Space velocity, definition

Soil-bearing pressures, 99-100 Soldering, 448 Solid-waste disposal, 88-90 Space velocity, definition of 728 Specific gravities of liquids, 882 of solids, 883... 

Vocabulary of Terms Used in Reactor Design. There are several terms that will be used extensively throughout the remainder of this text that deserve definition or comment. The concepts involved include steady-state and transient operation, heterogeneous and homogeneous reaction systems, adiabatic and isothermal operation, mean residence time, contacting and holding time, and space time and space velocity. Each of these concepts will be discussed in turn. 

Like the definition of the space time, the definition of the space velocity involves the volumetric flow rate of the reactant stream measured at some reference condition. A space velocity of 10 hr-1 implies that every hour, 10 reactor volumes of feed can be processed. 

Experimental conditions were 727, 756, and 794 K isothermal reactor temperature 827-, 1220-, and 2619-kPa hydrogen pressure 138- and 345-kPa hydrocarbon pressure and 1 to 26 liquid hourly space velocity. (See Section II for definition.) Charge stocks consisted of three C6 component blends (blends included 53/19/23/5, 25/75/0/0, and 0/0/50/50 wt. % hexane/methylcyclopentane/cyclohexane/benzene), C6 to C7 component naphthas (322-366 K TBP Kirkuk, Mid-Continent, and Nigerian), a C6 to C8 component naphtha (322-416 K TBP Mid-Continent), and C6 to C12 component naphthas (322-461 K TBP Arab Light, Mid-Continent, and... 

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 situation is more complicated if expansion or contraction of a volume element does occur and the volumetric flowrate is not constant throughout the reactor. The ratio VJv, where v is the volume flow into the reactor, no longer gives the true residence time or contact time. However, the ratio VJv may still be quoted but is called the space time and its reciprocal v/V, the space velocity. The space velocity is not in fact a velocity at all it has dimensions of (time) 1 and is therefore really a reactor volume displacement frequency. When a space velocity is quoted in the literature, its definition needs to be examined carefully sometimes a ratio Vi/ V, is used, where V/ is a liquid volume rate of flow of a reactant which is metered as a liquid but subsequently vaporised before feeding to the reactor. 

According to this definition, the relative activities of two catalysts can be obtained without knowing function /, but they may be readily compared by fixing the temperature and varying the Weight Hourly Space Velocity (WHSV), to obtain a chosen degree of conversion [54]. This can be done with the MFBR system [34, 49], where space velocity can be varied individually for each reactor across the 48 library members. 

Conversion efficiency is definitely affected by the large void fraction, which is apparent in the results from changes in the total throughput, or space velocity (0.56 versus 1.11 sec ), shown in Fig. 7. In this comparison, the concentration of unconverted hexane increased tenfold when the flow rate was doubled. The impact of improvements in conductive heat transfer, combined with the mass transfer limitations associated with the cell size and honeycomb design, and a catalyst loading that was nearly one-half Chat of commercial pellet catalysts (average, 11.5% versus 19.2%) suggested that both carbon formation and steam/hydrocarbon reactions were better controlled with monolithic supports under the conditions employed. This comparison was made where the extent of the endothermic reaction is equal between the pellet bed and the hybrid cordierite/metal monolith bed. 

In order to account for variable particle numbers, we generalize the collision term iSi to include changes in IVp due to nucleation, aggregation, and breakage. These processes will also require models in order to close Eq. (4.39). This equation can be compared with Eq. (2.16) on page 37, and it can be observed that they have the same general form. However, it is now clear that the GPBE cannot be solved until mesoscale closures are provided for the conditional phase-space velocities Afp)i, (Ap)i, (Gp)i, source term 5i. Note that we have dropped the superscript on the conditional phase-space velocities in Eq. (4.39). Formally, this implies that the definition of (for example) [Pg.113]

The term "space velocity" has established itself as a measure for the exhaust gas flow referred to the catalyst volume. By this definition it is an indication for the residence time of exhaust gas molecules within the catalyst. As mentioned before the inverse residence time plotted versus the selectivity of the reactants gives a tool for the identification of reaction paths. Therefore, the conversion efficiency over the catalyst was measured at 225°C by varying the space velocity (test 5, Table From these data the selectivities S(N2), S(N02) and S(N20) were calculated by using the equations given in chapter 2 and plotted in Fieure 7. 

On other occasions the volume of catalyst instead of the mass of catalyst may be used in the denominators of equations (8.0.7) and (8.0.8). The units associated with a particular space velocity indicate the definition employed. 

Methane conversion versus the gaseous inlet hourly space velocity is shown in Fig. 10.8. Conversion efficiency is definitely affected by the space velocity (s.v.). Converted methane increases with s.v. as result of a better conductive heat and mass transfer up to a peak at around 5,000 h . Higher s.v. reduce the methane conversion from 49.5 to 46.5%. 

One possible method of comparing the activity of different catalysts is to provide the flow rate under which complete and stable conversion is achieved over the catalysts. When this flow rate is related to the catalyst volume or mass, it is termed space velocity. Numerous definitions may be applied to define space velocity... 

The Gas-Hourly Space Velocity (G H S V) is the simplest definition, and is frequently provided for commercial systems, because catalysts are usually used as fixed beds. 

It is easy to use this definition for large scale fixed beds, because the porosity of the catalyst and of the catalyst bed are easy to measure and in most instances the reactor housing contributes to the overall volume only to a minor extent. However, the definition becomes doubtful when the catalyst is coated onto a monolith or foam. Here the question arises as to which volume is then being referred to the volume of the catalyst coating itself, of the monohth void fraction (channels) or of the entire monolith All these questions need to be clarified before a fair comparison of catalytic activity is feasible when gas hourly space velocity is applied for the calculations. 

The best definition is the Weight Hourly Space Velocity (WHSV), which is defined as the ratio of the volume flow of feed calculated back to normalised conditions to the weight of the catalyst ... 

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. 

The first requirement is the definition of a low-dimensional space of reaction coordinates that still captures the essential dynamics of the processes we consider. Motions in the perpendicular null space should have irrelevant detail and equilibrate fast, preferably on a time scale that is separated from the time scale of the essential motions. Motions in the two spaces are separated much like is done in the Born-Oppenheimer approximation. The average influence of the fast motions on the essential degrees of freedom must be taken into account this concerns (i) correlations with positions expressed in a potential of mean force, (ii) correlations with velocities expressed in frictional terms, and iit) an uncorrelated remainder that can be modeled by stochastic terms. Of course, this scheme is the general idea behind the well-known Langevin and Brownian dynamics. 

A very fine space resolution is required to measure the gradient of turbulent velocity fluctuations and calculate turbulent dissipation directly from the definition [5, 6]. 

Periodicity in space means that it repeats at regular intervals, known as the wavelength, A. Periodicity in time means that it moves past a fixed point at a steady rate characterised by the period r, which counts the crests passing per unit time. By definition, the velocity v = A/r. It is custom to use the reciprocals of wavelength 1/X — (k/2-ir) or 9, known as the wavenumber (k = wave vector) and 1/t — v, the frequency, or angular frequency u = 2itv. Since a sine or cosine (harmonic) wave repeats at intervals of 2n, it can be described in terms of the function... 

To obtain a more compact expession for the Cartesian drift velocity, it is useful to generalize the underlying diffusion equation in the /-dimensional constraint surface to a diffusion equation in the unconstrained 3N dimensional space. To define a mobility tensor throughout the unconstrained space, we adopt Eq. (2.133) as the definition of the constrained Cartesian mobility everywhere. To allow Eqs. (2.133) and (2.134) to be evaluated away from the constraint surface, we must also define n = 0c /0R everywhere, and specify definitions of the... 

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