Space-time yield defined

However, these investigations also point out that we need a proper definition of space-time yields for micro reactors. This refers to defining what essentially the reaction volume of a micro reactor is. Here, different definitions lead to varying values of the respective space-time yields. Following another definition of this parameter for ethylene oxide formation, a value of only 0.13 t h m is obtained -still within the industrial window [159, 162, 163]. 

The above-mentioned space-time yields were referred solely to the reaction volume, i.e. the micro channel volume. When defining this quantity via an idealized reactor geometry, taking into accoimt the construction material as well, natarally the difference in space-time yield of the micro reactors from the laboratory bubble column becomes smaller. Still, the performance of the micro reactors is more than one order of magnitude better [38], The space-time yields for the micro reactors defined in this way ranged from about 200 to 1100 mol monofluorinated product... 

The design optimization of an electrolytic cell aims at a high throughput with a low energy consumption at the lowest feasible cost. The throughput of an electrochemical reactor is measured in terms of the space time yield, Yt, defined as the volumetric quantity of the metal produced per unit time per unit volume of the process reactor. This quantity is expressed as ... 

Based on this approach, a useful estimation was provided recently [35]. With regard to the reaction aspect, achievable space-time yields (STY) of currently operated catalytic reactors were considered, whereupon Weisz defined the following window of reality [36] ... 

The percentage of halogenated organics removal (PR, %), space-time yield (STY, kgm-3 h-1), current efficiency (CE, %) and energy consumption (ECN, kWh kg-1) are used to evaluate the HDH process performance. They are defined as ... 

Various catalysts are used in different commodity epoxidation processes at diverse conditions. Important characteristics are the space time yield based on the weight of epoxide formed per kilogram of catalyst, and the TOF as a measure of the intrinsic activity per active surface metal atom (Table 1.4). In Table 1.4 and other tables in this chapter, conversion is always based on the reactant olefin and not on the oxidant, unless specified. Selectivity refers to the epoxide product, and yield to the product of conversion and selectivity, unless defined otherwise. 

The essential task of the electrochemical engineer deals with the process optimization, that is the idea of defining the best economics in terms of compromises among the competing factors such as space-time yield energy consumption, product quality and materials of construction. 

High activity is reflected in either high space-time yield from comparatively small catalyst volumes or mild operating conditions that enhance selectivity and stability. Catalyst activity defined as -Ra)v ot Rb)v depends on pressure, temperature, and reactant concentrations. 

There are some major differences between electrochemical engineering and classical electrochemistry. In conventional electrochemistry the mechanism of the electrode process and its kinetics are often the factors of major concern whereas in electrochemical engineering the actual mechanistic details of the process are usually less important than its specificity or process efficiency. The rate of the process defined either as current efficiency or as a general measure of reactor efficiency, the space-time yield are the main performance criteria. This latter factor determines whether a process is economically or commercially viable since it can be used to compare performance of different electrode designs as well as comparing an electrochemical process with the space-time yields for alternate non-electrochemical technologies. 

Several definitions are often used to define the performance of an electrode or electrochemical reactor. These are the current efficiency, process efficiency and electrode/cell space-time yield. In some cases, particularly with high surface electrodes an... 

The most Important engineering parameter Is the space-time yield parameter which defines the electrode/cell performance. The Importance of this term Is that It can be normalized thus allowing direct comparison both between different reactor designs and between electrochemical and non-electrochemical processes. The space-time yield term Is defined for an electrode as ... 

The overall rate of chemical change at an electrode process depends on the current density, j (the cell current, /, divided by the electrode area. A) for the desired reaction and the selectivity of the chemical change. The latter is usually discussed in terms of the current efficiency, , for the desired chemical change and is defined as the fraction of the charge passed used in the desired reaction. The space-time yield, Tst for n electrolytic reactor may be written... 

Economic evaluation - one main challenge is to bring MES out of the laboratory to technical application. At the early stage of the development, a rough calculation should at least include the assessment of needed productivities at the given fixed and variable costs (e.g., for reactor, electrodes, membranes, reaction medium, pretreatment of the gas). The volumetric productivities (space-time yields), final product concentrations, and total process times determine the overall process performance. These parameters should be used to define operational windows for the production of bulk chemicals. Furthermore, this theoretical approach allows the identification of limiting process parameters. 

Reactor performance can be measured by a number of parameters. For electrolytic cells two important parameters are current efficiency C.E. and space-time yield Ygj.. (Current efficiency or current yield has been defined in Section 1.4.2.)... 

The space-time yield of a reactor is defined as the mass of product produced by unit reactor volume in unit time. For an electrochemical reactor, it is ... 

The productivity of a reactor is frequently represented by the space-time yield (STY), which is defined as the amount of product i produced per unit time and unit volume (in mol m ... 

Remark the space-time yield (STY) can also be defined based on the mass of the product, for example, as the mass of product that is produced per unit time and volume of catalyst. In any event it is important to mention the exact definition, for example, whether the volume of the catalytic fixed bed or the volume of the catalyst without the void space is used. 

It will be clear from the above that the space-velocity (equation (2.85)) and the space-time yield (equations(2.105) and (2.Ml)) are dependent upon the degree of conversion and upon the initial (batch reactor) or inlet (plug flow reactor) oramtrattolu. In order to compare the perfomuiaoe of reactors involving different (in> values, it is therefore useful to define mmiglbed s and... 

This gives a uniform description for the models <11> to <13>. The space-time-yield (specific absorption rate) can be calculated from the expressions given in Table 2 whereby the appropriate equations for Q are found from eq. (16) (model <11>), eq. (25) (model <12>), and eq. (13) (model <13>). The entire treatment can be extended to catalytic slurry reactors (BCSR) by considering the liquid/solid mass transfer resistance and pore diffusional effects. In this case the value of Q in the absorption rate expressions of Table 2 has to be replaced by Q which is defined by... 

Taking the indefinite time integral of k yields w = —771 — 772. Continuing, the phase space metric is defined, and the two conservation laws are... 

The main attraction of using the integral approach in conventional studies is that it avoids the need to measure rates of reaction. Instead, the output conversions from several isothermal runs at different space times are plugged into the integrated rate expression and the rate parameters are optimized as simple constants for a given temperature. Since there usually are few parameters in a rate expression, a few runs will suffice to define all the constants at one temperature certainly, fewer runs than would be necessary to make valid estimates of rates from a plot of X vs. x. Repeating this procedure at several temperatures yields a set of constants suitable for plotting on an Arrhenius plot. This procedure minimizes the number of isothermal runs necessary to obtain the rate parameters. 

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