclization reactions under mild reaction conditions. Perhaps the most spectacular case is the formation of an ylide from the >S-lactam given below using polymeric Hflnig base (diisopropylaminomethylated polystyrene) for removal of protons. The phosphorus ylide in hot toluene then underwent an intramolecular Wlttig reaction with an acetyl-thio group to yield the extremely acid-sensitive penicillin analogue (a penem I. Ernest, 1979). 

Once a fusion reaction has begun in a confined plasma, it is planned to sustain it by using the hot, charged-particle reaction products, eg, alpha particles in the case of D—T fusion, to heat other, colder fuel particles to the reaction temperature. If no additional external heat input is required to sustain the reaction, the plasma is said to have reached the ignition condition. Achieving ignition is another primary goal of fusion research. 

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