Selective oxidation reactor

Figure 23. Methanol—steam reformers, heat exchangers, combustor, and selective oxidation reactors the body materiai was stainless steel. ...

Evaluate alternate approaches for the major catalytic components (e.g., desulfurizer, reformer, shift reactor and selective oxidation reactor). 

The second stage contains a commercial low temperature shift (LTS). The shift reactors reduces the CO concentration in the reformate gas to approximately 2,000 ppm. Final reduction of CO is achieved in a preferential selective oxidation reactor. 

The proper design of fuel reformer systems must pay careful attention to the minimization of carbon monoxide before the processed fuel stream enters the fuel cell stack. Many reformer systems use a secondary preferential oxidation reactor that selectively oxidizes the carbon monoxide present in reformate streams. In most transportation applications the steam reformer and the selective oxidation reactors do not operate under steady state conditions large transients may occur which produce relatively large amounts of carbon monoxide. It is highly desirable to have a low-cost real-time carbon monoxide measurement system that provides feedback control to the fuel processing system in order to protect the PEM fuel cells from performance degrading concentrations of carbon monoxide. 

A compact CO selective oxidation reactor for solid polymer fuel cell powered vehide application, J. Power Sources 2000, 86, 214-222. 

The combustion reaction is realized in the firebox of the waste heat boiler within the Claus unit, while the Claus reaction and hydrolysis reactions take place in the Claus reactor filled with a catalyst. This process requires two or three Claus reactors. The sulfur recovery is 90%-96% in the first two reactors and is 95%-98% in the third reactors. Recently, super-Claus process has emerged. In the super-Claus process, the sulfur recovery can reach up to 99% and 99.5%, respectively, after adding a selective oxidation step or a hydrogenation reactor followed by a selective oxidation reactor on the basis of Claus. For the purpose of meeting the atmospheric emission standard, acidic gas in the outlet of reactor should be combusted at 1,200°C to transform the remaining H2S into SO2. 

In the Selective oxidation reactor a small amount of air (typically around 2%) is added to the fuel stream, which then passes over a precious metal catalyst. This catalyst preferentially absorbs the carbon monoxide, rather than the hydrogen, where it reacts with the oxygen in the air. In addition to the obvious problem of cost, these units need to be very carefully controlled. There is the presence of hydrogen, carbon monoxide, and oxygen, at an elevated temperature, with a noble metal catalyst. Measures must be taken to ensure that an explosive mixture is not produced. This is a special problem in cases where the flow rate of the gas is highly variable, such as with a PEMFC on a vehicle. 

Pu (86 years) is formed from Np. Pu is separated by selective oxidation and solvent extraction. The metal is formed by reduction of PuF with calcium there are six crystal forms. Pu is used in nuclear weapons and reactors Pu is used as a nuclear power source (e.g. in space exploration). The ionizing radiation of plutonium can be a health hazard if the material is inhaled. 

High purity acetaldehyde is desirable for oxidation. The aldehyde is diluted with solvent to moderate oxidation and to permit safer operation. In the hquid take-off process, acetaldehyde is maintained at 30—40 wt % and when a vapor product is taken, no more than 6 wt % aldehyde is in the reactor solvent. A considerable recycle stream is returned to the oxidation reactor to increase selectivity. Recycle air, chiefly nitrogen, is added to the air introducted to the reactor at 4000—4500 times the reactor volume per hour. The customary catalyst is a mixture of three parts copper acetate to one part cobalt acetate by weight. Either salt alone is less effective than the mixture. Copper acetate may be as high as 2 wt % in the reaction solvent, but cobalt acetate ought not rise above 0.5 wt %. The reaction is carried out at 45—60°C under 100—300 kPa (15—44 psi). The reaction solvent is far above the boiling point of acetaldehyde, but the reaction is so fast that Httle escapes unoxidized. This temperature helps oxygen absorption, reduces acetaldehyde losses, and inhibits anhydride hydrolysis. 

Ethylbenzene Hydroperoxide Process. Figure 4 shows the process flow sheet for production of propylene oxide and styrene via the use of ethylbenzene hydroperoxide (EBHP). Liquid-phase oxidation of ethylbenzene with air or oxygen occurs at 206—275 kPa (30—40 psia) and 140—150°C, and 2—2.5 h are required for a 10—15% conversion to the hydroperoxide. Recycle of an inert gas, such as nitrogen, is used to control reactor temperature. Impurities ia the ethylbenzene, such as water, are controlled to minimize decomposition of the hydroperoxide product and are sometimes added to enhance product formation. Selectivity to by-products include 8—10% acetophenone, 5—7% 1-phenylethanol, and <1% organic acids. EBHP is concentrated to 30—35% by distillation. The overhead ethylbenzene is recycled back to the oxidation reactor (170—172). 

A derivative of the Claus process is the Recycle Selectox process, developed by Parsons and Unocal and Hcensed through UOP. Once-Thm Selectox is suitable for very lean acid gas streams (1—5 mol % hydrogen sulfide), which cannot be effectively processed in a Claus unit. As shown in Figure 9, the process is similar to a standard Claus plant, except that the thermal combustor and waste heat boiler have been replaced with a catalytic reactor. The Selectox catalyst promotes the selective oxidation of hydrogen sulfide to sulfur dioxide, ie, hydrocarbons in the feed are not oxidized. These plants typically employ two Claus catalytic stages downstream of the Selectox reactor, to achieve an overall sulfur recovery of 90—95%. 

In the second section, unconverted hydrogen sulfide reacts with the produced sulfur dioxide over a bauxite catalyst in the Claus reactor. Normally more than one reactor is available. In the Super-Claus process (Figure 4-3), three reactors are used. The last reactor contains a selective oxidation catalyst of high efficiency. The reaction is slightly exothermic ... 

Figure 4-3. The Super Claus process for producing sulfur (1) main burner, (2,4, 6,8) condensers, (3,5) Claus reactors, (7) reactor with selective oxidation catalyst.

So far, consideration has been limited to chemistry physical constraints such as heat transfer may also dictate the way in which reactions are performed. Oxidation reactions are highly exothermic and effectively there are only two types of reactor in which selective oxidation can be achieved on a practical scale multitubular fixed bed reactors with fused salt cooling on the outside of the tubes and fluid bed reactors. Each has its own characteristics and constraints. Multitubular reactors have an effective upper size limit and if a plant is required which is too large to allow the use of a single reactor, two reactors must be used in parallel. 

Ethylene is selectively oxidized to ethylene oxide using a silver-based catalyst in a fixed-bed reactor. Ethylene and oxygen are supplied from the gas phase and ethylene oxide is removed by it. The catalyst is stationary. Undesired, kinetically determined by-products include carbon monoxide and water. Ideally, a pure reactant is converted to one product with no by-products. 

Selective oxidation of p-xylene was carried out over the temperature range of 450-590°C at an atmospheric pressure using an 8-channel parallel tubular reactor system made in-... 

Evaluation of acetaldehyde oxidation reactor especially by determining reaction conversion and selectivity. 

Catalytic activity for the selective oxidation of H2S was tested by a continuous flow reaction in a fixed-bed quartz tube reactor with 0.5 inch inside diameter. Gaseous H2S, O2, H2, CO, CO2 and N2 were used without further purification. Water vapor (H2O) was introduced by passing N2 through a saturator. Reaction test was conducted at a pressure of 101 kPa and in the temperature range of 150 to 300 °C on a 0.6 gram catalyst sample. Gas flow rates were controlled by a mass flow controller (Brooks, 5850 TR) and the gas compositions were analyzed by an on-line gas chromotograph equipped with a chromosil 310 coliunn and a thermal conductivity detector. 

A growing number of research groups are active in the field. The activity of reforming catalysts has been improved and a number of test reactors for fuel partial oxidation, reforming, water-gas shift, and selective oxidation reactions were described however, hardly any commercial micro-channel reformers have been reported. Obviously, the developments are still inhibited by a multitude of technical problems, before coming to commercialization. Concerning reformer developments with small-scale, but not micro-channel-based reformers, the first companies have been formed in the meantime (see, e.g., ) and reformers of large capacity for non-stationary household applications are on the market. 

There is not much to be said about the use of micro reactors for bulk chemicals and commodities. Worz et al. are so far the only ones who have disclosed their work on the potential of micro-structured reactors for the optimization of chemical processes performed on a large scale ofindustrial relevance [110,112,154,288-290]. This included a fast exothermic liquid/liquid two-phase reaction, which was used for the industrial production of a vitamin intermediate product, and a selective oxidation reaction for an intermediate, a substituted formaldehyde derivative. 

Kah, S., Honicke, D., Selective oxidation of 1-butene to maleic anhydride -comparison of the performance between microchannel reactors and fixed bed reactor, in Matlosz, M., Ehreeld, W., Baselt, J. P. (Eds.), Microreaction Technology - IMRET 5 Proc. of the 5th International Conference on Microreaction Technology, pp. 397-407, Springer-Verlag, Berlin (2001). 

The effect of Bi promotion for the selective oxidation of 1-octanol using H202 as oxidant is reported in Table 2. Since decomposition of H202 by Platinum Group Metals is rapid, H202 is fed continuously into the reactor over 2 hours. The results obtained demonstrate that the presence of Bi203 as an additive within the reaction mixture displays no significant influence on catalyst activity. However, Bi promoted Pt/C catalysts, prepared by co-precipitation of... 

Patience, G. S., and Mills, P. L., Modelling of Propylene Oxidation in a Circulating Fluidized-Bed Reactor, New Developments in Selective Oxidation II, p. 1 (1994)... 

Initial tests using the pulse reactor described in this paper have been done on the selective oxidation of methanol to formaldehyde using molybdate catalysts. 

In an experiment (Williams, 1996) to evaluate a catalyst for the selective oxidation of propene (C3H6) to various products, 1 g of catalyst was placed in a plug-flow reactor operated at 450°C and 1 bar. The feed consisted of propene and air (21 mole % 02,79% N2 (inert)). GC analysis of the inlet and outlet gas gave the following results, the outlet being on a water-free basis (H20 is formed in the oxidation) ... 

---