Yield space-time

When the space-time yield is referred to the total reactor volume (and not only to the micro-channel volume), the large share of inactive construction material has to be taken into account. Consequently, the space-time yields per micro-channel volume have to differ by orders of magnitude, e.g. more than a factor of 1000, from 

By an industrial investigation of a gas-phase reaction, the chlorination of alkanes, thermal management (faster temperature ramping, avoidance of overshoots) was improved and, hence, control over radical formation was exerted. As a result, a significant increase in space-time yield to about 430 g h 1 was achieved using a hybrid micro-reactor plant compared with the conventional performance of 240 g h [127, 161]. 

Even for the well-known ethylene oxide formation, improvements in space-time yield were reported. A value of 0.78 t/(h m ) using an oxidative modified silver was obtained, which exceeds considerably the industrial performance of 0.13-0.261 h m [159]. 

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]. 

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By-products include propylene dibromide, bis-(bromopropyl) ether, propylene glycol, and propionic acid. Bromide losses are to the brominated organics and bromate formation. Current efficiency is a function of ceU design and losses to bromate. Energy consumption decreases with an increase in electrolyte concentration and a decrease in current density. Space—time yield increases with current density. See Table 5 for performance data (see... 

Reactor Current efficiency, % Current density, A/m Space—time yield, kmol/(h-m") Eneigy, kWh /kg Reference... 

Most hydrogenations can be achieved satisfactorily near ambient temperature, but in industrial practice the temperature is usually elevated to obtain more economical use of the catalyst and increase the space-time yield of the equipment. Tn laboratory work, a convenient procedure is to begin at ambient temperature, if reasonable, and raise the temperature gradually within bounds, should the reaction fail to go or if it is proceeding too slowly. 

Determination of the actual cost of a hydrogenation process is difficult. Among the factors entering into the determination are catalyst cost, catalyst life, cost of materials, capital investment, actual yield, space-time yield, and purification costs, Considerable data are needed to make an accurate evaluation. 

Space time yield refers to the quantity of product that can be produced in a reactor in a given time. It is a function of both selectivity and activity. Maximum efficiency is reached when this number is high, but if production schedules are not full, lower numbers may be tolerated. Acceptable catalyst life can be extended if space-time yield demands are not heavy. Catalyst cost thus becomes a function of the demands put upon it. 

Platinum and rhodium sulfided catalysts are very effective for reductive alkylation. They are more resistant to poisoning than are nonsulfided catalysts, have little tendency to reduce the carbonyl to an alcohol, and are effective for avoidance of dehydrohalogenation in reductive alkylation of chloronitroaromatics and chloroanilines (14,15). Sulfided catalysts are very much less active than nonsulfided and require, for economical use, elevated temperatures and pressures (300-2(KX) psig, 50-l80 C). Most industrial reductive alkylations, regardless of catalyst, are used at elevated temperatures and pressures to maximize space-time yields and for most economical use of catalysts. 

Biocatalysts in nature tend to be optimized to perform best in aqueous environments, at neutral pH, temperatures below 40 °C, and at low osmotic pressure. These conditions are sometimes in conflict with the need of the chemist or process engineer to optimize a reaction with respect to space-time yield or high product concentration in order to facilitate downstream processing. Furthermore, enzymes and whole cells are often inhibited by products or substrates. This might be overcome by the use of continuously operated stirred tank reactors, fed-batch reactors, or reactors with in situ product removal [14, 15]. The addition of organic solvents to increase the solubility of substrates and/or products is a common practice [16]. 

The catalytic single-step Alfen process has a good space-time yield, and the process engineering is simple. The molecular weight distribution of the olefins of the single-step process is broader (Schulz-Flory type of distribution) than in the two-step Alfen process (Poisson-type distribution) (Fig. 2). As a byproduct 2-alkyl-branched a-olefins also are formed, as shown in Table 6. About... 

Heat transfer problems become more severe as reaction rates are increased and water-to-monomer ratios are reduced. In addition, as reactor sizes are increased for improved process economics, the amount of wall heat transfer surface area per unit volume will drop and result in a lower reactor space-time yield. 

Since a CSTR operates at or close to uniform conditions of temperature and composition, its kinetic and product parameters can usually be predicted more accurately and controlled with greater ease. The CSTR can often be operated at a selected conversion level to optimize space-time yield, or where a particular product parameter is especially favored. 

As an example for continuous process design, 2-keto-3-deoxy-D lycero-D-galacto-nonosouate (KDN) (S) has been produced on a 100-g scale from D-mannose and pyruvate using a pilot-scale EMR at a space-time yield of 375 gl d and an overall crystallized yield of 75% (Figure 10.6) [47]. Similarly, L-KDO (6) can be synthesized from L-arabinose [48]. 

In this way, the operational range of the Kolbe-Schmitt synthesis using resorcinol with water as solvent to give 2,4-dihydroxy benzoic acid was extended by about 120°C to 220°C, as compared to a standard batch protocol under reflux conditions (100°C) [18], The yields were at best close to 40% (160°C 40 bar 500 ml h 56 s) at full conversion, which approaches good practice in a laboratory-scale flask. Compared to the latter, the 120°C-higher microreactor operation results in a 130-fold decrease in reaction time and a 440-fold increase in space-time yield. The use of still higher temperatures, however, is limited by the increasing decarboxylation of the product, which was monitored at various residence times (t). 

The transformation from batch to continuous processing, the safe operation with bromine at temperatures over 170°C and the decrease of reaction time, i.e. increase of space-time yields, were drivers for the development here. 

Each catalyst was evaluated in the fixed bed reactor their relative performance via the metrics of VAM STY (Space-Time-Yield) and VAM SEL as derived from eqs. 1 and 2. 

Space time yields of products formed over H-mordenite and HZSM-5 from a methanol/isobutanol = 1/1 reactant mixture (1.72 mol/kg catal/hr of each) at 0.1 MPa. 

Catalyst Temp. Space Time Yields (mol/kg catal/hr)... 

Special attention was also paid to the search for operation in the explosive regime as micro reactors are said to have much greater safety here. In this way, improvements in terms of space-time yield were expected. 

An increase from 2 to 5 bar total pressure increases the space-time yield by about 20% (15 vol.-% ethylene, 85 vol.-% oxygen, 2-20 bar 0.235-3.350 s 11 h ) [4], At higher pressures, 10 and 20 bar, a decrease activity is observed. Since industrial processes occur at up to 30 bar, at first sight this result is surprising. The decreasing activity with pressure was partially explained by catalyst deactivation, probably as a consequence of the longer residence times applied. 

GP 2] [R 2[ The addition of the promoter 1,2-dichloroethane improves selectivity from 52 to 69%, but at the expense of reducing the space-time yield from 0.78 to... 

GP 2] [R 2] The definition of space-time yield in a micro reactor depends on the definition of the reactor volume . Owing to the large amoimt of construction material relative to the reaction channels and the neglect of some reactor parts ( abstraction to the real reaction zone ), several more or less useful definitions can be made. In the following, two definitions concerning the time yield divided by the pure reaction channel volume and the platelet volume were used. 

Following the first definition, a space-time yield of 0.781 h m using a OAOR-modified silver is obtained, which exceeds the industrial performance considerably (0.13-0.26 t h m ) [4]. Following the second definition and hence orienting more on outer than on inner dimensions, a space-time yield of 0.13 t h m is obtained, still within the industrial window. 

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