Gas-phase reaction with molar change

VARMOL - Gas-Phase Reaction with Molar Change... 

In the two preceding examples there was no volume change with reaction consequently, we could use concentration as our dependent variable. We now consider a gas-phase reaction with volume change taibng place in a PFR, Under these conditions, we must use the molar flow rates as our dependent variables. 

Volume variations with conversion are large for constant-pressure gas-phase reactions with change in mole number. Here, as a rule, operation at constant volume poses no difficulties. Liquid-phase reactions may also entail volume contraction or expansion. However, these are not related to changes in mole number and can be predicted only if information on partial molar volumes is at hand. Because liquids are essentially incompressible, even at elevated temperature, it is unsafe to conduct liquid-phase reactions without a gas cap in a closed reactor. Some variation of liquid-phase volume with conversion therefore is apt to occur. Fortunately, the variation at constant temperature is usually so small that it can be neglected in the evaluation or accounted for by a minor correction. 

P3-21b The gas-phase reaction between cMotine and methane to form carbon tetrachloride and hydrochloric acid is to be carried out at 75 C and at 950 kPa in a continuous-flow reactor. The vapor pressure of carbon tetrachloiide at 75°C is approximately 95 kPa. Set up a stoichiometric table for this reaction with phase change. Calculate the conversion of methane at which condensation begins. Plot the concentrations and molar flow rates of each species as well as the total molar flow rate as a function of conversion for a stoichiometric feed, The volumetric flow rate is 0.4 dm /s. 

Analysis In this example, we formed a stoichiometric table in terms of molar flow rates. We then showed how to express the concentrations of each species in a gas phase reaction in which there is a change in the total number of moles. Next, we plotted each species concentration as a function of conversion and noted that the concentration of the inert. Ni, was not constant but increased with increasing conversion because of the decrease in the total molar flow rate, Fj, with conversion. 

We now use a stoichiometric table to relate C and C,- to x . Since this gas-phase reaction takes place at constant pressure and temperature, with no change in the number of moles, the mass density is constant. Therefore, the stoichiometric table (Table 4-17) can be constructed directly in terms of concentrations, instead of starting with molar fl ow rates. 

Kinetic data on olefin polymerization by polymer-immobilized zirconocene are scarce. It is generally accepted that homogeneous metallocene catalysts contain uniform active sites however, if they are immobilized on a polymer support, the MWD polymer production becomes broader compared with a homogeneous catalyst [103]. Kinetic analysis of gas-phase ethylene polymerization catalyzed by (CH3)2[Ind]2ZrCl2 bound at a hydroxylated copolymer of styrene with divinylbenzene and previously activated with MAO (0.17 wt.% Zr) has been carried out [104]. The influence of temperature (333 to 353 K), ethylene partial pressure (2 to 6 atm) and MAO level (molar ratio of MAO to zirconium from 2600 to 10,700) were studied. The activity of the catalyst in the gas-phase process changed from 5 to 32 kg PE (g of Zr atm h)It is possible that there are two types of active site. They are stable to temperature and deactivated by the same mechanism. A first-order reaction takes place. The propagation rate constants of two active sites show a similar dependence on temperature. 

Analytical solution of the mole balance equations is only likely to be possible when a number of simplifying assumptions can be made such as those adopted previously where we assumed a single irreversible first-order reaction, no change in molar flow due to reaction, isothermal reactor, negligible variation in pressure, plug flow of gas in the bubble phase, and either perfect mixing or plug flow in the dense phase (see Ref. [46]). Assumptions must also be made with respect to the respective... 

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