Low reactor

Hydrocarbon, typically natural gas, is fed into the reactor to intersect with an electric arc stmck between a graphite cathode and a metal (copper) anode. The arc temperatures are in the vicinity of 20,000 K inducing a net reaction temperature of about 1500°C. Residence time is a few milliseconds before the reaction temperature is drastically reduced by quenching with water. Just under 11 kWh of energy is required per kg of acetylene produced. Low reactor pressure favors acetylene yield and the geometry of the anode tube affects the stabiUty of the arc. The maximum theoretical concentration of acetylene in the cracked gas is 25% (75% hydrogen). The optimum obtained under laboratory conditions was 18.5 vol % with an energy expenditure of 13.5 kWh/kg (4). 

Catalyst Development. Traditional slurry polypropylene homopolymer processes suffered from formation of excessive amounts of low grade amorphous polymer and catalyst residues. Introduction of catalysts with up to 30-fold higher activity together with better temperature control have almost eliminated these problems (7). Although low reactor volume and available heat-transfer surfaces ultimately limit further productivity increases, these limitations are less restrictive with the introduction of more finely suspended metallocene catalysts and the emergence of industrial gas-phase fluid-bed polymerization processes. 

In the modern unit design, the main vessel elevations and catalyst transfer lines are typically set to achieve optimum pressure differentials because the process favors high regenerator pressure, to enhance power recovery from the flue gas and coke-burning kinetics, and low reactor pressure to enhance product yields and selectivities. 

A low reactor temperature may not fully vaporize the feed unvaporized feed droplets will aggregate to form coke around the feed nozzles on the reactor walls and/or the transfer line. A long residence time in the reactor and transfer line also accelerate coke buildup. 

Low Reactor temperature Long Residence Time in the Reactor and Main Column High Bottoms temperature Low Bottoms Pumparound Rate Cold exchanger tube wail V temperature ... 

Distillation required - energy intensive Low reactor utilization Energy-intensive diluent recovery... 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

Design of the reactor is no routine matter, and many alternatives can be proposed for a process. In searching for the optimum it is not just the cost of the reactor that must be minimized. One design may have low reactor cost, but the materials leaving the unit may be such that their treatment requires a much higher cost than alternative designs. Hence, the economics of the overall process must be considered. 

The activation energy of the second reaction is larger than the first, so low reactor temperatures favor the yield of the desired component C. However, low reactor temperatures reduce the conversion of A for a fixed batch time. 

If the reactions had higher activation energies and were more sensitive to temperature, the reaction rates could become so small with the low reactor temperatures that the process could quench (with nothing reacting and all temperatures dropping to the feed temperature). 

Evidently, changes in the reactor size impact on the above findings allowing an increase in the reactor holdup leads to an increase in the single-pass conversion and reduces the flow rate of the material recycle stream. While plant configurations with low reactor capacity are preferred in processes featuring multiple reactions with valuable intermediate products (Luyben 1993b), the optimal sizes... 

Now the liquid level loops must also be modified, since we no longer can specify production rate and reactor level control cannot use TV This is easily accomplished by using low level override controllers on each of the three levels. Low stripper level pinches product base product flowrate B. Low separator level pinches separator liquid flowrate L. Low reactor level pinches the condenser cooling water flowrate CWc. In an override situation the level control structure has been reversed from the basic structure and now levels are held in the direction of flow. 

Chemical reactions occurring in the gas-phase can be more or less important in CVD, depending on the system, and can often be analyzed in detail. Gas-phase reactions are more likely to be important with the use of high temperatures and high total reactor pressures, but less likely to be important at low reactor pressures. Many CVD systems are operated in ways that minimize gas-phase reactions in order to avoid particle formation that could interfere with the desired film deposition. Note that the absence of homogeneous nucleation of particles is not synonymous with the absence of gas-phase chemical reactions. In contrast, other CVD systems utilize gas-phase reactions to convert reactant molecules that are relatively unreactive at the surface into more reactive species. Examples where this strategy is used include the combustion CVD processes discussed in Chapter 4 and plasma-enhanced CVD processes. 

In the reference test, the low reactor exit temperature at the constant plant-energy input conditions indicates the expected higher heat losses in a short-duration reactor. The corresponding lower overall temperature profile through the test-reactor length reduces the process kinetic time-at-temperature. The associated gas-phase chemical kinetics at the lower residence times are believed to be responsible for the slight discrepancies in the reference test gas yields. Also, the "true enthalpy used for cracking is lower than that indicated by the measured reactor temperature. 

The optimum reactor conditions thus usually favor low reactor conversion to give high selectivity for the desired products when all of these costs are taken into account. 

The method may be of interest because it uses the least expensive chiral ammonia equivalent available (Table 8.2) in essentially equimolar quantities (1.05 equiv) as compared to the limiting reagent, the aldehyde. Additionally, this one-pot process has significant yield advantages over the two-step process (formation of the (R)- or (S)-N-a-methylbenzyl aldimine, isolation of this chiral aldimine, followed by carbanion addition). The reactions are fast (3 h at -78°C), but the low temperature requirements, low reactor volume to product ratio, and the need for 3 equiv of a cuprate (CuBr based) will be considered restrictive. Many times the cuprate can be reduced to 2.0 equiv... 

Interestingly, mass spectroscopic analyses on the bottoms product over a range of conversions in their pilot plant showed that low space velocities, and therefore low reactor temperatures, consistently favored the formation of two of the high VI components, the isoparaffins (VI approximately 155) and the monocycloparaffins (VI approximately 142) (Figure 7.20). 

The BASF15 and IFP12 processes are reported to use sulfided NiO/MoO catalysts, and catalysts of this general sulfided base metal type can be expected to be employed by all licensors. While noble metal or nickel catalysts result in low reactor temperatures, their use is unlikely in this application. Sulfur levels in the feeds make for short catalyst life unless they are from dewaxing a hydro-crackate. 

Neutron counters located close to a reactor core are subjected to both neutron and gamma bombardment. Although a neutron counter—e.g., a °B counter—is mainly sensitive to neutrons, it responds to gammas too. At low reactor power, when the neutron flux is small, the neutron signal is overshadowed by a signal due to gammas emitted from fission products that had been accumulated from earlier reactor operation. To eliminate the effect of the gammas, a compensated ion chamber is used. 

---