Conversion and Selectivity

The fractional conversion of a given reactant, XA, is defined for a batch system as 

For a well-mixed flow system at steady state, the fractional conversion XA is the ratio of the number of moles of A converted to the moles A fed to the system 

This definition is identical to that of the batch case. Fractional yield is defined by 

Moles of A transformed into a given product C Total moles of A reacted 

Again it is important that both the particular reactant and product, concerned, should be stated, when defining a fractional yield. 

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As with the other surface reactions discussed above, the steps m a catalytic reaction (neglecting diffiision) are as follows the adsorption of reactant molecules or atoms to fomi bound surface species, the reaction of these surface species with gas phase species or other surface species and subsequent product desorption. The global reaction rate is governed by the slowest of these elementary steps, called the rate-detemiming or rate-limiting step. In many cases, it has been found that either the adsorption or desorption steps are rate detemiining. It is not surprising, then, that the surface stmcture of the catalyst, which is a variable that can influence adsorption and desorption rates, can sometimes affect the overall conversion and selectivity. 

Even ia 1960 a catalytic route was considered the answer to the pollution problem and the by-product sulfate, but nearly ten years elapsed before a process was developed that could be used commercially. Some of the eadier attempts iacluded hydrolysis of acrylonitrile on a sulfonic acid ion-exchange resia (69). Manganese dioxide showed some catalytic activity (70), and copper ions present ia two different valence states were described as catalyticaHy active (71), but copper metal by itself was not active. A variety of catalysts, such as Umshibara or I Jllmann copper and nickel, were used for the hydrolysis of aromatic nitriles, but aUphatic nitriles did not react usiag these catalysts (72). Beginning ia 1971 a series of patents were issued to The Dow Chemical Company (73) describiag the use of copper metal catalysis. Full-scale production was achieved the same year. A solution of acrylonitrile ia water was passed over a fixed bed of copper catalyst at 85°C, which produced a solution of acrylamide ia water with very high conversions and selectivities to acrylamide. 

Ratio and Multiplicative Feedforward Control. In many physical and chemical processes and portions thereof, it is important to maintain a desired ratio between certain input (independent) variables in order to control certain output (dependent) variables (1,3,6). For example, it is important to maintain the ratio of reactants in certain chemical reactors to control conversion and selectivity the ratio of energy input to material input in a distillation column to control separation the ratio of energy input to material flow in a process heater to control the outlet temperature the fuel—air ratio to ensure proper combustion in a furnace and the ratio of blending components in a blending process. Indeed, the value of maintaining the ratio of independent variables in order more easily to control an output variable occurs in virtually every class of unit operation. 

If the production of vinyl chloride could be reduced to a single step, such as dkect chlorine substitution for hydrogen in ethylene or oxychlorination/cracking of ethylene to vinyl chloride, a major improvement over the traditional balanced process would be realized. The Hterature is filled with a variety of catalysts and processes for single-step manufacture of vinyl chloride (136—138). None has been commercialized because of the high temperatures, corrosive environments, and insufficient reaction selectivities so far encountered. Substitution of lower cost ethane or methane for ethylene in the manufacture of vinyl chloride has also been investigated. The Lummus-Transcat process (139), for instance, proposes a molten oxychlorination catalyst at 450—500°C to react ethane with chlorine to make vinyl chloride dkecfly. However, ethane conversion and selectivity to vinyl chloride are too low (30% and less than 40%, respectively) to make this process competitive. Numerous other catalysts and processes have been patented as weU, but none has been commercialized owing to problems with temperature, corrosion, and/or product selectivity (140—144). Because of the potential payback, however, this is a very active area of research. 

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... 

A comparison of the operations discussed in Section III with regard to applications in a particular chemical process should be based, at least in part, on the analysis of a theoretical model of the type discussed in Section IV. At the present stage of development, only an approximate estimate of reaction conversion and selectivity will be obtained in this way, and the analysis must in most cases be supplemented with qualitative considerations. The analysis is necessary, however, if optimum choice of operation and optimum design of the chosen operation are to be achieved. 

There are two important types of ideal, continuous-flow reactors the piston flow reactor or PFR, and the continuous-flow stirred tank reactor or CSTR. They behave very diflerently with respect to conversion and selectivity. The piston flow reactor behaves exactly like a batch reactor. It is usually visualized as a long tube as illustrated in Figure 1.3. Suppose a small clump of material enters the reactor at time t = 0 and flows from the inlet to the outlet. We suppose that there is no mixing between this particular clump and other clumps that entered at different times. The clump stays together and ages and reacts as it flows down the tube. After it has been in the piston flow reactor for t seconds, the clump will have the same composition as if it had been in a batch reactor for t seconds. The composition of a batch reactor varies with time. The composition of a small clump flowing through a piston flow reactor varies with time in the same way. It also varies with position down the tube. The relationship between time and position is... 

The molecular weight distribution (MWD) is of vital importance for polymers of all types. It determines the ease of manufacture, the ease of fabrication, and the end-use properties of the polymer. A proper kinetic description of a polymerization requires determination of the molecular weight distribution of the polymer in addition to the usual concepts of conversion and selectivity. 

Trade et al. [56] used cationic bis(oxazoHne)-Cu(II) complexes, intercalated into lamellar clays by electrostatic interactions, as catalysts for C - C bond formation reactions. Interestingly, the heterogeneous catalysts led to higher conversions and selectivities than their homogeneous coimterparts. 

Steady performance data from the second reactor are shown in Figure 11.10, where the pressure drop did not rise exponentially and the conversion and selectivity remained at 75 and 83%, respectively. The reactor was further analyzed after operation, shown in Figure 11.11, to confirm the lack of carbon deposition. Reactor models were pivotal to developing a robust design for this high-temperature and... 

Gas holdup and liquid circulation velocity are the most important parameters to determinate the conversion and selectivity of airlift reactors. Most of the reported works are focused on the global hydrodynamic behavior, while studies on the measurements of local parameters are much more limited [20]. In recent years, studies on the hydrodynamic behavior in ALRs have focused on local behaviors [20-23], such as the gas holdup, bubble size and bubble rise velocity. These studies give us a much better understanding on ALRs. 

Conversion (C) of n-heptane, composition of products and selectivities of toluene and gas products at different temperatures are presented in Table 2 and Fig. 4. Clearly, the conversion of n-heptane and the selectivity of toluene increase with temperature, whereas the selectivity of gas products decreases. At the same temperature the conversion and selectivity of gas products on HYl are slightly lower than that on HYs, but the selectivity of toluene is higher. 

Evaluation of acetaldehyde oxidation reactor especially by determining reaction conversion and selectivity. 

Fig. 1. Change of R22 conversion and selectivity for TFE with TOS (solid line R22 conversion dot line selectivity for TFE).

Table 1. Effect of catalyst structure on EC conversion and selectivities to DMC and EG...

Fig. 3. CO conversion and selectivity with respect to reaction temperature in a microchannel reactor...

A microchannel reactor for CO preferential oxidation was developed. The reactor was consisted of microchannel patterned stainless steel plates which were coated by R11/AI2O3 catalyst. The reactor completely removed 1% CO contained in the Ha-rich reformed gas and controlled CO outlet concentration less than Ippm at 130 200°C and 50,000h. However, CH4 was produced from 180"C and CO selectivity was about 50%. For high performance of present PrOx reactor, reaction temperature should be carefully and uniformly controlled to reach high CO conversion and selectivity, and low CH4 production. It seems that the present microchaimel reactor is promising as a CO removal reactor for PEMFC systems. 

Fig.2. Effect of temperature on the conversion and selectivity in a CSTR at SOOpsig.

As illustrated in Fig. 1, the activated carbon displays the highest conversion and selectivity among all the catalysts during the initial reaction period, however, its catalytic activity continues to decrease during the reaction, which is probably caused by coke deposition in the micropores. By contrast, the reaction over the CNF composites treated in air and HN03 can reach a pseudo-steady state after about 200 min. Similiar transient state is also observed on the CNFs and the untreated composite. Table 3 collects the kinetic results after 300 min on stream over catalysts tested for the ODE, in which the activity is referred to the BET surface area. The air-treated composite gives the highest conversion and styrene selectivity at steady state. 

The addition of acetic acid (0.5 equiv. to the substrate) to the catalyst system led to increased activity (doubling of yield) by maintaining the selectivity with 1.2 equiv. H2O2 as terminal oxidant. Advantageously, the system is characterized by a certain tolerance towards functional groups such as amides, esters, ethers, and carbonates. An improvement in conversions and selectivities by a slow addition protocol was shown recently [102]. For the first time, a nonheme iron catalyst system is able to oxidize tertiary C-H bonds in a synthetic applicable and selective manner and therefore should allow for synthetic applications [103]. 

Subsequently the intercalates - without prior ejqposure to air - were reduced either by hydrogen gas or using potassium naphthalide in THF. (3U) Clearly the choice of intercalation and reduction temperature will control the nature of the final product. These materials were then e qposed to air. Some of the results obtained are given in Table IV. Clearly catalyst B in its hi conversion and selectivity to acetylene demonstrates mique properties. 

It is estimated that the heat produced at the entrance of the catalyst bed is reduced by approximately 80% when the present Ru/Ti07 catalyst is employed, due to suppression of combustion reactions. It should be stated that the quantity of heat produced depends on conversion and selectivity. Thus, regardless of the reaction scheme which is followed, if high... 

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