Catalyst contact time defined

Since the best results were obtained with the W and W-based oxide catalysts, the reaction was studied in more detail using 20 g portions of these catalysts. The reaction was performed at 230°C, with feed rates of pyruvic acid, air, and water = 10.5, 350, and 480 mmol/h. The contact time defined as volume of catalyst (ml)/rate of gaseous feed (ml/s) was about 5.2 s. The main products were citraconic anhydride and CO2. The amount of acetic acid was very small. No other products were detected except for very small amounts of CO, acetone, and acetaldehyde. A relatively large discrepancy was observed between the amount of consumed pyruvic acid and that of the sum of produced citraconic anhydride and acetic acid. This discrepancy was defined as "loss". 

This complex system would be difficult to solve directly. However, the problem is separable by taking advantage of the widely different time scales of conversion and deactivation. For example, typical catalyst contact times for the conversion processes are on the order of seconds, whereas the time on stream for deactivation is on the order of days. [Note Catalyst contact time is defined as the volume of catalyst divided by the total volumetric flow in the reactor at unit conditions, PV/FRT. Catalyst volume here includes the voids and is defined as WJpp — e)]. Therefore, in the scale of catalyst contact time, a is constant and Eq. (1) becomes an ordinary differential equation ... 

Another measure of the efficiency of ammonia conversion is the space velocity which may be used. Space velocity refers to the volume of reactants fed to a reactor per hour, divided by the volume of the reactor. For liquid reaction streams this relationship is straightforward. For gases, however, the space velocity is defined as being the volume of gases corrected to 0°C and 760 mm Hg (1 atm) passing through the reactor (or catalyst) volume/hour. This amounts to a measure of the gas-catalyst contact time for heterogeneous reaction (Eq. 11.7). 

A well-defined bed of particles does not exist in the fast-fluidization regime. Instead, the particles are distributed more or less uniformly throughout the reactor. The two-phase model does not apply. Typically, the cracking reactor is described with a pseudohomogeneous, axial dispersion model. The maximum contact time in such a reactor is quite limited because of the low catalyst densities and high gas velocities that prevail in a fast-fluidized or transport-line reactor. Thus, the reaction must be fast, or low conversions must be acceptable. Also, the catalyst must be quite robust to minimize particle attrition. 

This new technique incorporates a catalyzed short contact time (SCT) substrate into a shock tube. Fig. 13. These SCT reactors are currently used in industry for a variety of applications, including fuel cell reformers and chemical synthesis.The combination of a single pulse shock tube with the short contact time reactor enables the study of complex heterogeneous reactions over a catalyst for very well defined regimes in the absence of transport effects. These conditions initiate reaction in a real environment then abruptly terminate or freeze the reaction sequence. This enables detection of intermediate chemical species that give insight into the reaction mechanism occurring in the presence of the chosen catalyst. There is no limitation in terms of the catalyst formulations the technique can study. 

The yield and selectivity of a particular product were defined as mole percentage yield and selectivity on a carbon-accounted-for basis. As for the yield of "other acid", the yield was calculated basing on the asumption that the acid was acetic acid or maleic anhydride, because the main acids, besides benzoic acid, were found to be acetic acid and maleic anhydride. The contact time was defined as (volume of catalyst used [ml])/(total flow rate [ml/s]). 

Frequently, particularly from the viewpoint of the technological application of a heterogeneous catalytic reaction, the conditions of experiments in a flow reactor are characterized by space velocity or contact time values. Space velocity, V, is the ratio to the volume of the catalyst bed of the volume of a gas mixture, reduced to the normal conditions (0°C, 760 Torr), passed through the reactor per hour. If the reaction involves a volume change, inlet and outlet space velocities should be distinguished. The reciprocal of V is of the dimension of time. Contact time ( conventional contact time), rc, is a value proportional to V l. It is defined as the ratio of the catalyst volume to the volume of the gas mixture passed per unit time, the gas volume being not under normal conditions but at temperature and pressure in the reactor. Usually, tc is expressed in seconds. It follows from the definitions given that... 

The best per pass yield to C2 + C3 products (aldehydes plus acids with two and three C atoms) with the said catalyst was obtained at a propene conversion of 61.3% (selectivity to acrolein 83.7%), at the reaction temperature of 355 °C, with the following feed composition C3H6/H20/N2 11.6 10.0 78.4 (mol.%), with a gas contact time of 2.4 s. A decrease in solids circulation rate, while keeping gas residence time constant, led to a considerable decrease in propene conversion, while selectivity to C2 + C3 oxygenated products was not much affected by circulation rate. With a less concentrated feed, the amount of solid to be circulated for a defined olefin conversion is lower, but productivity also becomes lower. Other catalysts based on Bi/Mo/O or on V/Mo/W/Cu/O [72c] afforded conversions >70% and selectivity >90% industrial... 

It is common knowledge that the rate of epoxidation by alkylperoxy radicals is defined by their structure. The simplest representative and the parent radical of this class is the HO radical. In the case of propylene epoxidation with hydrogen peroxide in the gas phase at 500-580 °C and contact time r = 4.4 s, the epoxidating particle is, probably, the HO radical whose activity in the absence of catalysts is higher than the RO activity. 

In the following description, the contact time, r is defined by the relation, T — F/v (sec.), where F is the catalyst fraction void assumed to be... 

A kinetic study has been made of the liquid-phase metathesis of oct-l-ene over Re207/Al203 in a flow reactor Fig. 5.8 shows the fractional conversion X at four temperatures as a function of the contact time expressed in terms of W/F as defined in the caption. The data are best interpreted in terms of a model in which either product desorption or interconversion of the alkene/carbene complex is rate-determining. This model leads to eqn. (11), where r is the reaction rate per unit weight of catalyst, and from which the curves in Fig. 5.8 have been calculated to give the best fit to the data (Spronk 1992). 

However, rather than thinking of r as just a contact time, let us define it as / volume catalyst... 

Reaction Measurements. The NO decomposition activity of the catalysts was evaluated in a laboratory-scale reactor system described in detail elsewhere (7). An amount of 0.5 g of catalyst was placed in the reactor for NO conversion measurements. The contact time, W/F, defined as the ratio of catalyst weight in the reactor to the total flowrate of the feed gas stream, was 1.0 g s/cw (STP). A gas mixture of 2% NO, 0-5% 02,0-20% H2O and balance He was used in the tests. 

Figure 3.52 depicts the dependence of methanol conversion and product composition on contact time as contact time increases, (reciprocal of the catalyst load), the yield of gasoUne and aromatics increases. (In English terminology, the catalyst load is defined as (LHSV) liquid hourly space velocity ). 

The dispersion model with particle diffusion always assumes complete external contacting of particles by liquid which may not be the case in trickle flow. This means that the effective diffusional time constant is increased in trickle flow resulting in a reduced apparent effective diffusivity which is based on the total external surface area. Using this diffusivity in the expression for the Thiele modulus, and equating it to the modulus defined for trickle-bed operation by Dudukovic (130) results in the following estimate of the external catalyst contacting efficiency, UcE ... 

The size of the converter required for a given duty is determined primarily by the design space velocity for the specific reaction conditions. Space velocity is an indirect measure of the contact time between the gas and the catalyst. It is u.sually defined as the volume of gas at standard conditions pas.sing through a unit volume of catalyst per unit of time. However, space velocity is occasionally expressed in terms of actual flowing gas volume per volume of catalyst per unit of time, and it is extremely important that the applicable definition be known before a space velocity value is used for design. 

WHSV is defined as the ratio of mass flow rate of MeOH with mass of catalyst. It is inversely proportional to contact time. According to Ref. [66], the increase of contact time has the same effects on products distribution as the temperature does. Selectivity to light olefins increases... 

High catalyst to biomass ratios are necessary to ensure all of the primary pyrolysis vapors are adsorbed on the catalyst surface. Otherwise the catalyst in the reaction zone can become deactivated before aU the primary pyrolysis vapors are reformed by the catalyst, resulting in the produced bio-oU being a mixture of catalytic pyrolysis oil and noncatalytic pyrolysis oil. This can also be the case for short vapor residence times that prevent sufficient contact time for the reactions on the catalyst surface to take place. The catalyst to biomass ratio can be expressed as the weight hourly space velocity (WHSV, h ), which is defined as the ratio of the mass flow rate of feed (g/h) to the mass of catalyst in the reactor (g). This is one of the most important parameters in... 

The problem of the diolefin formation was first solved with the introduction of the DeFine process, in which diolefins are hydrogenated to monoolefins by a contact (H-14). This catalyst was developed in 1984 and used on a large scale for the first time in 1986. With its help the diolefins are practically quantitatively removed and the monoolefin yield increased by about 4-5% [58]. The advantages of the DeFine step are [58,92] ... 

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