Stirred reactor, liquid phase oxidation

The liquid-phase oxidation of toluene with molecular oxygen is another example of a well established process (Table 4, entry 40). A cobalt catalyst is used in the process and the reaction proceeds via a free-radical chain mechanism. Heat of reaction is removed by external circulation of the reactor content and both bubble columns or stirred tanks are employed. It is important to note that air distribution is critical to prevent the danger of a runaway. Another example of direct oxidation is the commercial production of nitrobenzoic acid by oxidation of 4-nitrotoluene with oxygen (Table 4, entry 41). 

Apart from gas-phase oxidation, liquid-phase oxidation with dichromate is an expedient process if the chromium-(III)-sulfate by-product can be used as a tanning agent. The largest anthraquinone plant Bayer/ Qsi Germany) with a capacity of 11,000 tpa applies this process. Batch oxidation is carried out with pulverized 94-95% anthracene in a stirred tank reactor with the addition of sodium dichromate and sulfuric add at 60 to 105 °C. The reaction takes between 25 and 30 hours. The yield of anthraquinone, which is produced in purities of around 95%, is over 90%. The purity can be increased to over 99% by recrystallization from nitrobenzene or cyclohexanol/cyclohexanone. 

The liquid-phase oxidation of o-xylene into o-methylbenzoic acid by means of air is to be carried out in a continuous stirred tank reactor at 13.8 bar and 160°C. A yearly production of o-methylbenzoic acid of 30,000 tons is required. For reasons of selectivity, the o-xylene conversion per pass has to be limited to 16 percent. An excess of 25 percent with respect to the stoichiometric requirements is chosen for the oxygen feed rate. The rate equation is of pseudo-first order with respect to oxygen ... 

The oxidation of n-octane and the epoxidation of 1-hexene were performed in a 25 ml Parr reactor using 30% aqueous H2O2 as an oxidant and acetone as solvent at 100 °C and 80 °C, respectively and stirred at 500 RPM. Prior to product analysis, the product mixtures were diluted with acetone in order to obtain a single liquid-phase. The products were analyzed on a HP 5890 Series II GC equipped with a 25 m long HP-FFAP (polar) capillary column. 

Oxidation procedure with measurement of the oxygen concentration in the liquid phase. A calculated amount of catalyst, in order to obtain 12.5 /jmol surface atoms noble metal, was introduced into the reactor. 50 ml of water was added and the system was flushed with nitrogen (500 ml/min) for 5 min to remove oxygen from the reactor. Then the reactor was flushed with hydrogen for 5 min, followed by 25 min at low gas flow and stirring at 1500 rpm. Finally the system was flushed with nitrogen for 5 min. 

Gas-liquid reactions form an integral part of the production of many bulk and specialty chemicals, such as the dissolution of gases for oxidations, chlorin-ations, sulfonations, nitrations, and hydrogenations. When the gaseous reactant must be transferred to the liquid phase, mass transfer can become the rate-limiting step. In this case, the use of high-intensity mixers (motionless mixers or ejectors) can increase the reaction rate. Conversely, for slow reactions a coarse dispersion of gas, as produced by a bubble column, will suffice. Because a large variety of equipment is available (bubble columns, sieve trays, stirred tanks, motionless mixers, ejectors, loop reactors, etc.), a criterion for equipment selection can be established and is dictated by the required rate of mass transfer between the phases. 

Usually, the typology of batch reactors also includes the semi-batch gas-liquid reactors, in which a gaseous phase is fed continuously in order to provide one of the reactants. A typical example is given by the reactors used both in different oxidative industrial processes and in the active sludge processes for the treatment of wastewater. It is possible to distinguish between the bubble columns (Fig. 7.1(c)), in which the gas rises undisturbed in the liquid phase, and the bubble stirred reactor, in which a mechanical mixer is added. Finally, the slurry reactors can be considered, in which the liquid phase contains a finely dispersed solid phase as well, which can act as a reactant or as a heterogeneous catalyst these reactors assume in general the features of Fig. 7.1(d). 

A continuous stirred tank reactor was used by Wen, McMichael, and Nelson(17) to study the oxidation of aqueous solutions of sodium and calcium sulfite. In their experiments, the oxidations were gas phase to liquid phase mass transfer limited (rate cc impeller speed) for sodium sulfite solutions of 0.1 Mol/i. at impeller speeds up to 700 RPM. Their results showed the oxidation was first order in oxygen, slightly less than zeroth order in hydronium ion concentration, and independent of the sulfite ion concentration. 

Characterization and Catalytic Activity. 2.2.2.2.1 Hydrogenolysis of Ethyl Dodecanoate on CuO-ZnO-A Os Catalysts. - The authors of this review have also investigated the liquid phase hydrogenolysis of ethyl dodecanoate over three commercial CuO-ZnO-AbOa (CZA-1, CZA-2 and CZA-3) and a laboratory made CuO-ZnO-AbOa catalyst (CZA-4) in a stirred tank reactor at 90 bar hydrogen pressure and 250 X-ray diffraction (XRD) was used to characterize both the oxide and form of the catalysts, as well as the reduced and then used catalysts (see previous paragraph. Table 26 and Table 27). ... 

Oxidation reaction experiments were performed in a 300 mL stainless-steel high pressure reactor vessel (Parr Instraments Co., USA, 5521) operated under isothermal batch mode at 413 K, 2 MPa of oxygen pressure and stirred at 500 rpm to optimize the mass transfer in the liquid phase. For every run a fresh feed of aqueous phenol solution of 20 mmol L and 4 g L of the catalyst was introduced to the reaction vessel. The liquid phase was analyzed by HPLC on a Pursuit XRs 5 C18 150 while the gas phase was analyzed in a GC equipped with a Porapak Q packed column. 

Skeletal catalysts are usually employed in slurry-phase reactors or fixed-bed reactors. Hydrogenation of cottonseed oil, oxidative dehydrogenation of alcohols, and several other reactions are performed in sluny phase, where the catalysts are charged into the liquid and optionally stirred (often by action of the gases involved) to achieve intimate mixing. Fixed-bed designs suit methanol synthesis from syngas and catalysis of the water gas shift reaction, and are usually preferred because they obviate the need to separate product from catalyst and are simple in terms of a continuous process. 

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