Catalyst unit

Typical applications in the chemical field (Beaver, op. cit.) include detarring of manufactured gas, removal of acid mist and impurities in contact sulfuric acid plants, recovery of phosphoric acid mists, removal of dusts in gases from roasters, sintering machines, calciners, cement and lime Idlns, blast furnaces, carbon-black furnaces, regenerators on fluid-catalyst units, chemical-recoveiy furnaces in soda and sulfate pulp mills, and gypsum kettles. Figure 17-74 shows a vertical-flow steel-plate-type precipitator similar to a type used for catalyst-dust collection in certain fluid-catalyst plants. 

The vapors from a fluid catalyst unit carry a small amount of fine catalyst particles which might clog the narrow clearances of a conventional bubble cap plate. 

The laboratory prototype of the Dinex electrochemically promoted catalyst unit is shown in Figure 12.12 and the assembled unit schematically in Fig. 12.13. It consists (Fig. 12.14) of a tubular bundle porous (ceramic foam) structure made of CeOa-GcfeOj (CGO) which is an O2" conductor with ionic conductivity significantly higher than YSZ at temperatures below 500°C... 

The components of the Ziegler-Natta catalyst unite in such a way that there is a vacant co-ordination site on the transition metal to which a molecule of monomer can bond, a second molecule of monomer then attaches itself to another vacant coordination site, from which it is able to react with the first molecule of coordinated monomer. This causes the second coordination site to become vacant once again, thereby allowing a further molecule... 

Glycerol was to be ethoxylated at 115-125°C in a circulating reaction system with separate reactor, heat exchanger and catalyst units. The valve at the base of the reactor was still closed, but an inoperative flow indicator failed to indicate absence of circulation and a total of 3 tonnes of the oxide, plus glycerol, was charged to the reactor. Upon subsequent opening of the valve, the reaction mixture passed through the heater, now at 200° C, and a runaway reaction developed, the reactor burst and an explosion followed. 

For the cases where gas phase cyclohexenes do not appear to be intermediates, the question arises as to the nature of the surface reaction. Thus, does cyclohexane simultaneously lose six hydrogen atoms via the sextet mechanism (T3) originally proposed by Balandin in 1929, or does the reaction take place in a stepwise fashion without desorption of intermediate products According to the sextet theory, the active catalyst unit is an aggregate of metal atoms which must be spaced within certain definite limits consistent with the geometry of the cyclohexane ring. While there... 

Dickey, J. B, and H. W. Coover jr. Polymerization with alkyl phosphite catalysts. United States Patent 2,652,393 (to Eastman Kodak Co.), September 15, 1953. 

Impurities such as diolefins and mercaptans in the feed to the reactor affect adversely the quality of the alkylate produced. This is considered to be more critical in sulfuric acid than in HF catalyst units. Also, certain sulfuric unit designs seem to tolerate more normal butane than others. Whereas the isobutane content of the recycle to a hydrofluoric acid unit can operate at purities of around 75% without affecting product qualities appreciably, the isobutane content of the recycle stream to most sulfuric acid units should be 90 % or better. 

Ohmic heating of catalyst is often used as a simple method of igniting the chemical reaction during reactor startup, for instance, in the oxidation of ammonia on platinum-rhodium gauze catalysts. Another application is the prevention of cold-start emissions from automotive catalysts responsible for much of the residual pollution still produced from this source (21). The startup times needed for the catalyst to attain its operating temperature can be cut by a factor of 5 or more by installing an electrically heated catalyst element with a metallic support upstream of the main catalyst unit. Direct electrical catalyst heating permits facile temperature control but requires a well-defined catalyst structure to function effectively. 

A much simpler surface reaction model used in conjunction with a heat balance for the entire catalyst unit (wire, foil, or pellet), rather than for a surface layer, was discussed by Ray and Hastings (296). This paper was the first of a series of publications from this research group, culminating in the classic papers of Jensen and Ray (243,297). They analyzed the simplest catalytic reaction conceivable that having a single adsorption step and a single reaction step,... 

Figure 2-7 Dinwraot G (an organometaHic catalyst) unit (two CSTRs and one tubular reactor in series) to dimeiise propylene into isohexanes. Instisit Fran ais du Ptetrdle process. [Ptioto courtesy of Editions Technip Institut Franjais du PetnSie).]...

The main feature of monolithic catalysts is a high ratio of geometric surface area to volume and a low pressure drop with distnbution of gas flow through a large number of parallel channels. The most significant application for structured catalyst units of this type is in the control of exhaust emissions from cars this area is discussed here under five subheadings. Nonautomotive applications are discussed later, in Section IV.B. 

When substrates have been coated, the active catalyst unit needs to be incorporated into a canister, which physically protects the catalyst and also enables it to be fit into the exhaust tram Since the catalyst is designed to last the life of the car, the quality of the steel used is important, for holes in the exhaust system could lead to emissions escaping before being converted over the catalyst In addition, the bulk of catalysts are based on ceramic substrates and have a significantly different thermal expansion charactenstic to the extenor steel shell. It is usual to have a special interlayer between the shell and the catalyst that has the flexibility to allow for differential expansion as the temperature... 

Early fluid-catalyst units employed a bank of many small-diameter cyclones in parallel, but this practice was superseded by the use of a smaller number of large-diameter cyclones (97). These are typically 3 to 5 ft. in diameter and 10 to 15 ft. high, although rough-cut separators with diameters up to 8 ft. have been reported (272). Two stages of cyclones in series are ordinarily used in the reactor. In units without Cottrells, two stages are usually employed also in the regenerator. Three... 

Commercial production of synthetic silica-alumina catalysts for use in fluid cracking was initiated in 1942. The synthetic catalysts were first manufactured in ground form, but means were later developed for production in MS (micro-spheroidal) form. First shipments of the MS catalyst were made in 1946. The synthetic catalysts contain 10 to 25% alumina. Synthetic silica-magnesia catalyst has also been used commercially in fluid-catalyst units (19,100). Magnesia content is 25 to 35% as MgO (276). 

Although the cracking reactions in fixed-bed, moving-bed, and fluid-catalyst units are essentially the same, slight deviations in results may be expected because of the following differences ... 

The carbon is almost completely removed during regeneration in the Houdry fixed-bed process, but not in circulating-catalyst processes. Thus, the carbon content of catalyst leaving the regenerator has been reported to be less than 0.5% in TCC units (158,241), and in the range from 0.3 to 1.0% (usually 0.3 to 0.7%) in fluid-catalyst units (305). 

In this equation the Rate is the molar TOF of the reaction, moles of product formed/mole of metal catalyst/unit time. The terms in [ ] are the STO measured site densities given in moles of site/mole of metal. The specific site TOFs, A, B and C, have units of moles of product/mole of site/unit time. Of these factors, the site densities are available from an STO characterization of the catalyst and the Rate is determined for the specific reaction nm over the STO characterized catalyst. When a series of at least three STO characterized catalysts is used for the same reaction, run under the same conditions, the specific site TOFs can be calculated from the simultaneous equations expressed as in Eqn. 3.6. When this approach was used in the hydrogenation of cyclohexene over a series of seven Pt/CPG catalysts specific site TOF values for the Mr and MH sites were found to be 2.1, 18.2 and 5.2 moles of product/mole of site/second, respectively.21 Not surprisingly, that site with the weakly held hydrogen was the most active and that on which the hydrogen was strongly held was the least active. 

Last but not the least, criterion for good performance of a catalyst system is its crush strength. A refinery catalyst unit is liable to encounter upsets during its run length. A quality that a refiner looks for in the catalyst is its ability to sustain such upsets without any adverse impact on its performance. 

Catalyst Unit cell formula Si/Al (%) Exchange Acidity (10 hV) Pore Volume (cmlg ) Micro Meso ... 

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