Space velocity conversion

Feed flow impact on membrane performance is analyzed in the range 25—110% of the design flow. It is expected that an increase of feed flow will reduce the feed conversion. In Fig. 10.11 methane conversions with and without membrane are compared versus the space velocity after 70 h of operation with ECN membrane without steam sweeping. It appears quite clearly that by increasing the space velocity conversion of methane decreases. 

Find et al. [25] developed a nickel-based catalyst for methane steam reforming. As material for the microstructured plates, AluchromY steel, which is an FeCrAl alloy, was applied. This alloy forms a thin layer of alumina on its surface, which is less than 1 tm thick. This layer was used as an adhesion interface for the catalyst, a method which is also used in automotive exhaust systems based on metallic monoliths. Its formation was achieved by thermal treatment of microstructured plates for 4h at 1000 °C. The catalyst itself was based on a nickel spinel (NiAl204), which stabUizes the catalyst structure. The sol-gel technique was then used to coat the plates with the catalyst slurry. Good catalyst adhesion was proven by mechanical stress and thermal shock tests. Catalyst testing was performed in packed beds at a S/C ratio of 3 and reaction temperatures between 527 and 750 °C. The feed was composed of 12.5 vol.% methane and 37.5 vol.% steam balance argon. At a reaction temperature of 700°C and 32 h space velocity, conversion dose to the thermodynamic equilibrium could be achieved. During 96 h of operation the catalyst showed no detectable deactivation, which was not the case for a commercial nickel catalyst serving as a base for comparison. 

The MTDP process, which is similar to the Tatoray process, produces an equilibrium composition of xylene isomers. A -xylene yield of 24% in the xylene product is formed at 42—48 wt % toluene conversion over the heterogeneous catalyst at 390—495°C, 4.2 MPa (600 psig), 1 2 Hquid hourly space velocity, and 4 H2/hydrocarbon molar feed ratio. A new ZSM-5 catalyst, which has higher activity and stability than the current catalyst, has been reported (93). 

These design fundamentals result in the requirement that space velocity, effective space—time, fraction of bubble gas exchanged with the emulsion gas, bubble residence time, bed expansion relative to settled bed height, and length-to-diameter ratio be held constant. Effective space—time, the product of bubble residence time and fraction of bubble gas exchanged, accounts for the reduction in gas residence time because of the rapid ascent of bubbles, and thereby for the lower conversions compared with a fixed bed with equal gas flow rates and catalyst weights. 

The quantity of catalyst used for a given plant capacity is related to the Hquid hourly space velocity (LHSV), ie, the volume of Hquid hydrocarbon feed per hour per volume of catalyst. To determine the optimal LHSV for a given design, several factors are considered ethylene conversion, styrene selectivity, temperature, pressure, pressure drop, SHR, and catalyst life and cost. In most cases, the LHSV is ia the range of 0.4—0.5 h/L. It corresponds to a large quantity of catalyst, approximately 120 m or 120—160 t depending on the density of the catalyst, for a plant of 300,000 t/yr capacity. 

In recent years alkylations have been accompHshed with acidic zeoHte catalysts, most nobably ZSM-5. A ZSM-5 ethylbenzene process was commercialized joiatiy by Mobil Co. and Badger America ia 1976 (24). The vapor-phase reaction occurs at temperatures above 370°C over a fixed bed of catalyst at 1.4—2.8 MPa (200—400 psi) with high ethylene space velocities. A typical molar ethylene to benzene ratio is about 1—1.2. The conversion to ethylbenzene is quantitative. The principal advantages of zeoHte-based routes are easy recovery of products, elimination of corrosive or environmentally unacceptable by-products, high product yields and selectivities, and high process heat recovery (25,26). 

The methanation reaction is carried out over a catalyst at operating conditions of 503—723 K, 0.1—10 MPa (1—100 atm), and space velocities of 500—25,000 h . Although many catalysts are suitable for effecting the conversion of synthesis gas to methane, nickel-based catalysts are are used almost exclusively for industrial appHcations. Methanation is extremely exothermic (AT/ qq = —214.6 kJ or —51.3 kcal), and heat must be removed efficiently to minimise loss of catalyst activity from metal sintering or reactor plugging by nickel carbide formation. 

The catalytic conversion of gas oil is weU approximated by a second-order reaction where sv = space velocity (12). 

Gas Phase. The gas-phase methanol hydrochlorination process is used more in Europe and Japan than in the United States, though there is a considerable body of Hterature available. The process is typicaHy carried out as foHows vaporized methanol and hydrogen chloride, mixed in equimolar proportions, are preheated to 180—200°C. Reaction occurs on passage through a converter packed with 1.68—2.38 mm (8—12 mesh) alumina gel at ca 350°C. The product gas is cooled, water-scmbbed, and Hquefied. Conversions of over 95% of the methanol are commonly obtained. Garnma-alurnina has been used as a catalyst at 295—340°C to obtain 97.8% yields of methyl chloride (25). Other catalysts may be used, eg, cuprous or zinc chloride on active alumina, carbon, sHica, or pumice (26—30) sHica—aluminas (31,32) zeoHtes (33) attapulgus clay (34) or carbon (35,36). Space velocities of up to 300 h , with volumes of gas at STP per hour per volume catalyst space, are employed. 

The size of the catalyst bed depends mainly on the degree of VOC reduction requited (14). VOC destmction efficiencies up to 95% can usually be attained using reasonable space velocities (14). However, the low GHSVs, and subsequently high catalyst volumes requited to achieve extremely high (eg, 99%) conversions, can sometimes make catalytic oxidation uneconomical. Conventional bed geometries may be found in the Hterature (14). 

If using GHSV = 10,000 h" , the feed is, F = V GHSV = 20 (104) cm or 200 liters/h. Therefore, the 12.600 normal liters in the cylinder will be enough for 60 hours of operation. During these 60 hours, effects of temperature and conversion (by changing space velocity) can be studied at the one, given gas composition in the cylinder. 

The space velocity SV is often used with conversion to descrihe die overall reactor performance. It is common in die petroleum and petrochemical industries to plot conversion against space velocity to descrihe die effect of feed rate on die performance of a flow system. 

The reactor feed may be preheated and the feed pressure may alter. The volumetric flowrate of gases depends on the reactor temperature and pressure at fixed mass flowrate. In many cases, the feed is liquid at room temperature, while the reaction mixture is a gas at the higher temperature in the reactor. In these instances, the performance of the reactor is represented as conversion and selectivity against the liquid hourly space velocity (LHSV). This is defined as... 

For this reason, operation around atmospheric pressures is typical. Space velocity should he high to avoid the reaction of ammonia with oxygen on the reactor walls, which produces nitrogen and water, and results in lower conversions. The concentration of ammonia must he kept helow the inflammahility limit of the feed gas mixture to avoid explosion. Optimum nitric acid production was found to he obtained at approximately 900°C and atmospheric pressure. 

Figure 9-5. Effect of temperature and liquid hourly space velocity on conversion. ...

The first step in E-cat testing is to bum the carbon off the sample. The sample is then placed in a MAT unit (Figure 3-13), the heart of which is a fixed bed reactor. A certain amount of a standard gas oil feedstock is injected into the hot bed of catalyst. The activity i.s reported as the conversion to 430°F (221°C) material. The feedstock s quality, reactor temperature, catalyst-to-oil ratio, and space velocity are four variables affecting MAT results. Each catalyst vendor uses slightly different operating variables to conduct micro activity testing, as indicated in Table 3-2. 

Molybdenum and tungsten are unique in that they are resistant to sulfur, and, in fact, are commonly sulfided before use. The Bureau of Mines tested a variety of molybdenum catalysts (32). They are moderately active but relatively high temperatures are required in order to achieve good conversion, even at low space velocities. Selectivity to methane was 79-94%. Activity is considerably less than that of nickel. Although they are active with sulfur-bearing synthesis gas, the molybdenum and tungsten catalysts are not sufficiently advanced to be considered candidates for commercial use. 

The space velocity was varied from 2539 to 9130 scf/hr ft3 catalyst. Carbon monoxide and ethane were at equilibrium conversion at all space velocities however, some carbon dioxide breakthrough was noticed at the higher space velocities. A bed of activated carbon and zinc oxide at 149 °C reduced the sulfur content of the feed gas from about 2 ppm to less than 0.1 ppm in order to avoid catalyst deactivation by sulfur poisoning. Subsequent tests have indicated that the catalyst is equally effective for feed gases containing up to 1 mole % benzene and 0.5 ppm sulfur (5). These are the maximum concentrations of impurities that can be present in methanation section feed gases. 

In Fig. 28, the abscissa kt is the product of the reaction rate constant and the reactor residence time, which is proportional to the reciprocal of the space velocity. The parameter k co is the product of the CO inhibition parameter and inlet concentration. Since k is approximately 5 at 600°F these three curves represent c = 1, 2, and 4%. The conversion for a first-order kinetics is independent of the inlet concentration, but the conversion for the kinetics of Eq. (48) is highly dependent on inlet concentration. As the space velocity increases, kt decreases in a reciprocal manner and the conversion for a first-order reaction gradually declines. For the kinetics of Eq. (48), the conversion is 100% at low space velocities, and does not vary as the space velocity is increased until a threshold is reached with precipitous conversion decline. The conversion for the same kinetics in a stirred tank reactor is shown in Fig. 29. For the kinetics of Eq. (48), multiple solutions may be encountered when the inlet concentration is sufficiently high. Given two reactors of the same volume, and given the same kinetics and inlet concentrations, the conversions are compared in Fig. 30. The piston flow reactor has an advantage over the stirred tank... 

Catalyst system Temperature (K) Conditions Pressure (10s Nm-2) Weight hourly space velocity (hr-1) Conversion" (%) Selectivity6 (%) Ethene/butene molar ratio" Reference... 

Zabor et al. (Zl) have described studies of the catalytic hydration of propylene under such conditions (temperature 279°C, pressure 3675 psig) that both liquid and vapor phases are present in the packed catalyst bed. Conversions are reported for cocurrent upflow and cocurrent downflow, it being assumed in that paper that the former mode corresponds to bubble flow and the latter to trickle-flow conditions. Trickle flow resulted in the higher conversions, and conversion was influenced by changes in bed height (for unchanged space velocity), in contrast to the case for bubble-flow operation. The differences are assumed to be effects of mass transfer or liquid distribution. 

In Fig. 1, a comparison can be observed for the prediction by the honeycomb reactor model developed with the parameters directly obtained from the kinetic study over the packed-bed flow reactor [6] and from the extruded honeycomb reactor for the 10 and 100 CPSI honeycomb reactors. The model with both parameters well describes the performance of both reactors although the parameters estimated from the honeycomb reactor more closely predict the experiment data than the parameters estimated from the kinetic study over the packed-bed reactor. The model with the parameters from the packed-bed reactor predicts slightly higher conversion of NO and lower emission of NHj as the reaction temperature decreases. The discrepancy also varies with respect to the reactor space velocity. 

Bubble columns and mechanically stirred reactors are the most common reactor types for slurry systems in laboratories, but they have many disadvantages from an industrialization perspective. Mechanically stirred reactors usually used for laboratorial studies are difficult to scale-up. In order to achieve good mixing and mass transfer between the gas and slurry phases, bubble column must be operated at a high space velocity, which leads to a relative low one-through conversion of the syngas. 

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